Most eligible patients with coronary heart disease and heart failure do not participate in cardiac rehabilitation. Covid-19 has exacerbated this, with a substantial drop in the number of patients participatingHome and telehealth based interventions are increasingly being used as alternatives to traditional centre based rehabilitation programmesOutcomes for patients participating in home based rehabilitation compare favourably with centre based programmes in terms of hospitalisations, quality of life, and costTelehealth based interventions are promising, but some patients may find these interventions challengingNovel ways of delivering rehabilitation have been employed during the covid-19 pandemic, including hybrid models that are likely to be offered as alternatives to centre based rehabilitation in future, enabling greater patient choice and greater uptake of cardiac rehabilitationBefore the covid-19 pandemic, 100 000 people were admitted to hospital with heart attacks and approximately 200 000 were diagnosed with heart failure annually in the UK.A 2020 European position paper, in keeping with other national and international guidelines,Box 1Post-acute coronary syndrome (ACS), post-primary coronary angioplasty, and coronary artery surgery:Patients with ACS Adults and adolescents with congenital heart diseaseAtrial fibrillationUnlike other international guidelines, evidence informing guidance from the National Institute for Health and Care Excellence is assessed based on Grading of Recommendations, Assessment, Development, and Evaluations (GRADE) criteria—the class/level approach is not used and therefore not referenced aboveBox 2Patient assessmentClinical historySymptomsPhysical examinationInvestigations: electrocardiogram, cardiac imaging, blood testsPhysical activity levelPeak exercise capacity—eg, bicycle ergometer, treadmill, validated walk testPhysical activity counsellingType and levelEducationBarriers to exerciseExercise for patients unable to engage in walking or cycling based activitiesExercise trainingIndividualised prescribingFrequency, intensity, time (duration), and type of exercise (FITT), or FITT related to mealtime (FITT +T)Diet/nutritional counsellingAssessmentEducationHealthy food choicesWeight control managementAssessmentEducationWeight reductionLipid managementAssessmentDiet, physical activity, and medicationBlood pressure managementAssessmentLifestyle intervention and medicationSmoking cessationSmoking status, including other tobacco productsEncouragement to stop smoking all tobaccoReferral for smoking cessationPsychosocial managementAssessment of psychosocial risk factorsReferral for behavioural and psychosocial interventionVocational reintegration/return to workEvaluation of programme resultsDetermination of success and failure of interventionsNew rehabilitative goalsCommunication regarding continuing careQuality assurance using systematic registrationStructured follow-upEven before the covid-19 pandemic, most patients in high and low to middle income countries were not offered cardiac rehabilitationThis clinical update describes pre-pandemic evidence for cardiac rehabilitation and considers how adoption of a broader range of evidence based delivery methods can improve uptake and patient outcomes during the pandemic and beyond.Historically, rates of referral to cardiac rehabilitation were suboptimal in the USMost patients referred for rehabilitation after a cardiac event were offered supervised, group based classes, which ranged in frequency, intensity, duration of exercise, and self-help guidance.Consequently, calls were made for alternatives to centre based cardiac rehabilitation.Box 3In 2017, WHO issued a statement committing to key actions to strengthen cardiac rehabilitation services and highlighted the evidence indicating the benefits of rehabilitation after acute myocardial infarction.The 2018 NICE guidance on chronic heart failure states that rehabilitation “should be provided in a format and setting  that is easily accessible for the person”The NHS Long Term Plan similarly underlines the importance of rehabilitation and has set an ambitious target of 85% of eligible patients being able to access care by 2028https://www.nice.org.uk/Media/Default/About/what-we-do/Into-practice/measuring-uptake/cvd-impact-report/nice-impact-cvd-management.pdfThe importance of cardiac rehabilitation and delivering cardiac rehabilitation during the covid-19 pandemic is discussed in a cardiovascular disease impact report by NICE in 2021. https://www.aacvpr.org/Learn/Learning-Center/Virtual-Rehab-Module-SeriesIn response to the covid-19 outbreak, the American Association of Cardiovascular and Pulmonary Rehabilitation has established the Innovative Delivery Model Collaborative to facilitate digital approaches and encourage home based delivery of rehabilitation. It offers webinars on virtual cardiac and pulmonary rehabilitation. A US scientific statement from 2019 noted that only a minority of eligible patients participate in rehabilitation and recommended home based interventions for low to moderate risk patients unable to access centre based programmesIn 2016, the Million Hearts Cardiac Rehabilitation Collaborative of 100 organisations developed a road map to increase participation in rehabilitation from 20% to 70% by 2022A 2020 European position paper focuses on centre based programmes but does not comprehensively consider home based interventionsA paper from the European Association of Preventive Cardiology on recent cardiac tele-rehabilitation studies provides a practical guide for the setup of rehabilitation services during the covid-19 pandemicThe emergence of covid-19 has prompted calls for accelerated introduction of alternative methods of delivery that include home based and tele-rehabilitation optionsBox 4Referral to cardiac rehabilitation can be increased through automatic referral at discharge, which can be enhanced by informing the patient of the benefits of the interventionEngagement with cardiac rehabilitation is determined by service level and patient level factors—for instance, age, sex, ethnicity, level of deprivation, timing of rehabilitation, and mechanism of referralEconomic evaluations of data from the British Heart Foundation National Audit of Cardiac Rehabilitation confirm uptake rates of 37.6% and 51.7% for the most and least deprived populations, respectivelyPatients who are given a firm date to attend their initial rehabilitation assessment are more than four times more likely to engage in rehabilitation programmesCovid-19 has exacerbated health inequalities, with poor outcomes in marginalised populations, and has prompted calls to establish equitable care models “that build culturally appropriate communication and outreach practices to communicate with patients beyond traditional phone calls and office visits”Lower rates of digital literacy and access have also been reported in other marginalised groups during the pandemic.The number of patients with heart failure in the UK participating in rehabilitation decreased from 4969 (<10% of eligible patients) before the pandemic (May 2019-January 2020) to 1474 (<5% of eligible patients) during the first wave (February-August 2020).Indeed, although overall uptake of cardiac rehabilitation has decreased, the proportion of patients receiving home based rehabilitation in the UK has increased more than threefold since the pandemic—from 22.2% to 72.4%Baseline assessment of exercise capacity is core to the effective and safe delivery of exercise interventions.Several publications from North America and Europe discuss how to adapt and use different baseline fitness tests to assess the relative risk of exercise.Box 5Make it easyUse tip sheets to help staff adjust to delivering care virtuallyAvoid becoming overwhelmed by the multitude of available resources by finding a single, comprehensive, verified online resource for patients and staffDon’t waitEncourage patients to attend at minimum intake assessments to discuss the merits of virtual cardiac rehabilitationFollow a shared decision making process for enrolment in virtual rehabilitation, to ensure patients understand potential risks and benefits of participating virtually versus choosing to delay careFocus on core components*Consider lifestyle risk management, psychosocial support, medical advice, educationProvide simple exercise prescriptions aimed at encouraging low-to-moderate physical activityObtain patient metricsExamples include a self-administered six minute walk test for exercise capacity, using patients’ personal scales and blood pressure cuffsOffer group sessionsReduce “labour intensive” one-to-one sessions when possible by providing group tele-/video-conferencing for educational sessions and patient supportEvaluateFormalise an evaluation process to assess the merits and efficacy of virtual careInvest in accessFor rural and/or under-resourced areas, consider purchasing tablets, smartphones, or other electronic options for loan to participants to enhance a one-to-one personal experience*Moulson et al suggest that it is possible to complete assessment of the core components digitally/virtually, including an exercise test, albeit with the limitationsThe UK, Australia, and Canada have been key adopters of home based cardiac rehabilitation.Box 6Reduced enrolment delaysExpanded capacity/accessIndividually tailored programmesFlexible, convenient schedulingMinimal travel/transportation barriersGreater privacy while receiving cardiac rehabilitationIntegration with regular home routineLack of reimbursement*Less intensive exercise trainingLess social supportLess patient accountabilityLack of published standards for home based cardiac rehabilitationLess face-to-face monitoringSafety concerns for high risk patients*Lack of reimbursement is an issue in some countries; however, the Centers for Medicare and Medicaid Services in the US introduced reimbursements for virtual cardiac rehabilitation in 2020The standards and core components  used in A meta-analysis of individual participant data and the updated Cochrane review showed that participation in home or centre based cardiac rehabilitation after heart failure resulted in a clinically meaningful improvement in health related quality of life, determined using the Minnesota Living with Heart Failure questionnaire score (7.1 (95% CI -3.7 to -10.5)), compared with no rehabilitation.Improvements in modifiable cardiovascular risk factors with cardiac rehabilitation are documented in a US scientific statement.Adherence to home based cardiac rehabilitation is comparable with that in centre based programmes; however, considerable variation is seen in reporting of adherence, and the US scientific statement noted limited ability to pool data to compare home and centre based programmes.A Cochrane review concluded that clinical benefits with centre and home based cardiac rehabilitation in patients with heart failure, myocardial infarction, and coronary revascularisation are similar and with equivalent costs.Box 7A systematic review that examined economic evaluations in cardiac rehabilitation published since 2001 concluded that cardiac rehabilitation was cost effective compared with usual careIncremental cost effectiveness ratios (ICERs) ranged from $1065 to $71 755  per quality adjusted life year (QALY), although no significant differences in costs and QALYs were reported in two studies that directly compared home and centre based rehabilitationTele-rehabilitation was used in four included studies and was considered to be cost effective in all, but with wide ranging ICERs  to $588 734 per QALY)The authors called for larger studies to strengthen the evidence base, observing a potential reduction in costs if tele-rehabilitation were more widely adopted and noting that patient adherence to digital technologies is variableHealth economic modelling based on a recent multicentre trial in patients with heart failure concluded that home based cardiac rehabilitation can be cost effective in the health service setting in the UKA facilitated home based cardiac rehabilitation intervention for people with heart failure and their caregivers: a research programme including the REACH-HF RCTTele-rehabilitation interventions are promising and are replacing centre based programmes as part of home based approaches during the pandemic.A systematic review and meta-analysis that included 30 telehealth trials of secondary prevention in patients with coronary heart disease reported statistically significant lower re-hospitalisation and cardiac events (RR 0.56 (95% CI 0.39 to 0.81), P<0.0001) in the intervention groups. The authors concluded that telehealth interventions could be offered to patients unable to attend centre based rehabilitation.Another meta-analysis, which reviewed 11 trials n=1189), reported that telehealth interventions were at least as effective as centre based programmes for improving modifiable cardiovascular risk factors and exercise capacity., reportePoor rates of participation in conventional cardiac rehabilitation programmes led to the development of home based and digitally delivered interventions, which are supported by emerging evidence. Covid-19 has provided an opportunity “to reimagine how cardiac rehabilitation is delivered.”How well do you know the local pathways to refer patients for home based cardiac rehabilitation after a heart attack or new diagnosis of heart failure?In patients with a new diagnosis of heart failure, how many have been referred to and participated in a cardiac rehabilitation programme in the past 12 months?The National Institutes for Health Research (NIHR) Health Technology Assessment (HTA) Programme is funding a study based in the UK: A randomised controlled trial of a facilitated home based rehabilitation intervention in patients with heart failure with preserved ejection fraction and their caregivers: the REACH-HFpEF Study (2021-2024)The Western Norway Health Authority has funded a study of 3000 patients offered cardiac rehabilitation after a percutaneous coronary intervention,The Improving ATTENDance in Cardiac Rehabilitation Trial (iATTEND) (2019-23)What is the impact ofhome based services in more diverse and higher risk groups of patients?hybrid models of cardiac rehabilitation, including components from both centre and home based settings?What are the barriers for using digital health technologies including wearable monitoring devices for cardiac rehabilitation in terms of factors that arepatient related?clinician related?legal and ethical issues?interoperability and technical issues?lack of reimbursement?What study designs should be used to evaluate multimorbidity cardiac rehabilitation programmes?What is the feasibility of delivering cardiac rehabilitation through primary care networks? Can we improve inequality in the uptake of cardiac rehabilitation by culturally adapting programmes for ethnic minority groups?Cindy Edgeler is married to Chris, a retired roadman with Cornwall Highways, who was admitted to hospital with a heart attack in 2012 (Box “A carer’s story by Cindy Edgeler”). Chris was a patient of coauthor HD, and both Cindy and Chris were members of the Patient and Public Involvement Group that was part of the Rehabilitation Enablement in CHronic Heart Failure (REACH-HF) National Institute for Health Research  funded clinical trial. Cindy and Chris reviewed a draft version of the clinical review. https://www.nice.org.uk/sharedlearning/delivering-rehabilitation-enablement-in-chronic-heart-failure-reach-hf-in-wirralWe also received feedback from patients supported by the Wirral Community Cardiology Service, who have been delivering REACH-HF since 2019 and throughout the COVID-19 outbreak. BMJ clinical review, which is available at: https://soundcloud.com/bmjpodcasts/cardiac-rehab-patient.Our patient coauthor KP has been a member of the REACH-HF study group for several years, having had a heart attack in 2008. KP reviewed and provided input to this manuscript and talks about his experience of receiving CR in a podcast from our 2015 Our patient advisers acknowledged the usefulness of the digital approach to cardiac rehabilitation, especially for those who struggle to get to centre based sessions. But they also highlighted the need for a more hybrid approach so that rehabilitation is as accessible as possible. The final manuscript was modified to reflect the patients’ views.My husband had a heart attack in 2012, which also left him with heart failure. After having stents fitted on the day he was admitted to hospital, he was well looked after on the critical care ward, and then came home.Although he received good aftercare, I felt that I had no support and was left to my own devices. I had many questions, worries, and fears. I would lie next to him at night while he slept and wonder if each breath would be his last. He would say: “Stop asking me if I'm alright.”There was no one I felt I could talk to, and I needed reassurance.A few months later, we were offered the Rehabilitation Enablement in CHronic Heart Failure (REACH-HF) home based cardiac rehabilitation programme and were presented with the Heart Failure Manual and Family & Friends Resource for Carers, which importantly linked me to a nurse facilitator. From the first meeting there was a stark contrast with our previous experience, in that it was not just about the care of my husband but also about my own wellbeing. The facilitator answered all of my questions, and I knew I had a point of call should anything give me cause for concern.I recall that I was concerned about my husband’s breathlessness on one home visit, and the nurse facilitator explained by giving me an analogy of: “You wouldn’t expect your car to pull away in fourth gear, you start in first and move slowly through the gears.” This made complete sense to me then.My husband has other comorbidities that affect his walking and general fitness, and this is why the home based exercises are so much better for our situation. We do the chair based exercise DVD together; we are able to pace ourselves and decide which level we start at. We continue with weekly weigh-ins to ensure we keep within healthy limits, and we found the progress tracker helped to monitor progress.The patient manual itself was a tool that we used—and still use—together. There is a separate manual/resource just for caregivers, which I find very useful.http://thebmjawards.bmj.com/showcase/.A video featuring Cindy and Chris Edgeler can be viewed at www.heart.org/en/health-topics/cardiac-rehab/what-is-cardiac-rehabilitationAmerican Heart Association: what is cardiac rehabilitation? www.heart.org/en/news/2019/05/13/experts-urge-expansion-of-home-based-cardiac-rehabilitationAdditional information on home based cardiac rehabilitation. www.henryford.com/services/cardiology/support/cardiac-rehab/home-based-cardiac-rehabilitationA telehealth cardiac rehabilitation programme for patients using a free app to use on a mobile phone/device. www.bhf.org.uk/informationsupport/heart-matters-magazine/activity/10-minute-workoutBritish Heart Foundation: online exercise videos. www.healtheuniversity.ca/en/CardiacCollegeOnline patient education sessions available in six languages. https://services.nhslothian.scot/TheHeartManual/Pages/default.aspxA choice of a book or digital format of a home based programme for patients recovering from a heart attack or revascularisation. www.heartandstroke.ca/what-we-do/webinars/cardiac-rehab-during-covid-19A webinar on virtual rehabilitation and self-management techniques during covid-19 for people living with heart conditions. Heart Manual, which requires registrationAll resources are open access with no registration, except the digital format of the We focused on new evidence on home based and telehealth based cardiac rehabilitation that would be of interest to patients, clinicians, and commissioners based on recent publications on the impact of covid-19 and our 2015 clinical review that was co-authored by HD, RST, and PD. RST is a contributor and editor for the Cochrane Heart Group and has led and conducted several systematic reviews of cardiac rehabilitation. These Cochrane reviews have been cited in various national guidelines, which we consulted when writing this article. We referred to annual reports of the National Audit of Cardiac Rehabilitation, which is led by PD, and the BHF website for statistics on coronary heart disease in the UK. We also consulted recent national statements and standards from the UK, US, and Europe, and HD used his personal archive of references.


Coronavirus disease 2019 (COVID-19) has become the fastest-spreading pandemic of the 21st century. Various vaccines have been made available via emergency use authorization. Currently, two mRNA vaccines are being offered internationally, BNT162b2 and mRNA-1273. In randomized trials of these vaccines, the incidence of Bell’s palsy in the vaccinated group does not statistically exceed the placebo group. The FDA recommends increased surveillance for Bell’s palsy as a potential side effect with the administration of the vaccines among larger populations globally. There have been a few case reports of Bell’s palsy associated with mRNA vaccines. Type I interferons have been proposed as the potential mechanism linking mRNA COVID-19 vaccines to Bell’s palsy. Here, we report the case of a 36-year-old previously healthy patient who developed symptoms of Bell’s palsy along with left-arm numbness, tingling, and subjective weakness masquerading as a subacute stroke after receiving the second dose of the mRNA-1273 vaccine. CT and MRI of the brain were unremarkable. He was discharged home with a diagnosis of Bell’s palsy and improved on follow-up. mRNA COVID-19 vaccines may be considered a risk factor for Bell’s palsy. Coronavirus disease 2019 (COVID-19) has become the fastest-spreading pandemic of the 21st century. Globally, the number of cases reached up to 175 million in June 2021 . SpecifiA few case reports of Bell’s palsy associated with mRNA vaccines have been published after the initial randomized trials. According to our literature search, until June 2021, there is only one reported case of Bell’s palsy after receiving the first dose of the mRNA-1273 vaccine . Here, wA 36-year-old gentleman came to our emergency department (ED) complaining of weakness over the left side of his face. He had received the second dose of the mRNA-1273 severe acute respiratory syndrome coronavirus 2 vaccine one day before the onset of his symptoms in his left deltoid muscle. His symptoms had been ongoing and progressive for the past three days. He mentioned slight difficulty in speaking and eating. He also had mild numbness and tingling of the left arm along with subjective weakness of the left upper limb. He had never experienced such symptoms earlier. There was no prior history of any medical illnesses. No preceding fever or upper respiratory symptoms were present. He denied ear pain, increased sound sensitivity, or a change in taste sensation.The patient was alert and oriented at the time of examination. He was speaking in full sentences and no slurring of speech was noted. Initial vitals were a heart rate of 66 beats per minute, a temperature of 37°C, maintaining oxygen saturation of 97% on room air, and blood pressure of 120/90 mmHg. Cranial nerve examination showed isolated VII nerve palsy of lower motor neuron type. The patient was unable to close his left eye completely. There was flattening of wrinkle lines above the left eyebrow and drooping of the left corner of the mouth. There was no objective weakness in all four limbs. He had slightly decreased sensations on his left upper limb which spared the lower limbs. His ear examination was normal bilaterally with no vesicles or scabbing and no parotid swelling in front of the ears. The rest of his physical examination was unremarkable.A neurology consult was obtained in the ED due to his left upper limb numbness and weakness along with facial palsy to rule out brainstem stroke. CT and MRI of the brain were done which were unremarkable Figures , 2. A diBell’s palsy, also known as idiopathic facial nerve paralysis, is considered to be the most common cause of acute facial paralysis of spontaneous origin. Potential contributors to the development of Bell’s palsy include immune, infective, and ischemic mechanisms; however, the exact cause remains unclear. Reactivation of herpes simplex virus infection centered around the geniculate ganglion is suggested as a possible cause .Escherichia coli toxin found in the vaccine with the facial nerve. Adverse reports of Bell’s palsy have also been published after the administration of the meningococcal conjugate vaccine [The occurrence of Bell’s palsy has been linked to vaccine administration in the past. A matched case-control study and case series in 2000-2001 found an increased incidence of Bell’s palsy among the receivers of intranasal inactivated influenza vaccine (with an odds ratio of 84%). This phenomenon was thought to be due to the interaction of heat-labile  vaccine .In the published data regarding the mRNA-1273 vaccine, among the 30,351 volunteers who participated in the trial, three cases of Bell’s palsy were reported in the vaccinated group on days 22, 28, and 32 days after vaccination. One incident occurred in the placebo group 17 days after vaccination. The trial concluded that the slight excess of Bell’s palsy in this trial and the BNT162b2 vaccine trial raised a concern that it may be more than a chance event, and this possibility bears close monitoring [Other COVID-19 vaccine trials have also reported Bell’s palsy as an adverse event. The single-dose Ad26.COV2.S vaccine used recombinant, replication-incompetent human adenovirus type 26 (Ad26) vector. In a clinical trial of the Ad26.COV2.S vaccine, three cases of Bell’s palsy were reported in the vaccine group compared with two cases in the placebo group. The observed frequency of Bell’s palsy in the vaccine and control group was consistent with the expected background rate in the general population .Soeiro et al. proposed type I interferons as the potential mechanism linking mRNA COVID-19 vaccines to Bell’s palsy. Therefore, the mRNA COVID-19 vaccine should be considered as an additional risk factor for Bell's palsy .Data from vaccine trials of mRNA covid vaccines, BNT162b2 and mRNA-1273, suggest that there is an increased propensity of Bell’s palsy cases in the vaccinated group. However, the causal relationship between the mRNA vaccine and Bell’s palsy development needs to be further investigated. Our case highlights the importance of vaccine history in patients presenting to the emergency department with Bell’s palsy. COVID-19 mRNA vaccines can be considered as an additional possible risk factor in the etiology of Bell’s palsy.


The role of endocrine disruptors (EDs) in the human prostate gland is an overlooked issue even though the prostate is essential for male fertility. From experimental models, it is known that EDs can influence several molecular mechanisms involved in prostate homeostasis and diseases, including prostate cancer (PCa), one of the most common cancers in the male, whose onset and progression is characterized by the deregulation of several cellular pathways including androgen receptor (AR) signaling. The prostate gland essentiality relies on its function to produce and secrete the prostatic fluid, a component of the seminal fluid, needed to keep alive and functional sperms upon ejaculation. In physiological condition, in the prostate epithelium the more-active androgen, the 5α-dihydrotestosterone (DHT), formed from testosterone (T) by the 5α-reductase enzyme (SRD5A), binds to AR and, upon homodimerization and nuclear translocation, recognizes the promoter of target genes modulating them. In pathological conditions, AR mutations and/or less specific AR binding by ligands modulate differently targeted genes leading to an altered regulation of cell proliferation and triggering PCa onset and development. EDs acting on the AR-dependent signaling within the prostate gland can contribute to the PCa onset and to exacerbating its development. The prostate is the main accessory gland of the male reproductive system, which plays an essential role in male reproduction. The prostatic fluid, secreted by the epithelium of the gland, contains essential molecules contributing not only to the prostate functionality but also to the steps associated with ejaculation and hence to male fertility, such as sperm motility and capacitation ,2. AmongPCa is a clinically heterogeneous disease that may be either aggressive, with a clinical progression to metastasis, or asymptomatic, with an indolent course ,5. PCa iCommonly, the diagnosis of PCa is made based on elevated plasmatic blood levels of PSA (>4 ng/mL). The androgenic hormones regulate normal prostate gland growth and function by interacting with the androgen receptor (AR), whose gene expression in PCa appears to be deregulated; this plays a central role in development and metastatic progression ,8. PCa iIn PCa, besides the nuclear AR, four distinct proteins play a role in the PCa growth. This includes the transient receptor potential melastatin 8 (TRPM8), the oxoeicosanoid receptor 1 (OXER1), the zinc transporter ZIP9/SLC39A , and the G-protein-coupled receptor family C group 6 member A (GPRC6A). These proteins have been proposed as alternative membrane androgen receptors (mARs) that could be involved in the survival of PCa cells ,14.Moreover, in addition to androgens, estrogenic hormones are also involved in PCa development. Indeed, the use of anti-estrogens has been recognized to have a therapeutic role in PCa management ,16. BothThe established risk factors for PCa are mainly the age and the race of the individuals. For example, African Americans are twice as likely as European individuals to develop or die from PCa . Other fThere are several in vitro studies in human prostate epithelial cells and in vivo studies in animal models as well as epidemiologic studies that indicate a possible association between EDs and PCa risk ,15,26,27Sex hormones are derived from cholesterol and belong to a class of chemical compounds known as steroid hormones. These classes of hormones are very important for male sexual development, reproduction and health and metabolic homeostasis. This is especially true for androgens . The majAbout 95% of T is produced and secreted by Leydig cells in the testis, and the remaining 5% is produced in adrenal glands by conversion of precursors  . T is covasa deferentia, seminal vesicles and ejaculatory ducts, whereas DHT is the active ligand in several other androgen-target tissues such as the prostate gland, scrotum, urethra, and penis .,37.N-terThe classical nuclear (or genomic) mechanism of the androgen action presents the inactive AR protein resides in the cytoplasm associated with a chaperone complex . Upon hormone binding to the AR with high affinity at the level of the LBD, the receptor–chaperone complex rearranges and the AR undergoes conformational modifications. Subsequently, AR translocation to the nucleus and its homodimerization allows the ligand-bound receptor to recognize target genes, such as PSA and transmembrane protease serine 2 (TMPRSS2), via specific promoter and enhancer sequences, known as androgen response elements (AREs) ,37. Targ2+ concentration, and cyclic adenosine monophosphate (cAMP) levels [In addition, DHT and T, similarly to some other steroid hormones, also present a non-genomic signaling/action, with a rapid response (seconds to minutes), the AR localized on the lipid membrane rafts microdomain, leading to extra-nuclear mechanisms that can directly involve extracellular signal-regulated kinase (ERK), the phosphatidyl-inositol 3-kinase (PI3K)/Akt pathway, G-protein coupled receptors (GPCRs), intracellular Ca) levels ,43.The sequence comparison between AR and ERs shows a similar sequence for the palmitoylation, a post-translational modification, present in both types of sex steroid receptors. The palmitoylation of a conserved motif in the ligand-binding domain is critical for the membrane localization of both ERs and AR ,44. The T877A located in the AR-LBD. This mutation consists of the substitution of an alanine (Ala) with a threonine (Thr), resulting in the loss of specificity for the agonist. Therefore, the mutated AR might be activated not only by androgens but also by other steroid hormones, such as progesterone, estrogens, as well as by DHT metabolic products, or antiandrogen compounds, such as flutamide (FTA) [AR deregulation plays a central role in the onset and progression of PCa to an advanced metastatic disease ,8. The mde (FTA) ,48,49,50R725L) [F876L, lead to a change in the LBD domain conferring resistance to enzalutamide (a well-known substance that inhibits the AR), whereas several other mutations increase the binding capacity of AR with co-regulators, thus increasing AR-transcriptional activity  [Specific AR germline polymorphisms have been associated with an increased risk of developing PCa, up to six-fold higher than the general population  ,52. Othetations) ,52. AR gtations) ,52.A specific group of AR mutations, the AR splice variants (AR-Vs), have been identified in PCa, even if many of them have also been identified in non-cancerous tissues. Indeed, these AR isoforms are naturally occurring splice variants encoded by alternative AR transcripts. They derived from cryptic exon downstream regulation of the sequence that codes for the DBD, which presents premature stop codons: most translated AR proteins retain NTD and DBD, but lack the LBD being constitutively active ,53. AR-VIn recent years, it has been demonstrated that androgens can modulate several cellular processes that are important for cell growth and that in PCa progression and metastasis certain receptors localized at the plasma membrane (membrane AR or mARs) and/or impinging on Wnt-pathway are differentially expressed in comparison to healthy prostate tissues ,49,50,56Four distinct proteins  localized at the plasma membrane of prostate epithelial cells have been proposed as mARs . TRPM8, 2+-selective cation androgen-regulated channel. TRPM8 is expressed throughout the male urogenital tract and in other different tissues such as the peripheral nervous system [TRPM8, a member of the melastatin-related transient receptor potential family, is a Cas system ,59,60,61s system ,14,59,62s system ,63. TRPMs system . TRPM8 is system . Loss ofs system .OXER1 is a G-protein coupled receptor usually activated by 5-lipoxygenase metabolites of arachidonic acid, 5-oxoeicosatretraenoic acid (5-oxo-ETE) and other 5-eicosatretraenoic acid derivatives. OXER1 is expressed in several human tissues, including PC cells ,14, and GPRC6A is also a G-protein coupled receptor belonging to the class C group 6 subtype A expressed in humans ,68. GPRC2+ transporter and as a mAR: this dual role is exerted through G proteins involved in apoptotic pathways activated by androgens [2+, where it led to high Zn2+ concentration-mediated apoptosis [ZIP9/SLC39A9 is a zinc-importing protein belonging to a 14-member family regulating zinc transport from the extracellular space to the cytoplasm ,14. ZIP9ndrogens ,14,70. Zpoptosis . Finallypoptosis .Recent studies have shown that dysregulation of the Wnt signaling pathway plays an important role in PCa progression ,75. Wnt Epidemiologic studies have linked farming to an increased PCa risk ,90,91. IHuman exposure to pesticides may occur through occupational exposure in the case of agricultural workers or through non-occupational pathways for residents living close to agricultural lands and for bystanders. The same type of direct or indirect exposure occurs with those who work with biocidal products. Instead, the exposure of the general population to pesticides occurs mainly through diet either eating food or drinking water contaminated with pesticides. Non-occupational exposure originating from pesticide residues (MRLs) in food, air, and drinking water generally involves lower doses. This type of exposure can be considered chronic (or semi-chronic). Pesticides/biocides have been defined as substances or mixtures of substances intended for controlling, preventing, destroying, repelling, or attracting any biological organism deemed to be a pest. There are substances that can be considered both pesticides and biocides but in general the use of pesticides is specific to crops or plants overall, whereas biocides can be used in all other fields. Insecticides, herbicides, defoliants, desiccants, fungicides, nematocides, avicides, rodenticides, and hospital disinfectant  are some of the many types of existing pesticides and biocides . PlasticBelow, some chemicals of interest with known endocrine interference activity on the prostate gland representative of pesticides, biocides and plasticizers will be discussed. According to an Italian report on the sales and use of fungicide and neonicotinoids, vinclozolin (VIN) and imidacloprid (IMI) were selected based on their high use . Among pVinclozolin (VIN) is a dicarboxymide fungicide widely used on fruit and vegetables. It has been demonstrated that it acts as an AR antagonist in vitro and/or in vivo leading to an interference with the action of androgens in developing, pubertal, and adult male rats ,101,102.Cypermethrin (CYPE) is a pyrethroid broad spectrum commonly used as insecticide. Consumption of food containing pyrethroid residues, particularly vegetables and fruit, is the primary route of exposure . This suImidacloprid (IMI) is a neonicotinoid widely used due to its selective poisonousness to insects . IMI wasDi(2-ethylhexyl) (DEHP) and dibutyl phthalate (DBP) are commonly used as plasticizers. Plasticizers are particular substances that increase the plasticity or viscosity of a material and are commonly used in food packaging and in medical devices . In in vAlthough the phthalate DEHP does not bind to AR, in utero exposure to phthalates disrupts the differentiation program of androgen-dependent tissues in male rat offspring ,133,134.pi–pi interactions. This binding energy of DEHP with AR has been shown to be similar to that of native ligand T, and the amino-acid residue interactions of DEHP metabolites had 91–100% similarity compared to that of T [An interesting recent in silico docking study has characterized the bond of DEHP and its metabolites with AR to predict the potential endocrine-disrupting effects of these metabolites in AR signaling . The dathat of T . Therefohat of T .−8 M). In the same study, the gene expression of ER and AR after treatment with DBP was evaluated, and a reduced expression of ERα was demonstrated, which has been shown to be involved in cell proliferation and carcinogenesis of the prostate [In a recent in vitro study , both DEprostate ,140.Bisphenol A (BPA) is a synthetic polymer compound used predominantly in the production of products made of polycarbonate plastic and epoxy resins that line food and beverage cans , and sigSince July 2017, the European Commission has indicated that further attention should be given to pesticides/biocides or plasticizer substances for which evaluation or re-evaluation is ongoing or for which confirmatory data have been requested. Too little is still known about many substances, but the potential associations between PCa risk in humans and EDs exposure cannot be underestimated.


Androgen deprivation therapy targeting the androgens/androgen receptor (AR) signaling continues to be the mainstay treatment of advanced-stage prostate cancer. The use of second-generation antiandrogens, such as abiraterone acetate and enzalutamide, has improved the survival of prostate cancer patients; however, a majority of these patients progress to castration-resistant prostate cancer (CRPC). The mechanisms of resistance to antiandrogen treatments are complex, including specific mutations, alternative splicing, and amplification of oncogenic proteins resulting in dysregulation of various signaling pathways. In this review, we focus on the major mechanisms of acquired resistance to second generation antiandrogens, including AR-dependent and AR-independent resistance mechanisms as well as other resistance mechanisms leading to CRPC emergence. Evolving knowledge of resistance mechanisms to AR targeted treatments will lead to additional research on designing more effective therapies for advanced-stage prostate cancer. According to an estimate by the American Cancer Society, approximately 191,930 new cases of prostate cancer will be diagnosed and 33,330 deaths will occur from this disease in 2020. The majority of deaths from prostate cancer is due to advanced-stage metastatic spread dependent on the androgen receptor (AR). Because of the crucial role of AR in the development and progression of prostate cancer, androgen deprivation therapy (ADT) is the standard-of-care therapy for relapsed or metastatic patients attained by either surgery or medical castration with luteinizing hormone-releasing hormone (LHRH) agonists or antagonists. The first-generation antiandrogens such as hydroxyflutamide and bicalutamide have been used over many years for the treatment of prostate cancer. Most tumors initially shrink in response to ADT, but eventually emerges as castration-resistant prostate cancer (CRPC) within 18-24 months. Detailed studies on CRPC have shown their dependency on the AR signaling axis despite systemic depletion of androgens by various mechanisms. Upon progression, CRPC eventually metastasizes to the bone and later to other distant organs. Patients with metastatic CRPC (mCRPC) exhibit poor prognosis and low predicted overall survival of less than 2 years from the initial time of progression; such patients account for a large portion of the prostate cancer-related deaths. Two novel agents, abiraterone acetate and enzalutamide, were developed as second-generation AR axis-targets agents that offer more effective inhibition of the AR pathway. Understanding the mechanisms of resistance to the second-generation antiandrogens is the key to develop more effective therapies. In this review, we will discuss the current knowledge regarding the pathways leading to castration resistance and the mechanisms of resistance against these agents.Prostate cancer remains the most commonly diagnosed malignancy and the second leading cause of cancer related deaths among men in the United States,6. AR is a nuclear ligand-activated transcriptional factor, a member of the steroid hormone nuclear receptor family, whose coding gene is located on the X chromosome at the locus Xq11-Xq12. AR consists of three main functional domains, the N-terminal transactivation domain (NTD) comprising of exon 1, a central DNA-binding domain (DBD) constitutes exons 2-3, and a C-terminal ligand-binding domain (LBD) spanning between exons 4-8 [Figure 1A and B].AR play an essential role in the development and function of a normal prostate gland in men. Aberrant AR expression is primarily involved in prostate carcinogenesis that are AR sensitive, but is also engaged in the metastatic growth of CRPC, in fact, the androgen axis continues to play an important role in the function and growth of CRPC. The initial N-C interaction occurs in the cytoplasm. AR binds with androgens testosterone and dihydrotestosterone. Of note, adrenal androgens have lower binding affinity with the AR. Upon binding of AR with androgens, the complex acts as a transcriptional activator. In the absence of androgens, heat shock protein (HSP) binds to AR, therefore, AR remains inactive in the cytoplasm. Binding of androgens with AR induces a conformational change, resulting in the dissociation of HSP from AR, and the AR dimer translocate to the nucleus where it binds to androgen responsive elements of genomic DNA and regulate several target genes involved in growth and proliferation [Figure 1C],10.The presence of a hinge region between the DBD and LBD is involved in nuclear localization and degradation. The N-terminus has a unique LxxLL-like motif, which binds to a hydrophobic cleft of the C-terminus generated by ligand binding to the receptor. This binding stabilizes the ligand binding caused by physical interactions between the N-terminal and C-terminal of the receptor-14. Hence, research on prostate cancer treatments have targeted agents involved in the androgen receptor signaling axis. Androgen deprivation therapy (ADT), which inhibits transcriptional activity of AR, is utilized in the treatment of advanced prostate cancer. The therapeutic strategies involved in ADT strive to reduce levels of circulating androgen by surgical or chemical castration, and use antiandrogen to prevent androgen from binding to the AR. Circulating androgen levels are reduced by more than 90% within 24 h of surgical castration. Chemical castration could be performed by using luteinizing hormone-releasing hormone (LHRH) agonist such as leuprolide acetate and goserelin acetate or with LHRH antagonist including degarelix. The first-generation nonsteroidal antiandrogens that are in clinical use include flutamide, nilutamide, and bicalutamide. Over past several years, these LHRH agonists and antiandrogens have been in the clinical use for the treatment of advanced-stage prostate cancer; however, this therapy is palliative and not curative. Various types of antiandrogens have been developed in past decades , bicalutamide binds with AR ligand binding site, and flutamide compete with DHT for AR and inhibit the translocation of AR into nucleus, preventing AR-based downstream signaling,18. Abiraterone acetate is a CYP17 enzyme inhibitor that inhibit the biosynthesis of testosterone. The search for more effective therapies to block the transcriptional activity of AR remains the focus, of which the antiandrogen enzalutamide and the CYP17 enzyme inhibitor abiraterone acetate are discussed in detail.Several reports suggest that both AR responsive or refractory prostate cancer harbor increased AR expression due to genetic amplification of AR genes and AR enhancer elements. These enzymes are also required for the synthesis of steroids including progesterone, glucocorticoids, mineralocorticoids, and estrogens. The major function of CYP17A1 enzyme is to convert pregnenolone to dehydroepiandrosterone. CYP17A1 inhibition ultimately reduces the number of circulating androgens available to activate the AR. Loss of CYP17A1 activity results in significant loss of androgen production, specifically in the peripheral organs, and loss of adrenal androgen production in particular. Inhibition of CYP17A1 results in higher levels of urinary metabolite 3α5α-17HP which is correlated with the excretion of androsterone. Overexpression or genomic changes in CYP17A1 contributes to abiraterone acetate resistance. Using a xenograft mouse model, Chang et al. demonstrated that the HSD3B1 (1245C) mutation contributes to abiraterone acetate-resistant progression to CRPC, though the clinical significance is not completely elucidated. Abiraterone acetate has been shown to be 10-30 times more effective than ketoconazole, a non-specific inhibitor of p450 enzymes used to rapidly reduce androgen production. Abiraterone acetate received FDA approval for the treatment of CRPC through COU-AA-301 and COU-AA-302 phase III clinical trials,27. The double-blinded, placebo-controlled COU-AA-301 trial was conducted in chemotherapy-pretreated metastatic CRPC patients demonstrating 3.9-month median overall survival for patients treated with abiraterone acetate. Initially, it remains an effective and essential paradigm in the treatment of metastatic CRPC, but AR reactivation drives prostate cancer to lethal CRPC phenotype in all patients despite effective testosterone suppression.Abiraterone acetate (Zytiga®) is a molecule whose structure is similar to pregnenolone. Abiraterone acetate selectively and irreversibly blocks the production of intratumoral androgen biosynthesis by inhibiting cytochrome P450 17A1 (CYP17A1), a cytochrome p450 enzyme required for the production of androgens through both 17α-hydroxylase and C17, 20-lyase activity. Enzalutamide was designed to overcome the limitations of first-generation agents such as bicalutamide or flutamide. Enzalutamide exhibits three mechanisms of action: first, it prevents ligand binding and AR activation by binding to the AR LBD; second, it prevents AR translocation to the nucleus; and third, it inhibits the transcription of target genes by preventing binding of AR to DNA. FDA approval of enzalutamide is based on the AFFIRM and PREVAIL phase III clinical trials. AFFIRM was a double-blind, placebo-controlled trial that demonstrated a median overall survival of 4.8 months longer in patients with metastatic CRPC who were treated with enzalutamide. Additionally, PREVAIL was also a double-blind, placebo-controlled trial that revealed a 2.2 month longer median overall survival in chemotherapy-naïve metastatic CRPC patients treated with enzalutamide,29.Enzalutamide  is another multi-targeted second-generation AR inhibitor impeding testosterone binding to AR, AR nuclear translocation, AR binding to DNA, and co-activator recruitment,30. Additionally, progression to AR independent forms of prostate cancer such as neuroendocrine prostate cancer is frequently observed post-enzalutamide treatment. This highlights a need to pursue development of novel strategies to target molecular mechanisms of castrate resistance. Despite the initial response of enzalutamide and abiraterone acetate in CRPC patients, the secondary resistance mechanisms unavoidably result in clinical progression of the disease. Thus, it is important to understand the mechanisms and pathways of resistance towards antiandrogens for which several preclinical models have been developed in order to investigate the underlying mechanisms.Therapeutic agents in prostate cancer can develop resistance by primary or acquired mechanisms. Genetic changes, including amplification, mutation, or translocation of driver genes leading to AR splice variants and point mutations, are examples of processes causing enzalutamide or abiraterone acetate drug resistanceSPDEF, NKX3.1, ELF3, IRF5, FOXA1, and G-protein coupled receptors were downregulated in enzalutamide resistant cells, compared to those of the parental LNCaP cells Recent studies from our laboratory focused on the generation of antiandrogen resistant cell lines. For these studies, we treated androgen responsive human prostate cancer LNCaP cells with increasing concentrations of enzalutamide (1~20 μmol/L) by passage in media containing enzalutamide for six months. The resistant cells generated from 20 µmol/L enzalutamide were maintained in media containing 5 µmol/L enzalutamide and referred to as LNCaP-enzalutamide resistant cells. The LNCaP parental cells and LNCaP-enzalutamide resistant cells were subjected to RNA isolation followed by RNA-Seq analysis. The RNA-Seq data of LNCaP-enzalutamide resistant cells identified 4,578 upregulated and 4,184 downregulated transcripts. These differentially expressed genes were overrepresented by genes related to fatty acid oxidation, drug resistance signaling, drug metabolism, glucose and bile acid biosynthesis, lipid metabolism, fatty acid α/β oxidation, type II diabetes mellitus, and NF-κB signaling pathway. In contrast, signaling pathways such as cell cycle, Wnt signaling, and DNA repair pathways were downregulated. In particular, prolong suppression of AR by enzalutamide resulted in perturbations in the AR signaling pathway The study provides information on some new lead molecules altered during enzalutamide resistance in prostate cancer.. Development of resistance is mediated by the AR, which include AR amplification, AR overexpression, AR somatic point mutations, constitutively active AR splice variants, and altered intratumoral androgen biosynthesis.Abiraterone acetate and enzalutamide are drugs frequently prescribed for advanced prostate cancer; however, at least 20%-40% of patients develop primary resistance. Ultimately, patients who exhibit clinical or biochemical responses to treatment with these agents eventually develop secondary resistance through complex pathwayset al. demonstrated 3-fold increase in AR expression following treatment with abiraterone acetate in CRPC xenograft models. Another study that aims to probe the genomic landscape used liquid biopsies and circulating tumor DNA (ctDNA) obtained from prostate cancer patients and identified that AR amplification results in less responsive to antiandrogens. Approximately 50% of patients pretreated with either enzalutamide or orteronel (a CYP17A1 inhibitor) prior to abiraterone acetate treatment exhibited evidence of AR amplification, and only 13% of those with AR amplification demonstrated a response with > 50% PSA decline after being treated with abiraterone acetate. Approximately 5%-30% of mutations in AR were noted in circulating tumor cells and circulating tumor DNA of CRPC patients. Studies have shown that AR point mutations confer resistance to enzalutamide and abiraterone acetate. The most frequently reported somatic mutation F877L/F876L has been identified in patients treated with enzalutamide and apalutamide. Binding to the mutated AR at ARN-509, these drugs act as agonists rather than antagonists. Other mutations associated with poor response to enzalutamide are L702H, conferring acquired responsiveness to glucocorticoids, and T878A/T877A, resulting in progesterone-mediated activation of the AR. The W742C/L mutation is also reported to be responsible for the bicalutamide resistance-34. Apart from the mutations in LBD, more than 30 mutations have been reported in the other parts of the receptor. Buchanan et al. identified somatic mutations within the N-terminal polyglutamine tract of AR. In the NTD of AR gene, there is a polymorphic trinucleotide repeat region (CAG)n which encodes for the polyglutamine tract in AR protein. In healthy individuals the number of these repeats ranges 6 to 39 and variation in the number of these repeats is found associated with onset of prostate cancer-39. Interruption of the polyglutamine tract with two leucine residues (AR-polyQ2L) is reported to reduce the ligand induced N- and C-terminal interaction and result in the higher activity of receptor in comparison to wild type receptor. This high activity of AR receptor is attributed to the enhanced interaction with co-activators such as androgen receptor-associated protein 24,40. There are evidences that suggest that enhanced interaction with co-activators strengthens the AR signaling in androgen depleted environment and in presence of weak AR agonist. This consequently results in the failure of antiandrogen therapy.Clinical evidence demonstrates that AR amplification is important in developing resistance to antiandrogens. A study by Mostaghel . Prostate cancer progression occurs when AR-Vs lack the LBD and remain constitutively active, and their expression is amplified during ADT,44. AR-V7 is one of the most widely studied splice variant in prostate cancer research. A study conducted by Chen et al. reported the high expression of AR-V7 in CRPC patients. Another study by Antonarakis et al. reported the high proportion of AR-V7 in circulating tumor cells and found it to be associated with the enzalutamide and abiraterone acetate resistance. Proteins encoded by AR-V7 lack the LBD domain of AR which is directly targeted by both enzalutamide and abiraterone acetate. Furthermore, AR-V7 proteins remain constitutively active in a ligand independent manner which gives AR-V7 expressing cells a selective advantage in an androgen depleted environment. Another important AR-V reported in prostate cancer is ARv567e. It lacks exons 5, 6, and 7 which encode the LBD of AR. Interestingly, ARv567e is reported to induce oncogenesis autonomously. While expression of AR-V7 is reported both in benign and malignant prostate cancer tissues, ARv567e expression has been associated with only malignant prostate cancer tissues-48. Tagawa et al., with a cohort of 54 CRPC patients, reported the presence of AR-V7 splice variant in 36 patients (67%), ARv567e in 42 patients (78%), and the presence of both variants in 29 (54%) patients, but in 5 patients, both variants were absent. Previously, it was supposed that AR splice variants activate AR signaling independent of full-length AR, but it was found that ARv567e binds with full length AR to initiate ligand independent AR signaling which results in the cellular proliferation in the absence of androgen. Apart from these two major AR-Vs other splice variants have been detected in CRPC such as AR-V1, AR-V2, AR-V3, AR-V4, AR-V5, AR-V6, AR-V8, and up to AR-V14. AR-V1, AR-V4, and AR-V6 can dimerize with AR-V7 and AR-FL and therefore nuclear localization of these variants is induced by AR-V7 and AR-FL. Nuclear localization of these additional AR-Vs results in the enhanced activation of canonical AR targets and splice variant specific targets which consequently increases the severity of prostate cancer. AR-8 is another important splice variant highly expressed in CRPC cell lines such as C4-2, C4-2B, and CWR22Rv1. AR-8 has been found truncated at C-terminal and it does not possess DBD or LBD this it lack of trans-activating function. AR-8 remains attached to the plasma membrane via its two palmitoylated cysteine residues and associates with AR-FL and EGFR to mediate Src-induced AR activation.Androgen receptor splice variants (AR-Vs) are also involved in the progression of prostate cancer, as well as the development of resistance to antiandrogens. These augmented intratumoral levels of androgens promote both paracrine and autocrine activation of the AR regardless of systemic hormone levels. Studies conducted in cell culture and in vivo models suggests that enzalutamide resistance may be related to overexpression of genes encoding for enzymes involved in androgen biosynthesis such as SRD51A, HSD3β1, and AKR1C3. One of these enzymes, AKR1C, is a potential target for pharmacological approaches against AR resistance. In both in vitro and in vivo preclinical models, indomethacin, an NSAID, inhibits AKR1C activity and re-sensitizes CRPC to enzalutamide.Testosterone and 5α-dihydrotestosterone are sex steroid hormones that are largely synthesized from cholesterol in the testes and partially in the adrenal glands. Intratumoral synthesis of these hormones increases in CRPC patients. Cholesterol and weak androgens synthesized in the adrenal glands, such as androstenedione and dehydroepiandrosterone, may act as precursors of testosterone and 5α-dihydrotestosterone-60. In addition, Ser-213 and Ser-791 phosphorylation is essential for AR degradation by MDM2 E3 ligase while Thr-850 phosphorylation may stabilize AR by recruiting RNF6. AR phosphorylation at Ser-578 by p21-activated kinase 6 (PAK6) promotes the association between AR and MDM2 to activate MDM2-mediated proteasomal degradation of AR. Reports suggest that RNF6 and SIAH2 may represent effective therapeutic targets. In contrast, proteases such as ubiquitin-specific peptidases or deubiquitinases hydrolytically cleave ubiquitin or ubiquitin-like proteins from their substrates affect AR transcriptional activity and stability. USP7, USP10, USP14, and USP26 are some deubiquitinases which affect AR stability and its binding and are thus important therapeutic targets to control CRPC progression.The half-life of AR increases upon binding with the androgen with reduction in its degradation. Both the ubiquitin-proteasome system and the autophagy-lysosome pathway play important roles in the degradation of AR protein. Reports suggest several E3 ubiquitin ligases ubiquitinate AR proteins with distinct AR domain interactions including UBE3A (E6-AP), RCHY1 (ARNIP), and CHIP binding to the AR N-terminal domain, RNF4 (SNURF) to the AR DNA-binding domain while RNF6 and SIAH2 bind to the AR ligand binding domain. Aberrant activation of PI3K-Akt has been implicated in prostate cancer development and progression and activation of this pathway may be responsible for antiandrogen drug resistance. Activation of PI3K signaling and loss of the tumor suppressor gene PTEN, the negative regulator of PI3K/Akt pathway, occurs prominently in metastatic prostate cancer,66. A phase II randomized clinical trial, those having PTEN loss demonstrated the trends for improvement with the addition of Akt inhibitor to abiraterone acetate/prednisone treatment. Studies on preclinical models have shown that the activation of PI3K/Akt pathway is critical to CRPC development. PI3K isoform, especially p110α, is associated with the insulin and growth factor pathways while p110β isoform is known to regulate cell mitosis and survival. Overexpression of p110β is specifically involved in prostate cancer growth and proliferation. Interaction of AR with Src kinase and p58α regulatory subunit of PI3K activates mitogen-activated protein kinase (MAPK) and Akt pathways, leading to increased cell proliferation and survival. In addition, activation of MAPK and Akt also enhance AR signals by phosphorylating the AR or transcriptional co-activators. During the castration stage, Akt directly phosphorylates AR at two locations on Ser-791 and Ser-217; however, its clinical significance is not been established. Chandrasekar et al. demonstrated higher level of Src activation (androgen-independent) was associated with worse prostate cancer phenotypes like unlimited cell growth, tumor migration and invasion, and inhibition of apoptotic pathways. Besides, several clinical trials are being conducted via combination therapy of molecular targeted drugs like tyrosine kinase inhibitors with second-generation antiandrogens drugs. Thomas et al. in an in vivo study showed synergistic targeting of the PI3K/Akt pathway, and they showed that the AR axis significantly delayed CRPC progression.The PI3K-Akt-mTOR pathways are well known to regulate all major cellular processes such as cell growth, development, proliferation, protein synthesis regulation, and programmed cell death. GRs complexed with heat shock proteins are found in the cytoplasm. GRs consist of four functional domains, similar to ARs, including DNA binding domain, ligand binding domain, N-terminal, and the hinge region. ARs and GRs share similar transcriptomes and response elements in target genes. The binding of glucocorticoids promotes homodimerization and translocation in the nucleus where GR mediates transcriptional activation and influences target gene expression. The role of glucocorticoids and GRs in prostate cancer is complex as it influences both harmful and beneficial effects. Venkitaraman et al. demonstrated that glucocorticoids inhibit lymphangiogenesis through vascular endothelial growth factor downregulation and inhibit prostate cancer cell proliferation through the glucocorticoid receptor induction. Induction of GR upregulates p21 and p27 expression and downregulate oncogenic molecules such as MAPKs, NF-κt, and STAT1. GR mediates a similar but distinct set of AR-target gene expression. Glucocorticoids initially have suppressive effects on prostate cancer and are often given in conjunction with early treatments of CRPC with chemotherapy and second-generation antiandrogen agents . Both AR and GR possess similar DNA binding domain structures. GR has been shown to share response elements with many AR regulated target genes which ultimately leads to its upregulation in patients treated with chemotherapy along with second-generation antiandrogens. Upregulated GR signaling mediates resistance to androgen-targeted agents and the mechanism is known as “glucocorticoids receptor take-over” pathway. A recent investigation by Puhr et al. on GR expression and its functional significance in both prostate cancer cell lines and prostate cancer patients found that GR expression is low in primary prostate cancer tissue but is significantly increased during long-term exposure to enzalutamide. Similar findings were reported by Arora et al. who demonstrated how GR overexpression conferred resistance to enzalutamide. Li et al. further determined that treatment with enzalutamide maintained the cortisol level and enhanced glucocorticoid signaling by inhibiting the 11β-hydroxysteroid dehydrogenase-2 (11β-HSD2).Glucocorticoid receptor (GR), a member of the steroid hormone nuclear receptors family whose structure and mechanism of action is similar to androgen receptor, is expressed in almost all human tissues. There are five distinct NF-κB proteins of which p65/p50 heterodimer has shown to be constitutively active in prostate cancer. Moreover, activation of NF-κB/p52 pathway has been implicated in the development of resistance to prostate cancer. Overexpression of p52 increases glucose uptake and produces higher ATP and lactate levels moderating enzalutamide resistant in CRPC cells. It has been shown that processing of p100 to p52 through molecules such as B-cell activating factor, CD40, lymphotoxin β, and STAT3 may activate AR-Vs and glucose metabolism frequently involved in CRPC progression. Activation of AR-Vs mediated by heterogeneous nuclear RNA-binding protein (hnRPA-1) leads to significant hyperplasia and induced castration resistance growth. This leads to inhibition of apoptosis and cell cycle and thus limiting sensitivity to second-generation antiandrogen therapy. Liu et al. showed that overexpression of androgen receptor splice variant-7 (AR-V7) could activate NF-κB which in turn upregulates interleukin IL-6 gene expression. The study indicates a positive interaction between AR-V7 expression and activated NF-κB/IL-6 signaling in CRPC pathogenesis. They also demonstrated that AR-V7-induced NF-κB activation and IL-6 gene transcription could be inhibited by melatonin in LNCaP and 22Rv1 cells. Nadiminty et al. demonstrated that the mechanism of resistance to enzalutamide to be mediated by AR splice variants that lack the ligand binding domain (LBD); the AR variants expression are increased by NF-κB signaling. This group further demonstrated that downregulation of NF-κB2/p52 expression in CRPC cells by short hairpin RNA abrogates splice variants expression. Downregulation of NF-κB inhibits AR-Vs expression and restores the sensitivity of CRPC to second-generation antiandrogen therapy and desensitizes cells to androgens. Another study reported that downregulation of hnRPA-1 may lead to the re-sensitization of enzalutamide resistant prostate cancers. NF-κB has been shown to regulate the expression of several cytokines, in particular IL-6, in normal tissues and cancer cells. IL-6 is highly expressed in CRPC, increasing the transcriptional activity of the AR in a ligand-independent manner. But experimental therapies against IL-6 and the clinical trials on patients with late-stage prostate cancer has not been yet reported. IL-6 acts through the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway which has been approached as anti-STAT3 therapeutics in several human cancers including prostate cancer. Treatment of prostate cancer cells with the JAK inhibitor AG490 leads to the re-sensitization of cells to enzalutamide. Liu et al. demonstrated that co-treatment with enzalutamide and AG490 has an inhibitory effect on cell growth and induces apoptosis. Another study demonstrated that combinational therapy of antiandrogens with NF-κB inhibitors efficiently inhibits tumor growth of human CRPC xenografts.The NF-κB family proteins are important component of the oncogenic pathway in multiple human malignancies-98. The phosphorylated form of FOXO proteins remains inactive in the cytoplasm. Dephosphorylation of these proteins occurs due to Akt activity inhibition by several cellular factors such as PTEN or PI3K inhibitors. Activated FOXO proteins translocate from the cytoplasm to the nucleus and subsequently bind to promoters of their target genes such as p27, FASL, Bim-1, p21, Cyclins , PGC1α, RAG1, RAG2, etc.. FOXO3a increases androgen receptor expression by direct binding to the AR gene promoter. However, FOXO1 decreases AR transactivation by engagement of histone deacetylase HDAC3 proteins. Inactivated FOXO proteins have been implicated in prostate cancer progression towards castration resistance. Das et al. demonstrated that activation of Akt induces FOXO3a inactivation, meaning that PI3K/Akt inhibitors would activate FOXO3a. Ketola et al. highlighted the role of FOXM1 as the key player of the most aggressive prostate cancer subtype 1 (PCS1) of CRPC. Akt pathway upregulates the FOXM1, making it a target of Akt inhibitors. FOXO1 also controls cell survival in hepatic cells via gluconeogenesis modulation by cooperating with PGC-1α.Studies reveal that members of FOXO family also develops antiandrogen resistance. Many pro-apoptotic proteins such as caspase-9, Bad, and FOXO subfamily members including FOXO1, FOXO3a, and FOXO4 are phosphorylated by Akt to maintain cell survival. Wnt signaling constitutes both canonical (β-catenin dependent) and non-canonical (β-catenin independent) signaling pathways that regulate cell fate, proliferation, differentiation, migration, and self-renewal. Some studies demonstrated that AR and Wnt/β-catenin pathways intersect each other. It has been reported that expression of β-catenin and AR protein shows significant correlation in CRPC tumors,109. AR and β-catenin may together lead to the expression of target genes promoting androgen-independent growth and progression. Aberrant Wnt signaling is involved against ADT resistance after and prior to antiandrogen therapy,112 and elevated level of β-catenin has been found in post-ADT specimens. CRPC patients more frequently develop genetic changes compared to treatment-naïve prostate cancer. Gain and loss of functional mutation in the β-catenin gene (CTNNB1) and APC encoding gene alter the Wnt signaling in CRPC,115. Wnt-pathway activating mutations have been associated with resistance to antiandrogens in CRPC tumors. Nuclear localization and cytoplasmic accumulation of β-catenin and abnormal β-catenin expression has been observed in the specimens obtained from CRPC patients,118. Chen et al. demonstrated that suppression of non-canonical Wnt pathway overcomes enzalutamide resistance in CRPC. A study reported that enzalutamide-resistant cells show upregulation of β-catenin and AR which may be partially due to the reduction of β-TrCP mediated-ubiquitination. The study also demonstrates the correlation between AR and β-catenin playing a critical role not only in prostate cancer initiation but also in chemotherapy resistance progression. In various models, synergistic combination of β-catenin inhibitor (ICG001) with enzalutamide inhibit cell proliferation, tumor growth, and stem-like markers expression. However, the role of Wnt/βcatenin signaling in promoting enzalutamide resistance in prostate cancer has not yet been reported. Reports suggests that cellular signaling pathways like PI3K-Akt, Wnt, glucocorticoid, NF-κB, FOXO, and in others like ONECUT2 are involved in second-generation antiandrogen resistance in AR-independent pathways.Genome-wide analysis of prostate cancer has found that aberrant Wnt/β-catenin signaling drives the metastatic growth of prostate cancer. Human and murine prostate cancer models have exhibited varying effects of IL-6 and/or STAT3 on tumor cell growth. Experimental treatments have been proposed that block the IL-6/STAT3 signaling pathway. The anti-IL-6 antibody siltuximab (CNTO 328) has been shown to delay development of castrate resistance in prostate cancer in vitro and in vivo. However, the anti-IL-6 antibody has not been validated clinically as a monotherapy in phase II clinical trials. IL-6 is postulated to be involved in regulation of cellular stemness via phosphorylation of STAT3 and it is also thought to play a role in the development of resistance to enzalutamide. Endogenous inhibitors of IL-6 suppress cytokine signaling and inhibit activated STAT. While they inhibit signal transduction through STAT3, they may also exert anti-apoptotic effects. Given the complexity of IL-6 interactions in prostate cancer, a customized approach is required to identify patients who will benefit from anti-IL-6 therapy in conjunction with standard treatments.The Janus kinase and signal transducer and activator of transcription (STAT) 3, and its downstream effector IL-6 lead to AR activation and catabolic process to maintain cellular homeostasis through degradation and recycling of cellular components. It is constitutively active at a low basal rate and is activated in response to stressors. This process allows cells to degrade cellular proteins and organelles through lysosomes to generate energy. Also, the physiological balance between autophagy and apoptosis are supposed to be lost in cancer. Conditions such as metabolic stress and hypoxia leads to consequent upregulation of autophagy which forms a good source of nourishment for highly proliferative tumor cells. Androgen deprivation has shown to induce autophagy, but the exact mechanism remains unknown. An RB1 study demonstrated that the autophagy inhibitors clomipramine and metformin significantly increased the cytotoxicity of enzalutamide in vitro. An in vivo study on enzalutamide/clomipramine found that the drug combination reduced prostate tumor size by 91% as compared to enzalutamide/metformin combination which reduced the tumor size by 78%. Heat shock chaperone like protein-clusterin is also known to mediate autophagy induced by AR antagonists such as enzalutamide. Synergistic administration of enzalutamide and clusterin inhibitor (OXG-011) enhanced apoptosis and delayed progression in both in vitro and in vivo prostate cancer models.The other mechanism by which CRPC develops resistance to enzalutamide treatment includes autophagy. Autophagy is an adaptive. Activation of pathways such as TGF-β and SMAD alters the activity of some transcriptional factors such as Snail and Twist, reducing the expression of E-cadherin, a key event in EMT. Post-enzalutamide treatment and ADT result in upregulation of Twist and this alteration, together with protein kinase C (PKC) activation, facilitates drug resistance. A study found that using a combination of Ro318220 (PKC inhibitor) with enzalutamide altered resistance in prostate cancer. Snail, another transcriptional factor, can promote resistance to enzalutamide and increase migration and invasion of prostate cancer cells.The epithelial-to-mesenchymal transition (EMT) is induced by ADT in metastatic prostate cancer, and studies demonstrate that this process promotes tumor progression and drug resistance. It is characterized by the loss of AR signaling during neuroendocrine trans-differentiation which results in resistance to enzalutamide therapy. Reports suggest that 30% of metastatic CRPC belongs to NEPC and 1% of primary prostate cancers are diagnosed as NEPC. Several reports suggest that N-Myc is a driver of NEPC. Genomic amplification and overexpression of N-Myc and Aurora kinase A are also associated with differentiation of prostate cancer to NEPC. A preclinical study reported that overexpression of N-Myc abrogated AR signaling and developed features similar to NEPC. Knockdown or inhibition of Aurora kinase A with inhibitor alisertib (formerly MLN8237) resulted in decreased N-Myc target gene expression and cell viability and destabilizes N-Myc protein levels,137. In another preclinical study, CD532, another Aurora kinase A inhibitor, was shown to reduce N-Myc protein levels indicating viable treatment options for patients with NEPC. Guo et al. demonstrated that ectopic expression of ONECUT2 (transcription factor) in prostate adenocarcinoma in combination with hypoxia suppresses androgen signaling. ONECUT2 drives neuroendocrine prostate cancer by regulating hypoxia signaling. It activates SMAD3 which regulates hypoxia signaling through modulating HIF-1α chromatin-binding. It is also reported that treatment with hypoxia-activated prodrug TH-302 potentially reduces NEPC tumor growth. A study reported that both tumor suppressor proteins TP53 and RB1 are important factors in NEPC differentiation. Another study reported that TP53 and RB1 androgen-dependent prostate cancer shift to androgen-independent NEPC after enzalutamide treatment. Loss of TP53 and RB1 and the phenotypic switch of prostate cancer is mediated by the expression of a transcription factor SOX2. Mu et al. showed that inhibition of SOX2 restored TP53 and RB1 function. Tan et al. observed that approximately 90% of NEPC cases was associated with loss of RB1 and 85% of cases with RB1 deletions. It has been suggested that another factor involved in the development of NEPC is the enhancer of zeste homolog 2 (EZH2), an epigenetic reprogramming gene which represses androgen and drives neuroendocrine prostate cancer. In preclinical studies, various EZH2 inhibitors like GSK-126/343/503 that targets EZH2 enzyme activity demonstrated growth inhibition in prostate cancer cell lines. Ku et al.’s study on double knockout mice demonstrated that blocking of EZH2 may re-sensitize to enzalutamide PTEN and RB.Neuroendocrine prostate cancer (NEPC) is an aggressive form of prostate cancer that may arise in patients who underwent treatment like hormonal therapies for prostate adenocarcinoma. These results suggest that CRPC progression and resistance to AR signaling pathways is facilitated by PD-L1 and PD-1. Mechanisms of primary resistance can be further elucidated through phase II clinical trials investigating the role of immunotherapy in CRPC, involving treatment with ipilimumab in combination with abiraterone acetate in treatment-naïve CRPC (NCT01848067). The second phase II clinical trial is currently evaluating combination immune checkpoint blockade with ipilimumab and the PD-1 inhibitor nivolumab in mCRPC patients positive for AR-V7 (NCT02601014).Several studies demonstrated that PD-L1 is considerably expressed in enzalutamide-resistant cell lines and enzalutamide-resistant tumors in animal models,146. Moreover, tumor samples containing T-E gene fusion were reportedly more aggressive in comparison to T-E fusion negative samples, suggesting distinct molecular and perhaps clinical subtypes. The first gene fusion in prostate cancer was reported was TMPRSS2-ETV1. Later on other fusion transcripts (TMPRSS2-ETV4 and TMPRSS2-ETV5) involving TMPRSS2 and ERG proteins have also been reported. Different mechanisms are reported to be involved in the formation of these fusion transcripts. For example, the fusions of TMPRSS2-ERG and TMPRSS2-ETV4 are caused by intra-chromosomal deletion and fusion or intra-chromosomal translocation. In CRPC, interstitial deletions are the most common cause of T-E fusion. This high frequency of T-E fusion at this part of chromosome 21 suggests that it is a hotspot for chromosomal rearrangements. T-E fusion results in androgen dependent upregulation of ERG as one of the most common genomic dysregulation in prostate cancer. Some studies have reported that improved response to abiraterone acetate is associated with the ERG gene rearrangement. ERG is also reported to guide the expression of steroidogenic enzyme AKR1C3, highly expressed in enzalutamide resistant cells. Further investigation revealed the presence of the ERG/AKR1C3/AR feed-forward loop which confers androgen synthesis, AR signaling, and resistance to AR targeting agents in CRPC. Estrogen receptor (ERα) is reported to upregulate the expression T-E fusion transcript. Based on this evidence, it can be postulated that ERα antagonists might offer better therapeutic outcome by lowering the expression level of T-E fusion transcripts. Attard et al. tested this hypothesis using abiraterone acetate and results showed that five out of six patients responded to this therapy. HDACs inhibitors are reported to inhibit genes involved in the formation T-E fusion transcript.TMPRSS2-ERG gene fusion (T-E fusion) is the most common fusion event associated with CRPC. Two different studies reported the presence of T-E fusion in 9 out of 15 and 14 out of 19 CRPC tumor samplesThe advent of new generation hormonal therapies set the stage for a new era in the treatment landscape of CRPC. Although these agents showed initial treatment benefit, a relevant proportion of patients do not benefit at all or acquire resistance during treatment. Increasing knowledge of the subject has provided information that prostate cancer is a heterogeneous disease with the coexistence of both AR-responsive and AR-refractory cancer cells responsible for antiandrogen resistance at various degrees. This heterogeneity might be a critical factor for different biological behaviors and particularly for different responses to new-generation hormonal therapies among various prostate cancer. The current mechanisms of enzalutamide resistance, the mechanisms and pathways involved to design approaches for overcoming resistance, and the problems and solutions associated with these mechanisms are summarized in Intense ongoing research is needed to discern pathways and mechanisms to improve drug sensitivity. The more complete understanding of these mechanisms of resistance through approaches like next generation sequencing, single cell sequence, proteomics, and others will enable the development of improved treatment strategies to overcome this resistance. Moreover, development and validation of assays in identifying mechanisms of resistance and their clinical implementation will be useful in providing relevant predictive biomarkers and will become essential tools assisting clinicians for personalized treatment. Improving our understanding of these AR resistance mechanisms and translating them into the next generation of AR targeting agents will be key to designing more effective therapies for advanced-stage prostate cancer.