New approaches for patients with advanced radioiodine-refractory thyroid cancer

The cumulative evidence over the past decades has shown that the incidence of differentiated thyroid carcinoma (DTC) has exponentially increased. Approximately 10% of patients with DTC exhibit recurrent or metastatic disease, and about two-thirds of the latter will be defined as refractory to radioactive iodine (RAIR) treatment. Since this condition implies 10-year survival rates less than 10% after detection, using available treatments, such as systemic and targeted therapies, have become increasingly relevant. The initiation of these treatments aims to reach stabilization, tumor volume reduction, and/or symptom improvement and it should be decided by highly specialized endocrinologists/oncologists on the basis of patient’s features. Considering that despite enlarged progression-free survival was proven, multikinase inhibitors remain non-curative, their benefits last for a limited time and the side effects potentially cause harm and quality of life reduction. In this context, molecular testing of cancer cells provides a promising spectrum of targeted therapies that offer increased compatibility with individual patient needs by improving efficacy, progression free survival, overall survival and adverse events profile. This review article aims to provide a summary of the current therapeutic strategies in advanced RAIR-DTC, including approved target therapies as well as those for off-label use, RAI resensitization agents, and immunotherapy.


INTRODUCTION
The cumulative evidence over the past decades has shown that the incidence of differentiated thyroid carcinoma (DTC) has increased exponentially, probably due to the rising detection of low-risk small carcinomas [1]. Nevertheless, approximately 10% of patients with DTC exhibit a more aggressive behavior in which persistent or recurrent distant metastatic disease is developed, and about two-thirds of them will be defined as refractory to radioactive iodine (RAI) treatment [2]. This condition cannot be defined by a single criterion, but it rather comprises a spectrum of scenarios included into any of the following: (1) Lack of initial RAI uptake in all or some of the metastatic foci in a whole-body scan (diagnostic or following a therapeutic dose) or lose of the ability to take up RAI after previous evidence of uptake ; (2) Disease progression in a patient who has received RAI; (3) Disease progression in a patient who has received 600 mCi of 131 I of cumulative activity; and/or (4) Locally advanced disease for whom surgical resection is not feasible and RAI uptake status cannot be assessed [2]. After the detection of radioiodine refractory (RAIR) disease, 10-year survival rates may decrease to less than 10% [2]. Therefore, using second-choice treatments, such as systemic and targeted therapy, in these patients has become increasingly relevant in recent years. This review article aims to provide a summary of the current therapeutic strategies for patients with RAIR thyroid cancer, including approved target therapies as well as those prescribed for off-label use, RAI resensitization agents, and immunotherapy.

MOLECULAR PATHWAYS OF THYROID CANCER
The underlying carcinogenic molecular pathways of differentiated thyroid cancer have been well defined. The MAPK signaling pathway is one of the most extensively studied [3]. Driver mutations such as in BRAF and RAS oncogenes, as well as fusions involving tyrosine kinase receptors, lead towards a constitutive activation of the downstream events resulting in cell proliferation, dedifferentiation, and cancer cell survival. These mutations could be targeted with specific therapies which result in cell growth inhibition [3,4]. Meanwhile, multikinase inhibitors (MKIs) confer their antitumor effect in radioiodine-refractory metastatic thyroid cancer by other effects, mainly through their anti-angiogenic activity [4]. The main molecular signaling pathways involved in thyroid carcinogenesis and the most significant inhibitors are summarized in Figure 1.

INITIATION OF SYSTEMIC THERAPY
The initiation of health agencies approved systemic therapy or the enrollment of a patient in a clinical trial should be managed by highly specialized endocrinologists/oncologists. The aim of this treatment will be to reach stabilization, tumor volume reduction, and/or symptom improvement[2,4]. Nevertheless, it should be decided on an individualized basis and under a coordinated decision taken together with patients, considering that target and multikinase inhibitor-based therapies remain non-curative and their benefits in terms of extending progression-free survival last for a limited time. Furthermore, the side effects of these therapies may have the potential to cause harm and significantly reduce the patient's quality of life [4]. Thus, the assessment of tumor burden, disease progression, symptoms, or a high risk of local complications is essential [2]. If available, genetic interrogation should be granted in order to initiate a selective TKI (either an approved drug or from clinical trials) in a patient with a progressive advanced RAIR-DTC that carries a specific target mutation [5]. If not genetic alterations are found, RAIR-DTC patients and those having tumor lesions in which the sum of diameter is larger than 2 cm and showing < 12-mo progression should be considered for multikinase inhibitors [2]. Additionally, all patients with DTC-related imminent symptoms and potentially symptomatic disease should be guaranteed treatment initiation[2]. This is a simplistic view but may help to decide the correct moment of treatment initiation when no other therapies are no longer amenable. A proposed decision-making algorithm for systemic therapy initiation in RAIR-DTC is shown in Figure 2. The main available agents studied in the treatment of RAIR-DTC are summarized in Table 1.

MULTIKINASE INHIBITORS (SORAFENIB AND LENVATINIB)
Multikinase inhibitors block several signaling pathways responsible for tumor proliferation and survival, with varying degrees of potency[2]. However, the main target for MKIs is the vascular endothelial growth factor receptor (VEGFR) and therefore the inhibition of tumor angiogenesis [3]. That is why they are also called antiangiogenic MKIs. These MKIs have demonstrated in phase III trials, an increase in the median progression free-survival (mPFS) from 11 to 18 mo, and objective responses of 12% to 64% [6,7]. We should consider that these drugs are usually tumoristatic and will eventually lose their effect due to on-target or off-target resistance, after which, another therapy will be needed. To date, only one post-hoc analysis of the SELECT study has demonstrated improved overall survival in a subgroup of patients receiving an Lenvatinib vs placebo[8].

Sorafenib
Sorafenib inhibits the VEGFR 1, 2, and 3, platelet-derived growth factor, RET, c-kit, and less potently, BRAF kinases[9]. In the phase III DECISION trial, patients treated with sorafenib (n = 207) had a significantly longer PFS over patients receiving placebo . This could suggest that delaying the initiation of sorafenib could not have a significant impact on the effectiveness. Also, in the same analysis, patients who continued receiving sorafenib after progression had a still longer PFS in comparison to patients who initially received placebo (6.7 vs 5.8) [11], meaning that sorafenib could still be an option in patients when an alternative drug is not available or not possible. In our real-life experience with sorafenib (n = 18), 72% had SD ≥ 6 mo and 11% demonstrated PR with a PFS of 16.5 mo [12]. The most frequent adverse events during sorafenib treatment were hand-foot skin reaction, diarrhea, fatigue, alopecia, weight loss, and rash[6,10,12]. HFS reaction and hypertension were the most frequent grade 3-4 AEs, reported from to and from to, respectively[6,10,12]. As reported with other MKIs, dose reductions and interruptions were frequent, however, drug withdrawal was uncommon[6,10,12]. The recommended initial dose of sorafenib is 400 mg twice a day [13]. In an exposure-response model, initial lower doses of sorafenib (600 or 400 mg/d) were associated with improved tolerability but reduced PFS. However, a strategy of 800 mg/d for an initial two cycles followed by dose reductions seemed likely to maintain efficacy while possibly mitigating some AEs [14]. The summary of the efficacy and safety of sorafenib in patients with thyroid cancer reported by clinical trials is shown in Table 2.  Table 3.

Cabozantinib
Cabozantinib is a RET, vascular endothelial growth factor receptor-2 (VEGFR2), and MET kinases inhibitor agent currently approved for the treatment of advanced medullary thyroid cancer [26]. However, it has also been studied in 15 patients with RAI-refractory DTC in a phase I clinical trial, with promising efficacy[27]. Ten of the included patients were previously treated with VEGF inhibitors, mostly sorafenib.
Cabozantinib was administered at a starting dose of 140 mg daily. A partial response was observed in 8 (53%) patients, 5 with prior VEGF inhibitors treatment. On the other hand, a phase II trial is currently ongoing, which involves a cabozantinib therapy in RAIR-DTC patients who experienced disease progression after second-or third-line VEGFR-targeted therapy [28]. Partial response was reached in 10 (40%) of the 25 enrolled patients, with a starting dose of 60-80 mg daily. The median PFS and OS were 12.7 and 34.7 mo, respectively [28]. Exelixis announced by the end of 2020 that, at a planned interim analysis, the phase III COSMIC-311 pivotal trial met the co-primary endpoint, demonstrating a significant reduction in the risk of disease progression or death of 78% of patients receiving cabozantinib compared to placebo (HR, 0.22, 96%CI: 0.13-0.36; P < 0.0001) in patients with RAIR differentiated thyroid cancer who have progressed after up to two prior VEGFR-targeted therapies. The safety profile was consistent with that previously observed for cabozantinib. In 2021, Exelixis ® announced that the United States Food and Drug Administration (FDA) approved cabozantinib as a second/third line additional treatment for patients with RAIR thyroid cancer [29]. With this third MKI approved, there will surely be a change in defining first and second line of treatment according to the drug potency.

Apatinib
Apatinib, also known as rivoceranib, is a tyrosine kinase inhibitor that selectively inhibits the VEGFR2. Apatinib inhibits VEGF-mediated endothelial cell migration and proliferation thus blocking new blood vessel formation in tumor tissue. This agent also mildly inhibits c-Kit and c-SRC tyrosine kinases [30]. A recent phase II study performed in 20 patients with advanced thyroid cancer showed promising results with an objective response rate (ORR) of 80%, a median PFS of 18.4 mo (95%CI: 9.

NTRK inhibitors (larotrectinib and entrectinib)
Neurotrophic tropomyosin receptor kinase (NTRK) fusions have been reported in In a pooled analysis of three-phase 1/2 clinical trials, out of 24 patients with DTC bearing an NTRK fusion who received larotrectinib, 79% experienced an objective response [33]. This drug showed durable responses with a median time of 35 mo in the overall group of patients with solid tumors [33]. Also, larotrectinib seems to be active within the central nervous system (CNS)[33], which makes it an indispensable option when brain metastases are present in patients harboring this fusion, knowing that they have a worse outcome in patients with differentiated thyroid cancer [34]. Most frequent adverse events were primarily grade 1 and 2 and included fatigue (30%), cough, constipation (27%), dizziness (25%), and alanine aminotransferase increase (25%). The most common grade 3 or worse treatment-emergent adverse events (regardless of attribution) were anemia (10%) and decreased neutrophil count (5%) [33]. We recently showed our experience with Larotrectinib in a patient with RAIR DTC who had a rapid progression on MKI therapy (sorafenib and lenvatinib), and who had a complete response to treatment including the disappearance of multiple CNS metastasis [35].
Entrectinib also blocks ROS1 and ALK and was specifically designed to have systemic activity and cross the blood-brain barrier. In an analysis of three-phase I or II trials, two out of four patients had a PR with entrectinib[36]. Most AE were grade 1-2 and included dysgeusia (47%), fatigue (28%), and constipation (28%). The most common grade 3 or 4 treatment-related AE were anemia (12%) and weight gain (10%) [36].

Resistance to larotrectinib and entrectinib
TRK fusion-positive cancers can develop resistance to TRK inhibition[37]. This resistance can be classified into off-target (new additional mutations that may occur in the tumor) or on-target (within the same altered receptor, due point mutations that lead to amino acid substitutions in the solvent front, the gatekeeper residue or the xDFG motif) [38]. Mutations in the NTRK kinase domain cause resistance to TRK inhibitors by interfering with binding of the inhibitor, altering the kinase domain conformation or altering ATP-binding affinity [38].
New drugs are currently in development for those patients who develop on-target resistance, among them, selitrectinib and repotrectinib. Due to their small size, these low molecular weight molecules are able to engage the ATP-binding pocket while avoiding the steric penalties of kinase domain substitutions [39,40]. Selitrectinib is currently the drug with which the most experience has been gained. Thirty-one patients with solid tumors with NTRK fusions, previously treated with a TRK inhibitor (larotrectinib, entrectinib or PLX7486) with a median duration of prior therapy of 11 mo (range 2-30 mo) received treatment with selitrectinib. In patients with TRK kinase domain mutations (the majority of which involved the solvent front), the ORR was 45%[41].
Mutation-specific kinase inhibitors -RET and NTRK inhibitors, as well as BRAF inhibitors-produced higher and durable objective responses[32,33,42,43]. Although prolongation of progression-free survival has not yet been demonstrated in phase III clinical trials, they seem to be promising options for RAIR thyroid cancer patients. In line with this, the implementation of molecular screening strategies seems to be necessary to improve the clinical course of these patients.

Resistance to selpercatinib and pralsetinib
Evidence on acquired resistance mechanisms to RET, both on target and off-target, is recently arising. Selpercatinib and pralsetinib were oriented to target gatekeeper mutations, such as RET V804 and S904F which was associated with resistance to RETtargeted kinase inhibitors, as vandetanib [46]. Nevertheless, five RET kinase domain mutations at three non-gatekeeper residues were identified from selpercatinib and pralsetinib-resistant medullary thyroid cancer cell lines in a recent experimental study [47]. Information on acquired resistance to these drugs obtained from studies on nonsmall cell lung cancer (NSCLC) is slightly more extensive. For example, it was found acquired RET G810R/C/S/V mutations in RET fusion-positive tumors from patients who developed resistance to selpercatinib[48] and pralsetinib treatments [49]. Other reports of acquired selpercatinib resistance with MET amplification were demonstrated, in which probably this could be overcome by combing selpercatinib with crizotinib [50]. On the other hand, it was postulated that the combination of pralsetinib or selpercatinib with a selective MET inhibitor -such as capmatinib, savolitinib, or tepotinib-could offer acceptable tolerability and efficacy in NSCLC patients [51].
These experimental findings have shown the imperative need to develop nextgeneration targeted RET agents focused on both gatekeeper and non-gate keeper mutations for on-and off-target resistance in order to develop and validate combination therapies.

OFF-LABEL DRUGS FOR DIFFERENTIATED THYROID CARCINOMA
Considering the time-limited benefits of FDA-approved kinase inhibitors treatment in RAI-refractory thyroid cancer, it became necessary to develop additional new January 24, 2022 Volume 13 Issue 1 therapeutic lines that would enhance compatibility with individual patient needs by improving efficacy and adverse events profile. Several targeting agents are being studied in advanced differentiated thyroid cancers, but none of them have been approved yet (Table 1). A summary of some relevant ongoing clinical trials for the treatment of advanced RAIR-DTC are shown in Table 4.

Combination of dabrafenib-trametinib
BRAF oncogene mutations are present in approximately 50% of PTCs, while it has been observed that it rises to over 90% when an anaplastic transformation emerges from a prior history of PTC [52]. Under this premise, a clinical study using the combination dabrafenib 150 mg twice daily + trametinib 2 mg daily (selective inhibitors of BRAF V600E kinase and MEK1-2 kinase, respectively) in 23 patients with locally advanced, unresectable, or metastatic ATC [53], prompted the rapidly FDA approval for these patients. This study showed an overall response rate of 61%, with complete and partial response rates of 4% and 57%, respectively. Progression free survival for at least 6 mo was seen in 64% of these patients and overall survival was 80% at 1 year. The most common adverse events were fatigue (44%), fevers (31%), and nausea (31%), and the most common grade 3 and 4 adverse event was anemia (13%) [53]. In our setting, where access to molecular tests and target therapies is not widely available yet, we have reported the cases of two patients with metastatic and locally unresectable ATC, in whom the use of dabrafenib-trametinib (D-T) provided a dramatic reduction of the cervical mass with a minimal residual loco-regional disease, and even allowed surgical resection on one of them. Besides, a partial and complete response to the pulmonary metastatic disease was also observed [54,55].
The combination of D-T was studied in a phase II clinical trial that included 53 patients with BRAF mutated RAIR-PTC with disease progression within the last year [56]. The participants were randomized to Arm A (dabrafenib 300 mg daily, n = 26) or Arm B (dabrafenib 150 mg daily + trametinib 2 mg daily, n = 27). Cross-over to Arm B was allowed at the time of progression. Out of 25% of patients had prior therapy with multi-kinase inhibitors. Preliminary results exhibited partial responses in 10 (38%) and 9 (33%) patients from Arm A and B, respectively. Progression-Free Survival for patients who received D-T was 11.4 mo, with a median follow-up of 13 mo. The treatment-related adverse events were similar to previously reported trials [56].

Dabrafenib and vemurafenib in RAIR-DTC
Dabrafenib and vemurafenib have been approved as single agents for the treatment of advanced melanoma, but they also have been evaluated in phase 2 trials in patients with BRAF V600E-mutated PTC [57,58]. Both BRAF inhibitors are effective also in papillary carcinoma, although the outcomes have not been as robust as for ATC, so neither are currently approved for this use. In general terms, objective responses were seen for up to half of patients treated with either vemurafenib or dabrafenib in different trials and clinical experiences [57][58][59]. Among them, a randomized, multiinstitutional, open-label phase 2 trial was conducted over two arms of patients with BRAF V600E-mutated PTC [57]. Arm A employed dabrafenib as a single agent and arm B, the combination of dabrafenib with trametinib. Partial responses were reached in 10 of 26 patients (38%) from arm A, and 9 of 17 (33%) from arm B, with median PFS of 11.4 and 15.1 mo, respectively. Common adverse events included fever, diarrhea, anemia, fatigue, nausea, alopecia and skin reactions [57]. Meanwhile, a nonrandomized, open-label, multicenter phase 2 vemurafenib trial was conducted in two cohorts of patients with BRAF V600E-mutated PTC. Cohort 1 was comprised of 26 patients who had never received multikinase VEGFR inhibitors, in which the best overall response (partial response) was reached in 12 patients (38%), with a median duration of PFS of 18.8 mo (14.2-26), and the median OS had yet to be reached. In cohort 2 were included 25 patients who previously received MKIs treatment. Partial response rates were seen in 27.3%, with a median PFS of 8.9 mo. The most common adverse events reported were rash, fatigue, weight loss, dysgeusia, and alopecia. Serious adverse events were seen in 62% and 68% of the patients in cohort 1 and 2, respectively, including benign and malignant skin lesions and cerebrovascular accidents, among others [58]. January 24, 2022 Volume 13 Issue 1

Selective mTOR inhibitors (everolimus, temsirolimus)
The mammalian target of rapamycin (mTOR) is a serine/threonine kinase that exerts as an essential regulator of cell growth-related processes [60]. Everolimus and temsirolimus are two mTOR inhibitors that demonstrated clinical benefits in other cancers like advanced renal carcinoma, metastatic breast cancer, and pancreatic neuroendocrine tumors, in which they were approved by the FDA [61]. Since the mTOR pathway is over-activated in thyroid cancer, some studies have tested these agents' effects on advanced thyroid cancer, with promising outcomes [62,63]. Everolimus was evaluated in a single-arm, multicentric phase II study that included 31 patients with aggressive RAIR-DTC, among other thyroid tumor histologies. There was one PR (3%) but 27 patients (82%) had SD, for a clinical benefit rate of 84.8% and a median PFS for 12.9 mo. Median OS was not reached and 2-year OS was 73.5% [62]. For its part, a phase 2 study that enrolled 36 patients with metastatic RAIR-DTC evaluated the efficacy of the combination of oral sorafenib (200 mg twice daily) and intravenous temsirolimus (25 mg weekly) [63]. A partial response was reached in 8 patients (22%), while stable and progression disease was seen in 21 (58%) and 1 (2%) patients, respectively. The mPFS at one year was 30.5% and the most common toxicities included hyperglycemia, fatigue, anemia, and oral mucositis. The authors concluded that this combination appears to have better response rates in patients with RAIrefractory thyroid cancer who received no prior treatment, regardless of whether RAS or RAF mutation was present [63].

Redifferentiation agents
It has been well described that activating BRAF mutations induce loss of differentiated features required for response to radioiodine treatment, while its blockade would restore radioiodine uptake in experimental models [64]. In patients with radioiodinerefractory differentiated carcinoma with somatic BRAF or RAS mutations, treatment with the specific targeted inhibitors may restore radioiodine responsiveness in up to two-thirds of patients, permitting iodine therapeutic administration leading to tumor shrinkage in up to one-third [57][58][59]. On the other hand, constitutive activation of MAPK pathway causes inhibition of a variety of thyroid genes, including NIS, leading to the investigation of selective MAPK blocking agents as Selumetinib, as redifferentiation agent [64,65].

Selumetinib
Selumetinib is a MEK1-2, RAS and BRAF V600E inhibitor which efficacy was evaluated in 32 RAIR-DTC patients enrolled in a multicenter, open-label, phase II trial [66]. There were 1 partial response (3%), 21 stable disease (54%), and 11 progressive diseases (28%). Median PFS was 32 wk, and it was seen that BRAF V600E mutants had a longer median PFS compared with patients with BRAF wild-type cancer (33 vs 11 wk, respectively). This suggest a potential beneficence of Selumetinib based on underlying genetic disorders. The most common adverse events included rash, fatigue, diarrhea, and peripheral edema [66]. A phase III trial is currently in progress which continues to explore selumetinib's redifferentiation benefits in a larger number of participants [66] (Table 2).

Immunotherapy
In recent years, there has been significant progress in the field of oncological immunotherapy. Several immunotherapeutic agents have now been approved by the FDA for the treatment of a variety of malignancies, including melanoma, non-small cell lung cancer, renal and breast carcinomas, among others [67]. In this line, several phase I studies research the use of immunotherapy in the treatment of advanced differentiated thyroid cancer focuses on restoring immune surveillance [68]. The recent identification of blocking antibodies of CTLA-4 and PD-1 to their corresponding ligands (CD80/86 and PD-L1/PD-L2 respectively) enhances the effector T cells and inhibits the regulatory suppressor cells. Thus, the evidence of PD-1 (+) T cell in thyroid tumors involved lymph nodes in PTC patients suggests the potential utility of immune checkpoint inhibitors like pembrolizumab (as a single agent or in combination with MKIs) for advanced thyroid cancers [68]. Only a few immunotherapy trials in patients with thyroid cancer have been published to date, but several trials are ongoing.

Pembrolizumab
Pembrolizumab is an anti-PD-1 monoclonal antibody that exhibits antitumor activity by blocking interaction between PD-1 and its ligands [68]. Patients with advanced thyroid cancer were enrolled in the nonrandomized, phase Ib KEYNOTE-028 trial conducted to evaluate its safety and antitumor activity in 22 patients with advanced papillary or follicular thyroid cancer. Pembrolizumab 10 mg/kg was administered every 2 wk up to 24 mo or until confirmed progression or intolerable toxicity. SD was achieved by 57% (4/7) of patients with follicular histology and 60% (9/15) of patients with papillary histology and two patients reached partial response for 8 and 20 mo. Median PFS was 7 mo and median overall survival was not reached. Diarrhea and fatigue were the most common adverse events [69]. This study suggests that pembrolizumab may be effective and have a favorable safety profile in PD-L1-positive thyroid cancer, providing a baseline for future research [69].
Other ongoing single-arm multicenter phase II study combine lenvatinib and pembrolizumab in patients with RAIR-DTC [70]. Patients were excluded if they had received previous VEGFR-directed multikinase therapy. The lenvatinib starting dose was 20 mg/d orally and pembrolizumab was 200 mg IV every 3 wk. The preliminary results showed that out of 29 evaluable patients, 18 (62%) had a partial response, 10 (35%) had stable disease and the clinical benefit rate was 97%. The PFS at 12 mo was 74%, and median PFS was not yet reached. The most common adverse events were hypertension (47%), weight loss (13%), maculopapular rash (13%), leukopenia (7%), diarrhea (7%), and oral mucositis (7%) [70]. While the results are promising, the continuation of this study will help determine the magnitude of the responses. January 24, 2022 Volume 13 Issue 1

CONCLUSION
In conclusion, therapeutic options for patients with advanced radioiodine-refractory differentiated thyroid carcinoma have been increasingly evolving and fine-tuned. While the introduction of new therapies for multiple molecular targets has made it possible to extend progression-free survival, their impact on overall survival is still unclear. Based on the improving knowledge of the underlying molecular mechanisms in these patients, novel agents under study bring us a new scope for the near future. Thus, increasingly tailored therapy focused on critical molecular pathways will be offered, allowing to overcome drug evasion mechanisms, enhance efficacy, minimize adverse events, and finally achieve an overall survival improvement in these patients.