between 1960 and 1981 , the national cancer institute ( nci ) and the u.s . department of agriculture ( usda ) collaborated on a plant screening program that collected and tested 115,000 extracts from 15,000 species of plants to identify naturally occurring compounds with anticancer activity . samples from a single pacific yew tree , taxus brevifolia , were obtained by usda botanist arthur barclay on the last day of his expedition in 1962 . after his return , crude extracts from bark , twigs , needles , and fruit were tested , and bark extract was found to be cytotoxic . mansukh wani and monroe wall , working under contract with the nci at the research triangle institute ( research triangle park , nc ) , received t. brevifolia samples in 1964 . by 1967 , they had isolated and identified the active ingredient from the bark of t. brevifolia and named it taxol , based on its species of origin and the presence of hydroxyl groups ( perdue and hartwell , 1969 ; wall and wani , 1995 ) . in 1971 , they published the structure of taxol ( wani et  al . , 1971 ) , and it entered the nci drug development program ( table 1 ) . walsh and goodman ( 2002a ) . 
 wall and wani ( 1995 ) . taxol showed mixed results in preclinical trials and was not uniformly considered the most promising plant product . the insolubility of taxol in water necessitated its formulation with polyethoxylated castor oil , which can cause severe anaphylactic reactions and further dampened enthusiasm . however , by 1978 taxol had shown efficacy in a subset of mouse tumor models , including p388 leukemia ( fuchs and johnson , 1978 ) , and it entered clinical trials in 1984 ( walsh and goodman , 2002a ; tuma , 2003 ; table 1 ) . several clinical trials were delayed because of a shortage of taxol , the only source of which at the time was the slow - growing t. brevifolia . despite the scarcity , a clinical study on ovarian cancer proceeded and eventually concluded that 30% of patients with advanced ovarian cancer responded to taxol therapy ( mcguire et  al . , 1989 ) . high demand for taxol resulted in severe depletion of t. brevifolia , since removing the bark killed the trees . in 1990 , the department of the interior was petitioned to include t. brevifolia on the list of endangered species , and the pacific yew act was passed in 1992 to safeguard the tree ( walsh and goodman , 1999 ) . in 1988 , it was estimated that the cost of manufacturing taxol from the existing t. brevifolia was 10 times the budget available for the project at the nci , and environmental concerns regarding the long - term prospects for t. brevifolia were growing . owing to the limited accessibility of taxol , as well as its unique structure and cytotoxic potential , at least 30 laboratories worldwide competed to develop a total synthesis . however , because of the complexity of the molecule , these efforts were not successful until 1994 ( holton et  al . ultimately , several methods for total synthesis were developed , all of which require roughly 40 steps of reactions , and a more practical , semisynthetic protocol became the standard approach for production . however , in 1989 , no immediate solution for obtaining large quantities of taxol was apparent , and the nci made the decision to transfer taxol to a pharmaceutical company for commercialization . the request for applications received four responses , and bristol - myers squibb ( bms ) was selected in 1991 . in 1992 , bms trademarked the name  taxol  and created the new generic name paclitaxel ( walsh and goodman , 1999 , 2002a , b ; table 1 ) . this occurred despite the fact that the term taxol had been used in > 600 manuscripts published over the course of 20 years . congressional hearings were held in 1991 and 1993 regarding the transition of taxol to taxol and paclitaxel ( table 1 ) . the hearings questioned granting bms a monopoly on a natural resource , as well as the higher price that was charged for drugs , like taxol , that were identified and developed with federal funding rather than private money . the subcommittee staff concluded that the agreements between nci and bms were  not sufficient to fully protect the public interest ( united states congress , house committee on small business , subcommittee on regulation , business opportunities , and energy , 1992 ) . taxol is the most profitable chemotherapy drug in history , and the only drug in clinical use identified by the plant screening program ( walsh and goodman , 1999 , 2002a , b ) . in 1977 , the nci sent samples of paclitaxel ( still referred to as taxol at that point ) to susan horwitz at albert einstein college of medicine ( new york , ny ) . in 1979 paclitaxel reduces the critical concentration of purified tubulin subunits necessary for polymerization into microtubules in vitro and increases the percentage of tubulin subunits that assemble . furthermore , microtubules polymerized in the presence of paclitaxel are protected from the disassembly normally induced by cold or calcium treatment ( schiff et  al . , 1979 ) . these effects were in stark contrast to previously identified microtubule poisons , including colchicine and vinca alkaloids , which prevent microtubule polymerization ( malawista and bensch , 1967 ; bensch and malawista , 1968 ; de brabander et  al . similar to its effects on purified tubulin , paclitaxel promotes microtubule polymerization and stabilization in living cells , where it is capable of antagonizing the effects of colchicine and vinca alkaloids ( schiff and horwitz , 1980 ; de brabander et  al . , 1981 ) . phenotypically , paclitaxel treatment arrests a diverse array of cell types in mitosis , in both animal tumor models and cell culture ( fuchs and johnson , 1978 ; schiff and horwitz , 1980 ; milas et  al . , 1995 ; jordan and wilson , 2004 ; orth et  al . , 2008 ) . a large majority of reports indicate that paclitaxel - arrested cells are in metaphase and contain near - normal , bipolar spindles . canonical images of paclitaxel treatment show cells with chromosomes aligned at the cell equator , although in some cases one or a few chromosomes have been reported to remain misaligned ( jordan et  al . , 1993 , 1996 ; a small number of studies have reported that certain concentrations of paclitaxel induce multipolar spindles ( chen and horwitz , 2002 ; hornick et  al . however , until recently , there was no evidence to suggest that these effects were clinically relevant . the dominant perception , by far , has been that the antitumor effects of paclitaxel occur due to its ability to arrest cells in metaphase on bipolar spindles . paclitaxel - induced mitotic arrest occurs due to activation of the mitotic checkpoint ( also known as the spindle assembly checkpoint ) , the major cell cycle control mechanism acting during mitosis to prevent chromosome missegregation . the mitotic checkpoint delays separation of the chromosomes , which enter mitosis as replicated pairs of sister chromatids , until each pair has made stable attachments to both poles of the mitotic spindle . chromatids connect to spindle microtubules through their kinetochores , protein complexes that assemble on centromeric regions of dna . unattached kinetochores , which have not made stable attachments to microtubules , activate a signal transduction cascade that delays mitotic progression by inhibiting the anaphase - promoting complex / cyclosome ( kops et  al . , 2005 ; paclitaxel treatment arrests cells in mitosis due to the presence of a small number of unattached kinetochores ( waters et  al . , 1998 ) . in addition to its utility in cancer therapy , paclitaxel is also widely used in cell biology . in untreated cells , bipolar attachment of sister chromatids places kinetochores under tension , which helps stabilize the interactions between kinetochores and spindle microtubules . paclitaxel treatment reduces the tension on kinetochores that maintain bipolar attachment ( waters et  al . , 1998 ) , and is a useful tool both for arresting cells in mitosis and for dissecting the contributions of tension versus attachment in satisfying the mitotic checkpoint ( maresca and salmon , 2010 ) . a pubmed search for paclitaxel ( or taxol ) returns 25,000 articles . despite this vast literature , until recently , the clinically relevant concentration for use in cell culture studies has been unclear . first , paclitaxel is dosed at various levels and on different schedules , depending on the disease and the chemotherapy regimen selected . second , the concentration of paclitaxel in the plasma changes over time as the drug is cleared , primarily by the liver . third , paclitaxel accumulates intracellularly in cancer cell lines by 50- to > 1000-fold , depending on cell type and the concentration added ( jordan et  al . , 1993 , 1996 ; yvon et  al . , 1999 ) . therefore the concentration of paclitaxel is almost certainly higher in the tumor than in the plasma , where it is typically measured , but there is no linear calculation to predict the fold concentration . fourth , intratumoral measurements require a biopsy after initiation of therapy , which is not readily accessible outside of a clinical trial . in the absence of data establishing the intratumoral concentration of paclitaxel , it was reasonable to infer that its antitumor effects were due to mitotic arrest . mitotic arrest results in either death during mitosis or an abnormal exit from mitosis , without chromosome segregation or cytokinesis , to form a tetraploid g1 cell ; this exit is known as mitotic slippage . after slippage , a functional mitotic checkpoint has been reported by numerous groups to be required for efficient cell killing in response to mitotic arrest . in contrast , cells in which the mitotic checkpoint is weakened have also been reported to be sensitive to paclitaxel . still other studies have found that the state of the mitotic checkpoint does not affect this sensitivity ( rieder and maiato , 2004 ; weaver and cleveland , 2005 ; yamada and gorbsky , 2006 ; ryan et  al . , 2012 ) . some have hypothesized that a weakened mitotic checkpoint confers only short - term resistance to mitotic arrest ( janssen et  al . , 2009 ) , whereas others have proposed that activation of the mitotic checkpoint followed by mitotic slippage results in optimal cell killing ( tao et  al . , 2005 ) . one popular hypothesis was that the duration of mitotic arrest is predictive of cell death , with cells that arrest longer being more likely to die . however , multiple studies observing individual cells have now shown that the length of time a cell spends in mitosis can not predict whether it will survive ( gascoigne and taylor , 2008 ; orth et  al . , 2008 ; shi et  al . , 2008 ) . the difficulty of predicting sensitivity to mitotic arrest was further demonstrated in a study using nontransformed , chromosomally stable cells . time - lapse microscopy was used to identify sister cells that resulted from a normal bipolar division without chromosome missegregation . unexpectedly , the fates of the sister cells in response to mitotic arrest were completely unrelated . if one cell died from mitosis , its sister was no more likely to die from mitosis than it was to slip into interphase ( and either die or survive ) . thus cell fate in response to mitotic arrest is stochastic and not determined genetically ( gascoigne and taylor , 2008 ) . the predominant hypothesis for the past several decades has been that paclitaxel kills tumor cells as a consequence of mitotic arrest . however , despite significant effects on mitosis that are sufficient to cause cell death , it has been suggested that paclitaxel causes death in tumors through effects on interphase cells . this proposal is largely based on the idea that the mitotic index in tumors is not sufficient to explain the efficacy of paclitaxel . human tumors have a slow doubling time , and calculations that predict mitotic index based on tumor doubling rates , without accounting for cell death , suggest that an insufficient number of cells pass through mitosis in the presence of paclitaxel to account for tumor shrinkage rates ( komlodi - pasztor et  al . , 2011 , 2012 ; however , cell death has been observed in a wide array of untreated patient tumors , and directly measured proliferative rates are much higher than those estimated based on tumor doubling rates ( kerr and searle , 1972 ; kerr et  al . , 1972 ; in addition , paclitaxel is retained in tumors for > 5 d ( mori et  al . , 2006 ; 2009 ) , permitting an extended window of time for cells to undergo one or more rounds of division in the presence of drug . mechanistically , it is unclear how paclitaxel might enact cell death in interphase without having affected a prior mitosis . it has been hypothesized that paclitaxel may interfere with cell signaling , trafficking , and microtubule - mediated transport ( herbst and khuri , 2003 ; komlodi - pasztor et  al . , 2011 ) . however , in cell culture , clinically relevant levels of paclitaxel do not cause death in interphase cells that have not previously undergone mitosis in the context of drug ( janssen et  al . , 2013 ; zasadil et  al . , observed using intravital microscopy , the mitotic index after treatment with doses of paclitaxel expected to cause mitotic arrest was quite low ( orth et  al . , 2011 ; janssen et  al . , 2013 ) , leading to the suggestion that the microenvironment allows paclitaxel to exhibit interphase effects not observed in culture . to better mimic the antineoplastic effects of paclitaxel in cell culture , we first collaborated with our physician colleagues to design a clinical trial to measure the intratumoral concentration of paclitaxel in primary breast tumors ( zasadil et  al . , 2014 ) . to remove as many confounding variables as possible , patients who had not received prior therapy and did not require concurrent therapy were enrolled . at 20 h after the initiation of the first dose of 175 mg / m paclitaxel the 20-h time point was selected because the mitotic index of breast cancer cells in culture is increased 15-fold between 16 and 32 h after paclitaxel administration , and we therefore predicted that mitotic arrest would be evident at this time point . to assess whether tumors responded , measurements were obtained by ultrasound and/or mammogram before treatment and after four standard cycles of paclitaxel . as predicted by prior cell culture experiments , the intratumoral concentration of paclitaxel ( 19 m ) was higher than the plasma concentration ( 80280 nm ) in all patients . however , contrary to expectations based on cell culture data , mitotic arrest was neither necessary nor sufficient for tumor shrinkage in response to paclitaxel ( zasadil et  al . , 2014 ) . as a second step in determining the appropriate dose of paclitaxel with which to treat our cultured cells , we determined the extent to which the drug was concentrated in breast cancer cell lines . , 1993 , 1996 ; yvon et  al . , 1999 ) , we found that paclitaxel accumulated to a differing extent in distinct cell lines . high - performance liquid chromatography analysis determined that treatment with low nanomolar concentrations of paclitaxel ( 510 nm for mda - mb-231 and 1050 nm for cal51 ) resulted in clinically relevant intracellular concentrations of 19 m . of interest , whereas higher concentrations of drug cause a robust mitotic arrest in these breast cancer cell lines , clinically relevant concentrations do not . importantly , a majority of mitotic cells in patient tumors treated with paclitaxel also exhibit multipolar spindles . after a brief delay , cultured cells in clinically relevant concentrations of paclitaxel proceed through mitosis on multipolar spindles and often segregate their chromosomes in three , four , or five different directions . however , a portion of the cytokinetic furrows usually fail , and most divisions in paclitaxel produce two or three daughter cells ( figure 1 ; zasadil et  al . , 2014 ) . clinically relevant concentrations of paclitaxel cells entering mitosis in the presence of concentrations of paclitaxel equivalent to those in human breast tumors form abnormal spindles that contain additional spindle poles . rather than mounting a long - term mitotic arrest , these cells enter anaphase and divide their chromosomes in multiple directions . however , a portion of the cytokinetic furrows often fail , and two or three daughter cells are usually produced . the resultant daughter cells are aneuploid , and a portion of these die ( red x ) , presumably due to loss of one or more essential chromosomes . because paclitaxel causes mitotic arrest at concentrations typically used in culture , and was believed to do so in human tumors , numerous other drugs that induce mitotic arrest without affecting microtubule dynamics have entered clinical trials . these include inhibitors of aurora a , cenp - e , eg5/ksp , and plk1 . the expectation for these drugs was that they would have the efficacy of paclitaxel without one of its major dose - limiting toxicities  peripheral neuropathy . peripheral neuropathy after paclitaxel therapy is presumed to result from impaired transport along the longest axons in the body , although data for this are scarce , and some chemotherapy drugs that do not affect microtubules , such as cisplatin , cause the same symptoms . disappointingly , despite causing an accumulation of mitotic figures , the new classes of antimitotic drugs have yet to match the efficacy of paclitaxel ( chakravarty et  al . this may be because the cytotoxic effect of paclitaxel in patient tumors is induction of multipolar divisions rather than mitotic arrest . these have identified a diverse array of candidates , including proteasome subunits , cyclin g1 , and solute carrier genes ( rouzier et  al . , 2005 ; , 2007 ; pusztai et  al . , 2009 ; juul et  al . , 2010 ; wertz et  al . , 2011 ; njiaju et  al . , 2012 ; russell et  al . , however , these discoveries have not yet led to a validated biomarker that predicts which patients will benefit from paclitaxel therapy . this may be due , at least in part , to an emphasis on higher concentrations of drug that cause mitotic arrest and rapid cell death . it is hoped that recognition of the clinically relevant mechanism of paclitaxel will facilitate identification of a biomarker capable of predicting which patients will benefit from its use . it is not surprising that the rapid , dramatic effects of higher paclitaxel concentrations on mitosis and cell death were originally believed to be responsible for its efficacy in cancer therapy . unfortunately , the barriers to acquisition of patient samples by basic scientists substantially delayed the finding that lower concentrations , which are slower to evoke cell death , are clinically relevant . however , newly available data demonstrate that , rather than causing mitotic arrest , intratumoral concentrations of paclitaxel cause cell death due to chromosome missegregation on multipolar spindles . it is hoped that , in addition to expediting identification of a predictive biomarker for paclitaxel treatment , this insight will also encourage collaboration between basic scientists and clinicians .