Current Pharmaceutical Design, 2002, 8, 1707-1712 1707

Epothilones: A Novel Class of Non-taxane Microtubule-stabilizing Agents

Ramin Altaha1, Tito Fojo2, Eddie Reed1 and Jame Abraham1*

1West Virginia University, Mary Babb Randolph Cancer Center, Robert C. Byrd Health Sciences Center,Morgantown, West Virginia, 26506, USA 2National Cancer Institute, Medicine Branch, Bethesda, Maryland, USA

Abstract: The epothilones are a novel class of non-taxane microtubule-stabilizing agents obtained from thefermentation of the cellulose degrading myxobacteria, Sorangium cellulosum. Preclinical studies have shownthat the epothilones are more potent than the taxanes and active in some taxane-resistant models. Similar topaclitaxel and other taxanes, the epothilones block cells in mitosis, resulting in cell death. The chiefcomponents of the fermentation process are epothilones A and B, with epothilones C and D found in smalleramounts. Trace amounts of other epothilones have also been detected. Pre-clinical studies have shown thatepothilone B is the most active form, exhibiting significantly higher antitumor activity than paclitaxel anddocetaxel. Several phase I and phase II clinical trials are ongoing with epothilone B and BMS 247550, anepothilone B analog. Preliminary reports indicate these agents are active against human cancers in heavily pre-treated patients. The epothilones appear to be well tolerated, with a side effect profile that is similar to thatreported with the taxanes. This article will review some basic aspects of epothilone chemistry and biology, andpre-clinical and preliminary clinical experience with epothilone B and its analog, BMS 247550.

INTRODUCTION are substrates for P-glycoprotein, a drug efflux pumpresponsible for the extrusion of many cytotoxic compoundsout of multidrug-resistant (MDR) cells, their clinical efficacymay be compromised [18]. The search for new agents hasattempted to identify compounds that are at least as potent,are not substrates for Pgp and are more soluble.

Vincristine, first used in clinical oncology in the 1950s,was the first chemotherapeutic agent to target themicrotubules. Since then the successful introduction ofsimilar agents has established tubulin and the microtubule asan effective target [1]. While vinca alkaloids causeddepolymerization of microtubules, the success of paclitaxelshowed that microtubule stabilization is an equally or moreeffective mechanism of action. Encouraged by the clinicalefficacy of the taxanes efforts have been directed at findingadditional microtubule-stabilizing agents. This search hasidentified several classes of compounds including theepothilones, discodermolide, sarcodyctin/eleutherobin, andlaulimalide [2-6]. As the first of these to be identified, theepothilones, obtained from the fermentation of the cellulosedegrading myxobacteria, Sorangium cellulosum, havereceived special attention [4,5]. As a group the epothilonesare more potent than the taxanes; active against tumorsrefractory to taxanes; and more soluble with enhanced oralbioavailability [5,6].

MICROTUBULES

The basic unit of microtubules is dimers of α and βtubulin that can add at either end of microtubules [19].Microtubules polymerize and depolymerize by the additionand loss of α/β tubulin dimers from their ends [19,20]. Theaddition of tubulin dimers is reversible and noncovalent, butrequires energy. The energy is provided by the hydrolysis ofexchangeable GTP that is bound to α/β dimers, and ishydrolyzed as the dimers are added. In contrast, removal ofα/β dimers from either end of a microtubule does not requireenergy. The ability to add and remove α/β dimers rendersmicrotubules intrinsically dynamic [20,21]. Dynamicinstability occurs because periods of slow growth canalternate with episodes of rapid shortening at microtubuleends. The transition from a state of polymerization to one ofdepolymerization is referred to as a catastrophe; while theopposite transition is known as a rescue [20-22]. Whilemicrotubules can be assembled from purified tubulin, therate of polymerization is less than that which occurs underphysiologic conditions [23-25]. The differences can beexplained in part by the participation under physiologicconditions of additional proteins that modulate microtubuledynamics. Proteins known as microtubule associatedproteins, or MAPs, increase the polymerization rate ofmicrotubules, while destabilizing proteins increase the rateof depolymerization [23,24].

Paclitaxel, a complex diterpene with a baccatin ringsystem, was discovered in 1971 [7]. Subsequently, docetaxelwas synthesized and developed for clinical use [8]. Paclitaxeland docetaxel have shown efficacy against ovarian, breast,head and neck, and lung cancer [9-15]. Their side effectsinclude neutropenia, peripheral neuropathy, and alopecia; andwith docetaxel, fluid retention [16,17]. Because the taxanes

*Address correspondence to this author at Section ofHematology/Oncology, P.O. Box 9162, West Virginia University,Morgantown, WV 26506, Phone: 304 293 4229, Fax: 304 293 2519, email:jabraham@hsc.wvu.edu

1381-6128/02 $35.00+.00 © 2002 Bentham Science Publishers Ltd.

1708 Current Pharmaceutical Design, 2002, Vol. 8, No. 19 Abraham et al.

Differences between the dynamics of interphase andmitotic microtubules reflect differing functions. Spindlemicrotubule dynamics must be rapid to build the spindleand move chromosomes accurately in a short time. Dynamicinstability builds the spindle and establishes attachments tokinetochores [24-27]. As a result of this, the exchange of themicrotubules which comprise the mitotic spindle occurswith half-times of about 15 seconds. In contrast, theinterphase microtubule network is less dynamic so as toprovide a stable framework for intracellular trafficking.Consequently, exchange of the interphase microtubulenetwork occurs with a half-time of ~3 minutes to severalhours.

microtubule stabilization does not lead to activation of theendotoxin-signaling pathways.

To identify tubulin residues important for epothilonebinding, Giannakakou et al. isolated two epothiloneresistant human ovarian carcinoma sublines derived in asingle step selection using either epothilone A or B [38]. Invivo polymerization studies revealed that these epothilone-resistant sublines had impaired epothilone- and taxane-driventubulin polymerization. This failure to polymerize after theaddition of microtubule stabilizing agents was ascribed toacquired mutations in β-tubulin (β274Thr -> IIe and β282Arg-> Gln) located near the taxane binding site. Molecularmodeling was used to identify a common pharmacophoreshared by the taxanes and the epothilones . Two possibleconformations were identified, with one of these favored asthe likely conformation interacting with β-tubulin. Althoughepothilones lack the overall bulk of taxanes, criticalhydrogen bonds formed by the C1-OH, C7-OH, and epoxideoxygen, enable epothilones to interact with high affinitywith tubulin, even though they have significantly lessmolecular volume than taxanes.

The taxanes and the epothilones are consideredmicrotubule-stabilizing agents [4,5,28]. At lowconcentrations, they stabilize microtubule dynamics withouta change in microtubule polymer mass, blocking mitoticspindle function. At high concentrations, they recruit tubulininto microtubules, resulting in an increase in themicrotubule polymer mass and the appearance ofmicrotubule bundles and large dense asters. The latter resultsin disruption of the mitotic spindle. The result of either ofthese is mitotic arrest.

SYNTHETIC EPOTHILONESAlthough microtubule active agents target both

interphase and spindle microtubules, it is generally thoughtthat their cytotoxicity is a consequence of their effect on themitotic spindle. However, their effect on non-dividing cells,most notably nerve cells, suggest that targeting interphasemicrotubules can also lead to toxicity.

In addition to epothilones A and B, other naturallyoccurring epothilones have been identified [39-41]. Amongthese, epothilones C and D are most abundant. Furthermore,because of promising antitumor activity, numerousepothilone analogs have been synthesized [42,43]. Thesediffer mainly in the presence of a hydrogen or methyl groupat C12 and C21 and in the nature of the chemical bondbetween C12 - C13.Epothilones

The epothilones, are 16-membered macrolides producedby the gram-negative myxobacterium, sorangiumcellulosum. The epothilones were first shown to have anarrow anti-fungal spectrum, and only later were found tostabilize microtubules. The chief components of thefermentation process of sorangium cellulosum areepothilones A and B [29-31]. The epothilones were the firstclass of compounds to be described following the originaldiscovery of paclitaxel that mimicked the microtubulestabilizing effect of the taxanes [6,32-34]. While they sharewith the taxanes a similar mechanism of action they lackobvious structural similarity. Although electroncrystallographic studies have identified the taxane-bindingsite on β-tubulin, similar data are not available forepothilones [6,28,35,36]. However, the observation that theepothilones act as competitive inhibitors of paclitaxelbinding to microtubules has been interpreted as evidence thattheir binding site on the microtubule is similar.

Although epothilone B is one of the most potentepothilones in vitro, it remains to be determined whether itis the optimal candidate for cancer therapy [7]. Chou et al.divide the epothilone structure into three zones. Structuralchanges in the C1 - C8 acyl sector, are not tolerated, sincethey adversely affect in vitro cytotoxicity and microtubulestabilizing ability [37]. This stands in contrast to the C9 -C15 O-alkyl sector and the C15-pendant aryl sector, whereconsiderable modification of structure is tolerated. Inaddition, Chou et al. were able to demonstrate in both invitro and in vivo experiments that Z-12-13-desoxyepothilone B (desoxyepothilone B) had superior therapeuticefficacy and less toxicity compared to the parent, epothiloneB, as well as epothilone A, desoxyepothilone A andpaclitaxel. These results indicate that the epoxide portion ofthe epothilone ring is not essential for activity. They do not,however, clarify which agent is likely to have the besttherapeutic index in humans.

The epothilones, posses several qualities that make themattractive when compared to the taxanes. They are water-soluble, can be produced in larger quantities throughbacterial fermentation and retain activity against multidrug-resistant (MDR) cell lines and tumors [5,37]. Unlikepaclitaxel, however, they lack endotoxin activity. The latteroccurs as a result of paclitaxel mediated macrophageactivation that results in the synthesis of pro-inflammatorycytokines and nitric oxide. However, epothilone-mediated

BMS-247550 is a semi-synthetic analog of the naturalepothilone B. BMS-247550 has been developed by Bristol-Myers Squibb for use in treatment of cancer [42]. Themolecular formula of BMS-247550 is C27H42N2O5S. Thekey difference between BMS-247550 and epothilone B is thereplacement of the macrolide-ring oxygen atom with anitrogen atom to give the corresponding macrolactam.Although the molecular structural differences between BMS-247550 and epothilone B are subtle, accumulating evidence

Epothilones: A Novel Class of Non-taxane Microtubule-stabilizing Agents Current Pharmaceutical Design, 2002, Vol. 8, No. 19 1709

indicates they may be clinically important as discussedbelow. BMS-247550 is orally efficacious, with anti-tumoractivity after oral administration comparable to that producedby parenteral administration of the drug. Synergistic activityof BMS-247550 with a number of antineoplastic agents hasbeen demonstrated in vitro [42,43].

8 mg/m2. Other grade 3 non-hematologic toxicities includedfatigue (n=4), nausea/vomiting (n=2) and grade 2 peripheralneuropathy (n=3). Myelosuppression was infrequently noted,and was grade 1 or less in those patients in whom it wasrecorded. Despite the lack of myelosuppression, antitumoractivity was observed, with a partial response (PR) in onepatient with an unknown primary, and stable disease in11patients, including significant responses not reachingcriteria for PR in 4 patients (1 breast, 3 colorectal cancers).CLINICAL EXPERIENCE WITH EPOTHILONE B

AND BMS 247550Six phase I clinical trials using BMS 247550 have been

reported. In the study by Mani et al., BMS 247550 wasadministered as a one-hour infusion every 3 weeks [46]. Oftwelve patients enrolled, ten had received prior chemotherapyincluding 2 patients who had received taxane/platinum basedregimens. Five dose levels ranging from 7.4 mg/m2 to 59.2mg/m2 were evaluated. The maximum tolerated dose wasdetermined to be 50 mg/m2, with dose-limitingneutropenia/sepsis, fatigue, neuropathy, anorexia, andweakness noted. Responses were observed in taxane-refractory breast and ovarian cancer.

Numerous phase I and phase II clinical trials are ongoingusing both the naturally occurring epothilone B, and theepothilone analog, BMS 247550. Additional trials usingepothilone D may start in the future. Extensive pre-clinicaldata together with preliminary clinical experience indicatesthat epothilone B and BMS 247550 have antitumor activityand appear to be well tolerated. We will review thepublished clinical data on epothilone B and BMS 247550.Data is available in abstract form (Table 1).

Two phase I clinical trials have been publishedadministering epothilone B on either a weekly schedule oran every three weeks schedule. In one of the epothilone Btrials, Rubin et al. enrolled 36 patients at doses rangingfrom 0.3 to 3.6 mg/m2 administered intravenously everyweek [44]. Three of six patients treated with 3.6 mg/m2developed dose limiting diarrhea, while only one of the sixpatients treated at a dose of 2.5 mg/m2, had dose-limitingdiarrhea. Other toxicities, including myelosuppression, wereminimal. Among these heavily pretreated patients twopartial responses (in patients with breast and ovarian cancer),and three minimal responses (in patients with colon, lungand ovarian cancer) were reported.

Awada et al. conducted a phase I study administeringBMS-247550 as a weekly 30-minute infusion [47]. At thetime of publication, 20 patients had received doses of 1, 2.5,5, 10, 20, and 25 mg/m2/week with accrual ongoing at the30 mg/m2 dose level. Following two cases of grade 3-4hypersensitivity reactions (HSR), all subsequent patientswere pre-medicated with diphenhydramine and ranitidine,without further cases of HSR. Although no first course doselimiting toxicity was observed up to the 25 mg/m2 doselevel, grade 1 - 2 fatigue, anorexia, arthralgia/myalgia, andneutropenia were reported more frequently at the 20 and 25mg/m2 dose levels. Disease stabilization was observed in 5patients, including 3 patients pretreated with a taxane.

In the other epothilone B trial, Calvert et al. treatedpatients with advanced solid tumors with epothilone B everythree weeks [45]. Forty-two patients were enrolled, includingpatients with colorectal (13), breast (5); ovarian (4), and non-small cell lung cancers (3) as well as a broad range of othertumor types. The starting dose was 0.3 mg/m2 with the first6 dose levels given as 30-minute infusions every 21 daysand subsequent dose levels as a 5 to 10 minute infusionevery 21 days. Dose limiting toxicity (diarrhea) was seen at

Spriggs et al.. completed accrual to a phase I trial ofBMS-247550 administered once every 3 weeks as a one-hourinfusion [48]. Thirty-one patients were treated at dose levelsof 7.4, 15, 30, 50, 57 and 65 mg/m2. At doses below 50mg/m2, no dose limiting toxicity (DLT) was observed. Likepaclitaxel, BMS-247550 is formulated in Cremophor-EL®,and a hypersensitivity reaction was observed in one patienttreated at 30 mg/m2, prompting the institution of a

Table 1.

Author Epothilone Number Schedule MTD DLT Responses

Rubin et al.[44] Epothilone B 36 weekly 2.5mg/m2 diarrhea 2 PR

Calvert, et al.[45] Epothilone B 42 every 21 d NA diarrhea 1CUP

Manni et al.[46] BMS 247550 12 every 21 d 50mg/m2 neutropenia 3PR

Awada et al. [47] BMS 247550 20 weekly NA NA NA

Spriggs et al. [48] BMS 247550 31 every 21 d 50mg/m2 neuropathy 1CR3PR

LoRusso et al. [49] BMS247550 7 every 21 d NA NA NA

Fojo et al. [50] BMS247550 10 weekly 6mg/m2 Neutropenia 4 PR

Hao et al. [51] BMS247550 10 weekly NA NA 2 PR

MTD, maximum tolerated dose- DLT, dose related toxicity- PR, partial response- CUP, Carcinoma of unknown primary. NA not available

1710 Current Pharmaceutical Design, 2002, Vol. 8, No. 19 Abraham et al.

prophylactic regimen consisting of oral H1/H2 blockers,without subsequent events. First-course DLT wasexperienced by 2 of 3 patients at 65 mg/m2 (grade 3neuropathy and prolonged grade 4 neutropenia in one patientand prolonged grade 4 neutropenia in the second patient) andby all 3 patients at 57 mg/m2 (grade 3 arthralgia andmyalgia in 2 patients and grade 4 neutropenia withpneumococcal sepsis and death in 1 patient). After themaximum tolerated dose (MTD) was established as 50mg/m2, a cohort of 10 additional patients was treated at thisdose. Severe toxicity, requiring a dose reduction, wasobserved in only one of these patients (febrile neutropenia).Other toxicities included fatigue, weakness, constipation,diarrhea, nausea, vomiting, rash, alopecia and low-gradeneuropathy. Anti-tumor activity seen among the 22 patientstreated at 50 mg/m2 and above included a complete responsein a patient with primary peritoneal disease who had priortreatment with paclitaxel, partial responses in two patientswith melanoma and a patient with NSCLC previouslytreated with docetaxel, and stable disease in 11 patients.

dosing during course one. Non-hematologic toxicities acrossall courses were grade 2 or less including fatigue (6patients), arthralgias (4), alopecia (4), nausea (4), vomiting(3), diarrhea (3), myalgias (3), peripheral neuropathy (3),dysguesia (2) and nail discoloration (2). Two ovarian cancerpatients had decreases in CA-125 of 82% and 39%, and onepatient with melanoma had a minor response.

Further data will be needed to discriminate between thetwo epothilones that have thus far been tested clinically.While it was thought that the chemical modification inBMS 247550 would result in a compound with improved invivo metabolic stability when compared to its naturalprecursor, epothilone B, the available data appears tocontradict this in some respects. The availablepharmacokinetics indicates that the clearance of EPO B (231ml/min) is substantially slower than the clearance of BMS247550 (630 ml/min). Similarly, the terminal t1/2 of 81hours for EPO B is substantially longer than the 35 hoursfor BMS 247550. Given this, it was not surprising that witha weekly administration schedule, the MTD of EPO B wasonly 2.5 mg/m2 3 weeks on, 1 week off.In a similar study, Lo Rosso et al. reported an ongoing

trial administering BMS 247550 again as a one-hourintravenous infusion, given every 3 weeks [49]. The resultsin the first seven patients enrolled have been reported.Starting with a dose of 7.4 mg/m2 escalation to 40 mg/m2

was achieved in the first seven patients. Minor toxicitiesincluding grade 1 fatigue (three patients) and grade 1 nausea(one patient) were observed. At the 28.6 mg/m2 dose, grade3/4 prolonged thrombocytopenia and neutropenia occurred ina heavily pretreated breast cancer patient with marrowinvolvement. No other myelotoxicity was noted.

CONCLUSION

The epothilones are a novel class of non-taxanemicrotubule stabilizing natural products. Both pre-clinicaland clinical studies have shown that epothilones haveantitumor activity, and in pre-clinical models are activeagainst paclitaxel-resistant and multidrug-resistant (MDR)cells. The epothilones are extremely flexible compounds andnumerous analogues have been synthesized. Epothilone Band its analogue, BMS 247550, have been most extensivelystudied.

Fojo et al. presented the results of a regimenadministering BMS 247550 as a one-hour infusion on days1 to 5 every 21 days [50]. A total of 21 patients wereenrolled. Sixteen patients had received prior taxane therapyincluding paclitaxel (13 patients), or both paclitaxel anddocetaxel (3 patients). Dose levels included 1.5, 3, 6 and 8mg/m2/d administered on each of five successive days. Allthree patients who received 8 mg/m2/d without GCSF incycle 1 experienced DLT (neutropenia). Subsequently, 6mg/m2/d was identified as the maximum tolerated dose(MTD) without GCSF, and is the recommended phase IIdose. Objective responses were observed in patients withbreast and cervical cancer; with >50% reduction in CA125 in2/12 patients with advanced ovarian cancer (all breast,cervical and ovary cancer patients had prior taxane therapy).Hypersensitivity reactions were not observed using apremedication regimen consisting of H1 and H2 antagonistswithout steroids prior to each dose of BMS-247550.

Several phase I clinical trials have reported dose limitingtoxicities and maximum tolerated doses of either epothiloneB or BMS 247550. Dose limiting toxicities with epothiloneB include diarrhea and fatigue, while with BMS 247550,neutropenia and peripheral neuropathy have been reported.The side effect profile of BMS 247550 differed withtreatment schedule. Neuropathy was more profound with theevery three-week administration schedule but neutropeniawas more common with weekly and daily times fiveschedules of adminstration. Since BMS-247550 isformulated in Cremophor EL® some hypersensitivityreactions were noted although pre-treatment H1 and H2blockers with or without steroids seems to have beensuccessful in preventing their occurrence. Responses havebeen noted at higher dose levels in a variety of cancers, bothin heavily pretreated patients and patients who failed orprogressed through prior taxane therapy. Further studies willbe needed to determine the role, if any, of EPO B,BMS247550 and other epothilones in clinical oncology.Nevertheless, it is encouraging that with these agents:activity has been demonstrated at tolerable dose levels inpatients, including many with taxane-resistant tumors. It isalso encouraging that this activity has been observed atdoses where toxicities are manageable. There is alsopreliminary evidence suggesting the epothilones may beindifferent to the co-administration of CYP3A4 agents, an

Finally, Hao et al. reported administering BMS 247550intravenously over 60 minutes weekly x 4 every 28 dayswithout a built in rest period [51]. Ten patients received 20courses of BMS 247550 at escalating doses of 2.5, 5, 10, 20and 30 mg/m2/wk, using an accelerated titration studydesign. 50% of patients had received prior taxanes. Allpatients were pre-medicated with dexamethasone 20 mg po12 and 6 hours prior to BMS 247550, and a combination ofdiphenhydramine 50 mg IV and ranitidine 50 mg IV 30minutes prior to treatment. Three patients had non-doselimiting grade 3-4 neutropenia that interrupted continuous

Epothilones: A Novel Class of Non-taxane Microtubule-stabilizing Agents Current Pharmaceutical Design, 2002, Vol. 8, No. 19 1711

observation that while not essential would make theirclinical use easier.

[16] Rowinsky, E.K., Eisenhauer, E.A., Chandhry, V., Arbuck,S.G., Donehower, R.C. Semin. Oncol., 1993, 20 , 1.

[17] Benber, R.A., Hamel, E., Handle, K.P., CancerChemotherapy, Principles Practice, Chabner, B.A., andCollins, J.M. (eds.), Philadelphia, 1990, pp. 253-274.

Preliminary results from clinical trials with epothilone Band the epothilone B analog, BMS 247550 are promising.Phase II studies have been initiated in variety of diseases.Additional information on dose levels, details of the sideeffect profiles and reports of anti-tumor activity are expectedfrom ongoing clinical trials. The challenges that remaininclude: (1) the demonstration of activity in breast, ovarianand NSCL cancers unresponsive to taxanes de-novo; (2) thedemonstration of activity in taxane-refractory solid tumors;and (3) the identification of an optimal schedule.

[18] Zhan, Z., Scala, S., Monks, A., Hose, C., Bates, S., Fojo, T.,Cancer Chemother. Pharmacol., 1997, 40 (3), 245.

[19] Desai, A., Mitchison, T.J. Annu. Rev. Cell Dev. Biol., 1997,13 , 83.

[20] Mitchison, T., Kirschner, M. Nature, 1984, 312, 237.

[21] Walker, R.A. J. Cell Biol., 1988, 107, 1437.

REFERENCES [22] Horio, T., Hotani, H. Nature, 1986, 321, 605.

[23] Cassimeris, L., Pryer, N.K., Salmon, E.D. J. Cell Biol.,1988, 107, 2223.

[1] Sackett, D.L., Fojo, T. Cancer Chemother. Biol. Resp.Modif., 1999, 18 , 59.

[24] Rusan, N.M., Fagerstrom, C.J., Yvon, A.M., Wads-Worth,P. Mol. Biol. Cell, 2001, 12 , 971.

[2] Kowalski, R.J., Giannakakou, P., Gunasekera, S.P.,Longley, R.E., Day, B.W., Hamel, E. Mol. Pharmacol.,1997, 52, 613. [25] Belmont, L.D., Hyman, A.A., Sawin, K.E., Mitchison, T.J.

Cell, 1990, 62 , 579.[3] Balachandran, R., ter Haar, E., Welsh, M.J., Grant, S.G.,Day, B.W. Anti-Cancer Drugs, 1998, 9, 67. [26] Hayden, J.H., Bowser, S.S., Rieder, C.L. J. Cell Biol., 1990,

111, 1039.[4] Bollag, D.M., Mc Queney, P.A., Zhu, J., Hensens, O.,Koupal, L., Liesch, J., Goetz, M., Lazarides, E., Woods,C.M. Cancer Res., 1995, 55 , 2325. [27] Kinoshitak K., Arnal, I., Desai, A., Drechsel, D.N., Hyman,

A.A. Science, 2001, 294, 1340.

[5] Kowalski, R.J., Giannakakou, P., Hamel, E. J. Biol. Chem.,1997, 272, 2534. [28] Giannakakou, P., Gussio, R., Nogales, E., Downing K.H.,

Zaharevitz, D., Bollbuck, B., Poy, G., Sackett, D.,Nicolaou, K.C., Fojo, T. Proc. Natl. Acad. Sci., 2000, 97 ,2904.

[6] Chou, T-C., Zhang, X-G., Harris, C.R., Kuduk, S.D., Balog,A., Savin, K.A., Bertino, J.R., Daneshefsky, S.J. Proc.Natl. Acad. Sci., 1998, 95 , 15798. [29] Bryan, J.,Shah, S., Ziermann, R., Goldman, R., Katz, L.,

Khosla, C. Gene, 2000, 249, 153.[7] Wani, M.C., Tylor, H.L., Wall, M.E., Coggon, P., McPhail,A.T. J. Am. Chem. Soc., 1971, 93 , 2325. [30] Gerth, K., Bedorf, N., Hoefle, G., Irschik, H., Reichenbach,

H. J. Antibiot., 1996, 49 , 560.[8] Lynch, T.J. Jr. Semin. Oncol., 2001, 28(3 Suppl .9), 5-9.

[31] Mann, J. Nature, 1997, 385, 117.[9] McGuire, W.P., Rowinsky, E.K., Rosenshein, N.B.,Grumbeine, F.C., Ettinger, D.S., Armstrong, D.K.,Donehower, R.C. Ann. Intern. Med., 1989, 111, 273. [32] Nicolaou, K.C., Winssinger, N., Vourloumis, D., Ohshima,

T., Pfefferkorn, J., Xu, J.Y., Li, T. J. Am. Chem. Soc., 1998,120, 10814.[10] Holmes, F.A., Walters, R.S., Theriault, R.L., Forman, A.D.,

Newton, L.K., Raber, M.N., Buzdar, A.U., Frye, D.K.,Hortobagyi, G.N. J. Natl. Cancer Inst., 1991, 83 , 1797. [33] Cowden, C.J., and Paterson, I. Nature, 1997, 387, 238.

[34] Service, R.F. Science, 1996, 274, 2009.[11] Forastiere, A.A. Semin. Oncol., 1993, 20 , 56.

[35] Nicolaou, K.C., Finlay, M.R., Ninkovic, S., King, N.P.,Yun, H., Li, T., Sarabia, F., Vourloumis, D. Chemistry andBiology, 1998, 5, 365.

[12] Hainsworth, J.D. Burris, H.A.III., Yardley, D.A, Bradof,J.E., Grimaldi, M., Kalman, L.A., Sullivan, T., Baker, M.,Erland, J.B., Greco, F.A. J. Clin. Oncol., 2001, 19(15),3500. [36] Hardt, I.H, Steinmetz, H., Gerth, K., Sasse, F.,

Reichenbach, H., Hofle, G. J. Nat. Prod., 2001, 64(7), 847.[13] Ettinger, D.S. Semin. Oncol., 1993, 20 , 46.

[37] Chou, T.C., Zhang, X.G., Balog, A., Su, D.S., Meng, D.,Savin, K., Bertino, J., Danishefsky, S. Proc. Natl. Acad.Sci., 1998, 95 , 9642.

[14] Kohn, E.C., Sarosy, G.A., Davis, P., Christian, M., Link,C.E., Ognibene, F.P, Sindelar, W.F., Jacob, J., Steinberg,S.M., Premkumar, A., Reed, E. Gynecol. Oncol., 1996, 62(2), 181. [38] Giannakakou, P., Sackett, D.L., Kang, Y.K., Zhan, Z.,

Buters, J.T., Fojo, T., Poruchynsky, M.S. J. Biol. Chem.,1997, 272, 17118.[15] Reed, E., Sarosy, G., Kohn, E., Christian, M., Link, C.J. Jr.,

Goldspiel, B., Davis, P., Jacob, J., Maher, M. Gynecol.Oncol., 1996, 61(3), 349.

1712 Current Pharmaceutical Design, 2002, Vol. 8, No. 19 Abraham et al.

[39] White, J.D., Carter, R.G., Sundermann, K.F., Wartmann, M.J. Am. Chem. Soc., 2001, 123, 5407.

[46] Mani, S., McDaid, H., Shen, H.J., Sparano, J.A., Hamilton,A., Runowicz, C., Hochster, H., Muggia, F., Fields, A.,Damle, B., Letrent, S., Lebwohl, D., Horwitz, S.B.Proceedings Amer. Soc. Clin. Oncol., 2001.[40] Altmann, K.H., Bold, G., Caravatti, G., Florsheimer, A.,

Guagnano, V., Wartmann, M. Bioorg. Med. Chem. Lett.,2000, 18, 10(24), 2765. [47] Awada, A., Bleiberg, H., de Valeriola, D., Huart, A.,

Cornez, N., Letrent, S., Martin, C., Piccart, M. ProceedingsAmer. Soc. Clin. Oncol., 2001.[41] Nicolaou, K.C., Hepworth, D., King, N.P., Finlay, M.R.,

Scarpelli, R., Pereira, M.M., Bollbuck, B., Bigot, A.,Werschkun, B., Winssinger, N. Chemistry 2000, 6, 2783. [48] Spriggs, D., Soignet, S., Bienvenu, B., Letrent, S.,

Lebwohl, D., Jones, S., Burris, H. Proceedings Amer. Soc.Clin. Oncol., 2001.[42] Lee, F.Y., Borzilleri, C.R., Fairchild, C.R., Kim, S.H., Long,

B.H., Reventos-Suarez, C., Vite, G.D., Rose, W.C., Kramer,R.A. Clin. Cancer Res., 2001, 7, 1429. [49] LoRusso, P.M., Wozniak, A.J., Flaherty, L.E., Shields,

A.F., Wright, J., Lebwohl, D. Proceedings Amer. Soc. Clin.Oncol., 2001.[43] Yamaguchi, H., Paranawithana, S.R., Lee, M.W., Huang, Z.,

Bhalla, K.N., Wang, H.G. Cancer Res., 2002, 62 , 466.[50] Fojo, A.T., Kotz, H., Abraham, J., Agrawal, M., Bates, S.E.,

Balis, F., Widemann, B., Bakke, S., Edgerly, M., Rutt, A.,Damle, B., Sonnichsen, D., Lebwohl, D, AACR-NCI-EORTC, Abstract, 2001.

[44] Rubin, E.H., Siu, L.L., Beers, S., Moore, M.J., Thompson,C., Becker, M., Chen, T.L., Cohen, P., Rothermel, J., Oza,A.M. Proceedings Amer. Soc. Clin. Oncol., 2001.

[45] Calvert, P.M., O’Neill, V., Twelves, C., Azzabi, A., Hughes,A., Bale, C., Robinson, A., Machan, M., Dimitrijevic, S.,Moss, D., Rothermel, J., Cohen, P., Chen, T., Man, A.,Calvert, A.H. Proceedings Amer. Soc. Clin. Oncol., 2001.

[51] Hao, D., Hammond, L.A., Berg, K.E., Bass, A., Mays, T.A.,Smith, L., Drengler, R., Rowinsky, E.K., AACR-NCI-EORTC, Abstract, 2001.

RELATED ARTICLES RECENTLY PUBLISHED INCURRENT PHARMCEUTICAL DESIGN

Fahy, J. and Hill, B. T. Tubulin-interacting Agents. Epilogue.Curr. Pharm. Des., 2001, 7(13), 1297-301.

Stachel, S. J.; Biswas, K., and Danishefsky, S. J. The Epothilones,Eleutherobins, and Related Types of Molecules. Curr.Pharm. Des., 2001, 7(13), 1277-90.

Uckun, F. M.; Mao, C.; Jan, S. T.; Huang, H.; Vassilev, A. O.;Navara, C. S., and Narla, R. K. Spongistatins as TubulinTargeting Agents. Curr. Pharm. Des., 2001, 7(13), 1291-6.