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1、Recent Advances in the Chemistry and Biology of New Generation TaxoidsIwao Ojima* and Manisha DasDepartment of Chemistry and Institute of Chemical Biology & Drug DiscoVery, State UniVersity of New York at Stony Brook,Stony Brook, New York 11794-3400ReceiVed October 15, 2008Among the numerous chemoth

2、erapeutic drugs, paclitaxel and docetaxel are among the most widely used against varioustypes of cancer. However, these drugs cause undesirable side effects as well as drug resistance. Therefore, it is essentialto develop “taxane” anticancer agents with better pharmacological properties and improved

3、 activity especially againstdrug-resistant cancers. Several laboratories have performed extensive SAR studies on paclitaxel. Our SAR studies haveled to the development of numerous highly potent novel second- and third-generation taxoids with systematic modificationsat the C-2, C-10, and C-3 position

4、s. The third-generation taxoids showed virtually no difference in potency againstdrug-resistant and drug-sensitive cell lines. Some of the new generation taxoids also exhibited excellent cytotoxicityagainst pancreatic cell lines expressing multidrug-resistant genes. We have also designed taxoids wit

5、h strategic fluorineincorporation to investigate their effects on the cytotoxicity and the blockage of known metabolic pathways. Furthermore,we have successfully employed computational biology analysis to design novel macrocyclic taxoids to mimic the bioactiveconformation of paclitaxel. This account

6、 describes our work on the design, synthesis, and biological evaluation of thesenovel taxoids, which has led to the discovery of very promising candidates for further preclinical studies.IntroductionCancer is one of the leading causes of death in the world and isthe leading cause of death for people

7、 under the age of 85 in theUnited States.1,2Paclitaxel and docetaxel are among the most widelyused chemotherapeutic drugs especially against some types ofcancer such as ovarian, breast, lung, and Kaposis sarcoma.3,4Theseanticancer agents bind to the ?-tubulin subunit of the tubulin dimerand accelera

8、te their polymerization, resulting in stabilized micro-tubules. This causes the arrest of the cell division cycle mainly atthe G2/M phase, resulting in apoptosis through the cell-signalingcascade.5,6Despite their potent antitumor activity, paclitaxel anddocetaxel cause undesirable side effects as we

9、ll as drug resistance.3Thus, it is essential to develop new taxane anticancer agents thatwill have fewer side effects, enhanced activity against drug-resistanthuman tumors, and superior pharmacological properties.Our SAR study on taxoids has indicated that (i) the C-3-phenylgroup can be replaced wit

10、h an alkenyl or alkyl group and (ii) theC-10 position can be modified with certain acyl groups that makethe compounds 1-2 orders of magnitude more potent than theparent drugs (paclitaxel and docetaxel) against drug-resistant humanbreast cancer cell lines. These highly potent taxoids were termed“seco

11、nd-generation taxoids”.7We have also found that substitution(MeO, N3, Cl, F, etc.) at the meta position of the C-2-benzoyl groupof the second-generation taxoids enhanced their activities comparedto the parent drugs against drug-resistant human breast cancer celllines.8,9Multidrug resistance to pacli

12、taxel is caused mainly by theoverexpression of ABC transporters, e.g., Pgp,10but there are othermechanisms of drug resistance such as the overexpression ofspecific tubulin isotypes.11-14The microtubules with altered?-tubulin isotype compositions respond differently to paclitaxel.15Recently, Ferlini

13、has reported that the C-seco-taxoid IDN 5390 is8-fold more active than paclitaxel against drug-resistant andpaclitaxel-resistant cell lines.16-18We have synthesized a seriesof IDN 5390 analogues with C-2-benzoate modifications at the metaposition to investigate their effects on cytotoxicity.Fluorine

14、 is an important heteroatom used in drug design19because of its favorable atomic properties that result in highermetabolic stability, often increased binding to target molecules, andincreased lipophilicity and membrane permeability among otherproperties in biologically active compounds. Accordingly,

15、 we havesynthesized fluoro-taxoids to investigate the effects of fluorineincorporation on the cytotoxicity and the blockage of knownmetabolic pathways.20-23We have also designed novel macrocyclic paclitaxel congenersto mimic the bioactive conformation of paclitaxel based oncomputational analysis.24T

16、his account describes our work on thesynthesis and biological evaluation of new generation taxoidsbearing various substituents at the C-2, C-10, C-3, and C-3 Npositions as well as macrocyclic taxoids.Synthesis of C-10-Modified TaxoidsSecond-generation taxoids such as 6, bearing various C-10aromatic

17、acyl groups, were synthesized from 10-deacetylbaccatinIII (DAB, 1), by applying the procedure developed by Georg25via7-TES-baccatin III 2, Ojima-Holton coupling7,26of 2 with N-t-Boc-?-lactam 3a,27,28deacetylation, C-10-acylation, and deprotec-tion by HF-pyridine (Schemes 1 and 2, Table 1).29Synthesi

18、s of C-2- and C-3-Modified TaxoidsSecond-generation taxoids 12 and 13 (Table 3), with differentsubstituents at the meta position of the C-2 benzoyl group, weresynthesized through the Ojima-Holton coupling7,30of baccatins11a-l (Scheme 3, Table 2) with ?-lactams 3a-f, as illustrated inScheme 4.29Bacca

19、tins 11a-l were synthesized via 7,10,13-tris-TES-2-debenzoyl-DAB 8,31,32C-2-modified tris-TES-DABs 9a-f,7-TES-2-modified DABs 10a-f, and selective acylation at the C-10position of 10a-f (Scheme 3, Table 2). Enantiopure ?-lactams 3a-fwith various C-4 substituents were prepared through efficient chira

20、lester enolate-imine cyclocondensations7,9,26,28,30,33or 2+2ketene-imine cycloaddition, followed by enzymatic optical resolu-tion.34Taxoid 13g was obtained by hydrogenation of 12g on Pd/C(Scheme 4).Synthesis of C-3N-Modified TaxoidsC-3N-Modified taxoids 15 and 16 were synthesized usingenantiopure ?-

21、lactams bearing various N-acyl or N-carbalkoxygroups (Scheme 5, Table 4).29These ?-lactams were prepared byreacting NH-free 3-TBSO- or 3-TIPSO-?-lactam with acid chloridesor chloroformates. The resulting ?-lactams 14a-h were coupledwith baccatin 11b followed by deprotection of silyl groups to afford

22、Dedicated to Dr. David G. I. Kingston of Virginia Polytechnic Instituteand State University for his pioneering work on bioactive natural products.* To whom correspondence should be addressed. Tel: +631-632-1339.Fax: +631-632-7942. E-mail: iojimanotes.cc.sunysb.edu.J. Nat. Prod. 2009, 72, 55456555410

23、.1021/np8006556 CCC: $40.75 2009 American Chemical Society and American Society of PharmacognosyPublished on Web 02/24/2009the desired taxoids 15a-l. Hydrogenation on Pd/C of selected 15gave the corresponding taxoids 16 in nearly quantitative yields.Synthesis of C-3-Difluoromethyl- andC-3-Trifluorom

24、ethyl-taxoidsA series of the second-generation taxoids 21 and 22 with C-3-CF2H- and C-3-CF3- groups, respectively, were synthesized from?-lactams 17 (Rf) CF2H or CF3) and baccatins 18 by means ofthe ?-lactam synthon method,35as shown in Scheme 6.36Synthesis of C-2-(3-Fluorobenzoyl)-C-seco-taxoidsRec

25、ently, a C-seco-taxoid, IDN5390 (Figure 1), was reportedto exhibit several times better potency than paclitaxel againstdrug-resistant ovarian cancer cell lines overexpressing the classIII tubulin isotype.16As a part of our SAR study on IDN5390,Scheme 1aaReagents and conditions: (i) (a) TESCl, imidaz

26、ole, (b) LiHMDS, AcCl, 95% in two steps; (ii) LiHMDS, THF, 95%; (iii) N2H4H2O, EtOH, 85%.Scheme 2aaReagents and conditions: (i) (a) RCl, TEA, DMAP, CH2Cl2or (b) ROH,DIC, DMAP, CH2Cl2or (c) RCl, LiHMDS, THF, -20 C (75-98%); (ii) HF/pyridine, pyridine/MeCN (80-97%).Table 1. Synthesis of Taxoids 6taxoi

27、dR6aBz6b2-MeO(C6H4)CO6c3-MeO(C6H4)CO6d4-MeO(C6H4)CO6e3,4-(MeO)2(C6H3)CO6f1-naphthoyl6g2-naphthoyl6hCbz6i2-MeO(C6H4)CH2CO6j3-MeO(C6H4)CH2CO6k4-MeO(C6H4)CH2CO6l(C6H5)(CH2)2CO6m2-MeO(C6H4)(CH2)2CO6n3-MeO(C6H4)(CH2)2CO6o4-MeO(C6H4)(CH2)2COTable 2. 2,10-Modified Baccatins 11baccatinR1R211aMeMeCO11bMeOMeC

28、O11cFEtCO11dClEtCO11eN3EtCO11fCH2dCH-EtCO11gMeOEtCO11hMeOc-PrCO11iMeOMeOCO11jMeOPhCH2OCO11kMeO2-MeO(C6H4)CO11lMeO4-MeO(C6H4)CH2COTable 3. Second- and Third-Generation Taxoids 12 and 13taxoidR1R2R312aMeMeCOMe2CdCH-12bMeOMeCOMe2CdCH-12cFEtCOMe2CdCH-12dClEtCOMe2CdCH-12eN3EtCOMe2CdCH-12fCH2dCH-EtCOMe2Cd

29、CH-12gMeOEtCOMe2CdCH-12hMeOc-PrCOMe2CdCH-12iMeOMeOCOMe2CdCH-12jMeOPhCH2OCOMe2CdCH-12kMeO2-MeO(C6H4)COMe2CdCH-12lMeO4-MeO(C6H4)CH2COMe2CdCH-12mMeOEtCOCH2dCHCH2-12nMeOEtCO(E)-CH3CHdCH-12oMeOEtCOCH2dCH(CH2)2-12pMeOEtCO(S)-2,2-Me2-c-Pr-13cFEtCOMe2CHCH2-13dClEtCOMe2CHCH2-13eN3EtCOMe2CHCH2-13gMeOEtCOMe2CH

30、CH2-Table 4. C-3N-Modified Second-Generation Taxoids 15 and 16taxoidR1R415aHcyclobutyl15bHcyclopentyl15cHcyclohexyl15dHcyclopent-1-enyl15eHcyclohex-1-enyl15fHcyclopentyloxy15gHcyclohexyloxy15hHcyclopropyl15iHcyclobutyl15jHcyclopentyl15kHcyclohexyl15lHcyclohexyloxy16bHcyclopentyl16cHcyclohexyl16fHcyc

31、lopentyloxy16gHcyclohexyloxy16hMeOcyclopropyl16kMeOcyclohexyl16mMeOcyclopentyloxyReViewsJournal of Natural Products, 2009, Vol. 72, No. 3555we investigated two fluorine-containing analogues, SB-T-10104(28a) and SB-T-10204 (28b) (Scheme 7). These two C-seco-fluorotaxoids, 28a and 28b, were synthesize

32、d through theOjima-Holton coupling of 7,9-di-TES-2-(3-fluorobenzoyl)-C-seco-baccatin 25 with ?-lactams 26a28and 26b,27respectively.Di-TES-C-seco-baccatin 25 was prepared from 2-(3-fluoroben-zoyl)-10-deacetylbaccatin 2337using Appendinos protocol38,39as follows: Baccatin 23 was oxidized with Cu(OAc)2

33、and air togive the corresponding 10-oxo-baccatin 24, which was thentreated withL-selectride, followed by TES protection, to afforddi-TES-C-seco-baccatin 25 (Scheme 7).Synthesis of C-3-Difluorovinyl-taxoidsOur recent metabolism studies on 3-isobutyl- and 3-isobutenyl-taxoids has disclosed that the me

34、tabolism of second-generationtaxoids (SB-T-1214, SB-T-1216, and SB-T-1103) is markedlydifferent from those of docetaxel and paclitaxel.40These taxoidsare metabolized by CYP 3A4 of the cytochrome P450 familyenzymes primarily at the two allylic methyl groups of the C-3-isobutenyl group and the methine

35、 moiety of the 3-isobutyl group(Figure 2). This is a sharp contrast from the known result that thetert-butyl group of the C-3N-t-Boc moiety is the single predominantmetabolic site for docetaxel.41This prompted us to design andScheme 3aaReagents and conditions: (i) TESCl, imidazole, DMF, RT, 96%; (ii

36、) sodium bis(2-methoxyethoxy)aluminum hydride, THF, -10 C, 97%; (iii) DIC, DMAP,CH2Cl2, 85-90%; (iv) HF/pyridine, pyridine/MeCN; (v) TESCl, imidazole, DMF, RT, 2 h 71-95% for two steps; (vi) LiHMDS, R2COCl, THF, 67-98%.Scheme 4aaReagents and conditions: (i) 3a-f (1.2-1.5 equiv), LiHMDS, THF, -40 C,

37、30 min; (ii) HF/pyridine, pyridine/MeCN, 0 C - RT, 18 h, 65-95% for two steps;(iii) H2/Pd-C, EtOAc/MeOH, RT, 24 h, 70-89%.Scheme 5aaReagents and conditions: (i) LiHMDS, THF, -40 C, 30 min; (ii) HF/pyridine, pyridine/MeCN, 0 C to RT, 18 h (61-86% for two steps); (iii) H2, Pd/C, EtOAc,RT, 24 h (95-98%

38、).556Journal of Natural Products, 2009, Vol. 72, No. 3ReViewssynthesize 3-difluorovinyl-taxoids, to block the allylic oxidationby CYP 3A4, which should enhance the metabolic stability andactivity in vivo.The novel (3R,4S)-1-t-Boc-3-TIPSO-4-difluorovinyl-?-lactam32(+) was coupled with baccatins 18 (S

39、cheme 8).42The ?-lactam32(+) was prepared from 4-formyl-?-lactam 29(+) by the Wittig-type reaction (Scheme 8). The Ojima-Holton coupling30,43,44of?-lactam 32(+) with modified baccatins 1837and the subsequentremoval of the silyl protecting groups gave the corresponding C-3-difluorovinyl-taxoids 33 in

40、 good to excellent yields (Scheme 8).42Design and Synthesis of Novel C-14-C-3BzN-LinkedMacrocyclic TaxoidsThe first cryo-electron microscopy (cryo-EM) structure ofpaclitaxel-bound Zn2+-stabilized R?-tubulin dimer (1TUB structure)reported in 1998 at 3.7 resolution,45which used the docetaxelcrystal st

41、ructure for display, opened a new era for the structuralbiology and medicinal chemistry of paclitaxel. The ITUB structurewas later refined to 3.5 resolution with a paclitaxel molecule in2001 (1JFF structure).46However, the resolution of these cryo-EM structures was not high enough to solve the bindi

42、ng conforma-tion of paclitaxel. Thus, a computational study of the electron-density map was performed, and the “T-Taxol” conformation wasproposed.47To prove the validity of the T-Taxol structure, rigidifiedpaclitaxel congeners were designed, synthesized, and assayed fortheir tubulin polymerization a

43、bility and cytotoxicity48-52based onthe T-Taxol structure. Among those T-Taxol mimics, C-4-C-3-linked macrocyclic taxoids showed higher activities than paclitaxelin the cytotoxicity and tubulin-polymerization assays.49,51Weproposed “REDOR-Taxol” as a valid microtubule-bound paclitaxelstructure in 20

44、0553based on the two13C-19F intramoleculardistances of the microtubule-bound 2-(4-fluorobenzoyl)paclitaxelexperimentally obtained by means of a REDOR NMR study,54MDScheme 6Figure 1Scheme 7aaReagents and conditions: (i) Cu(OAc)2, MeOH, 77-86%; (ii)L-selectride, THF, -78 C 50-70%, (iii) methyl imidazo

45、le, TESCl, DMF, 0 C, 50-80%; (iv)LiHMDS, THF, -40 C, 70-80%; (v) HF/pyridine, CH3CN/pyridine, 0 C to RT, 52%-92%.ReViewsJournal of Natural Products, 2009, Vol. 72, No. 3557analysis of paclitaxel conformers, photoaffinity labeling,55andmolecular modeling studies using the 1TUB coordinate.37TheREDOR-T

46、axol structure was further refined using the 1JFFcoordinate. The “REDOR-Taxol (1JFF)” is fully consistent withthe additional REDOR experiments by Schaefer and collaborators56and also accommodates highly active macrocyclic paclitaxelanalogues designed based on the T-Taxol structure.24The C-2-OH group

47、 interacts with His227as the hydrogen bond donor in theREDOR-Taxol,53while the H-bonding is between the C-2-OH andthe backbone carbonyl oxygen of Arg369in the T-Taxol.47Accord-ingly, we have designed novel macrocyclic taxoids 34a-c bylinking the C-14 and C-3BzN groups, which mimic the “REDOR-Taxol”

48、structure, to examine the level of biological activitycompared to that of paclitaxel (Figure 3).24As the overlays in Figure4 illustrate, 34a and 34c appear to mimic the REDOR-Taxolstructure very well, while the C-3N-benzoyl group of 34b deviatesfrom the rest.24?-Lactams 37a-d were prepared in excell

49、ent yields throughacylation of the corresponding enantiopure N-H-free ?-lactams with2-alkenylbenzoyl chlorides (Scheme 9). The 7-TES-14?-allyloxy-baccatin 40 was prepared by following the method previouslyreported by us from 14-OH-DAB.53The Ojima-Holton couplingof 40 with ?-lactams 37a-d gave the co

50、rresponding paclitaxel-dienes 41a-d bearing olefinic groups at the C-14 position as wellas the ortho position of the C-3BzN moiety (Scheme 10).24As Scheme 11 shows, the RCM reactions of paclitaxel-dienes41a and 41d catalyzed by the “first-generation Grubbs catalyst”proceeded smoothly at room tempera

51、ture to give the correspondingmacrocyclic taxoids 42a and 42d, respectively. The subsequentdeprotection of all silyl groups with HF-pyridine afforded thedesigned macrocyclic taxoids 34a and 34d, respectively, in highFigure 2. Primary sites of hydroxylation on the second-generation taxoids by the P45

52、0 family of enzymes.41Scheme 8aaReagents and conditions: (i) CBr2F2, HMPT, Zn, THF, 84%; (ii) CAN, H2O/CH3CN, -15 C, 92%; (iii) Boc2O, Et3N, DMAP, CH2Cl2, 96%; (iv) LiHMDS,THF, -40 C, (v) HF/Py, Py/CH3CN, overnight, 0 C to RT, 57-91%.Figure 3. Designed novel C-14-C-3BzN-linked macrocyclictaxoids.Fig

53、ure 4. Overlays of REDOR-Taxol (green) with 34a (cyan), 34b(red), and 34c (pink).558Journal of Natural Products, 2009, Vol. 72, No. 3ReViewsyields.24In both products, the E-isomer was formed exclusively.On the other hand, the RCM reaction of 41c proceeded slowly andgave a ca. 1:1 mixture of 42c-E an

54、d 42c-Z isomers, which wereseparated and deprotected to give 34c-E and 34c-Z, respectively,in high yields (Scheme 12).24The observed low reactivity of 41cand the lack of stereoselectivity in the alkene formation can beattributed to the larger ring size of the product(s).In contrast, the RCM reaction

55、 of paclitaxel-diene 41b did notproceed as anticipated and gave 42b unexpectedly, which wasdeprotected to afford macrocyclic taxoid 43b in fairly good yield(Scheme 13).24The1H and13C NMR and 2D NMR analysessuggested that 43b should possess a butenylene unit between theC-14-O and the ortho position o

56、f the C-3N-benzoyl moiety andthe newly formed double bond should be conjugated to the phenylgroup. This proposed structure was confirmed by the X-raycrystallographic analysis of 43b (SB-T-2054), as shown in Figure5.24A plausible mechanism for this Ru-catalyzed novel olefin-olefincoupling reaction wa

57、s proposed,24which includes the formationof a regioisomeric metalacyclobutene and its isomerization to thecorresponding -allylic intermediate, followed by reductiveelimination.Biological Activities of New Generation TaxoidsThe new generation taxoids were evaluated for their cytotoxicityagainst vario

58、us drug-sensitive and drug-resistant cancer cell lines.Table 5 summarizes the potencies of C-10-modified taxoids 6a-oagainst LCC6-WT and LCC6-MDR cell lines.29All taxoids exhibitsimilar or better activities than paclitaxel against LCC6-WT, whilemore than a half of this series of taxoids show 2 order

59、s of magnitudebetter activity than that of paclitaxel against LCC6-MDR. Thedramatic decrease in the R/S ratio for a majority of this series oftaxoids (R/S ratio at or below 3), which is an excellent indicatorof the level of drug resistance associated with drugs, is the mostScheme 9aaReagents and con

60、ditions: (i) acid chloride (2.0 equiv), Et3N (4 equiv),CH2Cl2, DMAP, overnight.Scheme 10aaReagents and conditions: (i) (a) Ac2O (10 equiv), CeCl37H2O (0.1 equiv),THF, RT, 2 h, (b) TESCl (3.0 equiv), imidazole (4.0 equiv), DMF, RT, 5 h(83% in 2 steps), (ii) allyl iodide (1.1 equiv), NaHMDS (1.1 equiv

61、), DMF, -40C, 1 h (82%), (iii) 37a-d (3.0 equiv), LiHMDS (1.5 equiv), THF, -30 to 0C, 2.5 h.Scheme 11aaReagents and conditions: (i) (a) Cl2Ru(dCHPh)(PCy3)2(0.2 equiv), CH2Cl2,overnight; (ii) HF/Py, Py, CH3CN, RT, overnight.Scheme 12aaReagents and conditions: (i) (a) Cl2Ru(dCHPh)(PCy3)2(0.2 equiv), C

62、H2Cl2,overnight, flash chromatography; (ii) HF/Py, Py, CH3CN, RT, overnight.ReViewsJournal of Natural Products, 2009, Vol. 72, No. 3559noteworthy feature. Taxoid 6d shows almost no difference againstdrug-resistant and drug-sensitive cell lines with the R/S ratio of1.2.Taxoids 6a and 6d, with a benzo

63、yl group and a 4-methoxyben-zoyl group at C-10, respectively, possess highest potencies againstLCC6-MDR, while taxoid 6k, with a 4-methoxyphenylacetyl, isthe least potent but possesses the highest potency against LCC6-WT. Taxoids with a substituted or unsubstituted benzoyl group(6a-e) or 2-phenylpro

64、panoyl group (6m-o) are highly potentagainst LCC6-MDR, while taxoids with arylacetyl substituents(6i-k) show reduced activity against LCC6-MDR. Elongation ofthe alkyl chain of 6i-k just by one carbon restores high potencyagainst LCC6-MDR. The results suggest that the C-10 substituentsare critical to

65、 the modulation of the Pgp efflux pump in taxoids.The activities of C-3N-modified 10-propanoyl-taxoids are sum-marized in Table 6.29Most of these C-3N-modified taxoids possessbetter potency against LCC6-WT and MCF7 cell lines and 1-2orders of magnitude higher potency against drug-resistant LCC6-MDR

66、and NCI/ADR cell lines, as compared with paclitaxel. Taxoids15d and 15e exhibit high potency against these cell lines and possesscytotoxicity comparable to their parent taxoids SB-T-12137andSB-T-1103.7The results show that the t-Boc group at the C-3Nposition, which is the “gold standard”, can be rep

67、laced bycycloalkenoyl groups without losing potency.Table 7 summarizes the cytotoxicity assay results of taxoids witha modified C2-benzoyl group at its meta position.29The majorityof these new second-generation taxoids (12 and 13) with a C-3N-t-Boc group show remarkable potency against drug-resistan

68、t (Pgp+)cancer cell lines, LCC6-MDR and NCI/ADR with R/S ratios lessthan 3 in many cases and less than 1 in three cases (12g for LCC6-WT: LCC6-MDR and MCF7:NCI/ADR as well as 13g for LCC6-WT: LCC6-MDR). It can be said that the Pgp-mediated MDR iscompletely circumvented by the new taxoids 12g and 13g

69、.Accordingly, we haVe defined these new generation taxoids, whichcan Virtually circumVent the Pgp-mediated MDR, as the “third-generation” taxoids.The potency decreases in the order F Cl N3 MeO .CH2dCH- against LCC6-WT (Pgp-) for the meta-substituted C2-Scheme 13aaReagents and conditions: (i) (a) Cl2

70、Ru(dCHPh)(PCy3)2(0.25 equiv 3),CH2Cl2, reflux, 5 days; (ii) HF/Py, Py, CH3CN, RT, overnight.Figure 5. X-ray crystal structure of 43b (SB-T-2054).Table5. CytotoxicityofSecond-GenerationTaxoidswithModifications at C-10 (IC50nM)aaConcentration of compound that inhibits 50% (IC50, nM) of thegrowthofhuma

71、ntumorcelllineaftera72hdrugexposure.bLCC6-WT: human breast carcinoma cell line (Pgp-).cLCC6-MDR:mdr1 transduced cell line (Pgp+).dResistance factor ) (IC50for drugresistant cell line, R)/(IC50for drug-sensitive cell line, S).560Journal of Natural Products, 2009, Vol. 72, No. 3ReViewsTable 6. Cytotox

72、icity of Second-Generation Taxoids with C-3N Modifications (IC50nM)ataxoidR1R2LCC6-WTbLCC6-MDRcR/SdMCF7eNCI/ADRfR/Sdpaclitaxel3.13461121.7300176SB-T-1213t-Boc2-Me-prop-1-enyl0.184.022SB-T-1103t-Boc2-Me-propyl0.355.11515acyclobutyl2-Me-prop-1-enyl1.334261.2413415bcyclopentyl2-Me-prop-1-enyl1.119171.1

73、222016bcyclopentyl2-Me-propyl6.364102.3291215ccyclohexyl2-Me-prop-1-enyl1.313101.0181816ccyclohexyl2-Me-propyl5.8325.51.5191315dcyclopent-1-enyl2-Me-prop-1-enyl1.214120.46.01515ecyclohex-1-enyl2-Me-prop-1-enyl1.2119.20.33.81315fcyclopentyloxy2-Me-prop-1-enyl0.9412131.48.66.116fcyclopentyloxy2-Me-pro

74、pyl2.6145.41.38.86.815gcyclohexyloxy2-Me-prop-1-enyl1.215131.48.96.416gcyclohexyloxy2-Me-propyl4.8183.81.41410aSee the footnote of Table 5.bSee the footnote of Table 5.cSee the footnote of Table 5.dSee the footnote of Table 5.eMCF7: human breastcarcinoma cell line.fNCI/ADR: multidrug-resistant human

75、 ovarian carcinoma cell line.Table 7. Cytotoxicity of Second-Generation Taxoids with C-2-meta Modifications (IC50nM)ataxoidR1R2R3R4LCC6 -WTbLCC6-MDRcR/SdMCF7eNCI/ADRfR/SdpaclitaxelPhPhMeCOH3.13461121.7300176docetaxelt-BuOPhHH1.01201201.0235235SB-T-1213t-BuOMe2CdCH-EtCOH0.184.022SB-T-1103t-BuOMe2CHCH

76、2-EtCOH0.355.121SB-T-1214t-BuOMe2CdCH-c-PrCOH0.203.92012at-BuOMe2CdCH-MeCOMe1.55.83.90.85.06.312bt-BuOMe2CdCH-MeCOMeO0.62.74.50.82.32.912ct-BuOMe2CdCH-EtCOF0.52.14.212dt-BuOMe2CdCH-EtCOCl0.81.31.6-12et-BuOMe2CdCH-EtCON30.91.21.30.91.11.212ft-BuOMe2CdCH-EtCOCH2dCH-2.97.12.412gt-BuOMe2CdCH-EtCOMeO1.00

77、.90.900.360.330.9212ht-BuOMe2CdCH-c-PrCOMeO1.02.92.912it-BuOMe2CdCH-MeOCOMeO0.61.62.70.41.43.512jt-BuOMe2CdCH-PhCH2OCOMeO1.21.81.50.21.57.512kt-BuOMe2CdCH-2-MeO(C6H4)COMeO0.40.92.31.13.33.012lt-BuOMe2CdCH-4-MeO(C6H4)CH2COMeO0.40.41.00.61.83.012mt-BuOCH2dCHCH2-EtCOMeO1.28.47.00.88.710.912nt-BuO(E)-CH

78、3CHdCH-EtCOMeO1.24.13.412ot-BuOCH2dCH(CH2)2-EtCOMeO0.95.46.02.07.73.912pt-BuO(S)-2,2-Me2-c-Pr-EtCOMeO0.481.12.30.61.52.513ct-BuOMe2CHCH2-EtCOF0.42.46.013dt-BuOMe2CHCH2-EtCOCl0.82.93.613et-BuOMe2CHCH2-EtCON31.12.42.21.02.12.113gt-BuOMe2CHCH2-EtCOMeO0.90.80.890.360.431.1915hc-PrMe2CdCH-EtCOMeO1.117.21

79、60.57.815.615ic-BuMe2CdCH-EtCOMeO1.6159.40.88.81115jc-PentylMe2CdCH-EtCOMeO1.111100.39.53215kc-HexylMe2CdCH-EtCOMeO6.9426.11.817.59.715lc-Hex-OMe2CdCH-EtCOMeO1.2314.812.01.44149.716hc-PrMe2CHCH2-EtCOMeO0.613220.411.83016kc-HexylMe2CHCH2-EtCOMeO1.012120.76.59.316mc-Pent-OMe2CHCH2-EtCOMeO0.762.63.40.1

80、71.186.9aSee the footnote of Table 5.bSee the footnote of Table 5.cSee the footnote of Table 5.dSee the footnote of Table 5.eSee the footnote ofTable 6.fSee the footnote of Table 6.ReViewsJournal of Natural Products, 2009, Vol. 72, No. 3561benzoyl moiety, reflecting the taxoidss ability to bind micr

81、otubules.The potency order changes to MeO N3 Cl F . CH2dCH-against LCC6-MDR (Pgp+), which reflects their effect on MDRreversal activity or simply indicates the extent of their interactionwith Pgp in the reverse order. The C-3 2-methylprop-1-enyl and2-methylpropyl groups are the best substituents for

82、 enhancedpotency so far. However 12m-p are also very active. All thesehighly potent new taxoids are very good candidates for furtherpreclinical studies.29Cytotoxicity of selected new generation taxoids SB-T-1214, 12g,and 13g was examined against paclitaxel-resistant cancer cells withpoint mutations

83、in tubulin to test their ability to deal with drugresistance other than MDR.29Two paclitaxel-resistant sublines1A9PTX10 and 1A9PTX22 with point mutations in the class I?-tubulin have been reported.57As Table 8 shows, all three taxoidsexhibit extremely potent activity, especially against drug-resista

84、ntcell lines 1A9PTX10 and 1A9PTX22, with 2 orders of magnitudehigher potency than paclitaxel. These results clearly indicate thatthese second- and third-generation taxoids possess the capabilityto effectively circumvent the paclitaxel drug resistance arising frompoint mutations in tubulins/microtubu

85、les besides MDR, whichmakes these new generation taxoids even more attractive.29Pancreatic cancer is refractory to conventional therapy, and amajor factor for such drug resistance is the expression of variousmultidrug resistance proteins.58-60Our RT-PCR analysis showedthat the CFPAC-1 and PANC-1 cel

86、l lines expressed the mdr1, mrp1,mrp2, and lrp genes, responsible for multidrug resistance, whilethe MIA PaCa-2 and BxPC-3 cell lines expressed the mrp1, mrp2,and lrp genes.61Two taxoids, SB-T-1214 and 12g, were evaluatedagainst four pancreatic cancer cell lines, MIA PaCa-2, CFPAC-1,BxPC-3, and PANC

87、-1. The results are shown in Table 9.29These taxoids exhibited excellent cytotoxicity against pancreaticcancer cell lines except for 12g against PANC-1. Also, the third-generation taxoid, 12g, which is more potent than SB-T-1214 (10times) against the Pgp+ (i.e., mdr1) NCI/ADR cell line (see Table7),

88、 shows a lower potency than SB-T-1214 against the BxPC-3and PANC-1 cell lines. This may indicate that 12g cannot modulatea combination of multidrug-resistant proteins as efficiently as Pgpalone, but it is still highly cytotoxic to PANC-1. SB-T-1214 exhibitsvery high potency against these cell lines

89、and did not show anyappreciable cytotoxicity against primary pancreatic ductal cells upto 10 M concentration. A preliminary in vivo efficacy assay (20mg/kg 3, 60 mg/kg total dose in Tween 80/EtOH/PBS) againsta pancreatic cancer CFPAC-1 xenograft in nude mice showed highefficacy with no trace of canc

90、er cells by histopathological analysisafter 8 weeks.29The antitumor activity of SB-T-1214, one of the leadingcandidates among the new generation taxoids studied in ourlaboratory, was assayed in vivo against a Pgp+ DLD-1 human colontumor xenograft in SCID mice.29The taxoid was administeredintravenous

91、ly at three doses three times using a 3-day regimen (q3d 3, on day 5, 8, and 11), starting from day 5 after DLD-1subcutaneous tumor implantation. As Table 10 shows, the optimalefficacy was obtained at 60 mg/kg total dose (20 mg/kg 3),wherein complete regression of DLD-1 tumors was achieved infive of

92、 five mice (tumor growth delay 150 days).29A systemictoxicity profile showed that there was only 3-5% weight loss duringthe period of day 15 to day 20, and the drug was very well toleratedby animals.29This promising result warrants further preclinicalevaluation of this taxoid.The second-generation f

93、luoro-taxoids 21 and 22 were evaluatedfor their cytotoxicity in vitro against various cell lines, as sum-marized in Tables 11 and 12, respectively.36The IC50values weredetermined through 72 h exposure of the fluoro-taxoids to the cancercells, following the procedure developed by Skehan et al.62These

94、fluoro-taxoids possess substantially higher potencies than those ofpaclitaxel and docetaxel against drug-sensitive cancer cell lines(except for a few cases), and their potency against multidrug-resistant cell lines is more impressive (2 orders of magnitude morepotent than paclitaxel on average). The

95、 potency of 3-CF2H-taxoids21 against MCF7-S and LCC6-WT appears to be higher and moreuniform with different substitution patterns as compared to that of3-CF3-taxoids 22, except for two cases (SB-T-12822-1: 0.19 nM,MCF7-S; SB-T-12824-1: 0.17 nM, MCF7-S). On the contrary, 22exhibit more uniform potenc

96、y against multidrug-resistant MCF7-Rand LCC6-MDR cell lines than 21. For 21, cytotoxicity againstthese cell lines depends on the nature of meta substituents of theC-2-benzoate moiety; that is, potency tends to increase in the orderF MeO Cl 15005/5SB-T-121412015023/5aTreatment given iv to SCID mice o

97、n day 5 after DLD-1 humancolon tumor implant and continued on days 8 and 11, with all drugsformulated in Tween/EtOH.bBased on comparison of each group vscontrol using the Cox-Mantel test.cNumber of animals who either diedor lost greater than 20% body weight.dSCID mice with no palpabletumors on day 1

98、67, the end of experiment.562Journal of Natural Products, 2009, Vol. 72, No. 3ReViewsThe C-3-substitutents of C-seco-fluorotaxoids 28a (2-methyl-propyl) and 28b (2-methylprop-1-enyl) also show interesting effectson potency, which is assumed to be related directly to theirinteraction with the class I

99、II ?-tubulin. Overall, it has been shownthat the introduction of one fluorine to the C-2-benzoate moiety ofthe C-seco-taxoid molecule substantially increases the potencyagainst both paclitaxel-sensitive and paclitaxel-resistant humanovarian cancer cell lines.The cytotoxicities of 3-difluorovinyl-tax

100、oids 33 were evaluatedagainst several cancer cell lines. As Table 14 shows, these taxoidsare exceedingly potent as compared to paclitaxel.42The metasubstitution of C-2-benzoate has a clear effect on the potency againstdrug-sensitive and drug-resistant MCF7 cell lines (entries 2-5 vsentries 6-11). Di

101、fluorovinyl-taxoids with 2,10-modifications (en-tries 6-11) also possess impressive potency. SB-T-12853 appearsparticularly promising against gastrointestinal (GI) cancer cell lines.The cytotoxicity of the novel macrocyclic taxoids was evaluatedagainst several drug-sensitive and drug-resistant cell

102、lines. As Table15 shows, taxoid 43b (SB-T-2054) was the most potent com-pound.24The results may suggest that 43b is very closely mimickingpaclitaxels bioactive conformation. The results appear to indicatehigh sensitivity of the potency to the subtle difference in the positionof the C-3N-benzoyl grou

103、p, the rigidity of the macrocyclic structure,and the ring size. Also, there is a marked difference between thepotency of the E-isomer and the Z-isomer of 34c.The activity of 43b (SB-T-2054) was also evaluated in an invitro tubulin polymerization assay using paclitaxel as the standardfor comparison.

104、As Figure 6 shows, 43b induced tubulin polym-erization in the absence of GTP in a manner similar to paclitaxel,and the microtubules formed with both were stable against Ca2+-induced depolymerization.24This result also supports our observa-Table 11. In Vitro Cytotoxicity (IC50nM)aof C-3-CF2H-taxoid (

105、21)taxoidRXMCF7-Sb(breast)MCF7-Rc(breast)R/ScLCC6-WTb(breast)LCC6-MDRc(breast)R/SdH460f(lung)HT-29g(colon)paclitaxel1.73001763.13461124.93.6docetaxel1.02152151.0SB-T-12841-1AcMeO0.344.16120.265.57210.380.52SB-T-12841-2AcF0.445.33130.5210.0190.200.35SB-T-12841-3AcCl0.406.48160.315.80190.491.94SB-T-12

106、841-1AcN30.321.685.30.221.577.10.480.57SB-T-12842-1Et-COMeO1.144.053.50.694.927.10.400.59SB-T-12842-2Et-COF0.537.24140.884.633.50.410.86SB-T-12842-3Et-COCl0.445.20120.524.719.10.300.43SB-T-12842-4Et-CON30.320.963.00.391.152.90.270.37SB-T-12843-1Me2N-COMeO0.454.51100.697.06100.400.43SB-T-12843-2Me2N-

107、COF0.528.13160.6910.6150.200.35SB-T-12843-3Me2N-COCl0.312.969.50.213.87180.360.58SB-T-12843-4Me2N-CON30.371.443.90.291.695.80.520.40SB-T-12844-1MeO-COMeO0.816.598.11.0310.29.90.300.44SB-T-12844-2MeO-COF0.5911.38190.8612.6150.300.43SB-T-12844-3MeO-COCl0.262.088.00.131.82140.250.29SB-T-12844-4MeO-CON3

108、1.692.561.50.262.067.90.230.36a-eSee footnote a of Table 5.fHuman non-small cell lung carcinoma.gHuman Caucasian colon adenocarcinoma.Table 12. In Vitro Cytotoxicity (IC50nM)aof C-3-CF3-Taxoids (22)taxoidRXMCF7-Sb(breast)MCF7-Rc(breast)R/SdLCC6-WTb(breast)LCC6-MDRe(breast)R/SdH460f(lung)HT-29g(colon

109、)paclitaxel1.73001763.13461124.93.6docetaxel1.02152151.0SB-T-12821-1AcMeO0.328.8280.333.99120.380.69SB-T-12821-2AcF0.455.58130.385.93160.491.11SB-T-12821-3AcCl0.405.04130.224.96230.50.85SB-T-12821-4AcN30.473.858.21.184.003.40.200.50SB-T-12822-1Et-COMeO0.192.16110.454.2490.410.54SB-T-12822-2Et-COF0.6

110、83.785.60.824.275.20.591.15SB-T-12822-3Et-COCl0.343.289.60.392.546.50.631.11SB-T-12822-4Et-CON30.381.614.21.092.562.30.200.40SB-T-12823-1Me2NCOMeO0.571.843.20.284.48160.350.68SB-T-12823-2Me2NCOF0.322.648.30.325.57170.50.76SB-T-12823-3Me2NCOCl0.121.028.50.272.559.40.420.45SB-T-12823-4Me2NCON30.472.61

111、5.61.273.522.80.300.50SB-T-12824-1MeOCOMeO0.172.88170.273.99150.380.53SB-T-12824-2MeOCOF0.314.88160.395.81150.610.85SB-T-12824-3MeOCOCl0.654.727.30.295.08180.430.68SB-T-12821-1MeOCON30.472.926.21.094.003.70.200.40a-gSee footnotes of Table 11.Table 13. In Vitro Cytotoxicity (IC50nM)aof C-seco-Fluorot

112、axoidsC-seco-taxoidA2780wtbA2780CIScA2780TOPdA2780ADReA2780TC1fA2780TC3fpaclitaxel1.72.27.2123910 02717 800IDN 539017.416.827.526172060223728a (SB-CST-10104)11.111.812.83726149746028b (SB-CST-10204)6.14.96.922184454745aSee footnote of Table 5.bHuman ovarian carcinoma wild type.cCisplatin-resistant A

113、2780.dTopotecan-resistant A2780.eAdriamycin-resistantA2780.fClone derived from chronic exposure of A2780 to paclitaxel and cyclosporine.ReViewsJournal of Natural Products, 2009, Vol. 72, No. 3563tion that 43b is closely mimicking paclitaxels bioactive confor-mation (see Figure 7).24ConclusionNew gen

114、eration taxoids with systematic and strategic modifica-tions were designed, synthesized, and examined for their potencyand efficacy in vitro and in vivo. As a result, it has been shownthat a number of these taxoids are exceptionally potent. Some ofthese taxoids exhibited very low resistance factors

115、i.e., virtuallyovercoming multidrug resistance completely. Thus, these taxoidshave been termed “third-generation” taxoids. Novel fluorine-containing taxoids were also investigated. For example, 3-difluorovinyl-taxoids have been designed to block the metabolichydroxylation by cytochrome P-450 enzymes

116、, and these were foundto exhibit exceptionally high potency against several cancer celllines. Novel C-14-C-3BzN-linked macrocyclic taxoids were de-signed to mimic the REDOR-Taxol structure. One of the macro-cyclic taxoids was found to possess virtually the same potency asthat of paclitaxel, which su

117、ggests that its structure almost perfectlymimics the bioactive conformation of paclitaxel. The encouragingprofiles of some of these new generation taxoids make them highlypromising candidates for further preclinical studies.Table 14. In Vitro Cytotoxicity (IC50nM)aof 3-Difluorovinyl-taxoids 33entryt

118、axoidRXMCF7-Sb(breast)MCF7-Rc(breast)R/SHT-29d(colon)PANC-1e(pancreatic)1paclitaxel1.23002503.625.72SB-T-12851AcH0.0990.959.60.411.193SB-T-12852c-Pr-COH0.126.0500.855.854SB-T-12853Et-COH0.121.2100.340.655SB-T-12854Me2N-COH0.134.3330.461.586SB-T-12852-1c-Pr-COMeO0.0920.485.27SB-T-12853-1Et-COMeO0.340

119、.571.78SB-T-12855-1MeO-COMeO0.0780.506.49SB-T-12851-3AcN30.0920.343.710SB-T-12852-3c-Pr-CON30.0920.454.911SB-T-12855-3MeO-CON30.0780.405.3a-dSee footnotes of Table 11.eHuman pancreatic carcinoma.Table 15. Cytotoxicity of C-14-C-3BzN-Linked Macrocyclic Taxoids (IC50nM)ataxoidsMCF-7bNCI/ADRcLCC6-WTdLC

120、C6-MDReA2780wtfHT-29gpaclitaxel1.853952.4511036.17.2834a (SB-T-2053)12.359212.230011429.243b (SB-T-2054)3.491832.0912931.017.034c-E165010 01020672285147552334c-Z1961,1353481,92419862.034d3981,000130618aSee footnote of Table 5.b,cSee footnote of Table 6.d,eSee footnote of Table 5.fSee footnote of Tab

121、le 13.gSee footnote of Table 11.Figure 6. Tubulin polymerization with 43b and paclitaxel: microtubule protein 1 mg/mL, 37 C, GTP 1 mM, or drug 10 M.Figure 7. Overlays of REDOR-Taxol (green) with 43b (SB-T-2054)(purple).564Journal of Natural Products, 2009, Vol. 72, No. 3ReViewsReferences and Notes(1

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