Synthesis and biological evaluation of 2-acyl analogues of paclitaxel (Taxol)

Discovery and development of botanical natural products and their analogues as therapeutics for ovarian cancer

Covering: 2015 through the end of July 2022Ovarian cancer is one of the most common cancers affecting the female reproductive organs and has the highest mortality rate among gynecological cancers. Although botanical drugs and their derivatives, namely members of the taxane and camptothecin families, represent significant therapeutics currently available for the treatment of ovarian cancer, new drugs that have alternative mechanisms of action are still needed to combat the disease. For this reason, many efforts to identify additional novel compounds from botanical sources, along with the further development of existing therapeutics, have continued to appear in the literature. This review is designed to serve as a comprehensive look at both the currently available small-molecule therapeutic options and the recently reported botanically-derived natural products currently being studied and developed as potential future therapeutics that could one day be used against ovarian cancer. Specifically, key properties, structural features, and biological data are highlighted that are important for the successful development of potential agents. Recently reported examples are specifically discussed in the context of "drug discovery attributes," including the presence of structure-activity relationship, mechanism of action, toxicity, and pharmacokinetic studies, to help indicate the potential for future development and to highlight where these compounds currently exist in the development process. The lessons learned from both the successful development of the taxanes and camptothecins, as well as the strategies currently being employed for new drug development, are expected to ultimately help guide the future development of botanical natural products for ovarian cancer.

My 60-Year Love Affair with Natural Products

The author describes his 60-year career in studying the chemistry of natural products, which includes structural, synthetic, and biosynthetic studies of natural products ranging from insect pigments, antibiotics, and fecal mutagens to taxol and other anticancer natural products as well as antimalarial natural products. One of the compounds discussed, napabucasin, is now an anticancer drug in phase III clinical trials.

S-(4-Methoxyphenyl)-4-methoxybenzenesulfonothioate as a Promising Lead Compound for the Development of a Renal Carcinoma Agent

Organosulfur compounds show cytotoxic potential towards many tumor cell lines. Disulfides and thiosulfonates act through apoptotic processes, inducing proteins associated with apoptosis, endoplasmic reticulum stress, and the unfolded protein response. Three p-substituted symmetric diaryl disulfides and three diaryl thiosulfonates were synthesized and analyzed for inhibition of tubulin polymerization and for human cancer cell cytotoxic activity against seven tumor cell lines and a non-tumor cell line. S-(4-methoxyphenyl)-4-methoxybenzenesulfonothioate (6) exhibited inhibition of tubulin polymerization and showed the best antiproliferative potential, especially against the 786-0 cell line, being six times more selective as compared with the non-tumor cell line. In addition, compound 6 was able to activate caspase-3 after 24 and 48 h treatments of the 786-0 cell line and induced cell-cycle arrest in the G2/M stage at the highest concentration evaluated at 24 and 48 h. Compound 6 was able to cause complete inhibition of proliferation, inducing the death of 786-0 cells, by increasing the number of cells at G2/M and greater activation of caspase-3.

Design, synthesis and SAR study of 3rd-generation taxoids bearing 3-CH3, 3-CF3O and 3-CHF2O groups at the C2-benzoate position

It has been shown that inclusion of CF₃O and CHF₂O groups to drug candidates often improve their pharmacological properties, especially metabolic stability, membrane permeability and PK profile. Moreover, the unique non-spherical structure of the OCHF₂ group can provide interesting and beneficial characteristics. Accordingly, new 3rd-generation taxoids, bearing 3-OCF₃ or 3-OCF₂H (and 3-CH₃ for comparison) at the C2 benzoate moiety, were synthesized and their potencies against drug-sensitive and drug-resistant cancer cell lines examined. In this study, our previous SAR studies on 3rd-generation taxoids were expanded to disclose that CH₃, CF₃O and CHF₂O groups are well tolerated at this position and enhance potency, especially against MDR-cancer cell lines so that these taxoids can virtually overcome MDR. These new taxoids exhibit up to 7 times higher cytotoxicity (IC₅₀) than paclitaxel against drug-sensitive cancer cell lines (MCF7 and LCC6-WT) and 2-3 orders of magnitude higher potency than paclitaxel against drug-resistant ovarian, breast and colon cancer cell lines with MDR-phenotype (NCI/ADR, LCC6-MDR and LDL-1), as well as pancreatic cancer cell line, CFPAC-1. Since it has been shown that a bulky group at this position reduces potency, it is noteworthy that rather bulky CF₃O and CHF₂O groups are well tolerated. Molecular modeling analysis indicated the favorable van der Waals interactions of CF₃O and CHF₂O groups in the binding site. It is also worthy of note that new taxoids, bearing a CHF₂O group at the C2 benzoate position (1-06 series), exhibited the highest potencies against MDR-cancer cell lines and cancer stem cell (CSC)-enriched cancer cell lines. These new 3rd-generation taxoids are promising candidates for highly potent chemotherapeutic agents, as well as payloads for tumor-targeting drug conjugates such as antibody-drug conjugates (ADCs).

Synthesis and Cytotoxicity of 7,9-O-Linked Macrocyclic C-Seco Taxoids

A series of novel 7,9-O-linked macrocyclic taxoids together with modification at the C2 position were synthesized, and their cytotoxicities against drug-sensitive and P-glycoprotein and βIII-tubulin overexpressed drug-resistant cancer cell lines were evaluated. It is demonstrated that C-seco taxoids conformationally constrained via carbonate containing-linked macrocyclization display increased cytotoxicity on drug-resistant tumors overexpressing both βIII and P-gp, among which compound 22b, bearing a 2-m-methoxybenzoyl group together with a five-atom linker, was identified as the most potent. Molecular modeling suggested the improved cytotoxicity of 22b results from enhanced favorable interactions with the T7 loop region of βIII.

Synthesis and biological evaluation of novel larotaxel analogues

Taxoids are a class of successful drugs and have been successfully used in chemotherapy for a variety of cancer types. However, despite the hope and promises that these taxoids have engendered, their utility is hampered by some clinic limitations. Extensive structure-activity relationship (SAR) studies of toxoids have been performed in many different laboratories. Whereas, SAR studies that based on the new-generation toxoid, larotaxel, have not been reported yet. In view of the advantages in preclinical and clinical data of larotaxel over former toxoids, new taxoids that strategicly modified at the C3'/C3'-N and C2 positions of larotaxel were designed, semi-synthesized, and examined for their potency and efficacy in vitro. As a result, it has been shown that the majority of these larotaxel analogues are exceptionally potent against both drug-sensitive tumor cells and tumor cells with drug resistance arising from P-glycoprotein over expression. Further in vivo antitumor efficacies investigations revealed A2 might be a potent antitumor drug candidate for further preclinical evaluation.

A Series of Enthalpically Optimized Docetaxel Analogues Exhibiting Enhanced Antitumor Activity and Water Solubility

A dual-purpose strategy aimed at enhancing the binding affinity for microtubules and improving the water solubility of docetaxel led to the design and synthesis of a series of C-2- and C-3'-modified analogues. Both aims were realized when the C-3' phenyl group present in docetaxel was replaced with a propargyl alcohol. The resulting compound, 3f, was able to overcome drug resistance in cultured P-gp-overexpressing tumor cells and showed greater activity than docetaxel against drug-resistant A2780/AD ovarian cancer xenografts in mice. In addition, the considerably lower hydrophobicity of 3f relative to both docetaxel and paclitaxel led to better aqueous solubility. A molecular model of tubulin-bound 3f revealed novel hydrogen-bonding interactions between the propargyl alcohol and the polar environment provided by the side chains of Ser236, Glu27, and Arg320.

Quest for Efficacious Next-Generation Taxoid Anticancer Agents and Their Tumor-Targeted Delivery

Paclitaxel and docetaxel are among the most widely used chemotherapeutic drugs against various types of cancer. However, these drugs cause undesirable side effects as well as drug resistance. Therefore, it is essential to develop next-generation taxoid anticancer agents with better pharmacological properties and improved activity especially against drug-resistant and metastatic cancers. The SAR studies by the authors have led to the development of numerous highly potent novel second- and third-generation taxoids with systematic modifications at the C-2, C-10, and C-3' positions. The third-generation taxoids showed virtually no difference in potency against drug-resistant and drug-sensitive cell lines. Some of the next-generation taxoids also exhibited excellent potency against cancer stem cells. This account summarizes concisely investigations into taxoids over 25 years based on a strong quest for the discovery and development of efficacious next-generation taxoids. Discussed herein are SAR studies on different types of taxoids, a common pharmacophore proposal for microtubule-stabilizing anticancer agents and its interesting history, the identification of the paclitaxel binding site and its bioactive conformation, characteristics of the next-generation taxoids in cancer cell biology, including new aspects of their mechanism of action, and the highly efficacious tumor-targeted drug delivery of potent next-generation taxoids.

Paclitaxel Biosynthesis: Adenylation and Thiolation Domains of an NRPS TycA PheAT Module Produce Various Arylisoserine CoA Thioesters

Structure-activity relationship studies show that the phenylisoserinyl moiety of paclitaxel (Taxol) is largely necessary for the effective anticancer activity. Several paclitaxel analogues with a variant isoserinyl side chain have improved pharmaceutical properties versus those of the parent drug. To produce the isoserinyl CoAs as intermediates needed for enzyme catalysis on a semibiosynthetic pathway to paclitaxel analogues, we repurposed the adenylation and thiolation domains (Phe-AT) of a nonribosomal peptide synthetase (TycA) so that they would function as a CoA ligase. Twenty-eight isoserine analogue racemates were synthesized by an established procedure based on the Staudinger [2+2] cycloaddition reaction. Phe-AT converted 16 substituted phenylisoserines, one β-(heteroaryl)isoserine, and one β-(cyclohexyl)isoserine to their corresponding isoserinyl CoAs. We imagine that these CoA thioesters can likely serve as linchpin biosynthetic acyl donors transferred by a 13-O-acyltransferase to a paclitaxel precursor baccatin III to make drug analogues with better efficacy. It was also interesting to find that an active site mutant [Phe-AT (W227S)] turned over 2-pyridylisoserine and the sterically demanding p-methoxyphenylisoserine substrates to their CoA thioesters, while Phe-AT did not. This mutant is promising for further development to make 3-fluoro-2-pyridylisoserinyl CoA, a biosynthetic precursor of the oral pharmaceutical tesetaxel used for gastric cancers.

Prodrug Strategies for Paclitaxel

Paclitaxel is an anti-tumor agent with remarkable anti-tumor activity and wide clinical uses. However, it is also faced with various challenges especially for its poor water solubility and low selectivity for the target. To overcome these disadvantages of paclitaxel, approaches using small molecule modifications and macromolecule modifications have been developed by many research groups from all over the world. In this review, we discuss the different strategies especially prodrug strategies that are currently used to make paclitaxel more effective.

Drug discovery targeting cell division proteins, microtubules and FtsZ

Eukaryotic cell division or cytokinesis has been a major target for anticancer drug discovery. After the huge success of paclitaxel and docetaxel, microtubule-stabilizing agents (MSAs) appear to have gained a premier status in the discovery of next-generation anticancer agents. However, the drug resistance caused by MDR, point mutations, and overexpression of tubulin subtypes, etc., is a serious issue associated with these agents. Accordingly, the discovery and development of new-generation MSAs that can obviate various drug resistances has a significant meaning. In sharp contrast, prokaryotic cell division has been largely unexploited for the discovery and development of antibacterial drugs. However, recent studies on the mechanism of bacterial cytokinesis revealed that the most abundant and highly conserved cell division protein, FtsZ, would be an excellent new target for the drug discovery of next-generation antibacterial agents that can circumvent drug-resistances to the commonly used drugs for tuberculosis, MRSA and other infections. This review describes an account of our research on these two fronts in drug discovery, targeting eukaryotic as well as prokaryotic cell division.

Design, synthesis and cytotoxicity of novel 3'-N-alkoxycarbonyl docetaxel analogs

By-product 9a exhibited potent cytotoxicity against both SK-OV-3 and A549 cell lines. The structure of 9a was characterized using 1D and 2D NMR experiments and confirmed by synthesis to afford a diastereomeric mixture (16a) that was identical to 9a, as well as a pair of diastereomers (R)-16b and (S)-16c. The preliminary SAR study demonstrated that analogs with an (R)-configuration were slightly more potent than analogs with an (S)-configuration. In addition, α,α-gem-dimethyl analogs 16 g-i were the most potent analogs in this series, exhibiting similar potency to docetaxel and greater potency than Taxol against the SK-OV-3 cell line. For the A549 cell line, analogs 16 g-i were more potent (>65-fold) than both docetaxel and Taxol.

Isolation, identification, semi-synthesis of aziditaxel derivatives and their biological evaluation

Two new taxoids (5 and 6) were obtained by isolating impurities in aziditaxel, and their structures were characterized based on data analysis of (1)H NMR, (13)C NMR, HPLC-MS, and through comparison with literature. In order to test their cytotoxicities against human nonsmall lung cancer cell lines (A549), sufficient amounts of compounds 5 and 6 were obtained by semi-synthesis and both of them showed equipotent cytotoxiesty compared with taxol, docetaxel, and aziditaxel.

Overcoming tumor drug resistance with C2-modified 10-deacetyl-7-propionyl cephalomannines: a QSAR study

The microtubule-stabilizing taxanes such as paclitaxel and docetaxel are the two most important anticancer drugs currently used in clinics for the treatment of various types of cancers. However, the major common drawbacks of these two drugs are drug resistance, neurotoxicity, substrate for drug transporter P-gp, cross-resistance with other chemotherapeutic agents, low oral bioavailability, and no penetration in the blood-brain barrier (BBB). These limitations have led to the search for new taxane derivatives with improved biological activity. In the present paper, we discuss the quantitative structure-activity relationship (QSAR) studies on a series of C2-modified 10-deacetyl-7-propionyl cephalomannines (IV) with respect to their binding affinities toward beta-tubulin and cytotoxic activities against both drug-sensitive and drug-resistant tumor cells, in which resistance is mediated through either P-gp overexpression or beta-tubulin mutation mechanisms, by the formulation of five QSARs. Hydrophobicity and molar refractivity of the substituents (pi(X) and MR(X)) are found to be the most important determinants for the activity. Parabolic correlations in terms of MR(X) (eqs 2 and 4 ) are encouraging examples in which the optimum values of MR(X) are well-defined. We believe that these two QSAR models may prove to be adequate predictive models that can help to provide guidance in design and synthesis, and subsequently yield very specific cephalomannine derivatives (IV) that may have high biological activities. On the basis of these two QSAR models, 10 cephalomannine analogues (IV-21 to IV-30) are suggested as potential synthetic targets. Internal (cross-validation (q(2)), quality factor (Q), Fischer statistics (F), and Y-randomization) and external validation tests have validated all the QSAR models.

Paclitaxel and docetaxel resistance: molecular mechanisms and development of new generation taxanes

Taxanes represent one of the most promising classes of anticancer agents. Unfortunately, their clinical success has been limited by the insurgence of cellular resistance, mainly mediated by the expression of the MDR phenotype or by microtubule alterations. However, the remarkable relevance of paclitaxel and docetaxel in clinical oncology stimulated intensive efforts in the last decade to identify new derivatives endowed with improved activities towards resistant tumor cells, resulting in a huge number of novel natural and synthetic taxanes. Among them, several structurally different derivatives were found to exhibit a promising behavior against the MDR phenotype in terms of either MDR inhibiting properties, or enhanced cytotoxicity compared to parental drugs, or both. On the other hand, only in more recent years have the first taxanes retaining activity against resistant cancer cells bearing alterations of the tubulin/microtubule system emerged. This review describes the main molecular mechanisms of resistance to paclitaxel and docetaxel identified so far, focusing on the advances achieved in the development of new taxanes potentially useful for the treatment of resistant tumors.

Design, synthesis, and biological evaluation of new-generation taxoids

Novel second-generation taxoids with systematic modifications at the C2, C10, and C3'N positions were synthesized and their structure-activity relationships studied. A number of these taxoids exhibited exceptionally high potency against multidrug-resistant cell lines, and several taxoids exhibited virtually no difference in potency against the drug-sensitive and drug-resistant cell lines. These exceptionally potent taxoids were termed "third-generation taxoids". 19 (SB-T-1214), 14g (SB-T-121303), and 14i (SB-T-1213031) exhibited excellent activity against paclitaxel-resistant ovarian cancer cell lines with mutations in beta-tubulin as well, wherein the drug resistance is mediated by the beta-tubulin mutation. These taxoids were found to possess exceptional activity in promoting tubulin assembly, forming numerous very short microtubules similar to those formed by discodermolide. Taxoids 19 and 14g also showed excellent cytotoxicity against four pancreatic cancer cell lines, expressing three to four multidrug-resistant genes. Moreover, taxoid 19 exhibited excellent in vivo efficacy against highly drug-resistant CFPAC-1 pancreatic as well as DLD-1 human colon tumor xenografts in mice.

Enhanced microtubule binding and tubulin assembly properties of conformationally constrained paclitaxel derivatives

Microtubule binding and tubulin assembly promotion by a series of conformationally restricted paclitaxel (PTX) derivatives was investigated. In these derivatives, the C-4 acetate of the taxane is tethered to the C-3' phenyl at ortho and meta positions with different length linkers. The apparent affinity of these derivatives for GMPCPP-stabilized microtubules was assessed by a competition assay, and their influence on microtubule polymerization was evaluated by measuring the critical concentration of GDP-tubulin in the presence of the respective molecule. In general, taxane derivatives with higher apparent affinity for microtubules induced tubulin assembly more efficiently. Among the derivatives, molecules with the shortest tether display the strongest affinity for microtubules. These derivatives exhibited enhanced microtubule stabilization properties and efficiently induced GDP-tubulin assembly into microtubules at low temperature of 12 degrees C and in the absence of Mg2+ ions in 0.1 M PIPES. Based on molecular dynamics simulations, we propose that the enhanced ability to assemble microtubules by these taxane derivatives is linked to their ability to effectively shape the conformation of the M-loop of tubulin for cross-protofilament interaction.

Overcoming Tumor Drug Resistance with High-Affinity Taxanes: A SAR Study of C2-Modified 7-Acyl-10-Deacetyl Cephalomannines

A series of C2-modified 10-deacetyl-7-propionyl cephalomannine derivatives was designed, prepared, and biologically evaluated. Some C2 meta-substituted benzoate analogues showed potent activity against both drug-sensitive and drug-resistant tumor cells in which resistance is mediated through either P-gp overexpression or beta-tubulin mutation mechanisms. The taxoid 15 b and related compounds are of particular interest, as they are much more cytotoxic than paclitaxel, especially against drug-resistant tumor cells; they are able to kill both drug-resistant and drug-sensitive cells (low R/S ratio), and they have high affinity for beta-tubulin. Our research results led to an important hypothesis, that is, a taxane with very high binding affinity for beta-tubulin is able to counteract drug resistance, which may assist in future taxane-based drug-discovery efforts.

Rotational-echo double-resonance NMR distance measurements for the tubulin-bound Paclitaxel conformation

The important anticancer drug Taxol (paclitaxel, PTX) owes its unique activity to its ability to bind to tubulin in a stoichiometric ratio and promote its assembly into microtubules. The conformation of the microtubule-bound drug has been the focus of numerous research efforts, since the inability of polymerized tubulin to form crystals precludes structure proof by X-ray crystallography. Likewise, although the alpha,beta-tubulin dimer structure has been solved by electron crystallography, the 3.7 A resolution is too low to permit direct determination of either ligand conformation or binding pose. In this article, we present experimental results from 2H{19F} REDOR NMR that provide direct confirmation that paclitaxel adopts a T-shaped conformation when it is bound to tubulin.

Bridging converts a noncytotoxic nor-paclitaxel derivative to a cytotoxic analogue by constraining it to the T-Taxol conformation

The synthesis of the bridged A-nor-paclitaxel 4 has been achieved from paclitaxel in a key test of the T-Taxol conformational hypothesis. Although the unbridged A-nor-paclitaxel 3 is essentially noncytotoxic, the bridged analogue 4 is strongly cytotoxic. This result provides strong evidence for the T-Taxol conformation as the bioactive tubulin-binding conformation of paclitaxel.

Synergistic effects of peloruside A and laulimalide with taxoid site drugs, but not with each other, on tubulin assembly

Previous studies on the drug content of pelleted tubulin polymers suggest that peloruside A binds in the laulimalide site, which is distinct from the taxoid site. In a tubulin assembly system containing microtubule-associated proteins and GTP, however, peloruside A was significantly less active than laulimalide, inducing assembly in a manner that was most similar to sarcodictyins A and B. Because peloruside A thus far seems to be the only compound that mimics the action of laulimalide, we examined combinations of microtubule-stabilizing agents for synergistic effects on tubulin assembly. We found that peloruside A and laulimalide showed no synergism but that both compounds could act synergistically with a number of taxoid site agents [paclitaxel, epothilones A/B, discodermolide, dictyostatin, eleutherobin, the steroid derivative 17beta-acetoxy-2-ethoxy-6-oxo-B-homo-estra-1,3,5(10)-trien-3-ol, and cyclostreptin]. None of the taxoid site compounds showed any synergism with each other. From an initial study with peloruside A and cyclostreptin, we conclude that the synergism phenomenon derives, at least in part, from an apparent lowering of the tubulin critical concentration with drug combinations compared with single drugs. The apparent binding of peloruside A in the laulimalide site led us to attempt construction of a pharmacophore model based on superposition of an energy-minimized structure of peloruside A on the crystal structure of laulimalide. Although the different sizes of the macrocycles limited our ability to superimpose the two molecules, atom correspondences that were observed were consistent with the difficulty so far experienced in creation of fully active analogs of laulimalide.

Synthesis and biological evaluation of novel taxoids designed for targeted delivery to tumors

The use of drug-antibody conjugates affords a method for the targeted delivery of anticancer drugs specifically to cancer cells. Monoclonal antibodies alone usually do not possess high therapeutic efficacy, however, they are capable of targeting tumor markers selectively. We have prepared taxoids with significantly higher cytotoxicity than paclitaxel and docetaxel. These taxoids now meet the high potency required for use in a targeted-delivery approach using monoclonal antibodies. The synthesis and biological evaluation of these taxoids are reported.

Synthesis of potent taxoids for tumor-specific delivery using monoclonal antibodies

The targeted delivery of taxoids, in the form of taxane-antibody immunoconjugates, requires the preparation of taxoids containing moieties suitable for their conjugation to monoclonal antibodies. A series of taxoids incorporating a disulfide-containing linker at various positions of the taxoid framework have been prepared to investigate the most suitable position for conjugation. A second series of taxoids modified at the C-2 position aimed at increasing the potency of these taxanes has also been prepared.

Molecular cloning and characterization of a cytochrome P450 taxoid 2alpha-hydroxylase involved in Taxol biosynthesis

The Taxol biosynthetic pathway, arising from the primary isoprenoid precursors isopentenyl diphosphate and dimethylallyl diphosphate in yew (Taxus), consists of approximately twenty steps, at least nine of which are thought to be cytochrome P450-mediated oxygenations. Several oxygenases involved in the early hydroxylation steps of the pathway have been identified and the corresponding genes have been cloned; however, defining the enzymes and their genes responsible for oxygenations in the central portion of the pathway is more difficult because neither the exact sequence of reactions nor the relevant intermediates are known. A surrogate substrate, (+)-taxusin (taxa-4(20),11(12)-dien-5alpha,9alpha,10beta,13alpha-tetraol tetraacetate), that was previously employed in the isolation of a taxoid 7beta-hydroxylase, was used here to functionally screen a family of cytochrome P450 oxygenases originating from a Taxus cell EST library. This in vivo screen in yeast led to the identification of a 1488bp cDNA clone (encoding a 495 residue protein) that was capable of producing 2alpha-hydroxytaxusin from taxusin with a K(m) value of 10.5 +/- 2.7 microM and k(cat) of about 0.05 s(-1) for the surrogate substrate. This structurally typical cytochrome P450 resembles most closely the previously isolated taxoid 7beta-hydroxylase, which also uses taxusin as a substrate, and both 2alpha- and 7beta-hydroxylases are capable of the reciprocal conversion of their respective pentaol tetraacetate products to the common hexaol tetraacetate. This C2-hydroxylase would appear to mediate the mid-pathway functionalization of the C2-position of the taxane core that ultimately bears a benzoyl group as an important Taxol pharmacophore. Overexpression of this cytochrome P450 taxoid 2alpha-hydroxylase in Taxus cells may improve Taxol yields and could prove useful in the production of other 2alpha-hydroxy taxoids as starting materials for subsequent acylation at this position.

Mylabris phalerlata induces apoptosis by caspase activation following cytochrome c release and Bid cleavage

Mylabris phalerata (MP) is an insect that has been used for the treatment of cancer in oriental medicine. In the present study, the butanol (BuOH) fraction of MP (BFMP) was examined to determine whether it can exert anti-cancer activity through an apoptotic pathway with little toxicity. BFMP was found to have a specific cytotoxic effect on human monocytic leukemic U937 cells (IC(50) = 140 microg/ml) rather than on peripheral blood mononuclear lymphocytes (PBML, IC(50) = over 500 microg/ml). BFMP also induced the morphological changes of apoptosis, such as chromatin condensation, cell shrinking and DNA fragmentation at a concentration of 31.25 microg/ml. In addition, BFMP significantly increased the portion of apoptotic annexin-V positive cells in a dose-dependent manner, and effectively activated caspases (cysteine aspartase) cascade involving caspases 8, 9 and 3. BFMP also effectively cleaved Bid, a death agonist member of the Bcl-2 family and (poly(ADP-ribose)polymerase) (PARP) and induced the subsequent release of cytochrome c from mitochondria into the cytosol. However, it did not affect Bcl-2 and Bax expression. Taken together, these data suggest that the BuOH extract of Mylabris phalerata can induce apoptosis in U937 cells by caspase cascade activation in conjunction with cytochrome c release, induced by a product of Bid. Therefore, we conclude that BFMP has anti-cancer activity, which is achieved through apoptosis and is associated with little toxicity.

Synthesis and biological evaluation of C-3'NH/C-10 and C-2/C-10 modified paclitaxel analogues

Concurrent modifications on the C-3'NH/C-10, and C-2/C-10 positions on paclitaxel were carried out as a way of investigating possible synergistic effects. The biological activities of these analogues were evaluated in both a microtubule assembly assay and human ovarian cancer (A2780) and prostate cancer (PC3) cytotoxicity assay. In some cases the doubly modified analogues were more active than would have been predicted based on the activity of the singly modified analogues, indicating probable synergistic effects.

Progress in immunoconjugate cancer therapeutics

Advances in immunoconjugate technology have revitalized the "magic bullet" concept of immunotherapeutics for the treatment of cancer. The growing availability of "human" antibodies, the increased epitope repertoire due to genomics and proteomics efforts, and advances in the means of identification and production of tumor-specific antibodies have greatly increased the potential for cancer therapeutic opportunities. Furthermore, the realization that effector molecule potency must be sufficiently high to be effective at concentrations that might realistically be delivered to the tumor site on an antibody carrier has greatly spurred the fields of medicinal chemistry and radionuclide chelate chemistry to produce such molecules.

Overcoming multidrug resistance in taxane chemotherapy

Paclitaxel (Taxol) and docetaxel (Taxotère) are currently two of the most important anticancer drugs in cancer chemotherapy. However, clinical treatment with these taxane agents often encounters undesirable side effects and multidrug resistance (MDR) caused by overexpression of P-glycoprotein (Pgp). Photoaffinity labeling of Pgp using photoreactive radiolabeled paclitaxel analogs along with molecular modeling has revealed a unique binding region for paclitaxel on the C-terminal half of Pgp. Highly efficient taxane-based MDR reversal agents (TRAs) have been developed. Extensive structure-activity relationship (SAR) studies have led to the development of new generation taxanes that possess 2-3 orders of magnitude higher potencies against human cancer cell lines expressing the MDR phenotype. One of these taxanes, SB-T-1 10131 (IDN5109, BAY59-8862), exhibits excellent activity against a variety of drug-sensitive and drug-resistant cancer cell lines as well as human tumor xenografts in mice. This taxane is orally active with excellent bioavailability, and is currently undergoing phase II human clinical trials. Novel taxane-antibody immunoconjugates have shown very promising results for tumor-specific delivery and release of an extremely cytotoxic taxane, wherein epidermal growth factor receptor is used as the specific antigen on the tumor surface of human squamous cancer xenograft in SCID mice.

Chemistry and chemical biology of taxane anticancer agents

Taxol (paclitaxel) and Taxotere (docetaxel) are currently considered to be among the most important anticancer drugs in cancer chemotherapy. The anticancer activity of these drugs is ascribed to their unique mechanism of action, i.e., causing mitotic arrest in cancer cells, leading to apoptosis through inhibition of the depolymerization of microtubules. Although both paclitaxel and docetaxel possess potent antitumor activity, treatment with these drugs often results in a number of undesirable side effects, as well as multidrug resistance (MDR). Therefore, it has become essential to develop new anticancer agents with superior pharmacological properties, improved activity against various classes of tumors, and fewer side effects. This paper describes an account of our research on the chemistry of paclitaxel and taxoid anticancer agents at the biomedical interface, including: 1. The structure-activity relationship (SAR) study of taxoids leading to the development of the "second-generation" taxoids, which possess exceptional activity against drug-resistant cancer cells expressing the MDR phenotype. 2. Development of fluorinated taxoids to study the bioactive conformation of paclitaxel and photoaffinity labeling taxoids for mapping of the drug-binding domain on both microtubules and P-glycoprotein. 3. The synthesis of novel macrocyclic taxoids for the investigation into the common pharmacophore for microtubule stabilizing anticancer agents.

Synthesis and biological evaluation of novel macrocyclic paclitaxel analogues

[reaction: see text] This work describes the synthesis of two novel macrocyclic taxoid constructs by ring-closing olefin metathesis (RCM) and their biological evaluation. Computational studies examine conformational profiles of 1 and 2 for their fit to the beta-tubulin binding site determined by electron crystallography. The results support the hypothesis that paclitaxel binds to microtubules in a "T" conformation.

Synthesis and bioactivity of 2,4-diacyl analogues of paclitaxel

The 2,4-diacyl paclitaxel analogues 8a-8r were prepared from paclitaxel by acylation of 4-deacetyl-2-debenzoylpaclitaxel 1,2-carbonate (3) followed either by hydrolysis of the carbonate and acylation or by direct treatment of the carbonate with an aryllithium. Some of the resulting derivatives showed significantly improved tubulin assembly activity and cytotoxicity as compared with paclitaxel; in some cases this improvement was especially significant for paclitaxel-resistant cell lines.

A steroid derivative with paclitaxel-like effects on tubulin polymerization

The endogenous estrogen metabolite 2-methoxyestradiol has modest antimitotic activity that may result from a weak interaction at the colchicine binding site of tubulin, but it nevertheless has in vivo antitumor activity. Synthetic efforts to improve activity led to compounds that increased inhibitory effects on cell growth, tubulin polymerization, and binding of colchicine to tubulin. This earlier work was directed at modifications in the steroid A ring, which is probably analogous to the colchicine tropolonic C ring. One of the most active analogs prepared was 2-ethoxyestradiol (2EE). We report here that different modifications in the steroid B ring of 2EE yield compounds with two apparently distinct modes of action. Simple expansion of the B ring to seven members resulted in a compound comparable to 2EE in its ability to inhibit tubulin polymerization and colchicine binding to tubulin. Acetylation of the hydroxyl groups in this analog and in 2EE essentially abolished these inhibitory properties. The introduction of a ketone functionality at C6, together with acetylation of the hydroxyls at positions 3 and 17, produced a compound with activity similar to that of paclitaxel, in that the agent enhanced tubulin polymerization into polymers that were partially stable at 0 degrees C. The acetyl group at C17, but not that at C3, was essential for this paclitaxel-like activity.

Equilibrium studies of a fluorescent paclitaxel derivative binding to microtubules

A fluorescent derivative of paclitaxel, 3'-N-m-aminobenzamido-3'-N-debenzamidopaclitaxel (N-AB-PT), has been prepared in order to probe paclitaxel-microtubule interactions. Fluorescence spectroscopy was used to quantitatively assess the association of N-AB-PT with microtubules. N-AB-PT was found equipotent with paclitaxel in promoting microtubule polymerization. Paclitaxel and N-AB-PT underwent rapid exchange with each other on microtubules assembled from GTP-, GDP-, and GMPCPP-tubulin. The equilibrium binding parameters for N-AB-PT to microtubules assembled from GTP-tubulin were derived through fluorescence titration. N-AB-PT bound to two types of sites on microtubules (K(d1) = 61 +/- 7.0 nM and K(d2) = 3.3 +/- 0.54 microM). The stoichiometry of each site was less than one ligand per tubulin dimer in the microtubule (n(1) = 0.81 +/- 0.03 and n(2) = 0.44 +/- 0.02). The binding experiments were repeated after exchanging the GTP for GDP or for GMPCPP. It was found that N-AB-PT bound to a single site on microtubules assembled from GDP-tubulin with a dissociation constant of 2.5 +/- 0.29 microM, and that N-AB-PT bound to a single site on microtubules assembled from GMPCPP-tubulin with a dissociation constant of 15 +/- 4.0 nM. It therefore appears that microtubules contain two types of binding sites for paclitaxel and that the binding site affinity for paclitaxel depends on the nucleotide content of tubulin. It has been established that paclitaxel binding does not inhibit GTP hydrolysis and microtubules assembled from GTP-tubulin in the presence of paclitaxel contain almost exclusively GDP at the E-site. We propose that although all the subunits of the microtubule at steady state are the same "GDP-tubulin-paclitaxel", they are formed through two paths: paclitaxel binding to a tubulin subunit before its E-site GTP hydrolysis is of high affinity, and paclitaxel binding to a tubulin subunit containing hydrolyzed GDP at its E-site is of low affinity.

Synthesis and structure-activity relationships of new second-generation taxoids

A series of second-generation taxoids bearing a substituent on the C-2-benzoyl group and modifications at C-3'/C-10 positions was synthesized. These taxoids exhibited 2-3 orders of magnitude higher potency than that of paclitaxel against drug-resistant human breast cancer cell lines. It is also noteworthy that three taxoids showed almost no difference in activity against drug-resistant and drug-sensitive cell lines, which are categorized as "advanced second generation taxoids".