Structure-activity relationships of new taxoids derived from 14 beta-hydroxy-10-deacetylbaccatin III

Synthesis and biological evaluation of C-13, C-14 modified taxane analogues from 1-deoxybaccatin VI

Four C-13, C-14 side chain modified 9(R)-hydroxy-1-deoxy-taxane analogues 15, 16, 19 and 22 were semi-synthesized from 1-deoxybaccatin VI. The in vitro antitumor activity of these compounds was evaluated against A549 and A2780 cell lines. The preliminary SAR analysis showed that introduction of oxygen-containing group on C-14 could improve the cytotoxic activities.

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.

New Taxane Derivatives: Synthesis of Baccatin[14,1-d]furan-2-one Nucleus and Its Condensation with the Norstatine Side Chain

New taxanes 15 and 18, containing the unsaturated and saturated baccatin[14,1-d]furan-2-one nucleus, respectively, were prepared starting from the readily available 13-oxo-7-Tes-baccatin III (3). Sequential formation of the enolate of 3 and reaction with ethyl glyoxylate gave the 13-oxo-7-Tes-baccatin[14,1-d]-3,4-dehydrofuran-2-one 4. The reduction of 4 can result in the formation of a mixture of compounds corresponding to 13-hydroxy alcohol 5 and 13-enol derivative 6. Both 5 and 6 were transformed into 13-oxo-7-Tes-baccatin[14,1-d]furan-2-one 8 by treatment with a base. Further reduction of 8 gave 13-hydroxy compound 9. Esterification of 6 and 9 with N,O-protected norstatine 12, followed by deprotection, gave the new promising anticancer taxanes 15 and 18, respectively.

Taxoid metabolism: Taxoid 14beta-hydroxylase is a cytochrome P450-dependent monooxygenase

The production of the anticancer drug Taxol in Taxus (yew) cell cultures is often accompanied by the formation of side-route polyoxygenated taxoid metabolites bearing a 14beta-hydroxyl group. The recent acquisition of several new semisynthetic taxoid intermediates enabled the screening of a family of Taxus cytochrome P450 cDNA clones for the 14beta-hydroxylase and additional taxoid oxygenases. The candidate cytochrome P450 clones were functionally expressed in yeast and tested by in vivo feeding of radiolabeled 5alpha-acetoxy-10beta-hydroxy taxadiene and 5alpha,13alpha-dihydroxy taxadiene. One clone efficiently and specifically transformed the 5alpha-acetoxy-10beta-ol, but not the 5alpha,13alpha-diol, to a more polar product with the chromatographic properties of a taxoid triol monoacetate, and the identity of this product was confirmed by spectroscopic means as 5alpha-acetoxy-10beta,14beta-dihydroxy taxadiene. Microsome preparation from the transformed yeast allowed characterization of this new hydroxylase, which was shown to resemble other cytochrome P450 taxoid hydroxylases with pH optimum at 7.5 and a K(m) value for the taxoid substrate of about 50 microM. Because Taxol is unsubstituted at C14, the 14beta-hydroxylase cannot reside on the pathway to the target drug but rather appears to be responsible for diversion of the pathway to 14-hydroxy taxoids that are prominent metabolites of Taxus cell cultures. Manipulation of this hydroxylase gene could permit redirection of the pathway to increase flux toward Taxol and could allow the preparation of 13alpha,14beta-hydroxy taxoids as new therapeutic agents.

Progress in the development of alternative pharmaceutical formulations of taxanes

The currently available taxanes paclitaxel (Taxol) and docetaxel (Taxotere) are clinically effective against advanced breast, ovarian and non-small cell lung cancer. Due to their low aqueous solubility, both taxanes posed difficulties to the pharmaceutical scientists with respect to the development of an intravenous dosage form. At present, paclitaxel is formulated in a mixture of 50:50% (v/v) Cremophor EL and dehydrated ethanol. However, this formulation vehicle is associated with a number of pharmacological, pharmacokinetic and pharmaceutical concerns amongst which serious hypersensitivity reactions. This review deals with the attempts made into the development of alternative dosage forms of paclitaxel devoid of the Cremophor EL/ethanol excipients and potential future taxane formulations.

Structural significance of the benzoyl group at the C-3'-N position of paclitaxel for nitric oxide and tumor necrosis factor production by murine macrophages

The antitumor agent paclitaxel (Taxol) mimics the actions of lipopolysaccharide (LPS) on murine macrophages (M phi). Recently, we have shown that the benzoyl group at the C-3' position of paclitaxel is the most important site to induce nitric oxide (NO) and tumor necrosis factor (TNF) production by C3H/HeN M phi (Biochem. Biophys. Res. Commun. 210, 678-686, 1996). In the present study, synthetic analogs of paclitaxel with replacement of the C-3'-N position were examined for their potencies to induce NO and TNF production by peritoneal M phi of LPS-responsive C3H/HeN mice and LPS-hyporesponsive C3H/HeJ mice, by human blood cells and human M phi. In this structure-activity relationship study, we found that (i) the p-substitution of the benzoyl group definitely affects the activity to activate C3H/HeN M phi, (ii) the analogs having a methyl or chloro group at the p-position exhibit stronger activity than that of paclitaxel, (iii) there is good correlation between NO and TNF production by the M phi in response to compounds, (iv) the compounds tested do not induce either NO or TNF production by C3H/HeJ M phi or TNF production by human cells, (v) a previous treatment of C3H/HeN M phi with the inactive compounds can hardly affect either paclitaxel- or LPS-induced TNF production by the M phi, (vi) paclitaxel and its analogs marginally affect LPS-induced TNF production by human blood cells, and (vii) there is no correlation between the NO/TNF inducibility to C3H/HeN M phi and growth inhibitory activity against M phi-like J774.1 and J7.DEF3 cells.

Syntheses and structure-activity relationships of the second-generation antitumor taxoids: exceptional activity against drug-resistant cancer cells

A series of new 3'-(2-methyl-1-propenyl) and 3'-(2-methylpropyl) taxoids with modifications at C-10 was synthesized by means of the beta-lactam synthon method using 10-modified 7-(triethylsilyl)-10-deacetylbaccatin III derivatives. The new taxoids thus synthesized show excellent cytotoxicity against human ovarian (A121), non-small-cell lung (A549), colon (HT-29), and breast (MCF-7) cancer cell lines. All but one of these new taxoids possess better activity than paclitaxel and docetaxel in the same assay, i.e., the IC50 values of almost all the taxoids are in the subnanomolar level. It is found that a variety of modifications at C-10 is tolerated for the activity against normal cancer cell lines, but the activity against a drug-resistant human breast cancer cell line expressing MDR phenotype (MCF7-R) is highly dependent on the structure of the C-10 modifier. A number of the new taxoids exhibit remarkable activity (IC50 = 2.1-9.1 nM) against MCF7-R. Among these, three new taxoids, SB-T-1213 (4a), SB-T-1214 (4b), and SB-T-1102 (5a), are found to be exceptionally potent, possessing 2 orders of magnitude better activity than paclitaxel and docetaxel. The observed exceptional activity of these taxoids may well be ascribed to an effective inhibition of P-glycoprotein binding by the modified C-10 moieties. The new taxoid SB-T-1213 (4a) shows an excellent activity (T/C = 0% at 12.4 and 7.7 mg/kg/dose, log10 cell kill = 2.3 and 2.0, respectively) against B16 melanoma in B6D2F1 mice via intravenous administration.

Antitumor efficacy of taxane liposomes on a human ovarian tumor xenograft in nude athymic mice

Taxanes such as paclitaxel (Taxol) and docetaxel (Taxotere) are promising agents for use against ovarian cancer and other malignancies. Recently, SB-T-1011, a semisynthetic taxane, has been prepared from 14-hydroxy-10-deacetylbaccatin III. SB-T-1011 shows similar or greater in vitro cytostatic activity than paclitaxel, depending on the tumor cell line. The administration of taxanes is problematic due to their low solubility in most pharmaceutically acceptable solvents; formulations used clinically contain Cremophor/ethanol (diluent 12) or polysorbate 80/ethanol, excipients which may cause serious adverse effects. To eliminate these vehicles, we have prepared paclitaxel liposome formulations. The objective of the present work was to evaluate the antitumor activity of paclitaxel and two semisynthetic analogs in Cremophor-based and liposomal formulations. Antitumor activity was evaluated against A121a, a taxane-sensitive human ovarian tumor, growing as subcutaneous xenografts in athymic nude mice. Free and liposomal formulations of each taxane showed similar antitumor effect. The antitumor activity of paclitaxel and SB-T-1011 was similar, and docetaxel was more potent than either paclitaxel or SB-T-1011. Overall, taxane liposomes were better tolerated and more easily administered iv than taxane formulated in Cremophor/ethanol.