Synthesis and evaluation of some water-soluble prodrugs and derivatives of taxol with antitumor activity

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Abstract
The synthesis and evaluation of some 2'- and 7-amino acid derivatives of taxol (1) are reported. Reaction of taxol with N-protected amino acids gave 2'-N-protected amino acid esters of taxol. However, deprotection of the amino group and subsequent isolation of products were complex and only successful when formic acid was used to deprotect a t-BOC protecting group. Esterification of taxol using N,N-dialkylated amino acids gave 2'-amino acid esters of taxol, 2'-(N,N-dimethylglycyl)taxol (4) and 2'-[3-(N,N-diethylamino)propionyl]taxol as its methanesulfonic acid salt (5b), in good yield. The 7-derivatives, 7-(N,N-dimethylglycyl)taxol (9) and 7-L-alanyltaxol (12), were prepared by two alternate methods. In the first approach, the 2'-hydroxyl group was protected using the [(2,2,2-trichloro-ethyl)oxy]carbonyl, or troc, protecting group followed by the esterification of the 7-hydroxyl and subsequent deprotection of the amino and troc groups. In the second approach, taxol was allowed to react with more than 2 molar equiv of the N-protected amino acids or N,N-dialkylated amino acids to give 2',7-diamino acid esters of taxol. For the protected amino acids, the deprotection of the amino group followed by removal of the 2'-substituent gave the 7-amino acid esters of taxol. The methanesulfonic acid salts of both 2'- and 7-amino acid esters showed improved solubility ranging from 2 to greater than 10 mg/mL. The 7-derivatives were effective in promoting microtubule assembly in vitro while 2'-derivatives showed little in vitro activity. The derivatives 2'-(N,N-dimethylglycyl)taxol (4) and 2'-[3-(N,N-diethylamino)propionyl]taxol (5) inhibited proliferation of B16 melanoma cells to an extent similar to that of taxol, while the other derivatives were about 50% as cytotoxic. In a mammary tumor screen, 2'-[3-(N,N-diethylamino)propionyl]taxol showed the greatest antitumor activity compared to the other analogues. The lower activities of the 7-derivatives in inhibiting tumor growth and melanoma cell proliferation (although they were almost as active as taxol in inducing microtubule assembly in vitro) may be due to differences in drug uptake by the cells. The similar cytotoxic and antitumor activities of the 2'-analogues and taxol can be explained by their conversion to taxol or an active taxol metabolite. Therefore, the 2'-analogues appear to behave as prodrugs and have the potential to be developed as chemotherapeutic agents.

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