Design and Synthesis of a Pro-Drug of Vinblastine Targeted at Treatment of Prostate Cancer with Enhanced Efficacy and Reduced Systemic Toxicity

Abstract

Chemotherapy of prostate cancer with antimitotic agents such as vinblastine and doxorubicin is only marginally effective, due to dose-limiting systemic toxicity. Herein we report the development of peptidyl conjugate 5 of the cytotoxic agent vinblastine (1), along with the results of its in vitro and in vivo evaluation as a pro-drug targeted at prostate cancer cells. Prostate-derived tumors are known to produce significant amounts of prostate specific antigen (PSA), a serine protease with chymotrypsin-like properties. Earlier work in these laboratories established that an appropriately engineered peptidyl pro-drug will release active cytotoxic agent strictly within the microenvironment of the tumor tissue (Garsky, V. M., et al. J. Med.Chem. 2001, 44, 4216-4224). Conjugate 5, which features an octapeptide segment attached by an ester linkage at the 4-position of vinblastine (1), undergoes rapid cleavage by PSA (T(1/2) = 12 min) between the Gln and Ser residues. In nude mouse xenograft studies, 5 reduced circulating PSA levels by 99% and tumor weight by 85% at a dose just below its MTD. By contrast, the putative end-point metabolite, the cytotoxic agent des-acetyl vinblastine (1b), was ineffective in reducing PSA levels and tumor burden at its maximum tolerated doses. Additional data from metabolism studies on 5 support the supervention of a novel in vivo processing mechanism, the spontaneous release of 1b from a dipeptidyl intermediate driven by favorable diketopiperazine formation.

Cyclization-activated Prodrugs

“…In vitro and in vivo evaluation showed the best derivative to be a conjugate bearing an octapeptide segment attached by an ester linkage to position 4 of vinblastine. This conjugate was found to undergo fast (t 1/2 =12 min) and specific cleavage by prostate enzymes of the Gln-Ser peptide bond and additional data from metabolism studies supported that the final spontaneous vinblastine release occurred from a dipeptidyl intermediate and was driven by DKP formation [57]. Scheme 10.…”

“…In this connection, peptide derivatives of some drugs have been prepared and evaluated as prodrug candidates where prodrug activation processes involved DKP formation. One example is that of peptide conjugates of the cytotoxic agent vinblastine, designed as potential prodrugs targeted at prostate cancer cells [57]. In vitro and in vivo evaluation showed the best derivative to be a conjugate bearing an octapeptide segment attached by an ester linkage to position 4 of vinblastine.…”

“…Actually, the example of pilocarpic acid double ester prodrugs of pilocarpine given in section 2 (Scheme 2) may fall in this category, as the nucleophilic hydroxyl can be released after enzymatic cleavage of the ester moiety [13]. Vinblastine peptide conjugates mentioned in 3.2.2.a) can also be included in this category, as the dipeptide intermediate undergoing intramolecular cyclization via DKP to release vinblastine is a product of enzymatic cleavage of a larger oligopeptide precursor [57].…”

Modifications on the Basic Skeletons of Vinblastine and Vincristine

“…Compound 6 can be considered as the active metabolite of vinblastine [6], because its activity is substantially higher than that of the prodrug vinblastine.…”

“…The same reaction was carried out by Brady [6] in 95% yield with a 1:3 hydrazine-methanol mixture at 20 °C for 20 h. The resulting 17-desacetylvinblastine (6) was allowed to react with amino acids protected on the N-terminal with Fmoc or Phth [(9-fluorenylmethyloxi)carbonyl) and phthtaloyl]; after coupling the products were deprotected and new derivatives 7 containing amino acids (e.g., glycine, leucine and proline) and different peptides attached to position 17 with an ester bond were obtained (Scheme 3). Compounds 7 showed significant effects against prostate cancer.…”