Cytotoxicity and effect on collagen biosynthesis of proline analogue of melphalan as a prolidase-convertible prodrug in cultured human skin fibroblasts

Proline prodrug of melphalan, prophalan-l, demonstrates high therapeutic index in a murine melanoma model

The therapeutic efficacy of prophalan-L, the L-proline prodrug of melphalan that demonstrated prolidase-dependent bioactivation to melphalan, was examined in vivo in a mouse melanoma model. Prophalan-L exhibited 2- to 2.5-fold higher hydrolytic and cytotoxic activity than prophalan-D, the D-analog, in B16-F10 murine melanoma cells in vitro. Prophalan-L cytotoxicity in B16-F10 cells was lower (GI50=221 microM) than that of melphalan (GI50=173 microM). The tumor growth profiles in C57BL/6J mice injected with B16-F10 cells and treated with melphalan (5.5 microg/g i.p.) and equimolar concentrations of the prodrugs demonstrated significant difference between the control (buffered saline) and melphalan or prophalan-L but no significant difference between control and prophalan-D or between melphalan and prophalan-L. Prophalan-L was significantly less toxic than melphalan, while no significant difference was observed in toxicity, measured as percent weight loss, between the prodrugs and saline control. Tumor reduction efficacy at high doses (12 microg/g i.p.) was similar for melphalan and prophalan-L; however, fatal toxicity was associated with melphalan while prophalan-L exhibited significantly lower systemic toxicity. An excellent correlation between GI50 and tumor reduction efficacy was observed for the tested drugs (r2=0.95). Prophalan-L thus demonstrates higher therapeutic index than melphalan in the murine melanoma model.

Prolidase, a Potential Enzyme Target for Melanoma: Design of Proline-Containing Dipeptide-like Prodrugs

Bioinformatics tools such as Perl, Visual Basic, Cluster, and TreeView were used to analyze public gene expression databases in order to identify potential enzyme targets for prodrug strategies. The analyses indicated that prolidase might be a desirable enzyme target based on its differential expression in melanoma cancer cell lines and its high substrate specificity for dipeptides containing proline at the carboxy terminus. RT-PCR expression of prolidase and hydrolytic activity against N-glycyl-l-proline (GLY-PRO), a standard substrate of prolidase, determined in tumor cell lines, exhibited a high correlation (r(2) = 0.95). These results suggest the possibility of targeting prolidase with prodrugs of anticancer agents for enhanced selectivity. The feasibility of such a scenario was tested by (a) synthesizing prodrugs of melphalan that comprised linkage of the carboxy terminus of the l-phenylalanine moiety of melphalan to the N-terminus of l and d stereoisomers of proline and (b) determining their bioconversion and antiproliferative activities in SK-MEL-5 cells, a melanoma cancer cell line with high expression levels of prolidase. The results of hydrolysis studies of the l- and d-proline prodrugs of melphalan, designated as prophalan-l and prophalan-d, respectively, indicated a approximately 7-fold higher rate of activation of prophalan-l compared to prophalan-d in SK-MEL-5 cell homogenates. Prophalan-l exhibited cytotoxicity (GI(50) = 74.8 microM) comparable to that of melphalan (GI(50) = 57.0 microM) in SK-MEL-5 cells while prophalan-d was ineffective, suggesting that prolidase-specific activation to the parent drug may be essential for cytotoxic action. Thus, melphalan prodrugs such as prophalan-l that are cleavable by prolidase offer the potential for enhanced selectivity by facilitating cytotoxic activity only in cells overexpressing prolidase.

Acetylsalicylic acid as a potential regulator of prolidase-convertible pro-drugs in control and neoplastic cells

Proline analogue of melphalan (Mel-pro) is one of the pro-drugs activated by prolidase, cytoplasmic imidodipeptidase highly expressed in some neoplastic tissues. In order to limit the action of prolidase on the pro-drug in normal cells, prolidase inhibitor, acetylsalicylic acid (ASA), was tested in fibroblasts (showing average prolidase activity for normal cells) and in MDA-MB 231 breast cancer cells (showing elevated activity of the enzyme). The effect of Mel-pro in the presence and absence of ASA on prolidase activity (colorimetric assay), DNA biosynthesis (3H-thymidine incorporation assay), cytotoxicity (tetrazoline assay) and ability to penetrate cell membrane (thin layer chromatography) in both type of cells was measured. It has been found that 5 mM ASA significantly decreased conversion of Mel-pro to Mel in cultured fibroblasts as well as it decreased cytotoxicity and the effect of this drug on DNA synthesis. In contrast, 5 mM ASA had relatively lower effect on the conversion of Mel-pro into Mel in MDA-MB 231 cells as well it had little effect on Mel-pro-induced inhibition of DNA synthesis and cell death. It suggests that ASA may serve as an inhibitor of prolidase-convertible pro-drugs in normal cells.