Proline analogue of melphalan (MEL-PRO) was synthesised as a prodrug susceptible to the action of ubiquitously distributed, cytosolic imidodipeptidase--prolidase [E.C.3.4.13.9]. Conjugation of melphalan (MEL) with proline (PRO) through an imido-bond resulted in formation of a good substrate for prolidase. The susceptibility of MEL-PRO to the action of prolidase was found to be similar, compared to glycyl-proline--the most abundant, endogenous substrate for prolidase and about 6-fold higher compared to its substrate--glycyl-hydroxyproline. We have compared the transport of MEL and its prodrug through cell membrane, their antimitotic activity, cytotoxicity and effect on collagen biosynthesis in cultured, normal human skin fibroblasts. The prodrug was found to be more effectively transported into the cells than the free drug. Moreover, a lower cytotoxicity, antimitotic activity and inhibitory effect on collagen biosynthesis of the prodrug, compared to the free drug were observed after 24 h of incubation. MEL and MEL-PRO at concentrations of 12 microM led to the decrease in cell viability in confluent human skin fibroblasts by about 40 and 20%, respectively, during 24 h of incubation. IC50 of MEL for DNA synthesis (measured by thymidine incorporation assay) was found at about 7 microM, while MEL-PRO used at this concentration produced about 35% reduction in thymidine incorporation. Similarly, MEL and MEL-PRO used at 7 microM concentrations inhibited collagen biosynthesis in fibroblasts cultured for 24 h to about 30 and 80% of control values, respectively. However, when the cells were cultured with the drugs for 72 h, similar effects of both drugs on DNA and collagen biosynthesis were observed. The data suggest that MEL-PRO may serve as a prolidase-convertible prodrug that evokes lower cytotoxicity, antimitotic activity, and lower inhibitory effect on collagen biosynthesis in fibroblast cultures, compared to the free drug.
Antineoplastic Agents, Alkylating Cell Survival