Human melanoma targeting with alpha-MSH-melphalan conjugate

A conjugate made of alpha-MSH as a drug carrier and melphalan has been designed in order to target human melanoma cells. Iodination of the alpha-MSH moiety led to a relatively stable tracer which could be easily separated and analysed by reverse phase high pressure liquid chromatography. The conjugate was found to be unstable at neutral pH and a serious denaturation can take place at concentrations exceeding 100 micrograms/ml, especially in plasma. Receptor-mediated cytotoxicity has been shown by the use of cultured alpha-MSH receptor positive/negative cells as well as in vivo B16 murine melanoma model. Body distribution and uptake of the labelled compound were unaltered as compared to those of labelled free hormone. alpha-MSH receptor recognition properties also remained unchanged with a better apparent affinity of the conjugate probably due to the alkylating activity of melphalan itself. Using human melanoma dendritic cells expressing more than 10,000 alpha-MSH binding sites per cell as an in vitro model, we were able to demonstrate higher cytotoxicities as compared to melphalan-treated cells. In contrast, melanoma cells with low receptivity did not show higher cytotoxicity. P388D1 mouse plasmocytoma cells lacking receptors were much more sensitive to melphalan than the conjugate. This phenomenon appeared to be related with the number of binding sites expressed at the time of the experiment as well as cell differentiation and the doubling time. Our findings strongly support the concept of a receptor-mediated cytotoxicity and may enable the in vivo melphalan delivery to target tissues to be increased, achieving an improvement of drug penetration inside melanoma cells.