Melanogenesis as a targeting strategy against metastatic melanoma: a reassessment.

Following the early study of Hochstein and Cohen in 1963 suggesting the potential toxicity of quinonoid melanin precursors, considerable interest has been focused on the possibility of exploiting the pigment-producing pathway as a targeting strategy against metastatic melanoma. Several melanocytotoxic phenolic pro-drugs have been developed, and some of them have reached the preclinical stage. Here we review the progress made so far and supply some insight into the chemical and biochemical mechanisms underlying the melanocytotoxic response.

The inherent cytotoxicity of melanin precursors: a revision.

The potential cytotoxicity of the melanogenic intermediates DOPA, (L-3,4-dihydroxyphenylalanine) and DHI (5,6-dihydroxyindole) has long been recognized and exploited as a targeting concept in experimental melanoma therapy. In recent years, however, a novel branchpoint in the melanin biosynthetic pathway has been shown to divert the metabolism of DOPAchrome to a carboxylated derivative termed DHICA (DHI-2-carboxylic acid) rather than to DHI. In order to evaluate the biological implications of this regulatory control, we have reexamined the inherent cytotoxicity of DHICA versus DHI on different cell lines. We found that under the usual conditions of the biological assay, the apparent cytotoxicity of the two indoles reflect their instability in the culture medium, the less stable DHI being generally more toxic than DHICA to melanoma cells and nonmelanocytic cells. Moreover, the observed cytotoxic effects increased with the time of incubation and were markedly reduced by the addition of catalase to the medium, suggesting that they were probably due to the generation of reactive oxygen species (particularly H2O2) during the autoxidation of the melanin precursors outside the cells. To circumvent this problem, we then tested the diacetylated derivatives of DHI and DHICA (DAI and DAICA) which are sufficiently stable until taken up into the cells whereupon they may be converted by endogenous esterases back to the parent indoles. Although DAI proved to be cytotoxic for nonmelanocytic cells, it had no detectable activity on melanoma cells, whereas DAICA showed no effect on any of the cells examined. These results, when combined with other studies, point to a reconsideration of the inherent cytotoxicity of the 5,6-dihydroxyindoles, as well as DOPA, to melanin producing cells.

Synthesis and cytotoxic properties of new N-substituted 4-aminophenol derivatives with a potential as antimelanoma agents.

New tyrosinase-targeted compounds based on structural variants of the prototype unit 4-aminophenol have been synthesized and screened for their potential as antitumour agents against malignant melanoma. Cytotoxicity assays showed that N-4-hydroxyphenylglycine (NHPG) and its alpha-methyl derivatives methylphenylglycine and dimethylphenylglycine exhibit significant antiproliferative effects on pigmented human melanoma cell lines (HBL), with inhibitory concentrations at 50% (IC50) around 80 micrograms/ml. A marked increase in cytotoxicity was observed with morpholine-containing 4-aminophenols, e.g. N-(2-morpholinoethyl)-4-aminophenol, which showed an IC50 of 20 micrograms/ml of HBL cells. Much more pronounced was the effect of the diacetoxy-derivative, DiAcMoAc, which showed an IC50 of 15 micrograms/ml on HBL cells and as low as 2 micrograms/ml on tyrosinase-containing, non-pigmented human melanoma cells (LND1), with a toxicity response of the same order of magnitude as that of melphalan. These results open interesting perspectives in the design of new targeted pro-drugs against malignant melanoma.