Synthesis and actions of a melanotropin conjugate, Ac‐[Nle⁴, Glu(gamma‐4′‐hydroxyanilide)⁵, D‐Phe⁷]α‐MSH_4–10‐NH₂, on melanocytes and melanoma cells in vitro

L‐Glutamic acid (γ‐4′‐hydroxyanilide) (GHB) is oxidized by tyrosinase to a quinone which inhibits DNA polymerase, RNA polymerase, and mitochondrial energy production within mushrooms. It was previously shown that GHB can kill B16 melanoma cells in culture, but lacks cytotoxicity for nontyrosinase‐containing cells. We have conjugated this drug to a superpotent melanotropic peptide and examined the bioactivity of this conjugate to melanoma cells. 4′‐Hydroxyaniline was attached to glutamic acid at position 5 in the superpotent melanotropin fragment analogue, Ac‐[Nle⁴, D‐Phe⁷]α‐MSH_4–10‐NH₂. The melanotropin:anilide conjugate, Ac‐[Nle⁴, Glu(γ‐4′‐hydroxyanilide)⁵, D‐Phe⁷]α‐MSH_4–10‐NH₂, was not cytotoxic to B16 or Cloudman S91 mouse melanoma cells in culture, as determined by cell counts and protein assays. Interestingly, we also found that GHB stimulated melanoma cell tyrosinase above control levels in both melanoma cell lines. In our study, GHB itself also was found not to be cytotoxic to B16 or S91 melanoma cells in culture. In the frog skin bioassay, the melanotropin conjugate was more potent than α‐MSH or Ac‐[Nle⁴, D‐Phe⁷]α‐MSH_4–10 in stimulating melanosome dispersion. These results demonstrate that putative chemotherapeutic ligands can be incorporated into active‐site fragment analogues of MSH without loss of biological activity.