Human
melanoma cells were treated in culture with the
histamine (H2) agonist S-(3-(N-N-dimethylamino)propyl)isothiourea (
dimaprit), a partial agonist, S-(2-(N,N-dimethylamino)ethyl)-isothiourea (
nordimaprit), and two analogues of
nordimaprit, S-(2-(N,N-diethylamino)ethyl)isothiourea (DENOR) and
S-(2-(N,N-diisopropylamino)ethyl)isothiourea (
DINOR), to investigate the effects on toxicity and
tyrosinase activity. Cell survival studies showed highest toxicity in the constitutively pigmented human
melanoma cell line MM418,
DINOR being the most effective agent. Toxicity was not blocked by the H2 antagonist
cimetidine.
Dimaprit and its derivatives decreased
tyrosinase activity in the amelanotic human
melanoma cell line MM96E and inhibited expression of a melanosomal
antigen. Loss of
tyrosinase activity could be prevented by
cimetidine and
ranitidine, an H2 antagonist. Although the
tyrosinase activity in MM418 cells was much more resistant to inhibition by these agents compared with that in MM96E cells, prolonged growth in the presence of non-toxic levels of
DINOR caused a decrease in
tyrosinase activity and subsequent depigmentation. Ultrastructural examination of the depigmented cells showed a decrease in the number of melanized melanosomes and the appearance of premelanosomes. These results indicate that bulky substituents on the tertiary
amine group in
nordimaprit significantly enhance potency for depigmentation and cell killing but only the former effect is mediated by the H2 receptor.