The tridecapeptide
afamelanotide (Scenesse®) is a congener of α-
melanocyte stimulating hormone (α-
MSH). Upon binding to the
melanocortin 1 receptor (MC1R) on the surface of pigment cells of the skin, the melanocytes, α-
MSH or
afamelanotide trigger the synthesis of cAMP, which stimulates the synthesis of
melanin and therefore induces skin tanning. In a recent trial,
afamelanotide administered as controlled release implants protected
erythropoietic protoporphyria (EPP) patients from sunlight induced phototoxic skin reactions. Administration of
biological therapeutic
peptides may elicit unwanted immunogenic responses in recipients of these products. Although in a previous study using ELISA technique we excluded any newly developed immunogenicity during prolonged exposure to
afamelanotide, we confirmed the previously published existence of low titers of
antibodies against α-
MSH in
drug-naïve individuals that cross-reacted with
afamelanotide. In order to investigate whether such
antibodies are neutralizing, i.e. could block the
biological effect of
afamelanotide, we developed a cell culture-based bioassay. The basis of our assay was the measurement of
afamelanotide-induced cAMP formation in a strain of the B16 mouse
melanoma cell line, G4F-7, expressing the transfected human MC1R. Average half-effective concentrations of the natural
hormone α-
MSH and its congener
afamelanotide were 38.8 ± 10.6 and 10.9 ± 7.17 nM (n=5), respectively.
Neutralizing antibodies would reduce the cAMP formation. Two neutralizing anti-α-
MSH antibodies served as positive controls. cAMP formation in the G4F-7 cells after addition of sera of
drug-naïve (n=6) and of
drug-exposed EPP patients (n=17) was significantly lower than after that from healthy volunteers (n=13). There was no difference between
drug-naïve and
drug-exposed patients. Using
forskolin as a
hormone-independent stimulator of cAMP formation, we excluded an unspecific interference of EPP sera with cAMP formation. We conclude that
afamelanotide even after prolonged application to EPP patients did not elicit
neutralizing antibodies. Further, the low titer immunoreactivity observed in sera of some
drug-naïve individuals had no effect on the
biological activity of
afamelanotide.