Erythropoietic Protoporphyria (EPP)
A rare genetic condition (approximately 1 in 75,000-200,000 people) caused by ferrochelatase enzyme deficiency, leading to protoporphyrin accumulation and extreme light sensitivity with painful phototoxic reactions. Afamelanotide is approved for EPP, providing photoprotection through increased eumelanin production.
Technical Context
EPP is caused by mutations in the FECH gene (encoding ferrochelatase, the final enzyme in the haem biosynthesis pathway) or rarely the ALAS2 gene (X-linked protoporphyria, XLP). Ferrochelatase deficiency → accumulation of protoporphyrin IX (PPIX) in erythrocytes, plasma, and skin. PPIX is a potent photosensitiser — when it absorbs light (peak absorption at 408nm/Soret band, with additional peaks at 505, 540, 575, 630nm), it generates reactive oxygen species → rapid endothelial cell damage, mast cell degranulation, and complement activation → burning pain, oedema, and erythema within minutes of sun exposure. Afamelanotide mechanism in EPP: SC implant (60-day release) → MC1R activation on melanocytes → increased eumelanin production → eumelanin absorbs the light wavelengths that would otherwise activate PPIX → reduced phototoxic reactions → increased pain-free time outdoors. Clinical benefit: Scenesse Phase III showed significantly increased time in direct sunlight without pain.