A randomized, crossover, placebo-controlled, multicenter trial of 16 mg s.c. afamelanotide for 60 days was conducted in 15 patients with erythropoietic protoporphyria (EPP). Daily endpoints comprised pain intensity and sunlight exposure time, while 30-day endpoints comprised adverse events and skin melanin density. Phototoxic reactions were recorded when pain scores exceeded 3 on an 11-point Likert scale. Maximum intensity and severity of reactions were significantly lower with afamelanotide than placebo (P = 0.05 and P = 0.008, respectively). Sunlight exposure was significantly longer with afamelanotide (P = 0.009). A new model variable, sunlight exposure x freedom-from-pain (ETFP), correlated positively with afamelanotide (P = 0.004) and was pronounced in summer (P < 0.001). ETFP ...