The effect of whole-body sunbed ultraviolet A exposure on the pharmacokinetics of the photolabile drug nifedipine

The calcium antagonist nifedipine absorbs ultraviolet A (UVA) radiation and readily photodegrades in vitro to a toxic nitroso-pyridine photoproduct. W e examined whether whole body exposure of normal subjects to sunbed UVA rad iation would affect the pharmacokinetics of nifedipine. Eight healthy, male , Caucasian volunteers (phototypes I-III) participated in this ethically ap proved, randomised, cross-over study. Each subject attended on 2 occasions, one week apart, and on each occasion was given a single oral dose (10 mg) of nifedipine following which blood samples were collected at 0, 0.5, 1. 1. 5, 2, 2.5, 3, 3.5, 4, 5, 6 and 7 h. During one of the visits, 15 min after nifedipine ingestion, a whole-body UVA (sunbed comprising Philips R-UVA lam ps) dose of 70% of the individual's predetermined minimal phototoxic dose w as delivered over a period of 17-36 min. Plasma nifedipine levels were meas ured using a standard reverse-phase high-performance liquid chromatography method. The area under the plasma concentration-time curve (AUC) of nifedip ine during the UVA irradiation session (median 206 ng . ml(-1). h(-1)) was significantly higher than during the non-irradiation control session (media n 174.5 ng . ml(-1). h(-1)) (P=0.03; 95% C.I. for difference in medians 9.9 to 55.9 ng . ml(-1). h(-1)). UVA irradiation did not significantly affect any of the other measured pharmacokinetic parameters (C-max, t(1/2), t(max) ). We demonstrate that sunbed UVA irradiation does not lead to in vivo phot odegradation of nifedipine in healthy humans after a single dose. The appar ent increase in AUC during UVA irradiation may be due to slightly slower me tabolism of nifedipine in the presence of toxic photoproduct(s) or due to b lood distribution changes affecting liver blood flow.