Photoinactivation of skin fibroblasts by fractionated treatment with 8-methoxypsoralen and UVA.

Guinea pig skin fibroblasts treated with low doses of 8-methoxypsoralen (8-MOP) and long-wave ultraviolet light (UVA) showed a dose-dependent inhibition of 3H-Thymidine incorporation as determined by liquid scintillation counting. The minimum incubation time necessary to obtain constant inhibition rates was 60 min. By washing the drug was removed from the reactive sites within 30 min. Repeated light exposure at a constant concentration of 8-MOP caused a cumulative inhibition of DNA synthesis. Irradiation of 8-MOP-plus-UVA treated cells, from which the drug was removed, produced a small increase in photoinhibition. Split dose treatment at various time intervals (ranging from 1--48 hr) revealed inhibitory rates, which correspond to the total amount of UVA applied. No recovery effects were seen in cultures treated by single or multiple applications of 8-MOP-plus-UVA.