Combined treatment of L5187Y-R and L5178Y-S cells with an antitumor platinum complex (cis-PAD) and ultraviolet light

L5178Y-R and L5178Y-S murine lymphoma cells, cross-sensitive to ultraviolet (UV) light and the antitumor platinum complex cis-PAD, were subjected to combined treatment using both these agents. The cis-PAD dose used for pre-treatment 1 hr before UV-irradiation reduced survival to about 10%. The results of such treatment were analyzed by constructing isobolograms at several log cell kill levels. In both L5178Y strains the same effect was seen: the results of combined treatment were more than additive at UV fluences; this reduced survival to not more than 0.001; sub-additive results occurred at survival levels below 0.001. The similarity of effects of combined treatment in two cell strains that differed in ability to repair ultraviolet (UV) and cis-PAD-inflicted damage, indicates that the described change from supra-additivity to sub-additivity may not depend on repair processes, but rather on the nature of DNA damage produced by both agents