Effects of methylated xanthines on mammalian cells treated with bifunctional alkylating agents

Caffeine has been previously reported to enhance the lethal potential of many DNA-damaging agents in rodent cells 1–5 . This effect has most commonly been ascribed to the binding of caffeine to single-stranded DNA 6 , and the resulting inhibition of post-replication repair 7–10 , which is associated with the synthesis of abnormally small nascent DNA fragments 7,11–13 . However, certain aspects of this theory remain unclear: (1) why does the addition of caffeine to damaged cells elevate the level of DNA synthesis when it supposedly blocks post-replication repair 10,14 , and (2) as pointed out by Cleaver 15 , why does caffeine continue to exert its synergistic lethal effects until completion of the S phase 16,17 , even though the size of newly synthesized DNA seems normal much earlier 18–20 ? The present studies with nitrogen mustard (HN 2 ) fail to demonstrate any effect of non-lethal concentrations of methylated xanthines (MXs) on removal of DNA damage or post-replication repair in conditions producing synergistic lethal effects. We demonstrate an influence by MXs on initiation of DNA synthesis in damaged replicons, and propose that this effect is primarily responsible for the synergistic lethal properties of these drugs.