Greater promotion in sister chromatid exchanges by trimethylpsoralen than by 8-methoxypsoralen in the presence of UV-light.

The number of sister chromatid exchanges (SCEs) was determined in normal human leukocytes following exposure in vitro to UV light (365 nm, UV-A) and either 8-methoxypsoralen (8-MOP) or 4,5'8 trimethylpsoralen (TMP). Both psoralens produced an increase in SCEs, proportional to UV dose and concentration of drug. Neither drug alone nor UV alone promoted SCEs. The number of SCEs per cell was greater in leukocytes irradiated in the presence of TMP than in the presence of equimolar 8-MOP. At equimolar concentrations, TMP formed more cross-links than did 8-MOP. The difference in the effects of these 2 agents on various cell functions may relate to quantitative and qualitative differences in the photoadducts they form.