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Oral 8-methoxypsoralen (8-MOP) plus high-intensity long-wavelength ultraviolet radiation (UV-A) is used clinically to induce remissions of psoriasis and mycosis fungoides, Leukocytes in 8-MOP containing blood receive UV-A exposure when circulating through the dermis during therapy. The present study utilizes an in vitro assay system to permit quantitation and correlation of multiple biological and physical alterations in human lymphoid cells induced by 8-MOP plus UV-A treatment. Additive inhibition of lymphoid cell DNA synthesis by 8-MOP (0.01 to 1 μg/ml) plus UV-A (1,000 to 29,000 J/m²) was accompanied by a synergistic potentiation of cell killing in the therapeutic exposure range. Reduction in tritiated thymidine (³HTdR) incorporation to 65-70% of control value was associated with normal survival; while ³HTdR incorporation of less than 50% of control induced by any 8-MOP plus UV-A combination tested was associated with less than 10% survival, 8-MOP-DNA-cross-links were detected by the alkaline elution assay only when ³HTdR incorporation was reduced to less than 50% of control. The relative number of crosslinks increased proportionately with further 8-MOP plus UV-A-induced reduction in ³HTdR incorporation. 8-MOP plus UV-A induced at most approximately a 2-fold increase in sister chromatid exchanges (SCE) per chromosome in lymphocytes or lymphoblastoid cells, Increasing 8-MOP plus UV-A exposure resulted in marked toxicity with few cells progressing to second division metaphases and no further increase in SCE's per chromosome, Addition of 13-cis retinoic acid (1μg/ml) to the lymphoblastoid cells prior to 8-MOP plus UV-A treatment did not significantly alter the ³HTdR incorporation or cell survival. These studies dermonstrate that in vitro exposure of human lymphoid cells to therapeutic levels of 8-MOP and UV-A may decrease cellular DNA synthesis, produce DNA 8-MOP interstrand cross-links, reduce cell viability and induce small increases in sister chromosome exchanges. [ABSTRACT FROM AUTHOR]