The effect of trimethoprim/sulfamethoxazole on Friend erythroleukemia cells.

Cultures of Friend erythroleukemia cells were subjected to the antibiotics trimethoprim (T) and sulfamethoxazole (S) at levels equal to or below the usual therapeutic range. At T 8 microgram/ml and S 40 microgram/ml, cell growth was arrested, cells appeared megaloblastic, and the examination of cell-cycle distribution by flow microfluorimetry revealed arrest in S phase. With a tenfold reduction in drug levels (T, 08 microgram/ml; S, 4 microgram/ml) cell growth was less markedly inhibited, morphology remained megaloblastic, and S-phase block was still dramatic. A further tenfold reduction (T, 0.08 microgram/ml; S, 0.4 microgram/ml), well below effective antibacterial levels, allowed normal cell growth and morphology but DNA synthesis was still inhibited. Additions of folinic acid at 100 ng/ml averted all drug effects. Thus T/S can affect cell replication even at levels well below those usually employed and could prolong the rate of recovery of hematopoietic cells in the myelosuppressed patient.