THE HYDROXYLAMINE OF SULFAMETHOXAZOLE SYNERGIZES WITH FK506 AND CYCLOSPORINE-A, INHIBITING T-CELL PROLIFERATION

We previously demonstrated the capacity of the hydroxylamine metabolit e of sulfamethoxazole (SMX-HA) to inhibit mitogen-induced T-cell proli feration. We studied the interaction of SMX-HA with the immuno-suppres sants cyclosporin A (CsA), FK506 and rapamycin. Human peripheral blood mononuclear leukocytes were treated with SMX-HA and combined in cultu re with CsA or FK506 or rapamycin. The cells were stimulated with phyt ohaemaglutinin, and phorbol myristate acetate and proliferation was de termined by cellular uptake of H-3-thymidine, Using median-effect anal ysis and concentration reduction index calculations to assess immunosu ppressive drug interactions, we produced synergistic immunosuppression by SMX-HA/CsA and SMX-HA/FK506. Concentration reductions at the 50% i nhibitory level of over 46-fold and 64-fold with CsA and FK506, respec tively, were observed with 25 mu M SMX-HA, and this effect was not ass ociated with reduced cell viability, SMX-HA failed to augment the supp ressive capacity of rapamycin in inhibiting mitogen-induced cellular p roliferation. SMX-HA at immunosuppressive concentrations also failed t o interfere with interleukin-2 mRNA transcription and interleukin-2 pr otein production, which suggests that signaling events proximal to cyt okine production are not affected by the metabolite, Synergy between S MX-HA/FK506 and SMX-HA/CsA suggests that the mechanism(s) of action of reactive sulfonamide metabolites may occur in later stages of lymphoc yte activation.