In vivo and in vitro effect of FK 506 on rat Leydig cell function.

FK 506 is a macrolide antibiotic with powerful immunosuppressive activities. It is known to prevent allograft rejection in rats, dogs, monkeys, baboons, and humans. 1.2 In vitro studies indicate that, like cyclosporine (Cy A), FK 506 acts specifically by inhibiting the transcription of a limited set of early T-cell activation genes, including those encoding interleukin-2 (IL-2), IL-3, IL-4, and interferon-gamma (IFN-y).3 There are reports indicating that CyA impairs testicular steroidogenesis and spermatogenesis in rats.4 •5 The immunosuppressive effect of FK 506 is many times more potent than that of Cy A and to date there are no published studies concerning toxicity of FK 506 on testicular functions. The present study was undertaken to assess the in vivo and in vitro effects of FK 506 on rat Leydig cell function.