FK-506, a novel immunosuppressant, has been isolated from the fermentation broth of Streptomyces tsukuhaensis No. 9993 as colorless prism and the molecular formula was determined as C44H69NOi2«H2O. The compoundsuppressed immuneresponses in vitro and in vivo with mice. This immunosuppressiveeffect was more potent than that of ciclosporin.Ciclosporin (CS), a fungal metabolite, is an effective immunosuppressant with low myelotoxicity0 and has been used successfully as the primary drug to suppress the rejection of transplants.2"0It is now well established that CS inhibits the production of T cell-derived soluble mediators such as interleukin 2 (IL-2), interleukin 3 (IL-3) and gamma-interferon (IFN-f) induced by antigens and lectins.5~8) The immunosuppressive agent which would attack specific target sites of cell are expected to provide a useful prototype o drugs for immunotherapy. Accordingly, we hav< tested a wide range of fermented broths fo specific inhibitory effects on IL-2 production. As a result, a strain of Streptomyces tsukuba ensis No. 9993 was found to produce the poten immunosuppressive agent, designated by th< code number of FK-506. Taxonomic study or this strain will be presented in a separate paper.9) In this paper, we describe the fermentation, isolation procedures and some chemical and biological properties of FK-506. As shown in Fig. 1, FK-506 is a neutral macrolide. Determination of the chemical structure of FK-506 will be published elsewhere.10) Materials and MethodsFermentati on A seed medium 100 ml containing glycerol 1 %, corn starch 1 %, glucose 0.5%, cotton seed meal 1 %, corn steep liquor 0.5% and calcium carbonate 0.2% at pH 6.5 was poured into a 500-ml Erlenmeyer flask and sterilized at 120°C for 30 minutes. A loopful of slant culture of S. tsukubaensis No.

The immuno-pharmacological profile of a novel immunosuppressive agent, produced by a streptomycete, is presented here. Weproceeded to test the effect of the agent on various in vitro immunesystems. It showed that mixed lymphocyte reaction, cytotoxic T cell generation, the production of T cell-derived soluble mediators such as interleukin 2 (IL-2), interleukin 3 and gamma-interferon and the expression of the IL-2 receptor were suppressed by this agent. The IC50 values of FK-506 and ciclosporin (CS) in all tests were approximately 0.1 nM and 10 nM, respectively. Therefore, the novel agent, FK-506 suppressed in vitro immunesystems at about hundred times lower concentration than CS.

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