Tacrolimus, previously known as FK506, is the active ingredient in Prograf. Tacrolimus is a macrolide immunosuppressant produced by Streptomyces tsukubaensis. It reduces peptidyl-prolyl isomerase activity by binding to the immunophilin FKBP-12 (FK506 binding protein) creating a new complex. This FKBP12-FK506 complex interacts with and inhibits calcineurin thus inhibiting both T-lymphocyte signal transduction and IL-2 transcription. Tacrolimus inhibits T-lymphocyte activation, although the exact mechanism of action is not known. Experimental evidence suggests that tacrolimus binds to an intracellular protein, FKBP-12. A complex of tacrolimus-FKBP-12, calcium, calmodulin, and calcineurin is then formed and the phosphatase activity of calcineurin inhibited. This effect may prevent the dephosphorylation and translocation of nuclear factor of activated T-cells (NF-AT), a nuclear component thought to initiate gene transcription for the formation of lymphokines (such as interleukin-2, gamma interferon). The net result is the inhibition of T-lymphocyte activation (i.e., immunosuppression). Prograf is indicated for the prophylaxis of organ rejection in patients receiving allogeneic liver transplants, kidney transplants, heart transplants. It has also been used in a topical preparation in the treatment of severe atopic dermatitis.

Ascomycin 19-epimer is a stereoisomer of ascomycin (immunomycin). Ascomycin was isolated in the early sixties as a fermentation product of Streptomyces hygroscopicus var. ascomycetus and probed for its antifungal activity. Ascomycin derivatives represent a novel class of anti-inflammatory macrolactams currently under development for the treatment of skin diseases.

Immunomycin (Ascomycin, FR-900520, FK520) is a macrocyclic lactone produced by S. hygroscopicus. It is well known as an antifungal and antibiotic agent. Immunomycin is an analog of FK506 and also contains a 23-membered ring. Both FK506 and Immunomycin bind to FKBP12 at the same binding site for hindering calcineurin. Thus, Immunomycin possesses immunosuppressive activity similar to that of FK506. It appears that Immunomycin inhibits the production of Th1 (interferon- and IL-2) and Th2 (IL-4 and IL-10) cytokines. Additionally, Immunomycin preferentially inhibits the activation of mast cells, an important cellular component of the atopic response. Immunomycin produces a more selective immunomodulatory effect in that it inhibits the elicitation phase of allergic contact dermatitis but does not impair the primary immune response when administered systemically. Immunomycin is a member of the polyketide group of compounds. The polyketide synthase (PKS) enzyme is used to produce polyketides. The preparation of polyketide compounds has low product yield with traditional chemical methodology using wild-type cells. Reeves et al. provide a novel method of preparing a polyketide using a host cell with a recombinant DNA vector. The recombinant DNA vector contains nucleic acids that can encode the Immunomycin PKS enzyme. Immunomycin synthesized using this technology yields PKS at higher levels than that achieved by PKS in nature.