MIXED AGONIST-ANTAGONIST ACTIVITY OF AN FK-506-RELATED IMMUNOSUPPRESSANT - BIOLOGICAL AND BIOCHEMICAL-CHARACTERIZATION

FK-506 blocks T cell activation by preventing lymphokine gene transcri ption through formation of a complex with FKBP12 that inhibits calcine urin phosphatase activity. Immunosuppressive FK-506 analogs (agonists) have been generated whose potency correlates with calcineurin inhibit ion. Nonimmunosuppressive antagonist analogs have also been identified , including L-685,818, which binds to FKBP12 but does not inhibit calc ineurin. We describe a novel property of FK-506 analog, characterized as a mixed agonist/antagonist immunosuppressive activity. It is displa yed by L-688,617, the 32 O-methoxyethoxymethyl derivative of the agoni st L-683,590 (C21-ethyl). Although it binds to FKBPI2 similarly to L-6 83,590, L-688,617 incompletely suppressed T cell proliferation induced by optimal activation and enhanced that induced by supraoptimal activ ation. In the latter situation, L-688,617 suppressed IL-2 production o nly partially but blocked activation-driven cell death. Moreover, a 10 00-fold molar excess of L-688,617 antagonized the immunosuppressive ac tivity of L-683,590. L-688,617 inhibited calcineurin phosphatase activ ity in cells only partially. The unique agonist/antagonist activity of L-688,617 may therefore reflect its high affinity for FKBP12, combine d with a reduced ability of the drug-FKBP12 complex to inhibit calcine urin function. However, in a cell-free system, L-688,617 completely bl ocked this function when a large excess of FKBP12 over calcineurin was present, suggesting that the intracellular concentration of FKBP12 ma y be a limiting factor that prevents full agonist activity of L-688,61 7 in cells.