IMMUNOLOGY, PHARMACOLOGY AND TOXICOLOGY OF THE IMMUNOSUPPRESSIVE CYCLIC PEPTIDE, DIDEMNIN B.

Abstract

Didemnin B (DB) is a seven amino acid cyclic polypeptide (NSC-325319), MW 1112, isolated from a Caribbean tunicate of the family didemnidae (Trididemnum genus). In vitro assays of murine splenic mononuclear cell (MNC) proliferation showed that DB potently inhibited the mixed lymphocyte reaction (IC₅₀ = < 10 pg/ml), concanavalin A (Con A; IC₅₀ = 50 pg/ml) and lipopolysaccharide (IC₅₀ = < 100 pg/ml) mitogenesis. Proliferation induced by phorbol esters, calcium ionophore and Con A were equipotently inhibited by DB, suggesting that the drug acts upon an intracellular pathway common to these mitogens. Since DB did not produce lymphocytotoxicity, nor inhibit ongoing DNA, RNA or protein synthesis at immunosuppressive concentrations, it was concluded that this agent may affect lymphocyte activation processes. Investigation of the mechanisms of DB action showed that it failed to alter interleukin 2 (Il-2) production by MNC in vitro. However DB was found to block binding of the hormone prolactin (PRL) to both human lymphocytes and a PRL-dependent cell line, the Nb 2 node lymphoma. As PRL plays an important regulatory role in the immune response, abrogation of PRL-MNC interaction may represent a site of DB immunosuppressive action. In vivo, DB demonstrated a potent, inhibitory effect upon alloantigen-driven proliferation in the murine graft-versus-host reaction but hemagglutinating antibody responses in mice to sheep red blood cells were strongly enhanced by DB treatment (4.6-fold). Further, DB treatment of mice in vivo was not bone marrow suppressive, but increased organ cellularity, ongoing DNA synthesis and circulating levels of lymphocytes and granulocytes. DB also exerted significant toxicity in vivo. High doses caused losses in body weight (18%) and mortality (20%), but no mortality occurred at doses of 0.1 mg/kg/day x 7 days. DB did not significantly alter indices of renal function but high dose treatment produced significant but reversible hepatotoxicity. DB also induced ornithine decarboxylase activity in various rat tissues, with adrenal > liver > kidney = spleen > thymus > heart. This correlated with increased organ to body weight ratios for liver and spleen, suggesting a trophic response of these organs to DB. The data presented here show that DB exerts potent inhibitory activity toward cell mediated immunity in vivo and in vitro suggesting that DB might also inhibit rejection of solid organ grafts. In addition, humoral responses and bone marrow function are markedly enhanced by DB treatment in vivo, suggesting that resistance to infectious organisms might be increased by this drug.