The in vitro and in vivo oxytocin/arginine vasopressin (OT/AVP) antagonist properties of two cyclic hexapeptides derived from a newly discovered natural product (L-156,373) of Streptomyces silvensis are described. In radioligand binding assays, L-156,373 [cyclo(L-Pro-D-Phe-N-OH-L-Ile-D-piperazyl-L-piperazyl-N-Me-D -Phe)] exhibited moderate affinity for rat uterine OT receptors (Ki, 150 nM), with some selectivity (approximately 20-fold) vs. liver AVP-V1 and kidney AVP-V2 receptors. Dehydroxylation of N-hydroxyisoleucine and oxidation of the piperazic acid residues of L-156-373 produced an interesting derivative, L-365,209. These structural modifications increased OT receptor affinity and selectivity by 20- and 2.5-5-fold, respectively. In the isolated rat uterus, L-365,209 was a potent (apparent dissociation constant, 1.7 nM) and competitive OT antagonist. L-365,209 also blocked the effects of AVP at both AVP-V1 (phosphatidylinositol turnover in rat hepatocytes) and AVP-V2 (adenylate cyclase in rat kidney medulla) receptors, but only at low micromolar concentrations. L-365,209, given iv to anesthetized rats, antagonized the action of exogenous OT on the uterus (ID50, 460 micrograms/kg) with a relatively long duration of action. L-365,209 represents a unique class of compounds that provides an entirely new approach for the design of antagonists for these neurohypophyseal hormones.

The recently discovered class of cyclic hexadepsipeptide antibiotics is exemplified by azinothricin1) and A83586C2). In our search for novel substances with C5a antagonistic properties, a member of this class, L-156,602, was isolated from a newly isolated strain of streptomycete and found to be a competitive inhibitor of the binding of the anaphylatoxin C5a to its receptor on human PMNs3). Because of its broad range of biological properties, C5a has been implicated as a causative or aggravating agent in a variety of inflammatory and allergic diseases4'5) and an inhibitor of such inflammatory actions would therefore be therapeutically beneficial in the treatment of such diseases. The producing organism, MA-6348, was isolated from a plant rhizosphere soil sample obtained from a Japanese garden. Comparison of spectral data suggested the compound to be identical to PD 124,966, but no structure has been published6'7). We report here primarily on the structure determination of L-156,602 including absolute stereochemistry, based on spectroscopic and X-ray diffraction analyses. The culture was characterized by the International Streptomyces Project procedures described by Shirling and Gottlieb8*. Pigment production,

A novel cytochalasin, L-696,47r4, (18-dehydroxy cytochalasin H) that inhibits HIV-1 protease was discovered in fermentations ofa bark-inhabiting Ascomycete, Hypoxylonfragiforme. The product was first identified from extracts of an agar medium.Fermentation studies on a numberof media indicated that the product can be made on several solid and liquid media. Optimumproduction was obtained from growth in a complex medium composed of glycerol, glucose, citrate, Ardamine, soybean meal, tomato paste, and inorganic salts. Other Hypoxylon spp., related species of Xylariales, and other fungi known to produce cytochalasins, were also surveyed for their ability to make L-696, 474. Only one other Hypoxylonfragiforme isolate was found to make this novel cytochalasin; none of the other cultures surveyed made L-696,474 or any other compoundswhich inhibit HIV-1 protease.The human immunodeficiency virus (HIV-1) is a retrovirus which causes acquired immunodeficiency syndrome (AIDS). HIV-1 protease has been identified as a potential therapeutic target to block the formation of infectious virus particles1}. During our screening of fungi for novel metabolites, an isolate of Hypoxylonfragiforme from Frost Valley, NewYork was observed to produce a novel cytochalasin which inhibits the HIV-1 protease; The purified compound, L-696,474, whose structure is shown in the accompanying paper2), has a molecular weight of 477 and an empirical formula of C3oH39N04. The cytochalasins are a group of fungal secondary metabolites which inhibit cell division and motility. These compounds are characterized by a highly substituted isoindolinone system, to which is fused a macrocyclic ring. The latter is either carbocyclic, a lactone or a cyclic carbonate. The cytochalasins have a phenyl group at position 10 while other similar compounds have indoyl or isopropyl groups at this position. Cytochalasins are distributed amongmanygroups of Ascomycotinaand/or their anamorphic stages, including the Xylariales, but are not found in bacteria or plants. These compounds have been extensively reviewed3'4).Wedescribe here the recovery from nature of the organism which produces the novel cytochalasin L-696,474, its identification, and comparative fermentation studies designed to optimize for production of this compound.Wealso report the results of a survey of other organisms, to determine if other cultures produce L-696,474 or similar cytochalasins, from among selected species of the Xylariales and other known cytochalasin-producing fungi. Details of the isolation, structure determination and biological properties of L-696,474 are considered in the following papers by Ondeyka et al.2) and Lingham et al5).