Structure elucidation of xanthone derivatives with CD4-binding activity from Penicilliurn

Fermentations of Penicillium glabrum (Wehmer) Westling produce a complex mixture of metabolites with potent CD4-binding activity in an ELISA based on the binding of the monoclonal antibody antiLeu 3a to soluble recombinant CD4. On purification this mixture was found to include the known polyketide metabolites citromycetin and anhydrofulvic acid and a series of novel xanthone derivatives with CD4-binding activity. The structure elucidation of these xanthone derivatives was achieved by a combination of mass spectrometric and NMR spectroscopic techniques including HMQC, HMBC and ROESY. These new C27/C28 polyketide metabolites probably arise from a dimerisation of two C14 polyketides such as the citromycetin precursor polivione. INTRODUCTION Extracts of aqueous culture filtrates of fermentations of Penicillium glabrum (Wehmer) Westling contained a complex mixture of metabolites with potent CD4-binding activity in an ELISA based on the binding of the monoclonal antibody anti-Leu 3a to soluble recombinant CD4.l CD4 is a glycoprotein expressed on the surface of mature helperhnducer T lymphocytes and has an essential role in many immune responses.2 It is also the cellular receptor for the human immunodeficiency virus (HIVJ2 Anti-Leu 3a blocks both CD4-dependant T cell responses and the binding of HIV to T cells. Compounds with activity in the anti-Leu 3a / CD4 ELISA therefore have potential as selective immunusuppressive agents and also as anti-HIV agents. The P. glabrum fermentation extracts were purified by preparative reversed phase HPLC and were found to contain the known compounds citromycetin, polivione and anhydrofulvic acid, which had low activity in the CD4-binding assay, and a series of novel xanthone derivatives with potent CD4binding activity. Citromycetin, and polivione, are known metabolites of P ~ e g ~ e n t a n s . ~ ~ ~ Polivione exists as a mixture of two slowly interconverting t au tomer~ .~ Anhydrofulvic acid, has not previously been reported as a fermentation product. It has been reported as a dehydration product of the Penicillium sp. metabolite fulvic acid5, This paper presents the structure elucidation of the xanthone derivatives 411F, 1, 411P, 2, and 4115, 3, by a combination of mass spectrometric and NMR spectroscopic techniques. Solutions of 4115 also contained the cyclised form 4. After this work was completed the first of these new xanthone derivatives. 4 1 lF, was reported independantly as vinaxanthone, a phospholipase C inhibitor produced by P vinaceum6 The biological properties of the compounds described here will be reported el sew her^.^ l R = C O z H 2 R = H 3