Leptomycin B is the first low molecular weight inhibitor of nuclear export to be identified. Although it cannot be used therapeutically, it should serve as a valuable tool for dissecting nuclear export pathways.

We report here on the characterization of the novel immunosuppressant Sanglifehrin A (SFA). SFA is a representative of a class of macrolides produced by actinomycetes that bind to cyclophilin A (CypA), the binding protein of the fungal cyclic peptide cyclosporin A (CsA). SFA interacts with high affinity with the CsA binding side of CypA and inhibits its peptidyl-prolyl isomerase activity. The mode of action of SFA is different from known immunosuppressive drugs. It has no effect on the phosphatase activity of calcineurin, the target of the immunosuppressants CsA and FK506 when complexed to their binding proteins CypA and FK binding protein, respectively. Moreover, its effects are independent of binding of cyclophilin. SFA inhibits alloantigen-stimulated T cell proliferation but acts at a later stage than CsA and FK506. In contrast to these drugs, SFA does not affect IL-2 transcription or secretion. However, it blocks IL-2-dependent proliferation and cytokine production of T cells, in this respect resembling rapamycin. SFA inhibits the proliferation of mitogen-activated B cells, but, unlike rapamycin, it has no effect on CD154/IL-4-induced Ab synthesis. The activity of SFA is also different from that of other known late-acting immunosuppressants, e.g., mycophenolate mofetil or brequinar, as it does not affect de novo purine and pyrimidine biosynthesis. In summary, we have identified a novel immunosuppressant, which represents, in addition to CsA, FK506 and rapamycin, a fourth class of immunophilin-binding metabolites with a new, yet undefined mechanism of action.

Historically, nature has provided the source for the majority of the drugs in use today. More than 20,000 microbial secondary metabolites have been described, but only a small percentage of these have been carried forward as natural product drugs. Natural products are in tough competition with large chemical libraries and with combinatorial chemistries. Hence, each step of a natural product program has to be more efficient than ever, starting from the collection of environmental samples and the selection of strains, to metabolic expression, genetic exploitation, sample preparation and chemical dereplication. This review will focus on approaches for diversifying microbial natural product strains and extract libraries, while decreasing genetic and chemical redundancy.

A novel class of macrolides for which the name sanglifehrins is proposed, has been discovered from actinomycete strains based on their high affinity binding for cyclophilin A (CypA), an immunophilin originally identified as a cytosolic protein binding cyclosporin A (CsA). The sanglifehrins were produced by Streptomyces sp. A92-3081 10. They were isolated and purified by extraction and several chromatographic, activity-guided steps.

A novel class of macrolides, the sanglifehrins, was discovered by screening of actinomycete strains with a cyclophilin-binding assay. The chemical structures and absolute stereochemistries of the sanglifehrins A, B, C and D were determined unambiguously by NMR-techniques and by X-ray crystallography of the complex with cyclophilin A.Sanglifehrin A consists of a 22-membered macrocycle containing a tripeptide subunit and features in position 23 a chain of nine carbon atoms bearing a spirocyclic substituent. Sanglifehrins A and B are genuine metabolites whereas sanglifehrins C and D are artefacts.Screening for cyclophilin binding metabolites from actinomycete strains led to the discovery of a novel class of compounds, which were named sanglifehrins. The taxonomy, fermentation, isolation and biological activity are described in the foregoing paper1}.In this part we describe the structure elucidation mainly done by NMR-analysis, and give the chemical characteristics of these newmicrobial compounds.Methods Spectroscopy The XHand 13C NMRspectra of the sanglifehrins were recorded in T>MSO-d6 on a Bruker Avance DMX-500spectrometer with TMSas internal standard. The 1H, 13C and 15N NMRshifts are listed in Table 2 and image, at a crystal-to-detector distance of 130mm.Data processing was done with the programs DENZO1.5.ll &Scalepack2), and the CCP43.0-package3). The overall Rsymon intensities for a total of 179130 measurements of 41 552 independent reflections in the resolution range 15A-1.6A (completeness 99.2%) was 7.5%.The structure was solved by molecular replacement and refined with the program X-PLOR, Version 3.14).