The Epothilones: An Outstanding Family of Anti-Tumor Agents

2. General Aspects (p. 5-6)

Gerhard Höfle

Helmholtz-Zentrum für Infektionsforschung (formerly: GBF, Gesellschaft für Biotechnologische Forschung), Braunschweig, Germany

2.1. History of Epothilone Discovery and Development

2.1.1. The Early Days

Epothilone is a microbial product, and thus its history may be traced back to the discovery of the respective microbe, Sorangium cellulosum, a bacterium belonging to the taxonomic group of myxobacteria, which originally has been described by Roland Thaxter in 1892 (1). Today this group of organisms comprises around 40 species, one of which is Sorangium cellulosum. For a long time, myxobacteria were only known for their gliding motility and sophisticated life cycle, although it had been occasionally speculated that they might produce secondarymetabolites like actinomycetes or bacilli (2).

In 1975 Hans Reichenbach and his group at the German Centre for Biotechnology (GBF, now called the Helmholtz Centre for Infection Research) set out to isolate strains of myxobacteria from soil samples collected all over the world, and to examine their secondary metabolism. In 1978, while work was already ongoing, I joined them and took over the chemistry part. In the same year the first structure of a myxobacterial metabolite, ambruticin, was published by a group from Warner-Lambert (3) making us very confident of being on the right track. Ambruticin had been isolated from a Sorangium cellulosum strain, and was identified as a unique cyclopropane polyketide structure exhibiting potentially useful antifungal properties. Ambruticin and its derivatives had been developed for medical application for some time, and recently gained new interest (4). Meanwhile we had been working at GBF quite successfully with the easily handled Myxococcus, Corallococcus, and Stigmatella strains, and only slowly shifted our focus to Sorangium. It took us considerable time to establish large-scale isolation and cultivation procedures of this slowly growing species. As soon as several hundred strains of Sorangium cellulosum had been accumulated by 1985, the screening for biological activity became productive, and a constant flow of unusual secondary metabolites came into our hands. Up to now, approximately 50 novel basic structures have been isolated from the various strains of this species, often with outstanding antifungal properties. The predominance of antifungal activity may be attributed to the fact that Sorangium cellulosum grows on cellulose as carbon source, and thus has to compete through chemical warfare (and other means) with fungi for its ecological niche.

In July 1985 Sorangium cellulosum, strain So ce90, the first producer of epothilone, was isolated by Hans Reichenbach from a soil sample collected at the banks of the river Zambesi in southern Africa in August 1980. Only two years after isolation, the strain was introduced in an antifungal screening of Sorangium strains by Klaus Gerth and identified as one of several hits in January 1987. Later, Florenz Sasse, responsible for cell culture tests, noticed high cytotoxicity of the culture extract. From these and other preliminary tests we were dealing with a new compound and Norbert Bedorf from the chemistry group immediately started to isolate the compound and elucidate its structure. Guided by biological activity, he isolated two closely related antifungal compounds later named epothilone A and B (5), and a structurally non-related family of polyene carboxylic acids, later named spirangiens inMay 1987 (5, 6).