Isolation and structural elucidation of stipiamide, a new antibiotic effective to multidrug-resistant cancer cells.

Sir: Resistant of tumor cells to multiple cytotoxic drugs is a major impediment to cancer chemotherapy. Multidrug-resistance (MDR) is characterized by decreased cellular sensitivity to anticancer agents due to the increased expression of a membrane glycoprotein with a MWof approximately 170 kd, termed P-glycoprotein1}. It is well known that colchicine-resistance cells have simultaneously acquired resistance to doxorubicin and vinblastine2). Thus, it could be expected that compoundsactive against colchicine-resistance cells will be useful for overcoming MDR to antitumor substances. By employing colchicine-resistance KB(CHR)3) cells, we undertook the screening for agents which were more active to KB(CHR) cells in the presence of colchicine*. As a result, a gliding bacterium isolated from a soil sample from Izu Peninsula, Japan was found to produce a newantibiotic named stipiamide. The producing organism was identified as Myxococcus stipitatus (AJ-12587) by taxonomic comparison with type strains of Myxococcus sp. as well as by its morphological, cultural and physiological characteristics. In this paper, we will report the fermentation, isolation and structure elucidation of stipiamide. The organism was cultivated in 5-liter Sakaguchi flasks containing 1 liter of the Casitone liquid medium consisting of Casitone (Difco) 2% and MgSO4 0.2% (pH 7.2). The flasks were incubated at 28°C for 4 days on a reciprocal shaker at 1 15 rpm. In this medium,the organism grewas a homogeneous cell suspension. The cells harvested by centrifugation from the whole broth (5 liters) were stirred with 500ml of acetone. The solvent extract was concentrated in vacuo to a small volume and the active material was extracted from the aqueous