Resorthiomycin

We have been studying the resistance mechanisms of various antitumor drugs and screening substances from natural and synthetic products which overcome resistance. In this paper, we described compounds mainly obtained from our screening systems. I. Circumvention of multidrug resistance (MDR). Lactoquinomycin was discovered from the culture broth of a strain of Streptomyces sp. and it showed preferential growth inhibition against multidrug-resistant L5178 Y cells. The mechanism of action of lactoquinomycin was studied. Another novel antibiotic, resorthiomycin, exhibited not only preferential inhibition against MDR tumor cells but also augmentation of cytotoxicity of several antitumor drugs. As synthetic potentiators, dipyridamole, cepharanthine, AHC-52 and analogs of dihydropyridines were described; all of them were thought to interact with a P-glycoprotein and inhibit active efflux of drugs from tumor cells. SDB-ethylenediamine was unique because it overcame MDR and also potentiates a wide range of antitumor drugs including 5-FU and bleomycin. II. E-64 was found to inhibit the activity of a bleomycin-inactivating enzyme. It potentiated the activity of peplomycin in vitro and in vivo. III. Cadeguomycin was discovered from the culture filtrate of a Streptomyces sp. and it potentiated Ara-C by inhibition of the activity of dCMP deaminase, an Ara-C-inactivating enzyme.

Resorthiomycin suppressed the clonogenic activity of a multidrug-resistant mutant cell line of Chinese hamster V79 cells more potently than its parental cells. Moreover, resorthiomycin at 40 micrograms/ml potentiated the cytotoxic activity of vincristine and actinomycin D on V79 cells over 3-fold. Uptake of [3H]actinomycin D into V79 cells was stimulated 2-fold by 40 micrograms/ml of resorthiomycin during 2 hours incubation. On the other hand, incorporation of [3H]thymidine and [3H]uridine into mouse leukemia L5178Y cells was inhibited in a dose-dependent manner at resorthiomycin concentrations ranging from 5 to 40 micrograms/ml. In ATP-depleted L5178Y cells, membrane transport of [3H]thymidine and 2-[3H]deoxyglucose was strongly suppressed by resorthiomycin. These results suggest that resorthiomycin acts on the plasma membrane and perturbes some membrane function.

Resorthiomycin was revealed to be a new antibiotic with a molecular weight of 284 and a chemical formula of C14H20O4S as determined by MS and elemental analysis. The structure of resorthiomycin was determined to be 6-acetyl-4-(3-hydroxybutyl)-2-methyl-5-methylthioresorcinol by IR spectrum and 1H and 13C NMR.