Saframycin C

The quinone antitumor antibiotics saframycins A and C bind to duplex DNA and G,C containing complementary oligodeoxyribonucleotides at pHs below 5.5 via selective protonation of the antibiotics at the N-12 positions in an equilibrium process. This binding shows minor groove specificity to T4 DNA for both antibiotics. Saframycin A exhibits an additional mode of covalent binding to DNA, which is reversible by heating, via an animal linkage with the NH2 group of guanine. Covalent binding of saframycin A is enhanced by prior reduction of the quinone moiety via a different mechanism with concomitant elimination of the nitrile group and plausibly involves an intermediate quinone methide. Both saframycin A and saframycin C when reduced in situ with NADPH or sodium borohydride cause single-strand breaks, but not double-strand breaks, in PM2 covalently closer circular DNA. This latter reaction proceeds via intermediate semiquinones, the identity and conformation of which are established by EPR. The DNA scission process which requires oxygen is mediated by the generation of O2-., H2O2 and OH., the latter of which species was identified by spin trapping. The strand scission of DNA is pH dependent and, unlike the binding of the antibiotics, is strongly inhibited by Mg2+ and Zn2+ ions.

The antitumor activity of saframycin was examined against four different experimental tumor systems in mice. Saframycin A and C inhibited the growth of L1210 cells in suspension culture completely at concentrations of 0.02 microgram/ml and 1.0 microgram/ml, respectively. The LD50's of saframycin A for ddY mice were 4.9 mg/kg (ip) and 3.3 mg/kg (iv), respectively. In C3H/He mice, the LD50's were 10.5 mg/kg (ip) and 9.7 mg/kg (iv), respectively. Saframycin A was highly active against Ehrlich ascites carcinoma and P388 leukemia, and moderately active against L1210 leukemia and B16 melanoma. The antitumor activity of saframycin A was 50 to 100 times greater than that of saframycin C. The survivors cured of Ehrlich ascites carcinoma by treatment with saframycin A developed a resistance to rechallenge with the same tumor. On the other hand, when carbazilquinone and adriamycin were used as reference drugs, the cured mice in these cases did not resist rechallenge with the same tumor. When saframycin A (5 mg/kg) was administered intraperitoneally into mice, the blood concentration of saframycin A was 4.6 microgram/ml after 30 min, and 2.8 microgram/ml after 1 hr, and the total recovery within 3 hr from the urine was 30%. Saframycin A was found to be distributed widely, though to different extents, in various organs when injected intraperitoneally into mice.

After a long and energetic world-wide search for antibiotics from actinomycetes, the chances of finding new classes of biologically active compounds from these organisms seem to have drastically diminished. In this paper, sophisticated procedures for finding new potentialities of antibiotic-producing actinomycetes and isolating a novel class of antitumor antibiotics, the saframycins, are described. These antibiotics are satellite antibiotics which are co-produced in minute quantities with streptothricin by a strain of Streptomyces lavendulae. Saframycins, cyano-containing saframycin A in particular, are promising antitumor antibiotics because of their low toxicity to immunologically competent cells and organs, especially the bone marrow. Significant amplification of the yield of saframycin A was attained by the quite unexpected observation that the addition of cyanide to the culture broth significantly increased its production. Subsequent isolation of the saframycin A precursor, decyanosaframycin A or saframycin S, enabled us to prepare two kinds of labeled saframycin A which were critical for the elucidation of the unique molecular action mechanism of the antibiotic. On the basis of studies on saframycin biosynthesis, a method of preparing new saframycin derivatives using resting cells of the producing strain has been developed, and new saframycins with amino functional groups on their N-pyruvoyl side chain were obtained. Their HCl salts are readily soluble in water and are expected to exhibit pharmacodynamic properties different from those of other saframycins.