Tiacumicins B and C are members of a novel group of 18-membered macrolide antibiotics with in vitro activity against Clostridium difficile. The MICs against 15 strains of C. difficile were 0.12 to 0.25 ,ug/mI for tiacumicin B, 0.25 to 1 ,ug/ml for tiacumicin C, and 0.5 to 1 ,ug/ml for vancomycin. The The tiacumicins are a group of 18-membered macrolide antibiotics originally isolated from the fermentation broth of Dactylosporangium aurantiacum subsp. hamdenensis (7,16). Tiacumicin B, the major antibiotic component produced by this culture, contains an unsaturated 18-membered macrolide ring with a seven-carbon sugar at carbon 11 and a 6-deoxy sugar at carbon 20 (Fig. la). This compound is apparently identical to one of the lipiarmycins, a previously described group of antibiotics produced by Actinoplanes deccanensis (1, 10, 12), and to clostomicin Bi, an antibiotic from Micromonospora echinospora (9). Tiacumicin C differs in the position of butyrate esterification on the seven-carbon sugar (Fig. lb). Tiacumicin C appears to be identical to clostomicin B2, a member of the clostomicin complex isolated from M. echinospora (9).A preliminary report on the biological properties of the tiacumicins indicated that tiacumicin B has moderate activity against pathogenic strains of Staphylococcus aureus, Streptococcus pyogenes, and Enterococcus faecium and demonstrated that it has limited, but potent, activity against several anaerobic bacteria (16). In the same study, tiacumicin C was found to have a similar spectrum of activity but to be less potent than tiacumicin B.The present study was designed to examine the in vitro activities of the tiacumicins against Clostridium difficile and to evaluate the efficacies of these compounds in treating antibiotic-associated pseudomembranous colitis caused by * Corresponding author. In vitro potency. MICs in Wilkins-Chalgren agar were determined by a standard twofold agar dilution procedure (13).Effect of serum and pH on in vitro activity. The effects of 50% hamster serum and at pHs 6.5, 7.3, and 8.0 on the in vitro potencies of the tiacumicins in Wilkins-Chalgren broth were determined by a microdilution method (13). To separate the effect of pH from the effect of serum, the pHs of the serum samples were adjusted to 7.3 prior to testing.Effect of hamster cecal contents on in vitro activity. The bioavailabilities of the tiacumicins in the presence of normal hamster cecal contents were determined by a previously described methodology (13). Briefly, MICs and MBCs of

Altromycin B belongs to the pluramycin family of antitumor antibiotics, which also includes kidamycin, hedamycin, pluramycin, neopluramycin, DC92-B, and rubiflavin A. These potent antitumor compounds react with DNA in as yet imprecisely determined ways. In the present investigation, we have used gel electrophoresis methods in combination with nuclear magnetic resonance and mass spectrometry to determine the structure of the altromycin B-DNA adduct. High-resolution gel electrophoresis demonstrated that guanine was the reactive base, and N7 was implicated from experiments in which N7-deazaguanine was used in place of guanine in a strand breakage assay. Experiments using supercoiled DNA demonstrated that altromycin B and related drugs intercalated into DNA, which implicated this as a common mechanism for binding of the pluramycin antibiotics to DNA. The altromycin B-guanine adduct was isolated from calf thymus DNA after thermal depurination of the alkylated DNA. Mass spectrometry confirmed that altromycin alkylated DNA through guanine, and 1H- and 13C-NMR was used to confirm the covalent linkage sites between altromycin B and guanine. On the basis of these results, we propose that altromycin B first intercalates into DNA via a threading mechanism, reminiscent of nogalamycin, to insert the disaccharide into the minor groove and position the epoxide in the major groove in proximity to N7 of guanine. Nucleophilic attack from N7 of guanine leads to an acid-catalyzed opening of the epoxide, resulting in the altromycin B-DNA adduct. On the basis of these results, a general mechanism for the interaction of the pluramycin family of antibiotics with DNA is proposed.

MICs of clarithromycin and its major human metabolite, 14-hydroxy-clarithromycin, for Haemophilus influenzae in combination were reduced two-to fourfold compared with the MICs of each compound alone. Serum reduced the MICs of the parent compound and metabolite two-to fourfold compared with the MICs in medium without serum. In serum spiked with clinically relevant concentrations of clarithromycin and 14-hydroxy-clarithromycin at a fixed ratio of 4:1, 15 of 16 strains (94%) were inhibited and killed by combinations containing 1.2 and 0.3 ,ug/ml, respectively. In time kill experiments, the combination of parent compound and metabolite at one-fourth and one-half of their individual MICs, respectively, reduced bacterial counts by >5 log CFU. The postantibiotic effect of darithromycin combined with 14-hydroxy-clarithromycin was twice that of clarithromycin when tested alone. When orally administered to gerbils with H. influenzae otitis media, the 14-hydroxy metabolite was significantly more active than clarithromycin in reducing bacterial counts from the middle ear. The in vivo activity of the two compounds in combination was synergistic or additive, depending on the level of H. influenzae present at the time treatment was initiated. Significant reductions in bacterial counts and increases in cure rates were observed when clarithromycin at 50 or 100 mg/kg of body weight was combined with 14-hydroxy-clarithromycin at 12 mg/kg or higher. Results from in vitro and in vivo combinations suggest that routine susceptibility tests and animal efficacy studies with clarithromycin alone may underestimate its potential efficacy against H. influenzae.Clarithromycin is an acid-stable 14-membered macrolide which achieves higher peak levels in human serum after oral dosing than does erythromycin and has a serum half-life which is twice that of erythromycin (L. T. Sennello, S. Chemother., abstr. no. 419, 1986). The MICs of clarithromycin for organisms such as staphylococci, streptococci (including Streptococcus pneumoniae and Streptococcus pyogenes), Listeria monocytogenes, diphtheroids, and Branhamella catarrhalis are twofold lower than those of erythromycin, while the MICs of clarithromycin for Legionella pneumophila are generally fourfold lower (5, 11). The MICs of clarithromycin for Haemophilus influenzae, however, are generally twofold higher than those of erythromycin. After oral dosing in mouse protection tests, clarithromycin is 2 to 10 times more active than erythromycin against S. pneumoniae, S. pyogenes, and Staphylococcus aureus (5). In guinea pigs experimentally infected with L. pneumophila, clarithromycin is significantly more effective than erythromycin in reducing bacterial counts from lungs and spleens. In addition, the in vitro postantibiotic effects of clarithromycin for S. pyogenes and Staphylococcus aureus are three times those of erythromycin and correlate with results from mouse protection tests in which the optimal dosing intervals of clarithromycin were once a day compared with three times a day for erythromy...

The antineoplastic quinobenoxazines A-62176 and A-74932 were shown to be potent inhibitors of mammalian DNA topoisomerase II in vivo. This was demonstrated by their selective inhibition of the SV40 DNA replication stages that require topoisomerase II. Neither drug stabilized a covalent complex of the enzyme with SV40 DNA, which suggests that they are not poisons of DNA topoisomerase II. A-77601, an analog having little antitumor activity, barely inhibited DNA topoisomerase II in vivo, even at high concentrations. These findings were supported by in vitro studies which showed that A-62176 and A-74932, but not A-77601, strongly inhibited the catalytic activity of mammalian DNA topoisomerase II. A-62176 did not cause topoisomerase II-mediated DNA strand breaks in vitro under conditions in which adriamycin produced extensive DNA breakage. The antineoplastic and topoisomerase inhibitory activities of the quinobenoxazines correlate with their ability to unwind DNA. A-62176 antagonized the poisoning of topoisomerase II by VM-26 in vivo and in vitro, but had no effect on DNA breakage induced by camptothecin, a DNA topoisomerase I poison. A-62176 and A-74932 thus inhibit DNA topoisomerase II reactions at a step prior to the formation of the "cleavable complex" intermediate. These findings indicate that stabilization of the DNA topoisomerase II-DNA cleavable complex is not necessary for the antitumor activity of this class of quinolones and that the catalytic inhibition of DNA topoisomerase II may contribute significantly to the anticancer activity of other DNA topoisomerase II inhibitors.

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