Major advances in antibacterial quinolone therapy

Molecular characterization of recombinant Pneumocystis carinii topoisomerase I: differential interactions with human topoisomerase I poisons and pentamidine

The Pneumocystis carinii topoisomerase I-encoding gene has been cloned and sequenced, and the expressed enzyme interactions with several classes of topoisomerase I poisons have been characterized. The P. carinii topoisomerase I protein contains 763 amino acids and has a molecular mass of ca. 90 kDa. The expressed enzyme relaxes supercoiled DNA to completion and has no Mg2+ requirement. Cleavage assays reveal that both the human and P. carinii enzymes form covalent complexes in the presence of camptothecin, Hoechst 33342, and the terbenzimidazole QS-II-48. As with the human enzyme, no cleavage is stimulated in the presence of 4',6'-diamidino-2-phenylindole (DAPI) or berenil. A yeast cytotoxicity assay shows that P. carinii topoisomerase I is also a cytotoxic target for the mixed intercalative plus minor-groove binding drug nogalamycin. In contrast to the human enzyme, P. carinii topoisomerase I is resistant to both nitidine and potent protoberberine human topoisomerase I poisons. The differences in the sensitivities of P. carinii and human topoisomerase I to various topoisomerase I poisons support the use of the fungal enzyme as a molecular target for drug development. Additionally, we have characterized the interaction of pentamidine with P. carinii topoisomerase I. We show, by catalytic inhibition, cleavage, and yeast cytotoxicity assays, that pentamidine does not target topoisomerase I.

Mutation of a conserved serine residue in a quinolone-resistant type II topoisomerase alters the enzyme-DNA and drug interactions

A Ser740 --> Trp mutation in yeast topoisomerase II (top2) and of the equivalent Ser83 in gyrase results in resistance to quinolones and confers hypersensitivity to etoposide (VP-16). We characterized the cleavage complexes induced by the top2(S740W) in the human c-myc gene. In addition to resistance to the fluoroquinolone CP-115,953, top2(S740W) induced novel DNA cleavage sites in the presence of VP-16, azatoxin, amsacrine, and mitoxantrone. Analysis of the VP-16 sites indicated that the changes in the cleavage pattern were reflected by alterations in base preference. C at position -2 and G at position +6 were observed for the top2(S740W) in addition to the previously reported C-1 and G+5 for the wild-type top2. The VP-16-induced top2(S740W) cleavage complexes were also more stable. The most stable sites had strong preference for C-1, whereas the most reversible sites showed no base preference at positions -1 or -2. Different patterns of DNA cleavage were also observed in the absence of drug and in the presence of calcium. These results indicate that the Ser740 --> Trp mutation alters the DNA recognition of top2, enhances its DNA binding, and markedly affects its interactions with inhibitors. Thus, residue 740 of top2 appears critical for both DNA and drug interactions.

Cloning and characterization of the Aspergillus nidulans DNA topoisomerase I gene

The topoisomerase I (TOP1) gene was cloned and sequenced from Aspergillus nidulans using the polymerase chain reaction (PCR). Genomic DNA was used as a template to obtain a 2987-bp gene containing five small introns. PCR from a cDNA library yielded a 2613-bp sequence which codes for an 871 amino acid protein. Comparison of the deduced amino acid sequence with other DNA topoisomerase I (topo I) protein sequences shows a somewhat higher degree of identity with other fungal amino acid sequences than with the human enzyme. Topo I is a ubiquitous enzyme which can be converted to a cytotoxic molecule in the presence of drugs that function as topo I poisons. The Aspergillus TOP1 cDNA will be used in an effort to identify novel cytotoxic antifungals which target this enzyme.

Enoxacin: a reappraisal of its clinical efficacy in the treatment of genitourinary tract infections

Enoxacin is a 6-fluoronaphthyridinone antibacterial agent with good in vitro activity against Neisseria gonorrhoeae and most Gram-negative urinary tract pathogens. It is less active in vitro against Acinetobacter spp., Pseudomonas aeruginosa, and most Gram-positive bacteria, than against Gram-negative organisms. Enoxacin is rapidly absorbed, with a high oral bioavailability (87 to 91%). Of the absorbed dose, 44 to 56% is excreted unchanged in the urine, with peak urinary concentrations (>500 mg/L within 4 hours) remaining high (>100 mg/L) for up to 24 hours, sufficient to inhibit most urinary tract pathogens. Single (400 mg) and multiple oral dose regimens (100 to 600 mg twice or 3 times daily for 5 to 14 days) of enoxacin are as effective for the treatment of patients with complicated or uncomplicated urinary tract infections as other antibacterial agents such as amoxicillin, cefuroxime axetil, cotrimoxazole (trimethoprim-sulfamethoxazole) or trimethoprim. Noncomparative data suggest that enoxacin is also an effective agent for the treatment of prostatitis. Single 400 mgoral doses of enoxacin produce >/- 95% bacteriological cure rates in gonococcal infections, comparable to those produced by single intramuscular doses of ceftriaxone 250 mg. Perioperative doses of oral enoxacin 200 mg provide effective prophylaxis against postoperative bacteriuria after transurethral resection of the prostate. Concomitant administration of enoxacin with a number of commonly used therapeutic agents (e.g. antacids, methylxanthines, warfarin) affects the pharmacokinetic properties of either enoxacin or the coadministered agents. Enoxacin is reasonably well tolerated, with the incidence of adverse experiences ranging from 0 to 24%. Adverse events are mainly gastrointestinal, neurological or dermatological and resolve with minimal intervention. Overall, although enoxacin exhibits a number of clinical characteristics that are similar to those of other agents for the treatment of genitourinary tract infections, the advantages offered by this agent generally do not outweigh those of alternative fluoroquinolone agents. Thus, it is likely to prove to be yet another addition to the list of agents available for the management of these infections.

A mutation in yeast TOP2 homologous to a quinolone-resistant mutation in bacteria. Mutation of the amino acid homologous to Ser83 of Escherichia coli gyrA alters sensitivity to eukaryotic topoisomerase inhibitors

In prokaryotic type II topoisomerases (DNA gyrases), mutations that result in resistance to quinolones frequently occur at Ser83 or Ser84 of the gyrA subunit. Mutations to Trp, Ala, and Leu have been identified, all of which confer high levels of quinolone resistance. Extensive segments of DNA gyrase are homologous to eukaryotic topoisomerase II, and Ser741 of yeast TOP2 is homologous to Ser83 of prokaryotic DNA gyrA. Introduction of the Ser741-->Trp mutation into yeast TOP2 confers resistance to 6,8-difluoro-7-(4'-hydroxyphenyl)-1-cyclopropyl- 4-quinolone-3-carboxylic acid (CP-115,953), a fluoroquinolone with substantial activity against eukaryotic topoisomerase II, whereas changing Ser741 to either Leu or Ala does not change sensitivity to quinolones. Interestingly, Ser741-->Trp in the yeast TOP2 also confers hypersensitivity to etoposide. Sensitivity to intercalating anti-topoisomerase II agents such as amsacrine is not changed by any of the three mutations. The topoisomerase II protein carrying the Ser741-->Trp mutation was overexpressed and purified. The purified mutant enzyme had enhanced levels of etoposide stabilized covalent complex as compared with the wild type enzyme and reduced cleavage with CP-115,953. Unlike the wild type enzyme, etoposide-stabilized cleavage is not readily reversible by heat. We suggest that Ser741 is near a binding site for both quinolones and etoposide and that the Ser741-->Trp mutation leads to a more stable ternary complex between etoposide, DNA, and the mutant enzyme.