Helmut Kropp

THE MECHANISM OF ACTION OF FOSFOMYCIN (PHOSPHONOMYCIN)

The discovery of fosfomycin, a new antibiotic produced by strains of Streptomyces, was announced under its former name phosphonomycin by Hendlin and colleagues in 1969.l The chemical structure shown in FIGURE 1 combines two unusual features: an epoxide ring, rare among antibiotics, and a carbon-phosphorus bond which is seen to occur here for the first time among the natural products of the bacteria. Most of the present account concerns the determination of the enzymatic step in cell wall biosynthesis that is ultimately blocked by fosfomycin. We compare in detail the action of that enzyme’s catalytic center upon the antibiotic and its normal substrate. We also describe the role of two stereospecific nutrient transport systems that by mediating the entry and accumulation of fosfomycin comprise the determining factors in the sensitivity of various bacteria to this polar antibiotic. Although the main conclusions of these mechanism of action studies were briefly stated by us in the initial announcement,’, the present publication is the first in which there appears any portion of the original experimental data that support those conclusions. Several reports have appeared from other laboratories 4, and from our own IJ in which certain basic findings and methodology were reproduced in the course of pursuing the independent goals of their studies. Their additional contributions to the understanding of fosfomycin action are cited at appropriate points in the succeeding text.

Metabolism of Thienamycin and Related Carbapenem Antibiotics by the Renal Dipeptidase, Dehydropeptidase-I

Thienamycin (THM), the N-formimidoyl thienamycin derivative MK0787, and related carbapenem antibiotics were metabolized extensively in mice, rats, rabbits, dogs, rhesus monkeys, and chimpanzees. Urinary recovery of THM ranged from a low of 5% in dogs to 58% in rhesus monkeys. Renal clearance rates in dogs and chimpanzees were unusually low, less than glomerular filtration rates. The reduction in clearance of THM and MK0787 from plasma of rats and rabbits after ligation of renal arteries indicate that the kidneys are responsible for 35 and 92%, respectively, of metabolic drug clearance. Degradation was detected only in kidney homogenates. The enzyme activity was membrane bound and sensitive to inhibitors of Zn-metalloenzymes such as EDTA. A renal dipeptidase, dehydropeptidase-I (DHP-I), EC 3.4.13.11, was found to be responsible for the metabolism of the THM-class antibiotics, which exhibit a structural homology to dehydropeptides. A parallel increase in specific activity against THM and the substrate of DHP-I, glycyldehydrophenylalanine, was observed during solubilization and purification of the enzyme from porcine and human renal cortex. DHP-I was found to catalyze the hydrolysis of the beta-lactam ring in THM and MK0787. The products of the enzyme reaction were identical by high-powered liquid chromatography to their respective metabolites found in the urine. Nonbasic N-acylated THM and natural N-acylated carbapenems (epithienamycins and olivanic acids) were degraded 4- to 50-fold faster than THM when exposed to the enzymatic hydrolysis of DHP-I. Good correlations were obtained between the increased susceptibility of the carbapenem antibiotics to DHP-I as measured in the in vitro enzyme assay and the generally lower recoveries of active antibiotic in the urine of test animals. Despite this unusual degree of metabolism localized in the kidney, the plasma half-life of MK0787 and its efficacy against experimental systemic infections in animals remain satisfactory.

Carbapenems, a new class of beta-lactam antibiotics: Discovery and development of imipenem/cilastatin

The discovery of Streptomyces cattleya and its antibiotic product, thienamycin, has ushered in a new era of beta-lactam agents, the carbapenems. Numerous carbapenems were subsequently discovered; however, none had the potency, broad-spectrum activity, and lack of cross-resistance exhibited by thienamycin. Chemical instability encountered with thienamycin was overcome by the N-formimidoyl derivative, imipenem. Imipenem is distinguished from other beta-lactams by its outstanding activity against gram-positive organisms as well as against Enterobacteriaceae, Pseudomonas aeruginosa, and Bacteroides. However, development was hindered by extensive renal metabolism of imipenem, resulting in low urinary concentrations of antibiotic. A renal dipeptidase, dehydropeptidase-I, was responsible for hydrolyzing imipenem and other carbapenems. To counter its action, a specific inhibitor, cilastatin, was developed. Coadministered with imipenem in a one-to-one ratio, cilastatin provides prolonged, reversible blockade of imipenem metabolism, dramatically improving urinary recoveries to therapeutically significant levels. Cilastatin also contributes to the safety of imipenem, since its coadministration prevents proximal tubular necrosis which has been observed in sensitive animals receiving imipenem alone in high doses. Thus, the combination imipenem and cilastatin overcame the pharmaceutical and metabolic challenges presented by thienamycin, and allowed for the evaluation in humans of the outstanding antimicrobial activity of this new class of beta-lactam antibiotics.

MK0787 (N-formimidoyl thienamycin): evaluation of in vitro and in vivo activities.

The practical application of thienamycin, a novel beta-lactam antibiotic with a broad activity spectrum, was compromised by problems of instability. MK0787, N-formimidoyl thienamycin, does not have this liability. As reported, bacterial species resistant to most beta-lactam antibiotics, such as Pseudomonas aeurginosa, Serratis, Enterobacter, Enterococcus, and Bacteroides spp., are uniformly susceptible to MK0787, usually at one-half the inhibitory level of thienamycin. Bactericidal activity usually occurs at the minimal inhibitory concentration endpoint. Activity was reduced only at the highest inoculum densities tested and by a lessor factor than was observed with reference beta-lactam antibiotic active against P. aeruginosa and beta-lactamase-bearing strains. MK0787 exhibits a broad spectrum of in vivo activity when evaluated parenterally for efficacy against systemic infections in mice. The order of potency in vivo, 0.03 to 0.06 mg/kg for gram-positive species and 0.65 to 3.8 mg/kg for gram-negative infections including Pseudomonas, exceeded that of thienamycin and was at least 10-fold superior to reference beta-lactam antibiotics including two recently developed agents with antipseudomonal activity, cefotaxime and LY127935.

Evaluation of water-soluble pneumocandin analogs L-733560, L-705589, and L-731373 with mouse models of disseminated aspergillosis, candidiasis, and cryptococcosis.

The activities of the water-soluble pneumocandin derivatives L-733560, L-705589, and L-731373 were evaluated in mouse models of disseminated aspergillosis, candidiasis, and cryptococcosis and were compared with those of commercially available antifungal agents. Pneumocandins are inhibitors of 1,3-beta-D-glucan synthesis. In the aspergillosis model, L-733560 and L-705589 significantly prolonged the survival of DBA/2N mice challenged intravenously with Aspergillus fumigatus conidia. L-733560 and L-705589 exhibited efficacies comparable to that of amphotericin B (AMB) with 90% effective doses of 0.48, 0.12, and 0.36 mg/kg of body weight, respectively. Two mouse models of disseminated candidiasis were used to evaluate these compounds. In both models, mice were challenged intravenously with Candida albicans. In a C. albicans survival model with DBA/2N and CD-1 mice, the efficacy of L-733560 was comparable to that of AMB, while L-731373 and L-705589 were somewhat less active. In a previously described C. albicans target organ kidney assay, the pneumocandin analogs and AMB at doses of > or = 0.09 mg/kg were effective in sterilizing kidneys, while fluconazole and ketoconazole were considerably less active and did not sterilize kidneys when they were used at concentrations of < or = 100 mg/kg. Although orally administered L-733560 showed activity in both candidiasis models, its efficacy was reduced compared with that of parenterally administered drug. In a disseminated cryptococcosis mouse model that measures the number of CFU of Cryptococcus neoformans per gram of brain and spleen, L-733560 at 10 mg/kg was ineffective in reducing the counts in organs, while AMB at 0.31 mg/kg sterilized the organs. These results indicate that the pneumocandins may be beneficial as potent parenterally administered therapeutic agents for disseminated aspergillosis and candidiasis.

In vitro evaluation of the pneumocandin antifungal agent L-733560, a new water-soluble hybrid of L-705589 and L-731373.

Lipopeptide L-733560 is a hybrid analog of L-731373 and L-705589. All are water-soluble semisynthetic pneumocandin Bo derivatives. In vitro susceptibility testing of L-705589, L-731373, and L-733560 against more than 200 clinical isolates consisting of eight Candida species, Cryptococcus neoformans, and three Aspergillus species was performed by the broth microdilution methods. All three pneumocandins exhibited potent anti-Candida activity and moderate anti-C. neoformans activity. However, anti-Aspergillus activity was demonstrated only by an agar disk diffusion method. Antifungal agent-resistant Candida species and C. neoformans showed susceptibility comparable to that of susceptible isolates. Growth inhibition kinetic studies against Candida albicans revealed fungicidal activity within 3 to 5 h. Drug combination studies with pneumocandins and amphotericin B revealed indifferent activity against C. albicans and additive effects against C. neoformans and Aspergillus fumigatus. The activities of the compounds were not dramatically affected by the presence of serum. Resistance induction studies showed that the susceptibility of C. albicans MY1055 was not significantly altered by repeated exposure to subinhibitory concentrations of L-733560. Erythrocyte hemolysis studies indicated minimal hemolytic potential with pneumocandins. Results from preclinical evaluations and development studies performed thus far indicate that the pneumocandins should be safe, broad-spectrum fungicidal agents and potent parenteral antifungal agents.

Differences in mode of action of (β-lactam antibiotics influence morphology, LPS release and in vivo antibiotic efficacy

Antibiotic mediated release of endotoxin (lipopolysaccharide, LPS) from Gram-negative bacteria is implicated in septic shock. The β-lactam (cell wall active) class, in particular, has been deemed responsible for release of greater quantities of LPS than other classes of antibiotics. However, it is becoming increasingly recognized that variations in the ability to liberate free LPS exist within the β-lactam subclasses. Until recently, LPS-release studies have primarily included the cephalosporin, monobactam, and penicillin β-lactams, but not the carbapenem subclass. We document here that carbapenems significantly liberate less LPS than other β-lactam subclasses, and that disparity in LPS release also occurs within the carbapenems as well (i.e. imipenem vs meropenem). The propensity to release LPS correlates with the cumulative PBP binding affinity of each β-lactam antibiotic regardless of subclass. A direct correlation has been established between antibiotic concentration, frequency of antibiotic exposure, differential LPS release, PBP specificity (morphology, rate of bacterial lysis or cell mass) and antibiotic efficacy in two mouse models of infection. In the first, differences in the pathogenic (endotoxic) potential of potently biologically active smooth LPS (S-LPS) and poorly biologically active rough chemotype LPS (R-LPS) expressing bacteria underscores the in vivo importance of endotoxin in mediating lethality following antibiotic chemotherapy. Endotoxin is, however, not innately toxic and thus its lethal effects are mediated through LPS stimulation of host cells (i.e. macrophages) to produce deleterious levels of cytokines. In the second experimental model, therefore, the toxic effects of antibiotic-liberated endotoxin have been abrogated by pretreatment of mice with carrageenan (CGN) prior to challenge with endotoxin or Gram-negative bacteria. CGN treatment eliminates or markedly reduces the numbers of LPS-responsive mediator cells thus affecting mortality. In both animal models, an in vivo role for LPS is demonstrated through differential changes in antibiotic efficacy (ED 50) in response to differential effects of LPS via modulation of the endotoxic sensitivity/responsiveness of the host. We conclude that antibiotic class, concentration, dosing intervals (timing) and perhaps, method of treatment (i.e. bolus vs infusion) may each be important in the survival of experimental animals severely infected with Gram-negative bacteria.

Evaluation of Experimental Therapeutics in a New Mouse Model of Helicobacter felis Utilizing 16S rRNA Polymerase Chain Reaction for Detection

BACKGROUND A new mouse model of Helicobacter felis infection, which mimics the human infection observed with H. pylori, has recently been developed utilizing polymerase chain reaction (PCR) based on the 16S rRNA gene sequence for detection of infection. METHODS We tested several therapeutic regimens in this model, including some currently utilized in the clinic and some shown ineffective in the clinic. RESULTS The therapeutic results obtained by PCR with this model are consistent with results observed in the published human H. pylori clinical trials and also with results obtained in another H. felis mouse model utilizing culture and histology. CONCLUSIONS These results support further use of this new model in screening for new therapeutic regimens for the management of Helicobacter disease.

The discovery and synthesis of 2-biphenylcarbapenems active against methicillin resistant staphylococci

Abstract The discovery and synthesis of the arylcarbapenem 2b possessing potent activity against highly resistant strains of methicillin resistant staphylococci are dislosed.

Pharmacokinetics of L-671,329 in rhesus monkeys and DBA/2 mice.

The time course of plasma drug levels and urinary recovery for two lipopeptide antifungal antibiotics, L-671,329 and cilofungin, were measured in male rhesus monkeys (Macaca mulatta) and in female DBA/2 mice. The antibiotics were administered intravenously at 10 mg/kg of body weight in phosphate-buffered saline-26% polyethylene glycol for the rhesus monkeys and in 5% dimethyl sulfoxide for the mice. Plasma and urine drug concentrations were determined by high-pressure liquid chromatography and/or a microbiological assay versus Aspergillus niger, and pharmacokinetic parameters were determined for both species. In each of the two rhesus crossover tests as well as in the mouse studies, the pharmacokinetics of the two compounds were similar; however, a marked difference was evident between species. The half-lives of L-671,329 and cilofungin in plasma were 39 and 34 min in the mice and averaged 1.8 and 2 h in the rhesus monkeys, respectively. In mice and rhesus monkeys, urinary recovery was less than 4% for both compounds.