Cefoxitin, N-formimidoyl thienamycin, clavulanic acid, and penicillanic acid sulfone as suicide inhibitors for different types of beta-lactamases produced by gram-negative bacteria

Imipenem-Cilastatin-Relebactam: A Novel β-Lactam-β-Lactamase Inhibitor Combination for the Treatment of Multidrug-Resistant Gram-Negative Infections

Imipenem-cilastatin-relebactam (IMI-REL) is a novel β-lactam-β-lactamase inhibitor combination recently approved for the treatment of complicated urinary tract infections (cUTIs) and complicated intraabdominal infections (cIAIs). Relebactam is a β-lactamase inhibitor with the ability to inhibit a broad spectrum of β-lactamases such as class A and class C β-lactamases, including carbapenemases. The addition of relebactam to imipenem restores imipenem activity against several imipenem-resistant bacteria, including Enterobacteriaceae and Pseudomonas aeruginosa. Clinical data demonstrate that IMI-REL is well tolerated and effective in the treatment of cUTIs and cIAIs due to imipenem-resistant bacteria. In a phase III trial comparing IMI-REL with imipenem plus colistin, favorable clinical response was achieved in 71% and 70% of patients, respectively. Available clinical and pharmacokinetic data support the approved dosage of a 30-minute infusion of imipenem 500 mg-cilastatin 500 mg-relebactam 250 mg every 6 hours, along with dosage adjustments based on renal function. In this review, we describe the chemistry, mechanism of action, spectrum of activity, pharmacokinetics and pharmacodynamics, and clinical efficacy, and safety and tolerability of this new agent. The approval of IMI-REL represents another important step in the ongoing fight against multidrug-resistant gram-negative pathogens.

Kinetics of Sulbactam Hydrolysis by β-Lactamases, and Kinetics of β-Lactamase Inhibition by Sulbactam

Sulbactam is one of four β-lactamase inhibitors in current clinical use to counteract drug resistance caused by degradation of β-lactam antibiotics by these bacterial enzymes. As a β-lactam itself, sulbactam is susceptible to degradation by β-lactamases. I investigated the Michaelis-Menten kinetics of sulbactam hydrolysis by 14 β-lactamases, representing clinically widespread groups within all four Ambler classes, i.e., CTX-M-15, KPC-2, SHV-5, and TEM-1 for class A; IMP-1, NDM-1, and VIM-1 for class B; Acinetobacter baumannii ADC-7, Pseudomonas aeruginosa AmpC, and Enterobacter cloacae P99 for class C; and OXA-10, OXA-23, OXA-24, and OXA-48 for class D. All of the β-lactamases were able to hydrolyze sulbactam, although they varied widely in their kinetic constants for the reaction, even within each class. I also investigated the inactivation kinetics of the inhibition of these enzymes by sulbactam. The class A β-lactamases varied widely in their susceptibility to inhibition, the class C and D enzymes were very weakly inhibited, and the class B enzymes were essentially or completely unaffected. In addition, we measured the sulbactam turnover number, the sulbactam/enzyme molar ratio required for complete inhibition of each enzyme. Class C enzymes had the lowest turnover numbers, class A enzymes varied widely, and class D enzymes had very high turnover numbers. These results are valuable for understanding which β-lactamases ought to be well inhibited by sulbactam. Moreover, since sulbactam has intrinsic antibacterial activity against Acinetobacter species pathogens, these results contribute to understanding β-lactamase-mediated sulbactam resistance in Acinetobacter, especially due to the action of the widespread class D enzymes.

Fluorescent TEM-1 β-lactamase with wild-type activity as a rapid drug sensor for in vitro drug screening

We report the development of a novel fluorescent drug sensor from the bacterial drug target TEM-1 β-lactamase through the combined strategy of Val²¹⁶→Cys²¹⁶ mutation and fluorophore labelling for in vitro drug screening. The Val²¹⁶ residue in TEM-1 is replaced with a cysteine residue, and the environment-sensitive fluorophore fluorescein-5-maleimide is specifically attached to the Cys²¹⁶ residue in the V216C mutant for sensing drug binding at the active site. The labelled V216C mutant has wild-type catalytic activity and gives stronger fluorescence when β-lactam antibiotics bind to the active site. The labelled V216C mutant can differentiate between potent and impotent β-lactam antibiotics and can distinguish active-site binders from non-binders (including aggregates formed by small molecules in aqueous solution) by giving characteristic time-course fluorescence profiles. Mass spectrometric, molecular modelling and trypsin digestion results indicate that drug binding at the active site is likely to cause the fluorescein label to stay away from the active site and experience weaker fluorescence quenching by the residues around the active site, thus making the labelled V216C mutant to give stronger fluorescence in the drug-bound state. Given the ancestor's role of TEM-1 in the TEM family, the fluorescent TEM-1 drug sensor represents a good model to demonstrate the general combined strategy of Val²¹⁶→Cys²¹⁶ mutation and fluorophore labelling for fabricating tailor-made fluorescent drug sensors from other clinically significant TEM-type β-lactamase variants for in vitro drug screening.

Recent developments in carbapenems

Carbapenems are beta-lactam antibiotics characterised by the presence of a beta-lactam ring with a carbon instead of sulfone in the 4-position of the thyazolidinic moiety. The first carbapenem to be utilised in therapy was imipenem, the N-formimidoyl derivative of thienamycin. Imipenem is coadministered with cilastatin, an inhibitor of human renal dehydropeptidase I, as imipenem is hydrolysed by this enzyme. Meropenem was the first carbapenem with a 1-beta-methyl group and 2-thio pyrrolidinyl moiety, which renders this antibiotic stable to renal dehydropeptidase I. Other carbapenems for parenteral administration later discovered include biapenem, panipenem, ertapenem, lenapenem, E-1010, S-4661 and BMS-181139. Carbapenems which are orally administered include sanfetrinem, DZ-2640, CS-834 and GV-129606. Carbapenems have an ultra-broad spectrum of antibacterial activity and stability to almost all clinically relevant beta-lactamases. This differentiates them from all other currently available classes of beta-lactam antibiotics. However, Class B beta-lactamases, along with some rare Class A and D enzymes, are able to hydrolyse these antibiotics. Although Class B enzymes are generally chromosomally-encoded (isolated from Stenotrophomonas maltophilia, Aeromonas spp., Bacillus cereus, Bacteroides fragilis, Flavobacterium spp. and Legionella gormanii), plasmid-metallo-beta-lactamases now are appearing in B. fragilis, Pseudomonas aeruginosa, Acinetobacter baumannii and members of Enterobacteriaceae such as Serratia marcescens and Klebsiella pneumoniae. The number of these enzymes compared to the number of other beta-lactamase types is still low, however, it is likely that they will spread due to the increased selective pressure of carbapenem use. The very broad spectrum of antimicrobial activity associated with a good clinical efficacy and a favourable safety profile makes the carbapenems valuable as 'first-line' antibiotics in initial empirical therapy for the treatment of severe infections.

Mechanism of beta-lactamase inhibition: differences between sulbactam and other inhibitors

Three different types of beta-lactamases--TEM-2 type penicillinase, a typical cephalosporinase of Citrobacter freundii, and a Proteus vulgaris cephalosporinase with broad substrate range--were studied to determine the inactivation and reactivation kinetics for beta-lactamase inhibitors of these enzymes. Sulbactam, cloxacillin sulfone, clavulanic acid, imipenem, and aztreonam were evaluated. On the basis of the kinetic parameters a minimum scheme for the inactivation of these beta-lactamases by each compound was proposed, and the difference in the features of each of these as progressive and competitive inhibitors were evaluated. The relationship between the kinetic parameters and the synergistic effects of the inhibitors in combination with traditional beta-lactam antibiotics on the bacterial strains producing these beta-lactamases was examined. A close relationship between the synergistic effect, expressed as the FIC index, and a proposed parameter, TN x Ki/Km, was demonstrated. The results of this analysis suggest that sulbactam is a beta-lactamase inhibitor applicable to a wide range of beta-lactamase types.

Chromosomal beta-lactamase expression and resistance to beta-lactam antibiotics in Proteus vulgaris and Morganella morganii

Indole-positive members of the Proteeae usually have inducible expression of chromosomal beta-lactamases. Mutants with stably derepressed beta-lactamase expression occur in inducible populations at frequencies in the range of 10(-6) to 10(-8). The contribution of these beta-lactamases to drug resistance was examined in Morganella morganii and Proteus vulgaris. The M. morganii enzyme was a high-molecular-weight (49,000) class I cephalosporinase with low Vmax rates for ampicillin, carbenicillin, and and broad-spectrum cephalosporins. The P. vulgaris enzyme had a lower molecular weight (32,000) and high Vmax rates for ampicillin, cephaloridine, cefotaxime, and ceftriaxone. Imipenem and cefoxitin inactivated the P. vulgaris enzyme but were low-Vmax, low-Km substrates for that of M. morganii. Despite these differences, the two beta-lactamases caused similar resistance profiles. Ampicillin and cephaloridine were strong inducers for both species, and beta-lactamase-inducible strains and their stably derepressed mutants were resistant, whereas basal mutants (those with low-level uninducible beta-lactamase) were susceptible to these two compounds. Mezlocillin, cefotaxime, ceftriaxone, and (usually) carbenicillin were almost equally active against beta-lactamase-inducible organisms and their basal mutants, but were less active against stably derepressed mutants. This behavior reflected the beta-lactamase lability of these drugs, coupled with their weak inducer activity below the MIC. Carbenicillin was a labile strong inducer for a single P. vulgaris strain, and inducible enzyme was protective against the drug in this atypical organism. Cefoxitin and imipenem, both strong inducers below the MIC, were almost equally active against beta-lactamase-inducible organisms and their basal and stably derepressed mutants.

Sulbactam/ampicillin. A review of its antibacterial activity, pharmacokinetic properties, and therapeutic use

Sulbactam is a semisynthetic beta-lactamase inhibitor which when combined with certain beta-lactam antibacterials extends their activity against bacteria that are normally resistant to the antibiotic due to production of beta-lactamases. In combination with ampicillin it extends the antibacterial activity of ampicillin to include beta-lactamase-producing strains which are otherwise resistant, including Bacteroides fragilis, and increases the susceptibility of many sensitive strains. Sulbactam is poorly absorbed after oral administration and sulbactam/ampicillin is therefore administered parenterally, although another linked sulbactam-ampicillin compound, sultamicillin, has been developed which is well absorbed after oral administration. The basic pharmacokinetic characteristics of sulbactam after parenteral administration are similar to those of ampicillin. Multiple-dose therapy with sulbactam/ampicillin is clinically and bacteriologically effective in infections of the urinary tract, skin and soft tissue, bones and joints, respiratory tract, ears, nose and throat, as well as intra-abdominal and obstetric and gynaecological infections and septicaemia. In addition, single intramuscular doses of sulbactam/ampicillin administered with oral probenecid are therapeutically effective in gonorrhoea, including infections due to penicillinase-producing and/or ampicillin-resistant Neisseria gonorrhoeae. In the prophylaxis of infectious complications of surgery sulbactam/ampicillin is superior to placebo and appears to be similar in efficacy to several alternative antibacterial regimens. Further studies involving larger numbers of patients are needed to clarify the comparative therapeutic and prophylactic efficacy of sulbactam/ampicillin and alternative antibacterial drugs. Nonetheless, sulbactam/ampicillin improves the therapeutic and prophylactic efficacy of an antibacterial of familiar safety, and must be seen as a worthwhile advance.

Identification of porins in outer membrane of Proteus, Morganella, and Providencia spp. and their role in outer membrane permeation of beta-lactams

Proteus mirabilis, Proteus vulgaris, Morganella morganii, Providencia rettgeri, and Providencia alcalifaciens, which were once classified into the same genus, Proteus, were studied. Cefoxitin-resistant mutants from these species were isolated, and it was confirmed that the resistance was attributed to the lack of an outer membrane protein, resulting in a significant decrease in the penetration of hydrophilic cephalosporins through the outer membrane. Comparison of the mutant strains with their parental strains in the diffusion rates of six monoanionic cephalosporins, a zwitterionic cephalosporin (cephaloridine), and a divalent anionic cephalosporin (cephalosporin C) suggested that each species had only one kind of porin protein, with molecular weights of 40,000 (Proteus mirabilis) or 37,000 (the other four species) and that the porins formed channels with cation selectivity, except for Proteus vulgaris. Porin proteins were purified from all the bacterial species except Providencia alcalifaciens, and the radius of the pores formed by the purified porins was estimated by the use of the liposome swelling assay. The pore radii were estimated to be approximately 0.59 nm (Proteus mirabilis), 0.63 nm (Proteus vulgaris), 0.58 nm (Providencia rettgeri), and 0.60 nm (M. morganii), similar to the size of the pore radius of Escherichia coli porins.

Imipenem/cilastatin. A review of its antibacterial activity, pharmacokinetic properties and therapeutic efficacy

Imipenem is the first available semisynthetic thienamycin and is administered intravenously in combination with cilastatin, a renal dipeptidase inhibitor that increases urinary excretion of active drug. In vitro studies have demonstrated that imipenem has an extremely wide spectrum of antibacterial activity against Gram-negative and Gram-positive aerobic and anaerobic bacteria, even against many multiresistant strains of bacteria. It is very potent against species which elaborate beta-lactamases. Imipenem in combination with equal doses of cilastatin has been shown to be generally well tolerated and an effective antimicrobial for the treatment of infections of various body systems. It is likely to be most valuable as empirical treatment of mixed aerobic and anaerobic infections, bacteraemia in non-neutropenic patients and serious hospital-acquired infections.

Novel carbapenem derivative SF2103A: studies on the mode of beta-lactamase inactivation

A novel carbapenem, SF2103A, is a strong inhibitor of various types of beta-lactamase. Equimolar concentrations of SF2103A completely inactivated the cephalosporinases of Proteus vulgaris and Citrobacter freundii and type Ib and type II penicillinases mediated by R plasmids in a progressive manner. The inactivation of the two penicillinases and P. vulgaris cephalosporinase was apparently irreversible; however, when the inactivated enzymes were separated from excess SF2103A by gel filtration, they showed very slow reactivation. The hydrolysis of SF2103A by these three beta-lactamases was below the limit of detection. It is concluded that SF2103A acts as a tight-binding competitive inhibitor for the penicillinases and P. vulgaris cephalosporinase. In contrast, the inactivation of C. freundii cephalosporinase by SF2103A was evidently reversible. The rate constant of reactivation of the enzyme was compatible with the turnover rate of the enzyme in the steady state of SF2103A hydrolysis. Thus, SF2103A simply acts as a poor substrate for C. freundii cephalosporinase.

Kinetic studies on the inhibition of Proteus vulgaris beta-lactamase by imipenem

Imipenem was found to inhibit Proteus vulgaris beta-lactamase in a progressive manner. Kinetic experiments confirmed that the inactivated enzyme was not completely recovered after intact imipenem had been exhausted.

Kinetic studies on inactivation of Citrobacter freundii cephalosporinase by sulbactam

The inactivation kinetics for inhibition by sulbactam (CP45,899) of Citrobacter freundii GN346 cephalosporinase were studied in detail and compared with those of type Ib penicillinase or TEM-2 beta-lactamase mediated by R plasmid RGN823. The rate constant for progressive inactivation of the cephalosporinase was significantly larger than that measured with the penicillinase. The number of sulbactam molecules required to cause complete inactivation of one cephalosporinase molecule (turnover number) was 80. The turnover number for the penicillinase was 5,200. The powerful inhibition by sulbactam of this cephalosporinase is similar to clavulanic acid inhibition of the penicillinase (turnover number, 115; reported by others). The affinity of sulbactam for the cephalosporinase, expressed as Ki, was 500 microM; this value was much higher than that for the penicillinase, which was estimated to be 0.5 microM. These results indicated that sulbactam is an effective progressive inactivator but a poor competitive inhibitor for the cephalosporinase. Our study also revealed that the cephalosporinase and sulbactam formed a long-lived inhibitor-enzyme complex which we termed the pseudo-irreversible complex. The half-life of the complex was 550 min at pH 7.0 and 30 degrees C.