Beta-lactamases are a family of enzymes involved in bacterial resistance to beta-lactam antibiotics. In bacterial resistance to beta-lactam antibiotics, the bacteria have beta-lactamase which degrade the beta-lactam rings, rendering the antibiotic ineffective. However, with beta-lactamase inhibitors, these enzymes on the bacteria are inhibited, thus allowing the antibiotic to take effect. Strategies for combating this form of resistance have included the development of new beta-lactam antibiotics that are more resistant to cleavage and the development of the class of enzyme inhibitors called beta-lactamase inhibitors.[1] Although β-lactamase inhibitors have little antibiotic activity of their own,[2] they prevent bacterial degradation of beta-lactam antibiotics and thus extend the range of bacteria the drugs are effective against.
The most important use of beta-lactamase inhibitors is in the treatment of infections known or believed to be caused by gram-negative bacteria, as beta-lactamase production is an important contributor to beta-lactam resistance in these pathogens. In contrast, most beta-lactam resistance in gram-positive bacteria is due to variations in penicillin-binding proteins that lead to reduced binding to the beta-lactam.[3] [4] The gram-positive pathogen Staphylococcus aureus produces beta-lactamases, but beta-lactamase inhibitors play a lesser role in treatment of these infections because the most resistant strains (methicillin-resistant Staphylococcus aureus) also use variant penicillin-binding proteins.[5] [6]
The Ambler classification system groups known beta-lactamase enzymes into four groups according to sequence homology and presumed phylogenetic relationships. Classes A, C and D cleave beta-lactams by a multi-step mechanism analogous to the mechanism of serine proteases. Upon binding, a serine hydroxyl group in the beta-lactamase active site forms a transient covalent bond to the beta-lactam ring carbonyl group, cleaving the beta-lactam ring in the process. In a second step, nucleophilic attack by a water molecule cleaves the covalent bond between the enzyme and the carbonyl group of the erstwhile beta-lactam. This allows the degraded beta-lactam to diffuse away and frees up the enzyme to process additional beta-lactam molecules.
Currently available beta-lactamase inhibitors are effective against Ambler Class A beta-lactamases (tazobactam, clavulanate, and sulbactam) or against Ambler Class A, C and some Class D beta-lactamases (avibactam). Like beta-lactam antibiotics, they are processed by beta-lactamases to form an initial covalent intermediate. Unlike the case of beta-lactam antibiotics, the inhibitors act as suicide substrates (tazobactam and sulbactam) which ultimately leads to the degradation of the beta-lactamase.[7] Avibactam on the other hand does not contain a beta-lactam ring (non beta-lactam beta-lactamase inhibitor), and instead binds reversibly.[8] [9]
Ambler Class B beta-lactamases cleave beta-lactams by a mechanism similar to that of metalloproteases. As no covalent intermediate is formed, the mechanism of action of marketed beta-lactamase inhibitors is not applicable. Thus the spread of bacterial strains expressing metallo beta-lactamases such as the New Delhi metallo-beta-lactamase 1 has engendered considerable concern.[10]
Currently marketed β-lactamase inhibitors are not sold as individual drugs. Instead they are co-formulated with a β-lactam antibiotic with a similar serum half-life. This is done not only for dosing convenience, but also to minimize resistance development that might occur as a result of varying exposure to one or the other drug. The main classes of β-lactam antibiotics used to treat gram-negative bacterial infections include (in approximate order of intrinsic resistance to cleavage by β-lactamases) penicillins (especially aminopenicillins and ureidopenicillins), 3rd generation cephalosporins, and carbapenems. Individual β-lactamase variants may target one or many of these drug classes, and only a subset will be inhibited by a given β-lactamase inhibitor.[9] β-lactamase inhibitors expand the useful spectrum of these β-lactam antibiotics by inhibiting the β-lactamase enzymes produced by bacteria to deactivate them.[11]
Bacteria that can produce beta-lactamases include, but are not limited to:
Some bacteria can produce extended spectrum β-lactamases (ESBLs) making the infection more difficult to treat and conferring additional resistance to penicillins, cephalosporins, and monobactams.[16] Boronic acid derivatives are currently under vast and extensive research as novel active site inhibitors for beta-lactamases because they contain a site that mimics the transition state that beta-lactams go through when undergoing hydrolysis via beta-lactamases. They have been found generally to fit well into the active site of many beta-lactamases and have the convenient property of being unable to be hydrolysed, and therefore rendered useless. This is a favorable drug design over many clinically used competing agents, because most of them, such as clavulanic acid, become hydrolysed, and are therefore only useful for a finite period of time. This generally causes the need for a higher concentration of competitive inhibitor than would be necessary in an unhydrolyzable inhibitor. Different boronic acid derivatives have the potential to be tailored to the many different isoforms of beta-lactamases, and therefore have the potential to reestablish potency of beta-lactam antibiotics.[17]