β-Lactamase inhibitor

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]

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.^{[citation needed]}

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]

• β-lactamase inhibitors with a β-lactam core:
  • Tebipenem is the first carbapenem to be administered orally in the form of tebipenem-pivoxil. Structural and kinetic studies of tebipenem is available with M. tuberculosis beta-lactamase (BlaC).^[12]
  • Clavulanic acid or clavulanate, usually combined with amoxicillin (Augmentin) or ticarcillin (Timentin)
  • Sulbactam, usually combined with ampicillin (Unasyn) or cefoperazone (Sulperazon)
  • Tazobactam, usually combined with piperacillin (Zosyn, Tazocin)
  • Enmetazobactam, usually combined with cefepime (Exblifep)
• β-lactamase inhibitors with a diazabicyclooctane core:
  • Avibactam, approved in combination with ceftazidime (Avycaz, Zavicefta), currently undergoing clinical trials for combination with ceftaroline
  • Durlobactam, approved in combination with sulbactam (Xacduro) to treat hospital‑acquired bacterial pneumonia and ventilator-associated bacterial pneumonia (HABP/VABP) pneumonia caused by Acinetobacter baumannii-calcoaceticus complex
  • Relebactam, used in combination with imipenem/cilastatin (Recarbrio).^[13][14]
• β-lactamase inhibitors with other types of non β-lactam cores:
  • Vaborbactam, used in combination with meropenem (Vabomere). Has a boronic acid core.^[15]

Bacteria that can produce beta-lactamases include, but are not limited to:^{[citation needed]}

• Staphylococcus
  • MRSA(Methicillin-resistant Staphylococcus aureus)
• Enterobacteriaceae:
  • Klebsiella pneumoniae
  • Citrobacter
  • Proteus vulgaris
  • Morganella
  • Salmonella
  • Shigella
  • Escherichia coli
• Haemophilus influenzae
• Moraxella catarrhalis
• Neisseria gonorrhoeae
• Pseudomonas aeruginosa
• Mycobacterium tuberculosis

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]

• Xu H, Hazra S, Blanchard JS (June 2012). "NXL104 irreversibly inhibits the β-lactamase from Mycobacterium tuberculosis". Biochemistry. 51 (22): 4551–7. doi:10.1021/bi300508r. PMC 3448018. PMID 22587688.
• Kurz SG, Wolff KA, Hazra S, Bethel CR, Hujer AM, Smith KM, Xu Y, Tremblay LW, Blanchard JS, Nguyen L, Bonomo RA (December 2013). "Can inhibitor-resistant substitutions in the Mycobacterium tuberculosis β-Lactamase BlaC lead to clavulanate resistance?: a biochemical rationale for the use of β-lactam-β-lactamase inhibitor combinations". Antimicrobial Agents and Chemotherapy. 57 (12): 6085–96. doi:10.1128/AAC.01253-13. PMC 3837893. PMID 24060876.