1
Relebactam (Recarbrio), A ß-Lactamase Inhibitor for the Treatment of cIAI/cUTI/HABP/VABP

Dexi Yang

1. Background

The discovery of antibiotics is revolutionary in chemotherapy against infectious diseases in modern medicine history. Unfortunately, after its golden era from the 1950s to 1970s, antimicrobial resistance among common bacterial pathogens became a new threat to public health. Recently, WHO enlisted antibiotic resistance in the top three public health threats. Infections caused by multidrug-resistant organism became a new economic burden in health-care system. In the United States alone, it costs over 20 billion dollars per year, and more than 23,000 people died of infection with antibiotic-resistant annually. With this continuing, CDC estimated that victims will culminate to more than 300 million globally with loss of over 100 trillion dollars by 2050. The stake is high enough to draw more attention to invent new therapeutics to treat infected patients.1

Of all known antimicrobial resistance, carbapenem resistance in gram-negative pathogens is the most critical. Clinically, carbapenems are considered the most active and potent agents against MDR gram-negative pathogens. They are the last silver bullets to kill superbugs. However, according to the global priority list of antibiotic-resistant bacteria published by WHO in 2017, three of the top four pathogens critical for developing antibiotics are carbapenem-resistant, and they are Enterobacteriaceae (CRE), Pseudomonas aeruginosa, and Acinetobacter baumannii.2,3 In the 1990s, research on MDR revealed that antibiotic resistance of gram-negative bacteria is mainly caused by three mechanisms. The major mechanism of resistance to carbapenems is the production of ß-lactamase. It has been identified that MDR gram-negative pathogens produces at least four classes of ß-lactamases: A, B, C, and D.4 They all can degrade antibacterial agents and make them ineffective. In addition to these enzymes, these pathogens also developed other mechanisms to make antibiotics less efficient. One is porin mutation caused by porin expression. This renders the outer bacterial membrane impermeable to antibacterial. The other is efflux pump upregulation. Via efflux pump, antibiotics are pumped out of the membrane of bacteria and lose their therapeutic efficacy.3,4 Based on all these three mechanisms, new generations of ß-lactamase inhibitors should not only have high potency to help carbapenem restore potency, but also possess appropriate physicochemical properties to increase permeability and decrease efflux rate.

Guided by this strategy, in the past decade, several combinations of antibiotic with ß-lactamase inhibitors, such as polymyxins, ceftazidime-avibactam, ceftolozane- tazobactam, metropenem-veborbactam, etc., have been discovered and approved by FDA.5-7 But most of them are only effective against a small portion of pathogens, and with no surprise, gradually loose efficacy due to evolved resistance. As a result, new therapeutic options against gram-negative organisms with resistance are still urgently needed.

Recarbrio was approved in July 2019 as an alternative treatment option of adults with complicated urinary tract infections (cUTI), including pyelonephritis, and complicated intra-abdominal infections (cIAI) caused by susceptible gram-negative bacteria.8,9 Recarbrio is a three-drug combination injection containing imipenem, cilastatin, and relebactam (1). Imipenem is a carbapenem that inhibits cross-linking of peptidoglycan during cell wall synthesis by deactivating penicillin binding proteins. It is co-administered with cilastatin, a dehydro-peptidase-I inhibitor to reduce renal metabolism of imipenem. Cilastatin itself does not have antibacterial activity. Relebactam (1) is a novel ß-lactamase inhibitor. It alone has no antibacterial activity either. Its function is to protect imipenem by inhibiting Ambler class A (e.g., KPCs), class C (e.g., AmpC) ß-lactamases, and PDC. In vitro, the addition of relebactam (1) significantly improves the antibacterial activity of imipenem by lowering the minimum inhibitory concentration of imipenem by 2- to 128-folds against ESBL or KPC producing enterobacterales, as well as MDR or imipenem-resistant isolates.10

In June 2020, FDA further approved a supplemental new drug application (sNDA) for Recarbrio for the treatment of patients 18 years of age and older with hospital-acquired bacterial pneumonia and ventilator-associated bacterial pneumonia (HABP/VABP), caused by a group of susceptible gram-negative microorganism.9

It worth mentioning that relebactam (1) is inactive against class B metallo-ß-lactamases (e.g., NDM, VIM, and IMP) and class D oxacillinases (e.g., OXA-48). This leaves space for further development of novel BLIs with expanded coverage of class B metallo-ß-lactamase and class D ß-lactamase to secure efficacy of new antibiotics.

2. Pharmacology

Imipenem (trade name Primaxin) is an intravenous ß-lactam antibiotic discovered by Merck scientists Burton Christensen, William Leanza, and Kenneth Wildonger in the mid-1970s.10 As a carbapenem, it is highly resistant to the ß-lactamases produced by MDR gram-negative bacteria. It inhibits cross-linking of peptidoglycan during cell wall synthesis by deactivating penicillin binding proteins, thereby causing bacterial cell lysis and death.11 Since it rapidly degraded by the renal enzyme dehydropeptidase-I when administered alone, imipenem is always co-administered with cilastatin, a dehydropeptidase-I inhibitor to reduce renal metabolism. Ever since its approval, it played a key role in treating infections caused by susceptible strains when other antibiotics failed. However, since more and more bacteria developed drug-resistance via production of ß-lactamase, imipenem became less effective in some patients. It is necessary to invent new ß-lactamase inhibitors as additive to restore the antibacterial activity of imipenem.

Relebactam (MK-7655) is a novel ß-lactamase inhibitor discovered by Merck scientists as part of a drug discovery program aimed at novel BLI in 2008. Over several lead series, a bridged class showed broad spectrum of activities. Its cyclic urea can open and bind covalently to an active site serine within Ambler class A, C, and D ß-lactamases. To be specific, the constrained five-membered urea bridge facilitates acylation reaction between the C-7 carbonyl and a serine residue within the ß-lactamase active site. Modeling studies suggested that the N,O-oxysulfate group can further stabilize the ring-opened acyl-ß-lactamase intermediate via hydrogen-bond formation with neighboring catalytic site residues. As a result, the covalent bond blocks the active site of the ß-lactamase, stops hydrolysis of imipenem, and restores its bactericidal activity.12

So far, it is active in inhibiting Ambler class A (e.g., KPCs) and class C (e.g., AmpC) actamases and PDC, but inactive against class B metallo-ß-lactamases (e.g., NDM, VIM and IMP) and class D oxacillinases (e.g., OXA-48).13 In vitro, the addition of relebactam (1) significantly improves the antibacterial activity of imipenem by lowering the minimum inhibitory concentration of imipenem by 2- to 128-folds against ESBL or KPC producing enterobacterales, or imipenem-resistant isolates. Neither imipenem nor relebactam is subject to efflux, which is an advantage against strains that overexpress efflux pumps.

3. Structure-Activity Relationship (SAR)

The lead compound for the relebactam (1) project has two sources: one is MK-8712, a monobactam ß-lactamase inhibitor only effective against class C ß-lactamase; the other is avibactam sodium (NXL-104), a covalent and reversible non-ß-lactam ß-lactamase inhibitor to ß-lactamase TEM-1 and CTX-M-15.14 In order to expand the coverage of novel BLI to both class C (PDC) P. aeruginosa and class A (KPC) enterobacterales class A and class C ß-lactamases, Merck scientists took a hybrid approach by incorporating novel heterocyclic amide side chains for MK-8712 with the bridged core of NXL-104. After overcoming many synthetic difficulties, a series of bridged bicyclic urea with basic heterocyclic side chains have been prepared for SAR study.

As shown in Table 1, selected compounds were evaluated in enzyme inhibition assays and in vitro synergy assays.12 The enzyme inhibition assays measured each compound's ability to inhibit the hydrolysis of nitrocefin by four ß-lactamase: one class A BL, two class C BLs and one class D BL. The synergy data reported the concentration of each compound to reduce the MIC of imipenem to 4 µg/mL against the strains of Pseudomonas, Klebsiella, and Acinetobacter.

Let us...