Carbapenems are a class of β-lactam antibiotics with a broad spectrum of antibacterial activity. They have a structure that renders them highly resistant to most β-lactamases. Carbapenem antibiotics were originally developed from the carbapenem thienamycin, a naturally derived product of Streptomyces cattleya.
Carbapenems are one of the antibiotics of last resort for many bacterial infections, such as Escherichia coli (E. coli) and Klebsiella pneumoniae. Recently, alarm has been raised over the spread of drug resistance to carbapenem antibiotics among these coliforms, due to production of carbapenemases, including the New Delhi metallo-β-lactamase, NDM-1. There are currently no new antibiotics in development to combat bacteria resistant to carbapenems, and worldwide spread of the resistance gene is considered a potential nightmare scenario.
Approved for clinical use
The following drugs belong to the carbapenem class and are approved for use by health authorities:
- Imipenem, in general given as part of Imipenem/cilastatin (FDA approval 1985)
- Imipenem can be hydrolysed in the mammalian kidney by a dehydropeptidase enzyme to a nephrotoxic metabolite, and so is given with a dehydropeptidase inhibitor, cilastatin
- Meropenem (FDA approval 1996)
- Ertapenem (FDA approval 2001, since approved for multiple indications)
- Doripenem (FDA Approval 2007)
- Panipenem/betamipron (Japanese approval 1993)
- Biapenem (Japanese approval 2001)
- Razupenem (PZ-601)
- PZ-601 is a carbapenem antibiotic currently being tested as having a broad spectrum of activity including strains resistant to other carbapenems. Despite early Phase II promise, Novartis (who acquired PZ-601 in a merger deal with Protez Pharmaceuticals) recently dropped PZ-601, citing a high rate of adverse events in testing.
- Tebipenem is the first carbapenem whose prodrug form, the pivalyl ester, is orally available.
- Thienamycin (thienpenem) the first discovered carbapenem
Spectrum of use
These agents have the broadest antibacterial spectrum compared to other β-lactam classes such as penicillins and cephalosporins. In addition, they are generally resistant to the typical bacterial enzyme, β-lactamase, which is one of the principal β-lactam resistance mechanisms of bacteria. Carbapenems circumvent β-lactamase by binding it with high affinity and acylating the enzyme, rendering it inactive. Carbapenems are active against both Gram-positive and Gram-negative bacteria, and anaerobes, with the exception of intracellular bacteria (atypicals), such as the Chlamydiae. Carbapenems also are thus far the only β-lactams capable of inhibiting l,d-transpeptidases 
Emergence of bacterial resistance
In the United States and United Kingdom, strains of carbapenem-resistant enteric bacteria have been isolated from patients having received recent medical care in Pakistan, Bangladesh, and India. These strains carry a gene called New Delhi metallo-β-lactamase (shortened NDM-1) that is responsible for the production of a metallo-β-lactamase enzyme that hydrolyses carbapenem. A clinical isolate of E. coli from the sputum sample of a patient admitted to a Beijing hospital was found to show unusual resistance to carbapenem that does not rely on the presence of carbapenemase. The isolate was determined to have four separate mutations to acquire the resistance to carbapenems. Two mutations removed the outer membrane proteins OmpF and OmpC to prevent the antibiotics from reaching the PBPs (penicillin binding proteins) in the inner membrane. A regulator gene marR was mutated and a normally non-translated membrane protein yedS was expressed; both were demonstrated to have effects on the ability of this strain of E.coli to resist carbapenems. The bacteria also increased the expression of a multidrug efflux pump.
In 2014, researchers in Canada found carbapenem-resistant bacteria in a sample of frozen squid sold at a Chinese grocery store in Saskatoon, Canada. The squid reportedly originated in South Korea and was purchased in January 2014. University of Saskatchewan researcher Joseph Rubin called the find "extremely disturbing".
In terms of structure, the carbapenems are very similar to the penicillins (penams), but the sulfur atom in position 1 of the structure has been replaced with a carbon atom, and an unsaturation has been introduced—hence the name of the group, the carbapenems.
The carbapenems are thought to share their early biosynthetic steps in which the core ring system is formed. Malonyl-CoA is condensed with glutamate-5-semialdehyde with concurrent formation of the five-membered ring. Next, a β-lactam synthetase uses ATP to form the β-lactam and the saturated carbapenam core. Further oxidation and ring inversion provides the basic carbapenem.
Due to their expanded spectra, the desire to avoid generation of resistance and the fact that, in general, they have poor oral bioavailability, they are administered intravenously in hospital settings for more serious infections. However, research is underway to develop an effective oral carbapenem.
When the first bacterial β-lactamases emerged in the late 1960s, and reduced the efficacy of penicillin, an intensive search for β-lactamase inhibitors was launched. By 1976 the first such inhibitors had been discovered - these olivanic acids were metabolites produced by the Gram-positive bacterium Streptomyces clavuligerus. Olivanic acids are formed around a carbapenem backbone (a carbon at the 1 position, substituents at C-2, a C-6 ethoxy, and sp2-hybridized C-3) and act as broad-spectrum β-lactams. Because of their chemical instability and poor penetration of the bacterial cell wall, research on these olivanic acids was not continued. However, shortly after, two superior β-lactamase inhibitors were discovered: (1) clavulanic acid from S. clavuligerus, the first clinically available β-lactamase inhibitor, and (2) thienamycin from Streptomyces cattleya. Thienamycin was the first carbapenem and would serve as a model for all subsequent carbapenems.
A carbapenem is defined as the 4:5 fused ring lactam of penicillins with a double bond between C-2 and C-3 but with the substitution of carbon for sulfur at C-1. The hydroxyethyl side chain of thienamycin is a radical departure from the structure of conventional penicillins and cephalosporins, all of which have an acylamino substituent on the β-lactam ring; the stereochemistry of this hydroxyethyl side chain is a key attribute of carbapenems and is important for activity. Remarkably, thienamycin demonstrated potent broad-spectrum antibacterial and β-lactamase inhibitory activity. This notable discovery was first reported at the 16th Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC) meeting in 1976. Although thienamycin is a “natural product” and the biosynthetic pathway was determined, yields from the purification process were low. With time, the synthetic preparation of thienamycin assumed greater importance, especially as a key derivative, imipenem, was discovered.—"Carbapenems: Past, Present, and Future" - Krisztina M. Papp-Wallace et al.
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