Carbapenems are the broadest-spectrum beta-lactam antibiotics available in clinical practice, with activity against most gram-positive cocci, gram-negative rods including Pseudomonas aeruginosa (for most agents), and anaerobes. Their exceptional breadth of coverage stems from structural features that distinguish them from penicillins and cephalosporins and confer resistance to the vast majority of clinically significant beta-lactamases.
Carbapenem Structure. The carbapenem scaffold shares the four-membered beta-lactam ring common to all beta-lactam antibiotics but differs from penicillins and cephalosporins in two critical ways. First, the ring fused to the beta-lactam is a five-membered ring with a double bond between carbons 2 and 3 and a carbon atom (rather than sulfur) at position 1 of the ring — creating the carbapenem (1-carbapenem) nucleus rather than the penam or cephem scaffolds. Second, carbapenems carry a 1-beta-methyl group (with the exception of imipenem) that provides steric protection against hydrolysis by most clinically significant class A, class C, and class D serine beta-lactamases. This combination of the unusual ring structure and the 1-beta-methyl substituent explains why carbapenems resist hydrolysis by the extended-spectrum beta-lactamases (ESBLs) and AmpC (class C cephalosporinase) enzymes that destroy penicillins and cephalosporins.1
Mechanism of Action and Penicillin-Binding Protein (PBP) Binding. Like all beta-lactams, carbapenems act by covalently acylating the active-site serine of penicillin-binding protein (PBP) transpeptidases, irreversibly inhibiting peptidoglycan cross-linking and ultimately causing bactericidal cell lysis through autolysin-mediated wall degradation. Carbapenems are distinguished by their broad PBP (penicillin-binding protein) target range: they bind the PBP (penicillin-binding protein) variants PBP1a, PBP1b, PBP2 (cell shape), and PBP3 (cell division) in Enterobacteriaceae, with particularly high affinity for PBP2, which is responsible for maintaining the ovoid cell shape and whose inhibition results in rapidly lethal spheroplast formation. In Pseudomonas aeruginosa, carbapenems bind PBP1a and PBP3 with high affinity; loss of the OprD (outer membrane porin D) porin reduces carbapenem entry and is one of the dominant mechanisms of selective carbapenem resistance in this organism. Against staphylococci, carbapenems bind PBP1 (transpeptidase), PBP2 (cell shape), and PBP3 (cell division) but have no affinity for PBP2a, confirming that methicillin-resistant Staphylococcus aureus (MRSA) is resistant to all carbapenems.2
Pharmacodynamic Properties. Carbapenems exhibit time-dependent bactericidal killing, with fT>MIC (percentage of the dosing interval that free drug concentration exceeds the minimum inhibitory concentration) as the key pharmacodynamic (PD) index. For most organisms, carbapenem bactericidal activity is optimized when fT>MIC exceeds 40% of the dosing interval; however, for organisms with elevated MICs (particularly Pseudomonas aeruginosa at the susceptibility breakpoint), extended infusion over 3-4 hours substantially improves pharmacodynamic target attainment. The post-antibiotic effect (PAE) of carbapenems against gram-negative organisms is minimal (approximately 0-2 hours), reinforcing the importance of maintaining drug concentrations above the MIC throughout the dosing interval rather than relying on residual post-dose suppression. Against gram-positive organisms, carbapenems have a more pronounced PAE (1-3 hours).3
Beta-Lactamase Stability and Its Limits. Carbapenems are hydrolyzed by two clinically significant classes of beta-lactamases: the class B metallo-beta-lactamases (MBLs), which include NDM (New Delhi metallo-beta-lactamase), VIM (Verona integron-encoded metallo-beta-lactamase), and IMP (imipenemase), and certain class A serine carbapenemases, principally KPC (Klebsiella pneumoniae carbapenemase). The class D OXA-type enzymes (OXA-23, OXA-48, OXA-58) also hydrolyze carbapenems, particularly imipenem, and represent a growing threat in Acinetobacter baumannii and Klebsiella pneumoniae. Aztreonam (a monobactam) is not hydrolyzed by class B MBLs, making aztreonam-avibactam or ceftazidime-avibactam plus aztreonam potential treatment options for NDM-producing organisms, though the combination must be carefully validated against the specific isolate.1
Not all carbapenem-resistant organisms are equally resistant or equally dangerous. Selective imipenem resistance in Pseudomonas aeruginosa caused by OprD porin loss without carbapenemase production often retains susceptibility to meropenem and doripenem, and may respond to alternative beta-lactams combined with pharmacodynamic optimization. True carbapenemase production (KPC, NDM, OXA-48) confers broad resistance to all carbapenems and most other beta-lactams and requires specifically tailored salvage therapy. Distinguishing these mechanisms by phenotypic testing (mCIM, eCIM) or genotypic confirmation (PCR, whole-genome sequencing) before committing to therapy is essential.
Four carbapenems are in widespread clinical use: imipenem-cilastatin, meropenem, ertapenem, and doripenem. While sharing a common mechanism of action, they differ in spectrum (particularly Pseudomonas and MRSA activity), pharmacokinetics (PK), CNS (central nervous system) penetration, seizure risk, and renal dosing requirements in ways that are clinically consequential and frequently tested at the resident and attending level.
Imipenem-Cilastatin. Imipenem is the oldest clinical carbapenem and is unique in its formulation: it is always administered in combination with cilastatin, a specific inhibitor of renal dehydropeptidase I (DHP-I), which otherwise rapidly hydrolyzes imipenem in the proximal tubule, reducing urinary concentrations to subtherapeutic levels and generating nephrotoxic metabolites. Cilastatin itself has no antibacterial activity; it is a pharmacokinetic (PK) enabler. Imipenem has the broadest gram-positive spectrum of the carbapenems, with activity against MSSA (methicillin-susceptible Staphylococcus aureus), Streptococcus species, and Enterococcus faecalis, though it lacks coverage of MRSA and Enterococcus faecium. Its Pseudomonas activity is inferior to meropenem and doripenem. The most clinically significant adverse effect unique to imipenem is seizure, occurring in approximately 0.9-3% of patients — substantially more frequently than with other carbapenems. Seizure risk is highest with renal impairment (imipenem accumulation), high doses, and pre-existing CNS pathology or seizure disorder. This seizure liability makes imipenem a poor choice for meningitis or patients with active seizure disorders.2
Meropenem. Meropenem is the most widely used carbapenem in clinical practice and is generally considered the reference agent for serious gram-negative infections. Unlike imipenem, meropenem carries a 1-beta-methyl group that confers resistance to renal DHP-I hydrolysis, so it does not require a DHP-I inhibitor and is administered alone. Meropenem has superior Pseudomonas aeruginosa and Acinetobacter baumannii activity compared to imipenem, and achieves significantly better CNS penetration (CSF [cerebrospinal fluid]-to-plasma ratio approximately 15-40% with inflamed meninges), making it the carbapenem of choice for meningitis caused by susceptible gram-negative organisms. Meropenem has substantially lower seizurogenic potential than imipenem. Standard dosing is 0.5-1 g every 8 hours for most indications; extended infusion at 2 g over 3 hours every 8 hours is used for organisms with elevated MICs (Pseudomonas aeruginosa MIC near 2-4 mcg/mL) to optimize fT>MIC. Meropenem undergoes approximately 70% renal elimination and requires dose adjustment when creatinine clearance (CrCl) falls below 26 mL/min.2
Ertapenem. Ertapenem is distinguished from imipenem and meropenem by one critical absence: it has no activity against Pseudomonas aeruginosa or Acinetobacter baumannii. This narrower gram-negative spectrum is a deliberate pharmacological feature — ertapenem's side chain reduces outer membrane permeability penetration in non-fermenting gram-negative rods — that makes it useful as a carbapenem-sparing agent for ESBL (extended-spectrum beta-lactamase) infections when Pseudomonas coverage is not required. Ertapenem has the longest half-life of the carbapenems (approximately 4 hours) due to extensive plasma protein binding (approximately 92-95%), enabling once-daily dosing, which is its principal pharmacokinetic advantage. It is used for outpatient parenteral antibiotic therapy (OPAT) for community-acquired ESBL infections, complicated urinary tract infections, and community-acquired intra-abdominal infections caused by susceptible organisms. Ertapenem is renally eliminated and requires dose adjustment in renal impairment (CrCl below 30 mL/min). Because of its long half-life and OPAT convenience, ertapenem is used for step-down therapy from meropenem in ESBL bacteremia after clinical stability is achieved.5
Doripenem. Doripenem has a spectrum similar to meropenem, with antipseudomonal activity and low seizure potential. It achieves good lung tissue concentrations and was evaluated for ventilator-associated pneumonia (VAP) caused by Pseudomonas aeruginosa. The DOREMI (doripenem versus imipenem) trial (a randomized controlled trial of doripenem versus imipenem for VAP) and subsequent studies showed non-inferiority of doripenem compared to imipenem, but doripenem failed to demonstrate superiority over other regimens in major trials. Doripenem has somewhat reduced in vitro potency against Pseudomonas aeruginosa compared to meropenem when used at standard doses; however, at higher doses used in some acutely ill patients, it may provide marginal benefit for high-MIC Pseudomonas. Its use in current practice has declined significantly relative to meropenem. Doripenem is approved for complicated urinary tract infections and complicated intra-abdominal infections but not for VAP in the United States after the DOREMI trial results were incorporated into regulatory consideration.4
| Agent | Pseudomonas | Formulation Note | Half-Life | Seizure Risk | Key Niche |
|---|---|---|---|---|---|
| Imipenem-cilastatin | Yes (less than meropenem) | Requires cilastatin (DHP-I inhibitor) | ~1 h | Highest (0.9-3%); avoid in CNS disease | Broad gram-pos + gram-neg; not meningitis |
| Meropenem | Yes (reference agent) | No cilastatin needed (1-beta-methyl stable) | ~1 h | Low; drug of choice for gram-neg meningitis | Broadest clinical use; gram-neg meningitis; ESBL bacteremia |
| Ertapenem | No | Once-daily (long t½ due to protein binding ~92%) | ~4 h | Low | ESBL without Pseudomonas; OPAT; community-acquired infections |
| Doripenem | Yes | Standard IV infusion | ~1 h | Low | Limited current use; cUTI, cIAI |
For Pseudomonas aeruginosa or Acinetobacter baumannii infections where the isolate MIC is at or near the susceptibility breakpoint (MIC 2-4 mcg/mL), standard 30-minute meropenem infusions may fail to achieve fT>MIC targets above 40% of the dosing interval. Extending the infusion to 3 hours (maintaining meropenem concentration above the MIC for a greater proportion of the interval) substantially increases pharmacodynamic target attainment without increasing the total daily dose. The typical extended-infusion regimen is meropenem 2 g infused over 3 hours every 8 hours. This strategy is particularly important in the ICU for Pseudomonas VAP or bacteremia with documented high MIC values.
Aztreonam is the only monobactam in clinical use and occupies a unique pharmacological niche: it is a beta-lactam antibiotic with a monocyclic (single-ring) beta-lactam structure that confers gram-negative-only activity, no cross-reactivity with penicillin allergy, and intrinsic resistance to most class A and class B beta-lactamases — though it is hydrolyzed by ESBLs (extended-spectrum beta-lactamases) and class C AmpC enzymes.
Aztreonam Chemistry and Mechanism. Aztreonam contains a single four-membered beta-lactam ring without a fused second ring, distinguishing it structurally from penicillins (penam bicyclic), cephalosporins (cephem bicyclic), and carbapenems (carbapenem bicyclic). The sulfonate group on the beta-lactam nitrogen activates the ring for PBP (penicillin-binding protein) binding. Aztreonam binds selectively and with very high affinity to penicillin-binding protein 3 (PBP3) of gram-negative bacteria, inhibiting cell division transpeptidation and causing bacterial filamentous elongation followed by lysis. Of clinical importance, aztreonam has essentially no affinity for gram-positive PBPs or anaerobic PBPs, explaining its complete lack of activity against gram-positive cocci, anaerobes, and Listeria monocytogenes. Its gram-negative spectrum covers most Enterobacteriaceae, Pseudomonas aeruginosa (when susceptible), and Haemophilus influenzae, closely paralleling the spectrum of ceftazidime.6
Key Clinical Property: Penicillin Allergy Safety. Because aztreonam's monocyclic structure is structurally distinct from the bicyclic structures of penicillins and cephalosporins, it does not share the immunogenic determinants responsible for IgE (immunoglobulin E)-mediated penicillin allergy. Patients with documented severe penicillin allergy including anaphylaxis can safely receive aztreonam in most circumstances. The one exception is the ceftazidime-aztreonam cross-reactivity: aztreonam and ceftazidime share identical R1 (position-1 substituent) side chains (the aminothiazolyl-methoxyimino group), and patients allergic specifically to ceftazidime (rather than to penicillins or most cephalosporins) may react to aztreonam. This makes aztreonam an important alternative gram-negative agent for severe penicillin-allergic patients who require coverage of Pseudomonas aeruginosa or Enterobacteriaceae when carbapenems are unavailable or contraindicated.7
Aztreonam Limitations: ESBL (Extended-Spectrum Beta-Lactamase) and AmpC Susceptibility. Aztreonam is hydrolyzed efficiently by ESBL enzymes (CTX-M [cefotaxime-Munich enzyme], TEM (Temoniera)-derived ESBLs, and SHV (sulhydryl-variable)-derived ESBLs) and by AmpC (class C) cephalosporinases. This means aztreonam monotherapy is not reliable for infections caused by ESBL-producing or AmpC-overproducing organisms, even when isolates may appear susceptible in vitro at low bacterial inocula. Aztreonam is also hydrolyzed by KPC (class A carbapenemase) but is not hydrolyzed by class B metallo-beta-lactamases (NDM [New Delhi metallo-beta-lactamase], VIM, IMP). This critical property forms the pharmacological rationale for aztreonam-avibactam and aztreonam plus ceftazidime-avibactam combination therapy for NDM-producing organisms: avibactam protects aztreonam from hydrolysis by the co-produced serine beta-lactamases (ESBL, AmpC, KPC), while aztreonam itself is intrinsically resistant to NDM hydrolysis.8
Aztreonam ADME (Absorption, Distribution, Metabolism, Excretion) and Adverse Effects. Aztreonam is administered intravenously or intramuscularly; oral bioavailability is negligible. It is approximately 56% protein-bound and achieves good tissue penetration including adequate CSF (cerebrospinal fluid) concentrations with inflamed meninges. Aztreonam is primarily renally eliminated (approximately 60-70% unchanged in urine) and requires dose adjustment in renal impairment (CrCl below 30 mL/min). An inhaled formulation of aztreonam lysine (AZLI [aztreonam lysine for inhalation]) is approved for pulmonary use in patients with cystic fibrosis (CF) colonized with Pseudomonas aeruginosa, where it achieves high local airway concentrations while minimizing systemic exposure. Adverse effects are uncommon; phlebitis at the injection site and mild transaminase elevations occur. Aztreonam does not cause seizures, does not require a DHP-I (dehydropeptidase I) inhibitor, and has no significant nephrotoxicity.6
NDM-producing organisms are resistant to virtually all beta-lactams including all carbapenems, ceftazidime-avibactam, and meropenem-vaborbactam (because avibactam and vaborbactam do not inhibit class B metallo-beta-lactamases). Aztreonam-avibactam exploits aztreonam's intrinsic NDM stability: avibactam inhibits the co-produced serine beta-lactamases (ESBLs, AmpC, KPC) that would otherwise hydrolyze aztreonam, restoring aztreonam activity against NDM-producers. This combination is available as a co-formulated product (aztreonam-avibactam) or as aztreonam administered alongside ceftazidime-avibactam. Clinical data are growing but still limited to case series and small prospective studies, confirming in principle the pharmacological rationale.
Carbapenem-resistant Enterobacteriaceae (CRE) and carbapenem-resistant Acinetobacter baumannii (CRAB) represent two of the most clinically challenging resistance phenotypes in current infectious disease practice. Their growing prevalence has driven the development of a pipeline of novel beta-lactam combinations, non-beta-lactam agents, and siderophore-conjugated cephalosporins that have substantially expanded therapeutic options since 2015, though all carry important limitations.
CRE Mechanisms and Epidemiology. Carbapenem resistance in Enterobacteriaceae arises through three distinct mechanisms that may coexist; organisms displaying resistance to three or more antibiotic classes meet international definitions of multidrug-resistant (MDR) pathogens.9 The first is carbapenemase production: enzymatic hydrolysis by KPC (class A), NDM/VIM/IMP (class B metallo-beta-lactamases), or OXA-48-type (class D) enzymes. The second is non-carbapenemase-mediated resistance: loss of outer membrane porins (OmpK35, OmpK36 in Klebsiella pneumoniae) combined with upregulation of ESBL (extended-spectrum beta-lactamase) or AmpC beta-lactamases, creating intermediate or low-level carbapenem resistance without a carbapenemase. The third is efflux pump upregulation, which contributes to but rarely causes high-level resistance alone. KPC-producing Klebsiella pneumoniae sequence type ST258 (ST258: the globally dominant KPC epidemic clone) is the dominant epidemic clone in the United States and much of Europe; NDM (New Delhi metallo-beta-lactamase)-producing organisms predominate in South Asia and are increasingly prevalent worldwide on mobile plasmids carried by diverse Enterobacteriaceae. The CDC (Centers for Disease Control and Prevention) has designated CRE as an urgent antimicrobial resistance threat, reflecting crude mortality rates of 40-50% for CRE bacteremia in some series.12
Novel Combination Therapies for CRE. Three novel carbapenem-based and beta-lactam combinations have been approved specifically for CRE infections. Ceftazidime-avibactam (discussed in Module 02) is active against KPC, AmpC, and OXA-48 but not against NDM or VIM (Verona integron-encoded metallo-beta-lactamase). Meropenem-vaborbactam (meropenem combined with the boronic acid inhibitor vaborbactam) covers KPC and AmpC but not NDM or OXA-48; the TANGO (Treatment of Complicated Urinary Tract Infections and Other Complicated Infections) II trial (a randomized controlled trial of meropenem-vaborbactam versus best available therapy) demonstrated superior 28-day all-cause mortality and clinical cure rates with meropenem-vaborbactam for KPC-CRE (KPC carbapenem-resistant Enterobacteriaceae) infections. Imipenem-cilastatin-relebactam (imipenem combined with the DBO [diazabicyclooctane] inhibitor relebactam) covers KPC and AmpC and has enhanced anti-Pseudomonas activity, making it useful for multidrug-resistant Pseudomonas aeruginosa in addition to KPC-CRE. None of these three combinations reliably treats NDM-producing organisms, the fastest-growing global resistance threat.10
Cefiderocol: Siderophore-Conjugated Cephalosporin. Cefiderocol is a novel cephalosporin conjugated to a catecholate siderophore moiety that allows it to hijack bacterial iron-uptake transport systems (TonB-dependent outer membrane transporters) for active translocation across the outer membrane, achieving periplasmic concentrations that circumvent standard porin-loss resistance mechanisms. Once in the periplasm, cefiderocol binds PBP3 (cell division transpeptidase) with high affinity and is highly stable to hydrolysis by all classes of beta-lactamases including NDM and OXA-23 (a class D carbapenemase). The CREDIBLE-CR (Carbapenem-Resistant and Difficult-to-Treat Infections) trial (a non-randomized cohort study) and the APEKS-NP (A Phase 3, Prospective, Exploratory, Randomized, Open-Label) trial (a randomized, double-blind trial of cefiderocol versus meropenem for nosocomial pneumonia) established cefiderocol's activity against carbapenem-resistant gram-negative pathogens including Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacteriaceae with MBLs. However, the CREDIBLE-CR trial reported higher all-cause mortality in the cefiderocol arm compared to best available therapy in a small subset of Acinetobacter baumannii patients, raising questions about clinical efficacy in this organism that remain under investigation.11
Carbapenem-Resistant Acinetobacter baumannii. Carbapenem-resistant Acinetobacter baumannii (CRAB) is designated a critical priority pathogen by the WHO (World Health Organization) and the CDC, reflecting the near-total resistance of many clinical isolates to all conventional antibiotics. Resistance is mediated predominantly by OXA-23 and OXA-58 (class D OXA-type carbapenemases) combined with porin loss and efflux pump upregulation. Therapeutic options for CRAB remain severely limited: sulbactam (which has intrinsic antibacterial activity against Acinetobacter through direct PBP1 and PBP3 binding) combined with durlobactam (a novel DBO inhibitor that protects sulbactam from OXA hydrolysis) has been approved as sulbactam-durlobactam (Xacduro) for CRAB infections. Other agents used in combination regimens include polymyxins (colistin, polymyxin B), high-dose ampicillin-sulbactam, and tetracyclines (minocycline, tigecycline); evidence for clinical superiority of any specific regimen remains limited given the difficulty of conducting randomized trials in this population.12
Optimal CRE therapy requires knowing the specific carbapenemase before initiating definitive treatment. For KPC-CRE: ceftazidime-avibactam, meropenem-vaborbactam, or imipenem-relebactam are all options; combinations vary by drug interaction profile and institutional availability. For NDM-CRE: aztreonam-avibactam or aztreonam plus ceftazidime-avibactam; cefiderocol may be used as salvage. For OXA-48-CRE: ceftazidime-avibactam covers OXA-48 but not metallo-enzymes; confirm phenotype before relying on it. For CRAB (OXA-23/OXA-58): sulbactam-durlobactam is first approved targeted agent; cefiderocol is an alternative. Genotypic confirmation by PCR or whole-genome sequencing is the standard of care before definitive therapy.
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