Meropenem

Drug Class & Structure

Meropenem is a carbapenem - a synthetic beta-lactam antibiotic and a derivative of thienamycin. Like other beta-lactams, it contains a beta-lactam ring, but the thiazolidine ring's sulfur atom has been externalized and replaced by a carbon atom. A key structural distinction from imipenem is that meropenem is not degraded by renal dehydropeptidase, so it does not require co-administration with cilastatin.

Mechanism of Action

Meropenem binds to penicillin-binding proteins (PBPs), disrupting bacterial cell wall synthesis and causing death of susceptible organisms. It is highly resistant to hydrolysis by most beta-lactamases (except carbapenemases and metallo-beta-lactamases, or MBLs).

Antimicrobial Spectrum

Meropenem's spectrum is broadly similar to imipenem, but it is slightly less active against gram-positives (especially Enterococcus) and more active against gram-negatives.

Category	Key Organisms Covered
Gram (+) cocci	S. aureus (not MRSA), Enterococcus faecalis, Streptococci, S. pneumoniae (including PCN-resistant)
Gram (+) bacilli	Listeria monocytogenes
Gram (-) rods	Enterobacterales (E. coli, Klebsiella, Enterobacter, Proteus, Serratia, Citrobacter), Pseudomonas aeruginosa, Acinetobacter spp.
Gram (-) cocci	Neisseria spp.
Anaerobes	Bacteroides fragilis, Clostridium, Fusobacterium, etc.
Other	Nocardia, some rapidly growing mycobacteria
NOT covered	MRSA, MRSE, Enterococcus faecium, carbapenemase-producing organisms, Spirochetes, Mycoplasma, Chlamydia

Pharmacokinetics (ADME)

Parameter	Detail
Route	IV only (not absorbed orally)
Distribution	Penetrates well into body tissues and fluids, including CSF - notably, meropenem reaches therapeutic CSF levels even without meningeal inflammation (unlike most beta-lactams)
Half-life	~1 hour
Elimination	Renally cleared via glomerular filtration; dose must be adjusted in renal insufficiency
No cilastatin needed	Meropenem is not hydrolyzed by renal dehydropeptidase
PD optimization	Standard infusion is 30 min; extended infusion over 3 hours increases time above MIC (T>MIC) and can help treat low-level resistant pathogens

Dosing

Indication	Dose
Standard (normal renal function)	1-2 g IV every 8 hours
Meropenem-vaborbactam	2 g meropenem + 2 g vaborbactam IV every 8 h
Renal impairment	Dose reduction required (based on CrCl)

Adverse Effects

Seizures - less likely than with imipenem; this is why meropenem is the preferred carbapenem for CNS infections/meningitis
Nausea, vomiting, diarrhea
Eosinophilia, neutropenia (less common)
Drug interaction: Meropenem (and carbapenems in general) significantly lowers serum valproic acid levels - the combination should be avoided as it can precipitate seizures in epileptic patients
Cross-reactivity with penicillins is very low (<1%) but use cautiously in patients with prior severe immediate-type penicillin allergy

Meropenem/Vaborbactam (Vabomere)

Vaborbactam is a boronic acid-based beta-lactamase inhibitor that restores meropenem's activity against many carbapenemase-producing organisms (particularly Class A enzymes like KPC), but not against metallo-beta-lactamase (MBL) producers. Reserved for multidrug-resistant gram-negative pathogens.

Therapeutic Uses

Indication	Notes
Nosocomial (hospital-acquired) infections	Respiratory, GI, urinary tract when cephalosporin- or penicillin-resistant organisms are suspected
CNS infections / Bacterial meningitis	Preferred carbapenem for meningitis (less proconvulsant, good CSF penetration)
Intra-abdominal infections	Excellent anaerobic + gram-negative coverage
Skin, soft tissue, bone/joint	Severe infections
Empiric coverage of resistant GNRs	Pseudomonas, Acinetobacter, carbapenem-susceptible ESBL producers
Meropenem-vaborbactam	Carbapenem-resistant Enterobacteriaceae (CRE) infections

Resistance Mechanisms

Carbapenemases (e.g., KPC, OXA-48): Hydrolyze the beta-lactam ring - most clinically important
Metallo-beta-lactamases (MBLs): NDM, VIM, IMP - not blocked by vaborbactam or other current beta-lactamase inhibitors (except cefiderocol, which remains active)
Porin loss + ESBL combination: Reduced uptake combined with enzyme activity
Efflux pumps: Particularly in Pseudomonas and Acinetobacter

Recent Evidence

A 2024 systematic review and meta-analysis (PMID 39273526) found that meropenem-vaborbactam demonstrated superior clinical outcomes compared to best-available therapy for carbapenem-resistant Enterobacteriaceae (CRE) infections. A 2024 systematic review (PMID 38971203) of PK/PD of novel beta-lactam combinations for carbapenem-resistant pneumonia supports extended infusion strategies to optimize meropenem target attainment.

Sources: Goodman & Gilman's Pharmacological Basis of Therapeutics; Lippincott Illustrated Reviews: Pharmacology; The Washington Manual of Medical Therapeutics; Harriet Lane Handbook 23rd ed.

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