Table 8:
Carbapenems
  
Imipenem/Cilastatin
(Primaxin)
Meropenem
(Merrem)
Ertapenem
(Invanz)
Mechanism of action Inhibits bacterial cell wall synthesis. Cilastatin blocks metabolism of imipenem. Inhibits bacterial cell wall synthesis Inhibits bacterial cell wall synthesis
Spectrum of activity Gram-negative
(including P aeruginosa); Gram-positive (including some enterococci); not MRSA; Anaerobes including
Bacteroides fragilis		 Gram-negative (including P aeruginosa); Gram-positive (including some enterococci); not MRSA; Anaerobes including
Bacteriodes fragilis		 Gram-negative—
not P aeruginosa;
Gram-positive, not enterococci, not MRSA; Anaerobes including
Bacteriodes fragilis
Half-life 1 h 1-1.5 h 4h
Metabolism In kidney by dehydropeptidase. Blocked by cilastatin Minor hepatic to inactive metabolite Minor hepatic to inactive metabolite
Excretion 70% unchanged in urine 75% unchanged in urine 38% unchanged in urine; 37% inactive metabolite; 10% feces
Clinical side effects GI, phlebitis, neurologic (seizures < 1%), hypotension Headache, rash, GI, phlebitis, hypotension Headache, rash, GI, phlebitis, hypotension
Laboratory abnormalities Increased LFTs, other rare events Increased LFTs, other rare events Increased LFTs, other rare events
Mechanism of drug interactions No CYP-450 interactions. Probenecid increases levels No CYP-450 interactions. Probenecid increases levels No CYP-450 interactions. Probenecid increases levels.
Other considerations Significant dosage adjustment in renal insufficincy Dose adjustment necessary in renal insufficiency Minor dose adjustment necessary in renal insufficiency
Approved indications Intra-abdominal infection, lower respiratory tract infection, UTI, gynecologic infection, bacterial septicemia, bone, joint, and skin infections, endocarditis, polymicrobic infections Intra-abdominal infection, bacterial meningitis Intra-abdominal infection, skin and skin structure, community-acquired pneumonia, UTI, pelvic infections
Usual dose* 500 mg q6h, adjusted for renal function 1 g q8h, adjusted for renal function 1 g once daily, adjusted for renal function
* Check product information for the specific dosing that corresponds with the specific indication.
MRSA = methicillin-resistant Staphylococcus aureus, GI = gastrointestinal, LFT = liver function test, UTI = urinary tract infection.

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