Correct Answer: C. Imipenem
Cilastatin is a dehydropeptidase-1 (DHP-1) inhibitor that is administered exclusively with imipenem to prolong its action and increase its bioavailability. Imipenem is a broad-spectrum β-lactam antibiotic that is rapidly metabolized by DHP-1 enzymes present in the proximal tubule of the kidney. Without cilastatin, approximately 70% of imipenem is inactivated by renal metabolism, resulting in subtherapeutic serum levels and reduced efficacy. Cilastatin competitively inhibits DHP-1, preventing imipenem degradation and allowing 70–90% of the drug to remain active. This combination (imipenem-cilastatin) is marketed as Primaxin in India and is the standard formulation. The 1:1 molar ratio of imipenem to cilastatin (typically 500 mg:500 mg) is critical for optimal efficacy. Cilastatin itself has no antimicrobial activity and serves purely as a pharmacokinetic enhancer. This combination is particularly valuable in treating serious gram-negative infections, anaerobic infections, and nosocomial infections where high serum levels are essential. The clinical significance is that cilastatin reduces the required imipenem dose while maintaining therapeutic concentrations, thereby reducing nephrotoxicity risk and improving cost-effectiveness in Indian clinical practice.
Why the other options are wrong
A. Meropenem — Meropenem is a carbapenem structurally similar to imipenem but is resistant to DHP-1 metabolism due to a methyl group at position 1 of the pyrrolidine ring. It undergoes minimal renal tubular degradation and achieves adequate serum levels without a dehydropeptidase inhibitor. Cilastatin is not used with meropenem. This is a common NBE trap—students confuse all carbapenems as requiring cilastatin. B. Piperacillin — Piperacillin is an extended-spectrum penicillin (not a carbapenem) that is metabolized via hepatic and renal mechanisms, but not by DHP-1. It is typically combined with tazobactam (a β-lactamase inhibitor) to extend its spectrum, not cilastatin. Cilastatin has no role in piperacillin pharmacokinetics. Students may confuse β-lactam combinations here. D. Cefazolin — Cefazolin is a first-generation cephalosporin that is renally excreted unchanged but is not metabolized by DHP-1. It requires no pharmacokinetic enhancer and is used as a single agent. The question specifically tests knowledge of the imipenem-cilastatin pairing, and cefazolin is included as a distractor for students unfamiliar with carbapenem metabolism.
High-Yield Facts
- Imipenem-cilastatin is the only carbapenem-DHP-1 inhibitor combination used clinically; meropenem and ertapenem do not require cilastatin.
- Cilastatin inhibits dehydropeptidase-1 in renal proximal tubules, preventing 70% renal inactivation of imipenem and increasing bioavailability from 5% to 75%.
- 1:1 molar ratio (e.g., 500 mg imipenem + 500 mg cilastatin) is the standard formulation; cilastatin has no antimicrobial activity.
- Primaxin (imipenem-cilastatin) is the Indian brand name; used for serious gram-negative, anaerobic, and nosocomial infections.
- Meropenem is resistant to DHP-1 due to a methyl substituent at position 1 of the pyrrolidine ring, making it the preferred carbapenem in meningitis and renal impairment.
Mnemonics
*I-C Rule: Imipenem needs Cilastatin Only Imipenem (the first carbapenem) requires Cilastatin as a DHP-1 inhibitor. Meropenem, ertapenem, and doripenem are structurally protected from DHP-1 degradation. Use this to quickly eliminate other carbapenems. DHP-1 Trap: Only Imipenem Gets Trapped Imipenem is trapped and degraded by DHP-1* in the kidney (70% loss). Cilastatin is the 'escape route' that blocks DHP-1. Other β-lactams don't use this pathway. Remember: Imipenem = Inactivated by DHP-1 → needs Inhibitor (cilastatin).
NBE Trap
NBE pairs imipenem with cilastatin to test whether students understand that not all carbapenems require DHP-1 inhibition. The trap is including meropenem (which does NOT need cilastatin) as a plausible option, exploiting the misconception that all carbapenems are metabolized identically. Students who memorize "carbapenems + cilastatin" without understanding the structural basis will fall for meropenem.
Clinical Pearl
In Indian tertiary care settings, imipenem-cilastatin (Primaxin) is reserved for serious nosocomial infections and polymicrobial sepsis where broad-spectrum coverage is critical. However, meropenem is increasingly preferred due to better CNS penetration and lack of seizure risk, making understanding the structural difference between these two carbapenems clinically essential for antibiotic stewardship.
_Reference: KD Tripathi Pharmacology Ch. 45 (β-lactam antibiotics); Harrison Principles of Internal Medicine Ch. 139 (Antimicrobial agents)_