Bioavailability of Parenteral Meropenem
Meropenem administered parenterally (IV or IM) has essentially complete bioavailability approaching 100%, and does not require dose adjustment based on route of administration. 1, 2
Intravenous Administration
- Meropenem exhibits complete bioavailability when given intravenously, as it bypasses first-pass metabolism and enters systemic circulation directly. 3, 4
- The drug demonstrates linear pharmacokinetics with predictable concentration-time profiles regardless of whether administered as a 5-minute bolus injection or 30-minute infusion. 3
- Peak plasma concentrations (Cmax) after IV administration are dose-dependent: approximately 25.8 µg/mL for 500 mg, 53.5 µg/mL for 1000 mg, and 131.7 µg/mL for 2000 mg doses. 5
- The elimination half-life after IV administration is approximately 1 hour in adults with normal renal function. 2, 4
Intramuscular Administration
- IM administration of meropenem achieves bioavailability of approximately 99.69%, which is essentially equivalent to IV administration. 1
- Maximum plasma concentrations after IM injection are lower than IV (27.21 µg/mL vs 101.02 µg/mL for comparable doses), but the total drug exposure (AUC) remains nearly identical. 1
- The elimination half-life is slightly prolonged with IM administration (approximately 2.1 hours) compared to IV (1.35 hours), reflecting the absorption phase. 1
Subcutaneous Administration
- SC administration demonstrates bioavailability of 96.52%, which remains clinically equivalent to IV and IM routes. 1
- This route may be considered when IV access is difficult and IM injections are not feasible, though it is less commonly used in clinical practice. 1
Dose Adjustment Considerations
- No dose adjustment is required based on route of administration (IV vs IM), as bioavailability is essentially complete for both routes. 1, 2
- Dose adjustment is required for renal impairment, as meropenem undergoes primarily renal elimination with approximately 70% excreted unchanged in urine. 2, 4
- The standard adult dosing is 500 mg to 2000 mg every 8 hours IV, with specific adjustments needed when creatinine clearance falls below 50 mL/min. 6
Clinical Implications
- Meropenem can be administered by bolus IV injection over 5 minutes, IV infusion over 30 minutes, or IM injection without significant differences in overall drug exposure. 3, 2
- The choice between IV bolus and infusion does not appreciably alter disposition pharmacokinetics, plasma clearance, or terminal half-life. 3
- For critically ill patients or those with infections caused by organisms with higher MICs, prolonged or continuous infusions may be preferred to optimize the time above MIC (%T>MIC), which is the key pharmacodynamic parameter for beta-lactams. 6
Common Pitfalls to Avoid
- Do not assume oral formulations exist—meropenem is only available for parenteral administration due to instability in the acidic gastric environment. 2
- Avoid underdosing in patients with augmented renal clearance (common in younger critically ill patients), as standard doses may result in subtherapeutic concentrations. 6
- Remember that meropenem is stable to human dehydropeptidase-I and does not require co-administration with a DHP-I inhibitor like cilastatin (unlike imipenem). 2