If ertapenem (Invanz) was administered at 21:00 and then discontinued, when should meropenem (Merrem) be started?

When to Start Meropenem After Ertapenem Discontinuation

You can start meropenem immediately after discontinuing ertapenem—there is no required waiting period between these two carbapenems.

Rationale

The transition from ertapenem to meropenem does not require a washout period for several key reasons:

Pharmacokinetic Considerations

• Ertapenem has a half-life of approximately 4 hours in adults with normal renal function 1
• By 24 hours after the 21:00 dose, ertapenem levels are essentially undetectable (plasma concentrations drop to 1-2 mcg/mL by 24 hours) 1
• However, waiting for complete elimination is unnecessary because both drugs are beta-lactam carbapenems with similar mechanisms of action and no known antagonistic interactions

Clinical Practice Guidelines

The French Society guidelines on beta-lactam optimization in critically ill patients emphasize that when switching between beta-lactams, a loading dose should be administered immediately 2. This applies regardless of prior beta-lactam exposure because:

• The loading dose accounts for the increased volume of distribution in critically ill patients
• Immediate therapeutic levels are needed to maintain antimicrobial coverage
• There is no safety concern with overlapping carbapenem exposure

Practical Approach

Start meropenem now with a loading dose:

• Administer 1-2 g IV as a loading dose
• Follow with standard dosing: 1 g every 8 hours (or 2 g every 8 hours for severe infections like meningitis or Pseudomonas) 3
• Consider extended infusion (3-4 hours) or continuous infusion in critically ill patients for optimal PK/PD target attainment 2

Important Caveats

Stability Concerns

• Meropenem has limited stability (6-12 hours at 25°C depending on concentration) 2
• If using continuous infusion, prepare fresh solutions every 6-12 hours
• This is a key difference from ertapenem, which has better stability

Drug Interaction Alert

• If the patient is on valproic acid, avoid this transition entirely 4
• Both ertapenem and meropenem dramatically reduce valproic acid levels by 60-70% within 24 hours 4
• Ertapenem has a more pronounced effect than meropenem, but both cause clinically significant reductions
• Consider alternative antibiotics or alternative seizure medications

Renal Dosing

• Adjust meropenem dose if creatinine clearance <50 mL/min
• No adjustment needed for the initial loading dose 2

The key message: Start meropenem immediately without delay to maintain continuous antimicrobial coverage.