When can you check a risperidone (Risperdal) long-acting injectable plasma level for Invega (paliperidone)?

When to Check Risperidone Long-Acting Injectable Plasma Levels for Invega

According to the FDA package insert, plasma levels for risperidone long-acting injectable should be checked after steady-state is achieved, which occurs after 4 injections or approximately 6-8 weeks after treatment initiation.

Understanding Risperidone Long-Acting Injectable Pharmacokinetics

Risperidone long-acting injectable (Risperdal Consta) has a unique pharmacokinetic profile that differs significantly from oral formulations:

• After initial injection, there is no immediate drug release 1
• Main release begins at weeks 2-3 post-injection 2
• Drug levels increase during weeks 3-4 2
• Levels are maintained during weeks 4-6 2
• Decline occurs between weeks 6-7 2
• With repeated injections every 2 weeks, steady-state levels are reached by weeks 6-8 2, 1

Optimal Timing for Plasma Level Monitoring

When monitoring risperidone long-acting injectable plasma levels:

• Wait until steady-state is achieved (after 4 injections) 1
• Trough levels should be measured just before the next scheduled injection
• Peak levels occur approximately 4-6 weeks after treatment initiation 3
• Therapeutic range is similar to oral risperidone plus 9-hydroxy-risperidone (active moiety): 20-60 ng/ml 4

Clinical Relevance of Plasma Level Monitoring

Plasma level monitoring may be particularly useful in specific clinical scenarios:

• Suspected non-response despite adequate dosing
• Concerns about medication adherence prior to injection
• Suspected drug-drug interactions affecting metabolism
• Patients with hepatic or renal impairment
• Patients experiencing unexpected side effects

Important Considerations

• Intra-individual variability in trough serum concentrations is approximately 30-35%, similar to oral risperidone 4
• Paliperidone (Invega) is the active metabolite of risperidone (9-hydroxy-risperidone)
• When interpreting levels, the active moiety (risperidone + 9-hydroxy-risperidone) should be considered 3
• D2 receptor occupancy correlates with plasma levels: 25-48% at 25mg dose, 59-83% at 50mg dose, and 62-72% at 75mg dose 3

Practical Approach to Monitoring

1. Initiate risperidone long-acting injectable at appropriate dose (typically 25mg every 2 weeks)
2. Continue oral supplementation for first 3 weeks after initial injection 2
3. Wait until steady-state is achieved (after 4 injections, approximately 6-8 weeks)
4. Draw blood sample immediately before next scheduled injection (trough level)
5. Compare results to therapeutic range of 20-60 ng/ml for the active moiety 4

This approach ensures accurate assessment of medication levels and helps optimize treatment outcomes while minimizing adverse effects.