Potential interactions of methylphenidate and atomoxetine with dextromethorphan

Area of Science:

• Pharmacology
• Drug Metabolism
• Cytochrome P450 Interactions

Background:

• Attention-deficit/hyperactivity disorder (ADHD) treatments like atomoxetine and dl-methylphenidate are commonly prescribed.
• Dextromethorphan serves as a probe substrate for evaluating cytochrome P450 2D6 (CYP2D6) enzyme activity.
• Understanding potential drug-drug interactions (DDIs) is crucial for patient safety and effective treatment.

Purpose of the Study:

• To investigate the potential for DDIs between ADHD medications (atomoxetine, dl-methylphenidate) and dextromethorphan.
• To determine if atomoxetine or dl-methylphenidate affect CYP2D6-mediated metabolism.
• To assess the mechanism of any observed interactions.

Main Methods:

• In vitro study using pooled human liver microsomes and recombinant CYP2D6 supersomes.
• Incubation of dextromethorphan with atomoxetine and dl-methylphenidate at varying concentrations.
• Measurement of dextromethorphan metabolism and assessment of inhibition by atomoxetine and dl-methylphenidate.

Main Results:

• Atomoxetine significantly inhibited dextromethorphan metabolism in both human liver microsomes and recombinant CYP2D6, suggesting a CYP2D6-mediated interaction.
• The inhibition by atomoxetine was competitive, as high dextromethorphan concentrations reversed the effect.
• Dl-methylphenidate did not inhibit dextromethorphan metabolism, and dextromethorphan did not affect dl-methylphenidate metabolism.

Conclusions:

• Atomoxetine has the potential to interact with dextromethorphan in vivo, likely through CYP2D6 inhibition in patients with ADHD.
• No significant in vivo interaction is predicted between dextromethorphan and dl-methylphenidate based on these findings.
• These results highlight the importance of considering CYP2D6 interactions when co-prescribing atomoxetine with CYP2D6 substrates.