Carisoprodol

Carisoprodol (brand name Soma) is a centrally acting skeletal muscle relaxant approved by the U.S. Food and Drug Administration to help relieve discomfort associated with acute, painful musculoskeletal conditions.

It is prescribed to alleviate muscle spasms and associated pain, often in conjunction with rest and physical therapy in adults.

Carisoprodol works by interrupting neuronal communication within the reticular formation and spinal cord, producing muscle relaxation and an increased range of motion; it is metabolized in the liver to the active compound meprobamate.

The primary gene involved in carisoprodol metabolism is CYP2C19, which encodes the cytochrome P450 2C19 enzyme responsible for converting carisoprodol into its active metabolite meprobamate and facilitating drug clearance.

Variations in the CYP2C19 gene can alter enzyme activity, affecting how quickly or slowly carisoprodol and its metabolite are processed; for example, certain genetic variants reduce enzyme function, leading to higher drug levels and an increased risk of adverse effects.

Individuals can be classified into phenotype groups—ultrarapid, rapid, normal, intermediate, and poor metabolizers—based on CYP2C19 activity; these phenotypes influence carisoprodol exposure, with poor metabolizers having higher systemic concentrations and increased risk of sedation and central nervous system side effects, while ultrarapid and rapid metabolizers typically process the drug efficiently, similar to normal metabolizers.

The clinical impact of CYP2C19 phenotypes on carisoprodol response can be summarized as follows:

Ultrarapid Metabolizer

• Effect on drug levels: Similar or slightly lower exposure due to faster clearance.
• Clinical consequence: Standard therapeutic response at usual doses.
• Side effects: Typical sedation and dizziness; mild to moderate, common occurrence.

Rapid Metabolizer

• Effect on drug levels: Normal exposure, with efficient metabolism.
• Clinical consequence: Standard efficacy and safety at recommended dosing.
• Side effects: Common drowsiness and headache; generally mild.

Normal Metabolizer

• Effect on drug levels: Expected exposure consistent with clinical trials.
• Clinical consequence: Usual onset of muscle relaxation and pain relief.
• Side effects: Common sedation, dizziness, and nausea; mild to moderate.

Intermediate Metabolizer

• Effect on drug levels: Slightly increased exposure due to reduced metabolism.
• Clinical consequence: Potential for enhanced sedation or prolonged effect.
• Side effects: Increased drowsiness and dizziness; mild to moderate.

Poor Metabolizer

• Effect on drug levels: Higher systemic concentrations due to slow clearance.
• Clinical consequence: Elevated risk of adverse reactions and prolonged sedation.
• Side effects: Pronounced CNS depression (excessive drowsiness, respiratory depression risk); moderate severity, less frequent.

Indeterminate/Not Available

• Effect: Unknown
• Clinical consequence: No specific guidance; follow standard dosing with clinical monitoring.
• Side effects: As per general population; variable presentation.

CYP2C19 Dosing Guideline

Phenotype	Clinical Consequence	Guideline Recommendation
Ultrarapid Metabolizer	No FDA guidance; exposure may be slightly lower but within normal range	Initiate standard starting dose
Rapid Metabolizer	No FDA guidance; normal metabolism expected	Initiate standard starting dose
Normal Metabolizer	No FDA guidance; normal metabolism expected	Initiate standard starting dose
Intermediate Metabolizer	No FDA guidance; slightly reduced clearance	Initiate standard starting dose
Poor Metabolizer	Higher systemic concentrations; increased risk of adverse reactions	Consider dose reduction and monitor closely
Likely Intermediate Metabolizer	Higher systemic concentrations; increased risk of adverse reactions	Consider dose reduction and monitor closely
Likely Poor Metabolizer	Higher systemic concentrations; increased risk of adverse reactions	Consider dose reduction and monitor closely
Indeterminate	Unknown impact	Initiate standard starting dose
Not available	Unknown impact	Initiate standard starting dose

While specific pharmacogenetic alternatives for carisoprodol are not detailed in FDA guidance, clinicians may consider other centrally acting muscle relaxants such as cyclobenzaprine, methocarbamol, or baclofen based on patient tolerance and clinical judgment. These examples are for informational purposes only and are not direct recommendations.

• FDA – Table of Pharmacogenetic Associations