Carisoprodol is a centrally acting muscle relaxant used to relieve acute back pain and other rheumatology disorders. Carisoprodolol is a Schedule IV controlled substance and overdosing can lead to CNS (central nervous system) respiratory depression, seizures and death.

Carisoprodol is mainly metabolized by the CYP2C19 cytochrome. In individuals who have little or no CYP2C19 activity (“CYP2C19 poor metabolizers”), standard doses of carisoprodol can lead to a 4-fold increase in exposure to carisoprodol and the associated toxic effects compared with normal metabolizers.

Approximately 3- 5% of Caucasians and Africans, and 15 -20% of Asians, are CYP2C19 poor metabolizers.

The FDA-approved drug label for carisoprodol states that caution should be used when administering carisoprodol to patients with reduced CYP2C19 activity and when co-administering drugs that inhibit or induce CYP2C19.

The most common loss-of-function variant is CYP2C19*2 (named by rs4244285 code in clinical data bases), which contains a c.681G>A variant in exon 5 that results in an aberrant splicing site. Frequencies for this SNP are ~15% in Caucasians and Africans, and ~29–35% in Asians.

CYP2C19*3 (rs4986893) which contains a c.636G>A variant in exon 4 that causes a premature stop codon. The CYP2C19*3 allele frequencies are ~2–9% in Asian populations, but rare in other racial groups. Other loss-of-function variants occur in less than 1% of the general population and include CYP2C19*4-*8.

Homozygous individuals for the wild type allele are normal metabolizers of carisoprodol, however, heterozygous presenting one functional (or wild type) allele and one non-functional allele are slow metabolizers and the dose of carisoprodol prescribed should be controlled. Homozygous individuals for one of the non-functional alleles or compound heterozygous for two or more non-functional alleles are poor metabolizers and are at higuer risk of suffering severe side effects even death and the dose prescribed must be strictly adjusted or even a change of treatment is recommended.

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