Most clinically used opioids are mu-opioid receptor agonists. Therefore, genetic variation of the OPRM1 gene that encodes the mu-opioid receptor is of interest. An amino acid changing polymorphism 118A>G (rs1799971) within the OPRM1 gene affects the function of the receptor.
We studied the association between the 118A>G polymorphism and oxycodone analgesia and pain sensitivity in 1,000 women undergoing breast cancer surgery. Preoperatively, experimental cold and heat pain sensitivity was tested. Postoperative pain was assessed at rest and during motion. I.v. oxycodone analgesia was titrated first by a research nurse and on the ward using a patient controlled analgesia device. The primary endpoint was the amount of oxycodone needed for the first state of adequate analgesia. The 118A>G polymorphism was genotyped using Sequenom MassArray. The association between this variant and the pain phenotypes was tested using linear regression.
The 118A>G variant was significantly associated with the amount of oxycodone requested for adequate analgesia (P=0.001, β=0.016). Oxycodone consumption was highest in the individuals having the GG genotype (0.16 mgkg-1) , lowest in the AA-group (0.12 mg kg-¹) while the AG-group was in between (0.13 mg kg-¹). The G allele was also associated with higher postoperative baseline pain ratings (P=0.001, β=0.44). No evidence of association with the other examined pain phenotypes was seen.
