Effect of μ-opioid receptor A118G polymorphism on the ED50 of epidural sufentanil for labor analgesia

BackgroundA common polymorphism of the μ-opioid receptor gene (OPRM1, p.118A/G), which has been shown to effect the response to neuraxial opioids, occurs in 30% of Caucasian women. This double-blind up-down sequential allocation study was designed to examine the effect of p.118A/G on the ED50 of epidural sufentanil for labor analgesia.MethodsNulliparous women were recruited at 35 weeks of gestation (n = 77) and genotyped for p.118A/G. Those subsequently requesting epidural labor analgesia were enrolled. Each woman received epidural sufentanil diluted with 0.9% saline to a volume of 5 mL. The initial sufentanil dose was 21 μg, with subsequent doses determined by the response of the previous patient (testing interval 1 μg). Efficacy was accepted if the visual analogue score decreased to <10 mm on a 100-mm scale within 30 min of drug administration.ResultsTwenty patients were excluded, leaving 57 women from whom data were analyzed: 33 in Group A (wild-type A118 homozygotes) and 24 in Group G (heterozygotes and homozygotes G118). The ED50 for epidural sufentanil was 25.2 μg in Group A (95% CI 23.2–26.4) and 20.2 μg in Group G (95% CI 14.2–23.6) (P = 0.03). The potency ratio for epidural sufentanil in Group G compared to Group A was 1.25 (95% CI 1.00–1.64).ConclusionWomen carrying the variant allele of p.118A/G of OPRM1 (G118) had a lower ED50 for epidural sufentanil given for early labor analgesia than women homozygous for the wild-type allele.