Arg16 homozygosity of the β₂-adrenergic receptor improves the outcome after β₂-agonist tocolysis for preterm labor

Background: β₂-Adrenergic receptor (β₂AR) agonists are not consistently successful when administered as tocolytic therapy. The β₂AR displays genetic variability; an arginine-to-glycine substitution at codon 16 (Arg16Gly) has been shown to increase receptor desensitization in response to agonist exposure, whereas a substitution of glutamate for glutamine at codon 27 (Gln27Glu) decreases down-regulation. We have demonstrated that homozygosity for Arg16 protects against preterm delivery. Our goal was to determine whether β₂-agonists are more effective in women with the Arg16 genotype and preterm labor. Methods: Sixty white women with preterm labor between 24 and 34 weeks' gestation were treated for 48 hours with intravenous hexoprenaline. The effect of tocolysis and outcome of pregnancy were recorded. The β₂AR genotypes at codons 16 and 27 of ADRB2 were determined. A control group of 116 women delivered at term was also genotyped. Results: Preterm labor was not associated with β₂AR genotype at codon 16 (17% of patients with preterm labor were Arg16 homozygotes versus 19% of control subjects) or codon 27. Gestation was significantly prolonged in Arg16 homozygotes (median, 69 days; interquartile range, 63-79 days) compared with the other 2 genotypes (median, 58 days; interquartile range, 2-72 days) (P = .04). Tocolysis was 100% successful in delaying delivery for 48 hours in Arg16 homozygotes (n = 10), just failing to achieve statistical significance (P = .069). In contrast, only 37 of 50 women carrying 1 or 2 glycine alleles (74%) had delivery delayed by more than 48 hours with tocolysis. Neonatal outcomes were significantly better in babies born to mothers homozygous for arginine than in women with 1 or 2 Gly16 alleles. Conclusions: This is the first study examining the pharmacogenetics of β₂AR agonist therapy for preterm labor. It appears that Arg16 homozygosity improves pregnancy outcome after β₂-agonist tocolysis. The relatively low frequency of Arg16 homozygotes in our population limited the power of this investigation. Future assessments of tocolytic therapy may need to assess β₂AR genotype.