Combinatorial pharmacogenetic interactions of bucindolol and β1, α2C adrenergic receptor polymorphisms

Abstract

Background: Pharmacogenetics involves complex interactions of gene products affecting pharmacodynamics and pharmacokinetics, but there is little information on the interaction of multiple genetic modifiers of drug response. Bucindolol is a β-blocker/sympatholytic agent whose efficacy is modulated by polymorphisms in the primary target (β(1) adrenergic receptor [AR] Arg389 Gly on cardiac myocytes) and a secondary target modifier (α(2C) AR Ins [wild-type (Wt)] 322-325 deletion [Del] on cardiac adrenergic neurons). The major allele homozygotes and minor allele carriers of each polymorphism are respectively associated with efficacy enhancement and loss, creating the possibility for genotype combination interactions that can be measured by clinical trial methodology.

Methodology: In a 1,040 patient substudy of a bucindolol vs. placebo heart failure clinical trial, we tested the hypothesis that combinations of β(1)389 and α(2C)322-325 polymorphisms are additive for both efficacy enhancement and loss. Additionally, norepinephrine (NE) affinity for β(1)389 AR variants was measured in human explanted left ventricles.

Principal findings: The combination of β(1)389 Arg+α(2C)322-325 Wt major allele homozygotes (47% of the trial population) was non-additive for efficacy enhancement across six clinical endpoints, with an average efficacy increase of 1.70-fold vs. 2.32-fold in β(1)389 Arg homozygotes+α(2C)322-325 Del minor allele carriers. In contrast, the minor allele carrier combination (13% subset) exhibited additive efficacy loss. These disparate effects are likely due to the higher proportion (42% vs. 8.7%, P = 0.009) of high-affinity NE binding sites in β(1)389 Arg vs. Gly ARs, which converts α(2C)Del minor allele-associated NE lowering from a therapeutic liability to a benefit.

Conclusions: On combination, the two sets of AR polymorphisms 1) influenced bucindolol efficacy seemingly unpredictably but consistent with their pharmacologic interactions, and 2) identified subpopulations with enhanced (β(1)389 Arg homozygotes), intermediate (β(1)389 Gly carriers+α(2C)322-325 Wt homozygotes), and no (β(1)389 Gly carriers+α(2C)322-325 Del carriers) efficacy.

Trial registration: ClinicalTrials.gov NCT00000560.

Conflict of interest statement

Competing Interests: Drs Bristow, Carr, Murphy, and Port and Mr Davis are employees of and own stock or stock options in ARCA biopharma, Inc., which owns the rights to bucindolol. Drs Fiuzat, Liggett, Lindenfeld, and Robertson are consultants of ARCA biopharma. Also, Drs Fiuzat and Liggett own stock or stock options in ARCA biopharma. Drs O'Connor, Carson, Anand, Plehn, Gottlieb, Silver, Miller, White, Lazzeroni, and Abraham and Mr Walsh, Ms Nelson, and Mr Medway have no conflicts to report. This does not alter the authors' adherence to all the PLOS ONE policies on sharing data and materials.

Figures

Figure 1. Time to all-cause mortality or cardiac transplantation for Group 1/2 (A), Group 3 (B), and Group 4 (C), and time to heart failure progression (combination endpoint of heart failure mortality, cardiac transplantation, heart failure hospitalization, or emergency department care that includes intravenous therapy not requiring hospitalization) for Group 1/2 (D), Group 3 (E), and Group 4 (F).

Abbreviations: AC, all-cause; BUC, bucindolol; Del, deletion; HF, heart failure; PBO, placebo.

Figure 2. Representative competition curves between 50 pM 125[I]CYP and L-NE at increasing concentrations, in the absence and presence of 30 µM Gpp(NH)p in membranes from a non-failing human left ventricle with 77% β1 AR that was β1389 Arg/Arg genotype (A) and in membranes from a non-failing human heart with 77% β1 AR that was β1389 Arg/Gly genotype (B); mean±SEM (%) of high-affinity L-NE binding sites identified in seven β1389 Arg/Arg, five β1389 Arg/Gly, and five β1389 Gly/Gly left ventricles (C).

Abbreviations: AR, adrenergic receptor; CYP, cyanopindolol; Gpp(NH)p, non-hydrolyzable guanine nucleotide; KH, dissociation constant; KL, low-affinity binding constant; L-NE, L-norepinephrine; SEM, standard error measurement.