Beta₂‐adrenergic ligand racemic formoterol exhibits enantioselective disposition in blood and skeletal muscle of humans, and elicits myocellular PKA signaling at therapeutic inhaled doses

Abstract

Abstract While studies have demonstrated substantial differences in beta 2 ‐adrenergic agonist enantiomer pharmacology, enantioselective disposition of long‐acting beta 2 ‐adrenergic ligand racemic ( rac )‐formoterol in blood is inadequately explored after inhaled therapy given analytical challenges. Furthermore, information on enantioselective disposition and partitioning of beta 2 ‐adrenergic agonist in skeletal muscle is absent despite its promising data on muscle anabolism in humans. Using a sensitive ultra‐high performance liquid chromatography−mass spectrometry (UPLC−MS/MS) assay, we determined disposition of ( R,R )‐formoterol and ( S,S )‐formoterol in plasma and skeletal muscle samples from 11 non‐asthmatic men who had inhaled rac ‐formoterol at therapeutic doses (2 × 27 μg). Mean (SD) concentrations of ( R,R )‐ and ( S,S )‐formoterol in plasma and in muscle biopsies of the vastus lateralis 1 hour after inhalation of formoterol were 31 (15) and 45 (18) pg × mL −1 for ( R,R )‐formoterol and ( S,S )‐formoterol, respectively, in plasma, and 0.56 (0.32) and 0.51 (0.29) pg × mg wet wt −1 , respectively, in muscle. Formoterol exhibited different enantioselective disposition in plasma and muscle ( p < 0.0001). In plasma, mean log ( R,R ):( S,S )‐formoterol ratio was lower than 0 [−0.17(0.07), p < 0.0001], whereas in muscle, mean log ( R,R ):( S,S )‐formoterol ratio was slightly higher than 0 [0.04(0.07), p < 0.05]. Log ( R,R ):( S,S )‐formoterol ratio in muscle was related to muscle fiber‐type composition. Furthermore, formoterol induced an approximately two‐fold increase in muscle p‐PKA Ser/thr phosphorylation ( p < 0.01), indicating a substantial beta 2 ‐adrenergic response. Collectively, these findings suggest that formoterol exhibits modest enantioselective disposition in plasma after inhaled therapy in humans, which appear related to a greater ( R,R )‐enantiomer disposition in skeletal muscle that may be dependent on fiber‐type composition.