Assessment of the enantiomeric purity of R- and S-N6-phenylisopropyladenosine (PIA): implications for adenosine receptor subclassification.

A chiral column high-performance liquid chromatographic method was developed for the assessment of the enantiomeric purity of the stereoisomers of N6-phenylisopropyladenosine (PIA). The observed chiral purity of R-PIA was greater than 99.9%, whereas S-PIA was found to contain 4.4% of the R-enantiomer. In radioligand binding studies, the observed affinity of S-PIA for the adenosine A1 receptor (IC50 240 nM) could entirely be attributed to its content of R-PIA (IC50 7.8 nM). Calculation of a theoretical IC50 of pure S-PIA for the A2 receptor yielded a value of 6700 nM, which was 35-fold higher than for R-PIA (190 nM). Concludingly, the utilization of enantiomeric impure S-PIA in the definition of adenosine receptor subclasses is questionable.