The affinity and selectivity of α-adrenoceptor antagonists, antidepressants and antipsychotics for the human α2A, α2B, and α2C-adrenoceptors and comparison with human α1 and β-adrenoceptors.

Abstract

α2‐Adrenoceptors, subdivided into α2A, α2B, and α2C subtypes and expressed in heart, blood vessels, kidney, platelets and brain, are important for blood pressure, sedation, analgesia, and platelet aggregation. Brain α2C‐adrenoceptor blockade has also been suggested to be beneficial for antipsychotic action. However, comparing α2‐adrenoceptor subtype affinity is difficult due to significant species and methodology differences in published studies. Here, 3H‐rauwolscine whole cell binding was used to determine the affinity and selectivity of 99 α‐antagonists (including antidepressants and antipsychotics) in CHO cells expressing human α2A, α2B, or α2C‐adrenoceptors, using an identical method to β and α1‐adrenoceptor measurements, thus allowing direct human receptor comparisons. Yohimbine, RX821002, RS79948, and atipamezole are high affinity non‐selective α2‐antagonists. BRL44408 was the most α2A‐selective antagonist, although its α1A‐affinity (81 nM) is only 9‐fold greater than its α2C‐affinity. MK‐912 is the highest‐affinity, most α2C‐selective antagonist (0.15 nM α2C‐affinity) although its α2C‐selectivity is only 13‐fold greater than at α2A. There are no truely α2B‐selective antagonists. A few α‐ligands with significant β‐affinity were detected, for example, naftopidil where its clinical α1A‐affinity is only 3‐fold greater than off‐target β2‐affinity. Antidepressants (except mirtazapine) and first‐generation antipsychotics have higher α1A than α2‐adrenoceptor affinity but poor β‐affinity. Second‐generation antipsychotics varied widely in their α2‐adrenoceptor affinity. Risperidone (9 nM) and paliperidone (14 nM) have the highest α2C‐adrenoceptor affinity however this is only 5‐fold selective over α2A, and both have a higher affinity for α1A (2 nM and 4 nM, respectively). So, despite a century of yohimbine use, and decades of α2‐subtype studies, there remains plenty of scope to develop α2‐subtype selective antagonists.