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, ³ H-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.

Keywords: affinity; antagonist; antidepressant; antipsychotic; hypertension; selectivity; α-adrenoceptor.

FIGURE 1

Inhibition of ³H‐rauwolscine binding to whole cells by BRL44408 (A–C), S32212 (D–F) or MK‐912 (G–I) to CHO‐α2A cells (A, D, G), CHO‐α2B cells (B, E, H) or CHO‐α2C cells (C, F, I). Bars represent total ³H‐rauwolscine and non‐specific binding (determined in the presence of 10μM RX821002. The concentration of ³H‐rauwolscine was (A) 0.99 nM, (B) 0.99 nM, (C) 0.99 nM, (D) 0.88 nM, (E) 0.88 nM, (F) 0.88 nM, (G) 0.86 nM, (H) 0.86 nM, and (I) 0.88 nM. Data points are mean ±s.e.mean of triplicate determinations

FIGURE 2

Inhibition of ³H‐rauwolscine binding to whole cells by dibenamine following pre‐incubation of dibenamine with sfm or 1mM thiosulphate to CHO‐α2A cells (A), CHO‐α2B cells (B), or CHO‐α2C cells (C). Bars represent total ³H‐rauwolscine binding and non‐specific binding as determined in the presence of 10 μM RX821002. The concentration of ³H‐rauwolscine was 0.74 nM in all cases. Data points are mean ±s.e.mean of triplicate determinations.

FIGURE 3

Inhibition of ³H‐rauwolscine (α2A, α2B, and α2C) or ³H‐CGP12177 (β1 and β2) binding to whole cells by (A–E) risperidone, (F–J) aripiprazole and (K–O) clozapine to CHO‐α2A cells, CHO‐α2B cells, CHO‐α2C cells, CHO‐β1 cells, CHO‐β2 cells. Bars represent total radioligand binding and non‐specific binding as determined in the presence of 10μM RX821002 (α2A, α2B, and α2C cells) or 10μM propranolol (β1 and β2 cells). The concentration of radioligand was (A) 0.54 nM, (B) 0.54 nM, (C) 0.54 nM, (D) 0.77 nM, (E) 1.00 nM, (F) 0.50 nM, (G) 0.50 nM, (H) 0.50 nM, (I) 0.72 nM, (J) 0.72 nM, (K) 0.50 nM, (L) 0.54 nM, (M) 0.54 nM, (N) 0.94 nM and (O) 0.72 nM. Data points are mean ±s.e.mean of triplicate determinations

FIGURE 4

Plot of log K_D values showing the relative selectivity and affinity for the single most selective ligand at each receptor. Thus SNAP5089 is the most α1A‐selective ligand and the length of the line represents the selectivity for α1A over the next closest adrenoceptor affinity. Terazosin, although the “most” α1B‐selective ligand has no selectivity. The selectivity of the three most selective α2 ligands is considerably less than that for α1A, α1D, β1, or β2. Compounds within the black circles represent compounds where the log K_D is greater than the −3, −4, or −5 stated but included here to demonstrate attempts were made measurement. Data for α1‐adrenoceptors are from. β3 data are included for CGP20712A and ICI118551 from