Tetrahydrobenzothiophenone derivatives as a novel class of adenosine receptor antagonists

Abstract

A novel class of non-nitrogen-containing heterocycles, the tetrahydrobenzothiophenones, was found to bind to adenosine receptors as antagonists in the micromolar range. Affinity was determined in radioligand-binding assays at rat brain A1 and A2a receptors. A structure-activity analysis indicated that a 3-thioether group is favored and affinity at A2a, but not at A1, receptors is highly dependent on this thioether substituent. A carboxylic acid-derived substituent is required at the 1-position of the thiophene ring, with esters being more potent in binding at A1 receptors than the corresponding carboxyl hydrazide or carboxylic acid derivatives. The methyl (15) and ethyl (16) esters are about equipotent at A1 but not at A2a receptors. A 4-keto group on the saturated ring is favored for receptor affinity. Dimethyl substitution at the 6-position of the saturated ring is allowed. One of the most potent derivatives was the nonselective compound ethyl 3-(benzylthio)-4-oxo-4,5,6,7-tetrahydrobenzo[c] thiophene-1-carboxylate (BTH4, 7; Figure 1), which antagonized adenosine agonist-induced inhibition of adenylyl cyclase in rat adipocyte membranes with a KB value of 1.62 +/- 0.73 microM and adenosine agonist-induced stimulation of adenylyl cyclase in pheochromocytoma cell membranes with a KB value of 9.19 +/- 0.98 microM. Displacement of radioligand binding by BTH4 (7) at cloned human A3 receptors was negligible but one slightly A3 selective compound (11, 3.9-fold over A1 and >7.5-fold over A2a) was found. A 1-methylpropyl thioether (17) was 29-fold selective for A1 and A2a receptors. BTH4 (7) alone, at 10 mg/kg, stimulated locomotor activity in mice but paradoxically acted, under certain circumstances, synergistically with an A1 selective agonist to depress locomotor activity. A pharmacophore model relating structural features of xanthine and non-xanthine adenosine antagonists to BTH4 (7) suggests a high degree of similarity in electrostatic surfaces, assuming that the thiophene ring superimposes the region of the uracil ring of xanthines.

Figure 1.

Non-xanthine antagonists of adenosine receptors.

Figure 2.

(A) Effects of BTH₄ (7) on agonist-elicited inhibition of adenylyl cyclase via rat A₁ receptors in adipocyte membranes (mean ± SEM for n = 3): (O) (R)-PIA alone and (●) (R)-PIA + 3 μM BTH₄ (7). (B) Effects of BTH₄ (7) on agonist-elicited stimulation of adenylyl cyclase via A_2a receptors in PC12 cell membranes (mean ± SEM for n = 3): (▲) NECA alone and (Δ) NECA + 30 μM BTH₄ (7).

Figure 3.

(A) Locomotor effects of BTH₄ (7), caffeine, and CHA: (□) BTH₄ (7), (O) caffeine, and (◊) CHA (an asterisk indicates significantly different from vehicle at p < 0.02; mean ± SEM of at least 4 and maximally 14 experiments). (B) Locomotor effects of 10 mg/kg BTH₄ (7) and 10 mg/kg caffeine administered 10 min prior to varying doses of CHA: (□) BTH₄ (7), (O) caffeine, (◊) CHA alone (an asterisk indicates significantly different from CHA alone at p < 0.02; mean ± SEM of at least 4 and maximally 13 experiments). (C) Locomotor effects of BTH₄ (7) and caffeine coadministered with 100 μg/kg CHA: (Δ) vehicle, (0) BTH₄ (7) 10 min prior to CHA, (◊) CHA 10 min prior to BTH₄ (7), and (O) CHA 10 min prior to caffeine (an asterisk indicates significantly different from a single dose of 100 μg/kg CHA at p < 0.02, and double asterisks indicate significantly different from reverse order of administration at p < 0.02; mean ± SEM of at least 3 and maximally 14 experiments).

Figure 4.

(Top) van der Waals volumes of BTH₄ (7) and XAC. (Bottom) Isopotential surfaces of BTH₄ (7) and XAC (red = 5 kcal/mol, yellow = 0, and blue = −5 kcal/mol).

Figure 5.

Pharmacophore models for XAC, BTH₄ (7), CP66713, and PIQA, showing distances, in-plane angles, and dihedral angles. Point e is defined as the center of the aromatic ring; all other positions are defined by the atomic coordinates.