6-phenyl-1,4-dihydropyridine derivatives as potent and selective A3 adenosine receptor antagonists

Abstract

An approach to designing dihydropyridines that bind to adenosine receptors without binding to L-type calcium channels has been described. 1,4-Dihydropyridine derivatives substituted with beta-styryl or phenylethynyl groups at the 4-position and aryl groups at the 6-position were synthesized and found to be selective for human A3 receptors. Combinations of methyl, ethyl, and benzyl esters were included at the 3- and 5-positions. Affinity was determined in radioligand binding assays at rat brain A1 and A2A receptors using [3H]-(R)-PIA [[3H]-(R)-N6-(phenylisopropyl)adenosine] and [3H]CGS 21680 [[3H]-2-[[4-(2-carboxyethyl)phenyl]ethylamino]-5'- (N-ethylcarbamoyl)adenosine], respectively. Affinity was determined at cloned human and rat A3 receptors using [125I]AB-MECA [N6-(4-amino-3-iodobenzyl)-5'-(N-ethylcarbamoyl)-adenosine]. Structure-activity analysis indicated that substitution of the phenyl ring of the beta-styryl group but not of the 6-phenyl substituent was tolerated in A3 receptor selective agents. Replacement of the 6-phenyl ring with a 3-thienyl or 3-furyl group reduced the affinity at A3 receptors by 4- and 9-fold, respectively. A 5-benzyl ester 4-trans-beta-styryl derivative, 26, with a Ki value of 58.3 nM at A3 receptors, was > 1700-fold selective vs either A1 receptors or A2A receptors. Shifting the benzyl ester to the 3-position lowered the affinity at A3 receptors 3-fold. A 5-benzyl, 3-ethyl ester 4-phenylethynyl derivative, 28, displayed a Ki value of 31.4 nM at A3 receptors and 1300-fold selectivity vs A1 receptors. The isomeric 3-benzyl, 5-ethyl diester was > 600-fold selective for A3 receptors. Oxidation of 28 to the corresponding pyridine derivative reduced affinity at A3 receptors by 88-fold and slightly increased affinity at A1 receptors.

Figure 1.

Structures of 1,4-dihydropyridines as potent calcium channel antagonists (1-3) and an adenosine receptor antagonist (4). K_i values (μM) are shown for rat brain A₁ receptors (rA₁) and for cloned human A₃ receptors (hA₃).

Scheme 1.

General Procedure for the Synthesis of 1,4-Dihydropyridine Derivatives 9-22 and 24-31

Scheme 2.

General Procedure for the Synthesis of β-Keto Esters

Scheme 3.

Procedure for the Synthesis of Ethyl 3-Aminocinnamate

Scheme 4.

Method for Reduction of the Nitro Group on 1,4-Dihydropyridines

Scheme 5.

General Procedure for the Oxidation of 1,4-Dihydropyridine Derivatives Using Tetrachlorobenzoquinone, 45