Synthesis and calcium channel antagonist activity of dialkyl 1,4-dihydro-2,6-dimethyl-4-[4-(1-methoxycarbonyl-1,4-dihydropyridyl)]-3 ,5- pyridinedicarboxylates.

A novel class of dialkyl 1,4-dihydro-2,6-dimethyl-4-[4-(1- methoxycarbonyl-1,4-dihydropyridyl)]-3,5-pyridinedicarboxylates (8-14) were synthesized and evaluated as calcium channel antagonists. The differences in activity among members of this new class of compounds was less than one log unit (IC50 range of 1.12 x 10(-6) to 8.57 x 10(-6) M), relative to the reference drug nifedipine (IC50 = 1.43 x 10(-8) M). The small differences in potency, irrespective of the size of the dialkyl (Me, Et, i-Pr, i-Bu) ester substituents, is attributed to the fact that the N-CO2Me substituent is too far removed from the C-3 and C-5 ester substituents to undergo non-bonded steric interactions. The 4-[4-(1-methoxycarbonyl-1,4-dihydropyridyl) moiety in this new class of compounds is bioisosteric with a C-4 4-nitrophenyl, or a 4-pyridyl, substituent in classical 1,4-dihydropyridines.

Synthesis, rotamer orientation, and calcium channel modulation activities of alkyl and 2-phenethyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(3- or 6-substituted-2-pyridyl)-5-pyridinecarboxylates.

A group of racemic alkyl and 2-phenethyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(3- or 6-substituted-2-pyridyl)-5-pyridinecarboxylates (13a-q) was prepared using a modified Hantzsch reaction that involved the condensation of a 3- or 6-substituted-2-pyridinecarboxaldehyde (7a-j) with an alkyl or 2-phenethyl 3-aminocrotonate (11a-d) and nitroacetone (12). Nuclear Overhauser (NOE) studies indicated there is a significant rotamer fraction in solution where the pyridyl nitrogen is oriented above the 1,4-dihydropyridine ring, irrespective of whether a substituent is located at the 3- or 6-position. A potential H-bonding interaction between the pyridyl nitrogen free electron pair and the suitably positioned 1,4-dihydropyridine NH moiety may stablize this rotamer orientation. In vitro calcium channel antagonist and agonist activities were determined using guinea pig ileum longitudinal smooth muscle (GPILSM) and guinea pig left atrium (GPLA) assays, respectively. Compounds having an i-Pr ester substituent acted as dual cardioselective calcium channel agonists (GPLA)/smooth muscle-selective calcium channel antagonists (GPILSM), except for the C-4 3-nitro-2-pyridyl compound which exhibited an antagonist effect on both GPLA and GPILSM. In contrast, the compounds with a phenethyl ester group, which exhibited antagonist activity (IC50 = 10(-5)-10(-7) M range) on GPILSM, were devoid of cardiac agonist activity on GPLA. Structure-activity relationships showing the effect of a substituent (Me, CF3, Cl, NO2, Ph) at the 3- or 6-position of a C-4 2-pyridyl moiety and a variety of ester substituents (Me, Et, i-Pr, PhCH2CH2-) upon calcium channel modulation are described. Compounds possessing a 3- or 6-substituted-2-pyridyl moiety, in conjuction with an i-Pr ester substituent, are novel 1,4-dihydropyridine calcium channel modulators that offer a new drug design approach directed to the treatment of congestive heart failure and may also be useful as probes to study the structure-function relationships of calcium channels.

Synthesis and calcium channel-modulating effects of alkyl (or cycloalkyl) 1,4-dihydro-2,6-dimethyl-3-nitro-4-pyridyl-5-pyridinecarboxylate racemates and enantiomers.

A group of racemic alkyl (or cycloalkyl) 1,4-dihydro-2,6- dimethyl-3-nitro-4-(2-, 3-, or 4-pyridyl)-5-pyridinecarboxylate isomers (6-14) were prepared using a modified Hantzsch reaction that involved the condensation of nitroacetone with an alkyl (or cycloalkyl) 3-aminocrotonate and 2-, 3-, or 4-pyridinecarboxaldehyde. Determination of their in vitro calcium channel-modulating activities using guinea pig ileum longitudinal smooth muscle (GPILSM) and guinea pig left atrium (GPLA) assays showed that the 2-pyridyl isomers acted as dual cardioselective calcium channel agonists (GPLA)/smooth muscle selective calcium channel antagonists (GPILSM). In contrast, the 3-pyridyl and 4-pyridyl isomers acted as calcium channel agonists on both GPLA and GPILSM. In the C-4 2-pyridyl group of compounds, the size of the C-5 alkyl (or cycloalkyl) ester substituent was a determinant of GPILSM antagonist activity where the relative activity profile was cyclopentyl and cyclohexyl > t-Bu, i-Bu, and Et > MeOCH2CH2 > Me. The point of attachment of the C-4 pyridyl substituent was a determinant of GPLA agonist activity where the potency order was generally 4- and 3-pyridyl > 2-pyridyl. (+)-Cyclohexyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-pyridyl)-5- pyridinecarboxylate [(+)-14a] was a less potent calcium antagonist (IC50 = 5.27 x 10(-6) M) than the (-)-enantiomer (IC50 = 7.48 x 10(-8) M) on GPILSM. In the GPLA assay, (+)-14a exhibited a much more potent agonist effect (EC50 = 8.45 x 10(-6) M) relative to the marginal agonist effect produced by (-)-14a. The C-4 2-pyridyl compounds (enantiomers) constitute a novel type of 1,4-dihydropyridine calcium channel modulator that could provide a new drug design concept directed toward the treatment of congestive heart failure, and for use as probes to study the structure-function relationships of calcium channels.

Syntheses, calcium channel agonist-antagonist modulation activities, and voltage-clamp studies of isopropyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-pyridinylpyridine-5-carboxylate racemates and enantiomers.

A novel group of racemic isopropyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-pyridinylpyridine-5-carboxylate isomers [(+/-)-12-14] were prepared using a modified Hantzsch reaction that involved the condensation of nitroacetone with isopropyl 3-aminocrotonate and 2-, 3-, or 4-pyridinecarboxaldehyde. Determination of their in vitro calcium channel-modulating activities using guinea pig ileum longitudinal smooth muscle (GPILSM) and guinea pig left atrium (GPLA) assays showed that the 2-pyridinyl isomer (+/-)-12 acted as a dual cardioselective calcium channel agonist (GPLA)/smooth muscle selective calcium channel antagonist (GPILSM). In contrast, the 3-pyridinyl [(+/-)-13] and 4-pyridinyl [(+/-)-14] isomers acted as calcium channel agonists on both GPLA and GPILSM. The agonist effect exhibited by (+/-)-12 on GPLA was inhibited by nifedipine and partially reversed by addition of extracellular Ca2+. In anesthetized rabbits, the 4-pyridinyl isomer (+/-)-14 exhibited a hypertensive effect that was qualitatively similar to that exhibited by the nonselective agonist Bay K 8644 and the 3-pyridinyl isomer (+/)-13, whereas the 2-pyridinyl isomer (+/-)-12 induced a hypotensive effect similar to that of the calcium channel antagonist nifedipine. Similar results were obtained in a spontaneously hypertensive rat model. In vitro studies showed that the (+)-2-pyridinyl enantiomer (+)-12A exhibited agonist activity on both GPILSM and GPLA, but that the (-)-2-pyridinyl enantiomer (-)-12B exhibited agonist activity on GPLA and antagonist activity on GPILSM. Whole-cell voltage-clamp studies using isolated guinea pig ventricular myocytes indicated that (-)-12B inhibited the calcium current (ICa), that (+)-12A increased slightly ICa, and that (+/-)-12 inhibited ICa but the latter inhibition was less than that for (-)-12B. (-)-12B effectively inhibited ICa at all membrane potentials examined (-40-50 mV), whereas (+)-12A exhibited a weak agonist effect near the peak of the I-V curve. The 2-pyridinyl isomers (enantiomers) 12 represent a novel type of 1,4-dihydropyridine calcium channel modulator that could provide a potentially new approach to drug discovery targeted toward the treatment of congestive heart failure and probes to study the structure-function relationships of calcium channels.

Specific inhibition of Ca-activated K channels in red cells by selected dihydropyridine derivatives.

1. Thirty two dihydropyridine derivatives were screened as potential inhibitors of the Ca-activated K-channel in human red cells. 2. Three derivatives (26, 29, 32 see Tables 1 and 2) with high activity were then characterized in detail, and also tested against the smooth muscle Ca-channel and shown to have varying potencies. 3. One of the more potent derivatives (32) and nitrendipine were also tested on the Ca-activated K-channel, Maxi-K channel, from mouse pancreatic beta-cells. 4. We conclude from our results that it may be possible to develop selective Gardos-channel inhibitors based on these molecules, which may be of benefit in the treatment of sickle cell disease.

Enantioselective syntheses and calcium channel modulating effects of (+)- and (-)-3-isopropyl 5-(4-methylphenethyll) 1,4-dihydro-2,6-dimethyl-4-(2-pyridyl)-3,5-pyridinedicarboxylates.

The (+)- and (-)-enantiomers of 3-isopropyl 5-(4-methylphenethyl) 1,4-dihydro-2,6-dimethyl-4-(2-pyridyl)-3,5-pyridinedicarboxylate were synthesized using an efficient highly enantioselective (ee > or = to 96%) variant of the Hantzsch dihydropyridine synthesis. The key step in this procedure involved the asymmetric Michael addition of a metalated chiral aminocrotonate, derived from D-valine or L-valine, respectively, to the Knoevenagel acceptor (Z)-2-isopropoxycarbonyl-1-(2-pyridyl)-but-1-en-3-one. Both enantiomers exhibited a dual cardioselective partial calcium channel agonist (positive inotropic)/smooth muscle selective calcium channel antagonist effect. The relative in vitro smooth muscle calcium channel antagonist activities of the (-):(+) enantiomers was 26:1. In contrast, the (+)-enantiomer exhibited a greater in vitro positive inotropic effect on guinea pig left atrium where the contractile force was maximally increased by 14.8% at a concentration of 1.63 x 10(-8)M.

Synthesis and beta-adrenergic antagonist activity of heterocyclic propanolamines.

The synthesis and beta-adrenergic antagonist activities of a group of 1-(heteroaryloxy)- and 1-(heteroaryl)-3-alkylamino-2-propanols is described. beta 1-Adrenolytic (atria) structure-activity correlations indicated that replacement of the 1-naphthyl moiety of propranolol by a 4-quinolyl- (10), 2-quinolyl- (24), or 2-pyrimidyl- (26) heterocyclic moiety resulted in a 10-16 fold reduction in activity. The 1-(4-quinolyloxy)- (10) and 1-(2-quinolyloxy)- (24-25) heteroaryl moieties provided more potent activity than the structurally related 1-[2-(2H-isoquinolin-1-one)]- (31-32) or 1-[3-(3H-quinazolin-4-one)]- (35-36) moieties. All compounds tested exhibited weak beta 2-adrenolytic activity on trachea. Although the most potent beta 1-antagonist, 1-(2-pyrimidyloxy)-3-isopropylamino-2-propanol (26), was 13-fold less potent than metoprolol, it exhibited a beta 1/beta 2 selectivity ratio superior to the cardioselective metoprolol.

Synthesis and calcium channel antagonist activity of dialkyl 1,4-dihydro-2,6-dimethyl-4-[3-(1-methoxy-carbonyl-4-substituted-1,4- dihydro-pyridyl)]-3,5-pyridinedicarboxylates.

A novel class of dialkyl 1,4-dihydro-2,6-dimethyl-4-(3-[1- methoxycarbonyl-4-(H, Me, n-Bu or Ph)-1,4-dihydropyridyl])-3,5-pyridinedicarboxylates (3-29) were synthesized and evaluated as calcium channel antagonists using the muscarinic receptor mediated Ca2+ dependent contraction of guinea pig ileal longitudinal smooth muscle (GPILSM). The differences in activity among members of this new class of compounds was less than one log unit (IC50 range of 8.25 x 10(-6) to 4.36 x 10(-7) M), relative to the reference drug nifedipine (IC50 = 1.43 x 10(-8) M). Compounds possessing symmetrical C-3(5) diethoxycarbonyl ester substituents generally exhibited optimum activity. The R3 substituent (H, Me, n-Bu, Ph) at the 4-position of the 4-[3-(1-methoxycarbonyl-1,4-dihydropyridyl)] moiety was a determinant of activity. In symmetrical diester compounds, a R3 H substituent provided optimum activity for Me, i-Bu and t-Bu dialkyl ester analogues, whereas a R3 Ph substituent provided optimum activity for Et and i-Pr symmetrical diesters. The test results indicate the 4-[3-(1-methoxycarbonyl-4-substituted-1,4-dihydropyridyl)] substituent in this new class of compounds is bioisosteric with a 4-(3-nitrophenyl), or a 4-(3-pyridyl) substituent in classical 1,4-dihydropyridine antagonists. Compounds possessing large symmetrical dialkyl ester substituents (i-Bu, t-Bu), in conjunction with a 4-[3-(1-methoxycarbonyl-4-methyl-1,4-dihydropyridyl)] substituent, permanently inhibited recovery of the KCl-induced response in GPILSM.

Synthesis and beta-adrenergic antagonist activity of novel (3-oxazolinyl-1,4-dihydropyridyl) propanolamines.

A novel class of 1-(1-[4-phenyl(n-butyl or methyl)-3-(4,4-dimethyloxazolin-2-yl)-1,4- dihydropyridyl])-3-tert-butyl(or isopropyl)amino-2-propanols (7-12) were synthesized for evaluation as beta-adrenergic antagonists. Replacement of the naphthyloxy moiety of propranolol by a 1-[1-(4-n-butyl)-3-(4,4-dimethyloxazolin-2-yl)-1,4-dihydropyrid yl] group resulted in a significant decrease in cardiac beta 1-adrenergic antagonist activity which indicates that this group is not a suitable isostere for an aryloxy moiety. 1-(1-[4-n-Butyl-3-(4,4-dimethyloxazolin-2-yl)-1,4-dihydropyridy l])-3- isopropylamino-2-propanol (10) showed a modest beta 2-adrenergic antagonist selectivity for trachea (beta 2/beta 1 = 3:1).

Synthesis and cardioselective beta-adrenergic antagonist activity of quinolyloxypropanolamines.

The synthesis and beta-adrenergic antagonist activities of (2R,2S)-, (2S)-, (2R)-1-(5-quinolyloxy)-3-alkylamino-2-propanols (10-21) and (2R,2S)-1-(5-isoquinolyloxy)-3-alkylamino-2-propanols (24-27) is described. beta 1-Adrenolytic (atria) structure-activity correlations indicated the N-alkyl substituent was a determinant of activity where the relative potency order was i-Pr and t-Bu > cyclohexyl. Compounds having the (2S)-configuration were more potent than the corresponding (2R)-analogs. The position of the heteroaryl nitrogen atom influences potency since the quinolyl analogs were ten-fold more active than the corresponding isoquinolyl isomers. All compounds in both the quinolyl and isoquinolyl series exhibited weak beta 2-adrenolytic activity (trachea), and high beta 1/beta 2 selectivity ratios when the N-alkyl substituent was i-Pr or t-Bu. The quinolyl ring system is an excellent bioisostere for the aryl moiety in aryloxypropanolamines since (2R,2S)- or (2S)-1-(5-quinolyloxy)-3-isopropyl (or t-Bu) amino-2-propanols exhibit very potent and highly cardioselective beta 1-adrenergic antagonist activities.

Synthesis and beta-adrenergic antagonist activity of novel (3-cyanodihydropyridyl)propanolamines.

The synthesis and beta-adrenergic antagonist activities of 1-(1-dihydropyridyl)-3-alkylamino-2-propanols (9-16) in which the aryloxy group of aryloxypropanolamines (1) is replaced by a 1-[2-t-(n-)butyl-3-cyano-1,2-dihydropyridyl] (9-12) or 1-[6-t-(n-)butyl-3-cyano-1,6-dihydropyridyl] (13-16) moiety is described. These replacements resulted in a substantial reduction in beta 1 (atria) and beta 2 (trachea) adrenergic antagonist activities relative to the reference drug metoprolol. Structure-activity correlations indicate the relative potency order is 1,2-dihydropyridyl greater than 1,6-dihydropyridyl, n-Bu greater than t-Bu for dihydropyridyl substituents, and that this class of compounds are non-selective beta-antagonists with a beta 1/beta 2 selectivity ratio close to unity.

Nucleoside uptake by red blood cells from a primitive vertebrate, the Pacific hagfish (Eptatretus stouti), is mediated by a nitrobenzylthioinosine-insensitive transport system.

Red blood cells from the Pacific hagfish (Eptatretus stouti) were found to possess a facilitated diffusion nucleoside transport system insensitive to inhibition by the nucleoside transport inhibitor nitrobenzylthioinosine (NBMPR). Uridine uptake by this route was saturable (apparent Km 0.14 mM; Vmax 2 mmol/l cells per h at 10 degrees C), inhibited by inosine and adenosine, and blocked both by the vasodilator dipyridamole and by the thiol-reactive agent p-chloromercuriphenylsulphonate. The properties of this carrier resemble closely those of NBMPR-insensitive nucleoside transport systems in some mammalian neoplastic cell lines and in rat red cells. The presence of this type of carrier in a primitive vertebrate suggests that such transporters have a broad biological distribution and that they pre-date or arose at an early stage of vertebrate evolution.

Evidence for bumetanide-sensitive, Na(+)-dependent, partial Na-K-Cl co-transport in red blood cells of a primitive fish.

Tracer uptake studies identified the major routes for K+ transport in hagfish red cells, resolving them into ouabain-sensitive, loop diuretic-sensitive, and residual components. The K1/2 values for ouabain, bumetanide, and furosemide were 10(-5), 6 x 10(-7), and 5 x 10(-6) M, respectively. The properties of the Na-K-Cl co-transporter were investigated further by varying K+, Na+, and Cl- concentrations. The measured K1/2 values were similar to those for human red cells. Finally, the stoichiometry of Na:K:Cl uptake was determined, giving 1:1 for K+:Cl-; in contrast, no significant Na+ flux could be measured, although Na+ content must be present for measurable bumetanide-dependent K+ or Cl- flux to occur. The Na-K-Cl transport therefore shows Na(+)-dependent KCl co-transport or partial flux of the system.

Synthesis and calcium channel antagonist activity of nifedipine analogues containing 4-pyridyl and 3-arylethyloxycarbonyl substituents.

A series of unsymmetrical 3-arylethyl 5-isopropyl ester analogues of nifedipine, in which the 2'-nitrophenyl group at the 4 position is replaced by 2'- or 3'-pyridyl, were prepared and evaluated as calcium channel antagonists. The point of attachment of the pyridyl substituent was a determinant of activity, 2'-pyridyl analogues always being more potent than corresponding 3'-pyridyl analogues. The introduction of a substituent at the para-position of the phenethyl group in 3-phenethyl ester analogues usually enhanced the activity. The most potent compound was 3-(4'-bromophenethyl) 5-isopropyl 1,4-dihydro-2,6-dimethyl-4-(2'-pyridyl)-3,5-pyridinedicarboxylate. It was 82-fold more potent than nifedipine, and it did not exhibit a negative inotropic effect on guinea pig left atrium. Desirable features in 1,4-dihydropyridine calcium antagonists of the unsymmetrical 3,5-diester type are therefore a 4-(2-pyridyl) substituent in conjunction with a hydrophobic 3-(4-substituted-phenethyl) ester substituent. The arylethyl ester and the 4-(2'-pyridyl) substituents appear to provide important interdependent contributions to the calcium channel antagonist activity.

Synthesis and calcium channel antagonist activity of 3-arylmethyl 5-isopropyl 1,4-dihydro-2,6-dimethyl-4-(pyridyl)-3,5-pyridinedicarboxylates.

Unsymmetrical aryl(heteroaryl)methyl isopropyl ester analogues of nifedipine, in which the 2-nitrophenyl group at C-4 is replaced by a 2- or 3-pyridyl substituent, were synthesized and evaluated as calcium-channel antagonists using guinea pig ileal longitudinal smooth muscle. The point of attachment of the C-4 pyridyl substituent was a determinant of activity where the relative potency order was 2-pyridyl greater than 3-pyridyl. Within the C-4 2-pyridyl series of compounds, and electronegative substituent such as a trifluoromethyl or bromo at the 4 position of the benzyl ester substituent or a nitrogen atom at the 1 position of a 4-pyridylmethyl ester substituent, enhanced activity relative to the unsubstituted benzyl ester analogue. In contrast, in the C-4 3-pyridyl class of compounds, a variety of aryl(heteroaryl)methyl ester substituents did not alter potency to any significant extent. A number of compounds in the C-4 2-pyridyl series possessing 4-pyridylmethyl, 4-trifluoromethylbenzyl, 4-bromobenzyl, and 3-pyridylmethyl ester substituents were approximately equipotent to nifedipine. The aryl(heteroaryl)methyl ester and C-4 2-pyridyl substituents therefore appear to provide important interdependent contributions to calcium-channel antagonist activity.

Synthesis and activity of 1,4-dihydropyridine analogues of histamine H2-receptor antagonists.

Compounds of the type Het-CH2-S-CH2-CH2-Y were prepared, in which Het was 2-, 3- or 4-pyridyl, and Y was a derivative of 2-cyano-1,1-iminodiamine or 2-nitro-1,1-ethenediamine in which the terminal nitrogen was incorporated into a 1,4-dihydropyridine ring (general structure 7; X = NCN or CHNO2). Pharmacological testing using the histamine-induced guinea pig atrial chronotropic response indicated that the pyridyl substituent position was a determinant of activity, the activity order within each isomeric series being usually 2-pyridyl greater than 3- and 4-pyridyl. There was no significant difference in activity between otherwise similar compounds derived from 2-cyano-1,1-iminodiamine (7, X = NCN) or 2-nitro-1,1-ethenediamine (7, X = CHNO2). All compounds had a substituent (R) attached to the C-4 position of the dihydropyridine ring, and the nature of the R substituent influenced the H2-antagonist activity, the relative activity order being usually n-Bu greater than Ph greater than Me. In general, the incorporation of the terminal nitrogen into a 1,4-dihydropyridyl ring system favoured biological activity, 1-(2-[(4-Pyridylmethylthio)ethylamino])-1-(1-[3-(4, 4-dimethyloxazolin-2-yl)-4-n-butyl-1,4-dihydropyridyl)-2- cyanoimine (7f) was the most potent H2-receptor antagonist exhibiting an activity approaching that of cimetidine.

Smooth muscle contractility and calcium channel density in hibernating and nonhibernating animals.

Hibernating animals consistently survive prolonged periods of cold with body temperatures near the freezing point. Previous studies have suggested that regulation of calcium influx may be a fundamental cellular mechanism for cold tolerance in hibernating species. The present study was undertaken to compare (i) the calcium dependence of contractility and (ii) [3H]nitrendipine binding in homogenates of ileal longitudinal smooth muscle from the nonhibernating guinea pig (Cavia porcellus) and a hibernator, the ground squirrel (Spermophilus richardsonii). The contractility studies indicate that both the activation threshold for calcium and the concentration-response curve were shifted to the right in ground squirrel when compared with guinea pig. The binding site density in ground squirrel muscle was about an order of magnitude less than in guinea pig (Bmax = 10 +/- 2 (n = 12) and 86 +/- 6 fmol/mg protein (n = 5), respectively). These results indicate that ground squirrel tissues are less sensitive to external calcium and clearly have fewer calcium channels than the smooth muscle of the non-hibernator. The results continue to support the hypothesis that cold tolerance in hibernating species involves calcium homeostatic control mechanisms.

Synthesis and calcium channel antagonist activity of alkyl cycloalkyl esters of nifedipine containing pyridinyl substituents.

Alkyl cycloalkyl esters of nifedipine (1a) analogues, in which the ortho-nitrophenyl group at position 4 is replaced by pyridinyl (5-19) were synthesized and evaluated as calcium channel antagonists using the muscarinic receptor-mediated Ca(2+)-dependent contraction of guinea pig ileal longitudinal smooth muscle. The relative activity profile for unsymmetrical esters (5-15), which indicated the effect of cycloalkyl ring size on activity, was cyclopentyl > cyclohexyl > cyclobutyl. In addition unsymmetrical esters, possessing one R2-cyclohexyl substituent, exhibited an activity profile dependent upon the nature of the R1-alkyl ester substituent with Me > Et, i-Pr, i-Bu > cyclohexyl > t-Bu. The point of attachment of the C-4 pyridinyl substituent was also a determinant of activity for unsymmetrical compounds (R1 = Me, i-Pr; R2 = cyclohexyl, cyclobutyl) where the relative potency order was 2-pyridinyl > 3-pyridinyl > 4-pyridinyl. In contrast, when the R1 and R2-substituents are larger in size (R1 = R2 = cyclohexyl or R1 = i-Pr, R2 = cyclopentyl) the relative activity profile was 3-pyridinyl = 4-pyridinyl > 2-pyridinyl. The C-3 and C-5 ester substituents therefore appear to provide important interdependent contributions to calcium channel antagonist activity, and hence to interaction with the 1,4-dihydropyridine receptor site.