J. Med. Chem. 1995,38, 2851-2859
2851
Syntheses, Calcium Channel Agonist-Antagonist Modulation Activities, and Voltage-Clamp Studies of Isopropyl 1,4-Dihydro-fL,G-dimethy1-3-nitro4-pyridinylpyridine-5-carboxylate Racemates and Enantiomers? Dean V O ,Wandikayi ~ C. Matowe,$Manian Ramesh,* Nadeem Iqbal,+Michael W. Wolowyk,*,§ Susan E. Howlett,l and Edward E. h a u s * , * Faculty of Pharmacy and Pharmaceutical Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2N8, and Department of Pharmacology, Dalhousie University, Halifan, Nova Scotia, Canada B3H 4H7 Received March 7, 1995*
A novel group of racemic isopropyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-pyridinylpyridine-5carboxylate isomers LW-12- 141 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 (5)-12acted as a dual cardioselectiue calcium channel agonist (GPLA)lsmooth muscle selective calcium channel antagonist (GPILSM). In contrast, the 3-pyridinyl [(f)-131and 4-pyridinyl [(f)-141isomers acted as calcium channel agonists on both GPLA and GPILSM. The agonist effect exhibited by (f)-12 on GPLA was inhibited by nifedipine and partially reversed by addition of extracellular Ca2+. In anesthetized rabbits, the 4-pyridinyl isomer (f)-14exhibited a hypertensive effect that was qualitatively similar to that exhibited by the nonselective agonist Bay K 8644 and the 3-pyridinyl isomer (f)-13, whereas the 2-pyridinyl isomer (d3-12induced a hypotensive effect similar to that of the calcium channel antagonist nifedipine. Similar results were obtained in a spontaneously hypertensive rat model. I n vitro studies showed that the (+)-2-pyridinyl enantiomer (+)-12A exhibited agonist activity on both GPILSM and GPLA, but that the (-1-2-pyridinyl enantiomer (-)-12B exhibited agonist activity on GPLA and antagonist activity on GPILSM. Whole-cell voltageclamp studies using isolated guinea pig ventricular myocytes indicated that (-)-12B inhibited the calcium current (Ita), that (+>-l2Aincreased slightly Ica, and that (5)-12inhibited Ica but the latter inhibition was less than that for (-)-12B. (-)-12B effectively inhibited Ica at all membrane potentials examined (-40-50 mV), whereas (+)-12Aexhibited 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. The design of cardioselective 1,4-dihydropyridine (DHP) calcium channel agonists (positive inotropes), that act at the L-type voltage dependent calcium channel, has provided a significant challenge to medicinal A number of 1,4-dihydropyridine calcium channel agonists such as the nitro compounds Bay K 8644 (1),5 PN 202-791 12, (+)-S-enanti~merI,~ and LY249933 (3,RJt-dia~tereomer),~ lactone CGP 28392 (4),8 5-unsubstituted H 160/51 (5),9J0 amide YC 170 (6),11J2dicarboxylic ester 7,13 and benzoylpyrrole FPL 64176 (8)14J5 have been reported (see Figure 1). Unfortunately, calcium channel agonists such as Bay K 8644 also increase calcium entry into vascular smooth muscle, causing vasoconstriction, which precludes their clinical use in treating congestive heart failure (CHF).16-21 Although extensive efforts have been undertaken to understand the molecular basis of action and improve the pharmacological profile of calcium channel agonists, t o the best of our knowledge, no cardioselective calcium channel stimulant has been reported to date.22 The Dedicated to the memory of Professor Michael W. Wolowyk. University of Alberta. Deceased January 31, 1992. Dalhousie University. @Abstractpublished in Advance ACS Abstracts, June 15, 1995. +
3 9
future clinical use of calcium channel agonists to treat CHF will therefore be dependent upon separating their vasoconstricting effects from their cardiostimulant prope r t i e ~ .The ~ ~ individual enantiomers of racemic 1,4dihydropyridine calcium channel agonists 1-7 often exert diametrically opposite effects or have vastly differing potencies on the same tissue. For example, Bay K 8644 is a racemate consisting of a (-)-$-enantiomer that acts as an agonist which is about 10-fold more potent as an activator than its (+)-R-antipode is as an a n t a g ~ n i s t .Consequently, ~~~~~ the racemate acts as a calcium channel a g o n i ~ t . The ~ ? ~two ~ enantiomers of PN 202-791 (2) show a 100-fold difference in activities favoring the (-)-R-antagonist, relative to the (+)-Sagonist e n a n t i ~ m e r .The ~ ~ R,R- and S,R-diastereomers of LY249933 (3)exhibit calcium agonist and antagonist effects, respectively.' Our goal was t o discover a dual cardioselective calcium channel agonisthascular selective smooth muscle calcium channel antagonist thirdgeneration DHP drug which would have a suitable therapeutic profile for treating CHF patients. In this report, the synthesis of racemic and chiral 1,CdihydroS,6-dimethyl-3-nitro-4-pyridinylpyridine-5-carboxylates, their in vitro calcium channel-modulating activities on guinea pig smooth muscle and heart, their cardiovascular effects on rabbit and spontaneously
0022-2623/95/1838-2851$09.00/00 1995 American Chemical Society
Vo et al.
2852 Journal of Medicinal Chemistry, 1995, Vol. 38, No. 15
COzhle OzN
0zN Ma
C02Et
COzPr-i OzN
I
I
I
H
H
H
3
2
1
H
Me
Me H
H
4
5
I
H
H
6
0
7
k
Figure 1. Some representative calcium channel agonists.
Scheme 1" P
9
10
11
A
(*)-12, R 2-pyridinyl (*)-13, R 3-pyridinyi (*)-14, R = 4-pyridinyl
Reagent and conditions: (a) EtOH, reflux, 12 h.
hypertensive rat, and their modulation of L-type voltage sensitive calcium channels in isolated guinea pig ventricular myocytes using the whole-cell voltage-clamp technique are described.
Chemistry The racemic isopropyl 1,4-dihydro-2,6-dimethyl-3nitro-4-pyridinylpyridine-5-carboxylates12-14 were prepared by a modified Hantzsch reaction. Thus, condensation of nitroacetone 11 with isopropyl 3-aminocrotonate (10)and either 2-, 3-, or Q-pyridinecarboxaldehyde (9) afforded the title compounds in 64-79% yields as illustrated in Scheme 1. The symmetrical 3,5diisopropyl1,4-dihydro-2,6-dimethyl-4-pyridinylpyridine3,5-dicarboxylateproduct, resulting from condensation of 9 with 10, was present as a minor product (
