Electrophysiologic properties of UK-66,914, a novel class III antiarrhythmic agent.

A class III antiarrhythmic agent that preferentially increases the effective refractory period without altering conduction velocity holds considerable promise for the treatment of life-threatening cardiac arrhythmias dependent on a reentrant mechanism. In the present study, the cellular electrophysiologic effects of a novel class III antiarrhythmic agent, UK-66,914, were evaluated. UK-66,914 prolonged action potential duration and extended the effective refractory period in isolated canine ventricular muscle and Purkinje fibers in a concentration-dependent manner, beginning at a threshold concentration of 0.1 microM. Analogous effects were found in isolated rabbit atrium beginning at a threshold concentration of 2 microM. At concentrations of UK-66,914 up to 20 microM there was no effect on the maximum rate of phase 0 depolarization (Vmax) or the amplitude of the action potential. In guinea pig papillary muscles. UK-66,914 at concentrations from 0.1 to 20 microM increased the effective refractory period at stimulation frequencies of 1 or 5 Hz, but did not slow conduction velocity. Therefore, UK-66,914 exhibits high selectivity for a class III antiarrhythmic effect in normal tissue. To elucidate the mechanisms responsible for the increase in effective refractory period, voltage clamp procedures were used in guinea pig ventricular myocytes. UK-66,914 reduced the amplitude of outward tail currents following depolarizing clamp steps with little effect either on the background K+ current or calcium currents, indicating that UK-66,914 selectively blocked the time-dependent potassium current. In anesthetized dogs, UK-66,914 (10 micrograms/kg to 1 mg/kg i.v.) prolonged both atrial and ventricular effective refractory periods, but in contrast to the studies performed in vitro, the minimum effective doses required to increase the effective refractory period in atria and ventricle were the same. Therefore, UK-66,914 is a potent selective class III antiarrhythmic agent, which owes its electrophysiologic profile to blockade of the time-dependent potassium current.

UK-68,798: a novel, potent and highly selective class III antiarrhythmic agent which blocks potassium channels in cardiac cells.

UK-68,798 increased the duration and effective refractory period of cardiac action potentials recorded in vitro from canine ventricular muscle and Purkinje fibers in a concentration dependent manner from 5 nM to 1 microM. The resting membrane potential, amplitude and maximum upstroke velocity of action potentials were unaffected by UK-68,798, indicating the selective class III antiarrhythmic properties of this agent. UK-68,798 (5 nM-1 microM) increased the effective refractory period of isolated guinea pig papillary muscles at stimulation frequencies of 1 Hz and 5 Hz without influencing the conduction velocity, further confirming that UK-68,798 is devoid of class I antiarrhythmic activity including block of the sodium channel. Studies using single voltage clamped guinea pig ventricular myocytes indicated that UK-68,798 at concentrations of 50 nM and 2 microM blocks a time-dependent K+ current, with no appreciable effects on the time-independent K+ current or the inward calcium current. UK-68,798 is therefore a highly selective K+ channel blocking agent with class III antiarrhythmic properties, a profile that holds considerable promise for the therapy of life-threatening cardiac arrhythmias.

Measurement of the class III antidysrhythmic drug, UK-68,798, in plasma by radioimmunoassay.

A sensitive radioimmunoassay (RIA) for the specific determination of 1-(4-methanesulphonamidophenoxy)-2-[N-(4-methanesulphonamido -phenethyl)-N- methylamino]ethane (UK-68,798), a novel class III antidysrhythmic agent, in human plasma is described. Specific antisera were raised in sheep using desmesyl-UK-68,798-succinate-ovalbumin conjugate as the antigenic hapten carrier protein. The antisera produced exhibited high specificity for UK-68,798 compared with known metabolites from animals, other antidysrhythmic agents and co-administered drugs. Good correlation was found in a comparison of the RIA method with a high-performance liquid chromatography (HPLC) method (r = 0.997) and a 10-fold lower limit of determination was observed for the RIA method compared with the HPLC method (0.05 and 0.5 ng ml-1, respectively). The RIA method was applied to the analysis of UK-68,798 in plasma obtained from human volunteers receiving the compound.

Long-acting dihydropyridine calcium antagonists. 6. Structure-activity relationships around 4-(2,3-dichlorophenyl)-3-(ethoxycarbonyl)-2-[(2-hydroxyethoxy)methyl]-5 -(methoxycarbonyl)-6-methyl-1,4-dihydropyridine.

The preparation of 4-(2,3-dichlorophenyl)-3-(ethoxycarbonyl)-2-[(2-hydroxyethoxy)methyl]-5- (methoxycarbonyl)-6-methyl-1,4-dihydropyridine (2) is described, and its potent calcium antagonist activity on rat aorta (IC50 = 4 x 10(-9) M) and marked tissue selectivity in vitro for vascular smooth muscle over cardiac smooth muscle are established. In order to exploit the excellent in vitro profile of compound 2, a range of analogues were prepared but none were found to have superior calcium antagonist potency and tissue selectivity. Compound 2 has excellent in vivo activity in the anesthetized dog (ED50 = 12 micrograms/kg for reduction of CVR) and a plasma half-life in the conscious dog of 7.2 h. The pharmacokinetic parameters of 2 are compared to those determined for the structurally related compounds amlodipine and felodipine. The plasma clearance for 2 (9.6 mL/min/kg) is similar to that of amlodipine and is consistent with the extended 2-substituent hindering approach to the cytochrome P-450 enzyme responsible for oxidation of the DHP ring to the corresponding pyridine.

Long-acting dihydropyridine calcium antagonists. 5. Synthesis and structure-activity relationships for a series of 2-[[(N-substituted-heterocyclyl)ethoxy]methyl]-1,4-dihydropyridine calcium antagonists.

The synthesis of a series of 1,4-dihydropyridines which have N-linked heterocycles at the terminus of an ethoxymethyl chain at the 2-position is described. The calcium antagonist activity on rat aorta of this class of DHPs is compared with their negative inotropic activity as determined by using a Langendorff-perfused guinea pig heart model. The compounds examined show a wide range of selectivity for vascular over cardiac tissue, with those analogues which possess an amide group at the terminus of the 2-substituent proving the most selective. From the in vitro data obtained for a series of 1,2,3-triazoles, it is possible to conclude that the SARs for binding to the calcium channels in vascular and cardiac tissue are different. One of the compounds, 2-amino-1-[2-[[4-(2,3-dichlorophenyl)-3-(ethoxycarbonyl)-5- (methoxycarbonyl)-6-methyl-1,4-dihydropyrid-2-yl]methoxy]ethyl]-4( 3H)- imidazolone (20b, UK-55,444), was identified as a potent (IC50 = 8 x 10(-9) M) calcium antagonist which is 40-fold selective for vascular over cardiac tissue and which has a significantly longer duration of action (greater than 3 h) than nifedipine in the anesthetized dog on intravenous administration.

Selective class III antiarrhythmic agents. 1 Bis(arylalkyl)amines.

A series of bis(arylalkyl)amines is described and their effects on prolonging effective refractory period in isolated cardiac tissue listed. Most compounds prolonged the cardiac action potential without significantly altering the maximum rate of depolarization and may be defined as selective class III antiarrhythmic agents. It was found that a particularly advantageous structural feature was to have a methanesulfonamido moiety on both of the aryl rings. Thus, compound 16 [1-(4-methanesulfonamidophenoxy)2-[N-(4-methanesulfonamidophene thyl)-N- methylamine]ethane] was selected for further investigations. The compound is highly potent and selective class III agent which acts by blockade of cardiac potassium channels.

Long-acting dihydropyridine calcium antagonists. 4. Synthesis and structure-activity relationships for a series of basic and nonbasic derivatives of 2-[(2-aminoethoxy)methyl]-1,4-dihydropyridine calcium antagonists.

The preparation of a series of 1,4-dihydropyridines (DHPs) which have polar, acyclic, nonbasic substituents on an ethoxymethyl chain at the 2-position is described. In addition, in order to assess the effects of incorporating a basic center into DHPs of this type, a series of glycinamides were also prepared. The calcium antagonist activity on rat aorta of both these classes of DHP is compared with their negative inotropic activity as determined by using a Langendorff perfused guinea pig heart model. A number of the compounds evaluated have activity of the same order as nifedipine although those with more extended substituents have lower potency, particularly when a basic substituent is present. The compounds examined displayed a wide variation in selectivity for vascular over cardiac tissue. A number of structure-activity relationship trends were identified and possible explanations to account for the differences in selectivity observed are advanced. One of the compounds, 2-[[2-[[4-(2-chlorophenyl)-3-(ethoxycarbonyl)-5-(methoxycarbonyl)-6- methyl-1,4-dihydropyrid-2-yl]methoxy]ethyl]amino]acetamide (26, UK-51,656), was identified as a potent (IC50 = 4 x 10(-9) M) calcium antagonist which is 20-fold selective for vascular over cardiac tissue and which has a markedly longer duration of action (greater than 5 h) than nifedipine in the anesthetized dog on intravenous administration. The pharmacokinetic half-life of 26 was established as 4.7 h and possible explanations are advanced to account for 26 having a shorter plasma half-life than amlodipine and a longer plasma half-life than felodipine.

Long-acting dihydropyridine calcium antagonists. 3. Synthesis and structure-activity relationships for a series of 2-[(heterocyclylmethoxy)methyl] derivatives.

The preparation of 1,4-dihydropyridines containing (heterocyclylmethoxy)methyl groups in the 2-position is described and the structural identification of certain of the compounds using 1H NMR spectroscopic methods is reported. The calcium antagonist activity of the compounds on rat aorta is listed and is compared with the negative inotropic potency as determined by using a Langendorff-perfused guinea pig heart model. Several compounds are more potent than nifedipine and show greater selectivity for the vasculature over the heart. One compound, 2-[(2-amino-4-hydroxypyrimidin-6-yl)methoxy]-4- (2,3-dichlorophenyl)-3-(ethoxycarbonyl)-5-(methoxycarbonyl)-6-methyl- 1,4-dihydropyridine (27, UK-56,593), was identified as a potent (IC50 = 1.6 x 10(-9) M), tissue-selective calcium antagonist which proved to have a markedly longer duration of action (greater than 4.5 h) than nifedipine in the anesthetized dog on intravenous administration.

Long acting dihydropyridine calcium antagonists. 2. 2-[2-Aminoheterocycloethoxy]methyl derivatives.

A series of [(2-aminoheterocycloethoxy)methyl]dihydropyridines were prepared as selective coronary vasodilators. Results showed that a wide variety of five- and six-membered heterocycles were acceptable at the 2-position of the dihydropyridine ring and in vitro potency and tissue selectivity was independent of the basicity of these heterocycles. The SAR indicated that activity was optimum when the largest ester group was placed at the 3 rather than 5 position. 2-[[2-[(3-Amino-1H-1,2,4-triazol-5-yl)amino]ethoxy]methyl]-4- (2,3-dichlorophenyl)-3-(ethoxycarbonyl)-5-(methoxycarbonyl)-6-methyl- 1,4-dihydropyridine (3b) (UK-52,831) emerged as a potent (IC50 = 6.3 X 10(-9) M) and tissue-selective calcium channel blocker with a duration of action greater than 7 h in the anaesthetized dog.

Calcium channel blocking properties of amlodipine in vascular smooth muscle and cardiac muscle in vitro: evidence for voltage modulation of vascular dihydropyridine receptors.

Amlodipine was twice as potent as nifedipine at inhibiting Ca2+-induced contractions in depolarised rat aorta (IC50 1.9 nM vs. 4.1 nM) but, unlike nifedipine, displayed a very slow onset of action. Contractions induced by depolarising steps with 45 mM K+ were much less potently blocked by amlodipine (IC50 19.4 nM), whereas the potency of nifedipine was little changed (IC50 7.1 nM). This difference may be explained by a modulated receptor hypothesis, similar to that described for cardiac muscle, in which block of vascular calcium channels by dihydropyridines is enhanced at depolarized membrane potentials, such voltage-dependence only being apparent with a slow-acting drug such as amlodipine. Recovery from amlodipine block of K+-responses in rat portal vein after drug washout was also very slow. Amlodipine and nifedipine blocked phenylephrine-induced contractions of the rat aorta with potencies similar to those against depolarisation-induced responses. Negative inotropic potencies of amlodipine and nifedipine in perfused guinea pig hearts were approximately one-tenth those against Ca2+-induced contractions in rat aorta. Amlodipine caused complete block of guinea pig papillary muscle single-cell slow action potentials at a concentration (5 microM) that had no effect on upstroke velocity of normal, fast potentials but reduced the duration of the plateau phase.

Long-acting dihydropyridine calcium antagonists. 1. 2-Alkoxymethyl derivatives incorporating basic substituents.

A series of dihydropyridines substituted at the 2-position by basic side chains are described and their potencies as calcium antagonists listed. One compound, 2-[(2-aminoethoxy)methyl]-4-(2-chlorophenyl)-3-ethoxycarbonyl-5- methoxycarbonyl-6-methyl-1,4-dihydropyridine (17, amlodipine) was found to be comparable in potency to nifedipine and to have an elimination half-life of 30 h in dogs. Oral bioavailability approached 100%, and hemodynamic responses were gradual in onset and long-lasting in effect. The two enantiomers have been prepared, and the bulk of the activity was found to reside with the (-) isomer, 18. X-ray crystallographic studies, carried out on a close analogue of 17, suggest the existence of a weak hydrogen bond between the side-chain oxygen and the proton on the ring nitrogen.

Selective thromboxane synthetase inhibitors. 4. 2-(1H-imidazol-1-ylmethyl) carboxylic acids of benzo[b]furan, benzo[b]thiophene, indole, and naphthalene.

The preparation of a series of 2-(1H-imidazol-1-ylmethyl)-substituted carboxylic acids of benzo[b]furan, benzo-[b]thiophene, indole, and naphthalene is described. All compounds showed a similar level of activity as TxA2 synthetase inhibitors in vitro, having IC50 values between 1 and 7 X 10(-8) M. In the cases examined, compounds had, at most, only negligible activity against PGI2 synthetase, cyclooxygenase, and steroid 11 beta-hydroxylase. The benzo[b]thiophenes generally showed the greatest potency in vivo, and compounds 72, 73, and 75 caused almost complete inhibition of thromboxane production for 6 h after oral administration of 0.5 mg/kg to conscious dogs. In the case of 73 and 75, thromboxane production was still inhibited by 80% after 24 h.

Selective thromboxane synthetase inhibitors. 3. 1H-imidazol-1-yl-substituted benzo[b]furan-, benzo[b]thiophene-, and indole-2- and -3-carboxylic acids.

The preparation of a series of 1H-imidazol-1-yl-substituted benzo[b]furan-, benzo[b]thiophene-, and indolecarboxylic acids is described. Most of the compounds were potent inhibitors of TxA2 synthetase in vitro, and the distance between the imidazole and carboxylic acid groups was found to be important for optimal potency. The most potent compound in vivo was 6-(1H-imidazol-1-ylmethyl)-3-methylbenzo[b]thiophene-2-carboxylic acid (71), which, in conscious dogs, showed a similar profile of activity to that of dazoxiben (1).

Selective thromboxane synthetase inhibitors. 2. 3-(1H-imidazol-1-ylmethyl)-2-methyl-1H-indole-1-propanoic acid and analogues.

The preparation of a series of 3-(1H-imidazol-1-ylmethyl)-1H-indole-1-alkanoic acids is described. Several compounds were found to be more potent thromboxane synthetase inhibitors than the corresponding analogues lacking an acidic substituent. In the cases examined, compounds had no significant activity against PGI2 synthetase or cyclooxygenase, and introduction of the carboxylic acid substituent led to a reduction in activity against adrenal steroid 11 beta-hydroxylase. Compound 21 strongly inhibited thromboxane formation after iv administration to anesthetized rabbits and oral administration to conscious dogs. The compound had a long duration of action, and marked inhibition of thromboxane production was observed 15 h after oral administration of 1 mg/kg to conscious dogs.

Selective thromboxane synthetase inhibitors. 1. 1-[(Aryloxy)alkyl]-1H-imidazoles.

1-(2-Phenoxyethyl)-1H-imidazole was found to be an inhibitor of thromboxane (TxA2) synthetase, but it also inhibited the adrenal cytochrome P-450 enzyme steroid 11 beta-hydroxylase. The preparation of a series of analogues is described, and activity against TxA2 synthetase, PGI2 synthetase, cyclooxygenase, and steroid 11 beta-hydroxylase is discussed. Potency against TxA2 synthetase was increased by introduction of a carboxyl group at a suitable distance from the imidazole ring. A distance of 8.1-8.8 A between N-1 of the imidazole and the carboxyl carbon was found to be optimal. Introduction of a carboxyl group also had the effect of reducing activity against steroid 11 beta-hydroxylase. The most potent and selective compound was found to be 4-[2-(1H-imidazol-1-yl) ethoxy]benzoic acid (14).

Cerebrovasodilatation through selective inhibition of the enzyme carbonic anhydrase. 3. 5-(Arylthio)-, 5-(arylsulfinyl)-, and 5-(arylsulfonyl)thiophene-2-sulfonamides.

A series of 5-(arylthio)-, 5-(arylsulfinyl)-, and 5-(arylsulfonyl)thiophene-2-sulfonamides is described and anticonvulsant activities are listed for the compounds. In most cases, the sulfones had the highest activity and the sulfides the least. Sulfones with 3- or 4-halo substituents generally had the highest activity, and one analogue, 5-[(4-fluorophenyl)sulfonyl]thiophene-2-sulfonamide (51, UK-17022), had an anticonvulsant ED50 fo 2 mg/kg when administered orally to mice. Compound 51 selectively increased cerebral blood flow in animals without an unacceptable level of diuresis.

3-(1-imidazolylmethyl) indoles: potent and selective inhibitors of human blood platelet thromboxane synthetase.

Several 3-(1-imidazolylmethyl) indoles were tested for inhibition of the microsomal enzymes which catalyse the biosynthesis of thromboxane A2, prostaglandin I2, and prostaglandin endoperoxides. These products were measured by bioassay to assess levels of enzyme activity. The highest activity against human blood platelet thromboxane A2-synthetase was obtained with 2-cyclopropyl-3(1-imidazolylmethyl) indole (IC50 1 x 10(-10) M). This compound also exhibited the highest activity against pig aorta prostaglandin I2-synthetase (IC50 8.4 x 10(-7) M). Of much more potential therapeutic interest, 2-isopropyl-3-(1-imidazolylmethyl) indole showed almost complete selectivity against thromboxane A2-synthetase. Both compounds exhibited IC50's of 2 x 10(-8) M against the latter enzyme but showed only weak effects (IC50's greater than 10(-4) M) against prostaglandin I2-synthetase and ram seminal vesicle PGH2-synthetase.

Promotion of carbohydrate oxidation in the heart by some phenylglyoxylic acids.

A series of phenylglyoxylic acids is described, many of which are able to promote carbohydrate oxidation in muscle tissue, thereby favorably altering the carbohydrate/fatty acid balance in situations where fatty acid utilization is elevated. Such situations are reported to occur in ischemic heart disease, particularly following myocardial infection. In an attempt to effectively deliver the phenylglyoxylic acids to the site of action within the cell, the L-(+)-phenylglycines were employed as prodrugs. These are known to be transaminated to phenylglyoxylic acids. L-(+)-2-(4-Hydroxyphenyl)glycine (25, oxfenicine) has been selected for clinical evaluation.

Cerebrovasodilatation through selective inhibition of the enzyme carbonic anhydrase. 2. Imidazo[2,1-b]thiadiazole and imidazo[2,1-b]thiazolesulfonamides.

A series of imidazo[2,1-b]thiadiazole and imidazo[2,1-b]thiazolesulfonamide carbonic anhydrase inhibitors is described and their anticonvulsant activities are listed. Many of the compounds have the same degree of ionization as acetazolamide and methazolamide, but their higher lipophilicity means that they are more able to penetrate into the central nervous system. One compound, 6-tert-butyl-2-sulfamoylimidazo[2,1-b]-1,3,4-thiadiazole (8, UK-15,454) had an anticonvulsant ED50 of 2.6 mg/kg when administered orally to mice. 8 selectively increased cerebral blood flow in animals without producing a high level of metabolic acidosis.