Effects of a new antiarrhythmic drug, SD-3212, on canine ventricular arrhythmia models

Arrhythmia Models for Drug Research: Classification of Antiarrhythmic Drugs

The aim of this study was to classify antiarrhythmic drugs based on their effectiveness on 6 in vivo arrhythmia models, mainly using dogs. The models were produced by two-stage coronary ligation, digitalis, halothane-adrenaline, programmed electrical stimulation in old myocardial infarction dogs, coronary artery occlusion/reperfusion, or chronic atrioventricular block. Na(+)-channel-blocking drugs suppressed two-stage coronary ligation and digitalis arrhythmias. Ca(2+)-channel blockers and beta-blockers suppressed halothane-adrenaline arrhythmia. Positive inotropic drugs aggravated halothane-adrenaline arrhythmia, but did not aggravate digitalis arrhythmia. K(+)-channel blockers suppressed programmed electrical stimulation induced arrhythmia, but induced torsades de pointes type arrhythmia in chronic atrioventricular block dogs and aggravated halothane-adrenaline arrhythmia. Na(+)/H(+)-exchange blockers suppressed coronary artery occlusion/reperfusion arrhythmias. This classification may be useful for predicting the clinical effectiveness in the preclinical stage of drug development.

Induction of apoptosis by 1,4-benzothiazine analogs in mouse thymocytes

1,4-benzothiazine (1,4-B) derivatives exert numerous effects in vivo and in vitro, including neurotoxicity and antitumor cytotoxicity. To analyze the mechanisms responsible for 1,4-B-induced cytotoxicity, we performed experiments to evaluate the possible apoptotic effect. For that purpose, we used mouse thymocytes, a cell population well sensitive to induction of apoptosis that has been used to assay apoptosis in many experimental systems. Results indicate that a number of 1,4-B analogs are able to induce both thymocyte apoptosis in vitro and thymus cell loss in vivo. Moreover, analysis of the structure-activity relationship indicate that the sulfur (S) oxidation state, the presence of the carbonyl group, and the nature and position of the side chain modulate the apoptotic efficacy. Moreover, results of in vitro experiments show that the 1,4-B-induced apoptosis associates with different biochemical events including phosphatidylcholine-specific phospholipase C activation, acidic sphingomyelinase activation and ceramide generation, loss of mitochondrial membrane potential (DeltaPsi(m)) and cytochrome c release, and caspase-8, -9, and -3 activation. These results indicate that 1,4-B analogs induce apoptosis through a complex of biochemical events.

Antifungal and immunomodulating activities of 1,4-benzothiazine azole derivatives: review

We reviewed the studies on the in vitro and in vivo antifungal activity of 1,4-benzothiazine azole derivatives (1,4-BT). A number of different 1,4-BT have been tested for anti-Candida activity, investigating their N-4 substitution, sulfur oxidation state, presence of the carbonyl group in C-3, insertion of the side chain on C-6, C-7 or C-8 of benzothiazine nucleus, the nature of azolic substituent (triazole or imidazole), which tend to differ. Moreover, benzoxazine analogues have been tested to evaluate the effect of sulfur bioisosteric substitution on their activity. We found that their antifungal activity correlates with well-defined chemical characteristics including the presence of ether substitution at the side chain. In fact, ether derivatives are the most active compounds in vivo, although they have little anti-Candida effect in vitro. This discrepancy could be attributed to the fact that 1,4-BT are metabolized to active antifungal compounds and may have in vivo activity through improvement of protective immune response and direct antifungal effects. In fact, 1,4-BT also show immunomodulating activity so that the direct antifungal activity, in combination with the capability to stimulate the immune response, could result in a significant increase in in vivo efficacy.

SD3212, a new antiarrhythmic drug, raises atrial fibrillation threshold in isolated rabbit hearts

SD3212 is a new antiarrhythmic drug which has class I, III, and IV effects. The purpose of this study was to elucidate the electrophysiological effects of this compound on a rabbit atrial fibrillation model, and to test a hypothesis that atrial fibrillation threshold is a quantitative indicator of atrial vulnerability. Whole hearts were excised from rabbits, and the aortas cannulated to perfuse the coronary arteries. Atrial fibrillation was induced with a burst stimulation of 50 Hz for 1 s while 3 microM acetylcholine (ACh) was perfused. When the right atrial appendage was paced at 200-ms intervals, SD3212 prolonged interatrial conduction time: control 30 +/- 1.2 ms, ACh 33 +/- 1.4 ms, ACh + SD 1 microM 37 +/- 2.4 ms, ACh + SD 3 microM 52 +/- 8.1 ms. The drug also prolonged the effective refractory period: control 80 +/-3.0 ms, ACh 48 +/- 3.8 ms, ACh + SD 1 microM 65 +/- 4.7 ms, ACh + SD 3 microM 98 +/- 15 ms. The rate of induction of atrial fibrillation by rapid pacing was 26% in Tyrode's solution, 85% in the presence of ACh, and 38% in the presence of ACh + SD 1 microM. The atrial fibrillation threshold decreased from 8.6 +/- 0.8mA (control) to 2.5 +/- 0.7 mA in the presence of ACh. It increased again to 7.8 +/- 1.0 mA in the presence of SD3212 (1 microM). SD3212 prolonged both the conduction time and refractory period. A reversed use-dependency was not prominent. These features caused antifibrillatory effects. Thus, the atrial fibrillation threshold seems to be a good quantitative indicator of atrial vulnerability.

Binding study of semotiadil and levosemotiadil with alpha(1)-acid glycoprotein using high-performance frontal analysis

High-performance frontal analysis (HPFA) was used to investigate the binding properties of human alpha(1)-acid glycoprotein (AGP) with semotiadil ((R)-isomer, Ca-channel blocker) and its antipode levosemotiadil ((S)-isomer, Ca- and Na-channel blockers). An on-line HPLC system consisting of a HPFA column, an extraction column, and an analytical HPLC column was used to determine the unbound concentrations of these enantiomers, and the experimental data were subsequently subjected to the Scatchard analyses to estimate their binding parameters. The binding affinity of the (R)-isomer (K = 3.17 x 10(7) M, n = 0.74) is approximately 1.2 times stronger than that of (S)-isomer (K = 2.59 x 10(7) M, n = 0.74). An enantioselective competitive binding study indicated that both enantiomers are bound at the same site on AGP molecules.

Enantioselective protein binding of semotiadil and levosemotiadil determined by high-performance frontal analysis

An on-line frontal analysis HPLC system was developed for the determination of the unbound concentrations of semotiadil, a new calcium antagonist with non-dihydropyridine structure, and its antipode (Levosemotiadil), and was applied to the enantioselective investigation of their plasma protein binding properties. This system consists of a high-performance frontal analysis (HPFA) column, an extraction column, and an analytical column, which are connected via two switching valves. After the direct injection of the sample solution into the HPFA column, the drug was eluted as a zonal peak with a plateau region. The unbound drug concentration was determined as the drug concentration in the plateau. As low as 1.04 nM of the unbound drug was determined with good reproducibility. Semotiadil (R-isomer) and levosemotiadil (S-isomer) are bound strongly and enantioselectively to human serum albumin (HSA) and human alpha 1-acid glycoprotein (AGP), and the enantioselectivity was reversed between these plasma proteins. While HSA binds S-isomer more strongly than the antipode, human AGP binds R-isomer more strongly. In human plasma, the unbound drug fraction was less than 1%, and the enantioselectivity was similar to that observed in AGP solution.

Electrophysiologic effects of SD-3212, a new class I antiarrhythmic drug, on canine atrial flutter and atrial action-potential characteristics

SD-3212 (levo-semotiadil fumarate) is a newly developed compound that exhibits potent antiarrhythmic activity because of its inhibitory action on sodium and calcium channels. In animal models, SD-3212 suppressed ventricular tachyarrhythmias, but the effects of this drug on atrial tachyarrhythmias have not been reported. We investigated the electrophysiologic effects of SD-3212 on canine atrial flutter induced after placement of the intercaval obstacle and on atrial action-potential characteristics. In all seven dogs, SD-3212 (1.9 +/- 0.3 mg/kg) terminated atrial flutter after significant increase in atrial flutter cycle length from 126 +/- 5 to 166 +/- 14 ms (increase, 31 +/- 8%; p < 0.005). SD-3212 increased right atrial effective refractory period (RAERP) significantly from 126 +/- 7 to 149 +/- 11 ms at a basic cycle length of 300 ms. The increases in RAERP after SD-3212 at basic cycle lengths of 300, 200, and 150 ms did not differ (increase, 18 +/- 4%, 17 +/- 3%, and 19 +/- 3%, respectively). Interatrial conduction time (IACT) was prolonged after SD-3212 from 63 +/- 4 to 81 +/- 6 ms (increase, 31 +/- 6%) at a basic cycle length of 150 ms. Prolongation of IACT was frequency dependent. The plasma concentration of SD-3212 after the termination of atrial flutter was 187 +/- 56 ng/ml in four dogs tested. In vitro study by using standard microelectrode techniques showed SD-3212 at concentrations of 1-3 microM significantly prolonged action-potential duration at 90% repolarization. Vmax was decreased by SD-3212 in a concentration-dependent manner (0.3-3 microM), and the inhibitory effect on Vmax was greatest at the highest stimulation frequency of 3.3 Hz. These results indicate that a new antiarrhythmic drug, SD-3212, is effective in interrupting canine atrial flutter, possibly by suppressing atrial conduction, and might be effective for the treatment of clinical atrial tachyarrhythmias.

SD-3212, a new class I and IV antiarrhythmic drug: a potent inhibitor of the muscarinic acetylcholine-receptor-operated potassium current in guinea-pig atrial cells

1. By use of patch-clamp techniques, the effects of SD-3212, a novel antiarrhythmic drug, on the calcium current (Ica), the sodium current (INa) and the muscarinic acetylcholine-receptor-operated potassium current (IK.ACh) were examined and compared with those of bepridil in guinea-pig single atrial cells. 2. SD-3212 inhibited ICa and INa in a concentration-dependent manner. The IC50 values of SD-3212 for inhibition of ICa and INa were 1.29 microM and 3.92 microM, respectively. The steady state inactivation curves of ICa and INa were shifted in the hyperpolarizing direction in the presence of 1 microM SD-3212. Similar inhibition of ICa and INa was also observed with bepridil. The IC50 values of bepridil for depression of ICa and INa were 1.55 microM and 4.43 microM, respectively. 3. The muscarinic acetylcholine-receptor-operated potassium current (IK.ACh) was activated by the extracellular application of 1 microM carbachol in the GTP-loaded cells or by the intracellular loading of GTP gamma S, a nonhydrolysable GTP analogue. SD-3212 potently inhibited the carbachol- and GTP gamma S-induced IK.ACh and the IC50 values were 0.38 microM and 0.20 microM, respectively. These IC50 values were very close and about 10 times lower than those for inhibiting ICa and INa. Bepridil also suppressed the carbachol- and GTP gamma S-induced IK.ACh with the IC50 values of 0.69 microM and 0.84 microM, respectively. 4. In guinea-pig atrial cells stimulated at 0.2 Hz, carbachol at a concentration of 1 microM markedly shortened action potential duration. Both SD-3212 (0.1-1 microM) and bepridil (1-10 microM) reversed the action potential shortening in a concentration-dependent manner. The antagonizing effect of SD-3212 on the carbachol-induced action potential shortening was more potent than that of bepridil. 5. These results suggest that SD-3212 inhibits IK.ACh by depressing the function of the potassium channel itself and/or associated GTP-binding proteins. SD-3212 is a unique antiarrhythmic drug, which potently inhibits IK.Ach in addition to its class I and IV effects. SD-3212 and bepridil may be useful for the termination and prevention of vagally-induced atrial flutter and fibrillation.

Electrophysiological effects of SD-3212, a new antiarrhythmic agent with vasodilator action, on guinea-pig ventricular cells

1. The effects of SD-3212 on transmembrane action potentials were examined in right ventricular papillary muscles and in single ventricular myocytes isolated from guinea-pig hearts. 2. In papillary muscles, SD-3212 > or = 3 microM caused a significant decrease in the maximum upstroke velocity (Vmax) of action potential without affecting resting membrane potential. The inhibition of Vmax was enhanced at higher stimulation frequencies. 3. In the presence of SD-3212, trains of stimuli at rates > or = 0.5 Hz led to a use-dependent inhibition of Vmax. The time constant for the recovery of Vmax from the use-dependent block was 1.3 s. 4. Voltage-dependence of Vmax inhibition by SD-3212 was investigated in single myocytes. The curves relating membrane potential and Vmax were shifted by SD-3212 (10 microM) in a hyperpolarizing direction by 6.2 mV. 5. In myocytes treated with SD-3212 (10 microM), the Vmax of test action potentials preceded by conditioning clamp to 0 mV was decreased progressively as the clamp pulse duration was prolonged. Vmax of test action potentials following a long (1 s) 0 mV clamp recovered at a time constant ranging from 1.01 to 1.22 s, being shorter at the more negative potential within a range from -70 to -90 mV. 6. These findings suggest that the primary electrophysiological effect of SD-3212 is a use- and voltage-dependent inhibition of sodium channels. From the onset and offset kinetics of the use-dependent block, SD-3212 is located between fast and intermediate kinetic Class-I drugs. From the state-dependence of sodium channel block, SD-3212 belongs to inactivated channel blockers.