Alan M. Ezrin

Inhibition of calmodulin and protein kinase C by amiodarone and other class III antiarrhythmic agents

SummaryClass III antiarrhythmic agents may prolong refractoriness via modulation of ion channels, which may be sensitive to Ca2+ regulatory proteins or enzymes. Accordingly, the purpose of this study was to quantitate the effects of several structurally diverse class III antiarrhythmic agents on calmodulin-regulated enzymes and protein kinase C activity, and to evaluate the ability of these agents and known calmodulin antagonists to prolong cardiac refractoriness in vivo. The rank order of potency (IC50;μM) of selected class III antiarrhythmic agents and reference calmodulin antagonists as inhibitors of calmodulin-regulated phosphodiesterase activity were: calmidazolium (0.12 μM)>amiodarone (0.62 μM)>desethylamiodarone (1.5 μM)>trifluoperazine (4.3 μM), bepridil (5 μM)>W-7 (7.5 μM), clofilium (13 μM). Similar concentration-related inhibition was evident in a second calmodulin-regulated system, inhibition of myosin light-chain phosphorylation and superprecipitation light-chain phosphorylation and superprecipitation of arterial actomyosin. Sotalol and tetraethylammonium were inactive at 100 μM. Protein kinase C activity was also inhibited by some of these agents; desethylamiodarone (IC50=11 μM) was more potent than the reference agent, H-7 (IC50=79 μM), or amiodarone (38% inhibition at 100 μM) and clofilium (32% inhibition at 100 μM). In vivo, the minimally effective doses required to increase ventricular effective refractory doses required to increase ventricular effective refractory periods in paced guinea pigs were (in mg/kg) bepridil, sotalol [1]>clofilium [3]>amiodarone [10]>W-7, desethylamiodarone [20]. No changes in refractory period were noted with maximum testable doses of calmidazolium or trifluoperazine. These studies show that some, but not all, class III antiarrhythmic agents are effective and potent calmodulin antagonists or protein kinase C inhibitors. Moreover, some calmodulin antagonists are effective at prolonging refractoriness in vivo. However, a lack of correlation between these agents suggests that these mechanisms are not solely responsible for the prolongation of refractoriness of all class III agents.

Calcium-Regulated Protein Kinases Low Km cGMP Phosphodiesterases: Targets for Novel Antihypertensive Therapy

The recent gain in knowledge over the last ten years on the intracellular mechanisms which regulate vascular smooth muscle tone has expanded opportunities for the potential discovery of novel vasodilator/antihypertensive agents. This review focuses on three intracellular enzyme systems: myosin light chain kinase (MLCK) and protein kinase C (PKC), which are Ca2+-regulated protein kinases implicated in the control of smooth muscle tone, and the cGMP phosphodiesterases (PDEs), which regulate the levels of cGMP in smooth muscle (Fig. 1).

Subchronic Oral and Intravenous (iv) Safety Evaluation and Pharmacokinetics in Rats and Dogs of Ipazilide Fumarate (Win 54, 177-4), an Antiarrhythmic Agent

Ipazilide fumarate (Win 54, 177-4) is a chemically novel antiarrhythmic agent that prolongs ventricular refractoriness and possesses antiectopic activity. Subchronic (29 days) nonclinical safety evaluation of ipazilide was conducted following oral and iv administration in Sprague-Dawley rats (20-320 mg/kg oral and 1.25-10 mg/kg iv) and 14 and 28 days in beagle dogs (3-30 mg/kg oral and 2.5-20 mg/kg iv). The pharmacokinetic parameters of ipazilide indicate that ipazilide is absorbed (tmax less than or equal to 1 hr) in fasted rats and dogs following single and repeated oral administrations. The apparent elimination half-life in the two species is approximately 1 hr (except in rats at a dosage of 320 mg/kg), suggesting rapid clearance. Increases in liver weights (rats 320 mg/kg) accompanied by the observation of centrilobular hypertrophy of hepatocytes were considered an expression of an adaptive metabolic response of the liver to ipazilide and may be associated with the induction of microsomal enzymes. Duodenal villous atrophy and epithelial hyperplasia (rats, 80 and 320 mg/kg) were interpreted to represent an irritant response to the drug. Local irritation was also observed at the injection site in rats and dogs. Dogs tolerated the oral and the iv administration of ipazilide at dosages of up to 30 and 20 mg/kg, respectively. Despite emesis (oral dogs), which was reduced in frequency following repeated treatment over several weeks, plasma levels in treated dogs (i.e., Cmax 4-5 micrograms/ml) were approximately twice that required to convert spontaneous arrhythmias in the conscious dog model 24 hr after myocardial infarction.(ABSTRACT TRUNCATED AT 250 WORDS)