Kinetics of ajmaline disposition and pharmacologic response in beagle dogs

The Mechanism of Ajmaline and Thus Brugada Syndrome: Not Only the Sodium Channel!

Ajmaline is an anti-arrhythmic drug that is used to unmask the type-1 Brugada syndrome (BrS) electrocardiogram pattern to diagnose the syndrome. Thus, the disease is defined at its core as a particular response to this or other drugs. Ajmaline is usually described as a sodium-channel blocker, and most research into the mechanism of BrS has centered around this idea that the sodium channel is somehow impaired in BrS, and thus the genetics research has placed much emphasis on sodium channel gene mutations, especially the gene SCN5A, to the point that it has even been suggested that only the SCN5A gene should be screened in BrS patients. However, pathogenic rare variants in SCN5A are identified in only 20–30% of cases, and recent data indicates that SCN5A variants are actually, in many cases, prognostic rather than diagnostic, resulting in a more severe phenotype. Furthermore, the misconception by some that ajmaline only influences the sodium current is flawed, in that ajmaline actually acts additionally on potassium and calcium currents, as well as mitochondria and metabolic pathways. Clinical studies have implicated several candidate genes in BrS, encoding not only for sodium, potassium, and calcium channel proteins, but also for signaling-related, scaffolding-related, sarcomeric, and mitochondrial proteins. Thus, these proteins, as well as any proteins that act upon them, could prove absolutely relevant in the mechanism of BrS.

Effect of experimental renal dysfunction on bioavailability of ajmaline in rats

The effect of renal dysfunction on the bioavailability of ajmaline has been investigated in rats, where experimental renal dysfunction was induced by subcutaneous injection of uranyl nitrate (10 mg kg(-1)). Renal dysfunction did not cause any change in the blood ajmaline concentration after intravenous administration (2 mg kg(-1)), but it increased the blood ajmaline concentration by approximately 2.8-fold after intraduodenal administration (10 mg kg(-1)). The availability of ajmaline in control rats was 16.7%, whereas the availability was increased to 41.1% in rats with renal dysfunction. The unbound fraction in the blood and the metabolic activity in the liver, was assessed with the 10000-g supernatant fraction and with isolated hepatocytes, respectively. The values were found to be similar in both groups. The blood concentration following intraportal infusion was only slightly increased in rats with renal dysfunction, but the hepatic first-pass extraction was infusion rate-dependent and saturable. The initial absorption rate of ajmaline from the small intestine in rats with renal dysfunction was significantly greater compared with control rats. These results indicated that the increased availability of ajmaline in renal dysfunction was mainly a result of partially saturated extraction in the liver, which was caused by an increased absorption rate in the intestine and non-linear extraction in the liver.

Hemodynamic and Inotropic Effects of Antiarrhythmic Drugs Used to Treat Paroxysmal Supraventricular Arrhythmias

Episodes of sustained paroxysmal supraventricular tachycardias can be terminated by antiarrhythmic drugs given intravenously. The cardiodepressive effects of these drugs are an important limitation of this therapeutic procedure. The dose-dependent circulatory and myocardial effects of the nucleoside adenosine (0.5, 2.0, 5.0 mg/kg/minute) and the class I antiarrhythmic drug ajmaline (1.0, 2.0, 4.0 mg/kg) were investigated in 73 open-chest rats. Hemodynamic measurements in the intact circulation and isovolumic registrations (peak isovolumic left ventricular systolic pressure and peak isovolumic dP/dtmax) were compared with saline controls. Adenosine has a short-lasting, negative, chronotropic effect that causes a dose-dependent reduction of cardiac output (-34%, -54%, -65% vs control). The peak isovolumic left ventricular systolic pressure (LVSP) is not changed significantly by adenosine (-6%, -4%, +5% vs control). The negative chronotropic effect of ajmaline with consecutive reduction of cardiac output is less pronounced (cardiac output: -18%, -20%, -38% vs control). The highest dose of ajmaline causes a significant reduction of peak isovolumic LVSP (-2%, -1%, -7% vs control). Adenosine has an impressive negative chronotropic effect with a consequent marked decrease of cardiac output. The reduction of cardiac output by adenosine is more pronounced compared with ajmaline. Nevertheless, adenosine has-in contrast to ajmaline-no cardiodepressive effects in vivo.

The impact of arteriovenous concentration differences on pharmacodynamic parameter estimates

In many pharmacodynamic investigations venous drug concentrations are measured and linked to effect-site concentrations by means of a traditional first-order effect-compartment model to estimate pharmacodynamic (PD) parameters. This analysis ignores the underlying physiology that arterial blood supplies both the venous sampling site and effect site. Recently, an extended effect-compartment model has been proposed that reflects physiology by postulating a first-order rate constant of equilibrium between arterial and effect-site concentrations (ke0) as well as first-order rate constant between arterial and venous concentrations (kv0). In the current paper, we evaluate the bias in PD parameter estimates if venous drug concentrations are measured and linked to effect-site concentrations by a traditional effect compartment as a function ke0, kv0, and the drug's elimination half-life (T1/2); we present an analytical solution to the differential equations characterizing the extended effect-compartment model; and we evaluate the performance of the extended effect-compartment model to estimate pharmacodynamic parameters on the basis of venous drug concentrations. Time profiles of venous drug concentrations and drug effect were simulated for a wide range of different values of the half-life of ke0 (T1/2,e0), the half-life of kv0 (T1/2,v0), and T1/2. The simulations showed that a significant bias (up to 90%) in PD parameter estimates occurred for certain values of T1/2,e0, T1/2,v0, and T1/2 if venous drug concentrations are linked to effect-site concentrations by a traditional effect-compartment model. This model misspecification is not apparent from the results of the fitting procedure. The extended effect-compartment model provided unbiased but imprecise PD parameter estimates. The extended effect-compartment model was also able to analyze instances in which the venous concentrations equilibrate slower with the arterial concentrations than the effect-site concentrations, and proteresis is observed in the concentration--effect relationship. It is concluded that if the apparent T1/2 of the drug in the time period in which the decline in pharmacological effect is most pronounced is greater than 5 times T1/2,e0 and T1/2,e0 is greater than T1/2,v0 there is no need to model the underlying arteriovenous equilibrium delay. Under these conditions a traditional first-order link between venous and effect-site concentrations will yield accurate and reliable (less than 10% bias) estimates of the PD parameters such as Emax, EC50 and N. If T1/2 is less than 5 times T1/2,e0 or if T1/2,v0 is greater than T1/2,e0, the underlying arteriovenous equilibration delay needs to be taken into account in the model to obtain unbiased estimates of the PD parameters. This applies for almost all values of T1/2.v0. Arteriovenous equilibration delay can be best taken into account by measuring arterial blood concentrations. If this is not possible, the extended effect-compartment link model can be used. However, a large number of effect measurements needs to be obtained to estimate the model parameters accurately.

Effect of ajmaline on sustained ventricular tachycardia induced by programmed electrical stimulation in conscious dogs after myocardial infarction

As yet the antiarrhythmic efficacy of ajmaline with regard to suppressing the induction of sustained ventricular tachycardia after myocardial infarction has not been determined. Therefore, programmed electrical stimulation was performed in 8 conscious, chronically instrumented mongrel dogs 8-20 days after a 4-hour occlusion of the left anterior descending coronary artery. At baseline all animals responded with sustained ventricular tachycardia. Thereafter, ajmaline was administered at two consecutive i.v. doses: a bolus of 0.7 mg kg-1 followed by infusion of 2 mg kg-1 h-1 and infusion of 4 mg kg-1 h-1. The induction of sustained ventricular tachycardia was prevented in 2/8 animals by 2 mg kg-1 h-1 ajmaline and in 1/8 animals by 4 mg kg-1 h-1 ajmaline. During sinus rhythm only 4 mg kg-1 h-1 ajmaline significantly increased QRS-duration and intraventricular activation times, but during rapid right ventricular pacing (cycle length = 330 ms) both doses of ajmaline increased QRS duration and intraventricular conduction times. 4 mg kg-1 h-1 ajmaline also increased the cycle length of induced sustained ventricular tachycardia. In 3 animals induction of sustained ventricular tachycardia during infusion of 4 mg kg-1 h-1 ajmaline was achieved by introduction of less extrastimuli than at baseline. Furthermore the coupling intervals of extrastimuli that induced sustained ventricular tachycardia were substantially prolonged by this dose. Inhomogeneity of conduction between left ventricular normal zone and left ventricular infarct zone was significantly increased by 4 mg kg-1 h-1 ajmaline during rapid right ventricular pacing, but not during sinus rhythm.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement of free drug in phase I: is it useful?

Early investigation of protein binding of a new drug is mandatory. The following questions have to be answered: is unbound fraction constant over tested concentrations? Which proteins are involved? What are the binding parameters? Can the drug compete with other therapeutic agents for the binding sites or in other words can drug displacements be predicted? What is the interindividual variability in protein binding? Is the binding stereoselective? All this information is necessary in predicting the pharmacokinetic behaviour of the drug and in assisting in the design of future pharmacokinetic protocols in phases II and III. The use of free drug concentration should also be considered when comparing the bioavailability of regular vs sustained release dosage forms of drugs exhibiting concentration-dependent binding and when studying concentration-effect relationships.

Pharmacokinetics and dromotropic activity of ajmaline in rats with hyperthyroidism

1. The pharmacokinetics and the dromotropic action (increased PQ interval) of intravenously administered ajmaline (2 mg kg-1) were studied in hyperthyroid rats with sinus tachycardia. The hyperthyroidism was induced by intraperitoneal injection of 3,5,3'-triiodo-L-thyronine (0.5 mg kg-1) for 4 days. 2. The change in the ajmaline concentration in whole blood could be described by a biexponential equation. The steady state distribution volume of ajmaline decreased from 4.81 l kg-1 in control rats to 3.80 l kg-1 in hyperthyroid rats and the total body blood clearance was slightly higher in hyperthyroid rats than in control rats. 3. Ajmaline exhibited a saturable binding to rat plasma proteins, and one kind of binding site was found in the observed range of concentrations. The binding capacity was 2 fold higher in hyperthyroid rats than in control rats. 4. On the basis of the plasma unbound concentration, ajmaline exhibited an increased negative dromotropic activity in hyperthyroid rats compared with control rats. 5. A positive correlation was found between the pacing rate and the dromotropic action of ajmaline on atrioventricular conduction in isolated perfused hearts. There was no significant difference in the rate-dependence of the effect of ajmaline on the heart between control and hyperthyroid rats. 6. Our findings suggest that the increased dromotropic activity of ajmaline is mainly due to the increased heart rate in hyperthyroid rats.