Efficacy of ajmaline and propafenone in patients with accessory pathways: a prospective randomized study

[Emergency management of regular supraventricular tachycardias]

With an estimated incidence of approximated 36 per 100,000 persons per year, paroxysmal supraventricular tachycardias form a relevant clinical set of problems. They occur based on different substrates with varied symptoms and electrocardiographic items. The 12-channel ECG depicts the background to determine the underlying pathomechanism. The sinus node and all components of the conduction system such as atrial myocardium can be involved. Vagal maneuvers, several pharmacological strategies and various ablation technology are available for acute therapy.

Class Ia anti-arrhythmic drug ajmaline blocks HERG potassium channels: mode of action

Ajmaline is a class Ia anti-arrhythmic drug used in several European countries and Japan as first-line treatment for ventricular tachyarrhythmia. Ajmaline has been reported to induce cardiac output (QT) prolongation and to inhibit cardiac potassium currents in guinea pig cardiomyocytes. In order to elucidate the molecular basis of these effects, we examined effects of ajmaline on human ether a-go-go related gene HERG potassium channels. Electrophysiological experiments were performed with human embryonic kidney (HEK) cells (whole-cell patch clamp) and Xenopus oocytes (double-electrode voltage clamp) expressing wild-type and mutant HERG channels. Ajmaline blocked HERG currents with an IC(50) of 1.0 micromol/l in HEK cells and 42.3 micromol/l in Xenopus oocytes. The onset of block was fast and reached steady-state conditions after 180 s. The inhibitory effect was completely reversible upon wash-out. In HERG mutant channels Y652A and F656A lacking aromatic residues in the S6 domain, the inhibitory effect of ajmaline was completely abolished. Ajmaline induced a small shift in HERG current half-maximal activation voltage towards more negative potentials. Ajmaline did not markedly affect HERG inactivation. Inhibitory effects were not voltage-dependent. Ajmaline block exhibited positive frequency dependence. Ajmaline blocked HERG channels in the open, but not in the closed states. Binding of ajmaline to inactivated HERG channels may also be possible. In inactivation-deficient HERG S620T channels, the sensitivity to ajmaline was markedly reduced. The IC(50) of HERG channel blockade in HEK cells lies within the range of unbound therapeutic plasma concentrations of ajmaline. Therefore, inhibitory effects on HERG channels may contribute to both the high anti-arrhythmic efficacy of ajmaline and to its pro-arrhythmic potential.

Deoxysarpagine hydroxylase--a novel enzyme closing a short side pathway of alkaloid biosynthesis in Rauvolfia

Microsomal preparations from cell suspension cultures of the Indian plant Rauvolfia serpentina catalyze the hydroxylation of deoxysarpagine under formation of sarpagine. The newly discovered enzyme is dependent on NADPH and oxygen. It can be inhibited by typical cytochrome P450 inhibitors such as cytochrome c, ketoconazole, metyrapone, tetcyclacis and carbon monoxide. The CO-effect is reversible with light (450 nm). The data indicate that deoxysarpagine hydroxylase is a novel cytochrome P450-dependent monooxygenase. A pH optimum of 8.0 and a temperature optimum of 35 degrees C were determined. K(m) values were 25 microM for NADPH and 7.4 microM for deoxysarpagine. Deoxysarpagine hydroxylase activity was stable in presence of 20% sucrose at -25 degrees C for >3 months. The analysis of presence of the hydroxylase in nine cell cultures of seven different families indicates a very limited taxonomic distribution of this enzyme.

Effects of intravenous dofetilide on induction of atrioventricular re-entrant tachycardia

OBJECTIVE

To assess the efficacy and safety of intravenous dofetilide in preventing induction of atrioventricular re-entrant tachycardia.

DESIGN

A multicentre, open, dose ranging trial. Fifty one patients with electrically inducible atrioventricular re-entrant tachycardia were allocated to one of five doses of dofetilide (1.5, 3, 6, 9, and 15 microgram/kg), two thirds of the dofetilide dose being given over a 15 minute loading period and the remainder over a 45 minute maintenance period.

MAIN OUTCOME MEASURE

Responders were defined as patients in whom dofetilide prevented reinduction of atrioventricular re-entrant tachycardia at the end of the infusion.

RESULTS

Intravenous dofetilide had no effect on tachycardia inducibility at the two lower doses (1.5 and 3 microgram/kg) but prevented the reinduction of tachycardia at the three higher doses (6, 9, and 15 microgram/kg) at a rate of 36% (11/31). There was a clear relation between plasma dofetilide concentrations and efficacy (p = 0.009). In non-responders, dofetilide increased the cycle length of induced atrioventricular re-entrant tachycardia. Dofetilide increased the atrial and ventricular effective refractory periods, as well as the antegrade and retrograde effective refractory period of the accessory pathway. Treatment related side effects were reported in four patients, one with a new sustained incessant supraventricular tachycardia.

CONCLUSIONS

Dofetilide shows promise as an agent for the prevention of atrioventricular re-entrant tachycardia in patients without structural heart disease.

Stereocontrolled total synthesis of alkaloid G via the oxy-anion Cope rearrangement and improved total synthesis of (+)-ajmaline

[figure: see text] The oxy-anion Cope rearrangement followed by protonation of the enolate which resulted under conditions of kinetic control has been employed to generate the key asymmetric centers at C(15), C(16), and C(20) in alkaloid G (1) and (+)-ajmaline (2) in a highly stereocontrolled fashion. The aldehyde 7b from this process has been converted into alkaloid G (1) and (+)-ajmaline (2) in 36% and 13% overall yields (11 reaction vessels from 3), respectively.

Effects of ajmaline on rate-dependent atrioventricular node properties. Potential role in experimental atrioventricular re-entrant tachycardia

Ajmaline is a well-known atrioventricular (AV) node depressant agent, but its effects on functional properties of the AV node and on experimental AV re-entrant tachycardia have not been explored. The aims of the present study were (1) to determine whether ajmaline administration modifies the rate-dependent properties of the AV node and (2) to correlate these changes with the actions of ajmaline on an in vitro model of AV re-entrant tachycardia. Selective stimulation protocols and mathematical formulations were used to quantify independently AV node recovery, facilitation, and fatigue in 10 isolated rabbit AV nodes. Ajmaline decreased facilitation and fatigue and had no significant effect on AV node recovery. The most important effect of ajmaline was rate-induced prolongation of AV node effective refractory period, resulting in a greater increase in tachycardia cycle length. AV re-entrant tachycardia was sustained when AV effective refractory period divided to tachycardia cycle length was less than 1, ajmaline suppressed AV re-entrant tachycardia by increasing the slope of the AV effective refractory period divided to tachycardia cycle length versus tachycardia rate relation, causing the critical ratio of 1 to be attained at a slower rate. A mathematical model incorporating quantitative descriptors of recovery, facilitation, and fatigue accounted for changes in nodal conduction time, AV effective refractory period, tachycardia cycle length, and AV effective refractory period divided to tachycardia cycle length under all conditions. It can be concluded that (1) ajmaline increases AV conduction time, decreases AV node fatigue, and facilitation, without altering AV node recovery. (2) Ajmaline significantly prolongs AV effective refractory period in a rate-dependent manner. (3) These changes play a role in ajmaline's actions on experimental AV re-entrant tachycardia. Ajmaline's ability to terminate re-entrant supraventricular tachycardia may be due, at least in part, to its ability to amplify the rate-induced prolongation of the nodal refractory period.

Rate-dependent effects of ajmaline and propafenone on atrioventricular conduction

The aim of the present study was to characterize the time dependence of the depressant effects of ajmaline and propafenone on the Ca(2+)-channel-dependent tissue of the atrioventricular node in isolated guinea pig hearts perfused by the method of Langendorff. Ajmaline at a concentration of 0.03 microM and propafenone at a concentration of 0.3 microM caused a significant and comparable prolongation of the His bundle and atrioventricular conduction time (AVCT). When the pacing cycle length was abruptly shortened from 240 to 180 ms, the mean time constant (tau on) of the rate-dependent AVCT prolongation was comparable for ajmaline and propafenone. In contrast, if the pacing cycle length was abruptly increased from 180 to 240 ms the mean time constant (tau off) for ajmaline was significantly higher than for propafenone. The rate-dependent increase of the atrioventricular effective refractory period was significantly more pronounced in the presence of ajmaline than of propafenone. Ajmaline and propafenone affect the Ca(2+)-channel-dependent tissue of the myocardium. The more pronounced rate-dependent effect of ajmaline on the atrioventricular effective refractory period may be explained by a slower dissociation kinetic from the channel.