Effects of a new calcium antagonist, mibefradil (Ro 40-5967), on silent ischemia in patients with stable chronic angina pectoris: a multicenter placebo-controlled study. The Mibefradil International Study Group

Additional antianginal and anti-ischemic efficacy of mibefradil in patients concomitantly treated with long-acting nitrates for chronic stable angina pectoris

BACKGROUND

Mibefradil, a newly approved antihypertensive and antianginal drug, is the first member of a new class of calcium antagonists (CAs), the tetralol derivatives, that selectively blocks T-type Ca2+ channels in contrast to classical CAs which, at therapeutic concentrations, block only L-type Ca2+ channels. Since patients with chronic stable angina pectoris typically may be treated with the combination of a long-acting nitrate and a CA, the additive efficacy and safety of mibefradil in combination with nitrate therapy needs to be determined.

HYPOTHESIS

This study was designed to assess the efficacy, tolerability, and safety of mibefradil when added to long-acting nitrate therapy in patients with chronic stable angina pectoris.

METHODS

Following a 1-week placebo run-in period, patients were randomized to receive either mibefradil 50 mg (n = 96) or placebo (n = 93) once daily in addition to their nitrate therapy. After 2 weeks of active treatment, patients receiving the mibefradil were force titrated to 150 mg once daily for an additional 2 weeks. Exercise tolerance tests (ETTs) were performed at the end of Weeks 2 and 4; patients also maintained an anginal diary during the 4-week treatment period.

RESULTS

After 2 weeks of treatment with 50 mg mibefradil (within the current recommended dose range), a statistically significant but modest increase in total exercise duration was observed (treatment effect 16.4 s, p = 0.04). Similarly, there was a significant increase in time to onset of ischemia (treatment effect 26 s, p = 0.008). The adverse event profile of the 50 mg dose was indistinguishable from placebo, indicating that this dose was extremely well tolerated. At Week 4, the mibefradil-treated patients were taking 150 mg, which is above the current recommended dose range. The increase in total exercise duration was larger for the mibefradil 150 mg group than for the placebo group. For the intent-to-treat population, this difference did not reach statistical significance, whereas in the standard population it did (treatment effect 21 s, p = 0.05). The other two ETT variables, time to onset of angina and time to onset of 1 mm ST-segment depression, demonstrated significantly greater effect with mibefradil 150 mg (treatment effects 40 s, p = 0.002, and 55 s, p < 0.001, respectively, for the intent-to-treat population). Mibefradil 150 mg was associated with more adverse events than placebo, specifically, dizziness, leg edema, and postural hypotension.

CONCLUSIONS

This study demonstrates that mibefradil 50 mg once daily in the setting of the background long-acting nitrate therapy produces additive antianginal and anti-ischemic effects and is extremely well tolerated.

Pharmacological modulation of ion channels and transporters

Ion channels and transporter proteins are prerequisites for formation and conduction of cardiac electrical impulses. Acting in concert, these proteins maintain cellular Na(+) and Ca(2+) homeostasis. Since intracellular Ca(2+) concentration determines contractile activation, we expect the majority of agents that modulate activity of ion channels and transporters not only to influence cellular action potentials but also contractile force. Drugs which block ion channels usually possess antiarrhythmic properties, those inhibiting the Na(+) pump have predominantly inotropic effects and those affecting Na(+),Ca(2+)- or Na(+),H(+)-exchanger protect against ischaemic cell damage. However, irrespective of their primary indication, all compounds targeted against ion channels and transporter proteins possess potential proarrhythmic activity.

Randomized comparison of T-type versus L-type calcium-channel blockade on exercise duration in stable angina: results of the Posicor Reduction of Ischemia During Exercise (PRIDE) trial

BACKGROUND

Mibefradil is a T-type calcium-channel antagonist and arterial vasodilator with negative chronotropic effects. It is not known if T-type calcium-channel blockade is superior to L-type calcium-channel blockade in patients with stable angina pectoris.

METHODS

A multicenter, randomized, double-blind trial was conducted in patients with documented coronary disease and stable angina to compare a 360 mg dose of diltiazem CD with 100 mg dose of mibefradil. The primary end point was change in time to symptom-limited exercise termination from baseline to 8 weeks. Secondary efficacy parameters included time to onset of persistent ST-segment depression, time to awareness of angina, and change in exercise duration from baseline to 2 and 4 weeks of treatment.

RESULTS

A total of 121 patients were randomized to mibefradil and 113 to diltiazem CD. At 8 weeks, the increase in exercise duration was 24.5 seconds greater in the mibefradil group (P =.017; 95% CI 4.4-44.7 seconds). At 8 weeks, time to development of > or =1 mm ST-segment depression was greater by 45.3 seconds (P =.0025; 95% CI 16.2-74.5) with mibefradil, but time to development of angina was not significantly different.

CONCLUSION

T-type calcium-channel antagonism with mibefradil improved treadmill exercise parameters compared with diltiazem in patients with chronic stable angina. Further investigation and development of antagonists of T-type calcium channels with fewer adverse drug interactions is warranted and may be promising in the management of ischemic heart disease.

Inhibition of T-type and L-type calcium channels by mibefradil: physiologic and pharmacologic bases of cardiovascular effects

Ca2+ channel antagonists of the dihydropyridine, benzothiazepine, and phenylalkylamine classes have selective effects on L-type versus T-type Ca2+ channels. In contrast, mibefradil was reported to be more selective for T-type channels. We used the whole-cell patch-clamp technique to investigate the effects of mibefradil on T-type and L-type Ca2+ currents (I(CaT) and I(CaL)) recorded at physiologic extracellular Ca2+ in different cardiac cell types. At a stimulation rate of 0.1 Hz, mibefradil blocked I(CaT) evoked from negative holding potentials (HPs) (-100 mV to -80 mV) with an IC50 of 0.1 microM in rat atrial cells. This concentration had no effect on I(CaL) in rat ventricular cells (IC50: approximately3 microM). However, block of I(CaL) was enhanced when the HP was depolarized to -50 mV (IC50: approximately 0.1 microM). Besides a resting block, mibefradil displayed voltage- and use-dependent effects on both I(CaT) and I(CaL). In addition, inhibition was enhanced by increasing the duration of the step-depolarizations. Similar effects were observed in human atrial and rabbit sinoatrial cells. In conclusion, mibefradil combines the voltage- and use-dependent effects of dihydropyridines and benzothiazepines on I(CaL). Inhibition of I(CaL), which has probably been underestimated before, may contribute to most of the cardiovascular effects of mibefradil.

Pharmacokinetic and pharmacodynamic aspects of concomitant mibefradil-digoxin therapy at therapeutic doses

This study investigated the effect of mibefradil on digoxin pharmacokinetics an pharmacodynamics. Following a loading dose of digoxin (0.375 mg, three times, day 1), 0.375 mg was administered once daily to 40 healthy subjects (days 2-15). Mibefradil was administered daily at 50 mg, 100 mg, or 150 mg (days 9-15). With co-administration of 50 mg or 100 mg mibefradil (the recommended doses), mean digoxin Cmax values increased 1.19- and 1.32-fold, respectively; Cmin values were 0.95- and 1.04-fold, respectively; mean AUC0-24 h increased 1.05- and 1.11-fold, respectively; and the total amount of digoxin excreted in urine remained unchanged. Digoxin monotherapy produced modest but transient prolongations of PQ interval, small decreases in heart rate, and no changes in blood pressure. With the addition of mibefradil, no effects on trough blood pressure or cardiac index were observed, but there was a further increase in PQ interval and decrease in heart rate. In a previous study, mibefradil had no significant effect on trough plasma digoxin concentration in patients with congestive heart failure and ischemia. Therefore, while the vast majority of patients should not need their digoxin dosages adjusted when given mibefradil, an occasional patient may require dose reductions based on clinical response and plasma digoxin.