The effect of tricyclic antidepressant drugs on the isolated perfused guinea-pig heart

Protriptyline, a tricyclic antidepressant, inhibits voltage-dependent K+ channels in rabbit coronary arterial smooth muscle cells

In this study, we explore the inhibitory effects of protriptyline, a tricyclic antidepressant drug, on voltage-dependent K+ (Kv) channels of rabbit coronary arterial smooth muscle cells using a whole-cell patch clamp technique. Protriptyline inhibited the vascular Kv current in a concentration-dependent manner, with an IC50 value of 5.05 ± 0.97 μM and a Hill coefficient of 0.73 ± 0.04. Protriptyline did not affect the steady-state activation kinetics. However, the drug shifted the steady-state inactivation curve to the left, suggesting that protriptyline inhibited the Kv channels by changing their voltage sensitivity. Application of 20 repetitive train pulses (1 or 2 Hz) progressively increased the protriptyline-induced inhibition of the Kv current, suggesting that protriptyline inhibited Kv channels in a use (state)-dependent manner. The extent of Kv current inhibition by protriptyline was similar during the first, second, and third step pulses. These results suggest that protriptyline-induced inhibition of the Kv current mainly occurs principally in the closed state. The increase in the inactivation recovery time constant in the presence of protriptyline also supported use (state)-dependent inhibition of Kv channels by the drug. In the presence of the Kv1.5 inhibitor, protriptyline did not induce further inhibition of the Kv channels. However, pretreatment with a Kv2.1 or Kv7 inhibitor induced further inhibition of Kv current to a similar extent to that observed with protriptyline alone. Thus, we conclude that protriptyline inhibits the vascular Kv channels in a concentration- and use-dependent manner by changing their gating properties. Furthermore, protriptyline-induced inhibition of Kv channels mainly involves the Kv1.5.

Investigation of connexin 43 uncoupling and prolongation of the cardiac QRS complex in preclinical and marketed drugs

BACKGROUND AND PURPOSE

Prolongation of the cardiac QRS complex is linked to increased mortality and may result from drug-induced inhibition of cardiac sodium channels (hNaV 1.5). There has been no systematic evaluation of preclinical and marketed drugs for their additional potential to cause QRS prolongation via gap junction uncoupling.

EXPERIMENTAL APPROACH

Using the human cardiac gap junction connexin 43 (hCx43), a dye transfer 'parachute' assay to determine IC50 values for compound ranking was validated with compounds known to uncouple gap junctions. Uncoupling activity (and hNaV 1.5 inhibition by automated patch clamp) was determined in a set of marketed drugs and preclinical candidate drugs, each with information regarding propensity to prolong QRS.

KEY RESULTS

The potency of known gap junction uncouplers to uncouple hCx43 was ranked (according to IC50 ) as phorbol ester>digoxin>meclofenamic acid>carbenoxolone>heptanol. Among the drugs associated with QRS prolongation, 29% were found to uncouple hCx43 (IC50 < 50 μM), whereas no uncoupling activity was observed in drugs not associated with QRS prolongation. In preclinical candidate drugs, hCx43 and hNaV 1.5 IC50 values were similar (within threefold). No consistent margin over preclinical Cmax (free) was apparent for QRS prolongation associated with Cx43 inhibition. However, instances were found of QRS prolonging compounds that uncoupled hCx43 with significantly less activity at hNaV 1.5.

CONCLUSION AND IMPLICATIONS

These results demonstrate that off-target uncoupling activity is apparent in drug and drug-like molecules. Although the full ramifications of Cx inhibition remain to be established, screening for hCx43 off-target activity could reduce the likelihood of developing candidate drugs with a risk of causing QRS prolongation.

Protriptyline block of the human ether-à-go-go-related gene (HERG) K+ channel

Protriptyline, a tricyclic antidepressant for psychiatric disorders, can induce prolonged QT, torsades de pointes, and sudden death. We studied the effects of protriptyline on human ether-à-go-go-related gene (HERG) channels expressed in Xenopus oocytes and HEK293 cells. Protriptyline induced a concentration-dependent decrease in current amplitudes at the end of the voltage steps and HERG tail currents. The IC(50) for protriptyline block of HERG current in Xenopus oocytes progressively decreased relative to the degree of depolarization, from 142.0 microM at -40 mV to 91.7 microM at 0 mV to 52.9 microM at +40 mV. The voltage dependence of the block could be fit with a monoexponential function, and the fractional electrical distance was estimated to be delta=0.93. The IC(50) for the protriptyline-induced blockade of HERG currents in HEK293 cells at 36 degrees C was 1.18 microM at +20 mV. Protriptyline affected channels in the activated and inactivated states, but not in the closed states. HERG blockade by protriptyline was use-dependent, exhibiting a more rapid onset and a greater steady-state block at higher frequencies of activation. Our findings suggest that inhibition of HERG currents may contribute to the arrhythmogenic side effects of protriptyline.

Paroxetine: a review of its pharmacology, pharmacokinetics and utility in the treatment of a variety of psychiatric disorders

Paroxetine is a selective serotonin re-uptake inhibitor (SSRI). In vitro studies show that it is able to produce a concentration-dependent competitive inhibition of serotonin uptake into brain synaptosomes. This effect can also be demonstrated following in vivo administration of the compound to animals. Paroxetine is almost completely absorbed following oral administration. However, the drug undergoes extensive first pass metabolism. As a result, less than 50% of a single dose of paroxetine reaches the general circulation. Paroxetine is primarily metabolised by the cytochrome P4502D6 isoenzyme. The compound has also been shown to inhibit the activity of this enzyme. As a result, plasma levels of compounds metabolised by the cytochrome P4502D6 isoenzyme can be increased in patients given paroxetine. Paroxetine has been extensively evaluated in clinical studies in depressed patients. The compound shows efficacy superior to placebo, and similar to that obtained with standard tricyclic or tetracyclic agents. Paroxetine also appears to be as efficacious as other SSRIs. The efficacy seen in short-term studies with paroxetine in the treatment of depression is maintained when the drug is given chronically. More recently, paroxetine has been shown to be efficacious in the treatment of panic disorder, obsessive-compulsive disorder, and social anxiety disorder. Nausea, headache and somnolence are the most common adverse events reported by patients given paroxetine. As with other selective serotonin re-uptake inhibitors, a significant percentage of men under therapy with paroxetine report abnormal ejaculation. Paroxetine is well-tolerated by elderly patients, and appears to be associated with few serious adverse events.

Hemodynamic and electrocardiographic effects of fluoxetine and its major metabolite, norfluoxetine, in anesthetized dogs

The cardiovascular effects of the selective serotonin uptake inhibitor, fluoxetine, and its N-desmethyl metabolite, norfluoxetine, were studied in anesthetized dogs during constant iv infusion of supratherapeutic doses (0.1 mg/kg/min for 50 min). Fluoxetine and norfluoxetine did not significantly affect mean blood pressure, pulmonary artery wedge pressure, or heart rate compared to a corresponding vehicle group. Cardiac output fell 15 to 20% during fluoxetine infusion due to nonsignificant decreases in both heart rate (10%) and stroke volume (5 to 10%). In contrast, the tricyclic antidepressant agent, amitriptyline, infused at the same dose, decreased both mean pressure and systemic vascular resistance (20%) and increased heart rate (20%). Pulmonary wedge pressure rose by 35%, and stroke volume fell by 20% suggesting impaired ventricular contractility. Both intramyocardial and infranodal conduction was slowed during amitriptyline infusion as indicated by increases in the QRS duration, and the PQ and HV interval. Fluoxetine and norfluoxetine had no influence on cardiac impulse conduction velocity as assessed by either surface or intracardiac conduction indices. Plasma concentrations of fluoxetine, norfluoxetine, and amitriptyline reached during infusion ranged from 1.0 to 2.5 micrograms/ml. Platelet [3H]serotonin uptake was inhibited by 95% during infusion of fluoxetine and about 75% during infusion of norfluoxetine or amitriptyline. These observations indicate that large iv doses of fluoxetine or norfluoxetine lack prominent cardiodepressant effects in dogs, suggesting a greater margin of safety for fluoxetine compared to tricyclic antidepressant drugs.

Cardiological effects of nomifensine, a new antidepressant

A cardiovascular study of a new antidepressant, nomifensine, revealed no significant cardiac effects in eight ambulant patients who received up to 200 mg of nomifensine per day for three weeks. In an electro-physiological study of cardiac conduction, nomifensine had significantly less effect on cardiac activity than amitriptyline and doxepin. Results of the clinical and the experimental studies suggest that nomifensine may be of value in treating the depressed patient with heart disease.