Mariano Ferrari

On the mechanism of spasmolytic effect of papaverine and certain derivatives

Abstract The authors have compared the spasmolytic activity and the effect on oxidative phosphorylation of papaverine and some of its derivatives (dihydropapaverine, ethaverine and eupaverin). With guinea pig ileum and rabbit duodenum, papaverine, dihydropapaverine and ethaverine faithfully mimic the effect of anoxia, cyanide, 2,4-DNP or other enzyme inhibitors, by suppressing the “tonic phase”, of acetylcholine, histamine, BaCl 2 -induced contraction, without affecting the “spike phase” according to West et al . 1 Papaverine, dihydropapaverine, ethaverine strongly inhibit the oxygen uptake of rat liver mitochondria oxidizing glutamate under phosphorylative conditions. This effect is not reversed by 2,4-DNP. With succinate as substrate the oxygen uptake is unaffected by these drugs. The results suggest that the inhibition takes place in the electron-transfer reactions chain between nicotinamide-adenine dinucleotide and cytochrome b. Papaverine and ethaverine show the greatest activity both in experiments with isolated gut and with rat liver mitochondria. Eupaverin demonstrated a peculiar behaviour, because it failed to give an “anoxia-like” effect on the isolated gut and its mechanism of action on rat liver mitochondria is rather different. Possible relations between these biochemical effects of the drugs and their spasmolytic activity are discussed.

Pharmacological properties of tetrahydropapaveroline.

Tetrahydropapaveroline (thp) exerts β‐sympathomimetic effects similar to those of isoprenaline. On guinea‐pig isolated atria, thp elicits positive inotropic and chronotropic activities which are not abolished by previous reserpinization of the animals; on isolated mammalian heart these effects are associated with an increase in coronary flow. In the dog, thp increases myocardial contractile force and rate, elicits a hypotensive effect and stimulates respiratory activity in normal and reserpinized animals; when injected intra‐arterially the drug causes vasodilatation. All the effects are prevented by the β‐adrenergic blocking agents propranolol, dichloroisoprenaline and pronethalol. Structure‐activity relationships between tetrahydroisoquinoline derivatives and their open‐ring phenylethylamine congeners, which are closely related to sympathomimetic drugs, are discussed.

Effects of pinacidil on guinea-pig isolated perfused heart with particular reference to the proarrhythmic effect.

1 The effects of pinacidil (10, 30, 50 μm) on contractility (+ dP/dtmax), coronary perfusion pressure (cP), and ECG intervals (PR, QRS, QT) have been studied on constant‐flow perfused guinea‐pig hearts, driven at four frequencies (2.5, 3, 3.5, 4 Hz). 2 Pinacidil decreased +dP/dtmax, cP and the QT interval in a dose‐dependent manner, whereas the PR interval was increased. QRS duration was not modified. All these effects were independent of driving frequency. Pinacidil decreased the interval from Q‐wave to T‐wave peak (QTpeak) to a greater extent than the QT interval, thus decreasing the QTpeak/QT ratio. This effect, unlike that on QT interval, was more evident at the highest frequency of stimulation. 3 In 4 out of 20 hearts treated with pinacidil sustained ventricular fibrillation (VF) occurred following a short run of premature ventricular beats (R on T phenomenon). 4 In separate experiments, an attempt to induce VF electrically was made at drug concentrations ranging from 10 μm to 100 μm (8 experiments for each concentration). In control conditions and at the lowest concentration of pinacidil tested (10 μm) VF could never be induced; in the presence of 30 μm pinacidil VF was induced in 5 out of 8 experiments. Drug concentrations higher that 50 μm permitted the induction of VF in every case. 5 Although the concentrations of pinacidil producing ventricular fibrillation are 30–40 times higher than those found in patients under long term treatment with this agent, it is suggested that caution should be used in prescribing this drug, at least in patients suffering from myocardial ischaemia.

Pharmacokinetics and Electrophysiological Effects of Intravenous Ajmaline

SummaryThe pharmacokinetics of ajmaline were studied in 10 patients with suspected paroxysmal atrioventricular block who received a 1 mg/kg intravenous dose over 2 minutes for diagnostic purposes (ajmaline test). Plasma concentration decay followed a triexponential time course with a final half-life much longer (7.3 ±3.6 hours) than that previously found by other investigators (about 15 minutes). Mean total plasma clearance and renal clearance were 9.76 ml/min/kg and 0.028 ml/min/kg, respectively. Although most of the dose was eliminated through the extrarenal route (only 3.5% of the intravenous dose was recovered in urine), no fluorescent metabolites could be detected either in plasma or urine. The steady-state volume of distribution averaged 6.17 L/kg, and plasma protein binding ranged between 29 and 46%.Three patients developed a transient atrioventricular block after ajmaline administration. In the remainder, the drug prolonged atrio-His bundle (AH interval), His bundle-ventricular (HV interval) and intraventricular (QRS interval) conduction times. Corrected ventricular repolarisation time (QTc interval) showed less marked changes, which were biphasic at times. The mean maximum ajmaline-induced increase in HV interval was 98%, in QRS was 58%, in AH was 30%, and in QTc was 17%.In most cases the time course of electrocardiographic changes lagged behind that of plasma concentrations, suggesting a delayed equilibrium of plasma concentrations with the site of action (hysteresis). Despite that, the pharmacokinetic-pharmacodynamic model, which accounted for hysteresis, failed to fit the experimental data adequately. The duration of electrophysiological effects was short (about 30 minutes) in comparison with the slow decay of plasma concentrations, so that a threshold concentration exists under which no drug effect can be detected. Accordingly, if suprathreshold ajmaline plasma concentrations were steadily maintained (as can occur, for example, after repeated doses of lorajmine, an ajmaline prodrug), the duration of the electrophysiological effects would be unpredictably longer.

PHARMACOLOGICAL PROPERTIES OF TETRAHYDROPAPAVEROLINE AND THEIR RELATION TO THE CATECHOLAMINES.

SIR,-Recent reports from this laboratory (Santi, Contessa & Ferrari, 1963 ; Santi, Ferrari & Contessa, 1964) have shown that papaverine is a powerful inhibitor of the aerobic oxidation of substrates linked to nicotinamide adenine dinucleotide (NAD) in rat liver mitochondria. The inhibition of oxidative phosphorylation which could be localised in the electron transfer step between NAD and cytochrome b might be important in understanding the mechanism of the spasmolytic effect of the drug. Since some effects of papaverine resemble those generally referred to stimulation of the so-called 8-receptors of adrenaline, Santi (1963) has put forward a working hypothesis based on the possibility that the adrenaline-like drugs produce an impairment of cellular energy sources. In this context, the conclusion by Holtz, Stock, & Westerman (1963) that a substance similar in structure to papaverine, tetrahydropapaveroline, could be formed by the condensation of the well-known precursor of adrenaline, dopamine, and dihydroxyphenylacetic aldehyde, is of interest. We have confirmed the results of Holtz & others (1963) and we believe that the pharmacological properties of tetrahydropapaveroline are in themselves very interesting in as much as this drug behaves in some respects like papaverine, in others like the catecholamines and particularly, isoprenaline. Some pharmacological properties of the drug were described several years ago by Laidlaw (1910). The spasmolytic activity of tetrahydropapaveroline, as seen on the isolated guinea-pig ileum, resembles that of eupaverin (1 -benzyl-3-ethyl-6,7-dimethoxyisoquinoline) rather than that of papaverine. It differs from papaverine in not inhibiting mitochondria1 respiration. Tetrahydropapaveroline stimulates the myocardium as was seen in vivo by measuring the contractile strength by means of the strain gauge technique described by Boniface, Brodie & Walton (1953), as well as in vitro on isolated guinea-pig atria. The latter effect is antagonised by dichloroisoprenaline (DCI). The action of tetrahydropapaveroline on the heart is presumably important in understanding its pharmacological activities. In the dog, 0.1 mg/kg injected intravenously greatly increases the contractile strength of the heart and also its frequency. At 0.02 pg/ml, it has a positive inotropic and chronotropic action; a similar effect may be shown on isolated atria of the previously reserpinised guinea-pig. In dogs and cats, tetrahydropapaveroline reduces blood pressure at concentrations 20-30 times lower than papaverine and 50 times greater than isoprenaline. The hypotensive effect mainly concerns diastolic pressure, whereas the systolic values remain unchanged, the differential increasing accordingly. The decrease of blood pressure must be presumed to be due to a peripheral vasodilatation, since when 5-10 pg of the drug was introduced into the femoral artery, a strong increase of blood flow was measured with a ShipleyWilson rotameter (1951). On the other hand, the vasodilator response of the blood vessels of the isolated rabbit ear according to the technique of Pissemski (1914) was present, although not intense. When administered intravenously to dogs and cats, the drug greatly stimulated respiration. Tetrahydropapaveroline, similarly to adrenaline (Ussing, 1960), and in contrast to papaverine, increases the short circuit current of the isolated frog skin as measured by the technique of Ussing & Zerahn (1951), modified by Vescovini & Marro (1960). Finally, the drug injected intraperitoneally produces an increase in the plasma level of free fatty acids in rats ; this effect is considered to be

Effects of papaverine and eupaverin on calcium uptake by isolated sarcoplasmic vesicles

Papaverine and eupaverin increase the rate of uptake of calcium by sarcoplasmic vesicles isolated from rabbit white skeletal muscle. The degree of activity of the above drugs is clearly affected by changes of ATP, oxalate and Ca2+ concentrations. The results are discussed in view of present knowledge about the effects of papaverine‐like drugs upon muscular contraction.

Pharmacodynamic variability of flecainide assessed by QRS changes

The effect of flecainide on the QRS interval was studied in 10 patients who were receiving long‐term oral treatment (50 to 150 mg twice daily) for arrhythmias that were refractory to other drugs. Total and free drug plasma levels and QRS durations were measured at intervals after the morning administration. Free drug plasma levels were linearly correlated with QRS duration in each patient and the slope of the line was widely variable in the population studied. Even after the data from one patient with an unusually high slope (0.454) was excluded from the analysis, the slope range was 0.0284 to 0.144. Pharmacodynamic variability could not be explained by heart rate changes, active metabolites, electrolyte disturbances, or free drug concentration. None of the pharmacokinetic parameters measured (average steady‐state concentration, fluctuation of maximum and minimum concentrations, time to peak concentration, final half‐life, and protein binding) showed an intersubject variability greater than 4.4 times. Our findings suggest that the determination of flecainide free plasma concentration may not be sufficient to forecast electrophysiologic effects in individual patients.

New Limited Sampling Strategy for Determining 5-fluorouracil Area Under the Concentration-time Curve After Rapid Intravenous Bolus

All limited sampling models so far proposed to determine the area under the concentration-time curve (AUC) of anticancer drugs can be applied only to the dosing/sampling schedule used to obtain the model. The authors have developed a new method to predict the AUC of 5-fluorouracil (5-FU) after rapid intravenous bolus administration of various doses, using as few as two plasma drug concentrations. The 5-FU AUC (AUCtrue) was first determined in 20 patients receiving adjuvant therapy for colorectal cancer, based on nine plasma drug concentrations obtained at 0, 2.5, 5, 10, 15, 20, 30, 45, and 60 minutes after drug administration. Ten patients received 375 mg/m2 5F-U + 100 mg/m2 leucovorin and 10 received 425 mg/m2 5-FU + 20 mg/m2 leucovorin. The kinetics of 5-FU was described by either a one- or two-compartment linear model, as needed. The AUC was then recalculated (AUCapprox) using a reduced number of plasma concentrations and a simple one-compartment model. The time combinations tested were 2.5, 5, 10, and 20; 2.5, 10, and 20; 5, 10, and 20; 5 and 20; and 2.5 and 20 minutes. The accuracy and precision of the method in predicting the AUCtrue were measured by calculating the mean prediction error (MPE%) and the mean absolute error (MAE%) of the AUCapprox. MPE% ranged between −0.8% and −8.3% and MAE% between 6.1% and 9.5%, depending on the time combination used. In general, all limited sampling models tested tended to underestimate the AUCtrue slightly, particularly when 5-FU kinetics followed a two-compartment model, but bias was still within acceptable limits. The best results were obtained with plasma concentrations measured at 2.5 and 20 minutes after drug bolus (MPE%, −0.8%; MAE%, 6.1%). Although 5-FU kinetics was dose-dependent, MPE% and MAE% were not significantly different between the two groups. These data show that 5-FU AUC can be reliably predicted by using just two plasma concentrations and a one-compartment model, even when different doses are used and plasma concentration decay is biexponential.

Amiodarone and amiodarone plus digitalis in the treatment of paroxismal supraventricular reciprocating tachyarrhythmias

Summary Therapeutic response, incidence of side effects and plasma levels of the drugs have been evaluated in patients with paroxysmal supraventricular reciprocating tachyarrhythmias treated with amiodarone (A) or amiodarone plus a glycoside (digoxin or medigoxin). 26 out 40 patients of group A (A.) and 15 out 23 of group AD (A. plus digitalis) remained free of tachyarrhythmic episodes during the follow-up period (12 month) and a significant overall decrease of paroxysms was recorded in the remaining patients. A. was withdrawn in 5 patients, for hyperthyroidism (3 cases), hypothyroidism (1 case) and photodermatosis mediate (1 case). Corneal deposits were frequent (68% of the patients) but scanty and not progressive. A significant correlation was found at steady state between A. plasma levels and digitalemia. However, unlike the results obtained by other AA during a short term A. treatment, digitalis plasma levels remained in the therapeutic range in all patients, without any evidence of toxicity.

Present and future trends in research and clinical applications of inodilators.

The rationale of combining vasodilatation with positive inotropic intervention in the treatment of chronic heart failure has found a new implementation in the “inodilator” drugs. Inodilators are characterized by the properties of exerting positive inotropic effect and inducing systemic vasodilatation. The cellular mechanisms involved in the regulation of contractility of cardiac and vascular muscle and the pathophysiological events occurring in heart failure are briefly discussed, and the pharmacological profile as well as the therapeutic use of these drugs are reviewed. On the basis of the mechanism of action, two groups of inodilators are distinguished, the phosphodiesterase inhibitors and the dopaminergic agents. The increase of [cAMP]1 induced by the phosphodiesterase inhibitors is responsible for their vasodilating effect and for the positive inotropic action, but many of them have in addition the ability to enhance the Ca2− sensitivity of cardiac contractile proteins. The complex organization and the cardinal role of the catecholaminergic receptor system in the control of cardiovascular function and its contribution to the pathophysiological events occurring in heart failure are the rational basis of the therapeutic use of dopaminergic agents. These drugs, acting on DA, β-, and α-receptors, exert not only positive inotropic and vasodilating effects, but also a diuretic action, and can reduce aldosterone and renin secretion, blunt an excessive sympathetic activity, and possibly promote the release of atrial natriuretic peptide. The multireceptor mechanism of dopamine-like drugs, which accounts for their favorable hemodynamic, neurohumoral, and diuretic effects, represents the most promising approach to inodilator therapy.