Comparative cardiac toxicity of the i.v. administered benzimidazole pyridazinon derivative Pimobendan and its enantiomers in female Beagle dogs.

The new positive-inotropic and vasodilatating drug Pimobendan (racemate), 4,5-dihydro-6-[2-(4-methoxyphenyl)-1H-benzimidazole-5-yl]-5-methyl-3 (2-H)-pyridazinone, and its enantiomers were investigated with regard to their cardiotoxicity in young adult female Chbb: Beagle dogs. The racemate and the (-)-isomer (eutomer) were intravenously injected once daily for 4 consecutive weeks at doses of 0.25, 0.75 and 2.25 mg/kg, and the (+)-isomer (distomer) at doses of 0.75, 2.25 and 6.75 mg/kg, respectively. Clinical signs, hematological, clinico-chemical, ophthalmologic and electrophysiological parameters were monitored. Plasma concentration-time profiles of the parent compound and the major metabolite UD-CG 212 were established on day 1 and in week 4 using an HPLC assay. Partial areas under the curves from 0.08 h to 1 h (AUC0.08-1 h) as well as the plasma concentration at time point 0.5 h/C0.5 h) were used for statistical calculations. Necropsy and histopathologic examination were performed after completion of the treatment period. Reduction of the blood pressure occurred already at low dosages of the racemate and the eutomer, but only in high dose distomer-treated animals. A tendency to tachycardia developed only in high dose females receiving the racemate. Consequently, with respect to the pharmacological effects and the adverse events, the racemate is equivalent to the eutomer. Plasma concentrations of parent compound and metabolite were dose-linear for racemate, eutomer and distomer within the dose range 0.25-2.25 mg/kg.d at both time points. There were no significant effects of form or repeated administration. A moderate increase of AUC0.08 1 h and C0.5 h could be seen on day 23 for the distomer indicating a stereoselektive metabolism of the latter. Histologic changes of the valvular and parietal endocardium being termed jet lesion were observed after administration of the racemate (> or = 0.75 mg/kg.d) and the eutomer (> or = 0.25 mg/kg.d) at distinctly lower doses than after the distomer (> or = 2.25 mg/kg.d). Furthermore, extent and severity of the morphologic lesions were found to be higher in dogs exposed to the racemate or the eutomer than in those receiving the distomer. The results gave evidence that the so-called cardiotoxicity by Pimobendan in dogs resulted from the exaggerated pharmacodynamic effect but not from the chemical nature of the compound per se. They corroborate also the previously raised assumption that the exaggerated pharmacodynamic activity of cardiotonic compounds in the broadest sense accounts for their morphologic adverse effects in experimental animals.

[The normal electrocardiogram of miniature swine]. / Uber das normale Elektrokardiogramm des wachen Miniaturschweines.

Mean normal values for heart rate of 111 (64-190) actions/min, for duration of the PR-interval of 88 (63-120) ms, for the QRS-complex of 36 (26-46) ms and the QT-interval of 252 (183-353) ms were determined from the ECG in 204 male and female Göttinger mini-pigs (mean body weight 12.4 kg). The mean total amplitudes (voltages) of QRSI-III amount to 0.8, 0.9 and 1.1 mV. Considering leads I, II and III positive P-waves are most frequently found in II with 91% and negative ones in III with 23%. Positive T-waves appear in III with 86% and negative ones in I with 88% most frequently. The main polarity of QRS is most frequently positive in I with 56% and negative in III with 48%. The incidence of Q is highest in lead I with 71%, that one of S in II with 55%. The T-waves are mostly discordant with QRS, especially in I with 54%, more seldom concordant yet frequently in II with 30%. A significant negative correlation (r = -0.77) is evident between heart rate and QT-duration, slight negative correlations also exist between heart rate and PR- as well as QRS-duration. The mean angle a QRS from I, II, III and aVR, aVL, aVF in the frontal plane (direction of the cardiac vector; n = 35) amounts to -47 degrees (-150 degrees to +165 degrees). The position of the heart axis is more sagittal with a characteristic deviation of the electrical from the anatomical axis of the heart. Concerning the observed parameters there is also a less intraindividual variability in investigations in series besides the considerable interindividual one. Examples for physiological specialities or variations from the norm of rhythm and configuration of the ECG typical for the miniature pig are presented and discussed.

[The voltage of R-waves in the normal electrocardiogram of the conscious beagle]. / Uber die Voltage der R-Zacken im normalen Elektrokardiogramm wacher Beagle-Hunde.

Mean values for body weight of 12.5 kg, heart weight of 103 g, heart rate of 109 actions/min and from ECG lead II an R-wave amplitude (voltage) of 1.6 mV (0.5-2.7 mV) were calculated in 118 healthy untreated beagle dogs (58 males, 60 females). The values of these parameters are somewhat higher in the males than in the females. The body postures of the dogs (standing, sitting, lying) during the recording of the ECGs scarcely influence the amplitude of RII but do distinctly alter those of R1 and RIII corresponding to the related changes in the direction of the electrical heart axis. Body and heart weights are positively correlated (r = +0.74) within the whole sample. There are, however, no correlations between heart weight and rate, heart rate and RII-amplitude nor between amplitude RII and heart weight within the range of the physiological values investigated making use of interindividual consideration. In 35 beagle dogs the smallest voltage changes were found in RII after recordings in series compared intraindividually. However, significant variations in RI and RIII were apparent. Furthermore normal ECGs with low voltage of RII or QRSII were revealed as mostly secondary low voltages due to sagittal type by our special lead "DV-L" (dorsoventral left). ECGs with high voltage of RII, which showed a lower mean amplitude in DV-L, served as control test. The results are commented on in detail. Possible relationships between heart weight, volume of ventricles, and heart rate on the one hand and the voltage of R or QRS on the other hand are discussed.

[Animal experimental testing of injection solutions for intra-arterial tolerability]. / Zur tierexperimentellen Prüfung von Injektionslösungen auf intraarterielle Verträglichkeit.

Severe damage, even necrosis, has been noted in the past in man after accidental intra-arterial (i.a.) application of injectable preparations intended for intravenous (i.v.) or other parenteral administration. Various experimental models in animals have been developed to test an eventual i.a. incompatibility. Reasons are given for the methodology used by us in the central artery of the rabbit's ear with and without occlusion (harder and softer test variant). Herewith preparations of acetamidoeugenol and thiopental were tested to work out an appropriate measure. In comparison we investigated a series of "placebo solutions" with solvents or other excipients used in the formulation of injection solutions. The "placebos" investigated were then classified with regard to their i.a. compatibility, because in many cases an intra-arterial intolerance can be attributed to them as much as to the active principle. The choice of experimental animal model, the transferability of results to man as well as possible mechanisms taking place in an accidental i.a. application and in the development of ensuing damage are discussed.

[Animal studies on the tolerability of AR-L 115 BS (author's transl)]. / Tierexperimentelle Untersuchung zur Verträglichkeit von AR-L 115 BS.

Acute and subacute toxicity studies as well as reproduction-toxicologic investigations were carried out on the positive-inotropic substance 2[(2-methoxy-4-methylsulfinyl)phenyl]-1H-imidazo[4,5-b]pyridine (AR-L 115 BS) in various species. No undesirable side-effects or organ damage were observed in baboons after oral application of 2.5, 10 and 30 mg/kg AR-L 115 BS for 13 weeks. Beagle dogs, female miniature pigs and rats had brown-coloured urine after the substance. In miniature pigs and rats this was only observed at high dosage (mini-pig: 10 and 30 mg/kg; rat: 200 mg/kg). Changes in the ECG could be seen in the dose range of 20--40 mg/kg (dog i.v.), 30 mg/kg (dog p.o) and 200 mg/kg (rat p.o). Non-inflammatory mitral valve alterations (i.v. and p.o) as well as spot- and ring-shaped endocardial scleroses underneath the aortic valves in the region of the left cardiac outflow tract (30 mg/kg, p.o.) were found especially in female beagle dogs in the same dose range. The alterations are regarded as being of haemodynamic origin in view of the increased myocardial contractility caused by AR-L 115 BS. Phonocardiographic findings support this view. Besides, several dogs (30 mg/kg) and rats (200 mg/kg) showed myocardial scars. All male miniature pigs which had received 30 mg/kg AR-L 115 BS demonstrated an increase in heart and mitral valve weights. The results of the reproduction-toxicologic investigations gave no indication of an embryotoxic or mutagenic effect for the substance AR-L 115 BS. After comparing the results it can be concluded that the dog is the most sensitive of the species under investigation and that the results obtained in this species should not be generalised. Biochemical tests showed very similar metabolic patterns for AR-L 115 BS in baboons and man. From this it can be deduced that the results obtained in baboons are most likely to be transferable to man.

[Animal studies on the tolerability of NAB 365 (clenbuterol) (author's transl)]. / Tierexperimentelle Untersuchungen zur Verträglichkeit von NAB 365 (Clenbuterol)

The findings of acute, subacute and chronic toxicity studies in rats and dogs with the new beta-mimetic agent 4-amino-alpha-[(tert.-butylamino)methyl]-3,5-dichlorobenzyl alcoholhydrochloride (NAB 365, clenbuterol) are reported. The longest period of investigation, with daily oral administration of the substance, was 18 months (rats) and 12 months (dogs). No abnormalities of any kind were found in the rat studies which could be attributed to the substance. The administration of clenbuterol to dogs produced microscopically small lesions in the myocardium which were not dose-dependent and which were localised entirely in the papillary muscle of the left ventricle. The reason for these lesions was that dogs react to the substance with severe tachycardia, as a consequence of a reduction in diastolic blood pressure produced even by very low oral doses. Oxygen deficiency occurs which affects particularly the papillary muscle. The response to hypoxia is better capillarisation. This causes an improvement in the oxygen supply, which in turn prevents the spread of existing necroses and the development of further lesions.

Prostaglandins, oxygen tension and smooth muscle tone.

1. By using indomethacin to inhibit their intramural synthesis, we have investigated the contribution of prostaglandins to the maintenance of (a) the intrinsic tone of isolated smooth muscle preparations and (b) contractions produced by drugs or high oxygen concentration.2. When treated with indomethacin, the rat stomach strip and chick rectum preparation slowly relaxed, whether they were bathed in Krebs solution or blood. Although their sensitivity to added prostaglandin was somewhat enhanced, they became insensitive to changes in oxygen or glucose concentration. However, another smooth muscle preparation, the rat colon, was neither relaxed by indomethacin nor contracted by high oxygen concentration.3. These results support the hypothesis that intramural generation of prostaglandin maintains the tone of some smooth muscle preparations.4. Contractions of the guinea-pig isolated colon were induced by histamine. These contractions were normally well maintained but in Krebs solution lacking either oxygen or glucose, only the initial spike contraction remained. In the presence of indomethacin the histamine contraction was also poorly sustained, but maintenance was restored by a low concentration of prostaglandin E(2).5. Thus, the effects on smooth muscle of oxygen or glucose lack may also be mediated by reduction in the synthesis or effects of an intramural prostaglandin. Extension of this hypothesis to intestinal and vascular smooth muscle in vivo is discussed.