Evaluation of chronic drug-induced electrophysiological and cytotoxic effects using human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs)

Introduction: Cardiotoxicity is one of the leading causes of compound attrition during drug development. Most in vitro screening platforms aim at detecting acute cardio-electrophysiological changes and drug-induced chronic functional alterations are often not studied in the early stage of drug development. Therefore, we developed an assay using human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) that evaluates both drug-induced acute and delayed electrophysiological and cytotoxic effects of reference compounds with clinically known cardiac outcomes. Methods: hiPSC-CMs were seeded in 48-well multielectrode array (MEA) plates and were treated with four doses of reference compounds (covering and exceeding clinical free plasma peak concentrations -fC_max values) and MEA recordings were conducted for 4 days. Functional-electrophysiological (field-potentials) and viability (impedance) parameters were recorded with a MEA machine. Results: To assess this platform, we tested tyrosine-kinase inhibitors with high-cardiac risk profile (sunitinib, vandetanib and nilotinib) and low-cardiac risk (erlotinib), as well as known classic cardiac toxic drugs (doxorubicin and BMS-986094), ion-channel trafficking inhibitors (pentamidine, probucol and arsenic trioxide) and compounds without known clinical cardiotoxicity (amoxicillin, cetirizine, captopril and aspirin). By evaluating the effects of these compounds on MEA parameters, the assay was mostly able to recapitulate different drug-induced cardiotoxicities, represented by a prolongation of the field potential, changes in beating rate and presence of arrhythmic events in acute (<2 h) or delayed phase ≥24 h, and/or reduction of impedance during the delayed phase (≥24 h). Furthermore, a few reference compounds were tested in hiPSC-CMs using fluorescence- and luminescence-based plate reader assays, confirming the presence or absence of cytotoxic effects, linked to changes of the impedance parameters measured in the MEA assay. Of note, some cardiotoxic effects could not be identified at acute time points (<2 h) but were clearly detected after 24 h, reinforcing the importance of chronic drug evaluation. Discussion: In conclusion, the evaluation of chronic drug-induced cardiotoxicity using a hiPSC-CMs in vitro assay can contribute to the early de-risking of compounds and help optimize the drug development process.

Role of mixed ion channel effects in the cardiovascular safety assessment of the novel anti-MRSA fluoroquinolone JNJ-Q2

BACKGROUND AND PURPOSE

JNJ-Q2, a novel broad-spectrum fluoroquinolone with anti-methicillin-resistant Staphylococcus aureus activity, was evaluated in a comprehensive set of non-clinical and clinical cardiovascular safety studies. The effect of JNJ-Q2 on different cardiovascular parameters was compared with that of moxifloxacin, sparfloxacin and ofloxacin. Through comparisons with these well-known fluoroquinolones, the importance of effects on compensatory ion channels to the cardiovascular safety of JNJ-Q2 was investigated.

EXPERIMENTAL APPROACH

JNJ-Q2 and comparator fluoroquinolones were evaluated in the following models/test systems: hERG-transfected HEK293 cells sodium channel-transfected CHO cells, guinea pig right atria, arterially perfused rabbit left ventricular wedge preparations and in vivo studies in anaesthetized guinea pigs, anaesthetized and conscious telemetered dogs, and a thorough QT study in humans.

KEY RESULTS

The trend for effects of JNJ-Q2 on Tp-Te, QT, QRS and PR intervals in the non-clinical models and the plateau in QTc with increasing plasma concentration in humans are consistent with offsetting sodium and calcium channel activities that were observed in the non-clinical studies. These mixed ion channel activities result in the less pronounced or comparable increase in QTc interval for JNJ-Q2 compared with moxifloxacin and sparfloxacin despite its greater in vitro inhibition of I(Kr).

CONCLUSIONS AND IMPLICATIONS

Based on the non-clinical and clinical cardiovascular safety assessment, JNJ-Q2 has a safe cardiovascular profile for administration in humans with comparable or reduced potential to prolong QT intervals, compared with moxifloxacin. The results demonstrate the importance of compensatory sodium and calcium channel activity in offsetting potassium channel activity for compounds with a fluoroquinolone core.

Repolarization reserve determines drug responses in human pluripotent stem cell derived cardiomyocytes

Unexpected induction of arrhythmias in the heart is still one of the major risks of new drugs despite recent improvements in cardiac safety assays. Here we address this in a novel emerging assay system. Eleven reference compounds were administrated to spontaneously beating clusters of cardiomyocytes from human pluripotent stem cells (hPSC-CM) and the responses determined using multi-electrode arrays. Nine showed clear dose-dependence effects on field potential (FP) duration. Of these, the Ca(2+) channel blockers caused profound shortening of action potentials, whereas the classical hERG blockers, like dofetilide and d,l-sotalol, induced prolongation, as expected. Unexpectedly, two potent blockers of the slow component of the delayed rectifier potassium current (I(Ks)), HMR1556 and JNJ303, had only minor effects on the extracellular FP of wild-type hPSC-CM despite evidence of functional I(Ks) channels. These compounds were therefore re-evaluated under conditions that mimicked reduced "repolarization reserve," a parameter reflecting the capacity of cardiomyocytes to repolarize and a strong risk factor for the development of ventricular arrhythmias. Strikingly, in both pharmacological and genetic models of diminished repolarization reserve, HMR1556 and JNJ03 strongly increased the FP duration. These profound effects indicate that I(Ks) plays an important role in limiting action potential prolongation when repolarization reserve is attenuated. The findings have important clinical implications and indicate that enhanced sensitization to repolarization-prolonging compounds through pharmacotherapy or genetic predisposition should be taken into account when assessing drug safety.

Establishment of multiple sublineages of H5N1 influenza virus in Asia: implications for pandemic control

Preparedness for a possible influenza pandemic caused by highly pathogenic avian influenza A subtype H5N1 has become a global priority. The spread of the virus to Europe and continued human infection in Southeast Asia have heightened pandemic concern. It remains unknown from where the pandemic strain may emerge; current attention is directed at Vietnam, Thailand, and, more recently, Indonesia and China. Here, we report that genetically and antigenically distinct sublineages of H5N1 virus have become established in poultry in different geographical regions of Southeast Asia, indicating the long-term endemicity of the virus, and the isolation of H5N1 virus from apparently healthy migratory birds in southern China. Our data show that H5N1 influenza virus, has continued to spread from its established source in southern China to other regions through transport of poultry and bird migration. The identification of regionally distinct sublineages contributes to the understanding of the mechanism for the perpetuation and spread of H5N1, providing information that is directly relevant to control of the source of infection in poultry. It points to the necessity of surveillance that is geographically broader than previously supposed and that includes H5N1 viruses of greater genetic and antigenic diversity.

A new method to calculate the beat-to-beat instability of QT duration in drug-induced long QT in anesthetized dogs

INTRODUCTION

Instability of QT duration is a marker to predict Torsade de Pointes (TdP) associated with both congenital and drug-induced long QT syndrome. We describe a new method for the quantification of instability of repolarization.

METHODS

Female, adult beagle dogs anesthetized with a potent morphinomimetic were treated with either solvent (n=7) or dofetilide (n=7). Poincaré plots with QT(n) versus QT(n+1) were constructed to visualize the beat-to-beat variation in QT intervals from the lead II ECG. Short-term instability (STI), long-term instability (LTI) and total instability (TI) were quantified by calculating the distances of 30 consecutive data-points from the x and y-coordinate to the "centre of gravity" of the data cluster. Dofetilide at 0.0025 to 0.04 mg/kg i.v. (plasma concentrations of 4+/-0.6 to 41+/-2.7 ng/ml), dose-dependently prolonged QT and QTcV (at 0.04 mg/kg i.v.: QT: 280+/-ms versus 236+/-5 ms with solvent; p<0.05 and QTcV: 290+/-9 ms versus 252+/-4 ms with solvent; p<0.05). Concomitantly, the compound induced an increase in the instability parameters in a similar dose-dependent manner (at 0.04 mg/kg i.v.: TI: 6.8+/-0.9 ms versus 1.7+/-0.3 ms; p<0.05, LTI: 3.6+/-0.5 ms versus 1.0+/-0.2 ms; p<0.05 and STI: 4.2+/-0.6 ms versus 1.0+/-0.2 ms; p<0.05). The increases induced by dofetilide were associated with a high incidence of early afterdepolarizations (EADs) in the endocardial monophasic action potential (in 6 out of the 7 compound-treated animals versus 0 out of the 7 solvent animals; p<0.05).

CONCLUSION

Quantification of beat-to-beat QT instability by our method clearly detects changes in short-term, long-term and total instability induced by dofetilide, already at pre-arrhythmic doses. Dofetilide administration to anesthetized dogs prolongs ventricular repolarization, concomitantly increases beat-to-beat QT instability and induces early after depolarizations (EADs). As such, the use of these parameters in this in vivo model shows clear potential for risk identification in cardiovascular safety assessment.

Both β-adrenergic receptor stimulation and cardiac tissue type have important roles in elucidating the functional effects of IKs channel blockers in vitro

INTRODUCTION

Conflicting results associated with the use of I(Ks) blockers on the action potential duration (APD) have raised a question as to whether the variable results arise from the use of different cardiac tissues, beta-adrenergic stimulation, or by the "selectivity" of the chosen I(Ks) blockers.

METHODS

We used the highly selective I(Ks) blocker (-)-[3R, 4S] chromanol 293B [(-) chromanol] to mimic drug-induced long QT1 in isolated rabbit Purkinje fibers, papillary muscles, and ventricular trabeculae using the conventional microelectrode technique.

RESULTS

I(Ks) block with (-) chromanol at 1 x 10(-5) M did not significantly change the APD at different stimulation rates in all three cardiac tissues. Isoproterenol (Iso:1 x 10(-7) M) shortened APD(90), and (-) chromanol (1 x 10(-5) M) largely prevented this shortening in isolated papillary muscles at 1 Hz [-3% with Iso combined (-) chromanol group versus -16% with iso group; p<0.05] and also at 2 Hz (+7% versus -25% with Iso group; p<0.05), but did not significantly prevent this shortening in isolated Purkinje fibers. In isolated trabeculae, (-) chromanol combined with Iso significantly prolonged the APD(90) by 15% at 1 Hz (versus -10% with Iso group; p<0.05) and by 5% at 2 Hz (versus -11% with Iso group; p<0.05).

DISCUSSION

Our study shows that only during beta-adrenoceptor stimulation, pharmacological inhibition of the I(Ks) current plays an important role in the APD recorded from isolated ventricular trabeculae and papillary muscles, but not from Purkinje fibers. These results indicate that the APD prolonging effects of I(Ks)channel blockers during beta-adrenergic receptor stimulation can only be detected from isolated rabbit papillary muscles and ventricular trabeculae, but not Purkinje fibers.

QT and JT dispersion in the drug-induced long QT syndrome in anaesthetized rabbits is accurately detected by a three-lead surface ECG measurement

INTRODUCTION

QT dispersion (QTd) can be measured from three leads of the ECG in patients with myocardial ischemia. However, whether QT and JT dispersion (QTd, JTd) can be calculated from a three-lead of the ECG in drug-induced long QT syndrome (LQTS) in animals remains elusive. Therefore, we determined to what extent a three-lead measurement of the surface ECG accurately detects dispersion of QT and JT in comparison with multi-lead assessments in anaesthetized rabbits, challenged with methoxamine and additionally infused intravenously with solvent or dofetilide.

METHODS

Using several ECG leads in anaesthetized rabbits challenged intravenously with an alpha(1)-adrenoceptor agonist methoxamine, we assessed the QT and JT interval, as well as QT and JT dispersion, at baseline and in response to solvent or dofetilide (0.02 or 0.04 mg/kg/min iv for 60 min), an I(Kr) blocker. For that purpose, we recorded and analyzed the surface ECG and assessed QT and JT dispersion by four methods: (1) 12-lead ECG; (2) six precordial leads (V1-V6); (3) three leads most likely to contribute to the dispersion (aVF, V1, and V4); (4) three quasi-orthogonal leads (aVF, I, and V2). QT and JT dispersion were significantly lower in 6- and 3-lead measurements than in 12-lead measurement, both at baseline and during infusion of solvent or dofetilide. At 5 and 10 min of infusion, dofetilide at 0.02 or 0.04 mg/kg/min iv markedly increased QT and JT dispersion by 100% to 500% in all four ECG lead combinations. This dose regimen of dofetilide markedly prolonged QT and JT intervals in lead II, and was associated with high incidences of polymorphous ventricular tachycardia (PVT: 30% at 0.02 mg/kg/min; 100% at 0.04 mg/kg/min) and of ventricular fibrillation (VF: 17% with 0.02 mg/kg/min; 58% with 0.04 mg/kg/min).

CONCLUSIONS

Our present study shows that the measurement of QT and JT dispersion in three surface ECG leads only (aVF, I, V2 or aVF, V1 V4), instead of 12 ECG leads, is an appropriate approach to assess drug-induced heterogeneity or dispersion of ventricular repolarization in anaesthetized rabbits, both at baseline and during arrhythmogenic sensitization with methoxamine and challenged with dofetilide.

Female gender is a risk factor for drug-induced long QT and cardiac arrhythmias in an in vivo rabbit model

INTRODUCTION

Clinical observations and in vitro experimental data indicate that females have a longer QT interval than males, which is associated with a higher risk of drug-induced cardiac arrhythmias. Little is known about this gender difference in anesthetized animals, which may affect the outcome of in vivo drug tests.

METHODS AND RESULTS

We evaluated potential gender differences in ventricular repolarization (QT, QTc, JT, and JTc interval) and its dispersion, as well as in its response to dofetilide, an IKr blocker, in anesthetized rabbits challenged with the alpha1-adrenoceptor agonist methoxamine. A 12-lead ECG was recorded during the experiments. At baseline, there were no significant gender differences in ventricular repolarization values in male and female rabbits under anesthesia. Dofetilide (0.04 mg/kg/min IV for 60 min; n = 10 per gender) produced marked prolongation of the ventricular repolarization time and its dispersion, associated with a high incidence of polymorphic ventricular tachycardia (PVT; 100% in females vs 80% in males) and ventricular fibrillation (VF; 80% in females vs 50% in males; P > 0.05). QT and JT interval at 2 minutes as well as QT and JT dispersion at 10 and 30 minutes during dofetilide infusion were significantly higher in female than in male rabbits. After 30 minutes of dofetilide infusion, 10 of 10 female rabbits had severe cardiac arrhythmias (complete AV block, PVT, or VF), so ECG parameters were impossible to assess (vs 3/10 males with severe cardiac arrhythmias; P < 0.05). During dofetilide infusion, female rabbits developed complete AV block, PVT, or VF at doses about 50% lower than those given to males.

CONCLUSION

The present study indicates that female rabbits are more susceptible to drug-induced long QT and cardiac arrhythmias than are male rabbits; therefore, female rabbits are more appropriate for testing drug-induced cardiac arrhythmias.

Species plays an important role in drug-induced prolongation of action potential duration and early afterdepolarizations in isolated Purkinje fibers

INTRODUCTION

Although isolated Purkinje fibers (PFs) often are used to evaluate the electrophysiologic effects of new drugs in terms of prolongation of action potential duration (APD) and induction of early afterdepolarizations (EADs), species differences in this respect remain elusive. We evaluated potential species-specific differences in drug-induced prolongation of APD and EADs in isolated PF from various species.

METHODS AND RESULTS

Using a microelectrode technique, PFs (n = 7 to 11 per species) were isolated from hearts of rabbits, guinea pigs, dogs, swine, goats, or sheep, superperfused in Tyrode's solution with dofetilide (1 x 10(-8) M) or quinidine (1 x 10(-5) M) for 25 minutes, and stimulated at 1 Hz for 20 minutes and at 0.2 Hz for another 5 minutes. Dofetilide increased APD at 90% repolarization (APD90) at 1 Hz by 83% (rabbit), 24% (guinea pig), 65% (dogs), 18% (swine), 61% (goat), and 30% (sheep), and prolonged APD90 at 0.2 Hz by 187% (rabbit), 31% (guinea pig), 154% (dog), 17% (swine), 61% (goat), and 8% (sheep). Similarly, quinidine changed APD90 by 93% (rabbit), 0% (guinea pig), 16% (dog), -3% (swine), 0% (goat), and -24% (sheep) at 1 Hz, and by 124% (rabbit), 15% (guinea pig), 53% (dog), 17% (swine), 11% (goat), and -39% (sheep) at 0.2 Hz in PF. During superfusion of dofetilide or quinidine, EADs occurred in most preparations in rabbit PFs at 0.2 Hz, but not in any of the PFs from other species at 0.2 Hz.

CONCLUSION

Our study demonstrates that species plays an important role in the response of PF to drug-induced prolongation of APD and EADs. Rabbit PFs constitute the most sensitive model for detecting drug-induced, potential long APD and proarrhythmogenic effects in vitro.

Nonselective I(Kr)-blockers do not induce torsades de pointes in the anesthetized rabbit during alpha1-adrenoceptor stimulation

Selective I(Kr)- (the rapid component of the delayed rectifier potassium current) blockers are known to induce torsades de pointes (TdPs) in anesthetized rabbits during alpha1-adrenoreceptor stimulation. However, effects of nonselective I(Kr)-blockers, which produce TdPs in other animal models and in humans, are not known in this model. We examined two nonselective I(Kr)-blockers (quinidine, 1.25 mg/kg/min i.v [n = 7]; and terfenadine, 0.31 mg/kg/min i.v. [n = 7]) for their effects on electrocardiographic parameters and on incidence of cardiac arrhythmias in anesthetized rabbits during alpha1-adrenoceptor stimulation with methoxamine. We compared the drugs with two highly selective I(Kr)-blockers (dofetilide, 0.04 mg/kg/min i.v. [n = 7]; and clofilium, 0.08 mg/kg/min i.v. [n = 6]). Polymorphic ventricular tachycardia or TdPs were induced by dofetilide and clofilium at mean doses > or =0.33 mg/kg and 0.4 mg/kg i.v., in all animals tested (vs. none in solvent; p < 0.05). TdPs usually developed into ventricular fibrillation and developed after prolongation of QT/JT interval and of QT dispersion. Terfenadine and quinidine significantly increased PQ, QT, and QTc interval and largely increased QRS duration and QT dispersion. These compounds elicited intraventricular conduction defects and cardiac arrest, due to asystole, in all animals tested (vs. 0% in solvent; p < 0.05). Interestingly, these two nonselective I(Kr)-blockers did not produce TdPs or ventricular fibrillation in any animals tested. Our results thus indicate that selective I(Kr)-blockers elicit TdPs, whereas nonselective I(Kr)-blockers do not induce this type of arrhythmia in this rabbit model. Consequently, it should be noted that this rabbit model is not always useful to evaluate nonselective I(Kr)-blocker-induced TdPs and QT interval and QT dispersion, rather than TdPs, are also important indicators for drug-induced cardiac arrhythmias.

Are there sex-specific differences in ventricular repolarization or in drug-induced early afterdepolarizations in isolated rabbit purkinje fibers?

Women are known to have a longer QT interval than men and a greater propensity toward drug-induced "torsades de pointes" (TdPs). However, little is known about these sex differences in isolated cardiac tissues. We evaluated potential sex differences in repolarization in isolated rabbit Purkinje fibers using a microelectrode technique. Isolated male or female Purkinje fibers were perfused in a Tyrode's solution with solvent, dofetilide (1 x 10(-8) M) or quinidine (1 x 10(-5) M), and stimulated at 1 or 0.2 Hz. Female Purkinje fibers with solvent (n = 11) tended to have a longer duration of the action potential at 90% repolarization (APD90) than male fibers with solvent (n = 10): 331 (median) vs. 272 ms at 1 Hz (p > 0.05); 473 vs. 367 ms at 0.2 Hz (p < 0.05). Dofetilide (1 x 10(-8) M) significantly increased APD90 more in female Purkinje fibers (n = 11) than in male fibers (n = 10): 670 vs. 385 ms at 1 Hz, at 20 min after the infusion (p < 0.05), and 1,000 vs. 937 ms at 0.2 Hz at the end of the 25-min infusion (p < 0.05), respectively. Quinidine (1 x 10(-5) M) tended to increase APD90 more in female Purkinje fibers (n = 11) than in male fibers (n = 10): 705 vs. 500 ms at 1 Hz, at 20 min after the infusion (p > 0.05). Furthermore, dofetilide (1 x 10(-8) M) and quinidine (1 x 10(-5) M) elicited a higher incidence of early afterdepolarizations in female Purkinje fibers than in male fibers at 0.2 Hz (100 vs. 60%, p < 0.05; and 91 vs. 50%, p > 0.05). Our data indicate that female Purkinje fibers tend to have longer ventricular repolarization and are at higher risk of drug-induced early afterdepolarizations at a slow stimulation rate than male fibers. This may contribute to a sex difference in QT interval and to a greater tendency on the part of women to the development of drug-induced TdPs.

Reduction in QT dispersion and ventricular arrhythmias by ischaemic preconditioning in anaesthetized, normotensive and spontaneously hypertensive rats

QT dispersion is a marker for dispersion of ventricular repolarization and electrical instability of the heart. However, QT dispersion remains undocumented in both normotensive rats (NTRs) and spontaneously hypertensive rats (SHRs), in particular in conditions of myocardial ischaemia/reperfusion (isch./rep.) and ischaemic preconditioning (IP). Therefore, we assessed the effects of IP on the dynamic change of QT and QTc dispersion during isch./rep., and on isch.- and rep.-induced ventricular arrhythmias in both NTRs and SHRs. Isch. and rep. were produced by occlusion and release of a snare around the left coronary artery in all rats. The effect of IP (three cycles of 3 min coronary artery occlusion and 5 min rep.) on myocardial repolarization and on development of isch.- and rep.-induced ventricular arrhythmias was studied in 12 NTRs and 12 SHRs. Another 12 NTRs or 12 SHRs were subjected to 10 min of isch. followed by 10 min rep. without IP. SHRs have significantly longer QT- and QTc-intervals as well as QT and QTc dispersion before isch. compared to NTRs. Myocardial isch. and early rep. largely increased QT and QTc dispersion in both NTRs and SHRs and resulted in a high incidence of isch.- and rep.-induced ventricular tachycardia (VT) and fibrillation (VF). IP significantly reduced QT and QTc dispersion in SHRs before isch., and remarkably reduced the elevation of QT and QTc dispersion during a prolonged period of isch. and rep. in all rats. This protective effect on electrophysiology of IP was associated with an antiarrhythmic effect against both isch.- and rep.-induced ventricular arrhythmias in NTRs and SHRs. Our data indicate that: 1) SHRs have a significantly higher baseline dispersion of ventricular repolarization than NTRs; 2) IP provides protection against ventricular arrhythmias in SHRs; 3) the increasing QT dispersion provoked by myocardial isch. and rep. is associated with a high incidence of isch.- and rep.-induced ventricular arrhythmias and; 4) the reduction of QT dispersion by IP may be involved in its protective effect against isch.- and rep.-induced arrhythmias in both NTRs and SHRs.

Ischemia/reperfusion-induced arrhythmias in anaesthetized rats: a role of Na+ and Ca2+ influx

We hypothesized that by limiting the Na+ and Ca2+ loading by a blocker/inhibitor of the Na+ channel (lidocaine), Na+ overload (R56865: N-[1-[4-(4-fluorophenoxy)butyl]-4-piperidinyl]-N-methyl-2-benzothiazo lamine), Ca2+ channel (verapamil), Na+ -H+ exchange (ethylisobutyl amiloride) or of Na+ -Ca2+ exchange (No. 7943: 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate), it should be possible to reduce ischemia/reperfusion-induced arrhythmias. To test this hypothesis, we used anaesthetized rats subjected to 5 min of coronary artery occlusion followed by 10 min of reperfusion to study antiarrhythmic effects of above compounds on reperfusion-induced ventricular premature beats, ventricular tachycardia, and reversible and irreversible ventricular fibrillation. Compound or saline was administered as an intravenous bolus injection at 5 min before ischemia. Pretreatment with lidocaine (5 mg/kg), verapamil (0.63 mg/kg), R56865 (0.63 mg/kg) or ethylisobutyl amiloride (1.25 mg/kg) significantly reduced or abolished all types of ventricular arrhythmias. However, pretreatment with verapamil was associated with second or third degree heart block in 3 out of 12 animals. Pretreatment with No. 7943 did not significantly influence the ischemia/reperfusion-induced ventricular arrhythmias. The present results suggest that both intracellular Na+ -and Ca2+ -loading play important roles in reperfusion-induced ventricular arrhythmias and the inhibition of Na+ -Ca2+ exchange to limit Ca2+ loading probably does not play any important role in ischemia/reperfusion-induced arrhythmias in anaesthetized rats.

Time-related mapping of quantitative trait loci underlying tiller number in rice

Using time-related phenotypic data, methods of composite interval mapping and multiple-trait composite interval mapping based on least squares were applied to map quantitative trait loci (QTL) underlying the development of tiller number in rice. A recombinant inbred population and a corresponding saturated molecular marker linkage map were constructed for the study. Tiller number was recorded every 4 or 5 days for a total of seven times starting at 20 days after sowing. Five QTL were detected on chromosomes 1, 3, and 5. These QTL explained more than half of the genetic variance at the final observation. All the QTL displayed an S-shaped expression curve. Three QTL reached their highest expression rates during active tillering stage, while the other two QTL achieved this either before or after the active tillering stage.

Antifibrillary action of class I-IV antiarrhythmic agents in the model of ventricular fibrillation threshold of anesthetized guinea pigs

We compared the effects of class I-IV antiarrhythmic agents on the ventricular fibrillation threshold (VFT) induced by electrical stimulation directly on the myocardium in anesthetized, open-chest guinea pigs. VFT was assessed by determining the intensity (mA) of electrical current required to induce ventricular fibrillation (VF) and is expressed as a percentage change of the baseline premedication value. The following antiarrhythmic agents or their solvent were administered intravenously (i.v.) to pentobarbital-anesthetized animals (n = 6-12 per group): class I antiarrhythmic agent encainide (1.5 mg/kg); class II antiarrhythmic agents atenolol (2.5 mg/kg), metoprolol (2.5 mg/kg), and nebivolol (2.5 mg/kg); class III antiarrhythmic agents dofetilide (0.08 mg/kg), terikalant (0.04 mg/kg), and DL-sotalolol (10 mg/kg); and class IV antiarrhythmic agent verapamil (0.16 mg/kg). The antiarrhythmic compounds or their solvents resulted in the following changes in the VFT at 15 min after treatment: saline control, 1 +/- 14% (mean +/- SEM) from its baseline value; 10% hydroxypropyl-beta-cyclodextrine (CD), 4 +/- 13%; encainide, 183 +/- 46% (p < 0.05 vs. saline); atenolol, 66 +/- 23% (p > 0.05 vs. saline); metoprolol, 89 +/- 25% (p > 0.05 vs. saline); nebivolol, 224 +/- 58% (p < 0.05 vs. 10% CD); DL-sotalol, 485 +/- 119% (p < 0.05 vs. saline); dofetilide, 357 +/- 69% (p < 0.05 vs. saline); terikalant, 487 +/- 183% (p < 0.05 vs. saline), and verapamil, -17 +/- 21% (p > 0.05 vs. saline). At the doses used, all compounds significantly reduced heart rate (HR).(ABSTRACT TRUNCATED AT 250 WORDS)

Does the antiarrhythmic effect of ischemic preconditioning in rats involve the L-arginine nitric oxide pathway?

Ischemic preconditioning (PC) has been shown to limit ischemia- and reperfusion-induced arrhythmias. We wished to determine whether the antiarrhythmic effect of PC would be affected by inhibition of the L-arginine nitric oxide (NO) pathway in anesthetized rats. Ischemia and reperfusion were produced by occlusion and release of a snare around the left coronary artery in all rats. The effect of PC (three cycles of 2-min coronary artery occlusion and 5-min reperfusion) on development of reperfusion-induced arrhythmias after 5-min coronary artery occlusion was studied in 12 rats. In 24 other rats, the specific NO synthesis inhibitor NG-monomethyl-L-arginine (L-NMMA 10 mg/kg, n = 12) or the muscarinic receptor antagonist-NO synthesis inhibitor nitro-L-arginine methyl ester (L-NAME 10 mg/kg, n = 12), was administered intravenously (i.v.) before PC. In control groups, solvent (n = 15), L-NAME (10 mg/kg i.v., n = 12), L-NMMA (10 mg/kg i.v., n = 12), or L-arginine (L-Arg 100 mg/kg i.v., n = 12) was administered to rats 5 min before coronary artery occlusion without PC. PC significantly reduced the incidence of ventricular premature beats (VPBs) from 100% in the non-PC solvent group to 17%, decreased the incidence of ventricular tachycardia (VT) from 93 to 8%, and abolished the incidence of reversible and irreversible ventricular fibrillation (RVF and IVF: 87 and 47% in the non-PC solvent group, respectively). L-NAME and L-NMMA did not significantly affect the protective effect of PC on reperfusion-induced arrhythmias.(ABSTRACT TRUNCATED AT 250 WORDS)

Antiarrhythmic effects of nebivolol in experimental models in vivo

Antiarrhythmic effects of nebivolol, a cardioselective beta 1-adrenoceptor blocker facilitating vascular release of nitric oxide (NO), were investigated in different experimental models. In the reperfusion-induced arrhythmias after 5 min of left coronary artery ligation in rats, nebivolol 1.25 mg/kg intravenously (i.v.) reduced the incidence of ventricular tachycardia (VT) from 85% in the solvent group to 33% (p < 0.05) and decreased that of ventricular fibrillation (VF) to 20% vs. 67% in the solvent group, (p < 0.05). In ischemia-induced arrhythmias resulting from 20-min coronary artery ligation in rats, nebivolol 1.25 mg/kg i.v. reduced the incidence of VT to 50% as compared with 87% in the solvent group and decreased the incidence of VF to 17% as compared with 67% in the solvent group (p < 0.05). In electrically stimulated hearts in open-chest guinea pigs, nebivolol produced a dose-dependent (0.16-2.5 mg/kg i.v.) increase in VF threshold (VFT). In guinea pigs with ouabain-induced cardiotoxicity, nebivolol 1.25 mg/kg i.v. (n = 7) doubled the doses of ouabain required to provoke toxic ECG changes as compared with those in the solvent-pretreated group. In rats with aconitine-induced cardiac toxicity, nebivolol 1.25 mg/kg i.v. significantly reduced the incidence of ventricular arrhythmias. Our results demonstrate that nebivolol suppresses arrhythmias in various experimental models in vivo.

Time course of the effects of a single bolus injection of F(ab')2 fragments of the antiplatelet GPIIb/IIIa antibody 7E3 on arterial eversion graft occlusion, platelet aggregation, and bleeding time in dogs

The time course of the effects of a single intravenous bolus injection of 10 mg/kg aspirin or 0.8 mg/kg F(ab')2 fragments of the monoclonal antiplatelet glycoprotein IIb/IIIa receptor antibody 7E3 [7E3-F(ab')2] on arterial occlusion, platelet aggregation, and bleeding time was studied in 30 dogs with an everted (inside out) carotid arterial segment inserted into the femoral artery. In the absence of an antiplatelet agent, the eversion grafts occluded spontaneously with platelet-rich thrombus within 30 minutes. With aspirin, arterial occlusion persisting for 2 hours occurred in 5 of 10 dogs and cyclic occlusion and reflow in 4 animals; arterial occlusion was observed in all dogs at 24 hours. With 7E3-F(ab')2, arterial patency persisted throughout a 2-hour observation period in all of 10 dogs and for 24 hours in 4 of the 10 dogs. Contralateral eversion grafting 24 hours after aspirin or 7E3-F(ab')2 injection was associated with graft patency for 2 hours in 1 of 5 aspirin dogs and in 3 of 5 7E3-F(ab')2 dogs; patency persisted for 24 hours. In dogs grafted 48 hours after aspirin or 7E3-F(ab')2 injection, patency at 24 hours was seen in 0 of 5 dogs given aspirin and 3 of 5 dogs given 7E3-F(ab')2. The overall frequencies of arterial graft patency at 2, 24, 48, and 72 hours after study drug injection were significantly higher in the 7E3-F(ab')2 groups than in the aspirin groups (P < .0005, n = 10 in each group; P < .05, n = 15; P < .005, n = 15; and P = .05, n = 5, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Antithrombotic effects and bleeding time prolongation with synthetic platelet GPIIb/IIIa inhibitors in animal models of platelet-mediated thrombosis

Cyclic Arg-Gly-Asp (RGD) containing synthetic peptides such as L-cysteine, N-(mercaptoacetyl)-D-tyrosyl-L-arginylglycyl-L-alpha-aspartyl- cyclic (1-->5)-sulfide, 5-oxide (G4120) and acetyl-L-cysteinyl-L-asparaginyl-L-propyl-L-arginyl-glycyl-L-alpha- aspartyl-[0-methyltyrosyl]-L-arginyl-L-cysteinamide, cyclic 1-->9-sulfide (TP9201) bind with high affinity to the platelet GPIIb/IIIa receptor. The relationship between antithrombotic effect, ex vivo platelet aggregation and bleeding time prolongation with both agents was studied in hamsters with a standardized femoral vein endothelial cell injury predisposing to platelet-rich mural thrombosis, and in dogs with a carotid arterial eversion graft inserted in the femoral artery. Intravenous administration of G4120 in hamsters inhibited in vivo thrombus formation with a 50% inhibitory bolus dose (ID50) of approximately 20 micrograms/kg, ex vivo ADP-induced platelet aggregation with ID50 of 10 micrograms/kg, and bolus injection of 1 mg/kg prolonged the bleeding time from 38 +/- 9 to 1,100 +/- 330 s. Administration of TP9201 in hamsters inhibited in vivo thrombus formation with ID50 of 30 micrograms/kg, ex vivo platelet aggregation with an ID50 of 50 micrograms/kg and bolus injection of 1 mg/kg did not prolong the template bleeding time. In the dog eversion graft model, infusion of 100 micrograms/kg of G4120 over 60 min did not fully inhibit platelet-mediated thrombotic occlusion but was associated with inhibition of ADP-induced ex vivo platelet aggregation and with prolongation of the template bleeding time from 1.3 +/- 0.4 to 12 +/- 2 min.(ABSTRACT TRUNCATED AT 250 WORDS)

R 56 865, a Na+/Ca(2+)-overload inhibitor, protects against aconitine-induced cardiac arrhythmias in vivo

Disturbances in cellular Na+/Ca2+ homeostasis may play a central role in the pathogenesis of ventricular arrhythmias and cell damage induced by the alkaloids veratridine and aconitine in vitro. To test this hypothesis in vivo, the effects on aconitine-induced arrhythmias of intravenous (i.v.) pretreatment with R 56 865 (a Na(+)- and Ca(2+)-overload inhibitor) were compared with those of lidocaine, verapamil, and tetrodotoxin (TTX) in anesthetized rats (n = 10 for each compound). The i.v. bolus injection of aconitine (6.2, 12.5, or 25 micrograms/kg) induced ventricular premature beats (VPBs), ventricular tachycardia (VT), ventricular fibrillation (VF), and mortality in a dose-dependent manner. Because aconitine at a dose of 12.5 micrograms/kg i.v. resulted in a high incidence of ventricular arrhythmias as well as mortality, this dose was used in further tests. Pretreatment of rats with R 56 865 (1.25 mg/kg) significantly reduced the incidences of aconitine-induced VT and VF, as well as mortality, relative to the saline control group. Pretreatment with verapamil (0.32 mg/kg), was ineffective against aconitine-induced ventricular arrhythmias and mortality. Pretreatment with lidocaine (10 mg/kg) significantly reduced the incidence of VT and caused low but not significant reductions in the incidences of VF and mortality induced by aconitine. Pretreatment with a selective sodium channel blocker TTX (4 micrograms/kg) also significantly reduced the incidences of VT, VF, and mortality elicited by aconitine. These results suggest that intracellular Na+ loading plays an important role in aconitine-induced ventricular arrhythmias; the Ca(2+)-overload after Na+ loading elicited by aconitine is not likely to be mediated by increased Ca2+ influx through a slow channel.

Inhibition of Na+/Ca2+ overload with R 56,865 protects against cardiac arrhythmias elicited by ouabain in vivo in guinea-pigs

Several studies have suggested a central role for Na+/Ca2+ in the pathogenesis of ouabain-induced cardiac arrhythmias. To test this hypothesis, the effects on ouabain-induced arrhythmias of i.v. pretreatment with R 56,865, a Na+ and Ca2+ overload inhibitor, were compared with those of lidocaine, verapamil and tetrodotoxin in anesthetized guinea-pigs. Cardiac arrhythmias were induced by i.v. infusion of ouabain (10 micrograms/kg per min). All nine guinea-pigs pretreated with saline developed ventricular premature beats at an ouabain dose of 159 +/- 9 micrograms/kg (mean +/- S.E.M.), ventricular tachycardia at a dose of 190 +/- 10 micrograms/kg, ventricular fibrillation at a dose of 253 +/- 18 micrograms/kg and died at a dose of 269 +/- 16 micrograms/kg; none of the animals developed heart block or asystole. Pretreatment with R 56,865 (1.25 mg/kg, n = 6) significantly increased the ouabain doses required to induce ventricular premature beats, ventricular tachycardia, ventricular fibrillation and death relative to those for the saline group. Pretreatment with a Ca2+ entry blocker verapamil (0.32 mg/kg, n = 6) also significantly increased the ouabain doses required to provoke ventricular arrhythmias and death; this medication was associated with second or third degree heart block during ouabain infusion in four out of six animals. Pretreatment with lidocaine (10 mg/kg, n = 6) caused a significant increase in the dose of ouabain needed to initiate cardiac arrhythmias and to cause death. Pretreatment with a selective Na+ channel blocker tetrodotoxin (4 micrograms/kg, n = 6) also significantly increased the ouabain doses required to provoke ventricular premature beats, ventricular tachycardia, ventricular fibrillation, and death.(ABSTRACT TRUNCATED AT 250 WORDS)

Free polymerized hemoglobin versus hydroxyethyl starch in resuscitation of hypovolemic dogs

Polymerized bovine hemoglobin (PBH) was compared with hydroxyethyl starch (HES) in a hypovolemic shock model. Eighteen dogs were subjected to hemorrhage; systolic arterial blood pressure was maintained at 40 mm Hg for 30 min (mean blood pressure 37.8 +/- 4.7 [SD] mm Hg). Resuscitation was conducted by infusing their own shed blood (control group) or 6% HES (mol wt 200,000) in 0.9% NaCl (HES group) or PBH (PBH group), both in an equal amount to the shed blood. Directly after infusion, oxygen delivery and consumption returned to prehemorrhage levels in all three groups. In the HES group, the lowered arterial oxygen content was compensated by a 158% increase in cardiac output, in contrast to an increase of 31% and 9%, respectively, in the control and PBH groups. Early recovery from hypovolemic shock with regard to oxygen transport and delivery in the PBH group seemed to be comparable to the control group, without the increase in cardiac output seen with HES infusion.

G4120, an Arg-Gly-Asp containing pentapeptide, enhances arterial eversion graft recanalization with recombinant tissue-type plasminogen activator in dogs

The effects of G4120, a cyclic Arg-Gly-Asp (RGD) containing peptide which inhibits fibrinogen binding to the platelet receptor GPIIb/IIIa, on thrombolysis with recombinant tissue-type plasminogen activator (rt-PA) were investigated in a combined arterial and venous thrombosis model in heparinized dogs. The arterial thrombus model consisted of a 3 cm everted (inside-out) carotid arterial segment inserted into a transsected femoral artery which occludes within 30 min with platelet-rich material and which is resistant to recanalization with 0.5 mg/kg rt-PA. The venous thrombus was a 125I-fibrin labeled whole blood clot produced in the contralateral femoral vein. In 5 dogs given an intravenous bolus of 0.05 mg/kg G4120 followed by a continuous infusion of 0.05 mg/kg per hour for 3 h (group I), arterial occlusion persisted throughout a 4 h observation period and was still present at 24 h in all dogs; the extent of venous clot lysis after 120 min was 27 +/- 7%. In 5 dogs given the same infusion of G4120 in combination with 0.5 mg/kg rt-PA over 60 min, recanalization of the arterial graft occurred in all dogs, within 13 +/- 2 min and persisted throughout the observation period of 4 h (p = 0.01 versus G4120 or rt-PA alone); at 24 h, however, all grafts were occluded. Venous clot lysis in this group was 75 +/- 8% (p = 0.002 versus G4120 alone and p = NS versus rt-PA alone). Pathologic analysis revealed platelet-rich or mixed thrombus with platelet-rich and erythrocyte-rich zones.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative thrombolytic properties of tissue-type plasminogen activator (t-PA), single-chain urokinase-type plasminogen activator (u-PA) and K1K2Pu (a t-PA/u-PA chimera) in a combined arterial and venous thrombosis model in the dog

The chimeric molecule K1K2Pu, comprising the two kringle domains (K1 and K2) of tissue-type plasminogen activator (t-PA) and the COOH-terminal region with the serine protease domain (Pu) of urokinase-type plasminogen activator (u-PA), was previously shown to have a 5- to 10-fold reduced clearance rate with maintained specific thrombolytic activity, resulting in an increased thrombolytic potency in animal models of venous and arterial thrombosis. To document the thrombolytic potential of K1K2Pu, the thrombolytic potency and fibrin specificity were studied in a combined platelet-rich arterial eversion graft thrombosis and venous whole blood clot model in heparinized dogs (100 U/kg bolus and 50 U/kg per h infusion). Dose-response effects of bolus injections of K1K2Pu (0.032 to 0.25 mg/kg) were compared with those of recombinant t-PA (rt-PA) and of recombinant single chain u-PA (rscu-PA) (0.25 to 1.0 mg/kg each) in groups of five or six dogs, each given heparin with or without the thromboxane synthase inhibitor/prostaglandin endoperoxide receptor antagonist ridogrel. Heparin and ridogrel in the absence of a thrombolytic agent did not produce arterial reflow or venous clot lysis in five dogs. Addition of K1K2Pu, rt-PA or rscu-PA resulted in a dose-dependent induction of arterial reflow and of venous clot lysis in the absence of systemic fibrinolytic activation and fibrinogen breakdown. Consistent arterial reflow required 0.063 mg/kg of K1K2Pu and 0.5 mg/kg of rt-PA or of rscu-PA. The thrombolytic potency for venous clot lysis, expressed as percent lysis per mg compound administered per kg body weight, was (mean +/- SEM) 750 +/- 160 for K1K2Pu, 68 +/- 17 for rscu-PA (p less than 0.001 vs. K1K2Pu) and 110 +/- 29 for rt-PA (p less than 0.001 vs. K1K2Pu). The plasma clearance rates were significantly lower for K1K2Pu than for rscu-PA and rt-PA. In the absence of ridogrel, arterial reflow was significantly slower and was followed by cyclic reocclusion and reflow; however, venous clot lysis was unaffected. Template bleeding times were not significantly altered in the absence but were markedly prolonged in the presence of ridogrel. These results confirm and establish that, when given as a bolus injection, K1K2Pu has an approximately 10-fold higher thrombolytic potency for arterial and venous thrombolysis than does rt-PA or rscu-PA. Thrombolysis with K1K2Pu is obtained in the absence of systemic fibrinolytic activation and fibrinogen breakdown. These properties suggest that K1K2Pu offers potential for thrombolytic therapy by bolus administration in patients with thromboembolic disease.

Degradation of myocardial high-energy phosphates during twenty-four hours of cold storage. Effects of cardioplegic versus noncardioplegic arrest

Purine nucleotide catabolism was examined during 24 hours of cold (0.5 degree C) storage of human transplant recipient hearts, baboon hearts, and dog hearts. The hearts were excised either after cold hyperkalemic cardioplegic arrest or after simple hypothermic arrest (25 degrees C). In human myocardium, hypothermia alone preserved the adenosine triphosphate pool markedly. Even after 24 hours of cold storage, adenosine triphosphate was still 9.5 +/- 2.5 mumol/gm dry weight (58% of the preischemic value). The major fraction of catabolites remained nucleotides: adenosine triphosphate plus adenosine diphosphate plus adenosine monophosphate decreased only from 99% +/- 1% (preischemic value) to 80% +/- 13% of the total purine content. The remaining catabolites were mainly nucleosides (adenosine 0.2% +/- 0.1% and inosine 19% +/- 13% of the total purine content). Cardioplegic arrest before cold storage did not change the pattern of purine nucleotide catabolism in any respect (p greater than 0.05). In baboon myocardium, hypothermia alone preserved the adenosine triphosphate content somewhat less than in human myocardium. Adenosine triphosphate content after 24 hours was 5.2 +/- 1.6 mumol/gm dry weight (40% of the preischemic value). The catabolism of adenosine triphosphate, however, did not proceed far beyond the level of adenosine monophosphate, so that the sum of nucleotides remained the same as in human hearts. Adenosine was 0.2% +/- 0.3% and inosine 17% +/- 4% of the total sum of purines. Also in the baboon heart, cardioplegia did not influence the pattern of catabolism significantly (p greater than 0.05). In the dog myocardium, hypothermia alone did not protect against severe catabolism of adenosine triphosphate. The adenosine triphosphate content at 24 hours of storage was 3.5 +/- 2.5 mumol/g dry weight (25% of the preischemic value). Catabolism of adenosine triphosphate proceeded far beyond the level of the nucleotides (63% +/- 17% of the total sum of purines), resulting in an accumulation of adenosine and inosine (5% +/- 4% and 30% +/- 13% of the total sum of purines) and even of hypoxanthine (1% +/- 1% of the total sum of purines). In the dog heart cardioplegic arrest inhibited adenosine triphosphate catabolism considerably. Adenosine triphosphate content at 24 hours was 8.1 +/- 1.8 mumol/gm dry weight (56% of the preischemic value); 83% +/- 5% of the total purine content remained present as nucleotides, and the nucleoside content was reduced to 2% +/- 3% for adenosine and 11% +/- 6% for inosine.(ABSTRACT TRUNCATED AT 400 WORDS)

Thrombolytic and pharmacokinetic properties of chimeric tissue-type and urokinase-type plasminogen activators

BACKGROUND

Chimeric molecules comprising the A-chain of tissue-type plasminogen activator (t-PA) and the catalytic domain of urokinase-type plasminogen activator (u-PA) have intact enzymatic characteristics of u-PA, partial fibrin-binding properties of t-PA, and thrombolytic properties in animal models comparable with but not superior to those of single-chain u-PA (scu-PA). Deletion of the finger and growth factor domains (t-PA-delta FE/scu-PA-e) in such chimeras further reduces their affinity for fibrin.

METHODS AND RESULTS

A detailed investigation of the thrombolytic potency and the pharmacokinetics of t-PA and u-PA chimeras was performed in quantitative animal models for thrombolysis. In hamsters with pulmonary embolism, in rabbits with jugular vein thrombosis, and in baboons with femoral vein thrombosis, the thrombolytic potency (percent lysis per milligram of compound administered per kilogram of body weight) of t-PA-delta FE/scu-PA-e was significantly higher than that of recombinant scu-PA (rscu-PA, Saruplase) as shown by a maximal rate of 720 +/- 170% versus 45 +/- 5% lysis per milligram of compound per kilogram of body weight (mean +/- SEM, p less than 0.01) in hamsters, 210 +/- 18% versus 49 +/- 3% lysis per milligram of compound per kilogram of body weight (mean +/- SEM, p less than 0.01) in rabbits, and 310 +/- 73% versus 90 +/- 0.3% lysis per milligram of compound per kilogram of body weight (p less than 0.01) in baboons. However, the specific thrombolytic activity (percent lysis per microgram per milliliter steady-state plasma antigen level) of t-PA-delta FE/scu-PA-e was not significantly different from that of rscu-PA in hamsters (210 +/- 57% versus 160 +/- 27% lysis per microgram per milliliter antigen level) and was lower than that of rscu-PA in rabbits (37 +/- 4% versus 130 +/- 5% lysis per microgram per milliliter antigen level; p less than 0.01). In dogs with a combined femoral vein blood clot and a platelet-rich femoral arterial eversion graft thrombosis, 0.25 mg/kg body wt bolus injections of t-PA-delta FE/scu-PA-e produced significantly more venous clot lysis (90 +/- 5%, n = 10) than 0.25 mg/kg rscu-PA (26 +/- 3%, n = 10) (p less than 0.001) and, at the arterial side, more frequent (10 of 10 dogs versus three of 10 dogs) and more persistent (six of 10 dogs versus none of 10 dogs) recanalization (p = 0.002). After bolus injection in hamsters, rabbits, or baboons, t-PA-delta FE/scu-PA-e had a fourfold to sixfold longer initial half-life than rscu-PA and a slower plasma clearance of sixfold in hamsters, 10-fold in rabbits, and more than 10-fold in baboons.

CONCLUSIONS

These results indicate that t-PA-delta FE/scu-PA-e has a markedly enhanced thrombolytic potency toward venous and arterial thrombi caused by a delayed in vivo clearance with relatively maintained specific thrombolytic activity. These properties suggest that the chimera may be clinically useful for thrombolytic therapy by bolus administration in patients with thromboembolic disease.

Comparative thrombolytic properties of bolus injections and continuous infusions of a chimeric (t-PA/u-PA) plasminogen activator in a hamster pulmonary embolism model

The recombinant chimeric plasminogen activator, rt-PA-delta FE/scu-PA-e, consisting of amino acids 1 to 3 and 87 to 274 of tissue-type plasminogen activator (t-PA) and amino acids 138 to 411 of single-chain urokinase-type plasminogen activator (scu-PA), has a markedly increased thrombolytic potency following its continuous intravenous infusion in animal models of venous thrombosis (Collen et al, Circulation, in press). In the present study, the thrombolytic potencies of intravenous bolus injections of rt-PA-delta FE/scu-PA-e, of recombinant t-PA (rt-PA), and of recombinant scu-PA (rscu-PA), given alone or in combination, were compared with those of intravenous infusions in a hamster pulmonary embolism model. Dose-dependent clot lysis was obtained in the absence of systemic activation of the fibrinolytic system and fibrinogen breakdown. In bolus injection experiments, the maximal rate of clot lysis, expressed in percent clot lysis per milligrams per kilogram compound administered, was 120 +/- 10 for rt-PA, 54 +/- 8 for rscu-PA, and 2,100 +/- 500 for rt-PA-delta FE/scu-PA-e (P less than .01 v rt-PA or rscu-PA). Comparative results with continuous infusion over 1 hour were 270 +/- 64, 99 +/- 18, and 1,500 +/- 250 (P less than .01 v rt-PA or rscu-PA) percent lysis per mg/kg compound infused for rt-PA, rscu-PA, and rt-PA-delta FE/scu-PA-e, respectively. Thus, rt-PA and rscu-PA are more potent when administered as an infusion than as a bolus, whereas rt-PA-delta FE/scu-PA-e is at least as potent when administered as a bolus. Combined bolus injections of rt-PA and rscu-PA had a 2.2-fold synergistic effect on clot lysis, but no synergism was observed with combined bolus injections or with combined infusions of rt-PA and rt-PA-delta FE/scu-PA-e, or of rscu-PA and rt-PA-delta FE/scu-PA-e. The present study thus shows that rt-PA-delta FE/scu-PA-e is much more potent for clot lysis than rt-PA or rscu-PA when administered as a bolus injection, but no synergistic interaction is observed between the chimera and either rt-PA or rscu-PA.

Requirement of heparin for arterial and venous thrombolysis with recombinant tissue-type plasminogen activator

The effect of concomitant intravenous (IV) heparin (200 U/kg bolus, followed by 100 U/kg/h) on the efficacy of arterial and venous thrombolysis with IV recombinant tissue-type plasminogen activator (rt-PA; 0.5 mg/kg over 1 hour) was investigated in a combined femoral arterial and venous thrombosis model in the dog. The arterial model consisted of a high-grade stenosis, endothelial damage, and a thrombotic occlusion, and the venous model consisted of a 125I-fibrin-labeled blood clot. After a dose-finding pilot study in four dogs, a randomized, prospective, and blind study was performed in 20 animals pretreated with 2.8 mg/kg IV acetyl salicylic acid (ASA). The combination of rt-PA and heparin (group I, n = 10) induced early (less than 30 minutes) arterial reperfusion in seven dogs, late (greater than 30 minutes) reflow in two dogs, and persistent occlusion in one dog. rt-PA alone (group II, n = 10) was associated with early reperfusion in one dog, late reflow in three dogs, and persistent occlusion in six dogs (P = .018). Reocclusion occurred in five of nine reperfused dogs of group I and in one of four reperfused dogs of group II (P = not significant). Venous clot lysis amounted to 81% +/- 4% (mean +/- SEM) for group I and to 49% +/- 7% for group II (P less than .001). Template bleeding times increased moderately, but significantly, in group I (from 2.2 +/- 0.2 minutes at baseline to 7.0 +/- 1.4 minutes at 30 minutes, P = .006), but only marginally in group II (from 2.2 +/- 0.2 minutes to 3.6 +/- 0.7 minutes, P = .09). No systemic fibrinogen depletion was observed. Thus, the concommitant use of heparin with rt-PA accelerates arterial reperfusion and enhances venous thrombolysis in dogs pretreated with ASA. These results, obtained in a randomized prospective study design, add to a growing body of experimental and clinical evidence, indicating that thrombolytic therapy with rt-PA requires concomitant adjunctive IV heparin for optimal efficacy, even in the face of treatment with ASA.

Normothermic ischemic cardiac arrest in the isolated working rabbit heart: effects of dl-nebivolol and atenolol

The effect of pretreatment with selective beta 1-adrenoceptor blockers (dl-nebivolol or atenolol) on myocardial mechanical activity, mitochondrial function, morphology, and calcium cytochemistry was studied during normothermic ischemic arrest and reperfusion of isolated working rabbit hearts. The hearts subjected to 25 min of ischemia followed by 30 min of post-ischemic reperfusion showed typical signs of severe myocardial ischemic damage. The ultrastructural changes showed a good relation with the changes in mechanical activity and mitochondrial function. To determine whether these changes could be prevented or reduced by beta 1-adrenoceptor blockade, dl-nebivolol or atenolol (0.62 mg/liter) was added to the perfusate 30 min before the induction of ischemia. The results showed that dl-nebivolol exerted a protective effect on recovery of mechanical activity, on mitochondrial function during reperfusion as well as on the ultrastructure as examined at the end of the reperfusion period. On the other hand, atenolol failed to protect the myocardium against ischemia-reperfusion damage in the isolated working rabbit heart.

Effects of beta-adrenoceptor antagonists on cardiac function in ischemic-reperfused myocardium of the isolated working rabbit heart

The effects of beta-adrenoceptor antagonists (dl-nebivolol, atenolol and propranolol) and of 1-nebivolol on cardiodynamics and mitochondrial oxidative phosphorylation were studied in the isolated working rabbit heart subjected to normothermic global ischemia, followed, in some cases, by reperfusion. The hearts were pretreated with the different drugs (0.32 mg/l) 30 min before the start of ischemia, dl-Nebivolol and propranolol provided protection for both cardiodynamic and mitochondrial functions, as did l-nebivolol, which lacks beta-adrenoceptor blocking properties, while atenolol failed to protect mechanical activity and cardiac mitochondria against the effects of ischemia and post-ischemic reperfusion. Catecholamine depletion with reserpine did not have a beneficial effect on the recovery of cardiodynamic and mitochondrial function during post-ischemic reperfusion. It is concluded that the beneficial effects of beta-blockers on the ischemic and reperfused myocardium can not be explained by a specific beta-blocking action alone.

Cerebroprotective effects of flunarizine in an experimental rat model of cardiac arrest

A rat cardiopulmonary arrest model was used to study the effects of flunarizine on survival and on the development of postischemic brain damage. Ischemia was induced by a combination of hypovolemia and intracardiac injection of a cold potassiumchloride solution. To validate the model; survival rate and histological damage were assessed after ischemic periods ranging from 5 to 20 minutes. A 6-minute cardiac arrest period was withheld for further therapeutic investigations. In one group (n = 12), flunarizine was administered successively in doses of 0.5 mg/kg intravenous at 5 minutes, 10 mg/kg intraperitoneal at 1 hour, and 20 mg/kg orally at 16 and 24 hours after recirculation. The second group (n = 13) received only the vehicle. Flunarizine, although not affecting mortality; significantly reduced the mean number of ischemic neurons in CA1 hippocampus from 83% in the control to 44% in the drug-treated series (P = 0.014). The results are indicative of the usefulness of this cardiac arrest model to study morphologic aspects of cerebral injury. The results obtained with flunarizine show the effectiveness of this drug even when it is administered after a severe ischemic insult such as global complete ischemia.

Beta-blockade in the ischemic reperfused working rabbit heart: dissociation of beta-adrenergic blocking and protective effects

The cardioprotective effects of the beta-blocking drugs (dl-nebivolol, d-nebivolol, propranolol, atenolol, dilevalol and pindolol) were tested in the isolated working rabbit heart. The effects of l-nebivolol, having little beta-adrenoceptor blocking activity, were also studied. The hearts were subjected to 25 min ischemia, followed by 30 min of reperfusion. In solvent-treated hearts, ischemia resulted in a considerable loss of function. The mean functional recovery of one of the most sensitive parameters, i.e. aortic flow, was only 6%. Pretreatment either with dl-nebivolol, d-nebivolol, l-nebivolol or propranol significantly improved cardiodynamic function. Recovery after pretreatment with atenolol, dilevalol and pindolol (less than 10 mg/l) was not significantly improved when compared to solvent-treated hearts. The results suggest that the protective effects of some beta-blockers are most probably not related to beta-adrenoceptor blocking activity.

Comparison between epidural fentanyl, sufentanil, carfentanil, lofentanil and alfentanil in the rat: analgesia and other in vivo effects

The effects of epidural fentanyl, sufentanil, carfentanil, lofentanil and alfentanil were compared in the rat. All five opiates produced analgesia with the relative order of potency being lofentanil greater than carfentanil greater than sufentanil greater than fentanyl greater than alfentanil, and the relative order of duration at comparative doses lofentanil greater than sufentanil greater than carfentanil greater than fentanyl greater than alfentanil. Because epidural administered opiates still exert some effects within the brain, specificity ratios (defined as the ratio between the ED50s for a prolongation of the tail-withdrawal reaction latency and a blockade of the pinna reflex) were calculated. The relative order of specificity between the five opiates was fentanyl greater than lofentanil greater than sufentanil greater than carfentanil greater than alfentanil. These results are discussed in the context of the search for a safe and sufficiently long-acting lipid soluble epidural analgesic. It is concluded that of these five opiates, sufentanil appears to be the best drug of choice for epidural analgesia.