Roy Massingham

BRAIN DISTRIBUTION OF CETIRIZINE ENANTIOMERS: COMPARISON OF THREE DIFFERENT TISSUE-TO-PLASMA PARTITION COEFFICIENTS: Kp, Kp,u, AND Kp,uu

The objective of this study was to compare the blood-brain barrier (BBB) transport and brain distribution of levo- (R-CZE) and dextrocetirizine (S-CZE). Microdialysis probes, calibrated using retrodialysis by drug, were placed into the frontal cortex and right jugular vein of eight guinea pigs. Racemic CZE (2.7 mg/kg) was administered as a 60-min i.v. infusion. Unbound and total concentrations of the enantiomers were measured in blood and brain with liquid chromatography-tandem mass spectrometry. The brain distribution of the CZE enantiomers were compared using the parameters Kp, Kp,u, Kp,uu, and Vu,br. Kp compares total brain concentration to total plasma concentration, Kp,u compensates for binding in plasma, whereas Kp,uu also compensates for binding within the brain tissue and directly quantifies the transport across the BBB. Vu,br describes binding within the brain. The stereoselective brain distribution indicated by the Kp of 0.22 and 0.04 for S- and R-CZE, respectively, was caused by different binding to plasma proteins. The transport of the CZE enantiomers across the BBB was not stereoselective, since the Kp,uu was 0.17 and 0.14 (N.S.) for S- and R-CZE, respectively. The Kp,uu values show that the enantiomers are effluxed to a large extent across the BBB. The Vu,br of approximately 2.5 ml/g brain was also similar for both the enantiomers, and the value indicates high binding to brain tissue. Thus, when determining stereoselectivity in brain distribution, it is important to study all factors governing this distribution, binding in blood and brain, and the BBB equilibrium.

A study of compounds which inhibit vascular smooth muscle contraction

Abstract Several compounds have been examined for their inhibitory action on a rat aortic ring preparation stimulated by four different agonists, noradrenaline bitartrate (NA), Ba2+, Ca2+ and K+. The compounds tested include phentolamine, papaverine, cinnarizine, cyclandelate, SKF-525A, prenylamine, guancydine, verapamil and compound D-600. Phentolamine inhibited NA reponses at much lower concentrations than those required to antagonize K+, Ca2+ or Ba2+ responses. Guancydine was also more effective against NA but was much less potent than phentolamine in this respect. Papaverine, cinnarizine, cyclandelate, prenylamine and SKF-525A were non selective agents and inhibited responses to all agonists approximately equally at various concentrations. Verapamil and D-600 were highly potent compounds which were more effective against cation-induced responses, especially those to Ca2+, than against those induced by NA.

A comparison of the effects of verapamil and cinnarizine upon responses elicited by selective α1-and α2-adrenoceptor agonists in the autoperfused canine hindlimb

Abstract In an attempt to extend the hypothesis that activation of vascular postsynaptic α2-adrenoceptors requires an influx of Ca2+ ions, the effects of 2 calcium entry blocking drugs verapmil and cinnarizine have been examined as inhibitors of the pressor responses to methoxamine and B-HT 920 in autoperfused dog hindlimb preparations. Verapamil (0.1–1 mg i.a) selectively responses to B-HT 920 and had little or no effet upon responses to methoxamine, thus supporting this hypothesis. However cinnarizine, over the dose range studied (0.1–1 mg/kg i.a.) produced quantitatively similar inhibitions of the hindlimb responses to B-HT 920 and methoxamine. These results suggest that cinnarizine may have a different site of action to verapamil in resistance vessels of the dog hindlimb.

Peripheral and central H1 histamine receptor occupancy by levocetirizine, a non‐sedating antihistamine; a time course study in the guinea pig

The H1 receptor occupancy (H1RO) in brain is an indicator of central side effects of antihistamines. Here, we determined the kinetics of central and peripheral H1RO by levocetirizine in relation to its brain and plasma concentration, and investigated the role of the blood‐brain barrier in any delay in brain H1RO.

Effect of diltiazem upon contractile responses to phenylephrine, cirazoline, Sgd 101/75, St 587 and B-HT 920 in rabbit aorta and dog saphenous vein preparations

Diltiazem (10 microM) did not significantly affect concentration-response curves to the full, relatively selective alpha 1-adrenoceptor agonists phenylephrine and cirazoline in rabbit aorta and dog saphenous vein preparations. The effects of these 2 agonists remained resistant to diltiazem even in tissues pretreated with phenoxybenzamine (0.03 or 0.1 microM, 20 min) to reduce the alpha-adrenoceptor reserve. Sgd 101/75 and St 587 were partial agonists in both vascular preparations. The concentration-response curves to these relatively selective alpha 1-adrenoceptor agonists were also unaffected, or only slightly attenuated, by diltiazem. B-HT 920 at low concentrations preferentially stimulated the dog saphenous vein preparation and only at high concentrations elicited small contractions of the rabbit aorta. The responses to B-HT 920 were mediated by alpha 2-adrenoceptors in the vein and by alpha 1-adrenoceptors in the aorta yet concentration-response curves to this agonist were significantly attenuated by diltiazem in both tissues. The results indicate that the resistance of certain alpha-adrenoceptor-mediated responses in vascular preparations to calcium entry blockers need not be associated with the presence of a significant receptor reserve and that calcium dependency of a response may be determined by the agonist.

Failure of calcium channel blockade to reduce platelet-mediated cyclic flow variations in dogs with coronary stenosis and endothelial injury

Experimental canine coronary artery stenosis associated with endothelial injury results in a typical pattern of coronary flow characterized by gradual decreases in blood flow to almost zero values followed by abrupt restorations to original levels. Cyclic flow variations (CFVs) are the consequence of recurrent platelet aggregation at the site of the stenosis and subsequent dislodgement of the thrombus. The present study was designed to test the efficacy of diltiazem, nifedipine, and verapamil in inhibiting in vivo platelet aggregation as compared with that of aspirin and ketanserin, two potent reference compounds effective in this model. Except for aspirin, compounds were given as a slow intravenous infusion (i.v.) for 60 min to avoid hemodynamic changes. Diltiazem (0.01 mg/kg/min), nifedipine (3 micrograms/kg/min), and verapamil (0.01 mg/kg/min) were totally inactive against CFVs. A higher dose of verapamil (0.02 mg/kg/min) abolished CFVs in 3 of 4 dogs, but serious side effects were observed [atrioventricular (AV) block and death of 2 animals]. Aspirin (10 mg/kg bolus) caused complete inhibition of CFVs in 4 of 4 dogs, and ketanserin (0.01 mg/kg/min) abolished CFVs in 4 of 5 dogs. These data suggest that calcium channel blockade alone in contrast to cyclooxygenase inhibition or 5-HT2 antagonism cannot inhibit thrombus formation in this model.

Caffeine-induced contractions in rabbit isolated renal artery are differentially inhibited by calcium antagonists

Caffeine (1-60 mM) induced concentration-dependent, endothelium-independent phasic contractile responses in isolated rabbit renal artery ring preparations. For concentrations of caffeine over 2 mM, responses were mainly the result of intracellular calcium ion mobilization since they were relatively resistant to removal of calcium ions from the bathing medium. The L-type slow calcium channel blocker, nifedipine (10 microM), had no effect and high concentrations of verapamil and diltiazem (10-30 microM) only slight and inconsistent effects (not concentration-dependent) upon these caffeine responses. Likewise, the highly lipophilic calcium antagonists flunarizine and lidoflazine (3-30 microM) only slightly displaced caffeine concentration-response curves to the right and reduced the maximum response. These small inhibitory effects of flunarizine and lidoflazine were not augmented in a calcium-free medium. In contrast, the other lipophilic calcium antagonists, bepridil and fendiline (3-30 microM), produced marked, non-competitive type inhibition of caffeine responses, completely inhibiting responses to the alkaloid at the highest concentration. Furthermore, the inhibitory effects of bepridil and fendiline were markedly augmented in calcium-free medium. These results clearly differentiate bepridil and fendiline from the other calcium antagonists studied. In addition they provide further evidence for effects other than at the cell membrane which could theoretically contribute to the efficacy of bepridil and fendiline as anti-anginal agents.

Phenoxybenzamine-induced inhibition of cirazoline pressor responses in pithed rats pretreated with organic or inorganic calcium entry blocking drugs

In groups of propranolol-treated pithed rats pretreatment with either verapamil (1 mg/kg i.a., 20 min) or the inorganic calcium entry blocker (CEB), cobalt (23.8 mg/kg i.a., 20 min) reduced maximum obtainable pressor responses to the relatively selective alpha 2-adrenoceptor agonist B-HT 920 (0.1-1000 micrograms/kg i.v.) equally, by approximately 50%. Verapamil and cobalt at these doses had little or no effect upon pressor responses induced by the relatively selective alpha 1-adrenoceptor agonist cirazoline (0.1-1000 micrograms/kg i.v.). Phenoxybenzamine (0.1 mg/kg i.v., 15 min) displaced to the right and reduced by 44% the maximum obtainable pressor responses to cirazoline. Treatment of animals with the combination of either verapamil or cobalt followed by phenoxybenzamine, at the dose levels and pretreatment times given above, produced significantly greater inhibitions of cirazoline pressor responses (83% and 88% reduction in the maximum obtainable pressor responses to cirazoline respectively) than were observed following administration of phenoxybenzamine alone. Since yohimbine (1 mg/kg i.v.) did not significantly affect the residual responses to cirazoline following treatment with phenoxybenzamine the mechanism responsible for this interaction between CEBs and phenoxybenzamine is not mediated via postjunctional alpha 2-adrenoceptors. Additional studies are required to assess the involvement of a possible subtype of alpha 1-adrenoceptors which appear to mediate vascular responses sensitive to CEBs.

Protective effect of bepridil against veratrine-induced contracture in rat atria.

In isolated stimulated rat atria, superfusion with veratrine caused a marked contracture (VIC) which was absent in calcium-free medium and which was inhibited by tetrodotoxin (IC50VIC of 1.38 microM). Lowering the extracellular calcium concentration from 2.5 to 0.5 or 0.1 mM reduced the veratrine-induced contracture and delayed its onset. Superfusion of bepridil (1-10 microM) for 60 min before and during veratrine exposure markedly slowed the onset of contracture, reduced the maximum response (IC50VIC = 2.11 microM) and facilitated recovery upon washout of the alkaloid. The direct negative inotropic effect (NIE) of bepridil (IC50NIE = 10.96 microM) resulted in an VIC/NIE ratio of 5.19 for this drug. The protective effects of bepridil were rate-independent and were not modified by the presence of atropine (1.4 microM) and propranolol (0.3 microM) in the medium. Diltiazem, verapamil and nifedipine only reduced veratrine-induced contracture at concentrations much higher than those producing a negative inotropic effect, giving them negative NIE/VIC ratios of 0.31, 0.08 and 0.08 respectively. Like bepridil, flunarizine had a positive NIE/VIC ratio (15.87, IC50VIC = 3.71 microM). The lack of effect of the quaternary derivative of bepridil CERM 11888 indicated that intracellular sites of action may be involved in the activity of bepridil on veratrine-induced contracture. Given that veratrine-induced changes may mimic some of the pathological changes occurring in ischaemia, the results suggest that bepridil and flunarizine may be more effective than L-type, slow calcium ion-channel blockers in protecting against calcium overload during ischaemia and reperfusion injury.

Paracetamol potentiates isaxonine toxicity in vitro

The toxicity of isaxonine alone and in combination with the known glutathione depletor, paracetamol, was evaluated using rat hepatocyte primary cultures in vitro by measuring morphometric parameters and the leakage of intracellular lactate dehydrogenase into the culture medium. No cytotoxicity was observed with isaxonine at concentrations up to 10(-3) M, whereas paracetamol was cytotoxic at concentrations above 0.6 x 10(-3) M in the culture medium. Paracetamol cytotoxicity (0.6-3.3 x 10(-3) M) was enhanced in the presence of a non-cytotoxic concentration of isaxonine (10(-7) M). Furthermore cytotoxicity was observed when cells were exposed to a combination of non-cytotoxic concentrations of the paracetamol (0.3 x 10(-3) M) and isaxonine (10(-7) M). These findings demonstrate that isaxonine has no direct cytotoxic effect even at high concentrations. However co-administration of isaxonine with paracetamol greatly potentiates cytotoxicity. We suggest that this effect may be related to glutathione depletion within the cell but additional studies are required to verify this hypothesis.