A Specific Thromboxane Receptor Blocking Drug, AH23848, Reduces Platelet Deposition on Vascular Grafts in Man

The antithrombotic effect of a specific thromboxane A2 receptor blocking drug, AH23848, on radio-labelled platelet deposition in mature Dacron aorto-bifemoral grafts has been evaluated in patients. Thirty patients were randomly allocated to AH23848 70 mg, aspirin 300 mg plus dipyridamole 75 mg or placebo 8-hourly for 9 days. AH23848 inhibited platelet aggregarion induced by the thromboxane ,A2 mimetic U-46619; no such effect was observed with aspirin plus dipyridamole. 111In-platelet uptake was measured as the thrombogenicity index (TI) which is a measure of the daily rate of accumulation of platelets by the graft. The mean (s.e. mean) value of 0.193 (0.029) on placebo was significantly reduced to 0.115 (0.022) by AH23848 (p <0.05) but only to 0.175 (0.028) by aspirin plus dipyridamole. There was no difference in mean platelet life span between the three treatment groups. The pronounced antithrombotic effect of AH23848 implicates thromboxane ,A2 in the process of platelet deposition in arterial prostheses and demonstrates the considerable promise of thromboxane receptor blocking drugs as antithrombotic therapy.

Balancing the role of the dental school in teaching, research and patient care; including care for underserved areas

Inequalities within dentistry are common and are reflected in wide differences in the levels of oral health and the standard of care available both within and between countries and communities. Furthermore there are patients, particularly those with special treatment needs, who do not have the same access to dental services as the general public. The dental school should aim to recruit students from varied backgrounds into all areas covered by the oral healthcare team and to train students to treat the full spectrum of patients including those with special needs. It is essential, however, that the dental student achieves a high standard of clinical competence and this cannot be gained by treating only those patients with low expectations for care. Balancing these aspects of clinical education is difficult. Research is an important stimulus to better teaching and better clinical care. It is recognized that dental school staff should be active in research, teaching, clinical work and frequently administration. Maintaining a balance between the commitments to clinical care, teaching and research while also taking account of underserved areas in each of these categories is a difficult challenge but one that has to be met to a high degree in a successful, modern dental school.

Evaluation of an intranasal house dust mite provocation model as a tool in clinical research

BACKGROUND

As a result of the all-year-round exposure to house dust mite (HDM), perennial rhinitis patients never have a clear symptom-free period. In this study, we investigated whether, despite these symptoms, we can still use nasal HDM provocations to study perennial allergic rhinitis and the effects of treatment.

METHODS

In a parallel-group study, after 1 week treatment with either fluticasone propionate aqueous nasal spray (FPANS) or placebo, 20 patients, allergic to HDM, registered symptoms (nasal obstruction, rhinorrhoea, sneezing, pruritus and eye symptoms) using three different scoring methods [Lebel, categorical and visual analogue scale (VAS)] and peak nasal inspiratory flow (PNIF) after HDM provocations. Provocations were performed with 1000 biological units/ml and 24 h later with 10,000 biological units/ml of HDM. Before and after the provocations, nasal mucosa biopsies were taken for immunohistochemical staining to determine the number of eosinophils.

RESULTS

House dust mite provocations resulted in an increase in symptoms and a decrease in PNIF. Even at high-dose provocation, the FPANS group registered significantly lower symptoms than the placebo group for nasal blockage, sneezing, eye symptoms, and PNIF in both early and late phases. FPANS also suppressed rhinorrhoea during the late phase and the influx of eosinophils in the lamina propria.

CONCLUSION

Despite the high background of symptoms, allergic responses can be induced in this perennial rhinitis model. The VAS score seems most suited to detect these changes and the suppression of symptoms by 7 days of FPANS treatment. Epithelial eosinophilia at baseline was correlated positively with the severity of the reaction after the first provocation.

The metalloproteinase inhibitor GI5402 inhibits endotoxin-induced soluble CD27 and CD16 release in healthy humans

Metalloproteinases have been implicated in the cleavage of a number of cell surface immune receptors. Oral administration of the metalloproteinase inhibitor GI5402 attenuated the release of soluble CD27 and CD16 into the circulation after intravenous endotoxin injection in healthy humans.

The effect of a metalloproteinase inhibitor (GI5402) on tumor necrosis factor-alpha (TNF-alpha) and TNF-alpha receptors during human endotoxemia

Tumor necrosis factor-alpha (TNF-alpha) is released from the cell surface by cleavage of pro-TNF-alpha by metalloproteinases (MPs). In cell cultures, inhibition of MPs has been found not only to reduce the release of TNF-alpha, but also to enhance the surface expression of TNF-alpha and TNF-alpha receptors, which might lead to a proinflammatory effect. To determine the effect of MP inhibition during inflammation in humans, 7 healthy subjects were studied after intravenous injection of lipopolysaccharide (LPS; 4 ng/kg) preceded (-20 minutes) by an oral dose of the MP inhibitor GI5402 (100 mg) or matching placebo. GI5402 strongly reduced LPS-induced TNF-alpha release (P <.001), but did not influence the increase in monocyte-bound TNF-alpha. In addition, GI5402 attenuated the rise in plasma-soluble TNF-alpha receptors types I and II after LPS injection (both P <.001), but did not change the LPS-induced decreases in granulocyte and monocyte TNF-alpha receptor expression. These data suggest that MP inhibitors may be useful as a treatment modality in diseases in which excessive production of TNF-alpha is considered to play an important role. Furthermore, unlike in vitro, no evidence has been found in vivo with MP inhibition for a potential proinflammatory effect due to increases in membrane-bound TNF-alpha and TNF-alpha receptor number.

Prevention of intra-coronary thrombosis in the anaesthetised dog: the importance of thromboxane A2 and thrombin

Vapiprost (GR32191, a TxA2 antagonist), r-hirudin, aspirin, ticlopidine and aspirin plus ticlopidine were examined for their ability to prevent electrically-induced thrombosis in an artificially stenosed coronary artery in the anaesthetised dog. Drugs or vehicle were administered prior to a 2 h period of electrical damage which was followed by a further 2 h observation period. In all vehicle-treated animals, blood flow markedly declined with onset of the damaging current; 80% completely occluded. All treatments reduced the incidence of complete occlusion to a similar extent. Vapiprost and r-hirudin also largely prevented the decline in blood flow both during and following the damage period whilst aspirin and ticlopidine, either alone or in combination were much less effective. With r-hirudin treatment, marked cyclic changes in flow occurred throughout the experiment; these were abolished by administration of vapiprost. In this dog model, TxA2 and thrombin appear to work in concert to produce coronary thrombosis, ADP being of minor importance. The superior effect of vapiprost over aspirin suggests a beneficial role for endogenous prostacyclin.

Reduction in the number of thromboxane receptors on human platelets after exposure to GR32191

1. Exposure of human resuspended platelets in vitro for 30 min to the potent thromboxane A2 (TP)-receptor blocking drug GR32191, followed by its removal by dilution-dissociation, reduced the degree of subsequent binding to 2 nM [3H]-GR32191 by almost 50%. Exposure for longer periods (60 min) led to a further reduction. However, no change in the Kd of the radioligand was observed. 2. This effect of GR32191 could not be explained by persistent binding of drug to platelets since a dilution-dissociation stage, designed to remove all drug, was included prior to measurement of binding. 3. Using an alternative TP-receptor radioligand, [3H]-SQ29,548, to monitor receptor number, a reduction in Bmax was observed after GR32191 pre-treatment; the Kd value of the radioligand remained unchanged. 4. The effect was not a common property of TP-receptor blocking drugs since pre-exposure of platelets in vitro for 30 min to BM13.177 or SQ29,548 did not produce a fall in subsequent Bmax to [3H]-SQ29,548. 5. While the mechanism behind this apparent down-regulation of platelet TP-receptor is unknown, it may explain the long duration of action of GR32191 upon platelets in man which persists in the absence of detectable drug in the plasma.

Characteristics of the binding of [3H]-GR32191 to the thromboxane (TP-) receptor of human platelets

1. The interaction of the specific thromboxane (TP-) receptor blocking drug, [3H]-GR32191 with human intact platelets and platelet membranes has been investigated in vitro. 2. On intact platelets, association of specific [3H]-GR32191 binding at 37 degrees C was biphasic, with an initial rapid component and a slower secondary phase. Dissociation experiments indicated displacement from two sites with t1/2 values of 8.1 and 65.6 minutes. Kd values derived from the kinetic rate constants for the rapid onset/offset and slow onset/offset phases were 0.4 and 0.5 nM respectively. 3. Competition binding of [3H]-GR32191 and GR32191 on intact platelets gave an IC50 of 2.3 nM. Scatchard analysis indicated a single class of binding site with a Kd of 2.2 nM. Further analysis of the data yielded a Hill slope of -1.0 again indicating an interaction at a single binding site. Saturation binding experiments gave a similar estimate of the Kd value for [3H]-GR32191 to that obtained from competition binding experiments. A possible explanation for the biphasic interaction of the GR32191 in intact platelets may lie in restriction of its access to and egress from a population of TP-receptors. 4. In platelet membranes at 37 degrees C, specific [3H]-GR32191 binding was complete within 5 min with a calculated association rate constant of 3.2 x 10(8) M-1 min-1. Dissociation of [3H]-GR32191 was relatively slow, with measurable specific binding persisting for > 40 min. Analysis of these data yielded a t1/2 of 17.7 min and a dissociation rate constant of 0.04 min-1 and indicated dissociation from a single site. The ti for dissociation appeared to be related to the contact time of platelet membranes with [3H]-GR32191.Derivation of a Kd from the kinetic rate constants gave a value of 0.13 nM.5. Competition binding of [3H]-GR32191 and GR32191 to platelet membranes gave an IC50 value of 3.5 nM. Scatchard analysis of these data indicated a single binding site with a Kd of 2.1 nM. Saturation binding experiments with [3H]-GR32191 yielded similar IC50 and Kd values to those from competition experiments.6. In further competition binding experiments, the TP-receptor agonists U-46619, STA2, EP171 and 9,1 1-azo PGH2 and antagonists SQ29,548, BM 13.177 and EP092 all competed with specific [3H]-GR32191 binding on intact platelets and, where determined, on platelet membranes. All compounds fully displaced specific [3H]-GR32191 binding. However, where tested, the ICso values for a particular compound were always greater when [3H]-GR32191 was the radioligand than when [3H]-SQ29,548 was used. At the concentrations used in these studies (2 and 5 nM respectively), platelets appeared to bind approximately twice as much [3H]-GR32191 as [3H]-SQ29,548.7. In conclusion, the interaction of [3H]-GR32191 with human intact platelets was complex but the data were consistent with an action at a single class of binding site; from competition experiments this appears to be the functional TP-receptor. The interaction of the drug with this binding site is, however,characterized by a slow dissociation. This characteristic was confirmed in studies with platelet membranes and does not therefore appear to be an artefact of diffusion. Estimates of the Kd of the drug differed depending on the method of determination. Because of the slow dissociation of [3H]-GR32191,those relying upon equilibrium of the radioligand with competing agent may be unreliable. The rate of dissociation also appeared to be related to the contact time of drug with receptor. An explanation for this phenomenon may lie in the ability of GR32191 to induce a change in the conformational state or location of the human platelet TP-receptor.

The pharmacodynamics and pharmacokinetics of a novel thromboxane receptor blocking drug vapiprost (GR32191) after single intravenous doses in healthy subjects

1 The effect of single, serially increasing, intravenous doses of a specific thromboxane receptor blocking drug, vapiprost, upon platelet aggregation induced ex vivo by the thromboxane A2 mimetic, U-46619, was examined in 12 healthy males. 2 Subjects received either 1 (n = 1 subject), 2 (n = 6), 3 (n = 2), or 4 (n = 3) administrations of vapiprost within the dose range 0.125 to 16 mg and, in random order, placebo on separate study days at intervals of at least 48 h. 3 All doses of vapiprost produced an immediate antagonism of U-46619-induced platelet aggregation in whole blood. Both the magnitude and duration of the rightward displacement of the concentration-effect curves increased with dose. Although lower doses produced parallel displacements of these curves, with the higher doses the maximum response to U-46619 was reduced such that 50% platelet aggregation was not achieved. After the 16 mg dose of vapiprost, virtually complete suppression of platelet aggregation (up to a concentration of 30 microM) was seen. This degree of inhibition was maintained for 2 h after dosing, following which there was a gradual return to pre-dose U-46619 sensitivity over the next 12 to 24 h. U-46619-induced platelet aggregation was unaffected by placebo. 4 Across the dose range, vapiprost was rapidly cleared from plasma, with an elimination half-life of 69-84 min and a plasma clearance of 514-721 ml min-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Thromboxane (Tx) A2 receptor blockade and TxA2 synthase inhibition alone and in combination: comparison of anti-aggregatory efficacy in human platelets

1. The present study has compared the relative anti-aggregatory effect of various compounds which interfere with thromboxane (Tx) A2-dependent aggregation of human platelets in whole blood in vitro. These included the cyclo-oxygenase inhibitor aspirin, the TxA2 synthase inhibitor dazoxiben, the TxA2 (TP-) receptor blocking drug GR32191 and two compounds, R.68070 ((E)-5-[[[(3-pyridinyl) [3-(trifluoromethyl)phenyl]-methylen] amino]oxy] pentanoic acid) and CV-4151 [E)-7-phenyl-7-(3-pyridyl)-6-heptenoic acid), which possess both TP-receptor blocking and TxA2 synthase inhibitory activities in the same molecule. 2. GR32191, R.68070 and CV-4151 all antagonized aggregation to the TxA2 mimetic U-46619, with pA2 values of approximately 8.2, 5.4 and 4.8 respectively. This effect was specific, platelet aggregation induced by adenosine 5'-diphosphate (ADP) being unaffected by concentrations up to 10, 1000 and 300 microM respectively. In contrast, neither aspirin nor dazoxiben exhibited any measurable TP-receptor blocking activity. 3. The rank order of potency (pIC50) for inhibition of TxA2 formation in serum was R.68070 (7.4) greater than CV-4151 (6.9) greater than dazoxiben (5.7) greater than aspirin (5.3). In addition, all four drugs abolished collagen-induced platelet TxA2 formation. In contrast, GR32191 produced no consistent inhibition of TxA2 formation in either system up to concentrations of 10-30 microM. 4. The specificity of R.68070, CV-4151 and dazoxiben for TxA2 synthase was indicated by their ability to increase serum levels of prostaglandin E2 (PGE2) and PGD2 in parallel with decreases in TxA2 formation. This profile was not observed with aspirin or GR32191. However, high concentrations of R.68070 (100,microM) and CV-4151 (1000 microM) necessary for maximum TP-receptor blocking activity, produced substantially smaller increases in PGE2 and PGD2, consistent with an aspirin-like effect of these compounds upon cyclo-oxygenase. With dazoxiben (1000 microM), PGE2 and PGD2 levels remained elevated. 5. Aspirin inhibited collagen-induced platelet aggregation, the effect correlating with inhibition of TxA2 formation. Dazoxiben, whilst also achieving maximal inhibition of TxA2 formation, produced significantly less inhibition of aggregation than aspirin. In contrast, GR32191 (0.1-1O microM), at concentrations specific for TP-receptor blockade, produced a significantly greater antagonism of collagen-induced platelet aggregation than aspirin. This additional effect of GR32191 was absent in platelets pretreated with aspirin, indicating the probable involvement of an endogenous anti-aggregatory cyclo-oxygenase product in response to collagen stimulation. 6. R.68070 and CV-4151 also inhibited collagen-induced aggregation, with very high concentrations of R.68070 (100 microM) producing an effect equivalent to that of GR32191. 7. In contrast, the combination of GR32191 with either dazoxiben, R.68070 or CV-4151, at concentrations specific for TxA2 synthase, produced a synergistic inhibitory effect upon collagen-induced platelet aggregation which was greater than that achieved with either aspirin or any of the compounds used alone. Pretreatment of platelets with aspirin reversed this synergistic effect, consistent with it being dependent upon the formation and action of anti-aggregatory prostaglandins. 8. In conclusion, the present study has confirmed the superior platelet inhibitory profile of a combination of a TP-receptor blocking drug and a TxA2 synthase inhibitor to that of either activity alone. However, the maximum inhibitory effect of the currently available compounds, R.68070 and CV4151, which possess both activities in the same molecule, appears to be no greater in vitro than that obtained with the potent TP-receptor blocking drug, GR32191. This most probably reflects the inhibition by R.68070 and CV-4151 of platelet cyclo-oxygenase at the concentrations required for effective TP-receptor blockade which results in a reduction in the formation of anti-aggregatory prostanoids.

A specific thromboxane receptor blocking drug, AH23848, reduces platelet deposition on vascular grafts in man

The antithrombotic effect of a specific thromboxane A2 receptor blocking drug, AH23848, on radio-labelled platelet deposition in mature Dacron aorto-bifemoral grafts has been evaluated in patients. Thirty patients were randomly allocated to AH23848 70 mg, aspirin 300 mg plus dipyridamole 75 mg or placebo 8-hourly for 9 days. AH23848 inhibited platelet aggregation induced by the thromboxane A2 mimetic U-46619; no such effect was observed with aspirin plus dipyridamole. 111In-platelet uptake was measured as the thrombogenicity index (TI) which is a measure of the daily rate of accumulation of platelets by the graft. The mean (s.e. mean) value of 0.193 (0.029) on placebo was significantly reduced to 0.115 (0.022) by AH23848 (p less than 0.05) but only to 0.175 (0.028) by aspirin plus dipyridamole. There was no difference in mean platelet life span between the three treatment groups. The pronounced antithrombotic effect of AH23848 implicates thromboxane A2 in the process of platelet deposition in arterial prostheses and demonstrates the considerable promise of thromboxane receptor blocking drugs as antithrombotic therapy.

Preliminary clinical studies with thromboxane synthase inhibitors and thromboxane receptor blockers. A review

This review of the clinical studies of thromboxane synthase inhibitors (TXSIs) and thromboxane receptor blocking drugs (TXRBs) covers the years 1981 to the present. Clinical studies on TXSIs include those in normal volunteers as well as those in patients with angina, peripheral vascular disease and Raynaud's syndrome, pulmonary hypertension, cerebral vasospasm, hepatorenal syndrome, adult respiratory distress syndrome, and those on cardiopulmonary bypass and hemodialysis. The compounds studied include dazoxiben, dazmagrel, CGS 13080, CV 4151, OKY 1581, OKY 046, and U 63557A. In volunteers, single-dose studies have demonstrated inhibition of thromboxane A2 (TXA2) formation, with some small increases in bleeding time but no marked effect on platelet aggregation. In general, the compounds tested were ineffective in both chronic stable angina and vasospastic angina but caused symptomatic improvement in patients with unstable angina. The TXSIs studied were found to produce no consistent effects in any of the other clinical conditions. Since none of the compounds tested produced a sustained inhibition of TXA2 synthesis, the disappointing clinical results with this class of drugs may be due to an incomplete blockade of thromboxane synthase with the dosage regimens used. Possible alternative or additional reasons for the general lack of success with TXSIs could be that some of the diseases studied do not involve TXA2 or that accumulating prostaglandin endoperoxides in the presence of thromboxane synthase inhibition substitute for TXA2 in causing platelet aggregation. TXRBs rely for their efficacy only on blockade of the TXA2 receptor and antagonize the deleterious effects of both TXA2 and prostaglandin H2 equally, so they represent a simpler pharmacological approach than TXSIs. Such drugs include AH 23848, GR 32191, BM 13.177, BM 13.505, and SQ 28668. All of these compounds are inhibitors of platelet aggregation induced by TXA2 or by its stable mimetic, U-46619. AH 23848 was ineffective in patients with stable angina but did benefit patients with peripheral vascular disease. BM 13.177 has also proven effective in preventing restenosis after angioplasty, occlusion of coronary artery bypass grafts, and the deleterious effects of TXA2 in renal disease. From these preliminary studies, it would appear that TXRBs may offer greater clinical potential than TXSIs. Further studies currently underway with TXRBs to resolve this question include those in unstable angina, angioplasty, peripheral vascular disease, renovascular hypertension, and cyclosporine nephrotoxicity.

Preliminary assessment of a novel thromboxane A2 receptor-blocking drug, GR32191, in healthy subjects

The effects of GR32191, a novel, specific, thromboxane A2 receptor-blocking drug, on platelet aggregation induced by U-46619 and adenosine diphosphate (ADP) ex vivo, have been examined in healthy male subjects. In two placebo-controlled studies (the first, a single-dose, crossover study, and the second, a group comparative, multiple-dose study), concentration-effect curves for both agonists were constructed in whole blood by counting platelets electronically before and after dosing. Single oral doses of 0.125 and 0.25 mg/kg (four subjects) and 0.5 and 1.0 mg/kg (four subjects) produced dose-related shifts to the right in U-46619 concentration-effect curves. The elimination half-life of GR32191 (up to 8 hours postdrug) was approximately 2 hours. Multiple dosing with GR32191, 17.5 mg (equivalent to 0.25 mg/kg for a 70 kg subject), three times daily (three subjects) or every 12 hours (six subjects), resulted in a cumulative inhibitory effect on U-46619 platelet aggregation in the apparent absence of a build-up of plasma concentrations of GR32191. Primary platelet aggregation induced by ADP was unaffected by GR32191. Multiple dosing had no effect on lancet bleeding time, and no drug-related changes were seen in routine laboratory hematologic and biochemical screens. GR32191 was well tolerated, although a subject with a history of drug-induced rectal bleeding reported the same symptom while taking GR32191.

Pathophysiological actions of thromboxane A2 and their pharmacological antagonism by thromboxane receptor blockade with GR32191

With a growing general conviction that thromboxane A2 does have a pathological role in occlusive vascular disease, there is a current debate on the ideal type of drug treatment needed. The more widely accepted view now seems to be that drugs that antagonize the actions of thromboxane A2 by blocking its receptors have greater clinical potential than those that block its synthesis. However, this premise has yet to be proven clinically. The historical development of thromboxane receptor blockers as a new class of medicines and, in particular, that of GR32191, are described here. The clinical evaluation of GR32191 should determine the importance of thromboxane A2 in cardiovascular disease.

Differential ability of agonists to express distinct pools of fibrinogen (GpIIb/IIIa) receptors which can mediate the aggregation of human platelets

We have investigated the effect of two procedures that modify human platelet surface membrane glycoprotein (Gp) IIb and IIIa complexes upon whole blood platelet aggregation to a range of agonists. (A) Irreversible disruption of complexes by temporary (30 min) Ca2+-deprivation with EGTA at 37 degrees C. (B) Binding of a monoclonal antibody M148 to the complex. EGTA exposure abolished aggregation to ADP, adrenaline and PAF. In contrast, full aggregation curves to collagen and U-46619 could still be established. EGTA exposure reduced M148 binding to platelets by 80%. Excess M148 abolished aggregation to ADP, PAF, collagen and U-46619. However, upon removal of unbound antibody from platelets full aggregation curves to collagen and U-46619 but not to ADP and PAF could be re-established. Thus human platelet aggregation to ADP, PAF and adrenaline appears absolutely dependent upon surface membrane GpIIb/IIIa complexes. In contrast, collagen and U-46619 cause expression of an additional distinct pool of Gp complexes inaccessible to EGTA and M148 in unstimulated platelets which is intimately involved in aggregation to these agonists.

Effects of hypoxia, elevated K+ and acidosis on the potency of verapamil, diltiazem and nifedipine in the guinea-pig isolated papillary muscle

1 The effects of the structurally diverse calcium channel blockers verapamil, nifedipine and diltiazem were investigated on the force of contraction of guinea-pig, electrically stimulated papillary muscles in vitro. 2 Calcium channel blocking potency was assessed either as a direct negative inotropic effect or as the ability of the drugs to antagonise the positive inotropic effects of added calcium (Ca2+). By either method, the rank order of potency was found to be nifedipine greater than verapamil greater than diltiazem. 3 Various factors which mimic some of the consequences of acute ischaemia in vivo, namely low pH, hypoxia and elevated K+, in combination, but not singly, enhanced the negative inotropic potency of verapamil and to lesser extent that of diltiazem, but not that of nifedipine. 4 Whilst the various interventions, especially in combination, produced a profound negative inotropic effect themselves, this was not responsible per se for the potentiation of the negative inotropic effects of verapamil and diltiazem, since the negative inotropic effects of nifedipine and dinitrophenol were not potentiated under the 'ischaemic' conditions. 5 By use of antagonism of the positive inotropic action of exogenous Ca2+ as an alternative measure of potency, the differential influence of 'ischaemia' on calcium channel blocker potency was confirmed. The effect of verapamil was potentiated some 9 fold, that of diltiazem about 2 fold, and that of nifedipine was unchanged. 6 The differential effect of 'ischaemia' on the potencies of the calcium channel blockers was unexpected. Verapamil (but not nifedipine) is thought to bind to the calcium channel at a specific site not easily accessible from the extracellular space. 'Ischaemic' conditions may cause membranal perturbations which allow verapamil easier access to its binding site thus increasing its negative inotropic potency.

GR32191, a highly potent and specific thromboxane A2 receptor blocking drug on platelets and vascular and airways smooth muscle in vitro

1. The thromboxane A2 (TP)-receptor blocking activity and specificity of action of GR32191 ([1R-[1 alpha(Z),2 beta,3 beta,5 alpha]]-(+)-7-[5-([1,1'-biphenyl] -4-ylmethoxy)-3-hydroxy-2-(1-piperidinyl)cyclopentyl]-4-heptoni c acid has been evaluated in human platelets and various smooth muscle preparations, both vascular and non-vascular, from a range of species including man. 2. Utilising a platelet counting method to assess aggregation the drug was found to antagonise, in a surmountable manner, human platelet aggregation produced by the TP-receptor agonists, U-46619, EP171 and SQ26655, in whole blood and physiological buffer, with pA2 values of approximately 8.3 and 8.7 in the two media respectively. In the presence of GR32191 the rate of aggregation induced by U-46619 was slowed. 3. The effect of GR32191 upon U-46619-induced platelet shape change and aggregation in platelet-rich plasma was evaluated utilising a turbidometric technique. Both shape change and aggregation were antagonised by GR32191. At relatively high concentrations of the drug a slowing of aggregation and shape change to U-44619 was seen and an unsurmountable antagonism became apparent. 4. The action of GR32191 upon human platelets was specific with platelet aggregation induced by adenosine 5'-diphosphate, platelet activating factor, vasopressin and adrenaline and the inhibitory effects of prostacylin (PGI2), prostaglandin D2 (PGD2) and N-ethylcarboxamide-adenosine (NECA) being unaffected by concentrations of the drug as high as 10 microM. Furthermore, at concentrations of up to 100 microM, the drug itself produced no shape change or aggregation, of human platelets. 5. GR32191 also specifically and potently antagonised in a competitive, surmountable manner the contractile actions of U-46619 upon human vascular smooth muscle and antagonised U-46619-induced contractions of vascular and airways smooth muscle preparations from rat, dog, guinea-pig and rabbit with varying potency. This is discussed in terms of possible heterogeneity of TP-receptors. 6. GR32191 therefore represents a highly potent and specific TP-receptor blocking drug. This profile of action, coupled to its long duration of effect in man described elsewhere, make it an ideal drug tool for elucidating the physiological and pathophysiological role of thromboxane A2.

Use of an anti-platelet monoclonal antibody F(ab')2 fragment for imaging thrombus

Ten patients with suspected thrombus have been studied using 111In-labelled F(ab')2 fragments of P256, a monoclonal antibody which recognizes an epitope on the platelet membrane glycoprotein protein IIb/IIIa complex. The F(ab')2 fragment was radiolabelled with 111In via diethylenetriaminepenta-acetic acid to give a specific activity of up to 190 MBq (5 mCi) mg-1 without impairment of immunoreactivity. In vitro platelet aggregation studies showed that the F(ab')2 fragment caused less platelet aggregation than the whole antibody on a molar ratio and was without significant effect upon the sensitivity of platelets to a range of aggregating agents. Platelets were labelled in ten patients by intravenous injection of approximately 100 micrograms P256 F(ab')2. Of the ten patients studied, six showed localization of activity consistent with platelet accumulation. Localization was clearly seen when associated with thrombus of the lower limbs (three patients: deep vein thrombosis; one patient: aortofemoral graft), and was apparent although less marked in two other cases, one of aortic aneurysm and one of carotid stenosis. Use of radiolabelled P256 F(ab')2 offers a means of non-invasive detection of thrombus which, from in vitro studies, would appear to have less direct effect on platelet behaviour than the whole antibody.

AH6809, a prostaglandin DP-receptor blocking drug on human platelets

1. The effect of AH6809 (6-isopropoxy-9-oxoxanthene-2-carboxylic acid) has been studied upon the anti-aggregatory and aggregatory actions of various agents on human platelets in whole blood. 2. Prostaglandin D2 (PGD2), BW245C, 9 alpha, 11 beta-PGF2, PGI2 and 5'-N-ethylcarboxamide adenosine (NECA) all inhibited ADP-induced platelet aggregation in whole blood. The anti-aggregatory activity of PGD2, BW245C and 9 alpha, 11 beta-PGF2 but not PGI2 or NECA was antagonized by AH6809. NECA was antagonized by AH6809. 3. The antagonism of the anti-aggregatory activity of PGD2 by AH6809 was concentration-related and could be overcome by increasing the concentration of PGD2. Analysis of the data yielded an apparent pA2 for AH6809 of 5.35. 4. At approximately 10 fold higher concentrations than those required to antagonize the action of PGD2, AH6809 also antagonized the aggregatory effect of U-46619 in whole blood (pA2 = 4.45). However, concentrations of AH6809 up to 300 microM were without effect upon either ADP- or platelet activating factor (Paf)-induced aggregation (pA2 less than 3.5). 5. The potency of AH6809 against PGD2 and U-46619 was increased in a resuspended platelet preparation suggesting that the drug is extensively bound to plasma proteins. However, in resuspended platelets the specificity of AH6809 relative to that seen in whole blood was reduced since aggregation by ADP and Paf was also slightly antagonized. 6. In conclusion, AH6809 appears to be a weak but specific DP-receptor blocking drug on human platelets and should prove to be a useful drug tool for defining the involvement of endogenous PGD2 in platelet aggregation and classifying the mode of action of anti-aggregatory prostanoids.

Imaging thrombus with radiolabelled monoclonal antibody to platelets

Indium-111-hydroxyquinoline labelled platelets, though useful in the detection of thrombus, have not gained widespread use owing to the time and technical skill required for their preparation. A study was therefore conducted evaluating a new method of imaging thrombus with platelets radiolabelled with a 111In labelled monoclonal antibody, P256, directed to the platelet surface glycoprotein complex IIb/IIIa. When the number of receptors occupied by P256 was less than 3% of the total available on the platelet surface platelet function, as assessed by platelet aggregometry, was undisturbed. P256 was radiolabelled with 111In using diethylenetriaminepenta-acetic acid, which achieved a specific activity of 185 MBq (5 mCi)/mg. No impairment of immunoreactivity was detected at this specific activity. Platelets were labelled with radiolabelled monoclonal antibody in vitro in two patients at a receptor occupancy of 6% and in vivo--that is, by direct intravenous injection of P256--in six patients at a receptor occupancy of 1%. In vivo recovery and biodistribution kinetics suggested that after in vitro labelling platelets were minimally activated. The 111In kinetics recorded after intravenous P256 suggested rapid and efficient radiolabelling of platelets and gave no indication of platelet activation. Of the six patients who received intravenous P256, three had documented thrombus, two of whom gave positive results on P256 platelet scintigraphy. The third subject had chronic deep venous thrombosis and was scintigraphically negative. Imaging thrombus using a radiolabelled monoclonal antibody directed to platelets appears to offer great potential as a simple, non-invasive approach to the diagnosis of thrombosis.

Effect of the specific thromboxane receptor blocking drug AH23848 in patients with angina pectoris

The effect of the specific thromboxane receptor blocking drug AH23848 was investigated in two double blind placebo controlled studies in male patients with exercise induced angina pectoris and angiographically verified coronary lesions. In the first study cardiac pacing was performed in twenty patients after coronary angiography. Patients were then randomised into two groups and received either AH23848 (70 mg orally) or placebo. One hour later cardiac pacing was repeated. Neither treatment had any significant effect upon time to angina or the rate-pressure product at the onset of chest pain in these patients. In the second study twenty male patients were randomised to seven days' treatment with AH23848 (70 mg three times a day) or placebo followed by a crossover to the other treatment for a further seven days. Clinical assessment was performed before treatment and at the end of each treatment period. There was no significant difference between the placebo and AH23848 treatment periods in exercise tolerance, the rate-pressure product at angina after exercise testing, the number of ischaemic attacks as determined from 24 hour ambulatory electrocardiograms, the number of attacks of pain, or the number of glyceryl trinitrate tablets consumed. This lack of a clinical effect with AH23848 was seen despite a profound inhibition of ex vivo platelet aggregation stimulated by the thromboxane A2-mimetic U-46619. Because in experimental animals in vivo AH23848 blocks vascular thromboxane receptors as well as platelet thromboxane receptors the lack of effect of AH23848 in cardiac pacing and exercise induced angina is unlikely to be the result of inadequate blockade of thromboxane receptors. The lack of effect of the drug is more likely to indicate that thromboxane A2, is not a factor in the aetiology of the pain experienced by these patients during exercise or cardiac pacing.

Effects of intravenous infusions of prostaglandin D2 in man

Prostaglandin D2 (PGD2) was infused intravenously into normal male volunteers. Seven subjects received infusions of 16, 32, 64 ng/kg/min and six of these a further dose of 128 ng/kg/min. Each individual's maximum dose was limited by discomfort caused by intense facial flushing and nasal congestion. At these doses there was no significant effect on systolic or diastolic blood pressure nor on spirometric measurements. There was a small but statistically significant tachycardia at 64 and 128 ng/kg/min. Collagen- and adenosine diphosphate (ADP)-induced platelet aggregation ex vivo was not affected at any of the infusion rates. Infused PGD2 is unlikely to be a useful antithrombotic agent.

Studies on the characterisation of prostanoid receptors: a proposed classification

Comparison of rank orders of agonist potency of the naturally occurring prostanoids, PGD2, PGE2, PGF2 alpha and PGI2 as well as the stable TxA2 mimetic, U-46619, on a range of smooth muscle preparations provides evidence for the existence of distinct receptors for PGE2, PGF2 alpha and TxA2. Since others have provided evidence for the existence of distinct receptors for PGD2 and PGI2, we suggest that receptors exist for each of these naturally occurring 2-series prostanoids. Results obtained with two specific prostanoid receptor blocking drugs, SC-19220 and AH 19437, support and extend these conclusions. SC-19220 selectively blocks some but not all PGE-sensitive receptors, while AH 19437 selectively blocks all U-46619/TxA2-sensitive receptors. A nomenclature for prostanoid receptors is proposed, in which each receptor is designated the letter P preceded by a letter signifying the most potent natural prostanoid agonist at that receptor, such that receptors sensitive to PGs D2, E2, F2 alpha, I2 and TxA2 become DP-, EP-, FP-, IP and TP- receptors respectively. Where some sub-division is required within a receptor group, e.g. EP-receptors (SC-19220-sensitive and SC-19220-insensitive), subscript numbers may be used such that these are EP1 and EP2 subtypes. The resulting scheme is a working hypothesis and its confirmation requires the development of potent selective prostanoid receptor blocking drugs for each postulated type.

Comparison of the potencies of some prostaglandins as vasodilators in three vascular beds of the anaesthetised dog

Following close intra-arterial administration to the carotid and femoral arterial beds of the anaesthetised dog the rank order of potency for producing vasodilation was PGE greater than 11-deoxy PGE0 greater than PGA greater than PGI2 greater than PGB with the 1- and 2-series prostaglandins equipotent. PGI2 was about 60 times weaker than PGE1. In the mesenteric arterial bed the rank order of potency for producing vasodilation was the same except PGI2 was about equipotent with PGE1. The absolute potency of the prostaglandins, with the exception of PGI, was similar on all three vascular beds. A similar differential action of PGI2 relative to PGE1 was also observed following both left intraventricular and intravenous administration. We suggest that all three arterial beds contain PGE-receptors mediating vasodilatation at which the E-series prostaglandins are potent and PGI2 is weak. In addition the mesenteric bed contains PGI2-receptors which are absent or sparse in the carotid and femoral beds.

Brief marital therapy outcome: personality correlates

The purpose of the study was to investigate with regression analyses the relationship between personality and background variables and outcome of brief marital therapy. Twenty personality dimensions from the Personality Research Form and data on age, education, number of previous unions, number of children and number of sessions were used as independent variables. The dependent variable was provided by ratings of over-all improvement in affective communication: sexual, verbal, and social, provided by clinical records of 30 couples. The main result was that only the husband's traits were found to be important correlates of improvement in affective communication of both husbands and wives. The important traits involved husband's cognitive efficiency in the area of reflection and accurate judgement of situations, submissiveness, and sensitivity to social approval of his behaviour. The number of individual therapy sessions given to wives was also a notable predictor of improvement. Other personality and background variables were not significantly related to outcome.

A method for quantitating platelet aggregation and analyzing drug-receptor interactions on platelets in whole blood in vitro

A method for quantitating platelet aggregation in whole blood is described. The method is rapid and reproducible. By utilizing the Ultra-Flo 100 whole blood platelet count, platelet numbers before and after addition of an aggregating agent are counted electronically, without the need to separate platelets from other blood cell types. Although designed primarily for use with human blood, the Ultra-Flo can also be used successfully with blood from other species. The method has been used to study the antagonism of adrenaline-induced aggregation of platelets in human whole blood by the alpha-adrenoceptor blocking drug phentolamine. The potential of the method for studying drug-receptor interactions on platelets in vitro as well as rapid assessment of platelet function ex vivo is discussed.

Comparison of the actions of U-46619, a prostaglandin H2-analogue, with those of prostaglandin H2 and thromboxane A2 on some isolated smooth muscle preparations

1 The actions of the prostaglandin H2 (PGH2) analogue, U-46619, have been compared with those of PGH2 on thromboxane A2 (TxA2) on a range of isolated smooth muscle preparations in a superfusion cascade system. 2 U-46619 was a potent agonist on guinea-pig lung strip, dog saphenous vein and rat and rabbit aortae. In contrast, U-46619 was weak or inactive on guinea-pig ileum and fundic strip, cat trachea and dog and cat iris sphincter muscles, preparations on which either PGE2 or PGF2 alpha was the most potent agonist studied. 3 PGH2 was active on all of the preparations and displayed little selectivity. On some of the preparations, the actions of PGH2 may have been mediated indirectly by conversion to other prostanoids. 4 In contrast, TxA2 displayed the same pattern of selectivity as U-46619, being a potent agonist on the lung strip and vascular preparations but weak or inactive on the others. 5 It is suggested that U-46619 is a selective TxA2-mimetic and that it should therefore be a valuable tool in the study of the actions of TxA2.

Evidence from agonist and antagonist studies to suggest that the beta1-adrenoceptors subserving the positive inotropic and chronotropic responses of the heart do not belong to two separate subgroups

The positive inotropic and chronotropic responses of guinea-pig isolated left and right atria to 17 sympathomimetic amines were examined under conditions selected to control the pharmacological environment. Each agonist was compared with (--)-isoprenaline as the reference by constructing dose-response curves. Equiactive molar concentration ratios relative to (--)-isoprenaline were calculated at the EC50 for rate and tension responses. Statistical analysis revealed that (--)-noradrenaline and (+/-)-alpha-methylnoradrenaline were tension selective whereas (+/-)-alpha-ethylisoprenaline and N-methyldopamine were rate selective relative to (--)-isoprenaline. However, no structural trends emerged. The rank order of potency varied slightly between rate and tension, but an analysis of the regression and correlation coefficients indicated respectively that the equiactive molar concentration ratios and the rank orders could be considered identical on rate and tension. Antagonism of the (--)-isoprenaline-induced rate and tension responses by acebutolol, atenolol, practolol and propranolol was assessed from the pA2 values which were almost identical for both parameters with each antagnosti. It is concluded that the beta1-adrenoceptors mediating the positive inotropic and chronotropic responses do not warrant subdivision into two separate groups.

Analysis of the inotropic: chronotropic selectivity of dobutamine and dopamine in anaethetised dogs and guinea-pig isolated atria

The previously reported in vivo inotropic selectivity of dobutamine and dopamine compared with isoprenaline has been demonstrated in anaesthetised bivagotamised open chest dogs. Responses of heart rate, right ventricular tension, left ventricular dP/dtmax, and blood pressure were measured. Compared with isoprenaline, dobutamine and dopamine were 1.8 and 2.4 times more active in increasing cardiac contractility than rate. The inotropic selectivity of dopamine but not that of dobutamine was abolished by pretreatment of dogs with syrosingopine. In guinea-pig isolated atria, isoprenaline, dobutamine, and dopamine were all slightly rate selective although this was less for dopamine. In atria incubated with phenoxybenzamine from guinea-pigs pretreated with reserpine only the dopamine dose-response curves were displaced to the right indicating a considerable indirect sympathomimetic component. The in vivo inotropic selectivity of dopamine could therefore be explained on the basis of this indirect activity being manifested at lower doses in the myocardium where the stores of catecholamines are more abundant than at the sinoatrial node. However, it is concluded that the inotropic selectivity of dobutamine seen in vivo is not due to indirect activity, reflex effects, or to a difference in the beta-adrenoceptors mediating the rate and tension responses of the heart. Possible alternative explanations are discussed.

Selective reserpine-induced supersensitivity of the positive inotropic and chronotropic responses to isoprenaline and salbutamol in guinea-pig isolated atria

1 Dose-response curves for the positive inotropic and chronotropic responses to isoprenaline were obtained in atria from untreated guinea-pigs and those receiving various reserpine pretreatments. 2 Tension responses were unaffected, whereas rate responses were depressed by the lowest dose of reserpine (0.05 mg/kg i.p. at 24 hours). 3 With larger 24 h doses and a 3 day pretreatment, the rate and tension dose-response curves were progressively displaced to the left, indicating supersensitivity which was greater for tension at each pretreatment. 4 No supersensitivity to histamine or Ca2+ could be detected, leading to the conclusion that it was selective for the beta-adrenoceptor agonists possibly at the receptor level. 5 As an indication of the adrenergic neurone depleting effectiveness of each reserpine dosage, preparations were exposed to test doses of beta-phenylethylamine. 6 Salbutamol was a partial agonist in untreated atria, the maximum rate (63.3%) and tension (10.0%) responses being less than those for isoprenaline. In atria from reserpine pretreated animals the supersensitivity was revealed as an increase of this maximum compared with isoprenaline. 7 The significance of this observation in relation to the possible mechanism of the supersensitivity is discussed.

Differential blockade of guinea-pig atrial rate and force responses to (--)-noradrenaline by practolol - an uptake phenomenon

The relative blockade of rate and force responses of guinea-pig isolated atria to catecholamines by practolol was compared. pA2 determinations revealed that with (--)-noradrenaline as the agonist, practolol was more effective in antagonizing rate (pA2, 6.99) than force (pA2, 6.44). This difference was absent in either atria from reserpinized animals incubated with phenoxybenzamine and tropolone or atria incubated with cocaine. Similarly, when (--)-isoprenaline was the agonist, force and rate pA2 values were identical, suggesting that rate and force beta1-adrenoceptors do not differ. The preferential blockade of (--)-noradrenaline-induced rate responses in untreated atria was attributed to the operation of a saturable neuronal uptake mechanism which differs between left and right atria. To avoid high (--)-noradrenaline concentrations encountered during pA2 determinations, the % reduction of responses to single doses of (--)-noradrenaline by practolol were measured in untreated atria. Far from confirming the lack of receptor differentiation indicated by the pA2 determinations, this study revealed force to be more practolol sensitive. The relative merits of the two methods are discussed in an attempt to resolve the question whether rate and force beta1-adrenoceptors differ.