Dose antiplatelet agents; the relationship among side effects, and antithrombotic effectiveness.
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Intracellular calcium is a universal second messenger that can serve as a broad-based measure of receptor activity. Recent developments in multi-well plate fluorescence readers facilitate measurement of intracellular free-calcium levels and reduce reliance on slower, more cumbersome or expensive data collection methods. In this report, we describe a rapid and sensitive method to assay intracellular calcium ions in human embryonic kidney (HEK293) and Chinese hamster ovary (CHO) cells from multi-well plates using a fluorometer equipped with on-line injectors. We examine the compatibility of visible-light excitable dyes Calcium Green-1 and Oregon Green 488 BAPTA-1. Using this assay, we were able to detect and quantify activity from muscarinic and beta-adrenergic receptors endogenous to HEK293 cells and detect calcium signals generated by activation of Gi-coupled recombinant mu-opioid and dopamine D2L receptors, and the Gs-coupled melanocortin subtype 4 (MC4) receptor. Fluorescence signals, stable in HEK293 cells, required the use of Oregon Green 488 BAPTA-1 and an inhibitor of organic anion transport in CHO cells. Under appropriate conditions, both cell types can be used to collect complete concentration-response data for a variety of receptors (including a recombinant muscarinic M1 receptor expressed in CHO cells) from a single plate of dye-loaded cells.
Nonsteroidal anti-inflammatory drugs (NSAIDs) are now commonly used for the treatment of acute gout, but caution is required in view of their adverse effects, especially in the elderly. Colchicine is still an effective acute agent, but care must be taken to monitor toxicity. Intra-articular glucocorticosteroid therapy is useful and very safe; oral steroids and corticotrophin (adrenocorticotrophic hormone) may have a small role in acute therapy and seem safe when used over short time spans. Low dose colchicine may have a cost and toxicity advantage over NSAIDs in the prophylaxis of gout when commencing therapy aimed at reducing elevated plasma urate concentrations. Allopurinol is more frequently used than uricosuric agents such as probenecid, and toxicity may be largely avoided by tailoring dosage schedules according to renal function.
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The renewed interest in sulphinpyrazone in recent years has arisen from its potential to inhibit platelet aggregation. In vivo much of the activity is probably due to the thioether or sulphide metabolite which has a greater potency and a longer half-life than the parent compound. The sulphide metabolite is formed exclusively by the gut microflora in man. The pharmacokinetics of sulphinpyrazone (200 mg orally) have been studied, with particular attention to the formation of the sulphide metabolite, in groups of patients who might be expected to show abnormal formation of this active metabolite due to altered delivery of the drug to the lower gut or altered gut flora. Five patients studied 1 month after a myocardial infarction did not differ markedly from young, normal volunteers with respect to either sulphinpyrazone or its metabolite. Crohn's disease in the quiescent phase did not significantly alter the pharmacokinetics or metabolism of the drug, but 1 patient who had undergone a hemicolectomy formed negligible concentrations of the active metabolite. Antimicrobial therapy produced highly variable results with almost complete suppression of sulphide formation in some subjects but no apparent effect in others.
The defibrinating agent ancrod has had limited clinical trial, but appears to give no advantages over heparin. Intravenous infusion of dextran, a glucose polymer, has been shown to have an antithrombotic effect in many experimental models of thrombosis. However, the evidence that dextran is a clinically valuable antithrombotic drug is conflicting. A number of controlled randomized studies have shown that dextran can prevent postoperative venous thromboembolism when a large volume of dextran 40 or 70 was infused rapidly during and after surgery. However, blood volume expansion during dextran treatment prohibits its use in patients with reduced cardiac reserve, and infrequent though sometimes severe, allergic reactions have been reported. Evidence that dextran is of value for the treatment of venous or arterial thromboembolism comes from uncontrolled studies and is not convincing. Many compounds have been shown to inhibit platelet function in vitro but only five of these drugs have been extensively evaluated as prophylactic or therapeutic antithrombotic agents in man. These are aspirin, sulphinpyrazone, dipyridamole, hydroxychloroquine and clofibrate. They have been evaluated mainly in patients with cerebral vascular disorders, coronary artery disease, peripheral artery ischaemia, venous thromboembolism, prosthetic heart valves, and in patients with arteriovenous shunts. The evaluation of the clinical effect of the platelet function suppressing drugs is in its early stages, but they appear to differ from each other in the spectrum of their clinical effectiveness, and they may be more effective in arterial than in venous thromboembolic disorders. Their role in the management of cerebral vascular disease and coronary artery disease is still uncertain, and should be clarified by the results of a number of multi-centre, prospective, randomized studies which are currently in progress. Three types of thrombolytic drugs have been evaluated clinically; the plasminogen activators streptokinase and urokinase, proteolytic enzymes such as plasmin, and agents which increase the level of endogenous plasminogen activator (e.g. anabolic steroids). Of these, the plasminogen activators now have a definite place in clinical practice. The plasminogen activators accelerate the lysis of recent venous thrombi and pulmonary emboli, and of arterial thrombi or emboli. Thrombolytic therapy with these agents should be considered particularly in patients with recent major pulmonary embolism, as lysis of recent emboli is rapid and substantial. It should also be considered in patients with recent extensive venous thrombosis, because total lysis of venous thrombi has been reported to result in long-term preservation of valve function, and is likely to prevent postphlebitic syndrome, though this has not been proven. However, plasminogen activator therapy carries a higher risk of bleeding than heparin treatment...
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Interventional angiographic techniques are of increasing importance in the management of arteriosclerosis and its complications. Two areas of particular interest are the treatment of focal arterial stenoses by percutaneous transluminal angioplasty and the treatment of arterial thromboemboli with selective infusion of thrombolytic agents. The administration of multiple drugs, in various combinations, is a critical factor in the success of these interventions. To effectively use this new pharmacoangiography, it is important to understand both the pathophysiology of the atherosclerotic or thrombotic process being treated and the actions of the drugs used. Preserving the effect of angioplasty relies on preventing thrombus formation and preventing recurrence of the atherosclerotic obstruction. Heparin during the procedure is clearly useful for the former, and aspirin in small doses and other antiplatelet medications are indicated for the latter. The precise utility of long-term anticoagulation, of low molecular weight dextran, and of various antiplatelet regimens remains to be proven. The theoretical importance of these medications in improving long-term patency rests on the effects they have on platelet and vessel wall prostaglandins, on intimal smooth muscle cell proliferation, and on the thrombogenicity of injured arterial intima. Fibrinolytic therapy, with streptokinase and urokinase, has been used for many years. Selective intraarterial use, however, is a new and promising application. Intracoronary streptokinase infusion during acute myocardial ischemia appears to prevent or limit infarction in certain patients. Peripheral use for acute arterial occlusion, either in native vessels or in grafts, is an area of great promise. Key considerations are thrombus age, size, and location, and the status of the arterial flow proximal and distal to the obstruction.
Platelet function in vitro may be investigated in various ways. One of the accepted methods involves examination of platelet procoagulant release. Optimal conditions relating to maximal release have not been defined. Red cell stroma phospholipid can substitute for the platelet procoagulant in many in vitro tests (Quick et al., 1954, 1957; Margolis, 1961; Lovric & Margolis, 1964). Using a multiple channel coagulation meter, an assay system was developed for testing both red cell stroma and platelet procoagulants. Maximum procoagulant release was obtained by lysing in alkaline hypotonic solutions or by sonication. The system was tested in a one-stage kaolin-activated platelet-free plasma substrate, obtained from fresh pooled citrated plasma. With this method the amount of available procoagulant could be related to the numbers of red blood cells or platelets. The results indicated that one platelet provided procoagulant equivalent to that derived from 0.7 red cells. Procoagulant release was studied under various conditions and expressed in terms of stable (red cell) standard. Platelets subjected to hypotonic stress or factor 3 release (Hardisty test) were studied quantitatively. The addition of sulphinpyrazone (Anturan) had no observable effect on platelet release in this system.
Comparison of oral and i.v. administration of sulphinpyrazone (10 mg/kg) to rabbits showed that the oral route was associated with an incomplete bioavailability and a six-fold greater formation of the active sulphide metabolite. The bile was an important route of elimination of unchanged sulphinpyrazone in rabbits (18% of an i.v. dose in four hours). Only small amounts of the sulphide appeared in the bile after i.v. administration. Pretreatment with oral antibiotics decreased the area under the plasma concentration-time curve (AUC) for the sulphide but increased that of the parent drug. Excretion of the p-hydroxysulphide metabolite in urine was decreased 30-fold by antibiotic treatment. The contents of the caecum showed the greatest capacity for sulphinpyrazone reduction in vitro. The liver possessed a slight ability to reduce sulphinpyrazone in vitro under anaerobic, but not aerobic, conditions. The gut bacteria are the main site of reduction of sulphinpyrazone to the active sulphide metabolite in the rabbit. These findings contrast with those obtained for sulindac which was reduced extensively under both aerobic and anaerobic conditions by rabbit-liver soluble fraction in vitro. The sulphide metabolites of both sulphinpyrazone and sulindac were oxidized to the parent drug by rabbit-liver microsomes.
1. Incubation of human or rabbit faeces with sulphinpyrazone gave greater reduction under anaerobic than under aerobic conditions. Reduction of sulindac by human faeces was more extensive than that of sulphinpyrazone. 2. Growth of mixed cultures of intestinal bacteria in nutrient media containing antibiotics produced a marked inhibition in their ability to reduce sulphinpyrazone. Sulphide formation was inhibited by metronidazole and lincomycin for human faeces and by tetracycline for rabbit faeces/caecal contents. 3. The formation of the sulphides of sulindac and sulphinpyrazone ex vivo was decreased in faeces from patients treated with metronidazole. Metronidazole, but not tetracycline, decreased the extent of reduction of sulphinpyrazone by rabbits in vivo. No reduction of either substrate occurred on incubation with ileostomy effluent. These data indicate that anaerobic intestinal bacteria are important in the reduction of these sulphoxide-containing drugs. 4. However, when incubated anaerobically with over 200 strains of bacteria isolated from human faeces, sulphinpyrazone was reduced by most of the aerobic but not the anaerobic organisms. Sulindac was reduced more extensively by the same aerobes and by some anaerobes. 5. The discrepancy between the apparent importance of anaerobes in vivo and in vitro may be due to their very large number present in the hind gut and to the production of an anaerobic environment suitable for the enzymic activity of other organisms, such as aerobes or facultative anaerobes.
The poor correlation between clinical salicylate toxicity and serum blood levels is reapproached in light of recent evidence linking clinical severity with initial volume of distribution (Vd). It is recognized that two variables alter salicylate Vd in such manner that serum salicylate levels are misleading (thus, the change in Vd is not detected by present methods). These variables are serum protein binding and the pH-dependent ionized/un-ionized ratio in the unbound salicylate fraction. Measurements of salicylate concentration in the cerebro spinal fluid (CSF) would circumvent these variables, but would be clinically impractical. Thus, an alternative is sought to the inexact total serum salicylate levels and the impractical CSF salicylate levels for assessment of the severity of salicylate poisoning. This study indicates that, in dogs, serum unbound salicylate levels closely reflect CSF salicylate levels, even as a decrease in serum protein binding is in progress. However, serum unbound salicylate concentration does not reflect CSF salicylate concentration as a decrease in serum pH is elicited (CSF salicylate actually increased as serum unbound salicylate decreased). On the other hand, serum unbound salicylate measurement would seem preferable to total serum salicylate measurements now used in that the total value decreased markedly as either protein binding change or acidosis produced a change in distribution and the resultant increase in CSF salicylate.
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The authors report 22 cases of transient ischemic attacks (TIA's) manifested by amaurosis fugax or hemiparesis or paresthesia of less than 24 hours' duration. None of the patients demonstrated 1) evidence of atherosclerotic cerebral vascular disease on angiography, 2) evidence of intracranial lesion on brain scan, 3) cardiac source of emboli, 4) arteritis or collagen disease, or 5) history of migraine. The only abnormalities found to explain the TIA's were abnormally increased platelet adhesiveness and/or aggregation. All of these patients were followed from 1 to 5 years, and had repeated coagulation studies. Treatment with antiplatelet drugs showed an excellent clinical response with associated decrease in platelet adhesiveness and aggregation. Discontinuance of the antiplatelet drug resulted in a recurrence of the TIA's which coincided with an increase in aggregation and adhesiveness. In two cases the platelet morphology was studied by transmission and scanning electron microscopy. It appears that there is a specific group of patients with TIA's in whom the sole cause of the attack is an abnormality of platelet function. For these people there is a specific therapy and a method monitoring the treatment.
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