Blood coagulation and platelet function: Introduction

Pharmac

Yker Vol 5 I~ 225-227

1979

Pergamon Pre~a Ltd

Prmtzd m Great Brmun

BLOOD COAGULATION AND PLATELET FUNCTION: INTRODUCTION B B VARGAFFIG, I J C O N A R D 2 and M. SAMAMA 2 I Umt~ de Venms, lnstltut Pasteur, Pans, France, 2Laboratolre Central d'Hematologle, Hotel Dleu, Pans, France

1 G E N E R A L CONCEPTS The purpose of the Workshop's Group on Clotting and Platelets was to determine the best procedures to detect at an early stage (t e. dunng pharmacologtcal and biochemical testing) those drugs which may display unwanted effects on blood clotting. This involves the coagulaUon system, platelets, then" mteracUon with vessel wall and blood flow modificaUons. By definmon, drugs coming into the pharmacological screemng for toxicological purposes are those which are not expected to affect platelets, coagnlatmn, blood flow or the endothehum surface. The unwanted effects on clotting may mvolve 'antagonistic' effects (prohemorrhaglc) and 'agomsuc' effects (thrombogenic). Smce the former are likely to be uncovered dunng the bastc pharmacological procedures or during a subacute toxicological test with small groups of an|mals, the Workshop concentrated its attention on methods chosen to detect thrombogenic potency. A large number of test procedures ~s available by which isolated steps or the overall process of blood clotting can be measured. Most of these tests were onginally developed for the dtagnosis of the many diseases that are due to a lack or malfunct|on of one or several clotting factors, and the coagulopath|es, caused by abnormafiues of the blood platelets. Thus use of these tests for the assessment of hypercoagulabfllty, however, is problematic. The results of laboratory procedures m diseases that are presumably or probably accompanied by hypercoagulability of the blood (e.g. myocardial infarct, peripheral arterial and venous thrombosis, increased tendency to develop thrombosis in assoctauon with pregnancy, dtabetes melhtus, certain mahgnant tumors, etc.) have often been disappomung. It ~s understandable, therefore, that no routine procedures designed to detect thrombogenic~ty have so far been incorporated into the pre-clin|cal work-up of new drugs. 2. DRUGS AS CAUSE OF THROMBOSIS There Is much experimental evidence showing that chemical substances can, indeed, cause thrombosis Among the known thrombogenic agents are pamcularly those that acUvate the coagulaUon system, e.g thrombm, saturated long-chain fatty acids (Zbmden, 1973), and ellagic acid (Gn'olami et al., 1973). Thrombosis can also be induced by compounds causing platelet aggregaUon, e g. arach|domc acid (Vargaftig and Zirmis, 1973) and ADP (Born, 1962) and by agents that damage the vascular endothelium, e.g. homocystm (Harker et al., 1974) It is obvious that a substance having such drastic effects could never be used as a drug, and it zs unlikely that it would not be detected early by the presently used toxicologtcal test procedures. The undesirable effects of drugs on the coagnlaUon system and blood platelets are thought to be much more subtle Rather than to cause uneqmvocal thrombos~s m a high per cent of the patients, these compounds merely increase shghtly the statlsucal thrombosis risk within the exposed populauon (Zbmden, 1976) Whether or not thrombosis actually occurs m a given patient will thus also depend on many addluonal nsk factors, such as hls age, sex and welght, hls disease, hls eating and smolang habits, etc. It is thus most difficult to prove wlth clinlcal and eptdemlologlcal 225

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arguments that the drug therapy was indeed a significant causative factor in a paOent's thromboembohc accident. Drugs expected to increase the tendency to thrombosis may be divided into four groups, according to their site of action: those acting on the coagulatmn system, on platelets, on the vessel wall and on blood flow (Zbinden, 1976). It is probable that m most cases several mechanisms of action may come into play. A good and chnically tmportant example are the oral contraceptives with their complex action on hemostat~c mechamsms. Epldemlologlcal studies have shown that oral contraceptives are responsible for increasing the risk of arterial and venous thrombosis and that this risk is greater in women over 40 and in heavy smokers. Many biological studies have been performed m women during estroprogestagen admmlstraUon. They have demonstrated a variety of clear-cut effects many of which could potentially play a role m the Increase of the thrombosis nsk (Conard et al., 1976). The following factors have been especially mvesUgated: --Vascular lesions: endothelial proliferation and sub-endothelial fibrosis have been found. --Viscosity: viscosity and packed red cell volume are increased but results concernmg erythrocyte deformability are still unequivocal. --Platelet number and function: platelet number is not modified but results of platelet function are controversial. However, thrombin-induced platelet aggregation studied by Chandler's technique is absormal, and it is interesting to emphasize that this techmque is not independent of coagulation. ---Coagulatmn: 'hypercoagulability" is difficult to evaluate and no truly valid and simple method is at present available for measunng 'overall hypercoagulability'. Thrombm generaUon, studied by several groups, is accelerated and factor VII is constantly increased as is the activation of plasma coagulation after cold storage. A recent test detecting high molecular weight fibrinogen complexes is specially interesting in women taking the pill: these products, present in patients with venous thrombosis, are also found m women under oral contraceptives without clinzcal thrombotic symptoms, and they are five times more frequent than in normal controls. This tlme-consummg method has not yet been introduced as a routine procedure. Inh~bltors of coagulation and, more precisely, anti-thrombin III, have been extensively studied m oral contraception. Since families with AT III deficiency associated with thrombo-embolic episodes have been described, the decrease observed by many authors during estroprogestatlve administration has gained importance. Our personal results (Conard and Samama, unpubhshed) show that AT III decrease is frequent and more marked in serum than m plasma, suggesting it might be due, at least partly, to an increased consumpuon of AT III dunng the coagulation process, and may correspond to the increased thrombin generation observed. The exact relationship between this abnormality and thrombosis associated with the use of the pill Is not completely clear. However, a study of women subjected to emergency abdominal surgery has shown that isotopic thromboses (dmgnosed by the labeled fibrinogen test) were present only in those previously taking oral contraceptives. Moreover, their AT III level was lower than in the other women. Thus, decrease in AT III may play a role in venous thrombosis but arterial thrombosis and cerebro-vascular accidents have also been described in women taking the pill. mFibnnolysls: a decreased fibrinolytic activity would have been a predisposing factor to thrombosis but such an abnormality has not been observed in the treated women. However, It must be mentioned that few studies on fibrinolytic inhlbltors have been performed. 3. F U T U R E STRATEGY From the brief summary of the biological effects of oral contraceptives on blood coagulation it is clear that profound changes take place in the hemostatic system of

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women taking these drugs It is difficult, however, to determine whtch of these changes are the most important ones for the increased thrombosis task of these subjects On the other hand, many of the alterations observed m women taking oral contraceptives have also been demonstrated in ammals, and it is encouraging to see that experimental techmques are bemg developed which permR a quantitative assessment of certain processes that may be revolved in the development of thrombosis. The avadabihty of these test procedures Is a direct consequence of the rapidly expanding knowledge on the biochemical processes occumng dutang blood clotting and platelet aggregatmn The interdependence of the coagulaUon system and the platelets is now well recogmzed and has greatly advanced our knowledge of the thrombotic process It ~s thus clear that future toxicological approaches to the recogmtion of thrombogemc effects of drugs m toxicological tests must be based on modern concepts of hemostasls and platelet physiology. The members of the Working Group recogmze that a sunple screening system for the evaluaUon of potentml thrombogemc effects of drugs does not yet extst WRh then" contnbuuons they want to demonstrate, however, that many relevant properties of new drugs can now be assessed wRh relatively simple, reproducible and quanUtauve tests. At least some of these tests should be incorporated into the toxicological rouUne program so that future chmcal observaUons or climcal suspicions about the thrombogemc properttes of new drugs can be correlated with sound experimental data 4. RECOMMENDATIONS After extenswe discussions the Working Group has decided to incorporate m its report the following recommendations" On the problem of advice on standard tests for the evaluaUon of chemtcals, the Working Group felt that R should recommend certain tests as the mimmum toxicology screening procedures for abnormaliues of coagulation, platelets and/or platelet function. As a number of national regulatory authorities seem to be looking into the possibdiues of recommending minimum standards, as tt was felt appropriate for the group to recommend these standards before regulatory authonties do so. The one stage prothrombm tune test, the acuvated partml thromboplastm Ume and a platelet count are recommended as minimum procedures The group also recommends that test(s) for platelet functmn must be included ptaor to a drug being given to human volunteers. Other tests over and above those recommended should be done depending on the circumstances, 1.e. type of chemical being tested, faciliues available, etc. At this stage no recommendauons are being made on method procedures or use of standard reagents as it is at the moment imposstble to do so It ts hoped that m the future such standards wdl be recommended and some form of standardtzauon achieved. REFERENCES BORN, G V R (1962) AggregaUon of blood platelets by adenosme dlphosphate and tts reversal Nature 194 927-928 CON~,D, J , BoUSSEg, M J and SAM~A, M (1976) Contracepufs oraux et thromboses Eptd6nuolo~e--Anomahes btoiogcques Rev Med 34 1849-1862 GIROLAMI,A , BRUNETTI,A , CELIA, G , PEDRAZZOLI,S and BERNARDI,R (1973) D~sappearance of ellaglc acid mduced hypercoagulabdtty m the dog after Trasylol admmlstrauon Thrombos Diathes Haemorrh 29 384-392 HARKER, L A , StactcrF.g, S J , SCOTT, C R and Ross, R (1974) Homocysunetma Vascular injury and arterial thrombosis New Engl J Med 291 537-543 VARGAFrIG, B B and ZIRINIS, P (1973) Platelet aggregation induced by arachldomc is accompanied by release of potential inflammatory mediators distinct from PGF~ and PGF~ Nature (New Biol ) 244 11d---116 ZmNDEN, G 0973) Arznettmttelwtrkungen als Thromboseursachen Bull $chwelz Akad Med Wissensch 29 191-200 ZBINDEN, G (1976) Evaluation of thrombogemc effects of drugs Ann Rev Pharmac Toxlcol 16 177-188