Effects of Drugs on Platelet Function

EDWARD E. MORSE, M.D.

Department of Laboratory Medicine, University of Connecticut Health Center, Farmington, CT 06032

ABSTRACT

Numerous drugs and chemicals affect the function of human blood platelets. The mechanism of action of some medications is partly understood. Aspirin is the most frequently involved drug. It appears to interfere with the platelet release reaction by acetylation of a platelet membrane protein which may be involved in the synthesis of prostaglandins. Other anti-inflammatory drugs, including indomethacin, phenylbutazone, ibuprophen (Motrin) and clonixin, also interfere with the release reaction but have a shorter acting course than aspirin. Some drugs stimulate adenylcyclase (gliclazide) or block phosphodiesterase, (dipyridamole, caffeine) both of which actions lead to an increase in adenosine cyclic 3’:5’ monophosphate (cAMP) and decrease aggregation by adenosine diphosphate (ADP). These interactions should be known to clinical scientists since patients using these medicaments may manifest abnormal platelet function tests in the laboratory and mild hemor rhagic syndromes in the clinic.

Introduction ing. Unmasking platelet factor 3 without Research in platelet function during re actual separation from the platelet mem cent years has shown that platelets adhere brane results in local accumulation of clot to damaged blood vessel walls, particularly accelerating activity at the site of injury to the collagen exposed by the removal of with ultimate fibrin formation. endothelial cells.1'4 In subsequent reac The conformational changes in the tions adenosine diphosphate (ADP) is re platelet surface and the release of ADP, leased and induces aggregation of other catechol amines, and serotonin require the platelets at the site of injury.8,25 The re contraction of actinomyosin like fibrils lease reaction is accompanied by a change within the platelets and the utilization of in the platelet’s shape from a smooth disk to ATP.4 In vitro, the addition of collagen, a sphere with protruding pseudopods 2,25 ADP in various concentrations, or epi These conformational changes result in nephrine to platelet-rich citrate anticoagu greater availability of platelet factor 3 ac lated plasma produces the release reaction tivity which appears to reside in the phos and subsequent platelet aggregation. pholipid of the platelet membrane.4 Be Platelet aggregation can be measured by cause ADP itself is a potent inducer of the platelet aggregometer which records the release reaction, the process is self sustain decrease in optical density as the platelets

68 EFFECTS OF DRUGS ON PLATELET FUNCTION 69

aggregate (figure 1). Collagen induces and inhibit ADP induced aggregation and aggregation with a lag phase during which release.12,16 ADP is released from the platelets. ADP Aspirin is the leading drug interfering (10-5il/) induces immediate aggregation, with platelet function because it is so but in lower concentrations a short lag commonly used and because its effect on phase is seen with moderate primary the platelet appears to be permanent. The aggregation and a secondary aggregation entire cohort in the circulation at the time phase owing to the release of more en of aspirin ingestion appears to be involved dogenous ADP from the platelets. Still and new platelets must be produced tc lower concentrations of ADP induce slight primary aggregation which is reversible because no release of endogenous ADP occurs. Epinephrine also induces a mod erate primary aggregation, release of ADP and subsequent secondary aggregation. Platelet aggregation can be inhibited in this system by a number of drugs including antiinflammatory agents,18,19,20 antide pressants,12,23 adrenergic blocking agents9 anda number of miscellaneous com pounds such as ethanol3,7 clofibrate,16 di- pyridamol,16,28 and others.15,17,23,28 The mechanisms by which this inhibition oc curs is only partially understood forafewof these compounds (table I). Some drugs, aspirin for instance, clearly inhibit the release reaction induced by collagen while not interfering with aggre gation caused by exogenous added ADP.19,28 Recent evidence suggests that aspirin incorporates into a platelet mem brane protein and permanently inactivates the platelets in the circulation at the time of exposure.21 Other drugs inhibit the action of ADP induced aggregation. Drugs which in crease the levels of cyclic AMP in the platelet interfere with both ADP induced primary aggregation and with ADP in duced release reaction.16,22,23 At least two direct mechanisms for in creasing the cAMP concentration exist in platelets (figure 2), and other indirect mechanisms may also be found.23 Drugs which stimulate adenylcyclase activity in crease the conversion of ATP to cAMP.10 Similarly, drugs which inhibit cAMP TIME (Minutes) phosphodiesterase also increase cAMP F ig u r e 1. Aggregation of platelets. 7 0 MORSE

DRUGS THAT DRUGS THAT nor taken by, patients with hemophilia or STIMULATE INHIBIT ADENYLCYCLASE PHOSPHODIESTERASE other plasma coagulation defect, since the ATP <=— <► c-AMP <-=► AM P platelet is their primary and possibly only hemostatic mechanism.13 Furthermore, 1 patients who present with minor bleeding INHIBITS ADP AGGREGATION episodes, intermittent bruising, epistaxis OF PLATELETS and melena who are suspected of having a F i g u r e 2. Mechanisms for increasing cAMP hereditary platelet defect such as, throm concentration in platelets. basthenia, thrombocytopathy, storage overcome the defect.19,23,28 This, together pool disease or Portsmouth syndrome, with the evidence that14 C labeled acetyl should be carefully interrogated about from aspirin labels the platelet membrane prescription and proprietary preparations while14 C labeled salicylic acid from aspi they may be using. Many of the commonly rin does not, suggests a permanent change used non-prescription drugs contain aspi occurs in the platelet membrane from the rin, but give no clue by their names.* acétylation of a membrane component. Other antiinflammatory and analgesic Roth and Majerus have isolated a platelet agents, such as phenylbutazone,6,20,28 membrane protein (MW 55,000) labeled ibuprophen20 commonly known as Motrin with 14 C after exposure to 14 C acetyl and used in systemic lupus,5 and a new salicylic acid and speculate that the pro analgesic clonixin,26 also inhibit the re tein may be the active site of prostaglandin lease reaction and aggregation induced by synthetase.21,22,23,24 This interaction leads collagen but not the aggregation caused to a prolongation in bleeding time even in directly by addition of ADP. These drugs normal people.14 have a shorter lasting effect on the platelet Over 200 drug products on the present which is measured in hours rather than pharmaceutical market contain aspirin.11 days as contrasted with aspirin. Many of These drugs should not be prescribed for, these drugs “stabilize” membranes and their mechanism of action is thought to TABLE I be related to this membrane activity.15 Agents Which Interfere With P latelet Function Still other drugs interfere with platelet function by increasing cAMP directly or Mechanism Drug indirectly. Gliclazide, a new oral hypo

Inhibits release reaction: Aspirin18'19'20,23,27'29 glycemic agent, inhibits the release reac (antipyretic) tion to a moderate degree. More impor Clonixin2 5 (analgesic) tantly, however, it stimulates the activity Dipyridamole16 ' ■22 '2 7 of adenylcyclase.10 Thus, either ATP or Gliclazide1^ (antidiabetic) adenosine is rapidly converted to cAMP Ibuprophen2 0 (antiinflammatory) and aggregation is inhibited. Phenylbutazone2 3 , 2 Phentolamine is a blocking agent to the (antiarthritic) Stimulates adenylcyclase: Gliclazide1® adrenergic receptor sites and its presence (increases cyclic AMP) (antidiabetic) inhibits epinephrine induced aggrega Inhibits phosphodiesterase: Caffeine1 2 , 1 ^ ' 2 3 (increases cyclic AMP) (stimulant) tion. Some reduction in ADP induced Dipyridamole1 6 , 2 2 '2 7 Blocks collagen receptor: Imipramine• -917 '1' aggregation occurs with this drug and a (antidepressant) partial blockade of the ADP receptor site Amitriptyline2 ' 17 (ant idepre ssant) has been postulated.9 Blocks ADP receptor: Phentolamine9' (adrenergic blocker) Benadryl2 6 * Alka Seltzer, Arthra-Zene Caps, Babylove, Cope, (antihistamine) Coricidin, Midol, Percodan Demi, Stanback, Van quish and Quiet World. EFFECTS OF DRUGS ON PLATELET FUNCTION 71

Some drugs increase cAMP by inhibi and secondary aggregation and reduce tion of phosphodiesterase.12,16 Mills and platelet factor 3 availability in vivo.3"7 Smith have shown that dipyridamole and While no major changes in platelet struc other pyrimido-pyrimidine compounds ture or metabolism were detected in most potentiate the effects of prostaglandin E t patients receiving ethanol, at least one which stimulates adenylcyclase.16 D i patient who developed thrombo pyridamole and aspirin have such potent cytopenia during ethanol infusion effects on platelets they are being tried as showed significantly reduced intra antithrombotic agents in patients with a cellular ADP.7 Platelets from this patient tendency toward thrombosis.28 Unfortu showed giant granules, poor contractile nately, the doses of dipyridamole re activity and reduced aggregation. The au quired to produce an effect in vivo are so thors suggested an ethanol induced ADP high that vasodilation and hypotension storage pool defect in this patient. are frequent side effects. Other anti In summary, there are numerous drugs inflammatory agents, including in- and chemicals which affect platelet func domethacin,6,20,23,28 are almost as active tion. The mechanism of action of some of as aspirin inhibiting aggregation, but these medications is partially understood their effects are short lived and would be arid should be familiar to the clinical sci potentially important only in patients tak entist since use of these drugs may cause ing regular high doses. Phenylbutazone major or minor hemorrhagic syndromes is a relatively weak inhibitor, but it has in susceptible patients. occasionally been associated with bleed ing. References Antiinflammatory agents may inhibit 1. Ba u m g a r t n e r , H . R. and H audenschild , C.: the interaction between actomyosin and Adhesion of platelets to subendothelium. Ann. ATP,6 by increasing cAMP,16,22,23 or they NY Acad. Sci. 201:22-36, 1972. may stabilize the platelet membrane by 2. C a z e n a v e , J. P., Pa c k h a m , M . A ., G u c c io n e , M. A., and M u s t a r d , J. F.: Inhibition of platelet molecular interaction with membrane adherence to acollagen coated s urface by agents components,23 or they may block a recep that inhibit shape change and clot retraction. J. tor8 which is necessary for initiation of Lab. Clin. Med. 84:483-493, 1974. 3. COWAN, D. H. and GRAHAM, R. C .: Studies on the release reaction. the platelet defect in alcoholism. Thromb. Di- Local anesthetics, phenothiazines and ath. Haemorrh. 33:310-327, 1975. 4. D e y k in , D.: Emergingconcepts ofplateletfunc- tricyclic antidepressants such as imi- tion. New Eng. J. Med. 290:144-151, 1974. pramine (Tofranil) and amytryptaline 5. DUBOIS, E. L.: Ibuprofen for systemic lupus (Elavil) in concentrations of 10 to 50 /u-M erythematosis. New Eng. J. Med.2.93:779,1975. 6. GOROG, P. and KovACS, I.B.: Effect of anti inhibit the release reaction.17 Even bena- inflammatory compounds on actomyosin- dryl (50 /iM) inhibits the second wave of adenosinetriphosphate interaction. Biochem. ADP induced aggregation, but less so Pharmacol. 29:2289-2294, 1970. 7. H a u t , M. J. and COWAN, D. H.: The effect of than the phenothiazines and tricyclic ethanol on hemostatic properties of human antidepressants.27 While benadryl is not blood platelets. Amer. J. Med. 56:22-33, 1974. usually associated with bleeding 8. H o l m s e n , H ., D a y , H . J., and St o r m o r k e n , H.: The blood platelet release reaction. Scand J. episodes, O’Brien has shown that the Haemat. Suppl 8:3-26, 1969. bleeding time is prolonged when the 9. K u b is z , P.: Effect of phentolamine and prop- puncture sites have been previously in anolol, an alpha and beta receptor blocking agent, on epinephrine and ADP induced jected with benadryl.20 platelet factor 3, platelet acid phosphatase avail Finally, ethanol in susceptible patients ability and platelet factor 4 release. Acta Univ. Carol. 53:79-90, 1972. has been shown by Cowan et al to pro 10. L a g a r d e , M., D e C h a v a n n e , M., Th o u v e r e z , long the bleeding time, impair primary J. P., andVERRY, M.: Effects of gliclazide, anew 72 MORSE

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