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The Thrombolytic Activity of Tissue Plasminogen Activator

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The Thrombolytic Activity of Tissue Plasminogen Activator Endogenous Prostaglandin Endoperoxides and Prostacyclin Modulate the Thrombolytic Activity of Tissue Plasminogen Activator Effects of Simultaneous Inhibition of Thromboxane A2 Synthase and Blockade of Thromboxane A2/Prostaglandin H2 Receptors in a Canine Model of Coronary Thrombosis Paolo Golino, Mark Rosolowsky, Sheng-Kun Yao, Janice McNatt, Fred De Clerck, L. Maximilian Buja, and James T. Willerson The Texas Heart Institute and the Department ofInternal Medicine and Pathology, The University of Texas Health Science Center at Houston 77225, and the University of Texas Southwestern Medical Center at Dallas, Texas 75235 Abstract R 68070 and dazoxiben prevented the increase in plasma TxB2 levels, whereas 6-keto-PGFI. levels were significantly in- We tested the hypothesis that simultaneous inhibition ofTxA2 creased with respect to control and SQ 29548-treated dogs. synthase and blockade of TxA2/PHG2 receptors is more effec- Thus, simultaneous inhibition of TxA2 synthase and blockade tive in enhancing thrombolysis and preventing reocclusion of TxA2/PGH2 receptors is more effective than either inter- after discontinuation of tissue plasminogen activator (t-PA) vention alone in this experimental model in enhancing throm- than either intervention alone. Coronary thrombosis was in- bolysis and preventing reocclusion after t-PA administration. duced in 35 dogs by placing a copper coil into the left anterior (J. Clin. Invest. 1990. 86:1095-1102.) Key words: thromboly- descending coronary artery. Coronary flow was measured with sis * tissue plasminogen activator - prostaglandin endoperox- a Doppler flow probe. 30 min after thrombus formation, the ides * thromboxane A2 synthase inhibitors * thromboxane A2 animals received saline (controls, n = 10); SQ 29548 (0.4 receptor antagonists mg/kg bolus + 0.4 mg/kg per h infusion), a TxA2/PGH2 re- ceptor antagonist (n = 8); dazoxiben (5 mg/kg bolus + 5 Introduction mg/kg per h infusion), a TxA2 synthase inhibitor (n = 9); or R Early thrombolysis is now considered the treatment of choice 68070 (5 mg/kg bolus + 5 mg/kg per h infusion), a drug that for many patients with acute myocardial infarction. Attention blocks TxA2/PGH2 receptors and inhibits TxA2 synthase (n has been focused on thrombolytic agents with a more pro- = 8). Then, all dogs received heparin (200 U/kg) and a bolus of nounced thrombus selectivity, such as tissue-type plasminogen t-PA (80 ;&g/kg) followed by a continuous infusion (8 ag/kg per activator (t-PA).' t-PA offers the advantage of resulting in min) for up to 90 min or until reperfusion was achieved. The comparable reperfusion rates when given intravenously as time to thrombolysis did not change significantly in SQ compared with the intracoronary administration of strepto- 29548-treated dogs as compared with controls (42 ± 5 vs. 56 kinase and to cause less extensive systemic fibrinogenolysis ± 7 min, respectively, P = NS), but it was significantly short- (1-7). However, despite these advantages oft-PA over strepto- ened by R 68070 and dazoxiben (11 ± 2 and 25 ± 6 min, kinase, some problems with t-PA as a thrombolytic agent re- respectively, P < 0.001 vs. controls and SQ 29548-treated main. First, the incidence ofreperfusion varies between 60 and dogs). R 68070 administration resulted in a lysis time signifi- 80% of patients (1-7). Second, the time necessary to achieve cantly shorter than that observed in the dazoxiben-treated reperfusion from the beginning oft-PA administration is often group (P < 0.01). Reocclusion was observed in eight of eight at least 45 min (1-7). Finally, a substantial incidence of reoc- control dogs, five of seven SQ 29548-treated dogs, seven of clusion, which exists after the adminstration of t-PA, is dis- nine dazoxiben-treated dogs, and zero of eight R 68070-treated continued despite the use of heparin (1-7). Thus, substantial animals (P < 0.001). TxB2 and 6-keto-PGFia, measured in effort is being made to develop better thrombolytic agents blood samples obtained from the coronary artery distal to the and/or adjunctive therapies to be used in conjunction with thrombus, were significantly increased at reperfusion and at currently available thrombolytic agents. reocclusion in control animals and in dogs receiving SQ 29548. We (8) and others (9, 10) have demonstrated that intracor- onary platelet activation plays a major role in causing early reocclusion after discontinuation of t-PA in experimental Presented in part at the 62nd Scientific Session ofthe American Heart models of coronary thrombosis. Furthermore, intracoronary Association, New Orleans, LA, 1989. platelet activation and deposition on the thrombus may pro- Address correspondence and reprint requests to Dr. James T. Wil- ceed during administration of t-PA, thus increasing the total lerson, Department of Internal Medicine, The University of Texas Health Science Center at Houston, P.O. Box 20708, Houston, TX amount of thrombus to be lysed and prolonging the time to 77225. Dr. Golino's present address is Clinica Medica I, Cattedra di reperfusion. We have shown that TxA2 and serotonin coopera- Cardiologia, II Facolta' di Medicina, via S. Pansini 5, 80131 Napoli, tively mediate these phenomena, as TxA2 and serotonin antag- Italy. Dr. De Clerck's present address is Janssen Pharmaceuticals, onists given in combination, but not either intervention alone, Beerse, Belgium. markedly shorten the time to reperfusion and prevent or delay Receivedfor publication 23 February 1990 and in revisedform 17 reocclusion after discontinuation of t-PA (11). May 1990. TxA2, the major product of arachidonic acid metabolism J. Clin. Invest. © The American Society for Clinical Investigation, Inc. 1. Abbreviations used in this paper: ACT, activated coagulation time; 0021-9738/90/10/1095/08 $2.00 LAD; left anterior descending coronary artery; t-PA, tissue plasmino- Volume 86, October 1990, 1095-1102 gen activator. Modulation oft-PA Activity by Endogenous Prostaglandins 1095 in platelets, is a potent platelet agonist and vasoconstrictor ence of the intracoronary thrombus was documented for 30 min. The (12), and contributes in the pathophysiology of certain acute animals were then randomly assigned to one ofthe following groups: a coronary artery disease syndromes (13). One way to overcome group receiving saline (group I, n = 10) served as control; a group the deleterious effects of TxA2 is by inhibiting TxA2 synthase, received a bolus of SQ 29548 (0.4 mg/kg), a potent and selective the enzyme responsible for the conversion of PG endoperox- TxA2/PGH2 receptor antagonist (Squibb Pharmaceuticals, Princeton, NJ) (21, 22), followed by a continuous infusion (0.4 mg/kg per h) ides (PGG2/PGH2) to TxA2 (14). This approach has the ad- (group II, n = 8); a third group received a bolus ofdazoxiben (5 mg/kg), vantage over cyclooxygenase inhibition of leaving the biosyn- a selective TxA2 synthase inhibitor (Pfizer Pharmaceuticals, Groton, thesis of prostacyclin (PGI2) intact (15). PGI2 is the major CT) (23), followed by a continuous infusion (5 mg/kg per h) (group III, product of arachidonic acid metabolism in endothelial cells, n = 9); and a fourth group received a bolus ofR 68070 (5 mg/kg), a new and it is a potent anti-platelet substance ( 16). Indeed, the anti- drug with simultaneous TxA2/PGH2 receptor blocking and TxA2 syn- platelet effects of TxA2 synthase inhibitors may be potentiated thase inhibiting properties (Janssen Research Foundation, Beerse, by an increased production of anti-platelet prostaglandins Belgium) (24, 25), followed by a continuous infusion (5 mg/kg per h) formed by extra-platelet metabolism ofPG endoperoxides that (group IV, n = 8). Immediately after starting drug or saline infusions, accumulate when TxA2 synthase is inhibited (17, 18). How- all dogs received a bolus of heparin (200 U/kg) and a bolus of 80 /Ag/kg ever, PG endoperoxides may also activate a receptor on plate- of recombinant human tissue-type plasminogen activator (Knoll platelet activa- Pharmaceuticals Co., Whippany, NJ) followed by an infusion of 8 let membranes, shared with TxA2, mediating Ag/kg per min for up to 90 min or until reperfusion was achieved. tion (19, 20). Thus, there may be conflicting influences as Heparin administration was eventually repeated every hour, whereas regards the anti-thrombotic activity of TxA2 synthase inhibi- R 68070, SQ 29548, or dazoxiben infusion was maintained throughout tors. Addition of a TxA2 receptor antagonist to a TxA2 syn- the study. The time necessary to achieve adequate reperfusion (defined thase inhibitor should prevent the proaggregatory effects ofPG as LAD flow 2 70% of the baseline value), was identified and t-PA endoperoxides, while leaving intact the increased production infusion was discontinued at that time. of anti-platelet prostaglandins, thus resulting in a potentiation Hemodynamics and LAD blood flow velocity were monitored of antithrombotic activity. continuously until a persistent reocclusion occurred (defined as a To determine whether these mechanisms might affect the reocclusion lasting for at least 30 min) or until 90 min oftotal reperfu- thrombolytic activity of t-PA and the incidence ofreocclusion sion time. Prostaglandin measurements. To assess the effects of drug treat- after thrombolysis, we compared the effects of simultaneous ments on intracoronary PG production, TxB2, the stable metabolite of inhibition of TxA2 synthase and blockade of TxA2/PGH2 re- TxA2, and 6-keto-PGFIa, the stable metabolite of PGI2, were mea- ceptors on lysis time and incidence of reocclusion in a canine sured in blood samples obtained from the distal coronary catheter. model of coronary thrombosis. This study provides direct evi- Blood samples were taken before placement ofthe copper coil into the dence that this approach is more effective than either inter- LAD 1 min after reperfusion was achieved, and at the moment of vention alone in this model, and suggests a new class ofagents reocclusion or after 90 min of reperfusion. When a persistent reocclu- with potential therapeutic importance during coronary throm- sion occurred, LAD blood flow approached zero and flow from the bosis and thrombolysis.
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