Antithrombotic and anticoagulant effects of the direct thrombin inhibitor dabigatran, and its oral prodrug, dabigatran etexilate, in a rabbit model of venous thrombosis

BACKGROUND: Oral anticoagulant therapies targeted at thrombin are being developed to overcome limitations associated with current standard therapies. OBJECTIVES: This study was undertaken to assess and compare the antithrombotic and anticoagulant effects of the novel, selective and reversible, direct thrombin inhibitor (DTI), dabigatran, and its oral prodrug dabigatran etexilate, to that of unfractionated heparin (UFH), hirudin and melagatran using a rabbit model of venous thrombosis. METHODS: A rabbit model of venous thrombosis consisting of endothelial damage with blood flow reduction was used with minor modifications. RESULTS: All compounds demonstrated a dose-dependent reduction in thrombus formation following i.v. administration with complete or almost complete inhibition at the highest doses. Dabigatran (in the dose range 0.03-0.5 mg kg(-1)) had a 50% effective dose of 0.066 mg kg(-1). By comparison, UFH (5-50 U kg(-1)), hirudin (0.01-0.05 mg kg(-1)) and melagatran (0.01-0.3 mg kg(-1)) had a 50% effective dose of 9.8 U kg(-1), 0.016 mg kg(-1) and 0.058 mg kg(-1), respectively. Similarly, oral dabigatran etexilate (1-20 mg kg(-1)) inhibited thrombus formation in a dose-dependent manner. Maximum inhibition was achieved within 1 h of administration, suggesting a rapid onset of action. For both routes of administration, inhibition of thrombus formation directly correlated with prolongation of the activated partial thromboplastin time. CONCLUSIONS: These findings demonstrate the potent anticoagulant and antithrombotic activity of dabigatran as a selective thrombin inhibitor in a rabbit model of venous thrombosis. Notably, dose-dependent and long-lasting antithrombotic efficacy was observed after application of its oral form dabigatran etexilate, which is currently undergoing phase III clinical development.     

Pharmacological assessment of the antithrombotic activity of the peptide thrombin inhibitor, D-methyl-phenylalanyl-prolyl-arginal (GYKI-14766), in a canine model of coronary artery thrombosis.

The antithrombotic activity of the tripeptide thrombin inhibitor, D-methyl-phenylalanyl-prolyl-arginal (GYKI-14766), was compared to heparin in a model of canine coronary artery thrombosis. Thrombogenesis was initiated by electrolytic injury of the intimal surface of the left circumflex coronary artery. Drug administration was started 15 min before initiation of intimal injury. Clotting times and ex vivo platelet aggregation were determined on citrated blood samples. Gingival template bleeding times were determined. Clotting times (thrombin time; activated partial thromboplastin time, APTT; prothrombin time, PT) increased in a dose-dependent manner with both anticoagulants. The two anticoagulants selectively inhibited thrombin-induced platelet aggregation. GYKI-14766 and heparin were found to delay thrombosis significantly when compared to vehicle-treated animals; minimum effective antithrombotic doses were 0.25 mg/kg/h and 80 U/kg + 30 U/kg/h, respectively. GYKI-14766 (0.25 mg/kg/h) had no effect on template bleeding time, APTT or PT. Heparin (80 U/kg + 30 U/kg/h), however, was associated with a 2.5- to 3.0-min increase in template bleeding time, a 1.8-fold and 1.7-fold increase in APTT and PT, respectively. Antithrombotic efficacy was achieved at doses of GYKI-14766 that did not affect APTT, PT or template bleeding time, whereas antithrombotic efficacy observed with heparin was associated with significant increases in APTT, PT and template bleeding time. These data demonstrate that the tripeptide thrombin inhibitor, GYKI-14766, could potentially prove to be a safer and more effective antithrombotic agent than heparin.     

Antithrombotic therapy in atrial fibrillation: ximelagatran,an oral direct thrombin inhibitor

The oral direct thrombin inhibitor ximelagatran (Exanta?, AstraZeneca) is rapidly absorbed, is efficiently bioconverted to the active form, melagatran (AstraZeneca) and has shown efficacy and relative safety as an anticoagulant for prophylaxis and therapy of thromboembolism. Two Phase III trials, Stroke Prevention using an ORal Thrombin Inhibitor in atrial Fibrillation (SPORTIF V), have tested the hypothesis that oral ximelagatran, administered 36 mg twice daily without coagulation monitoring or dose adjustment, prevents stroke and systemic embolism at least as effectively as adjusted-dose warfarin (international normalized ratio, 2.0–3.0) in patients with nonvalvular atrial fibrillation. Both were randomized, multicenter trials (n > 3000 per trial) with blinded end-point assessment. The open-label SPORTIF III trial confirmed the noninferiority of ximelagatran versus warfarin. Publication of the full results from SPORTIF V is pending.     

Reduction in the incidence of thrombosis by the thromboxane synthetase inhibitor CGS 13080 in a canine model of coronary artery injury

The antithrombotic potential of the thromboxane (TX) synthetase inhibitor CGS 13080 (CGS) was studied in an anesthetized open-chest canine model of coronary artery intimal wall injury induced by the local application of a low amperage electrical current (100 microA for 6 hr). CGS was administered by i.v. infusion (1 mg/kg/min) beginning 30 min before applying the direct current to the intimal wall of the vessel. CGS did not alter basal values for heart rate, blood pressure or coronary blood flow. After 6 hr of current application to the vessel, 1 of 10 CGS-treated dogs exhibited complete thrombotic occlusion of the circumflex coronary artery compared to 8 of 10 nontreated control dogs (P less than .01). Thrombus masses within the injured left circumflex coronary artery were: Control, 25.9 +/- 4.5 mg (n = 10) and CGS, 11.0 +/- 2.8 mg (n = 10); P less than .01. The concentration of TXB2 determined ex vivo in serum from thrombin-activated whole blood was decreased by CGS administration: Control, 43.15 +/- 16.08 ng/ml (n = 9) vs. CGS, 1.72 +/- .69 ng/ml (n = 10); P less than .001. Ex vivo platelet aggregometry demonstrated that arachidonic acid (0.65 mM)-induced aggregation was reduced from a control value of 82.3 +/- 7.8% (n = 10) to 45.0 +/- 11.3% (n = 10) (P less than .05), whereas aggregation in response to ADP (5 micrograms/ml) or collagen (156 and 312 micrograms/ml) was unaffected. CGS was compared with two other TX synthetase inhibitors, U63557A and OKY1581, for the ability to divert cyclic endoperoxide metabolism to the synthesis of prostaglandin (PG) E2 and prostacyclin in response to stimulation of whole blood in vitro with collagen (25 micrograms/ml). CGS, U63557A and OKY 1581 were found to be equally effective with respect to PGE2 and 6-keto PGF1 alpha production in vitro. The data demonstrate that CGS is an effective antithrombotic agent in vivo and that it selectively inhibits arachidonic acid-induced platelet aggregation ex vivo and the formation of TXA2 in thrombin-activated whole blood.     

Combination of aspirin and metoclopramide produces a synergistic antithrombotic effect in a canine model of coronary artery thrombosis

Summary— We compared the antithrombotic properties of low doses of aspirin (0.03, 0.1 mg kg⁻¹ intravenously [iv]) and metoclopramide (0.1, 0.3 mg kg⁻¹ iv) alone or in combination. The animal model chosen for this study involved the generation of cyclic flow variations (CFV) in the circumflex coronary artery of anaesthetized dogs as a result of a critical coronary stenosis associated with a controlled arterial lesion at the site of stenosis. Subsequent regular CFV represent sequential thrombus formation and embolization in the damaged vessel. Neither aspirin nor metoclopramide alone demonstrated antithrombotic properties at the doses tested. However, the combination of aspirin 0.1 mg kg⁻¹ iv and metoclopramide 0.3 mg kg⁻¹ iv produced a significant antithrombotic effect, reducing the frequency of large CFV from 6.7 ± 0.5 to 0.8 ± 0.4 cycles h⁻¹ ( P < 0.01) and increasing minimum mean coronary blood flow from 5.0 ± 1.1 to 23.7 ± 2.6 mL min⁻¹ ( P < 0.01). This result apparently reflects an antithrombotic synergism between aspirin and metoclopramide since the effects of the combination were greater than the combined effects of the individual treatments. The antithrombotic influence of metoclopramide could be due to its 5HT₂-antagonist or α₂-antagonist properties, both of which would inhibit platelet aggregation. This demonstration of a synergistic antithrombotic action of the combination of aspirin and metoclopramide is of interest since these two agents are often combined in clinical use. Its therapeutic relevance, however, remains to be established.     

Pharmacological interruption of acute thrombus formation with minimal hemorrhagic complications by a small molecule tissue factor/factor VIIa inhibitor: comparison to factor Xa and thrombin inhibition in a nonhuman primate thrombosis model

This study was designed to evaluate the antithrombotic efficacy and bleeding propensity of a selective, small-molecule inhibitor of tissue factor/factor VIIa (TF/VIIa) in comparison to small-molecule, selective inhibitors of factor Xa and thrombin in a nonhuman primate model of thrombosis. Acute, spontaneous thrombus formation was induced by electrolytic injury to the intimal surface of a femoral blood vessel, which results in thrombus propagation at the injured site. The TF/FVIIa inhibitor 3-amino-5-[1-[2-([4-[amino(imino)methyl]benzyl]amino)-2-oxoethyl]-3-chloro-5-(isopropylamino)-6-oxo-1,6-dihydropyrazin-2-yl]benzoic acid dihydrochloride (PHA-927F) was fully effective in prevention of thrombosis-induced vessel occlusion at a dose of 400 microg/kg/min, i.v., in the arterial vasculature (femoral artery). Neither the effective dose nor multiples up to 4.4-fold the effective arterial plasma concentration elicited any significant effect on bleeding time or blood loss from either the bleeding time site or the surgical (femoral isolation) site. Small-molecule inhibitors of factor Xa or thrombin were effective arterial antithrombotic agents; however, in contrast to the TF/FVIIa inhibitor, they both elicited substantial increases in bleeding propensity at the effective dose and at multiples of the effective plasma concentration. These data indicate that TF/VIIa inhibition effectively prevented arterial thrombosis with less impact on bleeding parameters than equivalent doses of factor Xa and thrombin inhibitors.     

Antithrombotic properties of a direct thrombin inhibitor with a prolonged half-life and AT-mediated factor Xa inhibitory activity

Summary.  Rebound thrombin generation after successful thrombolysis might be related to (i) too short-term anticoagulant therapy and to (ii) the inability of heparin derivatives to inhibit clot-bound thrombin. To meet these shortcomings, a compound was synthesized, which consists of a pentasaccharide conjugated to a direct thrombin inhibitor. This compound (Org 42675) has a 10 times longer half-life compared with the original half-life of the direct thrombin inhibitor, while the thrombin inhibitory activity is maintained. An extra advantage of this product is the inhibitory activity on thrombin generation via antithrombin III (AT)-mediated factor (F)Xa inhibition. Org 42675 inhibited in vitro clot-bound thrombin with similar activity to the direct thrombin inhibitor argatroban. In experimental models in rats, Org 42675 showed on a molar base similar antithrombotic activity to unfractionated heparin, was more active than argatroban and was more active than fondaparinux sodium (AT-mediated FXa inhibitor) in arterial thrombosis. Finally, Org 42675 was far more active than the three reference compounds in an experimental thrombolysis model in rabbits. These properties of Org 42675, with its FXa and (clot-bound) thrombin inhibitory activity in combination with its long half-life, make this compound a powerful drug that is likely to be effective in the prevention of re-occlusion after successful thrombolysis in man.     

Active site-blocked factor IXa prevents intravascular thrombus formation in the coronary vasculature without inhibiting extravascular coagulation in a canine thrombosis model.

To assess the contribution of Factor IX/IXa, to intravascular thrombosis, a canine coronary thrombosis model was studied. Thrombus formation was initiated by applying current to a needle in the circumflex coronary artery. When 50% occlusion of the vessel developed, the current was stopped and animals received an intravenous bolus of either saline, bovine glutamyl-glycyl-arginyl-Factor IXa (IXai), a competitive inhibitor of Factor IXa assembly into the intrinsic Factor X activation complex, bovine Factor IX, or heparin. Animals receiving saline or Factor IX developed coronary occlusion due to a fibrin/platelet thrombus in 70 +/- 11 min. In contrast, infusion of IXai prevented thrombus formation completely (greater than 180 min) at doses of 460 and 300 micrograms/kg, and partially blocked thrombus formation at 150 micrograms/kg. IXai attenuated the accumulation of 125I-fibrinogen/fibrin at the site of the thrombus by approximately 67% (P less than 0.001) and resulted in approximately 26% decrease in serotonin release from platelets in coronary sinus (P less than 0.05). Hemostatic variables in animals receiving IXai, remained within normal limits. Animals given heparin in a concentration sufficient to prevent occlusive thrombosis had markedly increased bleeding, whereas heparin levels that maintained extravascular hemostasis did not prevent intracoronary thrombosis. This suggests that Factor IX/IXa can contribute to thrombus formation, and that inhibition of IXa participation in the clotting mechanism blocks intravascular thrombosis without impairing extravascular hemostasis.     

Intimatan prevents arterial and venous thrombosis in a canine model of deep vessel wall injury

Resistance of fibrin-bound thrombin to inactivation by the heparin/antithrombin III complex is considered a limitation in the use of heparin as an antithrombotic agent. Intimatan (dermatan 4,6-di-O-sulfate) is a heparin cofactor II agonist that inhibits both free and bound forms of thrombin. The present study examines the hypothesis that Intimatan prevents thrombotic occlusion in response to vascular wall injury in a canine model of carotid artery/jugular vein thrombosis. The left carotid artery and right jugular vein served as vehicle-treated control vessels, whereas the right carotid artery and left jugular vein were subjected to electrolytic injury after administration of Intimatan (9 mg/kg bolus + 300 microg/kg/min infusion, i.v.) or dalteparin (Fragmin) (400 IU/kg, s.c.). Intimatan significantly increased time to carotid artery (226.0 +/- 14.0 min) and jugular vein (240.0 +/- 0.0 min) thrombosis, compared with control vessels (carotid artery, 87.1 +/- 7.9 min; jugular vein, 60.6 +/- 7.4 min). Vessel patency was maintained in eight of eight jugular veins and seven of eight carotid arteries during treatment with Intimatan. Dalteparin significantly increased time to carotid artery thrombosis (122.1 +/- 17.5 min) compared with control (64.3 +/- 8.2 min), but did not change the time to thrombosis in the jugular vein. Only one carotid artery remained patent at the end of the dalteparin protocol. The two drugs produced minimal increases in bleeding times, and Intimatan increased the activated partial thromboplastin time above that observed with dalteparin. The results demonstrate that Intimatan is effective in preventing occlusive arterial and venous thrombosis in an experimental model of deep vascular wall injury.     

Acute antithrombotic effects of ximelagatran, an oral direct thrombin inhibitor, and r-hirudin in a human ex vivo model of arterial thrombosis

Summary. Background: Thrombin plays a major role in thrombus formation through activation of platelets and conversion of fibrinogen to fibrin. Objectives: To investigate the antithrombotic effects of the oral direct thrombin inhibitor (DTI) ximelagatran and the parenteral DTI r‐hirudin in humans. Subjects and methods: Healthy male volunteers randomized into four parallel groups each with 15 subjects received either ximelagatran (20, 40 or 80 mg orally) or r‐hirudin (0.4 mg kg^?1 intravenous bolus + infusion of 0.15 mg kg^?1 h^?1 for 2 h and 0.075 mg kg^?1 h^?1 for 3 h). Antithrombotic effects were assessed as changes in total thrombus area (TTA) and total fibrin area (TFA) from baseline, using the Badimon perfusion chamber model at baseline and 2 h and 5 h after drug administration. Results: Two hours postdosing, ximelagatran showed antithrombotic effects at both high and low shear rates (TTA% of mean baseline value ± SEM was 76 ± 13% and 71 ± 17% [both P < 0.05] for the 20‐mg dose, 85 ± 11% [P > 0.05] and 62 ± 15% [P < 0.05] for the 40‐mg dose and 60 ± 11% and 26 ± 7% [both P < 0.05] for the 80‐mg dose, respectively). r‐Hirudin also showed a significant antithrombotic effect at high and low shear rates (76 ± 11% [P = 0.05] and 57 ± 17% [P < 0.05] of baseline values, 2 h postdosing, respectively). The inhibitory effects on TFA were similar to those on TTA. Conclusions: The oral DTI ximelagatran shows antithrombotic effects under both high and low shear conditions. The antithrombotic effect of 40–80 mg ximelagatran appeared comparable to that of parenterally administered r‐hirudin, which has been previously demonstrated to be clinically effective in acute coronary syndromes.     

Potentiation of the antithrombotic effect of prostacyclin by simultaneous administration of aminophylline in a canine model of coronary artery thrombosis

The antithrombotic efficacy of prostacyclin (PGI2) when administered in conjunction with the phosphodiesterase inhibitor aminophylline was evaluated in a canine model in which coronary artery thrombosis was induced by electrical stimulation of the intimal surface of the left circumflex (LCX) coronary artery. Infusions of PGI2 (25 or 50 ng/kg/min) into the left atrial appendage and aminophylline (20 micrograms/kg/min) or ethylene diamine into the left jugular vein were initiated 10 min before the start of LCX coronary artery stimulation and continued for the 6-hr stimulation period. Every animal in the control (Tris buffer plus ethylene diamine, n = 7), PGI2 (25 ng/kg/min) only (n = 6) and aminophylline only (n = 7) groups developed completely occlusive coronary artery thrombi. In contrast, none of the animals receiving PGI2 (25 ng/kg/min) plus aminophylline or PGI2 (50 ng/kg/min) plus aminophylline underwent occlusive thrombus formation. The average thrombus mass developed in response to intimal injury of the LCX coronary artery was 57 +/- 14 mg (X +/- S.E.M.) in the control group. Aminophylline administration in conjunction with PGI2 infusion at doses of 25 and 50 ng/kg/min significantly reduced thrombus mass to 11 +/- 2 and 10 +/- 1 mg, respectively (P less than .05). PGI2 (25 ng/kg/min) plus aminophylline reduced mean arterial pressure by 12% from 116 +/- 5 to 102 +/- 4 mm Hg. These data demonstrate that the combined administration of aminophylline with low-dose PGI2 provides antithrombotic efficacy while minimizing the detrimental hemodynamic effects of large-dose PGI2 administration.     

Pharmacological profile of recombinant, human activated protein C (LY203638) in a canine model of coronary artery thrombosis

The antithrombotic activity of recombinant, human activated protein C (rh-APC, LY203638) was examined in a model of canine coronary artery thrombosis. Three doses of rh-APC (0.5, 1.0, and 2.0 mg/kg/h) were administered intravenously for 2 h. Whole blood clotting times (thrombin time, activated partial thromboplastin time), ex vivo platelet aggregation, and template bleeding times were determined. Activated partial thromboplastin time significantly increased 2- and 3.7-fold during the 2-h infusion of rh-APC (1.0 and 2.0 mg/kg/h, respectively); thrombin time did not change. Intravenous infusions of rh-APC (1.0 and 2.0 mg/kg/h) produced significant prolongations to occlusion, 186 +/- 21 and 190 +/- 22 min, respectively, compared with the vehicle and the 0.5 mg/kg/h group (86 +/- 12 and 93 +/- 17 min, respectively). Vessel patency was better at the end of the experiment in the intermediate- and high-dose groups (3 of 6 and 3 of 5 vessels, 1.0 and 2.0 mg/kg/h, respectively) compared with the vehicle and 0.5 mg/kg/h groups (0/5 and 0/6, respectively). Only the 1.0 mg/kg/h group was found to have significantly elevated template bleeding times, with peak increases seen 60 min into the drug infusion. All groups had returned to baseline values by the end of the study. There was no observed inhibition of platelet aggregation. These data demonstrate that recombinant, human activated protein C is an effective anticoagulant and antithrombotic agent in the dog.     

S35972, a direct‐acting thrombin inhibitor with high oral bioavailability and antithrombotic efficacy

Summary. Objectives: Dabigatran etexilate is the first oral thrombin inhibitor to demonstrate superior efficacy to warfarin for stroke prevention in patients with atrial fibrillation. This study describes the in vitro, ex vivo anticoagulant and in vivo antithrombotic effects of an oral thrombin inhibitor, S35972, in comparison with dabigatran etexilate. Methods: Enzyme assays with thrombin and related serine proteases were performed. Clotting times, including activated partial thromboplastin time (APTT) and thrombin time (TT), were measured in vitro in different species and ex vivo in dogs and rats to determine pharmacologic bioavailabilities. The formation of occlusive venous and arterial thrombi in the rat vena cava and aorta was induced with stasis plus thromboplastin or ferrous chloride, respectively. Results: S35972 inhibited human thrombin with an IC₅₀ of 3.7 nm , and did not inhibit other serine proteases. The anticoagulant activities of S35972 in vitro were comparable in dog and human plasmas, and the sensitivity of the clotting times to S35972 was TT > APTT > prothrombin time. In the fasted dog, oral administration of 3 mg kg^?1 S35972 increased TT rapidly and for at least 8 h, and its pharmacologic bioavailability was 75.4% ± 0.1%. In the rat venous thrombosis model, 3 mg kg^?1 oral S35972 or dabigatran etexilate significantly decreased the thrombus weight. In the rat aortic thrombosis model, oral S35972 at 10 mg kg^?1 significantly decreased thrombus weight, by approximately 50%, whereas, at this dose, no effect was obtained with dabigatran etexilate. Conclusions: S35972 is a non‐prodrug thrombin inhibitor with high selectivity, oral bioavailability, and antithrombotic efficacy.     

Nonpeptide factor Xa inhibitors II. Antithrombotic evaluation in a rabbit model of electrically induced carotid artery thrombosis

SK549 (mol. wt. 546 Da) is a synthetic, selective inhibitor of human coagulation factor Xa (fXa) (K(i) = 0.52 nM). This study compared the antithrombotic effects of SK549 and a series of benzamidine isoxazoline fXa inhibitors with aspirin, DuP 714 (a direct thrombin inhibitor), recombinant tick anticoagulant peptide, or heparin in a rabbit model of electrically induced carotid arterial thrombosis. Compounds were infused i.v. continuously from 60 min before electrical stimulation to the end of the experiment. Values of ED(50) (dose that increases the carotid blood flow to 50% of the control) were 0.12 micromol/kg/h for SK549, 0.56 micromol/kg/h for aspirin, 0.14 micromol/kg/h for DuP 714, 0.06 micromol/kg/h for recombinant tick anticoagulant peptide, and >100 U/kg/h for heparin. The EC(50) (plasma concentration that increased blood flow to 50% of the control) for SK549 was 97 nM. Unlike aspirin and heparin, SK549 was efficacious and, at 1.5 micromol/kg/h i.v. (n = 9), maintained carotid blood flow at 87 +/- 6% of control level for greater than 90 min. Unlike heparin, SK549 inhibited ex vivo fXa activity but not ex vivo thrombin activity. There was a highly significant correlation between K(i) (fXa) and ED(50) of a series of fXa inhibitors (r = 0. 85, P <.001). Therefore, these results suggest that SK549 is a novel, potent, and effective antithrombotic agent in a rabbit model of arterial thrombosis. It is likely that SK549 exerts its antithrombotic effect through selective inhibition of fXa. Furthermore, SK549 may be clinically useful for the prevention of arterial thrombosis.     

Antithrombotic drugs in coronary artery disease: risk benefit ratio and bleeding

Summary. The antithrombotic treatment of coronary artery disease is becoming increasingly complex. Aspirin is often combined with more potent antiplatelet agents such as thienopyridines and glycoprotein IIb/IIIa inhibitors. The classic anticoagulant unfractionated heparin is giving way to low‐molecular‐weight heparin, the pentasaccharide fondaparinux and the direct thrombin inhibitor bivalirudin. Warfarin (or another vitamin K antagonist) and antiplatelet agents are often required in combination for several months. Patients and physicians who have experienced major bleeding complications sometimes question the benefit of these treatment strategies. It is therefore crucial to try and weigh the impact on efficacy against safety. In this review the net benefit is discussed both numerically, comparing absolute reductions vs. increases in risks, and also by addressing the qualitative importance of each component in reaching the net benefit. Except for primary prophylaxis in patients at low‐moderate risk for coronary events, there is a net benefit of antithrombotic therapy. With increasing severity of the coronary condition the net benefit generally prevails even with an increasing number of antithrombotic drugs combined. However, as the patient slowly stabilizes after appropriate interventions, it is necessary to de‐escalate the treatment in accordance with decreasing net benefit of prolonged combination therapy.     

D-phenylalanine: a putative enkephalinase inhibitor studied in a primate acute pain model

D-Phenylalanine, along with morphine, acetylsalicylic acid and zomepirac sodium were evaluated for their antinociceptive actions in monkeys (M. fascicularis) trained to autoregulate nociceptive stimulation using a discrete-trials, aversive-threshold paradigm. Morphine sulfate produced dose-related increases in aversive threshold which were reversible after administration of naloxone (12.5 or 25 micrograms/kg i.m.). D-Phenylalanine (500 mg/kg p.o.) produced a small increase in aversive threshold which was not statistically significant and not naloxone reversible. Acetylsalicylic acid (200 mg/kg p.o.) but not zomepirac sodium (200 mg/kg p.o.) in combination with D-phenylalanine (500 mg/kg) produced a small statistically significant increase in aversive threshold. Our results argue against the hypothesis that D-phenylalanine is responsible for increasing aversive thresholds via opiate receptor mechanisms involving increased activity of enkephalins at synaptic loci. Previous studies by others in rats and mice showed that D-phenylalanine and acetylsalicylic acid produced increases in nociceptive thresholds which were naloxone reversible. Our failure to find opiate receptor mediated analgesia in a primate model with demonstrated opiate receptor selectivity and sensitivity is discussed in terms of previous basic and clinical research indicating an analgesic role for D-phenylalanine. Possible species difference in drug action is discussed in terms of inhibition by D-phenylalanine of carboxy-peptidase-like enkephalin processing enzymes as well as inhibition of carboxypeptidase-like enkephalin degrading enzymes.     

Magnetic resonance black-blood thrombus imaging can confirm chronic cerebral venous thrombosis: a case report and literature review

BackgroundCerebral venous thrombosis (CVT) is easily missed or misdiagnosed in clinical settings because of its high variability in terms of symptoms and radiological findings. Herein, we aimed to explore a promising modality for confirming presumed CVT in the hope to uncover its superior diagnostic performance to conventional imaging modalities.Case presentation: The patient complained of intolerable pain in her forehead and left eye. Her lumbar puncture opening pressure was 140 mmH₂O, and her cerebrospinal fluid composition was normal. No marked abnormalities were observed in routine brain images, including non-contrast computed tomography, magnetic resonance imaging, and contrast-enhanced magnetic resonance venography. However, chronic mural thrombi in the lumen of the left cortical veins, transverse/sigmoid sinus, and superior sagittal sinus were identified in magnetic resonance black-blood thrombus imaging (MRBTI) maps.ConclusionsMRBTI can be used to directly and non-invasively visualize thrombi, and may thus be a promising tool over alternative routine techniques for confirming the diagnosis of CVT.