Effect of low and high dose melagatran and other antithrombotic drugs on platelet aggregation.
BACKGROUND
Thrombin-induced aggregation of human platelets can be completely inhibited by melagatran, the bioactive form of ximelagatran, an oral direct thrombin inhibitor.


METHODS
The potential of melagatran to differentially inhibit alpha- and gamma-thrombins was tested with a synthetic thrombin substrate. Washed human platelets were also employed to determine if melagatran differentially inhibited alpha- and gamma-thrombin-induced platelet aggregation at distinct platelet thrombin receptors. In vitro studies were conducted with washed human platelets to determine thrombin-induced aggregation responses in the presence of varying doses of the anti-thrombotic drugs: melagatran, argatroban, heparin, and hirudin.


RESULTS
Melagatran rapidly inhibits the hydrolysis of a thrombin chromogenic substrate within 0-1 min with alpha-, beta- and gamma-thrombin being equally inhibited by high dose melagatran while alpha-thrombin was significantly more sensitive at low doses. The maximum level of melagatran inhibition of alpha- and gamma-thrombin-induced platelet aggregation requires platelets to be pre-incubated with the drug for 10-30 min. Melagatran appears to have no direct effect on the PAR-1 receptor. It does appear to have a direct effect on the GPIbalpha thrombin receptor activity as well as the PAR-4 receptor. Inhibition of platelet aggregation is dose dependent, however, at low melagatran doses (0.01-0.04 nM) platelets aggregate at significantly (P < 0.05) higher levels. The lower the level of thrombin-induced aggregation that was observed with control samples (aggregations from 10% to 39%), corresponded with a higher observed melagatran-induced stimulation with drug-treated platelets. The range of stimulation varies between several hundred percent at approximately 10% aggregation to around 20% at about 20-39% aggregation. Preliminary studies indicate that this in vitro stimulatory effect is abrogated in platelets derived from volunteers who took aspirin (81 mg/day) for 7 days. Three other anti-thrombotic drugs, argatroban, heparin and hirudin, were tested with low drug levels but none were found to consistently stimulate the reaction.


CONCLUSIONS
These results indicate that melagatran acts as both a direct thrombin inhibitor and indirectly by some interaction with the platelet membrane. While melagatran has been withdrawn from clinical use, its ability to differentially inhibit gamma-thrombin/PAR-4 versus alpha-thrombin/PAR-1 at low doses may warrant it, or less toxic analogs to be used in the future for as yet unknown disease states involving PAR-4.
Soslau
Gerald
G
Department of Biochemistry and Molecular Biology, Drexel University College of Medicine, 245 N 15th Street, Philadelphia, PA 19102, USA. gsoslau@drexelmed.edu
Ando
Aimee
A
Floyd
LaToya
L
Hong
Tom
T
Mathew
Lynn
L
Yen
Yvonne
Y
D000925:Anticoagulants / Q000008:administration & dosage*; D001384:Azetidines / Q000008:administration & dosage*; D001596:Benzylamines / Q000008:administration & dosage*; D006801:Humans; D010974:Platelet Aggregation / Q000187:drug effects*; D013917:Thrombin / Q000037:antagonists & inhibitors*
D000925:Anticoagulants; D001384:Azetidines; D001596:Benzylamines; C109573:melagatran; D013917:Thrombin
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