Screening of Anticoagulant & Antithrombotic Drugs

Screening of Anticoagulant and Antithrombotic Drugs

1. Definition, rationale & organising framework

• Screening of anticoagulant/antithrombotic drugs = the graded battery of in vitro, ex vivo and in vivo assays by which a candidate compound's ability to inhibit coagulation, platelet activation/aggregation, or established thrombi (thrombolysis) is detected, quantified and ranked against reference standards, with parallel assessment of its bleeding liability (Vogel 4e V1 Part II, p.711, p.759, p.783).
• The pathophysiological anchor is Virchow's triad (Virchow 1856): (1) obstruction/stasis of blood flow, (2) changes in blood constituents (hypercoagulability), (3) vessel-wall injury. Every experimental thrombosis model deliberately reproduces one, two or all three arms of the triad, which is why models differ by prothrombotic challenge, vessel type and species (Vogel 4e V1 Part II, p.721).
• Two thrombus phenotypes drive model choice (Vogel 4e V1 Part II, p.711, p.721):
  • Arterial ("white") thrombi — form under high shear; platelet-rich aggregates strengthened by a small fibrin fraction; modelled by vessel-wall injury and/or stenosis; predict antiplatelet efficacy.
  • Venous ("red") thrombi — form under low shear; fibrin-rich with few platelets; modelled by stasis and/or injection of a procoagulant; predict anticoagulant efficacy.
  • The dichotomy is not strict — platelets and the coagulation system reinforce each other, so a fibrin-directed drug may work in arterial models and vice versa (Vogel 4e V1 Part II, p.721).
• Hierarchy of testing (screening cascade): global in vitro coagulation/platelet assays → ex vivo tests on blood from dosed animals → in vivo thrombosis models → bleeding-time (hemostasis) models to define the safety/efficacy (bleeding-risk) ratio (Vogel 4e V1 Part II, p.759, p.783).
• Because thrombosis models are run in healthy animals, the underlying chronic human substrate (atherosclerosis, thrombophilia) is absent; therefore a new antithrombotic should be validated in more than one model, and a hemostatic (bleeding) parameter should be included wherever possible — the clinical usefulness of an antithrombotic is set by its safety/efficacy ratio regarding bleeding risk (Vogel 4e V1 Part II, p.721).
• Reference / standard drugs recur across the assays and serve as positive controls: heparin (unfractionated), LMWH, hirudin and synthetic direct thrombin inhibitors, warfarin/dicumarol, factor-Xa inhibitors (pentasaccharide, DX-9065a, antistasin, tick anticoagulant peptide, rivaroxaban/BAY 59-7939), aspirin, ticlopidine, prostacyclin/iloprost, NO donors, GPIIb/IIIa antagonists (abciximab/c7E3, tirofiban, roxifiban, orbofiban), and the thrombolytics t-PA and streptokinase (Vogel 4e V1 Part II, p.721, p.729, p.759).

GROUP A — IN VITRO & EX VIVO GLOBAL ASSAYS (blood-level / functional tests)