Inhibitors of coagulation factor Xa (fXa) have emerged as a new class of antithrombotics but lack effective antidotes for patients experiencing serious bleeding. We designed and expressed a modified form of fXa as an antidote for fXa inhibitors. This recombinant protein (r-Antidote, PRT064445) is catalytically inactive and lacks the membrane-binding γ-carboxyglutamic acid domain of native fXa but retains the ability of native fXa to bind direct fXa inhibitors as well as low molecular weight heparin-activated antithrombin III (ATIII). r-Antidote dose-dependently reversed the inhibition of fXa by direct fXa inhibitors and corrected the prolongation of ex vivo clotting times by such inhibitors. In rabbits treated with the direct fXa inhibitor rivaroxaban, r-Antidote restored hemostasis in a liver laceration model. The effect of r-Antidote was mediated by reducing plasma anti-fXa activity and the non-protein bound fraction of the fXa inhibitor in plasma. In rats, r-Antidote administration dose-dependently and completely corrected increases in blood loss resulting from ATIII-dependent anticoagulation by enoxaparin or fondaparinux. r-Antidote has the potential to be used as a universal antidote for a broad range of fXa inhibitors.

Up to 30% of acute myelogenous leukemia (AML) patients harbor an activating internal tandem duplication (ITD) within the juxtamembrane domain of the FLT3 receptor, suggesting that it may be a target for kinase inhibitor therapy. For this purpose we have developed CT53518, a potent antagonist that inhibits FLT3, platelet-derived growth factor receptor (PDGFR), and c-Kit (IC(50) approximately 200 nM), while other tyrosine or serine/threonine kinases were not significantly inhibited. In Ba/F3 cells expressing different FLT3-ITD mutants, CT53518 inhibited IL-3-independent cell growth and FLT3-ITD autophosphorylation with an IC(50) of 10-100 nM. In human FLT3-ITD-positive AML cell lines, CT53518 induced apoptosis and inhibited FLT3-ITD phosphorylation, cellular proliferation, and signaling through the MAP kinase and PI3 kinase pathways. Therapeutic efficacy of CT53518 was demonstrated both in a nude mouse model and in a murine bone marrow transplant model of FLT3-ITD-induced disease.

Based on genetic studies that establish the role of spleen tyrosine kinase (Syk) in immune function, inhibitors of this kinase are being investigated as therapeutic agents for inflammatory diseases. Because genetic studies eliminate both adapter functions and kinase activity of Syk, it is difficult to delineate the effect of kinase inhibition alone as would be the goal with small-molecule kinase inhibitors. We tested the hypothesis that specific pharmacological inhibition of Syk activity retains the immunomodulatory potential of Syk genetic deficiency. We report here on the discovery of (4-(3-(2H-1,2,3-triazol-2-yl)phenylamino)-2-((1R,2S)-2-aminocyclohexylamino) pyrimidine-5-carboxamide acetate (P505-15), a highly specific and potent inhibitor of purified Syk (IC 50 1-2 nM). In human whole blood, P505-15 potently inhibited B cell antigen receptor-mediated B cell signaling and activation (IC 50 0.27 and 0.28 M, respectively) and Fc receptor 1-mediated basophil degranulation (IC 50 0.15 M). Similar levels of ex vivo inhibition were measured after dosing in mice (Syk signaling IC 50 0.32 M). Syk-independent signaling and activation were unaffected at much higher concentrations, demonstrating the specificity of kinase inhibition in cellular systems. Oral administration of P505-15 produced dose-dependent anti-inflammatory activity in two rodent models of rheumatoid arthritis. Statistically significant efficacy was observed at concentrations that specifically suppressed Syk activity by ϳ67%. Thus specific Syk inhibition can mimic Syk genetic deficiency to modulate immune function, providing a therapeutic strategy in P505-15 for the treatment of human diseases.

We have previously found that the 4-[4-(N-substituted carbamoyl)-1-piperazinyl]-6,7-dimethoxyquinazolines can function as potent and selective inhibitors of platelet-derived growth factor receptor (PDGFR) phosphorylation. A series of highly potent, specific, orally active, small molecule kinase inhibitors directed against members of PDGFR receptor have been developed through modifications of the novel quinazoline template I. Systematic modifications in the A-bicyclic ring and D-rings of protype I were carried out to afford potent analogues, which display IC(50) values of <250 nM in cellular betaPDGFR phosphorylation assays. An optimized analogue in this series, 75 (CT53518), inhibits Flt-3, betaPDGFR, and c-Kit receptor phosphorylation with IC(50) values of 50-200 nM, whereas 15-20-fold less potent activity against CSF-1R was observed. This analogue also inhibits autophosphorylation of Flt-3 ligand-stimulated wild-type Flt-3 and a constitutively activated Flt-3/internal tandem duplication (ITD) with IC(50) values of 30-100 nM. Through this optimization process, 75 was found to be metabolically stable and has desirable pharmacokinetic properties in all animal species studied (F% > 50%, T(1/2) > 8 h). Oral administration of 75 promotes mice survival and significantly delayed disease progression in a Flt-3/ITD-mediated leukemia mouse model and shows efficacy in a nude mouse model of chronic myelomonocytic leukemia.

Direct factor Xa (FXa) inhibitors lack a specific reversal agent for emergencies such as major bleeding or urgent surgery. Andexanet alfa, a modified, catalytically inactive, recombinant human FXa derivative, reverses anticoagulant effect by binding and sequestering FXa inhibitors. This original report of safety and dose-finding, phase 1 and 2 randomized, double-blind, placebo-controlled studies, investigated various doses of andexanet in healthy volunteers. Phase 1 evaluated the safety and pharmacokinetics of andexanet (n = 24) or placebo (n = 8). In phase 2, andexanet (n = 36) or placebo (n = 18) was administered following steady-state apixaban dosing (5 mg twice daily for 6 days); safety, pharmacokinetics, and pharmacodynamics were assessed. Andexanet plasma concentration increased proportionally with dose, with rapid elimination (terminal elimination half-life, 4.35-7.5 hours). Following apixaban treatment, andexanet rapidly (≤2 minutes) and dose dependently reduced unbound apixaban concentration vs placebo (51% to 89% vs 5% reduction; all < .05), decreased anti-FXa activity (67.8% to 95.0% vs 7.1% reduction; all < .05), and restored thrombin generation in 67% to 100% vs 6% of subjects (all < .01), maintaining these effects during continuous 45- and 120-minute infusions. Andexanet was well tolerated. Nine subjects had mild/moderate infusion reactions not associated with hemodynamic changes or respiratory compromise that generally resolved without intervention or dose reduction. There were no thrombotic events or other serious safety issues. In conclusion, andexanet reversed apixaban-mediated effects on pharmacodynamic markers of anticoagulation in healthy volunteers within minutes after administration and for the duration of infusion. This trial was registered at www.clinicaltrials.gov as #NCT01758432.