A Subcutaneously Administered Investigational RNAi Therapeutic (ALN-AT3) Targeting Antithrombin for Treatment of Hemophilia: Interim Weekly and Monthly Dosing Results in Patients with Hemophilia A or B
Authors: Pasi K John , Georgiev Pencho , Mant Tim , Creagh Michael Desmond , Lissitchkov Toshko , Bevan David , Austin Steve , Hay Charles R , Brand Brigit , Kazmi Rashid , Chowdary Pratima , Ragni Margaret V , Chen Jihong , Akinc Akin , Sorensen Benny , Rangarajan Savita 
Published: October 2019

Abstract
Background: Hemophilia A and B are bleeding disorders characterized by a profound defect in thrombin generation (TG). Furthermore, in the presence of normal levels of endogenous anticoagulants a deficiency in factor VIII and IX results in major hemostatic imbalance and a bleeding phenotype. ALN-AT3 is a subcutaneously administered investigational RNAi therapeutic targeting the endogenous anticoagulant antithrombin (AT) that aims to restore the hemostatic balance by increasing TG.
Methods: We are conducting a phase 1 multi-center study (NCT02035605) in healthy volunteers and patients with moderate to severe hemophilia A or B. Part A of this study has been completed and assessed a single ascending dose study in healthy volunteers. Parts B and C are multiple ascending dose studies in patients with hemophilia who are receiving weekly or monthly dosing, respectively. Primary endpoints are safety and tolerability. Secondary endpoints include PK, AT knockdown; change in thrombin generation and whole blood clot formation as measured by Calibrated Automated Thrombin generation and ROTEM thromboelastometry. Exploratory endpoints include evaluations of bleed pattern and control.
Results: Part A enrolled 4 healthy volunteers, randomized (3:1) to 30 mcg/kg ALN-AT3 or placebo; no serious adverse events (SAE) or injection site reactions were observed. A total of 12 patients with severe hemophilia (10 hemohilia A; 2 hemophilia B) were enrolled in Part B and received 3 weekly subcutaneous doses of ALN-AT3 at 15 (n=3), 45 (n=6), and 75 (n=3) mcg/kg. Similar to part A, weekly administration of ALT-AT3 was generally safe and well tolerated in patients with hemophilia; no SAEs, discontinuations, clinical thromboembolic events or clinically significant D-dimer increases were reported. In the 75 mcg/kg dosing cohort, the mean maximum AT knockdown was 59% (p<0.05, relative to baseline), with nadir levels achieved between days 28 and 42. Maximum plasma AT knockdown of 86% was achieved, resulting in thrombin generation increases that correlated with AT knockdown and a bleed-free period of 114 days in the patient achieving the highest level of AT knockdown. The association between AT KD and TG was assessed in a post hoc exploratory analysis in which AT KD was categorized into tertiles. Part C aims to enroll several cohorts (n=3 per cohort) and will assess a monthly dosing schedule (x3 doses) of ALN-AT3. Patients in cohort 1 and 2 were dosed at 225 and 450 mcg/kg, respectively. Up to 4 additional cohorts may be enrolled within Part C. Updated safety, PK, AT knockdown, TG results as well as bleed patterns from Parts B and C will be presented.
Conclusions: Emerging clinical data suggest that targeting AT could be a promising approach for restoring hemostatic balance in hemophilia. The potential for low volume subcutaneous administration, monthly dosing, and applicability to patients with hemophilia A and B with and without inhibitors make ALN-AT3 a potentially encouraging investigational therapy.

Disclosures
Pasi: Octapharma: Research Funding; Biogen, Octapharma, Genzyme, and Pfizer: Consultancy, Honoraria. Off Label Use: ALN-AT3 is an investigational RNAi therapeutic targeting the endogenous anticoagulant antithrombin.. Mant:Quintiles: Employment, Equity Ownership. Creagh:Bayer Healthcare UK: Honoraria. Austin:SOBI: Other: member of advisory board and received educational support; Pfizer: Other: member of advisory board and received educational support; Novo Nordisk: Other: member of advisory board and received educational support; CSL Behring: Other: member of advisory board and received educational support; Bio Products Laboratory: Other: member of advisory board and received educational support; Bayer: Other: member of advisory board and received educational support; Baxter: Other: member of advisory board and received educational support. Brand:Alnylam: Honoraria. Chowdary:Bayer: Consultancy; Biogen Idec: Consultancy; Baxter: Consultancy; CSL Behring: Consultancy, Research Funding; Novo Nordisk: Consultancy, Research Funding; Pfizer: Consultancy, Research Funding; SOBI: Consultancy. Ragni:Tacere Benitec: Membership on an entity's Board of Directors or advisory committees; Alnylam: Research Funding; Bristol Myers Squibb: Research Funding; Biogen: Research Funding; Shire: Membership on an entity's Board of Directors or advisory committees, Research Funding; Dimension: Research Funding; Bayer: Research Funding; SPARK: Research Funding; Genentech Roche: Research Funding; Pfizer: Research Funding; Vascular Medicine Institute: Research Funding; Baxalta: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Biomarin: Research Funding; CSL Behring: Research Funding. Chen:Alnylam Pharmaceuticals: Employment, Equity Ownership. Akinc:Alnylam Pharmaceuticals: Employment, Equity Ownership. Sorensen:Alnylam Pharmaceuticals: Employment, Equity Ownership. Rangarajan:Octapharma: Other: Investigator.