A Subcutaneously Administered RNAi Therapeutic (ALN-AT3) Targeting Antithrombin for Treatment of Hemophilia: Interim Phase 1 Study Results in Healthy Volunteers and Patients with Hemophilia a or B

Introduction: Hemophilia A and B are congenital bleeding disorders caused by insufficient thrombin generation due to deficiency in factor VIII or IX, respectively. In the presence of normal levels of endogenous anticoagulants, deficiency of factor VIII or IX results in an imbalance of the hemostatic system toward a bleeding phenotype. ALN-AT3 is a subcutaneously administered investigational RNAi therapeutic aimed at reducing the levels of antithrombin (AT) with the purpose of increasing thrombin generation and thereby restoring hemostatic balance in hemophilia.

Material and methods: Preclinical studies in hemophilia mouse models were used to investigate the ability of ALN-AT3 to reduce AT, increase thrombin generation, restore thrombus formation after microvascular laser injury, and control traumatic bleeding after saphenous vein transection. A study in an induced hemophilia A model in non-human primates was used to investigate the ability of ALN-AT3 to enhance thrombin generation in the setting of inhibitors to factor VIII.

A Phase 1 clinical study in healthy volunteers and severe/moderate hemophilia A or B is ongoing. Part A, a single ascending dose study in healthy volunteers, has been completed and Part B, a multiple ascending dose study in hemophilia patients, is currently ongoing. Primary endpoints are safety and tolerability. Secondary endpoints are pharmacokinetics, relative reduction in levels of AT and change in thrombin generation as measured by Calibrated Automated Thrombin (CAT) generation.

Results: ALN-AT3 treatment resulting in residual AT levels of 20-40% in hemophilia A and B mouse models increased thrombin generation, restored real-time localized thrombus formation in the laser-injury model comparable to treatment with full-length recombinant factor VIII. ALN-AT3 controlled traumatic bleeding in the saphenous vein model with an increase in number of hemostatic events equivalent to that achieved with infusion of 25 IU/kg full-length recombinant factor VIII. ALN-AT3 treatment targeting 20% residual AT levels normalized thrombin generation in non-human primates with induced high titer inhibitor hemophilia A.

In Part A of the phase 1 study, a total of 4 human volunteer subjects were randomized to receive a single subcutaneous dose of ALN-AT3 0.03 mg/kg or placebo (3:1) and were followed for at least 70 days. No serious adverse events (SAEs) were observed. A total of 5 mild AEs were recorded. Four out of 5 mild AEs were considered unlikely/not related to study drug. One mild AE of temporary self-limiting headache was considered possibly related to study drug. In Part A, per protocol, the maximum allowable level of AT relative reduction was set at 40%. Results in healthy volunteers show that a single, low subcutaneous dose of ALN-AT3 at 0.03 mg/kg resulted in an up to 28-32% knockdown of AT activity at nadir, and the treatment effect was statistically significant relative to placebo (p < 0.01 by ANOVA). This led to a statistically significant (p < 0.01) increase in peak thrombin generation, that was temporally associated and consistent with the degree of AT knockdown. The AT reduction was stable and durable for up to 70 days post single dose. In the ongoing Part B, cohorts of 3 hemophilia patients will receive 3 weekly doses of ALN-AT3, with dosing commenced at 0.015 mg/kg. Up-to-date results from Part B will be presented.

Conclusion: Collectively, these data suggest that the use of a novel RNAi therapeutic targeting AT is a promising approach for restoring hemostatic balance in hemophilia, and potentially, other bleeding disorders. Further, the subcutaneous route of administration, infrequent dosing, and applicability to persons with hemophilia who have inhibitors, make this a particularly encouraging potential therapy.