Alnylam Pharmaceuticals, Inc., a leading RNAi therapeutics company, has filed a Clinical Trial Application (CTA) with the UK Medicines and Healthcare products Regulatory Agency (MHRA) to initiate a phase I clinical trial with ALN-AT3, a subcutaneously administered RNAi therapeutic targeting antithrombin (AT) for the treatment of haemophilia, including people with “inhibitors.”

Haemophilia is a hereditary disorder caused by genetic deficiencies in various blood clotting factors, resulting in recurrent bleeds into joints, muscles, and other major internal organs. By knocking down AT, ALN-AT3 administration is expected to increase thrombin generation and reduce disease burden, including annualized bleeding rate, severity of bleeding, and requirement for replacement factor or bypass agent, and to potentially improve quality of life. ALN-AT3 is a key program in the company’s “Alnylam 5x15” product development and commercialization strategy, in which the company aims to advance five RNAi therapeutic programmes toward genetically validated disease targets into clinical development, including programmes in advanced stages, by the end of 2015.

“Haemophilia is characterized by a genetic deficiency in clotting factors that ultimately leads to inadequate thrombin generation and a bleeding disorder. ALN-AT3 aims to correct the hemostatic imbalance in haemophilia by knocking down AT – an endogenous anticoagulant – thus increasing thrombin generation and improving hemostasis, and reducing disease burden. This innovative approach is strongly supported by clinical cases in people with haemophilia who have co-inherited prothrombotic traits, including AT deficiency, and are characterized by a milder bleeding phenotype,” said Akshay Vaishnaw, MD, Ph.D., executive vice president and chief medical officer at Alnylam. “This CTA for ALN-AT3 marks our second for an RNAi therapeutic that utilizes our proprietary GalNAc conjugate delivery platform, which we have now shown to be clinically validated. Our pre-clinical data with ALN-AT3 have demonstrated normalization of thrombin generation and improvement of hemostasis in hemophilia models. We have also demonstrated that ALN-AT3 has a wide therapeutic index in the context of hemophilia. We very much look forward to the continued advancement of ALN-AT3, including the start of our phase I clinical trial in early 2014 with data from haemophilia subjects expected by the end of that year.”

ALN-AT3 is a subcutaneously administered RNAi therapeutic that comprises an siRNA conjugated to a triantennary N-acetylgalactosamine (GalNAc) ligand. GalNAc-siRNA conjugates are a proprietary, clinically validated delivery platform and are designed to achieve targeted delivery of RNAi therapeutics to hepatocytes through uptake by the asialoglycoprotein receptor. Pre-clinical studies have shown that subcutaneous administration of ALN-AT3 can normalize thrombin generation and improve hemostasis in haemophilia mice and fully correct thrombin generation in a non-human primate (NHP) haemophilia “inhibitor” model. A single subcutaneous dose of ALN-AT3 that resulted in plasma AT reduction of approximately 90 per cent led to normalization of thrombin generation and improvement in hemostasis in haemophilia mice.

In wild type NHPs, repeat dosing with ALN-AT3 resulted in potent, titratable, and reversible silencing of plasma AT. Weekly subcutaneous doses of 0.50 mg/kg resulted in 90 per cent AT knockdown, while an ED50 knockdown was achieved at a dose as low as 0.125 mg/kg. Furthermore, in an NHP “inhibitor” model, in which a hemophilia A (HA) phenotype was induced via administration of a polyclonal anti-factor VIII antibody, ALN-AT3-treated animals showed the expected level of AT knockdown but also showed a statistically significant (p<0.01) dose-dependent increase in thrombin generation, fully restoring this hemostatic parameter back to normal levels. We believe these results demonstrate that ALN-AT3 can normalize thrombin generation in the absence of functional levels of factor VIII and/or in the presence of anti-factor VIII antibodies in a large animal model, providing key proof of concept for the programme. In addition, single doses of ALN-AT3 as high as 500 mg/kg – over 100-fold greater than doses that result in essentially complete ablation of AT – were well tolerated in HA mice: 100 per cent of animals survived, and there were no toxicologically significant findings in clinical or pathology exams.

As per the filed CTA, the phase I trial of ALN-AT3 will be conducted in the UK as a single- and multi-dose, dose-escalation study consisting of two parts. The first part will be a randomized, single-blind, placebo-controlled, single-dose, dose-escalation study, enrolling up to 24 healthy volunteer subjects. The primary objective of the first part of the study is to evaluate the safety and tolerability of a single dose of subcutaneously administered ALN-AT3. Secondary objectives include assessment of clinical activity as determined by knockdown of circulating AT levels. The second part of the study will be an open-label, multi-dose, dose-escalation study enrolling up to 18 people with moderate to severe hemophilia A or B. The primary objective of this part of the study is to evaluate the safety and tolerability of multiple doses of subcutaneously administered ALN-AT3 in hemophilia subjects. Secondary objectives include assessment of clinical activity as determined by knockdown of circulating AT levels and increase in ex vivo thrombin generation.

RNAi (RNA interference) is a revolution in biology, representing a breakthrough in understanding how genes are turned on and off in cells, and a completely new approach to drug discovery and development. Its discovery has been heralded as “a major scientific breakthrough that happens once every decade or so,” and represents one of the most promising and rapidly advancing frontiers in biology and drug discovery today which was awarded the 2006 Nobel Prize for Physiology or Medicine.