Leading RNA Interference Therapeutics Part 2: Silencing Delta-Aminolevulinic Acid Synthase 1, with a Focus on Givosiran

In November 2019 givosiran became the second small interfering RNA (siRNA)-based drug to receive US Food and Drug Administration (FDA) approval, it has been developed for the treatment of acute intermittent porphyria (AIP), a disorder characterized by life-threatening acute neurovisceral attacks. The porphyrias are a group of disorders in which enzymatic deficiencies in heme production lead to toxic accumulation of delta-aminolevulinic acid (ALA) and porphobilinogen (PBG), which are involved in the neurovisceral attacks. Givosiran acts as a conventional siRNA to trigger RNA interference (RNAi)-mediated gene silencing on delta-ALA synthase 1 (ALAS1), thus returning ALA and PBG metabolites to the physiological level to attenuate further neurotoxicity. Givosiran makes use of a new hepatic-delivery system that conjugates three GalNac (N-acetylgalactosamine) molecules to the siRNA passenger strand. GalNac binds to the liver asialoglycoprotein receptor, favoring the internalization of these GalNac-conjugated siRNAs into the hepatic cells. In a phase I study, subcutaneous monthly administration of givosiran 2.5 mg/kg reduced > 90% of ALA and PBG content. This siRNA is being analyzed in ENVISION (NCT03338816), a phase III, multicenter, placebo-controlled randomized controlled trial. In preliminary results, givosiran achieved clinical endpoints for AIP, reducing urinary ALA levels, and presented a safety profile that enabled further drug development. The clinical performance of givosiran revealed that suppression of ALAS1 by GalNac-decorated siRNAs represents an additional approach for the treatment of patients with AIP that manifests recurrent acute neurovisceral attacks.