SLN124, a GalNAc Conjugated 19-Mer Double-Stranded SiRNA Reduces Iron and Increases Hepcidin Levels of Healthy Volunteers in a Phase 1 Clinical Study

Background Hepcidin, a peptide hormone consisting of 25-amino-acids, is the central regulator of systemic iron homeostasis. It is synthesized predominantly in hepatocytes, and dysregulation of its production leads to a variety of disorders of iron metabolism, including iron overload as well as congenital or acquired iron-loading anaemias. These conditions are a major source of morbidity and mortality. SLN124 increases hepatic hepcidin synthesis and hence plasma hepcidin by silencing its repressor, TMPRSS6. SLN124 has been shown to lower serum iron levels for at least 6 weeks after single administration in mice and has also been shown to increase haemoglobin in a mouse model of beta-thalassemia. Here we report results from a randomised, double-blind, placebo-controlled phase 1, single ascending dose study to evaluate the safety, tolerability, pharmacokinetics (PK), and pharmacodynamics (PD) of subcutaneously (sc) administered SLN124 in healthy volunteers. Objectives The primary objective was to evaluate the safety and tolerability of single ascending doses of SLN124 in healthy subjects. In addition, PK parameters of SLN124 and PD biomarkers of iron metabolism were evaluated. Methods Each subject received a single dose of SLN124 or placebo given by sc injection into their abdomen. Dose levels of 1 mg/kg, 3 mg/kg and 4.5 mg/kg were evaluated. At each dose level, 6 subjects received SLN124 and 2 received matching placebo. Results Three cohorts of 8 subjects (6:2) were included in the study. The mean age of subjects was 31.3 years (SD 7.8) and 71% were male. There were no serious adverse events or severe treatment emergent adverse events (TEAEs) or TEAEs leading to withdrawal. The majority of TEAEs were mild, including transient mild injection site reactions, which resolved without intervention. No dose limiting toxicities were observed. Plasma hepcidin levels at baseline were (mean ± SD) 2.3±1.1, 2.5±1.8 and 2.7±2.0 nM in the three treatment groups, respectively. After a single administration of 1, 3 or 4.5 mg/kg of SLN124, levels were increased by 3.1±2.7, 5.8±2.6 and 7.8±2.9 nM on day 29 and by 2.6±2.6, 2.8±1.4 and 3.5±1.8 nM day 57 post dosing, respectively. Serum iron was reduced by mean (±SD) 32% (26), 40% (14) ,46% (21) on day 8 and by 42% (20), 48% (14) and 47% (26) on day 29 for single doses of 1, 3 and 4.5 mg/kg, respectively. Average percentage changes in all three treatment groups were also still reduced from baseline at day 57. The mean (±SD) percent transferrin saturation (TSAT) was reduced from baseline levels in all single dose administration groups with maximum decreases observed at day 29 but reductions still observed at day 57. (Table 1). The observed PK is consistent with the PK model based on preclinical data. Thus, whereas the effects of SLN124 on plasma hepcidin show clear increments across the dose range tested, the effect on lowering serum iron and transferrin saturation shows something of a plateau effect at the highest dose. In future studies in patients with raised baseline transferrin saturations, it will be of note to determine whether transferrin saturation continues to decrease at the highest SLN124 doses. Conclusion/summary In summary, SLN124, a GalNAc conjugated siRNA targeting TMPRSS6, effectively reduces serum iron levels and has the potential as a therapeutic for patients with alpha and beta-thalassemia, myelodysplastic syndromes, hereditary hemochromatosis and other related disorders. The effect on hepcidin levels at the end of study, day 57, suggest a prolonged duration of action. The encouraging efficacy signal on serum iron and transferrin saturation and the expected benign safety profile of SLN124 supports further development. The GEMINI II trial (NCT04718844) is currently recruiting and wil l evaluate single and multiple doses of SLN124 in patients withthalassemia and myelodysplastic syndromes. Figure 1 Figure 1. Disclosures Porter: bluebird bio, Inc.: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Agios: Consultancy, Honoraria; Protagonism: Honoraria; La Jolla Pharmaceuticals: Honoraria; Celgene (BMS): Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Silence Therapeutics: Honoraria, Membership on an entity's Board of Directors or advisory committees; Vifor: Honoraria, Membership on an entity's Board of Directors or advisory committees. Scrimgeour: Silence Therapeutics: Current Employment, Current holder of stock options in a privately-held company. Martinez: Silence Therapeutics: Current Employment, Current holder of stock options in a privately-held company. James: Silence Therapeutics: Consultancy. Aleku: Silence Therapeutics: Current Employment, Current holder of stock options in a privately-held company. Wilson: Silence Therapeutics: Consultancy. Muckenthaler: Silence Therapeutics: Research Funding. Schaeper: Silence Therapeutics: Current Employment, Current holder of stock options in a privately-held company. Campion: Silence Therapeutics: Current Employment, Current equity holder in publicly-traded company, Current holder of stock options in a privately-held company.