Targeting the Hepcidin Pathway with RNAi Therapeutics for the Treatment of Anemia

Abstract 688

Hepcidin, a liver-expressed peptide hormone, plays a central role in iron homeostasis by regulating the level of ferroportin. Ferroportin, the only known mammalian cellular iron exporter, is found on enterocytes, hepatocytes, and macrophages and allows the flux of iron from these cells into plasma. Upon binding, hepcidin induces the internalization and degradation of ferroportin, thereby modulating the absorption of iron from the diet and release of iron from cellular stores. Inappropriate hepcidin levels can therefore lead to dysregulation of iron homeostasis. In particular, inappropriately elevated hepcidin levels can limit iron needed for heme production, resulting in anemia. Elevated hepcidin is thought to play an important role in anemia of chronic disease and anemia of chronic kidney disease.

RNA interference (RNAi) is a fundamental cellular mechanism for silencing genes and other transcribed elements, including those currently “undruggable” by small molecule and antibody therapeutics. RNAi has already transformed biological research by serving as a powerful tool for studying gene function and is now poised to form the basis of a new class of therapeutics. Small interfering RNAs (siRNAs) were developed against HAMP, the hepcidin gene, as well as other targets involved in iron metabolism (including HJV, TFR2, HFE, Neogenin, BMP6, BMPRI, BMPRII, SMAD4, and IL6R), both as a means to identify a therapeutic for the treatment of anemia and as a means to further characterize the hepcidin signaling pathway.

Hepcidin was effectively silenced in mice, rats, and nonhuman primates, resulting in concomitant dose-dependent increases in serum iron, with approximately 100% transferrin saturation achievable after a single administration. Treatment with HAMP-targeting siRNA was able to blunt hepcidin induction, hypoferremia, and the onset of anemia in a turpentine-induced mouse model of inflammatory anemia. Silencing other members of the hepcidin pathway resulted in decreases in HAMP expression and increases in transferrin saturation to varying degrees. In particular, TFR2 was found to be an especially attractive target, leading to potent downregulation of HAMP, and rapid and durable increases in transferrin saturation. A single 0.1 mg/kg dose of a TFR2-targeting siRNA, resulted in ∼80% silencing of both TFR2 and HAMP and 100% transferrin saturation within 24 hours post-administration in mice. HAMP silencing and elevated transferrin saturation persisted for over two weeks, with levels returning to baseline after 4 weeks. In addition, TFR2 targeting resulted in the resolution of anemia in rodent models of anemia of inflammation. These data indicate that siRNAs directed at HAMP, TFR2, and other members of the hepcidin pathway represent an attractive novel therapeutic approach for the treatment of anemia of chronic disease and anemia of chronic renal disease.