Mechanistic Pharmacokinetics and Pharmacodynamics of GalNAc-siRNA: Translational Model Involving Competitive Receptor-Mediated Disposition and RISC-Dependent Gene Silencing Applied to Givosiran

Triantennary N-acetyl-D galactosamine (GalNAc)3-conjugated small interfering RNA (siRNA) have majorly advanced the development of RNA-based therapeutics. Chemically stabilized GalNAc-siRNAs exhibit extensive albeit capacity-limited (nonlinear) distribution into hepatocytes with additional complexities in intracellular liver disposition and pharmacology. A mechanism-based pharmacokinetic-pharmacodynamic (PK-PD) model of GalNAc-siRNA was developed to i) quantitate ASGPR-mediated disposition and downstream RNA-induced silencing complex (RISC)-dependent pharmacology following intravenous (IV) and subcutaneous (SC) dosing, ii) assess the kinetics of formed active metabolite, iii) leverage, as an example, published experimental data for givosiran, and iv) demonstrate PK translation across two preclinical species (rat and monkey) with subsequent prediction of human plasma PK. The structural model is based on competition between parent and formed active metabolite for occupancy and uptake via ASGPR into hepatocytes, intracellular sequestration and degradation, and downstream engagement of RNA-induced silencing complex (RISC) governing target mRNA degradation. The model jointly and accurately captured available concentration-time profiles of givosiran and/or AS(N-1)3' givosiran in rat and/or monkey plasma, liver, and/or kidney following givosiran administered both IV and SC. RISC-dependent gene silencing of ALAS1 mRNA was well-characterized. The model estimated an in vivo affinity (KD) value of 27.7 nM for GalNAc-ASGPR and weight-based allometric exponents of -0.27 and -0.24 for SC absorption and intracellular (endolysosomal) degradation rate constants. The model well-predicted reported givosiran plasma PK profiles in humans. PK simulations revealed net-shifts in liver-to-kidney distribution ratios with increasing IV and SC dose. Importantly, decreases in the relative liver uptake efficiency were demonstrated following IV and, to a lesser extent, following SC dosing explained by differential ASGPR occupancy profiles over time.