J Pharmacol Exp Ther. 2021 Aug 19. pii: JPET-AR-2021-000805. [Epub ahead of print]
Conjugation of small interfering ribonucleic acid (siRNA) to tris-N-acetylgalactosamine (tris-GalNAc) can enable highly selective, potent, and durable knockdown of targeted proteins in the liver. However, potential knowledge gaps between in vitro experiments, preclinical species, and clinical scenarios remain. A minimal physiologically based pharmacokinetic-pharmacodynamic (mPBPK-PD) model for GalNAc-conjugated siRNA (GalNAc-siRNA) was developed using published data for fitusiran (ALN-AT3), an investigational compound targeting liver antithrombin (AT), to delineate putative determinants governing the whole body-to-cellular PK-PD of GalNAc-siRNA and facilitate preclinical-to-clinical translation. The model mathematically linked relevant mechanisms: i) hepatic biodistribution, ii) tris-GalNAc binding to asialoglycoprotein receptors (ASGPRs) on hepatocytes, iii) ASGPR endocytosis and recycling, iv) endosomal transport and escape of siRNA, v) cytoplasmic RNA-induced silencing complex (RISC) loading, vi) degradation of target mRNA by bound RISC, and vii) knockdown of protein. Physiological values for 36 out of 48 model parameters were obtained from the literature. Kinetic parameters governing (GalNAc)3-ASGPR binding and internalization were derived from published studies of uptake in hepatocytes. The proposed model well characterized reported PK, RISC dynamics, and knockdown of AT mRNA and protein by ALN-AT3 in mice. The model bridged multiple PK-PD datasets in preclinical species (mice, rat, monkey) and successfully captured reported plasma PK and AT knockdown in a Phase-I ascending dose study. Estimates of in vivo potency (SC50 ) were similar (~2-fold) across species. Subcutaneous absorption and serum AT degradation rate constants scaled across species by body weight with allometric exponents of -0.29 and -0.22. The proposed mechanistic modeling framework characterizes the unique PK-PD properties of GalNAc-siRNA. Significance Statement (GalNAc)3-conjugated siRNA therapeutics enable liver-targeted gene therapy and precision medicine. Using a translational and systems-based minimal physiologically based pharmacokinetic-pharmacodynamic (mPBPK-PD) modeling approach, drug- and system-specific determinants influencing GalNAc-siRNA functionality in preclinical species (mice, rats, monkeys) and humans were investigated. The developed model successfully integrated and characterized relevant published in vitro-derived biomeasures, mechanistic PK-PD profiles in animals, and observed clinical PK-PD responses for an investigational GalNAc-siRNA (fitusiran). This modeling effort delineates the whole body-to-cellular disposition and liver-targeted pharmacodynamics of GalNAc-siRNA.
Keywords: RNA/siRNA; liver/hepatic; pharmacodynamics; pharmacokinetic/pharmacodynamic modeling/PKPD; pharmacokinetics; systems pharmacology