Prediction of Interspecies Translation for Targeting Delivery Coefficients of GalNAc-siRNA Silencing Apolipoprotein C-III Using a Mechanistic Minimal Physiologically Based Pharmacokinetic/Pharmacodynamic Model.

Background And Objective: The emerging N-acetylgalactosamine-small interfering RNA (GalNAc-siRNA) conjugates lead the way for liver-targeting delivery to exert gene-silencing therapeutic effects. To facilitate the drug development of GalNAc-siRNA, further detailed understanding of the key modality-specific mechanisms underlying the temporal discordance between pharmacokinetics and pharmacodynamics and how these processes can be extrapolated from animals to humans is needed.

Methods: A mechanistic minimal physiologically based pharmacokinetic/pharmacodynamic (mPBPK-PD) model for an investigational new apolipoprotein C-III (APOC3)-silencing GalNAc-siRNA (RBD5044) was developed using available pharmacokinetic/pharmacodynamic (PK/PD) data. The aim was to explore hepatic-targeting delivery processes, the PK/PD relationship, and interspecies translation.

Results: First, multiple PK/PD datasets from mice were satisfactorily fitted using the mPBPK-PD model. Second, we translated the mice model to the monkey model, validated it, and then extrapolated from mice and monkeys to humans to simulate the PK/PD characteristics. We then mechanistically summarized and proposed the essential in vivo delivery processes of GalNAc-siRNA after subcutaneous administration (termed "ADUEB": Absorption [into system circulation], Disposition [distribution to liver target and elimination], Uptake [into hepatocytes], Escape [from endosome and lysosome compartments], and Binding [with argonaute2 to form RNA-induced silencing complex]). The targeting delivery coefficients of these processes achieved with the model using RBD5044 and the published data of another GalNAc-siRNA (fitusiran) quantitatively reflected the delivery efficiency and rate-limiting factors in targeted hepatocytes.

Conclusion: This study successfully constructed the mPBPK-PD model and conducted interspecies extrapolation for a GalNAc-siRNA targeting APOC3. Promising quantitative insights into a hepatic-targeted GalNAc-siRNA delivery system are provided to characterize the unique temporal disconnection of PK/PD properties and evaluate the key in vivo delivery processes. It will promote model-informed strategies and quantitative mechanistic understanding to support efficient drug development, evaluation, and clinical application of this modality in the future.