Unconjugated antisense oligonucleotides (ASOs) and GalNAc-conjugated small interfering RNAs (siRNAs) have emerged as transformative therapeutic modalities, but their translation remains hindered by complex pharmacokinetics characterized by rapid plasma clearance, heterogeneous tissue uptake, and prolonged intracellular retention. Physiologically based pharmacokinetic (PBPK) modeling offers a mechanistic framework to integrate these processes across species, yet applications to oligonucleotides remain limited, and existing efforts are typically restricted to specific chemistries or routes of administration. This study aimed to develop a cross-species PBPK platform to characterize the plasma and tissue disposition of unconjugated ASOs and GalNAc-siRNAs in preclinical species and humans, and to evaluate the consistency of estimated drug-specific parameters with pre-specified allometric scaling relationships within the domain of compounds and routes considered. Published datasets spanning mice, rats, non-human primates, and humans were integrated for representative unconjugated ASOs (Malat-1, GluR1, p-tau, tominersen) and GalNAc-siRNAs (ALN-AT3, givosiran, fitusiran). The PBPK model incorporated 11 tissue compartments, cerebrospinal fluid substructures, tissue substructures, endosomal trafficking, non-specific binding in kidney and liver, and ASGPR-mediated internalization for GalNAc-siRNAs. Parameters for endocytosis, exocytosis, and degradation were estimated, and estimated values were compared with pre-specified allometric exponents to assess physiological plausibility (i.e., as a consistency check rather than independent inference of scaling laws). The model captured plasma, liver, kidney, and CNS profiles across species, reproducing rapid systemic clearance with prolonged tissue half-lives. Estimated parameter values for endocytosis, exocytosis, and non-specific kidney binding were consistent with pre-specified allometric exponents (b = -0.25 and -0.2, respectively), supporting the physiological plausibility of the estimates. Sensitivity analyses identified endocytosis and rate of degradation as dominant determinants of systemic and tissue disposition. This work establishes a translational PBPK platform for unconjugated ASOs and GalNAc-conjugated siRNAs, enabling quantitative interspecies extrapolation within the domain of the compounds, routes, and species studied. The framework provides a mechanistic basis to support dose selection, safety assessment, and clinical translation for these subclasses; extrapolation to other oligonucleotide subclasses (e.g., GalNAc-ASOs, unconjugated siRNAs, LNP-formulated oligonucleotides) will require additional datasets and dedicated validation.
