The potential toxicity of nucleic acid drugs can stem from several sources: on-target effects (pharmacological exaggeration), narrow off-target effects (unintended gene regulation due to sequence mismatches), and broad off-target effects (arising from the molecule's structure or physicochemical properties). Given that narrow off-target effects are often species-specific and challenging to fully simulate and predict in animal models, this article will not delve into them. Instead, it will focus on the design of animal studies for on-target and non-specific off-target toxicities, systematically discussing the fundamental principles and strategic considerations for animal species selection. Prisys Biotech emphasizes a scientifically rigorous approach to these selections to ensure robust preclinical data.
The Rationale for Using Two Animal Species
Similar to small molecule drugs, nucleic acid therapies typically require toxicity studies in two animal species during the non-clinical phase: one rodent species (e.g., mouse, rat) and one non-rodent species (e.g., dog, monkey). This "two-species" principle is primarily based on two factors:
Interspecies Differences in Toxicity Sensitivity: Nucleic acid drugs can induce non-specific off-target effects, and different animal species may exhibit significant variations in their toxicological responses.
Species-Specific Pharmacokinetics (PK): Many nucleic acid drugs contain artificially modified nucleotides. Their stability, distribution, and metabolic characteristics in vivo show notable species-dependent variations.
Therefore, even if short-term toxicity profiles appear similar across species, conducting long-term toxicity studies in two animal species is recommended to enhance the ability to identify potential toxicities. This practice aligns more closely with the recommendations in ICH M3(R2) for traditional small molecule drugs, rather than strictly applying the simplified principle from ICH S6(R1) which allows for a single animal species in long-term studies for some biologics.
Selecting Animal Species for On-Target Toxicity Assessment
On-target toxicity refers to the exaggerated pharmacological effects resulting from the drug acting on its intended target sequence. When evaluating this type of toxicity, it is crucial to ensure that the experimental animal species can exhibit the intended pharmacological activity of the nucleic acid drug:
If multiple animal species demonstrate the pharmacological effect, one rodent and one non-rodent species should be selected.
If only one species shows pharmacological activity, then that species alone should be used for on-target toxicity assessment.
If no conventional animal species can adequately model the pharmacological effect, the use of a "surrogate" molecule may be considered for supplementary evaluation. However, it's important to note the potential differences in pharmacodynamic and toxicological expression with surrogates, often yielding limited information.
If the on-target effects are anticipated to be minimal, one might opt not to use a surrogate and instead address any non-clinical gaps through careful, stepwise progression in clinical studies. This strategy necessitates a comprehensive preclinical assessment of target biology and mechanism of action by the development team to guide the selection of appropriate animal models. Prisys Biotech can assist in designing these crucial early-stage assessments.
Selecting Animal Species for Non-Specific Off-Target Toxicity Assessment
Broad off-target toxicity originates from the physicochemical properties of the drug molecule and its non-specific interactions with the immune system, plasma proteins, and other biological components, as well as its distribution and metabolism in different animal species. To assess this type of toxicity, the clinical candidate compound itself must be used, and toxicity studies should be conducted in two animal species to maximize the identification of potential adverse reactions. The study design should comprehensively consider the following:
Combined Assessment with On-Target Toxicity: If both selected species exhibit the pharmacological effect, both on-target and off-target toxicities can be evaluated concurrently.
Separate Assessment: If only one species shows pharmacological activity, another species lacking this pharmacological effect must be chosen for the dedicated off-target toxicity evaluation.
Reference to Class Effects: If the candidate drug shares high structural similarity (e.g., modification patterns, backbone structure) with a class of nucleic acid drugs with well-defined toxicity profiles, existing toxicity data can aid in animal species selection and result interpretation.
Matching Metabolism and PK Characteristics: If there are known interspecies differences in the metabolic pathways of the candidate drug compared to humans, species with metabolic profiles or PK characteristics most similar to humans should be prioritized. For example, some phosphorothioate-modified ASOs exhibit PK profiles in non-human primates (NHPs) that are more consistent with humans, whereas mice are often less representative. Prisys Biotech has extensive experience with NHP models for such PK assessments.
Establishing a Scientific and Translatable Animal Model Strategy
The selection of animal species for non-clinical toxicology studies of nucleic acid drugs must address two core assessment dimensions: on-target toxicity and broad off-target toxicity. The former can draw guidance from ICH S6(R1) for biologics, while the latter aligns more with the ICH M3(R2) framework for traditional small molecules.
For nucleic acid drugs targeting human-specific sequences, it is possible during the design phase to incorporate sequences that are common to both humans and animals, enabling the assessment of pharmacological effects in conventional animal models. This strategy reflects the high degree of tunability and engineering potential inherent in nucleic acid drug design.
Looking ahead, as more nucleic acid drugs are developed and our understanding of their mechanisms deepens, the principles for animal species selection will become increasingly refined. The integration of toxicology databases, structural analogy analysis, and emerging humanized model technologies holds the promise of further enhancing the predictive power of non-clinical study data for clinical safety, thereby building a more reliable safety assurance system for nucleic acid drug development. Prisys Biotech is committed to employing these evolving best practices to support the development of safe and effective nucleic acid therapies.
