Near Sequence Homology Does Not Guarantee siRNA Cross-Species Efficacy

Small interfering RNAs (siRNAs) represent a novel class of drugs capable of potent and sustained modulation of genes across various tissues. Preclinical development of siRNAs necessitates assessing efficacy and toxicity in animal models. While identifying therapeutic leads with cross-species activity can expedite development, it may compromise efficacy and be infeasible for certain gene targets. Here, we investigate whether deriving species-active siRNAs from potent human-targeting leads—an approach termed mismatch conversion—can yield potent compounds. We systematically altered potent siRNAs targeting human genes associated with diseases— SOD1 (ALS), JAK1 (inflammation), and HTT (HD)—to generate species-matching variants with full complementarity to their target in NHPs, mice, rats, sheep, and dogs. Variants potency and efficacy were measured in corresponding cell lines. We demonstrate that sequence, position, and number of mismatches significantly influence the ability to generate potent species-active compounds via mismatch conversion. Across tested sequences, mismatch conversion strategy ability to identify a species-active lead varied from 0% to 70%. For SOD1 , lead compounds identified from species-focus screening in mouse and dog cells were more potent than leads obtained from mismatch conversion. Thus, a focused screening of therapeutic lead and model compounds may represent a more reliable strategy for the clinical advancement of siRNAs.