Development of a Gryllus bimaculatus-Based Assay System for Evaluating Chemically Modified siRNAs

Small interfering RNAs (siRNAs) hold great therapeutic promise due to their ability to selectively silence disease-associated genes. Although chemically modified siRNAs have demonstrated clinical efficacy, their development remains hindered by challenges such as stability and delivery under physiological conditions. Furthermore, in vitro screening of chemically modified siRNAs using cultured cells is cost-effective but often fails to recapitulate in vivo complexity, limiting predictive accuracy. To address this, we have developed a transfection-free siRNA evaluation platform using Gryllus bimaculatus (Gb), an insect model with natural RNA uptake capacity. We first demonstrated that 21-nucleotide siRNAs induced RNA interference upon abdominal injection under conditions of free uptake. We then generated transgenic lines harboring an EGFP reporter fused to the therapeutic siRNA target sequence and integrated into the β-actin locus via clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein 9-mediated knock-in. Two transgenic strains (s1 and d1) were established and validated. We compared unmodified and chemically modified siRNAs designed using enhanced stabilization chemistry (ESC), a clinically validated modification pattern incorporating 2'-O-methyl, 2'-fluoro, and phosphorothioate modifications. While ESC-modified siRNAs showed reduced activity compared to unmodified natural siRNAs in conventional cell-based assays requiring transfection reagents, they exhibited consistent gene silencing in Gb, reflecting their enhanced biochemical stability under free uptake conditions at picomole-scale doses. These results establish Gb as a scalable, cost-effective, and biologically relevant platform for evaluating therapeutic siRNAs, particularly those incorporating chemical modifications.