Characterizing Antisense Oligonucleotide-Induced Histopathology Findings in Spinal Cord of Mauritius Cynomolgus Monkeys by Molecular Localization Investigation

The safety of a 2’- O-methoxyethyl antisense oligonucleotide (ASO) was investigated in Mauritius cynomolgus monkeys in a 41-week Good Laboratory Practice (GLP) toxicity study after multiple intrathecal (IT) administrations. Histopathological examination revealed ectopic formation of lymphoid follicles in the spinal cord (SC) at the injection site at all doses and the presence of granular material in neurons of the SC in high-dose animals. The granular material was seen in all the segments of the SC, but mainly in the lumbar segment and persisted at the end of the 26-week recovery period, while the lymphoid follicles showed a reversibility trend. Findings associated with repeated IT administration of ASOs have been described in nonhuman primate (NHP) toxicity studies, specifically in the brain, but findings in the SC are rarely reported. In the present study, we report a high incidence of findings in the SC compared to brain, especially in the lumbar segment in proximity to IT injection sites. An extensive panel of immunohistochemistry markers showed that the ectopic lymphoid follicle formation (LFF) had a cellular composition and organization consistent with tertiary lymphoid structure (TLS) without associated axonal damage in the adjacent nervous tissue. In situ hybridization with an miRNA probe complementary to the ASO revealed that the granular material represented a dose-dependent ASO accumulation in the cytoplasm of neurons without inducing cell death or apoptosis. Glial and ependymal cells in the SC also showed dose-dependent accumulation of the ASO preceding detection of granular material by hematoxylin and eosin (H&E). Based on these molecular localization data, the presence of LFF in SC suggests a chronic local immune activation. Considering the absence of neuronal dysfunction or injury and transient clinical signs previously reported with other 2’- MOE ASOs, the presence of TLS and ASO was considered non-adverse.