Nonclinical Safety Assessment of Lipid Nanoparticle-and Emulsion-Based Self-Amplifying mRNA Vaccines in Rats.

Vaccines containing mRNA with the capacity to self-amplify represent an alternative to the mRNA vaccines that came to prominence during the COVID-19 pandemic. To gain further insights on the safety profile of self-amplifying mRNA- (SAM-) vaccines, this preclinical toxicology study in rats evaluated the effect of (i) the type of delivery system (lipid nanoparticle [LNP] vs cationic nano-emulsion [CNE]); (ii) antigen-encoding sequence (rabies glycoprotein G vs SARS-CoV-2 Spike); and (iii) RNA amplification. Further analyses also evaluated gene expression in peripheral blood after vaccination, and the biodistribution of vaccine RNA. The SAM vaccines administered as two doses 2-weeks apart had acceptable safety profiles in rats, with respect to clinical signs, blood biochemistry, and macroscopic and microscopic pathology. A transient increase in ALT/AST ratio occurred only in female rats and in the absence of muscle and liver damage was dependent on RNA amplification and appeared related to the greater quantities of vaccine RNA in the muscle and livers of female rats vs male rats. The RNA and delivery-vehicle components, but not the nature of the antigen-coding sequence or the requirement for RNA amplification, affected aspects of the stimulation of innate-immune activity, which was consistent with the transient activation of type I and type II interferon signaling. The delivery vehicle, LNP, differed from CNE as vaccine RNA in CNE compositions appeared independently to stimulate innate-immune activity at 4 hours after vaccination. Our analysis supports further studies to assess whether these differences in innate-immune activity affect safety and efficacy of the SAM vaccine.

Artifactual Stratum Corneum Calcification of the Beagle Dog Tongue.

Examination of H&E-stained tongue samples from a 26-week intravenous infusion study of Beagle dogs, utilizing a compound with no recognized effect on mineral metabolism, exhibited superficial stratum corneum calcification in both treated and control animals. This resulted in the search for possible causes of the finding to help clarify confounding issues. Retrospective examination of 11 studies performed before the signal case indicated that the problem existed in the testing facility but was not recognized. Prior to 2008, this finding was not observed, perhaps indicating the requirement for a change in procedures or suppliers. Based on the hypothesis that the calcium salts were deposited from bone during processing, a series of tests was performed by fixing tongue and femur along with different tissues, processed routinely to slide, and stained with H&E and von Kossa stains. We conclude that the presence of superficial stratum corneum calcification of the tongue in dogs demonstrated in toxicology studies is an artifactual change related to the processing of tissues, specifically the fixation of tongue in the same container as bone and stomach. This change should not be confused with compound-related effects, even when the incidence varies between controls and treated animals.