The 2023 annual scientific meeting of the French Society of Toxicologic Pathology (Société Française de Pathologie Toxicologique, SFPT), entitled "mRNA-based technologies: preclinical development and therapeutic applications," was held in Lyon (France) on May 25 to 26, 2023. The aim of the meeting was to discuss the biology, immunology, and preclinical development of messenger RNA (mRNA)-based vaccines and therapeutics, including immuno-oncology and rare diseases, as well as the regulatory aspect of the COVID-19 vaccines and an overview of the principles and applications of in situ hybridization techniques. This article presents the summary of five lectures along with selected figures, tables, and key literature references on this topic.

Duchenne muscular dystrophy (DMD) is caused by mutations of the DMD gene that prevent the expression of functional dystrophin protein. BMN 351 is an antisense oligonucleotide (ASO) designed to induce skipping of exon 51 of dystrophin pre-mRNA and production of internally deleted but functional dystrophin. We determined whether extended-term BMN 351 dosing leads to exon skipping, dystrophin production, and improved motor function in hDMDdel52/mdx mice containing a human exon 52-deleted DMD transgene. Weekly intravenous doses of vehicle, 6 mg/kg BMN 351, or 18 mg/kg BMN 351 were administered for 25 weeks, and samples were analyzed 4 and 12 weeks post-dosing. BMN 351 produced dose-dependent exon skipping levels in the heart and quadriceps muscles, accompanied by dose-dependent increases in mean dystrophin levels of 17% to 55% 12 weeks post-dosing. Compared with vehicle-treated hDMDdel52/mdx mice, BMN 351 ameliorated DMD-related histopathologic changes in the gastrocnemius muscle and heart. Both BMN 351 doses preserved fine motor kinematics, which was worse in vehicle-treated hDMDdel52/mdx mice compared with wild-type 4 and 12 weeks post-dosing. Liver samples demonstrated findings consistent with ASO accumulation, to which mice are considered especially sensitive compared to humans and other non-clinical species. These results support further non-clinical and clinical development of BMN 351.

Exon skipping with antisense oligonucleotides (ASOs) can correct disease-causing mutations of Duchenne muscular dystrophy (DMD) through RNA-targeted splice correction. This correction restores the reading frame and supports expression of near full-length dystrophin. First-generation exon 51-skipping ASOs targeted the same binding site, with limited clinical efficacy. We characterized a novel binding site within exon 51 that induced highly efficient exon skipping. A precursor ASO (AON-C12) and clinical ASO (BMN 351) were designed using 2'-O-methyl-modified phosphorothioate (2'OMePS) RNA and locked nucleic acids. hDMDdel52/mdx mice were given AON-C12 or BMN 351 for 13 weeks and evaluated for molecular and phenotypic correction of dystrophin deficiency. BMN 351 treatment induced durable, dose-dependent levels of exon skipping and dystrophin production in all muscles evaluated. In the heart, 8 weeks after the last BMN 351 dose at 18 mg/kg, exon-skipped transcripts remained at 44.3% of total, and dystrophin levels were 21.8% of wild type. BMN 351 reached higher tissue concentrations and percent exon skipping in the heart than a clinically relevant peptide-conjugated phosphorodiamidate morpholino oligomer comparator. BMN 351 also improved gait scores and clinical and anatomical muscle pathology parameters compared with vehicle-treated hDMDdel52/mdx mice. The pharmacologic activity and safety of BMN 351 warrant further nonclinical and clinical development.

The following chapter is a review of the normal anatomical considerations specific to the non-human primate along with spontaneous diseases and selected xenobiotic-associated toxicities of their cardiovascular system. Most spontaneous diseases observed in the modern non-human primates used in laboratory research (e.g., congenital, degenerative, metabolic, and neoplastic) are quite rare with the exception of low-severity inflammatory infiltrates. The noninflammatory spontaneous diseases are generally only described in a limited number of case reports and are usually distinct enough to be readily distinguishable from xenobiotic-associated findings whereas the spontaneous inflammatory conditions of the cardiovascular system may overlap to a greater degree with xenobiotic-associated findings and thus prove to be more difficult diagnostic and interpretive challenge.

In this study, we assessed the toxicity and toxicokinetics of racemic vigabatrin and its S- and R-enantiomers (vigabatrin consists of 50:50% of the two enantiomers) by administering doses of the three test articles to male Long Evans rats via oral gavage. The animals were housed under high-intensity light conditions and the study consisted of an escalating dose phase and a 21-day fixed-dose phase. Systemic toxicity of vigabatrin appears to be due to the Vig-S-enantiomer only, as increasing doses of Vig-S or Vig-RS caused body weight loss, decreased food consumption, and affected activity. Administration of the Vig-R-enantiomer did not cause any such effects. Systemic exposure to R- and S-enantiomers was approximately linear with dose. Compared to administration of the racemate, there appeared to be a tendency for animals to take up higher amounts of Vig-R and lower amounts of Vig-S when administered as enantiomer. Bilateral retinal atrophy was observed in the fixed-dose phase in rats receiving Vig-S (either alone or as part of Vig-RS) and was characterized by irregular thinning and disorganization of the outer nuclear layer and thinning of the photoreceptor layer. The administration of the R-enantiomer alone did not cause any microscopic retinal change.

With advancements in whole slide imaging technology and improved understanding of the features of pathologist workstations required for digital slide evaluation, many institutions are investigating broad digital pathology adoption. The benefits of digital pathology evaluation include remote access to study or diagnostic case materials and integration of analysis and reporting tools. Diagnosis based on whole slide images is established in human medical pathology, and the use of digital pathology in toxicologic pathology is increasing. However, there has not been broad adoption in toxicologic pathology, particularly in the context of regulatory studies, due to lack of precedence. To address this topic, as well as practical aspects, the European Society of Toxicologic Pathology coordinated an expert international workshop to assess current applications and challenges and outline a set of minimal requirements needed to gain future regulatory acceptance for the use of digital toxicologic pathology workflows in research and development, so that toxicologic pathologists can benefit from digital slide technology.

The adult Göttingen Minipig is an acknowledged model for safety assessment of antisense oligonucleotide (ASO) drugs developed for adult indications. To assess whether the juvenile Göttingen Minipig is also a suitable nonclinical model for pediatric safety assessment of ASOs, we performed an 8-week repeat-dose toxicity study in different age groups of minipigs ranging from 1 to 50 days of age. The animals received a weekly dose of a phosphorothioated locked-nucleic-acid-based ASO that was assessed previously for toxicity in adult minipigs. The endpoints included toxicokinetic parameters, in-life monitoring, clinical pathology, and histopathology. Additionally, the ontogeny of key nucleases involved in ASO metabolism and pharmacologic activity was investigated using quantitative polymerase chain reaction and nuclease activity assays. Similar clinical chemistry and toxicity findings were observed; however, differences in plasma and tissue exposures as well as pharmacologic activity were seen in the juvenile minipigs when compared with the adult data. The ontogeny study revealed a differential nuclease expression and activity, which could affect the metabolic pathway and pharmacologic effect of ASOs in different tissues and age groups. These data indicate that the juvenile Göttingen Minipig is a promising nonclinical model for safety assessment of ASOs intended to treat disease in the human pediatric population.

The INHAND (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions Project (www.toxpath.org/inhand.asp) is a joint initiative of the Societies of Toxicologic Pathology from Europe (ESTP), Great Britain (BSTP), Japan (JSTP) and North America (STP) to develop an internationally accepted nomenclature for proliferative and nonproliferative lesions in laboratory animals. The purpose of this publication is to provide a standardized nomenclature for classifying microscopic lesions observed in most tissues and organs from the nonhuman primate used in nonclinical safety studies. Some of the lesions are illustrated by color photomicrographs. The standardized nomenclature presented in this document is also available electronically on the internet (http://www.goreni.org/). Sources of material included histopathology databases from government, academia, and industrial laboratories throughout the world. Content includes spontaneous lesions as well as lesions induced by exposure to test materials. Relevant infectious and parasitic lesions are included as well. A widely accepted and utilized international harmonization of nomenclature for lesions in laboratory animals will provide a common language among regulatory and scientific research organizations in different countries and increase and enrich international exchanges of information among toxicologists and pathologists.

Lysosomes have a central role in cellular catabolism, trafficking, and processing of foreign particles. Accumulation of endogenous and exogenous materials in lysosomes represents a common finding in nonclinical toxicity studies. Histologically, these accumulations often lack distinctive features indicative of lysosomal or cellular dysfunction, making it difficult to consistently interpret and assign adverse dose levels. To help address this issue, the European Society of Toxicologic Pathology organized a workshop where representative types of lysosomal accumulation induced by pharmaceuticals and environmental chemicals were presented and discussed. The expert working group agreed that the diversity of lysosomal accumulations requires a case-by-case weight-of-evidence approach and outlined several factors to consider in the adversity assessment, including location and type of cell affected, lysosomal contents, severity of the accumulation, and related pathological effects as evidence of cellular or organ dysfunction. Lysosomal accumulations associated with cytotoxicity, inflammation, or fibrosis were generally considered to be adverse, while those found in isolation (without morphologic or functional consequences) were not. Workshop examples highlighted the importance of thoroughly characterizing the biological context of lysosomal effects, including mechanistic data and functional in vitro readouts if available. The information provided here should facilitate greater consistency and transparency in the interpretation of lysosomal effects.

The identification of adverse health effects has a central role in the development and risk/safety assessment of chemical entities and pharmaceuticals. There is currently a need for better alignment regarding how nonclinical adversity is determined and characterized. The European Society of Toxicologic Pathology (ESTP) therefore coordinated a workshop to review available definitions of adversity, weigh determining and qualifying factors of adversity based on case examples, and recommend a practical approach to define and characterize adversity in toxicology reports, to serve as a valuable prerequisite for future organ- or lesion-specific workshops planned by the ESTP.

For pediatric indications requiring juvenile toxicity testing, the rat is the preferred species. However, for some drugs it might not be an appropriate model or regulatory agencies may also request a non-rodent species. Due to the relatively recent use of Göttingen minipigs, little background data are available. This shortage of historical data can raise concerns with respect to interpretation, thus potentially discouraging investigators. This article presents background data from 82 piglets collected at different ages. The data described show the normal variations and changes which are important in the interpretations of these studies. Age-related changes were observed for several cardiac and clinical pathology parameters and in the haematopoietic tissues. Therefore, all pigs were not considered equal. It can be concluded that these data can be used as guidance, to support the concurrent study control data but cannot completely replace them.

The identification of adverse health effects has a central role in the development and risk/safety assessment of chemical entities and pharmaceuticals. There is currently a need for better alignment regarding how nonclinical adversity is determined and characterized. The European Society of Toxicologic Pathology (ESTP) therefore coordinated a workshop to review available definitions of adversity, weigh determining and qualifying factors of adversity based on case examples, and recommend a practical approach to define and characterize adversity in toxicology reports, to serve as a valuable prerequisite for future organ- or lesion-specific workshops planned by the ESTP.

In this article we review the value and utility of the minipig as an animal model in regulatory toxicity testing. Our review is based on detailed consideration of the comparative biology of the minipig, and of the practical features of toxicity testing in the minipig. The minipig presents a favourable profile as a non-rodent toxicology model, in terms of the similarity to man and also in terms of applicability to different study types. Studies of general toxicology can be performed in the minipig by oral, cutaneous, parenteral and inhalation routes. For reproductive toxicology studies the minipig offers numerous advantages as a non-rodent model although the lack of placental transfer of macromolecules may limit the role of the minipig in reproductive testing of biotechnology products. For safety pharmacology studies the minipig is an advantageous model, particularly as regards the cardiovascular system. The immune system of the pig is better characterized than that of the dog, making the pig an interesting alternative model to the nonhuman primate for therapeutic approaches based on manipulation of the immune system. Overall, this review leads us to believe that the minipig might be a better non-rodent toxicology model than the dog. At the present time, however, insufficient comparative data is available to permit a rigorous evaluation of the predictivity of the minipig for human drug-induced toxicities and research is urgently needed to provide experimental data for evaluation of the hypothesis that minipig studies may better reflect human drug-induced toxicities than studies performed in traditional non-rodent toxicology models. It would be of particular value to gain a better vision of the potential utility of the minipig as a model for the safety testing of new biologics, where the minipig could potentially replace the use of non-human primates in the testing of some new products.

Drug-induced vascular injury (DIVI) is a recurrent challenge in the development of novel pharmaceutical agents. In recent years, DIVI has been occasionally observed in nonhuman primates given RNA-targeting therapeutics such as antisense oligonucleotide therapies (ASOs) during chronic toxicity studies. While DIVI in laboratory animal species has been well characterized for vasoactive small molecules, and immune-mediated responses against large molecule biotherapeutics have been well described, there is little published information regarding DIVI induced by ASOs to date. Preclinical DIVI findings in monkeys have caused considerable delays in development of promising new ASO therapies, because of the uncertainty about whether DIVI in preclinical studies is predictive of effects in humans, and the lack of robust biomarkers of DIVI. This review of DIVI discusses clinical and microscopic features of vasculitis in monkeys, their pathogenic mechanisms, and points to consider for the toxicologist and pathologist when confronted with ASO-related DIVI. Relevant examples of regulatory feedback are included to provide insight into risk assessment of ASO therapies.