Scientific and Regulatory Policy Committee Points to Consider*: Review of Scientific and Regulatory Policy Committee Points to Consider: Review of Current Practices for Ultrastructural Pathology Evaluations in Support of Nonclinical Toxicology Studies.

Anatomic pathology and clinical pathology end points are standard components of almost every nonclinical general toxicity study conducted during the risk assessment of novel pharmaceuticals and chemicals. On occasion, an ultrastructural pathology evaluation using transmission electron microscopy (TEM) may be included in nonclinical toxicity studies. Transmission electron microscopy is most commonly used when a light microscopic finding may require further characterization that could inform on the pathogenesis and/or mechanism of action. Regulatory guidance do not address the use of TEM in general study designs nor whether these assessments should be performed in laboratories conducted in compliance with Good Laboratory Practices. The Scientific and Regulatory Policy Committee of the Society of Toxicologic Pathology (STP) formed a Working Group to assess the current practices on the use of TEM in nonclinical toxicity studies. The Working Group constructed a survey sent to members of societies of toxicologic pathology in the United States, Europe, Britain, and Japan, and responses were collected through the STP for evaluation by the Working Group. The survey results and regulatory context are discussed, as are "points to consider" from the collective experience of the Working Group. This survey indicates that TEM remains an essential diagnostic option for complementing toxicologic pathology evaluations. *This Points to Consider article is a product of a Society of Toxicologic Pathology (STP) Working Group commissioned by the Scientific and Regulatory Policy Committee (SRPC) of the STP. It has been reviewed and approved by the SRPC and Executive Committee of the STP but it does not represent a formal Best Practice recommendation of the Society; rather, it is intended to provide key "points to consider" in designing nonclinical studies or interpreting data from toxicity and safety studies intended to support regulatory submissions. The points expressed in this document are those of the authors and do not reflect views or policies of the employing institutions. Readers of Toxicologic Pathology are encouraged to send their thoughts on these articles or ideas for new topics to the Editor.