Xylene: weight of evidence approach case study to determine the need for an extended one generation reproductive study with a developmental neurotoxicity animal cohort.

Xylene is a high production volume chemical that is widely used as a solvent and polymer precursor, and is currently undergoing substance evaluation under Registration, Evaluation, Authorization and Restriction of Chemicals (REACH). Xylenes recently received testing decisions on one-generation reproductive toxicity (EOGRT) studies with additional developmental neurotoxicity (DNT) cohorts for each of the three isomers. Xylene presents a unique opportunity to investigate the need for additional animal DNT toxicology testing because it is a legacy industrial chemical for which a significant amount of animal and human data already exists on its toxicity profile, including central nervous system effects. Therefore, to address the need for further vertebrate testing, a comprehensive weight of evidence (WOE) review of published and previously unpublished new studies of xylene substances was performed. Evidence topics included the pharmacokinetics, narcotic effects in humans and animals, narcotic mode of action (MOA), and strength of DNT signal for xylene. Pharmacokinetic data indicate minimal distribution of the unmetabolized parent compound to the fetus relative to parental brain tissue, and rapid metabolism of xylene to methyl hippuric acid (MHA), which is also rapidly excreted in both humans and animals. Xylene exposure has also resulted in transient, nonspecific neurological effects including delays in reaction time of human volunteers and reductions in motor activity of animals. This narcotic MOA for xylene occurs by the nonspecific perturbation of nervous cell membrane phospholipids, such that membrane-bound proteins and their respective functions are impaired. Furthermore, an in-depth review of the available DNT data indicates significant methodological deficiencies in several studies in the literature purported to provide evidence of a DNT concern following xylene exposure and no DNT concern reported in one reliable study. In conclusion, based on xylene's pharmacokinetics, narcotic effects on the central nervous system observed in animal and human studies, its narcotic MOA, and the lack of a robust signal from the published DNT studies, there is no trigger for the additional EOGRT study DNT cohort to be conducted for xylene. Further, the findings on narcotic effects and MOA underscore the difficulty in separating transient, acute intoxication effects via direct exposure of the offspring from investigating DNT effects (as investigated in a standard guideline (426) DNT study) in the EOGRT study, therefore producing unreliable data, which is ethically at odds with REACH and 3 R principles.