doi: medRxiv preprint NOTE: This preprint reports new research that has not been certified by peer review and should not be used to guide clinical practice.

Cell cycle abnormalities have been implicated in the pathology of neuropsychiatric disorders. Anti-obsessional agents like the serotonin reuptake inhibitor (SRI), fluoxetine, are known to cause cell cycle arrest. This study aimed at exploring cell cycle abnormalities in obsessive compulsive disorder (OCD) and the effect of in vitro Fluoxetine on lymphoblastoid cell lines (LCLs) derived from OCD patients and healthy controls. The patients had also been systematically characterized for SRI treatment response. LCLs were treated with 10&#956M of Fluoxetine for 24 hours and the percentage of cells in each phase of the cell cycle was determined by flow cytometry, after propidium iodide staining, and analysed using the software FlowJo. We observed a lower proportion of cells in the G2/M phase in OCD cases than controls. Among cases, clinical non-responders to SRI treatment had a lower proportion of cells in G2/M phase than the clinical responders. This suggests that there is a reduction of very specific cell cycle phase in OCD; and this difference is more pronounced in clinical non-responders. Although in-vitro fluoxetine treatment reduced the proportion of cells in G2/M phase overall , this was not specific to any group. The findings suggest that cell cycle G2/M phase dysregulation could be peripheral cellular phenotype for OCD, and a marker for SRI non-response in the clinic.