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The goal of this study is to examine whether measurements of inflammation from brain scanning and blood tests can predict how much benefit patients with obsessive-compulsive disorder (OCD) will have from treatment with an anti-inflammatory medication, celecoxib, when adding to ongoing medication-based treatment for OCD.

Existing evidence-based treatments for adults with obsessive-compulsive disorder (OCD), including cognitive behavioral therapy and first-line medication-based treatment, do not fully resolve symptoms for a majority of patients. This study proposes to test a new treatment strategy for OCD, examining inflammation as one factor that may contribute to the development of OCD and that may be targeted with medication-based treatment. Recent findings using both brain imaging and analyses from blood samples suggest that some individuals with OCD have increased levels of inflammation in the brain, which may have negative effects on brain function, contributing to symptoms of OCD. Moreover, early clinical studies suggest that some medications with anti-inflammatory properties may be beneficial in treating OCD symptoms. An important and untested question is whether the degree of inflammation assessed using these biological tools can predict how much benefit individuals with OCD will derive from treatment with an anti-inflammatory medication. The investigators will measure inflammation in the brain using PET imaging and measures of inflammation in the body using blood tests in adults with OCD. Patients will then undergo 8 weeks of treatment with an anti-inflammatory medication with some evidence of clinical effectiveness in OCD, celecoxib. The investigators predict that OCD patients with greater evidence of inflammation at baseline, assessed by brain imaging and blood tests, will derive the greatest benefit from anti-inflammatory treatment. In addition, the investigators will compare these inflammatory markers with data from a group of healthy volunteers (collected as part of another protocol, IRB #6786), to extend earlier work finding elevated inflammatory markers in adult OCD. Finally, the investigators will examine whether celecoxib treatment in OCD patients results in reductions in measures of inflammation measured from blood samples and whether the degree of anti-inflammatory effect observed biologically is related to improvement in OCD symptoms. The ultimate goal of this line of research is to pave the way for more individually-tailored, effective treatments for adult OCD based on improved understandings of pathological targets, and to validate an anti-inflammatory approach to the treatment of OCD in individuals with evidence of inflammation.

Inclusion Criteria:

• 18 to 55 years old
• Principal diagnosis of Obsessive-Compulsive Disorder (OCD)
• Ongoing OCD symptoms despite current treatment with psychiatric medication
• Capacity to provide informed consent

Exclusion Criteria:

• Psychiatric comorbidities that would increase risk of participation or impact research measures (e.g., suicidality)
• Medical or neurological conditions that would increase risk of participation or impact research measures
• Contraindications to an MRI or PET scan (e.g., metal implants)
• Current use of medications that may interact with study drug or impact research measures (e.g., antipsychotics, corticosteroids, immunosuppressant medications, or daily non-steroidal anti-inflammatory medications)
• Starting a new evidence-based therapy for OCD (e.g., exposure with response prevention) 4 weeks prior to study enrollment