Effects of Ondansetron in Obsessive-compulsive and Tic Disorders

Many psychiatric disorders are associated with altered sensory experiences arising from
within the body. Examples include increased experience of sensations or urges in muscles,
skins, joints or visceral organs in Tic/Tourette's Disorders, OCD patients with symptoms of
"not just right experiences" or disgust sensitivity, and other disorders such as
trichotillomania or excoriation disorder. In OCD, these sensory phenomena occur in
approximately half of patients, are associated with earlier age of onset, and may be harder
to treat with classic cognitive-behavioral approaches to OCD. Of interest, sensory phenomena
in OCD are associated with Tourette's syndrome and respond to pharmacological treatments
primarily used for tics. As such, abnormal sensory processing may be a basic mechanism that
links various psychiatric disorders.

The process of attending to body sensations is referred to as interoception, abnormality of
which may be related to sensory phenomena. Research has revealed a cortical interoceptive
circuit involving insula, anterior cingulate cortex (ACC), and sensorimotor cortex.
Ondansetron (OND) is a good candidate for the modulation of the above-described interoceptive
circuit. It is a selective 5-HT3 (serotonin) receptor antagonist that acts on both peripheral
and central receptors. OND has long been used to treat nausea and vomiting due to
chemotherapy, radiation therapy, anesthesia, and opioid-induced emesis. It has also been used
alone or as adjunctive therapy for the treatment of both OCD and Tourette's disorder, showing
some efficacy in small clinical trials. The mechanisms by which ondansetron improves symptoms
in OCD and tic disorders are unknown, although the investigator's earlier study found that
single doses of ondansetron reduce activation of insula and somatosensory cortex in healthy
controls. As a follow-up to this work, the current protocol will compare the effects of 24
mg/day of ondansetron vs. placebo for 4 weeks in patients with OCD or Tic Disorders on
symptoms and brain functioning.

Inclusion Criteria:

- Patients must be medically healthy, between 18 and 60 years of age

- Fluent (speaking and writing) in English

- Patients must have a current diagnosis of obsessive-compulsive disorder (OCD) or tic
disorder (OCD) according to Diagnostic and Statistical Manual of Mental Disorders
(DSM-5) criteria with moderate or greater disorder severity and moderate or greater
severity of sensory phenomena

- Patients must be unmedicated or taking antidepressants, stable for at least 6 weeks

Exclusion Criteria:

- Present or previous diagnosis of any psychosis, bipolar disorder, or major
developmental disorder (autism/Asperger's disorder, pervasive developmental disorder).
Present diagnosis of alcohol or substance use disorder (moderate or severe) will also
be exclusionary.

- Any disability or health problem that prevents them from completing study procedures
(e.g. color blindness, severe carpal tunnel syndrome, etc.).

- History of organic mental syndromes, head trauma, migraines, seizures, other central
nervous system (CNS) neurological disease, or significant medical illness other than
that listed above.

- Pregnant or nursing women will be excluded.

- Subjects with a medical condition or other predisposition that increases the risk of
adverse effects when taking ondansetron. These include, but are not limited to,
individuals with drug allergies or known hypersensitivity to ondansetron (or other
5-HT3 antagonists), heart disease, congestive heart failure, heart rhythm disorder,
congenital long QT syndrome, electrolyte abnormalities (e.g., hypokalemia,
hypomagnesemia) or hepatic impairment.

- Subjects who report taking apomorphine will be excluded.

- Subjects with abnormal EKG will either be excluded from participation, or referred to
a cardiologist for further assessment of eligibility.

- Subjects with abnormal liver function or electrolytes (as determined by blood test)
will be excluded from participation if a study team physician determines it is unsafe
for them to participate.

- Cross-reactivity with other 5-HT3 antagonists has been reported, so any individual
taking a 5-HT3 antagonist will be excluded.