This study focuses on people with obsessive-compulsive disorder (OCD) who do not respond well to standard treatments. Researchers aim to understand why some patients respond to medications or brain stimulation therapies, while others do not. The study will include 60 patients grouped by their treatment response:

1. Those who respond to medications
2. Those who respond to transcranial magnetic stimulation (TMS)
3. Those who do not respond to either Blood samples will be used to create nerve cells in the lab, allowing scientists to study how these cells react to treatments and brain stimulation. By combining clinical information with lab findings, the goal is to discover biological markers that predict which therapy will work best for each person. This research hopes to improve personalized treatment options for OCD.

Adults (18-65 years old) with obsessive-compulsive disorder (OCD) recruited at two specialized clinics in Milan (Luigi Sacco Hospital) and Monza (IRCCS Fondazione San Gerardo Monza). After a complete explanation of the study, everyone will provide written informed consent and have a diagnostic interview to confirm OCD. People with seizures/epilepsy or certain implanted devices (e.g., pacemaker, intracranial metal clips) will not be enrolled.

Procedures:

1. Initial assessments: Participants complete standard questionnaires and tests about OCD symptoms, mood, anxiety, and thinking (e.g., Y-BOCS, HDRS, HARS, MMSE, CANTAB, CGI). Basic information (age, sex, education, medical history, current medications) is collected in an anonymized electronic form.

2. Grouping by medication response:

   Responders (20 people): OCD symptoms improve with serotonin reuptake inhibitors (SRIs).

   Treatment-resistant (40 people): OCD symptoms did not improve after multiple SRI trials (including SSRIs and clomipramine at recommended doses).

3. Randomized brain stimulation for treatment-resistant OCD:

   The 40 treatment-resistant participants are randomly assigned (1:1) to Theta-Burst TMS (TBS) or Deep TMS (d-TMS), following established safety guidelines.

   Medications remain stable during TMS. Symptom changes are measured with the Y-BOCS at baseline and follow-up. Participants are classified as responders or non-responders to TBS or d-TMS.

   After treatment, people fall into five subgroups: SRI Responders, d-TMS Responders, TBS Responders, d-TMS Non-Responders, TBS Non-Responders.

4. Blood samples and lab modeling:

   A small blood sample is used to create patient-derived stem cells (hiPSCs) and then striatal neurons (medium spiny neurons, MSNs) in the lab.

   These neurons help us study how brain cells from different patient groups respond to medicines and stimulation.

5. Mimicking brain stimulation in the lab:

   Researchers use optogenetics (safe light pulses) to activate the lab-grown neurons, mirroring the effects of TMS.

   They also use well-known medicines that turn up or down dopamine signaling (e.g., D1/D2 agonists/antagonists) to see how neurons react.

   Changes in synaptic proteins and cell activity are measured with standard lab techniques.

6. Gene activity (transcriptomics):

Using RNA sequencing, the team looks at which genes are "on" or "off" in neurons from different patient groups-before and after light stimulation-to discover molecular signatures linked to treatment response or resistance.

Findings are checked and confirmed with additional lab tests.

Study results analysis

The study includes 60 participants in total. Data are analyzed with standard statistical methods to compare groups and treatments. Lab experiments use multiple independent preparations to ensure reliable results.

Study outcomes:

By combining careful clinical assessments with cutting-edge cell models, this study aims to discover biological markers that can predict who will benefit from medicines and from innovative TMS protocols (TBS or d-TMS)-helping move toward more personalized care for OCD.

Inclusion Criteria:

• diagnosis of Obsessive-Compulsive Disorder (according to DSM-5; APA, 2013); both sexes; age ≥ 18 years and ≤ 65 years, ability to provide valid written informed consent; for patients classified as responders to pharmacological treatment, a clinical history of at least one pharmacological trial with an SRI for a minimum of 6 weeks and evidence of treatment response, defined as a 30% reduction in the Yale-Brown Obsessive Compulsive Scale (Y-BOCS) score (Goodman et al., 1989) relative to the patient's baseline; for patients classified as resistant to pharmacological treatment [TR], clear evidence of treatment resistance, defined as the absence of a significant clinical response after at least two treatment trials with selective serotonin reuptake inhibitors (SSRIs) and one trial with clomipramine, each administered for a minimum of 12 weeks at the maximum recommended dose (Pallanti et al., 2002).

Exclusion Criteria:

• inability to provide informed consent; no clinical history of treatment with SRI medications; clinical history of epilepsy or seizures; presence of a pacemaker, removable metal prostheses, implanted medical pumps, or intracranial metal clips.

This is an open-label study with no masking. Participants, care providers, and investigators are aware of the assigned intervention (TBS or d-TMS), as the procedures differ in administration and cannot be feasibly blinded. No independent blinded outcome assessors are used.