Fluoxetine for the Modification of Colorectal Tumor Immune Cells Before Surgery in Patients With Colorectal Cancer

This study is testing whether fluoxetine (an antidepressant commonly known as Prozac) given before surgery can change the immune environment inside colorectal tumors in a way that might improve surgical outcomes. Researchers will analyze the tumor tissue after surgery to see if the drug had any effect on immune cells within the cancer. **You may be eligible if...** - You are 18 years old or older - You have been newly diagnosed with colorectal cancer (confirmed by biopsy) that will be treated with surgery and does not require pre-surgical chemotherapy - Your cancer has not spread to other organs - Your overall health status is acceptable (ECOG 0–2) - Your blood counts, liver, and kidney function are within acceptable range - Your heart rhythm (QTc interval) is normal **You may NOT be eligible if...** - You are currently taking antidepressants (SSRIs, SNRIs, MAOIs, or tricyclics) or lithium - You have a contraindication to fluoxetine (including a prolonged QTc on ECG) - You are currently taking tamoxifen, pimozide, or thioridazine - Your cancer has already spread (metastatic) - You have had another cancer within the past 5 years (skin cancer, excluding melanoma, is okay) Talk to your doctor to see if this trial is right for you.