Repurposing Drugs as Immunotherapeutic Agents: Changes in Colorectal Tumor Immune Cells After Targeting Serotonin

Inclusion Criteria: * Male or female ≥ 18 years of age at visit 1 * Previously untreated cytologically or histologically confirmed colorectal adenocarcinoma that will not need neoadjuvant therapy * Written informed consent (and assent when applicable) obtained from subject or subject's legal representative and ability for subject to comply with the requirements of the study * World Health Organization (WHO) Eastern Cooperative Oncology Group (ECOG) performance status 0-2 * Absolute neutrophil count (ANC) ≥ 1.5x10\^9/L * Platelets ≥ 100x10\^9/L * Hemoglobin ≥ 9 g/dL * Serum creatinine (sCr) ≤ 1.5 x upper limit of normal (ULN) * Creatinine clearance (Ccr) ≥ 40 mL/min (as calculated by Modified Cockcroft-Gault formula) * Serum total bilirubin ≤ 1.5 x ULN * Aspartate aminotransferase (AST)(serum glutamic-oxaloacetic transaminase \[SGOT\]) / alanine aminotransferase (ALT)(serum glutamate pyruvate transaminase \[SGPT\]) ≤ 2.5 x ULN * Baseline corrected QT (QTc) within normal limits per the Bazett formula. Any electrocardiogram (EKG) done prior to consent is acceptable for baseline QTc monitoring. * Normal QTc ranges from 350-450 ms for adult men and from 360-460 ms for adult women Exclusion Criteria: * Presence of a condition or abnormality that in the opinion of the investigator would compromise the safety of the participant or the quality of the data * A diagnosis of metastatic colorectal adenocarcinoma * Individuals who have received neoadjuvant chemotherapy prior to the planned colon cancer resection * Individuals with absolute or relative contraindications to fluoxetine * Baseline prolonged QTc * Concurrently taking tamoxifen, pimozide, or thioridazine * Individuals using other SSRIs, serotonin and norepinephrine reuptake inhibitors (SNRIs), monoamine oxidase inhibitors (MAOIs), lithium or other antidepressants at time of initial biopsy * Currently active second primary malignancy or history of malignancy less than 5 years prior to the time of study eligibility (Patients with history of skin cancers excluding melanoma will be eligible for participation)