Abstract Background: Post-mastectomy pain syndrome (PMPS) affects up to 60% of women undergoing mastectomy. Standard perioperative multimodal analgesia remains only moderately effective in preventing PMPS, and many patients continue to rely on opioids for their chronic pain. In the context of the opioid overdose crisis, alternative interventions are urgently needed. Ketamine targets risks factors for PMPS including acute pain and negative mood, making it an ideal candidate for the prevention of PMPS. Trial Design: This is a multisite, three-arm, double-blind, RCT to test the effectiveness of ketamine in reducing PMPS in women undergoing mastectomy for oncologic indication. Arm 1 consists of continuous perioperative ketamine infusion that begins during surgery and continues for 2 hours in the post-anesthesia care unit (PACU). Arm 2 consists of a single-dose of ketamine in the PACU given over 50-60 minutes. Arm 3 consists of placebo. Standard surgical and postsurgical care remain unchanged across all arms. Eligibility Criteria: Inclusion criteria are: women ≥18 years of age undergoing total mastectomy for oncologic indication +/- lymph node dissection and +/- immediate or delayed reconstruction with no distant metastases. Exclusion criteria include: (1) history of cognitive impairment (2) past ketamine or phencyclidine misuse or abuse, (3) schizophrenia or history of psychosis, (4) history of post-traumatic stress disorder, (5) known sensitivity or allergy to ketamine, (6) liver or renal sufficiency, (7) uncontrolled hypertension, chest pain, cardiac arrhythmia, stroke, head trauma, intracranial mass or hemorrhage, glaucoma, porphyria, uncontrolled thyroid disease, or other contraindication to ketamine, (8) lamotrigine alfentanil, physostigmine, or 4-aminopyridine use, (9) currently pregnant, (10) body mass index greater than 35, (11) non-English or non-Spanish speaker, (12) currently participating in another pain interventional trial, (13) patient has started or undergone hormone therapy for gender transition into male, or (14) patient is scheduled for bilateral (or greater) flap reconstruction. Specific Aims: The primary outcome is pain intensity on the Brief Pain Inventory short form scale at the surgical site three months after mastectomy. Secondary outcomes include pain severity and interference at the surgical site, incidence of PMPS, anxiety, and depression over 12 months after surgery. Tertiary outcomes include neuropathic symptoms, fatigue, sleep, physical function, and opioid use. Statistical Methods: We will test the differences in the primary outcome between 1) the continuous ketamine infusion and the control; and 2) the single-dose ketamine and the control, each at 0.025 significance level (adjusted for multiple comparisons using the Bonferroni correction), based on the two-sample t-tests (allowing unequal variances) if outcome variables are approximately normal, or Wilcoxon’s rank-sum tests otherwise. Accrual: The target accrual for this study is ~750. Recruitment began January 2022. Recruitment is expected to be complete by October 2025. As of July 14, 2022, 43 participants have been enrolled across all sites. If interested in the KALPAS Study, please contact kalpas@nyulangone.org, jing.wang@nyulangone.org, or lisa.doan@nyulangone.org This research is supported by the National Institutes of Health through the NIH HEAL Initiative under UH3CA261067. It is also supported by the NCATS Trial Innovation Network under award numbers U24TR001608 (CCC), U24TR001597 (DCC), U24TR001609 (SSC), U24TR001579 (RIC) Citation Format: Lisa Doan, Raven Perez, Jeana Chun, Randy Cuevas, Pabel Miah, Amber Guth, Karen Hiotis, Freya Schnabel, Michele Curatolo, Robert Edwards, Hyung Park, John Rotrosen, Deborah Axelrod, Jing Wang. Ketamine Analgesia for Long Lasting Pain Relief after Surgery (KALPAS) Study [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr OT3-20-01.