e21022 Background: The CPI ipilimumab, nivolumab, and pembrolizumab have been adopted into common use for patients (pts) with MM. While their clinical activity is undeniable, a substantial fraction of pts treated with these agents experience disease progression. Additional approaches are therefore needed. Intravenous IL-2 given via daily single intravenous infusions (pulses) has been developed to mitigate toxicity while maintaining anticancer activity against MM. Staccato pulse IL-2 is based on three prior observations. Daily IL-2 schedules have previously been demonstrated to induce Lymphokine Activated Killer cell (LAK) activity [Mitchell, 1989]. LAK generated by IL-2 then subsequently exposed to more IL-2 display enhanced cytotoxicity in vitro [Hank, 1990]. Increased numbers of LAK are seen in pts with melanoma treated with daily IL-2 [Quan, 2011]. Methods: In this retrospective study, ten pts with MM were treated with IL-2 18 Million IU/M2 intravenously over 15-30 minutes on days 1-3 and 21.6 Million IU/M2 intravenously over 15-30 minutes on day 5 on an outpatient basis. Cycles were repeated every 3 weeks. Results: Characteristics: 8 females/ 2 males, median age-56 (range: 21-74), median ECOG-1 (0-1); common disease sites: lymph nodes (12), subcutaneous (6), lungs (5), soft tissue (4). Prior CPI: Ipilimumab (8); Pembrolizumab (5); Nivolumab (2). Common toxicities: nausea/emesis (8), myalgia/arthralgia (5), sinus/catarrhal symptoms (5), fatigue (5), and hypotension requiring intravenous fluids (5). No pts required hospitalization for toxicity related to therapy. Median number of cycles: 3 (2-8). Four pts have had partial responses (total response rate = 40%; 95% CI: 10-70%). The median duration of response exceeds 7.4+ months. One pt having had resolution of prior lung, lymph node, and peritoneal metastases (partial response of 24.5+ months) underwent surgical resection of the residual intraabdominal disease focus and is free of disease at 36.5+ months. Overall, responses have occurred in lung, lymph nodes, subcutaneous, bones, peritoneum, small bowel, and soft tissue sites. Conclusions: Staccato pulse intravenous IL-2 has activity in melanoma pts previously treated with CPI.