Clinical outcomes of patients with metastatic renal cell carcinoma who discontinued nivolumab plus /- ipilimumab therapy electively or due to toxicity.

625 Background: Immune checkpoint inhibitor (ICI) based regimens are the standard of care for patients with metastatic renal cell carcinoma (mRCC). In other tumor types such as melanoma, there is evidence that cancer control can persist long after discontinuation of ICI therapy. Outcomes after ICI discontinuation for mRCC are less characterized. Our aim is to characterize the durability of treatment response in mRCC patients who discontinued ICI therapy electively or due to toxicity. Methods: We identified patients with mRCC who responded to nivolumab +/- ipilimumab and then discontinued ICI therapy either for elective reasons or due to toxicity. Patients who discontinued therapy due to progressive disease or who died within 3 months of their last ICI dose were excluded. Complete response (CR) duration was calculated from last ICI dose to the most recent documentation of CR status. Time to progression was defined as the time from last ICI dose to documentation of disease progression or initiation of next-line therapy. Durability of treatment response was calculated from the date of last ICI dose to the most recent follow up date at which sustained response was noted. Results: We identified a total of 56 patients treated with nivolumab +/- ipilimumab therapy who discontinued treatment for toxicity (n=30) or for elective reasons (n=26) and achieved CR or partial response/stable disease (PR/SD). Of the entire cohort, 19 patients (34%) achieved CR, and the majority (95%) were still alive and disease-free with a median follow up of 906 days (range 86-2143 days). Among all patients who discontinued treatment with best response of PR/SD, 49% experienced disease progression, with a median progression-free survival of 382 days (range 77-1267). 42% of patients who stopped ICI electively achieved CR and all (100%) were still alive with sustained CR (median follow-up = 735 days) at data collection. Among those with best response of PR/SD after elective discontinuation, about half (53%) maintained disease control while the rest had disease progression or death (47%). Conclusions: Patients with mRCC who achieve CR with nivolumab +/- ipilimumab and then subsequently discontinue therapy for reasons other than disease progression have excellent long-term cancer outcomes. Recurrence and subsequent progression are rare in this group. These results are similar to the durability of ICI treatment response observed in complete responders in melanoma. Outcomes were less favorable for patients who obtained PR/SD as best response prior to ICI discontinuation, although some patients had ongoing and durable cancer control for up to several years. More research is indicated to ascertain which disease/patient features predict long-term disease control after ICI discontinuation in patients who respond favorably to ICI therapy.