Background: In Ph3 trials, assessment for primary endpoint of OS necessitates extended follow-up (f/u) periods, larger pool of events, and higher associated costs. We sought to determine if shorter IEs like Time to Treatment Failure (TTF) and Time to Next Therapy (TTNT) are associated with OS in patients (pts) receiving ICI-based trt. Methods: We included all International mRCC Database Consortium (IMDC) pts who received contemporary approved first line(1L) ICI from 2013 to 2023. IEs were defined from ICI start until drug cessation or death for TTF, and initiation of next line or death for TTNT, or censored at date of last f/u. Associations of OS with TTF and TTNT status at 6-mo landmark were assessed using Cox regression adjusting for IMDC risk groups, metastatic sites, histology, age, and prior nephrectomy, stratified by treatment (trt) and yrs of ICI start. Endpoint associations across all f/u time were evaluated using Kendall’s Tau (KT) correlation by Clayton copula. A KT >0.49 indicates a strong correlation (Wicklin R., 2023).Results: The cohort consisted of 1667 pts with a median f/u of 15.4 mo (IQR: 7.1-28.6). Median age at 1L start was 63 yrs (IQR: 56-70), with 73% being male and 65% undergoing nephrectomy before starting 1L. 1132 patients received dual ICI, while 535 received an ICI+TKI combination. Pts who discontinued their 1L regimen within the 6-mo landmark demonstrated poor OS, with a hazard ratio (HR) of 2.74 (95% CI: 2.15-3.49). Additionally, those who transitioned to a 2L therapy within the first 6 mo showed worse OS, reflected by an HR of 2.82 (2.22-3.59). KT correlation with OS across all follow up was 0.49 (0.45, 0.52) for TTF and 0.67 (0.64, 0.69) for TTNT. Consistent results were seen across all subgroups with the strongest association in the ICI+TKI group (Table). Conclusions: TTNT demonstrated the strongest association with OS, particularly in the ICI+TKI subgroup, making it a potentially clinically meaningful intermediate endpoint for evaluating efficacy in ICI-based regimens.

*Excluding pts who died within 6 mo or had not been followed more than 6 mo.