Background: COMPARZ was a randomized, controlled, open-label, phase 3 trial that demonstrated comparable efficacy of first-line PAZ and SUN, but favorable safety and quality of life profiles for PAZ in patients (pts) with mRCC (NEJM 2013;369:722). We evaluated the relationship between dosing, safety, and efficacy in PAZ- and SUN-treated pts who did or did not undergo dose reduction or interruption resulting from adverse events (AEs) and other reasons. Methods: The AEs and median progression-free survival (mPFS) of PAZ and SUN were evaluated for pts with no, any, 1, and ≥2 dose reductions or dose interruptions lasting ≥7 days. Results: Similar percentages of pts in the PAZ and SUN groups had a dose interruption (44% vs 49%, respectively) or reduction (44% and 51%, respectively). The incidence of AEs in pts from the PAZ and SUN groups with dose modifications was higher compared to those with no dose modifications. Longer mPFS was observed in pts with dose modification (Table). Pts treated with PAZ or SUN with no dose reductions had mPFS of 7.3 months (mos) and 5.5 mos, respectively, whereas pts with any dose reduction had mPFS of 12.5 mos and 13.8 mos, respectively. Similarly, pts treated with PAZ or SUN with no dose interruptions lasting ≥7 days had mPFS of 8.2 mos and 5.6 mos, respectively, whereas those with any dose interruption lasting ≥7 days had mPFS of 12.6 mos and 13.8 mos, respectively. Pts with 2 or more dose interruptions or reductions had mPFS > 16 mos with both SUN and PAZ. Conclusions: Consistent with previous data for SUN, the current analyses showed longer mPFS with PAZ and SUN when dose modification is required to manage toxicity, suggesting that pts are not disadvantaged by such dose reductions or interruptions. Pts not requiring dose modification may have sub-optimal therapeutic drug exposure. Clinical trial information: NCT00720941