RECIST v1.1 and irRECIST outcomes in advanced HCC treated with pembrolizumab (pembro).

528 Background: IO can cause pseudoprogression (PP): apparent tumor growth followed by stability or favorable response. We assess PP in aHCC treated with pembro (KEYNOTE-224 [ph 2], NCT02702414; KEYNOTE-240 [ph 3], NCT02702401). Methods: aHCC pts with PD on/intolerance to sorafenib received pembro 200 mg IV Q3W until unacceptable toxicity, study withdrawal, 2 y of therapy, or RECIST v1.1 PD; if pt clinically stable at PD, physician could continue therapy and repeat scans to confirm PD per irRECIST. PP=RECIST v1.1 PD then irRECIST response other than PD. Data cutoff: Jan 02, 2019 (KEYNOTE-240); Feb 13, 2018 (KEYNOTE-224). Results: 245/382 pembro-treated pts had RECIST v1.1 PD: 138 irRECIST repeat scan; 105 PD; 33 (8.6%; 33/382) outcomes other than PD. Of 33 PP, 29 had SD, 3 PR, 1 CR (irRECIST; 29 had this at first irRECIST scan; 4 [2 SD, 2 PR] at subsequent scan). For initial RECIST v1.1 PD, 16/33 PP had PD at first postbaseline scan (pembro cycles 2-4); 17/33 PP had PD at pembro cycles 4-18. Median (range) time to RECIST v1.1 PD in the 33 PP was 80 (37-378) days. OS shown in Table. KEYNOTE-240 had 135 PBO-treated pts: 8 (5.9%) PP; small samples bar meaningful interpretation. Conclusions: PP in aHCC, per irRECIST, has a similar incidence to other cancers (eg, melanoma) and does not seem to correlate with OS. Data may help physicians assess when to continue pembro after PD. Clinical trial information: NCT02702414, NCT02702401. [Table: see text]