Background: ASPECCT met its primary endpoint of non-inferiority of overall survival (OS) of pmab vs cmab. We evaluate outcomes by hypomag, an on-treatment, anti-EGFR related adverse event that develops due to the inhibition of EGFR function. Conflicting reports have suggested hypomag is associated with survival. Methods: Patients (pts) with previously treated WT KRAS exon 2 mCRC were randomized 1:1 to receive pmab or cmab. The primary endpoint was non-inferiority of OS. Progression-free survival (PFS) and objective response rate (ORR) were secondary endpoints. Pts were categorized ± any grade hypomag during the study and data from the primary analysis was evaluated by treatment arm. Analysis of Mg supplementation during hypomag was not conducted. Results: 999 pts were randomized and treated: 499 pmab, 500 cmab. Any grade hypomag was 28.8% and grade ≥3 was 7.3% in the pmab arm vs 18.9% and 2.6% in the cmab arm, respectively. Median time to first hypomag onset was 82 days in the pmab arm and 57 days in the cmab arm. In the pmab arm, 1.0% of pts discontinued treatment and 5% of pts had dose modifications due to hypomag vs <0.5% and 3% in the cmab arm, respectively. Results are shown (Table). Conclusions: In ASPECCT, rates of hypomag were higher in the pmab vs the cmab arm. Pts who developed any grade hypomag with pmab or cmab had higher ORR, PFS, and OS compared with those pts who did not. Clinical trial information: NCT01001377

^aEvaluable pts per modified RECIST.