Background: ASPECCT demonstrated that panitumumab (pmab) was noninferior to cetuximab (cmab) for overall survival (OS). Ad hoc analyses from ASPECCT suggested that hypomag was associated with better outcomes for pmab and cmab (Price 2015). However, results from the phase 3 NCIC CTG/AGITG CO.17 trial indicated hypomag at day 28 was associated with worse outcomes for cmab (Vickers 2013). Methods: Patients (pts) with chemorefractory WT KRASexon 2 mCRC were randomized 1:1 to receive pmab or cmab. Ad hoc analyses by hypomag were performed from the final analysis of ASPECCT at week 5, consistent with the NCIC CTG/AGITG CO.17 trial analysis (Vickers, 2013). Results: 999 pts were treated: 496 received pmab and 503 received cmab. Any grade hypomag was 29.0% and grade ≥3 was 7.3% in the pmab arm vs 19.3% and 2.8% in the cmab arm, respectively. In the pmab arm, 1.2% of pts discontinued treatment and 5% of pts had dose modifications due to hypomag vs 0.4% and 3% in the cmab arm, respectively. Efficacy results by hypomag 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 longer median OS compared with those pts who did not. Consistent with previous analyses, development of hypomag at week 5 was associated with worse median OS for cmab. Clinical trial information: NCT01001377

*Refers to patients who developed hypomag anytime during the study.