Cetuximab (Cet) clearance and survival in patients (pts) with metastatic colorectal cancer (mCRC).
699 Background: Cet, a monoclonal antibody against EGFR, is a standard therapy for pts with RAS wild-type (WT) mCRC. Limited previous data suggest that Cet clearance correlates with progression-free survival (PFS). We performed a population pharmacokinetic (pop-pK) analysis of Cet in pts who participated in the randomized phase III NCIC CO.20 trial in KRAS WT mCRC patients. Methods: Standard Cet doses ± brivanib were administered. Using intermittent trough blood samples, pop-pK analysis was conducted to evaluate different models. Pts were divided into quartiles according to clearance parameters to assess the exposure-response relationship to response rate (RR), PFS and overall survival (OS). Clinical variables including demographic, laboratory, disease characteristics and co-administration of brivanib were evaluated as co-variates on Cet clearance. Results: In 701 pts, Cet elimination was best described as a one-compartment model with a non-linear saturable elimination process (defined by Vmax and Km). Mean values (± standard deviation) for pop-pk parameters were 2.7 ± 0.5 L/m2 for V, 2.5 ± 0.3 mg/h/m2 for Vmax, and 101.0 ± 0.05 mg/L/m2 for Km. Grouped into quartiles, Vmax and Km were significantly associated with OS, but not RR or PFS. The median OS for pts in the lowest quartile of Vmax was 12.0 ms versus (vs.) 6.9 ms for pts in the highest quartile ( p< 0.001), while the median OS was 11.6 ms in the highest Km quartile vs. 6.9 ms in the lowest Km quartile ( p< 0.001). When compared to the quartile with the combination of highest Vmax and lowest Km, pts in the quartile with the lowest Vmax and highest Km had longer PFS (5.0 vs. 3.7 ms, HR 0.75 (95% confidence interval (CI) 0.58-0.98, p= 0.032) and OS (11.7 vs. 6.6 ms, HR 0.59 (95% CI, 0.45-0.77, p< 0.001). Pts in the lower Vmax and higher Km quartiles also experienced less grade 3 toxicity. Neither clinical variables nor brivanib administration were associated with Cet clearance parameters. Conclusions: For KRAS WT mCRC, standard Cet dosing is not optimal for all pts. Pts with slower Cet clearance have significantly improved PFS and OS. Further studies are needed to optimize Cet doses based on individual pK assessments, and to identify novel factors associated with clearance.