Phase II Study of Panitumumab Monotherapy in Chemotherapy-Naïve Frail or Elderly Patients with Unresectable RAS Wild-Type Colorectal Cancer: OGSG 1602

Lessons learned: Panitumumab monotherapy showed favorable efficacy and feasibility in the treatment of frail or elderly patients with RAS WT unresectable CRC. It is especially effective for left-sided tumors; therefore, panitumumab as first-line treatment could be an additional therapeutic option for frail elderly patients, particularly in those who are unsuitable for upfront oxaliplatin-based or irinotecan-based combination regimens.

Background: First-line panitumumab monotherapy is expected to be well tolerated and improve survival in patients ineligible for intensive chemotherapy. However, its safety and efficacy in chemotherapy-naïve frail or elderly patients with unresectable RAS wild-type (WT) colorectal cancer (CRC) have not been studied. The aim of this phase II trial was to evaluate the efficacy and safety of panitumumab as first-line treatment.

Methods: We conducted a multicenter phase II study on patients aged ≥76 years or ≥65 years considered unsuitable for intensive chemotherapy. Panitumumab 6 mg/kg of intravenous infusion was administered every 2 weeks. The primary endpoint was disease control rate (DCR). Secondary endpoints included progression-free survival (PFS), overall survival (OS), response rate (RR), time to treatment failure (TTF), and incidence of grade 3 or 4 toxicities.

Results: Thirty-six patients [median age: 81 (range, 67-88) years] were enrolled between February 2017 and August 2018. Two patients were excluded from the analysis of efficacy: one due to lack of image examination at baseline and the other due to lack of a measurable lesion. Thirty-three (91.6%) patients had a performance status (PS) of 0 or 1, whereas two (5.6%) patients and one (2.8%) patient had a PS of 2 and 3, respectively. Twenty-eight patients (77.8%) had left-sided CRC, whereas eight (22.2%) had right-sided CRC. The RR was 50.0% (95% CI, 32.4-67.6), including three patients (8.8%) who had complete responses. 26.5% had stable diseases, resulting in a DCR of 76.5% (90% confidence interval [CI], 61.5-87.7). The RR of patients with left- and right-sided tumors was 65.4% (95% CI, 44.3-82.8) and 0.0% (95% CI, 0.0-36.9), respectively. Major grade 3 or 4 nonhematologic toxicities were rash (n=6, 16.7%), hypomagnesemia (n=4, 11.1%), fatigue (n=3, 8.3%), paronychia (n=2, 5.6%), and hyponatremia (n=2, 5.6%). The only grade 3 hematologic toxicity was neutropenia (n=1, 2.8%).

Conclusion: Panitumumab monotherapy showed favorable efficacy and feasibility in frail or elderly patients with RAS WT unresectable CRC. Survival analysis including OS, PFS and TTF is currently in progress.