Background: We previously reported the result of the phase II OGSG1602 study in which single agent of panitumumab (Pmab) demonstrated 76.5% and 50% of disease control rate (DCR), primary endpoint, and response rate (RR), respectively, in chemotherapy-naïve frail or elderly patietns (pts) with wild-type (wt) RAS unresectable colorectal cancer (CRC). Here, we reports the survival analysis including overall survival (OS) and progression free survival (PFS) in terms of sidedness and early tumor shrinkage (ETS). Methods: Thirty-six pts aged ≥76 years, or ≥65 considered unsuitable for intensive chemotherapy were enrolled and received Pmab 6 mg/kg intravenously every 2 weeks. Primary tumors located in the cecum to transverse colon were coded as right-sided tumors (RST), while tumors located from the splenic flexure to rectum were considered left-sided tumors (LST). Early tumor shrinkage (ETS) was determined as tumor reduction of 20% at week 8 compared to baseline. Results: Of total of 36 enrolled pts, 34 pts were included in the efficacy analysis, with pts with LST vs. RST being 26 vs. 8 cases, while pts who achieved ETS (ETS+) vs. those who did not achieve ETS (ETS-) being 15 vs. 19 cases. Among the evaluable 34 pts, the median PFS (mPFS) and median OS (mOS) were 6.0 months (95% Confidence Interval [CI]: 5.4-10.0) and 17.5 months (95%CI, 13.8-24.3), respectively, with the median follow-up of 17.0 months. For PFS, there were no significant differences between pts with LST vs. RST [6.6 months (95%CI, 5.4-11.5) vs. 4.9 months (95%CI, 1.9-NA), p = 0.120] but between pts with ETS+ vs. ETS- [10.4 months (95%CI, 7.4-NA) vs. months (95%CI, 2.1-7.9), p = 0.001]. Furthermore, OS was significantly different either in pts with LST vs. RST [19.3 months (95%CI, 14.2-NA) vs. 12.3 (95%CI, 9.9-NA), p = 0.043] or in pts with ETS+ vs. ETS- [ months (n = 15, 95%CI, 19.6-NA) vs. 10.1 months (n = 19, 95%CI, 6.8-21.8), p < 0.001]. Conclusions: Pmab monotherapy showed the favolable OS in the frail or elderly pts with RAS wt, unresectable CRC. Our data also confirmed the prognostic value of sidedness as well as predictive value of ETS in this setting. Clinical trial information: UMIN000024528.

HR; hazard ratio.