Treatment CHIR98014 holiday was not allowed. Median time to progression with first treatment with cetuximab was 10 months, the median interval time between last cycle of first cetuximab-based selleck chemicals therapy and first cycle of the following cetuximab retreatment was 6 months. Moreover, ORR was 53.8% with 19 partial responses (48.7%) and 2

complete responses (5.1%). The median time to progression (TTP) was 6.6 months, stable disease (SD) was obtained in 35.9% of patients and progression in 4 (10.2%), and 18 patients (46.1%) showed the same type of response (SD, partial response or complete response) during cetuximab retreatment when compared with the response obtained during the first cetuximab-based therapy. Then stable disease lasting at least 6 months and partial response during the first cetuximab-based therapy have been demonstrated to predict clinical benefit after cetuximab retreatment [30]. Conversely, a subsequent phase II prospective EGFR inhibitors cancer study, including twenty patients treated with panitumumab after progression on prior cetuximab-based therapy, did not show any response to panitumumab being stable disease (no progression for at least two cycles) the best response in 45% of patients [31]. This study showed that panitumumab has a minimal effect

after disease progression on cetuximab; however, no interval therapy or treatment holiday were permitted between cetuximab and panitumumab administration. Diaz Jr et al. evaluated the variation of circulating tumor DNA (ctDNA) in serum of 24 patient receiving single-agent therapy

with panitumumab. K-Ras mutations were recorded in 38% of cases between 5–6 months following treatment and mathematical modelling indicated that mutations were present in expanded subclones before the initiation of treatment. These results suggest that the emergence Parvulin of KRAS mutations is a mediator of acquired resistance to EGFR blockade [32]. Consistently, another small study showed that point mutations of K-Ras are casually associated with the onset of acquired resistance to anti-EGFR therapy. In fact analysis of metastasis from ten patients who developed resistance to cetuximab or panitumumab showed the emergence of K-Ras mutant alleles were detectable in the blood months before the radiographic documentation of disease progression, and the in vitro model confirmed the hypothesis of continuing mutagenesis under the pressure of anti-EGFR therapy [33]. These studies underlined the possibility of late acquisition of K-Ras secondary mutations under anti EGFR therapy but still do not confute the possibility of a rechallenge.