Dynamic molecular analysis and clinical correlates of tumor evolution within a phase II trial of panitumumab-based therapy in metastatic colorectal cancer

Abstract

Background: Mutations in rat sarcoma (RAS) genes may be a mechanism of secondary resistance in epidermal growth factor receptor inhibitor-treated patients. Tumor-tissue biopsy testing has been the standard for evaluating mutational status; however, plasma testing of cell-free DNA has been shown to be a more sensitive method for detecting clonal evolution.

Materials and methods: Archival pre- and post-treatment tumor biopsy samples from a phase II study of panitumumab in combination with irinotecan in patients with metastatic colorectal cancer (mCRC) that also collected plasma samples before, during, and after treatment were analyzed for emergence of mutations during/post-treatment by next-generation sequencing and BEAMing.

Results: The rate of emergence of tumor tissue RAS mutations was 9.5% by next-generation sequencing (n = 21) and 6.3% by BEAMing (n = 16). Plasma testing of cell-free DNA by BEAMing revealed a mutant RAS emergence rate of 36.7% (n = 39). Exploratory outcomes analysis of plasma samples indicated that patients who had emergent RAS mutations at progression had similar median progression-free survival to those patients who remained wild-type at progression. Serial analysis of plasma samples showed that the first detected emergence of RAS mutations preceded progression by a median of 3.6 months (range, -0.3 to 7.5 months) and that there did not appear to be a mutant RAS allele frequency threshold that could predict near-term outcomes.

Conclusions: This first prospective analysis in mCRC showed that serial plasma biopsies are more inclusive than tissue biopsies for evaluating global tumor heterogeneity; however, the clinical utility of plasma testing in mCRC remains to be further explored.

Clinicaltrials.gov identifier: NCT00891930.

Keywords: biomarkers and intervention studies; colorectal; gastrointestinal cancers; panitumumab; phase I–III trials.

Figure 1.

(A) Study schema and (B) patient tumor RAS status results determined by tissue and plasma testing biomarker analyses. The RAS status of pre-and post-treatment patient tumor samples were analyzed by each testing method. Total patient samples and the number of patients whose RAS status did (e.g. WT→MT, MT→WT) or did not (WT→WT, MT→MT) change pre- to post-treatment are indicated. The numbers in the central intersection of the Venn diagram represent those cases for whom comparisons of analyses by the three indicated methods and source of samples were available. Samples were categorized as discordant if the results varied between testing methods. BEAM, beads, emulsion, amplification, magnetics; CT, computed tomography; IGF-1 R, insulin-like growth factor receptor 1; MRI, magnetic resonance imaging; MT, mutant; ND, no data; NGS, next-generation sequencing; Q2W, every 2 weeks; Q4W, every 4 weeks; WT, wild-type. ^aBlood collection for plasma analysis at week 37 and every 4 weeks thereafter until disease progression intolerability. ^bBased on first restaging CT or MRI. ^cNine patients were either not doing or unevaluable for tumor response assessment. ^dOnly patients who had an objective response or stable disease after part 1 underwent a second biopsy. ^eA monoclonal antibody that inhibits the insulin-like growth factor 1 (IGF-1 R) pathway. ^fPatients without both before-treatment and after-treatment samples for both tests were not characterized as discordant. ^gPatient was characterized as RAS wild-type at baseline by NGS.

Figure 2.

Plasma testing schema and results. PD, progressive disease; RAS, rat sarcoma; t, time. ^aAll patents included were supposed to have a best response of at least stable disease; however, two patients who were mutant to mutant (progressive disease, n = 1; unevaluable, n = 1) and one patient who was wild-type to wild-type (progressive disease) were analyzed.

Figure 3.

(A) Maximum percentage reduction of the SLD of target lesions in the 30 patients with plasma wild-type RAS mCRC at baseline and evaluable RAS status post-baseline. (B) Time to progression in the 30 patients with wild-type RAS mCRC at baseline and evaluable RAS status post-baseline. (C) Select patient temporal analyses of reductions in SLD during the emergence of RAS mutations as evaluated by plasma BEAMing during treatment with panitumumab + irinotecan. Data from all patients with RAS mutations are shown in supplementary Figure S1, available at Annals of Oncology online. mCRC, metastatic colorectal cancer; PD, progressive disease; RAS, rat sarcoma; SLD, sum of longest diameters.

Figure 3.

(A) Maximum percentage reduction of the SLD of target lesions in the 30 patients with plasma wild-type RAS mCRC at baseline and evaluable RAS status post-baseline. (B) Time to progression in the 30 patients with wild-type RAS mCRC at baseline and evaluable RAS status post-baseline. (C) Select patient temporal analyses of reductions in SLD during the emergence of RAS mutations as evaluated by plasma BEAMing during treatment with panitumumab + irinotecan. Data from all patients with RAS mutations are shown in supplementary Figure S1, available at Annals of Oncology online. mCRC, metastatic colorectal cancer; PD, progressive disease; RAS, rat sarcoma; SLD, sum of longest diameters.