Evaluation of Emergent Mutations in Circulating Cell-Free DNA and Clinical Outcomes in Patients with Metastatic Colorectal Cancer Treated with Panitumumab in the ASPECCT Study

PURPOSE

Mutations in EGFR pathway genes are poor prognostic indicators in patients with metastatic colorectal cancer. Plasma analysis of cell-free DNA is a minimally invasive and highly sensitive method to detect somatic mutations in tumors.

EXPERIMENTAL DESIGN

Plasma samples collected from panitumumab-treated patients in the ASPECCT study at baseline and safety follow-up (SFU) were analyzed by a next-generation sequencing-based approach for extended RAS mutant allele frequency as a continuous variable and their association with clinical outcomes and the mutational prevalence of 63 cancer-related genes. The correlation between patient outcome and baseline mutational status of EGFR pathway genes was also examined.

RESULTS

Overall, 261 patients in the panitumumab arm had evaluable plasma samples. Patients with a higher RAS mutant allele frequency at baseline had worse clinical outcomes than those with a lower frequency (P < 0.001, Cox PH model); however, RAS mutations did not necessarily preclude patients from deriving benefits. The objective response rate (complete or partial response) was 10.8% for patients with baseline RAS mutations and 21.7% for those with BRAF mutations. The 63-gene panel analysis revealed an increase in tumor mutational burden from baseline to SFU (P < 0.001, Wilcoxon signed rank test). Baseline mutations in EGFR pathway genes, when analyzed both categorically and continuously, were associated with shorter survival.

CONCLUSIONS

When mutations in EGFR pathway genes were analyzed continuously, higher mutant allele frequency correlated with poorer outcomes. However, extended RAS mutation, by itself, did not preclude clinical responses to panitumumab in a monotherapy setting.

Impact of Emergent Circulating Tumor DNA RAS Mutation in Panitumumab-Treated Chemoresistant Metastatic Colorectal Cancer

Purpose: The accumulation of emergent RAS mutations during anti-EGFR therapy is of interest as a mechanism for acquired resistance to anti-EGFR treatment. Plasma analysis of circulating tumor (ct) DNA is a minimally invasive and highly sensitive method to determine RAS mutational status.Experimental Design: This biomarker analysis of the global phase III ASPECCT study used next-generation sequencing to detect expanded RAS ctDNA mutations in panitumumab-treated patients. Plasma samples collected at baseline and posttreatment were analyzed categorically for the presence of RAS mutations by the PlasmaSelect-R 64-gene panel at 0.1% sensitivity.Results: Among panitumumab-treated patients with evaluable plasma samples at baseline (n = 238), 188 (79%) were wild-type (WT) RAS, and 50 (21%) were mutant RAS Of the 188 patients with baseline ctDNA WT RAS status, 164 had evaluable posttreatment results with a 32% rate of emergent RAS mutations. The median overall survival for WT and RAS mutant status by ctDNA at baseline was 13.7 (95% confidence interval, 11.5-15.4) and 7.9 months (6.4-9.6), respectively (P < 0.0001). Clinical outcomes were not significantly different between patients with and without emergent ctDNA RAS mutations.Conclusions: Although patients with baseline ctDNA RAS mutations had worse outcomes than patients who were WT RAS before initiating treatment, emergent ctDNA RAS mutations were not associated with less favorable patient outcomes in panitumumab-treated patients. Further research is needed to determine a clinically relevant threshold for baseline and emergent ctDNA RAS mutations. Clin Cancer Res; 24(22); 5602-9. ©2018 AACR.

Abstract A032: Circulating tumor (ct)DNA mutations in EGFR pathway genes and clinical outcomes for patients with metastatic colorectal cancer (mCRC) treated with panitumumab from the ASPECCT study

Background: Mutations in RAS family genes are a negative predictor for response to anti-EGFR therapy. ASPECCT was the first prospective study to show that panitumumab was noninferior to cetuximab for overall survival (OS) in chemorefractory wild-type (WT) KRAS exon 2 mCRC. This analysis used next-generation sequencing of ctDNA before and after panitumumab therapy to explore mutations in EGFR pathway genes and their association with outcomes. Methods: Plasma samples collected at baseline (BL) and safety follow-up (SFU) were analyzed for ctDNA mutations using the PlasmaSelect-R™ 63-gene panel. Mutations for six major genes within the EGFR pathway (BRAF, KRAS, MAP2K1, NRAS, PIK3CA, and PTEN) were analyzed as categorical and continuous variables, and were evaluated for association with OS using univariate Cox proportional hazards (PH) model. Results: Of the 499 patients randomized to panitumumab, 208 had paired plasma samples at BL and SFU. Of the 113 (54.3%) patients who were WT for all genes at BL, 59 (52%) remained WT at SFU. At BL, 65 patients had single gene mutations in either BRAF (7.7%), KRAS (6.7%), PIK3CA (6.3%), NRAS (5.8%), PTEN (3.8%), or MAP2K1 (1.0%), and 30 patients had mutations in multiple genes (at &lt;2% prevalence). The majority of mutations at BL were in BRAF (16.3%), PIK3CA (14.4%), and KRAS (13.5%). At SFU, 62 patients gained single gene mutations in either KRAS (9.1%), BRAF (7.7%), PIK3CA (4.8%), MAP2K1 (3.8%), NRAS (3.4%), or PTEN (1.0%). The majority of mutations gained at SFU were in KRAS (22.6%) and BRAF (21.2%). Categorical analysis of mutations at BL found that patients with WT status for all six genes survived longer, and that KRAS and PTEN WT status had the most significant association to OS. Furthermore, continuous analysis indicated that patients with low mutation levels survived longer (table). Conclusions: Patients were more likely to have a mutation in a single gene at BL and in multiple genes at SFU. Based on the Cox PH model, KRAS had the smallest P value for correlation with OS at BL. Continuous analysis of ctDNA fraction suggests that patients with low-level KRAS mutations may still benefit from anti-EGFR therapy; however, a threshold for treatment benefit cannot be determined from a single-arm trial. These findings suggest that the tumor genetic landscape can become increasingly complex after treatment despite a single selection pressure. Categorical and Continuous Analysis for EGFR Pathway Genes and Median Overall Survival (mOS)*Categorical (Y/N)ContinuousGeneWild-typeMutantLow Risk†High Risk†Any/Sumn (%)113 (54.3)95 (45.7)153 (73.6)55 (26.4)mOS, months (95% CI)38.17 (31.58-48.08)21.92 (19.25-25.67)34.92 (29.25-38.67)18.42 (14.50-21.25)Cox PH P value2.803E-074.185E-09BRAFn (%)174 (83.7)34 (16.3)199 (95.7)9 (4.3)mOS, months (95% CI)31.58 (26.25-36.17)21.25 (15.67-28.67)29.25 (25.25-34.75)18.42 (6.08-21.92)Cox PH P value0.0030.014KRASn (%)180 (86.5)28 (13.5)192 (92.3)16 (7.7)mOS, months (95% CI)31.58 (27.33-36.17)19.92 (12.42-21.83)30.33 (26.67-35.33)15.67 (11.08-21.25)Cox PH P value1.597E-052.953E-06MAP2K1‡n (%)201 (96.6)7 (3.4)N/AN/AmOS, months (95% CI)29.17 (24.67-34.58)14.50 (0.00-43.42)Cox PH P value0.025NRASn (%)188 (90.4)20 (9.6)201 (96.6)7 (3.4)mOS, months (95% CI)29.17 (24.67-34.83)24.25 (16.33-33.67)29.17 (24.75-34.58)15.25 (0.00-45.58)Cox PH P value0.0450.002PIK3CAn (%)178 (85.6)30 (14.4)200 (96.2)8 (3.8)mOS, months (95% CI)29.50 (25.67-35.33)21.83 (15.67-29.42)29.17 (24.75-34.75)21.83 (6.08-33.17)Cox PH P value0.0090.021PTENn (%)190 (91.3)18 (8.7)197 (94.7)11 (5.3)mOS, months (95% CI)30.33 (26.25-35.50)19.75 (6.42-23.92)29.25 (25.25-34.83)15.25 (6.25-21.83)Cox PH P value6.907E-065.715E-05*A limitation of this analysis is the low number of mutants in some cases.†The cutoff was defined as the mutation level where the predicted hazard equals the baseline hazard.‡Insufficient numbers above cutoff to perform continuous analysis.CI, confidence interval; PH, proportional hazards; Y/N, yes/no

Citation Format: Michael Boedigheimer, Agnes Lee Ang, Tae Won Kim, Anne Thomas, Peter Gibbs, Paul Ruff, Kristina Hool, Timothy Price. Circulating tumor (ct)DNA mutations in EGFR pathway genes and clinical outcomes for patients with metastatic colorectal cancer (mCRC) treated with panitumumab from the ASPECCT study [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr A032.

Regional and practice setting differences in the management of EGFR rash among mCRC patients treated with panitumumab: Results of a national survey in the United States

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Background: Skin toxicity can be a limiting factor for the use of anti-EGFR therapies, such as panitumumab, and there are currently no standard practice guidelines for rash management in the United States (U.S.). This study aimed to evaluate if there were regional or practice setting differences in strategies used among oncologists to manage EGFR rash, including utilization of dermatologic and nursing support. Methods: 250 practicing oncologists who had treated at least three mCRC patients with panitumumab in the last year completed an online survey to report their opinions and perceptions regarding skin toxicity management strategies. Participants reported if they were affiliated with an academic/university or a community-based practice. Participants were stratified into years of practice post-fellowship ( < 10 and > 10 years) and geographic region of primary practice (West, Midwest, Northeast, Southern U.S.). Results: Oncologists surveyed did not consistently utilize dermatology support. 40% (n = 99) of practicing oncologists surveyed reported consulting a dermatologist “occasionally.” Less than 5% reported “always” consulting dermatology and 6% reported “never” utilizing dermatology support. Utilization of dermatology support varied significantly by region. In the Southern US more oncologists reported “never” consulting dermatology while in the Midwest more oncologists reported “always” utilizing dermatology support (p = 0.05). While dermatology was inconsistently utilized, oncologists frequently utilized nursing support to minimize and manage anti-EGFR skin toxicity. 73% (n = 182) of oncologists engaged nursing support to “monitor skin toxicity during treatment” and 70% (n = 175) of oncologists had nursing support to “educate on skin toxicity prior to starting treatment.” Conclusions: While nursing support is consistently utilized by oncologists in the management of EGFR rash in mCRC patients treated with panitumumab, use of dermatology support was inconsistent and varied significantly by region. This lack of consistency in toxicity management strategies highlights the need for increased physician education.

Clinical outcomes and emergent circulating tumor (ct)DNA RAS mutations and allele fraction for patients with metastatic colorectal cancer (mCRC) treated with panitumumab from the ASPECCT study

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Background: ASPECCT was a phase III clinical trial performed in the chemotherapy-refractory third-line mCRC setting (N = 1010). This analysis explores the relationship between circulating levels of mutations and clinical outcomes for panitumumab-treated subjects using univariate and multivariate models that treat total mutational load as a continuous measure. Methods: 238 subjects treated with panitumumab had paired plasma samples at baseline and post-treatment (PT). Samples were analyzed for mutations using the Plasma Select-R™ 63-gene panel (0.1% limit of detection). The fraction of mutant RAS reads was evaluated for association with tumor response (by RECIST) and overall survival using univariate and multivariate Cox proportional hazards models. Results: 52% of the subjects who were RAS wild-type by plasma at baseline never developed a RAS mutation. For those with mutant RAS ctDNA ( KRAS+ NRAS) detected at baseline or PT, there was an overall increase in RAS mutant DNA fraction at PT compared to baseline. By non-parametric analysis, there was no difference in the distribution of baseline mutant RAS fraction between those who achieved stable disease (SD) or those with progression ( P = 0.09). There was also no difference in the increase in mutant RAS fraction on therapy between subjects with SD or progressive disease (PD). In addition, RAS mutation was not required for progression: 48% of subjects with PD had no RAS mutant DNA detected. Conclusions: In this exploratory analysis, baseline plasma mutant RAS fraction is an unreliable predictor of subsequent tumor response. Subjects with objective response or SD may have stable or rising levels of mutant RAS DNA. Subjects without any detectable RAS mutation still experience PD. These findings suggest that detectable plasma ctDNA RAS mutations do not necessarily predict response to panitumumab and should be interpreted with caution. Further work is needed to establish clinically relevant and validated thresholds. Clinical trial information: NCT01001377.

Profiling circulating tumor (ct)DNA mutations after panitumumab treatment in patients with refractory metastatic colorectal cancer (mCRC) from the phase III ASPECCT study

3523

Background: ASPECCT was a clinical trial performed in the chemotherapy-refractory third-line mCRC setting (N = 1010). This biomarker analysis explores the mutational landscape in panitumumab monotherapy subjects. Analysis of plasma ctDNA at baseline and post-treatment (PT) by next-generation sequencing provides a snapshot of the main changes in key genes before and after therapy. Methods: CtDNA collected at baseline and PT was analyzed for mutations using the Plasma Select-R™ 63-gene panel (0.1% limit of detection). Gain or loss of mutation was defined at the amino acid level. Net change is the sum of mutations gained minus the sum of mutations lost. A single individual could have both net gain and/or net loss of mutations within a single gene. Results: Significant tumor clonal diversification was observed during therapy. In 238 subjects with paired plasma samples,29% of subjects had multiple mutations in the same gene at baseline and 41% of subjects had multiple mutations in the same gene PT. At least 10% of subjects demonstrated an on-therapy acquired mutation in at least one of the following genes: APC, EGFR, ALK, HER4, TP53, AR, KRAS, BRAF, PDGFRA, STK11, ESR1, FBXWT, and KIT (ordered by frequency). New mutations were noted both inside and outside the EGFR pathway. Unexpectedly, patients with a large decrease in mutant DNA burden after anti-EGFR treatment were also seen. EGFR pathway genes with significant net gain were: KRAS, EGFR, NRAS, BRAF, MAP2K1, PIK3CA, and AKT1. Non-EGFR pathway mutations gained included: APC, CDK6, SMARCB1, FBXW7, TERT, RB1, CTNNB1, and IDH1. Conclusions: This 63-gene plasma analysis suggests that there are significant dynamic changes in clonal mutational fraction under anti-EGFR selection. Our analysis reveals that increasing global tumor heterogeneity is associated with poorer overall survival. A subset of patients demonstrated an overall decrease in tumor heterogeneity on panitumumab therapy (28%), indicating that under anti-EGFR selective pressure mutational heterogeneity can also decrease. Clinical trial information: NCT01001377.

Preemptive versus reactive management of EGFR rash among mCRC patients treated with panitumumab: Results of a national survey of treating oncologists in the United States

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Background: The Skin Toxicity Evaluation Protocol with Panitumumab (STEPP) and the Japan Skin Toxicity Evaluation Protocol with Panitumumab (J-STEPP) were open-label, randomized trials that were designed to evaluate differences in preemptive versus reactive management of dermatologic toxicities associated with panitumumab among patients with mCRC. The objective of this study was to evaluate if there were differences in the knowledge and application of STEPP and J-STEPP among oncologists who self-identified as primarily managing EGFR rash preemptively or reactively. Methods: A total of 250 practicing oncologists who had treated at least three new or continuing mCRC patients with panitumumab in the last year completed an online survey to report their opinions and perceptions regarding the management strategies for mCRC patients who are treated with panitumumab. Preemptive treatment was described in the survey as beginning treatment “prior to the appearance of rash” and reactive treatment was defined as beginning treatment “after signs of rash.” Results: Of the 250 oncologists who participated in this study, n = 58 (23%) reported treating 100% of their patients preemptively and n = 38 (15%) reported treating 100% of their patients reactively. A significantly higher proportion of preemptive treaters than reactive treaters reported following the skin management strategies from STEPP or J-STEPP when managing panitumumab-related skin toxicity (31% vs 13%, p = 0.02). When asked if skin moisturizer, sunscreen, over-the-counter topical steroids, prescription topical steroids, oral antibiotics, topical antibiotics, or UV protective garments were most critical in the preemptive management of panitumumab-related skin toxicity, preemptive treaters more likely to report using oral antibiotics and reactive treaters more like to report using sunscreen (p = 0.05). Conclusions: The results of this survey highlight the wide variability in the management of EGFR-related rash among mCRC patients who are treated with panitumumab and they highlight the need for heighted education among oncologists who treat mCRC patients with panitumumab.

The treatment continuum of panitumumab, cetuximab, and bevacizumab in 1st through 3rd line by KRAS, NRAS, and BRAF mutation status among mCRC patients treated at community cancer centers in the United States

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Background: Our objective was to use real world data to evaluate of the treatment continuum of bevacizumab (Bmab), cetuximab (Cmab), panitumumab (Pmab) or no biologics across 1st, 2nd, and 3rd lines among metastatic colorectal cancer (mCRC) patients who were treated at community cancer centers in the United States. This objective was applied separately to patients with confirmed KRAS, NRAS, or BRAF wild-type (WT) or mutation (MUT) status, as well as patient who did not receive biomarker testing (UNK). Methods: This descriptive study utilized data from the Oncology Services Comprehensive Electronic Records (OSCER) database. It included 5,446 (2,064 WT, 1,807 MUT, and 1,575 UNK) patients diagnosed with mCRC between 1/1/2011 and 8/31/2015. Patients were stratified into the following mutually-exclusive categories in 1st, 2nd line, and 3rd line: Pmab only, Cmab only, Bmab only, or no biologic. Patients who survived and continued therapy were followed through the lines of therapy. Results: 23.8% of WT, 36.3% of MUT and 49.5% of UNK patients did not receive a biologic in 1st line. There were 1,003 WT patients who were treated with Bmab in 1st. Of those, n=587 (58.5%) survived and elected to continue treatment in 2nd line as follows: n=221 (37.6%) continued with Bmab, n=216 (36.8%) initiated Cmab, n=65 (11.1%) initiated Pmab, and n=85 (14.5%) received no biologic in 2nd line. Of the 221 WT patients who received Bmab in 1st and 2nd line, n=129 (58.4%) survived and initiated 3rd line treatment, of which n=20 (15.5%) received Bmab in 3rd line. There were 127/2,604 (4.9%) WT patients who were treated with Pmab in 1st line. Of those 127 patients, n=46 (36.2%) survived and elected to continue treatment in 2nd line as follows: n=11 (29.3%) continued treatment with Pmab in 2nd line, while n=14 (30.4%) initiated treatment with Bmab, n=4 (8.7%) initiated treatment with Cmab, and n=16 (36.4%) did not receive a biologic in 2nd line. Conclusions: A small proportion of patients continued treatment with the same biologic throughout subsequent lines of therapy. Many patients were not treated with a biologic in any line, even if they were confirmed WT.

A survey of medical oncologist’s opinions and perceptions regarding the management of dermatologic toxicities among mCRC patients treated with panitumumab in the United States

639

Background: Dermatologic toxicity can be a limiting factor for the use of anti-EGFR therapy such as panitumumab. There is a paucity of real world data regarding the management of dermatologic toxicity among metastatic colorectal cancer (mCRC) patients treated with panitumumab in the United States (U.S.). The objective of this study is to describe oncologist's opinions regarding timing of skin rash management in relation to the initiation of treatment and perceptions regarding rash management strategies. Methods: A total of 125 oncologists were recruited from a national database via a third independent party. Eligible oncologists (i.e.: licensed and practicing oncologist who had treated at least three new or continuing mCRC patients with panitumumab in the last year) completed an online survey to report their opinions regarding the grade and type of dermatologic toxicities seen and their perceptions about management strategies for mCRC patients who are treated with panitumumab. The timing of rash management initiation was defined as pre-emptive (prior to the appearance of the rash) or reactive (after any signs of skin rash). Results: Based upon their collective experience, oncologists expect that 44% of patients will develop acneiform rash while on treatment. More than half (58%) of the oncologists reported they did not follow any practice guidelines regarding the management of dermatologic toxicities. The oncologists reported that they pre-emptively initiated the management of dermatologic toxicities in 53% of their patients. Skin moisturizer and sunscreen were reported to be the most critical preemptive management approach, while skin moisturizer, over-the-counter topical steroids, and oral antibiotics were reported to be the most critical reactive management tools for Grades 1, 2, and 3, respectively. Conclusions: Despite evidence from randomized controlled trials, a majority of oncologists do not follow guidelines for dermatologic management of EGFR-I rash. There is a clear need for better physician education and awareness of mitigation strategies for skin toxicity management in mCRC patients treated with panitumumab.