Outcomes of IBD-associated colorectal cancer and implications in early-onset colorectal cancer

22

Background: Inflammatory bowel disease (IBD) increases the risk of developing colorectal cancer (CRC), and colitis-associated CRC (CA-CRC) mortality is on the rise. It has been postulated that CA-CRC may be contributing to the increasing prevalence of early-onset CRC (EOCRC) but supportive studies are currently lacking. Molecular and clinical differences between CA-CRC and sporadic-CRC (S-CRC) have been reported, however outcomes for CA-CRC remains unclear. Signet ring cell carcinoma (SRC) is a rare subtype of CRC which is seen at higher frequencies, along with mucinous histology, in both CA-CRC and EOCRC. In this study, we validate the association of SRC and mucinous (SRC/M) histology with CA-CRC and EOCRC, and utilize it to estimate the amount of EOCRC attributable to undiagnosed or subclinical IBD. Methods: A retrospective study was conducted using three independent mCRC patient datasets from MDACC. The mATTACC discovery cohort consisted of 32 IBD- and 425 S-mCRC patients enrolled in a prospective biomarker trial. Validation of tumor histology was completed with a tumor registry (n=1696), excluding the MSI-High samples, and a real-world evidence (RWE) cohort from MDACC containing 269 CA-mCRC and 29,596 S-mCRC patients, was used as our validation cohort. Results: In the mATTACC cohort SRC/M histology was found in 37.5% of CA-mCRC and 11.7% of S-mCRC, showing a strong association between SRC/M and CA-mCRC (OR = 4.54, 95% CI: 2.19-9.43). The RWE cohort confirmed the correlation of SRC/M with CA-mCRC (28.6%) relative to S-mCRC (11.4%) patients (OR = 3.13, 95%CI: 2.39-4.09). An association was found between SRC/M and EOCRC (OR = 1.35; 95% CI: 1.24-1.47). By comparing the prevalence of SRC/M in EOCRC and late-onset CRC and correcting by the proportion of CA-CRC cases with SRC/M histology, we estimate that between 8.28% to 10.15% of EOCRC may attributable to undiagnosed/subclinical IBD. Using the RWE cohort, median overall survival was determined to be lower for CA-mCRC (31m) relative to S-mCRC (39m; p=0.007), yielding a HR of 1.26 (95% CI: 1.06-1.48). CA-mCRC patients with EOCRC (25m) were also found to have significantly worse outcomes than S-mCRC patients (40m) with EOCRC (p=0.0005; HR = 1.61, 95%CI: 1.23-2.11). Within CA-mCRC, patients with SRC or SRC/M histology (21m) had decreased OS compared to mucinous histology (51m), indicating the poor prognosis of SRC in CA-mCRC (p=0.028; HR=0.53, 95% CI: 0.3-0.94). Conclusions: Tumor biology consistent with CA-CRC, including SRC/M histology, may be present in 8.3% – 10.2% of patients with EOCRC without a clinical diagnosis of IBD, and harbors worse outcomes. Although other confounding biology may be underlying this association, recognition of undiagnosed IBD in CRC patients, especially those with metastatic disease, is important as it may impact prognosis and treatment strategies for this high-risk patient population.

Phase I/II trial of encorafenib, cetuximab, and nivolumab in patients with microsatellite stable, BRAFV600E metastatic colorectal cancer

12

Background: Encorafenib (E) and cetuximab (C) offers short-lived response and survival benefit for patients (pts) with MSS, BRAFV600E metastatic colorectal cancer (CRC). BRAF + EGFR inhibition induced a transient MSI-H phenotype in preclinical models of MSS, BRAFV600E CRC and may prime these tumors for response to immunotherapy with anti-PD-1 antibodies like nivolumab (N). Methods: In this single-arm, single-institution, phase I/II clinical trial, pts with treatment-refractory MSS, BRAFV600E metastatic CRC were eligible. No prior BRAF inhibitors, anti-EGFR antibody, or immunotherapy was permitted. Pts received E (300 mg PO daily), C (500 mg/m2 IV q14 days), and N (480 mg IV q28 days). The primary endpoints were best overall response (RECIST 1.1) and safety/tolerability (CTCAE v5). A Simon two-stage design (H0: p≤.22; Ha: p≥.45, where p= percentage of pts with radiographic response) was employed using a one-sided α=.05 and β=.20. In the first stage, ≥ 4/15 responses were needed in order for the trial to enroll 11 additional pts. Median progression-free survival (PFS) and overall survival (OS) were estimated via Kaplan-Meier. Results: All 26 pts have been enrolled - 23 patients treated, and 21 evaluable for response so far. Median age is 59 years (range, 32-85), and 14 (54%) are female. No dose-limiting toxicities occurred. Grade 3-4 treatment-related adverse events (AE) occurred in 4/22 (18%) patients. Grade 3 AEs included colitis, maculopapular rash, leukocytosis, and elevated amylase/lipase (all N=1). Grade 4 AEs in a single patient were myositis/myocarditis. Overall response rate is 45% (95% CI, 23-68), and disease control rate is 95% (95% CI, 75-100). Median PFS is 7.3 months (95% CI, 5.5-NA). Median OS is 11.4 months (95% CI, 7.6-NA). For the 9 pts thus far with responses, median duration of response is 8.1 months (95% CI, 7.3-NA). Updated results will be presented. Conclusions: E + C + N is effective and well-tolerated for pts with MSS, BRAFV600E metastatic CRC. The E+C+N regimen met its predefined efficacy endpoint and suggests a role for immunotherapy as a novel combination approach for this specific subpopulation of MSS metastatic CRC. A follow-up randomized phase II trial (SWOG 2107) to evaluate encorafenib/cetuximab with or without nivolumab in pts with MSS, BRAFV600E metastatic CRC will activate in early 2022. Clinical trial information: NCT04017650.

Assessment of HER2 (ERBB2) amplification (HER2amp) using blood-based circulating tumor DNA (ctDNA) next generation sequencing (NGS) and correlation with tissue-based testing in metastatic colorectal cancer (mCRC)

3589

Background: HER2 amplified mCRC has emerged as a unique clinical subset, characterized by resistance to anti-EGFR therapy and response to anti-HER2 strategies. Accurate identification and quantification of HER2amp has predictive value for efficacy of anti-HER2 therapies and appropriate patient selection. Despite availability and use of various tumor tissue-based and blood-based assays for detecting HER2amp, data on cross-performance of these platforms are lacking. Methods: Leveraging a multicenter international consortium (Italy, Japan and USA), we generated a large cohort (N = 353) of mCRC patients (pts), tested for HER2amp using both tissue and blood. Tissue testing was done using immunohistochemistry (IHC), in-situ hybridization (ISH) and (NGS). ctDNA NGS was performed using CLIA-certified Guardant360 ctDNA assay, capable of detecting HER2 copy number (CN) variations. The primary endpoint was to correlate HER2 gene CNs in tissue (tCN) and plasma (pCN). Descriptive statistics, spearman correlation (r) and Fisher’s exact test were used. Results: Baseline tumors characteristics included right-sided primary in 234 (23%), proficient mismatch repair in 264 (98%) and RAS/BRAF wild type (WT) genotype in 194 (67%) pts. Tissue testing was done by IHC, ISH and NGS in 76%, 64% and 74% pts, respectively. A total of 177 pts had HER2amp detected by at least one test: 116 (66%), 157 (89%) and 96 (54%) of which had tissue +, ctDNA +, and both tissue and ctDNA + disease, respectively. Discordant cases consisted of 20 (6%) with positivity in tumor only and 61 (17%) in ctDNA only. Sensitivity, specificity, positive and negative predictive values of ctDNA assay (vis-à-vis tissue) were 83%, 74%, 61% and 90% respectively. Among HER2amp pts, median (range) HER2/CEP17 (ISH) ratio, tCN and pCN were 5.2 (2–12), 11.6 (2–700) and 3.5 (2–122), respectively. The pCN showed strong correlation with ISH ratio (r = 0.69) and tCN (r = 0.68) (P < 0.001). Median pCN differed significantly between pts with HER2 IHC 3+ (12.0), 2+ (2.2) and 0/1+ (2.0) tumors (P < 0.001). High HER2amp (pCN > 4.0) appeared to be enriched with tissue + cases (69% vs 8% [OR 24.6, P < 0.001]), tumor tissue HER2 + status (IHC3+ [75%] vs IHC2+ISH+ [50%] vs IHC2+/ISH- or IHC0/1+ [12%], P < 0.001), HER2 tCN > 6 (79% vs 31% [OR 8.7, P < 0.001]) and RAS/BRAF WT tumors (41% vs 17% [OR 3.5, P = 0.064) but not left sidedness (41% vs 38%; OR 1.1; P = 0.82). Conclusions: In this large diverse cohort of mCRC, we demonstrated correlation of HER2 tCN and pCN obtained by tissue-based and blood-based ctDNA assay. Further prospective efforts are needed to standardize this cross-platform quantification of HER2amp to facilitate robust clinical application of HER2 therapies. This effort shows the value of strategic international partnership in furthering research for rare cancer subsets.

Safety and efficacy of neoadjuvant pembrolizumab in mismatch repair deficient localized/locally advanced solid tumors

2520

Background: Pembrolizumab (Pembro), anti-PD1 therapy, is FDA approved for refractory microsatellite instability high (MSI-H)/deficient mismatch repair (dMMR) advanced/metastatic solid tumors. The robust activity of anti-PD1 therapy in these tumors argues for a neoadjuvant organ-sparing approach. However, the role of anti-PD1 monotherapy in the neoadjuvant setting is unknown. Methods: This is a phase 2 open-label, single center trial (NCT04082572) of MSI-H/dMMR non-metastatic solid tumors with localized unresectable or high risk resectable (defined as ≥ 20% recurrence) with measurable disease per RECISTv1.1 and ECOG PS 0/1. Treatment is Pembro 200mg every 3 wks for 8 cycles (6 months) followed by surgical resection with option to continue therapy for 18 cycles (12 months) followed by observation. First restaging is at 6 wks and includes baseline and 3-week 70-gene ctDNA assessment. To continue on study, patients are required to have PR/CR, SD with tumor shrinkage or SD with decline in ctDNA [highest variant allele frequency (VAF) baseline mutation]. The co-primary endpoints are safety and pathological complete response (pCR). Key secondary endpoints are response rate and organ-sparing at one year for patients who declined surgery. Results: Between 12/2019 and 2/2021, 32 pts were enrolled and treated. Enrolment goal of 35 anticipated to be met by 4/2021. Baseline characteristics included 13 females, median age of 63 yrs (range 26 - 91), Lynch syndrome in 12 pts, BRAF V600E mutation in 11 pts. Tumor type included 24 CRC and 8 non-CRC (1 endometrial, 1 gastric, 1 meningeal, 2 duodenal, 1 ampullary, 2 pancreatic). At baseline disease was resectable in 23 (72%). Among 30 evaluable pts, best overall response rate was 77%: 30% CR (n = 9), 47% PR (n = 14), 20% SD (n = 6), 3% PD (n = 1). Only one pt progressed after initial SD of -18%. Median follow-up is 6.1 months (range 0.1 - 14). Among the 6 (20%) pts who underwent surgery, pCR was seen in 3 (50%). A non-operative approach (pembro for 12 months) has been chosen in 15 pts and 1-year organ-sparing was seen in 2/2 evaluable pts. Treatment-related grade 3/4 immune adverse events (TRAE) were seen in 3 (9%) pts: grade 3 immune hepatitis (2) and grade 3 type 1 diabetes (1). Baseline ctDNA was positive in 17 (53%) pts with a median of 4 mutations per pt (1 - 35) and median highest VAF of 0.9% (range 0.3% to 38.2%). Among 26 pts with successful tumor tissue testing, median tumor mutations were 10.5, range 1 to 21 (Oncomine 134 gene panel). ctDNA decline at 3 weeks was seen in 14/17 (82%) patients. Luminal disease was present in 24 pts with endoscopic response of: CR in 13 (54%), major response 1, pending follow-up evaluation 6, not evaluated 3, and no response in 1. Conclusions: Neoadjuvant pembrolizumab is safe with encouraging clinical activity and this data suggests that a non-operative management for dMMR/MSI-H localized solid tumors should receive further investigation. Clinical trial information: NCT04082572.

Trastuzumab deruxtecan in patients with HER2-overexpressing locally advanced, unresectable, or metastatic colorectal cancer (mCRC): A randomized, multicenter, phase 2 study (DESTINY-CRC02)

TPS3620

Background: Trastuzumab deruxtecan (T-DXd) is an antibody–drug conjugate consisting of an anti-HER2 antibody (trastuzumab) linked to a potent topoisomerase I inhibitor (DXd). T-DXd has been approved to treat HER2-positive metastatic breast cancer (United States, Japan, Europe) and advanced gastric cancer (United States, Japan). It is currently being evaluated in other solid tumor types including colorectal cancer. The phase 2 DESTINY-CRC01 study included patients with RAS wild-type mCRC, with median 4 (range, 2-11) prior lines of therapy. Preliminary results in patients with HER2-overexpressing (IHC 3+ or IHC 2+/ISH+) mCRC showed T-DXd treatment (6.4 mg/kg intravenously [IV] every 3 weeks [Q3W]) resulted in a confirmed objective response rate (ORR) of 45.3% (24/53; 95% CI, 31.6%-59.6%) and a median progression-free survival (PFS) of 6.9 months (95% CI, 4.1 months-not evaluable; Siena J Clin Oncol. 2020;38[15]:4000). Activity was also seen in patients treated with prior anti-HER2 therapy. Although 5.4-mg/kg and 6.4-mg/kg doses of T-DXd have shown clinical efficacy in multiple cancer indications, the lower dose has not yet been tested in patients with HER2-overexpressing mCRC. Preliminary data also suggest T-DXd may be active in RAS mutant mCRC, unlike other anti-HER2 therapies. The DESTINY-CRC02 study aims to determine efficacy and safety of T-DXd in patients with HER2-overexpressing, RAS wild-type or mutant mCRC at 5.4-mg/kg and 6.4-mg/kg doses. Methods: DESTINY-CRC02 (NCT04744831) is a multicenter, randomized, double-blind, 2-arm, parallel phase 2 study that will be conducted in 2 stages. Eligible patients (≥18 years; ≥20 years in Japan, Taiwan, and Korea) will have HER2-overexpressing (IHC 3+ or IHC 2+/ISH+) locally advanced, unresectable or metastatic CRC and have previously received chemotherapy, anti-EGFR therapy, anti-VEGF treatment, and/or anti–PD-1/PD-L1 therapy, as clinically indicated. Prior anti-HER2 therapy will be allowed. In stage 1, patients will be randomly assigned 1:1 to receive T-DXd IV Q3W at a dose of 5.4 mg/kg (n = 40; arm 1) or 6.4 mg/kg (n = 40; arm 2). Randomization will be stratified by ECOG PS (0 or 1), HER2 status (IHC 3+ or IHC 2+/ISH+), and RAS status (wild-type or mutant). After stage 1 enrollment is complete, eligible patients in stage 2 (n = 40) will receive T-DXd 5.4 mg/kg until disease progression or other treatment discontinuation criteria are met. The study is actively enrolling and aims to enroll 120 patients across 60 sites. The primary objective is to assess efficacy of T-DXd at the 5.4-mg/kg and 6.4-mg/kg doses, with a primary end point of confirmed ORR by blinded independent central review. Secondary end points include investigator-assessed ORR, PFS, duration of response, disease control rate, clinical benefit rate, overall survival, pharmacokinetics, and safety. Clinical trial information: NCT04744831.

A novel clinical tool to estimate risk of false negative KRAS mutation in circulating tumor DNA testing

3594

Background: Recently, in metastatic colorectal cancer (mCRC), the detection of RAS mutations by circulating tumor (ct) DNA has recently emerged as a valid and non-invasive alternative approach, overall showing a high concordance with the standard tissue genotyping, giving information on response to EGFRi treatment and resistant mechanisms. However, RAS mutations may be missed due to low levels of any ctDNA in the blood (false-negative), and it has been difficult to distinguish this from patients without a RAS mutation in the tumor (true-negative). We propose a methodology that can be applied to multi-gene ctDNA testing panels to accurately distinguish true- and false-negative tests. Methods: 357 subjects with tissue and multi-panel ctDNA testing from MD Anderson (MDACC) were used as a training dataset and 295 subjects from Massachusetts General Hospital (MGH) dataset as the testing dataset. CtDNA panels contained between 65 and 70 genes, allowing evaluation of tumor ctDNA shedding from variant allele fraction (VAF) levels in the plasma from other genes (such as APC and TP53). Based on the relationship between KRAS and the VAFs of other gene, we established a Bayesian model providing a posterior probability of false negative in the ctDNA test, using thresholds of < 5% (low), 5-15% (medium), and > 15% (high). This model was validated on the MGH database. Results: Across both cohorts, 431 patients were ctDNA wild type for KRAS. Of those, 29 had tissue documenting a KRAS mutation for a false negative rate of 8%. The model provides the posterior probability that a KRAS mutation is indeed present in the tissue given the observed values of allele frequencies for other mutated genes in the plasma. In the validation cohort, a predicted low false negative had no false negatives (0/62, 95% CI 0%-5.8%), while a predicted medium false negative rate was associated with 3% false negative (1/32, 95% CI 0%-16%). In contrast, a high predicted false negative rate was associated with 5% false negative (5/100, 95% CI 1.6%-11%). The results demonstrate the ability of our tool to discriminate between subjects with true negative and false negatives, as a higher proportion of false negatives are observed at higher posterior probabilities. Conclusions: In conclusion, our approach provides increased confidence in KRAS ctDNA mutation testing in clinical practice, thereby facilitating the identification patients who will benefit from EGFR inhibition while reducing the risk of false negative tests. Extension of this methodology to NRAS and BRAF is possible, with clinical application enabled by a freely available online tool.

PERSPECTIVE: Tepotinib + cetuximab in patients (pts) with RAS/BRAF wild-type left-sided metastatic colorectal cancer (mCRC) and acquired resistance to anti-EGFR antibody therapy due to MET amplification (METamp)

TPS3616

Background: METamp is a secondary, or co-driving, genetic change in pts with mCRC and acquired resistance to anti-EGFR therapy, which can contribute to disease progression. In EGFR-resistant pts with mCRC and METamp, MET inhibition + an anti-EGFR agent may achieve disease control by targeting emerging MET pathway activation and maintaining EGFR pathway inhibition. Tepotinib is an oral, once-daily, highly selective, potent MET tyrosine kinase inhibitor (TKI), recently approved in the US for NSCLC harboring MET exon 14 skipping. Tepotinib + gefitinib demonstrated improved outcomes in pts with EGFR-mutant METamp NSCLC and acquired EGFR TKI resistance vs chemotherapy (INSIGHT: NCT01982955). In these pts, progression-free survival (PFS) was 16.6 vs 4.2 months (HR = 0.13; 90% CI: 0.04, 0.43) and overall survival (OS) was 37.3 vs 13.1 months (HR = 0.08; 90% CI: 0.01, 0.51). In pts with mCRC and acquired resistance to anti-EGFR antibody therapy due to METamp, tepotinib + anti-EGFR antibody cetuximab may be active and provide an effective therapeutic option. Methods: This Phase II, multicenter, single-arm, open-label study will assess preliminary safety and tolerability, antitumor activity, and explore pharmacokinetic (PK) profiles of tepotinib + cetuximab in pts with RAS/BRAF wild-type left-sided mCRC and acquired resistance to anti-EGFR antibody-targeted therapy due to METamp (NCT04515394). A safety run-in (6–12 pts) will evaluate the recommended Phase II dose of tepotinib to be used in combination with cetuximab (endpoint: dose-limiting toxicities). Enrollment is based on a confirmed advanced left-sided CRC diagnosis ( RAS/BRAF wild-type), documented previous anti-EGFR therapy and acquired resistance on most recent anti-EGFR antibody and METamp confirmed by liquid and/or tissue biopsy. Pts must be ≥18 years old, have ECOG PS of 0/1 and normal organ function. The study will screen sufficient pts to account for setting-specific heterogenecity in reported METamp incidence. Approximately 42 pts are planned to receive study treatment: ̃22 in Cohort A (second-line, outside US) and 20 in Cohort B (≥third-line, US only). Primary endpoint: investigator-assessed objective response (RECIST 1.1). Secondary endpoints are investigator-assessed duration of response (DoR), PFS (RECIST 1.1) and OS, tolerability and safety (NCI-CTCAE v5.0), and cetuximab immunogenicity (measured by antidrug antibody assays at the start and end of treatment). Additional endpoints include assessment of tepotinib and cetuximab PK profiles, and expression of biomarkers of resistance (from blood and/or tissue samples). Retrospective assessment of best overall response, DoR and PFS by an independent review committee may be conducted. No formal statistical hypothesis will be tested in this exploratory study. Clinical trial information: NCT04515394.

Single-arm, phase 2 study of regorafenib plus nivolumab in patients with mismatch repair-proficient (pMMR)/microsatellite stable (MSS) colorectal cancer (CRC)

3560

Background: The role of immunotherapy in the treatment of pMMR/MSS metastatic CRC is not established. A Japanese phase 1b trial in this setting showed the combination of regorafenib (multikinase inhibitor with immunomodulatory activity) plus nivolumab (anti PD-1) had encouraging activity and manageable safety (Fukuoka, 2020). This study further assessed the safety and efficacy of this combination. Methods: Patients (pts) from the US aged ≥18 years who progressed on/were intolerant to standard chemotherapy were enrolled. Regorafenib was given orally, once daily in 28-day (D) cycles (21D on/7D off) plus IV nivolumab 480 mg on D1. Regorafenib starting dose was 80 mg; if well tolerated, it could be escalated to 120 mg in Cycle 2. Primary endpoint was overall response rate (ORR; RECIST 1.1); secondary aims included disease control rate (DCR), overall survival (OS), progression-free survival (PFS), and safety (NCI-CTCAE v5.0 grade). Biomarker analysis was exploratory. Results: 70 pts (59% male) started treatment. At baseline, median age was 57 years (range 34–85), ECOG PS 0/1 was 51%/49%, 67% had liver metastases (mets), and the primary tumor site was right-sided colon in 36% and rectum in 17%. Median number of cycles was 3.0 (range 1–13); 41% of pts escalated regorafenib to 120 mg. Five pts (7.1%) had a partial response (PR) lasting ≥16 weeks (wks) and 22 (31.4%) had stable disease (SD); pts without liver mets had a higher ORR (21.7%). In pts with tumor samples (n = 40), higher baseline expression (IHC) of cytotoxic T cells (CD3+/CD8+/GranzymeB+), Tregs (FoxP3+), and macrophages (CD68+) trended with clinical benefit (PR/SD ≥16 wks/PFS); pts with liver mets had lower expression. Lower plasma levels of biomarkers of vascular biology (e.g. VEGF-D, Ang-2, VWF) trended with longer PFS. Grade (Gr) 3 treatment-emergent adverse events (TEAEs) occurred in 53% of pts and Gr 4 in 10%. Three pts had a Gr 5 TEAE: n = 1 related to the combination (sepsis); n = 1 related to nivolumab only by investigator (sepsis); n = 1 unrelated to treatment (respiratory failure). Most common Gr 3/4 TEAEs: maculopapular rash (14%), fatigue (7%), pneumonia (6%), increased bilirubin (6%). Conclusions: Combination treatment with regorafenib (up to 120 mg/day) and nivolumab (480 mg every 28D) has manageable safety. Efficacy of this combination in the North American population did not emulate results in the Japanese population. Absence of liver mets and expression of specific biomarkers indicate a better response and may warrant further analysis. Clinical trial information: NCT04126733. [Table: see text]

Rarity of acquired mutations (MTs) after first-line therapy with anti-EGFR therapy (EGFRi)

3514

Background: Colorectal cancers (CRC) lacking RAS MTs treated with EGFRi are thought to evolve by a repetitive process of genetic diversification and clonal evolution. Acquired MTs in KRAS, NRAS, BRAF, MAP2K1, and EGFR are known mechanisms of acquired resistance in the EGFRi refractory population. However, the prevalence of MTs in the first line (1L) setting is not well established as most experience with EGFRi has been beyond the 1L setting. Methods: We analyzed paired plasma samples from RAS/BRAF/EGFRWT mCRC patients (pts) enrolled in 3 large randomized phase 3 trials who had been treated with EGFRi and in whom paired baseline (BL) and time of progression (PRO) plasma samples had been collected for sequencing of ctDNA on a platform optimized for very low allele frequencies (Plasma Select-R™ and Resolution Bio™). Prevalence of MTs at BL and PRO from a 1L study (‘203; FOLFOX ± panitumumab) were compared with 2 studies in the third line setting (3L; ‘007; panitumumab + best supportive care [BSC] vs BSC; and 3L; ‘763; panitumumab vs. cetuximab), to assess the frequency of acquired resistance mutations via ctDNA analysis. Results: For pts with available paired plasma samples (n = 112 for ‘203; n = 89 for ‘007; n = 274 for ‘763), acquisition of at least one KRAS, NRAS, BRAF, MAP2K1, or EGFR MT was significantly less common in post-progression samples in the EGFR containing arms of the 1L ‘203 study compared to the 3L ‘763 and ‘007 studies (6.8% vs 50.4% vs 39.6%, respectively; p < 0.001). In the non EGFR containing arms of the ‘203 and ‘007 study, the rate of acquired MTs was 7.5% and 0%, respectively (p = 1). While this difference in the rate of acquired MTs between the EGFR and non EGFR containing arms was statistically significant for the 3L study (p < 0.001) it was not significant for the 1L study. Further, pts on both 3L studies treated with EGFRi who experienced CR, PR or SD acquired more MTs than those who had PD as best response (53.6% vs 33.3%, respectively; p < 0.001). This relationship was not significant in the 1L setting (7.7% vs 0%; p = 1). Subclonal MTs (rMAF < 25%) in KRAS, NRAS, EGFR, BRAF and MAP2K1 were present at BL in 129 pts (27%). Based on the hypothesis that EGFRi is selecting for rare existing mutated cells in the tumor, we would expect expansion of any preexisting subclones in the BL samples. However, in contrast to expectations, these subclones rarely expanded to become clonal at the time of progression (12.4%). Conclusions: In contrast to expectations, acquired KRAS, NRAS, BRAF, EGFR, or MAP2K1 MTs rarely develop after 1L therapy. While selective pressure appears to increase the frequency of acquired MTs in the 3L setting, preexisting subclonal MTs do not appear to be the dominant source of acquired MTs at progression, implying that there may also be a transient mutational process driving resistance rather than expansion of preexisting clones. These findings have significant implications for ongoing and planned EGFRi rechallenge studies.

Clinical outcomes following definitive treatment of young-onset, locally advanced rectal cancer: A single institution experience

e15601

Background: We evaluated demographic, treatment, and survival outcomes of adults age 18 to 49 years treated at our institution with long course chemoradiotherapy (CRT) followed by total mesorectal excision (TME) for locally advanced rectal cancer. Additionally, we compared outcomes between those age < 45 vs. > 45 years. Methods: The records of 219 patients diagnosed with non-metastatic, clinical T3, T4, or node positive rectal adenocarcinoma and treated between April 2000 and November 2017 were reviewed for age, sex, and presenting symptoms; clinical stage and microsatellite stable (MSS)/DNA mismatch repair (MMR) proficiency status; treatments delivered and sequence; pathologic response to pre-operative therapies; and the development of locoregional recurrence (LRR), distant metastasis (DM), and secondary pelvic malignancy. The Kaplan-Meier method and Log-Rank test were used to calculate and compare disease-free survival (DFS) and overall survival (OS) rates from the date of TME. Results: The median age at diagnosis was 44 years (range 19-49) and there was no sex predominance. Rectal bleeding was the most common presenting symptom (91%), with a median time to diagnosis of 5 months. Clinical tumor/nodal categories were T1-2 in 4%, T3 in 87%, T4 in 7%, N0 in 17%, and N1–2 in 80% of patients. MSS/MMR proficient disease was identified in 95% of tumors with status reported (n = 170). CRT followed by TME and post-operative chemotherapy was the most frequent treatment sequence (n = 196), with capecitabine (n = 176) and FOLFOX (n = 115) as the predominant concurrent and post-operative chemotherapies, respectively. Pathologic complete response at both primary and nodal sites occurred in 15% of all cases and 16% of MSS/MMR proficient cases. There was no difference in sex, tumor category, nodal category, MSS/MMR proficiency status, or pathologic complete response, by age ( < 45 years [n = 111] vs. > 45 years [n = 108]). At a median DFS follow-up time of 5.0 years, there were 11 LRR, 40 DM (including 11 DM detected prior to/at time of TME), and 1 synchronous presentation of LRR and DM. The 5-year rate of DFS was 70.4% for age < 45 years and 85.3% for age > 45 years ( P = 0.02). At an OS median follow-up time of 7.5 years, there were 38 deaths. The 5-year rate of OS was 87.7% for age < 45 years and 94.4% for age > 45 years ( P = 0.126). Two patients developed non-rectal pelvic malignancies. Conclusions: The outcomes reported here from one of the largest single-institution series for young-onset, locally advanced rectal cancer could serve as a benchmark to evaluate newer treatment approaches. Rectal bleeding was the leading presenting symptom, with approximately half-year delay from development of symptoms to diagnosis. Most tumors were MSS/MMR proficient. At 5 years’ follow-up time, the DFS rate was lower for patients age < 45 years when compared to those > 45 years. Secondary pelvic malignancies were a rare occurrence.

Serial circulating tumor DNA (ctDNA) monitoring in metastatic colorectal cancer (mCRC) reveals dynamic profile of actionable alterations

3572

Background: Serial ctDNA can measure dynamic changes in disease burden over time, however utility of serial profiling to detect changes in actionable alterations remains unclear. Methods: We evaluated 501 patients with ≥3 serial Guardant360 assays performed between 09/2016 and 11/2020 and compared MSI, fusion, amplification and single nucleotide variant (SNV) detection over time. This comprised 2147 assays with a median of 4 assays per patient (min 3, max 18) occurring an average of 163 days apart (+/- SD of 147 days). Maximum detected variant allele frequency in samples (maxVAF) was assessed for relation to changes in detected alterations as a surrogate for tumor volume. Results: Among 406 patients with assays assessable for MSI-status, 17 (4.2%) had MSI detected. New MSI detection on a subsequent assay always occurred with a rising maxVAF (3/3) that was also ≥0.7%, while loss of detectable MSI between assays always associated with falling maxVAFs (7/7) with 6/7 occurring when maxVAF fell below 0.4%. Fusions were noted in 9/501 (2%) patients. Among 3 patients who lost a detectable fusion, maxVAF decreased in 1 patient and changed ≤0.2% between assays in 2, while 2/3 patients with new fusions had rising maxVAFs and 1 patient had a falling maxVAF. Amplifications were detected in 242/501 patients (48%). While most genes had highly variable amplification detection between assays (9% serially detected), ERBB2 amplifications were more consistent and serially detected in 39% of detected cases (P < 0.0001). New detection of amplifications occurred more commonly in cases with rising maxVAF (OR 11.70, 95% CI 7.61-18.00, P < 0.0001) and loss of detectable amplifications occurred more between samples with falling maxVAF (OR 12.37, 95% CI 8.35-18.66, P < 0.0001). Change in maxVAF correlated with change in number of detected amplifications (r = 0.62, P < 0.0001), but only partially explained changes seen (R2= 0.39). Between serial assays, SNVs changed a median of 0 variants (IQR -1 to 1), however some patients had significant changes (max gain 21/max loss 18). Among 1646 serial time points, 454 (28%) had no change in SNVs, 674 (41%) gained SNVs, and 518 (31%) lost SNVs on subsequent assays. Gains were more common in samples with rising maxVAF (OR 7.76, 95% CI 6.18-9.73, P < 0.0001) while losses were more common when maxVAF fell (OR 6.90, 95% CI 5.47-8.66, P < 0.0001). The correlation between maxVAF change and SNV change was significant (r = 0.29, P < 0.0001), but minimally explained SNV changes (R2= 0.086) and was a much weaker association than noted for amplification changes. Conclusions: We noted significant differences in detection of actionable alterations across serial ctDNA assays. Increased ctDNA volume (higher maxVAF) due to tumor progression may explain some variation over time, but variability also occurs outside these changes, likely reflecting clonal evolution following therapy.

Trastuzumab deruxtecan (T-DXd; DS-8201) in patients (pts) with HER2-expressing metastatic colorectal cancer (mCRC): Final results from a phase 2, multicenter, open-label study (DESTINY-CRC01)

3505

Background: T-DXd is an antibody–drug conjugate of a humanized anti-HER2 antibody bound to a topoisomerase I inhibitor by a cleavable linker. The primary analysis of DESTINY-CRC01 (DS8201-A-J203; NCT03384940), a phase 2, open-label, multicenter study of T-DXd in pts with HER2-expressing mCRC showed promising antitumor activity and a manageable safety profile (cohort A median follow-up [FU], 27.1 weeks; Siena S, ASCO 2020). We present updated longer-term efficacy and safety data. Methods: Pts had centrally confirmed HER2-expressing, RAS wild-type mCRC that progressed after ≥2 prior regimens. 6.4 mg/kg of T-DXd was administered every 3 weeks (Q3W) in 3 cohorts (A: HER2 IHC3+ or IHC2+/ISH+; B: IHC2+/ISH−; C: IHC1+). The primary end point was confirmed objective response rate (ORR) by independent central review in cohort A. Secondary end points were disease control rate (DCR; CR + PR + SD), duration of response (DOR), progression-free survival (PFS), and overall survival (OS). Results: At data cutoff (Dec 28, 2020), 86 pts (A, 53; B, 15; C, 18) received T-DXd. Median age was 58.5 y (range, 27-79), 53.5% were male, and 90.7% had left colon or rectum cancer. Median prior regimens for metastatic disease was 4 (range, 2-11). All pts had prior irinotecan; 30.2% in cohort A had prior anti-HER2 therapy. Median (m) treatment duration (all pts) was 3.0 mo (95% CI, 2.1-4.1; cohort A, 5.1 mo [95% CI, 3.9-7.6]). In cohort A (median FU, 62.4 weeks), confirmed ORR was 45.3% (24/53 pts; 95% CI, 31.6-59.6), DCR was 83.0% (44/53 pts; 95% CI, 70.2-91.9), mDOR was 7.0 mo (95% CI, 5.8-9.5), mPFS was 6.9 mo (95% CI, 4.1-8.7) with 37 (69.8%) PFS events, and mOS was 15.5 mo (95% CI, 8.8-20.8) with 36 (67.9%) OS events. These results are consistent with the primary analysis. Confirmed ORR was 43.8% (7/16 pts; 95% CI, 19.8-70.1) in pts with prior anti-HER2 therapy, 57.5% (23/40 pts; 95% CI, 40.9-73.0) in pts with IHC3+ status, and 7.7% (1/13 pts; 95% CI, 0.2-36.0) in pts with IHC2+/ISH+ status. In cohorts B and C, mPFS was 2.1 mo (95% CI, 1.4-4.1) and 1.4 mo (95% CI, 1.3-2.1); mOS was 7.3 mo (95% CI, 3.0-NE) and 7.7 mo (95% CI, 2.2-13.9), respectively. Treatment-emergent adverse events (TEAEs) of grade (G) ≥3 occurred in 65.1% of pts (56/86); the most common TEAEs were hematologic and gastrointestinal. TEAEs leading to drug discontinuation occurred in 13 pts (15.1%). 8 pts (9.3%) had interstitial lung disease (ILD) adjudicated by an independent committee as related to T-DXd (4 G2; 1 G3; 3 G5). Conclusions: T-DXd at 6.4 mg/kg Q3W showed promising activity and durability with longer-term FU in this pt population. The safety profile was consistent with prior results; ILD continues to be recognized as an important identified risk that requires careful monitoring and intervention as needed. These results support continued exploration of T-DXd in pts with HER2-overexpressing mCRC. Clinical trial information: NCT03384940.

Comprehensive genomic profiling of malignant peritoneal mesothelioma (MPeM) reveals key genomic alterations (GAs) distinct from malignant pleural mesothelioma (MPM)

8557

Background: MPeM is a rare and aggressive cancer with very limited treatment options. Lack of dedicated research has impeded improvements in outcomes. Defining prevalent GAs is a critical unmet need for use of targeted therapies in these patients. Although MPeM is notably distinct from MPM vis-à-vis epidemiologic and clinical attributes, the genomic underpinings of these differences have yet to be established. We aimed at describing a comprehensive genomic profile (CGP) of MPeM in comparison to MPM. Methods: We performed a retrospective comparative analysis between 89 patients with MPeM and 241 patients with MPM (N = 330) who underwent CGP using CLIA certified next-generation sequencing assays. The cohort was generated using mesothelioma patients at MD Anderson Cancer Center (N = 223) and supplemented by additional mesothelioma patients (N = 107) from a publicly available database from Memorial Sloan Kettering Cancer Center, the MSK-IMPACT database. Essential clinicopathological variables were collected. Descriptive statistics, Fisher’s exact and Mann-Whitney tests were used for comparison. Kaplan-meier method and log rank tests were used for overall survival (OS) estimates. Results: MPeM cohort (vs. MPM) had more women (54% vs. 31%, P < 0.001) and younger age at diagnosis (56 vs. 69 years, P < 0.001). Histology was epithelioid, biphasic and sarcomatoid in 86%, 7% and 7% cases, a distribution similar to MPM cohort. At least 1 GA was found in 64 (72% vs. 82% in MPM, P = 0.044) of MPeM patients with a median of 1 (range 1 – 12) (vs. a median of 2, range 1 – 24, P < 0.001) GA per patient. A significantly lower proportion of MPeM patients had ≥ 3 mutations (14% vs. 26%, OR 2.1, P = 0.028) per patient. The most frequent mutations were present in the following genes: TP53 (24%), BAP1 (16%), NF2 (15%), MET (9%) and TRAF7, KIT and PIK3CA (each 6%). MPeM patients harbored more mutations in MET (9% vs. < 1%, P < 0.001) and TRAF7 (6% vs. < 1%, P = 0.02) but fewer mutations in BAP1 (16% vs. 32%, P = 0.003) and CDKN2B (0% vs. 5%, P = 0.041). The most common copy number variations (CNVs: amplifications or deletions) were seen in BAP1, MCL1, SETD2, WT1 (each 2%) and AURKA (1%) genes. Among genes with CNVs, MPeM had a lower rate of deletions in CDKN2A (1% vs. 6%, P = 0.040). Among more common GAs, only BAP1 mutations appeared to be associated with poor OS (45.7 vs. 127.1 months, HR 2.5, 95%CI: 0.6 – 10.1, P = 0.050) in patients with MPeM. Conclusions: In this large cohort with CGP, we identified potential molecular drivers in MPeM and demonstrated key genomic differences between MPeM and MPM. MPeM is frequently driven by GAs involved in cell cycle control, a potentially targetable pathway. Despite this insight from CGP, a large subset of patients do not have actionable GAs and for these patients, further collaborative trans-“omic” research efforts are needed to advance potential therapeutic options.

Tepotinib plus cetuximab in patients (pts) with RAS/BRAF wild-type left-sided metastatic colorectal cancer (mCRC) and acquired resistance to anti-EGFR antibody therapy due to MET amplification (METamp)

TPS149

Background: METamp is a secondary, or co-driving, genetic change in pts with mCRC and acquired resistance to anti-EGFR therapy, which can contribute to disease progression. In EGFR-resistant pts with mCRC and METamp, MET inhibition + an anti-EGFR agent could achieve disease control by targeting emerging MET pathway activation and maintaining EGFR pathway inhibition. Tepotinib is an oral, once-daily, highly selective, potent MET tyrosine kinase inhibitor (TKI). Tepotinib + the EGFR TKI gefitinib demonstrated improved outcomes in pts with EGFR-mutant METamp non-small cell lung cancer and acquired EGFR TKI resistance vs chemotherapy (INSIGHT: NCT01982955). In these pts, progression-free survival (PFS) was 16.6 vs 4.2 months (HR = 0.13; 90% CI: 0.04, 0.43) and overall survival (OS) was 37.3 vs 13.1 months (HR = 0.08; 90% CI: 0.01, 0.51). In pts with mCRC and acquired resistance to anti-EGFR antibody therapy due to METamp, tepotinib + anti-EGFR antibody cetuximab may be active and therefore provide an effective therapeutic option. Methods: This Phase II, multicenter, single-arm, open-label study will assess preliminary antitumor activity, safety and tolerability, and explore pharmacokinetic (PK) profiles of tepotinib + cetuximab in pts with RAS/BRAF wild-type left-sided mCRC and acquired resistance to anti-EGFR antibody-targeted therapy due to METamp. A safety run-in (6-12 pts) will evaluate the recommended Phase II dose of tepotinib to be used in combination with cetuximab (endpoint:dose-limiting toxicities). Enrollment is based on a confirmed advanced left-sided CRC diagnosis, with RAS/BRAF wild-type, documented previous anti-EGFR therapy and acquired resistance on the most recent anti-EGFR antibody and METamp confirmed by liquid and/or tissue biopsy. Pts must be ≥18 years old and have an Eastern Cooperative Oncology Group performance status of 0/1 and normal organ function. The study will screen sufficient pts to account for setting-specific METamp heterogenicity. Approximately 42 pts are planned to receive study treatment: ~22 in Cohort A (tepotinib second-line, outside US) and 20 in Cohort B (tepotinib ≥third-line, US only). Primary endpoint:investigator-assessed objective response (RECIST 1.1). Secondary endpoints are investigator-assessed duration of response (DoR) and PFS (RECIST 1.1), OS, tolerability and safety (NCI-CTCAE v5.0) and cetuximab immunogenicity (measured by antidrug antibody assays at the start and end of treatment). Additional endpoints include assessment of tepotinib and cetuximab PK profiles and expression of biomarkers of resistance (from blood and/or tissue samples). Retrospective assessment of best overall response, DoR and PFS by an independent review committee may be conducted. No formal statistical hypothesis will be tested in this exploratory study. Clinical trial information: NCT04515394.

Utility of circulating tumor DNA in the clinical management of patients with BRAFV600E metastatic colorectal cancer

119

Background: Molecular profiling is critical for oncologists in personalizing treatment decisions for patients (pts) with metastatic colorectal cancer (mCRC). In contrast to archival tumor tissue specimens classically used profiling, sequencing of circulating tumor DNA (ctDNA) is more sensitive at quantifying low mutation allele frequencies and characterize “real time” tumor biology. We assessed the relationship between detection of BRAFV600E mutations in ctDNA and the clinical management of pts with mCRC. Methods: We retrospectively analyzed mCRC patients evaluated at MD Anderson Cancer Center with BRAFV600E mutations on ctDNA. ctDNA was isolated and sequenced for somatic mutations using a 70-gene next-generation sequencing assay (MD Anderson/GuardantHealth LB70 panel). Variant allele frequency (VAF) was characterized as the ratio of mutant reads: total reads for a given gene. BRAFV600E mutations were classified as “clonal” if the relative VAF (rVAF) exceeded 50% of the maximum VAF. “Major” and “minor” subclonal mutations were called for a rVAF of 10-50% and < 10%, respectively. Associations between BRAFV600E clonality and treatment decision were performed using a Fisher’s exact test. Survival outcomes were estimated using the Kaplan-Meier method. Results: 64 patients with mCRC had a BRAFV600E mutation detected in ctDNA. Concordance between tissue and ctDNA for BRAFV600E mutation was occurred in 44/55 (80%) patients with evaluable tumor specimen. There were 9 patients with BRAFV600E mutations identified in the absence of evaluable tumor tissue. Median VAF for BRAFV600E in the ctDNA was 3.6% (interquartile range, 0.50 – 17%). The majority of patients had a clonal BRAFV600E mutation (50/64, 78%). There were 3 (5%) and 11 (17%) patients with major subclonal and minor subclonal BRAFV600E mutations, respectively. Among patients with minor subclonal BRAFV600E mutations, 91% (10/11) had developed resistance to anti-EGFR therapies for management of RASwild-type mCRC. Discordance between tissue and ctDNA BRAFV600E status was associated with minor subclones (odds ratio (OR) 56, p < .0001). Clonal BRAFV600E mutations in the ctDNA were associated with a higher likelihood for treatment with BRAF targeted therapies (OR 5.8, p = .008). Median progression-free survival among 37 evaluable patients was 6.4 months. Conclusions: Reported VAF in the ctDNA served to stratify BRAFV600E according to relative clonality. Lower VAF was linked to acquired resistance to anti-EGFR therapies, whereas higher VAF was associated with receipt of matched targeted therapies for BRAFV600E mCRC. ctDNA technologies for identifying BRAFV600E mutations are feasible and informative for conducting relevant molecular profiling for patients with mCRC.

Real-world genomic and treatment landscape in advanced colorectal cancer identifies treatment differences pre- and post-ctDNA genomic profiling

39

Background: Clinical insights gained from real-world data have led to numerous advances in oncology including new and expanded drug approvals and an understanding of real-world clinical utilization. In this precision oncology age, integrating real-world clinical data with genomic data can lead to further advancements. We aimed to understand the genomic and treatment landscape in advanced colorectal cancer (aCRC) by leveraging a uniquely large and detailed clinical-genomic database. Methods: The GuardantINFORM (Guardant Health) database comprises aggregated commercial payer health claims and de-identified records from over 100,000 individuals with comprehensive ctDNA results (Guardant360). GuardantINFORM was queried for patients (pts) with a diagnosis of CRC. Pts with fewer than two pharmacy claims prior to or after the first ctDNA test were excluded from the regimen analysis. Latest claims data was truncated as of 8/31/2020. Results: 10,845 pts had a total of 13,510 ctDNA tests (1 – 19 tests/pt). The first ctDNA test date was from 06/2014 - 06/2020. The majority of pts had one ctDNA test (86.7%) while 5% had three or more tests. 87.9% of ctDNA tests had at least one genomic alteration identified, with the distribution of alterations consistent with prior reports (Table). 78% of pts had at least two pharmacy claims before and/or after the first ctDNA test. Of those pts with at least one CRC treatment, the most commonly prescribed CRC regimens up to one year prior to the first ctDNA test were FOLFOX +/- bevacizumab (16%, 18%), FOLFIRI +/- bevacizumab (17%, 11%), capecitabine (15%), 5-FU (12%), and regorafenib (5.2%). Anti-EGFR mono and combination therapy was reported in 6% and 16% of pts pre ctDNA testing. Immune checkpoint inhibitor (ICPi) mono and combination therapy was reported in 2% and 0.5% of pts. The most commonly prescribed CRC regimens post first ctDNA test were capecitabine (16%), FOLIRI +/- bevacizumab (15%, 13%), tipiracil and trifluridine (15%), FOLFOX +/- bevacizumab (12%, 14%), 5-FU (11%), and regorafenib (10%). Anti-EGFR mono and combination therapy was reported in 8% and 18% of pts post ctDNA testing. ICPi mono and combination therapy was reported in 5% and 1% of pts. Conclusions: Using a large and uniquely detailed clinical-genomic dataset, we produced results that replicate the observed distribution of ctDNA identified mutations present in aCRC. This genomic information combined with real-world clinical data provides valuable insights into the variety of longitudinal treatments, including before and after comprehensive ctDNA genomic profiling, allowing for detailed outcomes research, especially focused on precision oncology. [Table: see text]

An open-label, phase II study of patritumab deruxtecan (HER3-DXd, U3-1402) in patients (pts) with previously treated advanced/metastatic colorectal cancer (CRC)

TPS157

Background: Patritumab deruxtecan (HER3-DXd; U3-1402) is a novel, investigational antibody drug conjugate comprising an anti-HER3 monoclonal antibody, a tetrapeptide-based linker, and a topoisomerase I inhibitor payload. Ongoing clinical trials of HER3-DXd in pts with metastatic breast cancer or non-small cell lung cancer have shown promising clinical activity and acceptable safety. HER3 (human epidermal growth receptor 3), a member of the tyrosine kinase receptor family, is overexpressed in most CRC tumors and associated with an adverse prognosis. Significant tumor regression with HER3-DXd has been observed in CRC murine xenograft models, regardless of KRAS mutation status. Here we introduce the design of a phase 2 study (U31402-A-U202) that is evaluating HER3-DXd in previously treated pts with advanced/metastatic CRC. Methods: U31402-A-U202 (NCT04479436) is an open-label, multicenter phase 2 study that will enroll 80 pts in the USA, Europe and Asia. Pts are enrolled who are aged ≥ 18 years with advanced/metastatic colorectal adenocarcinoma that is resistant/refractory/intolerant to ≥ 2 prior lines of therapy including a fluoropyrimidine, irinotecan, a platinum agent, an anti-EGFR agent (if clinically indicated), an anti-VEGF agent (unless contraindicated [CI]), and an immune checkpoint inhibitor (unless CI) for microsatellite instability-high CRC. Pts with current/previous interstitial lung disease or clinically severe pulmonary compromise are excluded. Archival tumor biopsy and pre-treatment tumor biopsy are collected from all pts at screening, with HER3 protein expression measured by immunohistochemistry (IHC). In part 1, results of the HER3 IHC assay from the pre-treatment tumor biopsy are used to assign pts into 1 of 2 cohorts (C). C1: HER3 high (IHC 3+, 2+), n = 24; C2: HER3 low/negative (IHC 1+, 0), n = 12. Pts receive 5.6 mg/kg HER3-DXd IV every 3 weeks. An interim futility analysis will be conducted separately for C1 and C2 and will determine enrollment in part 2, with 2 potential scenarios: enrollment continues irrespective of HER3 IHC status, or enrollment continues in HER3 high pts only. The primary objective is the evaluation of the antitumor activity of HER3-DXd as measured by objective response rate (ORR) (assessed by BICR according to RECIST v1.1). ORR will be summarized with the 2-sided 95% confidence interval. Secondary objectives include the evaluation of efficacy as measured by ORR (assessed by investigator according to RECIST v1.1), duration of response, time to tumor response, disease control rate, progression-free survival (assessed by investigator and BICR according to RECIST v1.1), overall survival, safety and tolerability, HER3 protein expression in tumor tissue and relationship with efficacy, and pharmacokinetic properties. Clinical trial information: NCT04479436.

Zanidatamab (ZW25) in HER2-positive biliary tract cancers (BTCs): Results from a phase I study

299

Background: Treatment options are limited for patients with unresectable, locally advanced or metastatic BTCs progressing after first line treatment. Standard second line chemotherapy yields objective response rates (ORR) of < 10% and median overall survival of these patients is < 6 months. Human epidermal growth factor receptor 2 (HER2) overexpression/ amplification is observed in 5–19% of BTCs. Zanidatamab is a bispecific HER2-targeted antibody that has demonstrated durable single agent activity with good tolerability in a range of HER2-overexpressing cancers. Methods: In the expansion cohort of this phase I study (NCT02892123), the primary objective is to characterize safety and tolerability of zanidatamab and secondary objectives include evaluation of anti-tumor activity. This cohort includes BTC patients with centrally confirmed HER2 overexpression (immunohistochemistry [IHC] 3+ or IHC 2+/ fluorescence in situ hybridization [FISH]+), disease progression after standard of care therapy, and measurable disease per RECIST 1.1. Zanidatamab is administered at the previously identified recommended dose of 20 mg/kg every 2 weeks (Q2W). Tumors are assessed every 8 weeks (response confirmed at ≥ 4 weeks). Results: As of the data cutoff date (Jul 28, 2020), 20 patients (median age: 63 years [range, 42–78]) with BTC (11 gallbladder cancers, 5 intra- and 4 extra-hepatic cholangiocarcinomas) have been treated with zanidatamab. The median number of prior systemic therapies was 2.5 (range, 1–8), including five patients who had received prior HER2-targeted therapy (trastuzumab). Fourteen (70%) patients experienced zanidatamab-related adverse events (AEs), all of which were grade 1 or 2 in severity. The most common (occurring in ≥ 20%) zanidatamab-related AEs were diarrhea (n = 9) and infusion-related reactions (n = 6). A single treatment-related serious AE of grade 2 fatigue was reported in one patient. Among patients evaluable for response (n = 17), the confirmed ORR was 47% (n = 8; 95% confidence interval [CI]: 23, 72), the disease control rate was 65% (n = 11; 95% CI: 38, 86) and the median duration of response was 6.6 months (95% CI: 3.2, not estimable). Conclusions: Zanidatamab is well tolerated with promising and durable anti-tumor activity in patients with HER2 overexpressing BTC. Based on these data, zanidatamab is now being evaluated in an ongoing global Phase 2b study in patients with advanced HER2+ BTC that have progressed after treatment with a gemcitabine-containing regimen (NCT04466891). Clinical trial information: NCT02892123.

A phase II trial of atezolizumab and bevacizumab in patients with relapsed/refractory and unresectable malignant peritoneal mesothelioma

9013

Background: Malignant peritoneal mesothelioma (MPeM) is an orphan malignancy. No recommended/FDA approved therapies exist for salvage treatment beyond first-line platinum and pemetrexed based chemotherapy. While immune checkpoint inhibition has shown preliminary efficacy in mesotheliomas, data and efficacy is limited in MPeM patients (pts) [objective response rate (ORR) ~ 11%; median progression-free survival (mPFS) ~ 4 months (m); median overall survival (mOS) ~ 11 m]. We aimed to prospectively assess the safety and efficacy of combined anti-PD1 (atezolizumab) and VEGF (bevacizumab) blockade (AtezoBev) in pts with MPeM. Methods: In this phase 2 study, eligible pts with histologically confirmed MPeM, ECOG PS 0-1, and prior platinum and pemetrexed treatment were treated with 1200 mg of atezolizumab and 15 mg/kg of bevacizumab IV every 21 days until disease progression, unacceptable toxicity, or withdrawal. Primary endpoint was confirmed ORR by RECIST 1.1 by independent radiology review. Duration of response (DOR), PFS and OS were pre-specified secondary endpoints. Results: Among 20 enrolled pts (3/2017 - 2/2019), median age was 63 (range, 33-87) years, 12 (60%) were female, 12 (60%) had PS 0, and 2 (10%) had biphasic MPeM. Among 20 evaluable pts (median cycles 14), confirmed ORR was 35% (7 pts; 95% CI: 15.4-59.2) (median DOR 8.8 m). Responses were ongoing in 5/7 (71.4%) pts at data cutoff. The median follow-up was 20.5 months. Six deaths were observed during follow-up, and the 1-year OS was 79% (95% CI: 52 – 91) (median OS ~ NR). Median PFS was estimated as 17.6 m (95% CI: 9.1 – NR). The 1-year PFS was 54% (95% CI: 28 – 74). Grade 3 (no grade 4/5) treatment-emergent adverse events occurred in 10 (50%) pts; most common being hypertension (40%) and anemia (10%). Two (10%) pts had grade 3 immune-related adverse events. Translational studies are ongoing. Conclusions: AtezoBev showed promising and durable efficacy in relapsed/refractory MPeM with acceptable safety profile. Ongoing multiomic analyses of pre and on-treatment tissue/liquid biopsies obtained on all these pts will provide additional insight into mechanisms and biomarkers of response and resistance. Clinical trial information: NCT03074513.

Final results of a randomized, open label, perioperative phase II study evaluating nivolumab alone or nivolumab plus ipilimumab in patients with resectable HCC

4599

Background: In resectable hepatocellular carcinoma (HCC) surgical resection is associated with high recurrence rates. However, there is no approved neoadjuvant or adjuvant therapies yet. Neoadjuvant immunotherapy effect has never been reported in this setting in HCC. Methods: This is a randomized phase II trial of nivolumab (Arm A) or nivolumab + ipilimumab (Arm B) as peri-operative treatment for patients (pts) with HCC who are eligible for surgical resection. Pts in Arm A are given nivolumab 240 mg iv, every 2 weeks (wks) for a total of 3 doses followed by surgery on week 6. Pts in Arm B are treated with nivolumab per same schedule as arm A plus concurrent ipilimumab 1 mg/kg on day 1. Adjuvant part of study starts 4 weeks after surgery, with Nivolumab at 480 mg iv every 4 weeks for 2 years in arm A. Pts in Arm B are treated with nivolumab per same schedule as arm A plus concurrent ipilimumab 1 mg/kg every 6 weeks times 4 doses after resection. The primary objective was the safety/tolerability of nivolumab +/- ipilimumab. Secondary objectives include overall response rate, pathologic complete response (pCR) rate and time to progression. Exploratory objectives include evaluating the pre- and post-treatment immunological changes in tumor tissues and peripheral blood. Results: 30 patients were enrolled, 2 patients withdrew consent, one patient was not eligible at time of therapy, and 27 randomized (13 to Arm A and 14 to Arm B). 21 patients proceeded with resection as planned and surgery was aborted for 6 patients; 1 for frozen abdomen due to old surgery, 2 for small residual volume, and 3 for progressive disease. Pts age ranged between 32-83 yo, 75 % were males, 7 pts had HCV, 7 had HBV and 7 had no hepatitis. Pathologic complete response (pCR) was observed in 5/21 pts (24% pCR rate) – 2 in Arm A and 3 Arm B, and 3/21 pts (16%) – 1 in Arm A, 2 in Arm B, achieved major pathologic response (necrosis effect of 50-99%). 5 patients in Arm B and 1 in Arm A experienced grade 3 or higher toxicity prior to surgery. No grade 4 or higher toxicity were observed and surgery was not delayed or cancelled due to oxicity. Conclusions: Our study reached its primary endpoint of safety. Importantly, we report a 40% pathologic response rate = pCR rate of 24%, and major necrosis rate of 16% for resectable HCC after preoperative immunotherapy in a randomized phase II pilot trial. After future validation, these promising results may contribute to a paradigm shift in the perioperative treatment of resectable HCC. Clinical trial information: NCT03222076 .

Clinical and pathologic factors associated with survival in BRAFV600E colorectal cancers

4047

Background: BRAFV600E mutations occur in fewer than 10% of all patients (pts) with metastatic colorectal cancer (mCRC) and arise from sessile serrated adenomas. Despite efficacy with targeted therapies against MAPK signaling and with immunotherapies in this population, survival outcomes for pts with BRAFV600E mCRC in general are poor. Characteristics distinguishing pts with BRAFV600E mCRC with favorable versus unfavorable outcomes have not been well annotated. Methods: Records of 188 pts with BRAFV600E mCRC evaluated at MD Anderson Cancer Center between 3/2010-1/2020 were reviewed. Pts with the shortest and longest metastatic survival (N = 25 for each group) were compared. Associations between prognostic group and clinical/pathologic features were measured by odds ratio and for median survival by log-rank testing. Results: Median metastatic survival differed between the 2 BRAFV600E mCRC populations (8.6 vs 84 months, p < .0001). Pts with poor survival more commonly had primary tumors arising from the hepatic flexure/proximal transverse colon (44% vs 16%, p = .04) and more frequent hepatic involvement (75% vs 28%, p = .001). Pts with favorable survival were more likely to develop metachronous metastases (52% vs 16%, p = .01), have fewer distant organ involvement (median 1 vs 2, p = .02), and undergo definitive locoregional therapy to metastatic disease (44% vs 0%, p = .01). Microsatellite instability (36% vs 4%, p = .008) and a history of tobacco use (44% vs 16%, p = .04) were associated with a favorable prognosis. Durable responses to MAPK-targeted therapies (5/25) and immunotherapy (3/25) were noted in the favorable group. Conclusions: Pts with BRAFV600E mCRC can achieve excellent long-term survival which belies conventional context and is driven by locoregional and systemic treatment options alike. Anatomic localization of the primary tumor and prior exposures may highlight environmental influences on tumor biology which account for the clinical heterogeneity of pts with BRAFV600E mCRC.

A phase II, multicenter, open-label study of trastuzumab deruxtecan (T-DXd; DS-8201) in patients (pts) with HER2-expressing metastatic colorectal cancer (mCRC): DESTINY-CRC01

4000

Background: T-DXd is an antibody-drug conjugate composed of an anti-HER2 antibody, cleavable tetrapeptide-based linker, and topoisomerase I inhibitor payload. Early studies have shown promising activity in advanced HER2-expressing tumors. DESTINY-CRC01 (DS8201-A-J203; NCT03384940) is a phase 2, open-label, multicenter study of T-DXd in pts with HER2-expressing mCRC. Methods: Pts with centrally confirmed HER2-expressing, RAS–wild type mCRC that progressed on ≥ 2 prior regimens received T-DXd 6.4 mg/kg every 3 weeks (q3w) in 3 cohorts (A: HER2 IHC 3+ or IHC 2+/ISH+; B: IHC 2+/ISH−; C: IHC 1+). The primary endpoint was confirmed objective response rate (ORR) by independent central review in cohort A; secondary endpoints included, disease control rate (DCR; CR + PR + SD), duration of response (DOR), progression-free survival (PFS), overall survival (OS), and ORR in cohorts B and C. Results: At data cutoff (Aug 9, 2019), 78 pts (A, 53; B, 7; C, 18) had received T-DXd. Median age was 58.5 y (range, 27-79 y), 52.6% of pts were male, and 89.7% had left colon or rectum cancer; median number of prior regimens was 4 (range, 2-11); all pts had prior irinotecan. Median treatment duration was 3.5 mo (95% CI, 2.1-4.3 mo; cohort A, 4.8 mo [95% CI, 3.9-5.8 mo]); 38.5% of pts remained on T-DXd treatment. The confirmed ORR was 45.3% (24/53 pts; 95% CI, 31.6%-59.6%) in cohort A, including 1 CR and 23 PRs; median DOR was not reached (95% CI, 4.2 mo-NE). The ORR in pts with prior anti-HER2 treatment was 43.8% (7/16 pts; 95% CI, 19.8%-70.1%). The DCR was 83.0% (44/53 pts; 95% CI, 70.2%-91.9%); median PFS was 6.9 mo (95% CI, 4.1 mo-NE); median OS was not reached. No responses were observed in cohorts B or C. Grade ≥ 3 treatment-emergent adverse events (TEAEs) occurred in 61.5% of pts (48/78); the most common (≥10%) were decreased neutrophil count (21.8%) and anemia (14.1%). Seven pts (9.0%) had TEAEs leading to drug discontinuation. Five pts (6.4%) had interstitial lung disease (ILD) adjudicated by an independent committee as related to T-DXd (2 grade 2; 1 grade 3; 2 grade 5 [the only drug-related deaths]). Conclusions: Overall, T-DXd 6.4 mg/kg q3w demonstrated remarkable activity in pts with HER2-expressing mCRC refractory to standard therapies, with a safety profile consistent with previous results. ILD is an important risk and requires careful recognition and intervention. Clinical trial information: NCT03384940 .

A randomized study evaluating tailoring of advanced/metastatic colorectal cancer (mCRC) therapy using circulating cell-free tumor DNA (ctDNA) (TACT-D)

TPS277

Background: Identifying non-responders to expensive salvage therapies with modest benefits and substantial treatment related adverse events (TRAEs) (e.g. regorafenib/TAS102 in mCRC) is necessary to maximize benefits and limit toxicities. Serial ctDNA sequencing is reliable for tracking tumor dynamics and appears to predict resistance to therapy earlier than radiographic progression. Methods: TACT-D is a randomized study (N = 100) to validate the ability of changes in ctDNA (ΔctDNA) to predict resistance early and in limiting toxicities. We hypothesize that increase in ctDNA (measured by variant allele fraction) at 2 weeks (wk) into treatment can predict resistance earlier than standard radiographic means [at 8-12 wk] and detecting resistance early can enable prompt change in therapy resulting in reduction of TRAEs. Pts with mCRC eligible for either regorafenib/TAS102 are randomized 2:1 to either standard of care (SOC) or ctDNA arm. On SOC arm, treatment is given as per current paradigm i.e. for 8 wk and then restaging. On ctDNA arm, decision to continue therapy is based on ctDNA change between baseline and 2 weeks [ΔctDNA = ctDNA (C1D15 – C1D1)]. Increase in ctDNA triggers early radiographic staging (4 wk). Treatment is continued for disease stability/regression and discontinued for progression. Study has 2 co-primary endpoints: 1) Association of Δ ctDNA and radiographic progression [62 pts on SOC arm, have 94% power (2-sided α 0.05) to detect difference of 95% vs. 58% in progressive disease between pts with increase vs decrease in ctDNA] and 2) Compare proportion of pts experiencing TRAEs within 4 months between study arms [67 in SOC arm and 33 in ctDNA arm have 82% power (2-sided α 0.05) to detect a 30% decrease in toxicity]. Key secondary endpoints include: patient-reported outcomes (MD Anderson Symptom Inventory and PRO-CTCAE), OS, clinical events of special interest (hospitalizations/ER visits/medical interventions such as blood transfusions/IV hydration), clinical trial referral and cost effectiveness. Study is now actively accruing pts (NCT03844620). Funding: MD Anderson Cancer Center, Houston, TX & Guardant Health Inc., Redwood City, CA. Clinical trial information: NCT03844620.

Female representation in clinical trials leading to FDA cancer drug approvals for gastrointestinal (GI) cancers between 2008 to 2018

809

Background: Proportionate representation of women in health research is an area for improvement. This study aims to assess the representation of women in gastrointestinal (GI) cancer clinical trials leading to FDA cancer drug approvals over the past 10 years. Methods: FDA cancer drug approvals between 07/2008-06/2018 were identified and trial reports supporting approvals sourced. The ratio of female to male (F:M) enrollment was compared with F:M cancer incidence in the U.S., and U.S. cancer prevalence and mortality. Results: Although F:M enrollment for all 229 trials leading to FDA cancer drug approvals in this period was similar to overall F:M cancer incidence in the U.S. (0.89 vs 0.86; Odds Ratio for female enrollment (OR) 1.05, 95% Confidence Interval (CI) 1.03-1.06, P<0.0001), in 17 trials that led to drug approvals in GI cancers there was lower F:M trial enrollment compared to cumulative U.S. incidence at those tumor sites (0.55 vs 0.79, OR 0.71, 95% CI 0.68-0.74, P<0.0001). F:M enrollment and U.S. incidence by the main GI tumor sites where approvals occurred is shown in Table. Female enrollment rates were also lower than U.S. female cancer mortality and prevalence rates in these tumor sites (P<0.0001 for all). Female enrollment in GI trials fell between 2008-2013 and 2014-2018 (38 vs 33%, OR 0.80, 95% CI 0.74-0.87, P<0.0001). Conclusions: Although disparity in female enrollment may be improving across combined FDA cancer drug approval trials, underrepresentation of females has persisted in GI cancer trials when compared to F:M cancer incidence, prevalence and mortality in the U.S. More work is required to determine the drivers of this disparity, in order to mitigate it. [Table: see text]

A phase II study of durvalumab (MEDI4736) (anti-PD-L1) and trametinib (MEKi) in microsatellite stable (MSS) metastatic colorectal cancer (mCRC)

152

Background: Monotherapy with immune checkpoint blockade (ICB) is ineffective for patients (pts) with MSS mCRC. Novel approaches to modulate the tumor microenvironment (TME) are needed. Here, we investigate whether the combination of trametinib (T) with durvalumab (D) can alter the immune TME by successfully priming and activating T-cells. Methods: An open-label, single center phase II trial with primary endpoint of immune-related response rate for T+D in refractory MSS mCRC pts (NCT03428126). T is 2mg/day orally starting 1 week prior to D, which is given 1500mg intravenously every 4 weeks. Dose de-escalation strategy performed to identify maximum tolerated dose (MTD). Simon 2-stage design utilized with plans to enroll 29 pts into the first stage, requiring response in 2 or more pts to proceed to stage 2 (n = 15). Results: Demographics for 29 treated pts: 48% female, median age 48 years (range 28-75), and median prior therapies was 2 (range 1-5). No grade (G) 4 treatment-related adverse events (TRAE). The most common G3 TRAE included autoimmune hepatitis (14%) and acneiform rash (10%). G1/2 TRAE included acneiform rash (69%), fatigue (24%) and anemia (21%). No fatal TRAE and 4 pts discontinued treatment due to TRAE. 1 of 29 pts had confirmed partial response (PR) lasting 9.3 months (mo) for an ORR of 3.4%. This pt had an ATM E221fs*14 mutation. 7 pts had stable disease (SD) with median time to progression (TTP) of 5.4 mo (range 3.9-9.3 mo). 1 pt remains on active therapy with SD ( > 10 mo). 5 pts (1 PR, 4 SD) demonstrated decrease in total CEA ng/mL (best percentage reduction: 94%, 95%, 42%, 34% and 21.6% respectively). Median TTP for the entire cohort was 3.2 mo (range 1.1-9.3 mo). Consensus molecular subtypes (CMS) were performed on the primary CRC in 23 pts: 12 CMS2, 2 CMS3, and 9 CMS4. 4 SD pts were CMS2, 1 SD pt was CMS4 and the CMS status of the pt with a PR was unknown. Conclusions: The combination of T+D did not meet efficacy criteria to proceed to the second stage of the study. Analysis of 15 paired on-treatment biopsies is ongoing and will be presented. Utilization of CMS characterization in mCRC clinical trials is feasible and may provide an improved biological understanding of treatment activity. Clinical trial information: NCT03428126.