4112 Background: BRAF V600 mt mCRC is an aggressive disease with poor OS under standard chemotherapy. Treatment with doublet and triplet targeted combinations, such as BRAF inhibitor+ antiEGFR+/- MEK inhibitor, has been shown to improve outcomes. Prognostic factors in this targeted treated population remain to be studied. Methods: Prospective international cohort of patients who received doublet or triplet anti-BRAF combinations in clinical trials or as compassionate use. Univariate Cox models for OS were constructed and the strongest predictors in stepwise variable selection were used to develop a prognostic score. The final multivariate model with selected predictors was stratified by prior lines. Results: In total, 42 patients were enrolled. Median age 60.7 y (33-83), 61% female, 61% right-sided tumors, 26% received 2 or more prior chemotherapy lines. One patient (2.6%) achieved complete response and 36% had partial response with median follow-up of 14.3 months. Median progression-free survival was 5.5 months (CI95% 4.4-10.4) and median OS (mOS) was 10.7 months (CI95% 8.4-22.1). In univariate models, ECOG performance status (1 vs 0), CEA levels (high - > 3.5 ng/mL- vs low - < 3.5 ng/mL), CA 19.9 (high vs. low), LDH (high vs. low), number of metastatic sites and presence of liver metastasis were significant prognostic factors. On the other hand, MSI status and peritoneal or nodal metastasis did not associate with outcome. In multivariable model, strongest determinants of OS were ECOG and baseline CEA levels. If high-risk for both factors (ECOG 1 and CEA high, 46% of the patients), mOS was 5.6 months (CI95% 4.2-NA); if intermediate-risk (either ECOG 1 or CEA high, 33%), mOS was 13.5 months (CI95% 10.6-NA); if low-risk (ECOG 0 and CEA low, 21%), mOS not reached (CI95% 16.5-NA). Differences between intermediate- and high-risk prognostic groups compared to low-risk were significant (HR = 5.9, p = 0.03; and HR = 25.9, p < 0.001, respectively). Conclusions: Patients characteristics such as ECOG and surrogates of tumor burden like CEA levels remain important OS determinants in BRAF V600 mt mCRC treated with doublet or triplet targeted therapy. In fact, there are not prognostic scores regarding BRAF mt mCRC treated with targeted therapies. Our study suggests that these prognostic factors may be considered as stratification factors in future clinical trials.

e15164 Background: Refining the selection of pts with HER2-positive mCRC for dual HER2 blockade is a challenge for precision oncology. Alterations in receptor tyrosin-kinase (RTK)/MAPK pathway and HER2 gene copy number (GCN) are promising predictive biomarkers for primary resistance or sensitivity, respectively. Methods: We conducted a multicentric case-control study to evaluate the negative predictive impact of a panel of candidate genomic alterations of primary resistance (PRESSING-HER panel) in pts with HER2-positive, RAS wt mCRC treated with trastuzumab-based dual HER2 blockade. Comprehensive genomic profiling was performed on archival pre-treatment tumor samples and screening sources were: the TRIUMPH trial (Oncomine Comprehensive Assay), the Italian-Spanish observational cohort (Foundation One CDx) and the MSKCC cohort (MSK-IMPACT). The panel grouped resistance alterations with a solid biological rationale as on-target, i.e. HER2 pathogenic mutations or rearrangements or off-target, i.e. mutations/amplifications in RTK/MAPK genes. Primary resistance was defined by PFS < 4 months (mos), sensitivity was defined by PFS ≥4 mos. Hypothesizing a prevalence of PRESSING-HER alterations equal to 5% and 30% among controls and cases, respectively, 35 cases and 35 controls were needed to reject the null hypothesis of equally prevalent alterations, with α and β errors of 0.05 and 0.20. HER2 GCN was derived from NGS and the cut-off was calculated by ROC curve analyses using 4-month PFS rate as endpoint. Results: Sixty-eight pts were evaluable (33 resistant and 35 sensitive). In the overall population, PRESSING-HER alterations were found in 14 (20.6%) pts and included HER2 mutations in 7 (50%), HER2 rearrangements in 4 (29%), BRAF class 1 mutations in 2 (14%) and EGFR amplification in 1 (7%). PRESSING-HER alterations were significantly more frequent in pts with primary resistant (12/21, 57%) versus (vs) sensitive tumors (2/35, 6%; P= 0.004). HER2 GCN had a median value of 34.5 (IQR: 18.7-78.5) and an optimal cut-off of 33. In multivariable analyses including the main baseline clinical prognostic features, PRESSING-HER alterations were independently associated with inferior PFS (median PFS 2.0 vs 5.4 mos; adjusted HR 3.5, 95%CI 1.7-7.0; P< 0.001) and OS (median OS 3.7 vs 14.8 mos; adjusted HR 3.9, 95%CI 1.8-8.3; P< 0.001). HER2 GCN < 33 was associated with significantly inferior median PFS (2.6 vs 5.4 mos; adjusted HR 1.8, 95%CI 1.0-3.1; P= 0.038) and numerically inferior OS (median OS 9.1 vs 15.4 mos; adjusted HR 1.3, 95%CI 0.7-2.5; P= 0.356). The predictive accuracy of PRESSING-HER was 66% and it was increased to 77% when combined with HER2 GCN. Conclusions: The combined assessment of HER2 on/off-target alterations and HER2 GCN stratifies patient outcomes to HER2 dual blockade.

3573 Background: Emerging evidence suggests that WNT mutations and liver/peritoneal metastases in mCRC may have an immunosuppressive role that could affect ICI activity. Aim: To evaluate the impact of WNT alterations and liver/peritoneal metastasis among patients with mCRC treated with ICi. Methods: Patients from Vall d’Hebron Hospital, with mCRC treated with ICI from 2017-2022 were included. WNT alterations (APC, AXIN1/2, CTNNB1, FBXW7, EPHB2, RNF43 and SOX9) were evaluated using NGS (tissue). Clinical outcomes were calculated using survival Kaplan-Meier curves. Patients' characteristics were collected retrospectively. Results: Overall, 104 patients were included (66 MSI patients and 38 MSS patients). Among MSI patients, median age was 63 years (22-95), with 53% female, and 64% of patients received immunotherapy in 1st or 2nd line. Regarding tumor characteristics, 72% were right-sided and 82% harbored WNT alterations. 75% of patients presented with liver/peritoneal metastases. Patients with WNT mutations and peritoneal/liver metastases exhibit lower ORR (46% vs 57% p 0.5 and 45% vs 75%, p 0.03 respectively). Peritoneal/liver metastases were associated with lower PFS (HR 3.6 CI95% 1.27-10.24 p 0.02 respectively). Overall, tumors with WNT pathway alterations tend to have shorter PFS and OS. Liver metastases were associated with lower OS (NR vs 34 months HR 2.49 CI95% 1.01-6.17 p 0.05). Table summarizes outcomes. Among MSS patients, median age was 57 years (41-75), with 25% female and 92% of patients receiving immunotherapy > = 3rd line. Regarding tumor characteristics, 78% were left-sided and 90% harboured WNT alterations. 79%, of patients presented with liver or peritoneal metastases. WNT mutations were not associated with ORR, patients without liver/peritoneal metastases tend to have higher ORR (12.5% vs 3.3% p 0.335). Patients with WNT alterations had worse PFS (9.23 vs 1.87 months, p 0.12), and liver metastases were associated with lower PFS (6.98 vs 1.79 months HR 2.87 CI95% 1.17-7.09 p:0.02). Regarding OS, tumors harboring WNT alterations have shorter OS (8.4 vs 12 months p 0.63). The presence of liver or peritoneal metastases was associated with lower OS (NR vs 7.85 months HR 3.69 CI95% 1.09-12.55 p: 0.04). Conclusions: In our cohort, WNT pathway mutations, and liver metastases were associated with worse ORR, PFS, and OS regardless of MSI status. These findings need further validation in a prospective cohort. [Table: see text]

3560 Background: Currently, there are no well-established biomarkers available to select mCRC patients (pts) who will benefit most from antiangiogenic therapy. RAS mutant (mt) allele fraction in plasma (plMAF) is an independent prognostic factor in mCRC. Preliminary data from our group suggests the possible predictive role of plMAF in RAS mt pts treated with 1st line chemotherapy (ct) +/- bevacizumab (bev). Methods: Data prospectively/retrospectively collected from RASmt mCRC pts who received 1st line ct+/-bev treatment in our center, selecting the subset of pts with plMAF sample evaluable with digital PCR (BEAMing) at baseline. Pts were stratified as high (≥ 5.8%) or low ( < 5.8%) plMAF, based on a previously established prognostic cutoff (Elez et al, Mol Onc 2019). We investigated the associations between clinic-pathological variables, overall survival (OS), and progression-free survival (PFS) stratified by plMAF RAS levels using Cox regression models. OS and PFS were calculated by Kaplan-Meier method. Murine PDX were developed from mCRC pts including models from patients with KRASG12/G13 mutations to explore recapitulation of the clinical findings. Results: From October ‘17 to December ‘21, 102 basal plasma samples were analyzed by BEAMing. 47 pts (46%) were classified as high, 39 pts (38%) as low, and 16 (16%) as no-mt detected by BEAMing (non-shedding). OS was significant longer in low plMAF pts than in high plMAF pts (median OS 15.9 vs 37.1 months (mo); HR 0.43; p = 0.001). In high plMAF pts, a trend towards a better PFS was observed in those pts treated with ct+bev compared to ct alone (median 9.4 vs 6.1 mo; HR 0.6; p = 0.18). No differences were observed in low plMAF pts treated with ct +/- bev (median 14.5 vs 14.9 mo; HR 1.2; p = 0.58). Results were not modified when adjusted by the presence of liver metastases. The multivariate PFS model showed no association between RAS plMAF and clinicopathological variables, except for treatment benefit with ct+bev and better outcomes in pts with resectable liver metastases. Interestingly, human ctDNA in one murine PDX model from non-shedding pt was not detectable, whereas human ctDNA was present in another PDX model with a plMAF of 36.2%. Conclusions: This study suggests that plMAF could be a promising predictive biomarker of response to bev in 1st line RASmt mCRC, but more pts need to be analyzed to confirm this effect. Our results confirm the prognostic role of RASmt plMAF in mCRC. plMAF could partially depend on tumor cell shedding degree and characteristics on the tumor vasculature architecture, this is being investigated in ongoing imaging studies and PDX models. These models will also help to understand the biology behind tumor response to bev and its connection with ctDNA shedding.

3569 Background: Encorafenib plus cetuximab is a standard option in the treatment of BRAF V600E mut mCRC pts pre-treated with at least one systemic therapy. RNF43 is a negative regulator of WNT pathway. A recent study showed that RNF43 mutation is associated with better outcome among pMMR/MSS BRAF V600E mut mCRC patients treated with TT but not in an independent cohort of pts not treated with TT (Elez et al. Nat Med 2022). However, no comparison is available between TT vs CT as second-line (2L) treatment for pMMR/MSS BRAF V600E mut mCRC according to RNF43 mutational status. Methods: The predictive impact of RNF43 mut was evaluated in a real-life dataset of 126 pMMR/MSS BRAF V600E mut mCRC pts treated with TT (consisting of BRAF inhibitor + anti-EGFR antibody ± MEK inhibitor) vs CT ± target agent as 2L treatment. A cohort of 36 pts receiving TT after 2L was also analyzed. Results: Thirty-one (25%) and 95 (75%) out of 126 pMMR/MSS BRAF V600E mut tumours were RNF43 mut and RNF43 wt, respectively. In the RNF43 mut group 14 (45%) received CT and 17 (55%) TT; in the RNF43 wt group, 56 (59%) and 39 (42%) received CT and TT, respectively. Among RNF43 mut pts, those treated with TT reported longer PFS (7.1 vs 3.0 months, HR: 0.35 95%CI: 0.16-0.76, p = 0.006) and higher ORR (42% vs 0%, p = 0.009) than those receiving CT. On the other hand, no significant difference was observed among RNF43 wt patients in terms of PFS (4.3 vs 3.7 months, HR: 0.69 95% CI: 0.45-1.05, p = 0.080) and ORR (28% vs 16%, p = 0.24). However, no significant interaction between treatment effect and RNF43 mutational status was reported in terms of PFS (pinteraction= 0.17) and ORR (pinteraction= 0.96). After excluding 36 pts in the CT group that received TT after 2L, no interaction effect was observed also in terms of OS (pinteraction= 0.53). However, among RNF43 mut pts, those treated with TT reported longer OS (16.5 vs 10.1 months; HR: 0.34 95% CI: 0.11-1.00, p = 0.049), while no significant difference was observed among RNF43 wt pts (10.6 vs 6.6 months; HR: 0.66 95% CI: 0.39-1.11; p = 0.12). In the group receiving TT after 2L, 9 (25%) out of 36 cases were RNF43 mut and achieved higher ORR (78% vs 26%, p = 0.014) and longer PFS (10.1 vs 4 months; HR: 0.35 95%CI: 0.14-0.88; p = 0.020) and OS (11.7 vs 7 months; HR: 0.35 95%CI: 0.15-0.82; p = 0.012) than RNF43 wt (N = 27). Conclusions: pMMR/MSS BRAF V600E mut mCRC patients achieve benefit from TT vs CT independently of RNF43 mutational status, but a higher magnitude of benefit from TT is observed among those with RNF43 mut tumors. These findings deserve confirmation in past and current randomized trials (i.e. BEACON and BREAKWATER).