Adjuvant TRastuzumab Deruxtecan Plus Fluoropyrimidine Versus Standard Chemotherapy In HER2-positive Gastric or Gastroesophageal Cancer Patients With Persistence of miNImal Residual Disease in Liquid Biopsy After Pre-operative chemoTherapy and Radical surgerY

Patients affected by localized/locally advanced GC/GEJC (Siewert I-II-III)/esophageal adenocarcinoma eligible for peri-operative chemotherapy and radical surgery and deemed fit enough to receive the standard treatment with the triplet FLOT (5-fluorouracil, oxaliplatin and docetaxel) will be evaluated for the enrollment into the observational phase of the study during the standard pre-operative therapeutic management according to the best clinical practice and local guidelines at their Centre.

OBSERVATIONAL PHASE The observational phase will include patients with localized/locally advanced HER2-positive GC/GEJC (Siewert I-II-III)/esophageal adenocarcinoma eligible for standard pre-operative chemotherapy with FLOT for 4 cycles and radical surgery. Patients will undergo an exploratory liquid biopsy (LB) to be centrally collected at baseline, i.e. during the baseline disease staging as per standard clinical practice and prior to the start of the pre-operative FLOT treatment, and a second exploratory LB to be centrally collected at the end of the pre-operative FLOT treatment before the standard surgery. Then, a third mandatory exploratory LB will be collected at 2-6 weeks after surgery and, during the same timepoint, the interventional liquid biopsy will be collected and centrally analyzed with the SignateraTM CE-marked assay as pre-screening test ® in order to determine whether post-operative ctDNA is positive or negative (LB#1: interventional for the potential enrollment in the TRINITY study). Patients will be permitted to be enrolled in the observational phase even after the start of pre-operative FLOT or even after surgery, the lack of availability of one or both pre-surgical exploratory LBs will not preclude the enrolment in the interventional study and postoperative SignateraTM will be offered as prescreening assay.

Patients eligible for the interventional TRINITY study will be those who underwent surgery after the completion of the standard FLOT pre-operative treatment, with locally-confirmed HER2 positive status both on archival pre-treatment tumor biopsy and on post-treatment surgical sample and with detected post-operative ctDNA at LB#1.

In any case of non-eligibility for the interventional study for not fulfilling the molecular pre-screening algorithm, patients will continue the standard treatment as per clinical practice with post-operative FLOT for 4 cycles and standard follow up. In this case, patients with prescreening failure will continue to be followed-up in the frame of the observational phase of the study with the collection of serial LBs at specific timepoints at 3, 6 and 12 months from the date of pre-screening failure (date of results of LB#1), respectively, and then in concomitance with follow-up visits scheduled as per standard clinical practice.

INTERVENTIONAL TRINITY STUDY Patients deemed eligible for the Screening in the interventional TRINITY trial will be screened from 2 and 12 weeks after the date of surgery, in order to start the post-operative treatment no later than 12 weeks after surgery, and will undergo all the screening tests comprising a CT scan including high resolution thorax-abdomen CT scan and echocardiography.