Overview

This phase II study will evaluate dostarlimab with a watch-and-wait approach for patients with localized mismatch repair deficiency (dMMR)/microsatellite instability (MSI) gastric or oeso-gastric junction adenocarcinoma.

The goal of the study is to determine whether the surgery could be avoided in patients with localized dMMR/MSI-H gastric/OGJ adenocarcinoma with complete response at endoscopy and biopsies free of tumoral cells after treatment with dostarlimab, with a watch-and-wait approaches.

Eligibility

Inclusion Criteria:

1. Capable of giving signed and dated informed consent,
2. An ECOG PS of 0-1,
3. ≥18 and ≤75 years old,

   The patient over 75 years of age is eligible only if all the following conditions are

   met

   • The patient's G8 questionnaire score is above 14 AND
   • The patient is eligible for surgery and has no contraindications to repeated UGI endoscopy with biopsies,

4. Histologically proven non-metastatic gastric or OGJ adenocarcinoma cT2 to T4, Nx, M0

   after computed tomography thorax-abdomen-pelvis (TAP-CT) and echo-endoscopy (EUS) according to the 7th Edition of the International Union Against Cancer; NB: Echo-endoscopy will be performed only if the tumor is not obstructive at UGI endoscopy ± a new UGI endoscopy with 10 biopsies, photos (if not done at the first UGI endoscopy done for diagnosis) and if possible (not mandatory) tumor tattooing/inking. If obstructive, the tumor will be classified as cT3 or cT4 (in the situation when the tumor was obstructive and prevented EUS, it was classified T3N+, if it did not invade the adjacent organs on CT scan, because obstructing tumours represented locally advanced disease in the vast majority of cases in previous studies). In this case a new UGI endoscopy must be done with 10 biopsies, photos (if not done at the first GGI endoscopy done for diagnosis) and if possible (not mandatory) tumor tattooing/inking to follow the location of the tumor.

5. No peritoneal carcinomatosis (optional coelioscopy; recommended in case of doubt/ suspicious on CT/ imaging),
6. No prior therapy (chemotherapy, radiotherapy, or immunotherapy) for localized gastric or OGJ adenocarcinoma,
7. Tumor status confirmed to be dMMR/MSI-H as follows:
   • MMR protein expression status will be evaluated by immunohistochemistry (IHC) with four antibodies (anti-hMLH1, anti-hMSH2, anti-hMSH6, anti-hPMS2) according to the local procedures. dMMR will be defined as loss of MLSH1 and PMS2, loss of MSH2 and MSH6, or loss of only one protein with presence of MSI-H.

             MSI analysis will be performed by polymerase chain reaction [PCR] using a pentaplex
             panel (BAT-25, BAT-26, NR-21, NR-24, and NR-27; PROMEGA). MSI-H is defined as
             instability in two or more of the five studied markers. For the purpose of this study
             samples with 2 unstable markers will also undergo MMR analysis by IHC. Agreement of
             Sponsor (GERCOR) on a dMMR/MSI status is mandatory to include the patient (the
             patient's file [an anonymized mail] has to be send to Sponsor). Approval/refusal email
             for inclusion of the patient will be send by the Sponsor within 24 hours of receipt of
             the Investigator email. In case of discrepancy between IHC and PCR, the final decision
             about the dMMR/MSI status will be taken by GERCOR or coordinating investigator,
          8. Hematological status: absolute neutrophil count (ANC) ≥1.5 x 109/L; platelets ≥100 x
             109/L; hemoglobin ≥9 g/dL,
          9. Adequate renal function: serum creatinine level ≤150 μM and clearance ≥50 ml/min
             (Modification of the Diet in Renal Disease [MDRD] or Cockcroft and Gault),
         10. Adequate liver function: ≤1.5 x upper limit of normal (ULN) of direct bilirubin ≤ ULN
             for participants with total bilirubin levels >1.5 x ULN (inclusion possible if known
             Gilbert syndrome), alkaline phosphatase <5 x ULN, alanine aminotransferase (ALT), and
             aspartate aminotransferase (AST) ≤2.5 x ULN,
         11. International normalized ratio (INR), prothrombin time (PT), and activated partial
             thromboplastin time (aPTT) ≤1.5 x ULN, except for the patient on anticoagulant therapy
             who must have PT-INR-aPTT within therapeutic range is deemed appropriate by the
             Investigator,
         12. Radiological tumor assessment at screening performed within 28 days before inclusion
             according to RECIST version 1.1 by chest, abdomen, and pelvis CT, showing the absence
             of metastatic or non-surgical disease,
         13. A female participant is eligible to participate if she is not pregnant or
             breastfeeding, and one of the following conditions applies:
               -  Is a woman of non-childbearing potential as defined: i/ ≥ 45 years of age and has
                  not had menses for >1 year, ii/ Amenorrheic for <2 years without a hysterectomy
                  and oophorectomy and have a follicle stimulating hormone (FSH) value in the
                  postmenopausal range upon pre-study (screening) evaluation, iii/
                  post-hysterectomy, post-bilateral oophorectomy, or post-tubal ligation documented
                  hysterectomy or oophorectomy must be confirmed with medical records of the actual
                  procedure or confirmed by an ultrasound, magnetic resonance imaging (MRI), or CT
                  scan. Tubal ligation must be confirmed with medical records of the actual
                  procedure, otherwise the patient must fulfill the criteria in Inclusion criteria
                  15. Information must be captured appropriately within the site's source
                  documents,
               -  Negative pregnancy blood test within 72 hours before the first dose of
                  dostarlimab, AND
               -  If woman of childbearing potential (WOCBP), female patient must be willing to use
                  a highly effective form of contraception from screening throughout the study
                  treatment and 4 months after the last dose of dostarlimab,
         14. Male participants are eligible to participate if they agree to the following during
             the study treatment and for 4 months after the last dose of dostarlimab:
               -  Refrain from donating sperm,
               -  Must use contraception/barrier as follows:
                    -  Agree to use a male condom when having sexual intercourse with a WOCBP who
                       is not currently pregnant.
                    -  Agree to use a male condom when engaging in any activity that allows for
                       passage of ejaculate to another person,
         15. Providing primary tumor tissue samples (processed as formalin-fixed, paraffin-embedded
             [FFPE] blocks or freshly frozen) acquired during UGI endoscopy together with images
             (mandatory), NB: The patient's agreement will be specifically requested for endoscopic
             images in the patient information note and informed consent for their use as clinical
             data that may be analyzed and presented in publications. These data will be used in
             the same manner as other personal data. The confidentiality of these data will be
             maintained,
         16. Willingness and capablility to comply with scheduled visits, treatment schedule,
             laboratory tests, tumor biopsies, and other requirements of the study,
         17. Registration in a National Health Care System (PUMa - Protection Universelle Maladie
             included).
        Exclusion Criteria:
          1. Prior concomitant unplanned antitumor therapy (e.g., chemotherapy, molecular targeted
             therapy, immunotherapy),
          2. Treatment with any investigational medicinal product within 28 days prior to study
             entry,
          3. Treatment anticoagulant or hemostasis disorder contraindicating - biopsies during
             endoscopy,
          4. Major surgical procedure within 28 days (4 weeks) prior to the first dose of study
             treatment,
          5. Other serious and uncontrolled non-malignant disease (including active infection) or
             is considered a poor medical risk due to a serious, uncontrolled medical disorder,
             nonmalignant systemic disease or active infection requiring systemic therapy. Specific
             examples include, but are not limited to, active, non-infectious pneumonitis;
             uncontrolled ventricular arrhythmia; recent (within 90 days) myocardial infarction;
             uncontrolled major seizure disorder; unstable spinal cord compression; superior vena
             cava syndrome; or any psychiatric or substance abuse disorders that would interfere
             with cooperation with the requirements of the study,
          6. Other concomitant or previous malignancy other than the disease under study, except as
             noted below:
             i/ adequately treated in-situ carcinoma of the uterine cervix, ii/ basal or squamous
             cell carcinoma of the skin, iii/ cancer from which the patients was in complete
             remission for ≥3 years,
          7. Metastases (M stage disease) whatever the location,
          8. Pregnancy or breastfeeding,
          9. Human immunodeficiency virus (HIV),
         10. Active hepatitis B virus (HBV, defined as having a positive hepatitis B surface
             antigen [HBsAg] test) or hepatitis C virus (HCV) prior to inclusion, Note: Patients
             with past HBV infection or resolved HBV infection (defined as having a negative HBsAg
             test and a positive antibody to hepatitis B core antigen antibody test) are eligible.
             Note: Patients positive for HCV antibody are eligible only if PCR testing is negative
             for HCV RNA.
         11. Patient under a legal protection regime (guardianship, curatorship, judicial
             safeguard) or administrative decision or incapable of giving his/her consent,
         12. Impossibility of submitting to the medical follow-up of the study for geographical,
             social, or psychiatric illness.
        Non-eligible to immunotherapy:
          1. Pyloric tumor, NB: tumors of the pylorus will be excluded because of the risk of high
             occlusion in case of pseudo progression and associated surgery,
          2. Any history of autoimmune disease including, but not limited to myasthenia gravis,
             myositis, autoimmune hepatitis, systemic lupus erythematosus, rheumatoid arthritis,
             inflammatory bowel disease, vascular thrombosis associated with antiphospholipid
             syndrome, Wegener's granulomatosis, Sjögren's syndrome, Guillain-Barré syndrome,
             multiple sclerosis, vasculitis, or glomerulonephritis, Note: History of
             autoimmune-related hypothyroidism on a stable dose of thyroid replacement hormone may
             be eligible.
             Note: Controlled Type 1 diabetes mellitus on a stable insulin regimen may be eligible.
          3. History of idiopathic pulmonary fibrosis (including pneumonitis), drug-induced
             pneumonitis, organizing pneumonia (i.e., bronchiolitis obliterans, cryptogenic
             organizing pneumonia), or evidence of active pneumonitis on screening chest imaging,
          4. Any live, attenuated vaccine within 14 days prior to the firs dose of study treatment
             or such administration is anticipated during the study,
          5. Prior therapy with any immune-checkpoint inhibitors, including antibodies or drugs
             targeting CD137, CTLA-4, PD-1, or PD-L1 or other checkpoint pathways,
          6. Prior allogeneic bone marrow transplantation or prior solid organ transplantation,
          7. Treatment with systemic corticosteroids or other systemic immunosuppressive
             medications (including but not limited to prednisone, dexamethasone, cyclophosphamide,
             azathioprine, methotrexate, thalidomide, and anti-tumor necrosis factor agents) within
             2 weeks prior to the first dose of adjuvant treatment or is required to receive
             systemic immunosuppressive medications during the study. Inhaled or topical steroids
             and adrenal replacement doses >10 mg daily prednisone equivalents are permitted in the
             absence of active autoimmune disease.
        Note: Patients who have received acute, low-dose, systemic immunosuppressant medications
        (e.g., a one-time dose of dexamethasone for nausea) may be enrolled into the study after
        approval of the Medical Contact. Note: Subjects are permitted the use of topical, ocular,
        intra-articular, intranasal, and inhalational corticosteroids (with minimal systemic
        absorption). Adrenal replacement steroid doses including doses >10 mg daily prednisone is
        permitted. A brief (less than 3 weeks) course of corticosteroids for prophylaxis (e.g.,
        contrast dye allergy) or for treatment of non-autoimmune conditions (e.g., delayed-type
        hypersensitivity reaction caused by a contact allergen) is permitted.