Zolbetuximab is being studied as a treatment for people with cancer in and around the stomach or cancer where the food pipe (esophagus) joins the stomach (gastroesophageal junction cancer). Most people with this type of cancer have a protein called Claudin 18.2 in their tumor. Zolbetuximab is thought to work by attaching to Claudin 18.2 in their tumor. This switches on the body's immune system to attack the tumor.

There is an unmet medical need to treat people with advanced cancer in and around the stomach or gastroesophageal junction cancer. This study will provide more information on zolbetuximab given by itself and in combination with other treatments in adults with advanced stomach or gastroesophageal junction cancer. The study is currently ongoing globally. People in this study will either be treated with zolbetuximab by itself, with zolbetuximab and chemotherapy, with zolbetuximab and a medicine called pembrolizumab, or zolbetuximab with chemotherapy and a medicine called nivolumab.

This study is ongoing, but enrollment in any of the treatment options has been completed. In addition, at this stage of the study, treatment in some of these treatment options has completed.

The main aim of this study is to check how well zolbetuximab controls tumors when given by itself.

Adults with cancer in and around the stomach or gastroesophageal junction cancer can take part. Their cancer is locally advanced unresectable or metastatic and has the CLDN18.2 marker in a tumor sample. Locally advanced means the cancer has spread to nearby tissue. Unresectable means the cancer cannot be removed by surgery.

Metastatic means the cancer has spread to other parts of the body. They may have been previously treated with standard therapies. People cannot take part if they need to take medicines to suppress their immune system, have blockages or bleeding in their gut, have specific uncontrollable cancers such as symptomatic or untreated cancers in the nervous system, have a specific heart condition or infections.

There are different treatments in the study. People who take part will receive just 1 of the treatments.

Treatment will be given in cycles. The treatment is given through a vein; this is called an infusion. Some people with advanced disease will have 1 infusion in 3 week (21-day) cycles. Some people will have several infusions in 6 week (42-day) cycles. Some people with cancer in and around the stomach or gastroesophageal junction who have surgery for their cancer will have a few infusions in 2-week (14-day) cycles. This will happen before and after they have surgery for their cancer.

People may receive chemotherapy for up to 6 months. Some people enrolled to received zolbetuximab and pembrolizumab, may have received pembrolizumab for up to 2 years.

People will visit the clinic on certain days during their treatment; there may be extra visits during the first cycle of treatment. The study doctors will check if people had any medical problems from zolbetuximab and the other study treatments. Also, people in the study will have a health check including blood tests. On some visits they will also have scans to check for any changes in their cancer. Tumor samples will be taken at certain visits with the option of giving a tumor sample after treatment has finished.

People will visit the clinic after they stop treatment. They will be asked about any medical problems and will have a health check including blood tests. After the clinic visits end some people will have a telephone health check every 3 months. The number of visits and checks done at each visit will depend on the health of each person and whether they completed their treatment or not.

This is a study to assess the antitumor activity of zolbetuximab, an Immunoglobulin (IgG1) chimeric monoclonal antibody directed against CLDN18.2, in subjects with recurrent locally advanced or metastatic gastric or gastroesophageal junction adenocarcinoma and locoregional gastric or GEJ adenocarcinoma whose tumors are CLDN18.2 positive. For each cohort, the study consists of the following periods: pre-screening; screening; treatment; and follow-up for disease progression (or post-treatment follow-up for disease recurrence, which will be conducted for Cohort 5). In addition, there will be a survival follow-up period for Cohorts 1A, 4B, and 5 participants only. Tolerability of zolbetuximab in combination with pembrolizumab in Japanese participant(s) will be evaluated in Cohort 3A DLT assessment. Tolerability of zolbetuximab in combination with mFOLFOX6 and nivolumab in Japanese subject(s) will be evaluated in Cohort 4B, if Japanese subjects are not enrolled in the Cohort 4A DLT assessment.

Inclusion Criteria:

• Female subject eligible to participate if she is not pregnant and at least one of the following conditions applies:

  • Not a woman of child-bearing potential (WOCBP) OR
  • WOCBP who agrees to follow the contraceptive guidance throughout the treatment period and for at least 9 months after the final oxaliplatin administration and 6 months after the final administration of all other study drugs.

• Female subject must agree not to breastfeed starting at screening and throughout the study period, and for 6 months after the final study drug administration.

• Female subject must agree not to donate ova starting at screening and throughout the study period, and for 9 months after the final oxaliplatin administration and 6 months after the final administration of all other study drugs.

• A sexually active male subject with a female partner(s) who is of child-bearing potential must agree to use contraception during the treatment period and for at least 6 months after the final study drug administration.

• Male subject must agree not to donate sperm starting at screening and throughout the study period, and for 6 months after the final study drug administration.

• Male subject with a pregnant or breastfeeding partner(s) must agree to remain abstinent or use a condom for the duration of the pregnancy or time partner is breastfeeding throughout the study period and for 6 months after the final study drug administration.

• Subject has histologically confirmed gastric or GEJ adenocarcinoma.

• Cohorts 1-4: Subject has radiographically-confirmed, locally advanced, unresectable or metastatic disease within 28 days prior to the first dose of study treatment.

• Subject's tumor is positive for CLDN18.2 expression demonstrating moderate to strong membranous staining as determined by central IHC testing.

• Subject agrees to not participate in another interventional study while on treatment.

• Subject has ECOG performance status 0 to 1.

• Subject has predicted life expectancy ≥ 12 weeks.

• Subject must meet all of the following criteria based on the centrally or locally analyzed laboratory tests collected within 14 days prior to the first dose of study treatment. In case of multiple central laboratory data within this period, the most recent data should be used.

  • Hemoglobin (Hgb) ≥ 9 g/dL (transfusion is allowed, but post-transfusion Hgb [24 hours or later following transfusion] must be ≥ 9 g/dL)
  • Absolute neutrophil count (ANC) ≥ 1.5 × 109/L
  • Platelets ≥ 100 × 10^9/L
  • Albumin ≥ 2.5 g/dL
  • Total bilirubin ≤ 1.5 × upper limit of normal (ULN)
  • Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) ≤ 2.5 × ULN in subjects without liver metastases (≤ 5 × ULN if liver metastases are present)
  • Cohorts 1-4: Estimated creatinine clearance ≥ 30 mL/min
  • Cohort 5: Serum creatinine ≤ 1.5 × ULN, or estimated creatinine clearance ≥ 50 mL/min for subjects with serum creatinine levels > 1.5 × ULN
  • Prothrombin time/international normalized ratio and partial thromboplastin time ≤ 1.5 × ULN (except for subjects receiving anticoagulation therapy)

Specific to Cohort 1A:

• Subject has measurable disease according to RECIST 1.1 within 28 days prior to the first dose of study treatment per investigator assessment. For subjects with only 1 evaluable lesion and prior radiotherapy ≤ 3 months before enrollment, the lesion must either be outside the field of prior radiotherapy or must have documented progression following radiation therapy.
• Subject has disease progression on or after at least 2 prior regimens for their advanced disease, including fluoropyrimidine and platinum-containing chemotherapy, and if appropriate, HER2/neu-targeted therapy and all associated side effects have resolved to grade 1 or less.
• Subject must have an additional available tumor specimen collected within 3 months prior to the first dose of study treatment.
• Subject must be an appropriate candidate for a tumor biopsy and is amenable to undergo a tumor biopsy during the screening period (if applicable) and treatment period as indicated in the Schedule of Assessments.

Specific to Cohort 2:

• Subject has measurable disease according to RECIST 1.1 within 28 days prior to the first dose of study treatment per investigator assessment. For subjects with only 1 evaluable lesion and prior radiotherapy ≤ 3 months before enrollment, the lesion must either be outside the field of prior radiotherapy or must have documented progression following radiation therapy.
• Subject has not received prior systemic anti-cancer therapy for their advanced disease (subject may have received neoadjuvant and/or fluorouracil-containing adjuvant chemotherapy as long as it has been completed ≥ 6 months before the first dose of study treatment).
• Subject has a gastric or GEJ tumor that is HER2-negative as determined by local or central testing.
• Subject must have an additional available tumor specimen collected within 3 months prior to the first dose of study treatment.
• Subject must be an appropriate candidate for a tumor biopsy and is amenable to undergo a tumor biopsy during the screening period (if applicable) and treatment period as indicated in the Schedule of Assessments.

Specific to Cohort 3A:

• Subject has radiologically evaluable disease (measurable and/or non-measurable) according to RECIST 1.1, per local assessment, ≤ 28 days prior to the first dose of study treatment. For subjects with only 1 evaluable lesion and prior radiotherapy ≤ 3 months before enrollment, the lesion must either be outside the field of prior radiotherapy or must have documented progression following radiation therapy.
• Subject has disease progression on or after at least 2 prior regimens for their advanced disease, including fluoropyrimidine and platinum-containing chemotherapy, and if appropriate, HER2/neu-targeted therapy.
• Subject has not received prior checkpoint inhibitor therapy.

Specific to Cohort 4A and 4B:

• Subject has radiologically evaluable disease.
• Subject has not received prior systemic anti-cancer therapy for their advanced disease.
• Subject has a gastric or GEJ tumor that is HER2-negative as determined by local or central testing.
• Subject has not received prior checkpoint inhibitor therapy.

Specific to Cohort 4B Only:

• Subject must have an additional available tumor specimen collected within 3 months prior to the first dose of study treatment.
• Subject must be an appropriate candidate for a tumor biopsy and is amenable to undergo a tumor biopsy during the screening period (if applicable) and treatment period.

Specific to Cohort 5 Only:

• Subject has new histologically confirmed primary gastric or GEJ adenocarcinoma that is amenable to curative resection.
• Subject has locoregional, resectable gastric or GEJ adenocarcinoma. GEJ may include type I-III Siewert classification. Clinical stage will be determined by endoscopic ultrasound (EUS) and/or CT or MRI. Diagnostic laparoscopy may be used as per institutional guidelines and clinical practices.
• Subject meets one of the following criteria of locoregional disease by clinical TNM staging:
• GEJ: cT2,N0 (high risk-lesions: ≥ 3 cm, poorly differentiated), cT1b-cT2,N+ or cT3-cT4a,Any N.
• Gastric: T2 to T4a, and/or N1-3,M0.
• Subject's tumor expresses CLDN18.2 in ≥ 75% of tumor cells demonstrating moderate to strong membranous staining as determined by central IHC testing

Exclusion Criteria:

• Subject has had prior severe allergic reaction or intolerance to known ingredients of zolbetuximab or other monoclonal antibodies, including humanized or chimeric antibodies.

• Subject has known immediate or delayed hypersensitivity or contraindication to any component of study treatment.

• Subject has received other investigational agents or devices concurrently or within 28 days prior to first dose of study treatment.

• Subject has received systemic immunosuppressive therapy, including systemic corticosteroids 14 days prior to first dose of study treatment.

• Subject has a complete gastric outlet syndrome or a partial gastric outlet syndrome with persistent recurrent vomiting.

• Subject has significant gastric bleeding and/or untreated gastric ulcers that would preclude the subject from participation.

• Subject has history of central nervous system metastases and/or carcinomatous meningitis from gastric/GEJ cancer.

• Subject has a known history of a positive test for human immunodeficiency virus (HIV) infection or known active hepatitis B (positive hepatitis B surface antigen [HBsAg]) or hepatitis C infection.

• Subject has had within 6 months prior to first dose of study treatment any of the following: unstable angina, myocardial infarction, ventricular arrhythmia requiring intervention or hospitalization for heart failure.

• Subject has active infection requiring systemic therapy that has not completely resolved within 7 days prior to the start of study treatment.

• Subject has active autoimmune disease that has required systemic treatment within the past 3 months prior to the start of study treatment.

• Subject has a clinically significant disease or co-morbidity that may adversely affect the safe delivery of treatment within this study or make the subject unsuitable for study participation.

• Subject has psychiatric illness or social situations that would preclude study compliance.

• Subject has had a major surgical procedure ≤ 28 days before start of study treatment.

• Subject is without complete recovery from a major surgical procedure ≤ 14 days before start of study treatment

  • Subject has received radiotherapy for locally advanced unresectable or metastatic gastric or GEJ adenocarcinoma ≤ 14 days (Cohorts 1 and 3A) and ≤ 28 days (Cohorts 2 and 4A or 4B) prior to start of study treatment and has NOT recovered from any related toxicity.
  • Subject has another malignancy, for which treatment is required.

• Cohort 2, 4 and 5 Only, subject has any of the following:

  • Prior severe allergic reaction or intolerance to any component of mFOLFOX6 or FLOT chemotherapeutics in this study
  • Known dihydropyrimidine dehydrogenase deficiency (DPD).
  • Known peripheral neuropathy > Grade 1 (absence of deep tendon reflexes as the sole neurological abnormality does not render the subject ineligible).
  • Sinusoidal obstruction syndrome, formerly known as veno-occlusive disease, if present, should be stable or improving.
  • History of clinically significant ventricular arrhythmias.
  • QTc interval > 450 msec for male subjects; QTc interval > 470 msec for female subjects.
  • History or family history of congenital long QT syndrome.
  • Cardiac arrhythmias requiring anti-arrhythmic medications (Subjects with rate controlled atrial fibrillation for > 1 month prior to first dose of study treatment are eligible).

• Cohorts 3A, 4A and 4B Only, subject has any of the following:

  • Ongoing or previous autoimmune disease or interstitial lung disease, active diverticulitis or gastrointestinal ulcerative disease, or solid organ or stem cell transplant (for Cohort 4) or other uncontrolled or clinically significant medical disorders.
  • Type 1 diabetes mellitus, endocrinopathies stably maintained on appropriate replacement therapy or skin disorders (e.g., vitiligo, psoriasis, or alopecia) not requiring systemic treatment are allowed.
  • Known history of serious hypersensitivity reaction to a known ingredient of pembrolizumab or nivolumab.

• Cohort 4B Only: Subjects has microsatellite instability-high or mismatch repair deficient tumors.

• Cohort 5 Only, subject has either of the following:

  • Subject cannot undergo curative resection per the investigator's judgment
  • Subject meets the following criterion of locoregional disease by clinical TNM staging: cT1N0.

Access to anonymized individual participant level data collected during the study, in addition to study-related supporting documentation, is planned for studies conducted with approved product indications and formulations, as well as products terminated during development. Studies conducted with product indications or formulations that remain active in development are assessed after study completion to determine if Individual Participant Data can be shared. Further details on Astellas' data sharing policy can be found at https://www.clinicaltrials.astellas.com/transparency/.

Access to participant level data is offered to researchers after publication of the primary manuscript (if applicable) and is available as long as Astellas has legal authority to provide the data.

Researchers must submit a proposal to conduct a scientifically relevant analysis of the study data. The research proposal is reviewed by an Independent Research Panel. If the proposal is approved, access to the study data is provided in a secure data sharing environment after receipt of a signed Data Sharing Agreement.

Participants will be enrolled to receive zolbetuximab as monotherapy or in combination with mFOLFOX6 (with or without Nivolumab) and in combination with pembrolizumab and in combination with FLOT in an unblinded fashion.