• For inclusion in the study, all of the following inclusion criteria must be fulfilled.

• Be willing and able to provide written informed consent.

• Be a female or male ≥ 18 and ≤ 75 years old at the time of signing of the prescreening ICF.

• For Part A and Part B Cohort MR1 only:

∙ Subjects with histologically/cytologically confirmed unresectable, locally advanced or metastatic adenocarcinoma of the GC/GEJC/EC for which standard treatment is considered intolerable, unlikely to confer significant clinical benefit, is no longer effective, or subject is ineligible or declines standard therapy.

‣ Subjects must have received prior therapy as follows:

⁃ Previous treatment must have included a fluoropyrimidine and/or platinum containing regimen. Subjects with HER2-neu-positive (HER2+) disease must have also received prior anti-HER2+ therapy.

• Neoadjuvant/adjuvant treatment will be considered as a prior regimen if disease progression occurred during treatment or within 6 months of cessation of treatment.

∙ For Part B Cohort MR2 only:

‣ Subjects with histologically/cytologically confirmed unresectable, locally advanced or metastatic PDAC for which standard treatment is considered intolerable, unlikely to confer significant clinical benefit, is no longer effective, or subject is ineligible or declines standard therapy.

‣ Subjects must have received prior therapy as follows:

⁃ Previous treatment must have included fluoropyrimidine and/or gemcitabine containing regimen.

• Neoadjuvant/adjuvant treatment will be considered as a prior regimen if disease progression occurred during treatment or within 6 months of cessation of treatment.

∙ For Part B Cohort CR1 only:

‣ Subjects with histologically/cytologically confirmed metastatic GC/GEJC/EC with RECIST v1.1 SD or PR at the initial response evaluation during first-line SOC treatment.

‣ Subjects with locally advanced, unresectable disease for whom radiation is not planned may be eligible with Sponsor approval.

‣ Subjects who develop metachronous metastatic disease after prior (neo)adjuvant treatment for previously localized disease are eligible if at least 6 months have elapsed since completion of prior chemotherapy (and disease status is satisfied, as above).

‣ Subjects must be clinically suitable to continue at least part of the initial first-line regimen during LB1908 manufacturing.

‣ Negative for human epidermal growth factor receptor 2 (HER2) or ineligible to receive HER2-directed therapy.

∙ Subjects must have received prior therapy as follows:

‣ Acceptable SOC first-line regimens include (but are not limited to) leucovorin/fluorouracil/oxaliplatin (FOLFOX; modifications allowed) and capecitabine/oxaliplatin (CAPOX), with or without an approved immune checkpoint inhibitor.

∙ Zolbetuximab is allowable as part of first-line SOC (subjects with prior zolbetuximab exposure require Sponsor approval prior to enrollment)

∙ No investigational therapies in first-line therapy, unless the investigational product is discontinued prior to enrollment on this trial.

• For Part B Cohort CR2 only:

∙ Subjects with metastatic PDAC with RECIST v1.1 SD or PR at the initial response evaluation during first-line SOC treatment.

∙ Subjects with locally advanced, unresectable disease for whom radiation is not planned may be eligible with Sponsor approval.

∙ Subjects who develop metachronous metastatic disease after prior (neo)adjuvant treatment for previously localized disease are eligible if at least 6 months have elapsed since completion of prior chemotherapy (and disease status is satisfied, as above).

∙ Subjects must be clinically suitable to continue at least part of the initial first-line regimen during LB1908 manufacturing.

∙ Subjects must have received prior therapy as follows:

‣ Acceptable SOC first-line regimens include (but are not limited to) leucovorin/fluorouracil/irinotecan/oxaliplatin (FOLFIRINOX), nabpaclitaxel/ gemcitabine, and liposomal irinotecan/fluorouracil/leucovorin/oxaliplatin (NALIRIFOX).

⁃ Prior investigational therapies are exclusionary.

∙ No investigational therapies in first-line therapy, unless the investigational product is discontinued prior to enrollment on this trial.

• Presence of CLDN18.2 positive tumors with staining intensity of ≥ 1+ in ≥ 50% of tumor cells by immunohistochemistry (IHC) (Performed during Prescreening).

‣ Subject has available formalin-fixed, paraffin-embedded (FFPE) tumor specimen in a tissue block or unstained serial slides accompanied by an associated pathology report prior to enrollment. Archival or fresh biopsy tissue is required.

⁃ If a subject had prior CLDN18.2 targeted therapy, subject may be required to have a formalin-fixed, paraffin-embedded tumor specimen in a tissue block or unstained serial slides accompanied by an associated pathology report that was obtained after exposure to CLDN18.2 targeted therapy, prior to enrollment, and fulfill criteria as outlined (\>50% expression) as directed by Sponsor.

• Presence of ≥ 1 radiologically measurable lesion per RECIST v1.1.

• Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.

• Life expectancy of at least 4 months per Investigator judgment.