A Study of MT-303 in Adults With Advanced or Metastatic GPC3-Expressing Cancers, Including HCC

This is a multicenter, open-label, Phase 1, first-in-human, dose-escalation study designed to assess the safety, tolerability and define the RP2D of MT-303 alone (Module 1) and in combination with Atezo/Bev (Module 2) in participants with advanced hepatocellular carcinoma expressing GPC3.

Participants will be enrolled into one of two treatment modules: * Module 1 (Monotherapy): Participants will receive MT-303. * Module 2 (Combination therapy): Participants will receive MT-303 in combination with atezolizumab + bevacizumab (Atezo/Bev). In Module 1 (Monotherapy), participants will receive MT-303 across five dose-escalation cohorts and in Module 2 (Combination therapy), participants will receive MT-303 in combination with Atezo/Bev across five dose-escalation cohorts. Additional cohorts in both modules may be scheduled based on emerging safety and PK data. Participants will be sequentially enrolled into Cohorts 1 through 5. Both modules will be enrolled concurrently, with Module 2 dosing beginning at one dose level below the known safe dose in Module 1. Safety Review Committee decisions will be informed by all available safety data from Modules 1 and 2.

Inclusion Criteria * Aged 18 years or older * Histological diagnosis of advanced/recurrent or metastatic and/or unresectable HCC. \[Note: participants with other tumor types expressing GPC3 may be eligible for Module 1 pending a discussion with the Medical Monitor. Only participants with HCC are eligible for Module 2. * Measurable lesion per RECIST 1.1 criteria * Eastern Cooperative Oncology Group (ECOG) performance status grade of 0 or 1 * Child-Pugh score: Class A * Adequate organ function General Exclusion Criteria * Known active CNS metastasis and/or carcinomatous meningitis. * Any acute illness including active infection * History of liver transplantation or on waiting list * Participants with untreated or incompletely treated varices with bleeding or high risk for bleeding * Uncontrolled pleural effusion, pericardial effusion, or ascites * History of symptomatic congestive heart failure * History of chronic or recurrent (within the last year) severe autoimmune or immune mediated disease requiring steroids or other immune-suppressive treatments. Additional Module 2 Exclusion Criteria: * Known fibrolamellar HCC, sarcomatoid HCC, or mixed cholangiocarcinoma and HCC. * Significant cardiovascular disease * History of severe hypersensitivity to atezolizumab and/or bevacizumab. * History of idiopathic pulmonary fibrosis * Prior history of hypertensive crisis or hypertensive encephalopathy.