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

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.