A Phase 1 Dose Escalation and Expansion Study of EO-3021, an Anti-claudin 18.2 (CLDN18.2) Antibody Drug Conjugate, in Patients with Solid Tumors Likely to Express CLDN18.2

• EO-3021 is an anti-CLDN18.2 antibody drug conjugate.

In this study, the sponsor and investigators want to learn:

• How much of the study agent can be given with an acceptable level of side effects
• The effects of the study agent (good and bad)
• much of the study agent is absorbed into the blood and how fast it is removed
• How the study agent is acting on your body
• Whether proteins in the blood and tissue samples can predict if someone has a better or worse response to the study agent

This is a dose escalation/expansion study that will be completed in two parts, Part A and Part B.

In Part A (dose escalation) the amount of the study  given will be increased in each group of research participants, to find the most appropriate dose for further study.  In Part B (dose expansion), a larger number of people will receive the study agent dose determined to be appropriate in the dose escalation part of the study.

About 3-13 people will receive each dose level, in Part A, starting with a low dose and going up to the highest dose being tested. It is expected that there will be four or more dose levels. The amount of study agent you receive at each study visit will be based on your body weight in kilograms (kg). If you are participating in Part A, you may receive 1.0 mg/kg, 2.0 mg/kg, 2.5 mg/kg, or 2.9 mg/kg. Your study doctor will tell you which dose you are receiving. The highest dose from Part A considered to be safe will be used as the dose in Part B.

Part A (dose escalation) will test EO-3021 in participants with gastric or gastroesophageal (GEJ) cancer that are likely to have a gene expression called CLDN18.2. Part B (dose expansion) will test EO-3021 in participants with different types of cancers that are either known to have this gene expression or that test positive for this gene expression. To find out if your cancer is positive for CLND18.2, your doctor will send your tumor tissue to be tested for CLDN18.2. Or, if you have a cancer that is already likely to express CLDN18.2, then you do not have to wait for more test results to be eligible for the study. The majority of gastric cancer including GEJ (cancer of the stomach) will have some level of CLDN18.2; however, if your cancer does not have CLDN18.2 (or is considered negative for CLDN18.2), you may not benefit from EO-3021.

The study regimen will be given in cycles. Each cycle is 21 days. You will receive the study agent on the first day of each 21-day cycle through an IV into your vein, given over 60-120 minutes depending on how you tolerate it. You may continue receiving EO-3021 as long as your study doctor thinks it is in your best interest and your disease does not progress, unless there are other reasons for stopping study treatment, or if you voluntarily withdraw your participation. Overall, you should expect to be on the study treatment for four months or longer.