This study is looking at a treatment for high-risk non-muscle invasive bladder cancer with a specific type called carcinoma in situ, which may occur with or without growths called papillary tumors. Non-muscle invasive bladder cancer means the cancer is present in the bladder lining but has not grown into the deeper muscle layer of the bladder wall. Carcinoma in situ is a flat form of cancer that stays in the inner layer of the bladder. The treatment being tested is called MK-3120, which is given directly into the bladder through a procedure called intravesical administration, meaning the medicine is placed inside the bladder rather than taken by mouth or injection into the bloodstream. The medicine is in the form of a suspension, which is a liquid mixture.

The purpose of this study is to evaluate the safety and tolerability of MK-3120 when used alone as a treatment. The study will look at how well people can handle the treatment and what side effects might occur. The study is divided into phases, with the first phase focusing on finding the right dose and understanding safety, while the second phase will look more closely at how well the treatment works. People taking part in this study will either have never received a treatment called BCG (Bacillus Calmette-Guérin), which is a common bladder cancer treatment, or they will have received BCG in the past but their cancer has returned. The study will track whether participants experience dose-limiting toxicity, which means side effects serious enough to require stopping or reducing the treatment dose, and will count how many people have adverse events or need to stop treatment because of side effects.

During the study, participants will receive MK-3120 directly into their bladder at scheduled times. The study will also measure the complete response rate, which means how many people have their cancer completely disappear after treatment. Before joining the study, participants must have had a procedure called transurethral resection of bladder tumor, which is a surgery to remove bladder tumors through the urethra, performed within a certain timeframe. The study will continue for several years to follow participants and gather information about the long-term effects of the treatment.