Phase 1/2 Study of Intravesical MK-3120 in BCG-Naïve or BCG-Exposed High-Risk Non-muscle Invasive Bladder Cancer

Inclusion Criteria: * Has histologically confirmed carcinoma in situ (CIS) +/- papillary high-risk non-muscle invasive bladder cancer (NMIBC), confirmed locally. * Is an individual whose most recent transurethral resection of bladder tumor (TURBT) was performed within 12 weeks before allocation and showed high-risk NMIBC histology. For individuals with papillary tumors (Ta and T1), a complete TURBT must have been performed, as characterized by attainment of a visually complete resection of all papillary tumors (Ta and T1). * Is either: a) Bacillus Calmette-Guérin (BCG)-naïve, defined as either having never received BCG or having received BCG more than 2 years before CIS +/- papillary high-risk NMIBC recurrence. Recurrence must be at least 24 months from the last exposure to BCG with evidence of complete response during the 2-year period post-BCG OR; b) BCG-exposed and received adequate BCG therapy and had recurrence of CIS +/- papillary high-risk NMIBC \>12 months but ≤24 months after the last BCG dose. * Human immunodeficiency virus (HIV)-infected participants must have well-controlled HIV on antiretroviral therapy. * Participants who are hepatitis B surface antigen positive are eligible if they have received hepatitis B virus (HBV) antiviral therapy for at least 4 weeks, and have undetectable HBV viral load prior to allocation. * Participants with history of hepatitis C virus (HCV) infection are eligible if HCV viral load is undetectable at screening. Exclusion Criteria: * Has history of or current locally advanced (ie, T2, T3, T4) or metastatic urothelial cancer (UC). * Has concurrent extravesical (ie, urethra, ureter, renal pelvis) non-muscle invasive UC or history of extravesical non-muscle invasive UC that recurred within the last 2 years. * Has active total bladder incontinence, active urinary tract infection, neurogenic bladder, or urethral stricture. * Has a condition that would prohibit normal voiding (or holding bladder voiding for 1 to 2 hours). * Has uncontrolled, significant cardiovascular disease or cerebrovascular disease, including New York Heart Association Class III or IV congestive heart failure, unstable angina, myocardial infarction, uncontrolled symptomatic arrhythmia, prolongation of QTcF interval to \>470 ms, and/or other serious cardiovascular and cerebrovascular diseases within the 6 months preceding study intervention. * Has history of documented severe dry eye syndrome, severe Meibomian gland disease and/or blepharitis, or severe corneal disease that prevents/delays corneal healing. * HIV-infected participants with a history of Kaposi's sarcoma and/or Multicentric Castleman's Disease. * Has known additional malignancy that is progressing or has required active treatment within the past 3 years. * Has known active central nervous system metastases and/or carcinomatous meningitis. * Has active infection requiring systemic therapy. * Has a history of (noninfectious) pneumonitis/interstitial lung disease (ILD) that required steroids, or has current pneumonitis/ILD. * Has not adequately recovered from major surgery or has ongoing surgical complications.