BCG in Combination With Durvalumab in Adult BCG-naïve, High-risk NMIBC Participants

Inclusion Criteria:

* BCG-naïve (defined as participants not having received prior intravesical BCG or who previously received but stopped BCG more than 3 years before study entry).
* Local histological confirmation (based on cytology and/or pathology report) of high-risk transitional cell carcinoma of the urothelium of the urinary bladder confined to the mucosa or submucosa.
* Complete resection of all Ta/T1 papillary disease prior to enrollment, with the transurethral resection of bladder tumor (TURBT) removing high-risk NMIBC (non-muscle invasive bladder cancer) performed not more than 4 months before enrollment in the study.
* No prior radiotherapy for bladder cancer.
* A life expectancy of at least 12 weeks (90 days).
* Adequate organ and marrow function
* World Health Organization/Eastern Cooperative Oncology Group performance status of 0 or 1 at screening
* No prior exposure to immune-mediated therapy of cancer
* A candidate for BCG treatment.
* Local histological confirmation (based on cytology and/or pathology report) of high-risk transitional cell carcinoma of the urothelium of the urinary bladder confined to the mucosa or submucosa. A high-risk tumor is defined as one of the following: T1 tumor; High-grade/G3 tumor; CIS.

Exclusion Criteria:

* Evidence of muscle-invasive, locally advanced, metastatic, and/or extra-vesical bladder cancer (ie, T2, T3, T4, and / or Stage IV).
* Predominantly variant histology such as micropapillary, plasmacytoid, nested, sarcomatoid, microcystic, squamous and adeno variants of UC representing \> 50% of tumor tissue or other than urothelial tumors as assessed by pathology.
* Evidence of lymphovascular invasion of bladder tumor, except if treatment with BCG is deemed to be the only clinically viable treatment.
* Immediate cystectomy is indicated.
* Known or documented absolute and/or relative contraindication of adjuvant intravesical BCG treatment.
* Concurrent extravesical, non-muscle-invasive transitional cell carcinoma of the urothelium.
* History of allogenic organ transplantation. Participants with any history of allogenic stem cell transplantation are also excluded.
* Active or prior documented autoimmune or inflammatory disorders.
* Participants with hypothyroidism stable on hormone replacement.
* History of active primary immunodeficiency.
* Active infection including hepatitis B (known positive HBV/HBsAg result), HCV, or HIV 1/2 (positive HIV) antibodies.
* Current or prior use of immunosuppressive medication within 14 days before the first durvalumab dose.
* Female participants who are pregnant or breastfeeding or male or female participants of reproductive potential who are not willing to employ highly effective birth control.
* Any concurrent chemotherapy, study intervention, biologic or hormonal therapy for cancer treatment; uncontrolled intercurrent illness;
* History of another primary malignancy except for Malignancy treated with curative intent and with no known active disease ≥ 2 years; Adequately treated nonmelanoma skin cancer or lentigo maligna without evidence of disease; Adequately treated CIS without evidence of disease; Prostate cancer of stage ≤ T2cN0M0 without biochemical recurrence or progression that in the opinion of the Investigator does not require active intervention.
* Previous or concurrent treatment with potent systemic immunostimulatory agents