The study will be comprised of 2 parts. The first part (dose escalation) will find the highest dose of enfortumab vedotin that does not cause unacceptable side effects in participants. The second part (dose expansion) will use the dose found in the first part to test how well the drug works.
All participants will receive enfortumab vedotin. Treatment on the study will occur during the induction and maintenance phases, and participants will enter a follow-up period after completion of the maintenance phase.
Inclusion Criteria:
* Histologically confirmed, non-muscle invasive urothelial carcinoma with carcinoma in situ (CIS) (with or without papillary disease)
* Predominant histologic component (\>50 percent) must be urothelial (transitional cell) carcinoma
* Participants must have high-risk Bacillus Calmette-Guerin (BCG) - unresponsive disease, defined as (where adequate BCG therapy is defined as one of the following: 5 of 6 doses of an initial induction course + at least 2 of 3 doses maintenance therapy or 5 of 6 doses of an initial induction course + at least 2 of 6 doses of a second induction course):
* Persistent or recurrent CIS alone or with recurrent Ta/T1 (noninvasive papillary disease/tumor invades the subepithelial connective tissue) disease within 12 months of completion of adequate BCG therapy.
* Recurrent high-grade Ta/T1 disease within 6 months of completion of adequate BCG therapy, or
* T1 high-grade disease at the first evaluation following an induction BCG course (at least 5 or 6 doses)
* Participant must be ineligible for or refusing a radical cystectomy
* All visible papillary Ta/T1 tumors must be completely resected within 60 days prior to enrollment.
* Eastern Cooperative Oncology Group Performance Status score of 0, 1, or 2.
Exclusion Criteria:
* Current or prior history of muscle-invasive urothelial carcinoma or metastatic disease.
* Nodal or metastatic disease as noted on computed tomography (CT) or magnetic resonance imaging (MRI) within 3 months prior to study treatment
* Concomitant upper tract urothelial carcinoma as noted on CT or MRI urogram performed within 3 months prior to study treatment
* Prior or concomitant urothelial carcinoma of the prostatic urethra within 6 months prior to study treatment
* Participants with tumor-related hydronephrosis
* Participant has received other systemic anticancer therapy including chemotherapy, biologic therapy, immunotherapy, targeted therapy, endocrine therapy, and/or investigational agent within 4 weeks or intravesical therapy within 6 weeks of first dose of study treatment
* Participant has had any prior radiation to the bladder for urothelial cancer
