Background: Intravesical BCG is the most effective treatment for high-grade non-muscle invasive bladder cancer (NMIBC), yet recurrences are common. Patients with BCG-relapsing NMIBC are often re-treated with BCG or BCG with interferon (IFN) with an expected response rate of only 40–60%. Several studies show that a major mechanism of resistance to BCG is high levels of myeloid-derived suppressor cells (MDSCs) and regulatory T cells (Tregs) in the pretreatment tumor microenvironment. Gemcitabine is a commonly used intravesical treatment for NMIBC that, in addition to direct anti-tumor cytotoxic effects, may also reduce MDSCs and Tregs. Prior trials combining BCG with intravesical mitomycin C have shown improved efficacy over BCG alone but with higher toxicity. While gemcitabine has been shown to be better tolerated than mitomycin as an intravesical treatment, no study has looked at combined BCG and intravesical gemcitabine. We hypothesize that combining BCG and intravesical gemcitabine will be well tolerated and result in higher response rates by reducing levels of MDSCs and Tregs. A novel aspect of our trial design is the use of a modified continual reassessment method to more accurately identify the maximum tolerated dose instead of the traditional 3 + 3 design used in most NMIBC phase I trials. Methods: This is an investigator-initiated phase I trial (NCT04179162) that will study the safety of alternating intravesical gemcitabine and BCG. Inclusion and exclusion criteria are designed so most patients who would ordinarily be re-treated with BCG or BCG/IFN would be eligible. Patients must have recurrent high-grade NMIBC within 24 months of their last BCG treatment without meeting the criteria for BCG-unresponsive NMIBC. Intravesical gemcitabine is given twice a week on weeks 1, 4, 7, and 10, for a total of 8 doses. BCG (50 mg) is given once a week on weeks 2, 3, 5, 6, 8, and 9, for a total of 6 doses. The trial is monitored using a modified continual reassessment method with increasing dose levels of gemcitabine (500 mg, 1,000 mg, 1,500 mg, and 2,000 mg) being evaluated. Adverse events are assessed using the Common Terminology Criteria for Adverse Events version 5.0. The primary objective is to determine the maximum tolerated dose of this combination to inform our planned phase II trial. Correlative studies will look at the immunomodulating effects of gemcitabine by evaluating changes in immune cell populations in serial blood and urine specimens. Tissue and urine will also be evaluated for molecular determinants of response and resistance to the combination. The trial is open to enrollment with 10 of 25 planned patients accrued to date. Clinical trial information: NCT04179162