Background: The standard of care for high-risk non-muscle invasive bladder cancer (NMIBC) after exposure to induction only BCG or relapse >12 months after “adequate” BCG is retreatment with BCG. However, ~50% of patients will relapse within 6 months. Thus, there is a critical need to develop novel combination therapies to improve BCG immunotherapy efficacy. Combination BCG with immune checkpoint inhibition is associated with a ~12-18% risk of serious treatment-related adverse events and BCG combined with Mitomycin C (MMC) has unacceptable urinary toxicity. Intravesical Gemcitabine (GEM) is a common treatment for NMIBC, is well tolerated, has better efficacy after BCG than MMC, and, based on preclinical data, may synergistically enhance the efficacy of BCG by augmenting the immune tumor microenvironment. We completed a prospective Phase I trial of chemoimmunotherapy with gemcitabine and BCG in BCG-relapsed/BCG-exposed NMIBC that was well-tolerated (no treatment related grade ≥3 adverse events) with promising early efficacy (95% [19/20] 6-month complete response rate). No dose limiting toxicities were seen. The maximum tolerated dose (MTD) was 2000 mg gemcitabine and 50 mg TICE BCG. Methods: To address the need for more effective and less toxic bladder preserving treatment options by enhancing the effectiveness of BCG, we launched an investigator-initiated phase I/II trial of combination intravesical gemcitabine with BCG in patients with BCG-exposed NMIBC (NCT04179162). The Phase II portion is currently enrolling patients with BCG-exposed carcinoma in-situ (CIS ±Ta/T1) that recurred within 24 months of last BCG (BCG-exposed/BCG-relapsing NMIBC). Patients with BCG-unresponsive disease, contraindication to BCG, or prostatic/ureteral urothelial disease are excluded. Induction intravesical GEM (2000mg, twice weekly) is given weeks 1, 4, 7, and 10 and intravesical BCG (40mg, Tice strain, weekly) is given weeks 2, 3, 5, 6, 8, and 9. Responders receive SWOG maintenance BCG. The primary endpoint is clinical complete response (CR) at 6 months (absence of high-grade disease on cystoscopy/TURBT and negative cytology) using a Simon’s optimal 2-stage design testing the null hypothesis of a true CR of 55% at 6 months (based on historical outcomes of BCG alone) against a 1-sided alternative. In the first stage, the study would stop if ≤ 9/15 patients had a CR. In the second stage, the null hypothesis will be rejected if ≥ 29 CRs are observed in 43 total patients (type I error rate 5% and power of 80% if the true CR rate is 75%). Phase I patients treated at the MTD (14/25) are included in Phase II. Secondary outcomes include recurrence-free, progression-free, and cystectomy-free survival. Correlative studies explore immune and molecular predictors of response and resistance to chemoimmunotherapy in tumor tissue, urine, and blood. Clinical trial information: NCT04179162.