Background: Despite undergoing transurethral resection and appropriate Bacillus Calmette–Guérin (BCG) administration for high-risk non-muscle invasive bladder cancer (NMIBC), the non-responder rate is approximately 25%. Although radical cystectomy is the gold standard of treatment recommended by various international guidelines, many patients are either unwilling or ineligible for this therapeutic option. In Argentina, keyhole limpet hemocyanin (immunocyanin KLH) is approved for those patients. The primary objectives of this study were to evaluate the overall treatment response, defined as disease-free survival (recurrence and/or local or distant progression), and adverse events. Methods: A retrospective, two-centre, single-arm clinical study with a quantitative exploratory and documentary approach was designed. Seventy patients diagnosed with NMIBC who were non-responders to appropriate BCG were evaluated between January 2001 and August 2022, with a minimum follow-up duration of one year. All patients completed an induction regimen of KLH and at least three maintenance treatments, which involved the intravesical administration of 20 mg of KLH in 60 mL of physiological saline weekly for six weeks during the induction phase, followed by monthly maintenance with the same dose. Cystoscopic controls were performed every three months during the first year and every three to four months in the second and third years. All suspicious lesions were completely resected, and cytological samples were systematically collected. Results: The mean age of the patients was 69.1 years (SD: 8.1). The mean follow-up duration from the first KLH dose was 60.9 months, with a median of 53 months. The estimated one-, two- and three-year disease-free survival rates were 57.1%, 33.5% and 30.1% (95% CI: 45.5–68.7, 22.3–44.7 and 19.1–41.1), respectively, with a median disease-free survival of 14 months (95% CI: 10.4–17.6). Multifocality, pT1 classification and lymphovascular invasion were identified as adverse factors in multivariate analysis. Toxicity was well tolerated, with six patients (8.6%) experiencing mild urinary infections during treatment (adverse event grade 2). Conclusions: The use of KLH in patients with NMIBC who are non-responders to BCG therapy shows favourable outcomes in terms of disease-free survival and exhibits an excellent safety profile. These findings support KLH as a potential treatment option for this challenging patient population.
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