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Superficial transitional cell carcinoma (STCC) of the bladder is usually managed with intravesical bacille Calmette-Guerin. Recombinant human interleukin-12 (rhIL-12) is a heterodimeric cytokine that induces T-cell and natural killer cell proliferation/activation and production of IFNγ. It has demonstrated preclinical in vivo bladder antitumor activity and no systemic toxicity. This phase 1 study evaluated intravesical rhIL-12 administration in patients with STCC (Tis, Ta, or T1) who had failed at least one prior intravesical therapy or had at least two recurrences of low-grade tumors. Eligible patients had adequate hematologic, renal, and hepatic function and Karnofsky performance status at least 70%. Cohorts of three patients received 5, 20, 50, 100, and 200 μg rhIL-12 (treated n = 15). Each patient received intravesical rhIL-12 weekly for 6 weeks, with a 2-hour bladder dwell. No patient experienced moderate, severe, or life-threatening systemic toxicity. Treatment-related adverse events included dysuria, urinary frequency, urinary urgency, asthenia, pain, hematuria, bladder spasms, and chills. Specific AE incidences were not dose-related. Among 12 patients without visible pretreatment lesions, 7 remained disease-free and 5 experienced recurrence of STCC within the 4-week follow-up period. Three patients with pretreatment Tis or Ta/T1 lesions had persistent disease at posttreatment follow-up. No patients with persistent tumor manifested antitumor response to rhIL-12. Two-hour dwell urine samples and 24- to 30-hour postdose serum samples showed negligible induction of IFNγ. In summary, intravesical rhIL-12 was well tolerated by patients with recurrent STCC but showed no clinically relevant evidence of antitumor or immunologic effects.