Purpose: Our purpose in the study was to establish the maximum tolerated dose and toxicity profile of SGN-10 (or BR96 sFv-PE40), a single-chain immunotoxin. SGN-10 is composed of the fused gene products encoding the translocating and ADP-ribosylating domains of Pseudomonas exotoxin (PE40) and the variable heavy (VH) and variable light (VL) regions of BR96 monoclonal antibody. This antibody is specific for a LewisY (LeY)-related carbohydrate antigen expressed on multiple carcinomas. Experimental Design: A total of 46 patients with LeY-positive metastatic carcinoma were enrolled in a Phase I dose-escalation study in cohorts of three to six patients who received SGN-10 at doses ranging from 0.024 to 0.962 mg/m2, administered on days 1, 4, 8, and 11, followed by 2 weeks of rest and a second cycle of therapy. Pharmacokinetics and human antibody response to SGN-10 were also determined. Results: The maximum tolerated dose of SGN-10 was 0.641 mg/m2 with gastrointestinal dose-limiting toxicity. Pharmacokinetic studies performed in eight patients at the 0.641-mg/m2 dose revealed a t[1/2] of 2.5 ± 0.3 h and a Cmax of 389 ± 112 ng/ml. Pharmacodynamic analyses demonstrated a rapid clearance of the drug by day 11 associated with an antitoxin human antitoxin antibody (HATA) response in most patients. Signs consistent with a modest vascular leak syndrome, specifically, transient hypoalbuminemia, were observed in patients treated with doses of ≥0.384 mg/m2. No complete or partial tumor responses were observed at an 8-week evaluation, although 31% of patients had stable disease. Conclusions: The maximal tolerated dose of SGN-10 given twice weekly for 2 weeks is 0.641 mg/m2 with gastrointestinal dose-limiting toxicity. The immunogenicity of the toxin moiety limits the ability of SGN-10 to circulate by day 11 of therapy. Studies are ongoing to evaluate strategies to ameliorate toxicities and to inhibit the development of the anti-SGN-10 immune response.