CAT-8015: A Second-Generation Pseudomonas Exotoxin A-Based Immunotherapy Targeting CD22-Expressing Hematologic Malignancies

Purpose: To compare the in vitro and in vivo efficacy of CAT-8015, a second-generation recombinant immunotoxin composed of disulfide-linked affinity matured V-H and V-L chains of the mouse anti-CD22 monoclonal antibody RFB4 fused to PE38, to the parental compound CAT-3888. Experimental Design: The biological activity of CAT-8015 was examined in vitro using B-cell tumor lines and in vivo in a JD38-based s.c. tumor model in NCr athymic mice. Pharmacokinetics and interspecies scaling of CAT-8015 were evaluated in mice, rats, and cynomolgus monkeys. The potential toxicity of CAT-8015 was assessed in monkeys in a toxicologic study and compared with CAT-3888. Results:The IC50 values of CAT-8015 in vitro using the EHEB, MEC1, Daudi, CA46, and JD38 cell lines ranged from 0.3 to 8.6 ng/mL. Pharmacokinetic studies with CAT-8015 were conducted in mouse, rat, and cynomolgus monkey. The t(1/2) was calculated to be 0.42, 0.61, and 0.79 hours and the Vss was 1.37, 5.57, and 140.3 mL in mouse, rat, and monkey, respectively. In vivo, when JD38 tumor-bearing animals were treated with CAT-8015 at doses >= 75 mu g/kg at 48-hour intervals for a total of three doses, a rapid reduction in tumor volume and in some cases complete remission in tumor growth was observed. The comparative toxicologic study showed comparable clinical and anatomic pathology changes for CAT-8015 and CAT-3888. Conclusions: CAT-8015 is a CD22-targeting immunotoxin that, in preclinical studies, has greatly improved efficacy compared with CAT-3888.