Evidence Of Efficacy Of Antibody Directed Enzyme Prodrug Therapy (Adept) In A Phase I Trial In Patients With Advanced Carcinoma.

Background: Antibody Directed Enzyme Prodrug Therapy (ADEPT) uses an antibody linked to an enzyme to deliver enzyme selectively to tumor deposits. When enzyme has localized to the tumor and cleared from the circulation, a low-toxicity prodrug is given. The prodrug is converted into an active cytotoxic drug by the enzyme in tumor deposits. Each targeted enzyme can convert many molecules of the prodrug and a bystander effect is generated. We have previously reported on dose-finding, immunogenicity, and toxicity in a Phase I clinical trial and that the components of ADEPT function as designed in patients (Mayer A, et al. Clin Cancer Res 2006;12:6509-16). Evidence of efficacy is now reported in relation to total prodrug dose (TPD), toxicity, and repeated treatment. Methods: A fixed dose of recombinant fusion protein (MFECP1) was administered intravenously (iv). It consisted of single chain Fv targeting carcinoembryonic antigen (CEA) and carboxypeptidase G2 (Tolner B, et al. Nature Protocols 2006;1:1006-21; Tolner B, et al. Nature Protocols 2006;1:1213-22). Bis-iodo phenol mustard prodrug was given iv when carboxypeptidase activity had cleared from the blood. Patients had measurable, previously treated, advanced malignancy. Either 1, 2, or 3 ADEPT treatments were given over 2 to 10 days. Results: 43 patients with CEA expressing tumors were treated (colorectal, gastro-esophageal, breast, unknown primary, gallbladder, peritoneal, appendix, pancreas). TPD per patient was escalated from 37 mg/m2 to 3226 mg/m2. Thrombocytopenia and neutropenia were the principal side effects, resulting in a maximum tolerated TPD dose of 1200 mg/m2 divided into 2 complete ADEPT treatments. Human anti-enzyme antibody developed in 40% of patients having a single treatment, in 75% of patients after 2 treatments, and in 100% of patients after 3 treatments; it is not yet known whether this will compromise repeated therapy. Efficacy was assessed by 18F FDG-PET using the criteria for partial response defined by Green et al. and shown to predict significant survival prolongation (Eur J Nucl Med Mol Imaging 2008;35:393-406). Four of 9 patients had partial response at TPD ≥ 900 mg/m2. There was a significant correlation between TPD and reduction of tumor FDG uptake (P=0.031). Response was corroborated by falls in serum CA19-9 ≥ 25% in the 3 patients with partial response who had raised levels before treatment. Stable disease by RECIST criteria was seen in 69% of patients having TPD ≥ 900 mg/m2. Conclusions: Evidence of efficacy was shown at a TPD ≥ 900 mg/m2 with clinically significant responses in 44% of patients. These results support the case for conducting a randomized Phase II clinical trial.