A CDR-Grafted (Humanized) Domain-Deleted Antitumor Antibody

While several murine monoclonal antibodies (MAbs) directed against carcinoma associated antigens have shown excellent tumor targeting properties in clinical trials, the use of radiolabeled MAbs for both diagnostic and therapeutic applications has been hindered by two factors: (a) the induction of host anti-immunoglobulin (Ig) responses and (b) slow plasma clearance of unbound radiolabeled MAb, resulting in bone marrow toxicity for therapeutic application, and long intervals between MAb administration and tumor detection for diagnostic applications. This report describes the development of the first recombinant Ig with properties designed to reduce or eliminate both of the above problems: a complementarity determining region (CDR)-grafted humanized (Hu) MAb with a CH2 domain deletion (ΔCH2). The MAb chosen for engineering was CC49, which is directed against a pancarcinoma antigen designated TAG-72 that is expressed on the majority of colorectal, gastric, breast, ovarian, prostate, pancreatic and lung carcinomas. When characterized for antigen binding in solid phase competition radioimmunoassays, the HuCC49ΔCH2 MAb completely inhibited the binding of murine (mu) CC49 andHuCC49for TAG-72. The relative affinity constants (K_a) of MAbs HuCC49ΔCH2, HuCC49 and muCC49 were 5.1× 10^-9, 2.1×10^-9 and 2.3×10^-9, respectively. The plasma clearance of ¹³¹ I-HuCC-49ΔCH2 was significantly faster than that of intact ¹²⁵I-HuCC49 after either i.v. or i.p. administration in athymic mice (p²0.05). Biodistribution studies in athymic mice bearing human colon carcinoma xenografts after i.v. or i.p. administration of ¹³¹I-HuCC49ΔCH2 and ¹²⁵I-HuCC49 demonstrated the efficient tumor localization and substantially lower percent of the injected dose (%ID/g) of the HuCC49ΔCH2 in normal tissues. This is reflected in the significantly higher radiolocalization indices (%ID/g in tumor divided by %ID/g in normal tissue) observed with the HuCC49ΔCH2 for most normal tissues tested (p²0.05). The differential between the rate of plasma clearance of HuCC49ΔCH2 and HuCC49 was even more pronounced in SCID mice, which have been shown to be an appropriate model to study the metabolism of human IgG. These studies thus describe the development of a recombinant Ig molecule which, for the first time, combines 1) the properties of more rapid blood clearance than an intact humanized Ig molecule--without loss of antigen binding affinity -- and 2) reduced potential for eliciting a human anti-murine antibody (HAMA) response in patients. These studies also demonstrate the potential utility of HuCC49ΔCH2 for i.p. as well as i.v. radioimmunodiagnosis and radioimmunotherapy in patients with TAG-72 positive tumors.