A CDR-GRAFTED (HUMANIZED) DOMAIN-DELETED ANTITUMOR ANTIBODY

While several murine monoclonal antibodies (MAbs) directed against car cinoma associated antigens have shown excellent tumor targeting proper ties in clinical trials, the use of radiolabelled MAbs for both diagno stic and therapeutic applications has been hindered by two factors: (a ) the induction of host anti-immunoglobulin (Ig) responses and (b) slo w plasma clearance of unbound radiolabelled MAb, resulting in bone mar row toxicity for therapeutic application, and long intervals between M Ab administration and tumor detection for diagnostic applications. Thi s report describes the development of the first recombinant Ig with pr operties designed to reduce or eliminate both of the above problems: a complementarity determining region (CDR)-grafted humanized (Hu) MAb w ith a CH2 domain deletion (Delta CH2). 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, breas t ovarian, prostate, pancreatic and lung carcinomas. When characterize d for antigen binding in solid phase competition radioimmunoassays, th e HuCC49 Delta CH2 MAb completely inhibited the binding of murine (mu) CC49 and HuCC49 for TAG-72. The relative affinity constants (K-a) of MAbs HuCC49 Delta CH2, HuCC49 and muCC49 were 5.1 x 10(-9), 2.1 x 10(- 9) and 2.3 x 1O(-9) respectively. The plasma clearance of I-131-HuCC49 Delta CH2 was significantly faster than that of intact I-125-HuCC49 a fter either i.v. or i.p. administration in athymic mice (p(2)0.05). Bi odistribution studies in athymic mice bearing human colon carcinoma xe nografts after i.v. or ip. administration of I-131-HuCC49 Delta CH2 an d I-125-HuCC49 demonstrated the efficient tumor localization and subst antially lower percent of the injected dose (%ID/g) of the HuCC49 Delt a CH2 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 HuCC49 Delta CH2 for most normal tissues tes ted (p(2)0.05). The differential between the rate of plasma clearance of HuCC49 Delta CH2 and HuCC49 was even more pronounced in SCID mice, which have been shown to be an appropriate model to study the metaboli sm of human IgG. These studies thus describe the development of a reco mbinant Ig molecule which, for the first time, combines 1) the propert ies 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. T hese studies also demonstrate the potential utility of HuCC49 Delta CH 2 for i.p. as well as i.v. radioimmunodiagnosis and radioimmunotherapy in patients with TAG-72 positive tumors.