Targeting of Human Breast Cancer by a Bispecific Antibody Directed against Two Tumour-Associated Antigens: ErbB-2 and Carcinoembryonic Antigen

Carcinoembryonic antigen (CEA) and ErbB-2 are expressed in about 50 and 30% of breast cancers, respectively. We hypothesised that targeting of these two antigens by a bispecific antibody (BAb) might provide efficient tumour uptake and prolonged tumour residence time. In the present study, we first studied the expression of CEA and ErbB-2 on primary breast tumours screened by immunohistochemistry. Of 106 primary breast cancers, 69 (65%) were positive for CEA, 20 (19%) were positive for ErbB-2, and 13 (12%) expressed both antigens. We then prepared and evaluated a BAb directed against CEA and ErbB-2. Using BIACORE technology, we showed that the BAb recognised both CEA and ErbB-2 with affinities of 0.9 × 10 and 0.8 × 10 M–1, respectively. In vivo, BAb tumour localisation was compared with that of its parental homodimeric F(ab′)2-ortho-phenylene- dimaleimide (PDM) fragments. Uptake of 125I-BAb was lower than that of 131I-35A7F(ab′)2-PDM in LS174T tumours, used as a model of CEA expressing tumours, and was similar to that of 131I-FWP51 F(ab′)2-PDM in SKOv3 tumours, used as a model of ErbB-2 expressing tumours. In a double-positive model, the SKOv3-CEA-1B9 tumour, BAb showed a similar uptake to that of 35A7 F(ab′)2-PDM and we demonstrated that, although BAb had double specificity, it internalised as a homodimeric anti-ErbB-2 antibody. BAb showed a greater uptake than that of FWP51 F(ab′)2-PDM and this difference was even more important 72 h after injection with an uptake of 7.3 ± 2.1 vs. 1.4 ± 0.5% of the injected dose per gram of tissue. The results obtained with the BAb in the double-positive tumour-bearing nude mice suggest that targeting two distinct tumour-associated antigens on the same cell could improve tumour localisation.