Metastatic human colonic carcinoma: Molecular imaging with pretargeted SPECT and PET in a mouse model

Purpose: To prospectively determine if a bispecific monoclonal antibody (MoAb) pretargeting method with a radiolabeled hapten peptide can depict small (<0.3 mm in diameter) microdisseminated human colon cancer colonies in the lungs of nude mice. Materials and Methods: Animal studies were approved in advance by animal care and use committees. Animals injected intravenously with a human colon cancer cell line to establish microdisseminated colonies in the lungs were pretargeted with TF2-a recombinant, humanized, anti-carcinoembryonic antigen (CEA) and anti-histamine-succinyl-glycine (HSG) bispecific MoAb; 21 hours later, a radiolabeled HSG peptide was given. Imaging and necropsy data for tumor-bearing animals given the anti-CEA bispecific MoAb (n = 38, all studies) were compared with those of animals given fluorine 18 (F-18) fluorodeoxyglucose (FDG) In = 15, all studies), peptide alone (n = 20, all studies), or an irrelevant anti-CD22 bispecific MoAb (n = 12, all studies). Uptake of these agents in the lungs of non-tumor-bearing animals enabled assessment of specificity (n = 15, 4, and 6 for TF2 pretarget, hapten peptide alone, and F-18-FDG, respectively). Results: TF2-pretargeting helped localize tumors in the lungs within 1.5 hours of the radiolabeled HSG peptide injection, while the peptide alone, irrelevant bispecific MoAb pretargeted peptide, and F-18-FDG failed. Necropsy data indicated that the signal in tumor-bearing lungs was five times higher than in blood within 1.5 hours, increasing to 50 times higher by 24 hours. Peptide uptake in tumor-bearing lungs pretargeted with TF2 was nine times higher than in non-tumor-bearing lungs, while it was only 1.5-fold higher with F-18-FDG or the peptide alone. Micro-positron emission tomographic (PET) images showed discrete uptake in individual metastatic tumor, colonies; autoradiographic data demonstrated selective targeting within the lungs, including metastases less than 0.3 mm in diameter. Conclusions: Bispecific antibody pretargeting is highly specific for imaging micrometastatic disease and may thus provide a complementary method to F-18-FDG at clinical examination. (C) RSNA, 2008.