Background: One of the main limitations of radioimmunotherapy (RIT) is the secondary toxicity related to the poor therapeutic indices achieved with labeled whole immunoglobulin (Ig)G or F(ab')2 fragments. To overcome this problem, we have developed a two-step targeting method, which we refer to as the Affinity Enhancement System (AES), using a radiolabeled bivalent hapten and a bispecific antibody recognizing the hapten and a target cell antigen. This method has been applied successfully to immunoscintigraphy in carcinoembryonic antigen (CEA)-expressing carcinoma patients and increased tumor to normal tissue uptake ratios have been achieved. The aim of the current study was to evaluate the application of AES to RIT of CEA-expressing solid tumors in an animal model.
Methods: Nude mice grafted with LS174T human colorectal carcinoma were treated either with 111 megabecquerels (MBq) of iodine-131 labeled bivalent diethylenetriamine pentaacetic acid (DTPA) hapten 20 hours after pretargeting by anti-CEA x anti-DTPA-indium bispecific antibody or 12 MBq of iodine-131 labeled anti-CEA IgG.
Results: Treatment with the IgG induced only a growth delay of 53 +/- 5 days but all tumors progressed. Treatment with the AES was highly efficient because tumor growth inhibition was achieved over 150 days. Hematologic and overall toxicity of both treatments were equivalent.
Conclusions: The long term tumor regression consecutive to AES RIT represents a very significant improvement over the use of directly labeled IgG. Toxicity consecutive to AES or IgG RIT were similar despite an administered activity nearly ten times higher with the AES. However, given the efficacy of the AES treatment, a lower dose may afford lower toxicity and significant antitumor effect.