Microscopic intratumoral dosimetry of radiolabeled antibodies is a critical determinant of successful radioimmunotherapy in B-cell lymphoma

Radioimmunotherapy is a highly effective treatment for some hematologic malignancies; however, the underlying mechanisms of tumor clearance remain poorly understood. We have previously shown that both targeted radiation using 131I-labeled anti-MHC class II (MHCII) monoclonal antibody (mAb) plus mAb signaling with unlabeled anti-idiotype are required for the long-term clearance of tumor in syngeneic murine lymphoma models. In this study, we have investigated how the microdistribution of the targeted radiation component of this combination affects the long-term clearance of lymphoma. 131I-labeled mAb targeting CD45 and MHCII antigens was found to deliver similar doses of radiation to tumor-bearing organ using conventional dosimetry (∼1.0 Gy per MBq when 131I was labeled to 500 μg mAb and given i.v. per mouse), but when used as radiation vectors in combination therapy only, 131I-anti-MHCII plus anti-idiotype produced long-term survival. The profound differences in therapy did not seem to be dependent on levels of 131I-mAb tumor-binding or antibody-dependent cytotoxicity. Instead, the microscopic intratumoral dosimetry seemed to be critical with the 131I-anti-MHCII, delivering more concentrated and therefore substantially higher radiation dose to tumor cells. When the administered activity of 131I-anti-CD45 was increased, a radiation dose response was shown in the presence of anti-idiotype and long-term survival was seen. We believe that these new insights should influence the selection of new antigen targets and the design of dosimetric methods in radioimmunotherapy of lymphoma. ©2007 American Association for Cancer Research.