Double-Tracer Dosimetry of Organs in Assessment of Bone Marrow Involvement by Two Monoclonal Antibodies

Abstract

Five patients with ductal breast cancer were studied using simultaneous administration of 99Tcm-labelled BW250/183 and 131I-labelled B72.3 monoclonal antibodies (MAbs). The distribution and dosimetry of these tracers were evaluated using the information from simultaneous anterior and posterior whole body scintigrams, together with 99Tcm and 131I standard activity sources, recorded on an average of 1, 4, 24, 90 and 224 h after injection. A method to eliminate 131I Scatter on 99Tcm-channel was developed. The geometric means of conjugate views and region-of-interest analysis were used to determine organ uptakes, mean residence times and absorbed radiation dose estimates of organs induced by the tracers. Organ uptakes (% of injected activity/ml) varied from 6.2 x 10(-3 /red marrow to 3.1 X 10(-2)/liver for 99Tcm-MAb and from 3.1 x 10(-2)/red marrow to 1.8 x 10(-1)/liver for 131I-MAb, one hour after injection. Calculated average residence times of organs for 99Tcm-labelled BW250/183 were in the range of physical mean-life of 99Tcm and from 71 to 95 h for 131I-B72.3 respectively. The average total absorbed dose from 99Tcm-MAb to the bone marrow was 0.01 and to the spleen 0.14 mGy/MBq and from 131I-MAb the corresponding values were 0.48 and 10.76 mGy/MBq. This double-tracer technique provides information from two antibodies having different kinetic behaviour and may facilitate in distinguishing various antigens in targeting and control MAb applications.