Localization and imaging of human plasmacytoma xenografts in severe combined immunodeficiency mice by a new murine monoclonal antibody, anti-HM1.24.

We have examined the localization in the tumor of anti-HM1.24, a murine monoclonal antibody which is specific for human plasma cell-associated antigen. The biodistribution and imaging were compared in severe combined immunodeficiency mice bearing human plasmacytoma (RPMI 8226) xenografts after intravenous injection of radiolabeled anti-HM1.24. The mean tumor uptake of 1 microCi of 125I-labeled anti-HM1.24 was 2.98% injected dose per g of tissue (%ID/g) at day 1 and increased to 5.50% ID/g at day 4, whereas tumor uptake values of control IgG was always less than 1.36% ID/g. The tumor localization indices ranged between 2.35 and 6.03 at day 1 to 4 after injection. Anti-HM1.24 showed no evidence of targeting to normal tissues except a splenic uptake, however, splenic targeting was abolished by the blocking of Fc receptors in the reticuloendothelium. In most cases, tumor-to-tissue ratios were greater than 2 at day 4, indicative of tumor selectivity for anti-HM1.24. Radioimaging with 10 microCi of 131I-labeled anti-HM1.24 showed that 25% of the total-body count were localized in the tumor and the tumors could be visualized without subtraction techniques at day 4. Furthermore, the penetration and binding of anti-HM1.24 to the tumor cells were confirmed by autoradiographic studies. These findings indicate that anti-HM1.24 is a potent agent for detection and targeting of human plasmacytoma.