Further investigations of morpholino pretargeting in mice-establishing quantitative relations in tumor

PURPOSE: This laboratory has previously published on phosphorodiamidate morpholino (MORF) pretargeting of tumor in which an anti-tumor antibody conjugated with MORF (a DNA analogue) is first administered, followed at a later time by the radiolabeled complementary MORF (cMORF) as the effector. In the present study, the pharmacokinetics of the antibody and effector were measured under different conditions in mice to establish their quantitative relationships with tumor accumulations by pretargeting. METHODS: [methods removed to fit abstract in Gene Tools database] RESULTS: (1) The biodistribution of antibody in percent accumulation (%ID or %ID/g) was largely independent of antibody dose but the absolute accumulation of antibody in tumor increased linearly with dose, showing no evidence of tumor saturation of CEA sites by MN14. Over 1-3 days post antibody administration, blood levels of radiolabeled antibody decreased as expected; however, tumor levels remained constant, thus showing an absence of antibody clearance in tumor over this period. (2) With fixed antibody-MORF dose and increasing number of injections of (99m)Tc-cMORF, cumulative percent blood levels steadily decreased in agreement with the values calculated based on the antibody-MORF in blood. In contrast, cumulative percent tumor levels stayed fairly constant over the first two injections. Thus the antibody-MORF in tumor became saturated with cMORF more slowly than that in blood owing to delivery differences. (3) As expected, percent blood levels decreased with increasing interval between injections of antibody-MORF and (99m)Tc-cMORF. The percent tumor accumulation, however, remained constant over the 3 day interval, thus demonstrating only slow loss of MORF expression in situ. The (99m)Tc-cMORF accumulation in tumor after saturation was mathematically determined based on the antibody-MORF concentration in tumor while the blood levels of (99m)Tc-cMORF were determined based on the concentration of antibody-MORF in blood. CONCLUSION: Contrary to conclusions arrived at in our earlier study, the results of this study show that tumor CEA sites were not saturated even at the highest antibody dose investigated, that accessibility of MORF sites in tumor by (99m)Tc-cMORF was unhindered and that the maximum percent tumor accumulation of (99m)Tc-cMORF depended only on the tumor delivery efficiency of (99m)Tc-cMORF.