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: A cytosine-free 18 mer cMORF was conjugated with MAG(3) for (99m)Tc labeling while the anti-CEA antibody MN14 was conjugated with DTPA for (111)In labeling and with MORF to impart binding affinity for radiolabeled cMORF. Mice bearing LS174T thigh tumors were used to study: (1) the pharmacokinetics of MN14-MORF by administering (111)In-MN14 at doses between 10 and 100 mug with sacrifice at 2 days and at 30 microg with sacrifice between 1 and 3 days; (2) the biodistribution of (99m)Tc-cMORF following one to four injections (containing 0.15 microg each and separated by 1 h) to animals having received 30 microg of antibody-MORF 2 days earlier and with sacrifice at 3 h after the final injection; and (3) the influence on the biodistribution of (99m)Tc-cMORF of a 2 to 4 day interval between the administration of 30 microg of antibody-MORF and 0.30 microg of (99m)Tc-cMORF.
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.