Radiolabeling morpholinos with 90Y, 111In, 188Re and 99mTc

Abstract This laboratory is investigating morpholinos (MORF), a DNA analogue, for radiopharmaceutical applications. While we routinely radiolabel with 99m Tc, we have now labeled MORFs with 111 In, 188 Re and 90 Y in anticipation of therapeutic studies. Methods: A 25 mer MORF with a primary amine on the 3′ equivalent end attached via a 10 member linker was conjugated with an isothiocyanate backbone derivative of DOTA (for labeling with 111 In and 90 Y) and with NHS-MAG 3 (for labeling with 188 Re and 99m Tc). The in vitro stability of labeled MORFs were investigated and biodistribution was carried out in normal mice. Results: As evident by size exclusion HPLC, ITLC and Sep-Pak analysis, all four radiolabeled MORFs were successfully radiolabeled. In each case, the labeled MORFs showed one sharp peak in HPLC that shifted completely to earlier retention times following addition of a polymer conjugated with the complementary MORF. In saline at room temperature and in 37°C serum, the radioactivity profile of 111 In, 188 Re and 99m Tc was unchanged over 48 h while over the same period, the 90 Y profile showed a pronounced lower molecular weight peak which did not shift and was shown to be most probably due to 90 Y-DOTA resulting from radiolysis. In addition, the recovery of 188 Re on HPLC decreased as samples aged probably due to oxidation to perrhenate which was retained by the HPLC column. The biodistributions at 1, 3 and 6 h in normal mice showed no important differences among all four labels with the exception that levels of radioactivity in stomach and thyroid were higher in the case of 188 Re due to in vivo oxidation of the radiolabel to perrhenate. Conclusions: When radiolabeled with DOTA, 90 Y-labeled MORF showed increased instabilities relative to that of 111 In and when radiolabeled with MAG 3 , 188 Re showed in vitro and in vivo instabilities compared to 99m Tc, but all labels were still largely intact after 48 h in saline or serum. Possibly because of the rapid clearance of MORFs, no important differences in biodistribution among 90 Y, 111 In and 99m Tc labels were evident in normal mice. These strategies for labeling MORF with 90 Y and 188 Re therefore appear to be suitable for therapeutic applications although both show some evidence of instabilities.