The influence of chain length and base sequence on the pharmacokinetic behavior of 99mTc-morpholinos in mice

Background. Despite in vivo use now over several years, in particular for nuclear medicine imaging, the influences on pharmacokinetics of chain length and base sequence of radiolabeled oligomers; has not been investigated. Methods. As test oligomer, morpholinos (MORFs), a DNA analogue, were radiolabeled with Tc-99m via MAG(3) and the pharmacokinctics in normal mice determined for 3 chain lengths (15,18 and 25 mer) and 2 base sequences (MORF and its complement cMORF). In addition, LS174T-tumor bearing nude mice received the anti-CEA antibody MN14 (Immunomedics) conjugated either with MORF15 or MORF18 and subsequently received Tc-99m-labeled cMORF15 or cMORF18 respectively in a pretargeting strategy. Results. In normal mice, after 1 hr, regardless of chain length or sequence, all labeled MORFs and cMORFs accumulated only slightly in all tissues (e.g. at 3 hr <0.15 ID%/g) except in kidneys. Besides being excessive, the kidneys were the only tissue with levels dependent upon chain length (e.g. at 1 hr, 5, 7 and 22 IDO/o/g for MORF15, 18 and 25, respectively) and sequence (e.g. at 3 hrs 9 ID%/g for MORF25 and 21 IDO/o/g for cMORF25). Identical biodistribution trends were observed in tumored mice with all tissues including tumor Showing levels independent of chain length or base sequence except for kidneys. Furthermore, while all other tissues cleared in the interval from 1-3 hrs, kidney levels remained constant in both normal and tumored animals. Largely because of these differences in kidneys, images obtained by pretargeting with Tc-99m-cMORF15 were superior compared to Tc-99m-cMORF18 (images of control animals not receiving the antibody showed no tumor at all). Conclusions. judged by radiolabel accumulations in tissue, the pharmacokinetics of Tc-99m labeled morpholinos were independent of chain length and base sequence. The only obvious exception was kidneys in which accumulations were significantly higher for the longer chain lengths and significantly different for cMORF vs MORF. These results show that chain length and base sequences may be varied to alter the pharmacokinetics of radiolabeled oligomers in nuclear medicine imaging studies.