An ⁸⁶Y-Labeled Mirror-Image Oligonucleotide: Influence of Y-DOTA Isomers on the Biodistribution in Rats

A mirror-image oligonucleotide (L-RNA) was radiolabeled with the positron emitting radionuclide ⁸⁶Y (t_1/2 = 14.7 h) via the bifunctional chelator approach. DOTA-modification of the L-RNA (sequence: 5’-Aminohexyl UGA CUG ACU GAC-3’; MW 3975) was performed using (S)-p-SCN-bz-DOTA.

⁸⁶Y radiolabeling of the DOTA-bz-L-RNA produced more than one species as evidenced by HPLC radiometric detection. For the identification of the ⁸⁶Y-labeled L-RNA the structural analogue ‘cold’ precursor [Y((S)-p-NH₂-bz-DOTA)]^- was synthesized. Two coordination isomers were separated via HPLC adopting the square antiprismatic (SAP) and the twisted square antiprismatic (TSAP) geometry, respectively. Their stereochemical configuration in the solution state was assessed by NMR and circular
dichroism spectroscopy. Both [Y((S)-p-NH₂-bz-DOTA)]^- isomers were converted into isothiocyanate derivatives [Y((S)-p-SCN-bz-DOTA)]^- and conjugated to the L-RNA. The identity of the [⁸⁶Y-DOTA-bz]-L-RNA species was finally established by comparison of the radiometric (⁸⁶Y) and UV-visible (‘cold’ Y) chromatographic profiles.

Biodistribution studies in Wistar rats showed minor changes in the biodistribution profile of the [⁸⁶Y((S)-p-NH₂-bz-DOTA)]^- complex isomers, while no significant differences where observed for the [⁸⁶Y-DOTA-bz]-L-RNA isomers. High renal excretions were found for the [⁸⁶Y((S)-p-NH₂-bz-DOTA)]^- complex isomers as well as for the L-RNA isomers.