Synthesis, biodistribution and metabolite analysis of different [⁸⁶Y]DOTA-L-RNA oligonucleotides in rats

Introduction:

Mirror-image oligonucleotides are constructed of an L-configured ribose or deoxyribose. This modification leads to an extraordinary high metabolic stability against enzymatic degradation in biological environments. The radiolabeling of an L-oligonucleotide with the positron emitter ⁸⁶Y (t_1/2 = 14.7 h) is done via the bifunctional chelator approach. The influence of two different [⁸⁶Y]DOTA chelates on the radiopharmacological properties of [⁸⁶Y]DOTA-L-oligonucleotides was investigated with an L-RNA 12mer 1 (sequence: 5'-Aminohexyl UGA CUG ACU GAC-3' MW 3975). Both ⁸⁶Y-labeled L-RNAs 3a, 5a were characterized by a comparative biodistribution study in Wistar rats and metabolite analyses.

Experimental:

The amine functionality of the 5'-hexylamine modified L-RNA 1 was used for conjugations with DOTA-NHS ester 2 and (S)-p-SCN-bz-DOTA 4, accomplished in the formation of a selective amide bond 3 and thiourea bond 5, respectively. After ⁸⁶Y-labeling of 3 and 5 biodistribution studies were performed for each compound 3a, 5a with eight Wistar rats.

Results and Discussion:

Both DOTA-modified L-RNAs 3, 5 were radiolabeled with ⁸⁶Y(III) (QSA Global, Germany) with radiochemical yields of 76 % and 85 %, respectively. High renal excretions were found for both ⁸⁶Y-labeled L-RNAs 3a, 5a whereas differences in the retention were observed for the radiolabeled compounds in the kidneys and the adrenal glands. The standardized uptake value (SUV) of compound 3a in the kidneys reached 10 ± 2.0 after 5 min and decreased to 6.1 ± 0.53 after 60 min. For compound 5a the SUV in the kidneys increased from 13 ± 1.6 after 5 min to 14 ± 1.1 after 60 min. Remarkably high SUVs (3.5 ± 0.48 and 3.2 ± 0.33) were also observed in the adrenal glands after 60 min for both compounds. The SUVs in other organs were below 1.0 after 60 min for 3a and 5a in this study. Over 85% intact 3a and 5a were found in urine samples of Wistar rats 60 min after application of 3a, 5a as confirmed by Radio-HPLC.

Conclusion:

Differences in the kidney excretion profile of 3a and 5a indicate an influence of the different chemical attachments of the [⁸⁶Y]DOTA chelates to L-RNA 1. The high metabolic stability of the L-RNA suggests the potential of L-oligonucleotides as molecular probes for PET.

Acknowledgement: This work was supported by the EU-FP6 integrated project BioCare contract no.: 505785.