Facile Synthesis of a PNA Oligomer containing 2,2´-Dipicolylamine as Chelator for ^99mTc and ¹⁸⁸Re

Since their discovery in 1991, Peptide Nucleic Acids (PNAs) have been used in molecular biology, mainly for detecting single base mismatches in oligonucleotide sequences. [1] Due to their non natural pseudo-peptide backbone, which replaces the phosphate-ribose backbone of natural DNA, PNAs are stable against enzymatic degradation. Furthermore, as they form very stable hybrids with both DNA and RNA, PNAs have also been thought to be useful for antisense and antigene therapies. Nuclear medicine is another field of research where PNAs have been used, as, for example, possible agents in the so-called “pretargeting approach”.[2] In this context, we are interested in the radiolabeling of PNA strands with therapeutically relevant nuclides, e.g. ¹⁸⁸Re. En route to explore the wide potential of this “pretargeting using PNA” principle, we prepared a new azido derivative of 2,2´-dipicolylamine, namely 2-azido-N,N-bis((pyridin-2-yl)methyl)ethanamine (Dpa-N₃). It was demonstrated that Dpa-N₃ could be efficiently labeled with both [Re(CO)₃(H₂O)₃]Br and [^99mTc(H₂O)₃(CO)₃]⁺ to give [Re(CO)₃(Dpa-N₃)]Br and [^99mTc(CO)₃(Dpa-N₃)]⁺, respectively. Furthermore, Dpa-N₃ was successfully coupled to a PNA oligomer. Subsequent labeling of Dpa-PNA with [^99mTc(H₂O)₃(CO)₃]⁺ afforded [^99mTc(CO)₃Dpa-PNA] in radiochemical yields > 90% (Figure 1). Biodistribution studies of [^99mTc(CO)₃Dpa-PNA] in Wistar rats showed a very fast blood clearance and modest accumulation in the kidneys. There was no significant activity in the thyroid and the stomach, demonstrating a high in vivo stability of the ^99mTc-labeled Dpa-PNA conjugate.

[1] P. E. Nielsen, M. Egholm, R. H. Berg, O. Buchhardt, Science, 1991, 254, 1497
[2] G. Mardirossian, K. Lei, M. Rusckowski, F. Chang, T. Qu, M. Egholm, D.J. Hnatowich, J Nucl Med, 1997, 38, 907
[3] G. Gasser, K. Jäger, M. Zenker, R.Bergmann, J. Steinbach, H. Stephan, N. Metzler-Nolte, 2010, submitted.