NOTA and NODAGA [Tc]Tc- and [Re]Re-Tricarbonyl Complexes: Radiochemistry and First Example of a [Tc]Tc-NODAGA Somatostatin Receptor-Targeting Bioconjugate.

With the long-term goal of developing theranostic agents for applications in nuclear medicine, in this work we evaluated the well-known NOTA and NODAGA chelators as bifunctional chelators (BFCs) for the [Tc/Re]Tc/Re-tricarbonyl core. In particular, we report model complexes of the general formula fac-[M(L)(CO)] (M = Re, Tc, Re) where L denotes NOTA-Pyr (1) or NODAGA-Pyr (2), which are derived from conjugation of NOTA/NODAGA with pyrrolidine (Pyr). Further, as proof-of-principle, we synthesized the peptide bioconjugate NODAGA-sst-ANT (3) and explored its complexation with the fac-[Re(CO)] and fac-[Tc][Tc(CO)] cores; sst-ANT denotes the somatostatin receptor (SSTR) antagonist 4-NO-Phe-c(DCys-Tyr-DTrp-Lys-Thr-Cys)-DTyr-NH. Rhenium complexes Re-1 through Re-3 were synthesized and characterized spectroscopically, and receptor binding affinity was demonstrated for Re-3 in SSTR-expressing cells (AR42J, IC = 91 nM). Radiolabeled complexes [Tc]Tc/[Re]Re-1/2 and [Tc]Tc-3 were prepared in high radiochemical yield (>90%, determined by radio-HPLC) by reacting [Tc]/[Re][Tc/Re(OH)(CO)] with 1-3 and correlated well with the respective Re-1 through Re-3 standards in comparative HPLC studies. All radiotracers remained intact through 24 h (Tc-labeled complexes) or 48 h (Re-labeled complexes) against 1 mM l-histidine and 1 mM l-cysteine (pH 7.4, 37 °C). Similarly, rat serum stability studies displayed no decomposition and low nonspecific binding of 9-24% through 4 h. Biodistribution of [Tc]Tc-3 in healthy CF-1 mice demonstrated a favorable pharmacokinetic profile. Rapid clearance was observed within 1 h post-injection, predominantly via the renal system (82% of the injected dose was excreted in urine by 1 h), with low kidney retention (% ID/g: 11 at 1 h, 5 at 4 h, and 1 at 24 h) and low nonspecific uptake in other organs/tissues. Our findings establish NOTA and NODAGA as outstanding BFCs for the fac-[M(CO)] core in the design and development of organometallic radiopharmaceuticals. Future in vivo studies of [Tc]Tc- and [Re]Re-tricarbonyl complexes of NODAGA/NOTA-biomolecule conjugates will further probe the potential of these chelates for nuclear medicine applications in diagnostic imaging and targeted radiotherapy, respectively.