1,4,7-Triazacyclononane-Based Chelators for the Complexation of [¹⁸⁶Re]Re- and [^99mTc]Tc-Tricarbonyl Cores

Metal complexes with the general formula [M^I(CO)₃(k³-L)]⁺, where M = Re, ¹⁸⁶Re, or ^99mTc and L = 1,4,7-triazacyclononane (TACN), NOTA, or NODAGA chelators, have previously been conjugated to peptide-based biological targeting vectors and investigated as potential theranostic radiopharmaceuticals. The promising results demonstrated by these bioconjugate complexes prompted our exploration of other TACN-based chelators for suitability for (radio)labeling with the [M(CO)₃]⁺ core. In this work, we investigated the role of the TACN pendant arms in complexation of the [M(CO)₃]⁺ core through (radio)labeling of TACN chelators bearing acid, ester, mixed acid-ester, or no pendant functional groups. The chelators were synthesized from TACN, characterized, and (radio)labeled with nonradioactive Re-, [¹⁸⁶Re]Re-, and [^99mTc]Tc-tricarbonyl cores. The nonfunctionalized (3), diacid (4), and monoacid monoester (7 and 8) chelators underwent direct labeling, while the diester (M-5 and M-6) complexes required indirect synthesis from M-4. All six chelators demonstrated stable radiometal coordination. The ester-bearing derivatives, which exhibited more lipophilic character than their acid-bearing counterparts, were prone to ester hydrolysis over time, making them less suitable for radiopharmaceutical development. These studies confirmed that the TACN pendant functional groups were key to efficient labeling with the [M(CO)₃]⁺ core, with ionizable pendant arms favored over nonionizable pendant arms.