H 2 CHX hox: Rigid Cyclohexane-Reinforced Nonmacrocyclic Chelating Ligand for [ nat/67/68 Ga]Ga 3 .

A rigid chiral acyclic chelator H ₂ CHX hox was synthesized and evaluated for Ga ³⁺ -based radiopharmaceutical applications; it was compared to the previously reported hexadentate H ₂ hox to determine the effect of a backbone reinforced from adding a chiral 1S,2S-trans-cyclohexane on metal complex stability, kinetic inertness, and in vivo pharmacokinetics. NMR spectroscopy and theoretical calculation revealed that [Ga( CHX hox)] ⁺ showed a very similar coordination geometry to that of [Ga(hox)] ⁺ , and only one isomer in solution was observed by NMR spectroscopy. Solution studies showed that the modification results in a significant improvement in the exceptionally high thermodynamic stability of [Ga(hox)] ⁺ with a 1.56 log unit increase in stability constant (log K _ML = 35.91(1)). More importantly, H ₂ CHX hox showed very fast Ga ³⁺ complexation at physiological pH 7.4, and acid-assisted Ga ³⁺ complex dissociation kinetic studies (pH 1) in comparison with H ₂ hox revealed a 50-fold increase of the dissociation half-life time from 73 min to 58 h. Fluorescence microscopy imaging study confirmed its cellular uptake and accumulation in endoplasmic reticulum and mitochondria. MTT studies indicated a quite low cytotoxicity of [Ga( CHX hox)] ⁺ over a large concentration range. Dynamic PET imaging studies showed no accumulation in muscle, lungs, bone, and brain, suggesting no release of free Ga ³⁺ ions. [ ⁶⁸ Ga][Ga( CHX hox)] ⁺ is cleared from the mouse via hepatobiliary and renal pathways. Compared to [ ⁶⁸ Ga][Ga(hox)] ⁺ , the increased lipophilicity of [ ⁶⁸ Ga][Ga( CHX hox)] ⁺ enhanced heart and liver uptake and decreased kidney clearance. [ ⁶⁷ Ga][Ga( CHX hox)] ⁺ SPECT/CT imaging and biodistribution study revealed good clearance from liver to gallbladder after 90 min and finally into feces after 5 h. No decomposition or transchelation was observed over the 5 h study. These results confirmed H ₂ CHX hox to be an obvious improvement over H ₂ hox and an excellent candidate in this new "ox" family for the development of radiopharmaceutical compounds.