H2CHXhox: Rigid Cyclohexane-Reinforced Nonmacrocyclic Chelating Ligand for [Ga]Ga

A rigid chiral acyclic chelator H2CHXhox was synthesized and evaluated for Ga-based radiopharmaceutical applications; it was compared to the previously reported hexadentate H2hox 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(CHXhox)] 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 (logKML = 35.91(1)). More importantly, H2CHXhox showed very fast Ga 3+ complexation at physiological pH 7.4, and acid-assisted Ga complex dissociation kinetic studies (pH 1) in comparison with H2hox 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(CHXhox)] over a large concentration range. Dynamic PET imaging studies showed no accumulation in muscle, lungs, bone, and brain, suggesting no release of free Ga ions. [Ga][Ga(CHXhox)] is cleared from the mouse via hepatobiliary and renal pathways. Compared to [Ga][Ga(hox)], the increased lipophilicity of [Ga][Ga(CHXhox)] enhanced heart and liver uptake and decreased kidney clearance. [Ga][Ga(CHXhox)] 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 H2CHXhox to be an obvious improvement over H2hox and an excellent candidate in this new “ox” family for the development of radiopharmaceutical compounds. ■ INTRODUCTION Ga is a clinically important isotope with a short half-life time (t1/2 = 68 min) and predominant β + decay yield (89%, 1.899 keV), suitable for imaging with small molecules and peptides. Ga based positron emission tomography (PET) imaging tracers have attracted increasing interest because of the commercially available Ge/Ga generator system which can be used for more than 1 year due to the long half-life time of the parent Ge (t1/2 = 270 days) and allow cost-effective use of Ga without the need for local cyclotron facilities. The most successful samples currently are Galabeled somatostatin analogues such as DOTATATE for neuroendocrine tumor (NET) imaging and [Ga]GaPSMA (prostate specific membrane antigen) tracers including [68Ga]Ga-HBED-CC-PSMA and [Ga]Ga-PSMA I&T, which have been studied widely and shown great potential in clinical imaging for prostate cancer diagnosis and staging. Over the past decade, numerous Ga chelators have been synthesized and tested for construction of Ga-based tracers. One major challenge is to maximize both rapid efficient radiolabeling under mild conditions (room temperature and near neutral pH) and high thermodynamic and kinetic stability, simultaneously. Many fast-labeling acyclic chelators suffer from their poor kinetic inertness compared with macrocyclic chelators such as DOTA, whereas Received: January 16, 2020 Article pubs.acs.org/IC © XXXX American Chemical Society A https://dx.doi.org/10.1021/acs.inorgchem.0c00168 Inorg. Chem. XXXX, XXX, XXX−XXX D ow nl oa de d vi a U N IV O F B R IT IS H C O L U M B IA o n M ar ch 1 6, 2 02 0 at 1 6: 10 :4 9 (U T C ). Se e ht tp s: //p ub s. ac s. or g/ sh ar in gg ui de lin es f or o pt io ns o n ho w to le gi tim at el y sh ar e pu bl is he d ar tic le s.