Four planar-chiral, enantiomerically pure ferrocene dibromides (3 R1R2 ; [(CHR 1 R 2 )BrH 3 C 5 ] 2 Fe) equipped with two CHR 1 R 2 groups in α position to bromine were synthesized. From the four C 2 symmetrical species, two are already known [CHR 1 R 2 = CHMe 2 (3 MeMe ), CHEt 2 (3 EtEt )] and two are new compoundsThe dibromides 3 R1R2 were in situ converted into dilithio ferrocene derivatives and reacted with Cl 2 BNiPr 2 resulting in mixtures of bora[1]-ferrocenophanes (4 R1R2 ) and 1,1′-bis(boryl)ferrocenes (5 R1R2 ). The aim of this investigation was to test the hypothesis that the alkyl group that is oriented approximately perpendicular to the Cp ring, i.e., R 2 , affects the outcome of the salt-metathesis reaction. The obtained product ratios 4 R1R2 :5 R1R2 were determined by 1 H NMR spectroscopy and revealed that systems with the same R 2 group gave similar 4 R1R2 :5 R1R2 ratios (1.0:0.51 and 1.0:0.49 for R 2 = Me; 1.0:0.30 and 1.0:0.27 for R 2 = Et), confirming the hypothesis. Shown by DFT calculations (B3PW91/6-311+G(d,p)), reaction paths resulting in either product 4 R1R2 or product 5 R1R2 are both concerted steps.

Herein we report a new high-denticity chelator based on the iron siderophore desferrioxamine (DFO). Our new chelatorDFO2is acyclic and was designed and synthesized with the purpose of improving the coordination chemistry and radiolabeling performance with radioactive zirconium-89. The radionuclide zirconium-89 ([89Zr]­Zr4+) has found wide use for positron emission tomography (PET) imaging when it is coupled with proteins, antibodies, and nanoparticles. DFO2 has a potential coordination number of 12, which uniquely positions this chelator for binding large, high-valent, and oxophilic metal ions. Following synthesis of the DFO2 chelator and the [natZr]­Zr-(DFO2) complex we performed density functional theory calculations to study its coordination sphere, followed by zirconium-89 radiolabeling experiments for comparisons with the “gold standard” chelator DFO. DFO (CN 6) can coordinate with zirconium in a hexadentate fashion, leaving two open coordination sites where water is thought to coordinate (total CN 8). DFO2 (potential CN 12, dodecadentate) can saturate the coordination sphere of zirconium with four hydroxamate groups (CN 8), with no room left for water to directly coordinate, and only binds a single atom of zirconium per chelate. Following quantitative radiolabeling with zirconium-89, the preformed [89Zr]­Zr-(DFO) and [89Zr]­Zr-(DFO2) radiometal–chelate complexes were subjected to a battery of in vitro stability challenges, including human blood serum, apo-transferrin, serum albumin, iron, hydroxyapatite, and EDTA. One objective of these stability challenges was to determine if the increased denticity of DFO2 over that of DFO imparted improved complex stability, and another was to determine which of these assays is most relevant to perform with future chelators. In all of the assays DFO2 showed superior stability with zirconium-89, except for the iron challenge, where both DFO2 and DFO were identical. Substantial differences in stability were observed for human blood serum using a precipitation method of analysis, apo-transferrin, hydroxyapatite, and EDTA challenges. These results suggest that DFO2 is a promising next-generation scaffold for zirconium-89 chelators and holds promise for radiochemistry with even larger radionuclides, which we anticipate will expand the utility of DFO2 into theranostic applications.

Positron emission tomography (PET) using radiolabeled, monoclonal antibodies has become an effective, noninvasive method for tumor detection and is a critical component of targeted radionuclide therapy. Metal ion chelator and bacterial siderophore desferrioxamine (DFO) is the gold standard compound for incorporation of zirconium-89 in radiotracers for PET imaging because it is thought to form a stable chelate with [89Zr]­Zr4+. However, DFO may not bind zirconium-89 tightly in vivo, with free zirconium-89 reportedly liberated into the bones of experimental mouse models. Although high bone uptake has not been observed to date in humans, this potential instability has been proposed to be related to the unsaturated coordination sphere of [89Zr]­Zr-DFO, which is thought to consist of the 3 hydroxamate groups of DFO and 1 or 2 water molecules. In this study, we have used a combination of X-ray absorption spectroscopy and density functional theory (DFT) geometry optimization calculations to further probe the coordination chemistry of this complex in solution. We find the extended X-ray absorption fine structure (EXAFS) curve fitting of an aqueous solution of Zr­(IV)-DFO to be consistent with an 8-coordinate Zr with oxygen ligands. DFT calculations suggest that the most energetically favorable Zr­(IV) coordination environment in DFO likely consists of the 3 hydroxamate ligands from DFO, each with bidentate coordination, and 2 hydroxide ligands. Further EXAFS curve fitting provides additional support for this model. Therefore, we propose that the coordination sphere of Zr­(IV)-DFO is most likely completed by 2 hydroxide ligands rather than 2 water molecules, forming Zr­(DFO)­(OH)2.

A series of new boron-bridged [1]ferrocenophanes ([1]FCPs) was prepared by salt-metathesis reactions between enantiomerically pure dilithioferrocenes and amino(dichloro)boranes (Et2 NBCl2 , iPr2 NBCl2 , or tBu(Me3 Si)NBCl2 ). The dilithioferrocenes were prepared in situ by lithium-bromine exchange from the respective planar-chiral dibromides (Sp ,Sp )-[1-Br-2-(HR2 C)H3 C5 ]2 Fe (R=Me or Et). In most of the cases, mixtures of the targeted [1]FCPs 4 and the unwanted 1,1'-bis(boryl)ferrocenes 5 were formed. The product ratio depends on the bulkiness of the amino group, the speed of addition of the amino(dichloro)borane, the alkyl group on Cp rings, and in particular on the reaction temperature. The formation of strained [1]FCPs is strongly favored by increased reaction temperatures. Secondly, CHEt2 groups at Cp rings favored the formation of the targeted [1]FCPs stronger than CHMe2 groups. These discoveries open up new possibilities to further suppress the formation of unwanted byproducts by a careful choice of the reaction temperature and through tailoring the bulkiness of CHR2 groups on ferrocene. Thermal ring-opening polymerizations of selected boron-bridged [1]FCPs gave metallopolymers with a Mw of 10 kDa (GPC).

Immuno-PET using desferrioxamine (DFO)-conjugated zirconium-89 ([89Zr]­Zr4+)-labeled antibodies is a powerful tool used for preclinical and clinical molecular imaging. However, a comprehensive study evaluating the variables involved in DFO-conjugation and 89Zr-radiolabeling of antibodies and their impact on the in vitro and in vivo behavior of the resulting radioimmunoconjugates has not been adequately performed. Here, we synthesized different DFO-conjugates of the HER2-targeting antibody (Ab)trastuzumab, dubbed T5, T10, T20, T60, and T200to indicate the molar equivalents of DFO used for bioconjugation. Next we radiolabeled the immunoconjugates with ([89Zr]­Zr4+) under a comprehensive set of reaction conditions including different buffers (PBS, chelexed-PBS, TRIS/HCl, HEPES; ± radioprotectants), different reaction volumes (0.1–1 mL), variable amounts of DFO-conjugated Ab (5, 25, 50 μg), and radioactivity (0.2–1.0 mCi; 7.4–37 MBq). We evaluated the effects of these variables on radiochemical yield (RCY), molar activity (A m)/specific activity (A s), immunoreactive fraction, and ultimately the in vivo biodistribution profile and tumor targeting ability of the trastuzumab radioimmunoconjugates. We show that increasing the degree of DFO conjugation to trastuzumab increased the RCY (∼90%) and A m/A s (∼194 MBq/nmol; 35 mCi/mg) but decreased the HER2-binding affinity (3.5×–4.6×) and the immunoreactive fraction of trastuzumab down to 50–64%, which translated to dramatically inferior in vivo performance of the radioimmunoconjugate. Cell-based immunoreactivity assays and standard binding affinity analyses using surface plasmon resonance (SPR) did not predict the poor in vivo performance of the most extreme T200 conjugate. However, SPR-based concentration free calibration analysis yielded active antibody concentration and was predictive of the in vivo trends. Positron emission tomography (PET) imaging and biodistribution studies in a HER2-positive xenograft model revealed activity concentrations of 38.7 ± 3.8 %ID/g in the tumor and 6.3 ± 4.1 %ID/g in the liver for ([89Zr]­Zr4+)-T5 (∼1.4 ± 0.5 DFOs/Ab) at 120 h after injection of the radioimmunoconjugates. On the other hand, ([89Zr]­Zr4+)-T200 (10.9 ± 0.7 DFOs/Ab) yielded 16.2 ± 3.2 %ID/g in the tumor versus 27.5 ± 4.1 %ID/g in the liver. Collectively, our findings suggest that synthesizing trastuzumab immunoconjugates bearing 1–3 DFOs per Ab (T5 and T10) combined with radiolabeling performed in low reaction volumes using Chelex treated PBS or HEPEs without a radioprotectant provided radioimmunoconjugates having high A m/A s (97 MBq/nmol; 17.5 ± 2.2 mCi/mg), highly preserved immunoreactive fractions (86–93%), and favorable in vivo biodistribution profile with excellent tumor uptake.