Room-Temperature Synthesis of Re(I) and Tc(I) Metallocarboranes

A series of carborane derivatives bearing guanidine substituents were prepared and characterized, and their reactivity toward Re­(I) and Tc­(I) in aqueous media was evaluated. Guanidinylation was achieved by treating 1-aminomethyl-1,2-<i>closo</i>-dodecaborane with <i>N</i>¹,<i>N</i>²-di-Boc-1<i>H</i>-pyrazole-1-carboxamidine, and the associated <i>N</i>-ethyl derivative, which produced the desired products in good (circa 50%) yield. These were deprotected and converted to the corresponding <i>nido</i>-carboranes, which, when combined with [M­(CO)₃(H₂O)₃]⁺ (M = Re and ^99mTc) at room temperature for 3 h or 35 °C for 1 h, afforded the corresponding η⁵-metallocarborane complexes. Corresponding reactions involving carboranes without basic substituents generally require microwave heating at temperatures greater than 150 °C. The rate, yields, and the temperature of the reaction appear to be dependent on the basicity of the guanidines tested. The biodistribution of two of the ^99mTc complexes, which are stable indefinitely in solution, were evaluated in vivo in CD1 mice and showed that the ^99mTc–carboranyl guanidine complexes clear key nontarget organs and tissues within one half-life (6 h) and have properties that are desirable for developing targeted molecular imaging probes

Room-Temperature Synthesis of Re(I) and Tc(I) Metallocarboranes

A series of carborane derivatives bearing guanidine substituents were prepared and characterized, and their reactivity toward Re­(I) and Tc­(I) in aqueous media was evaluated. Guanidinylation was achieved by treating 1-aminomethyl-1,2-<i>closo</i>-dodecaborane with <i>N</i>¹,<i>N</i>²-di-Boc-1<i>H</i>-pyrazole-1-carboxamidine, and the associated <i>N</i>-ethyl derivative, which produced the desired products in good (circa 50%) yield. These were deprotected and converted to the corresponding <i>nido</i>-carboranes, which, when combined with [M­(CO)₃(H₂O)₃]⁺ (M = Re and ^99mTc) at room temperature for 3 h or 35 °C for 1 h, afforded the corresponding η⁵-metallocarborane complexes. Corresponding reactions involving carboranes without basic substituents generally require microwave heating at temperatures greater than 150 °C. The rate, yields, and the temperature of the reaction appear to be dependent on the basicity of the guanidines tested. The biodistribution of two of the ^99mTc complexes, which are stable indefinitely in solution, were evaluated in vivo in CD1 mice and showed that the ^99mTc–carboranyl guanidine complexes clear key nontarget organs and tissues within one half-life (6 h) and have properties that are desirable for developing targeted molecular imaging probes

Technetium(I) Complexes of Bathophenanthrolinedisulfonic Acid

Bathophenanthrolinedisulfonate (BPS) complexes of technetium­(I) of the type [Tc­(CO)₃(BPS)­(L)]^<i>n</i> (L = imidazole derivatives) were synthesized and evaluated both in vitro and in vivo. [^99mTc­(CO)₃(BPS)­(MeIm)]⁻ (MeIm = 1-methyl-1<i>H</i>-imidazole) was prepared in near-quantitative yield using a convenient two-step, one-pot labeling procedure. A targeted analogue capable of binding regions of calcium turnover associated with bone metabolism was also prepared. Here, a bisphosphonate was linked to the metal through an imidazole ligand to give [^99mTc­(CO)₃(BPS)­(ImAln)]^2– (ImAln = an imidazole–alendronate ligand) in high yield. The technetium­(I) complexes were stable in vitro, and in biodistribution studies, [^99mTc­(CO)₃(BPS)­(ImAln)]^2– exhibited rapid clearance from nontarget tissues and significant accumulation in the shoulder (7.9 ± 0.2% ID/g) and knees (15.1 ± 0.9% ID/g) by 6 h, with the residence time in the skeleton reaching 24 h. A rhenium analogue, which is luminescent and has the same structure, was also prepared and used for fluorescence labeling of cells in vitro. The data reported demonstrate the potential of this class of compounds for use in creating isostructural optical and nuclear probes

Imidazole-Based [2 + 1] Re(I)/^99mTc(I) Complexes as Isostructural Nuclear and Optical Probes

The synthesis, stability, and photophysical properties of [2 + 1] Re­(I)/Tc­(I) complexes derived from bipyridine and a series of imidazole derivatives were investigated as a means of identifying complexes suitable for creating targeted isostructural optical/nuclear molecular imaging probes. To prepare the desired complexes, [Re­(CO)₃­(H₂O)₃]Br was combined with 2,2′-bipyridine (bipy) to give [Re­(CO)₃­(bipy)­Br], which in turn was converted to the desired complexes by treatment with functionalized imidazoles, yielding crystal structures of two new Re complexes. The corresponding ^99mTc complexes [^99mTc­(CO)₃­(bipy)­(L)]⁺ (L = imidazole derivatives) were prepared by combining [^99mTc­(CO)₃­(bipy)­(H₂O)]Cl with the same series of ligands and heating at 40 or 60 °C for 30 min. Quantitative transformation to the final products was confirmed in all cases by HPLC, and the nature of the complexes was verified by comparison to the authentic Re standards. Incubation in saline and plasma, and amino acid challenge experiments showed that N-substituted imidazole derivatives, bearing electron donating groups, exhibited superior stability to analogous metal complexes derived from less basic ligands. Imaging studies in mice revealed that with the appropriate choice of monodentate ligand, it is possible to prepare robust [2 + 1] Tc complexes that can be used as the basis for preparing targeted isostructural optical and nuclear probes

Isostructural Nuclear and Luminescent Probes Derived From Stabilized [2 + 1] Rhenium(I)/Technetium(I) Organometallic Complexes

A convenient method to prepare ^99mTc analogues of a class of rhenium­(I) luminophores was developed, creating isostructural pairs of nuclear and optical probes. A two-step procedure and a new one-pot procedure were used to produce a series of [2 + 1] complexes of the type [Tc­(CO)₃(bipy)­L]⁺ in greater than 80% yield. The plasma stability of the reported compounds was evaluated, where the basicity of the monodentate pyridine type ligand (L) has a significant impact with half-lives ranging from 2 to 20 h. The ability to generate the radioactive complexes makes it possible to quantitate cell uptake of Re luminophores, which was demonstrated in MCF-7 breast cancer cells using ^99mTc analogues of two Re­(I)-based mitochondrial targeting dyes

Synthesis, Radiolabeling, and In Vivo Imaging of PEGylated High-Generation Polyester Dendrimers

A fifth generation aliphatic polyester dendrimer was functionalized with vinyl groups at the periphery and a dipicolylamine Tc­(I) chelate at the core. This structure was PEGylated with three different molecular weight mPEGs (mPEG₁₆₀, mPEG₃₅₀, and mPEG₇₅₀) using thiol–ene click chemistry. The size of the resulting macromolecules was evaluated using dynamic light scattering, and it was found that the dendrimer functionalized with mPEG₇₅₀ was molecularly dispersed in water, exhibiting a hydrodynamic diameter of 9.2 ± 2.1 nm. This PEGylated dendrimer was subsequently radiolabeled using [^99mTc­(CO)₃(H₂O)₃]⁺ and purified to high (>99%) radiochemical purity. Imaging studies were initially performed on healthy rats to allow comparison to previous Tc-labeled dendrimers and then on xenograft murine tumor models, which collectively showed that the dendrimers circulated in the blood for an extended period of time (up to 24 h). Furthermore, the radiolabeled dendrimer accumulated in H520 xenograft tumors, which could be visualized by single-photon emission computed tomography (SPECT). The reported PEGylated aliphatic polyester dendrimers represent a new platform for developing tumor-targeted molecular imaging probes and therapeutics

Preparation and Evaluation of Radiolabeled Antibody Recruiting Small Molecules That Target Prostate-Specific Membrane Antigen for Combined Radiotherapy and Immunotherapy

The feasibility of developing a single agent that can deliver radioactive iodine and also direct cellular immune function by engaging endogenous antibodies as an antibody-recruiting small molecule (ARM) was determined. A library of new prostate-specific membrane antigen (PSMA)-binding ligands that contained antibody-recruiting 2,4-dinitrophenyl (DNP) groups and iodine were synthesized and screened in vitro and in vivo. A lead compound (<b>9b</b>) showed high affinity for PSMA and the ability to bind anti-DNP antibodies. Biodistribution studies of the iodine-125 analogue showed 3% ID/g in LNCaP xenograft tumors at 1 h postinjection with tumor-to-blood and tumor-to-muscle ratios of 10:1 and 44:1, respectively. The radiolabeled analogue was bound and internalized by LNCaP cells, with both functions blocked using a known PSMA inhibitor. A second candidate showed high tumor uptake (>10% ID/g) but had minimal binding to anti-DNP antibodies. The compounds reported represent the first examples of small molecules developed specifically for combination immunotherapy and radiotherapy for prostate cancer

Development of prostate specific membrane antigen targeted ultrasound microbubbles using bioorthogonal chemistry

<div><p>Prostate specific membrane antigen (PSMA) targeted microbubbles (MBs) were developed using bioorthogonal chemistry. Streptavidin-labeled MBs were treated with a biotinylated tetrazine (MB_Tz) and targeted to PSMA expressing cells using <i>trans</i>-cyclooctene (TCO)-functionalized anti-PSMA antibodies (TCO-anti-PSMA). The extent of MB binding to PSMA positive cells for two different targeting strategies was determined using an <i>in vitro</i> flow chamber. The initial approach involved pretargeting, where TCO-anti-PSMA was first incubated with PSMA expressing cells and followed by MB_Tz, which subsequently showed a 2.8 fold increase in the number of bound MBs compared to experiments performed in the absence of TCO-anti-PSMA. Using direct targeting, where TCO-anti-PSMA was linked to MB_Tz prior to initiation of the assay, a 5-fold increase in binding compared to controls was observed. The direct targeting approach was subsequently evaluated <i>in vivo</i> using a human xenograft tumor model and two different PSMA-targeting antibodies. The US signal enhancements observed were 1.6- and 5.9-fold greater than that for non-targeted MBs. The lead construct was also evaluated in a head-to-head study using mice bearing both PSMA positive or negative tumors in separate limbs. The human PSMA expressing tumors exhibited a 2-fold higher US signal compared to those tumors deficient in human PSMA. The results demonstrate both the feasibility of preparing PSMA-targeted MBs and the benefits of using bioorthogonal chemistry to create targeted US probes.</p></div

Triazole Appending Agent (TAAG): A New Synthon for Preparing Iodine-Based Molecular Imaging and Radiotherapy Agents

A new prosthetic group referred to as the triazole appending agent (TAAG) was developed as a means to prepare targeted radioiodine-based molecular imaging and therapy agents. Tributyltin-TAAG and the fluorous analogue were synthesized in high yield using simple click chemistry and the products labeled in greater than 95% RCY with ¹²³I. A TAAG derivative of an inhibitor of prostate-specific membrane antigen was prepared and radiolabeled with ¹²³I in 85% yield where biodistribution studies in LNCap prostate cancer tumor models showed rapid clearance of the agent from nontarget tissues and tumor accumulation of 20% injected dose g^–1 at 1 h. The results presented demonstrate that the TAAG group promotes minimal nonspecific binding and that labeled conjugates can achieve high tumor uptake and exquisite target-to-nontarget ratios

Comparison of the US signal enhancement obtained by direct targeting of PSMA⁺ LNCaP tumors using MB_Tz-TCO-J591 and MB_Tz-TCO-ARP.

<p>Data are the average ratio of US signals (n = 3, with SEM), for the targeting construct versus the control (MB_Tz alone). A statistically significant difference was not observed by one-way ANOVA (<i>P</i> = 0.07), due to variability of results from the mice injected with MB_Tz-TCO-ARP.</p