124I Radiolabeling of a AuIII‐NHC Complex for In Vivo Biodistribution Studies

Au^III complexes with N‐heterocyclic carbene (NHC) ligands have shown remarkable potential as anticancer agents, yet their fate in vivo has not been thoroughly examined and understood. Reported herein is the synthesis of new Au^III‐NHC complexes by direct oxidation with radioactive [¹²⁴I]I₂ as a valuable strategy to monitor the in vivo biodistribution of this class of compounds using positron emission tomography (PET). While in vitro analyses provide direct evidence for the importance of Au^III‐to‐Au^I reduction to achieve full anticancer activity, in vivo studies reveal that a fraction of the Au^III‐NHC prodrug is not immediately reduced after administration but able to reach the major organs before metabolic activation.     

In Vitro and In Vivo Characterization of 89Zirconium-Labeled Lintuzumab Molecule

Objective: Positron emission tomography (PET) imaging is a powerful non-invasive method to determine the in vivo behavior of biomolecules. Determining biodistribution and pharmacokinetic (PK) properties of targeted therapeutics can enable a better understanding of in vivo drug mechanisms such as tumor uptake, off target accumulation and clearance. Zirconium-89 (⁸⁹Zr) is a readily available tetravalent PET-enabling radiometal that has been used to evaluate the biodistribution and PK of monoclonal antibodies. In the current study, we performed in vitro and in vivo characterization of ⁸⁹Zr-lintuzumab, a radiolabeled anti-CD33 antibody, as a model to evaluate the in vivo binding properties in preclinical models of AML. Methods: Lintuzumab was conjugated to p-SCN-Bn-deferoxamine (DFO) and labeled with ⁸⁹Zr using a 5:1 µCi:µg specific activity at 37 °C for 1h. The biological activity of ⁸⁹Zr-lintuzumab was evaluated in a panel of CD33 positive cells using flow cytometry. Fox Chase SCID mice were injected with 2 × 10⁶ OCI-AML3 cells into the right flank. After 12 days, a cohort of mice (n = 4) were injected with ⁸⁹Zr-lintuzumab via tail vein. PET/CT scans of mice were acquired on days 1, 2, 3 and 7 post ⁸⁹Zr-lintuzumab injection. To demonstrate ⁸⁹Zr-lintuzumab specific binding to CD33 expressing tumors in vivo, a blocking study was performed. This cohort of mice (n = 4) was injected with native lintuzumab and 24 h later ⁸⁹Zr-lintuzumab was administered. This group was imaged 3 and 7 days after injection of ⁸⁹Zr-lintuzumab. A full ex vivo biodistribution study on both cohorts was performed on day 7. The results from the PET image and ex vivo biodistribution studies were compared. Results: Lintuzumab was successfully radiolabeled with ⁸⁹Zr resulting in a 99% radiochemical yield. The ⁸⁹Zr-lintuzumab radioconjugate specifically binds CD33 positive cells in a similar manner to native lintuzumab as observed by flow cytometry. PET imaging revealed high accumulation of ⁸⁹Zr-lintuzumab in OCI-AML3 tumors within 24h post-injection of the radioconjugate. The ⁸⁹Zr-lintuzumab high tumor uptake remains for up to 7 days. Tumor analysis of the PET data using volume of interest (VOI) showed significant blocking of ⁸⁹Zr-lintuzumab in the group pre-treated with native lintuzumab (pre-blocked group), thus indicating specific targeting of CD33 on OCI-AML3 cells in vivo. The tumor uptake findings from the PET imaging study are in agreement with those from the ex vivo biodistribution results. Conclusions: PET imaging of ⁸⁹Zr-lintuzumab shows high specific uptake in CD33 positive human OCI-AML3 tumors. The results from the image study agree with the observations from the ex vivo biodistribution study. Our findings collectively suggest that PET imaging using ⁸⁹Zr-lintuzumab could be a powerful drug development tool to evaluate binding properties of anti-CD33 monoclonal antibodies in preclinical cancer models.     

Establishing Radiolanthanum Chemistry for Targeted Nuclear Medicine Applications

We report the first targeted nuclear medicine application of the lanthanum radionuclides ^132/135La. These isotopes represent a matched pair for diagnosis via the positron emissions of ¹³²La and therapy mediated by the Auger electron emissions of ¹³⁵La. We identify two effective chelators, known as DO3Apic and macropa, for these radionuclides. The 18-membered macrocycle, macropa, bound ^132/135La with better molar activity than DO3Apic under similar conditions. These chelators were conjugated to the prostate-specific membrane antigen (PSMA)-targeting agent DUPA to assess the use of radiolanthanum for in vivo imaging. The ^132/135La-labeled targeted constructs showed high uptake in tumor xenografts expressing PSMA. This study validates the use of these radioactive lanthanum isotopes for imaging applications and motivates future work to assess the therapeutic effects of the Auger electron emissions of ¹³⁵La.     

Tuning Tetrazole Photochemistry for Protein Ligation and Molecular Imaging

Photochemistry provides a wide range of alternative reagents that hold potential for use in bimolecular functionalisation of proteins. Here, we report the synthesis and characterisation of metal ion binding chelates derivatised with disubstituted tetrazoles for the photoradiochemical labelling of monoclonal antibodies (mAbs). The photophysical properties of tetrazoles featuring extended aromatic systems and auxochromic substituents to tune excitation toward longer wavelengths (365 and 395 nm) were studied. Two photoactivatable chelates based on desferrioxamine B (DFO) and the aza-macrocycle NODAGA were functionalised with a tetrazole and developed for protein labelling with ⁸⁹Zr, ⁶⁴Cu and ⁶⁸Ga radionuclides. DFO-tetrazole ( 1 ) was assessed by direct conjugation to formulated trastuzumab and subsequent radiolabelling with ⁸⁹Zr. Radiochemical studies and cellular-based binding assays demonstrated that the radiotracer remained stable in vitro retained high immunoreactivity. Positron emission tomography (PET) imaging and biodistribution studies were used to measure the tumour specific uptake and pharmacokinetic profile in mice bearing SK-OV-3 xenografts. Experiments demonstrate that tetrazole-based photochemistry is a viable approach for the light-induced synthesis of PET radiotracers.     

Cyclometalated Palladium(II) N‐Heterocyclic Carbene Complexes: Anticancer Agents for Potent In Vitro Cytotoxicity and In Vivo Tumor Growth Suppression

Palladium(II) complexes are generally reactive toward substitution/reduction, and their biological applications are seldom explored. A new series of palladium(II) N‐heterocyclic carbene (NHC) complexes that are stable in the presence of biological thiols are reported. A representative complex, [Pd(C^N^N)(N,N′‐nBu₂NHC)](CF₃SO₃) ( Pd1 d , HC^N^N=6‐phenyl‐2,2′‐bipyridine, N,N′‐nBu₂NHC=N,N′‐di‐n‐butylimidazolylidene), displays potent killing activity toward cancer cell lines (IC₅₀=0.09–0.5 μm ) but is less cytotoxic toward a normal human fibroblast cell line (CCD‐19Lu, IC₅₀=11.8 μm ). In vivo anticancer studies revealed that Pd1 d significantly inhibited tumor growth in a nude mice model. Proteomics data and in vitro biochemical assays reveal that Pd1 d exerts anticancer effects, including inhibition of an epidermal growth factor receptor pathway, induction of mitochondrial dysfunction, and antiangiogenic activity to endothelial cells.     

Copper‐64‐Alloyed Gold Nanoparticles for Cancer Imaging: Improved Radiolabel Stability and Diagnostic Accuracy

Gold nanoparticles, especially positron‐emitter‐ labeled gold nanostructures, have gained steadily increasing attention in biomedical applications. Of the radionuclides used for nanoparticle positron emission tomography imaging, radiometals such as ⁶⁴Cu have been widely employed. Currently, radiolabeling through macrocyclic chelators is the most commonly used strategy. However, the radiolabel stability may be a limiting factor for further translational research. We report the integration of ⁶⁴Cu into the structures of gold nanoparticles. With this approach, the specific radioactivity of the alloyed gold nanoparticles could be freely and precisely controlled by the addition of the precursor ⁶⁴CuCl₂ to afford sensitive detection. The direct incorporation of ⁶⁴Cu into the lattice of the gold nanoparticle structure ensured the radiolabel stability for accurate localization in vivo. The superior pharmacokinetic and positron emission tomography imaging capabilities demonstrate high passive tumor targeting and contrast ratios in a mouse breast cancer model, as well as the great potential of this unique alloyed nanostructure for preclinical and translational imaging.     

N-Heterocyclic Carbene Iron Complexes as Anticancer Agents: In Vitro and In Vivo Biological Studies

Cisplatin and its derivatives are commonly used in chemotherapeutic treatments of cancer, even though they suffer from many toxic side effects. The problems that emerge from the use of these metal compounds led to the search for new complexes capable to overcome the toxic side effects. Here, we report the evaluation of the antiproliferative activity of Fe(II) cyclopentadienyl complexes bearing n-heterocyclic carbene ligands in tumour cells and their in vivo toxicological profile. The in vitro antiproliferative assays demonstrated that complex Fe1 displays the highest cytotoxic activity both in human colorectal carcinoma cells (HCT116) and ovarian carcinoma cells (A2780) with IC₅₀ values in the low micromolar range. The antiproliferative effect of Fe1 was even higher than cisplatin. Interestingly, Fe1 showed low in vivo toxicity, and in vivo analyses of Fe1 and Fe2 compounds using colorectal HCT116 zebrafish xenograft showed that both reduce the proliferation of human HCT116 colorectal cancer cells in vivo.     

Cu(I)-catalyzed (11)C carboxylation of boronic acid esters: a rapid and convenient entry to (11)C-labeled carboxylic acids, esters, and amides

Rapid and direct: the carboxylation of boronic acid esters with (11)CO(2) provides [(11)C]carboxylic acids as a convenient entry into [(11)C]esters and [(11)C]amides. This conversion of boronates is tolerant to diverse functional groups (e.g., halo, nitro, or carbonyl).     

One-Pot Synthesis of 11C-Labelled Primary Benzamides via Intermediate [11C]Aroyl Dimethylaminopyridinium Salts

Electrophilic ¹¹C-labelled aroyl dimethylaminopyridinium salts, obtained by carbonylative cross-coupling of aryl halides with [¹¹C]carbon monoxide, were prepared for the first time and shown to be valuable intermediates in the synthesis of primary [¹¹C]benzamides. The methodology furnished a set of benzamide model compounds, including the two poly (ADP-ribose) polymerase (PARP) inhibitors niraparib and veliparib, in moderate to excellent radiochemical yields. In addition to providing a convenient and practical route to primary [¹¹C]benzamides, the current method paves the way for future application of [¹¹C]aroyl dimethylaminopyridinium halide salts in positron emission tomography (PET) tracer synthesis.     

Heat‐Induced Radiolabeling of Nanoparticles for Monocyte Tracking by PET

Heat‐induced radiolabeling (HIR) yielded ⁸⁹Zr‐Feraheme (FH) nanoparticles (NPs) that were used to determine NP pharmacokinetics (PK) by positron emission tomography (PET). Standard uptake values indicated a fast hepatic uptake that corresponded to blood clearance, and a second, slow uptake process by lymph nodes and spleen. By cytometry, NPs were internalized by circulating monocytes and monocytes in vitro. Using an IV injection of HIR ⁸⁹Zr‐FH (rather than in vitro cell labeling), PET/PK provided a view of monocyte trafficking, a key component of the immune response.     

PET Imaging of Bacterial Infections with Fluorine‐18‐Labeled Maltohexaose

A positron emission tomography (PET) tracer composed of ¹⁸F‐labeled maltohexaose (MH¹⁸F) can image bacteria in vivo with a sensitivity and specificity that are orders of magnitude higher than those of fluorodeoxyglucose (¹⁸FDG). MH¹⁸F can detect early‐stage infections composed of as few as 10⁵ E. coli colony‐forming units (CFUs), and can identify drug resistance in bacteria in vivo. MH¹⁸F has the potential to improve the diagnosis of bacterial infections given its unique combination of high specificity and sensitivity for bacteria.     

A Highly Active N‐Heterocyclic Carbene Manganese(I) Complex for Selective Electrocatalytic CO2 Reduction to CO

We report here the first purely organometallic fac‐[Mn^I(CO)₃(bis‐^MeNHC)Br] complex with unprecedented activity for the selective electrocatalytic reduction of CO₂ to CO, exceeding 100 turnovers with excellent faradaic yields (η_CO≈95 %) in anhydrous CH₃CN. Under the same conditions, a maximum turnover frequency (TOF_max) of 2100 s^?1 was measured by cyclic voltammetry, which clearly exceeds the values reported for other manganese‐based catalysts. Moreover, the addition of water leads to the highest TOF_max value (ca. 320 000 s^?1) ever reported for a manganese‐based catalyst. A Mn^I tetracarbonyl intermediate was detected under catalytic conditions for the first time.     

18F Labeling of Arenes

Molecular imaging has witnessed an upsurge in growth, with positron emission tomography leading the way. This trend has encouraged numerous synthetic chemists to enter the field of ¹⁸F‐radiochemistry and provide generic solutions to address the well‐recognized challenges of late‐stage fluorination. This Minireview focuses on recent developments in the ¹⁸F‐labeling of aromatic substrates.     

One‐Pot,Direct Incorporation of [11C]CO2 into Carbamates

Why beat about the bush? An operationally simple and mild reaction based on the direct fixation of ¹¹CO₂ with 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) has been developed for the synthesis of ¹¹C‐labeled carbamates at 75 °C within 10 minutes in radiochemical yields above 70 % (see scheme). This strategy should be immediately useful for the construction of new radiotracers for positron emission tomography and other applications.

     

Difluorocarbene‐Derived Trifluoromethylthiolation and [18F]Trifluoromethylthiolation of Aliphatic Electrophiles

The first trifluoromethylthiolation and [¹⁸F]trifluoromethylthiolation of alkyl electrophiles with in situ generated difluorocarbene in the presence of elemental sulfur and external (radioactive) fluoride ion is described. This transition‐metal‐free approach is high yielding, compatible with a variety of functional groups, and operated under mild reaction conditions. The conceptual advantage of this exogenous‐fluoride‐mediated transformation enables unprecedented syntheses of [¹⁸F]CF₃S‐labeled molecules from most commonly used [¹⁸F]fluoride ions. The rapid radiochemical reaction time (≤1 min) and high functional‐group tolerance allow access to a variety of aliphatic [¹⁸F]CF₃S compounds in high yields.