89Zr-labeled ImmunoPET targeting the cancer stem cell antigen CD133 using fully-human antibody constructs.

BACKGROUND: Cancer stem cells play an important role in driving tumor growth and treatment resistance, which makes them a promising therapeutic target to prevent cancer recurrence. Emerging cancer stem cell-targeted therapies would benefit from companion diagnostic imaging probes to aid in patient selection and monitoring response to therapy. To this end, zirconium-89-radiolabeled immunoPET probes that target the cancer stem cell-antigen CD133 were developed using fully human antibody and antibody scFv-Fc scaffolds. RESULTS: ImmunoPET probes [89Zr]-DFO-RW03IgG (CA = 0.7 ± 0.1), [89Zr]-DFO-RW03IgG (CA = 3.0 ± 0.3), and [89Zr]-DFO-RW03scFv - Fc (CA = 2.9 ± 0.3) were radiolabeled with zirconium-89 (radiochemical yield 42 ± 5%, 97 ± 2%, 86 ± 12%, respectively) and each was isolated in > 97% radiochemical purity with specific activities of 120 ± 30, 270 ± 90, and 200 ± 60 MBq/mg, respectively. In vitro binding assays showed a low-nanomolar binding affinity of 0.6 to 1.1 nM (95% CI) for DFO-RW03IgG (CA = 0.7 ± 0.1), 0.3 to 1.9 nM (95% CI) for DFO-RW03IgG (CA = 3.0 ± 0.3), and 1.5 to 3.3 nM (95% CI) for DFO-RW03scFv - Fc (C/A = 0.3). Biodistribution studies found that [89Zr]-DFO-RW03scFv - Fc (CA = 2.9 ± 0.3) exhibited the highest tumor uptake (23 ± 4, 21 ± 2, and 23 ± 4%ID/g at 24, 48, and 72 h, respectively) and showed low uptake (< 6%ID/g) in all off-target organs at each timepoint (24, 48, and 72 h). Comparatively, [89Zr]-DFO-RW03IgG (CA = 0.7 ± 0.1) and [89Zr]-DFO-RW03IgG (CA = 3.0 ± 0.3) both reached maximum tumor uptake (16 ± 3%ID/g and 16 ± 2%ID/g, respectively) at 96 h p.i. and showed higher liver uptake (10.2 ± 3%ID/g and 15 ± 3%ID/g, respectively) at that timepoint. Region of interest analysis to assess PET images of mice administered [89Zr]-DFO-RW03scFv - Fc (CA = 2.9 ± 0.3) showed that this probe reached a maximum tumor uptake of 22 ± 1%ID/cc at 96 h, providing a tumor-to-liver ratio that exceeded 1:1 at 48 h p.i. Antibody-antigen mediated tumor uptake was demonstrated through biodistribution and PET imaging studies, where for each probe, co-injection of excess unlabeled RW03IgG resulted in > 60% reduced tumor uptake. CONCLUSIONS: Fully human CD133-targeted immunoPET probes [89Zr]-DFO-RW03IgG and [89Zr]-DFO-RW03scFv - Fc accumulate in CD133-expressing tumors to enable their delineation through PET imaging. Having identified [89Zr]-DFO-RW03scFv - Fc (CA = 2.9 ± 0.3) as the most attractive construct for CD133-expressing tumor delineation, the next step is to evaluate this probe using patient-derived tumor models to test its detection limit prior to clinical translation.

Tetrazine-Derived Near-Infrared Dye for Targeted Photoacoustic Imaging of Bone.

A near-infrared photoacoustic probe was used to image bone in vivo through active and bioorthogonal pretargeting strategies that utilized coupling between a tetrazine-derived cyanine dye and a trans-cyclooctene-modified bisphosphonate. In vitro hydroxyapatite binding of the probe via active and pretargeting strategies showed comparable increases in percent binding vs a nontargeted control. Intrafemoral injection of the bisphosphonate-dye conjugate showed retention out to 24 h post-injection, with a 14-fold increase in signal over background, while the nontargeted dye exhibited negligible binding to bone and signal washout by 4 h post-injection. Intravenous injection, using both active and pretargeting strategies, demonstrated bone accumulation as earlier as 4 h post-injection, where the signal was found to be 3.6- and 1.5-fold higher, respectively, than the signal from the nontargeted dye. The described bone-targeted dye enabled in vivo photoacoustic imaging, while the synthetic strategy provides a convenient building block for developing new targeted photoacoustic probes.

Pretargeted PET of Osteodestructive Lesions in Dogs.

The last decade has witnessed the creation of a highly effective approach to in vivo pretargeting based on the inverse electron demand Diels-Alder (IEDDA) click ligation between tetrazine (Tz) and trans-cyclooctene (TCO). Despite the steady progression of this technology toward the clinic, concerns have persisted regarding whether this in vivo chemistry will work in humans given their larger size and blood volume. In this work, we describe the use of a 64Cu-labeled Tz radioligand ([64Cu]Cu-SarAr-Tz) and a TCO-bearing bisphosphonate (TCO-BP) for the pretargeted positron emission tomography (PET) imaging of osteodestructive lesions in a large animal model: companion dogs. First, in a small animal pilot study, healthy mice were injected with TCO-BP followed after 1 or 6 h by [64Cu]Cu-SarAr-Tz. PET images were collected 1, 6, and 24 h after the administration of [64Cu]Cu-SarAr-Tz, revealing that this approach produced high activity concentrations in the bone (>20 and >15%ID/g in the femur and humerus, respectively, at 24 h post injection) as well as high target-to-background contrast. Subsequently, companion dogs (n = 5) presenting with osteodestructive lesions were administered TCO-BP (5 or 10 mg/kg) followed 1 h later by [64Cu]Cu-SarAr-Tz (2.2-7.3 mCi; 81.4-270.1 MBq). PET scans were collected for each dog 4 h after the administration of the radioligand, and SUV values for the osteodestructive lesions, healthy bones, and kidneys were determined. In these animals, pretargeted PET clearly delineated healthy bone and produced very high activity concentrations in osteodestructive lesions. Low levels of uptake were observed in all healthy organs except for the kidneys and bladder due to the renal excretion of excess radioligand. Ultimately, this work not only illustrates that pretargeted PET with TCO-BP and [64Cu]Cu-SarAr-Tz is an effective tool for the visualization of osteodestructive lesions but also demonstrates for the first time that in vivo pretargeting based on IEDDA click chemistry is feasible in large animals.

Synthesis and Preclinical Evaluation of [18F]SiFA-PSMA Inhibitors in a Prostate Cancer Model.

Positron emission tomography (PET) imaging of prostate-specific membrane antigen (PSMA) with gallium-68 (68Ga) and fluorine-18 (18F) radiotracers has aroused tremendous interest over the past few years. The use of organosilicon-[18F]fluoride acceptors (SiFA) conjugated to urea-based peptidomimetic PSMA inhibitors provides a "kit-like" multidose synthesis technology. Nine novel 18F-labeled SiFA-bearing PSMA inhibitors with different linker moieties were synthesized and analyzed for their in vitro binding against [125I]I-TAAG-PSMA in LNCaP cells. IC50 values ranged from 58-570 nM. Among all compounds, [18F]SiFA-Asp2-PEG3-PSMA (IC50 = 125 nM) showed the highest tumor uptake in LNCaP tumors (SUV60min 0.73). A substantial increase in molar activity (Am) (from 7.5 ± 0.5 to 86 ± 3 GBq/µmol) led to a significant increase in LNCaP tumor uptake (SUV60min 1.18; Δ 0.45 corresponding to +62%). In vivo blocking with DCFPyL resulted in -32% uptake after 60 min. The SiFA-isotopic exchange chemistry offers a method that is readily adaptable for a "kit-type" labeling procedure and clinical translation.

A Versatile Platform for the Development of Radiolabeled Antibody-Recruiting Small Molecules.

Building on clinical case reports of the abscopal effect, there has been considerable interest in the synergistic effects of radiation and immunotherapies for the treatment of cancer. Here, the first radiolabeled antibody-recruiting small molecule that can chelate a variety of cytotoxic radionuclides is described. The platform consists of a tunable antibody-binding domain against a serum antibody of interest (e.g., dinitrophenyl hapten) to recruit endogenous antibodies that activate effector cell function, a chelate capable of binding diagnostic and therapeutic radiometals, and a tetrazine for bioorthogonal coupling with trans-cyclooctene-modified targeting vectors. The dinitrophenyl-tetrazine ligand was shown to both affect dose-dependent antibody recruitment and immune cell function (phagocytosis) in vitro, and the bisphosphonate 177Lu-complex was shown to accumulate at sites of calcium accretion in vivo, which was achieved using both active and pretargeting strategies.

Imidazole fused phenanthroline (PIP) ligands for the preparation of multimodal Re(I) and 99mTc(I) probes.

A small library of [2 + 1] 99mTc(i) complexes based on phenyl-imidazole-fused phenanthroline (PIP) ligands were synthesized and evaluated as multimodal molecular imaging probes. Using either a two-step or a one-pot synthesis method, 99mTc-PIP complexes containing N-methylimidazole as the monodentate ligand were prepared and isolated in good (54 to 89%) radiochemical yield, with the exception of one derivative bearing a strongly electron-withdrawing substituent. The stability of the [2 + 1] complexes was assessed in saline and in cysteine and histidine challenge studies, showing 6 hours stability, making them suitable for in vivo studies. In parallel, the Re(i) analogues were prepared as reference standards to verify the structure of the 99mTc complexes. The optical properties were consistent with other previously reported [2 + 1] type Re(i) complexes that have been used as cellular dyes and sensors. To facilitate the development of targeted derivatives, a tetrazine-PIP ligand was also synthesized. The 99mTc complex of the tetrazine PIP ligand effectively coupled to compounds containing a trans-cyclooctene (TCO) group including a TCO-albumin derivative, which was prepared as a model targeting molecule. An added benefit of the Re-PIP-Tz construct is that the emission from the metal complex was quenched by the presence of the tetrazine. Following the addition of TCO, there was a 70-fold increase in fluorescence emission, which can in future be leveraged during in vitro studies to reduce background signal.

Tetrazine-Derived Near-Infrared Dye as a Facile Reagent for Developing Targeted Photoacoustic Imaging Agents.

A new photoacoustic (PA) dye was developed as a simple-to-use reagent for creating targeted PA imaging agents. The lead molecule was prepared via an efficient two-step synthesis from an inexpensive commercially available starting material. With the dye's innate albumin-binding properties, the resulting tetrazine-derived dye is capable of localizing to tumor and exhibits a biological half-life of a few hours, allowing for an optimized distribution profile. The presence of tetrazine in turn makes it possible to link the albumin-binding optoacoustic signaling agent to a wide range of targeting molecules. To demonstrate the utility and ease of use of the platform, a novel PA probe for imaging calcium accretion was generated using a single-step bioorthogonal coupling reaction where high-resolution PA images of the knee joint in mice were obtained as early as 1 h post injection. Whole-body distribution was subsequently determined by labeling the probe with 99mTc and performing tissue counting following necropsy. These studies, along with tumor imaging and in vitro albumin binding studies, revealed that the core PA contrast agent can be imaged in vivo and can be easily linked to targeting molecules for organ-specific uptake.

Evaluation of a 68Ga-Labeled DOTA-Tetrazine as a PET Alternative to 111In-SPECT Pretargeted Imaging.

The bioorthogonal reaction between a tetrazine and strained transcyclooctene (TCO) has garnered success in pretargeted imaging. This reaction was first validated in nuclear imaging using an 111In-labeled 1,4,7,10tetraazacyclododecane1,4,7,10tetraacetic acid (DOTA)-linked bispyridyl tetrazine (Tz) ([111In]In-DOTA-PEG11-Tz) and a TCO functionalized CC49 antibody. Given the initial success of this Tz, it has been paired with TCO functionalized small molecules, diabodies, and affibodies for in vivo pretargeted studies. Furthermore, the single photon emission tomography (SPECT) radionuclide, 111In, has been replaced with the ß-emitter, 177Lu and α-emitter, 212Pb, both yielding the opportunity for targeted radiotherapy. Despite use of the 'universal chelator', DOTA, there is yet to be an analogue suitable for positron emission tomography (PET) using a widely available radionuclide. Here, a 68Ga-labeled variant ([68Ga]Ga-DOTA-PEG11-Tz) was developed and evaluated using two different in vivo pretargeting systems (Aln-TCO and TCO-CC49). Small animal imaging and ex vivo biodistribution studies were performed and revealed target specific uptake of [68Ga]Ga-DOTA-PEG11-Tz in the bone (3.7 %ID/g, knee) in mice pretreated with Aln-TCO and tumor specific uptake (5.8 %ID/g) with TCO-CC49 in mice bearing LS174 xenografts. Given the results of this study, [68Ga]Ga-DOTA-PEG11-Tz can serve as an alternative to [111In]In-DOTA-PEG11-Tz.

Evaluation of the inverse electron demand Diels-Alder reaction in rats using a scandium-44-labelled tetrazine for pretargeted PET imaging.

BACKGROUND: Pretargeted imaging allows the use of short-lived radionuclides when imaging the accumulation of slow clearing targeting agents such as antibodies. The biotin-(strept)avidin and the bispecific antibody-hapten interactions have been applied in clinical pretargeting studies; unfortunately, these systems led to immunogenic responses in patients. The inverse electron demand Diels-Alder (IEDDA) reaction between a radiolabelled tetrazine (Tz) and a trans-cyclooctene (TCO)-functionalized targeting vector is a promising alternative for clinical pretargeted imaging due to its fast reaction kinetics. This strategy was first applied in nuclear medicine using an 111In-labelled Tz to image TCO-functionalized antibodies in tumour-bearing mice. Since then, the IEDDA has been used extensively in pretargeted nuclear imaging and radiotherapy; however, these studies have only been performed in mice. Herein, we report the 44Sc labelling of a Tz and evaluate it in pretargeted imaging in Wistar rats. RESULTS: 44Sc was obtained from an in house 44Ti/44Sc generator. A 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-functionalized tetrazine was radiolabelled with 44Sc resulting in radiochemical yields of 85-95%, a radiochemical purity > 99% at an apparent molar activity of 1 GBq/mmol. The 44Sc-labelled Tz maintained stability in solution for up to 24 h. A TCO-functionalized bisphosphonate, which accumulates in skeletal tissue, was used as a targeting vector to evaluate the 44Sc-labelled Tz. Biodistribution data of the 44Sc-labelled Tz showed specific uptake (0.9 ± 0.3% ID/g) in the bones (humerus and femur) of rats pre-treated with the TCO-functionalized bisphosphonate. This uptake was not present in rats not receiving pre-treatment (< 0.03% ID/g). CONCLUSIONS: We have prepared a 44Sc-labelled Tz and used it in pretargeted PET imaging with rats treated with TCO-functionalized bisphosponates. This allowed for the evaluation of the IEDDA reaction in animals larger than a typical mouse. Non-target accumulation was low, and there was a 30-fold higher bone uptake in the pre-treated rats compared to the non-treated controls. Given its convenient half-life and the ability to perform positron emission tomography with a previously studied DOTA-functionalized Tz, scandium-44 (t1/2 = 3.97 h) proved to be a suitable radioisotope for this study.

Improved radiosynthesis and preliminary in vivo evaluation of the 11C-labeled tetrazine [11C]AE-1 for pretargeted PET imaging.

Pretargeted nuclear imaging based on the ligation between tetrazines and nano-sized targeting agents functionalized with trans-cyclooctene (TCO) has recently been shown to improve both imaging contrast and dosimetry in nuclear imaging of nanomedicines. Herein, we describe the improved radiosynthesis of a 11C-labeled tetrazine ([11C]AE-1) and its preliminary evaluation in both mice and pigs. Pretargeted imaging in mice was carried out using both a new TCO-functionalized polyglutamic acid and a previously reported TCO-functionalized bisphosphonate system as targeting agents. Unfortunately, pretargeted imaging was not successful using these targeting agents in pair with [11C]AE-1. However, brain imaging in pig indicated that the tracer crossed the blood-brain-barrier. Hence, we suggest that this tetrazine scaffold could be used as a starting point for the development of pretargeted brain imaging, which has so far been a challenging task.

In vivo Biodistribution of Radiolabeled Acoustic Protein Nanostructures.

PURPOSE: Contrast-enhanced ultrasound plays an expanding role in oncology, but its applicability to molecular imaging is hindered by a lack of nanoscale contrast agents that can reach targets outside the vasculature. Gas vesicles (GVs)-a unique class of gas-filled protein nanostructures-have recently been introduced as a promising new class of ultrasound contrast agents that can potentially access the extravascular space and be modified for molecular targeting. The purpose of the present study is to determine the quantitative biodistribution of GVs, which is critical for their development as imaging agents. PROCEDURES: We use a novel bioorthogonal radiolabeling strategy to prepare technetium-99m-radiolabeled ([99mTc])GVs in high radiochemical purity. We use single photon emission computed tomography (SPECT) and tissue counting to quantitatively assess GV biodistribution in mice. RESULTS: Twenty minutes following administration to mice, the SPECT biodistribution shows that 84 % of [99mTc]GVs are taken up by the reticuloendothelial system (RES) and 13 % are found in the gall bladder and duodenum. Quantitative tissue counting shows that the uptake (mean ± SEM % of injected dose/organ) is 0.6 ± 0.2 for the gall bladder, 46.2 ± 3.1 for the liver, 1.91 ± 0.16 for the lungs, and 1.3 ± 0.3 for the spleen. Fluorescence imaging confirmed the presence of GVs in RES. CONCLUSIONS: These results provide essential information for the development of GVs as targeted nanoscale imaging agents for ultrasound.

[18F]-DCFPyL Positron Emission Tomography/Magnetic Resonance Imaging for Localization of Dominant Intraprostatic Foci: First Experience.

An ongoing prospective study is acquiring preoperative imaging data for men with prostate cancer (PCa) using the molecular imaging agent [18F]-DCFPyL targeted against prostate-specific membrane antigen (PSMA). To date, six men (of a planned accrual of 24) with clinically localized, biopsy-proven PCa have undergone preoperative [18F]-DCFPyL positron emission tomography (PET) imaging and multiparametric magnetic resonance imaging acquired using a hybrid PET/MRI system. Lesions identified by [18F]-DCFPyL uptake on PET/MRI were characterized in terms of maximum standardized uptake value (SUVmax) and volume using a boundary threshold of 40% SUVmax. Following surgery, all prostatectomy specimens were processed using a whole-mount technique for accurate deformable co-registration and correlation with PCa foci defined on digitized pathology images. Well-defined intraprostatic dominant lesions were identified by [18F]-DCFPyL PET/MRI (mean SUVmax 11.4±8.25; mean volume 2.2±2.4cm3) in all six men. Co-registered digitized whole-mount pathology for the first case revealed that intense [18F]-DCFPyL uptake (SUVmax 27±1.1cm3) and multiparametric MRI changes (Prostate Imaging Reporting and Data System score of 4) were highly correlated with a 0.5-cm3 dominant (largest) lesion with Gleason pattern 4 PCa in the right mid peripheral zone. A smaller focus (0.01cm3) of lower-grade PCa (Gleason pattern 3) had much lower uptake (SUV 2.7). These early prospective data show that dominant intraprostatic lesions could be identified in all six men using [18F]-DCFPyL as an imaging probe. Trial accrual will continue to quantify in terms of spatial concordance the ability of [18F]-DCFPyL to identify the location and characterize the grade of intraprostatic cancer foci in clinically localized PCa. PATIENT SUMMARY: Positron emission tomography using a novel probe called [18F]-DCFPyL directed against the prostate-specific membrane antigen protein was able to identify locations of prostate cancer in the prostate glands of men undergoing imaging before surgery. In the future, such imaging may allow better targeting of treatment to the portion of the prostate containing the most aggressive components of cancer rather than treating the whole prostate in a uniform fashion.

18F-Labeled perfluorocarbon droplets for positron emission tomography imaging.

INTRODUCTION: Nanoscale perfluorocarbon (PFC) droplets have been used to create imaging agents and drug delivery vehicles. However, development and characterization of new formulations of PFC droplets are hindered because of the lack of simple methods for quantitative and sensitive assessment of whole body tissue distribution and pharmacokinetics of the droplets. To address this issue, a general-purpose method for radiolabeling the inner core of nanoscale perfluorocarbon droplets with a hydrophobic and lipophobic fluorine-18 compound was developed, so that positron emission tomography (PET) and quantitative biodistribution studies can be employed to evaluate PFC nanodroplets in vivo. METHODS: A robust method to produce [18F]CF3(CF2)7(CH2)3F from a tosylate precursor using [18F]F- was developed. The product's effectiveness as a general label for different PFCs and its ability to distinguish the in vivo behavior of different PFC droplet formulations was evaluated using two types of PFC nanodroplets: fluorosurfactant-stabilized perfluorohexane (PFH) nanodroplets and lipid-stabilized perfluorooctylbromide (PFOB) nanodroplets. In vivo assessment of the 18F-labeled PFH and PFOB nanodroplets were conducted in normal mice following intravenous injection using small animal PET imaging and gamma counting of tissues and fluids. RESULTS: [18F]CF3(CF2)7(CH2)3F was produced in modest yield and was stable with respect to loss of fluoride in vitro. The labeled fluorocarbon was successfully integrated into PFH nanodroplets (~175 nm) and PFOB nanodroplets (~260 nm) without altering their mean sizes, size distributions, or surface charges compared to their non-radioactive analogues. No leakage of the radiolabel from the nanodroplets was detected after droplet formation in vitro. PET imaging and biodistribution data for the two droplet types tested showed significantly different tissue uptake and clearance patterns. CONCLUSION: A convenient method for producing 18F-labeled PFC droplets was developed. The results highlight the potential utility of the strategy for pre-clinical evaluation of different PFC droplet formulations through direct PFC core labeling using a fluorinated radiolabel.

67Ga-labeled deferoxamine derivatives for imaging bacterial infection: Preparation and screening of functionalized siderophore complexes.

INTRODUCTION: Deferoxamine (DFO) is a siderophore that bacteria use to scavenge iron and could serve as a targeting vector to image bacterial infection where current techniques have critical limitations. [67Ga]-DFO, which is a mimetic of the corresponding iron complex, is taken up by bacteria in culture, however in vivo it clears too rapidly to allow for imaging of infection. In response, we developed several new DFO derivatives to identify those that accumulate in bacteria, and at sites of infection, and that could potentially have improved pharmacokinetics. METHODS: A library of DFO derivatives was synthesized by functionalizing the terminal amine group of DFO using three different carbamate-forming reactions. Uptake of [67Ga]-DFO and the 67Ga-labeled derivatives by bacteria and the biodistribution of lead compounds were studied. RESULTS: 67Ga-labeled DFO derivatives were prepared and isolated in >90% radiochemical yield and >95% radiochemical purity. The derivatives had significant but slower uptake rates in Staphylococcus aureus than [67Ga]-DFO (6% to 60% of the control rate), with no uptake for the most lipophilic derivatives. Biodistribution studies in mice with a S. aureus infection in one thigh revealed that the ethyl carbamate derivative had an excellent infected-to-non-infected ratio (11:1), but high non-specific localization in the gall bladder, liver and small intestine. CONCLUSIONS: The work reported shows that it is possible to functionalize DFO-type siderophores and retain active uptake of the 67Ga-labeled complexes by bacteria. Novel 67Ga-labeled DFO derivatives were specifically taken up by S. aureus and selected derivatives demonstrated in vivo localization at sites of infection. ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE: 67Ga-labeled DFO derivatives were actively transported by bacteria using the iron-siderophore pathway, suggesting that it is possible to develop siderophore-based radiopharmaceuticals for imaging bacterial infection.

Preparation of tetrazine-containing [2 + 1] complexes of 99mTc and in vivo targeting using bioorthogonal inverse electron demand Diels-Alder chemistry.

The aim of this work was to synthesize and evaluate [2 + 1] 99mTc(i) polypyridine complexes containing tetrazines, which along with the corresponding Re(i) complexes, represent a new class of isostructural nuclear and turn-on luminescent probes that can be derivatized and targeted using bioorthogonal chemistry. To this end, [2 + 1] complexes of 99mTc(i) of the type [99mTc(CO)3(N^N)(L)] (N^N = bathophenanthroline disulfonate (BPS) or 2,2'-bipyridine (bipy)), where the monodentate ligand (L) was a tetrazine linked to the metal through an imidazole derivative, were prepared. The desired products were obtained in nearly quantitative radiochemical yield by adding [99mTc(CO)3(N^N)(OH2)]n to the imidazole-tetrazine ligand and heating at 60 °C for 30 min. Measurement of the reaction kinetics between the tetrazine and (E)-cyclooct-4-enol revealed a second-order rate constant of 8.6 × 103 M-1 s-1 at 37 °C, which is suitable for in vivo applications that require rapid coupling. Stability studies showed that the metal complexes were resistant to ligand challenge and exhibited reasonable protein binding in vitro. Biodistribution studies of the more water-soluble BPS derivative in normal mice, one hour after administration of a bisphosphonate derivative of trans-cyclooctene (TCO-BP), revealed high activity concentrations in the knee (9.3 ± 0.3 %ID g-1) and shoulder (5.3 ± 0.7 %ID g-1). Using the same pretargeting approach, SPECT/CT imaging showed that the [2 + 1] tetrazine complex localized to implanted skeletal tumors. This is the first report of the preparation of 99mTc complexes of BPS and demonstration that their tetrazine derivatives can be used to prepare targeted imaging probes by employing bioorthogonal chemistry.

Targeting Prostate-Specific Membrane Antigen (PSMA) with F-18-Labeled Compounds: the Influence of Prosthetic Groups on Tumor Uptake and Clearance Profile.

PURPOSE: Prostate-specific membrane antigen (PSMA) is an important biomarker expressed in the majority of prostate cancers. The favorable positron emission tomography (PET) imaging profile of the PSMA imaging agent 2-(3-(1-carboxy-5-[(6-[18F]fluoro-pyridine-3-carbonyl)-amino]-pentyl)-ureido)-pentane-dioic acid [18F]DCFPyL in preclinical prostate cancer models and in prostate cancer patients stimulated the development and validation of other fluorine-containing PSMA inhibitors to further enhance pharmacokinetics and simplify production methods. Here, we describe the synthesis and radiopharmacological evaluation of various F-18-labeled PSMA inhibitors which were prepared through different prosthetic group chemistry strategies. PROCEDURES: Prosthetic groups N-succinimidyl-4-[18F]fluorobenzoate ([18F]SFB), 4-[18F]fluorobenzaldehyde, and 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) were used for bioconjugation reactions to PSMA-binding lysine-urea-glutamate scaffold via acylation and oxime formation. All fluorine-containing PSMA inhibitors were tested for their PSMA inhibitory potency in an in vitro competitive binding assay in comparison to an established reference compound [125I]TAAG-PSMA. Tumor uptake and clearance profiles of three F-18-labeled PSMA inhibitors ([18F]4, [18F]7, and [18F]8) were studied with dynamic PET imaging using LNCaP tumor-bearing mice. RESULTS: F-18-labeled PSMA inhibitors were synthesized in 32-69 % radiochemical yields using (1) acylation reaction at the primary amino group of the lysine residue with [18F]SFB and (2) oxime formation with 4-[18F]fluorobenzaldehyde and [18F]FDG using the respective aminooxy-functionalized lysine residue. Compound 7 displayed an IC50 value of 6 nM reflecting very high affinity for PSMA. Compounds 4 and 8 showed IC50 values of 13 and 62 nM, respectively. The IC50 value of reference compound DCFPyL was 13 nM. Dynamic PET imaging revealed the following SUV60min for radiotracer uptake in PSMA(+) LNCaP tumors: 0.98 ([18F]DCFPyL), 2.11 ([18F]7), 0.40 ([18F]4), and 0.19 ([18F]8). CONCLUSION: The observed tumor uptake and clearance profiles demonstrate the importance of the selected prosthetic group on the pharmacokinetic profile of analyzed PSMA-targeting radiotracers. Radiotracer [18F]7 displayed the highest uptake and retention in LNCaP tumors, which exceeded uptake values of reference compound [18F]DCFPyL by more than 100 %. Despite the higher kidney and liver uptake and retention of compound [18F]7, the simple radiosynthesis and the exceptionally high tumor uptake (SUV60min 2.11) and retention make radiotracer [18F]7 an interesting alternative to radiotracer [18F]DCFPyL for PET imaging of PSMA in prostate cancer.

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

Prostate specific membrane antigen (PSMA) targeted microbubbles (MBs) were developed using bioorthogonal chemistry. Streptavidin-labeled MBs were treated with a biotinylated tetrazine (MBTz) and targeted to PSMA expressing cells using trans-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 in vitro flow chamber. The initial approach involved pretargeting, where TCO-anti-PSMA was first incubated with PSMA expressing cells and followed by MBTz, 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 MBTz prior to initiation of the assay, a 5-fold increase in binding compared to controls was observed. The direct targeting approach was subsequently evaluated in vivo 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.

Technetium(I) Complexes of Bathophenanthrolinedisulfonic Acid.

Bathophenanthrolinedisulfonate (BPS) complexes of technetium(I) of the type [Tc(CO)3(BPS)(L)]n (L = imidazole derivatives) were synthesized and evaluated both in vitro and in vivo. [99mTc(CO)3(BPS)(MeIm)]- (MeIm = 1-methyl-1H-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.

Preparation and Evaluation of 99mTc-labeled Tridentate Chelates for Pre-targeting Using Bioorthogonal Chemistry.

Pre-targeting combined with bioorthogonal chemistry is emerging as an effective way to create new radiopharmaceuticals. Of the methods available, the inverse electron demand Diels-Alder (IEDDA) cycloaddition between a radiolabeled tetrazines and trans-cyclooctene (TCO) linked to a biomolecule has proven to be a highly effective bioorthogonal approach to imaging specific biological targets. Despite the fact that technetium-99m remains the most widely used isotope in diagnostic nuclear medicine, there is a scarcity of methods for preparing 99mTc-labeled tetrazines. Herein we report the preparation of a family of tridentate-chelate-tetrazine derivatives and their Tc(I) complexes. These hitherto unknown compounds were radiolabeled with 99mTc using a microwave-assisted method in 31% to 83% radiochemical yield. The products are stable in saline and PBS and react rapidly with TCO derivatives in vitro. Their in vivo pre-targeting abilities were demonstrated using a TCO-bisphosphonate (TCO-BP) derivative that localizes to regions of active bone metabolism or injury. In murine studies, the 99mTc-tetrazines showed high activity concentrations in knees and shoulder joints, which was not observed when experiments were performed in the absence of TCO-BP. The overall uptake in non-target organs and pharmacokinetics varied greatly depending on the nature of the linker and polarity of the chelate.

Correction: A 99mTc-Labelled Tetrazine for Bioorthogonal Chemistry. Synthesis and Biodistribution Studies with Small Molecule trans-Cyclooctene Derivatives.

[This corrects the article DOI: 10.1371/journal.pone.0167425.].