HaloTag technology: a versatile platform for biomedical applications

Exploration of protein function and interaction is critical for discovering links among genomics, proteomics, and disease state; yet, the immense complexity of proteomics found in biological systems currently limits our investigational capacity. Although affinity and autofluorescent tags are widely employed for protein analysis, these methods have been met with limited success because they lack specificity and require multiple fusion tags and genetic constructs. As an alternative approach, the innovative HaloTag protein fusion platform allows protein function and interaction to be comprehensively analyzed using a single genetic construct with multiple capabilities. This is accomplished using a simplified process, in which a variable HaloTag ligand binds rapidly to the HaloTag protein (usually linked to the protein of interest) with high affinity and specificity. In this review, we examine all current applications of the HaloTag technology platform for biomedical applications, such as the study of protein isolation and purification, protein function, protein–protein and protein–DNA interactions, biological assays, in vitro cellular imaging, and in vivo molecular imaging. In addition, novel uses of the HaloTag platform are briefly discussed along with potential future applications.

Relationship between serum levels of endogenous secretory RAGE and blood pressure in male nondiabetic patients with obstructive sleep apnea

The interaction of advanced glycation end products (AGE) and their specific cell-surface receptor (RAGE) has an important role in the pathogenesis of cardiovascular disease and diabetic complications. Two isoforms of C-truncated RAGE, soluble RAGE (sRAGE) and endogenous secretory RAGE (esRAGE), may prevent activation of RAGE signaling by acting as decoys. This study investigated whether serum esRAGE and sRAGE levels are associated with blood pressure in nondiabetic patients with obstructive sleep apnea (OSA). Male nondiabetic patients (n=139) with OSA were enrolled. Serum esRAGE and sRAGE levels were examined using enzyme-linked immunosorbent assay. Three consecutive seated systolic blood pressure (SBP) and diastolic blood pressure (DBP) measurements were obtained at 5-min intervals in the morning. In univariate analysis, there was a significant correlation between serum esRAGE and SBP or DBP, but not between serum sRAGE and SBP or DBP. Multiple regression analysis showed that SBP was independently associated with waist circumference, HbA1c, minimum SaO2 and serum esRAGE, and that DBP was independently associated with low-density lipoprotein cholesterol, apnea-hypopnea index, serum AGE and body mass index, but not with serum esRAGE. These results indicated that serum esRAGE levels were inversely associated with blood pressure, especially SBP, in male nondiabetic patients with OSA. esRAGE may have a protective role against hypertension in patients with OSA, and it may be a novel biomarker for OSA patients at high risk of developing cardiovascular diseases.

3D fluoroscopic image estimation using patient-specific 4DCBCT-based motion models

3D fluoroscopic images represent volumetric patient anatomy during treatment with high spatial and temporal resolution. 3D fluoroscopic images estimated using motion models built using 4DCT images, taken days or weeks prior to treatment, do not reliably represent patient anatomy during treatment. In this study we developed and performed initial evaluation of techniques to develop patient-specific motion models from 4D cone-beam CT (4DCBCT) images, taken immediately before treatment, and used these models to estimate 3D fluoroscopic images based on 2D kV projections captured during treatment. We evaluate the accuracy of 3D fluoroscopic images by comparison to ground truth digital and physical phantom images. The performance of 4DCBCT-based and 4DCT-based motion models are compared in simulated clinical situations representing tumor baseline shift or initial patient positioning errors. The results of this study demonstrate the ability for 4DCBCT imaging to generate motion models that can account for changes that cannot be accounted for with 4DCT-based motion models. When simulating tumor baseline shift and patient positioning errors of up to 5 mm, the average tumor localization error and the 95th percentile error in six datasets were 1.20 and 2.2 mm, respectively, for 4DCBCT-based motion models. 4DCT-based motion models applied to the same six datasets resulted in average tumor localization error and the 95th percentile error of 4.18 and 5.4 mm, respectively. Analysis of voxel-wise intensity differences was also conducted for all experiments. In summary, this study demonstrates the feasibility of 4DCBCT-based 3D fluoroscopic image generation in digital and physical phantoms and shows the potential advantage of 4DCBCT-based 3D fluoroscopic image estimation when there are changes in anatomy between the time of 4DCT imaging and the time of treatment delivery.

PET Imaging of Abdominal Aortic Aneurysm with 64Cu-Labeled Anti-CD105 Antibody Fab Fragment

The critical challenge in abdominal aortic aneurysm (AAA) research is the accurate diagnosis and assessment of AAA progression. Angiogenesis is a pathologic hallmark of AAA, and CD105 is highly expressed on newly formed vessels. Our goal was to use (64)Cu-labeled anti-CD105 antibody Fab fragment for noninvasive assessment of angiogenesis in the aortic wall in a murine model of AAA.

METHODS

Fab fragment of TRC105, a mAb that specifically binds to CD105, was generated by enzymatic papain digestion and conjugated to NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid) for (64)Cu labeling. The binding affinity/specificity of NOTA-TRC105-Fab was evaluated by flow cytometry and various ex vivo studies. BALB/c mice were anesthetized and treated with calcium phosphate to induce AAA and underwent weekly PET scans using (64)Cu-NOTA-TRC105-Fab. Biodistribution and autoradiography studies were also performed to confirm the accuracy of PET results.

RESULTS

NOTA-TRC105-Fab exhibited high purity and specifically bound to CD105 in vitro. Uptake of (64)Cu-NOTA-TRC105-Fab increased from a control level of 3.4 ± 0.1 to 9.5 ± 0.4 percentage injected dose per gram (%ID/g) at 6 h after injection on day 5 and decreased to 7.2 ± 1.4 %ID/g on day 12, which correlated well with biodistribution and autoradiography studies (i.e., much higher tracer uptake in AAA than normal aorta). Of note, enhanced AAA contrast was achieved, due to the minimal background in the abdominal area of mice. Degradation of elastic fibers and highly expressed CD105 were observed in ex vivo studies.

CONCLUSION

(64)Cu-NOTA-TRC105-Fab cleared rapidly through the kidneys, which enabled noninvasive PET imaging of the aorta with enhanced contrast and showed increased angiogenesis (CD105 expression) during AAA. (64)Cu-NOTA-TRC105-Fab PET may potentially be used for future diagnosis and prognosis of AAA.

PET of c-Met in Cancer with ⁶⁴Cu-Labeled Hepatocyte Growth Factor

The hepatocyte growth factor (HGF) and its receptor, c-Met, are actively involved in tumor progression and metastasis and are closely associated with a poor prognostic outcome for cancer patients. Thus, the development of PET agents that can assess c-Met expression would be extremely useful for diagnosing cancer and subsequently monitoring response to c-Met-targeted therapies. Here, we report the characterization of recombinant human HGF (rh-HGF) as a PET tracer for detection of c-Met expression in vivo.

METHODS

rh-HGF was expressed in human embryonic kidney 293 cells and purified by nickel-nitrilotriacetic acid affinity chromatography. The concentrated rh-HGF was conjugated to 2-S-(4-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid and labeled with (64)Cu. c-Met binding evaluation by flow cytometry was performed on both U87MG and MDA-MB-231 cell lines, which have a high level and a low level, respectively, of c-Met. PET imaging and biodistribution studies were performed on nude mice bearing U87MG and MDA-MB-231 xenografted tumors.

RESULTS

The rh-HGF expression yield was 150-200 μg of protein per 5 × 10(6) cells after a 48-h transfection, with purity of approximately 85%-90%. Flow cytometry examination confirmed that rh-HGF had a strong and specific capacity to bind to c-Met. After (64)Cu labeling, PET imaging revealed specific and prominent uptake of (64)Cu-NOTA-rh-HGF in c-Met-positive U87MG tumors (percentage injected dose per gram, 6.8 ± 1.8 at 9 h after injection) and significantly lower uptake in c-Met-negative MDA-MB-231 tumors (percentage injected dose per gram, 1.8 ± 0.6 at 9 h after injection). The fact that sonication-denatured rh-HGF had significantly lower uptake in U87MG tumors, along with histology analysis, confirmed the c-Met specificity of (64)Cu-NOTA-rh-HGF.

CONCLUSION

This study provided initial evidence that (64)Cu-NOTA-rh-HGF visualizes c-Met expression in vivo, an application that may prove useful for c-Met-targeted cancer therapy.

ImmunoPET of tissue factor expression in triple-negative breast cancer with a radiolabeled antibody Fab fragment

PURPOSE

To date, there is no effective therapy for triple-negative breast cancer (TNBC), which has a dismal clinical outcome. Upregulation of tissue factor (TF) expression leads to increased patient morbidity and mortality in many solid tumor types, including TNBC. Our goal was to employ the Fab fragment of ALT-836, a chimeric anti-human TF mAb, for PET imaging of TNBC, which can be used to guide future TNBC therapy.

METHODS

ALT-836-Fab was generated by enzymatic papain digestion. SDS-PAGE and FACS studies were performed to evaluate the integrity and TF binding affinity of ALT-836-Fab before NOTA conjugation and (64)Cu-labeling. Serial PET imaging and biodistribution studies were carried out to evaluate the tumor targeting efficacy and pharmacokinetics in the MDA-MB-231 TNBC model, which expresses high levels of TF on the tumor cells. Blocking studies, histological assessment, as well as RT-PCR were performed to confirm TF specificity of (64)Cu-NOTA-ALT-836-Fab.

RESULTS

ALT-836-Fab was produced with high purity, which exhibited superb TF binding affinity and specificity. Serial PET imaging revealed rapid and persistent tumor uptake of (64)Cu-NOTA-ALT-836-Fab (5.1 ± 0.5 %ID/g at 24 h post-injection; n = 4) and high tumor/muscle ratio (7.0 ± 1.2 at 24 h post-injection; n = 4), several-fold higher than that of the blocking group and tumor models that do not express significant level of TF, which was confirmed by biodistribution studies. TF specificity of the tracer was also validated by histology and RT-PCR.

CONCLUSION

(64)Cu-NOTA-ALT-836-Fab exhibited prominent tissue factor targeting efficiency in MDA-MB-231 TNBC model. The use of a Fab fragment led to fast tumor uptake and good tissue/muscle ratio, which may be translated into same-day immunoPET imaging in the clinical setting to improve TNBC patient management.

Hexamodal imaging with porphyrin-phospholipid-coated upconversion nanoparticles

Hexamodal imaging using simple nanoparticles is demonstrated. Porphyrin-phospholipids are used to coat upconversion nanoparticles in order to generate a new biocompatible material. The nanoparticles are characterized in vitro and in vivo for imaging via fluorescence, upconversion, positron emission tomography, computed tomography, Cerenkov luminescence, and photoacoustic tomography.

Positron emission tomography imaging using radiolabeled inorganic nanomaterials

CONSPECTUS: Positron emission tomography (PET) is a radionuclide imaging technology that plays an important role in preclinical and clinical research. With administration of a small amount of radiotracer, PET imaging can provide a noninvasive, highly sensitive, and quantitative readout of its organ/tissue targeting efficiency and pharmacokinetics. Various radiotracers have been designed to target specific molecular events. Compared with antibodies, proteins, peptides, and other biologically relevant molecules, nanoparticles represent a new frontier in molecular imaging probe design, enabling the attachment of different imaging modalities, targeting ligands, and therapeutic payloads in a single vector. We introduce the radiolabeled nanoparticle platforms that we and others have developed. Due to the fundamental differences in the various nanoparticles and radioisotopes, most radiolabeling methods are designed case-by-case. We focus on some general rules about selecting appropriate isotopes for given types of nanoparticles, as well as adjusting the labeling strategies according to specific applications. We classified these radiolabeling methods into four categories: (1) complexation reaction of radiometal ions with chelators via coordination chemistry; (2) direct bombardment of nanoparticles via hadronic projectiles; (3) synthesis of nanoparticles using a mixture of radioactive and nonradioactive precursors; (4) chelator-free postsynthetic radiolabeling. Method 1 is generally applicable to different nanomaterials as long as the surface chemistry is well-designed. However, the addition of chelators brings concerns of possible changes to the physicochemical properties of nanomaterials and detachment of the radiometal. Methods 2 and 3 have improved radiochemical stability. The applications are, however, limited by the possible damage to the nanocomponent caused by the proton beams (method 2) and harsh synthetic conditions (method 3). Method 4 is still in its infancy. Although being fast and specific, only a few combinations of isotopes and nanoparticles have been explored. Since the applications of radiolabeled nanoparticles are based on the premise that the radioisotopes are stably attached to the nanomaterials, stability (colloidal and radiochemical) assessment of radiolabeled nanoparticles is also highlighted. Despite the fact that thousands of nanomaterials have been developed for clinical research, only very few have moved to humans. One major reason is the lack of understanding of the biological behavior of nanomaterials. We discuss specific examples of using PET imaging to monitor the in vivo fate of radiolabeled nanoparticles, emphasizing the importance of labeling strategies and caution in interpreting PET data. Design considerations for radiolabeled nanoplatforms for multimodal molecular imaging are also illustrated, with a focus on strategies to combine the strengths of different imaging modalities and to prolong the circulation time.

Red fluorescent zinc oxide nanoparticle: a novel platform for cancer targeting

Multifunctional zinc oxide (ZnO) nanoparticles (NPs) with well-integrated multimodality imaging capacities have generated increasing research interest in the past decade. However, limited progress has been made in developing ZnO NP-based multimodality tumor-imaging agents. Here we developed novel red fluorescent ZnO NPs and described the successful conjugation of 64Cu (t1/2=12.7 h) and TRC105, a chimeric monoclonal antibody against CD105, to these ZnO NPs via well-developed surface engineering procedures. The produced dual-modality ZnO NPs were readily applicable for positron emission tomography (PET) imaging and fluorescence imaging of the tumor vasculature. Their pharmacokinetics and tumor-targeting efficacy/specificity in mice bearing murine breast 4T1 tumor were thoroughly investigated. ZnO NPs with dual-modality imaging properties can serve as an attractive candidate for future cancer theranostics.

Iron oxide decorated MoS2 nanosheets with double PEGylation for chelator-free radiolabeling and multimodal imaging guided photothermal therapy

Theranostics for in vivo cancer diagnosis and treatment generally requires well-designed nanoscale platforms with multiple integrated functionalities. In this study, we uncover that functionalized iron oxide nanoparticles (IONPs) could be self-assembled on the surface of two-dimensional MoS2 nanosheets via sulfur chemistry, forming MoS2-IO nanocomposites, which are then modified with two types of polyethylene glycol (PEG) to acquire enhanced stability in physiological environments. Interestingly, (64)Cu, a commonly used positron-emitting radioisotope, could be firmly adsorbed on the surface of MoS2 without the need of chelating molecules, to enable in vivo positron emission tomography (PET) imaging. On the other hand, the strong near-infrared (NIR) and superparamagnetism of MoS2-IO-PEG could also be utilized for photoacoustic tomography (PAT) and magnetic resonance (MR) imaging, respectively. Under the guidance by such triple-modal imaging, which uncovers efficient tumor retention of MoS2-IO-(d)PEG upon intravenous injection, in vivo photothermal therapy is finally conducted, achieving effective tumor ablation in an animal tumor model. Our study highlights the promise of constructing multifunctional theranostic nanocomposites based on 2D transitional metal dichalcogenides for multimodal imaging-guided cancer therapy.

PET of follicle-stimulating hormone receptor: broad applicability to cancer imaging

Selective overexpression of follicle-stimulating hormone receptor (FSHR) inside the vascular endothelium of tumors has been confirmed to play critical roles in angiogenesis, tumor invasion, and metastases. The expression level of FSHR correlates strongly with the response of tumors to antiangiogenic therapies. In this study, an immunoPET tracer was developed for imaging of FSHR in different cancer types. A monoclonal antibody (FSHR-mAb) against FSHR was conjugated with S-2-(4-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid (p-SCN-Bn-NOTA) and used for subsequent (64)Cu-labeling. NOTA-FSHR-mAb preserved FSHR specificity/affinity, confirmed by flow cytometry measurements. (64)Cu-labeling was successfully conducted with decent yields (∼25%) and high specific activity (0.93 GBq/mg). The uptake of (64)Cu-NOTA-FSHR-mAb was 3.6 ± 0.8, 13.2 ± 0.7, and 14.6 ± 0.4 %ID/g in FSHR-positive CAOV-3 tumors at 4, 24, and 48 h postinjection, respectively (n = 3), significantly higher (p < 0.05) than that in FSHR-negative SKOV-3 tumors (2.3 ± 1.2, 8.0 ± 0.9, and 9.1 ± 1.3 %ID/g at 4, 24, and 48 h postinjection, respectively (n = 3)) except at 4 h p.i. FSHR-relevant uptake of (64)Cu-NOTA-FSHR-mAb was also readily observed in other tumor types (e.g., triple-negative breast tumor MDA-MB-231 or prostate tumor PC-3). Histology studies showed universal FSHR expression in microvasculature of these four tumor types and also prominent expression in tumor cells of CAOV-3, PC-3, and MDA-MB-231. Correlations between tumor FSHR level and uptake of (64)Cu-NOTA-FSHR-mAb were witnessed in this study. FSHR-specific uptake of (64)Cu-NOTA-FSHR mAb in different tumors enables its applicability for future cancer theranostic applications and simultaneously establishes FSHR as a promising clinical target for cancer.

⁵²Mn production for PET/MRI tracking of human stem cells expressing divalent metal transporter 1 (DMT1)

There is a growing demand for long-term in vivo stem cell imaging for assessing cell therapy techniques and guiding therapeutic decisions. This work develops the production of ⁵²Mn and establishes proof of concept for the use of divalent metal transporter 1 (DMT1) as a positron emission tomography (PET) and magnetic resonance imaging (MRI) reporter gene for stem cell tracking in the rat brain. ⁵²Mn was produced via proton irradiation of a natural chromium target. In a comparison of two ⁵²Mn separation methods, solvent-solvent extraction was preferred over ion exchange chromatography because of reduced chromium impurities and higher ⁵²Mn recovery. In vitro uptake of Mn-based PET and MRI contrast agents (⁵²Mn²⁺ and Mn²⁺, respectively) was enhanced in DMT1 over-expressing human neural progenitor cells (hNPC-DMT1) compared to wild-type control cells (hNPC-WT). After cell transplantation in the rat striatum, increased uptake of Mn-based contrast agents in grafted hNPC-DMT1 was detected in in vivo manganese-enhanced MRI (MEMRI) and ex vivo PET and autoradiography. These initial studies indicate that this approach holds promise for dual-modality PET/MR tracking of transplanted stem cells in the central nervous system and prompt further investigation into the clinical applicability of this technique.