In Vivo Labeling of Serum Albumin for PET

Protein click chemistry and its potential for medical applications

A revolution in chemical biology occurred with the introduction of click chemistry. Click chemistry plays an important role in protein chemistry modifications, providing specific, sensitive, rapid, and easy-to-handle methods. Under physiological conditions, click chemistry often overlaps with bioorthogonal chemistry, defined as reactions that occur rapidly and selectively without interfering with biological processes. Click chemistry is used for the posttranslational modification of proteins based on covalent bond formations. With the contribution of click reactions, selective modification of proteins would be developed, representing an alternative to other technologies in preparing new proteins or enzymes for studying specific protein functions in different biological processes. Click-modified proteins have potential in diverse applications such as imaging, labeling, sensing, drug design, and enzyme technology. Due to the promising role of proteins in disease diagnosis and therapy, this review aims to highlight the growing applications of click strategies in protein chemistry over the last two decades, with a special emphasis on medicinal applications.

Thyroid Cancers Exhibit Oncogene-Enhanced Macropinocytosis that Is Restrained by IGF1R and Promote Albumin-Drug Conjugate Response

PURPOSE

Oncogene-driven macropinocytosis fuels nutrient scavenging in some cancer types, yet whether this occurs in thyroid cancers with prominent MAPK-ERK and PI3K pathway mutations remains unclear. We hypothesized that understanding links between thyroid cancer signaling and macropinocytosis might uncover new therapeutic strategies.

EXPERIMENTAL DESIGN

Macropinocytosis was assessed across cells derived from papillary thyroid cancer (PTC), follicular thyroid cancer (FTC), non-malignant follicular thyroid, and aggressive anaplastic thyroid cancer (ATC), by imaging fluorescent dextran and serum albumin. The impacts of ectopic BRAFV600E and mutant RAS, genetic PTEN silencing, and inhibitors targeting RET, BRAF, and MEK kinases were quantified. BrafV600E p53-/- ATC tumors in immunocompetent mice were used to measure efficacy of an albumin-drug conjugate comprising microtubule-destabilizing monomethyl auristatin E (MMAE) linked to serum albumin via a cathepsin-cleavable peptide (Alb-vc-MMAE).

RESULTS

FTC and ATC cells showed greater macropinocytosis than non-malignant and PTC cells. ATC tumors accumulated albumin at 8.8% injected dose per gram tissue. Alb-vc-MMAE, but not MMAE alone, reduced tumor size by >90% (P < 0.01). ATC macropinocytosis depended on MAPK/ERK activity and nutrient signaling, and increased by up to 230% with metformin, phenformin, or inhibition of IGF1Ri in monoculture but not in vivo. Macrophages also accumulated albumin and express the cognate IGF1R ligand, IGF1, which reduced ATC responsiveness to IGF1Ri.

CONCLUSIONS

These findings identify regulated oncogene-driven macropinocytosis in thyroid cancers and demonstrate the potential of designing albumin-bound drugs to efficiently treat them.

Novel albumin-binding multifunctional probe for synergistic enhancement of FL/MR dual-modal imaging and photothermal therapy

The fluorescence/magnetic resonance (FL/MR) dual-modal imaging could provide accurate tumor visualization to guide photothermal therapy (PTT) of cancer, which has attracted widespread attention from scientists. However, facile and effective strategies to synergistically enhance fluorescence intensity, MR contrast and photothermal efficacy have rarely been reported. This study presents a novel multifunctional probe Gd-EB-ICG (GI) for FL/MR dual-modal imaging-guided PTT of cancer. GIs can self-assemble with endogenous albumin to form drug-albumin complexes (GIAs), which exhibit excellent biocompatibility. Albumin can protect GIAs from the recognition and clearance by the mononuclear phagocytic system (MPS). High plasma concentration and long half-life allow GIAs to accumulate continuously in the tumor area through EPR effect and specific uptake of tumor. Because of the prolonged rotational correlation time (τR) of Gd chelates, GIAs exhibited superior MR contrast performance over GIs with more than 3 times enhancement of longitudinal relaxation efficiency (r1). The fluorescence quantum yield and photothermal conversion efficiency of GIAs was also significantly improved due to the constrained geometry, disrupted aggregation and enhanced photothermal stability. This simple and feasible strategy successfully resulted in a synergistic effect for FL/MR dual-modal imaging and photothermal therapy, which can cast a new light for the clinical translation of multifunctional probes.

Preclinical PET Imaging and Toxicity Study of a 68Ga-Functionalized Polymeric Cardiac Blood Pool Agent

Cardiac blood pool imaging is currently performed almost exclusively with 99mTc-based compounds and SPECT/CT imaging. Using a generator-based PET radioisotope has a few advantages, including not needing nuclear reactors to produce it, obtaining better resolution in humans, and potentially reducing the radiation dose to the patient. When the shortlived radioisotope 68Ga is used, it can be applied repeatedly on the same day—for example, for the detection of bleeding. Our objective was to prepare and evaluate a long-circulating polymer functionalized with gallium for its biodistribution, toxicity, and dosimetric properties. A 500 kDa hyperbranched polyglycerol was conjugated to the chelator NOTA and radiolabeled rapidly at room temperature with 68Ga. It was then injected intravenously into a rat, and gated imaging allowed us to easily observe wall motion and cardiac contractility, confirming the suitability of this radiopharmaceutical for cardiac blood pool imaging. Internal radiation dose calculations showed that the radiation doses that patients would receive from the PET agent would be 2.5× lower than those from the 99mTc agent. A complete 14-day toxicology study in rats concluded that there were no gross pathology findings, changes in body or organ weights, or histopathological events. This radioactive-metal-functionalized polymer might be a suitable non-toxic agent to advance for clinical application.

Agglomerated serum albumin adsorbed protocatechuic acid coated superparamagnetic iron oxide nanoparticles as a theranostic agent

Iron oxide nanoparticles have been one of the most widely used nanomaterials in biomedical applications. However, the incomplete understanding of the toxicity mechanisms limits their use in diagnosis and treatment processes. Many parameters are associated with their toxicity such as size, surface modification, solubility, concentration and immunogenicity. Further research needs to be done to address toxicity-related concerns and to increase its effectiveness in various applications. Herein, colloidally stable nanoparticles were prepared by coating magnetic iron oxide nanoparticles (MIONPs) with protocatechuic acid (PCA) which served as a stabilizer and a linkage for a further functional layer. A new perfusion agent with magnetic imaging capability was produced by the adsorption of biocompatible passivating agent macro-aggregated albumin (MAA) on the PCA-coated MIONPs. PCA-coated MIONPs were investigated using infrared spectroscopy, thermogravimetric analysis and dynamic light scattering while adsorption of MAA was analysed by transmission electron microscopy, Fourier-transform infrared spectroscopy and x-ray diffraction methods. Magnetic measurements of samples indicated that all samples showed superparamagnetic behaviour. Cytotoxicity results revealed that the adsorption of MAA onto PCA-coated MIONPs provided an advantage by diminishing their toxicity against the L929 mouse fibroblast cell line compared to bare Fe₃O₄.

68Ga-Labeled Maleimide for Blood Pool and Lymph PET Imaging through Covalent Bonding to Serum Albumin In Vivo

This study aims to develop a novel ⁶⁸Ga-labeled tracer, which can covalently bind to albumin in vivo based on the maleimide-thiol strategy, and to evaluate its potential applications using positron emission tomography (PET). ⁶⁸Ga-labeled maleimide-monoamide-DOTA (denoted as [⁶⁸Ga]Ga-DM) was prepared conveniently with a high radiochemical yield (>90%) and radiochemical purity (>99%). Its molar activity was calculated as 249.60 ± 68.50 GBq/μmol, and the octanol-water partition coefficient (LogP) was -3.15 ± 0.08 with good stabilities. In vitro experiments showed that [⁶⁸Ga]Ga-DM can bind to albumin efficiently and rapidly, with a binding fraction of over 70%. High uptake and excellent retention in blood were observed with a long half-life (t _1/2Z) of 190.15 ± 24.14 min, which makes it possible for blood pool PET imaging with high contrast. The transient micro-bleeding in the rat model was detected successfully with PET imaging. In addition, the uptakes of [⁶⁸Ga]Ga-DM in the inflammatory popliteal lymph nodes depend on the severity (5.90% ID/g and 2.32% ID/g vs 1.01% ID/g for healthy lymph nodes at 0.5 h post-injection) indicating its feasibility for lymphatic imaging. In conclusion, a novel ⁶⁸Ga-labeled tracer was prepared with high efficiency and yield in mild conditions. Based on the promising properties of bonding covalently to albumin, great stability, high blood contrast with a long half-life, and well environmental tolerance, [⁶⁸Ga]Ga-DM could be developed as a potential tracer for PET imaging of blood pool, bleeding, and vascular permeability alteration diseases in the clinic.

Tailoring the NIR-II Photoluminescence of Single Thiolated Au25 Nanoclusters by Selective Binding to Proteins

Atomically precise gold nanoclusters are a fascinating class of nanomaterials that exhibit molecule-like properties and have outstanding photoluminescence (PL). Their ultrasmall size, molecular chemistry, and biocompatibility make them extremely appealing for selective biomolecule labeling in investigations of biological mechanisms at the cellular and anatomical levels. In this work, we report a simple route to incorporate a preformed Au₂₅ nanocluster into a model bovine serum albumin (BSA) protein. A new approach combining small-angle X-ray scattering and molecular modeling provides a clear localization of a single Au₂₅ within the protein to a cysteine residue on the gold nanocluster surface. Attaching Au₂₅ to BSA strikingly modifies the PL properties with enhancement and a redshift in the second near-infrared (NIR-II) window. This study paves the way to conrol the design of selective sensitive probes in biomolecules through a ligand-based strategy to enable the optical detection of biomolecules in a cellular environment by live imaging.

Relating18F-FDG image signal-to-noise ratio to time-of-flight noise-equivalent count rate in total-body PET using the uEXPLORER scanner

Objective.This work assessed the relationship between image signal-to-noise ratio (SNR) and total-body noise-equivalent count rate (NECR)-for both non-time-of-flight (TOF) NECR and TOF-NECR-in a long uniform water cylinder and 14 healthy human subjects using the uEXPLORER total-body PET/CT scanner.Approach.A TOF-NEC expression was modified for list-mode PET data, and both the non-TOF NECR and TOF-NECR were compared using datasets from a long uniform water cylinder and 14 human subjects scanned up to 12 h after radiotracer injection.Main results.The TOF-NECR for the uniform water cylinder was found to be linearly proportional to the TOF-reconstructed image SNR²in the range of radioactivity concentrations studied, but not for non-TOF NECR as indicated by the reducedR²value. The results suggest that the use of TOF-NECR to estimate the count rate performance of TOF-enabled PET systems may be more appropriate for predicting the SNR of TOF-reconstructed images.Significance.Image quality in PET is commonly characterized by image SNR and, correspondingly, the NECR. While the use of NECR for predicting image quality in conventional PET systems is well-studied, the relationship between SNR and NECR has not been examined in detail in long axial field-of-view total-body PET systems, especially for human subjects. Furthermore, the current NEMA NU 2-2018 standard does not account for count rate performance gains due to TOF in the NECR evaluation. The relationship between image SNR and total-body NECR in long axial FOV PET was assessed for the first time using the uEXPLORER total-body PET/CT scanner.

A Comparison of Evans Blue and 4-(p-Iodophenyl)butyryl Albumin Binding Moieties on an Integrin αvβ6 Binding Peptide

Serum albumin binding moieties (ABMs) such as the Evans blue (EB) dye fragment and the 4-(p-iodophenyl)butyryl (IP) have been used to improve the pharmacokinetic profile of many radiopharmaceuticals. The goal of this work was to directly compare these two ABMs when conjugated to an integrin αvβ6 binding peptide (αvβ6-BP); a peptide that is currently being used for positron emission tomography (PET) imaging in patients with metastatic cancer. The ABM-modified αvβ6-BP peptides were synthesized with a 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetracetic acid (DOTA) chelator for radiolabeling with copper-64 to yield [64Cu]Cu DOTA-EB-αvβ6-BP ([64Cu]1) and [64Cu]Cu DOTA-IP-αvβ6-BP ([64Cu]2). Both peptides were evaluated in vitro for serum albumin binding, serum stability, and cell binding and internalization in the paired engineered melanoma cells DX3puroβ6 (αvβ6 +) and DX3puro (αvβ6 −), and pancreatic BxPC-3 (αvβ6 +) cells and in vivo in a BxPC-3 xenograft mouse model. Serum albumin binding for [64Cu]1 and [64Cu]2 was 53−63% and 42−44%, respectively, with good human serum stability (24 h: [64Cu]1 76%, [64Cu]2 90%). Selective αvβ6 cell binding was observed for both [64Cu]1 and [64Cu]2 (αvβ6 (+) cells: 30.3−55.8% and 48.5−60.2%, respectively, vs. αvβ6 (−) cells <3.1% for both). In vivo BxPC-3 tumor uptake for both peptides at 4 h was 5.29 ± 0.59 and 7.60 ± 0.43% ID/g ([64Cu]1 and [64Cu]2, respectively), and remained at 3.32 ± 0.46 and 4.91 ± 1.19% ID/g, respectively, at 72 h, representing a >3-fold improvement over the non-ABM parent peptide and thereby providing improved PET images. Comparing [64Cu]1 and [64Cu]2, the IP-ABM-αvβ6-BP [64Cu]2 displayed higher serum stability, higher tumor accumulation, and lower kidney and liver accumulation, resulting in better tumor-to-organ ratios for high contrast visualization of the αvβ6 (+) tumor by PET imaging.

Dual Function of a in vivo Albumin-Labeling Tracer for Assessment of Blood Perfusion and Vascular Permeability in Peripheral Arterial Disease by PET

Background

Peripheral arterial disease (PAD) leads to tissue ischemia in the extremities. Enhanced vascular permeability plays a critical role in targeted delivery of drugs for effective therapeutic angiogenesis and resultant blood perfusion recovery. However, optimal tracers for evaluating this process in PAD patients are lacking. At this time, we employed a novel in vivo albumin-labeling tracer of dual function, termed as ¹⁸F-NEB, to assess blood perfusion as well as vascular permeability by positron emission tomography (PET).

Methods and Results

After successful establishment of mouse hindlimb ischemia (HI) model, static PET imaging was performed 15 min and 2 h post injection (p.i.) of ¹⁸F-NEB at 1, 3, 5, 7, 10 and 14 days post-surgery respectively. Gradual recovery of blood supply was detected by PET scan 15 min p.i. and collaborated by serial Laser Doppler. In addition, the highest vascular permeability observed by high local uptake of ¹⁸F-NEB at 2 h p.i. was consistent with histological examinations. Furthermore, we quantitatively evaluated the effect of vascular endothelial growth factor (VEGF) stimulus on vascular permeability and blood perfusion by PET scan using ¹⁸F-NEB probe in HI model, which were also confirmed by immunohistological results.

Conclusion

The application of ¹⁸F-NEB probe alone by PET can successfully achieve dual imaging of blood perfusion as well as vascular permeability at different time points p.i. and monitor their responses to therapy in PAD model. The simple labeling approach and multipurpose feature suggest the great promise of using this imaging probe in theranostic applications for treating ischemic disease.

Multi-Scale Imaging of Vascular Pathologies in Cardiovascular Disease

Cardiovascular disease entails systemic changes in the vasculature. The endothelial cells lining the blood vessels are crucial in the pathogenesis of cardiovascular disease. Healthy endothelial cells direct the blood flow to tissues as vasodilators and act as the systemic interface between the blood and tissues, supplying nutrients for vital organs, and regulating the smooth traffic of leukocytes into tissues. In cardiovascular diseases, when inflammation is sensed, endothelial cells adjust to the local or systemic inflammatory state. As the inflamed vasculature adjusts, changes in the endothelial cells lead to endothelial dysfunction, altered blood flow and permeability, expression of adhesion molecules, vessel wall inflammation, thrombosis, angiogenic processes, and extracellular matrix production at the endothelial cell level. Preclinical multi-scale imaging of these endothelial changes using optical, acoustic, nuclear, MRI, and multimodal techniques has progressed, due to technical advances and enhanced biological understanding on the interaction between immune and endothelial cells. While this review highlights biological processes that are related to changes in the cardiac vasculature during cardiovascular diseases, it also summarizes state-of-the-art vascular imaging techniques. The advantages and disadvantages of the different imaging techniques are highlighted, as well as their principles, methodologies, and preclinical and clinical applications with potential future directions. These multi-scale approaches of vascular imaging carry great potential to further expand our understanding of basic vascular biology, to enable early diagnosis of vascular changes and to provide sensitive diagnostic imaging techniques in the management of cardiovascular disease.

Radiolabeled albumin through SNAr of cysteines as a potential pretargeting theranostic agent

Human serum albumin has been functionalized with a radionuclide by combining SNAr conjugation to Cys34 with CuAAC and inverse-electron demand Diels–Alder reactions demonstrating a promising strategy for generating theranostics by bioconjugation.

11C-radiolabeled aptamer for imaging of tumors and metastases using positron emission tomography- computed tomography

Identification of primary tumors and metastasis sites is an essential step in cancer diagnostics and the following treatment. Positron emission tomography-computed tomography (PET/CT) is one of the most reliable methods for scanning the whole organism for malignancies. In this work, we synthesized an 11C-labeled oligonucleotide primer and hybridized it to an anti-cancer DNA aptamer. The 11C-aptamer was applied for in vivo imaging of Ehrlich ascites carcinoma and its metastases in mice using PET/CT. The imaging experiments with the 11C-aptamer determined very small primary and secondary tumors of 3 mm2 and less. We also compared 11C imaging with the standard radiotracer, 2-deoxy-2-[fluorine-18]fluoro-D-glucose (18F-FDG), and found better selectivity of the 11C-aptamer to metastatic lesions in the metabolically active organs than 18F-FDG. 11C radionuclide with an ultra-short (20.38 min) half-life is considered safest for PET/CT imaging and does not cause false-positive results in heart imaging. Its combination with aptamers gives us high-specificity and high-contrast imaging of cancer cells and can be applied for PET/CT-guided drug delivery in cancer therapies.

Feasibility of using a 99mTc-hydroxamamide complex containing an albumin binder moiety for in vivo albumin labeling-based tumor imaging

Human serum albumin (HSA), which is distributed throughout the blood, is used as a carrier for transporting drugs to tumors based on the enhanced permeability and retention (EPR) effect. To develop an agent for the in vivo radiolabeling of endogenous albumin, we designed and synthesized novel hydroxamamide (Ham)-based technetium-99m (^99mTc) complexes, which contained a monovalent or bivalent 4-(4-iodophenyl)butyric acid (IA) derivative as an albumin binder (ALB) moiety ([^99mTc]AB2 and [^99mTc]ALB2, respectively), and evaluated their utility for in vivo tumor imaging. In an in vitro HSA-binding assay, [^99mTc]AB2 and [^99mTc]ALB2 showed greater binding to HSA than [^99mTc]BHam, a ^99mTc-Ham complex without an ALB moiety. In an in vivo biodistribution assay, [^99mTc]ALB2 showed marked blood and tumor retention (25.13 and 4.61% injected dose (ID)/g, respectively, at 1 h postinjection), suggesting that the EPR effect had been induced. However, [^99mTc]AB2 showed no marked blood or tumor retention (4.16 and 0.75% ID/g, respectively, at 1 h postinjection), probably because the affinity of the monovalent IA derivative for albumin was insufficient to induce the EPR effect. These findings indicated that the multivalent interactions of [^99mTc]ALB2 had enhanced its affinity for albumin. ^99mTc-complexes containing multivalent ALB moieties may be useful for tumor imaging.

Synthesis and Application of a Long-Circulating Radiolabeled Peptide for Targeting of Osteosarcoma

PURPOSE

The small peptide TMTP1 (NVVRQ) has been proved to target a series of highly metastatic tumor cells. The aim of this study was to develop a new agent based on TMTP1 conjugated with Evans blue (EB), to increase tumor uptake and modify the pharmacokinetic characteristics of the resulting radiolabeled agent.

PROCEDURES

DOTA-EB-TMTP1 was prepared through conventional solid-phase peptide synthesis chemistry. Then, it was successfully labeled with Cu-64 to obtain [⁶⁴Cu]DOTA-EB-TMTP1. The tumor targeting properties were evaluated in vivo using 143B xenografts.

RESULTS

DOTA-EB-TMTP1 was successfully labeled with Cu-64 in a yield of 87.3 ± 5.2 %. In a small animal positron emission tomography/X-ray computed tomography (PET/CT) study in osteosarcoma 143B xenograft mice, [⁶⁴Cu]DOTA-EB-TMTP1 was found to rapidly accumulate in the tumor tissue. The tumor uptake increased over time and reached a plateau of 6.50 ± 0.88 % ID/g 8 h after tail vein injection. The radioactivity remained in the tumor tissue 48 h postinjection with a negligible decrease.

CONCLUSIONS

Overall, the introduction of the EB motif to TMTP1 significantly changed its pharmacokinetics in vivo, and this strategy fulfills the purpose of prolonging the blood circulation and enhancing the tumor uptake. [⁶⁴Cu]DOTA-EB-TMTP1 is a promising agent for osteosarcoma targeting. Moreover, our study highlights that DOTA-EB-TMTP1 is a good candidate for labeling with different radionuclides for potential theranostic applications.

[68Ga]ABY-028: an albumin-binding domain (ABD) protein-based imaging tracer for positron emission tomography (PET) studies of altered vascular permeability and predictions of albumin-drug conjugate transport

Background

Albumin is commonly used as a carrier platform for drugs to extend their circulatory half-lives and influence their uptake into tissues that have altered permeability to the plasma protein. The albumin-binding domain (ABD) protein, which binds in vivo to serum albumin with high affinity, has proven to be a versatile scaffold for engineering biopharmaceuticals with a range of binding capabilities. In this study, the ABD protein equipped with a mal-DOTA chelator (denoted ABY-028) was radiolabeled with gallium-68 (⁶⁸Ga). This novel radiotracer was then used together with positron emission tomography (PET) imaging to examine variations in the uptake of the ABD-albumin conjugate with variations in endothelial permeability.

Results

ABY-028, produced by peptide synthesis in excellent purity and stored at − 20 °C, was stable for 24 months (end of study). [⁶⁸Ga]ABY-028 could be obtained with labeling yields of > 80% and approximately 95% radiochemical purity. [⁶⁸Ga]ABY-028 distributed in vivo with the plasma pool, with highest radioactivity in the heart ventricles and major vessels of the body, a gradual transport over time from the circulatory system into tissues and elimination via the kidneys. Early [⁶⁸Ga]ABY-028 uptake differed in xenografts with different vascular properties: mean standard uptake values (SUV_mean) were initially 5 times larger in FaDu than in A431 xenografts, but the difference decreased to 3 after 1 h. Cutaneously administered, vasoactive nitroglycerin increased radioactivity in the A431 xenografts. Heterogeneity in the levels and rates of increases of radioactivity uptake was observed in sub-regions of individual MMTV-PyMT mammary tumors and in FaDu xenografts. Higher uptake early after tracer administration could be observed in lower metabolic regions. Fluctuations in the increased permeability for the tracer across the blood-brain-barrier (BBB) direct after experimentally induced stroke were monitored by PET and the increased uptake was confirmed by ex vivo phosphorimaging.

Conclusions

[⁶⁸Ga]ABY-028 is a promising new tracer for visualization of changes in albumin uptake due to disease- and pharmacologically altered vascular permeability and their potential effects on the passive uptake of targeting therapeutics based on the ABD protein technology.

68Ga-NEB PET/CT can be a new method for diagnosing chylous fistula: Case reports of a rare complication after breast cancer surgery

RATIONALE

We hypothesize that with the determination of lymph fistula location 3-dimensionally, application of appropriate pressure would promote fistula healing, and a secondary surgery may be avoided. Ga-labeled 1,4,7-triazacyclononane-N, N', N"-triacetic acid (NOTA) conjugated with truncated Evan blue (NEB) forms a complex with serum albumin in the interstitial fluid after it is locally injected and allows rapid visualization of the lymphatic system.

PATIENT CONCERNS

A 44-year-old woman had a chief complaint of left nipple discharge. A 38-year-old woman came to the hospital after sensing a right breast mass.

DIAGNOSES

The 2 patients were diagnosed with chylous fistula after breast cancer surgery based on the findings of a novel method, Ga-NOTA-Evans Blue (NEB) positron emission tomography/computed tomography.

INTERVENTIONS

We successfully obtained clear images to locate the fistula using Ga-NEB positron emission tomography/computed tomography (PET/CT) for both patients. The lymphatic vessels and lymph nodes could be clearly visualized owing to the Ga-NEB activity during PET/CT.

OUTCOMES

Three-dimensional positioning to locate the fistula could direct the application of the pressure dressing and reduce drainage markedly.

LESSONS

Ga-NEB PET/CT may be a new method for diagnosing chylous fistula and providing guidance for treatment.

A pocket-escaping design to prevent the common interference with near-infrared fluorescent probes in vivo

Near-infrared (NIR) fluorescent probes are among the most attractive chemical tools for biomedical imaging. However, their in vivo applications are hindered by albumin binding, generating unspecific fluorescence that masks the specific signal from the analyte. Here, combining experimental and docking methods, we elucidate that the reason for this problem is an acceptor (A) group-mediated capture of the dyes into hydrophobic pockets of albumin. This pocket-capturing phenomenon commonly applies to dyes designed under the twisted intramolecular charge-transfer (TICT) principle and, therefore, represents a generic but previously unidentified backdoor problem. Accordingly, we create a new A group that avoids being trapped into the albumin pockets (pocket-escaping) and thereby construct a NIR probe, BNLBN, which effectively prevents this backdoor problem with increased imaging accuracy for liver fibrosis in vivo. Overall, our study explains and overcomes a fundamental problem for the in vivo application of a broad class of bioimaging tools.

Combined 68Ga-NOTA-Evans Blue Lymphoscintigraphy and 68Ga-NOTA-RM26 PET/CT Evaluation of Sentinel Lymph Node Metastasis in Breast Cancer Patients

In this study, we applied a new strategy to identify sentinel lymph node (SLN) metastasis by combining ⁶⁸Ga-NOTA-Evans Blue (⁶⁸Ga-NEB) for SLN mapping and ⁶⁸Ga-NOTA-RM26 for LN metastasis detection in breast cancer patients. A total of 24 female patients with breast cancer diagnosed by core biopsy or suspected by mammography or ultrasonography were recruited and provided informed consent. All patients underwent ⁶⁸Ga-NEB and ⁶⁸Ga-NOTA-RM26 PET/CT imaging. Visual analysis of ⁶⁸Ga-NEB PET/CT images was used to determine SLNs, and then compared with the ⁶⁸Ga-NOTA-RM26 results and histopathological findings. SLNs were visualized in 24 of 24 patients (100.0%) within 4.0-10.0 (5.6 ± 1.4) min. All patients were pathologically diagnosed with breast cancer, and 12 patients had ipsilateral lymph node metastasis. By combining ⁶⁸Ga-NEB and ⁶⁸Ga-NOTA-RM26 images, 7/12 (58.3%) patients showed mild to intense uptake of ⁶⁸Ga-NOTA-RM26 in SLNs, 1/12 patient (8.3%) had moderate uptake of ⁶⁸Ga-NOTA-RM26 in the non-SLNs rather than SLN, indicating possible bypass lymphatic drainage, partially accounting for the false negatives in SLN biopsy during surgery. No false positives were found. The SUV_max of ⁶⁸Ga-NOTA-RM26 activity in metastatic SLNs was significantly higher than that in non-metastatic SLNs (2.2 ± 2.3 vs 0.7 ± 0.1, P = 0.047). This study manifests the value of combination of ⁶⁸Ga-NEB and ⁶⁸Ga-NOTA-RM26 dual tracer PET/CT in preoperative evaluation of SLN metastasis in breast cancer patients, especially in those patients with lymphatic obstruction and bypass drainage. In general, positive ⁶⁸Ga-NOTA-RM26 uptake in either SLN or other lymph nodes can apply lymph node dissection rather than intraoperative SLN biopsy.

Recent trends in protein and peptide-based biomaterials for advanced drug delivery

Engineering protein and peptide-based materials for drug delivery applications has gained momentum due to their biochemical and biophysical properties over synthetic materials, including biocompatibility, ease of synthesis and purification, tunability, scalability, and lack of toxicity. These biomolecules have been used to develop a host of drug delivery platforms, such as peptide- and protein-drug conjugates, injectable particles, and drug depots to deliver small molecule drugs, therapeutic proteins, and nucleic acids. In this review, we discuss progress in engineering the architecture and biological functions of peptide-based biomaterials -naturally derived, chemically synthesized and recombinant- with a focus on the molecular features that modulate their structure-function relationships for drug delivery.

Characterization of Brain Inflammation, Apoptosis, Hypoxia, Blood-Brain Barrier Integrity and Metabolism in Venezuelan Equine Encephalitis Virus (VEEV TC-83) Exposed Mice by In Vivo Positron Emission Tomography Imaging

Traditional pathogenesis studies of alphaviruses involves monitoring survival, viremia, and pathogen dissemination via serial necropsies; however, molecular imaging shifts this paradigm and provides a dynamic assessment of pathogen infection. Positron emission tomography (PET) with PET tracers targeted to study neuroinflammation (N,N-diethyl-2-[4-phenyl]-5,7-dimethylpyrazolo[1,5-a]pyrimidine-3-acetamide, [¹⁸F]DPA-714), apoptosis (caspase-3 substrate, [¹⁸F]CP-18), hypoxia (fluormisonidazole, [¹⁸F]FMISO), blood–brain barrier (BBB) integrity ([¹⁸F]albumin), and metabolism (fluorodeoxyglucose, [¹⁸F]FDG) was performed on C3H/HeN mice infected intranasally with 7000 plaque-forming units (PFU) of Venezuelan equine encephalitis virus (VEEV) TC-83. The main findings are as follows: (1) whole-brain [¹⁸F]DPA-714 and [¹⁸F]CP-18 uptake increased three-fold demonstrating, neuroinflammation and apoptosis, respectively; (2) [¹⁸F]albumin uptake increased by 25% across the brain demonstrating an altered BBB; (3) [¹⁸F]FMISO uptake increased by 50% across the whole brain indicating hypoxic regions; (4) whole-brain [¹⁸F]FDG uptake was unaffected; (5) [¹⁸F]DPA-714 uptake in (a) cortex, thalamus, striatum, hypothalamus, and hippocampus increased through day seven and decreased by day 10 post exposure, (b) olfactory bulb increased at day three, peaked day seven, and decreased day 10, and (c) brain stem and cerebellum increased through day 10. In conclusion, intranasal exposure of C3H/HeN mice to VEEV TC-83 results in both time-dependent and regional increases in brain inflammation, apoptosis, and hypoxia, as well as modest decreases in BBB integrity; however, it has no effect on brain glucose metabolism.

An Albumin Sandwich Enhances in Vivo Circulation and Stability of Metabolically Labile Peptides

The effectiveness of numerous molecular drugs is hampered by their poor pharmacokinetics. Different from previous approaches with limited effectiveness, most recently, emerging high-affinity albumin binding moieties (ABMs) for in vivo hitchhiking of endogenous albumin opens up an avenue to chaperone small molecules for long-acting therapeutics. Although several FDA-approved fatty acids have shown prolonged residence and therapeutic effect, an easily synthesized, water-soluble, and high-efficiency ABM with versatile drug loading ability is urgently needed to improve the therapeutic efficacy of short-lived constructs. We herein identified an ideal bivalent Evans blue derivative, denoted as N(tEB)₂, as a smart ABM-delivery platform to chaperone short-lived molecules, through both computational modeling screening and efficient synthetic schemes. The optimal N(tEB)₂ could reversibly link two molecules of albumin through its two binding heads with a preferable spacer, resulting in significantly extended circulation half-life of a preloaded cargo and water-soluble. Notably, this in situ dimerization of albumin was able to sandwich peptide therapeutics to protect them from proteolysis. As an application, we conjugated N(tEB)₂ with exendin-4 for long-acting glucose control in a diabetic mouse model, and it was superior to both previously tested NtEB-exendin-4 (Abextide) and the newly FDA-approved semaglutide, which has been arguably the best commercial weekly formula so far. Hence, this novel albumin binder has excellent clinical potential for next-generation biomimetic drug delivery systems.

Antidiabetic Effect of Abextide, a Long-Acting Exendin-4 Analogue in Cynomolgus Monkeys

Abextide, synthesized by conjugating an albumin-binding moiety-truncated Evans blue-to glucagon-like peptide 1 receptor (GLP-1R) agonist exendin-4, shows extended drug release and enhanced hypoglycemic effect in diabetic mice. The aim of this study is to evaluate the pharmacodynamics of Abextide in nonhuman primates. Two batches of elderly cynomolgus monkeys with naturally occurring diabetes are used for this study. During the whole experiment period, no abnormalities such as swelling at the injection site, lethargy, or hypoglycemia are observed in all animals. The monkeys in the Abextide group lose appetite after drug administration and then recover over time. In the single dose treatment, at day 1 and day 3 after treatment, decreased plasma glucose level is observed in the Abextide-treated group but not in placebo or Albiglutide-treated group. For monkeys that receive two doses of drug, the blood glucose level in all subjects in Abextide group decreases rapidly upon drug administration and return to a plateau by day 3. A similar pattern of response is seen after the second dose administration. The delayed drug release and hypoglycemic effect of Abextide make it potentially useful as an antidiabetic drug for weekly subcutaneous administration.

An Albumin-Binding T1- T2 Dual-Modal MRI Contrast Agents for Improved Sensitivity and Accuracy in Tumor Imaging

Magnetic resonance imaging (MRI) diagnosis is better assisted by contrast agents that can augment the signal contrast in the imaging appearance. However, this technique is still limited by the inherently low sensitivity on the recorded signal changes in conventional T₁ or T₂ MRI in a qualitative manner. Here, we provide a new paradigm of MRI diagnosis using T₁- T₂ dual-modal MRI contrast agents for contrast-enhanced postimaging computations on T₁ and T₂ relaxation changes. An albumin-binding molecule (i.e., truncated Evans blue) chelated with paramagnetic manganese ion was developed as a novel T₁- T₂ dual-modal MRI contrast agent at high magnetic field (7 T). Furthermore, the postimaging computations on T₁- T₂ dual-modal MRI led to greatly enhanced signal-to-noise ratios (SNR) and contrast-to-noise ratios (CNR) in both subcutaneous and orthotopic brain tumor models compared with traditional MRI methods. The T₁- T₂ dual-modal MRI computations have great potential to eliminate suspicious artifacts and false-positive signals in mouse brain imaging. This study may open new avenues for contrast-enhanced MRI diagnosis and holds great promise for precision medicine.

Development of 99mTc-Labeled Human Serum Albumin with Prolonged Circulation by Chelate-then-Click Approach: A Potential Blood Pool Imaging Agent

Technetium-99m-labeled human serum albumin (^99mTc-HSA) has been utilized as a blood pool imaging agent in the clinic for several decades. However, ^99mTc-HSA has a short circulation time, which is a critical shortcoming for a blood pool imaging agent. Herein, we developed a novel ^99mTc-labeled HSA with a long circulation time using click chemistry and a chelator, 2,2'-dipicolylamine (DPA), (^99mTc-DPA-HSA). Specifically, we examined the feasibility of copper-free strain-promoted alkyne-azide cycloaddition (SPAAC) for the incorporation of HSA to the [^99mTc (CO)₃(H₂O)₃]⁺ system by adopting a chelate-then-click approach. In this strategy, a potent chelate system, azide-functionalized DPA, was first complexed with [^99mTc (CO)₃(H₂O)₃]⁺, followed by the SPAAC click reaction with azadibenzocyclooctyne-functionalized HSA (ADIBO-HSA) under biocompatible conditions. Radiolabeling efficiency of azide-functionalized DPA (^99mTc-DPA) was >98%. Click conjugation efficiency of ^99mTc-DPA with ADIBO-HSA was between 76 and 99% depending on the number of ADIBO moieties attached to HSA. In whole-body in vivo single photon emission computed tomography images, the blood pool uptakes of ^99mTc-DPA-HSA were significantly enhanced compared to those of ^99mTc-HSA at 10 min, 2, and 6 h after the injection ( P < 0.001, 0.025, and 0.003, respectively). Furthermore, the blood activities of ^99mTc-DPA-HSA were 8 times higher at 30 min and 10 times higher at 3 h after the injection compared to those of conventional ^99mTc-HSA in ex vivo biodistribution experiment. The results exhibit the potential of ^99mTc-DPA-HSA as a blood pool imaging agent and further illustrate the promise of the pre-labeling SPAAC approach for conjugation of heat-sensitive biological targeting vectors with [^99mTc (CO)₃(H₂O)₃]⁺.

In vivo positron emission tomographic blood pool imaging in an immunodeficient mouse model using 18F-fluorodeoxyglucose labeled human erythrocytes

99m-Technetium-labeled (^99mTc) erythrocyte imaging with planar scintigraphy is widely used for evaluating both patients with occult gastrointestinal bleeding and patients at risk for chemotherapy-induced cardiotoxicity. While a number of alternative radionuclide-based blood pool imaging agents have been proposed, none have yet to achieve widespread clinical use. Here, we present both in vitro and small animal in vivo imaging evidence that the high physiological expression of the glucose transporter GLUT1 on human erythrocytes allows uptake of the widely available radiotracer 2-deoxy-2-(¹⁸F)fluoro-D-glucose (FDG), at a rate and magnitude sufficient for clinical blood pool positron emission tomographic (PET) imaging. This imaging technique is likely to be amenable to rapid clinical translation, as it can be achieved using a simple FDG labeling protocol, requires a relatively small volume of phlebotomized blood, and can be completed within a relatively short time period. As modern PET scanners typically have much greater count detection sensitivities than that of commonly used clinical gamma scintigraphic cameras, FDG-labeled human erythrocyte PET imaging may not only have significant advantages over ^99mTc-labeled erythrocyte imaging, but may have other novel blood pool imaging applications.

Mini-review: Targeted radiopharmaceuticals incorporating reversible, low molecular weight albumin binders

The combination of low molecular weight, reversible human serum albumin (HSA) binders with targeted radiopharmaceuticals in dual-targeted radioconjugates holds great promise, in particular for endoradiotherapy. Attachment of HSA-binders to radiopharmaceuticals extends their blood circulation time and results in an enhanced tumour uptake as well as often in an improved pharmacokinetic profile. In this mini-review, an overview of currently pursued approaches of this novel strategy is provided.

Radioiodinated Portable Albumin Binder as a Versatile Agent for in Vivo Imaging with Single-Photon Emission Computed Tomography

In this study, radioiodinated 4-( p-iodophenyl)butyric acid ([¹³¹I]IBA) was synthesized and evaluated as a portable albumin-binder for potential applications in single photon emission computed tomography imaging of blood pool, tumor, and lymph node with significantly improved pharmacokinetic properties. The [¹³¹I]IBA was prepared under the catalyst of Cu₂O/1,10-phenanthroline. After that, the albumin-binding capability of [¹³¹I]IBA was tested in vitro, ex vivo, and in vivo, respectively. [¹³¹I]IBA was obtained with very high radiolabeling yield (>99%) and good radiochemical purity (>98%) within 10 min. It binds to albumin effectively with high affinity (IC₅₀= 46.5 μM) and has good stability. The results of biodistribution indicated that the [¹³¹I]IBA was mainly accumulated in blood with good retention (10.51 ± 2.58%ID/g at 30 min p.i. and 4.63 ± 0.17%ID/g at 4 h p.i.). In the SPECT imaging of mice models with [¹³¹I]IBA, blood pool, lymph node, and tumors could be imaged clearly with high target-to-background ratio. Overall, the radioiodinated albumin binder of [¹³¹I]IBA with long blood half-life and excellent stability could be used to decorate diversified albumin-binding radioligands and developed as a versatile theranostic agent.

Aptamer-based targeted therapy

Precision medicine holds great promise to harness genetic and epigenetic cues for targeted treatment of a variety of diseases, ranging from many types of cancers, neurodegenerative diseases, to cardiovascular diseases. The proteomic profiles resulting from the unique genetic and epigenetic signatures represent a class of relatively well accessible molecular targets for both interrogation (e.g., diagnosis, prognosis) and intervention (e.g., targeted therapy) of these diseases. Aptamers are promising for such applications by specific binding with cognate disease biomarkers. Nucleic acid aptamers are a class of DNA or RNA with unique three-dimensional conformations that allow them to specifically bind with target molecules. Aptamers can be relatively easily screened, reproducibly manufactured, programmably designed, and chemically modified for various biomedical applications, including targeted therapy. Aptamers can be chemically modified to resist enzymatic degradation or optimize their pharmacological behaviors, which ensured their chemical integrity and bioavailability under physiological conditions. In this review, we will focus on recent progress and discuss the challenges and opportunities in the research areas of aptamer-based targeted therapy in the forms of aptamer therapeutics and aptamer-drug conjugates (ApDCs).

One-pot synthesis and biodistribution of fluorine-18 labeled serum albumin for vascular imaging

INTRODUCTION

Equilibrium single-photon radionuclide imaging methods for assessing cardiac function and the integrity of the vascular system have long been in use for both clinical and research purposes. However, positron-emitting blood pool agents that could provide PET equivalents to these (and other) clinical procedures have not yet been adopted despite technical imaging advantages offered by PET. Our goal was to develop a PET blood pool tracer that not only meets necessary in vivo biological requirements but can be produced with an uncomplicated and rapid synthesis method which would facilitate clinical translation. Herein, albumin labeled with fluorine-18 was synthesized using a one-pot method and evaluated in vitro and in vivo in rats.

METHODS

A ligand (NODA-Bz-TFPE), containing NODA attached to a tetrafluorophenylester (TFPE) via a phenyl linker (Bz), was labeled with aluminum fluoride (Al[18F]F). Conjugation of the serum albumin with the ligand (Al[18F]F-NODA-Bz-TFPE), followed by purification (size exclusion chromatography), yielded the final product (Al[18F]F-NODA-Bz-RSA/HSA). In vitro stability was evaluated in human serum albumin by HPLC. Rat biodistributions and whole-body PET imaging over a 4 h time course were used for the in vivo evaluation.

RESULTS

This synthesis exhibited an overall radiochemical yield of 45 ± 10% (n = 30), a 50-min radiolabeling time, a radiochemical purity >99% and apparent stability up to 4 h in human serum. Blood had the highest retention of Al[18F]F-NODA-Bz-RSA at all times with a blood half-life of 5.2 h in rats. Al[18F]F-NODA-Bz-RSA distribution in most rat tissues remained relatively constant for up to 1 h, indicating that the tissue radioactivity content represents the respective tissue plasma volume. Dynamic whole-body PET images were in agreement with these findings.

CONCLUSIONS

A new ligand has been developed and radiolabeled with Al[18F]F that allows rapid (50-min) preparation of fluorine-18 serum albumin in one-pot. In addition to increased synthetic efficiency, the construct appears to be metabolically stable in rats. This method could encourage wider use of PET to quantify cardiac function and tissue vascular integrity in both research and clinical settings.

Radioligand Therapy of Prostate Cancer with a Long-Lasting Prostate-Specific Membrane Antigen Targeting Agent 90Y-DOTA-EB-MCG

Several radioligands targeting prostate-specific membrane antigen (PSMA) have been clinically introduced as a new class of radiotheranostics for the treatment of prostate cancer. Among them, ((( R)-1-carboxy-2-mcercaptoethyl)carbamoyl)-l-glutamic acid (MCG) has been successfully labeled with radioisotopes for prostate cancer imaging. The aim of this study is to conjugate MCG with an albumin binding moiety to further improve the in vivo pharmacokinetics. MCG was conjugated with an Evans blue (EB) derivative for albumin binding and a DOTA chelator. PSMA positive (PC3-PIP) and PSMA negative (PC3) cells were used for both in vitro and in vivo studies. Longitudinal PET imaging was performed at 1, 4, 24, and 48 h post-injection to evaluate the biodistribution and tumor uptake of ⁸⁶Y-DOTA-EB-MCG. DOTA-EB-MCG was also labeled with ⁹⁰Y for radionuclide therapy. Besides tumor growth measurement, tumor response to escalating therapeutic doses were also evaluated by immunohistochemistry and fluorescence microscopy. Based on quantification from ⁸⁶Y-DOTA-EB-MCG PET images, the tracer uptake in PC3-PIP tumors increased from 22.33 ± 2.39%ID/g at 1 h post-injection (p.i.), to the peak of 40.40 ± 4.79%ID/g at 24 h p.i. Administration of 7.4 MBq of ⁹⁰Y-DOTA-EB-MCG resulted in significant regression of tumor growth in PSMA positive xenografts. No apparent toxicity or body weight loss was observed in all treated mice. Modification of MCG with an Evans blue derivative resulted in a highly efficient prostate cancer targeting agent (EB-MCG), which showed great potential in prostate cancer treatment after being labeled with therapeutic radioisotopes.

Evans Blue Derivative-Functionalized Gold Nanorods for Photothermal Therapy-Enhanced Tumor Chemotherapy

Chemotherapy is a standard care for cancer management, but the lack of tumor targeting and high dose-induced side effects still limit its utility in patients. Here, we report a chemotherapy combined with photothermal therapy (PTT) for enhanced cancer ablation by functionalization of gold nanorods (GNRs) with a novel small molecule named truncated Evans blue (tEB). On the basis of the high binding affinity of tEB with albumin, an Abraxane-like nanodrug, human serum albumin/hydroxycamptothecin (HSA/HCPT), was further complexed with GNR-tEB. This formed an HCPT/HSA/tEB-GNR (HHEG) with excellent biostability and biocompatibility. With photoacoustic and fluorescence imaging, we observed HHEG tumor targeting, which is mediated by enhanced permeability retention effect. The accumulation of HHEG peaked in tumor at 12 h postinjection. Moreover, HHEG can effectively ablate tumor growth with laser illumination via chemo/thermal therapy after intravenous administration into SCC7 tumor. This combination is much better than chemotherapy or PTT alone. Collectively, we constructed a chemo/thermal therapy nanostructure based on a tEB-modified GNR for better tumor treatment effect. The use of tEB in gold nanoparticles can facilitate many new approaches to design hybrid nanoparticles.

Harnessing albumin as a carrier for cancer therapies

Serum albumin, a natural ligand carrier that is highly concentrated and long-circulating in the blood, has shown remarkable promise as a carrier for anti-cancer agents. Albumin is able to prolong the circulation half-life of otherwise rapidly cleared drugs and, importantly, promote their accumulation within tumors. The applications for using albumin as a cancer drug carrier are broad and include both traditional cancer chemotherapeutics and new classes of biologics. Strategies for leveraging albumin for drug delivery can be classified broadly into exogenous and in situ binding formulations that utilize covalent attachment, non-covalent association, or encapsulation in albumin-based nanoparticles. These methods have shown remarkable preclinical and clinical successes that are examined in this review.

Evans Blue Dye: A Revisit of Its Applications in Biomedicine

Evans blue (EB) dye has owned a long history as a biological dye and diagnostic agent since its first staining application by Herbert McLean Evans in 1914. Due to its high water solubility and slow excretion, as well as its tight binding to serum albumin, EB has been widely used in biomedicine, including its use in estimating blood volume and vascular permeability, detecting lymph nodes, and localizing the tumor lesions. Recently, a series of EB derivatives have been labeled with PET isotopes and can be used as theranostics with a broad potential due to their improved half-life in the blood and reduced release. Some of EB derivatives have even been used in translational applications in clinics. In addition, a novel necrosis-avid feature of EB has recently been reported in some preclinical animal studies. Given all these interesting and important advances in EB study, a comprehensive revisiting of EB has been made in its biomedical applications in the review.

Safety, Pharmacokinetics, and Dosimetry of a Long-Acting Radiolabeled Somatostatin Analog 177Lu-DOTA-EB-TATE in Patients with Advanced Metastatic Neuroendocrine Tumors

Radiolabeled somatostatin analog therapy has become an established treatment method for patients with well to moderately differentiated unresectable or metastatic neuroendocrine tumors (NETs). The most frequently used somatostatin analogs in clinical practice are octreotide and octreotate. However, both peptides showed suboptimal retention within tumors. The aim of this first-in-humans study is to explore the safety and dosimetry of a long-acting radiolabeled somatostatin analog, ¹⁷⁷Lu-1, 4, 7, 10-tetra-azacyclododecane-1, 4, 7, 10-tetraacetic acid-Evans blue-octreotate (¹⁷⁷Lu-DOTA-EB-TATE). Methods: Eight patients (6 men and 2 women; age range, 27-61 y) with advanced metastatic NETs were recruited. Five patients received a single dose, 0.35-0.70 GBq (9.5-18.9 mCi), of ¹⁷⁷Lu-DOTA-EB-TATE and underwent serial whole-body planar and SPECT/CT scans at 2, 24, 72, 120, and 168 h after injection. The other 3 patients received intravenous injection of 0.28-0.41 GBq (7.5-11.1 mCi) of ¹⁷⁷Lu-DOTATATE for the same imaging acquisition procedures at 1, 3, 4, 24, and 72 h after injection. The dosimetry was calculated using the OLINDA/EXM 1.1 software. Results: Administration of ¹⁷⁷Lu-DOTA-EB-TATE was well tolerated, with no adverse symptoms being noticed or reported in any of the patients. Compared with ¹⁷⁷Lu-DOTATATE, ¹⁷⁷Lu-DOTA-EB-TATE showed extended circulation in the blood and achieved a 7.9-fold increase of tumor dose delivery. The total-body effective doses were 0.205 ± 0.161 mSv/MBq for ¹⁷⁷Lu-DOTA-EB-TATE and 0.174 ± 0.072 mSv/MBq for ¹⁷⁷Lu-DOTATATE. Significant dose delivery increases to the kidneys and bone marrow were also observed in patients receiving ¹⁷⁷Lu-DOTA-EB-TATE compared with those receiving ¹⁷⁷Lu-DOTATATE (3.2 and 18.2-fold, respectively). Conclusion: By introducing an albumin-binding moiety, ¹⁷⁷Lu-DOTA-EB-TATE showed remarkably higher uptake and retention in NETs as well as significantly increased accumulation in the kidneys and red marrow. It has great potential to be used in peptide receptor radionuclide therapy for NETs with lower dose and less frequency of administration.

A paclitaxel prodrug with bifunctional folate and albumin binding moieties for both passive and active targeted cancer therapy

Folate receptor (FR) has proven to be a valuable target for chemotherapy using folic acid (FA) conjugates. However, FA-conjugated chemotherapeutics still have low therapeutic efficacy accompanied with side effects, resulting from complications such as short circulation half-life, limited tumor delivery, as well as high kidney accumulation. Herein, we present a novel FA-conjugated paclitaxel (PTX) prodrug which was additionally conjugated with an Evans blue (EB) derivative for albumin binding. The resulting bifunctional prodrug prolonged blood circulation, enhanced tumor accumulation, and consequently improved tumor therapeutic efficacy. Methods: Fmoc-Cys(Trt)-OH was coupled onto PTX at the 7'-OH position for further synthesis of ester prodrug FA-PTX-EB. The targeting ability was investigated using confocal microscopy and flow cytometry. The pharmacokinetics of this bifunctional compound was also studied. Meanwhile, cell viability was evaluated in normal cells and three cancer cell lines by MTT assay. In vivo therapeutic effect was tested on FR-α overexpressing MDA-MB-231 tumor model. Results: Compared with free PTX, the FA-PTX, PTX-EB and FA-PTX-EB prodrugs increased circulation half-life in mice from 2.19 to 3.82, 4.41, and 7.51 h, respectively. Pharmacokinetics studies showed that the FA-PTX-EB delivered more PTX to tumors than FA-PTX and free PTX. In vitro and in vivo studies demonstrated that FA-EB-conjugated PTX induced potent antitumor activity. Conclusion: FA-PTX-EB showed prolonged blood circulation, enhanced drug accumulation in tumors, higher therapeutic index, and lower side effects than either free PTX or monofunctional FA-PTX and EB-PTX. The results support the potential of using EB for the development of long-acting therapeutics.

[AlCl3(BnMe2-tacn)] - a new metal chelate scaffold for radiofluorination by Cl/F exchange

Radiofluorination of a 2.63 μM solution (pH 4, NaOAc buffer) of [AlCl₃(BnMe₂-tacn)] via treatment with 2.99 mol. equiv. of [¹⁹F]KF doped with cyclotron-produced [¹⁸F]F^- target water, with heating to 80-100 °C for 1 h, gives up to 24% ¹⁸F incorporation. SPE purification of the [Al¹⁹F₂¹⁸F(BnMe₂-tacn)] radio-product gives >99% RCP, with excellent stability (>99% RCP after 3 h).

"Albumin Hitchhiking" with an Evans Blue Analog for Cancer Theranostics

Although ¹⁷⁷Lu-DOTA-TATE was recently approved in Europe for the treatment of certain neuroendocrine tumors, continued development and optimization has been ongoing to further improve the therapeutic efficacy of somatostatin receptor 2 targeted peptide receptor radionuclide therapy, as well as reducing the renal toxicity. In this work, the use of an Evans blue analog for “albumin hitchhiking” resulted in significant improvement in both the imaging performance and therapeutic efficacy of radiolabeled octreotate, as well as reducing the toxicity since much less radioactivity was used for therapy. Upon clinical translation, such “albumin hitchhiking” could make significant impact in the near future for cancer patient management.

Novel Approach for 99mTc-Labeling of Red Blood Cells: Evaluation of 99mTc-4SAboroxime as a Blood Pool Imaging Agent

Angiography with radiolabeled red blood cells (RBCs) plays an important role in diagnosis and prognosis in vascular diseases. Both in vitro and in vivo methods have been developed for ^99mTc-labeling of RBCs. However, these methods are complicated and lack reproducibility. Therefore, it is highly desirable to develop an alternative method for routine ^99mTc-labeling of RBCs. In this report, we present a novel approach for ^99mTc-labeling of RBCs. We prepared a new ^99mTc(III) radiotracer [^99mTcCl(CDO)(CDOH)₂B-4AS] (^99mTc-4ASboroxime: 4AS-B(OH)₂ = 4-aminosulfonylphenyl)boronic acid, and CDOH₂ = cyclohexanedione dioxime) in >95% radiochemical purity. Imaging and biodistribution studies were performed in Sprague-Dawley (SD) rats. It was found that the blood radioactivity was ∼6.0%ID/g (∼90% injected dose for 200-225 g SD rats) for ^99mTc-4ASboroxime with low uptake in the myocardium, kidneys, liver, lungs, and muscle, most likely due to lack of leakage of ^99mTc-labeled RBCs from the intravascular space. The blood radioactivity was almost unchanged over the 2 h period, suggesting that the binding of ^99mTc-4ASboroxime to blood components (cells, proteins, and plasma) is stable. The results from γ-counting of the isolated blood components showed that ^99mTc-4ASboroxime had >95% of blood radioactivity binding to RBCs, ∼1% to albumin, and ∼3% remaining free in blood plasma, demonstrating its RBC-specificity. The results from imaging studies in SD rats indicated that ^99mTc-4ASboroxime is predominantly distributed in the blood pool. Main blood vessels were well delineated in the head/neck and abdominal regions. This statement was further substantiated by the results from imaging studies in pigs. ^99mTc-4ASboroxime is an excellent blood pool agent with the potential for diagnosis and prognosis of vascular diseases.

Albumin/vaccine nanocomplexes that assemble in vivo for combination cancer immunotherapy

Subunit vaccines have been investigated in over 1000 clinical trials of cancer immunotherapy, but have shown limited efficacy. Nanovaccines may improve efficacy but have rarely been clinically translated. By conjugating molecular vaccines with Evans blue (EB) into albumin-binding vaccines (AlbiVax), here we develop clinically promising albumin/AlbiVax nanocomplexes that self-assemble in vivo from AlbiVax and endogenous albumin for efficient vaccine delivery and potent cancer immunotherapy. PET pharmacoimaging, super-resolution microscopies, and flow cytometry reveal almost 100-fold more efficient co-delivery of CpG and antigens (Ags) to lymph nodes (LNs) by albumin/AlbiVax than benchmark incomplete Freund's adjuvant (IFA). Albumin/AlbiVax elicits ~10 times more frequent peripheral antigen-specific CD8⁺ cytotoxic T lymphocytes with immune memory than IFA-emulsifying vaccines. Albumin/AlbiVax specifically inhibits progression of established primary or metastatic EG7.OVA, B16F10, and MC38 tumors; combination with anti-PD-1 and/or Abraxane further potentiates immunotherapy and eradicates most MC38 tumors. Albumin/AlbiVax nanocomplexes are thus a robust platform for combination cancer immunotherapy.

Statins Promote Cardiac Infarct Healing by Modulating Endothelial Barrier Function Revealed by Contrast-Enhanced Magnetic Resonance Imaging

OBJECTIVE

The endothelium has a crucial role in wound healing, acting as a barrier to control transit of leukocytes. Endothelial barrier function is impaired in atherosclerosis preceding myocardial infarction (MI). Besides lowering lipids, statins modulate endothelial function. Here, we noninvasively tested whether statins affect permeability at the inflammatory (day 3) and the reparative (day 7) phase of infarct healing post-MI using contrast-enhanced cardiac magnetic resonance imaging (MRI).

APPROACH AND RESULTS

Noninvasive permeability mapping by MRI after MI in C57BL/6, atherosclerotic ApoE^-/-, and statin-treated ApoE^-/- mice was correlated to subsequent left ventricular outcome by structural and functional cardiac MRI. Ex vivo histology, flow cytometry, and quantitative polymerase chain reaction were performed on infarct regions. Increased vascular permeability at ApoE^-/- infarcts was observed compared with C57BL/6 infarcts, predicting enhanced left ventricular dilation at day 21 post-MI by MRI volumetry. Statin treatment improved vascular barrier function at ApoE^-/- infarcts, indicated by reduced permeability. The infarcted tissue of ApoE^-/- mice 3 days post-MI displayed an unbalanced Vegfa(vascular endothelial growth factor A)/Angpt1 (angiopoetin-1) expression ratio (explaining leakage-prone vessels), associated with higher amounts of CD45⁺ leukocytes and inflammatory LY6C^hi monocytes. Statins reversed the unbalanced Vegfa/Angpt1 expression, normalizing endothelial barrier function at the infarct and blocking the augmented recruitment of inflammatory leukocytes in statin-treated ApoE^-/- mice.

CONCLUSIONS

Statins lowered permeability and reduced the transit of unfavorable inflammatory leukocytes into the infarcted tissue, consequently improving left ventricular outcome.

Transformative Nanomedicine of an Amphiphilic Camptothecin Prodrug for Long Circulation and High Tumor Uptake in Cancer Therapy

We report a camptothecin (CPT) prodrug that was well formulated in solution and rapidly transformed into long-circulating nanocomplexes in vivo for highly efficient drug delivery and effective cancer therapy. Specifically, using a redox-responsive disulfide linker, CPT was conjugated with an albumin-binding Evans blue (EB) derivative; the resulting amphiphilic CPT-ss-EB prodrug self-assembled into nanostructures in aqueous solution, thus conferring high solubility and stability. By binding CPT-ss-EB to endogenous albumin, the 80 nm CPT-ss-EB nanoparticles rapidly transformed into 7 nm albumin/prodrug nanocomplexes. CPT-ss-EB was efficient at intracellular delivery into cancer cells, released intact CPT in a redox-responsive manner, and exhibited cytotoxicity as potent as CPT. In mice, the albumin/CPT-ss-EB nanocomplex exhibited remarkably long blood circulation (130-fold greater than CPT) and efficient tumor accumulation (30-fold of CPT), which consequently contributed to excellent therapeutic efficacy. Overall, this strategy of transformative nanomedicine is promising for efficient drug delivery.

Quantification of Tumor Vascular Permeability and Blood Volume by Positron Emission Tomography

Purpose: Evans Blue (EB) is an azo dye that binds quantitatively with serum albumin. With an albumin binding, NOTA conjugated truncated Evan's blue (NEB) dye derived PET tracer, we aimed to establish a strategy for evaluating vascular permeability in malignant tumors via non-invasive PET. Experimental design: Sixty-minute dynamic PET using [¹⁸F]FAl-NEB was performed in three xenograft tumor models including INS-1 rat insulinoma, UM-SCC-22B human head and neck carcinoma and U-87 MG human glioblastoma. Tumor vascular permeability was quantified by the difference of the slopes between tumor and blood time-activity curve (TACs, expressed as P_s ). The method was further substantiated by EB extraction and colorimetric assay and correlates with that calculated from dynamic contrast enhanced magnetic resonance imaging (DCE-MRI). The changes in tumor vasculature at different time points were assessed with NEB PET in U-87 MG and UM-SCC-22B tumor models after treatment with bevacizumab or doxorubicin. Result: The P_s values calculated from tumor and blood TACs from multiple time-point static images are consistent with those from dynamic images. Moreover, the P_s showed a positive and significant correlation with extracted EB concentration and K_PS-MRI generated from DCE-MRI, which further confirmed the soundness of this methodology. The antiangiogenic effect of bevacizumab could be revealed by NEB PET in U-87 MG tumors as early as 8 hrs after therapy, demonstrated by a substantial decrease of P_s. On the contrary, there was no significant change of P_s in bevacizumab treated UM-SCC-22B tumors, compared with control group. However, the significant changes of P_s were overestimated in doxorubicin treated UM-SCC-22B tumors. Conclusions: We successfully developed a relatively convenient and novel strategy to evaluate vascular permeability and blood volume using NEB PET. This method will be advantageous in evaluating vascular permeability, promoting drug delivery, and monitoring tumor response to therapeutics that affect tumor angiogenesis.

Rethinking cancer nanotheranostics

Advances in nanoparticle synthesis and engineering have produced nanoscale agents affording both therapeutic and diagnostic functions that are often referred to by the portmanteau 'nanotheranostics'. The field is associated with many applications in the clinic, especially in cancer management. These include patient stratification, drug-release monitoring, imaging-guided focal therapy and post-treatment response monitoring. Recent advances in nanotheranostics have expanded this notion and enabled the characterization of individual tumours, the prediction of nanoparticle-tumour interactions, and the creation of tailor-designed nanomedicines for individualized treatment. Some of these applications require breaking the dogma that a nanotheranostic must combine both therapeutic and diagnostic agents within a single, physical entity; instead, it can be a general approach in which diagnosis and therapy are interwoven to solve clinical issues and improve treatment outcomes. In this Review, we describe the evolution and state of the art of cancer nanotheranostics, with an emphasis on clinical impact and translation.

Microsurgery guided by sequential preoperative lymphography using 68Ga-NEB PET and MRI in patients with lower-limb lymphedema

OBJECTIVE

The popularity of contemporary microsurgical techniques in treatment of lower-limb lymphedema calls for better visualization of the lymphatic system, both preoperatively and intra-operatively. The aim of this prospective study was to investigate the feasibility of a novel combination of ⁶⁸Ga-NEB positron emission tomography (PET) with magnetic resonance lymphography (MRL) in evaluating lymphedema and guiding surgical intervention.

METHODS

A total of 11 patients (F 9, M 2, age range 29-69 y) with lower-limb lymphedema classified into stage I to III were recruited. PET acquisition was performed at 30, 60 and 90 min after subcutaneous injection of the albumin-binding radiotracer ⁶⁸Ga-NEB into the bilateral first web spaces of the feet. All the patients were also subjected to ^99mTc-sulfur colloid (SC) lymphoscintigraphy for comparison. Gd-DTPA-enhanced magnetic resonance imaging (MRI) was performed using sequences specialized for lymphatic vessel scans. All the patients underwent surgical interventions within a week. The surgical approach includes the use of a linear marker for edema localization and indocyanine green (ICG) lymphography with a near-infrared surgical navigation system intra-operatively.

RESULTS

Lymph transport in lymphatic channels was clearly observed by visualization of ⁶⁸Ga-NEB activity in the lymphatic vessels and within lymph nodes for all 11 patients as well as the visualization of the edema section plane with dermal backflow (DB), abnormally increased and disconnected uptake along the lymphatic channels. Preoperative ⁶⁸Ga-NEB PET combined with MRL provides advantageous three-dimensional images, higher temporal resolution, significantly shorter time lapse before image acquisition after tracer injection and more accurate pathological lymphatic vessel distribution than ^99mTc-SC lymphoscintigraphy combined with MRI.

CONCLUSION

This study documented an effective imaging pattern to combine ⁶⁸Ga-NEB PET and MRL in patients with lower-limb lymphedema. This strategy demonstrated significant advantage over ^99mTc-SC lymphoscintigraphy/MRL in the evaluation of lymphedema severity, staging and pathological location of lymph vessels to make an individualized treatment plan. Dual ⁶⁸Ga-NEB PET/MRL is thus recommended before the operation for staging and therapy planning.