Synthesis and Evaluation of 64Cu-DOTA-NT-Cy5.5 as a Dual-Modality PET/Fluorescence Probe to Image Neurotensin Receptor-Positive Tumor

Chelator boosted tumor-retention and pharmacokinetic properties: development of 64Cu labeled radiopharmaceuticals targeting neurotensin receptor

PURPOSE

Accumulating evidence suggests that neurotensin (NTS) and neurotensin receptors (NTSRs) play key roles in lung cancer progression by triggering multiple oncogenic signaling pathways. This study aims to develop Cu-labeled neurotensin receptor 1 (NTSR1)-targeting agents with the potential for both imaging and therapeutic applications.

METHOD

A series of neurotensin receptor antagonists (NRAs) with variable propylamine (PA) linker length and different chelators were synthesized, including [64Cu]Cu-CB-TE2A-iPA-NRA ([64Cu]Cu-4a-c, i = 1, 2, 3), [64Cu]Cu-NOTA-2PA-NRA ([64Cu]Cu-4d), [64Cu]Cu-DOTA-2PA-NRA ([64Cu]Cu-4e, also known as [64Cu]Cu-3BP-227), and [64Cu]Cu-DOTA-VS-2PA-NRA ([64Cu]Cu-4f). The series of small animal PET/CT were conducted in H1299 lung cancer model. The expression profile of NTSR1 was also confirmed by IHC using patient tissue samples.

RESULTS

For most of the compounds studied, PET/CT showed prominent tumor uptake and high tumor-to-background contrast, but the tumor retention was strongly influenced by the chelators used. For previously reported 4e, [64Cu]Cu-labeled derivative showed initial high tumor uptake accompanied by rapid tumor washout at 24 h. The newly developed [64Cu]Cu-4d and [64Cu]Cu-4f demonstrated good tumor uptake and tumor-to-background contrast at early time points, but were less promising in tumor retention. In contrast, our lead compound [64Cu]Cu-4b demonstrated 9.57 ± 1.35, 9.44 ± 2.38 and 9.72 ± 4.89%ID/g tumor uptake at 4, 24, and 48 h p.i., respectively. Moderate liver uptake (11.97 ± 3.85, 9.80 ± 3.63, and 7.72 ± 4.68%ID/g at 4, 24, and 48 h p.i.) was observed with low uptake in most other organs. The PA linker was found to have a significant effect on drug distribution. Compared to [64Cu]Cu-4b, [64Cu]Cu-4a had a lower background, including a greatly reduced liver uptake, while the tumor uptake was only moderately reduced. Meanwhile, [64Cu]Cu-4c showed increased uptake in both the tumor and the liver. The clinical relevance of NTSR1 was also demonstrated by the elevated tumor expression in patient tissue samples.

CONCLUSIONS

Through the side-by-side comparison, [64Cu]Cu-4b was identified as the lead agent for further evaluation based on its high and sustained tumor uptake and moderate liver uptake. It can not only be used to efficiently detect NTSR1 expression in lung cancer (for diagnosis, patient screening, and treatment monitoring), but also has the great potential to treat NTSR-positive lesions once chelating to the beta emitter 67Cu.

Translocator Protein 18 kDa (TSPO): A Promising Molecular Target for Image-Guided Surgery of Solid Cancers

The translocator protein 18-kDa (TSPO) is a mitochondrial membrane protein that is previously identified as the peripheral benzodiazepine receptor (PBR). Furthermore, it plays a significant role in a diverse range of biochemical processes, including steroidogenesis, mitochondrial cholesterol transport, cell survival and death, cell proliferation, and carcinogenesis. Several investigations also reported its roles in various types of cancers, including colorectal, brain, breast, prostate, and lung cancers, as well as melanoma. According to a previous study, the expression of TSPO was upregulated in cancer cells, which corresponds to an aggressive phenotype and/or poor prognosis. Consequently, the potential for crafting diagnostic and prognostic tools with a focus on TSPO holds great potential. In this context, several radioligands designed to target this protein have been identified, and some of the candidates have advanced to clinical trials. In recent years, the use of hybrid probes with radioactive and fluorescence molecules for image-guided surgery has exhibited promising results in animal and human studies. This indicates that the approach can serve as a valuable surgical navigator during cancer surgery. The current hybrid probes are built from various molecular platforms, including small molecules, nanoparticles, and antibodies. Although several TSPO-targeted imaging probes have been developed, their development for image-guided surgery of cancers is still limited. Therefore, this review aims to highlight recent findings on the involvement of TSPO in carcinogenesis, as well as provide a new perspective on the potential application of TSPO-targeted hybrid probes for image-guided surgery.

Clinical translational barriers against nanoparticle-based imaging agents

Nanoparticle based imaging agents (NIAs) have been intensively explored in bench studies. Unfortunately, only a few cases have made their ways to clinical translation. In this review, clinical trials of NIAs were investigated for understanding possible barriers behind that. First, the complexity of multifunctional NIAs is considered a main barrier because it brings uncertainty to batch-to-batch fabrication, and results in sophisticated in vivo behaviors. Second, inadequate biosafety studies slow down the translational work. Third, NIA uptake at disease sites is highly heterogeneous, and often exhibits poor targeting efficiency. Focusing on the aforementioned problems, key design parameters were analyzed including NIAs' size, composition, surface characteristics, dosage, administration route, toxicity, whole-body distribution and clearance in clinical trials. Possible strategies were suggested to overcome these barriers. Besides, regulatory guidelines as well as scale-up and reproducibility during manufacturing process were covered as they are also key factors to consider during clinical translation of NIAs.

68Ga-DOTA-NT-20.3 Neurotensin Receptor 1 PET Imaging as a Surrogate for Neuroendocrine Differentiation of Prostate Cancer

Prostate-specific membrane antigen (PSMA)-negative neuroendocrine prostate cancer (PCa) is a subtype of PCa likely to be lethal, with limited clinical diagnostic and therapeutic options. High expression of neurotensin receptor subtype 1 (NTR1) is associated with neuroendocrine differentiation of PCa, which makes NTR1 a potential target for neuroendocrine PCa. In this study, the NTR1-targeted tracer ⁶⁸Ga-DOTA-NT-20.3 was synthesized, and its affinity to androgen-dependent (LNCap) and androgen-independent (PC3) xenografts was determined. Methods: ⁶⁸Ga-DOTA-NT-20.3 was labeled using an automated synthesizer module, and its stability, labeling yield, and radiochemical purity were analyzed by radio-high-performance liquid chromatography. Receptor binding affinity was evaluated in NTR1-positive PC3 cells by a competitive binding assay. The biodistribution of ⁶⁸Ga-DOTA-NT-20.3 in vivo was evaluated in PC3 and LNCap xenografts by small-animal PET imaging. NTR1 expression was identified by immunohistochemistry and immunofluorescence evaluation. Results: ⁶⁸Ga-DOTA-NT-20.3 was synthesized successfully, with a yield of 88.07% ± 1.26%, radiochemical purity of at least 99%, and favorable stability. The NTR1 affinity (half-maximal inhibitory concentration) for ⁶⁸Ga-DOTA-NT-20.3 was 7.59 ± 0.41 nM. Small-animal PET/CT of PC3 xenograft animals showed high-contrast images with intense tumor uptake, which revealed specific NTR1 expression. The tumors showed significant radioactivity (4.95 ± 0.67 percentage injected dose per gram of tissue [%ID/g]) at 1 h, which fell to 1.95 ± 0.17 %ID/g (P < 0.01, t = 8.72) after specific blockage by neurotensin. LNCap xenografts had no significant accumulation (0.81 ± 0.06 %ID/g) of ⁶⁸Ga-DOTA-NT-20.3 at 1 h. In contrast, ⁶⁸Ga-PSMA-11 was concentrated mainly in LNCap xenografts (8.60 ± 2.11 %ID/g), with no significant uptake in PC3 tumors (0.53 ± 0.05 %ID/g), consistent with the in vitro immunohistochemistry findings. Biodistribution evaluation showed rapid clearance from the blood and main organs (brain, heart, lung, liver, muscle, and bone), with significantly high tumor-to-liver (4.41 ± 0.73) and tumor-to-muscle (12.34 ± 1.32) ratios at 60 min after injection. Conclusion: ⁶⁸Ga-DOTA-NT-20.3 can be efficiently prepared with a high yield and high radiochemical purity. Its favorable biodistribution and prominent NTR1 affinity make ⁶⁸Ga-DOTA-NT-20.3 a potential radiopharmaceutical for the detection of PSMA-negative PCa and identification of neuroendocrine differentiation.

Dual-Labelling Strategies for Nuclear and Fluorescence Molecular Imaging: Current Status and Future Perspectives

Molecular imaging offers the possibility to investigate biological and biochemical processes non-invasively and to obtain information on both anatomy and dysfunctions. Based on the data obtained, a fundamental understanding of various disease processes can be derived and treatment strategies can be planned. In this context, methods that combine several modalities in one probe are increasingly being used. Due to the comparably high sensitivity and provided complementary information, the combination of nuclear and optical probes has taken on a special significance. In this review article, dual-labelled systems for bimodal nuclear and optical imaging based on both modular ligands and nanomaterials are discussed. Particular attention is paid to radiometal-labelled molecules for single-photon emission computed tomography (SPECT) and positron emission tomography (PET) and metal complexes combined with fluorescent dyes for optical imaging. The clinical potential of such probes, especially for fluorescence-guided surgery, is assessed.

PET/Fluorescence Imaging: An Overview of the Chemical Strategies to Build Dual Imaging Tools

Molecular imaging is a biomedical research discipline that has quickly emerged to afford the observation, characterization, monitoring, and quantification of biomarkers and biological processes in living organism. It covers a large array of imaging techniques, each of which provides anatomical, functional, or metabolic information. Multimodality, as the combination of two or more of these techniques, has proven to be one of the best options to boost their individual properties, hence offering unprecedented tools for human health. In this review, we will focus on the combination of positron emission tomography and fluorescence imaging from the specific perspective of the chemical synthesis of dual imaging agents. Based on a detailed analysis of the literature, this review aims at giving a comprehensive overview of the chemical strategies implemented to build adequate imaging tools considering radiohalogens and radiometals as positron emitters, fluorescent dyes mostly emitting in the NIR window and all types of targeting vectors.

A Novel PET Probe for Brown Adipose Tissue Imaging in Rodents

PURPOSE

Brown adipose tissue (BAT) has emerged as a promising target to counteract obesity and its associated metabolic disorders. However, the detection of this tissue remains one of the major roadblocks.

PROCEDURES

In this study, we assess the use of BODIPY 1 as a positron emission tomography (PET) imaging agent to image BAT depots in vivo in two mouse phenotypes: obesity-resistant BALB/c mice and the obesity-prone C57BL/6 mice. [¹⁸F]BODIPY 1 is a radioactive dye that processed both radioactivity for PET imaging and fluorescence signal for in vitro mechanism study.

RESULTS

Through the co-staining of cancer cells with BODIPY 1 and MitoTracker, we found BODIPY 1 mainly accumulated in cell mitochondria in vitro. Fluorescence imaging of primary brown and white adipocytes further confirmed BODIPY 1 had significantly higher accumulation in primary brown adipocytes compared with primary white adipocytes. We evaluated [¹⁸F]BODIPY 1 for BAT imaging in both obesity-resistant BALB/c mice and obesity-prone C57BL/6 mice. Indeed, [¹⁸F]BODIPY 1 was efficiently taken up by BAT in both mouse genotypes (6.40 ± 1.98 %ID/g in obesity-resistant BALB/c mice (n = 8) and 5.37 ± 0.82 %ID/g in obesity-prone C57BL/6 mice (n = 7)). Although norepinephrine stimulation could increase the absolute BAT uptake, the enhancement is not significant in both genotypes (p > 0.05) at current sample size. These results suggest BAT uptake of [¹⁸F]BODIPY 1 may be independent of BAT thermogenic activity. As a comparison, 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) PET imaging was performed in obesity-resistant BALB/c mice. Significantly increased uptake was observed in adrenergically activated BAT (10.08 ± 2.52 %ID/g, n = 3) but not in inactive BAT (3.803 ± 0.70 %ID/g; n = 3). Because [¹⁸F]BODIPY 1 maintained its fluorescent property, BAT tissue was excised and studied using fluorescence microscopy. Strong fluorescence signal was observed in BAT mouse that was injected with BODIPY 1.

CONCLUSIONS

Unlike [¹⁸F]FDG, [¹⁸F]BODIPY 1 showed prominent accumulation in BAT under both inactive and stimulated status. [¹⁸F]BODIPY 1 may serve as a valuable BAT PET agent to possibly assess BAT mitochondria density, thus BAT thermogenic capacity after further evaluation.

Therapeutic approaches targeting the neurotensin receptors

Introduction: Neurotensin is a gut-brain peptide hormone, a 13 amino acid neuropeptide found in the central nervous system and in the GI tract. The neurotensinergic system is implicated in various physiological and pathological processes related to neuropsychiatric and metabolic machineries, cancer growth, food, and drug intake. NT mediates its functions through its two G protein-coupled receptors: neurotensin receptor 1 (NTS1/NTSR1) and neurotensin receptor 2 (NTS2/NTSR2). Over the past decade, the role of NTS3/NTSR3/sortilin has also gained importance in human pathologies. Several approaches have appeared dealing with the discovery of compounds able to modulate the functions of this neuropeptide through its receptors for therapeutic gain.Areas covered: The article provides an overview of over four decades of research and details the drug discovery approaches and patented strategies targeting NTSR in the past decade.Expert opinion: Neurotensin is an important neurotransmitter that enables crosstalk with various neurotransmitter and neuroendocrine systems. While significant efforts have been made that have led to selective agonists and antagonists with promising in vitro and in vivo activities, the therapeutic potential of compounds targeting the neurotensinergic system is still to be fully harnessed for successful clinical translation of compounds for the treatment of several pathologies.

Fluorescence Image-Guided Surgery - a Perspective on Contrast Agent Development

In the past several decades, a number of novel fluorescence image-guided surgery (FGS) contrast agents have been under development, with many in clinical translation and undergoing clinical trials. In this review, we have identified and summarized the contrast agents currently undergoing clinical translation. In total, 39 novel FGS contrast agents are being studied in 85 clinical trials. Four FGS contrast agents are currently being studied in phase III clinical trials and are poised to reach FDA approval within the next two to three years. Among all novel FGS contrast agents, a wide variety of probe types, targeting mechanisms, and fluorescence properties exists. Clinically available FGS imaging systems have been developed for FDA approved FGS contrast agents, and thus further clinical development is required to yield FGS imaging systems tuned for the variety of contrast agents in the clinical pipeline. Additionally, study of current FGS contrast agents for additional disease types and development of anatomy specific contrast agents is required to provide surgeons FGS tools for all surgical specialties and associated comorbidities. The work reviewed here represents a significant effort from many groups and further development of this promising technology will have an enormous impact on surgical outcomes across all specialties.

Design of Bimodal Ligands of Neurotensin Receptor 1 for Positron Emission Tomography Imaging and Fluorescence-Guided Surgery of Pancreatic Cancer

Neurotensin receptor 1 (NTSR1) is overexpressed in most human pancreatic ductal adenocarcinomas. It makes it an attractive target for the development of pancreatic cancer imaging agents. In this study, we sought to develop a bimodal positron emission tomography (PET)/fluorescent imaging agent capable of specifically targeting these receptors. Starting from the structure of a known NTSR1 agonist, a series of tracers were synthesized, radiometalated with gallium-68, and evaluated in vitro and in vivo, in mice bearing an AsPC-1 xenograft. PET imaging allowed us to identify the compound [⁶⁸Ga]Ga-NODAGA-Lys(Cy5**)-AEEAc-[Me-Arg⁸,Tle¹²]-NT(7-13) as the one with the most promising biodistribution profile, characterized by high tumor uptake (2.56 ± 0.97%ID/g, 1 h post-injection) and rapid elimination from nontargeted organs, through urinary excretion. Fluorescence imaging gave similar results. On this basis, fluorescence-guided resection of tumor masses was successfully carried out on a preclinical model.

Evaluation of neurotensin receptor 1 as potential biomarker for prostate cancer theranostic use

INTRODUCTION

Despite recent developments in the diagnosis and treatment of prostate cancer, the advanced stages still have poor survival rates. This warrants further exploration of related molecular targets for patient screening, detection of metastatic disease, and treatment/treatment monitoring. Recent studies have indicated that neurotensin receptors (NTSRs) and their ligand neurotensin (NTS) critically affect the progression of prostate cancers. In this study, we evaluated the expression of neurotensin receptor1 (NTSR1) in patient tissues and performed NTSR1 PET imaging in a prostate cancer animal model.

METHODS

The NTSR1 expression was evaluated in 97 cases of prostate cancer and 100 cases of benign prostatic hyperplasia (BPH) of clinical patients by immunohistochemistry staining. The expression profile of PSMA and GRPR was also performed for comparison. The mRNA expression of NTSR1 in LnCap and PC-3 cells was measured by PCR. NTSR1 PET, and biodistribution studies were performed in PC-3 xenografts using ¹⁸F-DEG-VS-NT.

RESULTS

NTSR1 showed high or moderate expression in 91.8% of prostate cancer tissue, compared with PSMA (86.7%) and GRPR (65.3%). All examined PSMA-negative tissues showed positive NTSR1 expression, suggesting the potential complementary role of NTSR1 targeted imaging or therapy. Only 8% of BPH shows strong or moderate expression of NTSR1, which is significantly lower than that in prostate cancer (91.8%). PCR results indicated LNCap (an androgen-dependent prostate cancer cell) showed negative NTSR1 expression while PC-3 demonstrated positive expression (an androgen-independent prostate cancer cell), which correlated well with previously reported western blot results. In a preclinical animal model, NTSR1 targeted PET probe ¹⁸F-DEG-VS-NT demonstrated prominent tumor accumulation and low background.

CONCLUSION

We have demonstrated that NTSR1 is a promising molecular marker for prostate cancer based on patient tissue staining. The NTSR targeted probe ¹⁸F-DEG-VS-NT demonstrated high tumor to background contrast in animal models, which could be valuable in selecting patients for therapies targeting NTSR1 as well as monitoring therapeutic efficacy during treatment accordingly.

pH-dependent and cathepsin B activable CaCO3 nanoprobe for targeted in vivo tumor imaging

Background: The intraoperative visualization of tumor cells is a powerful modality for surgical treatment of solid tumors. Since the completeness of tumor excision is closely correlated with the survival of patients, probes that can assist in distinguishing tumor cells are highly demanded.

Purpose: In the present study, a fluorescent probe JF1 was synthesized for imaging of tumor cells by conjugating a substrate of cathepsin B (quenching moiety) to Oregon Green derivative JF2 using a self-immolative linker.

Methods: JF1 was then loaded into the folate-PEG modified CaCO₃ nanoparticles. The folate receptor-targeted, pH-dependent, and cathepsin B activable CaCO₃ nanoprobe was test in vitro and in vivo for tumor imaging.

Results: CaCO₃ nanoprobe demonstrated good stability and fast lighting ability in tumors under low pH conditions. It also showed lower fluorescence background than the single cathepsin B dependent fluorescent probe. The pH-dependent and cathepsin B controlled “turn-on” property enables precise and fast indication of tumor in vitro and in vivo.

Conclusion: This strategy of controlled drug delivery enables in vivo imaging of tumor nodules with a high signal-to-noise ratio, which has great potential in surgical tumor treatment.

Development of [18F]AlF-NOTA-NT as PET Agents of Neurotensin Receptor-1 Positive Pancreatic Cancer

Several studies have suggested that neurotensin receptors (NTRs) and neurotensin (NT) greatly affect the growth and survival of pancreatic ductal adenocarcinoma (PDAC). Developing NTR-targeted PET probes could therefore be important for the management of a pancreatic cancer patient by providing key information on the NTR expression profile noninvasively. Despite the initial success on the synthesis of ¹⁸F-labeled NT PET probes, the labeling procedure generally requires lengthy steps including azeotropic drying of ¹⁸F. Using a straightforward chelation method, here we report the simple preparation of aluminum-¹⁸F-NOTA-NT starting from aqueous ¹⁸F. The cell binding test demonstrated that [¹⁹F]AlF-NOTA-NT maintained high receptor-binding affinity to NTR1. This probe was then further evaluated in NTR1 positive pancreatic tumor models (AsPC-1 and PANC-1). After the administration of [¹⁸F]AlF-NOTA-NT, small animal PET studies showed a high contrast between tumor and background in both models at 1 and 4 h time points. A blocking experiment was performed to demonstrate the receptor specificity: the tumor uptake in AsPC1 without and with blocking agent was 1.0 ± 0.2 and 0.1 ± 0.0%ID/g, respectively, at 4 h post injection. In summary, a NTR specific PET agent, [¹⁸F]AlF-NOTA-NT, was prepared through the simple chelation method. This NTR-targeted PET probe may not only be used to detect NTR1 positive pancreatic tumors (diagnosis), but also it may be fully integrated to NTR target therapy leading to personalized medicine (theranostic).

Imaging Neurotensin Receptor in Prostate Cancer With 64Cu-Labeled Neurotensin Analogs

INTRODUCTION

Neurotensin receptor 1 (NTR-1) is expressed and activated in prostate cancer cells. In this study, we explore the NTR expression in normal mouse tissues and study the positron emission tomography (PET) imaging of NTR in prostate cancer models.

MATERIALS AND METHODS

Three ⁶⁴Cu chelators (1, 4, 7, 10-tetraazacyclododecane-1, 4, 7, 10-tetraacetic acid [DOTA], 1,4,7-triazacyclononane-N,N',N″-triacetic acid [NOTA], or AmBaSar) were conjugated to an NT analog. Neurotensin receptor binding affinity was evaluated using cell binding assay. The imaging profile of radiolabeled probes was compared in well-established NTR⁺ HT-29 tumor model. Stability of the probes was tested. The selected agents were further evaluated in human prostate cancer PC3 xenografts.

RESULTS

All 3 NT conjugates retained the majority of NTR binding affinity. In HT-29 tumor, all agents demonstrated prominent tumor uptake. Although comparable stability was observed, ⁶⁴Cu-NOTA-NT and ⁶⁴Cu-AmBaSar-NT demonstrated improved tumor to background contrast compared with ⁶⁴Cu-DOTA-NT. Positron emission tomography/computed tomography imaging of the NTR expression in PC-3 xenografts showed high tumor uptake of the probes, correlating with the in vitro Western blot results. Blocking experiments further confirmed receptor specificity.

CONCLUSIONS

Our results demonstrated that ⁶⁴Cu-labeled neurotensin analogs are promising imaging agents for NTR-positive tumors. These agents may help us identify NTR-positive lesions and predict which patients and individual tumors are likely to respond to novel interventions targeting NTR-1.

New extracellular factors in glioblastoma multiforme development: neurotensin, growth differentiation factor-15, sphingosine-1-phosphate and cytomegalovirus infection

Recent years have seen considerable progress in understanding the biochemistry of cancer. For example, more significance is now assigned to the tumor microenvironment, especially with regard to intercellular signaling in the tumor niche which depends on many factors secreted by tumor cells. In addition, great progress has been made in understanding the influence of factors such as neurotensin, growth differentiation factor-15 (GDF-15), sphingosine-1-phosphate (S1P), and infection with cytomegalovirus (CMV) on the 'hallmarks of cancer' in glioblastoma multiforme. Therefore, in the present work we describe the influence of these factors on the proliferation and apoptosis of neoplastic cells, cancer stem cells, angiogenesis, migration and invasion, and cancer immune evasion in a glioblastoma multiforme tumor. In particular, we discuss the effect of neurotensin, GDF-15, S1P (including the drug FTY720), and infection with CMV on tumor-associated macrophages (TAM), microglial cells, neutrophil and regulatory T cells (T_reg), on the tumor microenvironment. In order to better understand the role of the aforementioned factors in tumoral processes, we outline the latest models of intratumoral heterogeneity in glioblastoma multiforme. Based on the most recent reports, we discuss the problems of multi-drug therapy in treating glioblastoma multiforme.

Molecular Imaging of P-glycoprotein in Chemoresistant Tumors Using a Dual-Modality PET/Fluorescence Probe

Overexpression of P-glycoprotein (Pgp) has been considered a primary cause for multidrug resistance in a variety of cancers for three decades. However, clinical translation of Pgp targeted therapeutics has been hindered by lack of patient preselection based on the Pgp presence in tumors. We aim to develop a molecularly targeted probe for imaging tumoral Pgp in vivo with positron emission tomography (PET) and fluorescence, and to provide a tool for preselecting the patients with tumoral Pgp expression. Thus, a Pgp monoclonal antibody 15D3 was chemically modified with IRDye800 (IR800) and DOTA chelator. The specificity of the antibody conjugates DOTA-Pab-IR800 was verified in Pgp-expressing 3T3-MDR1 and control 3T3 cells. After radiolabeling with ⁶⁴Cu, the probe was applied in small animal PET imaging of Pgp in a mouse xenograft model of NCI/ADR-Res cells, which are chemoresistant through overexpression of Pgp. Quantification analysis of the PET images demonstrated that the tumor uptake of the radioactive probe was 9.9 ± 1.4, 12.1 ± 1.2, and 10.5 ± 1.0%ID/g at 4, 24, and 48 h post injection. The tumor-to-muscle ratio was 20.9 at 48 h post injection based on biodistribution studies. Fluorescence imaging was performed following PET experiments, and it demonstrated excellent tumor accumulation of this dual-modality probe in the NCI/ADR-Res tumors. Further, an image-guided surgery was successfully performed using the fluorescence modality of the probe, demonstrating potential utility of this probe in image-guided surgical removal of Pgp-positive drug resistant tumors in the patients. In conclusion, this study clearly demonstrated that the Pgp-targeted antibody probe, ⁶⁴Cu-DOTA-Pab-IR800, could provide a promising diagnosis tool for detection of Pgp-expressing tumors in vivo.

Evaluation of neurotensin receptor 1 as a potential imaging target in pancreatic ductal adenocarcinoma

Pancreatic cancer is one of the deadliest human malignancies and lack of effective diagnostic and therapeutic methods. Accumulating evidence suggests that the neurotensin (NT) and neurotensin receptors (NTRs) play key roles in pancreatic adenocarcinoma growth and survival. In this study, we not only evaluate the NTR1 expression in pancreatic cancer patient samples, but also explore the PET and fluorescence imaging of NTR1 expression in pancreatic cancer animal models. The NTR1 expression was evaluated by immunohistochemistry staining in clinical patient tissue samples with pancreatic ductal adenocarcinoma, insulinoma, and pancreatitis. The results showed 79.4% positive rate of NRT1 expression in pancreatic ductal adenocarcinoma, compared with 33.3 and 22.7% in insulinoma and pancreatitis samples, respectively. High NTR1 gene expression was also found in Panc-1 cells and confirmed by cell immunofluorescence. 64Cu-AmBaSar-NT and IRDye800-NT were synthesized as imaging probes and maintained the majority of NTR1-binding affinity. In vivo imaging demonstrated that 64Cu-AmBaSar-NT has prominent tumor uptake (3.76 ± 1.45 and 2.29 ± 0.10%ID/g at 1 and 4 h post-injection). NIR fluorescent imaging with IRDye800-NT demonstrated good tumor-to-background contrast (8.09 ± 0.38 × 108 and 6.67 ± 0.43 × 108 (p/s/cm2/sr)/(μW/cm2) at 30 and 60 min post-injection). Fluorescence guided surgery was also performed as a proof of principle experiment. In summary, our results indicated that NTR1 is a promising target for pancreatic ductal adenocarcinoma imaging and therapy. The imaging probes reported here may not only be considered for improved diagnosis of pancreatic ductal adenocarcinoma, but also has the potential to be fully integrated into patient screening and treatment monitoring of future NTR1 targeted therapies.

Molecular imaging probes derived from natural peptides

Covering: up to the end of 2015.Peptides are naturally occurring compounds that play an important role in all living systems and are responsible for a range of essential functions. Peptide receptors have been implicated in disease states such as oncology, metabolic disorders and cardiovascular disease. Therefore, natural peptides have been exploited as diagnostic and therapeutic agents due to the unique target specificity for their endogenous receptors. This review discusses a variety of natural peptides highlighting their discovery, endogenous receptors, as well as their derivatization to create molecular imaging agents, with an emphasis on the design of radiolabelled peptides. This review also highlights methods for discovering new and novel peptides when knowledge of specific targets and endogenous ligands are not available.

Radiolabeled theranostics: magnetic and gold nanoparticles

Introduction: Growing advances in nanotechnology have facilitated the applications of newly emerged nanomaterials in the field of biomedical/pharmaceutical sciences. Following this trend, the multifunctional nanoparticles (NPs) play a significant role in development of advanced drug delivery systems (DDSs) such as diapeutics/theranostics used for simultaneous diagnosis and therapy. Multifunctional radiolabeled NPs with capability of detecting, visualizing and destroying diseased cells with least side effects have been considered as an emerging filed in presentation of the best choice in solving the therapeutic problems. Functionalized magnetic and gold NPs (MNPs and GNPs, respectively) have produced the potential of nanoparticles as sensitive multifunctional probes for molecular imaging, photothermal therapy and drug delivery and targeting.

Methods: In this study, we review the most recent works on the improvement of various techniques for development of radiolabeled magnetic and gold nanoprobes, and discuss the methods for targeted imaging and therapies.

Results: The receptor-specific radiopharmaceuticals have been developed to localized radiotherapy in disease sites. Application of advanced multimodal imaging methods and related modality imaging agents labeled with various radioisotopes (e.g., ¹²⁵I, ¹¹¹In, ⁶⁴Cu, ⁶⁸Ga, ⁹⁹mTc) and MNPs/GNPs have significant effects on treatment and prognosis of cancer therapy. In addition, the surface modification with biocompatible polymer such as polyethylene glycol (PEG) have resulted in development of stealth NPs that can evade the opsonization and immune clearance. These long-circulating agents can be decorated with homing agents as well as radioisotopes for targeted imaging and therapy purposes.

Conclusion: The modified MNPs or GNPs have wide applications in concurrent diagnosis and therapy of various malignancies. Once armed with radioisotopes, these nanosystems (NSs) can be exploited for combined multimodality imaging with photothermal/photodynamic therapy while delivering the loaded drugs or genes to the targeted cells/tissues. These NSs will be a game changer in combating various cancers.

Analysis of argentinated peptide complexes using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry: Peptide = oxytocin, arg(8) -vasopressin, bradykinin, bombesin, somatostatin, neurotensin

RATIONALE

The increased use of silver nanoparticles (AgNPs) for various biological applications, and over-expression of various peptide receptors in different tumors/cancer cells, necessitate the need for dedicated investigations on the intrinsic binding ability of Ag with various biologically important peptides for better understanding of AgNPs-peptide interactions and for the future development of contrasting agents as well as drugs for imaging/biomedical applications.

METHODS

The [M+(Ag)n ](+) complexes are prepared and characterized using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS).

RESULTS

Silver complexes of the peptides [M+(Ag)n ](+) , where M = oxytocin, arg(8) -vasopressin, bradykinin, bombesin, somatostatin, and neurotensin, have been investigated for their intrinsic Ag(+) -binding ability. Unusual binding of up to seven Ag(+) with these small peptides is observed. The mass spectra show n = 1-5 for bombesin and somatostatin, n = 1-6 for bradykinin and arg(8) -vasopressin, and n = 1-7 for oxytocin and neurotensin. In addition, oxytocin and arg(8) -vasopressin show the formation of dimers and their complexes [M2 +(Ag)n ](+) with n = 1-8 and n = 1-5, respectively. The possible amino acid residues responsible for Ag(+) binding in each peptide have been identified on the basis of density functional theory (DFT)-calculated binding energy values of Ag(+) towards individual amino acids.

CONCLUSIONS

Mass spectrometric evidence indicates that the peptides, viz., oxytocin, arg(8) -vasopressin, bradykinin, bombesin, somatostatin, and neurotensin, show greater affinity for Ag(+) . Hence, they may be used as carriers for AgNPs in targeted drug delivery as well as an alternative for iodinated contrasting agents in dual energy X-ray imaging techniques. Radio-labeled Ag with these peptides can also be used in radio-pharmaceuticals for diagnostic and therapeutic applications. Copyright © 2016 John Wiley & Sons, Ltd.

Conformationally Strained trans-Cyclooctene (sTCO) Enables the Rapid Construction of (18)F-PET Probes via Tetrazine Ligation

The bioorthogonal reaction between tetrazines and trans-cyclooctenes is a method for the rapid construction of F-18 probes for PET imaging. Described here is a second generation ¹⁸F-labeling system based on a conformationally strained trans-cyclooctene (sTCO)—a dienophile that is approximately 2 orders of magnitude more reactive than conventional TCO dienophiles. Starting from a readily prepared tosylate precursor, an ¹⁸F labeled sTCO derivative (¹⁸F-sTCO) could be synthesized in 29.3 +/- 5.1% isolated yield and with high specific activity. Tetrazine ligation was carried out with a cyclic RGD-conjugate of a diphenyl-s-tetrazine analogue (RGD-Tz) chosen from a diene class with an excellent combination of fast reactivity and stability both for the diene as well as the Diels-Alder adduct. For both the tetrazine and the sTCO, mini-PEG spacers were included to enhance solubility and improve the in vivo distribution profile of the resulting probe. Extremely fast reactivity (up to 2.86 x 10⁵ M^-1s^-1 at 25 °C in water) has been observed in kinetic studies in the reaction of sTCO with diphenyl-s-tetrazine derivatives. A kinetic study on sTCO diastereomers in 55:45 MeOH:water showed that the syn-diastereomer displayed slightly faster reactivity than the anti-diastereomer. An ¹⁸F-sTCO conjugate with RGD-Tz demonstrated prominent and persistent tumor uptake in vivo with good tumor-to-background contrast. Unlike most radiolabeled RGD peptides, the tumor uptake of this PET agent increased from 5.3 +/- 0.2% ID/g at 1 h post injection (p.i.), to 8.9 +/- 0.5% ID/g at 4 h p.i., providing evidence for prolonged blood circulation. These findings suggest that tetrazine ligations employing ¹⁸F-sTCO should serve as a powerful and general platform for the rapid construction of peptide or protein derived PET agents.