PET Imaging of Integrin Positive Tumors Using F Labeled Knottin Peptides

Arginine-modified black phosphorus quantum dots with dual excited states for enhanced electrochemiluminescence in bioanalysis

The electrochemiluminescence (ECL) is generally emitted via radiative transition of singlet or triplet excited state (S₁ or T₁). Herein, an ECL mechanism with the transitions of both S₁ and T₁ of black phosphorus quantum dots (BPQDs) is found, and an arginine (Arg) modification strategy is proposed to passivate the surface oxidation defects of BPQDs, which could modulate the excited states for enhancing the ECL efficiency of BPQDs. The Arg modification leads to greater spatial overlap of highest and lowest occupied molecular orbitals, and spectral shift of radiative transitions, and improves the stability of anion radical of BPQDs. To verify the application of the proposed mechanism, it is used to construct a sensitive method for conveniently evaluating the inhibiting efficiency of cyclo-arginine-glycine-aspartic acid-d-tyrosine-lysine to cell surface integrin by using Arg containing peptide modified BPQDs as signal tag. The dual excited states mediated ECL emitters provide a paradigm for adjustable ECL generation and extend the application of ECL analysis.

Synthesis and evaluation of [18F]FP-Lys-GE11 as a new radiolabeled peptide probe for epidermal growth factor receptor (EGFR) imaging

INTRODUCTION

The epidermal growth factor receptor (EGFR) has emerged as an attractive target in the treatment of various cancers. Radiolabeled small molecules, antibodies, and peptides that specifically target EGFR are promising probes for tumor imaging to guide personalized treatment with EGFR-targeted drugs. This study aimed to radiolabel GE11 (an EGFR-specific targeting peptide) with 18-fluorine to develop a new EGFR-targeting positron emission tomography (PET) probe, [¹⁸F]FP-Lys-GE11, for imaging tumors overexpressing EGFR.

METHODS

[¹⁸F]FP-Lys-GE11 was produced by radiolabeling a GE11 peptide with the prosthetic group 4-nitrophenyl-2-[¹⁸F]fluoropropionate ([¹⁸F]NFP). Stability in PBS and mice serum, affinity for A431 cell line, U87 and PC-3 cells uptake and blocking studies, and biodistribution of [¹⁸F]FP-Lys-GE11 were determined. 2 h dynamic and static PET scans of probe for tumor-bearing mice normal and inhibition uptake were performed.

RESULTS

[¹⁸F]FP-Lys-GE11 was stable in PBS and mice serum. The Kd and Bmax values of probe for A431 were 42.43 ± 3.75 nM and 3383 ± 81.73 CPM, respectively. In cell uptake and blocking experiments, a significant reduction in radioactivity accumulation (over 4-fold) was observed by blocking U87 and PC-3 cells with unlabeled peptide. PET imaging of U87 and PC-3 tumor-bearing mice revealed clear tumor imaging (tumor radioactivity accumulation was 3.48 ± 0.44 and 3.68 ± 0.76%ID/g respectively, tumor-to-muscle ratio was 3.45 ± 0.43 and 3.64 ± 0.76 respectively). Blocking imaging revealed that the U87 tumor uptake was significantly inhibited (2.21 ± 0.41%ID/g). The biodistribution and dynamic PET imaging showed that [¹⁸F]FP-Lys-GE11 was mainly excreted by the kidneys and the rest was excreted through the bile and intestines.

CONCLUSION

The current results showed that [¹⁸F]FP-Lys-GE11was a good radiolabeled peptide probe for EGFR overexpression tumor's imaging.

Synthesis and Evaluation of 18F-Labeled Peptide for Gonadotropin-Releasing Hormone Receptor Imaging

The gonadotropin-releasing hormone (GnRH) receptor is overexpressed in the majority of tumors of the human reproductive system. The purpose of this study was to develop an ¹⁸F-labeled peptide for tumor GnRH receptor imaging. In this study, the GnRH (pGlu¹-His²-Trp³-Ser⁴-Tyr⁵-Gly⁶-Leu⁷-Arg⁸-Pro⁹-Gly¹⁰-NH₂) peptide analogues FP-d-Lys⁶-GnRH (FP = 2-fluoropropanoyl) and NOTA-P-d-Lys⁶-GnRH (P = ethylene glycol) were designed and synthesized. The IC₅₀ values of FP-d-Lys⁶-GnRH and NOTA-P-d-Lys⁶-GnRH were 2.0 nM and 56.2 nM, respectively. 4-Nitrophenyl-2-[¹⁸F]fluoropropionate was conjugated to the ε-amino group of the d-lysine side chain of d-Lys⁶-GnRH to yield the new tracer [¹⁸F]FP-d-Lys⁶-GnRH with a decay-corrected yield of 8 ± 3% and a specific activity of 20−100 GBq/µmol (n=6). Cell uptake studies of [¹⁸F]FP-d-Lys⁶-GnRH in GnRH receptor-positive PC-3 cells and GnRH receptor-negative CHO-K1 cells indicated receptor-specific accumulation. Biodistribution and PET studies in nude mice bearing PC-3 xenografted tumors showed that [¹⁸F]FP-d-Lys⁶-GnRH was localized in tumors with a higher uptake than in surrounding muscle and heart tissues. Furthermore, the metabolic stability of [¹⁸F]FP-d-Lys⁶-GnRH was determined in mouse blood and PC-3 tumor homogenates at 1 h after tracer injection. The presented results indicated a potential of the novel tracer [¹⁸F]FP-d-Lys⁶-GnRH for tumor GnRH receptor imaging.

Radiosynthesis and Preliminary Biological Evaluation of 18F-Fluoropropionyl-Chlorotoxin as a Potential PET Tracer for Glioma Imaging

Purposes

Chlorotoxin can specifically bind to matrix metalloproteinase 2 (MMP-2), which are overexpressed in the glioma. In this work, radiosynthesis of [¹⁸F]-fluoropropionyl-chlorotoxin ([¹⁸F]-FP-chlorotoxin) as a novel PET tracer was investigated, and biodistribution in vivo and PET imaging were performed in the C6 glioma model.

Procedures

[¹⁸F]-FP-chlorotoxin was prepared from the reaction of chlorotoxin with [¹⁸F]-NFB (4-nitrophenyl 2-[¹⁸F]-fluoropropionate), which was synthesized from multistep reactions. Biodistribution was determined in 20 normal Kunming mice. Small-animal PET imaging with [¹⁸F]-FP-chlorotoxin was performed on the same rats bearing orthotopic C6 glioma at different time points (60 min, 90 min, and 120 min) after injection and compared with 2-deoxy-2-[¹⁸F] fluoro-D-glucose ([¹⁸F]-FDG).

Results

[¹⁸F]-FP-Chlorotoxin was successfully synthesized in the radiochemical yield of 41% and the radiochemical purity of more than 98%. Among all the organs, the brain had the lowest and stable uptake of [¹⁸F]-FP-chlorotoxin, while the kidney showed the highest uptake. Compared with [¹⁸F]-FDG, a low uptake of [¹⁸F]-FP-chlorotoxin was detected in normal brain parenchyma and a high accumulation of [¹⁸F]-FP-chlorotoxin was found in the gliomas tissue. The glioma to normal brain uptake ratio of [¹⁸F]-FP-chlorotoxin was higher than that of [¹⁸F]-FDG. Furthermore, the uptake of [¹⁸F]-FP-chlorotoxin at 90 min after injection was better than that at 60 min after injection.

Conclusions

Compared with [¹⁸F]-FDG, [¹⁸F]-FP-chlorotoxin has a low and stable uptake in normal brain parenchyma. [¹⁸F]-FP-Chlorotoxin seems to be a potential PET tracer with a good performance in diagnosis of the glioma.

Exploring alternative antibody scaffolds: Antibody fragments and antibody mimics for targeted drug delivery

The field of targeted therapeutics has benefitted immeasurably from the development of high-affinity antibodies. These important ligands have facilitated the development of effective therapies, particularly when conjugated to potent cytotoxic payloads i.e. in antibody-drug conjugates (ADCs). The success of ADCs is evidenced by rapid adoption within the pharmaceuticals community; many major companies have dedicated ADC research programmes. However, despite the advantages, the field of ADCs has failed to live up to its full potential. Studies have emerged suggesting that traditional IgG scaffolds may not be the optimal format for targeted payload delivery. In response, the protein engineering community has begun to explore alternative high-binding protein scaffolds as antibody mimics. In this short review I will summarise the generation, modification, and application of emerging antibody fragments and synthetic antibody mimics, with a focus on their use as drug carriers. The review aims to highlight the advantages of antibody mimics, and how they could be employed to overcome the issues and limitations of traditional ADCs.

Beyond Antibodies as Binding Partners: The Role of Antibody Mimetics in Bioanalysis

The emergence of novel binding proteins or antibody mimetics capable of binding to ligand analytes in a manner analogous to that of the antigen-antibody interaction has spurred increased interest in the biotechnology and bioanalytical communities. The goal is to produce antibody mimetics designed to outperform antibodies with regard to binding affinities, cellular and tumor penetration, large-scale production, and temperature and pH stability. The generation of antibody mimetics with tailored characteristics involves the identification of a naturally occurring protein scaffold as a template that binds to a desired ligand. This scaffold is then engineered to create a superior binder by first creating a library that is then subjected to a series of selection steps. Antibody mimetics have been successfully used in the development of binding assays for the detection of analytes in biological samples, as well as in separation methods, cancer therapy, targeted drug delivery, and in vivo imaging. This review describes recent advances in the field of antibody mimetics and their applications in bioanalytical chemistry, specifically in diagnostics and other analytical methods.

Semi-automatic synthesis and biodistribution of N-(2-18F-fluoropropionyl)-bis(zinc (II)-dipicolylamine) (18F-FP-DPAZn2) for AD model imaging

Background

Phosphatidylserine (PS)-targeting positron emission tomography (PET) imaging with labeled small-molecule tracer is a crucial non-invasive molecule imaging method of apoptosis. In this study, semi-automatic radiosynthesis and biodistribution of N-(2-¹⁸F-fluoropropionyl)-bis(zinc(II)-dipicolylamine) (¹⁸F-FP-DPAZn2), as a potential small-molecule tracer for PET imaging of cell death in Alzheimer’s disease (AD) model, were performed.

Methods

¹⁸F-FP-DPAZn2 was synthesized on the modified PET-MF-2V-IT-I synthesizer. Biodistribution was determined in normal mice and PET images of AD model were obtained on a micro PET-CT scanner.

Results

With the modified synthesizer, the total decay-corrected radiochemical yield of ¹⁸F-FP-DPAZn2 was 35 ± 6% (n = 5) from ¹⁸F⁻ within 105 ± 10 min. Biodistribution results showed that kidney has the highest uptake of ¹⁸F-FP-DPAZn2. The uptake of radioactivity in brain kept at a relatively low level during the whole observed time. In vivo ¹⁸F-FP-DPAZn2 PET images demonstrated more accumulation of radioactivity in the brain of AD model mice than that in the brain of normal mice.

Conclusions

The semi-automatic synthetic method provides a slightly higher radiochemical yield and shorter whole synthesis time of ¹⁸F-FP-DPAZn2 than the manual operation method. This improved method can give enough radioactivity and high radiochemical purity of ¹⁸F-FP-DPAZn2 for in vivo PET imaging. The results show that ¹⁸F-FP-DPAZn2 seems to be a potential cell death tracer for AD imaging.

A Comprehensive Evaluation of the Activity and Selectivity Profile of Ligands for RGD-binding Integrins

Integrins, a diverse class of heterodimeric cell surface receptors, are key regulators of cell structure and behaviour, affecting cell morphology, proliferation, survival and differentiation. Consequently, mutations in specific integrins, or their deregulated expression, are associated with a variety of diseases. In the last decades, many integrin-specific ligands have been developed and used for modulation of integrin function in medical as well as biophysical studies. The IC₅₀-values reported for these ligands strongly vary and are measured using different cell-based and cell-free systems. A systematic comparison of these values is of high importance for selecting the optimal ligands for given applications. In this study, we evaluate a wide range of ligands for their binding affinity towards the RGD-binding integrins αvβ3, αvβ5, αvβ6, αvβ8, α5β1, αIIbβ3, using homogenous ELISA-like solid phase binding assay.

Designing repeat proteins for biosensors and medical imaging

Advances in protein engineering tools, both computational and experimental, has afforded many new protein structures and functions. Here, we present a snapshot of repeat-protein engineering efforts towards new, versatile, alternative binding scaffolds for use in analytical sensors and as imaging agents. Analytical assays, sensors and imaging agents based on the direct binding of analyte are increasingly important for research and diagnostics in medicine, food safety, and national security.

A highly fluorescent AIE-active theranostic agent with anti-tumor activity to specific cancer cells

A tetraphenylethene derivative with a structure resembling Tamoxifen is designed and synthesized as a theranostic agent for cell imaging and anti-breast cancer therapy. Its high brightness, excellent photostability and long-term cell tracing properties enable elucidation of its working mechanism and hence provide new insights into drug development.

Synthetic Cystine-Knot Miniproteins - Valuable Scaffolds for Polypeptide Engineering

Peptides with the cystine-knot architecture, often termed knottins, are promising scaffolds for biomolecular engineering. These unique molecules combine diverse bioactivities with excellent structural, thermal, and proteolytical stability. Being different in the composition and structure of their amino acid backbone, knottins share the same core element, namely cystine knot, which is built by six cysteine residues forming three disulfides upon oxidative folding. This motif ensures a notably rigid framework that highly tolerates both rational and combinatorial changes in the primary structure. Being accessible through recombinant production and total chemical synthesis, cystine-knot miniproteins can be endowed with novel bioactivities by variation of surface-exposed loops and incorporation of non-natural elements within their non-conserved regions towards the generation of tailor-made peptidic compounds. In this chapter the topology of cystine-knot peptides, their synthesis and applications for diagnostics and therapy is discussed.

Enzyme-Controlled Intracellular Self-Assembly of (18)F Nanoparticles for Enhanced MicroPET Imaging of Tumor

Herein, we report the development of a new "smart" radioactive probe (i.e., 1) which can undergo furin-controlled condensation and self-assembly of radioactive nanoparticles (i.e., 1-NPs) in tumor cells and its application for enhanced microPET imaging of tumors in nude mice co-injected with its cold analog (i.e., 1-Cold). Furin-controlled condensation of 1-Cold and self-assembly of its nanoparticles (i.e., 1-Cold-NPs) in vitro were validated and characterized with HPLC, mass spectra, SEM, and TEM analyses. Cell uptake studies showed that both 1 and 1-Cold have good cell permeability. TEM images of 1-Cold-treated MDA-MB-468 cells directly uncovered that the intracellular 1-Cold-NPs were at/near the location of furin (i.e., Golgi bodies). MTT results indicated that 50 µM 1-Cold did not impose cytotoxicity to MDA-MB-468 cells up to 12 hours. MicroPET imaging of MDA-MB-468 tumor-bearing mice indicated that mice co-injected with 1 and 1-Cold showed higher uptake and longer attenuation of the radioactivity in tumors than those mice only injected with same dosage of 1. Tumor uptake ratios of 1 between these two groups of mice reached the maximum of 8.2 folds at 240 min post injection. Biodistribution study indicated that the uptake ratios of 1 in kidneys between these two groups continuously increased and reached 81.9 folds at 240 min post injection, suggesting the formation of radioactive NPs (i.e., 1-NPs) in MDA-MB-468 tumors of mice co-injected with 1 and 1-Cold. And the nanoparticles were slowly digested and secreted from the tumors, accumulating in the kidneys. Our ''smart'' probe (i.e., 1), together with the strategy of co-injection, might help researchers trace the biomarkers of interest within a longer time window.

Cystine-knot peptides targeting cancer-relevant human cytotoxic T lymphocyte-associated antigen 4 (CTLA-4)

Cystine-knot peptides sharing a common fold but displaying a notably large diversity within the primary structure of flanking loops have shown great potential as scaffolds for the development of therapeutic and diagnostic agents. In this study, we demonstrated that the cystine-knot peptide MCoTI-II, a trypsin inhibitor from Momordica cochinchinensis, can be engineered to bind to cytotoxic T lymphocyte-associated antigen 4 (CTLA-4), an inhibitory receptor expressed by T lymphocytes, that has emerged as a target for the treatment of metastatic melanoma. Directed evolution was used to convert a cystine-knot trypsin inhibitor into a CTLA-4 binder by screening a library of variants using yeast surface display. A set of cystine-knot peptides possessing dissociation constants in the micromolar range was obtained; the most potent variant was synthesized chemically. Successive conjugation with neutravidin, fusion to antibody Fc domain or the oligomerization domain of C4b binding protein resulted in oligovalent variants that possessed enhanced (up to 400-fold) dissociation constants in the nanomolar range. Our data indicate that display of multiple knottin peptides on an oligomeric scaffold protein is a valid strategy to improve their functional affinity with ramifications for applications in diagnostics and therapy.

64Cu-Labeled Divalent Cystine Knot Peptide for Imaging Carotid Atherosclerotic Plaques

The rupture of vulnerable atherosclerotic plaques that lead to stroke and myocardial infarction may be induced by macrophage infiltration and augmented by the expression of integrin αvβ3. Indeed, atherosclerotic angiogenesis may be a promising marker of inflammation. In this study, an engineered integrin αvβ3-targeting PET probe, (64)Cu-NOTA-3-4A, derived from a divalent knottin miniprotein was evaluated in a mouse model for carotid atherosclerotic plaques.

METHODS

Atherosclerotic plaques in BALB/C mice, maintained on a high-fat diet, were induced with streptozotocin injection and carotid artery ligation and verified by MR imaging. Knottin 3-4A was synthesized by solid-phase peptide synthesis chemistry and coupled to 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA) before radiolabeling with (64)Cu. PET probe stability in mouse serum was evaluated. Mice with carotid atherosclerotic plaques were injected via the tail vein with (64)Cu-NOTA-3-4A or (18)F-FDG, followed by small-animal PET/CT imaging at different time points. Receptor targeting specificity of the probe was verified by coinjection of c(RGDyK) administered in molar excess. Subsequently, carotid artery dissection and immunofluorescence staining were performed to evaluate target expression.

RESULTS

(64)Cu-NOTA-3-4A was synthesized in high radiochemical purity and yield and demonstrated molecular stability in both phosphate-buffered saline and mouse serum at 4 h. Small-animal PET/CT showed that (64)Cu-NOTA-3-4A accumulated at significantly higher levels in the neovasculature of carotid atherosclerotic plaques (7.41 ± 1.44 vs. 0.67 ± 0.23 percentage injected dose/gram, P < 0.05) than healthy or normal vessels at 1 h after injection. (18)F-FDG also accumulated in atherosclerotic lesions at 0.5 and 1 h after injection but at lower plaque-to-normal tissue ratios than (64)Cu-NOTA-3-4A. For example, plaque-to-normal carotid artery ratios for (18)F-FDG and (64)Cu-NOTA-3-4A at 1 h after injection were 3.75 and 14.71 (P < 0.05), respectively. Furthermore, uptake of (64)Cu-NOTA-3-4A in atherosclerotic plaques was effectively blocked (∼90% at 1 h after injection) by coinjection of c(RGDyK). Immunostaining confirmed integrin αvβ3 expression in both the infiltrating macrophages and the neovasculature of atherosclerotic plaques.

CONCLUSION

(64)Cu-NOTA-3-4A demonstrates specific accumulation in carotid atherosclerotic plaques in which macrophage infiltration and angiogenesis are responsible for elevated integrin αvβ3 levels. Therefore, (64)Cu-NOTA-3-4A may demonstrate clinical utility as a PET probe for atherosclerosis imaging or for the evaluation of therapies used to treat atherosclerosis.

Comparison of three dimeric 18F-AlF-NOTA-RGD tracers

PURPOSE

RGD peptide-based radiotracers are well established as integrin αvβ3 imaging probes to evaluate tumor angiogenesis or tissue remodeling after ischemia or infarction. In order to optimize the labeling process and pharmacokinetics of the imaging probes, we synthesized three dimeric RGD peptides with or without PEGylation and performed in vivo screening.

PROCEDURES

Radiolabeling was achieved through the reaction of F-18 aluminum-fluoride complex with the cyclic chelator, 1,4,7-triazacyclononane-1,4,7-triacetic acid (NOTA). Three imaging probes were synthesized as (18)F-AlF-NOTA-E[c(RGDfK)]2, (18)F-AlF-NOTA-PEG4-E[c(RGDfK)]2, and (18)F-AlF-NOTA-E[PEG4-c(RGDfk)]2. The receptor binding affinity was determined by competitive cell binding assay, and the stability was evaluated by mouse serum incubation. Tumor uptake and whole body distribution of the three tracers were compared through direct tissue sampling and PET quantification of U87MG tumor-bearing mice.

RESULTS

All three compounds remained intact after 120 min incubation with mouse serum. They all had a rapid and relatively high tracer uptake in U87MG tumors with good target-to-background ratios. Compared with the other two tracers, (18)F-AlF-NOTA-E[PEG4-c(RGDfk)]2 had the highest tumor uptake and the lowest accumulation in the liver. The integrin receptor specificity was confirmed by co-injection of unlabeled dimeric RGD peptide.

CONCLUSION

The rapid one-step radiolabeling strategy by the complexation of (18)F-aluminum fluoride with NOTA-peptide conjugates was successfully applied to synthesize three dimeric RGD peptides. Among the three probes developed, (18)F-AlF-NOTA-E[PEG4-c(RGDfk)]2 with relatively low liver uptake and high tumor accumulation appears to be a promising candidate for further translational research.

A radiofluorinated divalent cystine knot peptide for tumor PET imaging

A divalent knottin containing two separate integrin binding epitopes (RGD) in the adjacent loops, 3-4A, was recently developed and reported in our previous publication. In the current study, 3-4A was radiofluorinated with a 4-nitrophenyl 2-¹⁸F-fluoropropinate (¹⁸F-NFP) group and the resulting divalent positron emission tomography (PET) probe, ¹⁸F-FP–3-4A, was evaluated as a novel imaging probe to detect integrin αvβ3 positive tumors in living animals. Knottin 3-4A was synthesized by solid phase peptide synthesis, folded, and site-specifically conjugated with ^18/19F-NFP to produce the fluorinated peptide ^18/19F-fluoropropinate-3-4A (^18/19F-FP–3-4A). The stability of ¹⁸F-FP–3-4A was tested in both phosphate buffered saline (PBS) buffer and mouse serum. Cell uptake assays of the radiolabeled peptides were performed using U87MG cells. In addition, small animal PET imaging and biodistribution studies of ¹⁸F-FP–3-4A were performed in U87MG tumor-bearing mice. The receptor targeting specificity of the radiolabeled peptide was also verified by coinjecting the probe with a blocking peptide cyclo(RGDyK). Our study showed that ¹⁸F-FP–3-4A exhibited excellent stability in PBS buffer (pH 7.4) and mouse serum. Small animal PET imaging and biodistribution data revealed that ¹⁸F-FP–3-4A exhibited rapid and good tumor uptake (3.76 ± 0.59% ID/g and 2.22 ± 0.62% ID/g at 0.5 and 1 h, respectively). ¹⁸F-FP–3-4A was rapidly cleared from the normal tissues, resulting in excellent tumor-to-normal tissue contrasts. For example, liver uptake was only 0.39 ± 0.07% ID/g and the tumor to liver ratio was 5.69 at 1 h p.i. Furthermore, coinjection of cyclo(RGDyK) with ¹⁸F-FP–3-4A significantly inhibited tumor uptake (0.41 ± 0.12 vs 1.02 ± 0.19% ID/g at 2.5 h) in U87MG xenograft models, demonstrating specific accumulation of the probe in the tumor. In summary, the divalent probe ¹⁸F-FP–3-4A is characterized by rapid and high tumor uptake and excellent tumor-to-normal tissue ratios. ¹⁸F-FP–3-4A is a highly promising knottin based PET probe for translating into clinical imaging of tumor angiogenesis.

99mTc-labeled cystine knot peptide targeting integrin αvβ6 for tumor SPECT imaging

Integrin α_vβ₆ is overexpressed in a variety of cancers, and its expression is often associated with poor prognosis. Therefore, there is a need to develop affinity reagents for noninvasive imaging of integrin α_vβ₆ expression since it may provide early cancer diagnosis, more accurate prognosis, and better treatment planning. We recently engineered and validated highly stable cystine knot peptides that selectively bind integrin α_vβ₆ with no cross-reactivity to integrins α_vβ₅, α₅β₁, or α_vβ₃, also known to be overexpressed in many cancers. Here, we developed a single photon emission computed tomography (SPECT) probe for imaging integrin α_vβ₆ positive tumors. Cystine knot peptide, S₀2, was first conjugated with a single amino acid chelate (SAAC) and labeled with [^99mTc(H₂O)₃(CO)₃]⁺. The resulting probe, ^99mTc-SAAC-S₀2, was then evaluated by in vitro cell uptake studies using two α_vβ₆ positive cell lines (human lung adenocarcinoma cell line HCC4006 and pancreatic cancer cell line BxPC-3) and two α_vβ₆ negative cell lines (human lung adenocarcinoma cell line H838 and human embryonic kidney cell line 293T). Next, SPECT/CT and biodistribution studies were performed in nude mice bearing HCC4006 and H838 tumor xenografts to evaluate the in vivo performance of ^99mTc-SAAC-S₀2. Significant differences in the uptake of ^99mTc-SAAC-S₀2 were observed in α_vβ₆ positive vs negative cells (P < 0.05). Biodistribution and small animal SPECT/CT studies revealed that ^99mTc-SAAC-S₀2 accumulated to moderate levels in antigen positive tumors (∼2% ID/g at 1 and 6 h postinjection, n = 3 or 4/group). Moreover, the probe demonstrated tumor-to-background tissue ratios of 6.81 ± 2.32 (tumor-to-muscle) and 1.63 ± 0.18 (tumor-to-blood) at 6 h postinjection in α_vβ₆ positive tumor xenografts. Co-incubation of the probe with excess amount of unlabeled S₀2 as a blocking agent demonstrated significantly reduced tumor uptake, which is consistent with specific binding to the target. Renal filtration was the main route of clearance. In conclusion, knottin peptides are excellent scaffolds for which to develop highly stable imaging probes for a variety of oncological targets. ^99mTc-SAAC-S₀2 demonstrates promise for use as a SPECT agent to image integrin α_vβ₆ expression in living systems.

One-step radiosynthesis of 4-nitrophenyl 2-[(18) F]fluoropropionate ([(18) F]NFP); improved preparation of radiolabeled peptides for PET imaging

The versatile (18) F-labeled prosthetic group, 4-nitrophenyl 2-[(18) F]fluoropropionate ([(18) F]NFP), was synthesized in a single step in 45 min from 4-nitrophenyl 2-bromopropionate, with a decay corrected radiochemical yield of 26.2% ± 2.2%. Employing this improved synthesis of [(18) F]NFP, [(18) F]GalactoRGD - the current 'gold standard' tracer for imaging the expression of αV β3 integrin - was prepared with high specific activity in 90 min and 20% decay corrected radiochemical yield from [(18) F]fluoride.

A novel aliphatic 18F-labeled probe for PET imaging of melanoma

Radiofluorinated benzamide and nicotinamide analogues are promising molecular probes for the positron emission tomography (PET) imaging of melanoma. Compounds containing aromatic (benzene or pyridine) and N,N-diethylethylenediamine groups have been successfully used for development of melanin targeted PET and single-photon emission computed tomography (SPECT) imaging agents for melanoma. The objective of this study was to determine the feasibility of using aliphatic compounds as a molecular platform for the development of a new generation of PET probes for melanoma detection. An aliphatic N,N-diethylethylenediamine precursor was directly coupled to a radiofluorination synthon, p-nitrophenyl 2-(18)F-fluoropropionate ((18)F-NFP), to produce the probe N-(2-(diethylamino)ethyl)-2-(18)F-fluoropropanamide ((18)F-FPDA). The melanoma-targeting ability of (18)F-FPDA was further evaluated both in vitro and in vivo through cell uptake assays, biodistribution studies, and small animal PET imaging in C57BL/6 mice bearing B16F10 murine melanoma tumors. Beginning with the precursor (18)F-NFP, the total preparation time for (18)F-FPDA, including the final high-performance liquid chromatography purification step, was approximately 30 min, with a decay-corrected radiochemical yield of 79.8%. The melanin-targeting specificity of (18)F-FPDA was demonstrated by significantly different uptake rates in tyrosine-treated and untreated B16F10 cells in vitro. The tumor uptake of (18)F-FPDA in vivo reached 2.65 ± 0.48 %ID/g at 2 h postinjection (p.i.) in pigment-enriched B16F10 xenografts, whereas the tumor uptake of (18)F-FPDA was close to the background levels, with rates of only 0.37 ± 0.07 %ID/g at 2 h p.i. in the nonpigmented U87MG tumor mouse model. Furthermore, small animal PET imaging studies revealed that (18)F-FPDA specifically targeted the melanotic B16F10 tumor, yielding a tumor-to-muscle ratio of approximately 4:1 at 1 h p.i. and 7:1 at 2 h p.i. In summary, we report the development of a novel (18)F-labeled aliphatic compound for melanoma imaging that can be easily synthesized in high yields using the radiosynthon (18)F-NFP. The PET probe (18)F-FPDA exhibits high B16F10 tumor-targeting efficacy and favorable in vivo pharmacokinetics. Our study demonstrates that aliphatic compounds can be used as a new generation molecular platform for the development of novel melanoma targeting agents. Further evaluation and optimization of (18)F-FPDA for melanin targeted molecular imaging are therefore warranted.

First experience of 18F-alfatide in lung cancer patients using a new lyophilized kit for rapid radiofluorination

(18)F-FPPRGD2, which was approved for clinical study recently, has favorable properties for integrin targeting and showed potential for antiangiogenic therapy and early response monitoring. However, the time-consuming multiple-step synthesis may limit its widespread applications in the clinic. In this study, we developed a simple lyophilized kit for labeling PRGD2 peptide ((18)F-AlF-NOTA-PRGD2, denoted as (18)F-alfatide) using a fluoride-aluminum complex that significantly simplified the labeling procedure.

METHODS

Nine patients with a primary diagnosis of lung cancer were examined by both static and dynamic PET imaging with (18)F-alfatide, and 1 tuberculosis patient was investigated using both (18)F-alfatide and (18)F-FDG imaging. Standardized uptake values were measured in tumors and other main organs at 30 min and 1 h after injection. Kinetic parameters were calculated by Logan graphical analysis. Immunohistochemistry and staining intensity quantification were performed to confirm the expression of integrin α(v)β(3).

RESULTS

Under the optimal conditions, the whole radiosynthesis including purification was accomplished within 20 min with a decay-corrected yield of 42.1% ± 2.0% and radiochemical purity of more than 95%. (18)F-alfatide PET imaging identified all tumors, with mean standardized uptake values of 2.90 ± 0.10. Tumor-to-muscle and tumor-to-blood ratios were 5.87 ± 2.02 and 2.71 ± 0.92, respectively.

CONCLUSION

(18)F-alfatide can be produced with excellent radiochemical yield and purity via a simple, 1-step, lyophilized kit. PET scanning with (18)F-alfatide allows specific imaging of αvβ3 expression with good contrast in lung cancer patients. This technique might be used for the assessment of angiogenesis and for planning and response evaluation of cancer therapies that would affect angiogenesis status and integrin expression levels.

Engineering agatoxin, a cystine-knot peptide from spider venom, as a molecular probe for in vivo tumor imaging

BACKGROUND

Cystine-knot miniproteins, also known as knottins, have shown great potential as molecular scaffolds for the development of targeted therapeutics and diagnostic agents. For this purpose, previous protein engineering efforts have focused on knottins based on the Ecballium elaterium trypsin inhibitor (EETI) from squash seeds, the Agouti-related protein (AgRP) neuropeptide from mammals, or the Kalata B1 uterotonic peptide from plants. Here, we demonstrate that Agatoxin (AgTx), an ion channel inhibitor found in spider venom, can be used as a molecular scaffold to engineer knottins that bind with high-affinity to a tumor-associated integrin receptor.

METHODOLOGY/PRINCIPAL FINDINGS

We used a rational loop-grafting approach to engineer AgTx variants that bound to αvβ3 integrin with affinities in the low nM range. We showed that a disulfide-constrained loop from AgRP, a structurally-related knottin, can be substituted into AgTx to confer its high affinity binding properties. In parallel, we identified amino acid mutations required for efficient in vitro folding of engineered integrin-binding AgTx variants. Molecular imaging was used to evaluate in vivo tumor targeting and biodistribution of an engineered AgTx knottin compared to integrin-binding knottins based on AgRP and EETI. Knottin peptides were chemically synthesized and conjugated to a near-infrared fluorescent dye. Integrin-binding AgTx, AgRP, and EETI knottins all generated high tumor imaging contrast in U87MG glioblastoma xenograft models. Interestingly, EETI-based knottins generated significantly lower non-specific kidney imaging signals compared to AgTx and AgRP-based knottins.

CONCLUSIONS/SIGNIFICANCE

In this study, we demonstrate that AgTx, a knottin from spider venom, can be engineered to bind with high affinity to a tumor-associated receptor target. This work validates AgTx as a viable molecular scaffold for protein engineering, and further demonstrates the promise of using tumor-targeting knottins as probes for in vivo molecular imaging.

Mesenchymal stem cell-based cell engineering with multifunctional mesoporous silica nanoparticles for tumor delivery

Stem cell engineering, the manipulation and control of cells, harnesses tremendous potential for diagnosis and therapy of disease; however, it is still challenging to impart multifunctionalization onto stem cells to achieve both. Here we describe a mesenchymal stem cell (MSC)-based multifunctional platform to target orthotopic glioblastoma by integrating the tumor targeted delivery of mesenchymal stem cells and the multimodal imaging advantage of mesoporous silica nanoparticles (MSNs). Rapid cellular uptake, long retention time and stability of particles exemplify the potential that the combination of MSNs and MSCs has as a stem cell-based multifunctional platform. Using such a platform, we verified tumor-targeted delivery of MSCs by in vivo multimodal imaging in an orthotopic U87MG glioblastoma model, displaying higher tumor uptake than particles without MSCs. As a proof-of-concept, this MSC platform opens a new vision for multifunctional applications of cell products by combining the superiority of stem cells and nanoparticles for actively targeted delivery.

Image guided biodistribution and pharmacokinetic studies of theranostics

Image guided technique is playing an increasingly important role in the investigation of the biodistribution and pharmacokinetics of drugs or drug delivery systems in various diseases, especially cancers. Besides anatomical imaging modalities such as computed tomography (CT), magnetic resonance imaging (MRI), molecular imaging strategy including optical imaging, positron emission tomography (PET) and single-photon emission computed tomography (SPECT) will facilitate the localization and quantization of radioisotope or optical probe labeled nanoparticle delivery systems in the category of theranostics. The quantitative measurement of the bio-distribution and pharmacokinetics of theranostics in the fields of new drug/probe development, diagnosis and treatment process monitoring as well as tracking the brain-blood-barrier (BBB) breaking through by high sensitive imaging method, and the applications of the representative imaging modalities are summarized in this review.

Quantitative analysis and comparison study of [18F]AlF-NOTA-PRGD2, [18F]FPPRGD2 and [68Ga]Ga-NOTA-PRGD2 using a reference tissue model

With favorable pharmacokinetics and binding affinity for α_vβ₃ integrin, ¹⁸F-labeled dimeric cyclic RGD peptide ([¹⁸F]FPPRGD2) has been intensively used as a PET imaging probe for lesion detection and therapy response monitoring. A recently introduced kit formulation method, which uses an ¹⁸F-fluoride-aluminum complex labeled RGD tracer ([¹⁸F]AlF-NOTA-PRGD2), provides a strategy for simplifying the labeling procedure to facilitate clinical translation. Meanwhile, an easy-to-prepare ⁶⁸Ga-labeled NOTA-PRGD2 has also been reported to have promising properties for imaging integrin α_vβ₃. The purpose of this study is to quantitatively compare the pharmacokinetic parameters of [¹⁸F]FPPRGD2, [¹⁸F]AlF-NOTA-PRGD2, and [⁶⁸Ga]Ga-NOTA-PRGD2. U87MG tumor-bearing mice underwent 60-min dynamic PET scans following the injection of three tracers. Kinetic parameters were calculated using Logan graphical analysis with reference tissue. Parametric maps were generated using voxel-level modeling. All three compounds showed high binding potential (Bp_ND = k₃/k₄) in tumor voxels. [¹⁸F]AlF-NOTA-PRGD2 showed comparable Bp_ND value (3.75±0.65) with those of [¹⁸F]FPPRGD2 (3.39±0.84) and [⁶⁸Ga]Ga-NOTA-PRGD2 (3.09±0.21) (p>0.05). Little difference was found in volume of distribution (V_T) among these three RGD tracers in tumor, liver and muscle. Parametric maps showed similar kinetic parameters for all three tracers. We also demonstrated that the impact of non-specific binding could be eliminated in the kinetic analysis. Consequently, kinetic parameter estimation showed more comparable results among groups than static image analysis. In conclusion, [¹⁸F]AlF-NOTA-PRGD2 and [⁶⁸Ga]Ga-NOTA-PRGD2 have comparable pharmacokinetics and quantitative parameters compared to those of [¹⁸F]FPPRGD2. Despite the apparent difference in tumor uptake (%ID/g determined from static images) and clearance pattern, the actual specific binding component extrapolated from kinetic modeling appears to be comparable for all three dimeric RGD tracers.