New Insights in the Design of Bioactive Peptides and Chelating Agents for Imaging and Therapy in Oncology

Synthesis, Radiolabeling, and Biodistribution Study of a Novel DOTA-Peptide for Targeting Vascular Endothelial Growth Factor Receptors in the Molecular Imaging of Breast Cancer

As angiogenesis plays a pivotal role in tumor progression and metastasis, leading to more cancer-related deaths, the angiogenic process can be considered as a target for diagnostic and therapeutic applications. The vascular endothelial growth factor receptor-1 (VEGR-1) and VEGFR-2 have high expression on breast cancer cells and contribute to angiogenesis and tumor development. Thus, early diagnosis through VEGFR-1/2 detection is an excellent strategy that can significantly increase a patient's chance of survival. In this study, the VEGFR1/2-targeting peptide VGB3 was conjugated with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), using 6-aminohexanoic acid (Ahx) as a spacer to prevent steric hindrance in binding. DOTA-Ahx-VGB3 was radiolabeled with Gallium-68 (68Ga) efficiently. An in vitro cell binding assay was assessed in the 4T1 cell line. The tumor-targeting potential of [68Ga]Ga-DOTA-Ahx-VGB3 was conducted for 4T1 tumor-bearing mice. Consequently, high radiochemical purity [68Ga]Ga-DOTA-Ahx-VGB3 (RCP = 98%) was prepared and stabilized in different buffer systems. Approximately 17% of the radiopeptide was internalized after 2 h incubation and receptor binding as characterized by the IC50 value being about 867 nM. The biodistribution and PET/CT studies revealed that [68Ga]Ga-DOTA-Ahx-VGB3 reached the tumor site and was excreted rapidly by the renal system. These features convey [68Ga]Ga-DOTA-Ahx-VGB3 as a suitable agent for the noninvasive visualization of VEGFR-1/2 expression.

18F-labeled PEGylated exendin-4 imaging noninvasively differentiates insulinoma from an accessory spleen: the first case report of [18F]FB(ePEG12)12-exendin-4 positron emission tomography/computed tomography for insulinoma

Background

Insulinomas are the most common functioning pancreatic neuroendocrine neoplasms, and these tumors induce hypoglycemia due to hyperinsulinemia. Hypoglycemia caused by insulinomas can cause seizures, coma or death due to the delayed diagnosis. The only curative treatment is surgical resection. To perform curative surgical resection of insulinomas, preoperative localization is crucial. However, localization of insulinomas is often challenging using conventional imaging methods such as computed tomography (CT) and magnetic resonance imaging. Although endoscopic ultrasound (EUS) fine-needle aspiration and selective arterial calcium stimulation test, which can reflect the endocrine character of the tumor, are performed in such cases, these modalities are invasive and require operator-dependent techniques. Additionally, somatostatin receptor (SSTR)-targeted imaging has a relatively low sensitivity for detecting insulinomas due to its low SSTR type 2 expression. Thus, there is an urgent need for developing a noninvasive diagnostic technique which is specific for detecting insulinomas. Consequently, glucagon-like peptide-1 receptor-targeted imaging has recently emerged and gained a wide interest. Recently, we have developed a novel 18F-labeled exendin-4-based probe conjugated with polyethylene glycol, [18F]FB(ePEG12)12-exendin-4 (18F-exendin-4), for positron emission tomography (PET) imaging. Here we report a case of insulinoma in which 18F-exendin-4 PET/CT noninvasively provided critical information for localization.

Case description

This is a case of a 58-year-old male with symptomatic hypoglycemia for 10 years; however, a preoperative diagnosis of insulinoma was not established due to the difficulty in differentiating it from an accessory spleen using conventional imaging. Moreover, the patient requested to avoid invasive diagnostic procedures including EUS. 18F-exendin-4 PET/CT revealed significant uptakes in the pancreatic tail whereas no apparent uptakes were observed in the spleen; thus, curative laparoscopic enucleation of the pancreatic tail was performed. The diagnosis of insulinoma was confirmed via histopathological examination. This is the first case report of insulinoma diagnosed using 18F-exendin-4 PET/CT.

Conclusion

In this case, PET information led to curative resection through enucleation of the pancreas. 18F-exendin-4 PET/CT may serve as a useful noninvasive clinical tool for insulinoma localization.

Clinical Advances and Perspectives in Targeted Radionuclide Therapy

Targeted radionuclide therapy has become increasingly prominent as a nuclear medicine subspecialty. For many decades, treatment with radionuclides has been mainly restricted to the use of iodine-131 in thyroid disorders. Currently, radiopharmaceuticals, consisting of a radionuclide coupled to a vector that binds to a desired biological target with high specificity, are being developed. The objective is to be as selective as possible at the tumor level, while limiting the dose received at the healthy tissue level. In recent years, a better understanding of molecular mechanisms of cancer, as well as the appearance of innovative targeting agents (antibodies, peptides, and small molecules) and the availability of new radioisotopes, have enabled considerable advances in the field of vectorized internal radiotherapy with a better therapeutic efficacy, radiation safety and personalized treatments. For instance, targeting the tumor microenvironment, instead of the cancer cells, now appears particularly attractive. Several radiopharmaceuticals for therapeutic targeting have shown clinical value in several types of tumors and have been or will soon be approved and authorized for clinical use. Following their clinical and commercial success, research in that domain is particularly growing, with the clinical pipeline appearing as a promising target. This review aims to provide an overview of current research on targeting radionuclide therapy.

Targeting Peptides: The New Generation of Targeted Drug Delivery Systems

Peptides can act as targeting molecules, analogously to oligonucleotide aptamers and antibodies. They are particularly efficient in terms of production and stability in physiological environments; in recent years, they have been increasingly studied as targeting agents for several diseases, from tumors to central nervous system disorders, also thanks to the ability of some of them to cross the blood-brain barrier. In this review, we will describe the techniques employed for their experimental and in silico design, as well as their possible applications. We will also discuss advancements in their formulation and chemical modifications that make them even more stable and effective. Finally, we will discuss how their use could effectively help to overcome various physiological problems and improve existing treatments.

[99mTc]-labeling and evaluation of a new linear peptide for imaging of glioblastoma as a αvβ3-positive tumor

PURPOSE

In this study, we designed a new linear 6-Hydrazinonicotinamide (HYNIC)-conjugated peptide (HYNIC-KRWrNM) (M-6) and labeled with technetium-99m for gamma imaging of glioblastoma as a α_vβ₃-positive tumor. We evaluated tumor targeting ability of this radio-peptide and compared with previous ^99mTc-labeled HYNIC-conjugated RGD analogue peptides.

PROCEDURES

One new linear peptide (HYNIC-KRWrNM) (M-6) was designed and labeled with technetium-99m in the presence of 2-[[1,3-dihydroxy-2-(hydroxymethyl) propan-2-yl] amino] acetic acid (Tricine)/Ethylenediamine-N,N'-diacetic acid (EDDA) as co-ligand system. Then, this ^99mTc-labeled peptide ([^99mTc]Tc-M-7) was evaluated for in vitro stability in saline and serum, specific binding assay, internalization, and binding affinity (K_d). In addition, we performed biodistribution study and planar imaging on nude mice bearing U87-MG xenograft as a α_vβ₃-positive tumor.

RESULTS

The radiochemical yield of [^99mTc]Tc-M-7 was obtained ˃95%. This ^99mTc-labeled peptide remained stable and intact in saline solution after 24 h incubation. In addition, metabolic stability of this ^99mTc-labeled peptide was obtained ˃60% after 4 h incubation in serum. The K_d value for [^99mTc]Tc-M-7 was obtained 5.2 ± 1.0 nM. Based on biodistribution results in nude mice bearing U87-MG xenograft, tumor/muscle activity ratio was 6.22 and decreased to 1.89 in blocking group at the same time point (4 h p.i.). The blocking experiment results also indicated that tumor uptake and kidney uptake were α_vβ₃-mediated. In comparison with previous HYNIC-conjugated RGD analogue peptides, kidneys had the highest uptake of this ^99mTc-labeled peptide (52.29 ± 11.48 at 1.5 h p.i. and 27.04 ± 0.66%ID/g at 4 h p.i.). Finally, similar to previous ^99mTc-labeled HYNIC-conjugated RGD analogue peptides, [^99mTc]Tc-M-7 showed acceptable tumor uptake after 4 h post-injection (based on ROI technique, target-to-background activity ratio = 3.80).

CONCLUSIONS

This small linear ^99mTc-labeled peptide, with high affinity to α_vβ₃ integrin, desirable water solubility, and cost efficient, demonstrates a potent tumor targeting ability as well as previous HYNIC-conjugated RGD analogue peptides. Hence, [^99mTc]Tc-M-7 can be of service to as a new candidate for early detection of α_vβ₃-positive tumors.

From Automated Synthesis to In Vivo Application in Multiple Types of Cancer-Clinical Results with [68Ga]Ga-DATA5m.SA.FAPi

Radiolabeled FAPI (fibroblast activation protein inhibitors) recently gained attention as widely applicable imaging and potential therapeutic compounds targeting CAF (cancer-associated fibroblasts) or DAF (disease-associated fibroblasts in benign disorders). Moreover, the use of FAPI has distinct advantages compared to FDG (e.g., increased sensitivity in regions with high glucose metabolism, no need for fasting, and rapid imaging). In this study, we wanted to evaluate the radiochemical synthesis and the clinical properties of the new CAF-targeting tracer [⁶⁸Ga]Ga-DATA^5m.SA.FAPi. The compound consists of a (radio)chemically easy to use hybrid chelate DATA.SA, which can be labeled at low temperatures, making it an interesting molecule for 'instant kit-type' labeling, and a squaric acid moiety that provides distinct advantages for synthesis and radiolabeling. Our work demonstrates that automatic synthesis of the FAP inhibitor [⁶⁸Ga]Ga-DATA^5m.SA.FAPi is feasible and reproducible, providing convenient access to this new hybrid chelator-based tracer. Our studies demonstrated the diagnostic usability of [⁶⁸Ga]Ga-DATA^5m.SA.FAPi for the unambiguous detection of cancer-associated fibroblasts of various carcinomas and their metastases (NSCLC, liposarcoma, parotid tumors, prostate cancer, and pancreas adenocarcinoma), while physiological uptake in brain, liver, intestine, bone, and lungs was very low.

Circumsporozoite Protein of Plasmodium berghei- and George Baker Virus A-Derived Peptides Trigger Efficient Cell Internalization of Bioconjugates and Functionalized Poly(ethylene glycol)-b-poly(benzyl malate)-Based Nanoparticles in Human Hepatoma Cells

In order to identify the peptides, selected from the literature, that exhibit the strongest tropism towards human hepatoma cells, cell uptake assays were performed using biotinylated synthetic peptides bound to fluorescent streptavidin or engrafted onto nanoparticles (NPs), prepared from biotin-poly(ethylene glycol)-block-poly(benzyl malate) (Biot-PEG-b-PMLABe) via streptavidin bridging. Two peptides, derived from the circumsporozoite protein of Plasmodium berghei- (CPB) and George Baker (GB) Virus A (GBVA10-9), strongly enhanced the endocytosis of both streptavidin conjugates and NPs in hepatoma cells, compared to primary human hepatocytes and non-hepatic cells. Unexpectedly, the uptake of CPB- and GBVA10-9 functionalized PEG-b-PMLABe-based NPs by hepatoma cells involved, at least in part, the peptide binding to apolipoproteins, which would promote NP’s interactions with cell membrane receptors of HDL particles. In addition, CPB and GBVA10-9 peptide–streptavidin conjugates favored the uptake by hepatoma cells over that of the human macrophages, known to strongly internalize nanoparticles by phagocytosis. These two peptides are promising candidate ligands for targeting hepatocellular carcinomas.

Synthesis, proteolytic stability, and in vitro evaluation of DOTA conjugated p160 peptide based radioconjugates: [177Lu]Lu-DOTA-p160

In this work, we describe the synthesis, in vitro stability, and preliminary biological evaluation of [¹⁷⁷Lu]Lu-DOTA-p160 peptide-based radiopharmaceuticals. Our findings highlight that all DOTA-p160-peptide radioconjugates exhibit favorable proteolytic and enzymatic stability with a prolonged half-life in human plasma and serum. Cell uptake studies carried out on MCF-7 cell line revealed saturable binding of the radioconjugates in the nanomolar range, thereby demonstrating their promising potential in the imaging and therapy of breast tumors.

Multifunctional self-assembled peptide nanoparticles for multimodal imaging-guided enhanced theranostic applications against glioblastoma multiforme

The synthesis of self-assembled peptide nanoparticles using a facile one-pot synthesis approach is gaining increasing attention, allowing therapy in combination with diagnosis. Their drawback is limited diagnostic potential, which can be improved after necessary modifications and efficacious functionalization. Herein, a cyclic heptapeptide having the Arg-Gly-Asp-Lys-Leu-Ala-Lys sequence was modified by conjugation of the ε-amino group of the terminal lysine residue with diethylenetriamine pentaacetic acid (DTPA) as a bifunctional chelating agent (BFC) for radiolabeling with a γ-emitting radionuclide (^99mTc, half-life 6.01 h; energy 140 keV). Further, the free amino group of the middle lysine residue was successfully conjugated with near-infrared fluorescence (NIRF) dye Cyanine5.5 N-succinimidyl ester (Ex/Em = 670/701 nm) by a co-assembly method to form newly designed novel NIRF dye conjugated self-assembled peptide-DTPA (Cy5.5@SAPD) nanoparticles. The fluorescent nanoparticle formation was confirmed by using a fluorescence spectrophotometer (Ex/Em = 650/701 nm), and the transmission electron microscope (TEM) images showed a size of ∼ 40 nm with a lattice fringe distance of 0.294 nm. Cytotoxicity and confocal laser scanning microscopy (CLSM) studies showed that these nanoparticles possess a high affinity for the α_vβ₃-integrin receptor overexpressed on brain tumor glioblastoma with an EC₅₀ = 20 μM. Moreover, these nanoparticles were observed to have potential to internalize into U87MG cells more prominently than HEK-293 cancer cells and induce apoptosis. The apoptosis assay showed 79.5% apoptotic cells after 24 h treatment of Cy5.5@SAPD nanoparticles. Additionally, these nanoparticles were also radiolabeled with ^99mTc for the single photon emission computed tomography (SPECT) imaging study in tumor-bearing female Balb/c mice. The excellent imaging feature of Cy5.5@SAPD-^99mTc nanoparticles as a multimodal (SPECT/NIRF) diagnostic probe, as well as noteworthy therapeutic potential was observed. The results suggested that our newly designed novel dual-targeting dual-imaging nanoparticles may serve as an admirable theranostic probe to treat brain tumor glioblastoma multiforme.

High-resolution positron emission microscopy of patient-derived tumor organoids

Tumor organoids offer new opportunities for translational cancer research, but unlike animal models, their broader use is hindered by the lack of clinically relevant imaging endpoints. Here, we present a positron-emission microscopy method for imaging clinical radiotracers in patient-derived tumor organoids with spatial resolution 100-fold better than clinical positron emission tomography (PET). Using this method, we quantify 18F-fluorodeoxyglucose influx to show that patient-derived tumor organoids recapitulate the glycolytic activity of the tumor of origin, and thus, could be used to predict therapeutic response in vitro. Similarly, we measure sodium-iodine symporter activity using 99mTc- pertechnetate and find that the iodine uptake pathway is functionally conserved in organoids derived from thyroid carcinomas. In conclusion, organoids can be imaged using clinical radiotracers, which opens new possibilities for identifying promising drug candidates and radiotracers, personalizing treatment regimens, and incorporating clinical imaging biomarkers in organoid-based co-clinical trials.

Artificial Intelligence and the Future of Diagnostic and Therapeutic Radiopharmaceutical Development:: In Silico Smart Molecular Design

Novel diagnostic and therapeutic radiopharmaceuticals are increasingly becoming a central part of personalized medicine. Continued innovation in the development of new radiopharmaceuticals is key to sustained growth and advancement of precision medicine. Artificial intelligence has been used in multiple fields of medicine to develop and validate better tools for patient diagnosis and therapy, including in radiopharmaceutical design. In this review, we first discuss common in silico approaches and focus on their usefulness and challenges in radiopharmaceutical development. Next, we discuss the practical applications of in silico modeling in design of radiopharmaceuticals in various diseases.

Hybridization of tumor homing and mitochondria-targeting peptide domains to design novel dual-imaging self-assembled peptide nanoparticles for theranostic applications

A novel hybridized dual-targeting peptide-based nanoprobe was successfully designed by using the cyclic heptapeptide. This peptide has Arg-Gly-Asp-Lys-Leu-Ala-Lys sequence, in which the RGD homing motif and KALK mitochondria-targeting motif were linked via amide bond. The designed peptide probe was further modified through covalent linkage to induce dual-imaging functionality, and self-assembled to form spherical nanoparticles. The novel Cy5.5-SAPD-^99mTc nanoparticles were tested for in vitro cytotoxicity, cellular uptake, and apoptosis-inducing functionalities. The cellular internalization, enhanced cytotoxicity and selective receptor binding capabilities against U87MG cells, excellent dual-imaging potential, improved apoptosis-inducing feature by damaging mitochondria, and in vivo preclinical investigations suggested that our newly designed novel hybridized peptide-based dual-imaging nanoparticles may serve as an admirable theranostic probe to treat brain tumor glioblastoma multiforme. This study describes the development of dual-targeting self-assembled peptide nanoparticles followed by modifications using NIRF dye and radiolabeled with ^99mTc for dual-imaging and enhanced therapeutic efficacy against brain tumor.

Distinctive detection of insulinoma using [18F]FB(ePEG12)12-exendin-4 PET/CT

Specifying the exact localization of insulinoma remains challenging due to the lack of insulinoma-specific imaging methods. Recently, glucagon-like peptide-1 receptor (GLP-1R)-targeted imaging, especially positron emission tomography (PET), has emerged. Although various radiolabeled GLP-1R agonist exendin-4-based probes with chemical modifications for PET imaging have been investigated, an optimal candidate probe and its scanning protocol remain a necessity. Thus, we investigated the utility of a novel exendin-4-based probe conjugated with polyethylene glycol (PEG) for [¹⁸F]FB(ePEG12)12-exendin-4 PET imaging for insulinoma detection. We utilized [¹⁸F]FB(ePEG12)12-exendin-4 PET/CT to visualize mouse tumor models, which were generated using rat insulinoma cell xenografts. The probe demonstrated high uptake value on the tumor as 37.1 ± 0.4%ID/g, with rapid kidney clearance. Additionally, we used Pdx1-Cre;Trp53^R172H;Rb^f/f mice, which developed endogenous insulinoma and glucagonoma, since they enabled differential imaging evaluation of our probe in functional pancreatic neuroendocrine neoplasms. In this model, our [¹⁸F]FB(ePEG12)12-exendin-4 PET/CT yielded favorable sensitivity and specificity for insulinoma detection. Sensitivity: 30-min post-injection 66.7%, 60-min post-injection 83.3%, combined 100% and specificity: 30-min post-injection 100%, 60-min post-injection 100%, combined 100%, which was corroborated by the results of in vitro time-based analysis of internalized probe accumulation. Accordingly, [¹⁸F]FB(ePEG12)12-exendin-4 is a promising PET imaging probe for visualizing insulinoma.

Production of a broad palette of positron emitting radioisotopes using a low-energy cyclotron: Towards a new success story in cancer imaging?

Over the last several years, positron emission tomography (PET) has matured as an indispensable component of cancer diagnostics. Owing to the large variability observed among the cancer patients and the need to personalize individual patient's diagnosis and treatment, the need for new positron emitting radioisotopes has continued to grow. This mini review opens with a brief introduction to the criteria for radioisotope selection for PET imaging. Subsequently, positron emitting radioisotopes are categorized as: established, emerging and futuristic, based on the stages of their advancement. The production methodologies and the radiochemical separation procedures for obtaining the important radioisotopes in a form suitable for preparation of radiopharmaceuticals for PET imaging are briefly discussed.

A perspective on the radiopharmaceutical requirements for imaging and therapy of glioblastoma

Despite numerous clinical trials and pre-clinical developments, the treatment of glioblastoma (GB) remains a challenge. The current survival rate of GB averages one year, even with an optimal standard of care. However, the future promises efficient patient-tailored treatments, including targeted radionuclide therapy (TRT). Advances in radiopharmaceutical development have unlocked the possibility to assess disease at the molecular level allowing individual diagnosis. This leads to the possibility of choosing a tailored, targeted approach for therapeutic modalities. Therapeutic modalities based on radiopharmaceuticals are an exciting development with great potential to promote a personalised approach to medicine. However, an effective targeted radionuclide therapy (TRT) for the treatment of GB entails caveats and requisites. This review provides an overview of existing nuclear imaging and TRT strategies for GB. A critical discussion of the optimal characteristics for new GB targeting therapeutic radiopharmaceuticals and clinical indications are provided. Considerations for target selection are discussed, i.e. specific presence of the target, expression level and pharmacological access to the target, with particular attention to blood-brain barrier crossing. An overview of the most promising radionuclides is given along with a validation of the relevant radiopharmaceuticals and theranostic agents (based on small molecules, peptides and monoclonal antibodies). Moreover, toxicity issues and safety pharmacology aspects will be presented, both in general and for the brain in particular.

Natural product-based Radiopharmaceuticals:Focus on curcumin and its analogs, flavonoids, and marine peptides

Natural products provide a bountiful supply of pharmacologically relevant precursors for the development of various drug-related molecules, including radiopharmaceuticals. However, current knowledge regarding the importance of natural products in developing new radiopharmaceuticals remains limited. To date, several radionuclides, including gallium-68, technetium-99m, fluorine-18, iodine-131, and iodine-125, have been extensively studied for the synthesis of diagnostic and therapeutic radiopharmaceuticals. The availability of various radiolabeling methods allows the incorporation of these radionuclides into bioactive molecules in a practical and efficient manner. Of the radiolabeling methods, direct radioiodination, radiometal complexation, and halogenation are generally suitable for natural products owing to their simplicity and robustness. This review highlights the pharmacological benefits of curcumin and its analogs, flavonoids, and marine peptides in treating human pathologies and provides a perspective on the potential use of these bioactive compounds as molecular templates for the design and development of new radiopharmaceuticals. Additionally, this review provides insights into the current strategies for labeling natural products with various radionuclides using either direct or indirect methods.

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Potential use of natural products for the development of diagnostic and therapeutic radiopharmaceuticals.

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Profile of potential natural products as molecular templates for the synthesis of new radiopharmaceuticals: Focus on curcumin and its closely related substances, flavonoids, and marine peptides.

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Radiolabeling strategies, challenges, and examples of natural product-based radiopharmaceuticals under investigation.

Development and characterization of two novel 68 Ga-labelled neuropeptide Y short analogues with potential application in breast cancer imaging

In vivo receptor targeting with radiolabelled peptide-based probes is an attractive approach for the development of novel radiotracers for molecular imaging. This work presents the development and characterization of two novel neuropeptide Y analogues labelled with a positron emitter 68 Ga, for potential use in breast cancer imaging. Both analogues share the same amino acid sequence and were derivatized with NOTA through either a lysine linker (L1) or an acetylated lysine (L2). In both cases, a single product with radiochemical purity higher than 95% was obtained. The two complexes were hydrophilic, showed remarkable in vitro stability, good cellular uptake, binding affinity in the nanomolar range and high cellular internalization rate. Biodistribution studies revealed low blood uptake and elimination through the urinary tract. The addition of an acetyl group in the spacer increased the lipophilicity of C2 and modified the reactivity of the ε-amino group of the lysine which resulted in lower protein binding and lower percentage of injected dose in bladder and urine. The tumour versus muscle ratio was (3.8 ± 0.4) for 68 Ga-L1 and (4.7 ± 0.4) for 68 Ga-L2. These results encourage performing further studies in order to complete the evaluation of both tracers as potential radiopharmaceutical for breast cancer imaging.

Molecular Imaging of Angiogenesis in Oncology: Current Preclinical and Clinical Status

Angiogenesis is an active process, regulating new vessel growth, and is crucial for the survival and growth of tumours next to other complex factors in the tumour microenvironment. We present possible molecular imaging approaches for tumour vascularisation and vitality, focusing on radiopharmaceuticals (tracers). Molecular imaging in general has become an integrated part of cancer therapy, by bringing relevant insights on tumour angiogenic status. After a structured PubMed search, the resulting publication list was screened for oncology related publications in animals and humans, disregarding any cardiovascular findings. The tracers identified can be subdivided into direct targeting of angiogenesis (i.e., vascular endothelial growth factor, laminin, and fibronectin) and indirect targeting (i.e., glucose metabolism, hypoxia, and matrix metallo-proteases, PSMA). Presenting pre-clinical and clinical data of most tracers proposed in the literature, the indirect targeting agents are not 1:1 correlated with angiogenesis factors but do have a strong prognostic power in a clinical setting, while direct targeting agents show most potential and specificity for assessing tumour vascularisation and vitality. Within the direct agents, the combination of multiple targeting tracers into one agent (multimers) seems most promising. This review demonstrates the present clinical applicability of indirect agents, but also the need for more extensive research in the field of direct targeting of angiogenesis in oncology. Although there is currently no direct tracer that can be singled out, the RGD tracer family seems to show the highest potential therefore we expect one of them to enter the clinical routine.

Synthesis of Poly(Malic Acid) Derivatives End-Functionalized with Peptides and Preparation of Biocompatible Nanoparticles to Target Hepatoma Cells

Recently, short synthetic peptides have gained interest as targeting agents in the design of site-specific nanomedicines. In this context, our work aimed at developing new tools for the diagnosis and/or therapy of hepatocellular carcinoma (HCC) by grafting the hepatotropic George Baker (GB) virus A (GBVA10-9) and Plasmodium circumsporozoite protein (CPB)-derived peptides to the biocompatible poly(benzyl malate), PMLABe. We successfully synthesized PMLABe derivatives end-functionalized with peptides GBVA10-9, CPB, and their corresponding scrambled peptides through a thiol/maleimide reaction. The corresponding nanoparticles (NPs), varying by the nature of the peptide (GBVA10-9, CPB, and their scrambled peptides) and the absence or presence of poly(ethylene glycol) were also successfully formulated using nanoprecipitation technique. NPs were further characterized by dynamic light scattering (DLS), electrophoretic light scattering (ELS) and transmission electron microscopy (TEM), highlighting a diameter lower than 150 nm, a negative surface charge, and a more or less spherical shape. Moreover, a fluorescent probe (DiD Oil) has been encapsulated during the nanoprecipitation process. Finally, preliminary in vitro internalisation assays using HepaRG hepatoma cells demonstrated that CPB peptide-functionalized PMLABe NPs were efficiently internalized by endocytosis, and that such nanoobjects may be promising drug delivery systems for the theranostics of HCC.

SPECT Imaging of SST2-Expressing Tumors with 99mTc-Based Somatostatin Receptor Antagonists: The Role of Tetraamine, HYNIC, and Spacers

[^99mTc]Tc-HYNIC-TOC is the most widely used ^99mTc-labeled somatostatin receptor (SST) agonist for the SPECT imaging of SST-expressing tumors, such as neuroendocrine tumors. Recently, radiolabeled SST antagonists have shown improved diagnostic efficacy over agonists. ^99mTc-labeled SST antagonists are lacking in clinical practice. Surprisingly, when [^99mTc]Tc-HYNIC was conjugated to the SST2 antagonist SS01, SST2 imaging was not feasible. This was not the case when [^99mTc]Tc-N4 was conjugated to SS01. Here, we assessed the introduction of different spacers (X: β-Ala, Ahx, Aun and PEG₄) among HYNIC and SS01 with the aim of restoring the affinity of HYNIC conjugates. In addition, we used the alternative antagonist JR11 for determining the suitability of HYNIC with ^99mTc-labeled SST2 antagonists. We performed a head-to-head comparison of the N4 conjugates of SS01 and JR11. [^99mTc]Tc-HYNIC-TOC was used as a reference, and HEK-SST2 cells were used for in vitro and in vivo evaluation. EDDA was used as a co-ligand for all [^99mTc]Tc-HYNIC conjugates. The introduction of Ahx restored, to a great extent, the SST2-mediated cellular uptake of the [^99mTc]Tc-HYNIC-X conjugates (X: spacer), albeit lower than the corresponding [^99mTc]Tc-N4-conjugates. SPECT/CT images showed that all ^99mTc-labeled conjugates accumulated in the tumor and kidneys with [^99mTc]Tc-HYNIC-PEG₄-SS01, [^99mTc]Tc-N4-SS01 and [^99mTc]Tc-N4-JR11 having notably higher kidney uptake. Biodistribution studies showed similar or better tumor-to-non-tumor ratios for the [^99mTc]Tc-HYNIC-Ahx conjugates, compared to the [^99mTc]Tc-N4 counterparts. The [^99mTc]Tc-HYNIC-Ahx conjugates of SS01 and JR11 were comparable to [^99mTc]Tc-HYNIC-TOC as imaging agents. HYNIC is a suitable chelator for the development of ^99mTc-labeled SST2 antagonists when a spacer of appropriate length, such as Ahx, is used.

Synthesis of a DOTA-C-glyco bifunctional chelating agent and preliminary in vitro and in vivo study of [68Ga]Ga-DOTA-C-glyco-RGD

The design of bifunctional chelating agents (BFCA) allowing straightforward radiometal labelling of biomolecules is a current challenge. We report herein the development of a bifunctional chelating agent based on a DOTA chelator linked to a C-glycosyl compound, taking advantage of the robustness and hydrophilicity of this type of carbohydrate derivative. This new BFCA was coupled with success by CuAAC with c(RGDfK) for α_vβ₃ integrin targeting. As attested by in vitro evaluation, the conjugate DOTA-C-glyco-c(RGDfC) demonstrated high affinity for α_vβ₃ integrins (IC₅₀ of 42 nM). [⁶⁸Ga]Ga-DOTA-C-glyco-c(RGDfK) was radiosynthesized straightforwardly and showed high hydrophilic property (log D_7.4 = −3.71) and in vitro stability (>120 min). Preliminary in vivo PET study of U87MG engrafted mice gave evidence of an interesting tumor-to-non-target area ratio. All these data indicate that [⁶⁸Ga]Ga-DOTA-C-glyco-c(RGDfK) allows monitoring of α_vβ₃ expression and could thus be used for cancer diagnosis. The DOTA-C-glycoside BFCA reported here could also be used with various ligands and chelating other (radio)metals opening a broad scope of applications in imaging modalities and therapy.

A carbohydrate containing [⁶⁸Ga]Ga-DOTA-RGD tracer was designed and demonstrated promising results for cancer diagnosis by positron emission tomography imaging.

Development of Radiogallium-Labeled Peptides for Platelet-Derived Growth Factor Receptor β (PDGFRβ) Imaging: Influence of Different Linkers

The purpose of this study is to develop peptide-based platelet-derived growth factor receptor β (PDGFRβ) imaging probes and examine the effects of several linkers, namely un-natural amino acids (D-alanine and β-alanine) and ethylene-glycol (EG), on the properties of Ga-DOTA-(linker)-IPLPPPRRPFFK peptides. Seven radiotracers, ⁶⁷Ga-DOTA-(linker)-IPLPPPRRPFFK peptides, were designed, synthesized, and evaluated. The stability and cell uptake in PDGFRβ positive peptide cells were evaluated in vitro. The biodistribution of [⁶⁷Ga]Ga-DOTA-EG₂-IPLPPPRRPFFK ([⁶⁷Ga]27) and [⁶⁷Ga]Ga-DOTA-EG₄-IPLPPPRRPFFK ([⁶⁷Ga]28), which were selected based on in vitro stability in murine plasma and cell uptake rates, were determined in BxPC3-luc-bearing nu/nu mice. Seven ⁶⁷Ga-labeled peptides were successfully synthesized with high radiochemical yields (>85%) and purities (>99%). All evaluated radiotracers were stable in PBS (pH 7.4) at 37 °C. However, only [⁶⁷Ga]27 and [⁶⁷Ga]28 remained more than 75% after incubation in murine plasma at 37 °C for 1 h. [⁶⁷Ga]27 exhibited the highest BxPC3-luc cell uptake among the prepared radiolabeled peptides. As regards the results of the biodistribution experiments, the tumor-to-blood ratios of [⁶⁷Ga]27 and [⁶⁷Ga]28 at 1 h post-injection were 2.61 ± 0.75 and 2.05 ± 0.77, respectively. Co-injection of [⁶⁷Ga]27 and an excess amount of IPLPPPRRPFFK peptide as a blocking agent can significantly decrease this ratio. However, tumor accumulation was not considered sufficient. Therefore, further probe modification is required to assess tumor accumulation for in vivo imaging.

New Frontiers in Molecular Imaging Using Peptide-Based Radiopharmaceuticals for Prostate Cancer

The high incidence of prostate cancer (PCa) increases the need for progress in its diagnosis, staging, and precise treatment. The overexpression of tumor-specific receptors for peptides in human cancer cells, such as gastrin-releasing peptide receptor, natriuretic peptide receptor, and somatostatin receptor, has indicated the ideal molecular basis for targeted imaging and therapy. Targeting these receptors using radiolabeled peptides and analogs have been an essential topic on the current forefront of PCa studies. Radiolabeled peptides have been used to target receptors for molecular imaging in human PCa with high affinity and specificity. The radiolabeled peptides enable optimal quick elimination from blood and normal tissues, producing high contrast for positron emission computed tomography and single-photon emission computed tomography imaging with high tumor-to-normal tissue uptake ratios. Owing to their successful application in visualization, peptide derivatives with therapeutic radionuclides for peptide receptor radionuclide therapy in PCa have been explored in recent years. These developments offer the promise of personalized, molecular medicine for individual patients. Hence, we review the preclinical and clinical literature in the past 20 years and focus on the newer developments of peptide-based radiopharmaceuticals for the imaging and therapy of PCa.

A Decade's Progress in the Development of Molecular Imaging Agents Targeting the Growth Hormone Secretagogue Receptor

The growth hormone secretagogue receptor 1a (GHSR), also called the ghrelin receptor, is a G protein-coupled receptor known to play an important metabolic role in the regulation of various physiological processes, including energy expenditure, growth hormone secretion, and cell proliferation. This receptor has been implicated in numerous health issues including obesity, gastrointestinal disorders, type II diabetes, and regulation of body weight in patients with Prader-Willi syndrome, and there has been growing interest in studying its mechanism of behavior to unlock further applications of GHSR-targeted therapeutics. In addition, the GHSR is expressed in various types of cancer including prostate, breast, and testicular cancers, while aberrant expression has been reported in cardiac disease. Targeted molecular imaging of the GHSR could provide insights into its role in biological processes related to these disease states. Over the past decade, imaging probes targeting this receptor have been discovered for the imaging modalities PET, SPECT, and optical imaging. High-affinity analogues of ghrelin, the endogenous ligand for the GHSR, as well as small molecule inhibitors have been developed and evaluated both in vitro and in pre-clinical models. This review provides a comprehensive overview of the molecular imaging agents targeting the GHSR reported to the end of 2019.

Overview of Radiolabeled Somatostatin Analogs for Cancer Imaging and Therapy

Identified in 1973, somatostatin (SST) is a cyclic hormone peptide with a short biological half-life. Somatostatin receptors (SSTRs) are widely expressed in the whole body, with five subtypes described. The interaction between SST and its receptors leads to the internalization of the ligand-receptor complex and triggers different cellular signaling pathways. Interestingly, the expression of SSTRs is significantly enhanced in many solid tumors, especially gastro-entero-pancreatic neuroendocrine tumors (GEP-NET). Thus, somatostatin analogs (SSAs) have been developed to improve the stability of the endogenous ligand and so extend its half-life. Radiolabeled analogs have been developed with several radioelements such as indium-111, technetium-99 m, and recently gallium-68, fluorine-18, and copper-64, to visualize the distribution of receptor overexpression in tumors. Internal metabolic radiotherapy is also used as a therapeutic strategy (e.g., using yttrium-90, lutetium-177, and actinium-225). With some radiopharmaceuticals now used in clinical practice, somatostatin analogs developed for imaging and therapy are an example of the concept of personalized medicine with a theranostic approach. Here, we review the development of these analogs, from the well-established and authorized ones to the most recently developed radiotracers, which have better pharmacokinetic properties and demonstrate increased efficacy and safety, as well as the search for new clinical indications.

Recovery of antioxidant and antimicrobial peptides through the reutilization of Nile perch wastewater by biodegradation using two Bacillus species

Nile perch wastewater was biodegraded using two Bacillus species to recover bioactive substances to enhance its reutilization value. The two Bacillus species successfully produced low-molecular-weight substances with a 47.8% degree of hydrolysis. The antioxidant activities of the Nile perch wastewater increased as the biodegradation proceeded, and the culture supernatant exhibited the highest DPPH (80.1%), ABTS (93.1%) and Fe²⁺ chelating (88.5%) antioxidant activities at 60 h. The antioxidant potential of the biodegraded Nile perch wastewater was found to be higher than those of other fish hydrolysates. Moreover, the biodegraded Nile perch wastewater exhibited effective antimicrobial activity against Vibrio vulnificus, exhibiting a minimal inhibitory concentration of 585 μg mL^-1. Two-dimensional thin layer chromatography analysis revealed the specific amino acids responsible for the antioxidant activity, and molecular-weight cut-off ultrafiltration revealed that the <2-kDa fraction exhibited the highest antioxidant activity with the lowest IC₅₀ values (0.43 and 0.22 mg mL^-1 for DPPH and ABTS antioxidant activities, respectively). This is the first report of the reutilization of Nile perch wastewater as a natural antioxidant and antimicrobial ingredient for nutraceuticals.

Current State of Radiolabeled Heterobivalent Peptidic Ligands in Tumor Imaging and Therapy

Over the past few years, an approach emerged that combines different receptor-specific peptide radioligands able to bind different target structures on tumor cells concomitantly or separately. The reason for the growing interest in this special field of radiopharmaceutical development is rooted in the fact that bispecific peptide heterodimers can exhibit a strongly increased target cell avidity and specificity compared to their corresponding monospecific counterparts by being able to bind to two different target structures that are overexpressed on the cell surface of several malignancies. This increase of avidity is most pronounced in the case of concomitant binding of both peptides to their respective targets but is also observed in cases of heterogeneously expressed receptors within a tumor entity. Furthermore, the application of a radiolabeled heterobivalent agent can solve the ubiquitous problem of limited tumor visualization sensitivity caused by differential receptor expression on different tumor lesions. In this article, the concept of heterobivalent targeting and the general advantages of using radiolabeled bispecific peptidic ligands for tumor imaging or therapy as well as the influence of molecular design and the receptors on the tumor cell surface are explained, and an overview is given of the radiolabeled heterobivalent peptides described thus far.

From Animal Poisons and Venoms to Medicines: Achievements, Challenges and Perspectives in Drug Discovery

Animal poisons and venoms are comprised of different classes of molecules displaying wide-ranging pharmacological activities. This review aims to provide an in-depth view of toxin-based compounds from terrestrial and marine organisms used as diagnostic tools, experimental molecules to validate postulated therapeutic targets, drug libraries, prototypes for the design of drugs, cosmeceuticals, and therapeutic agents. However, making these molecules applicable requires extensive preclinical trials, with some applications also demanding clinical trials, in order to validate their molecular target, mechanism of action, effective dose, potential adverse effects, as well as other fundamental parameters. Here we go through the pitfalls for a toxin-based potential therapeutic drug to become eligible for clinical trials and marketing. The manuscript also presents an overview of the current picture for several molecules from different animal venoms and poisons (such as those from amphibians, cone snails, hymenopterans, scorpions, sea anemones, snakes, spiders, tetraodontiformes, bats, and shrews) that have been used in clinical trials. Advances and perspectives on the therapeutic potential of molecules from other underexploited animals, such as caterpillars and ticks, are also reported. The challenges faced during the lengthy and costly preclinical and clinical studies and how to overcome these hindrances are also discussed for that drug candidates going to the bedside. It covers most of the drugs developed using toxins, the molecules that have failed and those that are currently in clinical trials. The article presents a detailed overview of toxins that have been used as therapeutic agents, including their discovery, formulation, dosage, indications, main adverse effects, and pregnancy and breastfeeding prescription warnings. Toxins in diagnosis, as well as cosmeceuticals and atypical therapies (bee venom and leech therapies) are also reported. The level of cumulative and detailed information provided in this review may help pharmacists, physicians, biotechnologists, pharmacologists, and scientists interested in toxinology, drug discovery, and development of toxin-based products.

Preparation and Biological Evaluation of ⁶⁷Gallium- Labeled Iranian Hemiscorpius Lepturus Scorpion Venom

Background

The Hemiscorpius lepturus (H. lepturus) is a deadly scorpion species living in the southern Iran.

Objective

H. lepturus induces delayed toxicity symptoms and understanding the long term biodistribution/biokinetic of the venom is of great interest in toxicology.

Methods

A Ga-67 labeled venom was prepared using a DOTA -conjugated venom followed by radiolabeling using ⁶⁷GaCl₃ at 40°C for 90 min. The purification of the radiolabeled venom was performed using size exclusion-chromatography (radiochemical purity 71%). The radiolabeled venom was stable in the final solution in the presence of human serum at 37°C for 72 hours. The tissue distribution was studied in blood, heart, liver, spleen, muscle, brain, kidney, intestine and skin tissues at the intervals of 1, 4, 24, 48 and 72 hours using tissue counting and SPECT imaging.

Results

The radiolabeled venom mixture obtained with an estimated molar activity of 0.52 MBq/µg. The main accumulation tissues during the first 72 hours were kidneys, blood, liver, intestines, stomach and skin, respectively. Therefore, it is likely that H. lepturus’ clinical effects and renal toxicity are primary and caused by direct effects of the H. lepturus venom.

Conclusion

The results have largely shown the direct clinical effects on the studied tissues during the 72-hour period and antivenom administration can strongly alleviate the toxicity effects as early as 72 hours in the management of the patients.

Neuroprotective and Memory-Enhancing Effects of Antioxidant Peptide From Walnut (Juglans regia L.) Protein Hydrolysates

Peptides have been reported to possess interesting biological properties. The present study was designed to evaluate neuroprotective and memory-enhancing effects of antioxidant peptide from walnut ( Juglans regia L.) protein hydrolysates. The neuroprotective effect of walnut peptide (WP) against oxidative stress on PC12 cells was evaluated. And zebrafish was used as the model to corroborate the effect. Its effect on learning and memory of mice using the Morris water maze and the step-down passive avoidance tests were performed. Moreover, the acute toxicity of WP was carried out to assess its safety profile. It was found that WP was able to suppress H2O2-induced cell death in PC12 cells. In the zebrafish model, WP had an obvious neuroprotective effect, and the ratio reached 42% at 222 µg/mL. The mechanism study revealed that WP could inhibit the activity of caspases 3/7 and 8, reduce the mRNA expression levels of Bax and glial cell line-derived neurotrophic factor, and improve the mRNA expression level of brain-derived neurotrophic factor significantly. Besides, the treatment of mice with WP shortened the escape latency and exhibited much longer target time and more crossing times significantly, compared with untreated control groups in the Morris water maze test. Similarly, the step-down passive avoidance test showed that WP could ameliorate memory impairments. The administrated dose (20.1 g/kg body weight [BW]) did not produce mortality or treatment-related adverse effects with regard to BW, general behavior, or relative organ weights of the tested male and female mice. The current results indicated that WP could exert neuroprotective effect, and attenuated learning and memory impairments. These ameliorating effects of WP may be useful for treatment of memory impairment in Alzheimer’s and its related diseases.

Radiochemical aspects of alpha emitting radionuclides for medical application

The use of α-emitting radionuclides in targeted alpha therapy (TAT) holds great potential for treatment of human diseases, such as cancer, due to the short pathlength and high potency of the α particle, which can localize damage to targeted cells while minimizing effects to healthy surrounding tissues. In this review several potential α-emitting radionuclides having emission properties applicable to TAT are discussed from a radiochemical point of view. Overviews of production, radiochemical separation and chelation aspects relative to developing TAT radiopharmaceuticals are provided for the α-emitting radionuclides (and their generator systems) 211At, 224Ra/212Pb/212Bi, 225Ac/213Bi, 227Th/223Ra, 230U/226Th, 149Tb and 255Fm.

Food-derived antithrombotic peptides: Preparation, identification, and interactions with thrombin

Thromboembolism and its sequelae have been the leading causes of morbidity and mortality throughout the world. Food-derived antithrombotic peptides, as potential ingredients in health-promoting functional foods targeting thrombus, have attracted increasing attention because of their high biological activities, low toxicity, and ease of metabolism in the human body. This review presents the conventional workflow of preparation, isolation and identification of antithrombotic peptides from various kinds of food materials. More importantly, to analyze the antithrombotic effects and mechanism of antithrombotic peptides, methods for interaction of anticoagulant peptides and thrombin, the main participant in thrombosis, were analyzed from biochemistry, solution chemistry and crystal chemistry. The present study is intended to highlight the recent advances in research of food-derived antithrombotic peptide as a novel vehicle in the field of food science and nutrition. Future outlooks are highlighted with the aim to suggest a research line to be followed in further studies with the introduced research approach.

Peptides as Therapeutic Agents for Inflammatory-Related Diseases

Inflammation is a physiological mechanism used by organisms to defend themselves against infection, restoring homeostasis in damaged tissues. It represents the starting point of several chronic diseases such as asthma, skin disorders, cancer, cardiovascular syndrome, arthritis, and neurological diseases. An increasing number of studies highlight the over-expression of inflammatory molecules such as oxidants, cytokines, chemokines, matrix metalloproteinases, and transcription factors into damaged tissues. The treatment of inflammatory disorders is usually linked to the use of unspecific small molecule drugs that can cause undesired side effects. Recently, many efforts are directed to develop alternative and more selective anti-inflammatory therapies, several of them imply the use of peptides. Indeed, peptides demonstrated as elected lead compounds toward several targets for their high specificity as well as recent and innovative synthetic strategies. Several endogenous peptides identified during inflammatory responses showed anti-inflammatory activities by inhibiting, reducing, and/or modulating the expression and activity of mediators. This review aims to discuss the potentialities and therapeutic use of peptides as anti-inflammatory agents in the treatment of different inflammation-related diseases and to explore the importance of peptide-based therapies.

Anti-EGFR Peptide-Conjugated Triangular Gold Nanoplates for Computed Tomography/Photoacoustic Imaging-Guided Photothermal Therapy of Non-Small Cell Lung Cancer

Non-small cell lung cancer (NSCLC) is difficult to cure because of the high recurrence rate and the side effects of current treatments. It is urgent to develop a new treatment that is safer and more effective than current treatments against NSCLC. Herein, we constructed anti-epidermal growth factor receptor (EGFR) peptide-conjugated PEGylated triangular gold nanoplates (TGN-PEG-P75) as a targeting photothermal therapy (PTT) agent to treat NSCLC under the guidance of computed tomography (CT) and photoacoustic (PA) imaging. The surface of TGNs is successfully conjugated with a novel peptide P75 that has the specific affinity to epidermal growth factor receptor (EGFR). It is found that the EGFR is overexpressed in NSCLC cells. The TGN-PEG-P75 has uniform edge length (77.9 ± 7.0 nm) and neutrally charged surface. The cell uptake experiments demonstrate remarkable affinity of the TGN-PEG-P75 to high EGFR expression cells than low EGFR expression cells (5.1-fold). Thanks to the strong near-infrared absorbance, high photothermal conversion efficiency, and the increased accumulation in tumor cells via the interaction of P75 and EGFR, TGN-PEG-P75 exhibits 3.8-fold superior therapeutic efficacy on HCC827 cells than TGN-PEG. The in vivo CT/PA dual-modal imaging of the TGN-PEG-P75 is helpful in selecting the optimal treatment time and providing real-time visual guidance of PTT. Furthermore, treatments on HCC827 tumor-bearing mouse model demonstrate that the growth of NSCLC cells can be effectively inhibited by the TGN-PEG-P75 through PTT, indicating the great promise of the nanoplatform for treating NSCLC in vivo.

Synthesis, Characterization, and Initial Biological Evaluation of [99m Tc]Tc-Tricarbonyl-labeled DPA-α-MSH Peptide Derivatives for Potential Melanoma Imaging

α-Melanocyte stimulating hormone (α-MSH) derivatives target the melanocortin-1 receptor (MC1R) specifically and selectively. In this study, the α-MSH-derived peptide NAP-NS1 (Nle-Asp-His-d-Phe-Arg-Trp-Gly-NH₂ ) with and without linkers was conjugated with 5-(bis(pyridin-2-ylmethyl)amino)pentanoic acid (DPA-COOH) and labeled with [^99m Tc]Tc-tricarbonyl by two methods. With the one-pot method the labeling was faster than with the two-pot method, while obtaining similarly high yields. Negligible trans-chelation and high stability in physiological solutions was determined for the [^99m Tc]Tc-tricarbonyl-peptide conjugates. Coupling an ethylene glycol (EG)-based linker increased the hydrophilicity. The peptide derivatives displayed high binding affinity in murine B16F10 melanoma cells as well as in human MeWo and TXM13 melanoma cell homogenates. Preliminary in vivo studies with one of the [^99m Tc]Tc-tricarbonyl-peptide conjugates showed good stability in blood and both renal and hepatobiliary excretion. Biodistribution was performed on healthy rats to gain initial insight into the potential relevance of the ^99m Tc-labeled peptides for in vivo imaging.

18F-AlF Labeled Peptide and Protein Conjugates as Positron Emission Tomography Imaging Pharmaceuticals

The clinical applications of positron emission tomography (PET) imaging pharmaceuticals have increased tremendously over the past several years since the approval of ¹⁸fluorine-fluorodeoxyglucose (¹⁸F-FDG) by the Food and Drug Administration (FDA). Numerous ¹⁸F-labeled target-specific potential imaging pharmaceuticals, based on small and large molecules, have been evaluated in preclinical and clinical settings. ¹⁸F-labeling of organic moieties involves the introduction of the radioisotope by C-¹⁸F bond formation via a nucleophilic or an electrophilic substitution reaction. However, biomolecules, such as peptides, proteins, and oligonucleotides, cannot be radiolabeled via a C-¹⁸F bond formation as these reactions involve harsh conditions, including organic solvents, high temperature, and nonphysiological conditions. Several approaches, including ¹⁸F-labeled prosthetic groups, silicon, boron, and aluminum fluoride acceptor chemistry, and click chemistry have been developed, in the past, for ¹⁸F labeling of biomolecules. Linear and macrocyclic polyaminocarboxylates and their analogs and derivatives form thermodynamically stable and kinetically inert aluminum chelates. Hence, macrocyclic polyaminocarboxylates have been used for conjugation with biomolecules, such as folate, peptides, affibodies, and protein fragments, followed by ¹⁸F-AlF chelation, and evaluation of their targeting abilities in preclinical and clinical environments. The goal of this report is to provide an overview of the ¹⁸F radiochemistry and ¹⁸F-labeling methodologies for small molecules and target-specific biomolecules, a comprehensive review of coordination chemistry of Al³⁺, ¹⁸F-AlF labeling of peptide and protein conjugates, and evaluation of ¹⁸F-labeled biomolecule conjugates as potential imaging pharmaceuticals.