Peptide PET Imaging: A Review of Recent Developments and a Look at the Future of Radiometal-Labeled Peptides in Medicine

Abstract

The development of peptide-based, radiometal-labeled PET imaging agents has seen an increase in attention due to the favorable properties the peptide backbone exhibits. These include high selectivity and affinity to proteins and cells directly linked to various types of cancers. In addition, rapid clearance from circulation and low toxicity allow for unique approaches to engineering a viable peptide-based imaging agent. Utilizing peptides as the backbone allows for various modifications to improve metabolic stability, target cell affinity, and image quality and imaging capabilities and reduce toxicity. Select radiolabeled peptides have already been FDA approved, with many more in late-stage trials. This review summarizes the current state of the radiometal-labeled PET peptide imaging field as well as explores methods used by researchers to modify peptides, concluding with a look at the future of peptide-based therapy and diagnostics.

1. Introduction

The success of cancer therapy relies on the speed and accuracy that tumor cells can be identified and treated.¹⁻⁴ A wider understanding of the nature of these tumor cells as well as their interactions with certain surrounding proteins allows for a variety of approaches to positively identify the presence of tumor cells at an early stage. Current forms of imaging include positron emission tomography (PET), magnetic resonance imaging (MRI), and computed tomography scans, each with their own advantages and uses.⁵⁻¹⁰ Many factors determine which type of imaging is used in the context of cancer therapeutics such as the type and location of the tumor cells and to contextualize the growth of the tumor cells.¹¹⁻¹⁴ However, PET scans are the most advantageous¹⁵⁻¹⁷ because they show changes on the cellular level and reveal tumor activity at an early stage.¹⁸ They can detect cancer, identify the stage of progression, determine if the tumor has metastasized, and characterize growth based on metabolic activity.¹⁹⁻²⁴

In PET scans, a radiopharmaceutical is injected into the patient. They can take many forms but must contain an atom that can undergo beta plus decay,^25,26 wherein a proton is converted into a neutron, releasing a positron and electron neutrino in the process. Positrons then interact with electrons and annihilate each other, releasing γ-rays.²⁷ These γ-rays are detected using scintillation cameras. Readings are taken from multiple angles to create a 3-dimensional map of where the gamma signals are coming from.^19,27

An important factor in the success of PET scans is the use of effective and highly target-specific probes that retain in vivo stability and have a sufficient half-life to allow for imaging.^28,29 These radiotracers bind to the target to form a three-dimensional image of the body showing the location of the target of interest.²⁸ A frequently used radiotracer is [¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG).³⁰⁻³³ Based on the structure of glucose, with a radioactive fluorine isotope (¹⁸F) at the 2 position, this radiotracer is widely used either as a single tracer or in combination with other targeted therapies,³⁴⁻³⁶ such as obtaining both PET and CT signals for a more sensitive and specific image synergizing each method’s advantages³⁷ or in cancer imaging due to the need of sugar for tumor growth.³⁸ However, [¹⁸F]FDG is susceptible to false positives, being prone to accumulate in infectious or inflammatory tissue, and false negatives, such as small-sized tumors and tumors with low glycolytic activity.^39,40 An alternative method to [¹⁸F]FDG imaging that has seen much interest lately is peptide-based PET imaging agents. Peptides have unique characteristics that make them well suited for imaging applications. Peptides can be selectively designed to bind target proteins, enzymes, or other biomolecules directly associated with tumor growth, which is an advantage that is not seen among other traditional forms of imaging.⁴¹⁻⁴³ This allows for quick binding and early detection of tumors, leading to fast and accurate diagnoses. Peptides also have the flexibility to utilize different chelators, radioisotopes, and radionuclides dependent on the specificity of the target.²⁰ An example of the advantageous nature of peptide-based PET imaging agents was demonstrated in a recent study of ∼130 patients with epithelial neoplasms.⁴⁴ [⁶⁸Ga] DOTATATE detected 95.1% of lesions, CT scans detected 45.3%, and other forms of CT/MRI only detected 30.9% of tumors.⁴⁴ These peptide-based PET imaging agents exhibit favorable characteristics such as high binding specificity and low toxicity, and although concerns about in vivo stability are valid, developments have been made to address these concerns. Many modifications to the peptide backbone have been explored that not only increase the stability of the peptide backbone against proteolytic cleavage but also sustain the high binding efficiency to the target biomolecules^41,45−47 (Figure 1).

Figure 1.

(Top) General application of a peptide-based molecular probe with an imaging or therapeutic radioisotope. These probes can be modified for many purposes. Shown is the schematic of a generic probe with each section color coded. (Bottom) For each section, two different modifications are shown to highlight the diversity of possible combinations.

In this review, new peptide-based PET imaging agents and their efficacy are explored, specifically radiometal-based imaging agents. Binding efficiencies as well as tumor-to-cell uptake ratios and percentage of detected tumors are compared with other traditional methods of imaging as well as explore other factors such as stability and half-life. Methods and techniques to increase the stability of the peptide backbone, such as using d-amino acids and PEGylation, as well as their effect on binding efficiency are explored. Finally, potential areas of improvement and growth are explored. We limited our scope to work from the last 3 years to maximize the current state of the field.

2. Peptide Screening Process

The process of identifying and developing a peptide candidate for either therapeutic or diagnostic purposes (or, at times, both) is not one that is always straightforward. Target tumor cells are selected based on unique or overexpressed receptors that can distinguish tumor cells from healthy cells. Peptide candidates can then be developed in silico, based on known structures with binding affinities to the overexpressed receptors or proteins, to confirm affinity and stability using programs such as Schrödinger or VMD (Virtual Molecular Dynamics).⁴⁸⁻⁵⁰ Peptide candidates can then be synthesized and tested in vitro and in vivo with a chelator bound using cell lines and animal models. Another screening method is using phage display technology, involving the use of bacteriophages, each with a unique peptide conformation, to rapidly screen and select peptides with favorable binding properties through iterative rounds of binding, washing, and amplification.⁵¹ A diverse peptide library can be made of synthetic or naturally derived peptides, each unique with mutations and alterations made to the peptide sequence to test for enhanced binding affinity to the target receptor or protein.

3. Chelators

An important aspect to explore when identifying potential peptide PET imaging agents is the chelator being used to bind to the radioisotope. Chelators are an important component of radiopharmaceuticals as they build a stable link between the radioisotope and the peptide, allowing the peptide to be tagged when using PET imaging.^52,53 Each radioisotope has its own physical properties that determine how suitable a chelator may be, properties such as half-life, size, and chemical stability.^52,54 In Table 1, chelators discussed in this review are listed as well as newer species that have not yet been widely implemented in peptide PET imaging; in addition, the radioisotope it binds and the peptides they are attached to are given. When developing a PET imaging agent, the type of chelator used must be considered, as different chelators have different properties best suited to different situations. These chelators are evaluated based on their thermodynamic stability, ability to form metal complexes, and the lack of interference the chelator has on the peptide’s ability to bind its target.

Table 1. Commonly used Chelators and Radioisotopes.

3.1. DOTA, NOTA, and Their Derivatives

DOTA and NOTA, as well as their derivatives, are well-known chelators that still see a large amount of use in the field of peptide PET imaging. DOTA, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid, and NOTA, 1,4,7-triazacyclononane-1,4,7-triacetic acid, can bind to a range of radioisotopes, making them favorable chelators for PET imaging (Table 1), and its derivatives bind strongly to ⁶⁸Ga, one of the more cost-effective radioisotopes, while NOTA and NOTA derivatives are more commonly used to bind ⁶⁴Cu and show higher in vitro cellular uptake in certain proteins.⁵⁵

3.2. DFO: N-[5-[[4-[5-[Acetyl(hydroxy)amino]pentylamino]-4-oxobutanoyl]-hydroxyamino]pentyl]-N′-(5-aminopentyl)-N′-hydroxybutanediamide

DFO is the most common chelator for ⁸⁹Zr, as it can strongly bind ⁸⁹Zr at room temperature with a high radiochemical yield.⁵⁶ Because ⁸⁹Zr has a much higher half-life (approximately 3 days) compared to other commonly used radioisotopes, DFO as a chelator is an important component of ⁸⁹Zr-peptide-based PET imaging agents. Different modifications to the DFO backbone, such as DFOSq, the squaramide ester of DFO, have been explored to yield more stable metal complexes.⁵⁷

3.3. THP: Tris(hydroxypyridinone)

Tris(hydroxypyridinone), or THP, has seen recent use in PET imaging. THP is a chelator that binds to ⁶⁸Ga and offers advantages over other ⁶⁸Ga chelators such as DOTA. The radiochemical synthesis of THP PET imaging agents is significantly faster and requires less demanding conditions and chemical processes compared to the DOTA equivalents. One study compares THP-TATE with DOTA-TATE, showing comparable specific activities (14.4 ± 0.8% ID g^–1 vs 11.5 ± 0.6% ID g^–1),⁵⁸ with high-quality images being produced. The simplicity of the labeling process provides faster radiosynthesis and formulation (<2 min).⁵⁸ THP-TATE and THP-PSMA are two examples of PET imaging agents currently being tested and compared to DOTA and NOTA equivalents, but there is still future potential for more THP-based peptide PET imaging agents to be developed.

3.4. HBED-CC: N,N′-Bis-[2-hydroxy-5-(carboxyethyl)benzyl]ethylenediamine-N,N′-diacetic Acid

HBED-CC is an acyclic, bifunctional compound used mainly for radiolabeling with ⁶⁸Ga. HBED-CC forms a very stable complex in vivo with ⁶⁸Ga and has a rapid and efficient labeling process at room temperature. One study compared the ⁶⁸Ga-HBED-CC-WL12 peptide PET imaging agent with other chelators, showing great image quality and advantages in evaluating the immune microenvironment, with excellent imaging quality in the first 20 min followed by continuous tumor uptake through 180 min.⁵⁹ HBED-CC chelator has begun to see more use in peptide PET imaging agents, although it is still expensive to synthesize, thus making it commercially limited. However, a new study has found a cost-effective method to synthesize HBED-CC derivatives with high purity and yield, allowing for more widescale use in peptide PET imaging agent development.⁶⁰

3.5. Versatile and Modifiable Diphosphine Chelators

A new study explores the use of two diphosphine chelators, 2,3-bis(diphenylphosphino)maleic anhydride (DP^Ph) and 2,3-bis(di-p-tolylphosphino)maleic anhydride (DP^Tol), with ⁶⁴Cu and ^99mTC as binding radioisotopes for both PET and SPECT imaging.⁶¹ Both DP^Ph and Dp^Tol readily bind to the primary amine groups of the RGD and PSMAt peptide, peptides commonly used in PET imaging. Both chelator–peptide complexes possess favorable properties for use as receptor-targeted imaging agents, exhibiting high stability in vitro as well as a radiochemical yield of above 95%. The phosphine substituents of both chelators can also be modified to increase chelator reactivity with the metal radioisotope as well as the radiotracer hydrophilicity for less off-target retention, introducing hydrophilic substituents that allow for clear rapid circulation.⁶¹ Faster circulation allows for less radioactivity in nontarget, healthy tissues, which contributes to a higher contrast image. Both DP^Ph and DP^Tol look to be promising chelator candidates for future peptide PET imaging use.

4. ⁶⁸Ga as a Radioisotope

⁶⁸Ga is often used as a radioisotope along with chelating agents such as DOTA and NOTA. It decays with a half-life of 67.6 min and a positron yield of 89.1%, allowing for a sufficient amount of time to undergo PET imaging and obtain usable results.⁶² Although the half-life is not as long as other radioisotopes, one advantage that ⁶⁸Ga has is availability. Unlike other radioisotopes, ⁶⁸Ga can be produced without a conventional cyclotron but rather by the use of a ⁶⁸Ge/⁶⁸Ga generator, which are commercially available.^62,63 Peptide-based PET imaging agents using ⁶⁸Ga as a radioisotope also report low to no toxicity to human patients.⁶⁴⁻⁶⁶

4.1. KDKPPR Peptide Coupled with Gallium-68 for Detection of Brain Metastases

Angiogenesis is a key mechanism in tumor growth.⁶⁷⁻⁶⁹ A key mediator of angiogenesis is the vascular endothelial growth factor (VEGF) and its receptors (VEGFR).^68,70,71 One potential target protein, neuropilin-1 (NRP-1), acts as a coreceptor to VEGF and is overexpressed in many cancers including brain⁷² and breast cancer.⁷³ Peptides with binding capabilities to NRP-1 have been synthesized for their use as PET imaging agents/therapies. One promising peptide sequence, KDKPPR, coupled with a bifunctional chelator for ⁶⁸Ga anchoring, NODAGA, showed promise as a possible imaging agent.⁷⁴ Binding assay data taken from surface plasmon resonance (SPR) experiments show a high affinity for NRP-1, and in vivo studies show that uptake in a brain model of metastasis in a nude rat was 7.6 times higher compared to the surrounding healthy brain. Standard uptake values (SUV) show a difference of 0.61 versus 0.08 SUV_mean for the tumor and healthy brain after 1 h of injection of [⁶⁸Ga]Ga-NODAGA-KDKPPR in rat subjects.⁷⁴ Using an endothelial cell line (HUVEC) and the MDA-MB-231 cancer line, no toxicity was observed from the KDKPPR peptide in the range of 0–10 μM 4 and 24 h after exposure.⁷⁴ [⁶⁸Ga]Ga-NODAGA-KDKPPR shows promising results both in vitro and in vivo.

4.2. NTS1R ⁶⁸Ga-PET Imaging Agent Targeting Overexpressed NTS Receptors in Tumor Cells

Neurotensin is a 13 amino acid peptide neurotransmitter in the central nervous system whose physiological activity is mediated by three receptors, NST receptor 1 (NST1R) along with NST2R and NST3R.⁷⁵ NST1R has been seen to be specifically expressed in certain types of tumors,⁷⁶ making it a promising target for PET imaging. In one study, different peptide sequences based on the 13 amino acid sequence of the neurotensin peptide were synthesized and compared. [⁶⁸Ga]UR-LS130 ([⁶⁸Ga]56) showed the highest in vivo stability and tumor to background ratio among other peptides tested and a high affinity for binding to NST1R.⁷⁷ Dynamic PET scans (PET scans registering tracer kinetics and data over time⁷⁸) also showed a clear tumor visualization in in vivo studies, confirming [⁶⁸Ga]56 as a potential PET imaging agent for NST1R. The [⁶⁸Ga]56 peptide (sequence of MRRPYIL) showed high stability estimated to be partly due to a β–β dimethylated tyrosine (Figure 2) that decreases the rate of enzymatic cleavage,⁷⁹ a modification not seen in the other peptide candidates. [⁶⁸Ga]56 also showed a tumor to background ratio of 16 ± 2.2% ID/g, the highest value among the other candidates.⁷⁷

Figure 2.

UR-LS130 peptide with an amino acid structure of MRRPYIL. The Tyr residue has a β–β demethylated R group which was found to have the highest in vivo stability and afforded a high tumor-to-muscle ratio.

4.3. [⁶⁸Ga]DOTA Peptides and [¹⁸F]FDG in the Evaluation of Neuroendocrine Neoplasms

Neuroendocrine neoplasms (NENs) are epithelial neoplasms that occur in various organs but more frequently in the endocrine and nervous systems. An increase in the prevalence of these tumors leaves these NENs as viable targets for diagnostic methods and imaging.⁸⁰ Most NENs express somatostatin receptors (SSTR) which are used as targets for radionuclide imaging.^44,81 Three [⁶⁸Ga]DOTA peptides, DOTA-TOC, DOTA-NOC, and DOTA-TATE, have been developed and recently used in clinical practice as imaging agents.^44,62 Each of these radiopeptides binds to various subtypes of SSTR, allowing these [⁶⁸Ga]DOTA peptides a wider capacity of imaging when compared to other conventional methods.

A study of 131 GEP-NENs patients shows that around 40% of the lesions detected by [⁶⁸Ga]DOTA-TATE PET/CT were missed on conventional imaging methods, including CT scans and MRI (Figure 3).⁴⁴ Along with the ability to bind with a range of SSTR subtypes, [⁶⁸Ga]DOTA peptides can also be used alongside [¹⁸F]FDG in a complementary role. Well-differentiated tumors tend to have a higher affinity toward [⁶⁸Ga]DOTA peptides with poorly differentiated tumors tending to have a higher affinity toward [¹⁸F]FDG than [⁶⁸Ga]DOTA peptides.^44,82 In well-differentiated tumors, SSTR expression is increased with a decrease in glycolytic activity and vice versa in poorly differentiated tumors,^44,83 allowing for the use of [⁶⁸Ga]DOTA peptide PET imaging along with [¹⁸F]FDG PET imaging for whole body characterization of tumor heterogeneity in GEP-NENs patients.

Figure 3.

PET/CT Imaging of NENs using ⁶⁸Ga-DOTA-NOC (A, B) and ¹⁸F-FDG (C). With the ⁶⁸Ga-DOTA-NOC imaging agent, we see three distinct areas of increased radiotracer uptake (indicated by the blue and white arrows in A and B, respectively) but no increased uptake of the ¹⁸F-FDG (C). Reprinted with permission from ref (43). Copyright 2003 Oxford Academic.

4.4. [⁶⁸Ga]GP12 Peptide PET Imaging Agent To Detect TIGIT Overexpression in Tumor Cells

An emerging target for peptide PET imaging agents is the T-cell immunoreceptor with Ig and ITIM domains or “TIGIT”. TIGIT has been found to be expressed on many types of T cells, including tumor-infiltrating T cells, regulatory T cells, and helper T cells, which are overexpressed in the tumor microenvironment.^84,85 Earlier studies utilized antibody-based TIGIT agents for PET imaging with ⁸⁹Zr and ⁶⁴Cu⁸⁶ however with mixed results due to the stability of the protein and the sustainability of the synthesis process. A 12 amino acid peptide [⁶⁸Ga]GP12 (GGYTGFWHRLNP) was synthesized with a NOTA group to allow for radiolabeling with ⁶⁸Ga. It showed a high affinity and specificity for TIGIT and demonstrated the capacity for PET imaging in both in vivo and ex vivo studies.⁸⁷ Compared to other peptide candidates, [⁶⁸Ga]GP12 showed the highest tumor uptake values (2.76 ± 0.68% ID/g) along with the highest tumor to background ratio of 12.94 ± 2.64 in in vivo studies.⁸⁷ In nonsmall lung cancer (NSCLC) patients, trial runs of the [⁶⁸Ga]GP12 peptide showed similar PET images of primary and metastatic lesions as those produced by [¹⁸F]FDG PET imaging.⁸⁷ The tumor uptake percentage of [⁶⁸Ga]GP12 (SUV_max = 4.82) was less than that of [¹⁸F]FDG (SUV_max = 9.45).⁸⁷

4.5. [⁶⁸Ga]DOTA-TBP-3 Peptide as a PET Imaging Agent To Detect TIGIT Overexpression in Tumor Cells

Another study uses [⁶⁸Ga]GP12 NOTA as a base to develop a simpler yet effective peptide imaging agent. The synthesis of a [⁶⁸Ga]DOTA-TBP-3 peptide was based on the d-peptide DTBP-3, which was found to effectively block TIGIT interaction with other ligands.⁸⁸ Western blot analysis showed a tumor uptake of 8.41 ± 1.22% in high TIGIT expressing cells and 2.56 ± 0.19% in low TIGIT expressing cells after 30 min.⁸⁹ [⁶⁸Ga]DOTA-TBP-3 was also found to noninvasively detect TIGIT expression in tumor bearing mice, showing a tumor to background ratio of 5.20 ± 0.16.⁸⁹ This peptide serves as a specific imaging agent for detecting TIGIT expression in vivo and has many promising inhibitory functions based on its tumor uptake and binding to the target protein.

4.6. ⁶⁸Ga Conclusion

⁶⁸Ga provides a reliable and cost-effective method to radiolabel peptides for use in PET imaging. Its availability through a ⁶⁸Ge/⁶⁸Ga generator and ability to bind multiple commonly used chelators provide researchers with easy access to radioisotopes for in vitro studies.⁹⁰ Multiple ⁶⁸Ga-based peptide PET imaging agents have recently passed FDA approval for use, such as [⁶⁸Ga]PSMA-11 and [⁶⁸Ga]Ga-DOTA-TOC.^62,91 New studies are also emerging that identify ⁶⁸Ga peptide PET imaging agents as cancer receptor antagonists, showing high specific binding to cancer receptors.^90,92,93 This could lead to the development of highly specific peptides with both imaging and therapeutic capabilities.

5. ⁶⁴Cu as a Radioisotope

⁶⁴Cu is often used as a radioisotope along with the chelators NOTA and DOTA, much like ⁶⁸Ga. Unlike ⁶⁸Ga, ⁶⁴Cu has a much longer half-life at around 12.7 h, although with a lower positron yield at around 18% (39% beta emission, 43% Auger electron emission).^94,95 Similarly, it is not as commercially available, as it must be synthesized via reactor-based and accelerator-based methods.⁹⁴ However, ⁶⁴Cu does have certain advantages, namely, a longer half-life, allowing for longer imaging as well as identifying tumor kinetics, and the potential to be a targeted radionuclide therapy due to the multiple decay modes.⁹⁴

5.1. CM-2 Peptide with DOTA ⁶⁴Cu for PET Imaging of CD133 Protein Expressed in Multiple Cancers

CD133 is a transmembrane protein recognized as a prominent marker for tumor cells because it is highly expressed in many human cancer types. Importantly, it is found in tumor progenitor cells known as cancer stem cells (CSCs),^96,97 which have been shown to drive tumor initiation and relapse, making them an excellent target for therapies.⁹⁸ CSCs maintain a high level of resistance to conventional therapies as well as detection methods due to their high adaptive abilities against harmful conditions⁹⁸ and heterogeneous nature among different tumor types.^96,99 The role of CD133 in tumor progression is not entirely known, yet its expression in many types of tumors makes it a promising biomarker for imaging and therapy. The peptide [⁶⁴Cu]CM-2 (sequence Ahx-CWRLRWHSPLKGM-OH, Figure 4) was synthesized with the chelator DOTA for ⁶⁴Cu labeling. This peptide, based on the structure of the CD133 binding peptide CM-1, includes a 6-aminohexanoic acid (Ahx) linker between the peptide N terminus and the DOTA for increased peptide stability,^96,100 a prevalent issue with the CM-1 peptide. [⁶⁴Cu]CM-2 retains the high binding specificity to the Cd133 protein seen with the CM-1 peptide while also increasing the stability (22.4% and 20.1% intact CM-1 vs 94.9% and 91.9% intact CM-2 after 1 and 2 h in mouse serum). Ex vivo studies confirm the PET imaging capabilities of the [⁶⁴Cu]CM-2 peptide, showing tumor uptakes of 3.77%, 4.23%, 6.89%, and 6.19% ID/g at 1, 2, 6, and 18 h.⁹⁶ Furthermore, there was a significant decrease in tumor uptake when running the [⁶⁴Cu]CM-2 peptide with excess antiCD133 antibody, suggesting a specificity toward CD133 uptake in the tumors. [⁶⁴Cu]CM-2 introduced very effective changes that boosted stability while retaining a high affinity for CD133 and has a half-life that can be used for effective PET imaging.

Figure 4.

Structure of CM-2 peptide. CM-2 contains a 6-aminohexanoic acid linker group between the DOTA chelator and the peptide backbone. This linker has shown an increase in in vivo stability when compared to non-Ahx CM-2 peptides.

5.2. [⁶⁴Cu]WL12 Peptides Targeting Programmed Death Ligand PDL1

Programmed death ligand 1 is expressed on tumor and inflammatory cells and is representative of immune checkpoint pathways negatively regulating T-cell function and inhibiting antitumor immunity.^101,102 PD-L1 has thus become a popular target for cancer therapy and PET imaging agents. Computational studies show similar interaction sites between PD-L1 monoclonal antibodies and a high-affinity PD-L1 binding peptide, WL12. WL12 (sequence AcYMeANPHLHypWSW(Me)MeNIe-MeNIe, Figure 5) was synthesized with a NOTA group to allow for ⁶⁴Cu binding.^103−105 In vivo studies show rapid uptake in PD-L1-positive tumors with tumor uptake values of 34.4 ± 3.1% and 24.2 ± 2.5% ID/g in the kidneys and liver. The tumor to background ratio for PD-L1-positive tumors was 25.6 ± 1.9, showing the consistent ability of [⁶⁴Cu]WL12 to provide clear PET images with high signal-to-noise ratios.¹⁰⁶ [⁶⁴Cu]WL12 identified variable and dynamic changes in PD-L1 expression and has a slightly lower binding affinity than PD-L1 antibodies, ensuring that [⁶⁴Cu]WL12 as a PET imaging agent does not interfere with the therapeutic efficacy of PD-L1 antibodies and therapies.^103,104

Figure 5.

Use of the [⁶⁴Cu]WL12 peptide to image tumor PD-L1 expression in vivo. This figure shows NSG mice with hPD-L1 (red arrow) and CHO tumors (blue arrow) being intravenously given 150 μCi of the [⁶⁴Cu]WL12 tracer. Images were taken 10, 30, 60, and 120 min after injection. Specific accumulation of [⁶⁴Cu]WL12 is shown. Reprinted with permission from ref (99). Copyright 2008 Springer Nature.

5.3. ⁶⁴Cu-Labeled Biterminally PEGylated Peptides Targeting Integrin α_vβ₆

Epithelial-specific integrin α_νβ₆ is upregulated in many cancers and is directly associated with tumor progression.¹⁰⁷ It is particularly upregulated during tumorigenesis and can serve as a prognostic marker for imaging and therapeutics.^107,108 Various peptides with specificity to integrin α_νβ₆ have been synthesized with one in particular garnering much attention as a possible PET imaging agent, A20FMDV2.^108,109 This 20 amino acid peptide (NAVPNLRGDLQVLAQKVART) showed preferential binding to integrin α_νβ₆ and was initially labeled with ¹⁸F. However, in vivo studies showed rapid levels of excretion as well as metabolic breakdown of the peptide.¹⁰⁸ Thus, the A20FMDV2 (Figure 6) peptide was synthesized with a DOTA and PCTA group for ⁶⁴Cu binding, allowing for an easier synthesis and longer half-life.¹¹⁰

Figure 6.

SARTATE peptide structure. [⁶⁴Cu]SARTATE utilizes the MeCOSar chelator for ⁶⁴Cu binding. The phenylalanine and tryptophan amino acid residues are in the d configuration, a technique to increase peptide stability. A S–S bridge is also seen between the two cystine residues.

The PCTA/DOTA [⁶⁴Cu]A20FMDV2 peptides were biterminally PEGylated, allowing for a more stable peptide and retaining a high affinity to integrin α_νβ₆ (PCTA 98.9% at 24 h in human serum and 62.4 ± 3.7 K_D to integrin α_νβ₆/DOTA 100% at 24 h in human serum and 60.4 ± 6.7 K_D to integrin α_νβ₆).¹⁰⁸ In vivo studies show that the tumor uptake of [⁶⁴Cu]Cu-PCTA-PEG28₂-A20FMDV2 (3.86 ± 0.58% ID/g) was higher than that of [⁶⁴Cu]Cu-DOTA-PEG28₂-A20FMDV2 (2.12 ± 0.83% ID/g) at 1 h.¹⁰⁸ Small animal PET imaging also shows good tumor uptake for both peptides with [⁶⁴Cu]Cu-DOTA-PEG28₂-A20FMDV2 showing a SUV_max of 0.44 ± 0.02 at 1 h and a steady increase to 0.69 ± 0.09 at 24 h compared to [⁶⁴Cu]Cu-PCTA-PEG28₂-A20FMDV2 showing a SUV_max of 0.46 ± 0.04 at 1 h and a steady increase to 0.74 ± 0.21 at 24 h.¹⁰⁸

Compared to the initial integrin α_νβ₆ binding peptide with the ¹⁸F group, both [⁶⁴Cu] A20FMDV2 peptides show better stability in human serum (from 72.5% to 98.9/100% after 1 h in human serum) and slightly better tumor uptake (from 2.64% to 2.12/3.86% ID/g).^108,111 [⁶⁴Cu]Cu-DOTA-PEG28₂-A20FMDV2 shows the most promising stability and tumor uptake data and has potential use as a PET imaging agent instead of the current ¹⁸F imaging agents.

5.4. [⁶⁴Cu]SARTATE PET Imaging for Neuroendocrine Tumors

Conventional [⁶⁸Ga]DOTA peptides (including DOTA-TATE) are commonly used for neuroendocrine neoplasia diagnosis and imaging.^44,112 The high sensitivity for tumor cells and the efficiency of PET scans have made this peptide the gold standard for PET imaging for neuroendocrine neoplasms (NENs).^112,113 However, one limitation that arises with the use of these [⁶⁸Ga]DOTA peptides is the half-life of the radiotracer. Since ⁶⁸Ga has a half-life of around 68 min, this does not give sufficient time to model the retention kinetics in individual patients, especially for cases that require significant clearance over time, such as the kidneys and liver.^114,115 This may also give rise to cases where lesions are not reported simply due to decay of the radiotracer.¹¹⁵ [⁶⁴Cu]SARTATE is an alternative peptide based on [⁶⁸Ga]DOTA-TATE that was recently proposed (Figure 7). It has the sequence Tyr³FCFWKTCT-OH with the chelator MeCOSar for ⁶⁴Cu binding.¹¹⁴ In vivo studies comparing the effectiveness of [⁶⁴Cu]SARTATE to the commonly used [⁶⁸Ga]DOTATATE show that [⁶⁴Cu]SARTATE was more effective at targeting lesions in patients with a mean SUV_max of 23.61 for [⁶⁸Ga]DOTATATE compared to a mean SUV_max of 33.32 for [⁶⁴Cu]SARTATE among the 10 patients in the study.¹¹⁴ [⁶⁴Cu]SARTATE also showed high tumor uptake values in organs that require a longer clearance over time (3.49% ID/g at 24 h in kidneys and 6.79% ID/g at 24 h in the liver).¹¹⁴ [⁶⁴Cu]SARTATE was well tolerated throughout the study and exhibited high lesion uptake and retention. Compared to conventional methods, [⁶⁴Cu]SARTATE showed comparable and/or superior lesion detection in all patients, more so in the liver. The longer half-life of ⁶⁴Cu also provides the opportunity to study the binding kinetics and retention at a longer period of time. The [⁶⁴Cu]SARTATE peptide was found to be a safe and effective imaging agent with high specificity and retention in tumor cells.^114,116

Figure 7.

l/d-Amino acid comparison.

5.5. ⁶⁴Cu Conclusion

⁶⁴Cu provides a different approach to PET imaging compared to other radioisotopes. The longer half-life and multiple forms of decay allow it to be used in many applications other than imaging, such as pharmacokinetics and theranostics.^95,117,118 Although ⁶⁴Cu production is more cumbersome than other radioisotopes, many ⁶⁴Cu peptide PET imaging agents are currently undergoing human studies^119,120 with the potential of FDA approval.

6. Other Metals as Radioisotopes

Other metals have seen use as radiotracers for peptide PET imaging. [⁸⁹Zr] exhibits a long half-life (78.4 h), and its positron emissions produce high-resolution PET images.⁴²⁸⁹Zr benefits from its long half-life when used in immuno-PET imaging, specifically tracking the target tumor over a longer course of time to characterize tumor heterogeneity, monitoring therapeutic response, and assessing possible disease recurrence.¹²¹ DFO is the most commonly used chelator; however, studies show that there is evidence of dissociation in vivo and free ⁸⁹Zr in bone.^42,57 Another metal that has seen promising results is ⁴⁴Sc. This radiotracer has a shorter half-life, at 3.97 h, but is more suited for targeting smaller molecules.¹²² Due to its nature, ⁴⁴Sc has a high-energy γ-radiation that is suitable for β + γ coincidence PET imaging, an extension of standard PET imaging that combines β and γ emissions for a more precise scan with a lower administered dose.¹²³

6.1. ⁸⁹Zr-Labeled Inhibitor of Prostate-Specific Membrane Antigen for PET Imaging in Prostate Cancer

Current applications of PET/CT imaging use ⁶⁸Ga- or ¹⁸F-specific prostate-specific membrane antigen (PSMA) ligands to target tumors localized in patients with biochemical recurrence of prostate cancer.^124,125 However, an estimated 20% of cases show undetected tumors using conventional methods for PSMA PET imaging¹²⁶ with an estimated 5–10% of primary prostate cancers not expressing PSMA and different tumors exhibiting different expressions of PSMA-positive cells.¹²⁷ To address this, a new PSMA binding peptide imaging agent was synthesized that exploits the longer half-life of Zr-89;¹²⁸⁸⁹Zr -PSMA-DFO has a tranexamic acid-A_DEuK backbone with N-succinimidedesferrioxamine (N-sucDf) for ⁸⁹Zr binding. Using in vivo data, the binding affinity of ⁸⁹Zr DFO was found to be comparable to conventional imaging agents in normal PSMA expressive tumors (⁸⁹Zr–Zr-PSMA-DFO at 4.97 ± 0.57 vs 5.15 ± 0.60 for conventional ⁶⁸Ga Ga-PSMA-11).¹²⁸ Tumor uptake experiments show comparable data between the two imaging agents with ⁸⁹Zr Zr-PSMA-DFO showing a tumor uptake value of 26.3 ± 5.3% ID/g after 2 h in tumor containing xenografts compared to 19.4 ± 2.4% ID/g after 2 h for ⁶⁸Ga Ga-PSMA-11.¹²⁸ Due to the long half-life of ⁸⁹Zr, tumor imaging remained visible over a period of 48 h. ⁸⁹Zr–Zr–PSMA-DFO showed no inferiority compared to other conventional antibody methods and can serve as a potential imaging agent and tool to run kinetics studies on PSMA tumor cells due to the favorable physical characteristics of ⁸⁹Zr.

6.2. [⁴⁴Sc]Sc-NODAGA-AMBA as a PET Probe for Preclinical Imaging of GRPR-Positive Prostate Cancer Cells

⁴⁴Sc is an attractive radioisotope with a higher half-life than ⁶⁸Ga (3.97 h vs 67.6 min) and a higher positron yield than ⁶⁴Cu (94.7% vs 18%).^122,129,130 Because of these decay characteristics, studies have been done to quantify the use of ⁴⁴Sc as a replacement for ⁶⁸Ga and ⁶⁴Cu. A recent study compared the [⁶⁸Ga]Ga-NODAGA-AMBA peptide with [⁴⁴Sc]Sc-NODAGA-AMBA.¹³⁰ These PET imaging peptides are based on the bombesin peptide (sequence of p-EQRLGNQWAVGHLM-NH₂¹³¹). This peptide exhibits high binding affinity to gastrin-releasing peptide receptors which are overexpressed in prostate cancer.^132,133 NODAGA was used to allow for both ⁴⁴Sc and ⁶⁸Ga binding. Ex vivo studies show a higher tumor uptake for [⁴⁴Sc]Sc-NODAGA-AMBA (4.56 ± 0.45% ID/g at 1 h) compared to [⁶⁸Ga]Ga-NODAGA-AMBA (3.78 ± 0.93% ID/g) as well as higher cellular uptake (5.65 ± 0.95 at 1 h vs 4.11 ± 0.79 at 1 h in receptor-positive PC-3 cells).¹³⁰ PET imaging studies show a SUV_mean value of 6.16 ± 1.24 for [⁴⁴Sc]Sc-NODAGA-AMBA vs a SUV_mean value of 5.50 ± 0.54 for [⁶⁸Ga]Ga-NODAGA-AMBA 1 h after injection.¹³⁰ [⁴⁴Sc]Sc-NODAGA-AMBA showed high binding affinity to GRPR-positive PC-3 prostate cancer cells, favorable radiotracer properties, and promising PET characteristics compared to conventional ⁶⁸Ga methods.

7. Methods To Enhance the Stability of the Peptide Backbone

One of the major challenges in developing peptide-based PET imaging agents is the stability of the peptide backbone in vivo. Peptides may exhibit a high binding efficiency to target cells/proteins, but they can be susceptible to proteases. To combat low in vivo stability, certain modifications to the peptide backbone and/or the amino acids have been explored. These can be used to increase the stability of peptide-based PET imaging agents. Enhancing the stability of the peptide allows for prolonged exposure to the target protein, ensuring a higher uptake rate. Having the peptide remain stable for as long as or longer than the half-life of the radioisotope can also result in clearer and more efficient PET scans and be more cost effective as the peptide degrading is no longer a limitation.

7.1. Stapled Peptides as Scaffolds for Peptide PET Imaging Agents

Stapled peptides are modified peptides with the aim of addressing the inherent challenges presented by peptide-based therapeutics and diagnostics. The development of stapled peptides involves introducing hydrocarbon bridges, or staples, across one or two helical turns of the peptide. Stapled peptides show an improvement in in vivo stability, cell permeability, and target binding affinity.¹³⁴

In recent years, there have been new studies exploring the use of stapled peptides as diagnostic tools. One study explores the development of a chromogranin A-derived peptide, peptide 5a, that binds to αvβ6 and αvβ8 with high affinity as well as exhibits inhibitory behavior of cell -mediated TGFβ activation, important in the inhibition of αvβ6-positive tumors.¹³⁵ Peptide 5a-NOTA showed high radiochemical purity, >95%, as well as a high specific activity and good stability after 4 h at room temperature. The study also showed the possible therapeutic potential of peptide 5a as tumor uptake of 5a-NOTA 2 h post injection was almost completely inhibited by prior administration of excess 5a.¹³⁵

7.2. Effect of l- to d-Amino acid Substitution on Stability and Activity of Antitumor peptide RDP215

One possible strategy to enhance the proteolytic stability of peptides is the substitution of l-amino acids with d-amino acids. l-Amino acids are commonly found in the human body and are the natural targets of enzymes and proteases, while d-amino acids are their enantiomeric counterparts which are present in trace amounts in humans¹³⁶ (Figure 8). Because the l-enantiomer makes up a significantly larger fraction found in nature, proteases are less likely to recognize the d-counterpart, making them highly resistant to degradation and cleavage.^137−139

Figure 8.

Cyclic RGD pentapeptide.

Two cationic peptides with tumor binding capabilities, RDP215 (sequence H-FWRIRIRR-P-RRIRIRWF-NH₂) and the d-amino acid variant 9D-RDP215 (sequence H-fWrIrIrr-P-rrIrIrWr-NH₂; lowercase represents d-amino acids), were developed and exposed to negatively charged phosphatidylserine (PS) cells, which mimic the cell membrane of certain cancer cells, e.g., melanoma and glioblastoma cells.^46,140 Both peptides were found to retain the specificity for cancer mimicking PS cells. However, the introduction of d-amino acids showed an increase in peptide stability in various ex vivo and in vitro studies.^141−143 Ex vivo studies show no degradation of 9D-RDP215 after a 7 day incubation in the presence of human serum, whereas RDP215 showed degradation after incubation for 1 day in the same conditions.⁴⁶ The d-peptide was found to remain effective and stable longer compared to the l-peptide variant. One drawback is the potential toxicity introduced with d-amino acids. 9D-RDP215 shows higher toxicity than RDP215 against non-neoplastic cells; however, 9D-RDP215 requires a much smaller amount to achieve a high killing efficacy compared with the l-amino acid variant.⁴⁶ The smaller amount needed mitigated the risk from the higher toxicity level of the 9D-RDP215 peptide.

7.3. Cyclic Peptides for Enhanced Stability and Cell Penetration

Cyclic peptides are formed by linking the N terminus with the C terminus of the peptide chain to create an amide bond between the amino and the carboxyl termini.¹⁴⁴ Cyclization of peptides is a possible route to enhance the stability and affinity for peptide agents as cyclic peptides exhibit favorable properties such as structural rigidity and receptor selectivity.^144−147 Along with the structural rigidity of a cyclic peptide, these peptides are not easily recognized by protease enzymes, due to the lack of termini, and are therefore less susceptible to cleavage in vivo.^146,148

Wu and Zhang use a cyclic peptide, [⁶⁴Cu]Cu-DOTA-E[c(RGDfK)₂] (sequence RGDfK, Figure 8), as a PET imaging agent for integrin α_vβ₃ receptors, synthesized with a DOTA group for ⁶⁴Cu binding. [⁶⁴Cu]Cu-DOTA-E[c(RGDfK)₂] showed high and specific tumor uptake in in vitro studies with a tumor uptake of 4.56 ± 0.51% ID/g after 24 h while also showing a high integrin α_vβ₃ affinity and long blood circulation time.^149,150

7.4. Bicyclic Peptides for Enhanced Stability and Target Retention

Bicyclic peptides are a class of polypeptides containing two macrocyclic rings with greater conformational rigidity and metabolic stability than linear and monocyclic peptides due to the increased rigidity and steric hindrance provided by the presence of two cyclic regions.¹⁵¹ One such bicyclic peptide, DOTA-BCY-C, was found to show impressive affinity to human and mouse MT1-MMP, a tumor-associated membrane type 1 matrix metalloproteinase, as well as target-dependent binding and internalization in cells with a dissociation constant (K_D) of 0.52 ± 0.24 nmol/L and 12% ID/g from in vivo imaging tests.¹⁵² Furthermore, BCY-C shows good proteolytic stability with no measurable degradation in mouse and human plasma over the course of 24 h.

7.5. Linkers To Enhance Stability

Linkers are molecules that separate multiple domains in proteins and peptides, space out two molecules to prohibit unwanted interactions, or increase resistance against proteases.^{41,96,153,154} One example of where linkers increase stability in peptides is the use of a 6-aminohexanoic acid (Ahx) linker, previously mentioned in the [⁶⁴Cu]CM-2 peptide. This modification increased the stability of the CM-2 peptide when compared to the unmodified CM peptide with 94.9% of the peptide still intact after 1 h of incubation in mouse serum as opposed to 22.4% of the unmodified peptide after 1 h of incubation.⁹⁶ Other linkers used to increase stability are polyethylene glycol (PEG)-based linkers, favored due to their low toxicity and effect on binding affinity.

8. Future Outlooks

Peptides have many favorable characteristics that make them attractive models for targeted therapies. High binding specificity to a wide variety of targets, a backbone that can be modified to increase stability and affinity, and the ability to attach chelators for PET imaging are some of the reasons peptide-based agents have seen an increase in development and research. Peptide-based PET imaging agents are beginning to gain FDA approval for use in detection of various cancers.^155,156 Because of the growing success of the field, new avenues to take advantage of peptide-based PET imaging agents are being explored.

8.1. Peptides as Theranostic Agents

Such avenues include the use of peptide-based molecules as theranostic agents and the use of image-guided therapy to enable both diagnosis and treatment.¹⁵⁷ Peptide-based molecules have already demonstrated their use as PET imaging agents, and the favorable characteristics listed above make them attractive models for lower risk therapies. The basis of peptide-based theranostics is to use the imaging component to identify and bind to a target and the therapy agent to treat the identified target with the goal being an enhanced therapy efficacy as well as lowering the risk of adverse effects due to the minimal toxicity of peptide agents.^157−159

Radioisotopes used for therapy differ than those used for diagnosis and imaging. Radioisotopes such as ⁶⁸Ga and ⁶⁴Cu are evaluated based on their ability to provide clear imaging over a certain period of time and being readily available for use. Radioisotopes used for therapy are evaluated based on, among other things, the decay method and type of emission released.¹⁶⁰ Radioisotopes used in therapy and being developed for theranostic use are primarily β-particle or α-particle emitters and Auger electrons.^160−162 Radioisotopes such as ¹⁷⁷Lu and ⁸⁶Y/⁹⁰Y decay through β emission and have the added benefit of binding to DOTA, a common chelator used in peptide PET imaging.¹⁶³

A classic example of theranostics is the use of both types of radioisotopes with an initial ⁶⁸Ga-labeled peptide-based agent for diagnosis followed by the same peptide-based agent labeled with ¹⁷⁷Lu or ⁸⁶Y/⁹⁰Y for peptide–receptor radionuclide therapy.¹⁶⁴

One peptide-based theranostic agent that has shown promise in recent clinical trials is [¹¹¹In]In-CP04, targeting the overexpressed cholecystokinin-2 receptors (CCK2R) on the membrane of medullary thyroid cancer (MTC) cells.^165−167 The CP04 peptide (sequence (e)₆AYGWMDF) is used as a PET imaging agent, showing high tumor uptake in in vitro studies (9.24 ± 1.35% ID/g)¹⁶⁸ and PET imaging capabilities,^165,169 but has recently been used in studies as a theranostic agent. A favorable biodistribution, tumor uptake, and low toxicity of CP04 peptide labeled with ¹⁷⁷Lu show a promising path to a potential theranostic agent.¹⁶⁵

8.2. Peptide–Receptor Radionuclide Therapy

Peptide–receptor radionuclide therapy (PRRT) is a targeted therapy for the treatment of neuroendocrine tumors through the use of radionuclide peptides selectively targeting the cancer cells.¹⁷⁰ The radioisotope of choice is usually lutetium-177 or yttrium-90 as the beta particles released by these radioisotopes causes DNA single-strand breaks, leading to cell death. However, actinium-255 also has been used in pilot studies to determine the effectiveness of alpha emitters, providing a targeted alpha therapy as a possible alternative to targeted beta therapy.

A previously mentioned peptide PET imaging agent, [⁶⁸Ga]DOTATATE, has been used as a theranostic agent for neuroendocrine tumors (NETs) with overexpressed somatostatin receptor subtype 2 (SSTR2). Lutathera, [¹⁷⁷Lu]DOTATATE, was recently approved by the FDA for treatment of SSTR-positive neuroendocrine tumors.¹⁷¹ The diagnostic characteristics of this peptide were explored earlier, showing a high affinity for binding to SSTR as well as a higher rate of lesion detection than conventional imaging techniques,⁴⁴ and because of the presence of a chelator that can also bind therapeutic radioisotopes, it has seen use as a theranostic agent. One study using [¹⁷⁷Lu]DOTATATE in 35 patients with NETs showed a radiological objective response rate of 9–35% (as complete or partial remission).^172,173 Current studies explore the effectiveness of using multiple therapeutic radioisotopes in combination, such as ¹⁷⁷Lu with ⁸⁶Y/⁹⁰Y-labeled DOTATATE or DOTATOC, and improvements to the peptide backbone for lower toxicity and higher stability.¹⁷⁴

[²⁵⁵Ac]Ac-DOTATATE has also seen use in research studies to identify the effectiveness of targeted alpha therapy. One study showed that targeted alpha therapies exhibit advantages over beta therapies, such as less dependence on tumor oxygen levels as well as the ability to break DNA double strands.¹⁷⁵ Another study showed a higher objective response rate for [²⁵⁵Ac]Ac-DOTATATE when compared to [¹⁷⁷Lu]DOTATATE (50% vs 25%).¹⁷⁶ Although these studies show positive results, there is a void in larger scale clinical studies and is a point for future research.

With a better understanding of the activity and function of peptides and the target tumor environment, there is potential for the increased development of theranostic peptide agents with high efficacy and minimal adverse side effects. Clinical studies using radioisotopes with therapeutic characteristics, while not yet at the desired effect, offer an optimistic outlook on the future of theranostic peptide agents, whether that be through using a combination of radioisotopes¹⁷⁷ or through developing agents for new targets.¹⁷⁸

9. Conclusion

In recent years, peptide PET imaging agents have seen a steady increase in FDA approval,^156,179,180 and significant strides have been made in the development of highly specific, stable, and nontoxic peptides for use in diagnosis (Table 2). Peptide therapeutics still suffer from issues such as proteolytic stability and high toxicity; however, research on applying known and newer methods to address these concerns has been highlighted in this review, including bicyclic peptides, stapled peptides, and other modifications. The peptides discussed in this review are just some of the many peptides developed in recent years. Different radioisotopes with different chelators along with different methods to increase stability, affinity, and lower toxicity are being developed for a wide variety of targets. Positive results in clinical trials also provide a positive outlook on the future of peptide PET imaging with many peptide-based molecules being developed as theranostic agents.

Table 2. Peptide PET Imaging Agents.

imaging agent	amino acid sequence	target protein
[68Ga]Ga-NODAGA-KDKPPR	KDKPPR	neuropilin-1 (NRP-1)
[68Ga]UR-LS130	MRRPYIL	neurotensin 1 receptor (NST1R)
[68Ga]DOTA-TOC	fCYwKTCT	somatostatin receptors (SSTR)
[68Ga]DOTA-NOC	fCFwKTCT	somatostatin receptors (SSTR)
[68Ga]DOTA-TATE	fCYwKTCT	somatostatin receptors (SSTR)
[68Ga]GP12	GGYTGFWHRLNP	T-cell immunoreceptors (TIGIT)
[64Cu]CM2	Ahx-CWRLRWHSPLKGM	CD133 transmembrane protein
[64Cu]WL12	cyc(AcYMeANPHLHypWSW(Me)MeNIe-MeNIe)	programmed death ligand 1 (PDL1)
[64Cu]PCTA/DOTA	NAVPNLRGDLQVLAQKVART	epithelial-specific integrin ανβ6
[64Cu]SARTATE	Tyr3FCFWKTCT	neuroendocrine neoplasms
[64Cu]DOTA-DPA	fhyqrdtpkstn	programmed death ligand 1 (PDL1)
[44Sc]Sc-NODAGA-AMBA	EQRLGNQWAVGHLM	gastrin-releasing peptide receptors

Glossary

Vocabulary

PET imaging

peptide positron emission tomography (PET) imaging is a molecular imaging technique wherein radiolabeled peptides or peptide–chelator complexes are bound to radioisotope to visualize and quantify biological processes at the molecular level

metal radioisotopes

metal radioisotopes are radioactive forms of metal atoms that decay and emit radiation. Usually for PET imaging the radiation type is beta plus decay wherein a proton is converted into a neutron, releasing a positron and electron neutrino. The interaction between the positron and an electron releases γ-rays, which are detected using scintillation cameras

peptide theranostics

peptide theranostics is the use of peptide-based molecules for both diagnostic and therapeutic purposes. Peptide theranostic agents are designed to bind to disease-associated targets and provide precise localization via imaging techniques such as PET as well as having therapeutic properties via mechanism inhibition or delivering therapeutic agents directly to the target

stapled peptides

stapled peptides are modified to address the inherent challenges presented by peptide-based therapeutics and diagnostics. The development of stapled peptides involves introducing hydrocarbon bridges, or staples, across one or two helical turns of the peptide

alpha therapy

alpha therapy is the use of use of alpha particles for therapeutic use. Alpha particles have a short wavelength but very high linear energy transfer, leading to high damage to nearby cells with minimal impact on surrounding tissues

Author Contributions

Research and review: M.S and L.S. Paper preparation and editing: M.S and L.S.

This work was supported by George Mason University and the state of Virginia.

The authors declare no competing financial interest.