18FDG-labeled LIKKPF: a PET tracer for apoptosis imaging

Innovative Peptide Bioconjugation Chemistry with Radionuclides: Beyond Classical Click Chemistry

Background: The incorporation of radionuclides into peptides and larger biomolecules requires efficient and sometimes biorthogonal reaction conditions, to which click chemistry provides a convenient approach. Methods: Traditionally, click-based radiolabeling techniques have focused on classical click chemistry, such as copper(I)-catalyzed alkyne-azide [3+2] cycloaddition (CuAAC), strain-promoted azide-alkyne [3+2] cycloaddition (SPAAC), traceless Staudinger ligation, and inverse electron demand Diels-Alder (IEDDA). Results: However, newly emerging click-based radiolabeling techniques, including tyrosine-click, sulfo-click, sulfur(VI) fluoride exchange (SuFEx), thiol-ene click, azo coupling, hydrazone formations, oxime formations, and RIKEN click offer valuable alternatives to classical click chemistry. Conclusions: This review will discuss the applications of these techniques in peptide radiochemistry.

Synthetic 18F labeled biomolecules that are selective and promising for PET imaging: major advances and applications

The concept of positron emission tomography (PET) based imaging was developed more than 40 years ago. It has been a widely adopted technique for detecting and staging numerous diseases in clinical settings, particularly cancer, neuro- and cardio-diseases. Here, we reviewed the evolution of PET and its advantages over other imaging modalities in clinical settings. Primarily, this review discusses recent advances in the synthesis of 18F radiolabeled biomolecules in light of the widely accepted performance for effective PET. The discussion particularly emphasizes the 18F-labeling chemistry of carbohydrates, lipids, amino acids, oligonucleotides, peptides, and protein molecules, which have shown promise for PET imaging in recent decades. In addition, we have deliberated on how 18F-labeled biomolecules enable the detection of metabolic changes at the cellular level and the selective imaging of gross anatomical localization via PET imaging. In the end, the review discusses the future perspective of PET imaging to control disease in clinical settings. We firmly believe that collaborative multidisciplinary research will further widen the comprehensive applications of PET approaches in the clinical management of cancer and other pathological outcomes.

Imaging of cell death in malignancy: Targeting pathways or phenotypes?

Cell death is fundamental in health and disease and resisting cell death is a hallmark of cancer. Treatment of malignancy aims to cause cancer cell death, however current clinical imaging of treatment response does not specifically image cancer cell death but assesses this indirectly either by changes in tumor size (using x-ray computed tomography) or metabolic activity (using 2-[18F]fluoro-2-deoxy-glucose positron emission tomography). The ability to directly image tumor cell death soon after commencement of therapy would enable personalised response adapted approaches to cancer treatment that is presently not possible with current imaging, which is in many circumstances neither sufficiently accurate nor timely. Several cell death pathways have now been identified and characterised that present multiple potential targets for imaging cell death including externalisation of phosphatidylserine and phosphatidylethanolamine, caspase activation and La autoantigen redistribution. However, targeting one specific cell death pathway carries the risk of not detecting cell death by other pathways and it is now understood that cancer treatment induces cell death by different and sometimes multiple pathways. An alternative approach is targeting the cell death phenotype that is "agnostic" of the death pathway. Cell death phenotypes that have been targeted for cell death imaging include loss of plasma membrane integrity and dissipation of the mitochondrial membrane potential. Targeting the cell death phenotype may have the advantage of being a more sensitive and generalisable approach to cancer cell death imaging. This review describes and summarises the approaches and radiopharmaceuticals investigated for imaging cell death by targeting cell death pathways or cell death phenotype.

Recent advance in treatment of atherosclerosis: Key targets and plaque-positioned delivery strategies

Atherosclerosis, a chronic inflammatory disease of vascular wall, is a progressive pathophysiological process with lipids oxidation/depositing initiation and innate/adaptive immune responses. The coordination of multi systems covering oxidative stress, dysfunctional endothelium, diseased lipid uptake, cell apoptosis, thrombotic and pro-inflammatory responding as well as switched SMCs contributes to plaque growth. In this circumstance, inevitably, targeting these processes is considered to be effective for treating atherosclerosis. Arriving, retention and working of payload candidates mediated by targets in lesion direct ultimate therapeutic outcomes. Accumulating a series of scientific studies and clinical practice in the past decades, lesion homing delivery strategies including stent/balloon/nanoparticle-based transportation worked as the potent promotor to ensure a therapeutic effect. The objective of this review is to achieve a very brief summary about the effective therapeutic methods cooperating specifical targets and positioning-delivery strategies in atherosclerosis for better outcomes.

Radiolabeled Peptides for Molecular Imaging of Apoptosis

Apoptosis is a regulated cell death induced by extrinsic and intrinsic stimulants. Tracking of apoptosis provides an opportunity for the assessment of cardiovascular and neurodegenerative diseases as well as monitoring of cancer therapy at early stages. There are some key mediators in apoptosis cascade, which could be considered as specific targets for delivering imaging or therapeutic agents. The targeted radioisotope-based imaging agents are able to sensitively detect the physiological signal pathways which make them suitable for apoptosis imaging at a single-cell level. Radiopeptides take advantage of both the high sensitivity of nuclear imaging modalities and favorable features of peptide scaffolds. The aim of this study is to review the characteristics of those radiopeptides targeting apoptosis with different mechanisms.

Targeting phosphatidylserine for radionuclide-based molecular imaging of apoptosis

One major characteristic of programmed cell death (apoptosis) results in the increased expression of phosphatidylserine (PS) on the outer membrane of dying cells. Consequently, PS represents an excellent target for non-invasive imaging of apoptosis by single-photon emission computed tomography (SPECT) and positron emission tomography (PET). Annexin V is a 36 kDa protein which binds with high affinity to PS in the presence of Ca²⁺ ions. This makes radiolabeled annexins valuable apoptosis imaging agents for clinical and biomedical research applications for monitoring apoptosis in vivo. However, the use of radiolabeled annexin V for in vivo imaging of cell death has been met with a variety of challenges which have prevented its translation into the clinic. These difficulties include: complicated and time-consuming radiolabeling procedures, sub-optimal biodistribution, inadequate pharmacokinetics leading to poor tumour-to-blood contrast ratios, reliance upon Ca²⁺ concentrations in vivo, low tumor tissue penetration, and an incomplete understanding of what constitutes the best imaging protocol following induction of apoptosis. Therefore, new concepts and improved strategies for the development of PS-binding radiotracers are needed. Radiolabeled PS-binding peptides and various Zn(II) complexes as phosphate chemosensors offer an innovative strategy for radionuclide-based molecular imaging of apoptosis with PET and SPECT. Radiolabeled peptides and Zn(II) complexes provide several advantages over annexin V including better pharmacokinetics due to their smaller size, better availability, simpler synthesis and radiolabeling strategies as well as facilitated tissue penetration due to their smaller size and faster blood clearance profile allowing for optimized image contrast. In addition, peptides can be structurally modified to improve metabolic stability along with other pharmacokinetic and pharmacodynamic properties. The present review will summarize the current status of radiolabeled annexins, peptides and Zn(II) complexes developed as radiotracers for imaging apoptosis through targeting PS utilizing PET and SPECT imaging.

Imaging Cell Death: Focus on Early Evaluation of Tumor Response to Therapy

Cell death plays a prominent role in the treatment of cancer, because most anticancer therapies act by the induction of cell death including apoptosis, necrosis, and other pathways of cell death. Imaging cell death helps to identify treatment responders from nonresponders and thus enables patient-tailored therapy, which will increase the likelihood of treatment response and ultimately lead to improved patient survival. By taking advantage of molecular probes that specifically target the biomarkers/biochemical processes of cell death, cell death imaging can be successfully achieved. In recent years, with the increased understanding of the molecular mechanism of cell death, a variety of well-defined biomarkers/biochemical processes of cell death have been identified. By targeting these established cell death biomarkers/biochemical processes, a set of molecular imaging probes have been developed and evaluated for early monitoring treatment response in tumors. In this review, we mainly present the recent advances in identifying useful biomarkers/biochemical processes for both apoptosis and necrosis imaging and in developing molecular imaging probes targeting these biomarkers/biochemical processes, with a focus on their application in early evaluation of tumor response to therapy.

Synthesis and biological evaluation of RGD conjugated with Ketoprofen/Naproxen and radiolabeled with [99mTc] via N4(GGAG) for αVβ3 integrin-targeted drug delivery

BACKGROUND

Integrins are interesting targets in oncology. RGD sequence has high affinity for α_Vβ₃ integrin receptors. Diagnostic/therapeutic agents can be selectively delivered into cancer cells overexpressing α_Vβ₃ integrin by using RGD as a carrier. Nonsteroidal anti-inflammatory drugs (NSAIDs) have shown anticancer properties in in vitro and in vivo studies. The anti-cancer properties of NSAIDs occur though COX-2 inhibition. Regarding the anti-cancer properties of NSAIDs and overexpression of COX-2 enzyme in cancer cells, targeted delivery of NSAIDs into cancer cells to maximize their efficiency and minimize their side effects may gain increased clinical interest.

OBJECTIVES

In this study, RGD was conjugated to ketoprofen/Naproxen to selectively transfer these non-selective COX inhibitors into cancer cells.

METHODS

Keto/Nap-RGD-N4 peptides were synthesized based on solid phase fmoc peptide synthesis. Radiolabeling with [^99mTc] via N4 (GGAG) ligand was done for biological evaluation. Affinity and specificity of Keto/Nap-RGD-N4 to integrin was determined using A2780, OVCAR-3, SKOV-3 and HT-1080 cell lines. Percentage of Intenalization was measured in A2780 cells. Biodistriburion was studied in normal and tumor model mice.

RESULTS

Radiolabeled compounds showed high affinity to cells expressing α_Vβ₃ integrin in comparison to cells not expressing α_Vβ₃. The affinity to A2780 was significantly higher than OVCAR-3 cells. The %internalization into A2780 cells was quite low. Compounds showed more than 50% inhibition on A2780 and OVCAR-3 cells, less than 10% on MCF-7 and HT-1080 cells and no cytotoxicity on fibroblast cells after 48 h incubation. Although uptake of radiolabeled compounds in tumor was high at 1 h post-injection, the tumor/blood ratio was less than 1.5 which made SPECT imaging impossible.

CONCLUSION

Provided that NSAID drugs are conjugated to RGD, there will be a selective delivery to target tissues as well as synergetic anti-tumor effects which reduce systemic doses and toxicity. Graphical abstract.

Design, Synthesis, Radiolabeling, and Biologic Evaluation of Three 18F-FDG-Radiolabeled Targeting Peptides for the Imaging of Apoptosis

Background: Early detection of apoptosis is very important for therapy and follow-up treatment in various pathologic conditions. Annexin V interacts strongly and specifically with phosphatidylserine, specific biomarkers of apoptosis with some limitations. Small peptides are suitable alternatives to annexin V. A reliable and noninvasive in vivo technique for the detection of apoptosis is in great demand. Based on our previous studies, three new peptide analogs of LIKKPF (Leu-Ile-Lys-Lys-Pro-Phe) as apoptosis imaging agents were developed. Materials and Methods: Aoa-LIKKP-Cl-F, Aoe-LIKKP-Pyr-F, and Aoe-LIKKP-Nap-F were synthesized, functionalized with aminooxy, and radiolabeled with ¹⁸F-FDG. Their biologic properties were evaluated in vitro using apoptotic Jurkat cells. ¹⁸F-FDG-Aoe-LIKKP-Pyr-F peptide was injected into normal and apoptotic mice models for biodistribution and in vivo positron emission tomography/computed tomography imaging studies. Results: ¹⁸F-FDG-Aoe-LIKKP-Pyr-F peptide showed higher affinity for apoptotic cells. The localization of peptide in apoptotic liver mice was confirmed in biodistribution and imaging studies. Conclusion: The results showed that Aoe-LIKKP-Pyr-F peptide is an auspicious agent for molecular imaging of apoptosis.