Early Prediction of Tumor Response to Treatment: Preclinical Validation of 99mTc-Duramycin

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.

Preclinical PET Imaging of Tumor Cell Death following Therapy Using Gallium-68-Labeled C2Am

There is an unmet clinical need for imaging agents capable of detecting early evidence of tumor cell death, since the timing, extent, and distribution of cell death in tumors following treatment can give an indication of treatment outcome. We describe here 68Ga-labeled C2Am, which is a phosphatidylserine-binding protein, for imaging tumor cell death in vivo using positron emission tomography (PET). A one-pot synthesis of 68Ga-C2Am (20 min, 25 °C, >95% radiochemical purity) has been developed, using a NODAGA-maleimide chelator. The binding of 68Ga-C2Am to apoptotic and necrotic tumor cells was assessed in vitro using human breast and colorectal cancer cell lines, and in vivo, using dynamic PET measurements in mice implanted subcutaneously with the colorectal tumor cells and treated with a TRAIL-R2 agonist. 68Ga-C2Am showed predominantly renal clearance and low retention in the liver, spleen, small intestine, and bone and generated a tumor-to-muscle (T/m) ratio of 2.3 ± 0.4, at 2 h post probe administration and at 24 h following treatment. 68Ga-C2Am has the potential to be used in the clinic as a PET tracer for assessing early treatment response in tumors.

Development of Duramycin-Based Molecular Probes for Cell Death Imaging

Cell death is involved in numerous pathological conditions such as cardiovascular disorders, ischemic stroke and organ transplant rejection, and plays a critical role in the treatment of cancer. Cell death imaging can serve as a noninvasive means to detect the severity of tissue damage, monitor the progression of diseases, and evaluate the effectiveness of treatments, which help to provide prognostic information and guide the formulation of individualized treatment plans. The high abundance of phosphatidylethanolamine (PE), which is predominantly confined to the inner leaflet of the lipid bilayer membrane in healthy mammalian cells, becomes exposed on the cell surface in the early stages of apoptosis or accessible to the extracellular milieu when the cell suffers from necrosis, thus representing an attractive target for cell death imaging. Duramycin is a tetracyclic polypeptide that contains 19 amino acids and can bind to PE with excellent affinity and specificity. Additionally, this peptide has several favorable structural traits including relatively low molecular weight, stability to enzymatic hydrolysis, and ease of conjugation and labeling. All these highlight the potential of duramycin as a candidate ligand for developing PE-specific molecular probes. By far, a couple of duramycin-based molecular probes such as Tc-99 m-, F-18-, or Ga-68-labeled duramycin have been developed to target exposed PE for in vivo noninvasive imaging of cell death in different animal models. In this review article, we describe the state of the art with respect to in vivo imaging of cell death using duramycin-based molecular probes, as validated by immunohistopathology.

Interest and Limits of [18F]ML-10 PET Imaging for Early Detection of Response to Conventional Chemotherapy

One of the current challenges in oncology is to develop imaging tools to early detect the response to conventional chemotherapy and adjust treatment strategies when necessary. Several studies evaluating PET imaging with 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG) as a predictive tool of therapeutic response highlighted its insufficient specificity and sensitivity. The [¹⁸F]FDG uptake reflects only tumor metabolic activity and not treatment-induced cell death, which seems to be relevant for therapeutic evaluation. Therefore, to evaluate this parameter in vivo, several cell death radiotracers have been developed in the last years. However, few of them have reached the clinical trials. This systematic review focuses on the use of [¹⁸F]ML-10 (2-(5-[¹⁸F]fluoropentyl)-2-methylmalonic acid) as radiotracer of apoptosis and especially as a measure of tumor response to treatment. A comprehensive literature review concerning the preclinical and clinical investigations conducted with [¹⁸F]ML-10 was performed. The abilities and applications of this radiotracer as well as its clinical relevance and limitations were discussed. Most studies highlighted a good ability of the radiotracer to target apoptotic cells. However, the increase in apoptosis during treatment did not correlate with the radiotracer tumoral uptake, even using more advanced image analysis (voxel-based analysis). [¹⁸F]ML-10 PET imaging does not meet current clinical expectations for early detection of the therapeutic response to conventional chemotherapy. This review has pointed out the challenges of applying various apoptosis imaging strategies in clinical trials, the current methodologies available for image analysis and the future of molecular imaging to assess this therapeutic response.

Targeting Phosphatidylethanolamine with Fluorine-18 Labeled Small Molecule Probe for Apoptosis Imaging

PURPOSE

Externalization of phosphatidylethanolamine (PE) in dying cells makes the phospholipid an attractive target for apoptosis imaging. However, no ideal PE-targeted positron emission tomography (PET) radiotracer was developed. The goal of the study was to develop a novel PE-targeted radiopharmaceutical to imaging apoptosis.

PROCEDURE

In this study, we have radiolabeled PE-binding polypeptide duramycin with fluorine-18 for PET imaging of apoptosis. Al[¹⁸F]F-NOTA-PEG₃-duramycin was synthesized via chelation reaction of NOTA-PEG₃-duramycin with Al[¹⁸F]F. PE-binding capacity of Al[¹⁸F]F-NOTA-PEG₃-duramycin was determined in a competitive radiometric PE-binding assay. The pharmacokinetic profile was evaluated in Kunming mice. The apoptosis imaging capacity of Al[¹⁸F]F-NOTA-PEG₃-duramycin was evaluated using in vitro cell uptake assay with camptothecin-treated Jurkat cells, along with in vivo PET imaging using erlotinib-treated nude mice.

RESULTS

The total synthesis procedure lasted for 30 min, with a decay-uncorrected radiochemical yield of 21.3 ± 2.6 % (n = 10). Compared with the control cells, the binding of Al[¹⁸F]F-NOTA-PEG₃-duramycin with camptothecin-induced apoptotic cells resulted in a tripling increase. A competitive radiometric PE-binding assay strongly confirmed the binding of Al[¹⁸F]F-NOTA-PEG₃-duramycin to PE. The biodistribution study showed rapid blood clearance, prominent kidney retention, and low liver uptake. In the in vivo PET/CT imaging, Al[¹⁸F]F-NOTA-PEG₃-duramycin demonstrated 2-fold increase in erlotinib-treated HCC827 tumors in nude mice.

CONCLUSION

Considering the facile preparation and improved biological properties, Al[¹⁸F]F-NOTA-PEG₃-duramycin seems to be a promising PET tracer candidate for imaging apoptosis in the monitoring of cancer treatment.

Molecular Imaging of Apoptosis: The Case of Caspase-3 Radiotracers

The molecular imaging of apoptosis remains an important method for the diagnosis and monitoring of the progression of certain diseases and the evaluation of the efficacy of anticancer apoptosis-inducing therapies. Among the multiple biomarkers involved in apoptosis, activated caspase-3 is an attractive target, as it is the most abundant of the executioner caspases. Nuclear imaging is a good candidate, as it combines a high depth of tissue penetration and high sensitivity, features necessary to detect small changes in levels of apoptosis. However, designing a caspase-3 radiotracer comes with challenges, such as selectivity, cell permeability and transient caspase-3 activation. In this review, we discuss the different caspase-3 radiotracers for the imaging of apoptosis together with the challenges of the translation of various apoptosis-imaging strategies in clinical trials.

Molecular Imaging of Apoptosis in Atherosclerosis by Targeting Cell Membrane Phospholipid Asymmetry

BACKGROUND

Apoptosis in atherosclerotic lesions contributes to plaque vulnerability by lipid core enlargement and fibrous cap attenuation. Apoptosis is associated with exteriorization of phosphatidylserine (PS) and phosphatidylethanolamine (PE) on the cell membrane. Although PS-avid radiolabeled annexin-V has been employed for molecular imaging of high-risk plaques, PE-targeted imaging in atherosclerosis has not been studied.

OBJECTIVES

This study sought to evaluate the feasibility of molecular imaging with PE-avid radiolabeled duramycin in experimental atherosclerotic lesions in a rabbit model and compare duramycin targeting with radiolabeled annexin-V.

METHODS

Of the 27 rabbits, 21 were fed high-cholesterol, high-fat diet for 16 weeks. Nine of the 21 rabbits received ^99mTc-duramycin (test group), 6 received ^99mTc-linear duramycin (duramycin without PE-binding capability, negative radiotracer control group), and 6 received ^99mTc-annexin-V for radionuclide imaging. The remaining normal chow-fed 6 animals (disease control group) received ^99mTc-duramycin. In vivo microSPECT/microCT imaging was performed, and the aortas were explanted for ex vivo imaging and for histological characterization of atherosclerosis.

RESULTS

A significantly higher duramycin uptake was observed in the test group compared with that of disease control and negative radiotracer control animals; duramycin uptake was also significantly higher than the annexin-V uptake. Quantitative duramycin uptake, represented as the square root of percent injected dose per cm (√ID/cm) of abdominal aorta was >2-fold higher in atherosclerotic lesions in test group (0.08 ± 0.01%) than in comparable regions of disease control animals (0.039 ± 0.0061%, p = 3.70·10^-8). Mean annexin uptake (0.060 ± 0.010%) was significantly lower than duramycin (p = 0.001). Duramycin uptake corresponded to the lesion severity and macrophage burden. The radiation burden to the kidneys was substantially lower with duramycin (0.49% ID/g) than annexin (5.48% ID/g; p = 4.00·10^-4).

CONCLUSIONS

Radiolabeled duramycin localizes in lipid-rich areas with high concentration of apoptotic macrophages in the experimental atherosclerosis model. Duramycin uptake in atherosclerotic lesions was significantly greater than annexin-V uptake and produced significantly lower radiation burden to nontarget organs.

18F-C2Am: a targeted imaging agent for detecting tumor cell death in vivo using positron emission tomography

INTRODUCTION

Trialing novel cancer therapies in the clinic would benefit from imaging agents that can detect early evidence of treatment response. The timing, extent and distribution of cell death in tumors following treatment can give an indication of outcome. We describe here an 18F-labeled derivative of a phosphatidylserine-binding protein, the C2A domain of Synaptotagmin-I (C2Am), for imaging tumor cell death in vivo using PET.

METHODS

A one-pot, two-step automated synthesis of N-(5-[18F]fluoropentyl)maleimide (60 min synthesis time, > 98% radiochemical purity) has been developed, which was used to label the single cysteine residue in C2Am within 30 min at room temperature. Binding of 18F-C2Am to apoptotic and necrotic tumor cells was assessed in vitro, and also in vivo, by dynamic PET and biodistribution measurements in mice bearing human tumor xenografts treated with a TRAILR2 agonist or with conventional chemotherapy. C2Am detection of tumor cell death was validated by correlation of probe binding with histological markers of cell death in tumor sections obtained immediately after imaging.

RESULTS

18F-C2Am showed a favorable biodistribution profile, with predominantly renal clearance and minimal retention in spleen, liver, small intestine, bone and kidney, at 2 h following probe administration. 18F-C2Am generated tumor-to-muscle (T/m) ratios of 6.1 ± 2.1 and 10.7 ± 2.4 within 2 h of probe administration in colorectal and breast tumor models, respectively, following treatment with the TRAILR2 agonist. The levels of cell death (CC3 positivity) following treatment were 12.9-58.8% and 11.3-79.7% in the breast and colorectal xenografts, respectively. Overall, a 20% increase in CC3 positivity generated a one unit increase in the post/pre-treatment tumor contrast. Significant correlations were found between tracer uptake post-treatment, at 2 h post-probe administration, and histological markers of cell death (CC3: Pearson R = 0.733, P = 0.0005; TUNEL: Pearson R = 0.532, P = 0.023).

CONCLUSION

The rapid clearance of 18F-C2Am from the blood pool and low kidney retention allowed the spatial distribution of cell death in a tumor to be imaged during the course of therapy, providing a rapid assessment of tumor treatment response. 18F-C2Am has the potential to be used in the clinic to assess early treatment response in tumors.

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.

The evolving roles of radiolabeled quinones as small molecular probes in necrotic imaging

Necrosis plays vital roles in living organisms which is related closely with various diseases. Non-invasively necrotic imaging can be of great values in clinical decision-making, evaluation of individualized treatment responses, and prediction of patient prognosis. This narrative review will demonstrate how the evolution of quinones for necrotic imaging has been promoted by searching for their active centers. In this review, we summarized the recent developments of various quinones with the continuous simplified π-conjugated cores in necrotic imaging and speculated their possible molecular mechanisms might be attributed to their intercalations with exposed DNA in necrotic tissues. We discussed their clinical challenges of necrotic imaging with quinones and their future translation studies deserved to be explored in personalized patient treatment.

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.

Early prediction of tumor response after radiotherapy in combination with cetuximab in nasopharyngeal carcinoma using 99m Tc-duramycin imaging

PURPOSE

^99mTc-duramycin imaging enables specific visualization of cell death qualitatively and quantitatively. This study aimed to investigate the potential of ^99mTc-duramycin imaging in the early prediction of the curative effect of radiotherapy in combination with or without cetuximab in a nasopharyngeal carcinoma (NPC) model.

METHODS

Male BALB/c mice bearing NPC xenografts were randomized into four groups (six mice each group). Group 1 received radiotherapy (RT, 15 Gy/mouse) in combination with cetuximab (CTX, 2 mg/mouse), group 2 received RT (15 Gy/mouse), group 3 was treated using CTX (2 mg/mouse), and group 4, the control group, was treated using a vehicle. ^99mTc-duramycin imaging was performed before treatment and 24 h after treatment to evaluate tumor response. Tumor uptake of ^99mTc-duramycin was validated ex vivo using γ-counting. Treatment response was further validated by cleaved caspase-3 (CC3) and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling (TUNEL). Another four groups were treated parallelly under the same conditions to observe treatment response by tumor volume changes.

RESULTS

After 24 h treatment, ^99mTc-duramycin uptake in the NPC tumor models were significantly higher in group 1 than in group 2 (P < 0.05), group 3 (P < 0.05), or group 4 (P < 0.05); the uptake also increased notably in comparison with baseline values (P < 0.05). Compared with group 4, group 2 and group 3 both showed significant ^99mTc-duramycin uptake in the tumors (P < 0.05). Although the ^99mTc-duramycin uptake of group 2 was moderately higher than group 3, there were no significant differences between these two groups (P >0.05). There was a strong positive correlation between tumor ^99mTc-duramycin uptake and CC3 (r = 0.893, p < 0.0001) and TUNEL (r = 0.918, P < 0.0001). Tumor volume decreased remarkably in the RT in combination with CTX group on day 5, in the RT alone group on day 7, and was inhibited on day 8 in the CTX alone group, whereas the tumors grew continuously in the control group.

CONCLUSIONS

We demonstrated that RT in combination with CTX treatment significantly improved disease control in a NPC xenograft model compared with monotherapy with either. ^99mTc-duramycin imaging might be able to reliably identify response to RT in combination with CTX as early as 24 h after therapy initiation in NPC xenograft models. This might help to isolate non-responding patients in a timely manner and avoid unnecessary side effects in the clinic in the future.

Synthesis and Evaluation of Diindole-Based MRI Contrast Agent for In Vivo Visualization of Necrosis

PURPOSE

Noninvasive imaging of cell necrosis can provide an early evaluation of tumor response to treatments. Here, we aimed to design and synthesize a novel diindole-based magnetic resonance imaging (MRI) contrast agent (Gd-bis-DOTA-diindolylmethane, Gd-DIM) for assessment of tumor response to therapy at an early stage.

PROCEDURES

The oil-water partition coefficient (Log P) and relaxivity of Gd-DIM were determined in vitro. Then, its necrosis avidity was examined in necrotic cells in vitro and in rat models with microwave ablation-induced muscle necrosis (MAMN) and ischemia reperfusion-induced liver necrosis (IRLN) by MRI. Visualization of tumor necrosis induced by combretastatin A-4 disodium phosphate (CA4P) was evaluated in rats bearing W256 orthotopic liver tumor by MRI. Finally, DNA binding assay was performed to explore the possible necrosis-avidity mechanism of Gd-DIM.

RESULTS

The Log P value and T1 relaxivity of Gd-DIM is - 2.15 ± 0.01 and 6.61 mM^-1 s^-1, respectively. Gd-DIM showed predominant necrosis avidity in vitro and in vivo. Clear visualization of the tumor necrosis induced by CA4P was achieved at 60 min after administration of Gd-DIM. DNA binding study indicated that the necrosis-avidity mechanism of Gd-DIM may be due to its binding to exposed DNA in necrotic cells.

CONCLUSION

Gd-DIM may serve as a promising necrosis-avid MRI contrast agent for early assessment of tumor response to therapy.

SPECT/CT imaging of chemotherapy-induced tumor apoptosis using 99mTc-labeled dendrimer-entrapped gold nanoparticles

Non-invasive imaging of apoptosis in tumors induced by chemotherapy is of great value in the evaluation of therapeutic efficiency. In this study, we report the synthesis, characterization, and utilization of radionuclide technetium-99m (99mTc)-labeled dendrimer-entrapped gold nanoparticles (Au DENPs) for targeted SPECT/CT imaging of chemotherapy-induced tumor apoptosis. Generation five poly(amidoamine) (PAMAM) dendrimers (G5.NH2) were sequentially conjugated with 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA), polyethylene glycol (PEG) modified duramycin, PEG monomethyl ether, and fluorescein isothiocyanate (FI) to form the multifunctional dendrimers, which were then utilized as templates to entrap gold nanoparticles. Followed by acetylation of the remaining dendrimer surface amines and radiolabeling of 99mTc, the SPECT/CT dual mode nanoprobe of tumor apoptosis was constructed. The developed multifunctional Au DENPs before and after 99mTc radiolabeling were well characterized. The results demonstrate that the multifunctional Au DENPs display favorable colloidal stability under different conditions, own good cytocompatibility in the given concentration range, and can be effectively labeled by 99mTc with high radiochemical stability. Furthermore, the multifunctional nanoprobe enables the targeted SPECT/CT imaging of apoptotic cancer cells in vitro and tumor apoptosis after doxorubicin (DOX) treatment in the established subcutaneous tumor model in vivo. The designed duramycin-functionalized Au DENPs might have the potential to be employed as a nanoplatform for the detection of apoptosis and early tumor response to chemotherapy.

Evaluation of [18F]CP18 as a Substrate-Based Apoptosis Imaging Agent for the Assessment of Early Treatment Response in Oncology

PURPOSE

The substrate-based positron emission tomography (PET) tracer [¹⁸F]CP18 is capable of detecting the activity of caspase-3/7, two key executioner proteases in the apoptosis pathway, through selective cleavage of the ligand by the activated proteases and subsequent accumulation in apoptotic cells. Using an in vitro and in vivo model of colorectal cancer (CRC), we investigated whether [¹⁸F]CP18 tracer accumulation provides a measure for apoptosis and reliably reflects early treatment response to chemotherapeutics.

PROCEDURES

[¹⁸F]CP18 cell uptake was assessed in treated Colo205 cells (saline, 5-fluorouracil (5-FU), irinotecan or their combination) and correlated with caspase-3/7 activity. [¹⁸F]CP18 imaging was performed in Colo205 xenografts, starting with a baseline μPET/micro X-ray computed tomography (​μCT) scan, followed by a 3-day treatment with saline (n = 5), 5-FU (low sensitivity, n = 4), irinotecan (high sensitivity, n = 5), or a combination of both (n = 7). The study was concluded with a second [¹⁸F]CP18 scan, 24 h after final treatment administration, followed by tumor removal for gamma counting (%ID/g) and for cleaved caspase-3 immunohistochemistry (apoptotic index/necrosis). Tumors were delineated on μCT images and, using the obtained volumes of interest, average percentage injected dose per cubic centimeter (%ID/cm³) was calculated from every μPET image.

RESULTS

In vitro, [¹⁸F]CP18 cell uptake was positively correlated with caspase-3/7 activity (r = 0.59, p = 0.003). A drug-dependent increase in [¹⁸F]CP18 tumor uptake compared to baseline was observed in animals treated with 5-FU (+14 ± 25 %), irinotecan (+56 ± 54 %), and their combination (+158 ± 69 %, p = 0.002). %ID/cm³ showed a positive relationship with both %ID/g (r = 0.83, p < 0.0001) and the apoptotic index (r = 0.60, p = 0.004), but not with tumor necrosis (r = 0.22, p = 0.36).

CONCLUSION

Both our in vitro and in vivo findings have shown the ability of [¹⁸F]CP18-PET to visualize therapy-induced cancer cell apoptosis and possibly serve as a biomarker for early therapy response.

Avenues to molecular imaging of dying cells: Focus on cancer

Successful treatment of cancer patients requires balancing of the dose, timing, and type of therapeutic regimen. Detection of increased cell death may serve as a predictor of the eventual therapeutic success. Imaging of cell death may thus lead to early identification of treatment responders and nonresponders, and to “patient‐tailored therapy.” Cell death in organs and tissues of the human body can be visualized, using positron emission tomography or single‐photon emission computed tomography, although unsolved problems remain concerning target selection, tracer pharmacokinetics, target‐to‐nontarget ratio, and spatial and temporal resolution of the scans. Phosphatidylserine exposure by dying cells has been the most extensively studied imaging target. However, visualization of this process with radiolabeled Annexin A5 has not become routine in the clinical setting. Classification of death modes is no longer based only on cell morphology but also on biochemistry, and apoptosis is no longer found to be the preponderant mechanism of cell death after antitumor therapy, as was earlier believed. These conceptual changes have affected radiochemical efforts. Novel probes targeting changes in membrane permeability, cytoplasmic pH, mitochondrial membrane potential, or caspase activation have recently been explored. In this review, we discuss molecular changes in tumors which can be targeted to visualize cell death and we propose promising biomarkers for future exploration.

Preclinical Evaluation of Radioiodinated Hoechst 33258 for Early Prediction of Tumor Response to Treatment of Vascular-Disrupting Agents

This study aimed to explore the use of ¹³¹I-Hoechst 33258 (¹³¹I-H33258) for early prediction of tumor response to vascular-disrupting agents (VDAs) with combretastatin-A4 phosphate (CA4P) as a representative. Necrosis avidity of ¹³¹I-H33258 was evaluated in mouse models with muscle necrosis and blocking was used to confirm the tracer specificity. Therapy response was evaluated by ¹³¹I-H33258 SPECT/CT imaging 24 h after CA4P therapy in W256 tumor-bearing rats. Radiotracer uptake in tumors was validated ex vivo using γ-counting, autoradiography, and histopathological staining. Results showed that ¹³¹I-H33258 had predominant necrosis avidity and could specifically bind to necrotic tissue. SPECT/CT imaging demonstrated that an obvious “hot spot” could be observed in the CA4P-treated tumor. Ex vivo γ-counting revealed ¹³¹I-H33258 uptake in tumors was increased 2.8-fold in rats treated with CA4P relative to rats treated with vehicle. Autoradiography and corresponding H&E staining suggested that ¹³¹I-H33258 was mainly localized in necrotic tumor area and the higher overall uptake in the treated tumors was attributed to the increased necrosis. These results suggest that ¹³¹I-H33258 can be used to image induction of cell necrosis 24 h after CA4P therapy, which support further molecular design of probes based on scaffold H33258 for monitoring of tumor response to VDAs treatment.

Noninvasive Whole-Body Imaging of Phosphatidylethanolamine as a Cell Death Marker Using 99mTc-Duramycin During TNF-Induced SIRS

Systemic inflammatory response syndrome (SIRS) is an inflammatory state affecting the whole body. It is associated with the presence of pro- and antiinflammatory cytokines in serum, including tumor necrosis factor (TNF). TNF has multiple effects and leads to cytokine production, leukocyte infiltration, and blood pressure reduction and coagulation, thereby contributing to tissue damage and organ failure. A sterile mouse model of sepsis, TNF-induced SIRS, was used to visualize the temporal and spatial distribution of damage in susceptible tissues during SIRS. For this, a radiopharmaceutical agent, ^99mTc-duramycin, that binds to exposed phosphatidylethanolamine on dying cells was longitudinally visualized using SPECT/CT imaging. Methods: C57BL/6J mice were challenged with intravenous injections of murine TNF or vehicle, and necrostatin-1 was used to interfere with cell death. Two hours after vehicle or TNF treatment, mice received ^99mTc-duramycin intravenously (35.44 ± 3.80 MBq). Static whole-body ^99mTc-duramycin SPECT/CT imaging was performed 2, 4, and 6 h after tracer injection. Tracer uptake in different organs was quantified by volume-of-interest analysis using PMOD software and expressed as SUV_mean After the last scan, ex vivo biodistribution was performed to validate the SPECT imaging data. Lastly, terminal deoxynucleotidyl-transferase-mediated deoxyuridine triphosphate nick-end labeling (TUNEL) staining was performed to correlate the obtained results to cell death. Results: An increased ^99mTc-duramycin uptake was detected in mice injected with TNF, when compared with control mice, in lungs (0.55 ± 0.1 vs. 0.34 ± 0.05), intestine (0.75 ± 0.13 vs. 0.56 ± 0.1), and liver (1.03 ± 0.14 vs. 0.64 ± 0.04) 4 h after TNF and remained significantly elevated until 8 h after TNF. The imaging results were consistent with ex vivo γ-counting results. Significantly increased levels of tissue damage were detected via TUNEL staining in the lungs and intestine of mice injected with TNF. Interestingly, necrostatin-1 pretreatment conferred protection against lethal SIRS and reduced the ^99mTc-duramycin uptake in the lungs 8 h after TNF (SUV, 0.32 ± 0.1 vs. 0.51 ± 0.15). Conclusion: This study demonstrated that noninvasive ^99mTc-duramycin SPECT imaging can be used to characterize temporal and spatial kinetics of injury and cell death in susceptible tissues during TNF-induced SIRS, making it useful for global, whole-body assessment of tissue damage during diseases associated with inflammation and injury.

Mechanistic Understanding of Lanthipeptide Biosynthetic Enzymes

Lanthipeptides are ribosomally synthesized and post-translationally modified peptides (RiPPs) that display a wide variety of biological activities, from antimicrobial to antiallodynic. Lanthipeptides that display antimicrobial activity are called lantibiotics. The post-translational modification reactions of lanthipeptides include dehydration of Ser and Thr residues to dehydroalanine and dehydrobutyrine, a transformation that is carried out in three unique ways in different classes of lanthipeptides. In a cyclization process, Cys residues then attack the dehydrated residues to generate the lanthionine and methyllanthionine thioether cross-linked amino acids from which lanthipeptides derive their name. The resulting polycyclic peptides have constrained conformations that confer their biological activities. After installation of the characteristic thioether cross-links, tailoring enzymes introduce additional post-translational modifications that are unique to each lanthipeptide and that fine-tune their activities and/or stability. This review focuses on studies published over the past decade that have provided much insight into the mechanisms of the enzymes that carry out the post-translational modifications.