Preclinical Evaluation of 18F-ML-10 to Determine Timing of Apoptotic Response to Chemotherapy in Solid Tumors

The therapeutic effect of a novel GAPDH inhibitor in mouse model of breast cancer and efficacy monitoring by molecular imaging

BACKGROUND

Breast cancer is a serious threat to women's health with high morbidity and mortality. The development of more effective therapies for the treatment of breast cancer is strongly warranted. Growing evidence suggests that targeting glucose metabolism may be a promising cancer treatment strategy. We previously identified a new glyceraldehyde-3-phosphate dehydrogenase (GAPDH) inhibitor, DC-5163, which shows great potential in inhibiting tumor growth. Here, we evaluated the anticancer potential of DC-5163 in breast cancer cells.

METHODS

The effects of DC-5163 on breast cancer cells were investigated in vitro and in vivo. Seahorse, glucose uptake, lactate production, and cellular ATP content assays were performed to examine the impact of DC-5163 on cellular glycolysis. Cell viability, colony-forming ability, cell cycle, and apoptosis were assessed by CCK8 assay, colony formation assay, flow cytometry, and immunoblotting respectively. The anticancer activity of DC-5163 in vivo was evaluated in a mouse breast cancer xenograft model.

RESULTS

DC-5163 suppressed aerobic glycolysis and reduced energy supply of breast cancer cells, thereby inhibiting breast cancer cell growth, inducing cell cycle arrest in the G0/G1 phase, and increasing apoptosis. The therapeutic efficacy was assessed using a breast cancer xenograft mouse model. DC-5163 treatment markedly suppressed tumor growth in vivo without inducing evident systemic toxicity. Micro-PET/CT scans revealed a notable reduction in tumor 18F-FDG and 18F-FLT uptake in the DC-5163 treatment group compared to the DMSO control group.

CONCLUSIONS

Our results suggest that DC-5163 is a promising GAPDH inhibitor for suppressing breast cancer growth without obvious side effects. 18F-FDG and 18F-FLT PET/CT can noninvasively assess the levels of glycolysis and proliferation in tumors following treatment with DC-5163.

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.

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.

Apoptosis Imaging in Oncology by Means of Positron Emission Tomography: A Review

To date, a wide variety of potential PET-apoptosis imaging radiopharmaceuticals targeting apoptosis-induced cell membrane asymmetry and acidification, as well as caspase 3 activation (substrates and inhibitors) have been developed with the purpose of rapidly assessing the response to treatment in cancer patients. Many of these probes were shown to specifically bind to their apoptotic target in vitro and their uptake to be enhanced in the in vivo-xenografted tumours in mice treated by means of chemotherapy, however, to a significantly variable degree. This may, in part, relate to the tumour model used given the fact that different tumour cell lines bear a different sensitivity to a similar chemotherapeutic agent, to differences in the chemotherapeutic concentration and exposure time, as well as to the different timing of imaging performed post-treatment. The best validated cell membrane acidification and caspase 3 targeting radioligands, respectively ¹⁸F-ML-10 from the Aposense family and the radiolabelled caspase 3 substrate ¹⁸F-CP18, have also been injected in healthy individuals and shown to bear favourable dosimetric and safety characteristics. However, in contrast to, for instance, the ^99mTc-HYNIC-Annexin V, neither of both tracers was taken up to a significant degree by the bone marrow in the healthy individuals under study. Removal of white and red blood cells from the bone marrow through apoptosis plays a major role in the maintenance of hematopoietic cell homeostasis. The major apoptotic population in normal bone marrow are immature erythroblasts. While an accurate estimate of the number of immature erythroblasts undergoing apoptosis is not feasible due to their unknown clearance rate, their number is likely substantial given the ineffective quote of the erythropoietic process described in healthy subjects. Thus, the clinical value of both ¹⁸F-ML-10 and ¹⁸F-CP18 for apoptosis imaging in cancer patients, as suggested by a small number of subsequent clinical phase I/II trials in patients suffering from primary or secondary brain malignancies using ¹⁸F-ML-10 and in an ongoing trial in patients suffering from cancer of the ovaries using ¹⁸F-CP18, remains to be proven and warrants further investigation.

Novel 18F-Labeled Isonicotinamide-Based Radioligands for Positron Emission Tomography Imaging of Glycogen Synthase Kinase-3β

Glycogen synthase kinase-3β (GSK-3β), a cytoplasmic serine/threonine protein kinase, is involved in several human pathologies including Alzheimer's disease, bipolar disorder, diabetes, and cancer. Positron emission tomography (PET) imaging of GSK-3β could aid in investigating GSK-3β levels under normal and pathological conditions. In this study, we designed and synthesized fluorinated PET radioligands starting with recently identified isonicotinamide derivatives that showed potent affinity to GSK-3β. After extensive in vitro inhibitory activity assays and analyzing U87 cell uptake, we identified [¹⁸F]10a-d as potential tracers with good specificity and high affinity. They were then subjected to further in vivo evaluation in rodent brain comprising PET imaging and metabolism studies. The radioligands [¹⁸F]10b-d penetrated the blood-brain barrier and accumulated in GSK-3β-rich regions, including amygdala, cerebellum, and hippocampus. Also, it could be specifically blocked using the corresponding standard compounds. With these results, this work sets the basis for further development of novel ¹⁸F-labeled GSK-3β PET probes.

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.

[18F]ML-10 PET imaging fails to assess early response to neoadjuvant chemotherapy in a preclinical model of triple negative breast cancer

PURPOSE

Pathological complete response to the neoadjuvant therapy (NAT) for triple negative breast cancer (TNBC) is predictive of prolonged patient survival. Methods for early evaluation of NAT efficiency are still needed, in order to rapidly adjust the therapeutic strategy in case of initial non-response. One option for this is molecular imaging of apoptosis induced by chemotherapy. Therefore, we investigated the capacity of [18F]ML-10 PET imaging, an apoptosis radiotracer, to detect tumor cell apoptosis and early predict the therapeutic response of human TNBC.

RESULTS

Initially, the induction of apoptosis by different therapies was quantified. We confirmed, in vitro, that paclitaxel or epirubicin, the fundamental cytotoxic drugs for breast cancer, induce apoptosis in TNBC cell lines. Exposure of TNBC models MDA-MB-231 and MDA-MB-468 to these drugs induced a significant increase (p < 0.01) of the apoptotic hallmarks: DNA fragmentation, membrane phospholipid scrambling, and PARP activation. Secondarily, apoptotic fraction was compared to the intracellular accumulation of the radiotracer. [18F]ML-10 accumulated in the apoptotic cells after 72 h of treatment by paclitaxel in vitro; this accumulation positively correlated with the apoptotic fraction. In vivo, [18F]ML-10 was rapidly cleared from the nontarget organs and mainly eliminated by the kidneys. Comparison of the in vivo [18F]FDG, [18F]FMISO, and [18F]ML-10 uptakes revealed that the tumor accumulation of [18F]ML-10 was directly related to the tumor hypoxia level. Finally, after the in vivo treatment of TNBC murine xenografts by paclitaxel, apoptosis was well induced, as demonstrated by the cleaved caspase-3 levels; however, no significant increase of [18F]ML-10 accumulation in the tumors was observed, either on day 3 or day 6 after the end of the treatment.

CONCLUSIONS

These results highlighted that PET imaging using [18F]ML-10 allows the visualization of apoptotic cells in TNBC models. Nevertheless, the increase of the chemotherapy-induced apoptotic response when using paclitaxel could not be assessed using this radiotracer in our mouse model.

Circulating biomarkers of cell death

Numerous disease states are associated with cell death. For many decades, apoptosis and accidental necrosis have been assumed to be the two ways how a cell can die. The recent discovery of additional cell death processes such as necroptosis, ferroptosis or pyroptosis revealed a complex interplay between cell death mechanisms and diseases. Depending on the particular cell death pathway, cells secrete distinct molecular patterns, which differ between cell death types. This review focusses on released molecules, detectable in the blood flow, and their potential role as circulating biomarkers of cell death. We elucidate the molecular background of different biomarkers and give an overview on their correlation with disease stage, therapy response and prognosis in patients.

Novel 18F-Labeled Radioligands for Positron Emission Tomography Imaging of Myelination in the Central Nervous System

Myelin is the protective sheath that surrounds nerves in vertebrates to protect axons, which thereby facilitates impulse conduction. Damage to myelin is associated with many neurodegenerative diseases such as multiple sclerosis and also includes spinal cord injury (SCI). The small size of the spinal cord poses formidable challenges to in vivo monitoring of myelination, which we investigated via conducting a structure-activity relationship study to determine the optimum positron-emitting agent to use for imaging myelin using positron emission tomography (PET). From these studies, [¹⁸F]PENDAS was identified as the lead agent to use in conjunction with PET imaging to delineate the integrity of spinal cord myelin. A subsequent in vivo PET imaging study of [¹⁸F]PENDAS in rats with SCI showed promising pharmacokinetic results that justify further development of imaging markers for diagnosing myelin-related diseases. Additionally, [¹⁸F]PENDAS could be valuable in determining the efficacy of therapies that are currently under development.

Evaluation of apoptosis imaging biomarkers in a genetic model of cell death

PURPOSE

We have previously developed the caspase-based radiotracer, 18F-ICMT-11, for PET imaging to monitor treatment response. We further validated 18F-ICMT-11 specificity in a murine melanoma death-switch tumour model with conditional activation of caspase-3 induced by doxycycline.

METHODS

Caspase-3/7 activity and cellular uptake of 18F-ICMT-11, 18F-ML-10 and 18F-FDG were assessed in B16ova and B16ovaRevC3 cells after death-switch induction. Death-switch induction was confirmed in vivo in xenograft tumours, and 18F-ICMT-11 and 18F-ML-10 biodistribution was assessed by ex vivo gamma counting of select tissues. PET imaging was performed with 18F-ICMT-11, 18F-ML-10 and 18F-FDG. Caspase-3 activation was confirmed by immunohistochemistry.

RESULTS

Significantly increased caspase-3/7 activity was observed only in B16ovaRevC3 cells after death-switch induction, accompanied by significantly increased 18F-ICMT-11 (p < 0.001) and 18F-ML-10 (p < 0.05) and decreased 18F-FDG (p < 0.001) uptake compared with controls. B16ova and B16ovaRevC3 tumours had similar growth in vivo; however, B16ovaRevC3 growth was significantly reduced with death-switch induction (p < 0.01). Biodistribution studies showed significantly increased 18F-ICMT-11 tumour uptake following death-switch induction (p < 0.01), but not for 18F-ML-10. Tumour uptake of 18F-ICMT-11 was higher than that of 18F-ML-10 after death-switch induction. PET imaging studies showed that 18F-ICMT-11 can be used to detect apoptosis after death-switch induction, which was accompanied by significantly increased expression of cleaved caspase-3. 18F-FDG signal decreased in tumours after death-switch induction.

CONCLUSIONS

We demonstrate that 18F-ICMT-11 can be used to detect caspase-3 activation in a death-switch tumour model, independent of the confounding effects of cancer therapeutics, thus confirming its specificity and supporting the development of this radiotracer for clinical use to monitor tumour apoptosis and therapy response.