Alpha Particle-Emitting Radiopharmaceuticals as Cancer Therapy: Biological Basis, Current Status, and Future Outlook for Therapeutics Discovery

Critical advances in radionuclide therapy have led to encouraging new options for cancer treatment through the pairing of clinically useful radiation-emitting radionuclides and innovative pharmaceutical discovery. Of the various subatomic particles used in therapeutic radiopharmaceuticals, alpha (α) particles show great promise owing to their relatively large size, delivered energy, finite pathlength, and resulting ionization density. This review discusses the therapeutic benefits of α-emitting radiopharmaceuticals and their pairing with appropriate diagnostics, resulting in innovative "theranostic" platforms. Herein, the current landscape of α particle-emitting radionuclides is described with an emphasis on their use in theranostic development for cancer treatment. Commonly studied radionuclides are introduced and recent efforts towards their production for research and clinical use are described. The growing popularity of these radionuclides is explained through summarizing the biological effects of α radiation on cancer cells, which include DNA damage, activation of discrete cell death programs, and downstream immune responses. Examples of efficient α-theranostic design are described with an emphasis on strategies that lead to cellular internalization and the targeting of proteins involved in therapeutic resistance. Historical barriers to the clinical deployment of α-theranostic radiopharmaceuticals are also discussed. Recent progress towards addressing these challenges is presented along with examples of incorporating α-particle therapy in pharmaceutical platforms that can be easily converted into diagnostic counterparts.

Simplified and highly-reliable automated production of [18F]FSPG for clinical studies

BACKGROUND

(S)-4-(3-¹⁸F-Fluoropropyl)-L-Glutamic Acid ([¹⁸F]FSPG) is a positron emission tomography (PET) tracer that specifically targets the cystine/glutamate antiporter (xc^-), which is frequently overexpressed in cancer and several neurological disorders. Pilot studies examining the dosimetry and biodistribution of [¹⁸F]FSPG in healthy volunteers and tumor detection in patients with non-small cell lung cancer, hepatocellular carcinoma, and brain tumors showed promising results. In particular, low background uptake in the brain, lung, liver, and bowel was observed that further leads to excellent imaging contrasts of [¹⁸F]FSPG PET. However, reliable production-scale cGMP-compliant automated procedures for [¹⁸F]FSPG production are still lacking to further increase the utility and clinical adoption of this radiotracer. Herein, we report the optimized automated approaches to produce [¹⁸F]FSPG through two commercially available radiosynthesizers capable of supporting centralized and large-scale production for clinical use.

RESULTS

Starting with activity levels of 60-85 GBq, the fully-automated process to produce [¹⁸F]FSPG took less than 45 min with average radiochemical yields of 22.56 ± 0.97% and 30.82 ± 1.60% (non-decay corrected) using TRACERlab™ FXFN and FASTlab™, respectively. The radiochemical purities were > 95% and the formulated [¹⁸F]FSPG solution was determined to be sterile and colorless with the pH of 6.5-7.5. No radiolysis of the product was observed up to 8 h after final batch formulation.

CONCLUSIONS

In summary, cGMP-compliant radiosyntheses and quality control of [¹⁸F]FSPG have been established on two commercially available synthesizers leveraging high activity concentration and radiochemical purity. While the clinical trials using [¹⁸F]FSPG PET are currently underway, the automated approaches reported herein will accelerate the clinical adoption of this radiotracer and warrant centralized and large-scale production of [¹⁸F]FSPG.

Simplified and Highly-reliable automated production of [18F]FSPG for clinical studies

Background (S)-4-(3- ¹⁸ F-Fluoropropyl)-L-Glutamic Acid ([ ¹⁸ F]FSPG) is a positron emission tomography (PET) tracer that specifically targets the cystine/glutamate antiporter (xc-), which is frequently overexpressed in cancer and several neurological disorders. Pilot studies examining the dosimetry and biodistribution of ([ ¹⁸ F]FSPG in healthy volunteers and tumor detection in patients with non-small cell lung cancer, hepatocellular carcinoma, and brain tumors showed promising results. In particular, low background uptake in the brain, lung, liver, and bowel was observed that further leads to excellent imaging contrasts of [ ¹⁸ F]FSPG PET. However, reliable production-scale cGMP-compliant automated procedures for [ ¹⁸ F]FSPG production are still lacking to further increase the utility and clinical adoption of this radiotracer. Herein, we report the optimized automated approaches to produce [ ¹⁸ F]FSPG through two commercially available radiosynthesizers capable of supporting centralized and large-scale production for clinical use. Results Starting with activity levels of 60-85 GBq, the fully-automated process to produce [ ¹⁸ F]FSPG took less than 45 minutes with average radiochemical yields of 22.56 ± 0.97% and 30.82 ± 1.60% (non-decay corrected) using TRACERlab™ FXFN and FASTlab™, respectively. The radiochemical purities were > 95% and the formulated [ ¹⁸ F]FSPG solution was determined to be sterile and colorless with the pH of 6.5-7.5. No radiolysis of the product was observed up to 8 hours after final batch formulation. Conclusions In summary, cGMP-compliant radiosyntheses and quality control of [ ¹⁸ F]FSPG have been established on two commercially available synthesizers leveraging high activity concentration and radiochemical purity. While the clinical trials using [ ¹⁸ F]FSPG PET are currently underway, the automated approaches reported herein will accelerate the clinical adoption of this radiotracer and warrant centralized and large-scale production of [ ¹⁸ F]FSPG.

Animal Models and Their Role in Imaging-Assisted Co-Clinical Trials

The availability of high-fidelity animal models for oncology research has grown enormously in recent years, enabling preclinical studies relevant to prevention, diagnosis, and treatment of cancer to be undertaken. This has led to increased opportunities to conduct co-clinical trials, which are studies on patients that are carried out parallel to or sequentially with animal models of cancer that mirror the biology of the patients' tumors. Patient-derived xenografts (PDX) and genetically engineered mouse models (GEMM) are considered to be the models that best represent human disease and have high translational value. Notably, one element of co-clinical trials that still needs significant optimization is quantitative imaging. The National Cancer Institute has organized a Co-Clinical Imaging Resource Program (CIRP) network to establish best practices for co-clinical imaging and to optimize translational quantitative imaging methodologies. This overview describes the ten co-clinical trials of investigators from eleven institutions who are currently supported by the CIRP initiative and are members of the Animal Models and Co-clinical Trials (AMCT) Working Group. Each team describes their corresponding clinical trial, type of cancer targeted, rationale for choice of animal models, therapy, and imaging modalities. The strengths and weaknesses of the co-clinical trial design and the challenges encountered are considered. The rich research resources generated by the members of the AMCT Working Group will benefit the broad research community and improve the quality and translational impact of imaging in co-clinical trials.

Feasibility of [18F]FSPG PET for Early Response Assessment to Combined Blockade of EGFR and Glutamine Metabolism in Wild-Type KRAS Colorectal Cancer

Early response assessment is critical for personalizing cancer therapy. Emerging therapeutic regimens with encouraging results in the wild-type (WT) KRAS colorectal cancer (CRC) setting include inhibitors of epidermal growth factor receptor (EGFR) and glutaminolysis. Towards predicting clinical outcome, this preclinical study evaluated non-invasive positron emission tomography (PET) with (4S)-4-(3-[18F]fluoropropyl)-L-glutamic acid ([18F]FSPG) in treatment-sensitive and treatment-resistant WT KRAS CRC patient-derived xenografts (PDXs). Tumor-bearing mice were imaged with [18F]FSPG PET before and one week following the initiation of treatment with either EGFR-targeted monoclonal antibody (mAb) therapy, glutaminase inhibitor therapy, or the combination. Imaging was correlated with tumor volume and histology. In PDX that responded to therapy, [18F]FSPG PET was significantly decreased from baseline at 1-week post-therapy, prior to changes in tumor volume. In contrast, [18F]FSPG PET was not decreased in non-responding PDX. These data suggest that [18F]FSPG PET may serve as an early metric of response to EGFR and glutaminase inhibition in the WT KRAS CRC setting.

18F-FSPG PET/CT Imaging of System xC- Transporter Activity in Patients with Primary and Metastatic Brain Tumors

Background The PET tracer (4S)-4-(3-[¹⁸F]fluoropropyl)-l-glutamate (¹⁸F-FSPG) targets the system x_C^- cotransporter, which is overexpressed in various tumors. Purpose To assess the role of ¹⁸F-FSPG PET/CT in intracranial malignancies. Materials and Methods Twenty-six patients (mean age, 54 years ± 12; 17 men; 48 total lesions) with primary brain tumors (n = 17) or brain metastases (n = 9) were enrolled in this prospective, single-center study (ClinicalTrials.gov identifier: NCT02370563) between November 2014 and March 2016. A 30-minute dynamic brain ¹⁸F-FSPG PET/CT scan and a static whole-body (WB) ¹⁸F-FSPG PET/CT scan at 60-75 minutes were acquired. Moreover, all participants underwent MRI, and four participants underwent fluorine 18 (¹⁸F) fluorodeoxyglucose (FDG) PET imaging. PET parameters and their relative changes were obtained for all lesions. Kinetic modeling was used to estimate the ¹⁸F-FSPG tumor rate constants using the dynamic and dynamic plus WB PET data. Imaging parameters were correlated to lesion outcomes, as determined with follow-up MRI and/or pathologic examination. The Mann-Whitney U test or Student t test was used for group mean comparisons. Receiver operating characteristic curve analysis was used for performance comparison of different decision measures. Results ¹⁸F-FSPG PET/CT helped identify all 48 brain lesions. The mean tumor-to-background ratio (TBR) on the whole-brain PET images at the WB time point was 26.6 ± 24.9 (range: 2.6-150.3). When ¹⁸F-FDG PET was performed, ¹⁸F-FSPG permitted visualization of non-¹⁸F-FDG-avid lesions or allowed better lesion differentiation from surrounding tissues. In participants with primary brain tumors, the predictive accuracy of the relative changes in influx rate constant K_i and maximum standardized uptake value to discriminate between poor and good lesion outcomes were 89% and 81%, respectively. There were significant differences in the ¹⁸F-FSPG uptake curves of lesions with good versus poor outcomes in the primary brain tumor group (P < .05) but not in the brain metastases group. Conclusion PET/CT imaging with (4S)-4-(3-[¹⁸F]fluoropropyl)-l-glutamate (¹⁸F-FSPG) helped detect primary brain tumors and brain metastases with a high tumor-to-background ratio. Relative changes in ¹⁸F-FSPG uptake with multi-time-point PET appear to be helpful in predicting lesion outcomes. Clinical trial registration no. NCT02370563 © RSNA, 2022 Online supplemental material is available for this article.

Co-Clinical Imaging Resource Program (CIRP): Bridging the Translational Divide to Advance Precision Medicine

The National Institutes of Health’s (National Cancer Institute) precision medicine initiative emphasizes the biological and molecular bases for cancer prevention and treatment. Importantly, it addresses the need for consistency in preclinical and clinical research. To overcome the translational gap in cancer treatment and prevention, the cancer research community has been transitioning toward using animal models that more fatefully recapitulate human tumor biology. There is a growing need to develop best practices in translational research, including imaging research, to better inform therapeutic choices and decision-making. Therefore, the National Cancer Institute has recently launched the Co-Clinical Imaging Research Resource Program (CIRP). Its overarching mission is to advance the practice of precision medicine by establishing consensus-based best practices for co-clinical imaging research by developing optimized state-of-the-art translational quantitative imaging methodologies to enable disease detection, risk stratification, and assessment/prediction of response to therapy. In this communication, we discuss our involvement in the CIRP, detailing key considerations including animal model selection, co-clinical study design, need for standardization of co-clinical instruments, and harmonization of preclinical and clinical quantitative imaging pipelines. An underlying emphasis in the program is to develop best practices toward reproducible, repeatable, and precise quantitative imaging biomarkers for use in translational cancer imaging and therapy. We will conclude with our thoughts on informatics needs to enable collaborative and open science research to advance precision medicine.

First-in-Human PET Imaging and Estimated Radiation Dosimetry of l-[5-11C]-Glutamine in Patients with Metastatic Colorectal Cancer

Altered metabolism is a hallmark of cancer. In addition to glucose, glutamine is an important nutrient for cellular growth and proliferation. Noninvasive imaging via PET may help facilitate precision treatment of cancer through patient selection and monitoring of treatment response. l-[5-¹¹C]-glutamine (¹¹C-glutamine) is a PET tracer designed to study glutamine uptake and metabolism. The aim of this first-in-human study was to evaluate the radiologic safety and biodistribution of ¹¹C-glutamine for oncologic PET imaging. Methods: Nine patients with confirmed metastatic colorectal cancer underwent PET/CT imaging. Patients received 337.97 ± 44.08 MBq of ¹¹C-glutamine. Dynamic PET acquisitions that were centered over the abdomen or thorax were initiated simultaneously with intravenous tracer administration. After the dynamic acquisition, a whole-body PET/CT scan was acquired. Volume-of-interest analyses were performed to obtain estimates of organ-based absorbed doses of radiation. Results: ¹¹C-glutamine was well tolerated in all patients, with no observed safety concerns. The organs with the highest radiation exposure included the bladder, pancreas, and liver. The estimated effective dose was 4.46E-03 ± 7.67E-04 mSv/MBq. Accumulation of ¹¹C-glutamine was elevated and visualized in lung, brain, bone, and liver metastases, suggesting utility for cancer imaging. Conclusion: PET using ¹¹C-glutamine appears safe for human use and allows noninvasive visualization of metastatic colon cancer lesions in multiple organs. Further studies are needed to elucidate its potential for other cancers and for monitoring response to treatment.

Nutrient partitioning in the tumor microenvironment

The tumor microenvironment (TME) includes cancer and infiltrating immune cells. Tumors canonically consume glucose through Warburg metabolism, a process forming the basis of cancer imaging by positron emission tomography (PET). Activated immune cells also rely on glucose, and impaired immune cell metabolism in the TME contributes to tumor progression. It remains uncertain, however, if immune cell metabolism is dysregulated in the TME by cell intrinsic programs or by competition with cancer cells for limiting nutrients. Here we used PET tracers to measure access and uptake of glucose and glutamine by specific cell subsets in the TME. Surprisingly, myeloid cells had the greatest capacity to uptake glucose in vivo, followed by T cells and cancer cells across a range of cancer models. Cancer cells, in contrast, demonstrated high glutamine uptake. This distinct nutrient partitioning was cell intrinsically programmed through mTORC1 signaling and glucose and glutamine-related gene expression. Inhibiting glutamine uptake enhanced glucose uptake across tumor resident cell types, suggesting that glutamine metabolism suppresses glucose uptake without glucose being limiting in the TME. Thus, cell intrinsic programs dictate the preferential immune and cancer cell acquisition of glucose and glutamine. Cell selective partitioning of these nutrients may be exploited to develop therapies and imaging strategies to enhance or monitor the metabolism and activities of specific cell populations in the TME.

Cell-programmed nutrient partitioning in the tumour microenvironment

Cancer cells characteristically consume glucose through Warburg metabolism1, a process that forms the basis of tumour imaging by positron emission tomography (PET). Tumour-infiltrating immune cells also rely on glucose, and impaired immune cell metabolism in the tumour microenvironment (TME) contributes to immune evasion by tumour cells2-4. However, whether the metabolism of immune cells is dysregulated in the TME by cell-intrinsic programs or by competition with cancer cells for limited nutrients remains unclear. Here we used PET tracers to measure the access to and uptake of glucose and glutamine by specific cell subsets in the TME. Notably, myeloid cells had the greatest capacity to take up intratumoral glucose, followed by T cells and cancer cells, across a range of cancer models. By contrast, cancer cells showed the highest uptake of glutamine. This distinct nutrient partitioning was programmed in a cell-intrinsic manner through mTORC1 signalling and the expression of genes related to the metabolism of glucose and glutamine. Inhibiting glutamine uptake enhanced glucose uptake across tumour-resident cell types, showing that glutamine metabolism suppresses glucose uptake without glucose being a limiting factor in the TME. Thus, cell-intrinsic programs drive the preferential acquisition of glucose and glutamine by immune and cancer cells, respectively. Cell-selective partitioning of these nutrients could be exploited to develop therapies and imaging strategies to enhance or monitor the metabolic programs and activities of specific cell populations in the TME.

Selective glutamine metabolism inhibition in tumor cells improves antitumor T lymphocyte activity in triple-negative breast cancer

Rapidly proliferating tumor and immune cells need metabolic programs that support energy and biomass production. The amino acid glutamine is consumed by effector T cells and glutamine-addicted triple-negative breast cancer (TNBC) cells, suggesting that a metabolic competition for glutamine may exist within the tumor microenvironment, potentially serving as a therapeutic intervention strategy. Here, we report that there is an inverse correlation between glutamine metabolic genes and markers of T cell-mediated cytotoxicity in human basal-like breast cancer (BLBC) patient data sets, with increased glutamine metabolism and decreased T cell cytotoxicity associated with poor survival. We found that tumor cell-specific loss of glutaminase (GLS), a key enzyme for glutamine metabolism, improved antitumor T cell activation in both a spontaneous mouse TNBC model and orthotopic grafts. The glutamine transporter inhibitor V-9302 selectively blocked glutamine uptake by TNBC cells but not CD8+ T cells, driving synthesis of glutathione, a major cellular antioxidant, to improve CD8+ T cell effector function. We propose a "glutamine steal" scenario, in which cancer cells deprive tumor-infiltrating lymphocytes of needed glutamine, thus impairing antitumor immune responses. Therefore, tumor-selective targeting of glutamine metabolism may be a promising therapeutic strategy in TNBC.

Combined blockade of EGFR and glutamine metabolism in preclinical models of colorectal cancer

Improving response to epidermal growth factor receptor (EGFR)-targeted therapies in patients with advanced wild-type (WT) RAS colorectal cancer (CRC) remains an unmet need. In this preclinical work, we evaluated a new therapeutic combination aimed at enhancing efficacy by targeting cancer cell metabolism in concert with EGFR. We hypothesized that combined blockade of glutamine metabolism and EGFR represents a promising treatment approach by targeting both the "fuel" and "signaling" components that these tumors need to survive. To explore this hypothesis, we combined CB-839, an inhibitor of glutaminase 1 (GLS1), the mitochondrial enzyme responsible for catalyzing conversion of glutamine to glutamate, with cetuximab, an EGFR-targeted monoclonal antibody in preclinical models of CRC. 2D and 3D in vitro assays were executed following treatment with either single agent or combination therapy. The combination of cetuximab with CB-839 resulted in reduced cell viability and demonstrated synergism in several cell lines. In vivo efficacy experiments were performed in cell-line xenograft models propagated in athymic nude mice. Tumor volumes were measured followed by immunohistochemical (IHC) analysis of proliferation (Ki67), mechanistic target of rapamycin (mTOR) signaling (pS6), and multiple mechanisms of cell death to annotate molecular determinants of response. In vivo, a significant reduction in tumor growth and reduced Ki67 and pS6 IHC staining were observed with combination therapy, which was accompanied by increased apoptosis and/or necrosis. The combination showed efficacy in cetuximab-sensitive as well as resistant models. In conclusion, this therapeutic combination represents a promising new precision medicine approach for patients with refractory metastatic WT RAS CRC.

TSPO-targeted PET and Optical Probes for the Detection and Localization of Premalignant and Malignant Pancreatic Lesions

PURPOSE

Pancreatic cancer is among the most aggressive malignancies and is rarely discovered early. However, pancreatic "incidentalomas," particularly cysts, are frequently identified in asymptomatic patients through anatomic imaging for unrelated causes. Accurate determination of the malignant potential of cystic lesions could lead to life-saving surgery or spare patients with indolent disease undue risk. Current risk assessment of pancreatic cysts requires invasive sampling, with attendant morbidity and sampling errors. Here, we sought to identify imaging biomarkers of high-risk pancreatic cancer precursor lesions.

EXPERIMENTAL DESIGN

Translocator protein (TSPO) expression, which is associated with cholesterol metabolism, was evaluated in premalignant and pancreatic cancer lesions from human and genetically engineered mouse (GEM) tissues. In vivo imaging was performed with [¹⁸F]V-1008, a TSPO-targeted PET agent, in two GEM models. For image-guided surgery (IGS), V-1520, a TSPO ligand for near-IR optical imaging based upon the V-1008 pharmacophore, was developed and evaluated.

RESULTS

TSPO was highly expressed in human and murine pancreatic cancer. Notably, TSPO expression was associated with high-grade, premalignant intraductal papillary mucinous neoplasms (IPMNs) and pancreatic intraepithelial neoplasia (PanIN) lesions. In GEM models, [¹⁸F]V-1008 exhibited robust uptake in early pancreatic cancer, detectable by PET. Furthermore, V-1520 localized to premalignant pancreatic lesions and advanced tumors enabling real-time IGS.

CONCLUSIONS

We anticipate that combined TSPO PET/IGS represents a translational approach for precision pancreatic cancer care through discrimination of high-risk indeterminate lesions and actionable surgery.

N-[18F]-Fluoroacetylcrizotinib: A potentially potent and selective PET tracer for molecular imaging of non-small cell lung cancer

N-[18F]fluoroacetylcrizotinib, a fluorine-18 labeled derivative of the first FDA approved tyrosine kinase inhibitor (TKI) for the treatment of Anaplastic lymphoma kinase (ALK)-rearranged non-small cell lung cancer (NSCLC), crizotinib, was successfully synthesized for use in positron emission tomography (PET). Sequential in vitro biological evaluation of fluoracetylcrizotinib and in vivo biodistribution studies of [18F]fluoroacetylcrizotinib demonstrated that the biological activity of the parent compound remained unchanged, with potent ALK kinase inhibition and effective tumor growth inhibition. These results show that [18F]fluoroacetylcrizotinib has the potential to be a promising PET ligand for use in NSCLC imaging. The utility of PET in this context provides a non-invasive, quantifiable method to inform on the pharmacokinetics of an ALK-inhibitor such as crizotinib prior to a clinical trial, as well as during a trial in the event of acquired drug resistance.

Abstract 4260: Molecular imaging of glutamine (Gln) metabolism in RAS wildtype (WT) metastatic colorectal cancer (mCRC)

Background: Gln metabolism plays a critical role in cancer. In CRC, the epidermal growth factor receptor (EGFR) and Gln cooperate to provide signal transduction and fuel for mitogen activated protein kinase-dependent cell growth. Our preclinical data illustrate that Gln abrogates EGFR inhibition and blockade of Gln metabolism restores EGFR sensitivity, forming the basis of a phase I/II clinical trial exploring dual EGFR and Gln metabolism in mCRC (NCT03263429). As a biomarker correlate, we are performing dual tracer PET imaging of Gln flux (11C-Gln) and Gln to glutamate conversion (18F-FSPG). We hypothesize that tumors dependent on Gln will exhibit high 11C-Gln uptake at baseline, while inhibition of Gln metabolism will reduce 18F-FSPG uptake. Notably, this is the first-in-human study with 11C-Gln.

Methods: Adult patients (pts) with previously treated anti-EGFR, RAS WT, mCRC are enrolling and receive panitumumab (6 mg/kg Day 1 and 15) and CB-839 (800 mg twice daily) in 28-day treatment cycles. 11C-Gln and 18F-FSPG scans are performed at baseline and Cycle 1 Day 28. Lesion to blood pool ratio (LBR) is calculated for each target lesion and compared with change in tumor size as measured by RECIST 1.1 after 2 cycles of therapy.

Results: The study is ongoing. To date, we have collected pre- and post-therapy PET data on 4 pts; individual lesion data for 2 pts are presented in the table. A mixed response with 11C-Gln uptake was observed for Pt #1, which could reflect different underlying lesion pathology; varying degree of lesion size change was also observed in this patient. The greatest decrease in 18F-FSPG uptake corresponded to the lesion with the smallest growth. For Pt #2, 11C-Gln uptake at baseline was highest in the lesion with the largest decrease in tumor size at Day 56; 18F-FSPG uptake was greatly reduced in all lesions, which corresponded to decreases in tumor size. Best overall response was progressive disease and partial response for Pt #1 and #2, respectively.

Gln PET as a function of lesion response11C-Gln18F-FSPG% Change in lesion size at Day 56Baseline LBRDay 28 LBR% ChangeBaseline LBRDay 28 LBR% ChangePatient#1L lung nodule+44%2.171.93-11.1%2.001.06-47.0%Posterior hepatic lobe+6%5.404.27-20.9%6.422.84-55.8%Segment IVA liver+107%5.114.88-4.5%5.583.90-30.1%Adrenal mass+152%3.504.4727.7%2.862.84-0.7%Patient#2R infrahilar pulmonary metastasis-15%2.471.74-29.6%3.131.72-45.0%L pulmonary metastasis-15%0.750.784.0%0.960.82-14.6%Hepatic Metastasis #1-100%8.976.81-24.1%7.003.50-50.0%

Conclusions: Quantitative biomarkers that predict response early in the course of therapy are a means to achieve the promise of precision medicine. Our preliminary clinical imaging data suggest that the employed investigational PET molecular imaging tracers have potential for prioritizing patients for combined EGFR/Gln targeted therapy and to predict response early in the treatment course. Additional PET data for all pts and summary statistics will be presented.

Citation Format: Jennifer Gray Whisenant, Gary Smith, Allison Cohen, Kristen K. Ciombor, Dana Cardin, Cathy Eng, Laura Goff, Satya Das, Tiffany Hickman, Anna Fisher, Adria Payne, Robert Coffey, G. Dan Ayers, Jordan D. Berlin, H. Charles Manning. Molecular imaging of glutamine (Gln) metabolism in RAS wildtype (WT) metastatic colorectal cancer (mCRC) [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4260.

Nutrient partitioning in the tumor microenvironment and FDG-PET imaging

The tumor microenvironment is composed of multiple cell types, including malignant cancer cells and tumor-infiltrating leukocytes (TIL). A possible mechanism of immunosuppression in the tumor microenvironment (TME) is cancer cells outcompeting anti-cancer TIL for nutrients such as glucose. 18F-deoxyglucose (FDG) positron emission tomography (PET) imaging is a staple of diagnosing and monitoring many types of cancer and is based on the Warburg model in which cancer cells within the tumor utilize glucose for growth and proliferation. Here, we use magnetic bead sorting to fractionate FDG-avid murine tumors and measure tumor cell-specific glucose uptake. We find that CD45+ immune cells are more FDG-avid than CD45− cancer cells. We further fractionate immune cell subsets into CD4/8+ T cells and CD11b+ myeloid cells to demonstrate that TIL T cells take up more glucose than CD45− cancer cells and resting splenic T cells. Strikingly, CD11b+ myeloid cells are the most FDG-avid cells in the TME. In MC38 colorectal cancer tumors, we show that CD11b+ F4/80 hi macrophages have high glucose uptake, and by extracellular flux analysis demonstrate higher metabolic activity than tumor T cells and CD45− cancer cells. Intriguingly, while glucose uptake is low in CD45− cancer cells, 18F-glutamine uptake is higher in cancer cells than immune cells. Our results illustrate a novel approach to measuring nutrient uptake in the TME and suggest that TIL are not starved of nutrients in the TME. Future work will determine the effects of immunotherapy and metabolism-targeted therapeutics on cell-specific glucose and glutamine uptake in the TME, as well as determine the immune contribution to cancer PET scans in the context of immunotherapy response.

Preclinical Evaluation of a Novel TSPO PET Ligand 2-(7-Butyl-2-(4-(2-[18F]Fluoroethoxy)phenyl)-5-Methylpyrazolo[1,5-a]Pyrimidin-3-yl)-N,N-Diethylacetamide (18F-VUIIS1018A) to Image Glioma

PURPOSE

There is an urgent need for the development of novel positron emission tomography (PET) tracers for glioma imaging. In this study, we developed a novel PET probe ([¹⁸F]VUIIS1018A) by targeting translocator protein (TSPO), an imaging biomarker for glioma. The purpose of this preclinical study was to evaluate this novel TSPO probe for glioma imaging.

PROCEDURES

In this study, we synthesized [¹⁹F]VUIIS1018A and the precursor for radiosynthesis of [¹⁸F]VUIIS1018A. TSPO binding affinity was confirmed using a radioligand competitive binding assay in C6 glioma cell lysate. Further, dynamic imaging studies were performed in rats using a microPET system. These studies include displacement and blocking studies for ligand reversibility and specificity evaluation, and compartment modeling of PET data for pharmacokinetic parameter measurement using metabolite-corrected arterial input functions and PMOD.

RESULTS

Compared to previously reported TSPO tracers including [¹⁸F]VUIIS1008 and [¹⁸F]DPA-714, the novel tracer [¹⁸F]VUIIS1018A demonstrated higher binding affinity and BP_ND. Pretreatment with the cold analog [¹⁹F]VUIIS1018A could partially block tumor accumulation of this novel tracer. Further, compartment modeling of this novel tracer also exhibited a greater tumor-to-background ratio, a higher tumor binding potential and a lower brain binding potential when compared with other TSPO probes, such as [¹⁸F]DPA-714 and [¹⁸F]VUIIS1008.

CONCLUSIONS

These studies illustrate that [¹⁸F]VUIIS1018A can serve as a promising TSPO PET tracer for glioma imaging and potentially imaging of other solid tumors.

Discovery of Furanone-Based Radiopharmaceuticals for Diagnostic Targeting of COX-1 in Ovarian Cancer

In vivo targeting and visualization of cyclooxygenase-1 (COX-1) using multimodal positron emission tomography/computed tomography imaging represents a unique opportunity for early detection and/or therapeutic evaluation of ovarian cancer because overexpression of COX-1 has been characterized as a pathologic hallmark of the initiation and progression of this disease. The furanone core is a common building block of many synthetic and natural products that exhibit a wide range of biological activities. We hypothesize that furanone-based COX-1 inhibitors can be designed as imaging agents for the early detection, delineation of tumor margin, and evaluation of treatment response of ovarian cancer. We report the discovery of 3-(4-fluorophenyl)-5,5-dimethyl-4-(p-tolyl)furan-2(5H)-one (FDF), a furanone-based novel COX-1-selective inhibitor that exhibits adequate in vivo stability, plasma half-life, and pharmacokinetic properties for use as an imaging agent. We describe a novel synthetic scheme in which a Lewis acid-catalyzed nucleophilic aromatic deiodo[¹⁸F]fluorination reaction is utilized for the radiosynthesis of [¹⁸F]FDF. [¹⁸F]FDF binds efficiently to COX-1 in vivo and enables sensitive detection of ovarian cancer in subcutaneous and peritoneal xenograft models in mice. These results provide the proof of principle for COX-1-targeted imaging of ovarian cancer and identify [¹⁸F]FDF as a promising lead compound for further preclinical and clinical development.

CB-839, panitumumab, and irinotecan in RAS wildtype (WT) metastatic colorectal cancer (mCRC): Phase I results

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Background: Epidermal growth factor receptor (EGFR) monoclonal antibodies (mAbs) are approved in RAS WT mCRC; however, patients (pts) will develop resistance to these agents. Alterations in glutamine (Gln) metabolism play a critical role in cancer cell growth. In cancers such as CRC, EGFR and Gln cooperate to provide signals and fuel for mitogen activated protein kinase-dependent cell growth. Our in vitro data show that Gln abrogates EGFR inhibition, and blockade of Gln transport restores sensitivity. We also observed a greater antitumor response in vivo with EGFR mAb plus CB-839, an inhibitor of a rate-limiting enzyme of Gln metabolism, than either agent alone. We designed a phase I/II study (NCT03263429) to evaluate CB-839 + panitumumab + irinotecan in anti-EGFR refractory RAS WT mCRC. Methods: Dose escalation used a Bayesian continual reassessment method targeting a 25% toxicity probability. CB-839 (600 mg or 800 mg twice daily [BID]) were evaluated with panitumumab (6 mg/kg) and irinotecan (180 mg/m2). Irinotecan was included in phase I to establish a future phase II dose of the triplet. Prior EGFR mAb treatment (tx) was not required for phase I. Dose-limiting toxicity (DLT) was any tx-related non-hematologic ≥Gr 3 toxicity (except fatigue, rash, or elevated liver enzymes) or ≥Gr 4 hematologic toxicity during the first 28 days. Results: Nine pts have been enrolled; 2 were not evaluable for DLT and replaced. Zero DLTs were observed at dose level 1 (n = 3) or 2 (n = 4); 2 more pts are needed to confirm the maximum tolerated dose (MTD). Most frequent toxicities were anemia and hypomagnesemia (88%) and elevated alkaline phosphatase, nausea, and rash (75%), most ≤Gr 2. One of 7 evaluable pts (14%) has an ongoing partial response, and 5 pts had stable disease (SD; 71%). Three pts have been on tx > 6 months, and 3 pts with prior EGFR mAb tx achieved SD. Conclusions: Triplet combination was tolerable at full doses of each drug, and preliminary antitumor activity was observed in a majority of pts. Phase II will begin after phase I completion and will evaluate efficacy of CB-839 (800 mg BID) and panitumumab (6 mg/kg). Imaging studies using investigational PET tracers to evaluate Gln metabolism as a function of tumor response are planned. Clinical trial information: NCT03263429.

Translocator Protein PET Imaging in a Preclinical Prostate Cancer Model

PURPOSE

The identification and targeting of biomarkers specific to prostate cancer (PCa) could improve its detection. Given the high expression of translocator protein (TSPO) in PCa, we investigated the use of [¹⁸F]VUIIS1008 (a novel TSPO-targeting radioligand) coupled with positron emission tomography (PET) to identify PCa in mice and to characterize their TSPO uptake.

PROCEDURES

Pten^pc-/-, Trp53^pc-/- prostate cancer-bearing mice (n = 9, 4-6 months old) were imaged in a 7T MRI scanner for lesion localization. Within 24 h, the mice were imaged using a microPET scanner for 60 min in dynamic mode following a retro-orbital injection of ~ 18 MBq [¹⁸F]VUIIS1008. Following imaging, tumors were harvested and stained with a TSPO antibody. Regions of interest (ROIs) were drawn around the tumor and muscle (hind limb) in the PET images. Time-activity curves (TACs) were recorded over the duration of the scan for each ROI. The mean activity concentrations between 40 and 60 min post radiotracer administration between tumor and muscle were compared.

RESULTS

Tumor presence was confirmed by visual inspection of the MR images. The uptake of [¹⁸F]VUIIS1008 in the tumors was significantly higher (p < 0.05) than that in the muscle, where the percent injected dose per unit volume for tumor was 7.1 ± 1.6 % ID/ml and that of muscle was < 1 % ID/ml. In addition, positive TSPO expression was observed in tumor tissue analysis.

CONCLUSIONS

The foregoing preliminary data suggest that TSPO may be a useful biomarker of PCa. Therefore, using TSPO-targeting PET ligands, such as [¹⁸F]VUIIS1008, may improve PCa detectability and characterization.

Ventral striatal dopamine transporter availability is associated with lower trait motor impulsivity in healthy adults

Impulsivity is a transdiagnostic feature of a range of externalizing psychiatric disorders. Preclinical work links reduced ventral striatal dopamine transporter (DAT) availability with heightened impulsivity and novelty seeking. However, there is a lack of human data investigating the relationship between DAT availability, particularly in subregions of the striatum, and the personality traits of impulsivity and novelty seeking. Here we collected PET measures of DAT availability (BP_ND) using the tracer ¹⁸F-FE-PE2I in 47 healthy adult subjects and examined relations between BP_ND in striatum, including its subregions: caudate, putamen, and ventral striatum (VS), and trait impulsivity (Barratt Impulsiveness Scale: BIS-11) and novelty seeking (Tridimensional Personality Questionnaire: TPQ-NS), controlling for age and sex. DAT BP_ND in each striatal subregion showed nominal negative associations with total BIS-11 but not TPQ-NS. At the subscale level, VS DAT BP_ND was significantly associated with BIS-11 motor impulsivity (e.g., taking actions without thinking) after correction for multiple comparisons. VS DAT BP_ND explained 13.2% of the variance in motor impulsivity. Our data demonstrate that DAT availability in VS is negatively related to impulsivity and suggest a particular influence of DAT regulation of dopamine signaling in VS on acting without deliberation (BIS motor impulsivity). While needing replication, these data converge with models of ventral striatal functions that emphasize its role as a key interface linking motivation to action.

Automated radiosynthesis of 5-[11C]l-glutamine, an important tracer for glutamine utilization

INTRODUCTION

The natural amino acid l-Glutamine (Gln) is essential for both cell growth and proliferation. In addition to glucose, cancer cells utilize Gln as a carbon source for ATP production, biosynthesis, and as a defense against reactive oxygen species. The utilization of [¹¹C]Gln has been previously reported as a biomarker for tissues with an elevated demand for Gln, however, the previous reports for the preparation of [¹¹C]Gln were found to be lacking several crucial aspects necessary for transition to human production. Namely, the presence of unreacted precursor and the use of non-commercialized, custom built, reaction platforms. Herein, we report the development and utilization of methodology for the automated production of [¹¹C]Gln that meets institutional criteria for human use.

METHODS

The preparation of [¹¹C]Gln was carried out on the GE FX₂N platform. Briefly, after trapping of [¹¹C]HCN with a solution of CsHCO₃ in DMF, the [¹¹C]CsCN was reacted with a commercially available precursor. This intermediate was then purified by HPLC and deprotected/hydrolyzed under acidic conditions. Following pH adjustment, the product was filtered to give the desired [¹¹C]Gln as a sterile injectable. The resulting product was then analyzed for quality assurance.

RESULTS

Automated production by this method reliably provides over 3.7 GBq (100 mCi) of [¹¹C]Gln. The resulting final drug product was found to have a >99% radiochemical purity, <5% of D-Gln present, no detectable impurities, and the total preparation time was roughly 45 min from the end-of-bombardment.

CONCLUSIONS

A fast, reliable and efficient automated radiosynthesis was developed using a commercially available module. Purifications used throughout allow for both a radiochemically and chemically pure final product solution of [¹¹C]Gln.

A simple microfluidic platform for rapid and efficient production of the radiotracer [18F]fallypride

Herein, we report the development of a simple, high-throughput and efficient microfluidic system for synthesizing radioactive [18F]fallypride, a PET imaging radiotracer widely used in medical research. The microfluidic chip contains all essential modules required for the synthesis and purification of radioactive fallypride. The radiochemical yield of the tracer is sufficient for multiple animal injections for preclinical imaging studies. To produce the on-chip concentration and purification columns, we employ a simple "trapping" mechanism by inserting rows of square pillars with predefined gaps near the outlet of microchannel. Microspheres with appropriate functionality are suspended in solution and loaded into the microchannels to form columns for radioactivity concentration and product purification. Instead of relying on complicated flow control elements (e.g., micromechanical valves requiring complex external pneumatic actuation), external valves are utilized to control transfer of the reagents between different modules. The on-chip ion exchange column can efficiently capture [18F]fluoride with negligible loss (∼98% trapping efficiency), and subsequently release a burst of concentrated [18F]fluoride to the reaction cavity. A thin layer of PDMS with a small hole in the center facilitates rapid and reliable water evaporation (with the aid of azeotropic distillation and nitrogen flow) while reducing fluoride loss. During the solvent exchange and fluorination reaction, the entire chip is uniformly heated to the desired temperature using a hot plate. All aspects of the [18F]fallypride synthesis were monitored by high-performance liquid chromatography (HPLC) analysis, resulting in labelling efficiency in fluorination reaction ranging from 67-87% (n = 5). Moreover, after isolating unreacted [18F]fluoride, remaining fallypride precursor, and various by-products via an on-chip purification column, the eluted [18F]fallypride is radiochemically pure and of a sufficient quantity to allow for PET imaging (∼5 mCi). Finally, a positron emission tomography (PET) image of a rat brain injected with ∼300 μCi [18F]fallypride produced by our microfluidic chip is provided, demonstrating the utility of the product produced by the microfluidic reactor. With a short synthesis time (∼60 min) and a highly integrated on-chip modular configuration that allows for concentration, reaction, and product purification, our microfluidic chip offers numerous exciting advantages with the potential for applications in radiochemical research and clinical production. Moreover, due to its simplicity and potential for automation, we anticipate it may be easily integrated into a clinical environment.

Evaluation of TSPO PET Ligands [18F]VUIIS1009A and [18F]VUIIS1009B: Tracers for Cancer Imaging

PURPOSE

Positron emission tomography (PET) ligands targeting translocator protein (TSPO) are potential imaging diagnostics of cancer. In this study, we report two novel, high-affinity TSPO PET ligands that are 5,7 regioisomers, [¹⁸F]VUIIS1009A ([¹⁸F]3A) and [¹⁸F]VUIIS1009B ([¹⁸F]3B), and their initial in vitro and in vivo evaluation in healthy mice and glioma-bearing rats.

PROCEDURES

VUIIS1009A/B was synthesized and confirmed by X-ray crystallography. Interactions between TSPO binding pocket and novel ligands were evaluated and compared with contemporary TSPO ligands using 2D ¹H-¹⁵N heteronuclear single quantum coherence (HSQC) spectroscopy. In vivo biodistribution of [¹⁸F]VUIIS1009A and [¹⁸F]VUIIS1009B was carried out in healthy mice with and without radioligand displacement. Dynamic PET imaging data were acquired simultaneously with [¹⁸F]VUIIS1009A/B injections in glioma-bearing rats, with binding reversibility and specificity evaluated by radioligand displacement. In vivo radiometabolite analysis was performed using radio-TLC, and quantitative analysis of PET data was performed using metabolite-corrected arterial input functions. Imaging was validated with histology and immunohistochemistry.

RESULTS

Both VUIIS1009A (3A) and VUIIS1009B (3B) were found to exhibit exceptional binding affinity to TSPO, with observed IC₅₀ values against PK11195 approximately 500-fold lower than DPA-714. However, HSQC NMR suggested that VUIIS1009A and VUIIS1009B share a common binding pocket within mammalian TSPO (mTSPO) as DPA-714 and to a lesser extent, PK11195. [¹⁸F]VUIIS1009A ([¹⁸F]3A) and [¹⁸F]VUIIS1009B ([¹⁸F]3B) exhibited similar biodistribution in healthy mice. In rats bearing C6 gliomas, both [¹⁸F]VUIIS1009A and [¹⁸F]VUIIS1009B exhibited greater binding potential (k ₃/k ₄)in tumor tissue compared to [¹⁸F]DPA-714. Interestingly, [¹⁸F]VUIIS1009B exhibited significantly greater tumor uptake (V _T) than [¹⁸F]VUIIS1009A, which was attributed primarily to greater plasma-to-tumor extraction efficiency.

CONCLUSIONS

The novel PET ligand [¹⁸F]VUIIS1009B exhibits promising characteristics for imaging glioma; its superiority over [¹⁸F]VUIIS1009A, a regioisomer, appears to be primarily due to improved plasma extraction efficiency. Continued evaluation of [¹⁸F]VUIIS1009B as a high-affinity TSPO PET ligand for precision medicine appears warranted.

Report from the SWOG Radiation Oncology Committee: Research Objectives Workshop 2017

The Radiation Therapy Committee of SWOG periodically evaluates its strategic plan in an effort to maintain a current and relevant scientific focus, and to provide a standard platform for future development of protocol concepts. Participants in the 2017 Strategic Planning Workshop included leaders in cancer basic sciences, molecular theragnostics, pharmaceutical and technology industries, clinical trial design, oncology practice, and statistical analysis. The committee discussed high-priority research areas, such as optimization of combined modality therapy, radiation oncology-specific drug design, identification of molecular profiles predictive of radiation-induced local or distant tumor responses, and methods for normal tissue-specific mitigation of radiation toxicity. The following concepts emerged as dominant questions ready for national testing: (i) what is the role of radiotherapy in the treatment of oligometastatic, oligorecurrent, and oligoprogressive disease? (ii) How can combined modality therapy be used to enhance systemic and local response? (iii) Can we validate and optimize liquid biopsy and other biomarkers (such as novel imaging) to supplement current response criteria to guide therapy and clinical trial design endpoints? (iv) How can we overcome deficiencies of randomized survival endpoint trials in an era of increasing molecular stratification factors? And (v) how can we mitigate treatment-related side effects and maximize quality of life in cancer survivors? The committee concluded that many aspects of these questions are ready for clinical evaluation and example protocol concepts are provided that could improve rates of cancer cure and quality of survival. Clin Cancer Res; 24(15); 3500-9. ©2018 AACR.

Activation of GRP/GRP-R signaling contributes to castration-resistant prostate cancer progression

Numerous studies indicate that androgen receptor splice variants (ARVs) play a critical role in the development of castration-resistant prostate cancer (CRPC), including the resistance to the new generation of inhibitors of androgen receptor (AR) action. Previously, we demonstrated that activation of NF-κB signaling increases ARVs expression in prostate cancer (PC) cells, thereby promoting progression to CRPC. However, it is unclear how NF-κB signaling is activated in CRPC. In this study, we report that long-term treatment with anti-androgens increases a neuroendocrine (NE) hormone - gastrin-releasing peptide (GRP) and its receptor (GRP-R) expression in PC cells. In addition, activation of GRP/GRP-R signaling increases ARVs expression through activating NF-κB signaling. This results in an androgen-dependent tumor progressing to a castrate resistant tumor. The knock-down of AR-V7 restores sensitivity to antiandrogens of PC cells over-expressing the GRP/GRP-R signaling pathway. These findings strongly indicate that the axis of Androgen-Deprivation Therapy (ADT) induces GRP/GRP-R activity, activation NF-κB and increased levels of AR-V7 expression resulting in progression to CRPC. Both prostate adenocarcinoma and small cell NE prostate cancer express GRP-R. Since the GRP-R is clinically targetable by analogue-based approach, this provides a novel therapeutic approach to treat advanced CRPC.

Evaluation of the novel TSPO radiotracer 2-(7-butyl-2-(4-(2-([18F]fluoroethoxy)phenyl)-5-methylpyrazolo[1,5-a]pyrimidin-3-yl)-N,N-diethylacetamide in a preclinical model of neuroinflammation

Translocator Protein (18 kDa, TSPO) is regarded as a useful biomarker for neuroinflammation imaging. TSPO PET imaging could be used to understand the role of neuroinflammation in brain diseases and as a tool for evaluating novel therapeutic effects. As a promising TSPO probe, [¹⁸F]DPA-714 is highly specific and offers reliable quantification of TSPO in vivo. In this study, we further radiosynthesized and evaluated another novel TSPO probe, 2-(7-butyl-2-(4-(2-[¹⁸F]fluoroethoxy)phenyl)-5-methylpyrazolo[1,5-a]pyrimidin-3-yl)-N,N-diethylacetamide ([¹⁸F]VUIIS1018A), which features a 700-fold higher binding affinity for TSPO than that of [¹⁸F]DPA-714. We evaluated the performance of [¹⁸F]VUIIS1018A using dynamic in vivo PET imaging, radiometabolite analysis, in vitro autoradiography assays, biodistribution analysis, and blocking assays. In vivo study using this probe demonstrated high signal-to-noise ratio, binding potential (BP_ND), and binding specificity in preclinical neuroinflammation studies. Taken together, these findings indicate that [¹⁸F]VUIIS1018A may serve as a novel TSPO PET probe for neuroinflammation imaging.

Pharmacological blockade of ASCT2-dependent glutamine transport leads to antitumor efficacy in preclinical models

The unique metabolic demands of cancer cells underscore potentially fruitful opportunities for drug discovery in the era of precision medicine. However, therapeutic targeting of cancer metabolism has led to surprisingly few new drugs to date. The neutral amino acid glutamine serves as a key intermediate in numerous metabolic processes leveraged by cancer cells, including biosynthesis, cell signaling, and oxidative protection. Herein we report the preclinical development of V-9302, a competitive small molecule antagonist of transmembrane glutamine flux that selectively and potently targets the amino acid transporter ASCT2. Pharmacological blockade of ASCT2 with V-9302 resulted in attenuated cancer cell growth and proliferation, increased cell death, and increased oxidative stress, which collectively contributed to antitumor responses in vitro and in vivo. This is the first study, to our knowledge, to demonstrate the utility of a pharmacological inhibitor of glutamine transport in oncology, representing a new class of targeted therapy and laying a framework for paradigm-shifting therapies targeting cancer cell metabolism.

Optimized Translocator Protein Ligand for Optical Molecular Imaging and Screening

Translocator protein (TSPO) is a validated target for molecular imaging of a variety of human diseases and disorders. Given its involvement in cholesterol metabolism, TSPO expression is commonly elevated in solid tumors, including glioma, colorectal cancer, and breast cancer. TSPO ligands capable of detection by optical imaging are useful molecular tracers for a variety of purposes that range from quantitative biology to drug discovery. Leveraging our prior optimization of the pyrazolopyrimidine TSPO ligand scaffold for cancer imaging, we report herein a new generation of TSPO tracers with superior binding affinity and suitability for optical imaging and screening. In total, seven candidate TSPO tracers were synthesized and vetted in this study; the most promising tracer identified (29, K_d = 0.19 nM) was the result of conjugating a high-affinity TSPO ligand to a fluorophore used routinely in biological sciences (FITC) via a functional carbon linker of optimal length. Computational modeling suggested that an n-alkyl linker of eight carbons in length allows for positioning of the bulky fluorophore distal to the ligand binding domain and toward the solvent interface, minimizing potential ligand-protein interference. Probe 29 was found to be highly suitable for in vitro imaging of live TSPO-expressing cells and could be deployed as a ligand screening and discovery tool. Competitive inhibition of probe 29 quantified by fluorescence and ³H-PK11195 quantified by traditional radiometric detection resulted in equivalent affinity data for two previously reported TSPO ligands. This study introduces the utility of TSPO ligand 29 for in vitro imaging and screening and provides a structural basis for the development of future TSPO imaging ligands bearing bulky signaling moieties.

Preclinical Evaluation of 4-[18F]Fluoroglutamine PET to Assess ASCT2 Expression in Lung Cancer

PURPOSE

Alanine-serine-cysteine transporter 2 (ASCT2) expression has been demonstrated as a promising lung cancer biomarker. (2S,4R)-4-[(18)F]Fluoroglutamine (4-[(18)F]fluoro-Gln) positron emission tomography (PET) was evaluated in preclinical models of non-small cell lung cancer as a quantitative, non-invasive measure of ASCT2 expression.

PROCEDURES

In vivo microPET studies of 4-[(18)F]fluoro-Gln uptake were undertaken in human cell line xenograft tumor-bearing mice of varying ASCT2 levels, followed by a genetically engineered mouse model of epidermal growth factor receptor (EGFR)-mutant lung cancer. The relationship between a tracer accumulation and ASCT2 levels in tumors was evaluated by IHC and immunoblotting.

RESULT

4-[(18)F]Fluoro-Gln uptake, but not 2-deoxy-2-[(18)F]fluoro-D-glucose, correlated with relative ASCT2 levels in xenograft tumors. In genetically engineered mice, 4-[(18)F]fluoro-Gln accumulation was significantly elevated in lung tumors, relative to normal lung and cardiac tissues.

CONCLUSIONS

4-[(18)F]Fluoro-Gln PET appears to provide a non-invasive measure of ASCT2 expression. Given the potential of ASCT2 as a lung cancer biomarker, this and other tracers reflecting ASCT2 levels could emerge as precision imaging diagnostics in this setting.

18F-FLT Positron Emission Tomography (PET) is a Pharmacodynamic Marker for EWS-FLI1 Activity and Ewing Sarcoma

Ewing sarcoma is a bone and soft-tissue tumor that depends on the activity of the EWS-FLI1 transcription factor for cell survival. Although a number of compounds have been shown to inhibit EWS-FLI1 in vitro, a clinical EWS-FLI1-directed therapy has not been achieved. One problem plaguing drug development efforts is the lack of a suitable, non-invasive, pharmacodynamic marker of EWS-FLI1 activity. Here we show that ¹⁸F-FLT PET (¹⁸F- 3′-deoxy-3′-fluorothymidine positron emission tomography) reflects EWS-FLI1 activity in Ewing sarcoma cells both in vitro and in vivo. ¹⁸F-FLT is transported into the cell by ENT1 and ENT2, where it is phosphorylated by TK1 and trapped intracellularly. In this report, we show that silencing of EWS-FLI1 with either siRNA or small-molecule EWS-FLI1 inhibitors suppressed the expression of ENT1, ENT2, and TK1 and thus decreased ¹⁸F-FLT PET activity. This effect was not through a generalized loss in viability or metabolic suppression, as there was no suppression of ¹⁸F-FDG PET activity and no suppression with chemotherapy. These results provide the basis for the clinical translation of ¹⁸F-FLT as a companion biomarker of EWS-FLI1 activity and a novel diagnostic imaging approach for Ewing sarcoma.

Identification and characterization of EGF receptor in individual exosomes by fluorescence-activated vesicle sorting

Exosomes are small, 40–130 nm secreted extracellular vesicles that recently have become the subject of intense focus as agents of intercellular communication, disease biomarkers and potential vehicles for drug delivery. It is currently unknown whether a cell produces different populations of exosomes with distinct cargo and separable functions. To address this question, high-resolution methods are needed. Using a commercial flow cytometer and directly labelled fluorescent antibodies, we show the feasibility of using fluorescence-activated vesicle sorting (FAVS) to analyse and sort individual exosomes isolated by sequential ultracentrifugation from the conditioned medium of DiFi cells, a human colorectal cancer cell line. EGFR and the exosomal marker, CD9, were detected on individual DiFi exosomes by FAVS; moreover, both markers were identified by high-resolution stochastic optical reconstruction microscopy on individual, approximately 100 nm vesicles from flow-sorted EGFR/CD9 double-positive exosomes. We present evidence that the activation state of EGFR can be assessed in DiFi-derived exosomes using a monoclonal antibody (mAb) that recognizes “conformationally active” EGFR (mAb 806). Using human antigen-specific antibodies, FAVS was able to detect human EGFR and CD9 on exosomes isolated from the plasma of athymic nude mice bearing DiFi tumour xenografts. Multicolour FAVS was used to simultaneously identify CD9, EGFR and an EGFR ligand, amphiregulin (AREG), on human plasma-derived exosomes from 3 normal individuals. These studies demonstrate the feasibility of FAVS to both analyse and sort individual exosomes based on specific cell-surface markers. We propose that FAVS may be a useful tool to monitor EGFR and AREG in circulating exosomes from individuals with colorectal cancer and possibly other solid tumours.

New structure-activity relationships of N-acetamide substituted pyrazolopyrimidines as pharmacological ligands of TSPO

Translocator protein (TSPO) represents an attractive target for molecular imaging and therapy due to its prevalence and critical roles played in oncology and other pathologies. Based upon our previously optimized pyrazolopyrimidine scaffold, we elucidated new structure activity relationships related to N,N-disubstitutions of the terminal acetamide on pyrazolopyrimidines and further explored the impacts of these substituents on lipophilicity and plasma protein binding. Several novel chemical probes reported here exhibited significantly increased binding affinity, suitable lipophilicity and protein binding compared with contemporary TSPO ligands. We illustrate that N,N-acetamide disubstitution affords opportunities to introduce diverse chemical moieties distal to the central pyrazolopyrimidine core, without sacrificing TSPO affinity. We anticipate that further exploration of N-acetamide substitutions may yield additional TSPO ligands capable of furthering the field of precision medicine.

Mouse Models of Breast Cancer: Platforms for Discovering Precision Imaging Diagnostics and Future Cancer Medicine

Representing an enormous health care and socioeconomic challenge, breast cancer is the second most common cancer in the world and the second most common cause of cancer-related death. Although many of the challenges associated with preventing, treating, and ultimately curing breast cancer are addressable in the laboratory, successful translation of groundbreaking research to clinical populations remains an important barrier. Particularly when compared with research on other types of solid tumors, breast cancer research is hampered by a lack of tractable in vivo model systems that accurately recapitulate the relevant clinical features of the disease. A primary objective of this article was to provide a generalizable overview of the types of in vivo model systems, with an emphasis primarily on murine models, that are widely deployed in preclinical breast cancer research. Major opportunities to advance precision cancer medicine facilitated by molecular imaging of preclinical breast cancer models are discussed.

2-Amino-4-bis(aryloxybenzyl)aminobutanoic acids: A novel scaffold for inhibition of ASCT2-mediated glutamine transport

Herein, we report the discovery of 2-amino-4-bis(aryloxybenzyl)aminobutanoic acids as novel inhibitors of ASCT2(SLC1A5)-mediated glutamine accumulation in mammalian cells. Focused library development led to two novel ASCT2 inhibitors that exhibit significantly improved potency compared with prior art in C6 (rat) and HEK293 (human) cells. The potency of leads reported here represents a 40-fold improvement over our most potent, previously reported inhibitor and represents, to our knowledge, the most potent pharmacological inhibitors of ASCT2-mediated glutamine accumulation in live cells. These and other compounds in this novel series exhibit tractable chemical properties for further development as potential therapeutic leads.

Preclinical TSPO Ligand PET to Visualize Human Glioma Xenotransplants: A Preliminary Study

Current positron emission tomography (PET) imaging biomarkers for detection of infiltrating gliomas are limited. Translocator protein (TSPO) is a novel and promising biomarker for glioma PET imaging. To validate TSPO as a potential target for molecular imaging of glioma, TSPO expression was assayed in a tumor microarray containing 37 high-grade (III, IV) gliomas. TSPO staining was detected in all tumor specimens. Subsequently, PET imaging was performed with an aryloxyanilide-based TSPO ligand, [¹⁸F]PBR06, in primary orthotopic xenograft models of WHO grade III and IV gliomas. Selective uptake of [¹⁸F]PBR06 in engrafted tumor was measured. Furthermore, PET imaging with [¹⁸F]PBR06 demonstrated infiltrative glioma growth that was undetectable by traditional magnetic resonance imaging (MRI). Preliminary PET with [¹⁸F]PBR06 demonstrated a preferential tumor-to-normal background ratio in comparison to 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG). These results suggest that TSPO PET imaging with such high-affinity radiotracers may represent a novel strategy to characterize distinct molecular features of glioma growth, as well as better define the extent of glioma infiltration for therapeutic purposes.

Molecular imaging of folate receptor β-positive macrophages during acute lung inflammation

Characterization of markers that identify activated macrophages could advance understanding of inflammatory lung diseases and facilitate development of novel methodologies for monitoring disease activity. We investigated whether folate receptor β (FRβ) expression could be used to identify and quantify activated macrophages in the lungs during acute inflammation induced by Escherichia coli LPS. We found that FRβ expression was markedly increased in lung macrophages at 48 hours after intratracheal LPS. In vivo molecular imaging with a fluorescent probe (cyanine 5 polyethylene glycol folate) showed that the fluorescence signal over the chest peaked at 48 hours after intratracheal LPS and was markedly attenuated after depletion of macrophages. Using flow cytometry, we identified the cells responsible for uptake of cyanine 5-conjugated folate as FRβ(+) interstitial macrophages and pulmonary monocytes, which coexpressed markers associated with an M1 proinflammatory macrophage phenotype. These findings were confirmed using a second model of acute lung inflammation generated by inducible transgenic expression of an NF-κB activator in airway epithelium. Using CC chemokine receptor 2-deficient mice, we found that FRβ(+) macrophage/monocyte recruitment was dependent on the monocyte chemotactic protein-1/CC chemokine receptor 2 pathway. Together, our results demonstrate that folate-based molecular imaging can be used as a noninvasive approach to detect classically activated monocytes/macrophages recruited to the lungs during acute inflammation.

Preclinical imaging evaluation of novel TSPO-PET ligand 2-(5,7-Diethyl-2-(4-(2-[(18)F]fluoroethoxy)phenyl)pyrazolo[1,5-a]pyrimidin-3-yl)-N,N-diethylacetamide ([ (18)F]VUIIS1008) in glioma

PURPOSE

Translocator protein (TSPO) concentrations are elevated in glioma, suggesting a role for TSPO positron emission tomography (PET) imaging in this setting. In preclinical PET studies, we evaluated a novel, high-affinity TSPO PET ligand, [(18)F]VUIIS1008, in healthy mice and glioma-bearing rats.

PROCEDURES

Dynamic PET data were acquired simultaneously with [(18)F]VUIIS1008 injection, with binding reversibility and specificity evaluated in vivo by non-radioactive ligand displacement or blocking. Compartmental analysis of PET data was performed using metabolite-corrected arterial input functions. Imaging was validated with histology and immunohistochemistry.

RESULTS

[(18)F]VUIIS1008 exhibited rapid uptake in TSPO-rich organs. PET ligand uptake was displaceable with non-radioactive VUIIS1008 or PBR06 in mice. Tumor accumulation of [(18)F]VUIIS1008 was blocked by pretreatment with VUIIS1008 in rats. [(18)F]VUIIS1008 exhibited improved tumor-to-background ratio and higher binding potential in tumors compared to a structurally similar pyrazolopyrimidine TSPO ligand, [(18)F]DPA-714.

CONCLUSIONS

The PET ligand [(18)F]VUIIS1008 exhibits promising characteristics as a tracer for imaging glioma. Further translational studies appear warranted.

Abstract P1-13-01: Phase I study of ARN-810, a novel and potent oral selective estrogen receptor degrader, in postmenopausal women with metastatic estrogen receptor positive (ER+), HER2- breast cancer

Background: Evidence that ER can signal in both ligand-dependent and independent manner in endocrine resistant breast cancer (BC) provides rationale for therapies that are not only functional antagonists of ER but also reduce ER levels, thus targeting both modes of signaling. Furthermore, mutations in ESR1 affecting the ligand-binding domain (LBD) that drive ER-dependent transcription and proliferation in the absence of estrogen suggest that LBD-mutant forms are involved in mediating clinical resistance and next generation ER modulators with robust activity in both wild type and mutant ER tumors are needed. ARN-810 is a novel, orally bioavailable, ER antagonist that induces proteasomal ER degradation in BC cell lines at picomolar concentrations and tumor regression in tamoxifen-sensitive and resistant BC xenograft models.

Methods: ARN-810 was tested using standard 3+3 dose escalation to assess safety, PK, and Recommended Phase 2 Dose (RP2D). Key eligibility criteria included ER+ (HER2-) metastatic BC progressing ≥ 6 months (m) on endocrine therapy and ≤ 2 prior chemotherapies. Pre- and on-study tumor biopsies were obtained when feasible. Pharmacodynamics was assessed by functional imaging with [18F]-fluoroestradiol (FES)-PET, tumor-based ER/PR/Ki67 IHC, and ER target gene expression. Plasma PK was assessed following a single dose and at steady-state. Anti-tumor activity was assessed by clinical benefit rate (CBR) [complete response, partial response, or stable disease ≥ 6m] and progression-free survival (PFS).

Results: From April 2013 to June 2014, 32 patients (pts) (median age 61 (range 43 – 75); median number of prior therapies = 3 (range 1 – 7); visceral metastases 54%) were enrolled at 5 doses (100, 200, 400, 600, 800 mg) and 2 different regimens (once [QD] and twice daily) given orally with and without fasting. Increases in ARN-810 exposure were dose-dependent with no apparent food effect. At 4 weeks of treatment, complete reduction in FES uptake consistent with full receptor saturation and/or degradation was seen in 95% pts (21/22 scanned to date), including 2 pts with ESR1 mutations, suggesting ARN-810 exhibits greater ER occupancy than that recently reported for fulvestrant 500 mg (van Krutchen et al, ASCO 2014). Evidence of reduced ER levels and Ki67 staining was observed on treatment. To date, 19 pts (59.4%) remain on study with a preliminary CBR of 41%. RP2D, PFS and gene expression results will be provided at time of meeting. The most common adverse events were grades 1/2 nausea, diarrhea, fatigue, and abdominal pain. There was 1 dose limiting toxicity (grade 3 diarrhea) at 800 mg QD which led to expansion of that cohort, while in parallel, evaluation of the other dose regimens continues. No patients have discontinued the study due to toxicity.

Conclusions: ARN-810 appears to be safe and tolerable, with predictable PK, promising anti-tumor activity, and pharmacodynamic evidence of target engagement, ER degradation and reduced tumor proliferation in heavily pre-treated metastatic ER+ BC. In Phase II, ARN-810 will be studied in patients previously treated with aromatase inhibitors and fulvestrant, including those with ESR1 mutations.

Citation Format: Aditya Bardia, Maura N Dickler, Ingrid A Mayer, Eric P Winer, Umar Mahmood, Gary Ulaner, H Charles Manning, Peter Rix, Jeffrey H Hager, Debasish Roychowdhury, Edna Chow Maneval, Carlos L Arteaga, Jose Baselga. Phase I study of ARN-810, a novel and potent oral selective estrogen receptor degrader, in postmenopausal women with metastatic estrogen receptor positive (ER+), HER2- breast cancer [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P1-13-01.

[(18)F]-FLT PET to predict early response to neoadjuvant therapy in KRAS wild-type rectal cancer: a pilot study

OBJECT

This pilot study evaluated the utility of 3'-deoxy-3'[18F]-fluorothymidine ([(18)F]-FLT) positron emission tomography (PET) to predict response to neoadjuvant therapy that included cetuximab in patients with wild-type KRAS rectal cancers.

METHODS

Baseline [(18)F]-FLT PET was collected prior to treatment initiation. Follow-up [(18)F]-FLT was collected after three weekly infusions of cetuximab, and following a combined regimen of cetuximab, 5-FU, and radiation. Imaging-matched biopsies were collected with each PET study.

RESULTS

Diminished [(18)F]-FLT PET was observed in 3/4 of patients following cetuximab treatment alone and in all patients following combination therapy. Reduced [(18)F]-FLT PET following combination therapy predicted disease-free status at surgery. Overall, [(18)F]-FLT PET agreed with Ki67 immunoreactivity from biopsy samples and surgically resected tissue, and was predictive of treatment-induced rise in p27 levels.

CONCLUSION

These results suggest that [(18)F]-FLT PET is a promising imaging biomarker to predict response to neoadjuvant therapy that included EGFR blockade with cetuximab in patients with rectal cancer.

Acetate dependence of tumors

Acetyl-CoA represents a central node of carbon metabolism that plays a key role in bioenergetics, cell proliferation, and the regulation of gene expression. Highly glycolytic or hypoxic tumors must produce sufficient quantities of this metabolite to support cell growth and survival under nutrient-limiting conditions. Here, we show that the nucleocytosolic acetyl-CoA synthetase enzyme, ACSS2, supplies a key source of acetyl-CoA for tumors by capturing acetate as a carbon source. Despite exhibiting no gross deficits in growth or development, adult mice lacking ACSS2 exhibit a significant reduction in tumor burden in two different models of hepatocellular carcinoma. ACSS2 is expressed in a large proportion of human tumors, and its activity is responsible for the majority of cellular acetate uptake into both lipids and histones. These observations may qualify ACSS2 as a targetable metabolic vulnerability of a wide spectrum of tumors.

Abstract 2058: Molecular imaging of glutaminolysis as a tool for evaluating therapeutic response in preclinical models of colorectal cancer

The metabolic repertoire of cancer cells diverge significantly from that of normal cells. Energy production in cancer cells tends to depend on aerobic glycolysis, a feature that is routinely monitored by positron emission tomography (PET) imaging using 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG). In addition to predilection for glycolysis, cancer cells may possess other unique metabolic characteristics, such as increased consumption of glutamine. As with glucose, glutamine also serves as a key carbon source for ATP production and biosynthesis. Given this, quantitative measures of glutaminolysis may reflect critical processes in oncology. Accordingly, PET agents targeting glutamine uptake, such as 4-[18F]fluoro-glutamine ([18F]4F-GLN), have been reported and used in preclinical models of cancer.

The goal of this study was to elucidate the feasibility of using [18F]4F-GLN PET to predict response to targeted therapy within the context of colorectal cancer (CRC). Initially we validated expression of SLC1A5, the primary plasma membrane transporter of glutamine, in tumor and normal colon specimens for 58 patients with primary and advanced CRC. We found that SLC1A5 expression was elevated in over 80%of primary tumor specimens but did not correlate with grade or gender. Furthermore, elevated SLC1A5 expression correlated strongly with elevated Ki67, a molecular marker of proliferation.

Given this, we evaluated [18F]4F-GLN PET in preclinical models of V600EBRAF-expressing CRC as a means of detecting anti-proliferation responses to targeted therapeutics. Simulating a clinical study within the context of the Vanderbilt GI SPORE, the regimen included an inhibitor of mutant BRAF, a PI3K/mTOR inhibitor, and a combination there of. Strikingly, [18F]4F-GLN PET was found to correlate more closely to markers of anti-proliferative responses in vivo than analogously performed [18F]FDG PET imaging studies. We believe that these findings not only provide a greater understanding of the role that glutaminolysis plays in CRC but also illuminate the potential impact that glutaminolysis derived PET could have towards guiding drug development clinical trials as an imaging metric of early therapeutic response detection.

Citation Format: Matthew R. Hight, Michael L. Schulte, Samir Saleh, Gregory D. Ayers, Frank L. Revetta, M. Kay Washington, Robert J. Coffey, H. Charles Manning. Molecular imaging of glutaminolysis as a tool for evaluating therapeutic response in preclinical models of colorectal cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2058. doi:10.1158/1538-7445.AM2014-2058

Abstract 4958: Fluorescent barcoding offers increased dimensionality in tracking tumor cells in vitro and in vivo

Fluorescent or luminescent imaging of tumors in animal models is a critical element in dynamic and longitudinal monitoring of tumor burden, tumor growth, and metastatic dissemination. Currently, most imaging strategies are limited to one or two different colors per animal. To improve utility of current imaging technologies, we have developed a library of fluorescent and luminescent tracking vectors designed to expand the color dimensions possible within a single model through a multi-label approach to fluorescent barcoding. The library was built using Multisite Gateway® Cloning technology, which offers a fast and simple recombination-based cloning approach that allows for efficient expression of several genes driven by one promoter on a single vector backbone. Our lentiviral-based vectors express firefly luciferase and one of seven spectrally unique fluorescent proteins covering the entire fluorescent protein color palette from blue to far-red. We have stably labeled bone-derived MDA-MB-231 cells with the vectors by viral transduction. Using these individually colored cell lines, we have demonstrated that the seven FPs are uniquely identifiable by spectral un-mixing with the Maestro Q Imaging System and by flow cytometry. The combination of luciferase and fluorescent labeling allows us to monitor tumor growth by luciferase activity and distinguish individual cell populations by their fluorescent label. We are currently verifying that the cells retain stable expression of the vectors in mouse models; preliminary results are promising. In the future, this system will be used to develop an elegant multi-label-based fluorescent barcoding strategy that will allow us to identify individual cell populations from a heterogeneous environment in vivo. By exploiting combinations of the seven fluorescent proteins, we could potentially create over 200 uniquely identifiable cells populations. This ability will not only reduce the number of animals necessary per experiment (as control and several experimental populations can be individually analyzed in a single mouse); but will also allow us to mimic the heterogeneous environment common to all human tumors.

Citation Format: Katie E. Hebron, Tatiana Ketova, Shanna Arnold, H. Charles Manning, Julie Sterling, Florent Elefteriou, Andries Zijsltra. Fluorescent barcoding offers increased dimensionality in tracking tumor cells in vitro and in vivo. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4958. doi:10.1158/1538-7445.AM2014-4958

Abstract 110: Preclinical evaluation of 7-chloro-N,N,5-trimethyl-4-oxo-3(6-[18F]fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide: A novel pyridazinoindole ligand for PET imaging of TSPO in cancer

There exists a continued need for the development of improved positron emission tomography (PET) biomarkers for glioma imaging to aid in tumor diagnosis, inform clinical outcome, and quantify response to therapeutic intervention. Translocator protein (TSPO) expression is elevated in cancer and has been linked with disease progression and diminished survival and tends to be a hallmark of aggressive tumors. Our laboratory has pursued development of TSPO PET ligands as candidates for molecular imaging of glioma, as TSPO ligand binding correlates with tumor grade in this setting.

We report the first exploration of the structure-activity relationship (SAR) around the pyridazinoindole ring and subsequent radiofluorination of a potent and novel pyridazinoindole-based TSPO-selective ligand for cancer imaging. Library and SAR development around the N3 position of the pyridazinoindole scaffold led to the discovery of 7-chloro-N,N,5-trimethyl-4-oxo-3(6-fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide (VUIIS8310), a novel TSPO ligand exhibiting a binding affinity comparable to SSR180575, yet bearing a fluorine atom for subsequent radiolabeling with fluorine-18 (18F) to give 7-chloro-N,N,5-trimethyl-4-oxo-3(6-[18F]fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide (18F-VUIIS8310).

Initial production feasibility and radiochemical development were evaluated with microfluidics and subsequently translated to traditional box-based methods. Subsequently, quantitative, preclinical elevation of 18F-VUIIS8310 was conducted in a rat glioma model, given our previous experience in this setting. Glioma-bearing rats were imaged in a microPET system, with dynamic PET acquisitions acquired simultaneously upon tracer injection. PET ligand reversibility and specificity were evaluated by ligand displacement studies that utilized VUIIS8310. 18F-VUIIS8310 exhibited elevated uptake and specific, displaceable binding in tumor, in contrast with normal brain, which exhibited very low uptake. 18F-VUIIS8310 uptake in the tumor, relative to normal brain, reached a tumor-to-normal brain ratio of greater than 10:1. Ex vivo histological analysis of resected tissue correlated well with PET imaging data.

These preclinical studies illuminate 18F-VUIIS8310 as a promising, novel TSPO PET ligand for imaging glioma and potentially other solid tumors. Importantly, the in vivo stability and high signal-to-noise achieved between tumor and surrounding normal brain suggest the potential of this PET ligand for early tumor detection within this setting. Future head-to-head comparisons between this tracer and emerging, potent TSPO ligands (J. Med. Chem. 2013, 56, 3429) are underway.

Citation Format: Yiu-Yin Cheung, Jason R. Buck, Michael L. Nickels, Dewei Tang, H. Charles Manning. Preclinical evaluation of 7-chloro-N,N,5-trimethyl-4-oxo-3(6-[18F]fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide: A novel pyridazinoindole ligand for PET imaging of TSPO in cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 110. doi:10.1158/1538-7445.AM2014-110