Patient-derived tumor xenograft study with CDK4/6 inhibitor plus AKT inhibitor for the management of metastatic castration-resistant prostate cancer

Metastatic castration-resistant prostate cancer (mCRPC) is an aggressive malignancy with poor outcomes. To investigate novel therapeutic strategies, we characterized three new metastatic prostate cancer PDTX models and developed 3D spheroids from each to investigate molecular targeted therapy combinations including CDK4/6 inhibitors (CDK4/6i) with AKT inhibitors (ATKi). Metastatic prostate cancer tissue was collected and three PDTX models were established and characterized using WES. PDTX 3-D spheroids were developed from these three PDTXs to show resistance patterns and test novel molecular targeted therapies. CDK4/6i's were combined with AKTi's to assess synergistic antitumor response to prove our hypothesis that blockade of AKT overcomes drug resistance to CDK4/6 inhibitor. This combination was evaluated in PDTX 3-D spheroids and in vivo experiments with responses measured by tumor volumes, PSA and Ga-68 PSMA-11 PET-CT imaging. We demonstrated CDK4/6i's with AKTi's possess synergistic antitumor activity in three mCRPC PDTX models. These models have multiple unique pathogenic and deleterious genomic alterations with resistance to single agent CDK4/6i's. Despite this, combination therapy with AKTi's was able to overcome resistance mechanisms. The IHC and Western blot analysis confirmed on target effects, while tumor volume, serum PSA ELISA, and radionuclide imaging demonstrated response to therapy with statistically significant SUV differences seen with Ga-68 PSMA-11 PET-CT. These preclinical data demonstrating antitumor synergy by overcoming single agent CDK 4/6i as well as AKTi drug resistance provide the rational for a clinical trial combining a CDK4/6i with an AKTi in mCRPC patients whose tumor expresses wild type RB1.

FDG PET/CT and thyroid biopsy leads to neurosarcoidosis diagnosis

We present a unique case of neurosarcoidosis diagnosed based on thyroid biopsy and FDG PET (Fluorodeoxyglucose positron emission tomography) imaging. A patient presented for a second opinion after being placed in hospice for rapidly progressing dementia, presumed to be due to Creutzfeldt Jakob disease despite negative workup and was unable to perform activities of daily life or communicate with his wife. The patient underwent a workup including whole-body FDG PET, which showed hypermetabolic lymph nodes as well as a hypermetabolic nodule in the thyroid. Biopsy of the lymph nodes was nondiagnostic, but the thyroid biopsy tissue yielded a diagnosis of sarcoid. After ruling out other causes and reviewing the tissue pathology, the patient was diagnosed with systemic sarcoidosis with neurological involvement and started on infliximab with rapid improvement.

Radiosynthesis and Preliminary Evaluation of [11C]SSI-4 for the Positron Emission Tomography Imaging of Stearoyl CoA Desaturase 1

Stearoyl CoA desaturase 1 (SCD1) is the rate-limiting enzyme for converting saturated fatty acids (SFAs) into monounsaturated fatty acids (MUFAs) and plays a key role in endogenous (de novo) fatty acid metabolism. Given that this pathway is broadly upregulated across many tumor types with an aggressive phenotype, SCD1 has emerged as a compelling target for cancer imaging and therapy. The ligand 2-(4-(2-chlorophenoxy)piperidine-1-carboxamido)-N-methylisonicotinamide (SSI-4) was identified as a potent and highly specific SCD1 inhibitor with a strong binding affinity for SCD1 at our laboratory. We herein report the radiosynthesis of [11C]SSI-4 and the preliminary biological evaluation including in vivo PET imaging of SCD1 in a human tumor xenograft model. Radiotracer [11C]SSI-4 was labeled at the carbamide position via the direct [11C]CO2 fixation on the Synthra MeIplus module in high molar activity and good radiochemical yield. In vitro cell uptake assays were performed with three hepatocellular carcinoma (HCC) cell lines and three renal cell carcinoma (RCC) cell lines. Additionally, in vivo small animal PET/CT imaging with [11C]SSI-4 and the biodistribution were carried out in a mouse model bearing HCC xenografts. Radiotracer [11C]SSI-4 afforded a 4.14 ± 0.44% (decay uncorrected, n = 10) radiochemical yield based on starting [11]CO2 radioactivity. The [11C]SSI-4 radiosynthesis time including HPLC purification and SPE formulation was 25 min from the end of bombardment to the end of synthesis (EOS). The radiochemical purity of [11C]SSI-4 was 98.45 ± 1.43% (n = 10) with a molar activity of 225.82 ± 33.54 GBq/μmol (6.10 ± 0.91 Ci/μmol) at the EOS. In vitro cell uptake study indicated all SSI-4 responsive HCC and RCC cell line uptakes demonstrate specific uptake and are blocked by standard compound SSI-4. Preliminary small animal PET/CT imaging study showed high specific uptake and block of [11C]SSI-4 uptake with co-injection of cold SSI-4 in high SCD1-expressing organs including lacrimal gland, brown fat, liver, and tumor. In summary, novel radiotracer [11C]SSI-4 was rapidly and automatedly radiosynthesized by direct [11C]CO2 fixation. Our preliminary biological evaluation results suggest [11C]SSI-4 could be a promising radiotracer for PET imaging of SCD1 overexpressing tumor tissues.

Evaluation of Hepatotoxicity from Peptide Receptor Radionuclide Therapy in Patients with Gastroenteropancreatic Neuroendocrine Tumors and a Very High Liver Tumor Burden

The aim of the current study was to describe the risk of hepatotoxicity for patients with gastroenteropancreatic neuroendocrine tumors undergoing peptide receptor radionuclide therapy (PRRT) with a very high liver tumor burden, defined as tumor involving more than 75% of the liver. Methods: We conducted a retrospective analysis of 371 patients who received at least 1 cycle of ¹⁷⁷Lu-DOTATATE at Mayo Clinic for advanced gastroenteropancreatic neuroendocrine tumors. We identified 15 total patients with more than 75% liver involvement on ⁶⁸Ga-DOTATATE PET/CT and with either a contrast-enhanced abdominal MRI or dual-phase abdominal CT examination. Results: Of the 15 patients with more than 75% liver involvement, 1 experienced hepatotoxicity (i.e., worsening liver enzymes or bilirubin) as defined by the Common Terminology Criteria for Adverse Events, version 5.0. No patients had grade 3-5 hepatotoxicity (i.e., clinical signs of liver failure). Conclusion: When considering the risk of liver injury from PRRT due to burden of disease, our data suggest that PRRT may be a safe option in patients with more than 75% liver involvement. Future efforts should be made to determine the safety profile of PRRT in patients with varying degrees of liver involvement.

Atypically Intense Pharmacologically Induced Brown Fat Activation on FDG PET/CT

ABSTRACT

Brown fat activation with increased radiotracer localization on FDG PET/CT studies is a well-known phenomenon. Activated brown adipose tissue (BAT) is usually seen in the supraclavicular region, but also in paraspinal and rarely in upper abdominal fat. Ours is a unique case of atypically intense, multilobular FDG uptake in activated BAT. Chart review revealed that the patient was receiving mirabegron, a known activator of brown fat. Methods of reducing brown fat uptake are known, but little information is reported on pharmacologic causes of increased uptake. Factors increasing FDG uptake in BAT should also be considered when interpreting PET/CT studies.

Nuclear Receptor Imaging In Vivo-Clinical and Research Advances

Nuclear receptors are transcription factors that function in normal physiology and play important roles in diseases such as cancer, inflammation, and diabetes. Noninvasive imaging of nuclear receptors can be achieved using radiolabeled ligands and positron emission tomography (PET). This quantitative imaging approach can be viewed as an in vivo equivalent of the classic radioligand binding assay. A main clinical application of nuclear receptor imaging in oncology is to identify metastatic sites expressing nuclear receptors that are targets for approved drug therapies and are capable of binding ligands to improve treatment decision-making. Research applications of nuclear receptor imaging include novel synthetic ligand and drug development by quantifying target drug engagement with the receptor for optimal therapeutic drug dosing and for fundamental research into nuclear receptor function in cells and animal models. This mini-review provides an overview of PET imaging of nuclear receptors with a focus on radioligands for estrogen receptor, progesterone receptor, and androgen receptor and their use in breast and prostate cancer.

Prognostic Evaluation of Metastatic Castration Resistant Prostate Cancer and Neuroendocrine Prostate Cancer with [68Ga]Ga DOTATATE PET-CT

Objectives: Prostate cancer is well known to express high levels of somatostatin receptors and preliminary data suggests that PET imaging with the somatostatin analog, [68Ga]Ga-DOTATATE, may allow for whole body staging of patients with metastatic castration resistant prostate cancer (mCRPC) and neuroendocrine prostate cancer (NePC). This study explores the utility of [68Ga]Ga-DOTATATE PET-CT to identify metastatic deposits in men with mCRPC and NePC and prognosticate disease progression. Methods: [68Ga]Ga-DOTATATE PET-CT was performed in 17 patients with mCRPC and of those, 2/17 had NePC. A semiquantitative analysis with standardized uptake values (SUV) (e.g., SUVmax, SUVmean) was performed for each metastatic lesion and reference background tissues. [68Ga]Ga-DOTATATE uptake in metastatic deposits was further classified as: mild (less than liver), moderate (up to liver average), or marked (greater than liver). Serial prostate-specific antigen measurements and patient survival were followed up to 3 years after PET imaging to assess response to standard of care treatment. Results: All patients had at least one metastatic lesion with identifiable [68Ga]Ga-DOTATATE uptake. Marked [68Ga]Ga-DOTATATE uptake was found in 7/17 patients, including both NePC patients, and all were non-responders to systemic therapy and died within the follow up period, with a mean time to death of 8.1 months. Three patients had mild [68Ga]Ga-DOTATATE uptake, and all were responders to systemic therapy and were alive 36 months after [68Ga]Ga-DOTATATE imaging. Conclusions: [68Ga]Ga-DOTATATE is able to identify mCRPC and NePC metastatic deposits, and lesions with [68Ga]Ga-DOTATATE uptake > liver may portend poor outcomes in patients with mCRPC.

FDG PET-MRI evaluation of synchronous gallbladder adenocarcinoma and POEMS syndrome

POEMS syndrome is a rare paraneoplastic syndrome associated with a plasma cell proliferative disorder. Gallbladder adenocarcinoma is a rare malignancy, with no association with POEMS syndrome. The plasma cell dyscrasia is routinely evaluated with advanced hybrid imaging to assess both anatomic and functional components. We present a case of a 59-year-old female with a known diagnosis of POEMS syndrome who underwent a whole-body restaging evaluation with hybrid positron emission tomography (PET) and magnetic resonance imaging (MR) to restage her plasma cell dyscrasia. She also had a prior diagnosis of gallbladder adenocarcinoma. Our case focuses on the value of PET/MR in this scenario as well as a rare case of osseous metastasis from gallbladder carcinoma.

JNMT continuing education: 177Lu PSMA therapy

Radiopharmaceutical therapy utilizing ¹⁷⁷Lu-PSMA is an effective treatment for prostate cancer which has recently been approved by the United States Food and Drug Administration. This method leverages the success of PSMA targeted PET imaging, enabling the delivery of targeted radiopharmaceutical therapy, This agent has demonstrated a clear benefit in large prospective clinical trials, and promises to become part of the standard armamentarium of treatment for patients with prostate cancer. In this review, the evidence supporting the use of this agent is highlighted, along with important areas now under investigation. Practical information on technology aspects, dose administration, nursing, and the role of the treating physician is highlighted. Overall, ¹⁷⁷Lu-PSMA treatment requires close collaboration between referring physicians, nuclear medicine, technologists, radiopharmacy, and nursing, to enable streamlined patient care.

Neuropsychiatric Manifestations of Lymphoma-Associated Cerebral Glucose Hypometabolism Can Be Reversed by Intensive Glucose Supplementation

Cerebral glucose hypometabolism (CGHM) is characterized by diffuse or focal reduction in uptake of glucose by the brain as determined on a FDG PET-CT. We report a case of lymphoma-associated cerebral glucose hypometabolism (LA-CGHM) in a patient with hepatosplenic T-cell lymphoma (HSTCL) whose neuropsychiatric symptoms were resolved with glucose supplementation. PET-CT scan showed diffuse cerebral hypometabolism in addition to focal hypermetabolism in the liver related to lymphomatous involvement. He responded rapidly to infusion of 10% dextrose with complete resolution of neurological symptoms on two separate occasions and was later maintained on oral glucose without relapse. While his neuropsychiatric symptoms improved, his aggressive lymphoma and chemo-refractory disease ultimately led to his demise. We suggest that LA-CGHM can cause neuropsychiatric manifestations which can be reversed by intensive glucose supplementation.

Unusual Increased Blood Pool Activity on 68Ga-DOTATATE PET/CT in a Patient With Metastatic Neuroendocrine Disease

ABSTRACT

68Ga-DOTATATE is a well-established, positron-emitting, somatostatin receptor-binding radiopharmaceutical. We present an unusual case of transiently increased blood pool uptake of 68Ga-DOTATATE in a patient with well-differentiated stage IV neuroendocrine tumor, with Ki-67 <2% (WHO grade 1) maintained on lanreotide. During serial 68Ga-DOTATATE PET/CT examinations, increased blood pool accumulation of presumably unbound 68Ga was demonstrated, which could impact the Kenning score and lead to a false treatment response assessment.

Advances and Future Directions in Molecular Breast Imaging

Molecular breast imaging (MBI) using ^99mTc-sestamibi has advanced rapidly over the past decade. Technical advances allow lower-dose, higher-resolution imaging and biopsy capability. MBI can be used for supplemental breast cancer screening with mammography for women with dense breasts, as well as to assess neoadjuvant therapy response, evaluate disease extent, and predict breast cancer risk. This article highlights the current state of the art and future directions in MBI.

Neuro-Oncology Practice Clinical Debate: FDG PET to differentiate glioblastoma recurrence from treatment-related changes

The ability to accurately differentiate treatment-related changes (ie, pseudoprogression and radiation necrosis) from recurrent glioma remains a critical diagnostic problem in neuro-oncology. Because these entities are treated differently and have vastly different outcomes, accurate diagnosis is necessary to provide optimal patient care. In current practice, this diagnostic quandary commonly requires either serial imaging or histopathologic tissue confirmation. In this article, experts in the field debate the utility of 2-deoxy-2[18F]fluoro-d-glucose positron emission tomography (FDG PET) as an imaging tool to distinguish tumor recurrence from treatment-related changes in a patient with glioblastoma and progressive contrast enhancement on magnetic resonance (MR) following chemoradiotherapy.

Exploratory study of 18F-fluciclovine pet/ct for response assessment to docetaxel in patients with metastatic castration-resistant prostate cancer

Monitoring therapeutic response in patients with metastatic castration-resistant prostate cancer (mCRPC) can be challenging. We set out to determine if ¹⁸F-fluciclovine PET/CT could be a useful imaging biomarker for response to docetaxel chemotherapy in patients with mCRPC. Seven patients with mCRPC had ¹⁸F-fluciclovine PET/CT scheduled at baseline and after 1 and 6 cycles of chemotherapy. The sum of SUVmax from the prostate/bed and up to 5 metastatic bone and soft tissue/visceral lesions were recorded. The SUVpeak of the hottest lesion (PERCIST-like) was also recorded. In comparison to the baseline scan, a decrease of ≥30% was considered response; new lesions or >30% increase was progressive disease; change of <30% was stable disease. Bone scintigraphy and CT were acquired at baseline and after the 6th cycle. Response assessment was based on the Prostate Cancer Clinical Trial Working Group 3 recommendations. All (7/7) enrolled patients completed the 1^st and 2^nd scans, while 4/7 patients completed all 3 scans. PET response correlated with PSA response in 3/7 (42.9%) patients after 1 cycle of docetaxel, and 3/4 (75%) patients after 6 cycles of docetaxel, respectively. Bone scan and CT correlated with PSA response in 1/4 (25%) patients. There was no significant correlation between baseline ¹⁸F-fluciclovine PET parameters or changes in PET parameters and time to PSA progression. In conclusion, this exploratory study showed that ¹⁸F-fluciclovine PET/CT has better correlation with PSA response than CT or bone scan in patients with mCRPC treated with docetaxel. ¹⁸F-fluciclovine PET/CT however did not predict time to PSA progression.

[18F]-Fluciclovine PET discrimination of recurrent intracranial metastatic disease from radiation necrosis

Background

Stereotactic radiosurgery (SRS) is often the primary treatment modality for patients with intracranial metastatic disease. Despite advances in magnetic resonance imaging, including use of perfusion and diffusion sequences and molecular imaging, distinguishing radiation necrosis from progressive tumor remains a diagnostic and clinical challenge. We investigated the sensitivity and specificity of ¹⁸F-fluciclovine PET to accurately distinguish radiation necrosis from recurrent intracranial metastatic disease in patients who had previously undergone SRS.

Methods

Fluciclovine PET imaging was performed in 8 patients with a total of 15 lesions that had previously undergone SRS and had subsequent MRI and clinical features suspicious for recurrent disease. The SUVmax of each lesion and the contralateral normal brain parenchyma were summated and evaluated at four different time points (5 min, 10 min, 30 min, and 55 min). Lesions were characterized as either recurrent disease (11 of 15 lesions) or radiation necrosis (4 of 15 lesions) and confirmed with histopathological correlation (7 lesions) or through serial MRI studies (8 lesions).

Results

Time activity curve analysis found statistically greater radiotracer accumulation for all lesions, including radiation necrosis, when compared to contralateral normal brain. While the mean and median SUV_max for recurrent disease were statistically greater than those of radiation necrosis at all time points, the difference was more significant at the earlier time points (p = 0.004 at 5 min–0.025 at 55 min). Using a SUV_max threshold of ≥ 1.3, fluciclovine PET demonstrated a 100% accuracy in distinguishing recurrent disease from radiation necrosis up to 30 min after injection and an accuracy of 87% (sensitivity = 0.91, specificity = 0.75) at the last time point of 55 min. However, tumor-to-background ratios (TBR_max) were not significantly different between recurrent disease and radiation necrosis at any time point due to variable levels of fluciclovine uptake in the background brain parenchyma.

Conclusions

Fluciclovine PET may play an important role in distinguishing active intracranial metastatic lesions from radiation necrosis in patients previously treated with SRS but needs to be validated in larger studies.

Co-existent Epicardial Paraganglioma and Anterior Mediastinal Thymoma

Thymoma and paraganglioma are known causes of mediastinal masses, the latter being extremely rare. Thymomas arise from remnant thymic tissue in the anterior mediastinum; whereas, thoracic paragangliomas arise from para-aortic or para-vertebral sympathetic chain ganglion (derivatives of embryonic neural crest) in the middle or posterior mediastinum. We report a case of a middle-aged woman with two mediastinal masses, originally believed to be a single tumor or primary malignancy with adjacent metastasis on Computed Tomography (CT) that were further delineated with Magnetic Resonance Imaging (MRI) and [68Ga]-DOTA-(Tyr3)-octreotate (DOTA-TATE) Positron Emission Tomography-Computed Tomography (PET-CT) and surgical pathology as two distinct entities: left epicardial paraganglioma and anterior mediastinal thymoma. A comprehensive discussion of both entities is included.

Fifty Years of Radiopharmaceuticals

To celebrate the 50th anniversary of the founding of the SNMMI Technologist Section in 1970, the Radiopharmaceutical Sciences Council board of directors is pleased to contribute to this celebratory supplement of the Journal of Nuclear Medicine Technology with a perspective highlighting major developments in the radiopharmaceutical sciences that have occurred in the last 50 years.

The role of 68Ga-DOTATATE for evaluation of patients with metastatic castration-resistant prostate cancer (mCRPC)

217

Background: Neuroendocrine transformation (NE) of prostate cancer is difficult to diagnose and treat. 68Ga-DOTATATE PET is useful to detect well-differentiated neuroendocrine tumors, but its application in prostate cancer is not well understood. We evaluated DOTATATE PET in patients with mCRPC and suspected or known NE. Methods: DOTATATE PET/CT were performed in 17 patients with mCRPC. Summed SUVmax of up to 5 bone and 5 soft tissue lesions in each patient were correlated with histologic presence of NE and response to standard of care treatment and clinical outcome (NCT03448458). Results: Mean age was 62.3±9.2 years. Median PSA was 39.0 ng/ml. 2/17 patients had biopsy proven NE. 1/17 was found to have squamous cell cancer and therefore excluded from analysis. 2 patients with NE had platinum-based chemotherapy. Of the 14 patients without NE, 7/14 received enzalutamide or abiraterone, and 7/14 received taxane based chemotherapy after DOTATATE PET. On PET, all patients had at least one lesion (median 7 [interquartile range 6-9]) with uptake. 11/16 patients had bone and visceral/nodal lesions, 4/16 had only bone lesions, and 1 had only nodal disease. Summed SUVmax was significantly higher in 2/16 patients with proven NE compared to the 14/16 non-NE patients (99.1± 16.5 ng/ml vs 48.4± 40.6 ng/ml; p=0.04). There was no correlation between summed SUVmax and PSA. Follow-up was available in 15/16 patients. SUVmax was higher in patients who did not respond to treatment, however, the trend was non-significant (69.5 ± 46.6 vs 39.3 ± 29.6; p=0.20). There was no significant association between survival and DOTATATE uptake. 6/16 patients had next generation sequencing. Of these, 1 patient without NE had BRCA2 mutation, and also had the highest summed uptake in this study. Conclusions: DOTATATE uptake is higher in mCRPC patients with NE or with BRCA mutation. Uptake was higher in patients who did not respond to standard of care treatment, though this was a nonsignificant trend, possibly due to small sample size. Larger studies are therefore recommended. Clinical trial information: NCT03448458.

82Rubidium chloride PET discrimination of recurrent intracranial malignancy from radiation necrosis

BACKGROUND

Accurate identification and discrimination of post treatment changes from recurrent disease remains a challenge for patients with intracranial malignancies despite advances in molecular and magnetic resonance imaging. We have explored the ability of readily available Rubidium-82 chloride (82RbCl) PET to identify and distinguish progressive intracranial disease from radiation necrosis in patients previously treated with radiation therapy.

METHODS

Six patients with a total of 9 lesions of either primary (n=3) or metastatic (n=6) intracranial malignancies previously treated with stereotactic radiation surgery (SRS) and persistent contrast enhancement on MRI underwent brain 82RbCl PET imaging. Two patients with arteriovenous malformations previously treated with SRS, also had brain 82RbCl PET imaging for a total of 11 lesions studied. Histological confirmation via stereotactic biopsy/excisional resection was obtained for 9 lesions with the remaining 2 classified as either recurrent tumor or radiation necrosis based on subsequent MRI examinations. 82RbCl PET time activity curve analysis was performed which comprised lesion SUVmax, contralateral normal brain SUVmax, and tumor to background ratios (TBmax).

RESULTS

82RbCl demonstrates uptake greater than normal brain parenchyma in all lesions studied. Time activity curves demonstrated progressive uptake of 82RbCl in all lesions without evidence of washout. While recurrent disease demonstrated a greater mean SUVmax compared to radiation necrosis, no statistically significant difference between lesion SUVmax nor TBmax was found (p>0.05).

CONCLUSIONS

82RbCl PET produces high-contrast uptake of both recurrent disease and radiation necrosis compared to normal brain. However, no statistically significant difference was found between recurrent tumor and radiation necrosis.

[18F]Fluciclovine PET discrimination between high- and low-grade gliomas

BACKGROUND

The ability to accurately and non-invasively distinguish high-grade glioma from low-grade glioma remains a challenge despite advances in molecular and magnetic resonance imaging. We investigated the ability of fluciclovine (18F) PET as a means to identify and distinguish these lesions in patients with known gliomas and to correlate uptake with Ki-67.

RESULTS

Sixteen patients with a total of 18 newly diagnosed low-grade gliomas (n = 6) and high grade gliomas (n = 12) underwent fluciclovine PET imaging after histopathologic assessment. Fluciclovine PET analysis comprised tumor SUVmax and SUVmean, as well as metabolic tumor thresholds (1.3*, 1.6*, 1.9*) to normal brain background (TBmax, and TBmean). Comparison was additionally made to the proliferative status of the tumor as indicated by Ki-67 values. Fluciclovine uptake greater than normal brain parenchyma was found in all lesions studied. Time activity curves demonstrated statistically apparent flattening of the curves for both high-grade gliomas and low-grade gliomas starting 30 min after injection, suggesting an influx/efflux equilibrium. The best semiquantitative metric in discriminating HGG from LGG was obtained utilizing a metabolic 1 tumor threshold of 1.3* contralateral normal brain parenchyma uptake to create a tumor: background (TBmean1.3) cutoff of 2.15 with an overall sensitivity of 97.5% and specificity of 95.5%. Additionally, using a SUVmax > 4.3 cutoff gave a sensitivity of 90.9% and specificity of 97.5%. Tumor SUVmean and tumor SUVmax as a ratio to mean normal contralateral brain were both found to be less relevant predictors of tumor grade. Both SUVmax (R = 0.71, p = 0.0227) and TBmean (TBmean1.3: R = 0.81, p = 0.00081) had a high correlation with the tumor proliferative index Ki-67.

CONCLUSIONS

Fluciclovine PET produces high-contrast images between both low-grade and high grade gliomas and normal brain by visual and semiquantitative analysis. Fluciclovine PET appears to discriminate between low-grade glioma and high-grade glioma, but must be validated with a larger sample size.

Update on 18F-Fluciclovine PET for Prostate Cancer Imaging

PET is a functional imaging method that can exploit various aspects of tumor biology to enable greater detection of prostate cancer than can be provided by morphologic imaging alone. Anti-1-amino-3-¹⁸F-flurocyclobutane-1-carboxylic acid (¹⁸F-fluciclovine) is a nonnaturally occurring amino acid PET radiotracer that was recently approved by the U.S. Food and Drug Administration for detection of suspected recurrent prostate cancer. The tumor-imaging features of this radiotracer mirror the upregulation of transmembrane amino acid transport that occurs in prostate cancer because of increased amino acid metabolism for energy and protein synthesis. This continuing medical education article provides an overview on ¹⁸F-fluciclovine PET diagnostic capabilities for primary and metastatic disease, including reviews of published comparisons to conventional imaging and other molecular imaging agents. Additionally, the imaging procedure and image interpretation are detailed, including physiologic and pathologic uptake patterns and pitfalls.

Fluorine-18 labeling and biodistribution studies on peroxisome proliferator-activated receptor-gamma ligands: potential positron emission tomography imaging agents

INTRODUCTION

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is an important regulator of lipid metabolism; it controls the differentiation of preadipocytes and is also found at high levels in small metastatic tumors. In this report, we describe the radiochemical synthesis and evaluation of two (18)F-labeled analogs of the potent and selective PPARgamma agonist farglitazar.

MATERIALS AND METHODS

The isomeric aromatic fluorine-substituted target compounds [(2S)-(2-benzoylphenylamino)-3-(4-(2-[2-(4-[(18)F]fluorophenyl)-5-methyloxazol-4-yl]ethoxy)-phenyl)propionic acid ([(18)F]-1) and (2S)-[2-(4-fluorobenzoyl)phenylamino]-3-(4-[2-(5-methyl-2-phenyloxazol-4-yl)ethoxy]-phenyl)propionic acid ([(18)F]-2)] were prepared in fluorine-18-labeled form, respectively, by radiofluorination of an iodonium salt precursor or by Ullmann-type condensation with 2-iodo-4'-[(18)F]fluorobenzophenone after nucleophilic aromatic substitution with [(18)F]fluoride ion. Each compound was obtained in high specific activity and good radiochemical yield.

RESULTS AND DISCUSSION

(18)F-1 and (18)F-2 have high and selective PPARgamma binding affinities comparable to that of the parent molecule farglitazar, and they were found to have good metabolic stability. Tissue biodistribution studies of (18)F-1 and (18)F-2 were conducted, but PPARgamma-mediated uptake of both agents was minimal.

CONCLUSION

This study completes our first look at an important class of PPARgamma ligands as potential positron emission tomography (PET) imaging agents for breast cancer and vascular disease. Although (18)F-1 and (18)F-2 have high affinities for PPARgamma and good metabolic stability, their poor target-tissue distribution properties, which likely reflect their high lipophilicity combined with the low titer of PPARgamma in target tissues, indicate that they have limited potential as PPARgamma PET imaging agents.

Synthesis of 7alpha-(fluoromethyl)dihydrotestosterone and 7alpha-(fluoromethyl)nortestosterone, structurally paired androgens designed to probe the role of sex hormone binding globulin in imaging androgen receptors in prostate tumors by positron emission tomography

Although prostate cancer growth is regulated by androgens through the androgen receptor (AR), in vitro assays of AR levels in prostate tumors have limited prognostic value. This might be improved by direct measurement of tumor AR in vivo using positron emission tomography (PET) imaging with fluorine-18-labeled androgens. Most AR PET imaging agents have been designed to limit steroid binding to serum proteins, but there is evidence that binding to sex hormone binding globulin (SHBG) might enhance tumor uptake. To probe the role of SHBG in prostate tumor uptake of PET imaging agents, we have synthesized two fluoro steroids, 7alpha-(fluoromethyl)dihydrotestosterone (7alpha-FM-DHT) and 7alpha-(fluoromethyl)nortestosterone (7alpha-FM-norT), by a route amenable to their labeling with [18F]fluoride ion. Both compounds have high affinity for AR, but 7alpha-FM-norT has much lower affinity for SHBG. Thus, these two fluoro steroids are well matched in terms of their site of fluorine labeling, similarity of structure, and equivalent AR binding affinity-but contrasting SHBG binding-and therefore can be used as agents for evaluating the role of SHBG binding in the target tissue uptake of AR PET imaging agents in humans.

Synthesis and Biological Evaluation of [18F]Bicalutamide, 4-[76Br]Bromobicalutamide, and 4-[76Br]Bromo-thiobicalutamide as Non-Steroidal Androgens for Prostate Cancer Imaging

Androgen receptors (AR) are overexpressed in most primary and metastatic prostate cancers. To develop a nonsteroidal AR-mediated imaging agent, we synthesized and radiolabeled several analogs of the potent antiandrogen bicalutamide: [18F]bicalutamide, 4-[76Br]bromobicalutamide, and [76Br]bromo-thiobicalutamide. Two of these analogs, 4-[76Br]bromobicalutamide and [76Br]bromo-thiobicalutamide, were found to have a substantially increased affinity for the androgen receptor (AR) compared to that of bicalutamide. The synthesis of [18F]bicalutamide utilized a pseudocarrier approach to effect addition of a carbanion generated from tracer-level amounts of a radiolabeled precursor to an unlabeled carbonyl precursor. 4-[76Br]Bromobicalutamide and [76Br]bromo-thiobicalutamide were labeled through electrophilic bromination of a tributylstannane precursor. The former could be prepared in high specific activity, and its tissue distribution was tested in vivo. Androgen target tissue uptake was evident in castrated adult male rats; however, in DES-treated, AR-positive, tumor-bearing male mice, tumor uptake was low.

Synthesis and biological evaluation of a fluorine-18-labeled nonsteroidal androgen receptor antagonist, N-(3-[18F]fluoro-4-nitronaphthyl)-cis-5-norbornene-endo-2,3-dicarboxylic imide

INTRODUCTION

Androgen receptor (AR), which is overexpressed in most prostate cancers, is the target of androgen ablation and antiandrogen therapies: it is also the target for the receptor-mediated imaging of AR-positive prostate cancer using radiolabeled ligands. Previous AR imaging agents were based on a steroidal core labeled with fluorine. To develop a novel class of nonsteroidal imaging agents, with binding and pharmacological characteristics that are more similar to those of clinically used AR antagonists, we synthesized N-(3-fluoro-4-nitronaphthyl)-cis-5-norbornene-endo-2,3-dicarboxylic imide (3-F-NNDI), an analog of recently reported AR antagonist ligands.

METHODS

3-F-NNDI was synthesized in six steps starting with 1-nitronaphthalene, with fluorine incorporation as the final step. The labeling of 3-F-NNDI with fluorine-18 was achieved through a novel, extremely mild, S(N)Ar displacement reaction of an o-nitro-activated arene trimethylammonium salt, and 3-[(18)F]F-NNDI was prepared in high specific activity.

RESULTS AND DISCUSSION

3-F-NNDI was found to have an AR-binding affinity similar to that of its parent compound. In vitro assays demonstrated high stability of the labeled compound under physiological conditions in buffer and in the blood. Androgen target tissue uptake in diethylstilbestrol-pretreated male rats, however, was minimal, probably because of extensive metabolic defluorination the radiolabeled ligand.

CONCLUSIONS

This study is part of our first look at a novel class of nonsteroidal AR antagonists as positron emission tomography (PET) imaging agents that are alternatives to steroidal AR agonist-based imaging agents. Although 3-[(18)F]F-NNDI has significant affinity for AR, it showed limited promise as a PET imaging agent because of its poor target tissue distribution properties.

Synthesis and biological evaluation of a nonsteroidal bromine-76-labeled androgen receptor ligand 3-[76Br]bromo-hydroxyflutamide

INTRODUCTION

Androgen receptors (ARs) are overexpressed in normal tissues and in most primary and metastatic prostate cancers. In our efforts to develop a nonsteroidal AR-specific imaging agent, we synthesized (+/-)-3-[(76)Br]bromo-hydroxyflutamide ((76)Br-), an analog of hydroxyflutamide, the active metabolite of the AR antagonist ligand flutamide.

MATERIALS AND METHODS

(76)Br- was synthesized in three steps, starting with commercially available compounds. Labeling of (76)Br- was achieved through the nucleophilic opening of an epoxide intermediate, and a labeled compound was obtained in high specific activity and good radiochemical yield.

RESULTS AND DISCUSSION

(+/-)-3-Bromo-hydroxyflutamide has a significantly higher affinity for ARs compared to hydroxyflutamide, its parent compound. The androgen target-tissue uptake of (76)Br- in diethylstilbestrol-treated male rats was examined; however, AR-mediated uptake was minimal due most likely to the rapid metabolic debromination of the radiolabeled ligand.

CONCLUSIONS

This study is part of our first look at a novel class of nonsteroidal AR antagonists as positron emission tomography (PET) imaging agents, which are alternatives to steroidal AR agonist-based imaging agents. Although (76)Br- has a significant affinity for ARs, it showed limited promise as a PET imaging agent because of its poor target-tissue distribution properties.