Assessment of PET tracer uptake in hormone-independent and hormone-dependent xenograft prostate cancer mouse models

Preclinical Imaging of Prostate Cancer

Prostate cancer remains a major cause of mortality and morbidity, affecting millions of men, with a large percentage expected to develop the disease as they reach advanced ages. Treatment and management advances have been dramatic over the past 50 years or so, and one aspect of these improvements is reflected in the multiple advances in diagnostic imaging techniques. Much attention has been focused on molecular imaging techniques that offer high sensitivity and specificity and can now more accurately assess disease status and detect recurrence earlier. During development of molecular imaging probes, single-photon emission computed tomography (SPECT) and positron emission tomography (PET) must be evaluated in preclinical models of the disease. If such agents are to be translated to the clinic, where patients undergoing these imaging modalities are injected with a molecular imaging probe, these agents must first be approved by the FDA and other regulatory agencies prior to their adoption in clinical practice. Scientists have worked assiduously to develop preclinical models of prostate cancer that are relevant to the human disease to enable testing of these probes and related targeted drugs. Challenges in developing reproducible and robust models of human disease in animals are beset with practical issues such as the lack of natural occurrence of prostate cancer in mature male animals, the difficulty of initiating disease in immune-competent animals and the sheer size differences between humans and conveniently smaller animals such as rodents. Thus, compromises in what is ideal and what can be achieved have had to be made. The workhorse of preclinical animal models has been, and remains, the investigation of human xenograft tumor models in athymic immunocompromised mice. Later models have used other immunocompromised models as they have been found and developed, including the use of directly derived patient tumor tissues, completely immunocompromised mice, orthotopic methods for inducing prostate cancer within the mouse prostate itself and metastatic models of advanced disease. These models have been developed in close parallel with advances in imaging agent chemistries, radionuclide developments, computer electronics advances, radiometric dosimetry, biotechnologies, organoid technologies, advances in in vitro diagnostics, and overall deeper understandings of disease initiation, development, immunology, and genetics. The combination of molecular models of prostatic disease with radiometric-based studies in small animals will always remain spatially limited due to the inherent resolution sensitivity limits of PET and SPECT decay processes, fundamentally set at around a 0.5 cm resolution limit. Nevertheless, it is central to researcher's efforts and to successful clinical translation that the best animal models are adopted, accepted, and scientifically verified as part of this truly interdisciplinary approach to addressing this important disease.

Absorbed dose calculation for a realistic CT-derived mouse phantom irradiated with a standard Cs-137 cell irradiator using a Monte Carlo method

Computed tomography (CT) derived Monte Carlo (MC) phantoms allow dose determination within small animal models that is not feasible with in-vivo dosimetry. The aim of this study was to develop a CT-derived MC phantom generated from a mouse with a xenograft tumour that could then be used to calculate both the dose heterogeneity in the tumour volume and out of field scattered dose for pre-clinical small animal irradiation experiments. A BEAMnrc Monte-Carlo model has been built of our irradiation system that comprises a lead collimator with a 1 cm diameter aperture fitted to a Cs-137 gamma irradiator. The MC model of the irradiation system was validated by comparing the calculated dose results with dosimetric film measurement in a polymethyl methacrylate (PMMA) phantom using a 1D gamma-index analysis. Dose distributions in the MC mouse phantom were calculated and visualized on the CT-image data. Dose volume histograms (DVHs) were generated for the tumour and organs at risk (OARs). The effect of the xenographic tumour volume on the scattered out of field dose was also investigated. The defined gamma index analysis criteria were met, indicating that our MC simulation is a valid model for MC mouse phantom dose calculations. MC dose calculations showed a maximum out of field dose to the mouse of 7% of Dmax. Absorbed dose to the tumour varies in the range 60%-100% of Dmax. DVH analysis demonstrated that tumour received an inhomogeneous dose of 12 Gy-20 Gy (for 20 Gy prescribed dose) while out of field doses to all OARs were minimized (1.29 Gy-1.38 Gy). Variation of the xenographic tumour volume exhibited no significant effect on the out of field scattered dose to OARs. The CT derived MC mouse model presented here is a useful tool for tumour dose verifications as well as investigating the doses to normal tissue (in out of field) for preclinical radiobiological research.

Molecular Imaging Assessment of Androgen Deprivation Therapy in Prostate Cancer

Hormonal therapy has long been recognized as a mainstay treatment for prostate cancer. New generation imaging agents have provided unprecedented opportunities at all phases along the natural history of prostate cancer. We review the literature on the effect of androgens and androgen deprivation therapy on prostate tumor at its various biological phases using the new generation molecular imaging agents in conjunction with positron emission tomography.

Iodinated Choline Transport-Targeted Tracers

We present a novel series of radioiodinated tracers and potential theranostics for diseases accompanied by pathological function of proteins involved in choline transport. Unlike choline analogues labeled with ¹¹C or ¹⁸F that are currently used in the clinic, the iodinated compounds described herein are applicable in positron emission tomography, single-photon emission computed tomography, and potentially in therapy, depending on the iodine isotope selection. Moreover, favorable half-lives of iodine isotopes result in much less challenging synthesis by isotope exchange reaction. Six of the described compounds were nanomolar ligands, and the best compound possessed an affinity 100-fold greater than that of choline. Biodistribution data of ¹²⁵I-labeled ligands in human prostate carcinoma bearing (PC-3) mice revealed two compounds with a biodistribution profile superior to that of [¹⁸F]fluorocholine.

A comparative study of peptide-based imaging agents [68Ga]Ga-PSMA-11, [68Ga]Ga-AMBA, [68Ga]Ga-NODAGA-RGD and [68Ga]Ga-DOTA-NT-20.3 in preclinical prostate tumour models

INTRODUCTION

Peptide-based imaging agents targeting prostate-specific membrane antigen (PSMA) have revolutionized the evaluation of biochemical recurrence of prostate cancer (PCa) but lacks sensitivity at very low serum prostate specific antigen (PSA) levels. Once recurrence is suspected, other positron emission tomography (PET) radiotracers could be of interest to discriminate between local and distant relapse. We studied [¹⁸F]fluorodeoxyglucose ([¹⁸F]FDG) targeting glucose metabolism, [¹⁸F]fluorocholine ([¹⁸F]FCH) targeting membrane metabolism and peptide-based imaging agents [⁶⁸Ga]Ga-PSMA-11, [⁶⁸Ga]Ga-AMBA, [⁶⁸Ga]Ga-NODAGA-RGD and [⁶⁸Ga]Ga-DOTA-NT-20.3 targeting PSMA, gastrin releasing peptide receptor (GRPr), αvβ3 integrin and neurotensin type 1 receptor (NTSR1) respectively, in different PCa tumour models.

METHODS

Mice were xenografted with 22Rv1, an androgen-receptor (AR)-positive, PCa cell line that expresses PSMA and PC3, an AR-negative one that does not express PSMA. PET imaging using the different radiotracers was performed sequentially and the uptake characteristics compared to one other. NTSR1 and PSMA expression levels were analysed in tumours by immunohistochemistry.

RESULTS

[¹⁸F]FDG displayed low but sufficient uptake to visualize PC3 and 22Rv1 derived tumours. We also observed a low efficacy of [¹⁸F]FCH PET imaging and a low [⁶⁸Ga]Ga-NODAGA-RGD tumour uptake in those tumours. As expected, an elevated tumour uptake was obtained for [⁶⁸Ga]Ga-PSMA-11 in 22Rv1 derived tumour although no uptake was measured in the androgen independent cell line PC3, derived from a bone metastasis of a high-grade PCa. Moreover, in PC3 cell line, we obtained good tumour uptake, high tumour-to-background contrast using [⁶⁸Ga]Ga-AMBA and [⁶⁸Ga]Ga-DOTA-NT-20.3. Immunohistochemistry analysis confirmed high NTSR1 expression in PC3 derived tumours and conversely high PSMA expression in 22Rv1 derived tumours.

CONCLUSION

PET imaging using [⁶⁸Ga]Ga-AMBA and [⁶⁸Ga]Ga-DOTA-NT-20.3 demonstrates that GRPr and NTSR1 could represent viable alternative targets for diagnostic or therapeutic applications in PCa with limited PSMA expression levels. More preclinical and clinical studies will follow to explore this potential.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT

Peptide-based imaging agents targeting PSMA represent a major progress in the evaluation of biochemical recurrence of PCa but sometimes yield false negative results in some lesions. Continuing efforts have thus been made to evaluate other radiotracers. Our preclinical results suggest that [⁶⁸Ga]labelled bombesin and neurotensin analogues could serve as alternative PET radiopharmaceuticals for diagnostic or therapy in cases of PSMA-negative PCa.

Bridging from Brain to Tumor Imaging: (S)-(-)- and (R)-(+)-[18F]Fluspidine for Investigation of Sigma-1 Receptors in Tumor-Bearing Mice

Sigma-1 receptors (Sig1R) are highly expressed in various human cancer cells and hence imaging of this target with positron emission tomography (PET) can contribute to a better understanding of tumor pathophysiology and support the development of antineoplastic drugs. Two Sig1R-specific radiolabeled enantiomers (S)-(-)- and (R)-(+)-[18F]fluspidine were investigated in several tumor cell lines including melanoma, squamous cell/epidermoid carcinoma, prostate carcinoma, and glioblastoma. Dynamic PET scans were performed in mice to investigate the suitability of both radiotracers for tumor imaging. The Sig1R expression in the respective tumors was confirmed by Western blot. Rather low radiotracer uptake was found in heterotopically (subcutaneously) implanted tumors. Therefore, a brain tumor model (U87-MG) with orthotopic implantation was chosen to investigate the suitability of the two Sig1R radiotracers for brain tumor imaging. High tumor uptake as well as a favorable tumor-to-background ratio was found. These results suggest that Sig1R PET imaging of brain tumors with [18F]fluspidine could be possible. Further studies with this tumor model will be performed to confirm specific binding and the integrity of the blood-brain barrier (BBB).

[68Ga]pentixafor for CXCR4 imaging in a PC-3 prostate cancer xenograft model - comparison with [18F]FDG PET/CT, MRI and ex vivo receptor expression

Purpose

The aim was to characterize the properties of [⁶⁸Ga]Pentixafor as tracer for prostate cancer imaging in a PC-3 prostate cancer xenograft mouse model and to investigate its correlation with [¹⁸F]FDG PET/CT, magnetic resonance imaging (MRI) and ex vivo analyses.

Methods

Static [⁶⁸Ga]Pentixafor and [¹⁸F]FDG PET as well as morphological/ diffusion weighted MRI and ¹H MR spectroscopy was performed. Imaging data were correlated with ex vivo biodistribution and CXCR4 expression in PC-3 tumors (immunohistochemistry (IHC), mRNA analysis). Flow cytometry was performed for evaluation of localization of CXCR4 receptors (in vitro PC-3 cell experiments).

Results

Tumor uptake of [⁶⁸Ga]Pentixafor was significantly lower compared to [¹⁸F]FDG. Ex vivo CXCR4 mRNA expression of tumors was shown by PCR. Only faint tumor CXCR4 expression was shown by IHC (immuno reactive score of 3). Accordingly, flow cytometry of PC-3 cells revealed only a faint signal, cell membrane permeabilisation showed a slight signal increase. There was no significant correlation of [⁶⁸Ga]Pentixafor tumor uptake and ex vivo receptor expression. Spectroscopy showed typical spectra of prostate cancer.

Conclusion

PC-3 tumor uptake of [⁶⁸Ga]Pentixafor was existent but lower compared to [¹⁸F]FDG. No significant correlation of ex vivo tumor CXCR4 receptor expression and [⁶⁸Ga]Pentixafor tumor uptake was shown. CXCR4 receptor expression on the surface of PC-3 cells was existent but rather low possibly explaining the limited [⁶⁸Ga]Pentixafor tumor uptake; receptor localization in the interior of PC-3 cells is presumable as shown by cell membrane permeabilisation. Further studies are necessary to define the role of [⁶⁸Ga]Pentixafor in prostate cancer imaging.

What Medical, Urologic, and Radiation Oncologists Want from Molecular Imaging of Prostate Cancer

As molecular imaging better delineates the state of prostate cancer, clinical management will evolve. The currently licensed imaging modalities are limited by lack of specificity or sensitivity for the extent of cancer and for predicting outcome in response to therapy. Clinicians want molecular imaging that-by being more reliable in tailoring treatment and monitoring response for each patient-will become a key facet of precision medicine, surgery, and radiation therapy. Identifying patients who are candidates for specific or novel treatments is important, but equally important is the finding that a given patient may not be a good candidate for single-modality therapy. This article presents prostate cancer scenarios in which managing clinicians would welcome molecular imaging innovations to help with decision making. The potential role of newer techniques that may help fill this wish list is discussed.

An Assessment of Early Response to Targeted Therapy via Molecular Imaging: A Pilot Study of 3'-deoxy-3'[(18)F]-Fluorothymidine Positron Emission Tomography 18F-FLT PET/CT in Prostate Adenocarcinoma

Fluorothymidine is a thymidine analog labeled with fluorine-18 fluorothymidine for positron emission tomography (¹⁸F-FLT-PET) imaging. Thymidine is a nucleic acid that is used to build DNA. Fluorine-18 fluorothymidine (¹⁸F-FLT) utilizes the same metabolic pathway as does thymidine but has a very low incidence of being incorporated into the DNA (<1%). ¹⁸F-FLT-PET could have a role in the evaluation of response to targeted therapy. We present here a pilot study where we investigated cellular metabolism and proliferation in patients with prostate cancer before and after targeted therapy. Seven patients with Stage IV prostate adenocarcinoma, candidates for targeted therapy inhibiting the hepatocyte growth factor/tyrosine-protein kinase Met (HGF/C-MET) pathway, were included in this study. The HGF/C-MET pathway is implicated in prostate cancer progression, and an evaluation of the inhibition of this pathway could be valuable. ¹⁸F-FLT was performed at baseline and within four weeks post-therapy. Tumor response was assessed semi-quantitatively and using visual response criteria. The range of SUVmax for ¹⁸F-FLT at baseline in the prostate varied from 2.5 to 4.2. This study demonstrated that ¹⁸F-FLT with positron emission tomography/computerized tomography (¹⁸F-FLT PET/CT) had only limited applications in the early response evaluation of prostate cancer. ¹⁸F-FLT PET/CT may have some utility in the assessment of response in lymph node disease. However, ¹⁸F-FLT PET/CT was not found to be useful in the evaluation of the prostate bed, metastatic skeletal disease, and liver disease.

Preclinical Applications of 3'-Deoxy-3'-[18F]Fluorothymidine in Oncology - A Systematic Review

The positron emission tomography (PET) tracer 3'-deoxy-3'-[18F]fluorothymidine ([18F]FLT) has been proposed to measure cell proliferation non-invasively in vivo. Hence, it should provide valuable information for response assessment to tumor therapies. To date, [18F]FLT uptake has found limited use as a response biomarker in clinical trials in part because a better understanding is needed of the determinants of [18F]FLT uptake and therapy-induced changes of its retention in the tumor. In this systematic review of preclinical [18F]FLT studies, comprising 174 reports, we identify the factors governing [18F]FLT uptake in tumors, among which thymidine kinase 1 plays a primary role. The majority of publications (83 %) report that decreased [18F]FLT uptake reflects the effects of anticancer therapies. 144 times [18F]FLT uptake was related to changes in proliferation as determined by ex vivo analyses. Of these approaches, 77 % describe a positive relation, implying a good concordance of tracer accumulation and tumor biology. These preclinical data indicate that [18F]FLT uptake holds promise as an imaging biomarker for response assessment in clinical studies. Understanding of the parameters which influence cellular [18F]FLT uptake and retention as well as the mechanism of changes induced by therapy is essential for successful implementation of this PET tracer. Hence, our systematic review provides the background for the use of [18F]FLT in future clinical studies.

Evaluation of the Metabolic Activity of Echinococcus multilocularis in Rodents Using Positron Emission Tomography Tracers

PURPOSE

2-Deoxy-2-[(18)F]fluoro-D-glucose ([(18)F]FDG) has been used as a standard clinical positron emission tomography (PET) tracer for the follow-up of the rare but life-threatening parasitic disease alveolar echinococcosis (AE). Given that the disease is endemic in many countries in the northern hemisphere and the diagnosis is still challenging, the aim of our study was to evaluate further clinically relevant PET tracers as possible diagnostic tools for AE in vitro and in vivo.

PROCEDURES

Various clinically used PET tracers were evaluated in vitro and assessed in an in vivo AE animal model based on PET/magnetic resonance (MR) measurements.

RESULTS

In vitro binding assays displayed high uptake of [(18)F]FDG in a cell suspension of E. multilocularis tissue, whereas 3'-deoxy-3'-[(18)F]fluorothymidine ([(18)F]FLT) and [(11)C]choline were found to be taken up strongly by E. multilocularis vesicles. [(18)F]FDG and [(18)F]FLT displayed an elevated uptake in vivo, which appeared as several foci throughout the parasite tissue as opposed to [(18)F]fluoro-azomycinarabinofuranoside ([(18)F]FAZA) and [(11)C]choline.

CONCLUSIONS

Our data clearly demonstrate that the clinically applied PET tracer [(18)F]FDG is useful for the diagnosis and disease staging of AE but also has drawbacks in the assessment of currently inactive or metabolically weak parasitic lesions. The different tested PET tracers do not show the potential for the replacement or supplementation of current diagnostic strategies. Hence, there is still the need for novel diagnostic tools.

Radiation treatment monitoring using multimodal functional imaging: PET/CT ((18)F-Fluoromisonidazole & (18)F-Fluorocholine) and DCE-US

Background

This study aims to assess the effect of radiation treatment on the tumour vasculature and its downstream effects on hypoxia and choline metabolism using a multimodal approach in the murine prostate tumour model CWR22. Functional parameters derived from Positron Emission Tomography (PET)/Computer Tomography (CT) with ¹⁸F-Fluoromisonidazole (¹⁸F-FMISO) and ¹⁸F-Fluorocholine (¹⁸F-FCH) as well as Dynamic Contrast-Enhanced Ultrasound (DCE-US) were employed to determine the relationship between metabolic parameters and microvascular parameters that reflect the tumour microenvironment. Immunohistochemical analysis was employed for validation.

Methods

PET/CT and DCE-US were acquired pre- and post-treatment, at day 0 and day 3, respectively. At day 1, radiation treatment was delivered as a single fraction of 10 Gy. Two experimental groups were tested for treatment response with ¹⁸F-FMISO and ¹⁸F-FCH.

Results

The maximum Standardized Uptake Values (SUVmax) and the mean SUV (SUVmean) for the ¹⁸F-FMISO group were decreased after treatment, and the SUVmean of the tumour-to-muscle ratio was correlated to microvessel density (MVD) at day 3. The kurtosis of the amplitude of the contrast uptake A was significantly decreased for the control tumours in the ¹⁸F-FCH group. Furthermore, the eliminating rate constant of the contrast agent from the plasma k_el derived from DCE-US was negatively correlated to the SUVmean of tumour-to-muscle ratio, necrosis and MVD.

Conclusions

The present study suggests that the multimodal approach using ¹⁸F-FMISO PET/CT and DCE-US seems reliable in the assessment of both microvasculature and necrosis as validated by histology. Thus, it has valuable diagnostic and prognostic potential for early non-invasive evaluation of radiotherapy.

Molecular Imaging of Prostate Cancer

Prostate cancer is the most common noncutaneous malignancy among men in the Western world. The natural history and clinical course of prostate cancer are markedly diverse, ranging from small indolent intraprostatic lesions to highly aggressive disseminated disease. An understanding of this biologic heterogeneity is considered a necessary requisite in the quest for the adoption of precise and personalized management strategies. Molecular imaging offers the potential for noninvasive assessment of the biologic interactions underpinning prostate carcinogenesis. Currently, numerous molecular imaging probes are in clinical use or undergoing preclinical or clinical evaluation. These probes can be divided into those that image increased cell metabolism, those that target prostate cancer-specific membrane proteins and receptor molecules, and those that bind to the bone matrix adjacent to metastases to bone. The increased metabolism and vascular changes in prostate cancer cells can be evaluated with radiolabeled analogs of choline, acetate, glucose, amino acids, and nucleotides. The androgen receptor, prostate-specific membrane antigen, and gastrin-releasing peptide receptor (ie, bombesin) are overexpressed in prostate cancer and can be targeted by specific radiolabeled imaging probes. Because metastatic prostate cancer cells induce osteoblastic signaling pathways of adjacent bone tissue, bone-seeking radiotracers are sensitive tools for the detection of metastases to bone. Knowledge about the underlying biologic processes responsible for the phenotypes associated with the different stages of prostate cancer allows an appropriate choice of methods and helps avoid pitfalls.

18F-fluorodeoxyglucose PET/CT for detection of disease in patients with prostate-specific antigen relapse following radical treatment of a local-stage prostate cancer

The present study aimed to retrospectively review the contribution of ¹⁸F-fluorodeoxygluose-positron emission tomography/computed tomography (¹⁸F-FDG PET/CT) in the assessment of biochemical recurrence in patients with a diagnosis of local-stage prostate cancer (PCa) who underwent radical prostatectomy (RP) or received external beam radiation therapy (EBRT). A total of 28 patients who underwent RP or received EBRT for PCa between July 2007 and April 2013, and who underwent ¹⁸F-FDG PET/CT scanning for re-staging due to biochemical recurrence were included in the present study. The mean age of the patients was 65.07 years and the standard deviation was 7.51 years (range, 51-82 years). Of the 28 patients, 23 (82.1%) underwent RP and 5 (17.9%) received definitive EBRT. Prior to scanning, all patients were required to fast for 6 h, and ~1 h after the intravenous injection of 555 MBq ¹⁸F-FDG, whole-body PET scans were performed from the skull base to the upper thighs. Whole-body CT scans were performed in the craniocaudal direction. ¹⁸F-FDG PET images were reconstructed using CT data for attenuation correction. Histopathology examination or clinical follow-up was used to confirm any suspicious recurrent or metastatic lesions. The sensitivity, specificity, positive predictive value, negative predictive value and accuracy of ¹⁸F-FDG PET/CT were 61.6, 75.0, 61.6, 75.0 and 71.4%, respectively. ¹⁸F-FDG PET/CT can detect local and distant metastases with a high accuracy in the assessment of biochemical recurrence, thus detecting occult metastases and allowing the re-staging of PCa in the patients receiving definitive treatment. It is considered that ¹⁸F-FDG PET/CT may be useful in re-assessing the patients with PCa receiving definitive treatment.

Incidental detection of prostate-specific antigen-negative metastatic prostate cancer initially presented with solitary pulmonary nodule on fluorodeoxyglucose positron emission tomography/computed tomography

A 71-year-old male patient with solitary pulmonary nodule underwent fluorodeoxyglucose positron emission tomography/computed tomography (FDG PET/CT) showing slightly increased FDG uptake in this nodule. In addition, PET/CT detected hypermetabolic sclerotic bone lesions in the right second rib and 7^th thoracic vertebrae, which were interpreted as possible metastases, and mildly increased FDG uptake in the prostate gland highly suspicious of malignancy. The patient's prostate-specific antigen (PSA) level was within normal range (3.8 ng/dL). The histopathological examination of the lung nodule and right second rib lesion proved metastases from prostate cancer, then the prostate biopsy-confirmed prostate adenocarcinoma. The unique feature of this case is to emphasize the importance of performing PET/CT for solitary pulmonary nodule in detecting PSA-negative metastatic prostate cancer. This case indicated that it should be kept in mind that, even if the PSA is negative, a lung metastasis of prostate cancer may be an underlying cause in patients evaluated for solitary pulmonary nodule by FDG PET/CT.

[(18)F]-Fluorodeoxyglucose Positron Emission Tomography/Computed Tomography of LAPC4-CR Castration-Resistant Prostate Cancer Xenograft Model in Soft Tissue Compartments

Preclinical xenograft models have contributed to advancing our understanding of the molecular basis of prostate cancer and to the development of targeted therapy. However, traditional preclinical in vivo techniques using caliper measurements and survival analysis evaluate the macroscopic tumor behavior, whereas tissue sampling disrupts the microenvironment and cannot be used for longitudinal studies in the same animal. Herein, we present an in vivo study of [¹⁸F]-fluorodeoxyglucose (FDG) positron emission tomography (PET)/computed tomography (CT) designed to evaluate the metabolism within the microenvironment of LAPC4-CR, a unique murine model of castration-resistant prostate cancer. Mice bearing LAPC4-CR subcutaneous tumors were administered [¹⁸F]-FDG via intravenous injection. After a 60-minute distribution phase, the mice were imaged on a PET/CT scanner with submillimeter resolution; and the fused PET/CT images were analyzed to evaluate tumor size, location, and metabolism across the cohort of mice. The xenograft tumors showed [¹⁸F]-FDG uptake that was independent of tumor size and was significantly greater than uptake in skeletal muscle and liver in mice (Wilcoxon signed-rank P values of .0002 and .0002, respectively). [¹⁸F]-FDG metabolism of the LAPC4-CR tumors was 2.1 ± 0.8 ID/cm³*wt, with tumor to muscle ratio of 7.4 ± 4.7 and tumor to liver background ratio of 6.7 ± 2.3. Noninvasive molecular imaging techniques such as PET/CT can be used to probe the microenvironment of tumors in vivo. This study showed that [¹⁸F]-FDG-PET/CT could be used to image and assess glucose metabolism of LAPC4-CR xenografts in vivo. Further work can investigate the use of PET/CT to quantify the metabolic response of LAPC4-CR to novel agents and combination therapies using soft tissue and possibly bone compartment xenograft models.

Imaging in prostate carcinoma

Imaging plays a central role in the detection, diagnosis, staging, and follow-up of prostate carcinoma. This article discusses the role of multiple imaging modalities in the diagnosis and staging of prostate cancer, with attention to imaging features of localized and metastatic disease, imaging adjuncts to improve prostate biopsy, and potential imaging biomarkers. In addition, the role of imaging in the management of prostate cancer, with emphasis on surveillance, evaluation of response to new therapies, and detection of recurrent disease is described. Lastly, future directions in prostate cancer imaging are presented.

Pharmacokinetics and PET imaging properties of two recombinant anti-PSMA antibody fragments in comparison to their parental antibody

BACKGROUND

Radioimmunoimaging with disease-specific tracers can be advantageous compared to that with nonspecific tracers for the imaging of glucose metabolism and cell proliferation. Monoclonal antibodies (mAbs) or their fragments are excellent tools for immuno-positron emission tomography (PET). In this study, PSMA-specific mAb 3/F11 and its recombinant fragments were compared for the imaging of prostate cancer in xenografts.

METHODS

Recombinant anti-PSMA antibody fragments D7-Fc and D7-CH3 were constructed by genetically fusing the binding domains of mAb 3/F11 (D7) to the human IgG3 CH3 or CH2-CH3 (Fc) domain. The fragments and the mAb 3/F11 were DOTA conjugated, tested in vitro, and radiolabeled with (64) Cu. PSMA-positive C4-2 and PSMA-negative DU 145 prostate cancer xenografts were used for PET-MR imaging and for ex vivo biodistribution.

RESULTS

The constructs showed strong and specific binding to PSMA-positive C4-2 cells in vitro which did not decrease after DOTA conjugation. Both tested fragments showed stable accumulation in PSMA-positive C4-2 tumors at all measured time points but reduced uptake compared to the full-length antibody. Other organs and PSMA-negative tumors showed a very low tracer uptake only 3 hr after injection, with the exception of the kidneys, which demonstrated high radioactivity uptake due to rapid renal clearance of the mAb fragments.

CONCLUSION

Stable tumor uptake and fast serum clearance of the tested radiolabeled fragments was observed in this preclinical study compared to the full length mAb. Since the fragments show rapid and specific tumor uptake, the tested fragments might serve as tools for theranostic imaging with suitable isotopes for radioimmunotherapy.

Evaluation of positron emission tomographic tracers for imaging of papillomavirus-induced tumors in rabbits

In this study, simultaneous positron emission tomography (PET)/magnetic resonance (MR) imaging was employed to evaluate the feasibility of the PET tracers 2-deoxy-2-18F-fluoro-d-glucose (18F-FDG), 11C-choline, and 18F-fluorothymidine (18F-FLT) to detect papillomavirus-induced tumors in an established rabbit model system. The combined PET/MR allowed the analysis of tracer uptake of the tumors using the morphologic information acquired by MR. New Zealand White rabbits were infected with cottontail rabbit papillomavirus genomes and were imaged for up to 10 months with a simultaneous PET/MR system during the course of infection. The uptake characteristics of the PET tracers 11C-choline and 18F-FLT of tumors and reference tissues were examined relative to the clinical standard, 18F-FDG. Tracer biodistribution of various organs was measured by gamma-counting after the last PET scan and compared to the in vivo PET/MR 18F-FDG uptake. Increased tracer uptake was found 2 months postinfection in primary tumors with 18F-FDG and 11C-choline, whereas 18F-FLT failed to detect the tumors at all measured time points. Our data show that the PET tracer 18F-FDG is superior for imaging papillomavirus-induced tumors in rabbits compared to 11C-choline and 18F-FLT. However, 11C-choline imaging, which has previously been applied to detect various tumor entities in patients, appears to be an alternative to 18F-FDG.

Preclinical evaluation of a novel c-Met inhibitor in a gastric cancer xenograft model using small animal PET

PURPOSE

Here, we describe the efficacy of the novel small molecule c-Met inhibitor BAY 853474 in reducing tumor growth in the Hs746T gastric cancer xenograft model and tested the suitability of 2-deoxy-2-[(18)F]fluoro-D-glucose ([(18)F]FDG) versus 3'-deoxy-3'-18F-fluorothymidine ([(18)F]FLT) for response monitoring in a gastric cancer xenograft mouse model using small animal PET.

PROCEDURES

The c-Met inhibitor or vehicle control was administered orally at various doses in tumor-bearing mice. Glucose uptake and proliferation was measured using PET before, 48 and 96 h after the first treatment. The PET data were compared to data from tumor growth curves, autoradiography, Glut-1 and Ki-67 staining of tumor sections, and biochemical analysis of tissue probes, i.e., c-Met and ERK phosphorylation and cyclin D1 levels.

RESULTS

BAY 853474 significantly reduces tumor growth. [(18)F]FDG uptake in Hs746T tumors was significantly reduced in the groups receiving the drug, compared with the control group. The [(18)F]FLT uptake in the tumor tissue was completely absent 96 h after treatment. Autoradiographic, immunohistochemical, and biochemical analyses confirmed the PET findings. Treatment with the c-Met inhibitor did not affect body weight or glucose levels, and no adverse effects were observed in the animals.

CONCLUSION

These preclinical findings suggest that clinical PET imaging is a useful tool for early response monitoring in clinical studies.

Update on positron emission tomography for imaging of prostate cancer

Prostate cancer is the most common non-cutaneous malignancy among men in the Western world, and continues to be a major health problem. Imaging has recently become more important in the clinical management of prostate cancer patients, including diagnosis, staging, choice of optimal treatment strategy, treatment follow up and restaging. Positron emission tomography, a functional and molecular imaging technique, has opened a new field in clinical oncological imaging. The most common positron emission tomography radiotracer, 18F-fluorodeoxyglucose, has been limited in imaging of prostate cancer. Recently, however, other positron emission tomography tracers, such as 11C-acetate and 11C- or (18) F-choline, have shown promising results. In the present review article, we overview the potential and current use of positron emission tomography or positron emission tomography/computed tomography imaging employing the four most commonly used positron emission tomography radiotracers, 18F-fluorodeoxyglucose, 11C-acetate and 11C- or 18F-choline, for imaging evaluation of prostate cancer.

Evaluation of androgen-induced effects on the uptake of [18F]FDG, [11C]choline and [11C]acetate in an androgen-sensitive and androgen-independent prostate cancer xenograft model

Background

Androgen deprivation (AD) is generally used as a first-line palliative treatment in prostate cancer (PCa) patients with rising prostate-specific antigen (PSA) after primary therapy. To acquire an accurate detection of tumour viability following AD with positron emission tomography (PET), an androgen-independent uptake of tracers would be advantageous. Several metabolic PET tracers are employed for detecting recurrent PCa. We evaluated the effect of AD on the uptake of 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG), [¹¹C]choline and [¹¹C]acetate in vivo.

Methods

An [¹⁸F]FDG, [¹¹C]choline and [¹¹C]acetate baseline micro(μ)PET/μ computed tomography (CT) scan was subsequently performed in xenografts of androgen-sensitive (LAPC-4) and androgen-independent (22Rv1) tumours in nude mice. An untreated control group was compared to a surgical castration group, i.e. androgen-deprived group. μPET/μCT imaging with the above-mentioned tracers was repeated 5 days after the start of treatment. The percentage change of SUV_max and SUV_meanTH in the tumours was calculated.

Results

AD did not significantly affect the uptake of [¹⁸F]FDG and [¹¹C]choline in LAPC-4 tumours as compared with the uptake of both tracers in untreated tumours. In control 22Rv1 tumours, [¹¹C]choline and [¹⁸F]FDG uptake increased over time. However, compared with the uptake in control tumours, AD significantly decreased the uptake of [¹¹C]choline and tended to decrease [¹⁸F]FDG uptake. [¹¹C]acetate uptake remained unaffected by AD in both PCa xenograft models.

Conclusions

[¹⁸F]FDG and especially [¹¹C]choline PET, which is currently used for the detection of recurrent PCa, could miss or underestimate the presence of local recurrent PCa following AD therapy. [¹¹C]acetate uptake occurs independently of androgens and thus may be more favourable for detecting tumour viability during or following AD.

Imaging evaluation of prostate cancer with 18F-fluorodeoxyglucose PET/CT: utility and limitations

Prostate cancer is a major public health problem in developed countries. The remarkable biological and clinical heterogeneity of prostate cancer provides unique opportunities as well as challenges for the diagnostic imaging evaluation of this prevalent disease. The disease is characterized by a natural history that ranges from localized slowly growing hormone-dependent tumor progressing to metastatic hormone-refractory disease. PET is an ideal imaging tool for noninvasive interrogation of the underlying tumor biology. (18)F-FDG is the most common PET radiotracer used for oncological applications based upon elevated glucose metabolism in malignant tissue in comparison to normal tissue. FDG uptake in prostate cancer depends on tumor differentiation with low accumulation in well-differentiated tumors and high uptake in aggressive poorly differentiated tumors. Cumulative current evidence suggests that FDG PET may be useful in detection of disease in a small fraction of patients with biochemical recurrence, in the imaging evaluation of extent and treatment response in metastatic disease and in prediction of patient outcome.

Molecular imaging of prostate cancer: PET radiotracers

OBJECTIVE

Recent advances in the fundamental understanding of the complex biology of prostate cancer have provided an increasing number of potential targets for imaging and treatment. The imaging evaluation of prostate cancer needs to be tailored to the various phases of this remarkably heterogeneous disease.

CONCLUSION

In this article, I review the current state of affairs on a range of PET radiotracers for potential use in the imaging evaluation of men with prostate cancer.