Contribution of perfusion to the 11 C-acetate signal in brown adipose tissue assessed by DCE-MRI and 68 Ga-DOTA PET in a rat model.

PURPOSE: Determine if dynamic contrast enhanced (DCE) -MRI and/or 68 gallium 1,4,7,10-tetraazacyclododecane N, N', N″, N‴-tretraacetic acid (68 Ga-DOTA) positron emission tomography (PET) can assess perfusion in rat brown adipose tissue (BAT). Evaluate changes in perfusion between cold-stimulated and heat-inhibited BAT. Determine if the 11 C-acetate pharmacokinetic model can be constrained with perfusion information to improve assessment of BAT oxidative metabolism. METHODS: Rats were split into three groups. In group 1 (N = 6), DCE-MRI with gadobutrol was compared directly to 68 Ga-DOTA PET following exposure to 10 °C for 48 h. 11 C-Acetate PET was also performed to assess oxidation. In group 2 (N = 4), only 68 Ga-DOTA PET was acquired following exposure to 10 °C for 48 h. Finally, in group 3 (N = 10), perfusion was assessed with DCE-MRI in rats exposed to 10 °C or 30 °C for 48 h, and oxidation was measured with 11 C-acetate. Perfusion was quantified with a two-compartment pharmacokinetic model, while oxidation was assessed by a four-compartment model. RESULTS: DCE-MRI and 68 Ga-DOTA PET provided similar perfusion measures, but a decrease in the perfusion signal was noted with longer imaging sessions. Exposure to 10 °C or 30 °C did not affect the perfusion measures, but the 11 C-acetate signal increased in BAT at 10 °C. Without prior information about blood volume, the 11 C-acetate compartment model overestimated blood volume and underestimated oxidation in 10 °C BAT. CONCLUSION: Precise assessment of oxidation via 11 C-acetate PET requires prior information about blood volume which can be obtained by DCE-MRI or 68 Ga-DOTA PET. Since perfusion can change rapidly, simultaneous PET-MRI would be preferred.

[18F]-fluoroestradiol quantitative PET imaging to differentiate ER+ and ERα-knockdown breast tumors in mice.

INTRODUCTION: The purpose of this study was to develop a noninvasive model in tumor-bearing mice to investigate the use of 16α-[(18)F]fluoro-17ß-estradiol (FES) positron emission tomography (PET) imaging as a tool to discriminate between tumors having different estrogen receptor (ER) α status. METHODS: MC7-L1 and MC4-L2 murine mammary adenocarcinoma cell lines (ER+) received a small hairpin RNA targeting the ERα gene by lentiviral infection. In vitro assessment of ERα levels of the new cell lines (MC7-L1 and MC4-L2 ERα-knockdown; ERαKD), compared to the parental cell lines, was performed by immunoblotting (-75% ERα protein) and binding assays (-50% estrogen binding). These cell lines were implanted subcutaneously in Balb/c mice and allowed to grow up to a volume of at least 20 mm(3). FES and [(18)F]fluorodeoxyglucose (FDG) PET images were acquired to measure FES and FDG uptake in the various tumors. RESULTS: FES uptake as assessed by PET imaging was 1.06±0.21 percent injected dose per gram of tissue (%ID/g) for MC7-L1 tumors and 0.47±0.08 %ID/g for MC7-L1 ERαKD tumors. MC4-L2 tumors had a FES uptake of 1.03±0.30 %ID/g, whereas its ERαKD equivalent was 0.51±0.19 %ID/g. Each ERαKD tumor had a significantly lower %ID/g value, by ~50%, than its ER+ counterpart. Biodistribution studies confirmed these findings and gave %ID/g values that were not significantly different from PET imaging data. FDG PET showed no significant uptake difference between the ER+ and ERαKD tumors, indicating that the metabolic phenotype of the ERαKD cell lines was not altered. CONCLUSION: FES PET imaging was able to reliably differentiate between tumors having differences in their ERα expression in vivo, in a mouse model. Quantitative data obtained by FES PET were in concordance with biodistribution studies and in vitro assays. It is concluded that FES PET imaging can likely be used to monitor subtle ER status changes during the course of hormone therapy.

Irradiation of normal mouse tissue increases the invasiveness of mammary cancer cells.

PURPOSE: Treatment of breast tumours frequently involves irradiating the whole breast to reach malignant microfoci scattered throughout the breast. In this study, we determined whether irradiation of normal tissues could increase the invasiveness of breast cancer cells in a mouse model. MATERIALS AND METHODS: Non-irradiated MC7-L1 mouse mammary carcinoma cells were injected subcutaneously in irradiated and non-irradiated thighs of Balb/c mice. The invasion volume, tumour volume, blood vessel permeability and interstitial volumes were monitored by magnetic resonance imaging (MRI). Slices of normal tissue invaded by cancer cells were examined by histology. Activity of matrix metalloproteinase -2 and -9 (MMP -2 and -9) in healthy and irradiated tissues was determined, and the proliferation index of the invading cancer cells was evaluated. RESULTS: Three weeks after irradiation, enhancement of MC7-L1 cells invasiveness in irradiated thighs was already detected by MRI. The tumour invasion volume continued to extend 28- to 37-fold compared to the non-irradiated implantation site for the following three weeks, and it was associated with an increase of MMP-2 and -9 activities in healthy tissues. The interstitial volume associated with invading cancer cells was significantly larger in the pre-irradiated sites; while the blood vessels permeability was not altered. Cancer cells invading the healthy tissues were proliferating at a lower rate compared to non-invading cancer cells. CONCLUSION: Implantation of non-irradiated mammary cancer cells in previously irradiated normal tissue enhances the invasive capacity of the mammary cancer cells and is associated with an increased activity of MMP-2 and -9 in the irradiated normal tissue.

Behavioral, medical imaging and histopathological features of a new rat model of bone cancer pain.

Pre-clinical bone cancer pain models mimicking the human condition are required to respond to clinical realities. Breast or prostate cancer patients coping with bone metastases experience intractable pain, which affects their quality of life. Advanced monitoring is thus required to clarify bone cancer pain mechanisms and refine treatments. In our model of rat femoral mammary carcinoma MRMT-1 cell implantation, pain onset and tumor growth were monitored for 21 days. The surgical procedure performed without arthrotomy allowed recording of incidental pain in free-moving rats. Along with the gradual development of mechanical allodynia and hyperalgesia, behavioral signs of ambulatory pain were detected at day 14 by using a dynamic weight-bearing apparatus. Osteopenia was revealed from day 14 concomitantly with disorganization of the trabecular architecture (µCT). Bone metastases were visualized as early as day 8 by MRI (T(1)-Gd-DTPA) before pain detection. PET (Na(18)F) co-registration revealed intra-osseous activity, as determined by anatomical superimposition over MRI in accordance with osteoclastic hyperactivity (TRAP staining). Pain and bone destruction were aggravated with time. Bone remodeling was accompanied by c-Fos (spinal) and ATF3 (DRG) neuronal activation, sustained by astrocyte (GFAP) and microglia (Iba1) reactivity in lumbar spinal cord. Our animal model demonstrates the importance of simultaneously recording pain and tumor progression and will allow us to better characterize therapeutic strategies in the future.

A new tool for molecular imaging: the microvolumetric beta blood counter.

UNLABELLED: Radiotracer kinetic modeling in small animals with PET allows absolute quantification of physiologic and biochemical processes in vivo. It requires blood and tissue tracer concentrations as a function of time. Manual sampling, the reference method for blood tracer concentration measurements, requires fairly large amounts of blood besides being technically difficult and time-consuming. An automated microvolumetric beta blood counter (microBC) was designed to circumvent these limitations by measuring the blood activity in real time with PET scanning. METHODS: The microBC uses direct beta-particle detection to reduce its footprint and is entirely remote controlled for sampling protocol selection and real-time monitoring of measured parameters. Sensitivity has been determined for the most popular PET radioisotopes ((18)F, (13)N, (11)C, (64)Cu). Dispersion within the sampling catheter has been modeled to enable automatic correction. Blood curves obtained with the microBC were compared with manual samples and PET-derived data. The microBC was used to estimate the myocardial blood flow (MBF) of mice injected with (13)N-ammonia and to compare the myocardial metabolic rate of glucose (MMRG) of rats injected with (18)F-FDG for arterial and venous cannulation sites. RESULTS: The sensitivity limit ranges from 3 to 104 Bq/microL, depending on the isotope and the catheter used, and was found to be adequate for most small-animal studies. Automatic dispersion correction appears to be a good approximation of dispersion-free reference curves. Blood curves sampled with the microBC are well correlated with curves obtained from manual samples and PET images. With correction for dispersion, the MBF of anesthetized mice at rest was found to be 4.84 +/- 0.5 mL/g/min, which is comparable to values found in the literature for rats. MMRG values derived from the venous blood tracer concentration are underestimated by 60% as compared with those derived from arterial blood. CONCLUSION: The microBC is a compact automated counter allowing real-time measurement of blood radioactivity for pharmacokinetic studies in animals as small as mice. Reliable and reproducible, the device makes it possible to increase the throughput of pharmacokinetic studies with reduced blood sample handling and staff exposure, contributing to speed up new drug development and evaluation.