Accumulation Of Radiotracers Research Articles

Blocking Studies to Evaluate Receptor-Specific Radioligand Binding in the CAM Model by PET and MR Imaging.

Simple SummaryIn the development of new targeted radiopharmaceuticals, it is mandatory to demonstrate their target-specific binding. Rodents are still primarily used for these experiments. With respect to the 3Rs principles, the demand for alternative methods to reduce the number of animal experiments is continuously increasing. In the present study, we investigated whether radiotracer uptake specificity can be evaluated by blocking studies in the CAM model. PET and MR imaging were used to visualize and quantify ligand accumulation. It was demonstrated that the CAM model could be used to evaluate the target-specific binding of a radiopharmaceutical. Due to intrinsic limitations of the CAM model, animal testing will still be required at more advanced stages of compound development. Still, the CAM model could significantly reduce the number of experiments through early compound pre-selection.Inhibition studies in small animals are the standard for evaluating the specificity of newly developed drugs, including radiopharmaceuticals. Recently, it has been reported that the tumor accumulation of radiotracers can be assessed in the chorioallantoic membrane (CAM) model with similar results to experiments in mice, such contributing to the 3Rs principles (reduction, replacement, and refinement). However, inhibition studies to prove receptor-specific binding have not yet been performed in the CAM model. Thus, in the present work, we analyzed the feasibility of inhibition studies in ovo by PET and MRI using the PSMA-specific ligand [18F]siPSMA-14 and the corresponding inhibitor 2-PMPA. A dose-dependent blockade of [18F]siPSMA-14 uptake was successfully demonstrated by pre-dosing with different inhibitor concentrations. Based on these data, we conclude that the CAM model is suitable for performing inhibition studies to detect receptor-specific binding. While in the later stages of development of novel radiopharmaceuticals, testing in rodents will still be necessary for biodistribution analysis, the CAM model is a promising alternative to mouse experiments in the early phases of compound evaluation. Thus, using the CAM model and PET and MR imaging for early pre-selection of promising radiolabeled compounds could significantly reduce the number of animal experiments.

The Effects of Monosodium Glutamate on PSMA Radiotracer Uptake in Men with Recurrent Prostate Cancer: A Prospective, Randomized, Double-Blind, Placebo-Controlled Intraindividual Imaging Study

The prostate-specific membrane antigen (PSMA) is an excellent target for theranostic applications in prostate cancer. However, PSMA-targeted radioligand therapy can cause undesirable effects due to high accumulation of PSMA radiotracers in salivary glands and kidneys. This study assessed orally administered monosodium glutamate (MSG) as a potential means of reducing kidney and salivary gland radiation exposure using a PSMA-targeting radiotracer. Methods: This prospective, double-blind, placebo-controlled study enrolled 10 patients with biochemically recurrent prostate cancer. Each subject served as his own control. PET/CT imaging sessions using 2-(3-{1-carboxy-5-[(6-18F-fluoro-pyridine-3-carbonyl)-amino]-pentyl}-ureido)-pentanedioic acid (18F-DCFPyL) were performed 3-7 d apart, after oral administration of either 12.7 g of MSG or placebo. Data from the 2 sets of images were analyzed by placing regions of interest on lacrimal, parotid, and submandibular glands; left ventricle; liver; spleen; kidneys; bowel; urinary bladder; gluteus muscle; and malignant lesions. The results from MSG and placebo scans were compared by paired analysis of the region-of-interest data. Results: In total, 142 pathologic lesions along with normal tissues were analyzed. As hypothesized a priori, there was a significant decrease in SUVmax corrected for lean body mass (SULmax) on images obtained after MSG administration in the parotids (24% ± 14%, P = 0.001), submandibular glands (35% ± 11%, P < 0.001), and kidneys (23% ± 26%, P = 0.014). Significant decreases were also observed in the lacrimal glands (49% ± 13%, P < 0.001), liver (15% ± 6%, P < 0.001), spleen (28% ± 13%, P = 0.001), and bowel (44% ± 13%, P < 0.001). A mildly lower blood pool SULmean was observed after MSG administration (decrease of 11% ± 13%, P = 0.021). However, significantly lower radiotracer uptake in terms of SULmean, SULpeak, and SULmax was observed in malignant lesions on scans performed after MSG administration than on the placebo studies (SULmax median decrease, 33%; range, -1% to 75%; P < 0.001). No significant adverse events occurred after placebo or MSG administration, and vital signs were stable. Conclusion: Orally administered MSG significantly decreased salivary gland, kidney, and other normal-organ PSMA radiotracer uptake in human subjects, using 18F-DCFPyL as an exemplar. However, MSG caused a corresponding reduction in tumor uptake, which may limit the benefits of this approach for diagnostic and therapeutic applications.

Synthesis and evaluation of a (18) F-labeled 4-phenylpiperidine-4-carbonitrile radioligand for σ1 receptor imaging.

We report the design and synthesis of several 4-phenylpiperidine-4-carbonitrile derivatives as σ1 receptor ligands. In vitro radioligand competition binding assays showed that all the ligands exhibited low nanomolar affinity for σ1 receptors (Ki (σ1 ) = 1.22-2.14 nM) and extremely high subtype selectivity (Ki (σ2 ) = 830-1710 nM; Ki (σ2 )/Ki (σ1 ) = 680-887). [(18) F]9 was prepared in 42-46% isolated radiochemical yield, with a radiochemical purity of >99% by HPLC analysis after purification, via nucleophilic (18) F(-) substitution of the corresponding tosylate precursor. Biodistribution studies in mice demonstrated high initial brain uptakes and high brain-to-blood ratios. Administration of SA4503 or haloperidol 5 min prior to injection of [(18) F]9 significantly reduced the accumulation of radiotracers in organs known to contain σ1 receptors. Two radioactive metabolites were observed in the brain at 30 min after radiotracer injection. [(18) F]9 may serve as a lead compound to develop suitable radiotracers for σ1 receptor imaging with positron emission tomography.

Comparison of the Therapeutic Response to Treatment with a 177Lu-Labeled Somatostatin Receptor Agonist and Antagonist in Preclinical Models.

Peptide receptor scintigraphy and peptide receptor radionuclide therapy using radiolabeled somatostatin receptor (SSTR) agonists are successfully used in the clinic for imaging and treatment of neuroendocrine tumors. Contrary to the paradigm that internalization and the resulting accumulation of radiotracers in cells is necessary for efficient tumor targeting, recent studies have demonstrated the superiority of radiolabeled SSTR antagonists for imaging purposes, despite little to no internalization in cells. However, studies comparing the therapeutic antitumor effects of radiolabeled SSTR agonists versus antagonists are lacking. The aim of this study was to directly compare the therapeutic effect of (177)Lu-DOTA-octreotate, an SSTR agonist, and (177)Lu-DOTA-JR11, an SSTR antagonist. We analyzed radiotracer uptake (both membrane-bound and internalized fractions) and the produced DNA double-strand breaks, by determining the number of p53 binding protein 1 foci, after incubating SSTR2-positive cells with (177)Lu-diethylene triamine pentaacetic acid, (177)Lu-DOTA-octreotate, or (177)Lu-DOTA-JR11. Also, biodistribution studies were performed in tumor-xenografted mice to determine the optimal dose for therapy experiments. Afterward, in vivo therapy experiments comparing the effect of (177)Lu-DOTA-octreotate and (177)Lu-DOTA-JR11 were performed in this same animal model. We found a 5-times-higher uptake of (177)Lu-DOTA-JR11 than of (177)Lu-DOTA-octreotate. The major part (88% ± 1%) of the antagonist uptake was membrane-bound, whereas 74% ± 3% of the total receptor agonist uptake was internalized. Cells treated with (177)Lu-DOTA-JR11 showed 2 times more p53-binding protein 1 foci than cells treated with (177)Lu-DOTA-octreotate. Biodistribution studies with (177)Lu-DOTA-JR11 (0.5 μg/30 MBq) resulted in the highest tumor radiation dose of 1.8 ± 0.7 Gy/MBq, 4.4 times higher than the highest tumor radiation dose found for (177)Lu-DOTA-octreotate. In vivo therapy studies with (177)Lu-DOTA-octreotate and (177)Lu-DOTA-JR11 resulted in a tumor growth delay time of 18 ± 5 and 26 ± 7 d, respectively. Median survival rates were 43.5, 61, and 71 d for the control group, (177)Lu-DOTA-octreotate group, and the (177)Lu-DOTA-JR11-treated group, respectively. On the basis of these results, we concluded that the use of radiolabeled SSTR antagonists such as JR11 might enhance peptide receptor scintigraphy and peptide receptor radionuclide therapy of neuroendocrine tumors and provide successful imaging and therapeutic strategies for cancer types with relatively low SSTR expression.

Lymphoid metastasis of rat My2/De leukemia

By grafting spontaneous leukemia tumor cells, the myeloid My2/De leukemia rat model was established. Death was caused by impaired functions of heavily infiltrated organs. In vitro culturing of tumor cells, blood and bone marrow counts and cytochemic reactions indicated the leukemic the origin resembling human myeoloblastic leukemia. Metastatic spread was followed after i.v. and i.p. injection, and by implantation of leukemia cells under the renal capsule of rats. Primary tumor and metastasis formation was visualized by (18)FDG or (11)C-methionine administration and MiniPET. The accumulation of radiotracers was measured in different organs and expressed as Differential Absorption Ratios (DARs). Subrenal implantation of My2/De cells resulted in their appearance in other abdominal organs and in parathymic lymph nodes. The release of tumor cells from the primary kidney to the peritoneum was mimicked by the i.p. administration of ink particles. Ink particles deposited in the abdominal organs and in the thoracal lymph nodes, preferentially in parathymic lymph nodes, confirming the notion of lymphatic spread of metastasis.

Non-invasive imaging of glioma vessel size and densities in correlation with tumour cell proliferation by small animal PET and MRI

Angiogenesis is a key event in the progression of glioblastomas (GBM). Our goal was to measure different anatomical and physiological parameters of GBM vessels using steady-state contrast-enhanced magnetic resonance imaging (SSCE-MRI), together with the assessment of biochemical parameters on GBM proliferation and angiogenesis using [(11)C]methyl-L-methionine (MET) and 3'-deoxy-3'-[(18)F]fluorothymidine (FLT) and positron emission tomography (PET). We focused on how these anatomical and biochemical read-outs correlate with one another and with immunohistochemistry. SSCE-MRI together with (11)C-MET and (18)F-FLT PET were performed 3 weeks after intracranial implantation of human GBM spheroids in nude rats (n = 8). Total cerebral blood volume (tCBV), blood volume present in microvessels (μCBV), vessel density and size were calculated. Rats were treated with bevacizumab (n = 4) or vehicle (n = 4) for 3 weeks. Imaging was repeated at week 6, and thereafter immunohistochemistry was performed. Three weeks after implantation, MRI showed an increase of vessel density and μCBV in the tumour compared to the contralateral brain. At week 6, non-treated rats showed a pronounced increase of (11)C-MET and (18)F-FLT tumour uptake. Between weeks 3 and 6, tCBV and vessel size increased, whereas vessel density and μCBV decreased. In rats treated with bevacizumab μCBV values were significantly smaller at week 6 than in non-treated rats, whereas the mean vessel size was higher. Accumulation of both radiotracers was lower for the treated versus the non-treated group. Most importantly, non-invasive measurement of tumour vessel characteristics and tumour proliferation correlated to immunohistochemistry findings. Our study demonstrates that SSCE-MRI enables non-invasive assessment of the anatomy and physiology of the vasculature of experimental gliomas. Combined SSCE-MRI and (11)C-MET/(18)F-FLT PET for monitoring biochemical markers of angiogenesis and proliferation in addition to vessel anatomy could be useful to improve our understanding of therapy response of gliomas.

Mammary Cancer Bone Metastasis Follow-up Using Multimodal Small-Animal MR and PET Imaging

Despite tremendous progress in the management of breast cancer, the survival rate of this disease is still correlated with the development of metastases-most notably, those of the bone. Diagnosis of bone metastasis requires a combination of multiple imaging modalities. MR imaging remains the best modality for soft-tissue visualization, allowing for the distinction between benign and malignant lesions in many cases. On the other hand, PET imaging is frequently more specific at detecting bone metastasis by measuring the accumulation of radiotracers, such as (18)F-sodium fluoride ((18)F-NaF) and (18)F-FDG. Thus, the main purpose of this study was to longitudinally monitor bone tumor progression using PET/MR image coregistration to improve noninvasive imaging-assisted diagnoses. After surgical implantation of mammary MRMT-1 cells in a rat femur, we performed minimally invasive imaging procedures at different time points throughout tumor development. The procedure consisted of sequential coregistered MR and PET image acquisition, using gadolinium-diethylenetriaminepentaacetic acid (DTPA) as a contrast agent for MR imaging and (18)F-FDG, (11)C-methionine, and (18)F-NaF as molecular tracers for PET imaging. The animals were then euthanized, and complementary radiologic (micro-CT scans) and histologic analyses were performed. In this preclinical study, we demonstrated that coregistered MR and PET images provide helpful information in a rat mammary-derived bone cancer model. First, MR imaging provided a high-definition anatomic resolution that made the localization of bone resorption and tumor extension detectable between days 9 and 18 after the injection of cancer cells in the medullary channel of the femur. Indeed, the calculation of mean standardized uptake value (SUVmean) and maximal SUV (SUVmax) in bone and soft-tissue regions, as defined from the gadolinium-DTPA contrast-enhanced MR images, showed (18)F-NaF uptake modifications and increased (18)F-FDG or (11)C-methionine uptake in the bone and surrounding soft tissues. (18)F-FDG and (11)C-methionine were compared in terms of the magnitude of change in their uptake and variability. We observed that (11)C-methionine SUVmean variations in the tumor were more important than those of (18)F-FDG. We also found fewer interindividual variations using SUVmean as a quantitative parameter than SUVmax. This preclinical evaluation demonstrated that a PET/MR image coregistration protocol provided a powerful tool to evaluate bone tumor progression in a rat model of bone metastasis and that this protocol could be translated to improve the clinical outcome for metastatic breast cancer management.

Synthesis and biological evaluation of 18F labeled fluoro-oligo-ethoxylated 4-benzylpiperazine derivatives for sigma-1 receptor imaging

We report the synthesis and evaluation of a series of fluoro-oligo-ethoxylated 4-benzylpiperazine derivatives as potential σ1 receptor ligands. In vitro competition binding assays showed that 1-(1,3-benzodioxol-5-ylmethyl)-4-(4-(2-fluoroethoxy)benzyl)piperazine (6) exhibits low nanomolar affinity for σ1 receptors (Ki=1.85±1.59nM) and high subtype selectivity (σ2 receptor: Ki=291±111nM; Kiσ2/Kiσ1=157). [18F]6 was prepared in 30–50% isolated radiochemical yield, with radiochemical purity of >99% by HPLC analysis after purification, via nucleophilic 18F− substitution of the corresponding tosylate precursor. The logDpH 7.4 value of [18F]6 was found to be 2.57±0.10, which is within the range expected to give high brain uptake. Biodistribution studies in mice demonstrated relatively high concentration of radiotracers in organs known to contain σ1 receptors, including the brain, lungs, kidneys, heart, and spleen. Administration of haloperidol 5min prior to injection of [18F]6 significantly reduced the concentration of radiotracers in the above-mentioned organs. The accumulation of radiotracers in the bone was quite low suggesting that [18F]6 is relatively stable to in vivo defluorination. The ex vivo autoradiography in rat brain showed high accumulation of radiotracers in the brain areas known to possess high expression of σ1 receptors. These findings suggest that [18F]6 is a suitable radiotracer for imaging σ1 receptors with PET in vivo.

An Unusual Case of Extraosseous Accumulation of Bone Scan Tracer in a Renal Calculus - Demonstration by SPECT-CT

Extraosseous localization of radioisotope, used in bone scan, in a variety of physiological and pathological conditions is a well-known phenomenon. The causes of extraosseous accumulation of bone-seeking radiotracers should be kept in mind when bone-imaging studies are reviewed to avoid incorrect interpretations. We report an extremely rare occurrence of extraosseous accumulation of bone scintigraphy tracer in a renal calculus, in a patient with adenocarcinoma of prostate, that was demonstrated by Single Photon Emission Computed Tomography and Computed Tomography (SPECT-CT) fusion imaging.

Pgp inhibition by UIC2 antibody can be followed in vitro by using tumor-diagnostic radiotracers, 99mTc-MIBI and 18FDG

P-glycoprotein (Pgp, ABCB1) is one of the active efflux pumps that are able to extrude a large variety of chemotherapeutic drugs from the cells, causing the phenomenon of multidrug resistance. It has been shown earlier that the combined application of a class of Pgp modulators (e.g. cyclosporine A and SDZ PSC 833) used at low concentrations and UIC2 antibody is a novel, specific, and effective way of blocking Pgp function ( Goda et al., 2007). In the present work we study the UIC2 antibody mediated Pgp inhibition in more detail measuring the accumulation of tumor diagnostic radiotracers, 2-[ 18F]fluoro-2-deoxy- d-glucose ( 18FDG) and [ 99mTc]hexakis-2-methoxybutyl isonitrile ( 99mTc-MIBI), into Pgp + (A2780AD) and Pgp − (A2780) human ovarian carcinoma cells. Co-incubation of cells with UIC2 and cyclosporine A (CSA, 2 μM) increased the binding of UIC2 more than 3-fold and reverted the rhodamine 123 (R123), daunorubicin (DNR) and 99mTc-MIBI accumulation of the Pgp + 2780AD cells to approx. the same level as observed in Pgp − cells. Similarly, 50 μM paclitaxel (Pacl) increased UIC2 binding, and consequently reinstated the uptake of R123, DNR and 99mTc-MIBI into the Pgp + cells. Blocking Pgp by combined treatments with CSA + UIC2 or Pacl + UIC2 also decreased the glucose metabolic rate of the A2780AD Pgp + cells measured in 18FDG accumulation experiments suggesting that the maintenance of Pgp activity requires a considerable amount of energy. Similar treatments of the A2780 Pgp − cells did not result in significant change in the R123, DNR, 99mTc-MIBI and 18FDG accumulation demonstrating that the above effects are Pgp-specific. Thus, combined treatment with the UIC2 antibody and Pgp modulators can completely block the function of Pgp in human ovarian carcinoma cells and this effect can be followed in vitro by using tumor-diagnostic radiotracers, 99mTc-MIBI and 18FDG.

Radiogene Nephropathie

SummaryPatient-individual dosimetric analyses are a useful tool in external beam radiotherapy (EBR) to protect patients from side effects such as radiogenic nephropathy. At this point in time, individual dosimetry is not used as a standard in patient treated with radiolabelled antibody fragments or polypeptides. The reasons are a number of problems, which make patient dosimetry more challenging than in EBR. While in EBR, the dose is distributed evenly in the organ and the organ volume can exactly be determined, in internal radiotherapy the tracer is not evenly distributed within the organ leading to a non-uniform dose distribution. In addition, the dose rate of the most commonly used radionuclides is lower than in EBR and the range of their radiation differ, so that the radiobiological effects are differing considerably in comparison to EBR. Conclusion: More complex models have to be used for clinical kidney dosimetry in internal radiotherapy.In this paper, we give a concise overview of the reasons for accumulation of radiotracers in the kidney, the most recent developments in kidney dosimetry, and approaches to reduce the kidney uptake of radiotracers in order to avoid radiogenic nephropathy.

Daunorubicin and doxorubicin inhibit the [ 11C]choline accumulation in cancer cells

We studied how very short (10–40 min) incubation with anthracycline derivatives modifies the accumulation of PET tumor-diagnostic radiotracers in cancer cells. The human ovarian A2780 and A2780AD, human B lymphoid JY, human epidermoid KB-3-1 and KB-V-1, and smooth muscle DDT1 MF-2 cells were pre-incubated with daunorubicin and doxorubicin, and the uptake of [ 18F]FDG and [ 11C]choline was measured. Anthracycline treatment decreased remarkably the [ 11C]choline accumulation in a concentration dependent manner, while it did not modify significantly the [ 18F]FDG uptake of the cells.

Intestinal accumulation of 99mTc-MDP in a patient with protein-losing enteropathy

A 36-year-old woman was hospitalized because of severe diffuse joint pain. Hypoproteinemia was found in the routine laboratory work-up. Prompted by the joint pain, a 99m Tc-MDP bone scan was done: the planar whole-body views did not reveal any skeletal abnormality. However, on the anterior view, unexpected abdominal activity was noted in the ascending and transverse colon (arrows). The diagnosis of protein-losing enteropathy was established with a positive 99m Tc-human serum albumin scintigraphy. Intestinal accumulation of bone-seeking radiotracers is exceedingly rare; furthermore, its mechanism remains to be elucidated [1, 2].

Multidrug resistance mediated by ABC transporters in osteosarcoma cell lines: mRNA analysis and functional radiotracer studies

Drug resistance remains a significant impediment to successful chemotherapy and constitutes a major prognostic factor in osteosarcoma (OS) patients. This study was designed to identify the role and prognostic significance of multidrug-resistance (MDR)-related transporters, such as multidrug resistance protein 1 (MDR1), multidrug-resistance-associated protein (MRP1) and breast-cancer-related protein (BCRP), in OS using cationic lipophilic radiotracers. We evaluated the chemosensitivity of four OS cell lines (Saos-2, 143B, MNNG/HOS and U-2OS) to doxorubicin (DOX), cisplatin (CIS) and methotrexate. The expression of MDR-related transporters was analyzed at mRNA level by quantitative polymerase chain reaction and at functional level by 99mTc sestamibi and 99mTc tetrofosmin. The effectiveness of MDR modulators [cyclosporin A (CsA) and imatinib] on transporter inhibition and on the reversal of resistance was also assessed. MNNG/HOS and U-2OS cells expressing high levels of MDR1 were highly resistant to DOX and showed reduced accumulation and higher efflux for radiotracers. Although MRP1 was uniformly expressed in all cells, only U-2OS was resistant to CIS. CsA restored sensitivity to DOX and CIS, and enhanced the accumulation and efflux half-life of radiotracers in MDR1-expressing cell lines. The chemosensitivity of OS cells to DOX was strongly dependent on mRNA MDR1 expression and could be circumvented by adding CsA. The kinetic parameters of radiotracers correlated with MDR1 expression levels, hence predicting DOX resistance. We concluded that sensitivity to chemotherapy is strongly dependent on the expression of MDR1 transporter and that radiotracer studies could prove clinically useful in predicting chemotherapy response and in evaluating the efficacy of MDR-reversing agents.

Paclitaxel modifies the accumulation of tumor-diagnostic tracers in different ways in P-glycoprotein-positive and negative cancer cells

Aim To study how paclitaxel treatment modifies the accumulation of tumor-diagnostic radiotracers in P-glycoprotein (P-gp) positive and negative cancer cells. Methods The accumulations of different P-gp substrates, including rhodamine 123, daunorubicin and [ 99mTc]hexakis-2-methoxybutyl isonitrile ( 99mTc-MIBI), were measured in P-gp-positive (A2780AD) and P-gp-negative human ovarian carcinoma cells (A2780) and JY human lymphoid B cells. The uptakes of the tumor-diagnostic tracers 11C-choline and 2-[ 18F]fluoro-2-deoxy- d-glucose ( 18FDG) were measured in the same cell lines. The P-gp expression and function were demonstrated by flow-cytometry. Results The 18FDG measurements revealed that the glucose metabolic rate was significantly higher ( p < 0.01) in the P-gp-positive A2780AD cells than in the P-gp-negative cells. Paclitaxel (1–70 μM) increased the 18FDG uptake (up to 200%) of both P-gp-positive and P-gp-negative cells, whereas it did not modulate their 11C-choline uptake. Paclitaxel reinstated the 99mTc-MIBI accumulation of the A2780AD cells (to 1500% of the control) in a concentration-dependent manner, while it increased the uptake of the P-gp-negative cells to a lesser extent (to a maximum of 200% of the control). Conclusion Paclitaxel modifies the uptake of tumor-diagnostic tracers in both P-gp-dependent and independent manners. Interpretation of the multifactorial effects of paclitaxel may promote a correct in vivo diagnosis of P-gp-positive and P-gp-negative tumors.

Accumulation of metal radiotracers by Mytilus edulis

Accumulation of metal radiotracers by Mytilus edulis