Datta E. Ponde

In Vivo Imaging of β-Cell Mass in Rats Using 18F-FP-(+)-DTBZ: A Potential PET Ligand for Studying Diabetes Mellitus

Recent studies on gene expression of β-cell mass (BCM) in the pancreas showed that vesicular monoamine transporter 2 (VMAT2) is highly expressed in the BCM (mainly in the islets of Langerhans). Imaging pancreatic BCM may provide an important tool for understanding the relationship between loss of insulin-secreting β-cells and onset of diabetes mellitus. In this article, 9-fluoropropyl-(+)-dihydrotetrabenazine (FP-(+)-DTBZ), which is a VMAT2 imaging agent, was evaluated as a PET agent for estimating BCM in vivo. Methods: Organ biodistribution after an intravenous injection of 18F-FP-(+)-DTBZ (active isomer) and 18F-FP-(−)-DTBZ (inactive isomer) was evaluated in normal rats. The specificity of uptake of 18F-FP-(+)-DTBZ was assessed by a pretreatment (3.8 mg of (+)-DTBZ per kilogram and 3.5 mg of FP-(+)-DTBZ per kilogram, intravenously, 5 min prior) or coadministration (2 mg of (+)-DTBZ per kilogram). PET studies were performed in normal rats. Results: The in vivo biodistribution of 18F-FP-(+)-DTBZ in rats showed the highest uptake in the pancreas (5% dose/g at 30 min after injection), whereas 18F-FP-(−)-DTBZ showed a very low pancreas uptake. Rats pretreated with FP-(+)-DTBZ displayed a 78% blockade of pancreas uptake. PET studies in normal rats demonstrated an avid pancreas uptake of 18F-FP-(+)-DTBZ. Conclusion: The preliminary data obtained with 18F-FP-(+)-DTBZ suggest that this fluorinated derivative of DTBZ shows good pancreas specificity and has the potential to be useful for quantitative measurement of VMAT2 binding sites reflecting BCM in the pancreas.

Embryonic Stem Cell Grafting in Normal and Infarcted Myocardium: Serial Assessment with MR Imaging and PET Dual Detection

PURPOSEnTo use magnetic resonance (MR) imaging and positron emission tomography (PET) dual detection of cardiac-grafted embryonic stem cells (ESCs) to examine (a) survival and proliferation of ESCs in normal and infarcted myocardium, (b) host macrophage versus grafted ESC contribution to serial MR imaging signal over time, and (c) cardiac function associated with the formation of grafts and whether improvement in cardiac function is related to cardiac differentiation of ESCs.nnnMATERIALS AND METHODSnAll animal procedures were approved by the institutional animal care and use committee. Murine ESCs were stably transfected with a mutant version of herpes simplex virus type 1 thymidine kinase, HSV1-sr39tk, and also were labeled with superparamagnetic iron oxide (SPIO) particles. Cells were injected directly in the border zone of the infarcted heart or in corresponding regions of normal hearts in athymic rats. PET and MR imaging were performed longitudinally for 4 weeks in the same animals.nnnRESULTSnESCs survived and underwent proliferation in the infarcted and normal hearts, as demonstrated by serial increases in 9-(4-[(18)F]fluoro-3-hydroxymethylbutyl) guanine PET signals. In parallel, the hypointense areas on MR images at the injection sites decreased over time. Double staining for host macrophages and SPIO particles revealed that the majority of SPIO-containing cells were macrophages at week 4 after injection. Left ventricular ejection fraction increased in the ESC-treated rats but decreased in culture media-treated rats, and border-zone function was preserved in ESC-treated animals; however, cardiac differentiation of ESCs was less than 0.5%.nnnCONCLUSIONnDual-modality imaging permits complementary information in regard to cell survival and proliferation, graft formation, and effects on cardiac function.nnnSUPPLEMENTAL MATERIALnhttp://radiology.rsnajnls.org/cgi/content/full/250/3/821/DC1.

Death and Proliferation Time Course of Stem Cells Transplanted in the Myocardium

PurposeNoninvasive positron emission tomography (PET) imaging of reporter gene is combined with quantitative real-time polymerase reverse transcription (RT-PCR) method to study the time course of death and proliferation of stem cells transplanted in the myocardium.MethodsMale murine embryonic stem cells (ESCs) were stably transfected with a mutant version of herpes simplex virus type 1 thymidine kinase (HSV1-sr39tk) reporter gene; 5u2009×u2009106 such cells were injected into the myocardium of female athymic rats. While the transplanted cells was monitored by in vivo 9-(4-[F-18]fluoro-3-hydroxymethylbutyl)guanine ([F-18]FHBG) PET imaging of the heart, their absolute number was estimated by RT-PCR from hearts harvested at 3–5xa0h, 24xa0h, daysxa04, 7, and 14 after transplantation.Results(1) Forty percent of injected cells were retained in the heart while majority of injected cells were lost within a few hours after injection. Cell death was peaked at 24xa0h when 18% of donor cells retained in the heart were dead. (2) The substantial cell loss was reversed by significant proliferation of ESCs. This led to the recovery of cell number to 3.4 million (70% of injected dose) at dayxa04 and first visual observation of in vivo [F-18] signal in the heart. (3) A robust correlation (R2u2009=u20090.9) between percent of injected dose per gram of tissue derived from in vivo PET signal and the number of donor cells estimated by RT-PCR was revealed.ConclusionsThe time course of transplanted stem cells surviving in the heart reveals a process of substantial cell loss within 24xa0h of injection and subsequent recovery of cell number through proliferation. Such proliferation can be noninvasively monitored by reporter gene imaging.

Development of anti-EGF receptor peptidomimetics (AERP) as tumor imaging agent.

EGFR is over-expressed in several solid tumors including breast, prostate, pancreas, and lung cancers and is correlated to the metastatic potential of the tumor. Anti-EGFR receptor-binding peptidomimetics (AERP) were examined to assess the small molecules potential use as tumor-specific imaging agents. The aim of this work was to design and characterize the binding specificity of the radiolabeled peptidomimetics to EGFR over-expressing cell lysate and to A431 xenograft tumors. Our newly designed peptidomimetic, AERP, was conjugated to DTPA and labeled with (99m)Tc. The in vivo tumor accumulation of [(99m)Tc] DTPA-AERP-2 was 1.6±0.1%ID/g and tumor to muscle ratio was 5.5. Our studies suggest that this novel peptidomimetic, AERP-2, warrants further development as an EGFR specific tumor-imaging agent.

Quantitative, noninvasive, in vivo longitudinal monitoring of gene expression in the brain by co-AAV transduction with a PET reporter gene

In vivo imaging of vector transgene expression would be particularly valuable for repetitive monitoring of therapy in the brain, where invasive tissue sampling is contraindicated. We evaluated adeno-associated virus vector expression of a dopamine-2 receptor (D2R) mutant (D2R80A) by positron emission tomography in the brains of mice and cats. D2R80A is inactivated for intracellular signaling and binds subphysiologic amounts of the radioactive [18F]-fallypride analog of dopamine. The [18F]-fallypride signal bound to D2R80A in the injection site was normalized to the signal from endogenous D2R in the striatum and showed stable levels of expression within individual animals. A separate adeno-associated virus type 1 vector with identical gene expression control elements, expressing green fluorescent protein or a therapeutic gene, was coinjected with the D2R80A vector at equal doses into specific sites. Both transgenes had similar levels of gene expression by immunohistochemistry, in situ hybridization, and quantitative PCR assays, demonstrating that D2R80A is a faithful surrogate measure for expression of a gene of interest. This dual vector approach allows the D2R80A gene to be used with any therapeutic gene and to be injected into a single site for monitoring while the therapeutic gene can be distributed more widely as needed in each disease.

Radiosynthesis of [11C] N-Methyl and N,N′-Dimethyl Ethanolamine for Measuring Phospholipid Metabolism Using PET Imaging

Phosphoglycerides are important in the formation of cellular and organelle membranes in rapidly growing tumors. The most abundant phosphoglycerides in higher plants and animals are phosphatidylethanolamine (PtdEt) and phosphatidylcholine (PtdCho), which contain the amino alcohols ethanolamine and choline, respectively. Recently, P magnetic resonance spectroscopy (MRS) in vivo and in vitro has revealed an elevated level of choline metabolites in tumors, which are precursors and catabolites of the most abundant phospholipids in the cell membrane of all eukaryotic cells. It is thought that this elevation is the result of increased uptake of choline, a precursor of the biosynthesis of phosphatidylcholine. Based on this hypothesis, Hara et al. employed [C]-choline for imaging prostate cancer and various other neoplastic diseases. Earlier [C]-choline was developed by Friedland et al. and utilized for measuring