Imaging HDACs In Vivo: Cross-Validation of the [11C]Martinostat Radioligand in the Pig Brain.

PURPOSE: With the emerging knowledge about the impact of epigenetic alterations on behavior and brain disorders, the ability to measure epigenetic alterations in brain tissue in vivo has become critically important. We present the first in vivo/in vitro cross-validation of the novel positron emission tomography (PET) radioligand [11C]Martinostat in the pig brain with regard to its ability to measure histone deacetylase 1-3 (HDAC1-3) levels in vivo. PROCEDURES: Nine female Danish landrace pigs underwent 121-min dynamic PET scans with [11C]Martinostat. We quantified [11C]Martinostat uptake using both a simple ratio method and kinetic models with arterial input function. By the end of the scan, the animals were euthanized and the brains were extracted. We measured HDAC1-3 protein levels in frontal cortex, cerebellum vermis, and hippocampus and compared the protein levels and regional outcome values to the [11C]Martinostat PET quantification. RESULTS: [11C]Martinostat distributed widely across brain regions, with the highest uptake in the cerebellum vermis and the lowest in the olfactory bulbs. Based on the Akaike information criterion, the quantification was most reliably performed by Ichise MA1 kinetic modeling, but since the radioligand displayed very slow kinetics, we also calculated standard uptake value (SUV) ratios which correlated well with VT. The western blots revealed higher brain tissue protein levels of HDAC1/2 compared to HDAC3, and HDAC1 and HDAC2 levels were highly correlated in all three investigated brain regions. The in vivo SUV ratio measure correlated well with the in vitro HDAC1-3 levels, whereas no correlation was found between VT values and HDAC levels. CONCLUSIONS: We found good correlation between in vivo measured SUV ratios and in vitro measures of HDAC 1-3 proteins, supporting that [11C]Martinostat provides a good in vivo measure of the cerebral HDAC1-3 protein levels.

18F-Labelling of electron rich iodonium ylides: application to the radiosynthesis of potential 5-HT2A receptor PET ligands.

18F-Labelling of aromatic moieties was limited to electron deficient aromatic systems for many years but recent developments have provided access to the direct labelling of electron rich aromatic systems. Herein we report the synthesis and 18F-labelling of iodonium ylide precursors in the pursuit of 18F-labelled 5-HT2A receptor agonist PET-ligands. Subsequent evaluation in pigs showed high brain uptake of the PET ligands but a blocking dose of ketanserin did not significantly reduce the signal in relevant brain regions - indicating that the ligands do not interact specifically with the 5-HT2A receptor in vivo.

Autoradiographic imaging and quantification of the high-affinity GHB binding sites in rodent brain using 3H-HOCPCA.

GHB (γ-hydroxybutyric acid) is a compound endogenous to mammalian brain with high structural resemblance to GABA. GHB possesses nanomolar-micromolar affinity for a unique population of binding sites, but the exact nature of these remains elusive. In this study we utilized the highly selective GHB analogue, 3-hydroxycyclopent-1-enecarboxylic acid (HOCPCA) as a tritiated version (3H-HOCPCA) to radioactively label the specific GHB high-affinity binding site and gain further insight into the density, distribution and developmental profile of this protein. We show that, in low nanomolar concentrations, 3H-HOCPCA displays excellent signal-to-noise ratios using rodent brain autoradiography, which makes it a valuable ligand for anatomical quantification of native GHB binding site levels. Our data confirmed that 3H-HOCPCA labels only the high-affinity specific GHB binding site, found in high density in cortical and hippocampal regions. The experiments revealed markedly stronger binding at pH 6.0 (Kd 73.8 nM) compared to pH 7.4 (Kd 2312 nM), as previously reported for other GHB radioligands but similar Bmax values. Using 3H-HOCPCA we analyzed the GHB binding protein profile during mouse brain development. Due to the high sensitivity of this radioligand, we were able to detect low levels of specific binding already at E15 in mouse brain, which increased progressively until adulthood. Collectively, we show that 3H-HOCPCA is a highly sensitive radioligand, offering advantages over the commonly used radioligand 3H-NCS-382, and thus a very suitable in vitro tool for qualitative and quantitative autoradiography of the GHB high-affinity site.

Is the electron radiation length constant at high energies?

Experimental results for the radiative energy loss of 149, 207, and 287 GeV electrons in a thin Ir target are presented. From the data we conclude that at high energies the radiation length increases in accordance with the Landau-Pomeranchuk-Migdal (LPM) theory and thus electrons become more penetrating the higher the energy. The increase of the radiation length as a result of the LPM effect has a significant impact on the behavior of high-energy electromagnetic showers.

Relation between non-bicarbonate buffer value and tolerance to cellular acidosis: a comparative study of myocardial tissue.

There is a large variation in the tolerance of myocardial tissue to cellular acidosis. Assuming the cytoplasmic acid-base status to be mainly a result of intracellular processes, this variation could be produced by variations in the tissue non-bicarbonate buffer value. In the myocardial tissue from nine vertebrate species, the non-bicarbonate buffer value did not correlate either with ability to develop tension under hypercapnic acidiosis or with the indirectly estimated capacity for anaerobic glycolysis. Therefore, differences in myocardial tolerance to acidosis must be explained either by an active pH regulation or by other compensatory mechanisms.