Neuropharmacological studies with SPECT in neuropsychiatric disorders

Brain-targeting by optimized 99mTc-olanzapine: in vivo and in silico studies

Purpose: Olanzapine (OLZ) is an atypical antipsychotic agent that is characterized by low brain porousness. The present work aimed to develop radiolabeled olanzapine (OLZ) without colloidal impurities and evaluate its biodistribution following intravenous (I.V.) and intranasal (I.N.) administration as a potential agent for brain diagnosis. Materials and methods: OLZ was radiolabeled with technetium-99m by using sodium dithionite as the reducing agent. Biodistribution of ^99mTc-OLZ complex in mice following I.V. and I.N. administrations was examined. Furthermore, a molecular docking study was performed.Results: Sodium dithionite labeling procedure resulted in highest radiochemical yield (96.30 ± 0.09%) and in vitro stability in serum up to 8 h. Biodistribution study of ^99mTc-OLZ complex showed high brain uptake following I.N. (6.2 ± 0.12% ID/g) and I.V. (5.5 ± 0.09% ID/g) at 0.5 and 1 h post administration (P.I.), respectively. Docking into two brain targets predicts higher affinity of ^99mTc-OLZ than free OLZ. Additionally, docking to P-glycoproteins shows less affinity for the radiolabelled OLZ and hence it is expected to be associated with better brain exposure than free OLZ.Conclusion: These chemical and preliminary biological merits strongly suggest that the ^99mTc-OLZ complex with new reducing agent could be used as a potential diagnostic agent for brain.

Electrophilic Aromatic Synthesis of Radioiodinated Aripiprazole: Experimental and DFT Investigations

BACKGROUND

Aripiprazole is a quinolinone derivative. It shows a high affinity for neurotransmitters dopamine and serotonin receptors, which can overcome the blood-brain barrier (BBB) to reach the central nervous system (CNS) to exert therapeutic effects. Its radioiodination may lead to high radiochemical yield and improved its affinity. Aripiprazole radioiodination is an aromatic electrophilic substitution.

OBJECTIVE

Herein, we investigate the favorable atom site of the aromatic electrophilic substitution of aripiprazole by calculating the Fukui indices of heavy atoms and ESP charges of the parent molecule.

METHODS

The calculations have been carried out at the B3LYP/LanL2DZ level of theory. The iodinated aripiprazole structure is confirmed by comparing the experimental and the predicted 1H NMR chemical shifts of the parent molecule and its iodinated forms.

RESULTS

Finally, the electronic properties of aripiprazole and its iodinated form were calculated at the same level of theory. Nucleophilic Fukui indices and ESP charges calculations confirm that C8 is the most favorable site of the electrophilic substitution. The calculated electronic properties (e.g, gap energy, electron affinity, and electronegativity) of aripiprazole and its iodinated form reveal the higher reactivity of iodinated aripiprazole compared with aripiprazole.

CONCLUSION

This may explain the higher affinity of iodinated aripiprazole and the increase of its radiochemical yield.

Association between triallelic polymorphism of the serotonin transporter and [18F]MPPF binding potential at 5-HT1A receptors in healthy subjects

Previous [(11)C]WAY100-635 PET studies have demonstrated that the short (S) and long (L) alleles of the serotonin transporter gene-linked polymorphic region (5-HTTLPR) were associated with distinct patterns of 5-HT(1A) receptor distribution in human. However, these studies reported discordant findings and did not take into account the recent description of two functional variants of the L allele (L(A)/L(G)). To further explore this issue, we investigated the triallelic functional polymorphism of the 5-HTTLPR in 38 healthy volunteers who underwent a [(18)F]MPPF PET study of 5-HT1A receptors. We used a simplified reference tissue model to generate parametric images of [(18)F]MPPF binding potential (BP(ND)), and compared these data among the different genotypes using statistical parametric mapping and region of interest of the raphe nuclei. Homozygote carriers of the S allele demonstrated greater [(18)F]MPPF BP(ND) than carriers of the L(A) allele, but this association was only found in women. Differences in [(18)F]MPPF BP(ND) between women with and without L(A) allele were observed over large clusters encompassing the right and left temporal lobes, cingulate and perisylvian regions, as well as the right precuneus and frontal dorso-lateral cortex, and the left orbitofrontal cortex. In contrast, no difference was found between groups in the raphe nuclei. The greater [(18)F]MPPF BP(ND) observed in women homozygote carriers of the S allele could either reflect a greater 5-HT1A receptor density or a lower extracellular concentration of 5-HT. Our data suggest that any future PET studies of 5-HT1A receptors should incorporate the 5-HTTLPR polymorphism status of the population studied.

Single-photon emission computed tomography in neurotherapeutics

The measurement of regional cerebral blood flow (rCBF) by single-photon emission computed tomography (SPECT) is a powerful clinical and research tool. There are several clinical applications now documented, a substantial number under active investigation, and a larger number yet to be studied. Standards regarding patient imaging environment and image presentation are becoming established. This article reviews key aspects of SPECT functional brain imaging in clinical practice, with a particular emphasis on therapeutics, including 1) the quality of the tomographic device, 2) the radiopharmaceutical employed, 3) environmental conditions at the time of radiotracer administration, 4) characteristics of the subject, 5) the format used for image presentation, and 6) the essential components of image processing necessary to the achievement of high-quality SPECT brain images. Next, a brief description of relevant radiation safety issues is provided. Finally, applications in molecular imaging, especially in small animal imaging for research as well as drug discovery and development are discussed. The gamut of SPECT studies from currently routine clinical applications to molecular imaging offers a wonderful frontier for opportunities to employ functional brain imaging in neurotherapeutics.