An Efficient and Practical Radiosynthesis of [¹¹C]Temozolomide

Temozolomide (TMZ) is a prodrug for an alkylating agent used for the treatment of malignant brain tumors. A positron emitting version, [¹¹C]TMZ, has been utilized to help elucidate the mechanism and biodistribution of TMZ. Challenges in [¹¹C]TMZ synthesis and reformulation make it difficult for routine production. A highly reproducible one-pot radiosynthesis of [¹¹C]TMZ with a radiochemical yield of 17 ± 5% and ≥97% radiochemical purity is reported

An Efficient and Practical Radiosynthesis of [¹¹C]Temozolomide

An Efficient and Practical Radiosynthesis of [¹¹C]Temozolomid

Imaging Evaluation of 5HT_2C Agonists, [¹¹C]WAY-163909 and [¹¹C]Vabicaserin, Formed by Pictet–Spengler Cyclization

The serotonin subtype 2C (5HT_2C) receptor is an emerging and promising drug target to treat several disorders of the human central nervous system. In this current report, two potent and selective 5HT_2C full agonists, WAY-163909 (<b>2</b>) and vabicaserin (<b>3</b>), were radiolabeled with carbon-11 via Pictet–Spengler cyclization with [¹¹C]­formaldehyde and used in positron emission tomography (PET) imaging. Reaction conditions were optimized to exclude the major source of isotope dilution caused by the previously unknown breakdown of <i>N</i>,<i>N</i>-dimethylformamide (DMF) to formaldehyde at high temperature under mildly acid conditions. In vivo PET imaging was utilized to evaluate the pharmacokinetics and distribution of the carbon-11 labeled 5HT_2C agonists. Both radiolabeled molecules exhibit high blood–brain barrier (BBB) penetration and nonspecific binding, which was unaltered by preadministration of the unlabeled agonist. Our work demonstrates that Pictet–Spengler cyclization can be used to label drugs with carbon-11 to study their pharmacokinetics and for evaluation as PET radiotracers

Synthesis and Imaging Validation of [¹⁸F]MDL100907 Enabled by Ni-Mediated Fluorination

Several voids exist in reliable positron emission tomography (PET) radioligands for quantification of the serotonin (5HT) receptor system. Even in cases where 5HT radiotracers exist, challenges remain that have limited the utility of 5HT imaging in clinical research. Herein we address an unmet need in 5HT_2a imaging using innovative chemistry. We report a scalable and robust synthesis of [¹⁸F]­MDL100907, which was enabled by a Ni-mediated oxidative fluorination using [¹⁸F]­fluoride. This first demonstration of a Ni-mediated fluorination used for PET imaging required development of a new reaction strategy that ultimately provided high specific activity [¹⁸F]­MDL100907. Using the new synthetic strategy and optimized procedure, [¹⁸F]­MDL100907 was evaluated against [¹¹C]­MDL100907 for reliability to quantify 5HT_2a in the nonhuman primate brain and was found to be superior based on a single scan analysis using the same nonhuman primate. The use of this new 5HT_2a radiotracer will afford clinical neuroscience research the ability to distinguish 5HT_2a receptor abnormalities binding between healthy subjects and patients even when group differences are small

Late Stage Benzylic C–H Fluorination with [¹⁸F]Fluoride for PET Imaging

We describe the first late-stage ¹⁸F labeling chemistry for aliphatic C–H bonds with no-carrier-added [¹⁸F]­fluoride. The method uses Mn­(salen)­OTs as an F-transfer catalyst and enables the facile labeling of a variety of bioactive molecules and building blocks with radiochemical yields (RCY) ranging from 20% to 72% within 10 min without the need for preactivation of the labeling precursor. Notably, the catalyst itself can directly elute [¹⁸F]­fluoride from an ion exchange cartridge with over 90% efficiency. Using this feature, the conventional and laborious dry-down step prior to reaction is circumvented, greatly simplifying the mechanics of this protocol and shortening the time for automated synthesis. Eight drug molecules, including COX, ACE, MAO, and PDE inhibitors, have been successfully [¹⁸F]-labeled in this way

Synthesis and Evaluation of Methylated Arylazepine Compounds for PET Imaging of 5‑HT_2c Receptors

The serotonin 5-HT_2c receptor is implicated in a number of diseases including obesity, depression, anxiety, and schizophrenia. In order to ascribe the role of 5-HT_2c in these diseases, a method for measuring 5-HT_2cdensity and function in vivo, such as with positron emission tomography (PET), must be developed. Many high-affinity and relatively selective ligands exist for 5-HT_2c but cannot be accessed with current radiosynthetic methods for use as PET radiotracers. We propose that <i>N</i>-methylation of an arylazepine moiety, a frequent structural feature in 5-HT_2c ligands, may be a suitable method for producing new radiotracers for 5-HT_2c. The impact of <i>N</i>-methylation has not been previously reported. For the agonists that we selected herein, <i>N-</i>methylation was found to increase affinity up to 8-fold without impairing selectivity. Compound 5, an <i>N</i>-methylated azetidine-derived arylazepine, was found to be brain penetrant and reached a brain/blood ratio of 2.05:1. However, our initial test compound was rapidly metabolized within 20 min of administration and exhibited high nonspecific binding. <i>N</i>-Methylation, with 16 ± 3% isolated radiochemical yield (decay corrected), is robust and may facilitate screening other 5-HT_2c ligands as radiotracers for PET

Development of a Fluorinated Class‑I HDAC Radiotracer Reveals Key Chemical Determinants of Brain Penetrance

Despite major efforts, our knowledge about many brain diseases remains remarkably limited. Epigenetic dysregulation has been one of the few leads toward identifying the causes and potential treatments of psychiatric disease over the past decade. A new positron emission tomography radiotracer, [¹¹C]­Martinostat, has enabled the study of histone deacetylase in living human subjects. A unique property of [¹¹C]­Martinostat is its profound brain penetrance, a feature that is challenging to engineer intentionally. In order to understand determining factors for the high brain-uptake of Martinostat, a series of compounds was evaluated in rodents and nonhuman primates. The study revealed the major structural contributors to brain uptake, as well as a more clinically relevant fluorinated HDAC radiotracer with comparable behavior to Martinostat, yet longer half-life

Brain-Penetrant LSD1 Inhibitors Can Block Memory Consolidation

Modulation of histone modifications in the brain may represent a new mechanism for brain disorder therapy. Post-translational modifications of histones regulate gene expression, affecting major cellular processes such as proliferation, differentiation, and function. An important enzyme involved in one of these histone modifications is lysine specific demethylase 1 (LSD1). This enzyme is flavin-dependent and exhibits homology to amine oxidases. Parnate (2-phenylcyclopropylamine (2-PCPA); tranylcypromine) is a potent inhibitor of monoamine oxidases, and derivatives of 2-PCPA have been used for development of selective LSD1 inhibitors based on the ability to form covalent adducts with flavin adenine dinucleotide (FAD). Here we report the synthesis and in vitro characterization of LSD1 inhibitors that bond covalently to FAD. The two most potent and selective inhibitors were used to demonstrate brain penetration when administered systemically to rodents. First, radiosynthesis of a positron-emitting analogue was used to obtain preliminary biodistribution data and whole brain time–activity curves. Second, we demonstrate that this series of LSD1 inhibitors is capable of producing a cognitive effect in a mouse model. By using a memory formation paradigm, novel object recognition, we show that LSD1 inhibition can abolish long-term memory formation without affecting short-term memory, providing further evidence for the importance of reversible histone methylation in the function of the nervous system