Yiyun Huang

Pet imaging of serotonin 1A receptor binding in depression

BACKGROUND The serotonin-1A (5HT1A) receptor system has been implicated in the pathophysiology of major depression by postmortem studies of suicide victims and depressed subjects dying of natural causes. This literature is in disagreement, however, regarding the brain regions where 5HT1A receptor binding differs between depressives and controls and the direction of such differences relative to the normal baseline, possibly reflecting the diagnostic heterogeneity inherent within suicide samples. PET imaging using the 5HT1A receptor radioligand, [11C]WAY-100635, may clarify the clinical conditions under which 5HT1A receptor binding potential (BP) is abnormal in depression. METHODS Regional 5HT1A receptor BP values were compared between 12 unmedicated depressives with primary, recurrent, familial mood disorders and 8 healthy controls using PET and [carbonyl-11C]WAY-100635. Regions-of-interest (ROI) assessed were the mesiotemporal cortex (hippocampus-amygdala) and midbrain raphe, where previous postmortem studies suggested 5HT1A receptor binding is abnormal in depression. RESULTS The mean 5HT1A receptor BP was reduced 41.5% in the raphe (p < .02) and 26.8% in the mesiotemporal cortex (p < .025) in the depressives relative to the controls. Post hoc comparisons showed the abnormal reduction in 5HT1A receptor BP was not limited to these regions, but extended to control ROI in the occipital cortex and postcentral gyrus as well. The magnitude of these abnormalities was most prominent in bipolar depressives (n = 4) and unipolar depressives with bipolar relatives (n = 4). CONCLUSIONS Serotonin-1A receptor BP is abnormally decreased in the depressed phase of familial mood disorders in multiple brain regions. Of the regions tested, the magnitude of this reduction was most prominent in the midbrain raphe. Converging evidence from postmortem studies of mood disorders suggests these reductions of 5HT1A receptor BP may be associated with histopathological changes involving the raphe.

Test–retest variability of serotonin 5-HT2A receptor binding measured with positron emission tomography and [18F]altanserin in the human brain

The role of serotonin in CNS function and in many neuropsychiatric diseases (e.g., schizophrenia, affective disorders, degenerative dementias) support the development of a reliable measure of serotonin receptor binding in vivo in human subjects. To this end, the regional distribution and intrasubject test–retest variability of the binding of [18F]altanserin were measured as important steps in the further development of [18F]altanserin as a radiotracer for positron emission tomography (PET) studies of the serotonin 5‐HT2A receptor. Two high specific activity [18F]altanserin PET studies were performed in normal control subjects (n = 8) on two separate days (2–16 days apart). Regional specific binding was assessed by distribution volume (DV), estimates that were derived using a conventional four compartment (4C) model, and the Logan graphical analysis method. For both analysis methods, levels of [18F]altanserin binding were highest in cortical areas, lower in the striatum and thalamus, and lowest in the cerebellum. Similar average differences of 13% or less were observed for the 4C model DV determined in regions with high receptor concentrations with greater variability in regions with low concentrations (16–20%). For all regions, the absolute value of the test–retest differences in the Logan DV values averaged 12% or less. The test–retest differences in the DV ratios (regional DV values normalized to the cerebellar DV) determined by both data analysis methods averaged less than 10%. The regional [18F]altanserin DV values using both of these methods were significantly correlated with literature‐based values of the regional concentrations of 5‐HT2A receptors determined by postmortem autoradiographic studies (r2 = 0.95, P < 0.001 for the 4C model and r2 = 0.96, P < 0.001 for the Logan method). Brain uptake studies in rats demonstrated that two different radiolabeled metabolites of [18F]altanserin (present at levels of 3–25% of the total radioactivity in human plasma 10–120 min postinjection) were able to penetrate the blood–brain barrier. However, neither of these radiolabeled metabolites bound specifically to the 5‐HT2A receptor and did not interfere with the interpretation of regional [18F]altanserin‐specific binding parameters obtained using either a conventional 4C model or the Logan graphical analysis method. In summary, these results demonstrate that the test–retest variability of [18F]altanserin‐specific binding is comparable to that of other PET radiotracers and that the regional specific binding of [18F]altanserin in human brain was correlated with the known regional distribution of 5‐HT2A receptors. These findings support the usefulness of [18F]altanserin as a radioligand for PET studies of 5‐HT2A receptors. Synapse 30:380–392, 1998.

An efficient synthesis of the precursors of [11C]MDL 100907 labeled in two specific positions

An efficient, integrated route for the synthesis of two precursors of [11C]MDL 100907 labeled in the 2′- or 3′-methoxy position is reported. The synthesis involved a one-pot, two-step process to transform the intermediate esters to ketones and subsequent resolution of the racemic alcohols to their respective enantiomers. The resolved, enantiomerically pure phenol precursors were reacted with high specific activity [11C]methyl iodide to produce [11C]MDL 100907 labeled in two specific positions. Copyright

CHAPTER 44 – Characterization of the Radiolabeled Metabolites of [18F]Altanserin: Implications for Kinetic Modeling

Altered serotonergic neurotransmission has been implicated in many neuropsychiatric illnesses, and evidence supporting a role for 5-HT2A receptors in these disorders has motivated the development of a 5-HT2A-selective radiotracer. [18F]Altanserin has demonstrated many favorable characteristics for in vivo imaging of 5-HT2A receptors, including reversibility, saturability, and high specific binding. However, previous studies indicated that radiolabeled metabolites of [18F]altanserin may cross the blood-brain barrier (BBB). The goal of this work was to identify and characterize the radiolabeled metabolites of [18F]altanserin in rat plasma and verify BBB penetration. Identification of the metabolites of [18F]altanserin was aided by reference to those of ketanserin, a close structural analog. Two metabolites of [18F]altanserin were identified: [18F]altanserinol and [18F]4-(4-fluorobenzoyl)piperidine ([18F]4-FBP). After [18F]altanserin injection, the radioactivity in rat plasma was separated using HPLC methods and was compared to authentic standards of altanserinol and 4-FBP. Radiosynthetic methods were also developed that yielded [18F]altanserinol and [18F]4-FBP, which were injected into rats to study their brain uptake and regional brain distribution. These two putative metabolites were found to cross the BBB and resulted in uniformly distributed brain radioactivity concentrations that were unaffected by pretreatment with 5-HT2A antagonists. Injection of [18F]4-FBP resulted in threefold greater brain radioactivity concentrations than [18F]altanserinol. [18F]4-FBP was further metabolized in rats, and its secondary metabolite also crossed the BBB, but exhibited no specific binding to 5-HT2A receptors. Hence, these metabolites contributed only to uniformly distributed, nonspecific binding throughout the rat brain. The in vivo specific binding of [18F]altanserin in positron emission tomography studies is quantifiable, as the contribution of these metabolites to the signal from the brain can be estimated and corrections performed if necessary.

Stereoconservative synthesis of the enantiomerically pure precursors of [11C](+)-McN 5652 and [11C](-)-McN 5652

A stereoconservative synthetic route for the preparation of the precursors of [11C](+)-McN 5652 and [11C](−)-McN 5652, also known as [11C]McN-5652-X and [11C]McN-5652-W, is reported. The key steps involved the transformation of the (+)-trans-4-bromophenyl or (−)-trans-4-bromophenyl analogues of (+)-McN 5652 or (−)-McN 5652 to triisopropylsilyl-protected thioethers and conversion of the thioethers to benzoyl thioesters via a one-pot deprotection and esterification sequence. Subsequent formation of a tartrate salt of the benzoyl thioesters provided stable, convenient precursors for the routine radiosynthesis of [11C](+)-McN 5652 and [11C](−)-McN 5652. The radiosyntheses were accomplished by S-methylation with high specific activity [11C]methyl iodide of the phenylsulfides, which were generated in situ from the tartrate salt of the benzoyl thioesters. HPLC analyses of the final products demonstrated that all of the reactions were stereoconservative, with no detectable isomerization or racemization taking place in any of the reaction sequences. [11C](+)-McN 5652 and [11C](−)-McN 5652 were obtained with end of synthesis (EOS) yields of ∽120 mCi and EOS specific activities of ∽4000 Ci/mmol following radiosynthesis and purification procedures that required approximately 25 min. The chemical and radiochemical purities of the final products were greater than 95%.

CHAPTER 64 – [18F]Altanserin PET Studies of Serotonin-2A Binding: Examination of Nonspecific Component

Previous analyses of the regional brain kinetics of [ 18 F]altanserin ([ 18 F]ALT) in humans, using positron emission tomography (PET), indicated the existence of a slow nonspecific binding component. These analyses involved (1) compartmental models that did not account for blood–brain barrier (BBB) passage of radiolabeled metabolites and (2) metabolite models that did. Both analyses yielded regional binding parameters that were highly correlated with the known rank order of human serotonin-2A (5HT 2 A ) receptors. In this work, the slow nonspecific binding component was further examined by applying conventional and metabolite methods to PET studies of [ 18 F]ALT and two putative metabolites: [ 18 F]altanserinol ([ 18 F]ALT-ol) and [ ,8 F]4-(4-fluorobenzoyl)piperidine ([ 18 F]4-FBP). High specific activity [ 18 F]ALT, [ 18 F]ALT-ol, and [ 18 F]4-FBP PET studies were performed in baboons at baseline and after 5HT 2 A antagonist pretreatment with ketanserin or SR 46349B (blocking data). Radioligand distribution volumes (DV) were estimated using model-based methods. Following [ 18 F]ALT-ol or [ 18 F]4-FBP injection, the radioactivity concentration in brain increased, indicating that [ 18 F]ALT-ol and [ 18 F]4-FBP (and/or their secondary metabolites) crossed the BBB, yielding cerebellar and frontal DV values that were equivalent to [ 18 F]ALT cerebellar and blocking DV values. Hence, the metabolite-generated brain radioactivity was nonspecifically distributed. Conventional models performed statistically better than metabolite models, although a greater dynamic range in the regional binding parameters was obtained by the latter method. These results support the quantification of [ 18 F]ALT data by standard kinetic methods.