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[18F]MPPF Binding to 5-HT1A Receptors in the Human Brain

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[18F]MPPF Binding to 5-HT1A Receptors in the Human Brain On the Quantification of [18F]MPPF Binding to 5-HT1A Receptors in the Human Brain Jan Passchier, Aren van Waarde, Willem Vaalburg, and Antoon T.M. Willemsen PET Center, Groningen University Hospital, Groningen; Groningen University Institute for Drug Exploration, Groningen, The Netherlands symptoms that are observed in these disorders (5–9). Previous studies have shown that 4-(2Ј-methoxyphenyl)-1- Clearly, the ability to determine receptor binding noninva- [2Ј-(N -2Љ-pyridinyl)-p -[18F]fluorobenzamido]ethylpiperazine sively would enhance the prospect of early diagnosis and 18 ([ F]MPPF) binds with high selectivity to serotonin (5-HT1A) assessment of disease progression as well as offer a poten- receptors in man. However, in these studies, the calculation of tial way to evaluate therapy and early (phases 1 and 2) drug the binding potential (BP, which equals receptor density divided testing. PET is a unique technique that provides the possi- by equilibrium dissociation constant) used a metabolite-cor- rected arterial input. The aim of this study was to determine bility of imaging and quantifying metabolic processes and whether metabolite correction and arterial sampling are essen- receptor binding. Animal studies have shown the selectivity tial for the assessment of BP. Methods: Five analytic methods of 4-(2Ј-methoxyphenyl)-1-[2Ј-(N-2Љ-pyridinyl)-p-[18F]flu- 18 using full datasets obtained from 6 healthy volunteers were orobenzamido]ethylpiperazine ([ F]MPPF) for the 5-HT1A compared. In addition, the clinical applicability of these meth- receptor (10–14). Studies on volunteers have shown that ods was appraised. Three methods were based on Logan anal- regional uptake agreed well with known receptor distribu- ysis of the dynamic PET data using metabolite-corrected and tion (14–20). (The various radioligands for the 5-HT uncorrected arterial plasma input and cerebellar input. The 1A other 2 methods consisted of a simplified reference tissue receptor have been reviewed (21).) Using a metabolite- model and standard compartmental modeling. Results: A high corrected input function in a Logan analysis (22), binding correlation was found between BP calculated with Logan anal- potentials (BPs, which equal receptor density [Bmax] divided ysis using the metabolite-corrected plasma input (used as the by equilibrium dissociation constant [KD]) were estimated reference method for this study) and Logan analysis using either in several regions from their respective distribution volumes the uncorrected arterial plasma input (r 2 ϭ 0.95, slope ϭ 0.85) (DVs) (BP Ϸ DVregion/DVcerebellum Ϫ 1). The resulting ap- 2 or cerebellar input (r ϭ 0.98, slope ϭ 0.91). A high correlation parent BP closely correlated with previous PET studies was also found between our reference method and the simpli- using [carbonyl-11C]WAY 100635 (17,18,23–25) and with fied reference tissue model (r 2 ϭ 0.94, slope ϭ 0.92). In con- trast, a poor correlation was observed between our reference ex vivo autoradiography (26). Unfortunately, the require- method and the standard compartmental model (r 2 ϭ 0.45, ment of arterial sampling and metabolite analyses hampers slope ϭ 1.59). Conclusion: These results indicate that neither the clinical applicability of this method. metabolite analysis nor arterial sampling is necessary for clinical The aim of this study was to determine whether metab- evaluation of BP in the human brain with [18F]MPPF. Both the olite correction and arterial sampling are essential for the Logan analysis method with cerebellar input and the simplified assessment of BP. Therefore, full datasets obtained from reference tissue method can be applied clinically. healthy volunteers were analyzed by several methods. In Key Words: serotonin receptor; binding potential; PET; addition, the clinical applicability of the analysis methods modeling was appraised. J Nucl Med 2001; 42:1025–1031 MATERIALS AND METHODS Volunteers Approval was gained from the medical ethics committee of the The serotonin (5-HT1A) receptor plays an important role in the pathophysiology of a variety of psychiatric and neu- Groningen University Hospital for this study. Six volunteers (2 men, 4 women; age range, 19–65 y) were included after written rodegenerative diseases such as depression, schizophrenia, informed consent had been obtained and an independent physician and dementia (1–4). Autoradiography studies have shown had confirmed their suitability to take part in the study according changes in the receptor density that may be related to the to the following inclusion and exclusion criteria: age between 18 and 65 y; healthy according to medical examination; no history of neurodegenerative or psychiatric disorders; no use of neuroleptic Received Nov. 13, 2000; revision accepted Mar. 5, 2001. drugs, sedative drugs, or antidepressants; no use of corticosteroids For correspondence or reprints contact: Antoon T.M. Willemsen, PhD, PET Center, Groningen University Hospital, P.O. Box 30001, 9700 RB Groningen, or agents that suppress adrenal function; and no pregnancy or The Netherlands. possibility of pregnancy. 18 [ F]MPPF BINDING TO 5-HT1A RECEPTORS • Passchier et al. 1025 Experimental Procedure min), and 30 samples were collected during 15 min using a fraction Radiochemistry. [18F]MPPF was prepared by nucleophilic sub- collector. The retention time of the parent compound was 9 min. stitution of the aromatic nitro group (a comparable method has been described elsewhere (11,12)). Quality control was performed Data Analysis using reverse-phase high-performance liquid chromatography Regions of Interest. The image obtained by summation of the (Nova-Pak; Waters, Milford, MA) with the following parameters: [18F]MPPF frames (5–60 min) was used to determine areas with 150 ϫ 3.9 mm, acetonitrile/tetrahydrofuran/0.01N NaOAc ϭ 28: enhanced uptake, in particular the medial temporal cortex and the 18 6:65, pH 5, retention time (tR)of[ F]MPPF ϭ 5 min, tR of the insular cortex. In addition, the image obtained by summation of the 15 nitro precursor of MPPF ϭ 7 min. The levels of the nitro precursor [ O]H2O scan (0–3.5 min) was used to obtain anatomic informa- were ϽϽ 1 mg/L, and no detectable amounts of WAY-100634 tion on areas with moderate uptake, in particular the frontal, (nonradioactive compound that is formed by hydrolysis of the cingulate, and lateral temporal cortices. Regions of interest (ROIs) nitroprecursor during the synthesis, with affinity for both the were drawn manually in the transaxial orientation using a contour 5-HT1A and the ␣1-adrenergic receptor (27)) were observed. tool (Clinical Applications Programming Package; CTI). Partial- The radiochemical purity of the product was Ͼ99%. volume correction was not applied. PET Measurements. PET was performed on an ECAT EXACT BPs. Five methods for the calculation of BP were compared. HRϩ (CTI, Knoxville, TN/Siemens Medical Systems, Inc., Hoff- Three methods were based on Logan analysis of the dynamic PET man Estates, IL) with a resolution of 4 to 5 mm full width at half data for the calculation of DVs using metabolite-corrected and maximum, giving 63 slices with a center-to-center distance of uncorrected arterial plasma input or cerebellar input, from which 2.425 mm. The volunteers were positioned in the camera using a the BP can be calculated using BP ϭ DVregion/DVcerebellum Ϫ 1 head restraint, and a transmission scan was obtained using 3 (21,28). The other 2 methods consisted of a simplified reference 68Ge/68Ga rod-sources for consequent attenuation correction of the tissue model (29,30) and standard compartmental modeling dynamic PET data. After the transmission scan, the volunteers (23,24,31). All calculations were performed in MATLAB (The received an intravenous bolus injection (10 s) of 1,850 MBq MathWorks, Inc., Natick, MA), with the Logan-based analysis 15 [ O]H2O in 10 mL saline (0.9%). Radioactivity in the brain was using least squares linear regression and the simplified reference measured during 15 min. Eight consecutive frames were acquired. tissue model and standard compartmental modeling using the 15 At least 15 min after administration of [ O]H2O, during which the Levenberg–Marquart optimization algorithm. Kruskal–Wallis volunteer remained in the camera, 69 Ϯ 17 MBq [18F]MPPF nonparametric ANOVA with Bonferroni post hoc multiple com- (specific activity Ͼ 110 TBq/mmol at the time of injection) in 10 parisons testing was performed to analyze the differences between mL phosphate buffer (Na3PO4 ϭ 9.0 mmol/L; Na2HPO4 ϭ 1.3 the various methods (SPSS, version 10; SPSS Inc., Chicago, IL). mmol/L; 0.9% saline) at pH 7.4–7.6 with 7%–8% ethanol were Two-tailed probability values of Ͻ0.05 were considered signifi- administered intravenously over 1 min. Twenty-one consecutive cant. frames were acquired, consisting of six 10-s frames, three 30-s frames, two 60-s frames, two 120-s frames, two 180-s frames, RESULTS three 300-s frames, and three 600-s frames. Blood samples were taken manually from the radial artery at 30-s intervals for the first Intravenously injected [18F]MPPF was rapidly distributed 5 min, followed by 6 samples from 10 to 60 min at 10-min throughout the brain. ROI-derived time–activity curves intervals, giving a total of 16 samples. After centrifugation, showed a rapid decrease of radioactivity in the cerebellum 250-␮L samples of plasma were counted in a calibrated ␥ counter and a slower decrease in target areas (i.e., frontal, cingulate, (CompuGamma 1282 CS; LKB-Wallac, Turku, Finland) to obtain insular, lateral temporal, and medial temporal cortices). An the arterial input function. Whole-blood samples (250 ␮L) were example of time–activity curves for the cerebellum and the also counted to estimate binding of [18F]MPPF to erythrocytes. frontal, insular, and medial temporal cortices in a single Metabolites. Seven of the 16 arterial plasma samples (1 mL, obtained at 1, 2, 5, 10, 20, 40, and 60 min) were deproteinized with volunteer is shown in Figure 1A.
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