Determination of [ 18 F]FCWAY, [ 18 F]FP-TZTP, and their metabolites in plasma using rapid and efficient liquid-liquid and solid phase extractions Ying Ma*, Dale O. Kiesewetter, Lixin Lang, Margaret Der, Bill Huang, Richard E. Carson, William C. Eckelman PET Department, The Warren G Magnuson Clinical Center, National Institutes of Health, Bethesda, MD, USA Received 24 September 2002; received in revised form 11 November 2002; accepted 14 December 2002 Abstract Liquid-liquid and solid phase extraction methods were developed for the accurate and rapid quantitation of radioactive components in human plasma following injection of two PET ligands. A solid phase extraction (SPE) method was developed for the determination of the 5HT 1A receptor ligand [N-{2-[4-(2-methoxyphenyl) piperazino]ethyl}-N-(2-pyridinyl) trans-4-[ 18 F]fluorocyclohexanecarboxamide (FC- WAY), and its acidic metabolite, 4-[ 18 F]fluorocyclohexane carboxylic acid (FC). In both cases, the extraction method was much faster and easier to use, yet provided results comparable to HPLC and TLC methods. In addition, an easy to perform two-step liquid-liquid extraction was developed for quantitation of 3-(3-((3-[ 18 F]fluoropropyl)thio)-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-methylpyridine ([ 18 F]FP- TZTP), a selective M2 muscarinic agonist. Published by Elsevier Inc. All rights reserved. 1. Introduction The conversion of radioactivity data obtained from positron emission tomography (PET) into biologically rel- evant parameters is one of the major areas of research [1,12]. Application of tissue compartment models to PET data from receptor-binding ligands can result in extraction of parameters proportional to receptor concentration and binding affinity [1]. The mathematical solution to tissue compartment models requires knowledge of the concentra- tion of unmetabolized radioligand in the plasma as a func- tion of time (time-activity curve, TAC). The identity of the source of radioactivity in the tissue must also be known. If a metabolite contributes to the tissue uptake, the metabolite TAC also must be known [4]. Human studies with the PET 5-HT 1A ligand, [ 18 F]FC- WAY (N-{2-[4-(2-methoxyphenyl) piperazino]ethyl}-N- (2-pyridinyl) trans 4-fluorocyclohexanecarboxamide, Fig. 1), have uncovered data analysis challenges. These include brain uptake of a metabolite, [ 18 F]FC (trans-4-fluorocyclo- hexanecarboxylic acid, Fig. 1), and skull uptake of the secondary metabolite [ 18 F]fluoride [3]. The plasma TAC for parent FCWAY and metabolite FC are required to produce quantitative images of the FCWAY distribution volume (V) and K 1 [3]. The analysis of PET data from studies of our muscarinic ligand, [ 18 F]FP-TZTP (3-(3-(3-fluoropropyl- thio)-1,2,5-thiadiazol-4-yl) 1,2,5,6-tetrahydro-1-methyl- pyridine, Fig. 5), also requires a plasma TAC for the amount of parent [2,6]. Traditionally, the concentration of parent ligand is deter- mined by a single extraction of the plasma by an organic solvent or solid phase extraction followed by HPLC or TLC analysis of the extract for the proportion of parent ligand [4,6,9]. The percent extraction of radioactivity from plasma times the percent of parent in the extract gives the fraction of extracted parent in the plasma. With correction for ex- traction efficiency and radioactivity decay, the concentra- tion of radioactive parent ligand can be calculated. The HPLC and TLC analyses limit the number of samples that can be analyzed for an imaging study because the decay of this radionuclide is time limiting. We have previously identified the metabolites of [ 18 F] FCWAY and [ 18 F]FP-TZTP produced by human hepato- cytes using HPLC/MS/MS [10,11]. The metabolic profile * Corresponding author. Tel.: +1-301-435-2229; fax: +1-301-402- 3521. E-mail address: yma@mail.cc.nih.gov (Y. Ma). Nuclear Medicine and Biology 30 (2003) 233–240 www.elsevier.com/locate/nucmedbio 0969-8051/03/$ – see front matter Published by Elsevier Inc. All rights reserved. doi:10.1016/S0969-8051(02)00452-3