Determination of plasma protein binding of positron emission tomography radioligands by high-performance frontal analysis

Positron emission tomography (PET) is an imaging technique based on the use of radioligands labeled with short lived radionuclides, such as C-11 (t(1/2) = 20.4 min) and F-18 (t(1/2) = 109.8 min), which as a consequence often requires rapid plasma protein binding analysis methods. In addition, PET radioligands can suffer from non-specific binding to the membrane when ultrafiltraion, which is the most commonly used method for measuring protein binding in PET, is employed. In this study a high-performance frontal analysis (HPFA) method based on incorporation of a gel filtration column (discovery (R) BIO GFC 100, 50 mm x 4.6 mm, 100 A) into a radio-LC system with phosphate buffered saline (PBS, pH 7.4) at a flow rate of 3 ml/min as mobile phase was developed and investigated for four PET radioligands. The minimum injection volume (MIV) of plasma, which is a crucial factor in HPFA, was determined to be 200 mu l (human), 500 mu l (monkey), 700 mu 1 ( human) and 1000 mu l (monkey) for these four radioligands. The MIV values increased as a higher fraction of the radioligand was present in the protein-free form. The protein binding results obtained were in good agreement with ultrafiltration and the method did not suffer from non-specific binding. The short analysis time (<12 min) allowed multiple protein binding measurements during time course of a human [C-11]PBR28 PET study. (C) 2014 Elsevier B.V. All rights reserved.