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

• A method to determine plasma protein binding of PET radioligands was developed.
• The method was based on high-performance frontal analysis (HPFA).
• The method was simple and rapid considering the short half lives of radionuclides.
• Good agreement with results from ultrafiltration method was achieved.

Positron emission tomography (PET) is an imaging technique based on the use of radioligands labeled with short lived radionuclides, such as 11C (t½ = 20.4 min) and 18F (t½ = 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® BIO GFC 100, 50 mm × 4.6 mm, 5 μm, 100 Å) 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 μl (human), 500 μl (monkey), 700 μl (human) and 1000 μl (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 [11C]PBR28 PET study.

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