Determination Of Receptor Occupancy In The Presence Of Mass Dose: [C-11]Gsk189254 Pet Imaging Of Histamine H-3 Receptor Occupancy By Pf-03654746

Measurements of drug occupancies using positron emission tomography (PET) can be biased if the radioligand concentration exceeds tracer levels. Negative bias would also arise in successive PET scans if clearance of the radioligand is slow, resulting in a carryover effect. We developed a method to (1) estimate the invivo dissociation constant K-d of a radioligand from PET studies displaying a non-tracer carryover (NTCO) effect and (2) correct the NTCO bias in occupancy studies taking into account the plasma concentration of the radioligand and its invivo K-d. This method was applied in a study of healthy human subjects with the histamine H-3 receptor radioligand [C-11]GSK189254 to measure the PK-occupancy relationship of the H-3 antagonist PF-03654746. From three test/retest studies, [C-11]GSK189254 K-d was estimated to be 9.5 +/- 5.9pM. Oral administration of 0.1 to 4mg of PF-03654746 resulted in occupancy estimates of 71%-97% and 30%-93% at 3 and 24h post-drug, respectively. NTCO correction adjusted the occupancy estimates by 0%-15%. Analysis of the relationship between corrected occupancies and PF-03654746 plasma levels indicated that PF-03654746 can fully occupy H-3 binding sites (ROmax=100%), and its IC50 was estimated to be 0.144 +/- 0.010ng/mL. The uncorrected IC50 was 26% higher.