[11C]PBR28, a radioligand targeting the translocator protein (TSPO), does not produce a specific binding signal in approximately 14% of healthy volunteers. This phenomenon has not been reported for [11C]PK11195, another TSPO radioligand. We measured the specific binding signals with [3H]PK11195 and [3H]PBR28 in brain tissue from 22 donors. Overall, 23% of the samples did not generate a visually detectable specific autoradiographic signal with [3H]PBR28, although all samples showed [3H]PK11195 binding. There was a marked reduction in the affinity of [3H]PBR28 for TSPO in samples with no visible [3H]PBR28 autoradiographic signal ( K i=188±15.6 nmol/L), relative to those showing normal signal ( K i=3.4±0.5 nmol/L, P<0.001). Of this latter group, [3H]PBR28 bound with a two-site fit in 40% of cases, with affinities ( K i) of 4.0±2.4 nmol/L (high-affinity site) and 313±77 nmol/L (low-affinity site). There was no difference in Kd or Bmax for [3H]PK11195 in samples showing no [3H]PBR28 autoradiographic signal relative to those showing normal [3H]PBR28 autoradiographic signal. [3H]PK11195 bound with a single site for all samples. The existence of three different binding patterns with PBR28 (high-affinity binding (46%), low-affinity binding (23%), and two-site binding (31%)) suggests that a reduction in [11C]PBR28 binding may not be interpreted simply as a reduction in TSPO density. The functional significance of differences in binding characteristics warrants further investigation.

This work examines the inter-relationship between the unbound drug fractions in blood and brain homogenate, passive membrane permeability, P-glycoprotein (Pgp) efflux ratio, and log octanol/water partition coefficients (cLogP) in determining the extent of central nervous system (CNS) penetration observed in vivo. The present results demonstrate that compounds often considered to be Pgp substrates in rodents (efflux ratio greater than 5 in multidrug resistant Madin-Darby canine kidney cells) with poor passive permeability may still exhibit reasonable CNS penetration in vivo; i.e., where the unbound fractions and nonspecific tissue binding act as a compensating force. In these instances, the efflux ratio and in vitro blood-brain partition ratio may be used to predict the in vivo blood-brain ratio. This relationship may be extended to account for the differences in CNS penetration observed in vivo between mdr1a/b wild type and knockout mice. In some instances, cross-species differences that might initially seem to be related to differing transporter expression can be rationalized from knowledge of unbound fractions alone. The results presented in this article suggest that the information exists to provide a coherent picture of the nature of CNS penetration in the drug discovery setting, allowing the focus to be shifted away from understanding CNS penetration toward the more important aspect of understanding CNS efficacy.Within the modern drug discovery paradigm, drug metabolism and pharmacokinetics (DMPK) play an integral role in the process of compound selection and progression. Much of the impact of DMPK has been caused by its transformation from a largely descriptive discipline to that of a predictive science, fuelled by advances in bioanalysis and in vitro techniques. Hence discovery DMPK provides a powerful means for assessing the risks of taking potential assets into development.Nevertheless, the development of molecules targeted at the central nervous system (CNS) remains a significant challenge caused by the increased regulation and protection afforded to the brain over other organs of the body. The major knowledge gaps are 1) understanding the physicochemical features that determine CNS penetration, 2) understanding the impact of the blood-brain barrier (BBB) on CNS uptake, and 3) providing a coherent measure of CNS penetration that can be related to drug efficacy. Regarding the latter point, although it is important to develop a link between the pharmacokinetics of a molecule and the biophase, arguably the critical issue is one of sufficient access of free drug to the requisite site of action.Numerous models and measures of CNS uptake are available to assist in the search for centrally active agents. In situ brain perfusion techniques have highlighted the good correlation between increasing lipophilicity and CNS permeability. Polar drugs that are subject to paracellular absorption such as atenolol (logD oct,7.4 Ϫ2.1; Artursson, 1990) and sumatriptan (logD oct,7.4 Ϫ1.5;Pascual and Munoz, 2005) show...

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Purpose Integrin α v β 6 belongs to the RGD subset of the integrin family, and its expression levels are a prognostic and theranostic factor in some types of cancer and pulmonary fibrosis. This paper describes the GMP radiolabelling of the synthetic 20 amino acid peptide A20FMDV2 (NAVPNLRGDLQVLAQKVART), derived from the foot-and-mouth disease virus, and characterises the use of [ 18 F]FB-A20FMDV2 as a high affinity, specific and selective PET radioligand for the quantitation and visualisation of α v β 6 in rodent lung to support human translational studies. Methods The synthesis of [ 18 F]FB-A20FMDV2 was performed using a fully automated and GMP-compliant process. Sprague-Dawley rats were used to perform homologous (unlabelled FB-A20FMDV2) and heterologous (anti-α v β 6 antibody 8G6) blocking studies. In order to generate a dosimetry estimate, tissue residence times were generated, and associated tissue exposure and effective dose were calculated using the Organ Level Internal Dose Assessment/Exponential Modelling (OLINDA/EXM) software. Results [ 18 F]FB-A20FMDV2 synthesis was accomplished in 180 min providing~800 MBq of [ 18 F]FB-A20FMDV2 with a molar activity of up to 150 GBq/μmol and high radiochemical purity (> 97%). Following i.v. administration to rats, [ 18 F]FB-A20FMDV2 was rapidly metabolised with intact radiotracer representing 5% of the total radioactivity present in rat plasma at 30 min. For the homologous and heterologous block in rats, lung-to-heart SUV ratios at 30-60 min post-administration of [ 18 F]FB-A20FMDV2 were reduced by 38.9 ± 6.9% and 56 ± 19.2% for homologous and heterologous block, respectively. Rodent biodistribution and dosimetry calculations using OLINDA/EXM provided a whole body effective dose in humans 33.5 μSv/MBq. Conclusion [ 18 F]FB-A20FMDV2 represents a specific and selective PET ligand to measure drug-associated αvβ6 integrin occupancy in lung. The effective dose, extrapolated from rodent data, is in line with typical values for compounds labelled with fluorine-18 and combined with the novel fully automated and GMP-compliant synthesis and allows for clinical use in translational studies.