Test-retest reliability and convergent validity of (R)-[11C]PK11195 outcome measures without arterial input function

Purpose: The PET radioligand (R)-[C-11]PK11195 is used to quantify the 18-kDa translocator protein (TSPO), a marker for glial activation. Since there is no brain region devoid of TSPO, an arterial input function (AIF) is ideally required for quantification of binding. However, obtaining an AIF is experimentally demanding, is sometimes uncomfortable for participants, and can introduce additional measurement error during quantification. The objective of this study was to perform an evaluation of the test-retest reliability and convergent validity of techniques used for quantifying (R)-[C-11]PK11195 binding without an AIF in clinical studies. Methods: Data from six healthy individuals who participated in two PET examinations, 6weeks apart, were analyzed. Regional non-displaceable binding potential (BPND) values were calculated using the simplified reference tissue model, with either cerebellum as reference region or a reference input derived using supervised cluster analysis (SVCA). Standardized uptake values (SUVs) were estimated for the time interval of 40-60min. Results: Test-retest reliability for BPND estimates were poor (80% of ICCs <0.5). BPND estimates derived without an AIF were not correlated with BPND, total or specific distribution volume from the 2TCM using an AIF (all R-2<12%). SUVs showed moderate reliability but no correlation to any other outcome measure. Conclusions: Caution is warranted when interpreting patient-control comparisons employing (R)-[C-11]PK11195 outcome measures obtained without an AIF

Pulmonary PET imaging confirms preferential lung target occupancy of an inhaled bronchodilator

Background: Positron emission tomography (PET) is a non-invasive molecular imaging technique that traces the distribution of radiolabeled molecules in experimental animals and human subjects. We hypothesized that PET could be used to visualize the binding of the bronchodilator drug ipratropium to muscarinic receptors (MR) in the lungs of living non-human primates (NHP). The objectives of this study were two-fold: (i) to develop a methodology for quantitative imaging of muscarinic receptors in NHP lung and (ii) to estimate and compare ipratropium-induced MR occupancy following drug administration via intravenous injection and inhalation, respectively. Methods: A series of PET measurements (n=18) was performed after intravenous injection of the selective muscarinic radioligand C-11-VC-002 in NHP (n=5). The lungs and pituitary gland (both rich in MR) were kept in the field of view. Each PET measurement was followed by a PET measurement preceded by treatment with ipratropium (intravenous or inhaled). Results: Radioligand binding was quantified using the Logan graphical analysis method providing the total volume of distribution (V-T). Ipratropium reduced the V-T in the lung and pituitary in a dose-dependent fashion. At similar plasma ipratropium concentrations, administration by inhalation produced larger reductions in V-T for the lungs. The plasma-derived apparent affinity for ipratropium binding in the lung was one order of magnitude higher after inhalation (K-iih=1.01nM) than after intravenous infusion (K-iiv=10.84nM). Conclusion: Quantitative muscarinic receptor occupancy imaging by PET articulates and quantifies the therapeutic advantage of the inhaled route of delivery and provides a tool for future developments of improved inhaled drugs

[C-11]PBB3 binding in A beta(-) or A beta(+) corticobasal syndrome

Corticobasal syndrome (CBS) is associated with 4-repeat tauopathy and/or Alzheimers disease pathologies. To examine tau and amyloid-beta (A beta) deposits in CBS patients using positron emission tomography (PET). Eight CBS patients and three healthy individuals lacking amyloid pathology underwent PET with [C-11]PBB3 for tau imaging, and [C-11]AZD2184 for A beta. Subcortical and cortical binding of [C-11]PBB3 was compared between A beta(-) and A beta(+) CBS patients and reference group. Postmortem analysis was done in one CBS patient. Three CBS patients were considered A beta(+). Total binding was higher in all patients compared to the reference group. Similar regional binding profiles of [C-11]PBB3 in A beta(+) and A beta(-) CBS patients were found. Elevated [C-11]PBB3 binding in pallidum was observed in all CBS patients. Cortical [C-11]PBB3 binding was higher in A beta(+) compared to A beta(-) patients. Postmortem analysis of a CBS patient revealed corticobasal degeneration neuropathology and [C-11]PBB3 autofluorescence in some tau-positive structures. [C-11]PBB3 is elevated in CBS patients with binding in relevant areas capturing some, but not all, 4-repeat tauopathy in CBS

In vivo visualization of β-cells by targeting of GPR44

GPR44 expression has recently been described as highly β-cell selective in the human pancreas and constitutes a tentative surrogate imaging biomarker in diabetes. A radiolabeled small-molecule GPR44 antagonist, [11C]AZ12204657, was evaluated for visualization of β-cells in pigs and nonhuman primates by positron emission tomography as well as in immunodeficient mice transplanted with human islets under the kidney capsule. In vitro autoradiography of human and animal pancreatic sections from subjects without and with diabetes, in combination with insulin staining, was performed to assess β-cell selectivity of the radiotracer. Proof of principle of in vivo targeting of human islets by [11C]AZ12204657 was shown in the immunodeficient mouse transplantation model. Furthermore, [11C]AZ12204657 bound by a GPR44-mediated mechanism in pancreatic sections from humans and pigs without diabetes, but not those with diabetes. In vivo [11C]AZ12204657 bound specifically to GPR44 in pancreas and spleen and could be competed away dose-dependently in nondiabetic pigs and nonhuman primates. [11C]AZ12204657 is a first-in-class surrogate imaging biomarker for pancreatic β-cells by targeting the protein GPR44