Comparison of [(11)C]-(R)-PK 11195 and [(11)C]PBR28, two radioligands for translocator protein (18 kDa) in human and monkey: Implications for positron emission tomographic imaging of this inflammation biomarker

Neuroimage. 2010 Feb 15;49(4):2924-32. doi: 10.1016/j.neuroimage.2009.11.056. Epub 2009 Dec 4.


Ten percent of humans lack specific binding of [(11)C]PBR28 to 18 kDa translocator protein (TSPO), a biomarker for inflammation. "Non-binders" have not been reported using another TSPO radioligand, [(11)C]-(R)-PK 11195, despite its use for more than two decades. This study asked two questions: (1) What is the cause of non-binding to PBR28? and (2) Why has this phenomenon not been reported using [(11)C]-(R)-PK 11195?

Methods: Five binders and five non-binders received whole-body imaging with both [(11)C]-(R)-PK 11195 and [(11)C]PBR28. In vitro binding was performed using leukocyte membranes from binders and non-binders and the tritiated versions of the ligand. Rhesus monkeys were imaged with [(11)C]-(R)-PK 11195 at baseline and after blockade of TSPOs.

Results: Using [(11)C]PBR28, uptake in all five organs with high densities of TSPO (lung, heart, brain, kidney, and spleen) was 50% to 75% lower in non-binders than in binders. In contrast, [(11)C]-(R)-PK 11195 distinguished binders and non-binders in only heart and lung. For the in vitro assay, [(3)H]PBR28 had more than 10-fold lower affinity to TSPO in non-binders than in binders. The in vivo specific binding of [(11)C]-(R)-PK 11195 in monkey brain was approximately 80-fold lower than that reported for [(11)C]PBR28.

Conclusions: Based on binding of [(3)H]PK 11195 to leukocyte membranes, both binders and non-binders express TSPO. Non-binding to PBR28 is caused by its low affinity for TSPO in non-binders. Non-binding may be differentially expressed in organs of the body. The relatively low in vivo specific binding of [(11)C]-(R)-PK 11195 may have obscured its detection of non-binding in peripheral organs.

Publication types

  • Comparative Study
  • Evaluation Study
  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Acetamides / pharmacokinetics*
  • Adult
  • Animals
  • Biomarkers / metabolism
  • Carbon Radioisotopes / pharmacokinetics
  • Female
  • Haplorhini
  • Humans
  • Inflammation / diagnostic imaging*
  • Inflammation / metabolism*
  • Isoquinolines / pharmacokinetics*
  • Male
  • Positron-Emission Tomography / methods*
  • Pyridines / pharmacokinetics*
  • Radiopharmaceuticals / pharmacokinetics
  • Receptors, GABA / metabolism*
  • Reproducibility of Results
  • Sensitivity and Specificity
  • Whole Body Imaging / methods


  • Acetamides
  • Biomarkers
  • Carbon Radioisotopes
  • Isoquinolines
  • N-(2-methoxybenzyl)-N-(4-phenoxypyridin-3-yl)acetamide
  • Pyridines
  • Radiopharmaceuticals
  • Receptors, GABA
  • TSPO protein, human
  • PK 11195