title: An admonition when measuring the lipophilicity of radiotracers using counting techniques.
abstract: Log P measurements are a fundamental physico-chemical parameter and one of the cornerstones of structure activity relationships in medicinal chemistry. Despite the attractiveness of the method, the use of counting techniques to measure the log P of lipophilic radiotracers is fraught with pitfalls due to the amplifying effects of small quantities of radioactive impurities. For example, a radiotracer with a log P of 4 containing only 0.1% of a radioactive impurity with a log P of -1 will have an apparent log P of 3 if measured using conventional shake-flask partition techniques, counting the radioactivity in each phase. However, pre-washing the radiotracer-containing organic phase with aqueous phase can, in many cases, allay doubts about the validity of such measurements.
authors.affiliation: The Vivian M. Rackoff PET Centre, Centre for Addiction and Mental Health, Toronto, Ont., Canada. aaw@camhpet.on.ca
authors.forename: A A
authors.initials: AA
authors.lastname: Wilson
authors.forename: L
authors.initials: L
authors.lastname: Jin
authors.forename: A
authors.initials: A
authors.lastname: Garcia
authors.forename: J N
authors.initials: JN
authors.lastname: DaSilva
authors.forename: S
authors.initials: S
authors.lastname: Houle
mesh_terms: D020003:1-Octanol; D000818:Animals; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D004340:Drug Contamination; D006801:Humans; D008055:Lipids / Q000737:chemistry; D021281:Quantitative Structure-Activity Relationship; D011868:Radioisotopes / Q000737:chemistry* / Q000302:isolation & purification; D012588:Scintillation Counting; D012997:Solvents; D014867:Water
chemical_list: D008055:Lipids; D011868:Radioisotopes; D012997:Solvents; D014867:Water; D020003:1-Octanol