title: The p53 inhibitor pifithrin-alpha forms a sparingly soluble derivative via intramolecular cyclization under physiological conditions.
abstract: The transcription factor p53 coordinates cell cycle arrest and apoptosis in response to DNA damage. Pifithrin-alpha (PFT-alpha) is a small molecule inhibitor of p53 activity that is frequently used in cell culture studies of p53 function. Here we report an investigation of the stability of this compound. PFT-alpha rapidly converts to a planar tricyclic derivative, with a half-life of 4.2 h under physiological conditions. This spontaneous conversion greatly alters the structural and physicochemical properties of the drug. PFT-alpha has a pKa of 9.11 and is an ionic species in physiological medium, whereas the tricyclic derivative has a pKa of 4.36 and exists as the neutral free base at pH 7. The tricyclic derivative is very hydrophobic, with a log P of 4.26. Although PFT-alpha is generally used at 10-30 microM concentration, the aqueous solubility of its derivative is only 0.2 microM, and it can form a visible precipitate under conditions of typical use. The conversion of PFT-alpha proceeds via an intramolecular cyclization reaction involving the imine and carbonyl groups. Modification of the carbonyl function creates a stable analogue of PFT-alpha that remains soluble indefinitely. These results provide a strategy for the rational design of PFT-alpha analogues that exhibit predictable stability, hydrophobicity, and aqueous solubility.
authors.affiliation: Department of Chemistry, University of Nevada, Las Vegas, Nevada 89154, USA. rogary@ccmail.nevada.edu
authors.forename: Ronald K
authors.initials: RK
authors.lastname: Gary
authors.forename: Derek A
authors.initials: DA
authors.lastname: Jensen
mesh_terms: D052160:Benzothiazoles; D001692:Biological Transport; D002460:Cell Line; D006801:Humans; D007700:Kinetics; D012995:Solubility; D013844:Thiazoles / Q000138:chemical synthesis / Q000493:pharmacokinetics*; D014050:Toluene / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000493:pharmacokinetics; D016159:Tumor Suppressor Protein p53 / Q000037:antagonists & inhibitors / Q000378:metabolism*
chemical_list: D052160:Benzothiazoles; D013844:Thiazoles; D016159:Tumor Suppressor Protein p53; D014050:Toluene; C121565:pifithrin