Screening and Characterization of Allosteric Small Molecules Targeting Bruton's Tyrosine Kinase

Bruton's Tyrosine Kinase (Btk) is a nonreceptor tyrosine kinase that belongs to the TEC family. Mutations in the Btk gene cause X-linked agammaglobulinemia (XLA) leading to an arrest in B-cell development. Btk is also a drug target for B-cell lymphomas that rely on an intact B-cell receptor signaling cascade for survival. All FDA approved drugs for Btk target the ATP binding site of the catalytic kinase domain, leading to potential adverse events due to off-target inhibition. In addition, acquired resistance mutations occur in a subset of patients, rendering available Btk inhibitors ineffective. Therefore, allosteric sites on Btk should be explored for drug development to target Btk more specifically and in combination with active site inhibitors. Virtual screening against nonactive site pockets and in vitro experiments resulted in a series of small molecules that bind to Btk outside of the active site. We characterized these compounds using biochemical and biophysical techniques and narrowed our focus to compound "C2". C2 activates full-length Btk and smaller multidomain Btk fragments but not the isolated kinase domain, consistent with an allosteric mode of action. Kinetic experiments reveal a C2-mediated decrease in K_m and an increase in k_cat leading to an overall increase in the catalytic efficiency of Btk. C2 is also capable of activating the Btk XLA mutants. These proof-of-principle data reveal that Btk can be targeted allosterically with small molecules, providing an alternative to active site Btk inhibitors.