The structural basis for high affinity binding of α1-acid glycoprotein to the potent antitumor compound UCN-01

The α1-acid glycoprotein (AGP) is an abundant blood plasma protein with important immunomodulatory functions coupled to endogenous and exogenous ligand binding properties. Its affinity for many drug-like structures, however, means AGP can have a significant effect on the pharmokinetics and pharmacodynamics of numerous small molecule therapeutics. Staurosporine, and its hydroxylated forms UCN-01 and UCN-02, are kinase inhibitors that have been investigated at length as anti-tumour compounds. Despite their potency, these compounds display poor pharmokinetics due to binding to both AGP variants, AGP1 and AGP2. Recent renewed interest in UCN-01 as a cytostatic protective agent prompted us to solve the structure of the AGP2/UCN-01 complex by X-ray crystallography, revealing for the first time the precise binding mode of UCN-01. Solution NMR suggests AGP2 undergoes a significant conformational change upon ligand binding, but also that it uses a common set of sidechains with which it captures key groups of UCN-01 and other small molecule ligands. We anticipate that this structure and supporting NMR data will facilitate rational re-design of small molecules that could evade AGP and therefore improve tissue distribution.