Karen Elizabeth Lackey

A unique structure for epidermal growth factor receptor bound to GW572016 (Lapatinib): relationships among protein conformation, inhibitor off-rate, and receptor activity in tumor cells.

GW572016 (Lapatinib) is a tyrosine kinase inhibitor in clinical development for cancer that is a potent dual inhibitor of epidermal growth factor receptor (EGFR, ErbB-1) and ErbB-2. We determined the crystal structure of EGFR bound to GW572016. The compound is bound to an inactive-like conformation of EGFR that is very different from the active-like structure bound by the selective EGFR inhibitor OSI-774 (Tarceva) described previously. Surprisingly, we found that GW572016 has a very slow off-rate from the purified intracellular domains of EGFR and ErbB-2 compared with OSI-774 and another EGFR selective inhibitor, ZD-1839 (Iressa). Treatment of tumor cells with these inhibitors results in down-regulation of receptor tyrosine phosphorylation. We evaluated the duration of the drug effect after washing away free compound and found that the rate of recovery of receptor phosphorylation in the tumor cells reflected the inhibitor off-rate from the purified intracellular domain. The slow off-rate of GW572016 correlates with a prolonged down-regulation of receptor tyrosine phosphorylation in tumor cells. The differences in the off-rates of these drugs and the ability of GW572016 to inhibit ErbB-2 can be explained by the enzyme-inhibitor structures.

CHAPTER 27:Lessons Learned From the Discovery and Development of Lapatinib/Tykerb

Tykerb (lapatinib) is a successful medicine for the treatment of a subset of solid tumours driven by aberrant signalling in tumour cells that could be selectively targeted over healthy, normal tissue. The lessons learnt from the discovery and development of lapatinib can serve as a key learning tool in pharmaceutical project management as much as a study in scientific insight. The research team operated without the benefit of protein crystal structure information whilst the knowledge of the ERBB family of biological targets and the required inhibition profile of an effective molecule grew as the project progressed. In brief, because it was a new area of oncology with huge potential for dramatically improved treatments, it was a question of knowing when to be flexible and when to be rigid; of knowing what to change and what to retain. Fundamentals like retaining a strong link between in vitro and in vivo assays were constant throughout the lifetime of the project; progression criteria could be more flexible. Outstanding synthetic design was core to the success of the project, allowing the rapid evaluation of diversely functionalized, high quality small molecules, thus creating a robust drug discovery knowledge base. In the earlier days of the project, the emphasis was on data acquisition and SAR, whereas the success of the selection of GW2016 for clinical study lay in the application of the acquired understanding for the candidate target profile developed throughout the project.