A Qualified Success: Discovery of a New Series of ATAD2 Bromodomain Inhibitors with a Novel Binding Mode Using High-Throughput Screening and Hit Qualification

The bromodomain of
ATAD2 has proved to be one of
the least-tractable proteins within this target class. Here, we describe
the discovery of a new class of inhibitors by high-throughput screening
and show how the difficulties encountered in establishing a screening
triage capable of finding progressible hits were overcome by data-driven
optimization. Despite the prevalence of nonspecific hits and an exceptionally
low progressible hit rate (0.001%), our optimized hit qualification
strategy employing orthogonal biophysical methods enabled us to identify
a single active series. The compounds have a novel ATAD2 binding mode
with noncanonical features including the displacement of all conserved
water molecules within the active site and a halogen-bonding interaction.
In addition to reporting this new series and preliminary structure–activity
relationship, we demonstrate the value of diversity screening to complement
the knowledge-based approach used in our previous ATAD2 work. We also
exemplify tactics that can increase the chance of success when seeking
new chemical starting points for novel and less-tractable targets.