A novel series of
non-amidine-based C1s inhibitors have
been explored.
Starting from high-throughput screening hit3, isoquinoline
was replaced with 1-aminophthalazine to enhance C1s inhibitory activity
while exhibiting good selectivity against other serine proteases.
We first disclose a crystal structure of a complex of C1s and a small-molecule
inhibitor (4e), which guided structure-based optimization
around the S2 and S3 sites to further enhance C1s inhibitory activity
by over 300-fold. Improvement of membrane permeability by incorporation
of fluorine at the 8-position of 1-aminophthalazine led to identification
of(R)-8as a potent, selective, orally
available, and brain-penetrable C1s inhibitor.(R)-8significantly inhibited membrane attack complex formation
induced by human serum in a dose-dependent manner in an in vitro assay
system, proving that selective C1s inhibition blocked the classical
complement pathway effectively. As a result,(R)-8emerged as a valuable tool compound for both in vitro
and in vivo assessment.