Proteasomes contribute to maintaining protein homeostasis and their inhibition is
beneficial in certain types of cancer and in autoimmune diseases. However, the inhibition
of the proteasomes in healthy cells leads to unwanted side-effects and significant
effort has been made to identify inhibitors specific for the immunoproteasome, especially
to treat diseases which manifest increased levels and activity of this proteasome
isoform. Here, we report our efforts to discover fragment-sized inhibitors of the
human immunoproteasome. The screening of an in-house library of structurally diverse
fragments resulted in the identification of benzo[d]oxazole-2(3H)-thiones, benzo[d]thiazole-2(3H)-thiones,
benzo[d]imidazole-2(3H)-thiones, and 1-methylbenzo[d]imidazole-2(3H)-thiones (with
a general term benzoXazole-2(3H)-thiones) as inhibitors of the chymotrypsin-like (β5i)
subunit of the immunoproteasome. A subsequent structure-activity relationship study
provided us with an insight regarding growing vectors. Binding to the β5i subunit
was shown and selectivity against the β5 subunit of the constitutive proteasome was
determined. Thorough characterization of these compounds suggested that they inhibit
the immunoproteasome by forming a disulfide bond with the Cys48 available specifically
in the β5i active site. To obtain fragments with biologically more tractable covalent
interactions, we performed a warhead scan, which yielded benzoXazole-2-carbonitriles
as promising starting points for the development of selective immunoproteasome inhibitors
with non-peptidic scaffolds.
