Bortezomib (BTZ), a registered
proteasome inhibitor (PI) for
multiple myeloma, has also been proposed as a potential
antirheumatic agent. Its reported side effects, however, make it unappealing for long-term administration, and resistance may also develop. To overcome this, second-generation PIs became available. Here, we investigated whether a novel class of
peptide epoxyketone-based PIs, including
carfilzomib, N-((S)-3-methoxy-1-(((S)-3-methoxy-1-(((S)-1-((R)-2-methyloxiran-2-yl)-1-oxo-3-phenylpropan-2-yl)amino)-1-oxopropan-2-yl)amino)-1-oxopropan-2-yl)-2-methylthiazole-5-carboxamide (ONX0912), and
(S)-3-(4-methoxyphenyl)-N-((S)-1-((S)-2-methyloxiran-2-yl)-1-oxo-3-phenylpropan-2-yl)-2-((S)-2-(2-morpholinoacetamido)propanamido)propanamide (ONX0914), might escape two established BTZ-resistance mechanisms: 1) mutations in the
proteasome β5 subunit (PSMB5) targeted by these PIs, and 2)
drug efflux mediated by
ATP-binding cassette transporters. THP1 myeloid sublines with acquired resistance to BTZ (54- to 235-fold) caused by mutations in the PSMB5 gene displayed marked cross-resistance but less pronounced cross-resistance to
carfilzomib (9- to 32-fold), ONX0912 (39- to 62-fold), and ONX0914 (27- to 97-fold). As for
ATP-binding cassette transporter-mediated efflux, lymphoid CEM/VLB cells with
P-glycoprotein (Pgp)/multidrug resistance 1 overexpression exhibited substantial resistance to
carfilzomib (114-fold), ONX0912 (23-fold), and ONX0914 (162-fold), whereas less resistance to BTZ (4.5-fold) was observed. Consistently, β5 subunit-associated
chymotrypsin-like
proteasome activity was significantly less inhibited in these CEM/VLB cells. Ex vivo analysis of peripheral blood mononuclear cells from
therapy-naive patients with
rheumatoid arthritis revealed that, although basal Pgp levels were low,
P-glycoprotein expression compromised the inhibitory effect of
carfilzomib and ONX0914. However, the use of P121 (
reversin 121), a Pgp transport inhibitor, restored parental cell inhibitory levels in both CEM/VLB cells and peripheral blood mononuclear cells. These results indicate that the pharmacologic activity of these PIs may be hindered by drug resistance mechanisms involving PSMB5 mutations and PI extrusion via Pgp.