Noncompetitive Modulation of the Proteasome by Imidazoline Scaffolds Overcomes Bortezomib Resistance and Delays MM Tumor Growth in Vivo

Abstract

Multiple myeloma is a malignant disorder of differentiated B-cells for which standard care involves the inhibition of the proteasome. All clinically used proteasome inhibitors, including the chemotherapeutic drug bortezomib, target the catalytic active sites of the proteasome and inhibit protein proteolysis by competing with substrate binding. However, nearly all (~97%) patients become intolerant or resistant to treatments within a few years, after which the average survival time is less than one year. We describe herein the inhibition of the human proteasome via a non-competitive mechanism by the imidazoline scaffold, TCH-13. Consistent with a mechanism distinct from competitive inhibitors, TCH-013 acts additively with and overcomes resistance to bortezomib. Importantly, TCH-013 induces apoptosis a panel of myeloma and leukemia cell lines, but in contrast, normal lymphocytes, primary bone marrow stromal cells and macrophages are resistant to its cytotoxic effects. TCH-013 was equally effective in blocking MM cell growth in co-cultures of MM cells with hBMSC isolated from CD138 negative BM samples of MM patients. The cellular activity translated well in vivo where TCH-013 delayed tumor growth in an MM xenograft model to a similar extent as bortezomib.

Small Molecule Modulation of Proteasome Assembly

“…Given the antitumor efficacy of this molecule class, we turned our attention to proteins that are overexpressed in certain cancers. 58,59 For these studies, we tested TCH-165 in HEK293T cells expressing ornithine decarboxylase (ODC), labeled with GFPSpark at its N-terminus. 30 GFPSpark is a structured 28 kDa protein and is not degraded by the 20S, whereas the disordered C-terminal of ODC is a well-known 20S substrate (Figure 3A).…”

“…58 The parent compound of that study was found not to interact with the catalytic sites of the 20S proteasome but to modulate protein proteolysis through an unknown mechanism. Our efforts to optimize the activity of the imidazoline scaffold generated the small molecule TCH-165 (Figure 1).…”

Substituted quinolines as noncovalent proteasome inhibitors

“…Examples of nonpeptidic noncovalent proteasome modulators include the phakellins, 25 oxadiazoles, 26 hydroxyureas, 24 imidazolines 27,28 sulfone or piperazine agents, 29 and tamoxifen derivatives. 30 …”

“…37 Importantly, 5-AHQ was found effective against bortezomib resistant cell lines, thus exemplifying another advantage of using mechanistically distinct classes of proteasome inhibitors. 27,38,39 It is therefore tempting to speculate that at least some of these noncovalent, nonpeptidic quinolines may occupy a common allosteric binding site. 35 …”

Analysing the effect of I1 imidazoline receptor ligands on DSS-induced acute colitis in mice

“…Furthermore, various novel imidazoline scaffolds were shown to inhibit human proteasome (Lansdell et al 2013), which in turn prevented NF-κB mediated gene transcription in cell cultures (Sharma et al 2004;Kahlon et al 2009) and the production of proinflammatory cytokines tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) in human blood (Kahlon et al 2009). Although in the majority of these studies other molecular targets than IRs were found to mediate the anti-inflammatory action of imidazoline drugs, the intriguing hypothesis of Molderings et al (2007a, b) that I1-IRs may belong to the family of S1P receptors and represent mixtures of homo and/or heterodimers of S1P1-3 receptors suggests that also pharmacological modulation of I1-IRs may result in reduced inflammation.…”