Next Phase Newsletter

Juan Carlos Lopez

Andy Marshall

​Drug development efforts targeting the constitutive 26S proteosome have led to the development of several important multiple myeloma (MM) and mantle cell lymphoma treatments, including the first landmark FDA approval of Millennium Pharmaceuticals’ (now Takeda) dipeptide boric acid Velcade (bortezomib) in 2003 and second-generation molecules, such as Amgen/Ono Pharmaceutical’s irreversible inhibitor Kyprolis (carfilzomib) and Takeda’s orally available inhibitor Ninlaro (ixazomib). Second-generation versions of these ‘pan-proteosome’ drugs have longer duration of effect, reduced peripheral neuropathy and increased safety in renally impaired patients, but may cause gastrointestinal and cardiac toxicity. This toxicological profile has shifted attention to developing inhibitors selective for an alternative form of the core 20S proteosome—the immunoproteasome, which processes peptides for presentation to CD8⁺ T cells in the MHC-I complex and is constitutively expressed only in hematopoietic cells, induced in immune cells stimulated in the presence of IFN-γ, and upregulated in certain cancers like MM.

Currently, Kezar Life Sciences’ is furthest along in development; in April, it completed a phase 2a trial in autoimmune hepatitis of zetomipzomib (KZ-616), a small-molecule that inhibits both the immunoproteasome core particle component beta subunit 8 (PSMB8; LMP7/β5i) and PSMB9 (LMP2/β1i). Merck kGaA (Darmstadt, Germany) is also pushing forward with a phase 1 clinical program of M3258, a small-molecule inhibitor specific for PSMB8 and intended for use in MM (Principia Biopharma’s selective PSMB8 inhibitor was swallowed up by Sanofi in 2020 when the pharma acquired the San Francisco-based biotech’s Bruton’s tyrosine kinase inhibitor program). Elsewhere, Leiden University startup iProtics recently received a €200K grant from the Dutch Biotech Booster to develop selective immunoproteosome inhibitors, while Auburn University spinout Inhiprot (West Lebanon, NH) received SBIR funding to develop a dual PSMB6/PSMB9 inhibitor for MM. Now, a new study reveals immunoproteosome targeting may also have benefits in neuroinflammatory diseases like multiple sclerosis.

The work, published in Cell and led by Catherine Meyer-Schwesinger and Manuel Friese, from University Medical Center Hamburg-Eppendorf, identifies a neuron-intrinsic mechanism of neurodegeneration in multiple sclerosis (MS) driven by the immunoproteasome.

Under healthy conditions, neurons utilize the constitutive proteasome subunit PSMB5 to regulate proteostasis and degrade 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3 (PFKFB3), a potent stimulator of glycolysis. This degradation is key because neurons rely more on the pentose phosphate pathway than on glycolysis to produce antioxidants like NADPH and glutathione for protection against oxidative stress.

However, Meyer-Schwesinger, Friese and their colleagues show that, during neuroinflammation, chronic exposure to interferon-γ leads to the induction of the immunoproteasome in neurons, triggering the replacement of constitutive proteosome PSMB5 (β5c) with PSMB8 (β5i). This subunit swap in neurons reduces proteasomal activity, resulting in accumulation of PFKFB3, which in turn enhances glycolysis, diminishes the activity of the pentose phosphate pathway, and impairs redox homeostasis — conditions that sensitize neurons to oxidative injury and ferroptosis.

The team showed that this mechanism was operational in both experimental autoimmune encephalomyelitis (EAE; a mouse model of MS) and brain tissue from MS patients. Moreover, neuron-specific knock-out of Psmb8 or pharmacological inhibition using the small-molecule PSMB8 inhibitor ONX-0914 (originally developed at Onyx Pharmaceuticals/Proteolix) protected neurons in vivo from inflammation-induced damage. Similarly, blocking PFKFB3 with the small-molecule inhibitor PFK-158 or through conditional knockout in neurons reduced disease severity in EAE, prevented neuronal and synaptic loss, and reduced markers of oxidative stress and lipid peroxidation.

It is important to highlight that, unlike cancer or immune cells, neurons do not upregulate PSMB8 in response to a series of MS-related cytokines. So, the neuron-specific effect reported in this study might only become active upon chronic neuroinflammation (i.e. chronic exposure to interferon-γ). Understanding this mechanism might reveal new targets related to the immunoproteosome in the treatment of MS.

This brings us to challenges for translational efforts seeking to develop immunoproteosome inhibitors against MS. Several important neuronal processes, such as synaptic transmission and calcium signaling, are tightly linked to proteasome function; thus, pan-proteosome inhibitors like Velcade could be detrimental to the CNS. The saving grace of approved proteosome inhibitors is that current chemotypes (boronate-based peptides or epoxyketone-based binders) do not cross the blood brain barrier, at least in healthy individuals. Thus, any MS program might need to use intrathecal injection for compounds derived from existing chemical series or engage a medicinal-chemistry effort to design molecules that can breach the BBB and retain potency.

The gambit for immunoproteosome-selective drugs is that they avoid inhibiting constitutive 26S proteosome activity in most tissues (and non-inflammed CNS), which is what makes Velcade and its derivatives so difficult for patients to tolerate; an immunoproteosome inhibitor should therefore have a more favorable safety profile. But so far, immunoproteosome-targeting drugs have had their own share of toxicity problems in the clinic.

Last October, Kezar abandoned its program for zetomipzomib in lupus nephritis after the FDA placed a clinical hold on the trial after 4 patient deaths. The agency placed a second partial hold on the company’s autoimmune hepatitis trial in 24 patients last November due to concerns about steroid control and injection site reactions in 4 patients who were waiting to roll over into the open-label extension arm. Concerns about compromised immune surveillance of acute or latent viral infections due to hobbled antigen processing and presentation would also need to be explored.

In sum, the new work provides strong evidence that the immunoproteosome plays a key role not only in inflammation or infiltration of immune cells, but also in a metabolic switch in neurons which is a key driver of vulnerability in MS. It will be interesting to see whether either targeting immunoproteosome component PSMB8 or taking a completely different tack, blocking PFKFB3, will prove more practical as a neuroprotective strategy in MS.

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