FV-162 is a novel,orally bioavailable,irreversible proteasome inhibitor with improved pharmacokinetics displaying preclinical efficacy with continuous daily dosing

Approved proteasome inhibitors have advanced the treatment of multiple myeloma but are associated with serious toxicities, poor pharmacokinetics, and most with the inconvenience of intravenous administration. We therefore sought to identify novel orally bioavailable proteasome inhibitors with a continuous daily dosing schedule and improved therapeutic window using a unique drug discovery platform. We employed a fluorine-based medicinal chemistry technology to synthesize 14 novel analogs of epoxyketone-based proteasome inhibitors and screened them for their stability, ability to inhibit the chymotrypsin-like proteasome, and antimyeloma activity in vitro. The tolerability, pharmacokinetics, pharmacodynamic activity, and antimyeloma efficacy of our lead candidate were examined in NOD/SCID mice. We identified a tripeptide epoxyketone, FV-162, as a metabolically stable, potent proteasome inhibitor cytotoxic to human myeloma cell lines and primary myeloma cells. FV-162 had limited toxicity and was well tolerated on a continuous daily dosing schedule. Compared with the benchmark oral irreversible proteasome inhibitor, ONX-0192, FV-162 had a lower peak plasma concentration and longer half-life, resulting in a larger area under the curve (AUC). Oral FV-162 treatment induced rapid, irreversible inhibition of chymotrypsin-like proteasome activity in murine red blood cells and inhibited tumor growth in a myeloma xenograft model. Our data suggest that oral FV-162 with continuous daily dosing schedule displays a favorable safety, efficacy, and pharmacokinetic profile in vivo, identifying it as a promising lead for clinical evaluation in myeloma therapy.The ubiquitin–proteasome system is responsible for the regulation and degradation of the majority of the intracellular proteins in eukaryotic cells.¹ The 26S proteasome is a multi-subunit protein complex that mediates the proteolytic degradation and turnover of damaged, misfolded or excess proteins that have been polyubiquitylated in the cytoplasm and nucleus.^{1, 2} The 26S proteasome consists of a 20S core particle, capped by 19S regulatory particles.^{3, 4} The 19S particle participates in the recognition, processing, unfolding, and translocation of ubiquitylated protein substrates into the 20S core.⁵ Substrates are then degraded inside the chamber of the barrel-like 20S core particle, where the active sites of multiple β1, β2, and β5 subunits catalyze caspase-like (C-L), trypsin-like (T-L), or chymotrypsin-like (CT-L) proteolysis, respectively.^{6, 7} Inhibition of the 26S proteasome activity leads to disruption of the cell cycle and induction of apoptosis.⁸Cancer cells have an increased dependency on the integrity of the ubiquitin–proteasome system machinery compared with normal cells in preclinical studies. This finding is predominantly evident in hematological malignancies, identifying the 26S proteasome as a promising anticancer therapeutic target.^{9, 10, 11, 12} In particular, cells derived from multiple myeloma are notably sensitive to proteasome inhibition, at least in part, owing to their characteristically high rates of immunoglobulin protein biosynthesis and increased proteasome activity.^{13, 14, 15} The continuous activity of the proteasome in myeloma cells makes them particularly susceptible to prolonged inhibition.¹⁶Bortezomib, the first proteasome inhibitor approved for clinical use, is a dipeptide boronic acid that reversibly binds to the active site of the β5 and β1 subunit to competitively inhibit proteasome function.^{9, 10, 17} By inhibiting the proteasome, bortezomib acts through multiple cellular pathways that ultimately result in cell cycle arrest and apoptosis.¹⁸ Bortezomib is currently approved for the treatment of newly diagnosed, relapsed or refractory multiple myeloma and mantle cell lymphoma.¹⁸ Carfilzomib was subsequently developed as a second-generation inhibitor that belongs to the epoxyketone class and irreversibly binds to the active site of the β5 subunit of the proteasome. Carfilzomib is structurally and mechanistically distinct from bortezomib and overcomes bortezomib resistance in multiple myeloma cell lines and in primary multiple myeloma cells from patients.^{17, 19} Carfilzomib is currently also approved for relapsed and refractory multiple myeloma. ONX-0912 (also known as oprozomib) is another epoxyketone class oral proteasome inhibitor that is an analog of carfilzomib.^{20, 21} Similar to carfilzomib, ONX-0912 promotes cell death in primary myeloma cells from patients who relapsed after treatment with bortezomib.²⁰ ONX-0912 has advanced into phase I/II trials in hematological and solid malignancies.^{22, 23, 24}Despite their clinical efficacy, treatment with proteasome inhibitors is associated with a number of toxicities, including neuropathy, thrombocytopenia, and cardiotoxicity.^{25, 26, 27} The toxicity of currently available proteasome inhibitors necessitates administering the drugs in intermittent dosing schedules, typically biweekly. Although intermittent dosing permits proteasome activity in normal tissues during dose holidays, it has been shown to be sub-optimal for therapy in malignant cells.¹⁶ Moreover, infrequent administration at relatively high exposures may give rise to undesirable and potentially unnecessary toxicities in normal cells. Potentially, by moderating exposures, an optimized oral proteasome inhibitor with continuous daily dosing could be developed that exploits the high proteasome dependency in malignant cells while sparing normal cells.In the present study, we report the development of FV-162, a novel, metabolically stable and orally bioavailable epoxyketone-based proteasome inhibitor. FV-162 displays potent anticancer activity and maintains a wide differential activity between malignant and normal cells despite a continuous daily dosing schedule in multiple myeloma cell lines, primary patient cells, and animal models. Overall, our results show that FV-162 inhibits the proteasome, displays metabolic stability, and has a favorable toxicity profile.