Design and discovery of novel dipeptide boronic acid ester proteasome inhibitors, an oral slowly-released prodrug for the treatment of multiple myeloma

Multiple myeloma (MM), the second most common hematological malignancy, is a disease characterized by a clonal expansion of malignant plasma cells that accumulate in the bone marrow. Ixazomib citrate was the first commercially available oral proteasome inhibitor for the treatment of MM. However, it immediately hydrolyzed into the active form on exposure to aqueous solution and so it was a pseudo prodrug. Herein, a series of dipeptide boronic acid esters as novel oral proteasome inhibitors were designed, synthesized and biologically investigated for the inhibition of the β5 subunit of 20S proteasome. Based on the enzymatic results, structure-activity relationships (SAR) were discussed in detail. Some potent compounds were further evaluated to inhibit the proliferation of MM cell line RPMI-8226. The results showed that some compounds were active against RPMI-8226 with IC50 values of less than 10 nM. The solution stability showed that ixazomib citrate was completely hydrolyzed to its active form ixazomib within 2 min in the simulated gastric juice. However, among the screened compounds, prodrug 18u was stable enough in simulated gastric juice and simulated intestinal juice, and its hydrolysis rate was 59.7% and 3.6% after 2 h, respectively. In addition, 18u exhibited good microsome stabilities and pharmacokinetic properties and displayed strong antiproliferative activity against the RPMI-8226 cell line (5.6 nM). Furthermore, compound 18u exhibited strong in vivo anticancer efficacy in human MM (RPMI-8226) xenograft mouse model.