Venetoclax, bortezomib and S63845, an MCL1 inhibitor, in multiple myeloma

Venetoclax, an orally available BCL2-selective inhibitor, has demonstrated promising single-agent anti-tumour activity in myeloma especially patients with t(11;14). Herein, whether venetoclax sensitivity could be enhanced or restored in combination with bortezomib or S63845, a novel MCL1-selective inhibitor, was examined in human myeloma cell lines (HMCLs), including bortezomib-resistant HMCLs.

By MTS assay, half-maximal inhibitory concentration (IC₅₀ ) and hence sensitivity/resistance to venetoclax, bortezomib and S63845 were determined.

Venetoclax (IC₅₀ ≥100 nm), bortezomib (IC₅₀ ≥50 nm) and S63845 (IC₅₀ ≥100 nm) resistance was observed in nine (75%), three (25%) and six (50%) HMCLs, respectively. Moreover, venetoclax sensitivity was independent of bortezomib (R²  = 0.1107) or S63845 (R²  = 0.0213) sensitivity. Venetoclax sensitivity correlated with high mRNA ratio of BCL2/MCL1 (P = 0.0091), BCL2/BCL2L1 (P = 0.0182) and low MCL1 expression (P = 0.0091). In HMCLs sensitive to both venetoclax and bortezomib/S63845, venetoclax combined with S63845 showed stronger synergistic effect than combined with bortezomib. Moreover, in venetoclax-resistant HMCLs, S63845, but not bortezomib, significantly restored venetoclax sensitivity. Conversely, bortezomib combined with S63845 did not result in augmented bortezomib sensitivity or abolishment of bortezomib resistance.

Regardless of t(11;14), combination of venetoclax with S63845 is a promising strategy in enhancing venetoclax sensitivity or overcoming venetoclax resistance in myeloma therapy, hence warrant future clinical studies.