The Novel HDAC Inhibitor UCL67022 Is Highly Potent in Multiple Myeloma and Non-Hodgkin’s Lymphoma and Is Enhanced by Bortezomib.

Introduction: Histone deacetylases (HDACs) regulate the acetylation state of nucleosomal histones and represent a novel target in haematological malignancies. Inhibition of HDACs results in the induction of apoptosis in multiple myeloma (MM) and lymphoma (NHL) cells, associated with the down-regulation of signaling pathways involving IL-6 and IGF-1 and the inactivation of BCL-6. We have therefore investigated the activity of a novel hydroxamic acid HDAC inhibitor (UCL67022) in comparsion with SAHA, alone and in combination with the proteasome inhibitor bortezomib.

Methods: HDAC activity in partially purified rat liver homogenate and intact CEM cells was determined using a fluorescent HDAC substrate. The human MM cell lines RPMI 8226/S and U266, and the NHL cell lines SUD-4, CRL, DOHH2, DHL-4, 5, 6, 7, GRANTA-519 and JEKO-1 were used to investigate effects on viable cell number using an ATP-dependent bioluminescence method. Activity against primary malignant cells from patients with MM, DLBCL, FL or CLL was also studied. Western blot analysis was used to investigate changes in acetylated histone-H3 and a-tubulin.

Results:UCL67022 showed more potent HDAC inhibitory activity in liver extracts and whole CEM cells than SAHA (IC₅₀ 0.05 vs 0.39uM in liver and 0.11 vs 0.33uM in CEM cells). UCL67022 was more potent than SAHA in reducing viable cell number in U266 (EC₅₀ 0.14 vs 1.8 uM) and RPMI 8226/S (0.05 vs 0.78 uM) cells. Similarly NHL cell lines were 10–20 fold more sensitive to UCL67022, with median EC₅₀ values of 0.05uM (range 0.03–0.10uM) vs 0.81uM, (range 0.68–1.28uM). In 2 primary MM samples UCL67022 showed increased activity with EC₅₀ values of 0.7uM and 0.11uM vs 12.2uM and 1.1uM for SAHA. 3 FL patient samples and 1 CLL sample were 3.7-fold more sensitive to UCL67022 than SAHA (median EC₅₀ 4.7uM vs 17.5uM respectively). Western blot analysis showed a 10-fold difference in histone H3 acetylation between the two compounds, with acetylation returning to pre-treatment levels by 24hr with 3uM SAHA but remaining elevated out to 48hr with 0.3uM UCL67022. Increased acetylation of a-tubulin confirmed the inhibition of HDAC6. Combination with bortezomib at 1, 2 and 4nM in MM cells showed increased antiproliferative activity with both SAHA and UCL67022, with synergistic responses observed in the U226 cell line and in 1 primary sample and additive effects in the others.

Conclusions: These data demonstrate the activity of the highly potent, novel HDAC inhibitor UCL67022 in MM and NHL cell lines and primary patient cells. Activity was increased in co-exposures with bortezomib possibly due to inhibition of HDAC6 and subsequent aggresome formation, suggesting a therapeutic advantage with the combination.