Histone deacetylase (HDAC) inhibitors have emerged as a class of novel and promising anti-cancer agents and their activities are currently being investigated in both the pre-clinical and clinical settings. Using an unbiased, ultrahigh throughput screening system a novel mercaptoketone-based HDAC inhibitor KD5170 was identified. This novel non-hydroxamic acid non-benzamide compound was profiled for its anti-myeloma activity in vitro and in vivo. KD5170 inhibited the proliferation of multiple myeloma (MM) cell lines and the viability of CD138+ primary MM cells by induction of apoptosis. This was accompanied by an increase of acetylation of histones and activation of caspase-3, -8 and -9. Treatment with KD5170 caused a loss of mitochondrial membrane potential measured by JC-1 staining and resulted in release of apoptogenic factors such as cytochrome c, smac, and AIF from mitochondria. Furthermore, KD5170 induced oxidative stress and oxidative DNA damage in myeloma cells, as evidenced by the upregulation of heme oxygenase-1 and H2A.X phosphorylation, respectively. Combination of KD5170 with the proteasome inhibitor bortezomib or TNF-related apoptosis-inducing ligand synergistically enhanced the anti-myeloma activity. Resistance of MM cells to KD5170 was associated with activation of the ERK/MAPK pathway under treatment with KD5170. Pretreatment with the MAPK-inhibitor restored the sensitivity to KD5170, suggesting that the combination of KD5170 with a MAPK inhibitor (U0126) could overcome drug resistance. Growth of myeloma tumor xenografts in KD5170-treated nude mice was significantly inhibited (1401 mm3 ± 596 versus 442 mm3 ± 221; P = .009) and survival was prolonged (9 days and 17.5 days; P= .0095) compared to vehicle-treated mice. Accordingly, our in vivo data showed that histone acetylation was markedly upregulated in tela in spleen or tumor tissues of the animals treated with KD5170 as early as 2 hours. In conclusion, our data show that KD5170 has anti-myeloma activity in vitro and in vivo, mediated by the induction of apoptosis due to DNA damage and mitochondrial signaling.

Author notes

Disclosure:Employment: Christian A. Hassig, Joseph E. Payne, Nicholas D. Smith, and Jeffrey H. Hager are employees of Kalypsys, Inc., developers of KD7150. Research Funding: Funding to carry out this research was provided by Kalypsys, Inc.