Potent Cytotoxicity Against Multiple Myeloma and Acute Myeloid Leukemia Cells by a New Histone Deacetylase Inhibitor ITF2357.

ITF2357 (Italfarmaco, Italia) is a novel hydroxamic acid-based HDAC inhibitor (HDACi) that has shown reduced toxicity in phase I studies. We have investigated the cytotoxic and anti-proliferative activities of ITF2357 in multiple myeloma (MM) and acute myelogenous leukaemia cells (AML). ITF2357 had a strong cytotoxic activity in 8/9 MM and 6/6 AML cell lines, with a mean IC₅₀ of about 0.2 μM, a concentration largely attained following oral administration of safe doses of ITF2357 to healthy individuals. In contrast SAHA, the prototypic hydroxamic HDACi, showed an IC₅₀ of about 1 μM or above in all cases. The cytotoxic activity of ITF2357 was due to induction of apoptosis, as documented by detection of annexin V and cleaved caspase 3. The ITF2357 induced hyperacetylation of histones in cell lines resistant or sensitive to the cytoytoxic activity of the drug is under investigation in order to further define its mechanism of action. ITF2357 had also more potent cytotoxic activity compared to SAHA against freshly isolated CD138⁺ purified MM cells and AML samples, with an IC₅₀ of about 0.1 μM in 3/3 MM and 13/15 AML cases. Sensitive AML cases included five cases of FAB M1, five M2 and three M4. Three of the sensitive cases carried a t(8;21) translocation, 2 an inv(16), 6 had a normal and 2 a complex karyotype. Furthermore four of the sensitive AML also carried a flt3 internal tandem duplication and 3 a type A mutation in the nucleophosmin 1 gene (NPM1). The 2 more resistant AML cases (one M1 and one M5, both with normal karyotype) showed nonetheless a response to the drug with an IC₅₀ of about 0.5 μM. We have also developed a culture system to grow freshly isolated AML cells for at least 3 weeks in vitro on human mesenchymal cells (MSC). Interestingly, ITF2357 was cytotoxic for primary AML cells stimulated to grow in optimal conditions on MSCs, at the same dose as in standard short term cultures. In contrast ITF2357 was not cytotoxic for MSCs even at the 1 μM concentration. The strong cytotoxic activity of ITF2357 on MM and AML has provided the framework for ongoing phase I studies of ITF2357 in these malignancies.