Phase I Study of the Oral Histone Deacetylase Inhibitor SB939 In Patients with Advanced Hematologic Malignancies

SB939 is a novel orally bioavailable inhibitor of class 1, 2 and 4 histone deacetylases. In human tumor cell lines SB939 inhibits proliferation and promotes apoptosis at an IC₅₀ of 0.1 – 1.3mM. Antitumor activity has been demonstrated in xenograft models of AML (MV4-11) and B-cell lymphoma (Ramos), as well as solid tumors. A phase I, open label, dose escalation study in patients with advanced hematologic malignancies was conducted to assess the safety, maximum tolerated dose (MTD), pharmacokinetics, pharmacodynamics and preliminary efficacy of SB939.

SB939 was administered orally every other day 3 times a week for 3 consecutive weeks, in a 4-week cycle. Cohorts of patient were treated with escalating doses of SB939 starting from 10 mg. The MTD was defined as the lowest dose level with less than 2 DLTs. The recommended Phase 2 dose level was defined as one dose level below the MTD. PK and PD (Acetylated Histone 3 in PBMCs) samples were collected in the first cycle.

A total of 44 patients were enrolled. 23 patients during dose escalation at dose levels of 10 mg (n=1), 20 mg (n=1), 40 mg (n=6), 60 mg (n=3), 80 mg (n=3), 100 mg (n=3) and 120 mg (n=6). An additional 21 patients were enrolled as part of a cohort expansion at 100 mg. The median age was 70 yr (range 37–84 yr), 57% were male, 61% were caucasian and 27% asian. Median number of prior therapies was 2 (range 0–9), 16 % had a prior transplant. 89% had ECOG performance score of 0–1. The median number of doses received was 17. DLTs included prolonged QTc at 40 mg and neutropenic sepsis at 120 mg. The MTD as defined was not reached; 120 mg was declared as MTD due to the requirement for dose reduction after multiple cycles of treatment. 100 mg was determined to be the recommended Phase II dose. 24 patients, MDS (n=11), AML (n=12), and lymphoma (n=1) were treated at the 100 mg dose level. SB939 was generally well tolerated. Grade 1–2 events included nausea (45%), fatigue (44%), diarrhea (36%), anorexia (34%) and vomiting (30%). Grade 3–4 adverse events included thrombocytopenia (39%), anemia (23%), pneumonia (23%), febrile neutropenia (20%), fatigue (16%), hypokalemia (11%), and neutropenic sepsis (11%). Samples for pharmacokinetics were drawn prior to dosing and 0.5, 1, 1.5, 2, 3, 4, 6, 8, 24 ± 2 and 30 ± 2 hours after dosing on days 1 and 15 of Cycle 1. Levels of SB939 in plasma were determined using a validated LC-MS/MS method and Non-Compartmental Analysis used WinNonlin, version 5.2 (Pharsight). SB939 was rapidly absorbed with mean T_max ranging between 0.5–1.3 h; the mean elimination half-life ranged between 6–17 hrs. The C_max and AUC _(0-∞) increased dose-proportionally in the range of doses tested. There was no accumulation of SB939 on day 15 following repeated dosing. Concentrations above IC₅₀ of SB939 for HDAC 1, 2, and 4 were reached at all doses and increased acetylation of H3 was observed in PBMCs across all dose levels. 1 PR (80 mg) and 1 CR (120 mg) were observed in 2 patients with AML with durations of 362 and 206 days respectively. Stable disease for more than 2 cycles was seen in 7 patients, 3 with IPSS intermediate or high risk MDS (duration 72–134 d) and 4 with AML (duration 56–354 d).

SB939 demonstrated excellent PK properties and target inhibition and was generally very well tolerated. Toxicities were mild to moderate and similar to some but not all toxicities seen with other HDAC inhibitors. The MTD as defined for this regimen of SB939 in patients with hematologic malignancies was not reached and 100mg is the recommended dose, indicating a favorable therapeutic index. Response data particularly in higher risk MDS and AML encourage further exploration of the therapeutic benefit of SB939 in combination with other anti-cancer therapies.

Ethirajulu:S*BIO: Employment. Zhu:S*BIO: Employment.