Pracinostat (SB939)

Pracinostat, also known as SB939, is a potent and orally available inhibitor of histone deacetylase (HDAC) with a relatively stronger selectivity (more than 1000-fold) for class I, class II and class IV HDACs rather than class III HDACs. Pracinostat potently suppresses proliferation in a wide range of cancer cell lines, including colon cancer, ovarian cancer, prostate carcinomas, acute myeloid leukaemia (AML) and B cell lymphoma. Recent study results have shown that SB939 induces the accumulation of acetylated histone H3 (AcH3) and acetylated α-tubulin and increases the expression of the cyclin dependent kinase inhibitor p21 in cancer cells.

Reference

Razak AR, Hotte SJ, Siu LL, Chen EX, Hirte HW, Powers J, Walsh W, Stayner LA, Laughlin A, Novotny-Diermayr V, Zhu J, Eisenhauer EA. Phase I clinical, pharmacokinetic and pharmacodynamic study of SB939, an oral histone deacetylase (HDAC) inhibitor, in patients with advanced solid tumours. Br J Cancer. 2011;104(5):756-762.

A phase I study of histone deacetylase inhibitor, pracinostat (SB939), in pediatric patients with refractory solid tumors: IND203 a trial of the NCIC IND program/C17 pediatric phase I consortium.[Pubmed:23893953]

Pediatr Blood Cancer. 2013 Nov;60(11):1868-74.

BACKGROUND: Pracinostat (SB939) is a potent oral inhibitor of class 1, 2, and 4 histone deacetylases (HDAC). The adult recommended phase II dose (RP2D) is 60 mg po three times per week (t.i.w.) for 3 weeks every 4 weeks. This study assessed the toxicities and pharmacokinetics of pracinostat and determined the RP2D in children with refractory solid tumors. METHODS: Pediatric patients with refractory solid tumors were treated with oral pracinostat t.i.w. for 3 consecutive weeks, followed by 1 week off dosing. Three dose levels-25, 35, and 45 mg/m(2) were evaluated using a standard 3 + 3 cohort design. Pharmacokinetic (PK) studies were optional. RESULTS: Twelve patients were enrolled. The most common diagnosis was Ewing sarcoma. Most adverse events (AEs) were hematological with five (40%) patients experiencing grade 3 neutropenia. Non-hematological AEs were generally grade 1. No dose limiting toxicities occurred. More hematological and non-hematological AEs occurred at 45 mg/m(2) : Two of five patients experienced Grade 3 neutropenia and one each Grade 3 thrombocytopenia and leucopenia, Grade 1 fatigue and anorexia occurred in three. The RP2D was declared to be 45 mg/m(2) (comparable to an adult dose of 80 mg). One patient had a best response of stable disease (duration of 2.9 months). Three patients on 25 mg/m(2) and one each on 35 and 45 mg/m(2) participated in the PK study. No dose related changes in Cmax or AUC occurred. CONCLUSIONS: Pracinostat is reasonably well tolerated in children with refractory solid tumors. The RP2D is 45 mg/m(2) .

Preclinical metabolism and disposition of SB939 (Pracinostat), an orally active histone deacetylase inhibitor, and prediction of human pharmacokinetics.[Pubmed:21873472]

Drug Metab Dispos. 2011 Dec;39(12):2219-32.

The preclinical absorption, distribution, metabolism, and excretion (ADME) properties of Pracinostat [(2E)-3-[2-butyl-1-[2-(diethylamino) ethyl]-1H-benzimidazol-5-yl]-N-hydroxyarylamide hydrochloride; SB939], an orally active histone deacetylase inhibitor, were characterized and its human pharmacokinetics (PK) was predicted using Simcyp and allometric scaling. SB939 showed high aqueous solubility with high Caco-2 permeability. Metabolic stability was relatively higher in dog and human liver microsomes than in mouse and rat. The major metabolites formed in human liver microsomes were also observed in preclinical species. Human cytochrome P450 (P450) phenotyping showed that SB939 was primarily metabolized by CYP3A4 and CYP1A2. SB939 did not significantly inhibit human CYP3A4, 1A2, 2D6, and 2C9 (>25 muM) but inhibited 2C19 (IC(50) = 5.8 muM). No significant induction of human CYP3A4 and 1A2 was observed in hepatocytes. Plasma protein binding in mouse, rat, dog, and human ranged between approximately 84 and 94%. The blood-to-plasma ratio was approximately 1.0 in human blood. SB939 showed high systemic clearance (relative to liver blood flow) of 9.2, 4.5, and 1.5 l . h(-1) . kg(-1) and high volume of distribution at steady state (>0.6 l/kg) of 3.5, 1.7, and 4.2 l/kg in mouse, rat, and dog, respectively. The oral bioavailability was 34, 65, and approximately 3% in mice, dogs, and rats, respectively. The predicted oral PK profile and parameters of SB939, using Simcyp and allometric scaling, were in good agreement with observed data in humans. Simcyp predictions showed lack of CYP3A4 and 2C19 drug-drug interaction potential for SB939. In summary, the preclinical ADME of SB939 supported its preclinical and clinical development as an oral drug candidate.

The oral HDAC inhibitor pracinostat (SB939) is efficacious and synergistic with the JAK2 inhibitor pacritinib (SB1518) in preclinical models of AML.[Pubmed:22829971]

Blood Cancer J. 2012 May;2(5):e69.

Acute myeloid leukemia (AML) is currently treated with aggressive chemotherapy that is not well tolerated in many elderly patients, hence the unmet medical need for effective therapies with less toxicity and better tolerability. Inhibitors of FMS-like tyrosine kinase 3 (FLT3), JAK2 and histone deacetylase inhibitors (HDACi) have been tested in clinical studies, but showed only moderate single-agent activity. High efficacy of the HDACi pracinostat treating AML and synergy with the JAK2/FLT3 inhibitor pacritinib is demonstrated. Both compounds inhibit JAK-signal transducer and activator of transcription (STAT) signaling in AML cells with JAK2(V617F) mutations, but also diminish FLT3 signaling, particularly in FLT3-ITD (internal tandem duplication) cell lines. In vitro, this combination led to decreased cell proliferation and increased apoptosis. The synergy translated in vivo in two different AML models, the SET-2 megakaryoblastic AML mouse model carrying a JAK2(V617F) mutation, and the MOLM-13 model of FLT3-ITD-driven AML. Pracinostat and pacritinib in combination showed synergy on tumor growth, reduction of metastases and synergistically decreased JAK2 or FLT signaling, depending on the cellular context. In addition, several plasma cytokines/growth factors/chemokines triggered by the tumor growth were normalized, providing a rationale for combination therapy with an HDACi and a JAK2/FLT3 inhibitor for the treatment of AML patients, particularly those with FLT3 or JAK2 mutations.