Pracinostat is an orally available histone deacetylase (HDAC) inhibitor that is being developed for advanced hematologic diseases such as myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). HDACs belong to a larger set of proteins collectively known as epigenetic regulators that can alter gene expression by chemically modifying DNA or its associated chromosomal proteins. Abnormal activity of these regulators is believed to play an important role in cancer and other diseases.
Pracinostat has been tested in multiple Phase I and Phase II clinical trials in hematologic and solid tumor indications. Pracinostat has been generally well tolerated in more than 300 adult and pediatric patients, with manageable side effects often associated with drugs of this class, such as fatigue. The results of these studies suggest that Pracinostat has potential best-in-class pharmacokinetic properties when compared to the two approved oral HDAC inhibitors.
Pracinostat has demonstrated clinical evidence of single-agent activity in patients with AML and myelofibrosis. In a Phase I dose-escalation study, two out of 14 evaluable patients with AML (14%) achieved a complete remission (CR), with the responses enduring for more than 206 and 362 days, respectively. These results were presented at the American Society of Hematology (ASH) Annual Meeting in December 2010. In addition, a Phase II clinical trial of Pracinostat showed that eight out of 22 patients with intermediate or high-risk myelofibrosis (36%) demonstrated a clinical response, including two patients showing a clinical improvement (anemia response) and six patients experiencing some reduction in splenomegaly. These results were published in the September 2012 issue of Leukemia Research.
Pracinostat has also shown evidence of synergistic activity when used in combination with the hypomethylating agent azacitidine (marketed as Vidaza®). In December 2012, data from a pilot study of Pracinostat in combination with azacitidine in patients with advanced MDS were presented at the ASH Annual Meeting. Data from nine patients treated at the MD Anderson Cancer Center showed an overall response rate of 89%. Combined with the results from an additional patient treated at the University of Wisconsin-Madison, a CR, the trial showed an overall response rate of 90% (nine out of 10). The combination of Pracinostat and Vidaza was well tolerated in the study; the most frequent side effects were nausea and fatigue.
In August 2014, we completed enrollment in a randomized Phase II clinical study of Pracinostat in combination with azacitidine in intermediate-2 and high-risk patients with previously untreated MDS. The double-blind, placebo-controlled study enrolled 102 evaluable patients, randomized one-to-one, at 19 sites in the U.S. The primary endpoint of the study is CR. Secondary endpoints include overall response rate, hematologic improvement, duration of response, progression-free survival, rate of leukemic transformation, overall survival and safety. We announced top-line data from this study in March 2015.
We reached a clinical response milestone in an open-label Phase II study of Pracinostat in hypomethylating agent (HMA)-refractory MDS in December 2014. Of the first 28 patients who received Pracinostat in combination with azacitidine or decitibine (marketed as Dacogen®) after progressing while being treated with the same HMA alone, three have now achieved clinical responses -- one partial response (PR) and two marrow complete responses (mCR) -- exceeding the pre-specified clinical improvement rate for expansion of study enrollment. We have now completed enrollment in this study and will continue to follow patients for response and survival. The combination of Pracinostat and azacitidine or decitabine has been generally well-tolerated in the study, with no unexpected toxicities. The most common treatment-emergent adverse events include anemia, fatigue and gastrointestinal disorders.
In February 2014, the U.S. Food & Drug Administration (FDA) granted orphan drug designation to Pracinostat for the treatment of AML. The designation provides orphan status to drugs defined by the FDA as those intended for the safe and effective treatment, diagnosis or prevention of rare diseases that affect fewer than 200,000 people in the U.S. Orphan designation qualifies us for certain development incentives, including tax credits for qualified clinical testing, prescription drug user fee exemptions and seven-year marketing exclusivity upon FDA approval. We also intend to seek orphan drug designation for Pracinostat in combination with azacitidine for the treatment of AML.
In November 2014, we completed enrollment in an open-label Phase II study of Pracinostat in combination with azacitidine in elderly patients with newly diagnosed AML. The study enrolled a total of 50 patients at 15 clinical sites in the U.S. Updated results from the study were presented at the European Hematology Association (EHA) Annual Congress in June 2015. To date, 54% of patients (27 out of 50) have achieved the primary endpoint of the study, including 42% (21 out of 50) who achieved a CR. Most responses occur within the first two cycles and continue to improve with ongoing therapy. Median overall survival has not yet been reached in the study, with 32 patients (64%) still living (range, 8-17 months). The 60-day mortality rate, often used as a benchmark in AML clinical studies, was 10% (5 out of 50). The combination of Pracinostat and azacitidine has been well-tolerated in the study. The most common treatment-emergent adverse events include febrile neutropenia, thrombocytopenia, nausea and fatigue.
While these data are encouraging, longer follow-up is necessary to get an accurate overall survival estimate of the combination. We believe this survival estimate will be critical in determining the development path forward for Pracinostat.