Phase 1 Study of Bisthianostat, an Orally Efficacious Pan-HDAC Inhibitor: Part Results of Safety, Pharmacokinetics and Efficacy in Patients with Relapsed or Refractory Multiple Myeloma

Background: Multiple myeloma (MM) is the second most common hematological malignancy. This disease remains incurable as nearly all patients will relapse and become refractory to established MM therapy. Thus, new treatment option for relapsed or refractory (R/R) MM is needed, particularly those with different mechanisms of action. One such approach is to inhibit histone deacetylase (HDAC) and produce synergistic anti-myeloma activity via mechanisms of epigenetic modulations. In 2015, panobinostat was approved by US FDA as the first HDACi to treat R/R MM in combination with bortezomib and dexamethasone.

Bisthianostat is a novel bisthiazole-based HDACi evolved from the thiazole-thiazoline cap group in natural product Largazole (Nan et al., ACS Med Chem Lett. 2014). It is orally available and displayed inhibition against a series of MM cell lines. Here we presented preliminary in-human findings from CH-020PI study, an ongoing phase 1 study of bisthianostat. (Trial registered at ClinicalTrial.gov: NCT03618602)

Methods: CH-020PI is a first-in-human study to investigate the safety, tolerability, pharmacokinetics, and efficacy of bisthianostat in R/R MM patients. It is a single center, open-label, single arm, dose escalating phase I study. A standard 3+3 cohort design with 100mg as the starting dose was used to determine the maximum tolerated dose of bisthianostat. This study comprised two phases: a pharmacokinetics phase and an expansion phase. In the pharmacokinetics phase, a single-dose of bisthianostat was administered on day 1, and then multiple-dose was administered on a twice-weekly schedule for 4 consecutive weeks. Patients in the expansion phase received continuous bisthianostat twice weekly until progressive disease or unacceptable toxicities.

Results: Until 30 June 2019, 8 patients were enrolled at 3 dose levels from 100 to 400mg. The median age at enrollment was 62 years (range, 51-70 years). The median number of previous lines of therapy was 5 (range, 2-6). Per protocol, all of 8 patients were evaluable for pharmacokinetics, toxicities and efficacy. In the pharmacokinetic evaluation, for all the 8 patients tested at day 1, the peak concentration of bisthianostat was reached within 2.3 hours; half life time were around 4 hours; bisthianostat uptake represented by AUC_last were in good proportion to the level of dose as 100, 200 and 400mg, respectively. Similar results were observed at day 28. Any grade hematological treatment-related adverse events (AEs) occurred in 4 of 8 patients (50%), while grade 3/4 hematological AEs occurred in 2 (25%) patients. Any grade non-hematological treatment-emergent AEs were observed in 3 (37.5%) patients; no grade 3/4 non-hematological AEs were reported. No patient discontinued the treatment of bisthianostat due to AEs. Except patient 007 (200mg cohort) experienced a grade 2 nausea, no patients experienced diarrhea, nausea, or vomiting. It is worthy to note that gastrointestinal toxicity is common with the use of panobinostat, a FDA-approved HDAC inhibitor. Overall single-agent efficacy was modest, and stable disease (SD) was observed in 4 (50%) patients. At the time of data cut-off for statistical analysis, no dose-limiting toxicity has been observed.

Conclusions: Bisthianostat proved to be well absorbed and tolerated. It exhibited modest anti-tumor efficacy in our cohort of heavily pretreated patients with R/R MM. This phase I clinical trial is currently ongoing, and future trials should compare different doses and schedules of the combination in order to optimize the treatment tolerability and enhance its efficacy.