Early Effects of a Repurposed Therapeutic PB-04 in Beta Thalassemia Intermedia and in Transgenic Sickle Mice

Abstract

Background: Biochemical, epidemiologic, genetic, and clinical data over 7 decades have shown that renewed or persistent expression of fetal globin reduces clinical severity in beta-thalassemias (BTI) and hemoglobinopathies (sickle cell disease, SCD). Hydroxyurea (HU) and epigenetic therapies which act through HDAC and DNMT-1 inhibition have demonstrated beneficial clinical activity; butyrate, a Class I HDAC inhibitor which suppresses BCL11A, increased HbF by 3-fold and total hemoglobin (Hb) by 1 gm/dl in SCD and eliminated transfusions in beta thalassemia patients, and decitabine strongly induces HbF, increases total Hb, and reduces biomarkers of sickling. Active erythropoiesis, basal EPO levels, available iron, and select mutations or modifiers are associated with responses. Additional small molecule therapies, especially those with additive activity with HU, are considered desirable for many patients world-wide. Methods: Seeking oral noncytotoxic therapeutic candidates with low development risks, we utilized a high-throughput screen with a gamma globin promoter-reporter and cytotoxicity readout for unrecognized fetal globin-inducing activity in libraries of all FDA- and EMA-approved or clinical stage therapeutics (for any medical indication). PB-04, a candidate with a benign long-term safety profile (used to inhibit L-dopa metabolism in Parkinson's disease), does not inhibit proliferation of erythroid progenitors and suppresses or displaces 4 repressors of the fetal globin gene promoter, BCL11A, HDAC3, KLF1, LSD-1, indicating targeted molecular activity. PB-04 has additive effects with HU in inducing F-cells in sickle cell erythroid progenitors. PB-04 was evaluated in 3 predictive animal models and in 3 escalating dose cohorts in adult subjects with BTI. Results: In 3 animal models, PB-04 increased fetal globin mRNA by 12-to 20-fold and F-cells by 20% in anemic baboons, F-cells and mean Hb F/cell by 2 to 7-fold in beta-YAC mice, and fetal globin mRNA by 1.8-fold and % F-cells by 2.4-fold without toxicity in Townes SCD mice, similar to the changes induced by HU (both given 5 days/wk for 4 wks). In six beta thalassemia (BTI) subjects who completed ten 12-week courses at 1, 3, or 5 mg/kg doses given once/day, 3 times/wk, increases from baseline in F-reticulocytes (mean 5.2-fold, range 1.5-15.7), F-cells, (mean 4.4-fold, range 2.3 to 7.3-fold), and HbF/cell (MFI), (mean 6.6-fold, range 3 to 11.3-fold), were observed (p<0.03 for all parameters). Increases were observed even in subjects with low baseline HbF levels. HbF increases by HPLC were 2.7 to 14%. Treatment emergent adverse effects (TEAEs) have been Gr 1-2 and largely transient in these subjects receiving 12 weeks' therapy. Other dosing regimens are being investigated. Conclusions: Initial studies in beta globin disorders indicate PB-04 induces fetal globin protein expression assayed by F-reticulocytes, F-cells, and mean HbF/cell in BTI patients and in transgenic SCD mice without toxicity. These findings indicate PB-04, which has complementary activity to all the approved SCD therapeutics, merits clinical evaluation in patients with sickle cell disease.