H3B-5942 belongs to a class of ERα antagonists referred to as selective estrogen receptor covalent antagonists (SERCA).

Erα (WT):1 nM (Ki), Erα (Y537S):0.41 nM (Ki)

H3B-5942 dosed once (q.d.×1) orally at 30 to 300 mg/kg showed a dose-proportional increase in plasma and tumor exposure and a concomitant dose-proportional decrease in expression of the ERα target genes PGR and NPY1R in the ERαY537S/WT ST941 tumor model.?Single or repeat dosing of H3B-5942 at 200 mg/kg suppressed a large panel of direct ERα target genes, with q.d.×1 dosing maintaining target gene suppression for up to 72 hours after dose, and q.d.×3 (3 daily doses) dosing demonstrating greatest suppression in PGR and NPY1R

Single or repeat dosing of H3B-5942 at 200 mg/kg suppressed a large panel of direct ERα target genes, with q.d.×1 dosing maintaining target gene suppression for up to 72 hours after dose, and q.d.×3 (3 daily doses) dosing demonstrating greatest suppression in PGR and NPY1R .

ERαWT (297–554) and mutant (297–554) proteins were incubated in 50 mmol/L Tris pH 8.0, 150 mmol/L NaCl, 5% glycerol, and 1 mmol/L TCEP with a 2-fold excess of compound (2 μmol/L H3B-5942:1 μmol/L ERα protein solution) at 4°C overnight.?Mass analyses were carried out (ESI source, 4.0 kV ionization voltage, 250°C capillary temperature, 10 arb sheath gas, S-lens RF level 65) coupled with an Accela Open AS 1250.?Samples (10 μL) were desalted on a C4 column ( 2.1 × 150 mm, 2.6 μm) with a gradient from 5% to 95% B over 10 minutes.?Eluent A consisted of 0.1% formic acid in water, and eluent B consisted of 0.1% formic acid in acetonitrile.?The flow was set to 400 nL/minute.?All solvents were LC/MS grade .?The mass spectrometer was run in positive mode collecting full scan at R = 70,000 from m/z 500 to m/z 2,000.?Data were collected with the Xcalibur 3.1 software.

Animals were selected based on TV and randomized into treatment groups of 6 to 8 animals per group.?Single-agent or combination treatments were started on day 0 and continued for the duration of the study.?H3B-5942 was administered orally, tamoxifen was given Q2D, fulvestrant was given?, and palbociclib was administered orally ?Each treatment was administered based on BW (10 mL/kg).?H3B-5942 was formulated daily in 10% 2-Hydroxypropyl-β-CycloDextrin (HPβCD) in 5% dextrose, tamoxifen was formulated in 95% peanut oil/5% ethanol (EtOH), clinical-grade fulvestrant was administered, and palbociclib was formulated in 25 mmol/L sodium bicarbonate, 15 mmol/L lactic acid solution with 2% Cremophor EL.?The BW measurements were performed daily, and tumor measurements were recorded twice a week.