Abstract 1319: A novel formulation of CX-5461, a small-molecule inhibitor of rRNA synthesis, and its use for treatment of acute myeloid leukemia models

Abstract

Abstract CX-5461 is a RNA polymerase I inhibitor currently in Phase I clinical trial in Australia for patients with advanced hematologic malignancies. In the pre-clinical setting, CX-5461 is efficacious in a wide range of hematologic and solid tumor models when given orally or intraperitoneally. Currently, the compound is given intravenously (iv) in the first-in-human clinical trial. CX-5461 is solubilized in low pH (3.5) 50 mM sodium phosphate as it is sparingly soluble in water (<1 mg/mL) and at physiological pH. The use of such low pH phosphate buffers affects infusion tolerance. While CX-5461 has nanomolar range activity in vitro, in vivo studies are conducted with either high dosages or frequent dosing. The chemical structure of CX-5461 also suggests potential drug instability under physiological conditions. We have addressed these solubility and stability issues by developing a lipid-based nanoparticulate (LNP) formulation which relies on a previously unrecognized interaction between CX-5461 and copper. Copper (Cu) coordination with CX-5461 was demonstrated using 1H NMR and UV-Vis. Cu-LNPs were prepared with 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and cholesterol (CHOL) (55:45 molar ratio) using extrusion methods. CX-5461 was loaded into Cu-LNPs at 60°C for 30 minutes and the external buffer was exchanged to Hepes Buffered Saline (pH 7.4). Pharmacokinetic (PK) studies were performed in CD-1 mice given a dose of 30 mg/kg (iv) and plasma concentrations of CX-5461 were determined by HPLC. Efficacy studies were performed in RAG2M mice with established subcutaneous (sc) MV-4-11 tumors and in a MV-4-11 bone marrow engraftment model (30 mg/kg given iv, Q4Dx3). Our 1H NMR data suggests that CX-5461 binds to copper at a 1:1 molar ratio. Copper complexation does not affect the anticancer activity of CX-5461 as determined in several cancer cell lines in vitro. Using copper-complexation as a driving force, CX-5461 was efficiently loaded into LNPs (∼100 nm) prepared to contain copper (300 mM CuSO4). Resulting LNPs have a drug-to-lipid-ratio of 0.2. The LNP formulation has significantly improved the PK profile of CX-5461 and increased the total exposure to drug by an order of magnitude (AUC = 4156 μg-hr/mL for LNP and 319 μg-hr/mL for free CX-5461). The LNP formulation of CX-5461 is more active than the current low-pH formulation of CX-5461 when given iv in leukemia xenograft models. Specifically 37 days after sc tumor cell inoculation the tumor sizes were 720 mm3 and 240 mm3 in control and LNP-CX5461-treated animals respectively. The clinical formulation administered at the same dose exhibited tumor volumes that were comparable to the untreated controls. We have generated the first LNP formulation of CX-5461 using the copper complexation technology developed in our laboratory. LNP-CX-5461 is suitable for iv administration and is more efficacious than the current formulation in our pre-clinical studies. Citation Format: Ada W.Y. Leung, Malathi Anantha, Kathleen E. Prosser, Mohamed Wehbe, Charles J. Walsby, Marcel B. Bally. A novel formulation of CX-5461, a small-molecule inhibitor of rRNA synthesis, and its use for treatment of acute myeloid leukemia models. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1319.