APTO-253 Induces KLF4 to Promote Potent in Vitro Pro-Apoptotic Activity in Hematologic Cancer Cell Lines and Antitumor Efficacy As a Single Agent and in Combination with Azacitidine in Animal Models of Acute Myelogenous Leukemia (AML)

APTO-253, a small molecule that mediates anticancer activity through induction of the Krüppel-like factor 4 (KLF4) tumor suppressor, is being developed clinically for the treatment of acute myelogenous leukemia (AML) and high risk myelodysplastic syndromes (MDS). APTO-253 was well tolerated in a Phase I study in patients with solid tumors using a dosing schedule of days 1, 2, 15, 16 of a 28 day cycle (2T-12B-2T-12B), but recent scientific observations guided APTO-253 toward AML and high risk MDS. Indeed, suppression of KLF4 was reported as a key driver in the leukemogenesis of AML and subsets of other hematologic diseases. The vast majority (~90%) of patients with AML aberrantly express the transcription factor CDX2 in human bone marrow stem and progenitor cells (HSPC) (Scholl et al., J Clin Invest. 2007, 117(4):1037-48). The CDX2 protein binds to CDX2 consensus sequences within the KLF4 promoter, thereby suppressing KLF4 expression in HSPC (Faber et al., J Clin Invest. 2013, 123(1):299-314). Based on these observations, the anticancer activity of APTO-253 was examined in AML and other hematological cancers. APTO-253 showed potent antiproliferative activity in vitro against a panel of blood cancer cell lines, with ηM IC₅₀values in AML (6.9 - 305 ηM), acute lymphoblastic leukemia and chronic myeloid leukemia (39 – 250 ηM), non-Hodgkin’s lymphoma (11 – 190 ηM) and multiple myeloma (72 – 180 ηM). To explore in vivo efficacy, dose scheduling studies were initially conducted in the H226 xenograft model in mice. In the H226 model, APTO-253 showed improved antitumor activity when administered for two consecutive days followed by a five day break from dosing (2T-5B) each week, i.e. on days 1,2, 8,9, 15,16, 22,23, compared to the 2T-12B-2T-12B schedule. The 2T-5B schedule was used to evaluate antitumor activity of APTO-253 in several AML xenograft models in mice. In Kasumi-1 AML and KG-1 AML xenograft models, APTO-253 showed significant antitumor activity (p = 0.028 and p=0.0004, respectively) as a single agent when administered using the 2T-5B schedule each week for four weeks compared to control animals. Mice treated with APTO-253 had no overt toxicity based on clinical observations and body weight measurements. Mice bearing HL-60 AML xenograft tumors were treated with APTO-253 for one day or two consecutive days per week for three weeks, either as a single agent or combined with azacitidine, or with azacitidine alone twice per week (on days 1,4, 8, 11, 15 and 18). APTO-253 as a single agent inhibited growth of HL-60 tumors to approximately the same extent as azacitidine. Furthermore, both once weekly and twice weekly dosing of APTO-253 in combination with azacitidine resulted in significantly enhanced antitumor activity relative to either single agent alone (p = 0.0002 and p = 0.0006 for 1X and 2X weekly APTO-253 treatment, respectively, compared to control). Likewise, using a THP-1 AML xenograft model, APTO-253 administered as a single agent using the 2T-5B per week schedule showed significant efficacy, similar to that of azacitidine, while the combination of APTO-253 and azacitidine demonstrated greatly improved antitumor effects relative to either drug alone. APTO-253 was effective and well tolerated as a single agent or in combination with azacitidine in multiple AML xenograft models, plus APTO-253 does not cause bone marrow suppression in animal models or humans. Taken together, our results indicate that APTO-253 may serve as a targeted agent for single agent use and may provide enhanced efficacy to standard of care chemotherapeutics for AML and other hematological malignancies.