Clinical Pharmacokinetics of Apto-253 Support Its Use As a Novel Agent for the Treatment of Relapsed or Refractory Hematologic Malignancies
Authors: Rice William G. , Lightfoot Jeff , Zhang Hongying , Cheng Tiffany , Vellanki Avanish , Peralta Robert , Howell Stephen B , Local Andrea , Chau Fannie , Esquivies Luis 
Published: November 2019

Abstract
APTO-253 is a novel anticancer small molecule currently in a multicenter open-label, Phase I dose escalation study in patients with relapsed or refractory hematologic malignancies. APTO-253 has potent cytotoxic activity against leukemia, lymphoma and myeloma cell lines IC50s of 6.9 - 305 nM. APTO-253 produces significant tumor growth inhibition in the KG-1, THP-1 and Kasumi-1 xenograft models of human acute myeloid leukemia (AML). The anticancer activity of APTO-253 is mediated through induction of Krüppel-like factor 4 (KLF4), a tumor suppressor that is epigenetically silenced in many solid tumors and hematologic cancers. KLF4 expression is often downregulated in AML due to repressive binding of CDX2 to the KLF4 promoter. Increased levels of CDX2 are found in the majority of patients with AML and ALL, as well as in 40% of MDS patients, whereas CDX2 is not expressed in normal hematopoietic cells. Treatment of cultured AML cells with APTO-253 reverses the KLF4 silencing, resulting in induction of p21 and cell death by apoptosis. KG-1 AML cells treated with APTO-253 showed time- and concentration-dependent induction of KLF4 and a concentration-dependent increase in p21 mRNA levels following induction of KLF4. APTO-253 treatment of KG-1 cells for 24 h induced a 14-fold increase in KLF4 mRNA and 16-fold increase in p21 mRNA over basal levels. Following washout of APTO-253, the level of KLF4 mRNA decayed to approximately 50% of maximum induction level over a 24 h period.
The potential for APTO-253 as a therapeutic option in AML was further supported by safety and pharmacokinetics data from an earlier Phase I trial of APTO-253 in patients with advanced or metastatic solid tumors during which APTO-253 was administered at doses of 20 - 387 mg/m2 using a dosing schedule of days 1 and 2, and 15 and 16 of a 28 day cycle. APTO-253 showed a dose-dependent increase in Cmax and AUC, and as the dose was escalated from 80 to 176 mg/m2 the Cmax ranged from 1,800 - 4,960 nM on day 1 and 1,600 - 6,100 nM on day 2. These results suggest that exposure levels from these doses of APTO-253 should be sufficient for single agent activity in patients with AML and other hematologic malignancies.
APTO-253 demonstrated a favorable safety profile when tested against 5 major cytochrome P450 enzymes (1A2, 2C19, 2C9, 2D6 and 3A4), against a panel of proteins and receptors, and in the hERG tail current density assay. Metabolic profiling of APTO-253 at 50 μM in human liver microsomes showed no glutathione or glucuronide conjugation and only a minor hydroxylated metabolite. Finally, 1 μM APTO-253 did not inhibit kinases in a safety panel (40 kinases) or in a broad oncology panel (98 kinases), demonstrating that APTO-253 activity is not driven by kinase inhibition.
Taken together, our results demonstrate that APTO-253 has substantial potential for the treatment of AML and other hematologic malignancies and is of particular interest due to its ability to modulate the expression of the KLF4 master transcription factor that plays a central role in restraining the growth of leukemic cells.

Disclosures
Howell: Aptose Biosciences: Consultancy, Equity Ownership; Angstrom: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Abeoda: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; InhibRx: Equity Ownership.