On April 17, 2018 Aptose Biosciences Inc. (NASDAQ:APTO) (TSX:APS) reported the presentation of preclinical data exploring the mechanism of action of APTO-253, the company’s clinical stage product candidate (Press release, Aptose Biosciences, APR 17, 2018, View Source;p=RssLanding&cat=news&id=2343016 [SID1234525432]). The data, demonstrating heightened sensitivity of BRCA1 or BRCA2 mutated cancer cells to APTO-253, were presented in a poster Tuesday, April 17 at the 2018 American Association for Cancer Research (AACR) (Free AACR Whitepaper) Annual Meeting being held April 14-18, in Chicago, IL.

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The poster, entitled APTO-253 is a new addition to the repertoire of drugs that can exploit DNA BRCA1/2 deficiency, explored the mechanism of action of APTO-253, a small molecule with anti-proliferative activity against cell lines derived from a wide range of human malignancies. This study investigated the mechanism of action of APTO-253 to identify synthetic lethal interactions that can guide combination drug studies.

The research team found that APTO-253 stabilizes certain quadruplex DNA structures, causes DNA damage, and exhibits synthetic lethality comparable to olaparib – an FDA-approved targeted therapy that acts against cancers in people with hereditary BRCA1 or BRCA1 mutations, including some ovarian, breast and prostate cancers – albeit through a different mechanism. Unlike other drugs for which loss of this DNA repair function results in hypersensitivity, APTO-253 does not produce myelosuppression even at the maximum tolerated dose. The observations reported also identify γH2AX as a potential biomarker of clinical effect and open the window to more detailed studies of how APTO-253 promotes DNA damage and how it might be used clinically to treat patients with tumors harboring deficiencies in DNA repair.

The presentation will be published in the AACR (Free AACR Whitepaper) Conference Proceedings. The poster can also be accessed here or at the Publications & Presentations section of the Aptose website, www.aptose.com.
"We have clarified the mechanism of APTO-253 during the past year or so, including its mechanism to inhibit expression of the MYC gene, an oncogene that promotes tumor growth and resistance to drugs in AML and other cancers," said William G. Rice, Ph.D., Chairman, President and Chief Executive Officer. "AML remains our primary focus for APTO-253, and we hope to re-initiate dosing of AML patients with APTO-253 in an open phase Ib trial during the 2nd quarter of 2018. In the current presentation at AACR (Free AACR Whitepaper), we report that cancer cells deficient in the BRCA1/2 DNA repair functions are hyper-sensitive to APTO-253, analogous to the FDA-approved PARP inhibitor olaparib, but acting through a different mechanism. The findings reveal potential new solid tumor indications for APTO-253. Importantly, APTO-253 does not produce myelosuppression even at the maximum tolerated dose, which significantly distinguishes it from other cancer chemotherapies."

About APTO-253
APTO-253 is a clinical-stage small molecule targeted therapeutic agent that inhibits expression of the c-Myc oncogene, leading to cell cycle arrest and programmed cell death (apoptosis) in human-derived solid tumor and hematologic cancer cells, without causing general myelosuppression of the healthy bone marrow. The c-Myc oncogene is overexpressed in hematologic cancers, including acute myeloid leukemia (AML). Aptose researchers have reported the ability of APTO-253 to induce cell death, or apoptosis, in multiple blood cancer cell lines including AML, as well as in vitro synergy with various classes of conventional approved and investigational therapies for AML or myelodysplastic syndromes (MDS). New findings reveal that APTO-253 might also serve certain solid tumor patients with BRCA1/2 mutations, but without causing toxicity to the normal bone marrow functions.