Abstract P6-02-02: Exploiting Isobutyl-deoxynyboquinone-induced DNA damage responses and metabolic changes for breast cancer therapy
Authors: Ilcheva Mariya , Cao Lifen , Burma Sandeep , Boothman David 
Published: May 2015

Abstract
During oncogenic or cellular stress new genes are frequently (over)-expressed that could be exploited for targeted cancer therapy. The enzyme, NAD(P)H:quinone oxidoreductase-1 (NQO1) is over-expressed in most solid cancers, including 60% of primary and metastatic breast cancers regardless of subtype. Normally, NQO1 detoxifies quinones resulting in the formation of stable hydroquinones that are subsequently excreted from the cell. However, NQO1 bioreduction can turn certain rare quinones, such as β-lapachone and deoxynyboquinone (DNQ), into potent cell death-inducing compounds. These agents cause severe DNA damage, poly(ADP-ribose)polymerase-1 (PARP1) hyperactivation, NAD+/ATP loss, and programmed necrosis of NQO1-expressing cancer cells. Although ß-lapachone (ARQ761) is in current clinical trials at UTSW, more potent tumor-selective NQO1 compounds are needed. Based on its structure and mode of action, isobutyl-DNQ (IB-DNQ) was recently added to the spectrum of NQO1 substrates as a more selective and potent anti-cancer agent whose mechanism of action remains to be elucidated.
Although NQO1 expression is a major determinant of IB-DNQ-induced lethality, previously published results from our group showed that increased catalase expression could cause marked cytoprotection. We conducted a screen for NQO1:Catalase ratios in 266 breast tumor samples, and 143 normal breast samples, for a total of 409 specimens. We found that NQO1 expression was significantly elevated in breast tumors compared to normal tissue. In contrast, catalase expression was suppressed in breast tumors versus adjacent normal tissue. These results predict that normal tissue, which typically has higher catalase levels than cancer cells, could be selectively spared from IB-DNQ-induced toxicity. Thus, NQO1:Catalase ratios favor use of IB-DNQ in breast cancers to exploit this large therapeutic window.
Since NQO1 bioactivatable drugs synergize with agents that damage DNA, we hypothesized that certain cancer vulnerabilities (e.g., BRCA1-deficient breast cancers) that have elevated endogenous DNA damage would synergize with IB-DNQ. Exposure of breast cancer cells with IB-DNQ induced DNA damage, PARP1 hyperactivation, dramatic loss of essential nucleotides (NAD+/ATP), and µ-calpain-mediated programmed necrosis with 10X greater potency than ß-lapachone. IB-DNQ-induced DNA double-strand breaks (DSBs) that occurred in cells in S/G2 phases were mainly repaired by error-free homologous recombination (HR), and therefore BRCA1-deficient cancers, being HR defective, would be particularly vulnerable to IB-DNQ treatment. Indeed, HCC1937 breast cancer cells, deficient in BRCA1, were extremely sensitive to low dose IB-DNQ due to the overwhelming levels of IB-DNQ-induced DNA damage and their inability to repair it due to their compromised HR. In fact, IB-DNQ was far superior to PARP inhibitors in targeting BRCA1-deficient cells. Studies in vivo showed equivalent antitumor efficacy of IB-DNQ to β-lapachone and DNQ, but with much greater potency at lower doses. These findings offer preclinical ‘proof-of-concept’ for IB-DNQ as a potent chemotherapeutic agent for the treatment of breast cancers, especially those deficient in BRCA1. This research was supported by grant CA102972 to DAB.
Citation Format: Mariya Ilcheva, Lifen Cao, Sandeep Burma, David Boothman. Exploiting Isobutyl-deoxynyboquinone-induced DNA damage responses and metabolic changes for breast cancer therapy [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P6-02-02.