A Novel TNBC Therapeutic Opportunity: Cystine Addiction

Institution: University of California, San Francisco
Investigator(s): Luika Timmerman, Ph.D. -
Award Cycle: 2014 (Cycle 20) Grant #: 20IB-0119 Award: $187,330
Award Type: IDEA
Research Priorities
Detection, Prognosis and Treatment>Innovative Treatment Modalities: search for a cure

Initial Award Abstract (2014)

Overview. Fifteen to 20% of breast tumors do not express the estrogen receptor, progesterone receptor, or the Her2 receptor tyrosine kinase. They are termed Triple Negative (TNBC). Patients with TNBC are generally younger, disproportionately of African-American and Hispanic heritage, with more rapid disease progression, and poorest clinical outcomes. No therapeutic that specifically targets TNBC exists. Patients are treated with chemotherapies that affect all cells of the body, with doses limited by toxic effects on normal tissues.

Work from our laboratory is establishing a new therapeutic paradigm: that targeting abnormal, tumor specific metabolic behaviors and proteins will successfully produce novel, potent drugs. This idea grows out of experiments in which we identified a new cystine transporter on the surface of about 1/3 of TNBC. This transporter, named xCT, must be functional for these TNBC to thrive, because they have lost the ability to produce their own cystine. Nor can they acquire cystine from the environment by using other transporters. Few normal cells in the body express xCT. In animal experiments, normal cells that do express xCT do not appear to be harmed if it is blocked or if the DNA encoding it is removed from the genome. This makes xCT an ideal, tumor-specific drug target. We are translating these findings into an urgently-needed new drug which may be the first specific therapeutic for TNBC.

We first visualized using this drug alone to treat susceptible TNBC, but by understanding the biology of cystine use in tumor cells, we have come to an exciting hypothesis (below) with potentially huge clinical impact. TNBC need increased amounts of cystine from their environment in order to make a molecule called glutathione. Tumors use glutathione for two important activities related to therapeutic resistance: 1. Control of reactive oxygen species (ROS), which are potent mutagens created by radiation therapy and some chemotherapeutics, and 2. Direct chemotherapeutic inactivation. Historically, drugs that directly inhibit the cellular enzymes that synthesize glutathione have been developed by other investigators. But those studies had to be abandoned since normal cells in the body also make and use those enzymes and the drugs had unacceptably toxic side effects. In contrast, we are targeting a transporter with a highly restricted expression pattern that provides a nutrient (cystine) required for these enzymes to make glutathione.

Central Hypothesis. We hypothesize that an xCT inhibitor, used in combination therapies will specifically sensitize tumors to treatment with a variety of commonly-used chemotherapeutics and/or radiation therapy, while having little or no effect on normal cells of the body. This would reduce the dose of therapeutics that physicians need to give to patients, and lower the treatment side effects, while maintaining or increasing the effects on tumor cells. We will also produce critical reagents to clearly identify patient populations that will benefit from an xCT inhibitor, including specific ethnic minorities and/or age groups, who may disproportionately over-express xCT and be the patients most likely to benefit from anti-xCT therapy. This research is directly in line with the CBCRP Priority Issue IV: Detection, Prognosis and Treatment: Delivering Clinical Solutions.

Methods. We will use cells from a panel of 50 breast cancer cells in experiments in vitro and in xenografts, to identify which of four common TNBC therapeutics synergize with xCT inhibition. We will generate reagents to specifically identify xCT and therefore patients that are good subjects for our drug. We will analyze expression datasets from patient tumors to compare expression patterns of xCT before and after treatment, to identify biomarkers of xCT susceptibility, and association with common breast cancer disparity groups. We will test several new improved versions of our xCT inhibitor.

Innovative Elements. This project would be the first CBCRP funded effort that attacks this completely new class of breast tumor cell features: their abnormal metabolic requirements for viability. Our studies may ultimately produce a novel therapeutic with a novel mechanism of action, and may potentiate many different standard chemotherapeutics and/or radiation therapy. Our evidence suggests that xCT may be also be overexpressed by cancer stem cells and some estrogen receptor positive tumors. Thus an xCT inhibitor may ultimately have a large clinical impact, and importantly, be the first specific therapeutic for TNBC, the most underserved population of breast cancer patients. They deserve our best efforts.

Final Report (2017)

Triple negative breast cancer (TNBC) patients receive a variety of non-specific chemotherapeutics and/or radiation therapy since their tumors do not express molecules targeted by tumor-specific therapeutics. These nonspecific therapeutics interact with all cells in the body, thus they can produce severe, long lasting side effects and toxicities. We study an amino acid transporter named xCT, which cells use to acquire the amino acid cystine from the environment. Cystine is used by cells to inactivate chemotherapeutics and radiation therapy. The vast majority of normal cells in the body do not express or need xCT, however about of TNBC cells do express xCT and require it for growth. We find that blocking xCT by chemical and genetic means potentiates the effects of radiation therapy and chemotherapy, specifically in xCT+ TNBC tumor cells. Development of a clinicallyapproved drug that targets xCT will allow cancer patients with xCT-positive tumors to receive lower chemotherapeutic and radiation therapy doses, and reduce the side effects they incur.

We accomplished all goals of our study except for analysis of potential xCT induction by chemotherapeutic and radiation therapy on normal tissues, comparing treatment with or without erastin co-treatment. These studies were not conducted because we unexpectedly found that injected erastin was precipitating and remaining local to the injection site, rather than being carried correctly to distal, normal tissues. However we have strong evidence demonstrating that xCT inhibition in vitro, and via injection near the xenograft tumor can potentiate the effects of chemotherapy such as doxorubicin. We also find that erastin pretreatment can potentiate the effects of radiation therapy in vitro and in xenografts.

Future activities: Most importantly, this research provides strong evidence that development of a clinicallyapproved xCT inhibitor would have a large benefit for TNBC and other xCT-positive tumor types. We will pursue development of such a drug with future funding. We will also develop a more detailed understanding of the molecular mechanisms of synergy/cooperation between xCT inhibition and doxorubicin, and xCT inhibition and radiation therapy. Finally, we aim to develop a modern, genomics-era portrait of how tumors become resistant to common breast therapeutics. This would allow us to systematically address this critical clinical problem.

Conference Abstract (2016)

A Novel TNBC Therapeutic Opportunity: Cysteine Addiction

Cobler, Lara, Ph.D.; Velarde, Nona; Samson, Susan MA, M.P.H (Advocate); Timmerman, Luika A., Ph. D.
University of California Helen Diller Family Comprehensive Cancer Center, San Francisco, CA 94115

Background: Between 650,000 and 850,000 patients/ year receive chemotherapy to treat their tumors, including essentially all triple negative breast cancer (TNBC) patients. To date, over 100 different types of chemotherapeutics are now commonly prescribed. These drugs are largely non-specific, thus they affect every cell in the body, not just tumor cells. Predictably this means that they can have severe, even life threatening side effects as normal cells are poisoned along with tumor cells during treatment. Based on extensive research, physicians prescribe these drugs at doses that maximize the damage to tumor cells and minimize the damage to normal cells in the body. This is known as the therapeutic window. However the prevalence of harmful side effects observed clinically indicates that the therapeutic window can be small, and most likely varies a bit between different tissues, patients, and tumors. Despite these drawbacks, many lives have been saved by chemotherapeutic use and these drugs remain the mainstay tools used to combat tumors such as TNBC.

Hypothesis: If tumors such as TNBC could specifically be made more sensitive to chemotherapeutics, then doctors could prescribe lower drug doses to kill tumors. These lower doses would produce fewer side effects on normal tissues making therapy easier to tolerate, with fewer long-term deleterious effects. Alternatively, for really tough tumors, specifically making the tumors more sensitive to chemotherapeutics would increase the ability of chemotherapy to kill or slow tumor growth, without increasing the effects on normal tissues.

Approach and results: Tumors use a molecule named glutathione to inactivate many types of chemotherapy. They also use a molecule named xCT, which is present on tumor cells such as TNBC, to produce glutathione by importing cystine from the environment. We tested whether chemotherapeutics commonly used to treat TNBC were made more potent by co-treating cells with chemical inhibitors of xCT. In ongoing cell culture studies we find that in fact that chemotherapy such as doxorubicin is made more potent by combination treatment with an xCT inhibitor. We are now working to understand the molecular underpinnings of the drug synergy we find and are testing these results using tumors grafted into animals, to determine whether this is as promising as our cell culture studies suggest. While our work is particularly relevant to triple negative breast cancer, it also illustrates how important a clinically approved xCT inhibitor could be for the treatment of many types of cancer, since xCT is active on many types of solid tumors.

Due to the translational potential of this novel therapeutic opportunity, we engaged early with the advocacy community. We focused on three major imperatives for our advocate: collaboration, the acquisition of scientific competence, and the representation of program priorities. The research advocate meets regularly with study investigators, reviews research activities, and raises broad policy issues and TNBC challenges, including disparities issues. Forging credibility in reframing what is at stake on biomedical matters, the advocate attempts to spark innovation, democratize science, and support smarter interventions that expedite the incredible potential of future investments in bioscience.