Pharmacogenetics in Esophageal Cancer

Pharmacogenetics in Esophageal Cancer Xifeng Wu,a Charles Lu,b Silvia S. Chiang,a and Jaffer A. Ajanic The current oncology practice of treating cancer with aggressive doses of radiation and chemotherapy is potentially disastrous, as response and side effects vary depending on several factors including pharmacogenetics. This study is the first to evaluate esophageal cancer treatment with a pharmacogenetic paradigm and its application of pharmacogenetic analysis to multiple genes in each drug action pathway as a means of developing a more accurate and consistent risk prediction model. This study has enrolled 235 patients with resectable adenocarcinoma or squamous cell carcinoma of the esophagus who had been treated with chemoradiation followed by esophagectomy. The preliminary finding that methylenetetrahydrofolate reductase polymorphisms modify 5-fluorouracil response supports the hypothesis that response or resistance to therapy in esophageal cancer patients may be modulated by genetic variants involved in the metabolism or mechanism of chemotherapy drug action. Our ongoing esophageal cancer research aims to determine individual pharmacogenetic profiles to identify patients most likely to have chemotherapeutic benefit and patients with the highest risk of suffering genotoxic side effects. These profiles will ideally lead to individualized therapies, improved treatment outcomes, and a movement toward clinically applied pharmacogenetics. This emergent area of biomedicine could lead to substantially improved clinical outcomes for patients with adenocarcinoma or squamous cell carcinoma of the esophagus. Semin Oncol 32(suppl 9):S87-S89 © 2005 Elsevier Inc. All rights reserved.

The Need for Improved Cancer Therapy

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n the current model of cancer treatment, the oncologist treats the patient with aggressive doses of radiation and chemotherapeutic agents, attempting to kill as many cancer cells as possible, while limiting damage to healthy, normally functioning cells. With some consideration to body size and performance status, oncologists treat all cancer patients who have the same diagnosis with very similar treatments; however, like a sledgehammer, this methodology is unrefined, unwieldy, and potentially disastrous because treatment response and side effects vary widely. Cancer treatment may cause more harm than therapeutic benefit in many patients. By using pretreatment genetic profiling, oncologists can much more effectively predict patients who will respond poorly to and/or suffer severe side effects from certain treat-

From The University of Texas M.D. Anderson Cancer Center, Houston, TX. aDepartments of Epidemiology. bThoracic/Head & Neck Oncology. cGI Medical Oncology. Dr Wu has no significant financial relationships to disclose. Address reprint requests to Xifeng Wu, MD, PhD, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Blvd, Houston, TX 77030; e-mail: [email protected]

0093-7754/05/$-see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1053/j.seminoncol.2005.04.031

ments. In the pursuit of maintaining quality of life in cancer patients, chemotherapy drugs are administered, but are often prone to a narrow therapeutic index that could possibly make them ineffective and/or toxic. Although oncologists typically adjust treatment based on poor response and/or severe side effects, delayed or interrupted treatment could also negatively impact patient response. By identifying interindividual genetic variations to drug response, the clinical application of pharmacogenetics will allow physicians to tailor therapy to maximize efficacy while minimizing adverse reactions. Side effects from chemotherapy are frequent, but for certain individuals the damage caused by the drugs outweighs any therapeutic benefit. While some patients obtain the drugs’ desired benefits, in terms of efficacy, others have minimal or no therapeutic response. Factors that influence response to chemotherapeutic agents include age, gender, ethnicity, performance status, presence of comorbid conditions, drug interactions, prior therapy, and genetics. Genetic differences account for an estimated 20% to 95% variability in treatment efficacy and toxicity, and evidence supporting the importance of genetics continues to accumulate.1 One of the most intensively studied examples of pharmacogenetic variation CYP2D6 is a member of the cytochrome P-450 family that codes for the most important phase I (activation) drug S87

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S88 metabolism enzymes. This variation may contain a polymorphism that leads to poor codeine metabolism. Studies have shown that 5% to 10% of Caucasians possess this genetic mutation, resulting in an inability to derive analgesic benefit from codeine.2,3 An example of genetic influence on drug toxicity is the interaction of single nucleotide polymorphisms in the thiopurine methyltransferase enzyme with 6-mercaptopurine, a chemotherapy agent used in the treatment of childhood acute lymphoblastic leukemia. This interaction leads to fatal bone marrow suppression. Therefore, genetic testing is necessary to prevent the lethal prescription of this drug to leukemia patients.2-4 Like pharmacology, pharmacogenetics is divided into two research areas: pharmacodynamics and pharmacokinetics. Pharmacodynamics denotes the pharmacologic actions of a drug on the body. Drug targets, broadly defined as proteins that mediate drug effects and sometimes, drug transporters, belong in this category. On the other hand, pharmacokinetics denotes the effects of the body on the drug–in other words, the pathways used by the body to absorb, distribute, metabolize, and eliminate the drug. These categories, although not always discrete, help organize concepts and findings within the broad and complex discipline of pharmacogenetics.

Esophageal Carcinoma Cancer of the esophagus is a highly lethal malignancy typically diagnosed at an advanced stage that has a 5-year survival rate of less than 10% for all patients and less than 20% for surgically resected patients. The most common disease subtypes are squamous cell carcinoma and adenocarcinoma. The American Cancer Society has estimated that 14,520 new cases and 13,570 deaths will occur from this disease in 2005.5 Surgery remains the standard of care for potentially resectable patients, while definitive chemoradiation (chemoRT) is generally prescribed for patients who either have locoregional disease or are unfit for surgery. ChemoRT may also be used preoperatively to shrink the cancer, especially if cancer size or location would complicate the surgery. A number of studies have suggested that chemoRT may serve as the primary therapy in lieu of surgery, conferring comparable survival rates and fewer treatment complications.6,7 Unfortunately, in either case, prognoses are dismal, with at least 85% of all patients dying from this cancer within 5 years of diagnosis.8 Drug resistance presents a major obstacle to treatment, but pharmacogenetics may shed light on the poorly understood mechanisms underlying resistance to both chemotherapy and radiation. Common chemotherapeutic agents used alone or in combination to treat esophageal carcinoma include cisplatin, the taxanes, and 5-fluorouracil (5-FU). We hypothesized that specific genotypes in genes involved in the metabolism, transportation, and targets of these drugs may negatively impact patient response and disposition. Our ultimate goal is to determine individual pharmacogenetic profiles to identify patients with the greatest likelihood of deriving therapeutic benefit from chemoRT, as well as patients with the highest risk of suffering genotoxic side effects. These

profiles will ideally lead to individually tailored therapies and improved treatment outcomes. Specific polymorphisms that were examined in our study include those in nucleotide excision repair (NER) genes. NER, a component of cellular DNA repair capacity, entails the removal of carcinogenic DNA adducts or DNA-bound chemotherapeutic agents. Successful genome repair via NER and other mechanisms lead to a reduced risk of carcinogenesis; however, it impairs clinical response to cisplatin which depends on the unsuccessful repair of the DNA adducts it forms to induce the apoptosis of malignant cells. Therefore, we hypothesized that polymorphisms that modify NER capacity affect both esophageal cancer risk and cisplatin response.

Study Population and Approach For this ongoing study, we recruited 235 patients with resectable adenocarcinoma or squamous cell carcinoma of the esophagus who had been treated with chemoRT followed by esophagectomy at the University of Texas M. D. Anderson Cancer Center (Houston, TX) between 1985 and 2003. We have determined the frequencies of polymorphisms in pathways that are relevant to the activity and/or disposition of the platinum analogs, 5-FU, and the taxanes. In addition to NER, other pathways in drug metabolism include multidrug resistance, detoxification, damage recognition, cell cycle, and apoptosis. We are in the process of correlating pathologic complete response, recurrence, and survival with our comprehensive panel of genetic markers. The novelty and significance of our study is two-fold: (1) it is the first to approach esophageal cancer treatment with a pharmacogenetic paradigm, and (2) rather than evaluate associations between drug response and single polymorphisms, it applies pharmacogenetic analysis to multiple genes in each drug action pathway, resulting in a more accurate and consistent risk prediction model.

Preliminary Findings Methylenetetrahydrofolate reductase (MTHFR) plays an important role in DNA methylation, catalyzing 5,10-methylenetetrahydrofolate (5,10-MTHF) to 5-methyltetrahydrofolate and allowing for homocysteine re-methylation to methionine. More importantly for our study, MTHFR is also involved in dTMP production and DNA synthesis–and thus, is implicated in response to 5-FU. Through a series of steps, impaired MTHFR protein function ultimately leads to decreased levels of dTMP, a necessary component of DNA. Specifically, decreased MTHFR activity causes the concentration of 5,10-MTHF to rise which results in increased blocking of the thymidylate synthase enzyme.1 Thymidylate synthase inhibition leads to DNA damage and triggers the desired goal of cell death. Several studies have suggested that single nucleotide polymorphisms in the MTHFR gene, C667T (Ala222Val) and A1298C (Glu427Ala) modify 5-FU response,4,9-11 but overall findings have been contradictory. Previous studies have yielded inconsistent results most likely because the

Pharmacogenetics in esophageal cancer complexity of drug action mechanisms warrants genetic analysis of the entire pathway, rather than a single variant. Although our study has found that human cell lines with the 1298C variant allele show improved 5-FU response compared with the wild-type (P ⬍.05), we plan to analyze this finding together with other polymorphisms involved in the mode of action of 5-FU to construct a more accurate and consistent risk prediction model. The advancement of pharmacogenetics depends on the implementation of this type of nuanced, whole-pathway approach. Nonetheless, the preliminary finding that MTHFR polymorphisms modify 5-FU response is informative because it supports our hypothesis that response or resistance to therapy in esophageal cancer patients may be modulated by genetic variants involved in the metabolism or mechanism of chemotherapy drug action. Our study is also investigating the effect of polymorphisms in a thorough selection of genes involved in cisplatin metabolism and drug action. Cisplatin destroys malignant cells by forming intra- and inter-strand DNA adducts, inhibiting DNA replication, and triggering apoptosis after unsuccessful genomic repair. Resistance to this drug results from the blockage of its entry into the cell and from the removal of adducts by cellular DNA repair capacity. Because the success of cisplatin therapy hinges on causing irreparable genomic damage that leads to cell death, more efficient DNA repair capacity (although useful in the prevention of carcinogenesis) is postulated to result in shorter survival times for patients receiving cisplatin. Although this hypothesis has been tested in a large number of studies, their conclusions are often contradictory, and no studies have been published on esophageal cancer. Our analysis will focus on cisplatin transport, detoxification, excision repair, and apoptosis (and whether polymorphisms of the genes involved in these pathways modify cisplatin resistance). Our long-term objective is to develop a quantitative, multivariate, risk assessment model that incorporates and prioritizes these relevant molecular and epidemiologic predictors of clinical outcome.

Working Toward Clinically Applied Pharmacogenetics and Conclusions As with any scientific innovation, the successful clinical application of pharmacogenetics follows a sequence of research steps, beginning with the identification of polymorphisms and their functional effects in relevant genes. The clinical impact of the polymorphisms are then studied; first by either

S89 administering the drug to healthy volunteers or applying it to uses not typical of the clinical setting. Next, the drug is tested in a relevant clinical population in a manner that reflects clinical practice. Variable alleles must be determined to cause a significant difference in drug response; otherwise, the information would not be clinically predictive of treatment response and toxicity. In the final phase, clinical studies determine whether pharmacogenetic-guided care leads to increased survival and decreased toxicity compared with standard care. A number of challenges must be met before the clinical application of pharmacogenetics becomes a reality. Access to substantial patient populations are required to conduct clinical trials that have adequate samples sizes. Also, we must continue to untangle the ethical, social, and legal dilemmas surrounding genetic testing. Finally, pharmacogeneticguided care, like all medical treatments, must receive approval by the US Food and Drug Administration. Our ongoing esophageal cancer research aims to contribute to the greater movement toward clinically applied pharmacogenetics. For patients who suffer from adenocarcinoma or squamous cell carcinoma of the esophagus, this emergent area of biomedicine could lead to substantially improved clinical outcomes.

References 1. Ulrich CM, Robien K, McLeod HL: Cancer pharmacogenetics: Polymorphisms, pathways and beyond. Nature 3:912-920, 2003 2. Weinshilboum R: Inheritance and drug response. N Engl J Med 348: 529-537, 2003 3. Johnson JA: Pharmacogenetics: Potential for individualized drug therapy through genetics. Trends Genet 19:660-666, 2003 4. Nagasubramanian R, Innocenti F, Ratain MJ: Pharmacogenetics in cancer treatment. Annu Rev Med 54:437-452, 2003 5. Jemal A, Murray, T, Ward E, et al: Cancer Statistics, 2005. CA Cancer J Clin 55:10-30, 2005 6. Enzinger PC, Mayer RJ: Esophageal cancer. N Engl J Med 349:22412251, 2003 7. Geh JI: The use of chemoradiotherapy in oesophageal cancer. Eur J Cancer 38:300-313, 2002 8. Munro AJ: Oesophageal cancer: A view over overviews. Lancet 364: 566-568, 2004 9. Sohn K-J, Croxford R, Yates Z, et al: Effect of the methylenetetrahydrofolate reductase C677T polymorphism on chemosensitivity of colon and breast cancer cells to 5-fluorouracil and methotrexate. J Natl Cancer Inst 96:134-144, 2004 10. Etienne M-C, Ilc K, Formento J-L, et al: Thymidylate synthase and methylenetetrahydrofolate reductase gene polymorphisms: Relationships with 5-fluorouracil sensitivity. Br J Cancer 90:526-534, 2004 11. Cohen V, Panet-Raymond V, Sabbaghian N, et al: Methylenetetrahydrofolate reductase polymorphism in advanced colorectal cancer: A novel genomic predictor of clinical response to fluoropyrimidine-based chemotherapy. Clin Cancer Res 9:1611-1615, 2003