21-Gene Assay to Inform Chemotherapy Benefit in Node-Positive Breast Cancer.

BACKGROUND: The recurrence score based on the 21-gene breast-cancer assay has been clinically useful in predicting a chemotherapy benefit in hormone-receptor-positive, human epidermal growth factor receptor 2 (HER2)-negative, axillary lymph-node-negative breast cancer. In women with positive lymph-node disease, the role of the recurrence score with respect to predicting a benefit of adjuvant chemotherapy is unclear. METHODS: In a prospective trial, we randomly assigned women with hormone-receptor-positive, HER2-negative breast cancer, one to three positive axillary lymph nodes, and a recurrence score of 25 or lower (scores range from 0 to 100, with higher scores indicating a worse prognosis) to endocrine therapy only or to chemotherapy plus endocrine (chemoendocrine) therapy. The primary objective was to determine the effect of chemotherapy on invasive disease-free survival and whether the effect was influenced by the recurrence score. Secondary end points included distant relapse-free survival. RESULTS: A total of 5083 women (33.2% premenopausal and 66.8% postmenopausal) underwent randomization, and 5018 participated in the trial. At the prespecified third interim analysis, the chemotherapy benefit with respect to increasing invasive disease-free survival differed according to menopausal status (P = 0.008 for the comparison of chemotherapy benefit in premenopausal and postmenopausal participants), and separate prespecified analyses were conducted. Among postmenopausal women, invasive disease-free survival at 5 years was 91.9% in the endocrine-only group and 91.3% in the chemoendocrine group, with no chemotherapy benefit (hazard ratio for invasive disease recurrence, new primary cancer [breast cancer or another type], or death, 1.02; 95% confidence interval [CI], 0.82 to 1.26; P = 0.89). Among premenopausal women, invasive disease-free survival at 5 years was 89.0% with endocrine-only therapy and 93.9% with chemoendocrine therapy (hazard ratio, 0.60; 95% CI, 0.43 to 0.83; P = 0.002), with a similar increase in distant relapse-free survival (hazard ratio, 0.58; 95% CI, 0.39 to 0.87; P = 0.009). The relative chemotherapy benefit did not increase as the recurrence score increased. CONCLUSIONS: Among premenopausal women with one to three positive lymph nodes and a recurrence score of 25 or lower, those who received chemoendocrine therapy had longer invasive disease-free survival and distant relapse-free survival than those who received endocrine-only therapy, whereas postmenopausal women with similar characteristics did not benefit from adjuvant chemotherapy. (Funded by the National Cancer Institute and others; RxPONDER ClinicalTrials.gov number, NCT01272037.).

Trastuzumab Emtansine for Residual Invasive HER2-Positive Breast Cancer.

BACKGROUND: Patients who have residual invasive breast cancer after receiving neoadjuvant chemotherapy plus human epidermal growth factor receptor 2 (HER2)-targeted therapy have a worse prognosis than those who have no residual cancer. Trastuzumab emtansine (T-DM1), an antibody-drug conjugate of trastuzumab and the cytotoxic agent emtansine (DM1), a maytansine derivative and microtubule inhibitor, provides benefit in patients with metastatic breast cancer that was previously treated with chemotherapy plus HER2-targeted therapy. METHODS: We conducted a phase 3, open-label trial involving patients with HER2-positive early breast cancer who were found to have residual invasive disease in the breast or axilla at surgery after receiving neoadjuvant therapy containing a taxane (with or without anthracycline) and trastuzumab. Patients were randomly assigned to receive adjuvant T-DM1 or trastuzumab for 14 cycles. The primary end point was invasive disease-free survival (defined as freedom from ipsilateral invasive breast tumor recurrence, ipsilateral locoregional invasive breast cancer recurrence, contralateral invasive breast cancer, distant recurrence, or death from any cause). RESULTS: At the interim analysis, among 1486 randomly assigned patients (743 in the T-DM1 group and 743 in the trastuzumab group), invasive disease or death had occurred in 91 patients in the T-DM1 group (12.2%) and 165 patients in the trastuzumab group (22.2%). The estimated percentage of patients who were free of invasive disease at 3 years was 88.3% in the T-DM1 group and 77.0% in the trastuzumab group. Invasive disease-free survival was significantly higher in the T-DM1 group than in the trastuzumab group (hazard ratio for invasive disease or death, 0.50; 95% confidence interval, 0.39 to 0.64; P<0.001). Distant recurrence as the first invasive-disease event occurred in 10.5% of patients in the T-DM1 group and 15.9% of those in the trastuzumab group. The safety data were consistent with the known safety profile of T-DM1, with more adverse events associated with T-DM1 than with trastuzumab alone. CONCLUSIONS: Among patients with HER2-positive early breast cancer who had residual invasive disease after completion of neoadjuvant therapy, the risk of recurrence of invasive breast cancer or death was 50% lower with adjuvant T-DM1 than with trastuzumab alone. (Funded by F. Hoffmann-La Roche/Genentech; KATHERINE ClinicalTrials.gov number, NCT01772472 .).

Real-world outcomes in young women with breast cancer treated with neoadjuvant chemotherapy.

Breast cancer in young women has been shown to have an aggressive behavior and worse prognosis. Studies evaluating young women enrolled in clinical trials of neoadjuvant chemotherapy have shown that age is a determinant factor in the achievement of a pathological complete response (pCR). In this study, we sought to analyze the outcomes of young patients treated with neoadjuvant chemotherapy at a single institution. 1639 patients treated with neoadjuvant chemotherapy were included. 316 patients ≤40 years were compared with 1323 patients aged >40 years regarding the achievement of a pCR (defined as no invasive residual tumor in the breast or lymph nodes). Disease-free survival (DFS) and overall survival were compared between groups according to pCR status and subtype, defined by hormone receptor (HR) and HER2 status. Young women were more likely to have a pCR than their older counterparts (37.4 vs. 26.3 %, P < 0.001). This difference was significant both for HR+/HER2- and triple-negative (TN) tumors. Young age and achieving less than pCR were associated with a greater chance of recurrence for the entire population. Age was not an independent factor for recurrence in TN and HER2+ disease. However, being younger than 40 increased recurrence risk in HR+/HER2- tumors. The achievement of a pCR was not associated with improved DFS in young women with HR+/HER2- tumors. Although young women have a high rate of pCR, they also have a worse prognosis. In a real-world clinical setting, the achievement of a pCR was an independently significant protective factor for recurrence across all subtypes and ages, except for HR+, HER2- disease in young women.

Navigating HER2-Low Testing in Invasive Breast Cancer: Update Recommendations for Pathologists.

The article discusses the importance of accurately distinguishing HER2-low from HER2-negative breast cancer, as novel ADCs have demonstrated activity in a large population of patients with HER2-low-expressing BC. While current guidelines recommend a dichotomous classification of HER2 as either positive or negative, the emergence of the HER2-low concept calls for standardization of HER2 testing in breast cancer, using currently available assays to better discriminate HER2 levels. This review covers the evolution and latest updates of the ASCO/CAP guidelines relevant to this important biomarker in breast cancer, including still-evolving concepts such as HER2 low, HER2 heterogeneity, and HER2 evolution. Our group presents the latest Mexican recommendations for HER2 status evaluation in breast cancer, considering the ASCO/CAP guidelines and introducing the HER2-low concept. In the era of personalized medicine, accurate HER2 status assessment remains one of the most important biomarkers in breast cancer, and the commitment of Mexican pathologists to theragnostic biomarker quality is crucial for providing the most efficient care in oncology.

Physicians' attitudes and perceived barriers to adherence to the national breast cancer clinical practice guidelines in Mexico: a survey study.

BACKGROUND: Adherence to clinical practice guidelines improves outcomes for patients with breast cancer. However, their implementation may not be feasible in low- and middle-income countries. This study aimed to evaluate physicians' adherence, attitudes, and barriers towards the Colima Consensus, which is the Mexican national breast cancer clinical practice guideline. METHODS: A cross-sectional, 31-item survey was e-mailed to Consensus attendees and members of the Mexican Society of Oncology and Mexican Mastology Association. Descriptive statistics, univariate, and multivariate analysis were used to analyze the associations between participants' characteristics, adherence, attitudes, and barriers. RESULTS: Of 439 respondents, 78% percent adhered to Consensus recommendations and 94% believed it was applicable to their clinical practice. Forty percent reported using the Consensus as their sole breast cancer guideline. This was associated with being a surgical oncologist (OR 3.3, 95% CI 2.0-5.3) and practicing at a public hospital (OR 2.1, 95% CI 1.2-3.7). The most common barriers to adherence were lack of resources and logistical problems. Regarding attitudes towards the Consensus, 90% considered it a good educational tool, 89% considered it a reliable source of information, and 90% thought it improved quality of care. CONCLUSIONS: We showed high levels of adherence and positive attitudes towards the Colima Consensus, with a significant proportion of physicians using it as their only guideline. Lack of resources and logistical issues were the main barriers to adherence. Our results highlight the relevance of local breast cancer guidelines and suggest a need for the creation of resource-stratified guidelines.

Capecitabine Plus Aromatase Inhibitor as First Line Therapy for Hormone Receptor Positive, HER2 Negative Metastatic Breast Cancer.

(1) Background: recent evidence suggests that long low-dose capecitabine regimens have a synergistic effect with endocrine therapy as aromatase inhibitors (AIs), and might increase overall survival for hormone-receptor-positive, HER2-negative, metastatic breast cancer compared to both treatments. We performed a retrospective study to confirm the efficacy and expand the safety data for capecitabine plus AI (a combination henceforth named XELIA) for this indication. (2) We conducted a single-center retrospective cohort study of 163 hormone receptor-positive metastatic breast cancer patients who received either the XELIA regimen, capecitabine, or an aromatase inhibitor (AI) as single agents in first-line treatment. The primary endpoint was progression-free survival, and the secondary endpoints were overall survival, best objective response, and toxicity incidence. (3) Results: the median progression-free survival for patients receiving XELIA, AI, and capecitabine was 29.37 months (20.91 to 37.84; 95% CI), 20.04 months (7.29 to 32.80; 95% CI) and 10.48 (8.69 to 12.28; 95% CI), respectively. The overall response rate was higher in the XELIA group (29.5%) than in the AI (14.3%) and capecitabine (9.1%) groups. However, the differences in overall survival were not statistically significant. Apart from hand-foot syndrome, there were no statistically significant differences in adverse events between the groups. (4) Conclusions: this retrospective study suggests that progression-free survival and overall response rates improved with the XELIA regimen compared to use of aromatase inhibitors and capecitabine alone. Combined use demonstrated an adequate safety profile and might represent an advantageous treatment in places where CDK 4/6 is not available. Larger studies and randomized clinical trials are required to confirm the effects shown in our study.

[Evaluation of breast cancer treatment at a tertiary-level institution with Popular Health Insurance in Mexico]. / Evaluación del tratamiento del cáncer de mama en una institución del tercer nivel con el Seguro Popular, México.

BACKGROUND: In our country breast cancer represents a major health problem. Only 45% of all population has access to health services, the consequence is delay in diagnosis and treatment. In Mexico, 66% of all new cases of breast cancer are diagnosed in locally advanced stages. From May 2007 the Health System Protection Against Catastrophic Expenses, called Seguro Popular (SP), breast cancer was included in covering the treatment of this neoplasm in any patient without access to social security. OBJECTIVE: To evaluate the results and impact of SP in the adjuvant and neoadjuvant treatment of a group of patients diagnosed with breast cancer at an institution of national reference. MATERIAL AND METHODS: We analyzed a group of 259 patients in stages (I-IIIC). The clinical stages I and II (55 patients) were treated with adjuvant chemotherapy FAC -T (fluorouracil 500 mg/m2, adriamycin 50 mg/m2 and cyclophosphamide 500 mg/m2 (FAC) followed by 12 weeks of paclitaxel 80 mg/m2 +/- trastuzumab loading dose of 4 mg/kg followed by 2 mg/kg); 204 patients in locally advanced stages (IIB-IIIC) received FAC-T +/- trastuzumab followed by surgery. Adjuvant treatment consisted of endocrine therapy for hormone-sensitive patients and radiotherapy 50 cGy according to international standards. RESULTS: The age at diagnosis was 47 years (range 23-68). 80% of them were locally advanced stages (IIB-IIIC) and were treated in a neoadjuvant setting, 20% was in early stages, treated with surgery and adjuvant chemotherapy The disease-free survival and overall survival at 30 months was 85.7 and 90% respectively. Overall pathologic complete response was obtained in 15% of cases. In the subgroup analysis showed that 41% of patients HER2 (+), 29% of triple-negative patients and 9% of hormone-sensitive tumors achieved complete pathological response (p = 0.0001). CONCLUSION: This is the first analysis of efficacy of adjuvant and neoadjuvant treatment in breast cancer since the introduction of popular secure non-entitled population. It is clear that treatment efficacy is similar to that reported in the literature, with 15% of pRC and survival to 30 months in 94-80%. The coverage of health expenditures treats a larger number of patients optimally. Along with this, efforts should be made to reduce the high frequency of diagnosis at advanced stage.

Extracellular vesicles from blood of breast cancer women induce angiogenic processes in HUVECs.

Breast cancer is the most frequent malignancy among women in developed countries and the main cause of death related to cancer in women worldwide. Extracellular vesicles (EVs) are vesicles with a variable size enclosed within a phospholipid bilayer that contain a variety of molecules with biological activity. Cancer cells release EVs that induce proliferation, escape from apoptosis, reprogramming energy metabolism, invasion and metastasis. In this study we studied whether EV fractions deprived of platelet EVs from breast cancer women (BC EVs) can mediate cell processes related with angiogenesis in human umbilical vein endothelial cells (HUVECs). Our findings demonstrate that BC EVs enhance migration, invasion and formation of new tubules in HUVECs, compared with EV fractions deprived of platelet EVs from healthy women (Ctrl EVs). In summary, we demonstrate, for the first time, that BC EVs induce cellular processes in HUVECs that participate in angiogenesis.

Lymphoepithelioma-like carcinoma of breast: A case report and review of the literature.

Lymphoepithelioma is a rare clinical entity, first was described in nasopharynx, and have been reported in another organs. Lymphoepithelioma-like carcinoma (LEC) of the breast is an undifferentiated carcinoma. Approximately 32 cases have been described, and differential diagnoses include primary breast lymphoma and medullary carcinoma. We present a 57-year-old woman, diagnosed with LEC of the breast, treated with mastectomy, followed by adjuvant chemotherapy, radiotherapy, and hormone therapy, with a disease-free interval of 2 years. There is scarce evidence related to the best treatment choice in this histological type.

MicroRNA-143 is Associated With Pathological Complete Response and Regulates Multiple Signaling Proteins in Breast Cancer.

Almost 55% to 80% of patients with breast cancer have an unfavorable pathological complete response to chemotherapy. MicroRNAs are small noncoding RNAs involved in cancer progression; however, their utility as predictors of pathological complete response to neoadjuvant chemotherapy is unclear. Here, we investigated if miR-143 could discriminate between pathological complete response and no-polymerase chain reaction of patients with locally advanced triple negative breast cancer that have received a fluorouracil-cisplatin/paclitaxel-based neoadjuvant treatment. Data showed that miR-143 exhibited a significant low expression ( P < .0006) in patients that achieved pathological complete response in comparison to nonresponder group. Receiver operating characteristic curve analysis suggested that miR-143 could be a good predictor of pathological complete response (area under curve = 0.849, P < .0006). Moreover, Kaplan-Meier analysis indicated that before neoadjuvant therapy low levels of miR-143 were associated to increased disease free survival. To gain insights into cellular functions of miR-143, we firstly showed that miR-143 was severely repressed in breast cancer cell lines and tumors in comparison to normal mammary cells and tissues. Ectopic restoration of miR-143 using RNA mimics inhibited both cell proliferation and migration and sensitized breast cancer cells to cisplatin therapy in vitro. To decipher the signaling networks regulated by miR-143, we used a high-throughput enzyme-linked immunosorbent assay-based phosphorylation antibody array. Phospho-proteomic profiling revealed that miR-143 coordinately reduced the protein levels and phosphorylation status of multiple oncoproteins involved in AKT, WNT/ß-catenin, SAPK/JNK, FAK, and JAK/STAT signaling pathways. Moreover, low miR-143 and high GSK3-ß, RAF1, paxillin, and p21CIP1 expression levels in a large cohort of patients with breast cancer were associated with worst outcome. In summary, miR-143 could be a potential predictor of response to neoadjuvant therapy and it may function as a divergent regulator of diverse signaling networks to suppress cell proliferation and migration in breast cancer.

Clinical characteristics and outcomes of older women with breast cancer in Mexico.

INTRODUCTION: Although the epidemiology of breast cancer in older women has been widely described before, little is known about the clinical characteristics and prognosis of older patients living in developing countries. Here, we studied older women with breast cancer treated at a public cancer center in Mexico City, and compared their outcomes with their younger counterparts. MATERIALS AND METHODS: We retrospectively analyzed a database of 5488 women treated for breast cancer at a single institution. We compared clinical characteristics, treatment and survival between women aged <65 and ≥65 years of age. Survival analyses were performed for each molecular subtype. RESULTS: 851 women (15.5%) were ≥65 years of age, of which 45% presented with Stages III-IV disease. Compared with their younger counterparts, older women had lower grade disease, a larger proportion of hormone receptor positive tumors, and were less likely to receive both chemotherapy and radiotherapy. At 5 years, no differences in both disease free and overall survival were found between younger and older women in a multivariate model including stage, grade, tumor subtype and treatment received. CONCLUSIONS: In contrast with reports from high-income countries, older women with breast cancer in developing nations present with more advanced disease requiring more aggressive treatment. Strategies aimed at earlier detection, improved access to care, and downstaging among older adults are greatly needed in Mexico and in the rest of the developing world.

Molecular Subtypes and Prognosis in Young Mexican Women With Breast Cancer.

INTRODUCTION: Young age represents an adverse prognostic factor in breast cancer (BC), and young women present with more advanced and aggressive disease. In Latin America, BC is increasing in young women, and there is a lack of information regarding the characteristics and outcomes of this patient population. PATIENTS AND METHODS: We retrospectively analyzed a database of 4315 women treated for BC at a single institution. We compared clinical characteristics, treatment, and survival between women ≤ 40 and > 40 years of age. Survival analyses were performed for each molecular subtype. RESULTS: A total of 662 women (15.3%) were ≤ 40 years old. Younger women had more advanced disease, higher grade, and a larger proportion of luminal B and triple-negative tumors (P < .001). At 5 years, both disease-free and overall survival (OS) were lower in younger women, although there were no differences after adjusting for stage. Five-year OS was worse for young women with hormone receptor-positive, human epidermal growth factor receptor 2-negative subtype (82% vs. 87.1%; P = .03), but not for those with human epidermal growth factor receptor 2-positive or triple-negative disease. This difference can be attributed to luminal B tumors, which showed a worse 5-year OS in younger women (79.1% vs. 85.2%; P = .03). CONCLUSION: Although young Mexican patients with BC have more aggressive disease at presentation than older women, only those with luminal B tumors have a worse survival after adjusting for stage. Strategies aimed at downstaging the disease and at improving the treatment of luminal B tumors in this population are needed.

Clinical and Epidemiological Profile of Breast Cancer in Mexico: Results of the Seguro Popular.

Purpose One half of the Mexican population lacks comprehensive health care coverage. In 2003, a reform to the General Health Law was approved that led to the creation of the System of Social Protection in Health and made universal health coverage mandatory. The main innovation of this reform was Seguro Popular, which provided coverage for breast cancer. Here we report the outcomes of women with breast cancer treated at a cancer center in Mexico under Seguro Popular. Materials and Methods This was a retrospective cohort study that included all patients with breast cancer treated in the Instituto Nacional de Cancerología in Mexico City between January 2007 and December 2013 with Seguro Popular coverage. Demographic and clinical information were collected and survival outcomes were analyzed. Results A total of 4,300 women with breast cancer were included in this analysis. Most patients had locally advanced disease at diagnosis (53%, n = 2,293), and 13% (n = 558) presented with stage IV disease. Neoadjuvant chemotherapy was administered to 1,834 patients (52%), with a pathologic complete response in 25.1% (n = 460). Median follow-up was 40.5 months. Five-year survival for the entire cohort was 82% (95% CI, 81% to 84%). Five-year survival was 97% for early-stage disease (95% CI, 95% to 98%), 82% for locally advanced disease (95% CI, 80% to 84%), and 36% for metastatic disease (95% CI, 30% to 42%). Conclusion This represents the first description of a cohort of patients with breast cancer treated in Mexico under Seguro Popular. Seguro Popular has allowed our institution, and other Mexican centers, to establish efficient standardized mechanisms to treat patients with breast cancer.

Outcomes of Hispanic women with lymph-node positive, HER2 positive breast cancer treated with neoadjuvant chemotherapy and trastuzumab in Mexico.

INTRODUCTION: Evidence regarding the outcomes of Hispanic women with breast cancer is lacking. We analyzed women with HER2+ disease treated with trastuzumab-based neoadjuvant chemotherapy in Mexico. METHODS: 244 patients were included. Outcomes were compared between patients who achieved pathologic complete response (pCR) (n = 119), or less than pCR (n = 125). Patients with noninvasive (ypT0/is, ypN0) residual disease were also analyzed. RESULTS: 119 (48.8%) patients achieved pCR. pCR was the only factor associated with improved 3 year survival (98.1% vs 92.3%: P = 0.02). Survival was better in patients with ypT0/is, ypN0 response than in those with residual invasive disease (p < 0.01). 3 year survival was 98.1% for patients with pCR and 92.6% for patients with ypTis, ypN0 response (p = 0.64). CONCLUSIONS: Response rates to trastuzumab based neoadjuvant chemotherapy in Hispanics mimic that of other ethnic groups. This underlines the fact that access to treatment, rather than ethnicity, is the main prognostic factor in this population.

Thalidomide combined with neoadjuvant chemotherapy in angiosarcoma of the breast with complete pathologic response: case report and review of literature.

BACKGROUND: Primary angiosarcoma of the breast is a rare malignancy. CASE REPORT: We report on a 41-year-old female patient who initially presented with locally advanced disease. Core biopsy showed angiosarcoma of the breast, grade 1, CD31-positive. The patient was treated with neoadjuvant systemic chemotherapy based on cisplatin, doxorubicin, and paclitaxel, given concurrently with thalidomide. After treatment completion, the patient underwent radical mastectomy. Pathologic complete response in the breast and axillary lymph nodes was achieved. The patient has no evidence of disease recurrence 6 months after her initial diagnosis. CONCLUSION: Anti-angiogenic therapy may be considered as part of the management of primary angiosarcoma of the breast.

Phase II randomized, double-blind, placebo-controlled study of whole-brain irradiation with concomitant chloroquine for brain metastases.

BACKGROUND AND PURPOSE: Chloroquine (CLQ), an antimalarial drug, has a lysosomotropic effect associated with increased radiationsensibility, which is mediated by the leakage of hydrolytic enzymes, increased apoptosis, autophagy and increased oxidative stress in vitro. In this phase II study, we evaluated the efficacy and safety of radiosensibilization using CLQ concomitant with 30 Gray (Gy) of whole-brain irradiation (WBI) to treat patients with brain metastases (BM) from solid tumors. METHODS: Seventy-three eligible patients were randomized. Thirty-nine patients received WBI (30 Gy in 10 fractions over 2 weeks) concomitant with 150 mg of CLQ for 4 weeks (the CLQ arm). Thirty-four patients received the same schedule of WBI concomitant with a placebo for 4 weeks (the control arm). All the patients were evaluated for quality of life (QoL) using the EORTC Quality of Life (QoL) Questionnaire (EORTC QLQ-C30) (Mexican version) before beginning radiotherapy and one month later. RESULTS: The overall response rate (ORR) was 54% for the CLQ arm and 55% for the control arm (p=0.92). The progression-free survival of brain metastases (BMPFS) rates at one year were 83.9% (95% CI 69.4-98.4) for the CLQ arm and 55.1% (95% CI 33.6-77.6) for the control arm. Treatment with CLQ was independently associated with increased BMPFS (RR 0.31,95% CI [0.1-0.9], p=0.046).The only factor that was independently associated with increased overall survival (OS) was the presence of< 4 brain metastases (RR 1.9, 95% CI [1.12-3.3], p=0.017). WBI was associated with improvements in cognitive and emotional function but also with worsened nausea in both patients groups. No differences in QoL or toxicity were found between the study arms. CONCLUSION: Treatment with CLQ plus WBI improved the control of BM (compared with the control arm) with no increase in toxicity; however, CLQ did not improve the RR or OS. A phase III clinical trial is warranted to confirm these findings.

Genetic determinants of cancer drug efficacy and toxicity: practical considerations and perspectives.

Drug-metabolizing enzymes are responsible for the activation or detoxification of cytotoxic drugs. Allelic variants are present with a variable frequency in different populations around the world and have an important role in the therapeutic index of such drugs. It is known that polymorphisms in thiopurine methyltransferase and dihydropyrimidine dehydrogenase have been associated with altered drug metabolism and increased risk of severe toxicity from 6-mercaptopurine and 5-fluorouracil, respectively. Additionally, a variant number of dinucleotide-repeat sequences in the promotor for uridine 5'-diphosphate glucuronosyltransferase 1A1 influences the glucuronidation of SN-38, the active metabolite of irinotecan, which is associated with severe toxicity, including diarrhea and neutropenia. In the same way, polymorphisms in thymidylate synthase have been associated with pyrimidine-associated toxicity and also with response to chemotherapy. The examples shown in this review demonstrate the usefulness of pre-screening patients for well-characterized polymorphism to identify the best-tolerated and most-effective treatment.