6. Clinical Use of Selective Estrogen Receptor Modulators and Aromatase Inhibitors in Prevention and Adjuvant Treatment of Breast Cancer

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1 6. Clinical Use of Selective Estrogen Receptor Modulators and Aromatase Inhibitors in Prevention and Adjuvant Treatment of Breast Cancer Saad J. Sirop, MD James N. Ingle, MD Matthew P. Goetz, MD Introduction About 65% to 75% of breast cancer expresses estrogen receptors (ERs) or progesterone receptors (PRs). In this group of patients, endocrine therapy represents the most important treatment modality. Tamoxifen, the selective estrogen receptor modulator (SERM), has been studied and utilized in breast cancer for the last 40 years. When administered to women with ER-positive breast cancer for 5 years after surgery, tamoxifen almost halves the annual recurrence rate and reduces the breast cancer mortality rate by one-third in both premenopausal and postmenopausal women ( 1 ). While tamoxifen is US Food and Drug Administration (FDA) approved for the treatment of metastatic breast cancer and the adjuvant treatment of postmenopausal ER-positive breast cancer, tamoxifen s continued importance is reflected by its status as the only hormonal agent approved by the FDA for the prevention of premenopausal breast cancer, the treatment of ductal carcinoma in situ, and the adjuvant and metastatic treatment of premenopausal invasive breast cancer. Over the last decade, third-generation aromatase inhibitors (AIs) (anastrazole, letrozole and exemestane) have been tested in all clinical settings (metastatic, adjuvant, and prevention), and in general, AIs are superior to tamoxifen with regard to treatment of breast cancer. However, metaanalyses demonstrate no difference in breast cancer mortality or overall survival comparing tamoxifen and AIs as monotherapy ( 2 ). Because both Translational Endocrinology & Metabolism, Volume 3, Number 1,

2 of these classes of drugs will continue to play an important role in the treatment and prevention of breast cancer, this review focuses on the clinical uses, common side effects, pharmacology, and pharmacogenomics of SERMs and AIs. Additionally, it reviews the clinical application of these drugs in the adjuvant setting and in the prevention of breast cancer. Selective Estrogen Receptor Modulators The SERM tamoxifen has been used in the treatment of breast cancer for the last 40 years. It was first FDA approved for metastatic breast cancer in It received FDA approval for use in postmenopausal women with node-positive disease in 1986, for premenopausal and postmenopausal women in the adjuvant setting in 1990, in prevention of breast cancer in 1998, and for resected ductal carcinoma in situ in Tamoxifen as Adjuvant Therapy in Breast Cancer Multiple trials have evaluated tamoxifen as adjuvant therapy in breast cancer. These trials are well summarized in a meta-analysis from the Early Breast Cancer Trialists Cancer Group ( 1 ). This meta-analysis included all trials of early stage breast cancer that randomized women to receive tamoxifen versus no tamoxifen regardless of whether chemotherapy was given. Eleven trials used tamoxifen for 1 year (7862 patients); 32 trials used it for 2 years ( patients), and 20 trials ( patients) used a median of 5 years of tamoxifen. Of these 20 trials, 14 used adjuvant chemotherapy. Analysis of clinical trials where tamoxifen was used for a median of 5 years showed a 50% reduction of cancer recurrence risk in ER+ patients during the first 5 years and a 33% reduction during years 5 to 9 (average risk reduction was 39%, hazard ratio [HR] 0.61, P < ). Breast cancer mortality was also reduced by 30% at year 15, with an absolute difference of 9%. During years 5 to 9, recurrence rates were reduced by 33% despite the fact that tamoxifen use at that time was similar between the 2 arms. Recurrence rates were similar after 10 years, indicating sustained gain of benefits after 10 years. In a multivariate model, risk reduction was independent of age, grade, nodal status, tumor grade, or use of chemotherapy. Tamoxifen was associated with substantial benefits in the 14 trials where chemotherapy was given. In node-negative patients, there was a 39% relative risk reduction in the tamoxifen group for the first 5 years and 140 Translational Endocrinology & Metabolism: Breast Cancer Update

3 8% risk reduction for years 5 to 9. This was more pronounced in patients with node-positive disease with a relative risk (RR) reduction of 40% during the 5 five years and 20% during years 5 to 9 Analysis of the relevance of the ER and PR status with regard to tamoxifen efficacy was recently updated in the Oxford overview ( 1 ). If the ER measurement was less than 10 fmol/mg cytosol protein (ie, ER-poor disease), there was no apparent benefit from addition of tamoxifen. However, for weakly positive ER (defined as fmol/mg), there was substantial benefit (RR 0.67, standard error [SE] 0.08), and the proportional effect at much higher ER was only slightly better (RR 0.52, SE 0.07 for ER 200 fmol/mg, trend in RR with ER [if ER 10 fmol/mg] p =0.002). Unlike the ER, PR status was not an independent predictor of response to tamoxifen. Patients with ER+, PR- breast cancer (2310 patients) had similar benefits of tamoxifen therapy (RR 0.5, 0.84, and 0.92 at years 0-4, years 5-9 and 10+ years, respectively) compared to ER+, PR+, but exhibited higher recurrence rates in both tamoxifen and control arms (indicating more Analysis of clinical trials where aggressive biology, likely luminal B subtype). tamoxifen was used for a Currently, the American Society of Clinical Oncology clinical guidelines committee 50% reduction of cancer median of 5 years showed a recommends 5 years of tamoxifen in women recurrence risk in ER+ patients who are pre or peri-menopausal ( 3 ). For during the first 5 years and a postmenopausal women, tamoxifen is still 33% reduction during years recommended as adjuvant therapy, but only 5 to 9 (average risk reduction in sequence with AIs. was 39%, hazard ratio [HR] 0.61, P< ). SERMs in the Chemoprevention of Breast Cancer Tamoxifen Four large randomized clinical trials have evaluated the role of tamoxifen in breast cancer prevention (summarized in Table 6-1 ). The Royal Marsden Tamoxifen Prevention Trial ( 4 ) was the first trial to evaluate the role of tamoxifen in chemoprevention of breast cancer. It was a smaller trial and included 2494 healthy women (age years) with family history of breast cancer and randomized them to tamoxifen versus placebo for 8 years. At a median follow-up of 13.2 years (maximum follow-up of 20 years), there continued to be no statistically significant reduction in the risk of breast cancer. However, there was a 39% risk reduction in the incidence of ER+ invasive breast cancer (HR 0.61, P =0.005), with an absolute risk reduction of 26 ER+ breast cancer cases per 1000 women treated. Clinical Use of SERMs and AIs in Breast Cancer Treatment 141

4 TABLE 6-1. SERMs and AIs in Prevention of Breast Cancer Study Randomization Sample size Population Results Comments Tamoxifen NSABP-P1 Placebo vs tamoxifen for 5 years 13,388(6681 tamoxifen vs 6707 placebo) High risk (age >60, 5-year predicted risk of breast cancer of 1.66% for age 35-59, history of LCIS) 49% reduction in invasive breast cancer; 50% reduction in noninvasive breast cancer (at 5 years) Results persisted at the 7-year follow-up update. No reduction in ER negative cancer RMTPT Placebo vs tamoxifen for 8 years 2494(1250 tamoxifen vs 1244 placebo) Healthy women ages with family history of breast cancer No difference in breast cancer incidence between both groups at 5 years; 39% reduction in ER+ invasive cancer at a median follow-up of 13.2 years Women on HT were included. IBIS-I Trial Tamoxifen vs placebo for 5 years 7154(3579 tamoxifen vs 3575 placebo) High-risk women (2-fold RR increased in women y, 4-fold RR increase in women y, and 10-fold RR increase in women y) 32% reduction in risk of breast cancer in tamoxifen group, 31% reduction in risk of invasive ER+ breast cancer (at 5 years) Majority of patients remained unblinded. Number Needed to Treat continued 142 Translational Endocrinology & Metabolism: Breast Cancer Update

5 TABLE 6-1. (Continued) Study Randomization Sample size Population Results Comments IRTPT Tamoxifen vs placebo for 5 years 5408(2,700 Tam vs 2,708 placebo) Average cancer risk women with prior hysterectomy years 39% reduction in PR+ breast cancer; no difference in overall incidence after 11.2 years of follow-up 48% of participitants had had bilateral oophorectomy; women on HT were included Raloxifene MORE Raloxifene (120 mg or 60 g) vs placebo for 4 years 7705(5219 raloxifene and 2578 placebo) Postmenopausal women with osteoporosis 76% reduction in breast cancer risk at 3 years Development of breast cancer was a secondary outcome CORE (amended trial of MORE) Raloxifene 60 mg vs placebo for 4 additional years 4011(2721 raloxifene and 1286 placebo) Amended design of MORE 59% reduction in overall breast cancer risk and 66% reduction in ER+ breast cancer risk Breast cancer risk was assessed at entry RUTH Raloxifene 60 mg vs placebo (5044 raloxifene and 5057 placebo) Coronory artery diease or multiple risks for coronory artery disease 44% reduction in invasive breast cancer risk and 55% reduction in ER+ breast cancer risk Primary outcomes were breast cancer and coronory events Number Needed to Treat continued Clinical Use of SERMs and AIs in Breast Cancer Treatment 143

6 TABLE 6-1. (Continued) Study Randomization Sample size Population Results Comments Number Needed to Treat STAR Raloxifene 60 mg vs tamoxifen 20 mg daily (9872 raloxifene and 9875 tamoxifen) Postmenopausal women with increased risk of breast cancer (>1.66% using Gail model) At the most recent follow-up, tamoxifen is superior to raloxifene in terms of breast cancer prevention Incidence of uterine cancer significantly lower in Raloxifene arm (RR 0.55; 95% CI ) Exemestane MAP3 Exemestane vs placebo 4560 (2285 exemestane, 2275 placebo) 1 of the following: age >60, risk >1.66% (Gail model), ADH, LH, LCIS or DCIS with mastectomy 65% risk reduction in invasive breast cancer Study follow-up has been halted 94(3 y) Abbreviations: ADH, atypical ductal hyperplasia; AIs, aromatase inhibitors; CORE, Continuing Outcomes Relevant to Evista; DCIS, ductal carcinoma in situ; ER, estrogen receptor, HT, hormonal therapy; IBIS-I, International Breast Cancer Intervention Study; IRTPT, Italian Randomized Tamoxifen Prevention Trial; LCIS, lobular carcinoma in situ;lh, lobular hyperplasia; MORE, Multiple Outcomes for Raloxifene Evaluation; NSABP-P1, National Surgical Adjuvant Breast and Bowel Project P1; RMTPT, Royal Marsden Tamoxifen Prevention Trial; RR, relative risk; RUTH,Raloxifene for Use of the Heart; SERMs: selective estrogen receptor modulators; STAR, Study of Tamoxifen and Raloxifene. 144 Translational Endocrinology & Metabolism: Breast Cancer Update

7 The National Surgical Adjuvant Breast and Bowel Project (NSABP) P-1 ( 5 ) trial randomized women who were high risk for developing breast cancer (defined as age more than 60, age with a 5-year predicted risk of breast cancer of 1.66%, or a history of lobular carcinoma in situ) to either 5 years of tamoxifen or placebo. At the first analysis (69-month follow-up), tamoxifen was associated with 49% reduction in the risk of invasive breast cancer (43.4% vs 22% at 69 months follow-up, P <0.0001). Tamoxifen also reduced the incidence of noninvasive breast cancer by 50%. The risk reduction was noted in younger as well as older women. The reduction of invasive breast cancer was primarily in hormone-positive breast cancer (69% reduction; 5.02 cases per 1000 women in the placebo group compared with 1.58 cases per 1000 women in the tamoxifen group). There was no difference in the incidence of hormonenegative breast cancer. The International Breast Cancer Intervention Study ( 6 ) (IBIS-I) trial randomized 7154 women to either 5 years of tamoxifen or placebo. This trial included high-risk women, defined as a 2-fold increase in the RR for women between In the NSABP P-1, tamoxifen 45 and 70 years, 4-fold increase in relative was associated with 49% risk in women between 40 and 44 years, reduction in the risk of invasive and a 10-fold increase for women between breast cancer (43.4% vs 22% 35 and 39 years. At 5-year follow up, tamoxifen use was associated with 32% reduction P<0.0001). at 69 months follow-up, in the overall risk of breast cancer and 31% reduction in ER+ breast cancer. These results persisted at 8 and 10 years of follow-up. At the 10-year follow-up analy sis, 142 of 3579 women in the tamoxifen group developed breast cancer, compared to 195 of 3575 women in the placebo group. This would result in an absolute risk reduction of 15 breast cancers per 1000 women treated. Similar to the results of NSABP-P1 trial, there was no difference in the incidence of ER negative breast cancer comparing the 2 groups. Also, a decrease in ductal carcinoma in situ (DCIS) was observed over the 10 years of the study but was not statistically significant (RR 0.63, 95% CI ). The Italian Randomized Tamoxifen Prevention Trial ( 7 ) randomized 5408 women with prior hysterectomy at average risk of breast cancer to either 5 years of tamoxifen or placebo. Of note, 48% of women in this study had had oophorectomy, and the use of hormonal therapy was allowed. At a mean follow-up of 11.2 years, there was no statistically significant difference in the overall incidence of breast cancer. However, there was a 39% statistically significant reduction in the incidence of progesteronepositive breast cancer (27 in the tamoxifen arm vs 44 in the placebo arm). Clinical Use of SERMs and AIs in Breast Cancer Treatment 145

8 Raloxifene The first trial evaluating raloxifene in prevention of breast cancer was the Multiple Outcomes for Raloxifene Evaluation (MORE) trial ( 8 ), in which 7705 postmenopausal women with osteoporosis were randomly assigned to receive raloxifene or placebo for 4 years. Breast cancer risk was not assessed during study entry, and development of breast cancer was a secondary outcome. Women treated with raloxifene had a statistically significant reduction in risk of invasive breast cancer (RR 0.28) and in risk of ER+ breast cancer (RR 0.16). The Continuing Outcomes Relevant to Evista ( 9 ) (CORE) trial is an amended design of the MORE trial that included 4011 MORE participants and kept them in the original assignment except that raloxifene dose was changed to 60 mg daily (the FDA approved dose for osteoporosis). Also, breast cancer risk was assessed at entry using the Gail model. Raloxifene use was associated with a 59% reduction in risk of invasive breast cancer risk and a 66% reduction in risk of estrogen-positive breast cancer. The absolute In the STAR trial at the most risk reduction was 12 cases per 1000 women recent update (median 81 for invasive breast cancer and 10 cases per months follow-up), tamoxifen 1000 women for estrogen-positive breast was superior to raloxifene in cancer at the end of 8 years of treatment. preventing breast cancer. The The Raloxifene for Use of the Heart ( 10 ) incidence of invasive breast (RUTH) trial randomized women with cancer was 310 cases per coronory artery disease or with multiple 9754 women in the raloxifene risk factors to raloxifene 60 mg daily or placebo. The primary outcomes were invasive per 9736 women in the group compared to 247 cases breast cancer and coronary events. Raloxifene was associated with a 44% reduction 95% CI ). tamoxifen group (RR 1.24, in risk of invasive breast cancer and a 55% reduction in risk of estrogen-positive breast cancer. The absolute risk reduction was 7 cases per 1000 women for invasive breast cancer and 7 cases per 1000 women for estrogen-positive breast cancer. The Study of Tamoxifen and Raloxifene ( 11 ) (STAR) trial compared tamoxifen to raloxifene in the prevention of breast cancer. It randomized postmenopausal women at increased risk of breast cancer (defined as a 5-year risk of 1.66% per Gail model) to either raloxifene 60 mg daily or tamoxifen 20 mg daily. The primary endpoint was reduction in risk of breast cancer. At the most recent update (median 81 months follow-up), tamoxifen was superior to raloxifene in preventing breast cancer. The incidence of invasive breast cancer was 310 cases per 9754 women in the 146 Translational Endocrinology & Metabolism: Breast Cancer Update

9 raloxifene group compared to 247 cases per 9736 women in the tamoxifen group (RR 1.24, 95% CI ). There were more noninvasive breast cancers in the raloxifene group as well, but the difference was not statistically significant. Pharmacology and Metabolism of Tamoxifen The pharmacology of tamoxifen is complex. Tamoxifen is a weak antiestrogen that is extensively metabolized, and its metabolites exhibit similar, less, or more potent antiestrogenic activity. Furthermore, tamoxifen can exhibit either antiestrogenic or proestrogenic properties, depending on the target tissue and the presence/absence of coactivators/corepressors. Tamoxifen undergoes extensive primary and secondary metabolism in people ( Figure 6-1 ), and the concentrations of tamoxifen and its metabolites demonstrate great interindividual and intraindividual variation ( ). While 4-hydroxy tamoxifen is used most commonly as an in vitro surrogate for tamoxifen, given its 100-fold greater affinity to the Recent evidence suggests that another hydroxylated ER and 100 times greater potency in suppressing cell proliferation compared to tamoxifen metabolite, 4 hydroxy-ndesmethyl tamoxifen ( 17, 18, 22 ), human pharmacology studies dem onstrate that the concentrations (endoxifen), substantially contributes to tamoxifen of 4-hydroxy tamoxifen are consistently less activity. than 10 nm, and therefore unlikely to be clinically important ( 19 ). This hypothesis was confirmed in a secondary analysis of the Women s Healthy Eating and Living Study (WHEL), where 4-hydroxy tamoxifen steady-state concentrations were not associated with breast cancer recurrence ( 20 ). Recent evidence suggests that another hydroxylated metabolite, 4 hydroxy-n-desmethyl tamoxifen (endoxifen), substantially contributes to tamoxifen activity ( 21 ). Endoxifen results from the CYP2D6 -mediated oxidation of N-desmethyl tamoxifen, and its steady-state plasma concentrations are 5- to 10-fold higher than 4-hydroxy (4-OH) tamoxifen. While endoxifen is identical to 4-OH tamoxifen in its ER binding affinity and ability to suppress estradiol (E2)-stimulated cell proliferation ( 14, 17 ), recent data suggest that the mechanism of action of these 2 SERMs may differ, given that unlike 4-OH tamoxifen or the parent drug tamoxifen, endoxifen uniquely targets ER α for proteosomal degradation ( 23 ). In people, the most important factor contributing to the variability in endoxifen concentrations is related to genetic variation in the rate-limiting enzyme responsible for the formation of endoxifen, CYP2D6. Tamoxifen-treated Clinical Use of SERMs and AIs in Breast Cancer Treatment 147

10 OCH 2 CH 2 N CH 3 CH 3 CH 3 CH 2 c c CH 3 CH 2 c c CYP3A4 CYP3A5 Tamoxifen CYP2D6 CYP2C9 CYP2C19 CYP1A2 OCH 2 CH 2 N N-desmethyltamoxifen CYP2D6 CH 3 H CYP2B6 CYP2C9 CYP2C19 OCH 2 CH 2 N CYP2D6 CH 3 H CYP3A4 CYP2B6 CYP2C9 CYP2C19 CH 3 CH 2 c c CYP3A4 OH OCH 2 CH 2 N 4-hydroxytamoxifen CH 3 CH 3 SULT1A1 UGTs CH 3 CH 2 c c OH 4-hydroxy-N-desmethyltamoxifen (Endoxifen) SULT1A1 UGTs FIG 6-1. Primary and secondary metabolism of tamoxifen by the cytochrome P450 system. The relative contribution of each pathway to the overall oxidation of tamoxifen is shown by the thickness of the arrow. [Originally published in Sideras K, Ingle JN, Ames MM, et al. J Clin Oncol Jun 1; Epub 2010 May 3.] 148 Translational Endocrinology & Metabolism: Breast Cancer Update

11 women with CY2D6 genetic variants associated with reduced or absent CYP2D6 activity or who concomitantly take medications that inhibit CYP2D6 activity have significantly lower concentrations of endoxifen ( 15, 19, 24, 25 ). Cytochrome P450 2D6 Genetic Polymorphisms The CYP2D6 gene is highly polymorphic, currently with over 100 different alleles known, many of which are associated with increased, decreased, or abolished function of the final gene product. The CYP2D6 phenotypes associated with these different alleles include poor, intermediate, extensive, and ultrarapid metabolizers. Some of the most common and important variant alleles distributed in different ethnic groups are listed in Table 6-2, and all variant alleles are presented on the home page of the human CYP allele nomenclature committee ( se/cypalleles/cyp2d6.htm ). Carriers of any 2 of approximately 20 known null alleles are phenotypic poor metabolizers, representing 7% to 10% of the European and North American Caucasian population. One of the most important functionally altered null variants includes CYP2D6*4 (15% 21% in Caucasians). Alleles associated with reduced enzyme activity include CYP2D6*10 (up to 57% in Asians) ( 27 ), and CYP2D6*17 (20% 34% TABLE 6-2. CYP2D6 Common Variant Alleles and Distribution Among Ethnic Groups CYP2D6 Alleles Enzyme Activity Ethnicity CYP2D6*4 CYP2D6*10 CYP2D6*5 CYP2D6*17 CYP2D6*41 CYP2D6*1 CYP2D6*2N Allele associated with absent activity Allele associated with reduced activity Allele associated with absent activity Allele associated with reduced activity Allele associated with reduced activity Allele associated with normal enzyme activity Allele associated with increased enzyme activity Abbreviation: CYP2D6, cytochrome P-456 2D6. 15%-20% of Caucasians 4% of African Americans 57% of Asians 2%-3% of Caucasians 3% in Caucasians 6% in African Americans 20%-30% of African Americans 8%-10% of Caucasians 33%-43% of Asians 32% of African American 36% of Caucasians 22% of Caucasians 14% of African Americans Clinical Use of SERMs and AIs in Breast Cancer Treatment 149

12 in Africans and African Americans) ( 28 ). Individuals at the high end of the activity spectrum (ultrarapid metabolizers) carry gene duplications and multiduplications of functional alleles, which lead to higher CYP2D6 expression and enzyme activity, with relatively low frequency observed in Caucasians and Asians ( 29 ), but commonly observed in Ethiopians (up to 30%). Many studies have evaluated the role of genetic variation in CYP2D6 and other enzymes involved the biotransformation and conjugation of endoxifen ( 21 ). While multiple studies have demonstrated that reductions in CYP2D6 metabolism are associated with worse diseasefree survival (DFS) and a lower incidence of hot flashes in postmenopausal women treated with adjuvant tamoxifen ( ), many others have been negative ( ). This heterogeneity is well illustrated in the data recently presented from 3 large adjuvant clinical breast cancer trials. While no association of CYP2D6 genotype with breast cancer recurrence was observed in subset of patients enrolled in the Arimidex, Tamoxifen, Alone or in Combination (ATAC) and Breast International Group 1-98 (BIG 1-98) clinical trials ( 47, 48 ), CYP2D6 genotype was associated with an increase in the odds of breast cancer recurrence in a case-control analysis from the Austrian Breast and Colorectal Cancer Study Group (ABCSG) 8 trial ( 49 ). However, this latter observation was only observed in patients who received tamoxifen monotherapy, and not in those who received anastrozole following tamoxifen ( 49 ), suggesting that crossover to an active drug not metabolized by CYP2D6 (anastrozole) may affect the association between CYP2D6 genetic variation and tamoxifen clinical outcome. In the only study evaluating the role of steady-state endoxifen concentrations with clinical outcome, low endoxifen concentrations (<5.9 nm) were associated with a higher risk of recurrence ( 20 ). In the setting of tamoxifen chemoprevention, while there have been conflicting reports ( 50 ), the largest data set examined from the NSABP P1 and P2 tamoxifen prevention trials ( 51 ) demonstrated no association between CYP2D6 genotype, medication use, or CYP2D6 metabolizer status with occurrence of breast cancer ( 51 ). Polymorphism of UGT2B15, Sulfotransferase 1A1, and CYP2C19 Both tamoxifen and its active metabolites undergo conjugation by uridine diphosphate-glucuronosyltransferase (UDP-UGT) and sulfotransferase 1A1 (SULT1A1) ( 52 ). This process is thought to substantially reduce the activity 150 Translational Endocrinology & Metabolism: Breast Cancer Update

13 of these metabolites by reducing their ability to interact with the ER. While multiple studies have evaluated the role of polymorphisms in the conjugating enzymes ( 41, 53, 54 ), no consistent results have been demonstrated. This is most likely due to the fact that CYP2D6 is the sole ratelimiting enzyme in the formation of endoxifen, whereas there are multiple conjugating enzymes able to inactivate endoxifen. In summary, the biotransformation of tamoxifen is complex, with multiple enzymes contributing to the bioactivation of tamoxifen into hydroxylated metabolites with substantially greater antitumor activity, but highly variable in vivo concentrations. Since CYP2D6 genetic variation accounts for only 30% to 40% of the variability in endoxifen concentrations ( 19 ), additional research is needed to understand unexplained variability in endoxifen concentrations, especially given the recent data demonstrating the potential importance of steady-state endoxifen concentrations as it relates to tamoxifen efficacy ( 20 ). In the meantime, first-in-human studies are underway at both the Mayo Clinic (NCT ) and the National Cancer Institute (NCT ) using a novel formulation of endoxifen (endoxifen hydrochloride) for the treatment of hormonally responsive malignancies. Aromatase Inhibitors The third-generation AIs anastrozole, exemestane, and letrozole have been established to be of value for postmenopausal women with breast cancer in the advanced/metastatic, adjuvant, and high-risk (prevention) settings. An American Society of Clinical Oncology (ASCO) practice guideline has recommended the use of AIs at some point during adjuvant endocrine therapy either as initial therapy or following some period of tamoxifen therapy ( 3 ). The history of research into the aromatase enzyme and its inhibitors has recently been reviewed by 5 pioneers in the field ( 56 ). AI Pharmacology The AIs can be placed into 2 categories: steroidal as in the case of exemestane, and nonsteroidal as in the case of anastrozole and letrozole. The elimination half-life for exemestane is about 24 hours, but it is longer for both anastrozole and letrozole. Exemestane is metabolized extensively by CYP3A4, whereas letrozole is metabolized by CYP3A4 and CY2A6. Anastrozole is metabolized by N-dealkylation, hydroxylation, and glucuronidation. The pharmacology, in vitro aromatase inhibition, and in vivo aromatase Clinical Use of SERMs and AIs in Breast Cancer Treatment 151

14 inhibition have recently been reviewed ( 57 ). Exemestane irreversibly binds to the substrate-binding site of aromatase, whereas anastrozole and letrozole bind reversibly to the P450 site of the aromatase com plex. In general, the nonsteroidal AIs have greater potency both in vitro and in vivo, consistent with the requirement for higher doses of exemestane. However, at the clinically used doses, the mean percent inhibition of aromatization for the 3 drugs is very high, ranging from 96.7% for anastrozole to over 99.1% for letrozole ( 57 ). Clinical Trials of AIs as Adjuvant Therapy in Early Stage Breast Cancer The third-generation AIs anastrozole, exemestane and letrozole have become firmly established as important agents in adjuvant endocrine therapy for postmenopausal women with early stage ER+ breast cancer. The clinical trials that have provided the basis for this ASCO guideline and ongoing trials have recently been reviewed ( 58 ) and are summarized in Table 6-3. Two large trials, and BIG 1-98, have examined AIs as initial therapy in comparison with tamoxifen, with a meta-analysis revealing an absolute 2.9% decrease in breast cancer recurrence at 5 years that was statistically significant but not a significant improvement in breast cancer mortality ( 2 ). However, a recent update of the BIG 1-98 trial (median follow-up of 8.7 years) demonstrated a statistically significant improvement in DFS (HR 0.86, SE ) and overall survival (HR 0.87, SE ) in favor of the letrozole arm based on intention-to-treat analysis ( 59 ). Five clinical trials have examined the value of switching to an AI after several years of tamoxifen. These trials are the Intergroup Exemestane Study (IES), the ABCSG 8 trial, the German ARNO (Arimidex-Nolvadex) 95 trial, the Italian Tamoxifen Anastrozole (ITA) trial, and a trial involving Japanese women designated as N-SAS BC03. The first 4 of these trials were included in a meta-analysis ( 2 ) that revealed a statistically significant decrease in breast cancer recurrence (3.1%) and improvement in breast cancer mortality (0.7%) for those women who switched to an AI after several years of tamoxifen, compared with those women who took 5 years of tamoxifen. BIG 1-98 included a comparison of 2 different sequences of tamoxifen and letrozole with letrozole monotherapy. Neither the sequence of tamoxifen followed by letrozole nor the sequence of letrozole followed by tamoxifen was superior to monotherapy with letrozole ( 59 ). The Tamoxifen Exemestane Adjuvant Multinational (TEAM) trial involved a 152 Translational Endocrinology & Metabolism: Breast Cancer Update

15 randomization between tamoxifen for 2.5 to 3 years followed by a switch to exemestane versus 5 years of exemestane therapy. There was no significant difference between the 2 treatment arms in terms of DFS, time to recurrence, or overall survival ( 60 ). Several trials have examined the value of AIs after about 5 years of tamoxifen. The largest of these trials was MA.17, which was a placebo-controlled trial of letrozole in women who had received 5 years of tamoxifen therapy ( 61 ). This trial established the value of letrozole as extended adjuvant endocrine therapy, demonstrating a significantly better DFS and distant DFS. Overall survival (OS) was not significantly different between the letrozole and placebo arms. Two smaller studies also examined the use of AIs in the extended adjuvant setting, the NSABP B-33 trial ( 62 ) and ABCSG-6a ( 63 ), and found similar benefit for the use of AIs in this setting. In the premenopausal setting, ABCSG trial 12 randomized premenopausal women treated with goserelin to either tamoxifen or anastrozole (and also with or without zoledronic acid) and reported no difference between tamoxifen or anastrozole with regard to DFS ( 64 ). However, a recent update now demonstrates significantly worse overall survival for the anastrozole arm ( 65 ). One potential reason for this discrepancy has been hypothesized to be related to body mass index ( 66 ). Overweight patients treated with anastrozole had a 60% increase in the risk of disease recurrence (HR 1.60, 95% CI , P =0.02) and more than a doubling in the risk of death (HR 2.14, 95% CI , P =0.01) compared with normal weight patients treated with anastrozole. In the overweight group, patients treated with anastrozole had a nearly 50% increase in the risk of disease recurrence (HR 1.49, 95% CI , P =0.08) and a 3-fold increase in the risk of death (HR 3.03, 95% CI , P =0.004) compared with patients treated with tamoxifen ( 66 ). Finally, Suppression of Ovarian Function Trial (SOFT) and Tamoxifen and Exemestane Trial (TEXT) are 2 important ongoing trials that will provide additional information on the value of AIs in premenopausal women treated with ovarian function suppression. As noted in Table 6-3, 3 clinical trials are examining the duration of AI therapy, and 1 is examining the value of intermittent AI therapy. Results, however, are still pending from these trials. AIs in Chemoprevention of Breast Cancer The first report of an AI as treatment for prevention of breast cancer was recently published ( 67 ). Exemestane was evaluated in a randomized, double-blinded, placebo-controlled trial for breast cancer prevention in Clinical Use of SERMs and AIs in Breast Cancer Treatment 153

16 TABLE 6-3. Trials of AIs as Adjuvant Therapy in Women with Early Breast Cancer 1. Studies of aromatase inhibitors as initial therapy a. In postmenopausal women ATAC: tamoxifen vs anastrozole vs combination BIG 1-98: tamoxifen vs letrozole MA.27: anastrozole vs exemestane FACE: anastrozole vs letrozole b. In premenopausal women ABCSG-12: goserelin plus tamoxifen or anastrozole, each with or without zoledronic acid SOFT: tamoxifen vs ovarian function suppression (OFS) + tamoxifen vs OFS + exemestane TEXT: OFS + tamoxifen vs OFS + exemestane 2. Studies of sequential use of aromatase inhibitors after <5 years (usually 2-3 years) of tamoxifen vs tamoxifen for a total of 5 years of therapy IES: after 2-3 years of tamoxifen: tamoxifen vs exemestane ABCSG-8: tamoxifen (5 years) vs tamoxifen (2 years) then anastrozole (3 years) ITA: after 2-3 years of tamoxifen): tamoxifen vs anastrozole ARNO 95: after 2 years of tamoxifen: tamoxifen vs anastrozole NSAS BC-03: after 1-4 years of tamoxifen: tamoxifen vs exemestane 3. Studies of sequencing of aromatase inhibitors and tamoxifen vs aromatase monotherapy for a total of 5 years of therapy BIG 1-98: tamoxifen to letrozole and letrozole to tamoxifen, both compared with letrozole monotherapy TEAM: tamoxifen (2.5-3 years) then exemestane vs exemestane monotherapy 4. Studies of aromatase inhibitors after about 5 years of tamoxifen MA.17: letrozole vs placebo (both for 5 years) NSABP B33: exemestane vs placebo (both for 5 years) ABCSG-6a: anastrozole (3 years) vs observation LATER *: letrozole vs placebo (both for 5 years) 5. Studies of duration of aromatase inhibitor therapy (after some period of aromatase inhibitor therapy) MA.17R: letrozole vs placebo (both for 5 years) continued 154 Translational Endocrinology & Metabolism: Breast Cancer Update

17 TABLE 6-3. (Continued) NSABP B42: exemestane vs placebo (both for 5 years) ABCSG-16: anastrozole (3 years) vs anastrozole (5 years) 6. Study of intermittent aromatase inhibitor therapy SOLE: Arm: continuous letrozole vs intermittent letrozole *Prior endocrine therapy may have included an aromatase inhibitor. postmenopausal women (MAP.3 trial). The trial was a placebo control trial, because some women were not interested in taking tamoxifen despite its proven ability to decrease breast cancer in high-risk women. The trial was designed to detect a 65% difference in the incidence of invasive breast cancer, an amount based on the results showing superiority of AIs, when compared with tamoxifen, in decreasing contralateral breast cancers. The trial included postmenopausal women with at least 1 risk factor (prior history of ductal carcinoma in situ, atypical ductal hyperplasia, or lobular carcinoma in situ; age more than 60; Gail risk score more than 1.66%). The study included 4560 women with a median follow-up of 35 months. There was a 65% relative risk reduction in breast cancer in the exemestane arm 11 events in the exemestane arm versus 32 events in the placebo arm (HR 0.35, 95%CI , P =0.002) (summarized in Table 6-1 ) Exemestane was well tolerated, with minimal difference in quality of life. However, in a nested substudy of the MAP.3 trial that included nonosteoporotic postmenopausal women from 5 centers (351 women), 2 years of exemestane significantly accelerated age-related bone loss. Therefore, bone density should be regularly monitored in women receiving exemestane for the chemoprevention of breast cancer ( 68 ). Pharmacogenomics of AIs Ma et al ( 69 ) sequenced the aromatase gene in 60 patients from each of 4 ethnic groups (Caucasian Americans, African Americans, Chinese Americans, and Mexican Americans) and found 88 polymorphisms resulting in 44 haplotypes. Functional genomic studies were performed with 4 nonsynonymous coding single nucleotide polymorphisms (csnps) that were identified, and a significant correlation between level of activity and immunoreactive protein was identified. Three of the 4 csnps had levels Clinical Use of SERMs and AIs in Breast Cancer Treatment 155

18 of immunoreactive protein that was significantly lower than wild-type aromatase enzyme. The finding of substantial genetic variability in the aromatase gene within ethnic groups and substantial variation between the 4 ethnic groups provide a strong impetus to explore the pharmacogenomics of AIs. Although the AIs have demonstrated clear efficacy, many patients experience a recurrence of their cancer, and there is substantial variability in terms of tolerability, particularly with respect to certain adverse events such as musculoskeletal complaints, which can be so severe that patients will withdraw from therapy. This variability is possibly consistent with pharmacogenomic differences between patients that, if understood, would offer the potential for individualizing therapy and ensuring that patients would be able to receive optimal therapy. Considering anastrozole, Partridge et al ( 70 ) examined adherence, defined as the proportion of days patients had medications available over an observation period, to this agent when given as adjuvant therapy for early breast cancer. Patients who adhered to medications on fewer than 80% of days were considered nonadherent. The mean adherence ranged from 82% to 88% in year 1 and decreased to 62% to 79% in year 3, emphasizing the substantial problem associated with poor adherence. Contributing to this poor adherence, most certainly, are musculoskeletal adverse events (MSAEs), which can occur in about half of patients treated with AIs ( 71 ). Addressing this MSAE issue, the first major pharmacogenomic study in women receiving AIs was conducted in women receiving anastrozole as adjuvant therapy for early breast cancer. This case-control genomewide association study (GWAS) was performed utilized MSAEs as the phenotype identified in the Breast Intergroup MA.27 adjuvant therapy trial ( 72 ). This study involved 293 patients with MSAEs and 585 controls who did not have this side effect. This GWAS identified SNPs associated with MSAEs and with a gene ( TCL1A ). Notably, extensive functional genomic studies that were conducted in the context of this study revealed that 1 SNP (rs ) created an estrogen response element and that TCL1A expression was estrogen dependent, was associated with the variant SNP genotypes in estradiol-treated lymphoblastoid cells transfected with estrogen receptor alpha and was directly related to interleukin 17 receptor A (IL-17RA) expression. The novel methodology employed in using functional genomic studies along with the GWAS prompted an accompanying editorial to state that this study established a new pharmacogenomic paradigm in breast cancer treatment ( 73 ). 156 Translational Endocrinology & Metabolism: Breast Cancer Update

19 A more recent cross-sectional study in 390 Caucasian patients examined polymorphisms in CYP191A ( 74 ). They found women carrying at least 1 8-repeat allele had a significantly lower risk (adjusted odds ratio [OR] 0.41, p =0.008) of AI-associated arthralgias and concluded that this supports the hypothesis that the host hormonal environment contributes to the pathophysiology of the arthralgias. Adverse Events of SERMs and AIs The toxicity of AIs compared to tamoxifen has been extensively evaluated and recently updated in a meta-analysis ( 75 ). Table 6-4 reviews HR and number needed to harm (NNH) to develop 1 event, comparing tamoxifen to AIs. The major toxicities are described. Cardiovascular Events Longer durations of AI use are associated with an increased odds of cardiovascular disease compared with tamoxifen. Combined analysis of the 2 trials that evaluated up-front AI versus up-front tamoxifen (ATAC and BIG 1-98) demonstrated a statistically significant association between AI use and cardiovascular disease (OR 1.30, 95% CI , P =0.01). Combined analysis of trials that evaluated switching versus up-front tamoxifen demonstrated a nonstatistically significant association between TABLE 6-4. Adverse Events of Tamoxifen and AIs Tamoxifen AIs HR/OR NNH HR/OR NNH Death without recurrence NA NA Cardiovascular events (grade 3 and 4) NA Endometrial cancer NA VTE NA Cerebrovascular disease NA NA Fractures NA Hypercholesterolemia NA 2.36 Cataracts NA Abbreviations: AI, aromatase inhibitors; NNH, number needed to harm to cause 1 event; HR, hazards ratio; NA, not applicable/not significant; OR, odds ratio; VTE, venous thromboembolism. Clinical Use of SERMs and AIs in Breast Cancer Treatment 157

20 AI use and cardiovascular disease (OR 1.15, 95% CI , P =0.20). With regard to raloxifene, there were no increased risks for cardiovascular events compared to placebo in the RUTH trial ( 10 ). Endometrial Cancer In a meta-analysis of the tamoxifen prevention trials ( 76 ), tamoxifen use was associated with more than doubling on the risk of endometrial cancer (RR 2.4, P =0.0005). Most uterine cancers were stage 1 adenocarcinomas. Clinical trials comparing AIs to tamoxifen consistently demonstrated a reduction in endometrial cancer risk (66% risk reduction in a pooled analysis of adjuvant clinical trials, 0.4% absolute risk reduction). There was no increased risk of uterine cancer associated with raloxifene use in the CORE ( 9 ), MORE ( 8 ), or RUSH ( 10 ) trials. In a recent update from the STAR ( 11 ), there was a statistically significant decrease in the risk of uterine carcinoma comparing raloxifene to tamoxifen (RR 0.55, 95% CI , P =0.003). An important consideration with regard to the development of uterine carcinoma appears to be age. For example, in the NSABP P1 prevention trials, there was no increased incidence of uterine carcinoma in premenopausal women ( 5 ). This finding was recently confirmed in the updated Oxford overview ( 1 ), which demonstrated that the uterine cancer risk was strongly correlated with age, with little absolute risk for entry age younger than 45 years or 45 to 54 years. However, for entry age 55 to 69 years, the 5-year incidence was 3.8% in the tamoxifen group versus 1.1% in the control group (absolute increase 2.6% SE 0.6, 95% CI ). Therefore, it is critical that premenopausal women should be counseled that there is no increased risk for uterine carcinoma. The current recommendations in the United States are for baseline gynecological examination before starting tamoxifen and annually thereafter. Routine endometrial biopsies or other imaging modalities are not recommended. Venous Thromboembolism In a meta-analysis of the tamoxifen prevention trials ( 76 ), there was 1.9-fold increased risk of venous thromboembolism (VTE) compared to placebo (HR 1.9, P ). In the IBIS-I trial ( 6 ), surgery and immobilization were independent factors associated with increased risk for thrombosis. A meta-analysis of adjuvant hormonal trials in early stage breast cancer comparing AIs to tamoxifen showed 45% reduced risks of VTE with AIs compared to tamoxifen (HR 0.55, P =0.001) ( 75 ). As with endometrial 158 Translational Endocrinology & Metabolism: Breast Cancer Update

21 carcinoma, age appears to be an important predictor of venous thrombotic events. In the NSABP P1 clinical trial, for women aged 49 years or younger, the number of deep venous thrombosis (DVT) cases was 8 in the placebo group versus 11 in the tamoxifen group. However, in women 50 years of age or older, the number of cases was 14 versus 24, with an RR of 1.71 (95% CI ). When comparing SERMs, a recent update of the STAR trial demonstrated a significantly decreased risk of VTE events in favor of raloxifene (HR 0.75, 95% CI, ) ( 11 ). Cerebrovascular Events There was no significant difference in risks of cerebrovascular events between tamoxifen and AIs in the meta-analysis of adjuvant hormonal trials in early breast cancer ( 75 ). However, there was increased risk of cerebrovascular events compared to placebo in the tamoxifen prevention trials. In a metaanalysis of 9 tamoxifen randomized trials (4 prevention trials and 5 adjuvant trials), tamoxifen was associated with increased risk of strokes (HR 1.82) ( 77 ). The apparent discrepancy may relate to the type of cerebral vascular accident (CVA) that is observed. Notably, while tamoxifen may increase the risk of thromboembolic stroke, the observation of no difference comparing AIs with tamoxifen is interesting, and suggests that there may be dif ferences with regard to the type of stroke observed (ischemic versus thromboembolic). In the STAR trial, comparing tamoxifen and raloxifene, there was no increase in the incidence of stroke or transient ischemia attack ( 78 ). Fractures A potential benefit of tamoxifen and raloxifene relates to bone health. In the NSABP-P1 ( 79 ) prevention study, tamoxifen was associated with a 32% risk reduction in hip, spine, and radius fractures. However, compared to tamoxifen, AIs were associated with a 47% increase in risk of fractures (HR 1.47, P <0.001) in the recent meta-analysis of adjuvant hormonal trials ( 75 ). Although both tamoxifen and raloxifene are associated with reductions in fractures (compared to placebo), there were no differences in bone protection comparing these 2 drugs in the STAR trial ( 78 ). Dyslipidemia AIs are associated with a statistically significant increase in risk of hypercholesterolemia when compared to tamoxifen. In a pooled analysis of 4 adjuvant studies where dyslipidemia was assessed, there was more than a Clinical Use of SERMs and AIs in Breast Cancer Treatment 159

22 2-fold increase (HR 2.36, P <0.001) ( 75 ). The risk appeared to be more pronounced when AIs were used as initial therapy. Cataracts In the NSABP-P1 ( 79 ) tamoxifen prevention trial, cataracts and cataract surgery were reported in 3% of women on tamoxifen versus 1.5% in women on placebo (HR 1.57). The absolute risk increase was 14 cases per 1000 women on tamoxifen. In the ATAC trial ( 80 ), cataracts were reported in 7% in women on tamoxifen versus 6% in women on aromatase inhibitors. Notably, comparing SERMs, cataracts appear to be a tamoxifenspecific side effect. In an updated analysis of the STAR trial, the rate of cataract development (RR 0.80, 95% CI ) and the rate of cataract surgery (RR 0.79, 95% CI ) are about 20% less in the raloxifene group compared to the tamoxifen group ( 11 ). Vasomotor Symptoms In the NSABP -P1 ( 79 ) tamoxifen prevention trial, hot flashes were reported in 78% of women on tamoxifen. Similar findings were noted in the other tamoxifen prevention trials. When comparing tamoxifen with anastrozole, more vasomotor symptoms were reported with tamoxifen compared to anastrozole (39.7% vs 34.3%, p < ( 80 ). Interestingly, a retrospective analysis of the ATAC trial ( 81 ) as well as the WHEL trial ( 82 ) suggested that the presence of vasomotor symptoms was associated with a lower incidence of breast cancer events compared to women who did not report such symptoms. Musculoskeletal Side Effects As previously noted, musculoskeletal events are the most common adverse events associated with AIs. Arthralgia was reported in 15% of patients taking anastrozole in the ATAC trial ( 80 ), compared to 11% in patients on tamoxifen. Arthritis was reported in 17% of patients on anastrozole. Overall, musculoskeletal events (arthralgia, arthritis, backache, bone pain, or carpel tunnel syndrome) were reported in 36% of patients on AIs compared to 29% of patients on tamoxifen. Similar to the associations between vasomotor symptoms and hormonal efficacy, the presence of MSAE was associated with a lower incidence of disease recurrence in both the tamoxifen and anastrozole-treated patients in the ATAC trial ( 81 ). 160 Translational Endocrinology & Metabolism: Breast Cancer Update

23 Breast Cancer in Men In contrast to breast cancer in women, breast cancer in men is rare. Approximately, 2140 new cases are diagnosed annually, and 450 deaths occur ( 83 ). This accounts for less than 0.5% of cancer deaths. The median age of onset is 65 years. Identifiable risk factors include gynecomastia, cryptorchidism, chronic liver disease, and Klinefelter syndrome ( 84 ). Patients with Klinefelter syndrome have a 20-fold increase in the incidence of breast cancer and a significant increase in mortality. Inherited mutations in BRCA also significantly increase the risk of male breast cancer, and surveillance programs are recommended for male BRCA carriers. Furthermore, testing for BRCA is recommended in a new diagnosis of male breast cancer. The most common histological type in male breast cancer is invasive ductal carcinoma (90%). About 90% of cases express ERs, and more than 80% express PRs ( 85 ). HER-2 over-expression is reported in about 10% of cases in multiple series ( 86 ). Male breast cancer is treated in a similar way to female breast cancer. Modified radical mastectomy is the most commonly used procedure. However a recent analysis of the surveillance, epidemiology and end results (SEER) database showed that around 20% of male breast cancer was treated with breast conservation. While adjuvant tamoxifen has not been evaluated in randomized trials, data from retrospective studies showed survival benefits, compared to matched controls. In one report of 38 patients with male breast cancer treated with tamoxifen, the 5-year overall survival improved from 44% to 61% ( 87 ). Whereas data are limited for AIs in men with breast cancer, there are case reports indicating benefit, and AIs are commonly used for the treatment of metastatic disease following disease progression on tamoxifen. Summary Both SERMs and AIs have proven to be effective for both the prevention and treatment of breast cancer. Notably, the adverse effect profile is substantially different comparing these 2 classes of drugs. These differences in toxicity can be used to individualize therapy with the knowledge that the slight improvement in breast cancer outcomes (comparing up-front AI use versus tamoxifen) has not yet yielded a difference in breast cancer mortality or survival. Emerging data regarding the pharmacogenomic variability may lead to the ability to select the best drug as it relates to efficacy and adverse effects. Clinical Use of SERMs and AIs in Breast Cancer Treatment 161