Population Pharmacokinetics (PK) and Pharmacodynamics (PD) Analysis of LY3015014, a Monoclonal Antibody to Protein Convertase Subtilisin/Kexin Type 9 (PCSK9) in Healthy Subjects and Hypercholesterolemia Patients.

PURPOSE: LY3015014 is a humanized immunoglobulin G4 (IgG4) monoclonal antibody that binds to the catalytic domain of PCSK9 and reduce low-density lipoprotein cholesterol (LDL-C) in patients with hypercholesterolemia that is poorly controlled by maximally tolerated statin therapy. The objective of this pharmacokinetic/pharmacodynamics (PK/PD) analysis was to characterize the PK and PD properties of LY3015014 and assess the effect of covariates on the LY3015014 PK-PD profiles. METHODS: Single and multiple dose data from three phase1 studies in healthy subjects (n = 133), as well as a phase 2 study in hypercholesterolemia patients (n = 527) were combined into a single dataset for analysis. In this dataset, healthy subjects received single intravenous (IV) doses of 0.03 to 10 mg/kg, or multiple subcutaneous (SC) doses of 1.0 to 3.0 mg/kg, administered every 2 to 4 weeks, while patients received 20 to 300 mg every 4 weeks or 100 to 300 every 8 weeks. PK/PD analysis was performed using NONMEM (ICON, software version 7.0 level 3). PK and PD modeling were conducted sequentially, with PK parameters fixed during the development of the PK/PD model. PD parameters and estimated intersubject and intrasubject variability were obtained based on pharmacological drug exposure-response relationships. Age, baseline total PCSK9, body weight, diabetes diagnosis, hypercholesterolemia disease status, dose, ezetimibe administration, gender, ethnic origin, metabolic syndrome, and satin administration were evaluated as potential covariates in the PK model. Baseline total PCSK9, baseline LDL-C, diabetes diagnosis, disease status, ezetimibe administration, gender, ethnic origin, metabolic syndrome, and Statin administration were evaluated as potential covariates in the PD model. RESULTS: LY3015014 PK profile was consistent across all the studies and between healthy subjects and hypercholesterolemia patients. The PK time course data were well described by a two compartment PK model with first order absorption, and covariates identified for PK parameters included weight on both clearance (CL) and central volume (V2), dose on CL, race on bioavailability (F), and age on V2. The PD (LDL-C) was described using an indirect response model with LY3015014 acting to stimulate the elimination of LDL-C. Covariates identified to have a statistically significant impact on PD were coadministration of statins, baseline LDL-C, metabolic syndrome status and gender. CONCLUSIONS: The population PK/PD model adequately describes the PK and PD profiles of LY3015014. Identification of clinically significant covariates will support the design and dose selection for the pivotal registration studies, ensuring that patients are dosed appropriately.

Evacetrapib alone or in combination with statins lowers lipoprotein(a) and total and small LDL particle concentrations in mildly hypercholesterolemic patients.

BACKGROUND: Potent CETP inhibitors reduce plasma concentrations of atherogenic lipoprotein biomarkers of cardiovascular risk. OBJECTIVES: To evaluate the effects of the cholesteryl ester transfer protein (CETP) inhibitor evacetrapib, as monotherapy or with statins, on atherogenic apolipoprotein B (apoB)-containing lipoproteins in mildly hypercholesterolemic patients. METHODS: VLDL and LDL particle concentrations and sizes (using nuclear magnetic resonance spectroscopy) and lipoprotein(a) concentration (using nephelometry) were measured at baseline and week 12 in a placebo-controlled trial of 393 patients treated with evacetrapib as monotherapy (30 mg/d, 100 mg/d, or 500 mg/d) or in combination with statins (100 mg plus simvastatin 40 mg/d, atorvastatin 20 mg/d, or rosuvastatin 10 mg/d; Clinicaltrials.gov Identifier: NCT01105975). RESULTS: Evacetrapib monotherapy resulted in significant placebo-adjusted dose-dependent decreases from baseline in Lp(a) (up to -40% with evacetrapib 500 mg), total LDL particle (LDL-P) (up to -54%), and small LDL particle (sLDL) (up to -95%) concentrations. Compared to statin alone, coadministration of evacetrapib and statins also resulted in significant reduction from baseline in Lp(a) (-31%), LDL-P (-22%), and sLDL (-60%) concentrations. The percentage of patients with concentrations above optimal concentrations for LDL-P (>1000 nmol/L) and sLDL (>600 nmol/L) decreased from 88% and 55% at baseline, respectively, to 20% and 12% at week 12, for patients treated with evacetrapib plus statins. Evacetrapib, alone or with statins, significantly increased LDL-P size. CONCLUSIONS: Evacetrapib, as monotherapy or with statins, significantly reduces the concentrations of atherogenic apoB-containing lipoproteins, including Lp(a), LDL-P, and sLDL.

Safety and efficacy of LY3015014, a monoclonal antibody to proprotein convertase subtilisin/kexin type 9 (PCSK9): a randomized, placebo-controlled Phase 2 study.

AIMS: The objective of this study was to evaluate the efficacy, safety, and tolerability of LY3015014 (LY), a neutralizing antibody of proprotein convertase subtilisin/kexin type 9 (PCSK9), administered every 4 or 8 weeks in patients with primary hypercholesterolaemia, when added to a background of standard-of-care lipid-lowering therapy, including statins. METHODS AND RESULTS: Double-blind, placebo-controlled trial randomized 527 patients with primary hypercholesterolaemia from June 2013 to January 2014 at 61 community and academic centres in North America, Europe, and Japan. Patients were randomized to subcutaneous injections of LY 20, 120, or 300 mg every 4 weeks (Q4W); 100 or 300 mg every 8 weeks (Q8W) alternating with placebo Q4W; or placebo Q4W. The primary endpoint was percentage change from baseline in low-density lipoprotein cholesterol (LDL-C) by beta quantification at Week 16. The mean baseline LDL-C by beta quantification was 136.3 (SD, 45.0)mg/dL. LY3015014 dose-dependently decreased LDL-C, with a maximal reduction of 50.5% with 300 mg LY Q4W and 37.1% with 300 mg LY Q8W compared with a 7.6% increase with placebo maintained at the end of the dosing interval. There were no treatment-related serious adverse events (AEs). The most common AE terms (>10% of any treatment group) reported more frequently with LY compared with placebo were injection site (IS) pain and IS erythema. No liver or muscle safety issues emerged. CONCLUSIONS: LY3015014 dosed every 4 or 8 weeks, resulted in robust and durable reductions in LDL-C. No clinically relevant safety issues emerged with the administration of LY. The long-term effects on cardiovascular outcomes require further investigation.

Cholesterol Efflux Capacity and Pre-Beta-1 HDL Concentrations Are Increased in Dyslipidemic Patients Treated With Evacetrapib.

BACKGROUND: Potent cholesteryl ester transfer protein (CETP) inhibitors have been shown to substantially increase high-density lipoprotein cholesterol (HDL-C) and apolipoprotein A-I levels as monotherapy and combined with statins. However, data on the effects of this class of drugs on macrophage cholesterol efflux capacity (CEC), a functional assay that characterizes a key step in the process of reverse cholesterol transport, are limited. OBJECTIVES: This study assessed the impact of evacetrapib, statins, or combination therapy on CEC. METHODS: We analyzed samples from 377 subjects with elevated low-density lipoprotein cholesterol (LDL-C) or low HDL-C levels who were enrolled in a phase 2 trial of evacetrapib. Percent changes from baseline in CEC (total, non-ABCA1-, and ABCA1-specific) and HDL subpopulations were evaluated after 12 weeks of treatment with placebo, statin monotherapy, evacetrapib monotherapy, or evacetrapib combined with statins. Pre-beta-1 HDL levels were quantified by immunofixation and nondenaturing 2-dimensional gel electrophoresis (2DGE). RESULTS: Relative to placebo, evacetrapib monotherapy increased dose-dependent total and non-ABCA1-specific CEC up to 34% and 47%, respectively. Evacetrapib monotherapy also increased ABCA1-specific CEC up to 26%. Relative to statin monotherapy, evacetrapib with statins also increased total, non-ABCA1-, and ABCA1-specific CEC by 21%, 27%, and 15%, respectively. In contrast, rosuvastatin and simvastatin significantly reduced total and ABCA1-specific CEC, whereas atorvastatin had no significant effect. Consistent with ABCA1-specific CEC, evacetrapib monotherapy and evacetrapib combined with statins significantly increased pre-beta-1 HDL levels as measured by either method. CONCLUSIONS: Evacetrapib, as monotherapy and combined with statins, not only increased total CEC, but also increased ABCA1-specific CEC and pre-beta-1 HDL. The mechanisms by which potent CETP inhibition increases ABCA1-specific CEC and pre-beta-1 HDL require further study. (A Study of LY2484595 in Patients With High LDL-C or Low HDL-C; NCT01105975).

CYP-mediated drug-drug interactions with evacetrapib, an investigational CETP inhibitor: in vitro prediction and clinical outcome.

AIMS: Evacetrapib is a cholesteryl ester transfer protein (CETP) inhibitor under development for reducing cardiovascular events in patients with high risk vascular disease. CETP inhibitors are likely to be utilized as 'add-on' therapy to statins in patients receiving concomitant medications, so the potential for evacetrapib to cause clinically important drug-drug interactions (DDIs) with cytochromes P450 (CYP) was evaluated. METHODS: The DDI potential of evacetrapib was investigated in vitro, followed by predictions to determine clinical relevance. Potential DDIs with possible clinical implications were then investigated in the clinic. RESULTS: In vitro, evacetrapib inhibited all of the major CYPs, with inhibition constants (K(i)) ranging from 0.57 µM (CYP2C9) to 7.6 µM (CYP2C19). Evacetrapib was a time-dependent inhibitor and inducer of CYP3A. The effects of evacetrapib on CYP3A and CYP2C9 were assessed in a phase 1 study using midazolam and tolbutamide as probe substrates, respectively. After 14 days of daily dosing with evacetrapib (100 or 300 mg), midazolam exposures (AUC) changed by factors (95% CI) of 1.19 (1.06, 1.33) and 1.44 (1.28, 1.62), respectively. Tolbutamide exposures (AUC) changed by factors of 0.85 (0.77, 0.94) and 1.06 (0.95, 1.18), respectively. In a phase 2 study, evacetrapib 100 mg had minimal impact on AUC of co-administered simvastatin vs. simvastatin alone with a ratio of 1.25 (1.03, 1.53) at steady-state, with no differences in reported hepatic or muscular adverse events. CONCLUSIONS: Taken together, the extent of CYP-mediated DDI with the potential clinical dose of evacetrapib is weak and clinically important DDIs are not expected to occur in patients taking concomitant medications.

Effects of the cholesteryl ester transfer protein inhibitor evacetrapib on lipoproteins, apolipoproteins and 24-h ambulatory blood pressure in healthy adults.

OBJECTIVES: We investigated the safety, tolerability, pharmacokinetics and pharmacodynamics of evacetrapib. METHODS: Healthy volunteers received multiple daily doses of evacetrapib (10-600 mg) administered for up to 15 days in a placebo-controlled study. KEY FINDINGS: Mean peak plasma concentrations of evacetrapib occurred at 4-6 h and terminal half-life ranged 24-44 h. Steady state was achieved at approximately 10 days; all subjects had undetectable levels of evacetrapib 3 weeks after their last dose. The trough inhibition of cholesteryl ester transfer protein (CETP) activity was 65 and 84% at 100 and 300 mg, respectively. At the highest dose (600 mg), evacetrapib significantly inhibited CETP activity (91%), increased HDL-C (87%) and apo AI (42%), and decreased LDL-C (29%) and apo B (26%) relative to placebo. For the highest dose tested, levels of evacetrapib, CETP activity, CETP mass, HDL-C and LDL-C returned to levels at or near baseline after a 2-week washout period. Evacetrapib at the highest dose tested did not produce any significant effect on 24-h ambulatory systolic or diastolic blood pressure. CONCLUSIONS: Multiple doses of evacetrapib potently inhibited CETP activity, leading to substantial elevations in HDL-C and lowering of LDL-C. Evacetrapib was devoid of clinically relevant effects on blood pressure and mineralocorticoid levels.

Efficacy, safety, tolerability, and pharmacokinetic profile of evacetrapib administered as monotherapy or in combination with atorvastatin in Japanese patients with dyslipidemia.

The cholesteryl ester transfer protein (CETP) inhibitor evacetrapib has been previously shown to increase high-density lipoprotein cholesterol (HDL-C) and decrease low-density lipoprotein cholesterol (LDL-C) levels, as monotherapy or in combination with statins. In this study, 165 Japanese patients with elevated LDL-C or low HDL-C levels were randomly assigned to receive placebo, evacetrapib monotherapy 30 mg, 100 mg, or 500 mg, atorvastatin 10 mg, or evacetrapib 100 mg in combination with atorvastatin 10 mg. After 12 weeks, evacetrapib monotherapy increased HDL-C levels by 74%, 115%, and 136% and decreased LDL-C levels by 15%, 23%, and 22% and CETP activity by 50%, 83%, and 95% (for the 30-mg, 100-mg, and 500-mg dose groups, respectively) versus placebo. In combination with atorvastatin 10 mg, evacetrapib 100 mg increased HDL-C levels by 103% and decreased LDL-C levels by 15% and CETP activity by 68% versus atorvastatin alone. After a 4- to 6-week washout, HDL-C, LDL-C, and CETP mass and activity returned to baseline levels in the evacetrapib-treated groups, and most patients had evacetrapib concentrations below the quantitation limit. Evacetrapib monotherapy or in combination with atorvastatin was not likely to be associated with any significant change in blood pressure and did not have any adverse effects on mineralocorticoid or glucocorticoid measures. Notably, plasma evacetrapib concentrations were mostly undetectable, and all pharmacodynamic biomarkers (HDL-C and LDL-C levels and CETP mass and activity) returned to baseline after a 4- to 6-week washout. In conclusion, evacetrapib as monotherapy or in combination with atorvastatin effectively decreased CETP activity and LDL-C levels and increased HDL-C levels after 12 weeks in Japanese patients with dyslipidemia.

Evacetrapib at a supratherapeutic steady state concentration does not prolong QT in a thorough QT/QTc study in healthy participants.

PURPOSE: To evaluate whether evacetrapib prolongs QT intervals in healthy participants. METHODS: This was a single-center, randomized, active and placebo-controlled, 3-period, 6-sequence, and crossover study. Participants were randomized to 1 of 6 treatment sequences in which they received 1 of 3 treatments: evacetrapib 1200 mg daily for 10 days (supratherapeutic dose), moxifloxacin 400 mg for 1 day (positive control), or placebo for 10 days in each of the 3 separate treatment periods. Electrocardiographic parameters were recorded at time points specified in the protocol. The primary end point was the comparison of evacetrapib effect on the population-corrected QT interval (QTcP) to that of placebo at 7 time points following dosing on day 10. An upper limit of the 2-sided 90% confidence interval (CI) <10 milliseconds confirmed the absence of significant effect. Pharmacokinetic parameters were also calculated. RESULTS: Subjects were predominantly male (73.2%) with a mean age of 43.1 years and a mean body mass index of 25.9 kg/m(2). For the primary analysis, the upper bound of the 2-sided 90% CI for the mean difference between evacetrapib and placebo was <10 milliseconds at all time points on day 10. Following administration of moxifloxacin, the QTcP increased by ≥5 milliseconds at all time points (2, 3, and 4 hours postdose). Maximum plasma concentrations of evacetrapib occurred at a median time of approximately 2 hours, and the mean apparent elimination half-life was approximately 41 hours. The area under the curve and Cmax achieved in this study were both ∼5-fold the values that are expected with the dose level being studied in a phase 3 cardiovascular outcome study. A 1200-mg supratherapeutic dose of evacetrapib was considered to be well tolerated after 10 days of daily dosing in healthy participants. CONCLUSIONS: Evacetrapib is not associated with QT interval prolongation, even at supratherapeutic doses.

Effects of the CETP inhibitor evacetrapib administered as monotherapy or in combination with statins on HDL and LDL cholesterol: a randomized controlled trial.

CONTEXT: Interest remains high in cholesteryl ester transfer protein (CETP) inhibitors as cardioprotective agents. Few studies have documented the efficacy and safety of CETP inhibitors in combination with commonly used statins. OBJECTIVE: To examine the biochemical effects, safety, and tolerability of evacetrapib, as monotherapy and in combination with statins, in patients with dyslipidemia. DESIGN, SETTING, AND PARTICIPANTS: Randomized controlled trial conducted among 398 patients with elevated low-density lipoprotein cholesterol (LDL-C) or low high-density lipoprotein cholesterol (HDL-C) levels from April 2010 to January 2011 at community and academic centers in the United States and Europe. INTERVENTIONS: Following dietary lead-in, patients were randomly assigned to receive placebo (n = 38); evacetrapib monotherapy, 30 mg/d (n = 40), 100 mg/d (n = 39), or 500 mg/d (n = 42); or statin therapy (n = 239) (simvastatin, 40 mg/d; atorvastatin, 20 mg/d; or rosuvastatin, 10 mg/d) with or without evacetrapib, 100 mg/d, for 12 weeks. MAIN OUTCOME MEASURES: The co-primary end points were percentage changes from baseline in HDL-C and LDL-C after 12 weeks of treatment. RESULTS: The mean baseline HDL-C level was 55.1 (SD, 15.3) mg/dL and the mean baseline LDL-C level was 144.3 (SD, 26.6) mg/dL. As monotherapy, evacetrapib produced dose-dependent increases in HDL-C of 30.0 to 66.0 mg/dL (53.6% to 128.8%) compared with a decrease with placebo of -0.7 mg/dL (-3.0%; P < .001 for all compared with placebo) and decreases in LDL-C of -20.5 to -51.4 mg/dL (-13.6% to -35.9%) compared with an increase with placebo of 7.2 mg/dL (3.9%; P < .001 for all compared with placebo). In combination with statin therapy, evacetrapib, 100 mg/d, produced increases in HDL-C of 42.1 to 50.5 mg/dL (78.5% to 88.5%; P < .001 for all compared with statin monotherapy) and decreases in LDL-C of -67.1 to -75.8 mg/dL (-11.2% to -13.9%; P < .001 for all compared with statin monotherapy). Compared with evacetrapib monotherapy, the combination of statins and evacetrapib resulted in greater reductions in LDL-C (P <.001) but no greater increase in HDL-C (P =.39). Although the study was underpowered, no adverse effects were observed. CONCLUSIONS: Compared with placebo or statin monotherapy, evacetrapib as monotherapy or in combination with statins increased HDL-C levels and decreased LDL-C levels. The effects on cardiovascular outcomes require further investigation. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT01105975.

The metabolic syndrome and development of cognitive impairment among older women.

BACKGROUND: Several studies support a role for cardiovascular risk factors in cognitive aging. The metabolic syndrome, a constellation of cardiovascular risk factors, is common in elderly people. A growing but conflicting body of literature suggests that the metabolic syndrome may be associated with cognitive impairment. OBJECTIVE: To investigate the association between the metabolic syndrome and its components and incident cognitive impairment in older women. DESIGN: We prospectively determined if the metabolic syndrome and its components were associated with a 4-year risk of developing cognitive impairment (dementia, mild cognitive impairment, or low global cognitive test score). SETTING: The study was conducted at 180 clinical centers in 25 countries. PARTICIPANTS: A total of 4895 older women (mean age, 66.2 years) with osteoporosis who were part of an ancillary study to determine clinically relevant cognitive impairment were included in this study. These women were free of baseline cognitive impairment and had metabolic syndrome component measures. MAIN OUTCOME MEASURES: Clinically significant cognitive impairment was defined to include women with clinically adjudicated dementia or MCI and women who had a Short Blessed test score greater than 6 (consistent with impairment), but whose cases were not clinically adjudicated. Logistic regression analysis was used to examine the association between presence of the metabolic syndrome and development of clinically significant cognitive impairment. RESULTS: A total of 497 women (10.2%) had the metabolic syndrome and, of these, 36 (7.2%) developed cognitive impairment compared with 181 (of 4398 or 4.1%) without the syndrome (age-adjusted odds ratio, 1.66; 95% confidence interval, 1.14-2.41). The mean (SD) number of metabolic syndrome components for all women was 1.0 (1.1); 518 women (10.6%) were obese, 895 (18.3%) had hypertriglyceridemia, 1200 (24.5%) had low high-density lipoprotein cholesterol levels, 1944 (39.7%) had high blood pressure, and 381 (7.8%) had high fasting blood glucose levels. There was a 23.0% age-adjusted increase in the risk of developing cognitive impairment (odds ratio, 1.23; 95% confidence interval, 1.09-1.39) per unit increase in the number of components. Further multivariable adjustment somewhat reduced the effect. CONCLUSION: We found an association between the metabolic syndrome and the number of components and risk of developing cognitive impairment in older women. Additional studies are needed to determine if screening and close management of these at-risk elderly women would diminish the incidence of cognitive impairment.

Post hoc analysis of data from the Multiple Outcomes of Raloxifene Evaluation (MORE) trial on the effects of three years of raloxifene treatment on glycemic control and cardiovascular disease risk factors in women with and without type 2 diabetes.

BACKGROUND: The long-term effects of the selective estrogen-receptor modulator raloxifene hydrochloride on glycemic control and markers of cardiovascular disease risk in postmenopausal women with type 2 diabetes mellitus are unknown. OBJECTIVE: The aim of this analysis was to compare the effects of 3-year treatment with raloxifene 60 mg/d versus placebo on glycemic control and markers of cardiovascular disease risk in osteoporotic postmenopausal women with and without type 2 diabetes. METHODS: In this analysis, we included women from the Multiple Outcomes of Raloxifene Evaluation trial (a multicenter, double-masked trial) who were randomized to receive raloxifene 60 mg/d (n = 2557) or placebo (n = 2576). Baseline and 36-month fasting plasma glucose (FPG) and total cholesterol (TC) were measured for all participants. Glycated hemoglobin (HbA1c), high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), triglycerides (TGs), apolipoprotein (apo) A-I, apo B, and fibrinogen were assessed in approximately 1800 participants from selected larger sites. RESULTS: At baseline, 202 of all 5133 women (3.9%) had type 2 diabetes. Of the approximately 1800 women who were assessed for HbA1c, LDL-C, TGs, apo A-I, apo B, and fibrinogen, 70 (3.9%) had type 2 diabetes at baseline. Compared with placebo, raloxifene did not significantly affect HbA1c, FPG, HDL-C, or TGs in women with or without diabetes. Raloxifene produced statistically significant reductions in TC, LDL-C, and fibrinogen both in women with diabetes (all P < or = 0.004) and without diabetes (all P < 0.001). Raloxifene significantly increased apo A-I (P < 0.001) and reduced apo B (P < 0.001) in women without diabetes. In the raloxifene-treated group, body weight increased by a mean 0.31 kg (P < 0.001) in women without diabetes. CONCLUSIONS: In osteoporotic postmenopausal women with or without type 2 diabetes, raloxifene 60 mg/d did not affect glycemic control and had favorable effects on TC, LDL-C, and fibrinogen levels.

Serum estradiol level and risk of breast cancer during treatment with raloxifene.

CONTEXT: As endogenous estradiol increases, risk of breast cancer increases. Raloxifene competes with endogenous estrogen for binding to estrogen receptors in breast tissue. A woman's estradiol level may alter the effects of raloxifene on breast cancer and other outcomes. OBJECTIVE: To test the hypothesis that raloxifene reduces breast cancer risk more in women with relatively high estradiol levels than in women with very low estradiol levels. DESIGN: Analysis of the Multiple Outcomes of Raloxifene Evaluation, a randomized, double-blind, placebo-controlled trial conducted from 1994 to 1999. SETTING: One hundred eighty community settings and medical practices in 25 countries including the United States. PARTICIPANTS: A total of 7290 postmenopausal women aged 80 years or younger with osteoporosis who had baseline serum estradiol concentrations measured by a central laboratory using a sensitive assay. Women with a history of breast cancer or estrogen use were excluded. INTERVENTION: Participants were randomly assigned to receive 60 mg/d or 120 mg/d of raloxifene (n = 4843) or matching placebo (n = 2447) for 4 years. MAIN OUTCOME MEASURE: New cases of histopathologically confirmed breast cancer in the treatment and placebo groups, stratified by estradiol levels. RESULTS: In the placebo group, women with estradiol levels greater than 10 pmol/L (2.7 pg/mL) had a 6.8-fold higher rate of breast cancer (3.0% per 4 years; 95% confidence interval [CI], 1.8%-4.1%) than that of women with undetectable estradiol levels (0.6% per 4 years; 95% CI, 0%-1.1%; P =.005 for trend). Women with estradiol levels greater than 10 pmol/L in the raloxifene group had a rate of breast cancer that was 76% (95% CI, 53%-88%) lower than that of women with estradiol levels greater than 10 pmol/L in the placebo group (absolute rate reduction, 2.2% [95% CI, 1.0%-3.5%; number needed to treat = 45]). In contrast, women with undetectable estradiol levels had similar breast cancer risk whether or not they were treated with raloxifene (risk difference, -0.1%; 95% CI, -0.8% to 0.6%; P =.02 for the interaction). In this cohort, treating women with estradiol levels greater than 10 pmol/L with raloxifene for 4 years would have avoided 47% of breast cancer cases. CONCLUSIONS: Measurement of estradiol level by sensitive assay in postmenopausal women identifies those at high risk of breast cancer who may benefit most from raloxifene. If confirmed, this suggests that measuring estradiol and treating women with high estradiol levels could substantially reduce the rate of breast cancer among postmenopausal women.