S0226: Fulvestrant Plus Anastrozole in Hormone Receptor–Positive Metastatic Breast Cancer

⚠️ Updated analysis. The primary endpoint (progression-free survival) was previously reported (Mehta et al., N Engl J Med 2012). This analysis reports updated PFS and final overall survival at a median follow-up of 7 years in patients without disease progression (maximum 12 years).

Primary endpoint (PFS): 15.0 vs 13.5 months — HR 0.81 (95% CI 0.69–0.94; P=0.007) Overall survival: 49.8 vs 42.0 months — HR 0.82 (95% CI 0.69–0.98; P=0.03) Safety signal: Grade 5 pulmonary emboli (2 patients) and cerebrovascular ischemic event (1 patient) in the combination group

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Clinical Bottom Line

S0226 demonstrated that combining fulvestrant with anastrozole as first-line endocrine therapy for postmenopausal women with hormone receptor–positive metastatic breast cancer resulted in significantly longer progression-free survival and overall survival compared with anastrozole alone. With extended follow-up, the survival advantage was maintained, translating to a clinically meaningful improvement in median overall survival despite substantial crossover from the control arm.

This trial is historically important as one of the first to demonstrate that dual endocrine blockade — combining an aromatase inhibitor with an estrogen receptor antagonist — could improve outcomes over aromatase inhibitor monotherapy in the first-line metastatic setting. However, the treatment landscape has evolved substantially since S0226 was designed and enrolled. CDK4/6 inhibitors combined with endocrine therapy have become the standard first-line approach for HR+/HER2− metastatic breast cancer, and S0226 predates that era entirely. Additionally, the fulvestrant maintenance dose used in S0226 was lower than the current approved standard dose.

The combination was well tolerated with no significant increase in grade 3 toxicity compared with anastrozole alone. The key safety concern is the occurrence of fatal thromboembolic and cerebrovascular events in the combination arm, though these were rare. The benefit appeared concentrated in patients without prior adjuvant endocrine therapy — an important consideration for patient selection that warrants attention.

Trial Overview

Study Design

• Trial name and registration: S0226 (NCT00075764)
• Design: Investigator-initiated, multicenter, randomized, open-label trial; updated follow-up analysis. Phase was not reported in this publication.
• Randomization: 1:1 ratio; stratified by adjuvant tamoxifen use
• Setting: Hormone receptor–positive metastatic breast cancer; first-line therapy for metastatic disease
• Enrollment period and sites: June 2004 through June 2009

This publication reports the updated follow-up analysis of the S0226 trial. The primary endpoint of progression-free survival was previously reported [4]. The current analysis reports updated PFS and final overall survival outcomes, with the number of deaths increasing from 330 to 508 and PFS events increasing from 565 to 647 since the original report.

Patient Population

• Key eligibility: Postmenopausal women with estrogen-receptor–positive or progesterone-receptor–positive metastatic breast cancer; Zubrod's performance-status score of 0 to 2; no previous chemotherapy, hormonal therapy, or immunotherapy for metastatic disease; previous adjuvant tamoxifen allowed (stratification factor); neoadjuvant or adjuvant chemotherapy or aromatase inhibitor therapy completed more than 12 months before enrollment
• Biomarker selection: Estrogen-receptor–positive or progesterone-receptor–positive
• Sample size flow: 707 randomized → 12 ineligible and 1 withdrew consent → 694 eligible patients analyzed (345 anastrozole alone, 349 combination). In the anastrozole-alone arm, 341 received treatment (4 did not); in the combination arm, 348 received treatment (2 did not).

Baseline Characteristics

Baseline characteristics were reported in detail in the original publication [4]. Limited baseline data are available from this publication:

Treatment Protocol

Control Arm: Anastrozole Alone (n=345 analyzed; safety population n=338)

Standard-dose anastrozole alone. - Dose and schedule: Standard-dose anastrozole (dose not specified in this publication) - Treatment duration: Not reported in this publication - Median treatment duration: Not reported in this publication - Crossover at progression: At the time of progression, in the absence of visceral crisis, crossover to fulvestrant was strongly encouraged

Experimental Arm: Anastrozole Plus Fulvestrant (n=349 analyzed; safety population n=348)

Anastrozole plus fulvestrant. - Dose and schedule: Standard-dose anastrozole plus fulvestrant at a loading dose of 500 mg on day 1, with 250 mg administered on days 14 and 28 and then 250 mg administered as maintenance therapy every 28 days - Treatment duration: Not reported in this publication - Median treatment duration: Not reported in this publication - Median relative dose intensity: Not reported in this publication

Important dosing note: The fulvestrant maintenance dose used in this trial was 250 mg every 28 days, which is lower than the current FDA-approved standard dose of 500 mg every 28 days (after the loading regimen). The 500-mg dose was not approved at the time the trial was designed. At least 5 patients who crossed over and at least 9 patients in the combination group received the 500-mg maintenance dose after FDA approval of the higher dose.

Crossover

• Permitted: Yes — crossover to fulvestrant was strongly encouraged at progression for patients in the anastrozole-alone group, in the absence of visceral crisis
• N crossed over: 45% of patients in the anastrozole-alone group crossed over to receive fulvestrant (including at least 5 patients who received the 500-mg maintenance dose)
• Trigger: Disease progression
• Analysis approach: Intention-to-treat; crossover patients analyzed in their original randomization arm

Efficacy Outcomes

The primary endpoint of this trial (progression-free survival) was reported in the original publication [4]. The current analysis, with a median follow-up of 7 years in patients without disease progression (maximum 12 years), reports updated PFS and final overall survival.

Primary Endpoint: Progression-Free Survival

Definition: Time from randomization to progression or death from any cause Analysis population: Intention-to-treat (694 eligible patients) Statistical method: Stratified log-rank test, Cox regression Median follow-up: 7 years (in patients without disease progression); maximum 12 years

Anastrozole alone: Median PFS 13.5 months; 329 events in 345 patients Anastrozole plus fulvestrant: Median PFS 15.0 months; 318 events in 349 patients Comparison: HR 0.81 (95% CI, 0.69 to 0.94; P=0.007)

Total PFS events: 647 (increased from 565 in the original report).

PFS by Prior Tamoxifen Use (Prespecified Stratified Analysis)

Among 414 women (60%) who had not received tamoxifen previously: - Anastrozole alone (n=206): Median PFS 12.7 months; 198 events - Combination (n=208): Median PFS 16.7 months; 183 events - HR: 0.73 (95% CI, 0.60 to 0.89)

Among 280 women (40%) with previous adjuvant tamoxifen exposure: - Anastrozole alone: Median PFS 13.9 months

• Combination: Median PFS 13.6 months

• HR: 0.93 (95% CI, 0.73 to 1.19)

Key Secondary Endpoint: Overall Survival

Definition: Time from randomization to death from any cause (implied) Analysis population: Intention-to-treat (694 eligible patients) Formally tested: Yes

Anastrozole alone: Median OS 42.0 months; 261 deaths in 345 patients Anastrozole plus fulvestrant: Median OS 49.8 months; 247 deaths in 349 patients Comparison: HR 0.82 (95% CI, 0.69 to 0.98; P=0.03)

Total deaths: 508 (increased from 330 in the original report).

5-year overall survival rates:

OS by Prior Endocrine Therapy (Prespecified Stratified Analysis)

Among patients with no previous endocrine therapy (206 anastrozole, 208 combination): - Anastrozole alone: Median OS 40.3 months; 155 deaths - Combination: Median OS 52.2 months; 139 deaths - HR: 0.73 (95% CI, 0.58 to 0.92)

Among patients with previous endocrine therapy (139 anastrozole, 141 combination): - Anastrozole alone: Median OS 43.5 months; 106 deaths - Combination: Median OS 48.2 months; 108 deaths - HR: 0.97 (95% CI, 0.74 to 1.27)

Interaction P=0.09.

Safety

Safety Population

Anastrozole alone: n=338 Anastrozole plus fulvestrant: n=348

Safety Summary

Fewer than 4 standard summary safety fields were reported in this publication. The following narrative summarizes the available safety data:

Grade 3 toxic effects occurred in 51 of 348 patients (15%) in the combination-therapy group and in 43 of 338 patients (13%) in the anastrozole-alone group (P=0.47). Grade 3 toxic effects included musculoskeletal pain, fatigue, hot flashes, mood alterations, and gastrointestinal symptoms, at frequencies of 1 to 4%.

Few patients discontinued treatment owing to adverse events or side effects: 5 patients in the anastrozole-alone group and 12 in the combination-therapy group.

Standard summary safety metrics — including rates of any adverse event, grade ≥3 adverse events (combined), serious adverse events, treatment-related adverse events, treatment-related deaths, dose reductions, and dose interruptions — were not reported in this publication. Detailed safety data were published in the original report [4].

Adverse Events of Special Interest

⚠️ Pulmonary Emboli (Grade 5): Critical Safety Signal

• Grade 5 (fatal): 2 patients in the combination-therapy group
• Grade 4: 1 patient in the combination-therapy group
• Clinical implication: Fatal thromboembolic events occurred exclusively in the combination arm. While rare, clinicians should be aware of this risk, particularly in patients with pre-existing thromboembolic risk factors.

Cerebrovascular Ischemic Event (Grade 5)

• Grade 5 (fatal): 1 patient in the combination-therapy group

Additional Grade 4 Events

• Neutropenia or lymphopenia (grade 4): 1 patient in the combination-therapy group
• Thromboembolism (grade 4, new since prior report): 1 patient in the anastrozole-alone group

Since the initial report, no additional toxic effects of grade 4 or 5 were reported in the combination-therapy group. Previously reported grade 4 toxic effects in the anastrozole-alone group included thrombosis or embolism, arthralgia, thrombocytopenia, and dyspnea (in one patient each).

Deaths

• Total deaths: 508 (261 anastrozole alone, 247 combination)
• Treatment-related deaths: Not reported in this publication as a separate category. Grade 5 adverse events in the combination arm included pulmonary emboli (2 patients) and a cerebrovascular ischemic event (1 patient).

Subgroup Analyses

Prespecified Stratified Analyses

The trial was stratified by adjuvant tamoxifen use. The PFS and OS results by prior endocrine therapy status are presented above in the Efficacy section. The key finding was a significant interaction trend: patients without prior endocrine therapy appeared to derive substantially greater benefit from combination therapy than those with prior exposure (OS interaction P=0.09).

Post Hoc Subgroup Analyses of Overall Survival

Endocrine-sensitive subgroup detail:

• Anastrozole alone: Median OS 42.3 months (95% CI, 38.9 to 47.8)
• Combination: Median OS 50.7 months (95% CI, 46.6 to 58.3)
• HR: 0.79 (95% CI, 0.65 to 0.95)

Endocrine-refractory subgroup detail:

• Anastrozole alone: Median OS 39.2 months (95% CI, 30.2 to 50.0)
• Combination: Median OS 35.1 months (95% CI, 26.8 to 50.1)
• HR: 1.08 (95% CI, 0.65 to 1.80)

Patients with ≥10 years from diagnosis to first metastasis:

• Anastrozole alone: Median OS 49.7 months

• Combination: Median OS 65.4 months

• HR: 0.69 (95% CI, 0.49 to 0.98)

These post hoc subgroup analyses were not prespecified and were not powered for formal statistical comparisons. Confidence intervals were not adjusted for multiple comparisons. Results should be interpreted as hypothesis-generating.

Key Comparator Trials

Cross-trial comparisons are limited by differences in patient populations, trial designs, and endpoints. These data are presented for context, not for direct statistical comparison.

Contextual Notes

S0226 and the FACT trial both tested the combination of anastrozole plus fulvestrant 250 mg versus anastrozole alone in the first-line metastatic setting [5]. The FACT trial did not show a benefit for the combination, a discrepancy attributed to differences in prior endocrine therapy exposure between the two trial populations. In S0226, 60% of patients had not received prior adjuvant endocrine therapy, and the benefit of combination therapy was concentrated in this subgroup. The FACT trial had a higher proportion of patients with prior endocrine therapy exposure.

The FALCON trial [6] subsequently established fulvestrant 500 mg monotherapy as superior to anastrozole in the first-line setting for patients without prior endocrine therapy — using the higher, now-standard fulvestrant dose. The fulvestrant maintenance dose in S0226 was 250 mg, which is lower than current standard dosing.

Critically, the treatment landscape for HR+/HER2− metastatic breast cancer has been transformed since S0226 by the introduction of CDK4/6 inhibitors (palbociclib, ribociclib, abemaciclib) combined with endocrine therapy. CDK4/6 inhibitor combinations are now the preferred first-line endocrine-based therapy per NCCN guidelines [3]. S0226 provides important historical data on dual endocrine blockade but does not address the current standard-of-care comparator.

Grey Zones and Unanswered Questions

• Fulvestrant dose below current standard: The fulvestrant maintenance dose in S0226 was 250 mg every 28 days, lower than the current FDA-approved standard of 500 mg. Whether the results would be different — potentially more favorable — with the 500-mg dose is unknown and cannot be inferred from this trial. This dosing difference limits the direct applicability of S0226 results to current practice.

• Benefit restricted to endocrine-naive patients: The survival benefit was concentrated among patients without prior adjuvant endocrine therapy (OS HR 0.73) while patients with prior endocrine therapy showed no benefit (OS HR 0.97; interaction P=0.09). The interaction did not reach statistical significance, so whether prior endocrine therapy truly abolishes the benefit of combination therapy remains uncertain. This has implications for patient selection but is not definitive.

• Open-label design: S0226 was open-label, which may introduce bias in progression assessment (investigator-assessed PFS) and in subsequent treatment decisions. This is a relevant limitation for interpreting both PFS and the pattern of crossover.

• Crossover confounding OS: 45% of anastrozole-alone patients crossed over to fulvestrant at progression, and postprogression survival was reported as similar in both groups. This substantial crossover rate would be expected to attenuate the observed OS difference, potentially underestimating the true survival benefit of upfront combination therapy. However, it also makes the OS benefit more difficult to interpret definitively.

• Relevance in the CDK4/6 inhibitor era: S0226 was designed and enrolled before CDK4/6 inhibitors were available. The current standard first-line therapy is a CDK4/6 inhibitor plus endocrine therapy. Whether adding fulvestrant to an aromatase inhibitor offers benefit in combination with a CDK4/6 inhibitor, or whether dual endocrine blockade is relevant when CDK4/6 inhibition is already in use, is not addressed by this trial.

• Endocrine-refractory population: The small endocrine-refractory subgroup (9–12% of patients) showed no benefit and a numerically unfavorable HR of 1.08. While this subgroup was small and the analysis exploratory, it raises concern about using this combination in patients with primary endocrine resistance.

Clinical Implications

Where This Fits in the Treatment Sequence

S0226 tested first-line endocrine therapy for HR+/HER2− metastatic breast cancer. The combination of anastrozole plus fulvestrant demonstrated superior PFS and OS compared with anastrozole alone. However, the current (2026) standard first-line therapy for this population is a CDK4/6 inhibitor combined with endocrine therapy [3]. S0226 predates the CDK4/6 inhibitor era and does not address how dual endocrine blockade compares to or interacts with CDK4/6 inhibitor–based regimens. The trial's primary clinical contribution is in establishing that more complete estrogen receptor blockade improves outcomes — a principle that informs current treatment strategies — rather than providing a regimen for routine first-line use today.

In settings where CDK4/6 inhibitors are unavailable (resource-limited settings or contraindications), the combination of an aromatase inhibitor plus fulvestrant may be considered for endocrine therapy–naive postmenopausal women, though the fulvestrant dose should be 500 mg per current labeling [2].

Practical Considerations

• Fulvestrant dose: Current standard fulvestrant dosing is 500 mg intramuscularly on days 1, 15, and 29, then every 28 days. The 250-mg maintenance dose used in S0226 is no longer recommended.
• Patient selection: The benefit appeared concentrated in patients without prior adjuvant endocrine therapy. Patients with prior tamoxifen exposure showed no PFS or OS benefit from the combination. This pattern aligns with the FALCON trial results suggesting greater benefit of fulvestrant-based therapy in endocrine-naive patients.
• Tolerability: The combination was well tolerated with no significant increase in grade 3 toxicity (15% vs 13%; P=0.47). Discontinuation due to adverse events was low in both arms (12 vs 5 patients).

Impact of Crossover on Results

• 45% of patients crossed from anastrozole alone to fulvestrant after disease progression
• OS analysis used intention-to-treat: crossover patients remained in their original randomization arm
• The paper noted that postprogression survival was similar in both groups, suggesting that crossover may have contributed to attenuating the OS difference
• This conservative ITT analysis likely underestimates the true OS benefit of upfront combination therapy
• No adjusted crossover analysis (e.g., RPSFT) was reported

Post-Progression Options

Subsequent therapy rates were not reported in this publication. In the anastrozole-alone arm, 45% crossed over to fulvestrant at progression. Current post-progression options for HR+/HER2− metastatic breast cancer include fulvestrant (if not previously used), CDK4/6 inhibitor combinations (if not previously used), PI3K inhibitors (alpelisib for PIK3CA-mutated tumors), mTOR inhibitors (everolimus), and chemotherapy.

Unanswered Questions

The two most practice-relevant open questions are: (1) whether dual endocrine blockade with an aromatase inhibitor plus fulvestrant (at the current 500-mg dose) has a role in the CDK4/6 inhibitor era — either as an alternative for patients who cannot receive CDK4/6 inhibitors or as a backbone for triple combination regimens; and (2) whether the endocrine-sensitivity biomarker signal (benefit concentrated in endocrine-naive patients) should guide treatment selection when considering endocrine combination strategies.

Regulatory and Guideline Status

Regulatory

• FDA: Fulvestrant (Faslodex) is approved at 500 mg for the treatment of HR+/HER2− advanced breast cancer in postmenopausal women, including in combination with CDK4/6 inhibitors. The specific combination of anastrozole plus fulvestrant as tested in S0226 does not have a distinct FDA-approved indication; each agent is approved individually.
• EMA: Fulvestrant is approved for locally advanced or metastatic breast cancer.

⚠️ Regulatory status verified as of March 2026. Confirm current approval status and labeled indications before clinical use at FDA.gov.

Guidelines

• NCCN: CDK4/6 inhibitor plus endocrine therapy is the preferred first-line approach for HR+/HER2− metastatic breast cancer [3]. Endocrine monotherapy (aromatase inhibitor, fulvestrant, or tamoxifen) is listed as an alternative for patients with contraindications to CDK4/6 inhibitors or with low disease burden. The combination of an aromatase inhibitor plus fulvestrant is not a standard recommended first-line regimen in current NCCN guidelines.

Companion Diagnostics

ER/PR positivity by immunohistochemistry is required per standard practice. No novel companion diagnostic is required.

About the Author

Andrew Stevenson is the founder and systems architect of kill-cancer.com, a clinical intelligence platform delivering structured, source-traced oncology trial analysis to practicing clinicians. He holds 17 Google technical certifications in data systems, automation, and applied AI — the engineering foundation behind the extraction and verification pipeline that produces every article on this platform. Every number traces directly to its source publication. Zero calculation. Zero editorializing. Zero hallucination.

Five siblings lost to cancer built the urgency. The engineering builds the trust.

📧 andrew@kill-cancer.com 🌐 kill-cancer.com 💬 kill-cancer.com/forum

Disclaimer

This article is intended for healthcare professionals only. It is not medical advice and should not be used as a substitute for professional clinical judgment. Treatment decisions should be made in consultation with qualified healthcare providers based on individual patient circumstances.

All trial data presented in this article are sourced directly from the published clinical trial report and its supplementary materials. Numbers are reproduced exactly as reported; no calculations, derivations, or estimates have been performed.

Trial results are presented as reported in the source publication. Updated data, label changes, or guideline revisions published after the source article may alter clinical applicability.

Comparator trial data presented in Section 8 are sourced from their respective published reports and are provided for contextual purposes only. Cross-trial comparisons have inherent limitations and should not be interpreted as direct statistical comparisons.

References

1. Mehta RS, Barlow WE, Albain KS, et al. Overall survival with fulvestrant plus anastrozole in metastatic breast cancer. N Engl J Med. 2019;380:1226-1234. doi:10.1056/NEJMoa1811714
2. Faslodex (fulvestrant) prescribing information. U.S. Food and Drug Administration. Accessed March 2026.
3. NCCN Clinical Practice Guidelines in Oncology: Breast Cancer. Version 1.2026. Accessed March 2026.
4. Mehta RS, Barlow WE, Albain KS, et al. Combination anastrozole and fulvestrant in metastatic breast cancer. N Engl J Med. 2012;367:435-444.
5. Bergh J, Jönsson PE, Lidbrink EK, et al. FACT: an open-label randomized phase III study of fulvestrant and anastrozole in combination compared with anastrozole alone as first-line therapy for patients with receptor-positive postmenopausal breast cancer. J Clin Oncol. 2012;30(16):1919-1925.
6. Robertson JFR, Bondarenko IM, Trishkina E, et al. Fulvestrant 500 mg versus anastrozole 1 mg for hormone receptor-positive advanced breast cancer (FALCON): an international, randomised, double-blind, phase 3 trial. Lancet. 2016;388(10063):2997-3005.