Raising the bar for using surrogate endpoints in drug regulation and health technology assessment

Era of surrogate endpoints and accelerated approvals: a comprehensive review on applicability, uncertainties, and challenges from regulatory, payer, and patient perspectives

PURPOSE

The regulatory landscape in rare diseases and oncology has evolved to address unmet medical needs by implementing expedited approval pathways. The US FDA's Accelerated Approval and the EMA's Conditional Marketing Authorization facilitate earlier patient access to therapies through reliance on surrogate endpoints derived from early-phase clinical trials. The review aims to provide a comprehensive review of the role and utilization of surrogate endpoints in accelerated drug approvals, highlighting their strengths, limitations, and the varying perspectives of stakeholders on their validity and utility.

METHODS

This article reviews existing literature and regulatory guidelines to assess the effectiveness and challenges associated with surrogate endpoints in expedited approval pathways. It also examines the post-approval commitment adherence required by regulatory bodies, exploring discrepancies among stakeholder perspectives.

RESULTS

Findings indicate that while surrogate endpoints enable faster market access, uncertainties remain regarding post-approval commitments and their consistency. Differences in stakeholder opinions also persist, reflecting varying levels of confidence in the validity and applicability of surrogate endpoints.

CONCLUSION

Surrogate endpoints play a crucial role in accelerating drug approvals in areas with high unmet needs, yet challenges around post-approval commitments and stakeholder acceptance suggest the need for enhanced regulatory clarity and ongoing assessment of surrogate endpoint validity.

Surrogate End Points for Overall Survival in Neoadjuvant Randomized Clinical Trials for Early Breast Cancer

PURPOSE

To assess trial-level surrogacy value for overall survival (OS) of the pathologic complete response (pCR) and invasive disease-free survival (iDFS) in randomized clinical trials (RCTs) for early breast cancer (BC).

METHODS

Individual patient data of neoadjuvant RCTs with available data on pCR, iDFS, and OS were included in the analysis. We used the coefficient of determination R2 from weighted linear regression models to quantify the association between treatment effects on OS and on the surrogate end points.

RESULTS

Eleven RCTs, for a total of 15 treatment comparisons and 12,247 patients, were included in the analysis. There was a weak association between hazard ratios (HRs) for OS and odds ratio of pCR overall (R2, 0.07; 95% CI, 0.00 to 0.48), as well as in all the subgroups explored. Overall, the R2 for the association between HR OS and HR iDFS was 0.46 (95% CI, 0.08 to 0.71), which is just below the cutoff of 0.5 for moderate surrogacy. In the majority of subgroups explored, the R2 ranged from 0.5 to <0.7, while in hormone receptor-/human epidermal growth factor receptor 2- subtype, histologic grade 1-2 tumors, and lobular tumors, surrogacy was strong (ie, R2 ≥0.7). The surrogacy value of iDFS for OS was affected by follow-up (FUP) length: R2 substantially increased up to 36 months of FUP, with little further improvement after 48 months of FUP.

CONCLUSION

iDFS with sufficient FUP is an acceptable surrogate end point to confidently anticipate final OS results of neoadjuvant RCTs for early BC. This recommendation holds true across many subgroups, with the notable exception of HR+ disease. There is definite need to reassess whether OS is the optimal end point for treatment efficacy measurement in HR+ early BC.

Use of surrogate endpoints in health technology assessment: a review of selected NICE technology appraisals in oncology

OBJECTIVES

Surrogate endpoints, used to substitute for and predict final clinical outcomes, are increasingly being used to support submissions to health technology assessment agencies. The increase in the use of surrogate endpoints has been accompanied by literature describing the frameworks and statistical methods to ensure their robust validation. The aim of this review was to assess how surrogate endpoints have recently been used in oncology technology appraisals by the National Institute for Health and Care Excellence (NICE) in England and Wales.

METHODS

This article identifies technology appraisals in oncology published by NICE between February 2022 and May 2023. Data are extracted on the use and validation of surrogate endpoints including purpose, evidence base, and methods used.

RESULTS

Of the 47 technology appraisals in oncology available for review, 18 (38 percent) utilized surrogate endpoints, with 37 separate surrogate endpoints being discussed. However, the evidence supporting the validity of the surrogate relationship varied significantly across putative surrogate relationships with 11 providing randomized controlled trial evidence, 7 providing evidence from observational studies, 12 based on the clinical opinion, and 7 providing no evidence for the use of surrogate endpoints.

CONCLUSIONS

This review supports the assertion that surrogate endpoints are frequently used in oncology technology appraisals in England and Wales and despite the increasing availability of statistical methods and guidance on appropriate validation of surrogate endpoints, this review highlights that use and validation of surrogate endpoints can vary between technology appraisals, which can lead to uncertainty in decision making.

How are patient inputs considered in HTA? A thematic document analysis of NICE ultra-rare disease appraisals

Patient organisations are increasingly involved in HTA. Given this, it is important to understand what these organisations contribute and how their voices are accounted for in the decision-making process. This study characterises inputs from patient organisations and/or their nominated patient experts in technology appraisals for ultra-rare diseases in England and Wales and seeks to understand how these are considered in NICE final recommendations. We thematically analysed all HST appraisals completed between January 2022 and August 2024 (N = 15). We appraised inputs from patient organisations' and experts' written submissions, the novelty of patient inputs, as well as financial ties between contributing organisations and the manufacturer of the technology being appraised. We compared themes identified with those found in the Final Evaluation Determination documents to understand how and to what extent patients' inputs were considered in NICE final recommendations. We found that patient submissions mainly focused on disease aspects (54%). Patients raised concerns on access challenges, caregiver burden, and mental health impacts. Most patient themes overlapped with manufacturers' submissions (82%) and doctors' testimonies (45%), with most novel insights focusing on access issues and mental health. Patient organisations reported receiving funding from the technology manufacturer in most appraisals, with amounts ranging from £5,000 to £74,113. Approximately half of patient inputs were explicitly mentioned in NICE final decision documents, with some considerations being neglected despite being raised by patients. While NICE incorporates many issues of importance to patients, there is room for improvement to ensure all aspects patients deem important are captured. Further research could pinpoint optimal areas for patient contributions and assess their impact.

UK Stakeholder Perspectives on Surrogate Endpoints in Cancer, and the Potential for UK Real-World Datasets to Validate Their Use in Decision-Making

Duration of overall survival in patients with cancer has lengthened due to earlier detection and improved treatments. However, these improvements have created challenges in assessing the impact of newer treatments, particularly those used early in the treatment pathway. As overall survival remains most decision-makers' preferred primary endpoint, therapeutic innovations may take a long time to be introduced into clinical practice. Moreover, it is difficult to extrapolate findings to heterogeneous populations and address the concerns of patients wishing to evaluate everyday quality and extension of life. There is growing interest in the use of surrogate or interim endpoints to demonstrate robust treatment effects sooner than is possible with measurement of overall survival. It is hoped that they could speed up patients' access to new drugs, combinations, and sequences, and inform treatment decision-making. However, while surrogate endpoints have been used by regulators for drug approvals, this has occurred on a case-by-case basis. Evidence standards are yet to be clearly defined for acceptability in health technology appraisals or to shape clinical practice. This article considers the relevance of the use of surrogate endpoints in cancer in the UK context, and explores whether collection and analysis of real-world UK data and evidence might contribute to validation.

A Systematic Review of Potential Biomarkers for Bacterial Burden and Treatment Efficacy Assessment in Tuberculosis Platform-Based Clinical Trials

Adaptive platform trials can be more efficient than classic trials for developing new treatments. Moving from culture-based to simpler- or faster-to-measure biomarkers as efficacy surrogates may enhance this advantage. We performed a systematic review of treatment efficacy biomarkers in adults with tuberculosis. Platform trials can span different development phases. We grouped biomarkers as: α, bacterial load estimates used in phase 2a trials; β, early and end-of treatment end points, phase 2b-c trials; γ, posttreatment or trial-level estimates, phase 2c-3 trials. We considered as analysis unit (biomarker entry) each combination of biomarker, predicted outcome, and their respective measurement times or intervals. Performance metrics included: sensitivity, specificity, area under the receiver-operator curve (AUC), and correlation measures, and classified as poor, promising, or good. Eighty-six studies included 22 864 participants. From 1356 biomarker entries, 318 were reported with the performance metrics of interest, with 103 promising and 41 good predictors. Group results were: α, mycobacterial RNA and lipoarabinomannan (LAM) in sputum, and host metabolites in urine; β, mycobacterial RNA and host transcriptomic or cytokine signatures for early treatment response; and γ, host transcriptomics for recurrence. A combination of biomarkers from different categories could help in designing more efficient platform trials. Efforts to develop efficacy surrogates should be better coordinated.

Progress and Challenges of the New Conditional Approval Process in China: A Pooled Analysis From 2018 to 2021

PURPOSE

To speed the review and approval of drugs and address pressing medical needs, China began to advocate for the implementation of the conditional approval process in 2017. We aimed to assess the implementation of the conditional approval process in China and further analyze its potential problems and future challenges.

METHODS

This study examined the new drug approval with conditions in China between 2018 and 2021, based on an analysis of drug technical review documents from the Center for Drug Evaluation (CDE). Using publicly available information, we further analyzed the characteristics and results of pivotal clinical trials of conditionally approved drugs, postmarketing study requirements and progress.

FINDINGS

Between 2018 and 2021, China conditionally approved 50 drugs, with 80% (40/50) being antineoplastic agents. Premarketing pivotal trials predominantly used single-arm clinical trials (83.7%, 41/49), while postmarketing trials mainly employed randomized controlled clinical trials (81.0%, 34/42). In oncology drugs, conditionally approved drugs with progression-free survival (PFS) and overall survival (OS) as primary endpoints achieved significant clinical value in terms of efficacy. However, there were also pivotal clinical trials with response rate (RR) as the primary endpoint that demonstrated lower clinical benefits (8.9% of drugs with RR below 20%). Safety analysis revealed substantial variations in the proportions of grade ≥3 adverse events (AEs) and serious adverse events (SAEs) across pivotal trials (Grade ≥ 3 AEs: 9.0%-99.0%; SAEs: 8.0%-83.0%). For nononcology drugs, pivotal trials also demonstrated an acceptable risk-benefit ratio but exhibited methodological issues. Meanwhile, Most postmarketing studies lacked completion date restrictions (43.2%, 17/47), and no requirements were specified for the transition to full approval. Furthermore, surrogate endpoints were primarily utilized both pre- and postmarketing, but the rational selection of surrogate endpoints remains to be investigated.

IMPLICATIONS

The conditional approval process expedites patient access to drugs for serious diseases. However, challenges pertaining to evidence assessment during approval and design flaws in postmarketing studies exist in China's conditional approval system, necessitating future improvements.

Experimental Medicine Approaches in Early-Phase CNS Drug Development

Traditionally, Phase 1 clinical trials were largely conducted in healthy normal volunteers and focused on collection of safety, tolerability, and pharmacokinetic data. However, in the CNS therapeutic area, with more drugs failing in later phase development, Phase 1 trials have undergone an evolution that includes incorporation of novel approaches involving novel study designs, inclusion of biomarkers, and early inclusion of patients to improve the pharmacologic understanding of novel CNS-active compounds early in clinical development with the hope of improving success in later phase pivotal trials. In this chapter, the authors will discuss the changing landscape of Phase 1 clinical trials in CNS, including novel trial methodology, inclusion of pharmacodynamic biomarkers, and experimental medicine approaches to inform early decision-making in clinical development.

Systemic therapy for early-stage breast cancer: learning from the past to build the future

The treatment of breast cancer has improved dramatically over the past century, from a strictly surgical approach to a coordinated one, including local and systemic therapies. Systemic therapies for early-stage disease were initially tested against observation or placebo only in adjuvant trials. Subsequent clinical trials focusing on treatment 'fine-tuning' had a marked increase in cohort size, duration and costs, leading to a growing interest in the neoadjuvant setting in the past decade. Neoadjuvant trial designs have the advantages of enabling the direct evaluation of treatment effects on tumour diameter and offer unique translational research opportunities through the comparative analysis of tumour biology before, during and after treatment. Current technologies enabling the identification of better predictive biomarkers are shaping the new era of (neo)adjuvant trials. An urgent need exists to reinforce collaboration between the pharmaceutical industry and academia to share data and thus establish large databases of biomarker data coupled with patient outcomes that are easily accessible to the scientific community. In this Review, we summarize the evolution of (neo)adjuvant trials from the pre-genomic to the post-genomic era and provide critical insights into how neoadjuvant studies are currently designed, discussing the need for better end points and treatment strategies that are more personalized, including in the post-neoadjuvant setting.

Surrogacy of Pathologic Complete Response in Trials of Neoadjuvant Therapy for Early Breast Cancer: Critical Analysis of Strengths, Weaknesses, and Misinterpretations

Importance

The pathologic complete response (pCR) is supported by regulatory agencies as a surrogate end point for long-term patients' clinical outcomes in the accelerated approval process of new drugs tested in neoadjuvant randomized clinical trials (RCTs) for early breast cancer (BC). However, a meaningful association between pCR and patients' survival has been proven only at the patient level (ie, significantly better survival of patients who achieved pCR compared with those who did not), but not at trial level (ie, poor association between degree of improvement in pCR rate and survival reported across trials).

Observations

We critically discuss the potential reasons of such discrepancy between pCR surrogacy value at the patient and trial level, as well as the relevant implications for both clinical research and drug regulatory policy. We also describe alternative surrogate end points, including combined end points that jointly analyzed pathological response and event-free survival data, or the assessment of circulating tumor DNA (ctDNA). Such proposed surrogate end points could overcome limits of pCR and provide a reasonable trade-off between the 2 conflicting needs to have access to effective therapies rapidly, and to reliably assess patients' clinical benefit.

Conclusions and Relevance

Using surrogate end points to grant drug approvals is justified only when they can provide accurate prediction of a drug's effect on the long-term patient outcomes. Evidence currently available does not support pCR used alone as a reliable surrogate end point in regulatory neoadjuvant RCTs for BC. The surrogacy value at trial level of potentially more robust surrogate end points needs to be urgently tested.

The Validity of Surrogate Endpoints in Sub Groups of Metastatic Colorectal Cancer Patients Defined by Treatment Class and KRAS Status

Background and Aim: Findings from the literature suggest that the validity of surrogate endpoints in metastatic colorectal cancer (mCRC) may depend on a treatments' mechanism of action. We explore this and the impact of Kirsten rat sarcoma (KRAS) status on surrogacy patterns in mCRC. Methods: A systematic review was undertaken to identify randomized controlled trials (RCTs) for pharmacological therapies in mCRC. Bayesian meta-analytic methods for surrogate endpoint evaluation were used to evaluate surrogate relationships across all RCTs, by KRAS status and treatment class. Surrogate endpoints explored were progression free survival (PFS) as a surrogate endpoint for overall survival (OS), and tumour response (TR) as a surrogate for PFS and OS. Results: 66 RCTs were identified from the systematic review. PFS showed a strong surrogate relationship with OS across all data and in subgroups by KRAS status. The relationship appeared stronger within individual treatment classes compared to the overall analysis. The TR-PFS and TR-OS relationships were found to be weak overall but stronger within the Epidermal Growth Factor Receptor + Chemotherapy (EGFR + Chemo) treatment class; both overall and in the wild type (WT) patients for TR-PFS, but not in patients with the mutant (MT) KRAS status where data were limited. Conclusions: PFS appeared to be a good surrogate endpoint for OS. TR showed a moderate surrogate relationship with PFS and OS for the EGFR + Chemo treatment class. There was some evidence of impact of the mechanism of action on the strength of the surrogacy patterns in mCRC, but little evidence of the impact of KRAS status on the validity of surrogate endpoints.

Value Insider Season 1 Episode 1: The Importance of Payers and HTA: How Did We End Up Here? (Introduction to Value) [Podcast]

The importance of payers and HTA: How did we end up here? In this episode of the Value Insider podcast, host Mike Chambers speaks with Prof. Mike Drummond about Health Technology Assessment (HTA) and how value is defined. Prof. Drummond is professor of Health Economics of York University, former president of the International Society of Pharmacoeconomics and Outcomes Research (ISPOR), and author of two major textbooks in the field, as well as over 700 publications, and has acted as consultant to the WHO as well as European Union with regards to value assessment. Starting from the very beginning, Prof. Drummond explains in a simple yet engaging way why demonstrating value of new interventions has become so important.

Implications of Oncology Trial Design and Uncertainties in Efficacy-Safety Data on Health Technology Assessments

Background: Advances in cancer medicines have resulted in tangible health impacts, but the magnitude of benefits of approved cancer medicines could vary greatly. Health Technology Assessment (HTA) is a multidisciplinary process used to inform resource allocation through a systematic value assessment of health technology. This paper reviews the challenges in conducting HTA for cancer medicines arising from oncology trial designs and uncertainties of safety-efficacy data. Methods: Multiple databases (PubMed, Scopus and Google Scholar) and grey literature (public health agencies and governmental reports) were searched to inform this policy narrative review. Results: A lack of robust efficacy-safety data from clinical trials and other relevant sources of evidence has made HTA for cancer medicines challenging. The approval of cancer medicines through expedited pathways has increased in recent years, in which surrogate endpoints or biomarkers for patient selection have been widely used. Using these surrogate endpoints has created uncertainties in translating surrogate measures into patient-centric clinically (survival and quality of life) and economically (cost-effectiveness and budget impact) meaningful outcomes, with potential effects on diverting scarce health resources to low-value or detrimental interventions. Potential solutions include policy harmonization between regulatory and HTA authorities, commitment to generating robust post-marketing efficacy-safety data, managing uncertainties through risk-sharing agreements, and using value frameworks. Conclusion: A lack of robust efficacy-safety data is a central problem for conducting HTA of cancer medicines, potentially resulting in misinformed resource allocation.

Overall Survival Benefits of Cancer Drugs Approved in China From 2005 to 2020

IMPORTANCE

Of approximately 9 million patients with cancer in China in 2020, more than half were diagnosed with late-stage cancers. Recent regulatory reforms in China have focused on improving the availability of new cancer drugs. However, evidence on the clinical benefits of new cancer therapies authorized in China is not available.

OBJECTIVE

To characterize the clinical benefits of cancer drugs approved in China, as defined by the availability and magnitude of statistically significant overall survival (OS) results.

DESIGN, SETTING, AND PARTICIPANTS

This mixed-methods study comprising a systematic review and cross-sectional analysis identified antineoplastic agents approved in China between January 1, 2005, and December 31, 2020, using publicly available data and regulatory review documents issued by the National Medical Products Administration. The literature published up to June 30, 2021, was reviewed to collect results on end points used in pivotal trials supporting cancer drug approvals.

MAIN OUTCOMES AND MEASURES

The primary outcome measure was a documented statistically significant positive OS difference between a new cancer therapy and a comparator treatment. Secondary outcome measures were the magnitude of OS benefit and other primary efficacy measures in pivotal trials.

RESULTS

Between 2005 and 2020, 78 cancer drugs corresponding to 141 indications were authorized in China, including 20 drugs (25.6%) (for 30 indications) approved in China only. Of all indications, 26 (18.4%) were evaluated in single-arm or dose-optimization trials, most of which were authorized after 2017. By June 30, 2021, 34 drug indications (24.1%) had a documented lack of OS gain. For 68 indications (48.2%) that had documented evidence of OS benefit, the median magnitude of OS improvement was 4.1 (range, 1.0-35.0) months. After a median follow-up of 1.9 (range, 1.0-11.1) years from approval, OS data for 13 indications (9.2%) were either not reported or were still not mature. Fewer than one-third of cancer drug indications approved in China only had documented evidence of OS benefits (9 of 30 [30.0%]), whereas more than one-half of the cancer drug indications also available in the US or Europe had OS benefits (59 of 111 [53.1%]).

CONCLUSIONS AND RELEVANCE

In this study, almost half of cancer drug indications approved in China had demonstrated OS gain. With the increase of cancer drug approvals based on single-arm trials or immature survival data in recent years, these findings highlight the need to routinely monitor the clinical benefits of new cancer therapies in China.

Aspiring to Reasonableness in Accelerated Approval: Anticipating and Avoiding the Next Aducanumab

The US Food and Drug Administration's decisions about drug approval-though guided by science, as well as relevant statutes, regulations, and guidance documents-reflect normative judgments about how the agency should exercise its discretion. This is particularly true in the context of the "accelerated approval" pathway, where the agency must balance speeding to market drugs for patients with unmet needs before they have been proven to work and ensuring confidence about the benefits and risks of those drugs. A key challenge in evaluating normative judgments such as these is that reasonable people can disagree, rendering it difficult to proclaim with certainty that a particular decision is right or wrong. Therefore, we propose that it is preferable to ask whether a decision is reasonable. A decision is reasonable when it transparently, comprehensively, and fairly balances the interests of affected parties, within the parameters of the decision maker's legal authority. If a decision achieves these three qualities, it can be viewed as legitimate and worthy of trust and confidence, regardless of whether one agrees with the particular outcome. We recommend that the Food and Drug Administration adopt procedural protections to promote reasonableness in four domains affecting accelerated approval decisions: pathway gatekeeping, endpoint selection, stakeholder engagement, and deliberation. This should aid the agency in minimizing controversies, such as that surrounding the 2021 approval of aducanumab (Aduhelm; Biogen).

For Whom the Price Escalates: High Price and Uncertain Value of Cancer Drugs

The price of cancer drugs has skyrocketed, yet it is not clear whether their value is commensurate with their price. More cancer drugs are approved under expedited review, which considers less rigorous clinical evidence, yet only 20% of them show an overall survival gain in the confirmatory trial. Moreover, clinical data are often generated based on small, single-arm studies with surrogate outcomes, challenging economic evaluation. With their high price and uncertain (marginal) clinical value, cancer drugs are frequently rejected by health technology assessment (HTA) bodies. Therefore, agencies, including the UK's National Institute for Health and Care Excellence (NICE), have adopted cancer drug funds (CDF) or risk-sharing schemes to provide extra access for expensive cancer drugs which fail to meet NICE's cost effectiveness threshold. With rising pricing and fewer new cancer medications with novel mechanisms of action, it is unclear if newly marketed cancer therapies address unmet clinical needs or whether we are paying too much. Transparency, equity, innovativeness, and sustainability are all harmed by a "special" approach for cancer medications. If early access is allowed, confirmatory trials within a certain time frame and economic evaluation should be conducted, and label changes or disinvestment should be carried out based on those evaluations.

Evaluation of pathological complete response as surrogate endpoint in neoadjuvant randomised clinical trials of early stage breast cancer: systematic review and meta-analysis

OBJECTIVE

To evaluate pathological complete response as a surrogate endpoint for disease-free survival and overall survival in regulatory neoadjuvant trials of early stage breast cancer.

DESIGN

Systematic review and meta-analysis.

DATA SOURCES

Medline, Embase, and Scopus to 1 December 2020.

ELIGIBILITY CRITERIA FOR STUDY SELECTION

Randomised clinical trials that tested neoadjuvant chemotherapy given alone or combined with other treatments, including anti-human epidermal growth factor 2 (anti-HER2) drugs, targeted treatments, antivascular agents, bisphosphonates, and immune checkpoint inhibitors.

DATA EXTRACTION AND SYNTHESIS

Trial level associations between the surrogate endpoint pathological complete response and disease-free survival and overall survival.

METHODS

A weighted regression analysis was performed on log transformed treatment effect estimates (hazard ratio for disease-free survival and overall survival and relative risk for pathological complete response), and the coefficient of determination (R2) was used to quantify the association. The secondary objective was to explore heterogeneity of results in preplanned subgroups analysis, stratifying trials according treatment type in the experimental arm, definition used for pathological complete response (breast and lymph nodes v breast only), and biological features of the disease (HER2 positive or triple negative breast cancer). The surrogate threshold effect was also evaluated, indicating the minimum value of the relative risk for pathological complete response necessary to confidently predict a non-null effect on hazard ratio for disease-free survival or overall survival.

RESULTS

54 randomised clinical trials comprising a total of 32 611 patients were included in the analysis. A weak association was observed between the log(relative risk) for pathological complete response and log(hazard ratio) for both disease-free survival (R2=0.14, 95% confidence interval 0.00 to 0.29) and overall survival (R2 =0.08, 0.00 to 0.22). Similar results were found across all subgroups evaluated, independently of the definition used for pathological complete response, treatment type in the experimental arm, and biological features of the disease. The surrogate threshold effect was 5.19 for disease-free survival but was not estimable for overall survival. Consistent results were confirmed in three sensitivity analyses: excluding small trials (<200 patients enrolled), excluding trials with short median follow-up (<24 months), and replacing the relative risk for pathological complete response with the absolute difference of pathological complete response rates between treatment arms.

CONCLUSION

A lack of surrogacy of pathological complete response was identified at trial level for both disease-free survival and overall survival. The findings suggest that pathological complete response should not be used as primary endpoint in regulatory neoadjuvant trials of early stage breast cancer.