Abstract PO1-03-03: STEEP criteria v2.0 validation: A multi-trial analysis using GEICAM and TRIO adjuvant trials to evaluate surrogate endpoints for overall survival
Authors: López-Tarruella Sara , Pollan Marina , Urriticoechea Ander , Carrasco Eva , Spera Gonzalo , Martín Miguel , Bermejo Begoña , Chap Linnea , Ruíz - Borrego Manuel , Crown John , García-Sáenz Jose Angel , Chan Arlene , Guerrero Angel , Bee Valerie , Calvo Lourdes , Fresco Rodrigo , Blasco Andrea , Hernando Andrés , Slamon Dennis 
Published: May 2024

Abstract
Background: Standardized definitions of efficacy endpoints are needed to improve clinical trial designs, data reporting and interpretation of results, and to allow cross-trial comparisons. STEEP (Standardized Definitions for Efficacy Endpoints) criteria were proposed in 2007 by an expert panel from the National Cancer Institute (NCI) to provide common definitions for clinical trials in early breast cancer (BC). Invasive disease-free survival (IDFS) was selected as a candidate surrogate endpoint for overall survival (OS) in adjuvant trials and has been widely used in the last 15 years. Advances in BC systemic treatment led to review these criteria (STEEP v2.0), with the definition of a new modified endpoint: invasive BC-free survival (IBCFS). IBCFS excludes second primary non-breast invasive cancers from IDFS which may better detect efficacy effects when the toxicity profile of the intervention is well-known and the risk of second primary cancer is small. Methods: We proposed a multi-trial analysis of 8 adjuvant chemotherapy (CT) trials from GEICAM and TRIO to evaluate the different surrogate endpoints defined in the STEEP v2.0. Trial-level surrogacy was assessed through correlation between the ln hazard ratio (HR) of OS and the ln HR of the different studied endpoints across trials. The correlation was estimated by the weighted, by trial size, least squares regression (WLS) line and evaluated comparing the R2 values between OS and the corresponding endpoint tested.
Primary analyses evaluated the correlation of OS with IDFS/IBCFS. Secondary analyses evaluated the correlation of OS with other STEEP v2.0 endpoints (distant disease-free survival [DDFS], distant relapse-free survival [DRFS], recurrence-free survival [RFS], recurrence-free interval [RFI] and distant recurrence-free interval [DRFI]) and explored the surrogacy between OS and all endpoints in specific groups of patients (pts) according to their risk of relapse: hormone receptors positive (HR+)/HER2-negative (HER2-) pts with intermediate/high and high-risk and triple negative (TN) pts. Results: A total of 14,473 pts were included. Median age was 50 years (range, 20–82). All were female, and 51.8% were postmenopausal. With a median follow-up of 115 months (range, 0.0–188) we found 2,504 OS events, 4,146 IDFS events (60.9% being distant recurrences) and 3,810 IBCFS events (62.4% being distant recurrences).
For each trial, HR of OS, IDFS and IBCFS indicated same conclusions. If HR&lt; 1 for OS, it was also for IDFS and IBCFS and the same was true when HR &gt;1. R2 results of the weighted linear correlations between the ln HR of OS versus (vs) IDFS and OS vs IBCFS were 0.86 and 0.89, respectively. Confidence intervals (CI) at 95% were both overlapping. All other efficacy endpoints showed strong correlation with OS in the overall analysis.
R2 values for HR+/HER2- intermediate/high and high-risk pts were 0.89 (OS vs IDFS) and 0.84 (OS vs IBCFS). For TN pts the correlations were weaker, being 0.63 (OS vs IDFS) and 0.73 (OS vs IBCFS). All WLS R2 values are described in the table. Conclusions: All tested STEEP v2.0 endpoints were strongly correlated with OS (R2 close to 1), meaning that their use as surrogate endpoints for adjuvant CT trials is appropriate. Notably, no differences were found between them in this multi-trial analysis. Therefore, none of them is more appropriate than other to discriminate the value of a new intervention vs comparator in adjuvant CT studies with mature data and long-term follow-up.
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Citation Format: Sara López-Tarruella, Marina Pollan, Ander Urriticoechea, Eva Carrasco, Gonzalo Spera, Miguel Martín, Begoña Bermejo, Linnea Chap, Manuel Ruíz - Borrego, John Crown, Jose Angel García-Sáenz, Arlene Chan, Angel Guerrero, Valerie Bee, Lourdes Calvo, Rodrigo Fresco, Andrea Blasco, Andrés Hernando, Dennis Slamon. STEEP criteria v2.0 validation: A multi-trial analysis using GEICAM and TRIO adjuvant trials to evaluate surrogate endpoints for overall survival [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-03-03.