Abstract PD1-7: Phosphatase and tensin homolog deleted from chromosome 10 (PTEN) evaluation in the NeoALTTO trial (BIG 01-06)

Abstract

Abstract Introduction Assessment of PTEN might be an important tool in identifying patients unlikely to derive substantial benefit from trastuzumab and lapatinib-based therapies. Current studies failed to demonstrate an unequivocal predictive role of PTEN in human epidermal growth factor receptor-2 (HER2)-positive breast cancer both in neo-adjuvant and adjuvant settings. Lack of standardization of PTEN status determination in formalin fixed paraffin embedded (FFPE) tissue samples together with the small datasets analyzed in previous studies may have contributed to conflicting PTEN results and high variability of PTEN loss rates reported (22%-64%). Methods In the present study we have conducted a preliminary analysis investigating the influence of the antibodies, scoring methods and cutoff criteria used, together with the impact of inter-observer variability on PTEN status determination. Core biopsies from 429 FFPE baseline samples from early-stage HER2-positive breast cancer patients prospectively collected in the neoALTTO trial were stained with two widely used anti-PTEN monoclonal antibodies (clone 6H2.1 from DAKO and clone 138G6 from Cell Signaling Technology-CST). Two different pathologists independently scored the slides using the H-score system. Two different cut-off criteria were used to define PTEN loss of expression: H-score < 50 and ≤10% positive staining (any level of intensity). Results Out of 429 processed samples, 363 and 369 were evaluable for observer 1 (CST and DAKO, respectively) and 367 were evaluable for observer 2, with an average missing rate of 15%. A strong linear correlation was observed between CST and DAKO antibodies for both observers (R2 > 0.7). After setting a cutoff for PTEN loss, the observed discordance rate was below 10%, with the number of discordant cases ranging from 28 to 33 (depending on the cut-off criteria and observer). Inter-observer correlation was strong with both antibodies (R2 = 0.8). Based on the cutoff criteria and antibodies used, inter-observer discordance rates ranged between 6% and 10%; the lowest rate observed at ≤10% cutoff threshold with CST antibody. PTEN was classified as “loss” in 23%, 19%, 16%, and 13% of cases by observer 1 and in 32%, 29%, 24%, 19% of cases by observer 2 using DAKO H-score<50, CST H-score<50, DAKO≤10%, and CST≤10% criteria, respectively. Conclusion Our preliminary analysis showed high inter-antibody and inter-observer concordant rates for PTEN status determination by IHC. However, the lack of a standardized definition of PTEN cutoff thresholds for defining negativity and positivity generated loss call rates ranging from 13% to 32% within the same dataset. Correlative analyses with pCR and PIK3CA status are ongoing to determine the impact of PTEN expression and its variability in predicting response to single-agent or dual HER2 targeting. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr PD1-7.