Abstract PD07-02: βIII Tubulin Expression by Immunohistochemistry (IHC) and mRNA in a Randomized Phase 2 Neoadjuvant Breast Cancer (BC) Study of Ixabepilone (ixa) Compared to Paclitaxel (pac)
Authors: Horak C , Pusztai L , Mudenda B , Xing G , Trifan O , Saura C , Tseng L-M , Chan S , Welcher R , Liu D. 
Published: April 2011

Background: Overexpression of βIII tubulin has been shown to correlate with poor prognosis and reduced response to taxanes. Ixa exhibits activity in βIII overexpressing tumor models and BC patients (pts) with high βIII mRNA levels. We report the prevalence of βIII by a validated IHC assay and the correlation of βIII protein and mRNA expression in BC tissue samples. Methods: In this neoadjuvant BC study CA163-100, 295 patients were randomized 1:1 to ixa (40 mg/m2 IV x 4 cycles) or pac (80 mg/m2 IV wkly x 12) after all pts received 4 cycles of doxorubicin (60 mg/m2) plus cyclophosphamide (600 mg/m2). Pre-chemotherapy formalin-fixed paraffin-embedded pt tissue sections were assessed for intensity and percentage of tumor cell staining of βIII by IHC. RNA was extracted from additional pre-chemotherapy core needle biopsy specimens and profiled for gene expression using Affymetrix HG-U133Av2 gene chips. Tumors were classified as basal-like, Her2-enriched, luminal-like A and B and normal-like using a 50-gene subtype predictor. In addition, Her2 and hormone receptor status was used for tumor subtyping. Results: βIII staining data were obtained for 261 pts, of which 247 were randomized to either ixa or pac arms of study CA163-100. The prevalence of βIII positivity was 39.1% (102/261) using the prespecified cut-off of ≥50% (2+, 3+) positive cells. The distribution of IHC scores was bimodal. 64% of specimens classified as βIII negative had no observable staining, while 77% of βIII positive specimens had a majority of tumor cells staining at the highest intensity (3+). IHC and gene expression profiling data were available for 235 pts and a correlation between βIII tubulin protein and mRNA expression was observed. The relative βIII mRNA expression levels were significantly higher in pts classified as βIII positive vs. βIII negative by IHC (P&lt;0.0001) and 80% of tumors classified as βIII tubulin positive by IHC, had high relative TUBB3 mRNA levels, using a median cut-off. An association between βIII IHC status and breast cancer subtypes was also seen with 69% (62/90) of βIII positive specimens also classified as basal-like and conversely, 55% (62/113) of basal-likes classified as βIII positive. Nearly half (48%) the specimens in this cohort were basal-like, a classification that was quite concordant with triple negative status and as such, triple negative tumors were also enriched for βIII positivity. Conclusion: βIII is reliably measured by a validated IHC assay, providing a bimodal distribution of scores in this BC study. Correlative analyses of βIII status by BC subtype suggest that βIII positivity is significantly associated with aggressive basal-like/TN breast cancers. The clinical utility of βIII as a potential tool to select patients who may benefit from ixa in this study will be presented separately.

Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr PD07-02.