Mechanisms of resistance to trastuzumab as neoadjuvant therapy in women with HER2-overexpressing operable breast cancer.

Abstract

e11060 Background: Despite clinical usefulness of trastuzumab, intrinsic or acquired resistance is a common clinical phenomenon. Potential mechanisms of resistance include aberrant downstream signaling caused by loss of PTEN and/or PIK3CA mutation, alternative signaling from IGF-1R and masking with MUC4. Methods: Patients with operable breast cancer received 12 cycles of weekly paclitaxel (80 mg/m2 IV) plus weekly trastuzumab (4mg/kg loading dose followed by 2 mg/kg IV) before surgery. All tumors were HER2-positive by immunohistochemistry (IHC) or fluorescence in situ hybridization. Expressions of ER, PgR, Ki67, PTEN, phosphorylated IGF-1R (pIGF-1R) and MUC4 were performed by IHC in core needle biopsy samples at baseline. ER and PgR status was assessed using Allred score. For Ki67 labeling index, a total of 400 cells were counted from three consecutive high-power magnifications. PTEN, pIGF-1R and MUC4 expression level was scored semiquantitatively based on staining intensity. PIK3CA mutation status was evaluated by sequencing of PIK3CA exons 9 and 20 using PCR amplification and direct sequencing. Results: Thirty-seven patients were enrolled and assessable for clinical and pathologic responses. The pCR rate was 48.6% (18/37). ER and PgR were positive in 18 (48.6%) and 16 (43.2%) patients, respectively. Negative, moderate and strong membranous expression of MUC4 was observed in 3 (8.1%), 18 (48.6%) and 12 (32.4%) patients, respectively. Membranous staining for pIGF1-R was negative in 24 (64.9%) patients. PTEN loss was observed in 33.3% (8/24) of the tumor examined. PIK3CA sequence analysis of the 13 tumors identified 2 mutations in exon 20 and 2 mutations in exon 9, corresponding to a PIK3CA mutation frequency of 30.8%. MUC4 and pIGF1-R status did not affect the pCR rate. PTEN loss and/or PIK3CA mutation were not significantly associated with pCR rate. pCR rate was significantly correlated with PgR negativity (p=0.004) and higher Ki67 (p=0.01). Conclusions: These data indicate that aberrant downstream signaling caused by loss of PTEN and/or PIK3CA mutation, alternative signaling from IGF-1R and masking with MUC4 were not important mechanisms of resistance to trastuzumab.