Estimation of hormone receptor status and HER2 in cytologic cell blocks from breast cancer using the novel rabbit monoclonal antibodies (SP1, SP2, and SP3).

The determination of estrogen (ER) and progesterone (PR) receptor status has become standard practice in the evaluation of patients with invasive breast cancer, having important prognostic and therapeutic implications. HER2 assessment is important to evaluate the response to Herceptin (Trastuzumab) therapy for primary and metastatic breast cancer. This study is undertaken to compare rabbit monoclonal antibodies (RabMAb) for ER, PR, and HER2 against FDA-approved monoclonal and polyclonal antibodies (FDAMpab). Cell blocks from primary and metastatic/recurrent breast carcinomas of 52 breast cancer patients were used. Immunohistochemistry was performed, following optimized epitope retrieval, with a polymer based detection system using RabMAb: ER (SP1), PR (SP2), and HER2 (SP3). FDA approved Mpab (Dako) used were: ER (1D5); PR (PgR636); and HercepTest kit according to manufacturer's instructions. HER2 immunostain is correlated with FISH results. Overall, positive, and negative agreement is as follows: 88.5, 88.9, and 88.2% for ER; 84.6, 70.5, and 91.4% for PR; 58.3, 100, and 50% for HER2. There is substantial to moderate agreement between RabMAb and FDAMpab for ER (kappa = 0.75) and PR (kappa = 0.64), respectively. There is poor agreement (kappa = 0.25) between RabMAb (SP3) and FDApab (HercepTest). SP3 shows better concordance (93.8%) than HercepTest (46.9%) with FISH results. RabMAb SP clones are almost comparable with FDA-approved ER and PR, with fair to moderate agreement. Both are as sensitive as their FDA-approved clones. SP3, on the other hand, is superior to HercepTest for detecting HER2 overexpression, with an excellent concordance with FISH.