Dual HER 1-2 targeting of hormone-refractory prostate cancer by ZD1839 and trastuzumab

Abstract

Epidermal growth factor receptor (EGFR) and HER-2 are associated with a poor prognosis in various cancers, including prostate cancer. Inhibition of these receptors may provide a treatment for hormone-refractory prostate cancer. The presence of HER-2 (Western blot) and EGFR (5830 fmol/mg protein, ligand-binding assay) was assessed in the hormone-refractory human prostate cancer cell line, DU-145. Cells were exposed to the selective EGFR-TKI (EGFR tyrosine kinase inhibitor) gefitinib (‘Iressa™; ZD1839) and/or the HER-2-targeted monoclonal antibody trastuzumab (‘Herceptin^®’), for 96 h. Irradiation (RX) at 6 Gy the dose causing 50% growth inhibition, was applied 48 h after the start of drug treatment. There was a dose-related effect on cell survival for both ZD1839 and trastuzumab treatments. Combining ZD1839 and trastuzumab led to less than additive effects on cell survival. Chou and Talalay representations further characterised this less than additive effect on cell survival. The application of ZD1839 led to a marked elevation in the level of the negative regulator of cell division, p27. The ZD1839-trastuzumab combination had less of an impact on p27 expression compared with the effect of ZD1839 treatment alone. The lowest expression of the apoptotic-related protein, Bax, was observed in the presence of the drug combination. There was a significant interaction (synergism) between RX and either ZD1839 or trastuzumab treatments. In contrast, the drug combination with RX resulted in antagonistic cytotoxic effects. These results indicate an antagonistic interaction between EGFR and HER-2 targeting and provide molecular mechanisms supporting this observation. Data from DU-145 cells suggest that dual targeting of EGFR and HER-2 may be inappropriate for the treatment of hormone-refractory prostate cancer, especially in the context of their combination with RX.

Introduction

Prostate cancer remains one of the leading causes of cancer-related deaths in the United States of America (USA) and Europe [1]. In prostate cancer, a pejorative evolution follows a typical course of metastatic disease that is refractory to androgen ablation [2]. The progressive growth of many human carcinomas, including prostate [3], has been associated with expression of epidermal growth factor receptor (EGFR). The progression of prostate cancer to androgen-independence has been shown to be linked with the tumoral overexpression of EGFR [4]. In addition, it has been shown that androgen-independent prostate cancers express increased levels of the ErbB2 receptor protein [5], [6], [7]. Thus, both ErbB1 (EGFR) and ErbB2 inhibitors may play an important role in the therapeutic targeting of hormone-refractory prostate cancer. Accordingly, experimental data have shown that the EGFR tyrosine kinase inhibitor gefitinib (ZD1839; Iressa™) inhibited prostate cancer cell growth [8]. In addition, antitumour effects were observed in xenografts derived from androgen-independent prostate cancer cells treated with an anti-HER recombinant humanised monoclonal antibody [9]. However, another study by Agus and colleagues [10] pointed to the inefficacy of trastuzumab (Herceptin^®) on androgen-independent prostate xenograft tumour growth. Phase I clinical studies suggest a promising clinical activity of ZD1839 in advanced prostate cancer patients [11]. A combination therapy of docetaxel, estramustine and trastuzumab has led to a promising biological response (decrease in prostate-specific antigen (PSA)) in metastatic androgen-independent prostate cancer patients [12]. Based on the above findings, we felt it would be interesting to combine both EGFR and HER-2 targeting approaches in the treatment of androgen-independent prostate cancer. This led us to study the impact of a combined application of ZD1839 and trastuzumab on the androgen-refractory prostate cancer cell line, DU145, that expresses both EGFR and HER-2. The impact on cell survival when coupling irradiation with anti-HER receptor therapy was also examined, since irradiation remains one of the primary therapeutic approaches in prostate cancer.