Gene expression profiling and pathway analyses are a detrimental tool to unravel the pharmacological effect of chemotherapeutics on tumor cells. Moreover, gene expression profiling is an important tool to analyze pathways that may confer resistance to chemotherapeutic drugs commonly used in clinical practice for the treatment of several types of cancer. Topotecan is a clinically active anticancer agent, used as a second-line treatment or in combination with other drugs. Topotecan interacts with topoisomerase I and forms DNA double-strand breaks. On the other hand, Paclitaxel is a natural antitumor agent that has become the standardized first-line chemotherapy treatment for several types of cancer. Unfortunately, Paclitaxel is associated with the development of drug resistance. At present, studies suggest that Topotecan and Paclitaxel do not exhibit significant cross-resistance, suggesting that tumors resistant to one drug may still be susceptible to the other.

Here, we generated an in vitro Topotecan-resistant breast cancer cell line. MCF-7 cells were treated with 50 nM of Topotecan for two weeks with cell culture media changes every 3 to 4 days and Topotecan concentration increased 1.5-fold every two weeks until cells were resistant to 1.2 µM. Resistant cells were treated with 130 nM of paclitaxel for 15 days and then cell pellets were collected for ARN extraction and cDNA synthesis. A real time PCR array that evaluated 26 genes involved in DNA Damage signaling was performed.

Surprisingly, DNA damage signaling gene expressions were dowregulated between -3-to-10-fold change. Downregulation was significantly reduced after Paclitaxel treatment, however, DNA damage response signaling gene expression was not incresead more than 1.5 fold change upregulation.

This findings suggest cross-resistance in HER2 expressing breast cancer cells and that the use of Paclitaxel in combination with topotecan for the treatment of HER2 breast cancer is not a effective line for treatment.