Synergistic antitumor effect of novel mixed backbone oligonucleotides targeting protein kinase A in combination with cytotoxic drugs or biological agents.

Protein kinase A type I (PKAI) plays a key role in neoplastic transformation and conveys mitogenic signals of different growth factors and oncogenes. Inhibition of PKAI by antisense (AS) oligonucleotides targeting its Rla regulatory subunit results in cancer cell growth inhibition in vitro and in vivo. A novel class of mixed backbone oligonucleotides (MBOs), with improved pharmacokinetic properties and bioavailability, were tested on soft agar growth of several human cancer cell types. As compared to their respective mismatched controls, the AS Rla MBOs demonstrated a dose-dependent inhibition of colony formation in all cell lines. A non-inhibitory dose of each MBO was used to study whether any cooperative effect may occur between the AS and a series of cytotoxic drugs acting by different mechanisms. Treatment with these oligos resulted in a synergistic growth inhibition, which correlated with increased apoptosis, when combined with taxanes, platinum-based compounds and topoisomerase ll-selective drugs. When the MBOs administered either i.v. or per os were added to paclitaxel, a cooperative effect was also obtained in vivo, causing tumor growth inhibition and increase of survival in nude mice bearing human colon cancer xenografts. Based on our previous demonstrations of a structural and functional interaction between PKAI and the epidermal growth factor receptor (EGFR) and of the PKAI overexpression in cancer cells in which a TGFa-EGFR autocrine pathway is activated, we have studied whehter a combined blockade of both PKAI and EGFR may represent a therapeutic strategy. MAb C225 is a humanized monoclonal antibody which blocks activation of EGFR tyrosine kinase. We have demonstrated that the AS Rla MBO in combination with MAb C225 caused a clear cooperative growth inhibitory effect at all doses tested in the breast cancer cells ZR-75-1, and that a marked synergism of action could be obtained when the two compounds were combined with the docetaxel. Since this novel class of AS Rla MBOs is now entering clinical evaluation and MAb C225 is currently in phase II clinical trials, these combined treatments may.represent a novel therapeutic strategy to be evaluated in a clinical setting.