Type I
protein kinase A (PKAI) is overexpressed in the majority of human
tumors and plays a relevant role in neoplastic transformation, conveying mitogenic signals of different
growth factors and oncogenes. Inhibition of PKAI by
antisense oligonucleotides targeting its RIalpha regulatory subunit results in
cancer cell growth inhibition in vitro and in vivo. We have recently shown that a mixed backbone
oligonucleotide targeting RIalpha can cooperatively inhibit human
cancer cell growth when combined with selected cytotoxic drugs. In the present study, we have used
HYB 165, a novel
DNA/
RNA hybrid mixed backbone
oligonucleotide that exhibits improved pharmacokinetic and bioavailability properties in vivo and is presently undergoing Phase I trials. We have shown that
HYB 165 exhibits a dose-dependent inhibitory effect on ZR-75-1 cells and a cooperative activity with
docetaxel, a cytotoxic
drug active in
breast cancer. The antiproliferative activity is accompanied by increased apoptosis, as compared with each single agent. On the basis of our previous demonstration of a structural and functional relation between PKAI and
epidermal growth factor receptor, we have performed a double blockade of these pathways using
HYB 165 in combination with
monoclonal antibody (
MAb) C225, an anti-
epidermal growth factor receptor chimeric MAb. The two compounds determined a cooperative growth inhibitory effect on ZR-75-1 cells and increased apoptosis. To study whether different
biological agents and cytotoxic drugs can interact together, low doses of
HYB 165,
MAb C225, and
docetaxel were combined causing an even greater cooperative effect toward growth inhibition. Finally, we have demonstrated that each single agent is able to induce bcl-2 phosphorylation and that the three agents, used in combination at suboptimal doses, determine a greater degree of bcl-2 phosphorylation and cause apoptosis of the majority of ZR-75-1 cells. These findings provide the basis for a novel strategy of treatment of
breast cancer patients because both
HYB 165 and
MAb C225 are presently under clinical evaluation.