Multigene targeting of signal transduction pathways for the treatment of breast and prostate tumors

Abstract

Previous studies have demonstrated that monospecific antisense oligonucleotides (oligos) directed against mRNA encoding proteins associated with tumor growth, death, and survival are efficacious against breast and prostate tumors. Targeted proteins, associated with different signal transduction pathways, have included transforming growth factor-alpha [TGF-α (MR₁)], its binding site the epidermal growth factor receptor [EGFR (MR₂)] sharing sequence homology to the breast cancer prognostic marker Her-2/neu, an apoptosis inhibiting protein [bcl-2 (MR₄)], and the androgen receptor [AR (MR₅)]. In attempts to enhance antisense therapy, recent reports describe how two of the binding sites mentioned above can be sequentially placed within a single complementary (bispecific) strand and administered either in the presence or absence of additional therapeutic agents. When tested against breast and prostate tumor cell lines specific differences were noted: MCF-7 breast cancer cells were more receptive to the inhibitory effects of monospecific oligos, whereas PC-3 and LNCaP prostate cells were particularly responsive to bispecifics. In an effort to identify agents which enhance the activity of oligos and which possess less toxicity than traditionally employed chemotherapeutics, Rapamycin, an immunosuppressive agent known to regulate tumor growth and signal transduction mediated by the mTOR receptor, is compared to paclitaxel in combination therapy employing monospecific or bispecific oligos. Bispecifics were constructed recognizing the binding sites for TGF-α and EGFR mRNA [TGF-α/EGFR (MR₁₂) and EGFR/TGF-α (MR₂₁)]; another pair recognized binding sites for EGFR and bcl-2 [EGFR/bcl-2 (MR₂₄) and bcl-2/EGFR (MR₄₂)]; while a third pair employed only against the LNCaP prostate cell line recognized bcl-2 and the androgen receptor [bcl-2/AR (MR4₄₅) and AR/bcl-2 (MR₅₄)]. Oligo pairs differ in their 5′–3′ linear binding site orientations, and were tested in vitro against MCF-7 breast and PC-3 and LNCaP prostate tumor cell lines. Following cell attachment, incubations were done for 2 days with the agents followed by 2 days in their absence. Five experiments evaluated the effect of monospecific or bispecific antisense oligos in combination with an LD₅₀ dosage of either Rapamycin or paclitaxel and led to the conclusion that although these agents act via different mechanisms, they are comparable in effectiveness.