Background: Dysfunction in the regulation of apoptosis is associated with important pathologies, including cancers. While apoptosis is known to rely on the Bcl-2 family members and caspases, recent data suggest that the serine/threonine phosphatase PP2A is a key actor in regulating both Bcl-2 family members and caspases for cell life or cell death decision. Cell penetrating peptides are molecules that can translocate into cells without causing membrane damage, leading to their proposed use as vector for delivering therapeutic cargo. Focusing on both caspase-9 and PP2A, our goal was to develop cell penetrating peptides specifically designed against the caspase-9/PP2A interaction and evaluate their in vitro and in vivo therapeutic potential in various cancer cells and tumors.

Materials and Methods: We have published the Drug Phosphatase Technology (DPT) based on the design of cell penetrating peptides obtained by association of a cell permeable shuttle, DPT-sh1 to a sequence of interest. We have also shown a direct interaction of human caspase-9/human PP2Ac, detected by co-immunoprecipitation and confirmed by far western. We decided to generate a peptide containing the previously published penetrating sequence (DPT-Sh1) associated to the site of interaction of human caspase-9 (DPT-C9h), in order to disturb the interaction caspase-9/PP2Ac. Using both mouse and human breast (BC) and non-small cell lung (NSCLC) cancer cells and tumors, we have then investigated the effect of DPT-C9 (specific to the sequence of the mouse caspase-9) and DPT-C9h (specific to the sequence of the human caspase-9) peptides. In in vivo experiments, DPT-C9 and DPT-C9h peptides were intraperitonealy administered at doses ranging from 1 to 50 mg/kg/day for up to 6 weeks.

Results: We have demonstrated that both DPT-C9 and DPT-C9h peptides specifically blocked in vitro, in vivo, and ex vivo caspase-9/PP2A interaction, and induced caspase-9 activation and apoptosis in all tested cancer cell lines. In in vivo experiments, we have showed that DPT-C9 induced significant tumor growth inhibition (TGI) in mice bearing transgenic Polyoma Middle-T PymT mouse breast tumors and in mice bearing Kras mutated mouse NSCLC tumors. Similarly, DPT-C9h induced significant TGI in various primary human BC and NSCLC xenografts. Finally, no toxicity has been observed in treated mice, as well as no immunogenic response in immunodeficient mice.

Conclusions: We have identified an interaction between caspase-9/PP2A that may be used as a new therapeutic target. Using the cell penetrating peptide DPT-C9h blocking this interaction, we have demonstrated that DPT-C9h had an impressive in vitro and in vivo therapeutic effect and therefore constitutes a new therapeutic approach of human cancers.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A205.