Abstract P4-16-10: A novel paclitaxel-dendrimer conjugate demonstrates better efficacy in breast cancer models

Abstract

Abstract BACKGROUND: Breast cancer is the 2nd most common cause of cancer deaths in women. Chemotherapy is an important treatment modality. Yet, delivery of drugs such as paclitaxel (PTX) to cancer cells remains a challenge, mainly due to a drug's low solubility, rapid in vivo clearance, high systemic toxicity, and low drug delivery to tumor. Nanotechnology has been proposed as a solution to these problems. In active targeting, increased tumor specificity of nanoparticles is achieved through attachment of agents that target the molecular differences between normal and malignant cells. Dendrimers are a category of synthetic, monodisperse and multivalent nanopolymers, capable of high drug-loading and multiple covalent bonding of bioactive moieties. Conjugation using enzyme cleavable linkers allows for specific, targeted release. While PTX delivery through dendrimers has been established, such vehicles have not used active targeting for specific delivery to cancer cells. Thus, a need exists for specific delivery of PTX through an active-targeting dendrimer-based vehicle, while maintaining high-drug loading properties of dendrimers. This study reports the synthesis, characterization, and biological assessment of polyamidoamine (PAMAM) dendrimers conjugated to PTX via a peptide spacer cleavable by a cancer-upregulated enzyme, Cathepsin B (CatB). METHODS: PAMAM was conjugated to PTX via an enzyme-cleavable, peptide spacer resulting in formation of a PTX-laden dendrimer (PLD). Structure confirmation of PLD was done by 1H-NMR and mass spectrometry (MS). MDA-MB-231 breast cancer cells and MCF10A immortalized normal breast cells served as in vitro models for tumor efficacy and specificity. WST-1 based cytotoxicity assays were performed after 72h drug incubation at various concentrations of PTX or PTX-equivalent dendrimer. In vivo tumor efficacy of PLD as compared to PTX alone or vehicle alone was performed on MDA-MB-231 xenografts in athymic nude mice. RESULTS: MS data showed a broad peak centered at 24 kDa. NMR proton integration revealed a PTX:dendrimer ratio of 7:1. This ratio was used to deliver specific molar amounts of PTX in PLD form during assays. Cytotoxicity assays for the MDA-MB-231 demonstrated a marked shift in the relative IC50 values of PTX incorporated in PLD. The IC50 of paclitaxel for MDA-MB-231 cancer cells was 0.640 nM for PLD relative to 3.29 nM for PTX. By contrast, the IC50 of PLD for MCF10A normal cells was 2.45 nM as compared to 2.85 nM for PTX. This smaller shift in relative IC50 values correlates to reported CatB activity. When compared at 10 nM PTX/PLD, negative control PAMAM had no effect on cell growth, while PLD had a statistically-significant reduced proliferation compared PTX for MDA-MB-231 cells only. In vivo studies revealed PLD had a strong reduction in tumor volume growth compared to vehicle alone (p<0.05) up to 2 weeks after treatment while PTX treatment showed a smaller reduction (p>0.05). CONCLUSIONS: PTX when delivered through PLD showed increased cytotoxicity to MDA-MB-231 cancer cells but not normal MCF10A cells, correlating with previous reports on CatB activity. PLD demonstrates enhanced reduction in tumor growth as compared to PTX. This suggests successful development of a cancer-specific active targeting delivery vehicle. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P4-16-10.