Abstract Tumor stroma consists of genetically normal cells that express abnormal phenotype in the tumor microenviroment, and are drivers of tumor cell proliferation and migration processes. Stromal cells can represent a significant fraction of tumor mass, and represent a barrier to drug delivery, given that stromal fibroblasts exert contractile forces which reduce vascular perfusion and increase interstitial fluid pressure. In the field, the stroma is widely recognized as a target for therapy, both to normalize the reactive microenvironment, and to reduce drug delivery barriers, but with limited clinical success. We have developed a nanoparticle delivery system for docetaxel (Cellax), a conjugate of carboxymethylcellulose and PEG, which condenses into well defined 120 nm particles suitable for intravenous administration. In addition to enhanced pharmacokinetics, 5-10X improved tumor uptake, and enhanced anti-tumor efficacy compared to approved taxanes such as native DTX (Taxotere) and nab-PTX (an albumin-paclitaxel nano-formulation, Abraxane), we have observed an anti-stromal effect in breast and pancreatic models. In a 4T1 orthotopic breast cancer model in mice, 85% of Cellax particles were taken up by cancer-associated fibroblasts (CAF), a major component of stroma. Further, in 4T1 and MDA-MB-231 breast models, Cellax treatments reduced CAF by 82 and 70% respectively, while native DTX and Nab-paclitaxel exerted no anti-stromal activity. Concomitant effects were ∼70-fold increased perfusion, ∼3-fold decreased interstitial fluid pressure (IFP), and a 7-24 fold reduction in lung metastases. In a primary human pancreatic xenograft model (OCIP19), we measured a 50% reduction in αSMA+ cells (activated pancreatic stellate cells) and macrophage within 1 day of Cellax treatment, while nab-PTX only decreased the macrophage population 3-7 days post therapy. In addition, vascular perfusion in the Cellax treated pancreatic tumor was increased 12X relative to nab-PTX and control treatment groups, with significantly reduced metastases. Preliminary mechanistic studies indicate that albumin absorbs to Cellax particles, promoting interaction with SPARC-expressing stromal cells, leading to stroma-specific activity. Citation Format: Mark Ernsting, Mami Murakami, Elijus Undzys, Shyh-Dar Li. Stromal depletion by a docetaxel nanoparticle for enhanced therapy of breast and pancreatic cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4486. doi:10.1158/1538-7445.AM2014-4486