A Cellular Trojan Horse for Delivery of Therapeutic Nanoparticles into Tumors Mi-Ran Choi, ² Katie J. Stanton-Maxey, ² Jennifer K. Stanley, ² Carly S. Levin, ‡,§ Rizia Bardhan, ‡,§ Demir Akin, | Sunil Badve, ⊥ Jennifer Sturgis, # J. Paul Robinson, # Rashid Bashir, |,# Naomi J. Halas, ‡,§,+ and Susan E. Clare* ,² Department of Surgery, Indiana UniVersity School of Medicine, Indianapolis, Indiana 46202, Department of Chemistry, Rice UniVersity, Houston, Texas 77005, Department of Electrical and Computer Engineering, Rice UniVersity, Houston, Texas 77005, Birck Nanotechnology Center, Purdue UniVersity, West Lafayette, Indiana 47907, Department of Pathology and Laboratory Medicine, Indiana UniVersity School of Medicine, Indianapolis, Indiana 46202, Bindley BioScience Center, Purdue UniVersity, West Lafayette, Indiana 47907, and Laboratory for Nanophotonics, Rice UniVersity, Houston, Texas 77005 Received August 31, 2007; Revised Manuscript Received September 25, 2007 ABSTRACT Destruction of hypoxic regions within tumors, virtually inaccessible to cancer therapies, may well prevent malignant progression. The tumor’s recruitment of monocytes into these regions may be exploited for nanoparticle-based delivery. Monocytes containing therapeutic nanoparticles could serve as “Trojan Horses” for nanoparticle transport into these tumor regions. Here we report the demonstration of several key steps toward this therapeutic strategy: phagocytosis of Au nanoshells, and photoinduced cell death of monocytes/macrophages as isolates and within tumor spheroids. Nanotechnology shows great promise for the diagnosis and treatment of cancer. Nanoparticle-based therapeutics have been successfully delivered into tumors by exploiting the enhanced permeability and retention (EPR) effect, a property that permits nanoscale structures to be taken up passively into tumors without the assistance of antibodies or other targeting moieties. 1 Nanoparticle-based therapies have also been targeted to malignant cells by conjugating the nano- particles with antibodies or peptides specific to those cells. 2 An alternative strategy for delivering nanoparticle-based therapies within the tumor microenvironment, not previously explored, involves the uptake of nanoparticles within non- malignant cells which subsequently are recruited into the tumor. These nonmalignant cells, rather than being “innocent bystanders”, may actually be the active agents of metastatic disease. The centers of solid tumors are frequently observed to be largely necrotic, resulting from prolonged hypoxia: insuf- ficient availability of oxygen and glucose to meet the metabolic demands of the malignant cells. During tumor growth, the rapid proliferation of malignant cells places cells within the core of the tumor at increasingly larger distances from their nearest capillaries, drastically compromising blood flow to these cells. In addition to inducing necrosis, this isolation of cells with respect to the tumor vasculature renders the hypoxic areas of tumors inaccessible to virtually all molecular or nanoparticle-based therapies where delivery into the tumor is based on the EPR effect. This not only is a severe limitation to many standard therapeutic strategies such as chemotherapy 3 but also may limit the efficacy of many nanoparticle-based therapeutic approaches currently in de- velopment. In fact, one possible scenario for the progression of cancer to its latter, highly fatal stages is that cells which survive in these inaccessible hypoxic regions may themselves be the source of subsequent local recurrence and distant metastasis. It has been shown that hypoxia exerts a selective pressure on tumor cells in that only those with an aggressive phenotype (e.g., mutated p53) are able to survive in a low oxygen tension microenvironment. 4-6 One of the body’s responses to the presence of a malignant neoplasm is the recruitment of peripheral blood monocytes into the tumor, induced into the tumor mass by a chemoat- tractive gradient. Once the monocytes cross the endothelial basement membrane, they differentiate into macrophages. * Corresponding author. Phone: (317) 278-3907. Fax (317) 278-3185. E-mail: sclare@iupui.edu. ² Department of Surgery, Indiana University School of Medicine. ‡ Department of Chemistry, Rice University. § Laboratory for Nanophotonics, Rice University. | Birck Nanotechnology Center, Purdue University. ⊥ Department of Pathology and Laboratory Medicine, Indiana University School of Medicine. # Bindley BioScience Center, Purdue University. + Department of Electrical and Computer Engineering, Rice University. NANO LETTERS 2007 Vol. 7, No. 12 3759-3765 10.1021/nl072209h CCC: $37.00 © 2007 American Chemical Society Published on Web 11/03/2007