Altres ajuts: we are indebted to CIBER de Bioingeniería, Biomateriales y Nanomedicina (projects NANOREMOTE and VENOM4CANCER) to EV and AV respectively, Marató de TV3 foundation (TV32013-132031) and CIBER (NanoMets3) to RM. Protein production has been partially performed by the ICTS "NANBIOSIS", more specifically by the Protein Production Platform of CIBER in Bioengineering, Biomaterials & Nanomedicine (CIBER-BBN)/IBB, at the UAB SepBioES scientific-technical service (http://www.nanbiosis.es/unit/u1-protein-production-platform-ppp/), whereas the in vivo biodistribution assays were performed in the NANBIOSIS Nanotoxicology platform (http://www.nanbiosis.es/unit/u18-nanotoxicology-unit/). We are also indebted to Fran Cortes from the Cell Culture and Cytometry Units of the Servei de Cultius Cel·lulars, Producció d'Anticossos i Citometria (SCAC), and to the Servei de Microscòpia at the UAB. AV received an ICREA ACADEMIA award. U.U received a Sara Borrell postdoctoral fellowship from ISCIII, MVC was supported by Miguel Servet contract from ISCIII, and JSF received and AECC postdoctoral fellowship.

Sustained release of drug delivery systems (DDS) has the capacity to increase cancer treatment efficiency in terms of drug dosage reduction and subsequent decrease of deleterious side effects. In this regard, many biomaterials are being investigated but none offers morphometric and functional plasticity and versatility comparable to protein-based nanoparticles (pNPs). Here we describe a new DDS by which pNPs are fabricated as bacterial inclusion bodies (IB), that can be easily isolated, subcutaneously injected and used as reservoirs for the sustained release of targeted pNPs. Our approach combines the high performance of pNP, regarding specific cell targeting and biodistribution with the IB supramolecular organization, stability and cost effectiveness. This renders a platform able to provide a sustained source of CXCR4-targeted pNPs that selectively accumulate in tumor cells in a CXCR4 colorectal cancer xenograft model. In addition, the proposed system could be potentially adapted to any other protein construct offering a plethora of possible new therapeutic applications in nanomedicine.