Self-assembly of pH-responsive fluorinated dendrimer-based particulates for drug delivery and noninvasive imaging Jason M. Criscione a , Bonaire L. Le b , Eric Stern a , Matthew Brennan c , Christoph Rahner d , Xenophon Papademetris a, e , Tarek M. Fahmy a, b, * a Malone Engineering Center, 55 Prospect Street, New Haven, CT 06511, USA b Department of Chemical Engineering, Yale University, New Haven, CT 06511, USA c Department of Surgery, Yale University, New Haven, CT 06520, USA d Department of Cell Biology, Yale University, New Haven, CT 06520, USA e Department of Diagnostic Radiology, Yale University, New Haven, CT 06520, USA article info Article history: Received 30 January 2009 Accepted 13 April 2009 Available online 13 May 2009 Keywords: PAMAM Fluorinated dendrimers pH-responsive 19 F MRI Self-assembly abstract Dendrimers are nanoscale macromolecules with well-defined branching chemical structures. Control over the architecture and function of these structures has enabled many advances in materials science and biomedical applications. Though dendrimers are directly synthesized by iteration of simple repetitive steps, generation of the larger, more complex structures required for many biomedical applications by covalent synthetic methods has been challenging. Here we demonstrate a spontaneous self-assembly of poly(amidoamine) dendrimers into complex nanoscopic and microscopic particulates following partial fluorination of the constituent dendrimer subunits. These dense particulates exhibit a stimulus-induced response to low external pH that causes their disassembly over time, enabling controlled release of encapsulated agents. In addition, we show that these assemblies offer a sufficiently high density of fluorine spins to enable detection of their site-specific accumu- lation in vivo by 19 F magnetic resonance imaging ( 19 F MRI). Fluorinated dendrimer-based particulates present new features and capabilities important for a wide variety of emerging biomedical applications. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Dendrimers are a class of branched polymers distinguished by their repeating patterns emanating from a central core [1]. The well- defined architecture of these molecules obtained by control over their synthesis [2] has led to a diverse array of therapeutic and diagnostic applications [3–15]. Though dendrimers are directly synthesized by iteration of simple repetitive steps, generation of larger and more complex structures by covalent synthetic methods has proven far more challenging and expensive. Thus, assembling dendrimer or dendron subunits, either by self-assembly [16–18] or directed assembly [19–24], has become an attractive and inexpensive short cut to achieve these structures. However, fabrication of ordered, solid particulates from dendrimers for applications in materials science and biological systems has not been reported to date. We hypothesized that controlled fluorination of poly(amidoamine) (PAMAM) starburst dendrimers would mediate self-assembly through the ‘‘fluorophobic effect’’ [17,18,25]. This effectdthe tendency of fluorinated molecules to separate and assemble into a fluorous phase that is both hydrophobic and lipophobic [25] has unveiled a unique set of non-covalent interactions useful for engineering self-assembled systems. Exerting control over these interactions will enable the design of interesting new systems for therapeutic and diagnostic applications. Indeed, this effect has been observed in the self-assembly of semi- fluorinated monodendrons into supramolecular columnar dendrimers which exhibit liquid crystalline properties [17,18]. We chose PAMAM starburst dendrimers as our starting subunit because of their well-established properties and wide use in biomedical applications [1]. Additionally, the amphiphilic nature and easily modifiable surface of the PAMAM dendrimer makes it an attractive vehicle for targeted drug delivery and molecular imaging. The use of these systems for therapeutic delivery applications has been limited by the quantity and chemical nature of the encapsulated agent. While the nanoscopic size and amphiphilicity of dendrimers are important features for drug delivery, it would be attractive to be able to manipulate this system to enhance encapsulation and release of different agents. Self-assembly of partially fluorinated PAMAM into larger particulates preserves dendrimer integrity and creates a dense incorporation of fluorines sufficient to enable noninvasive observation by 19 F MRI. We therefore hypothesized that the self-assembly of * Corresponding author. Malone Engineering Center, 55 Prospect Street, New Haven, CT 06511, USA. Tel.: þ1 203 432 1043; fax: þ1 203 432 0030. E-mail address: tarek.fahmy@yale.edu (T.M. Fahmy). Contents lists available at ScienceDirect Biomaterials journal homepage: www.elsevier.com/locate/biomaterials 0142-9612/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.biomaterials.2009.04.014 Biomaterials 30 (2009) 3946–3955