Engineered nanoparticles have the potential to revolutionize the diagnosis and treatment of many diseases; for example, by allowing the targeted delivery of a drug to particular subsets of cells. However, so far, such nanoparticles have not proved capable of surmounting all of the biological barriers required to achieve this goal. Nevertheless, advances in nanoparticle engineering, as well as advances in understanding the importance of nanoparticle characteristics such as size, shape and surface properties for biological interactions, are creating new opportunities for the development of nanoparticles for therapeutic applications. This Review focuses on recent progress important for the rational design of such nanoparticles and discusses the challenges to realizing the potential of nanoparticles.

Transferrin receptor (TfR, CD71) has long been therapeutic target due to its over-expression on many malignant tissues. In this study, PRINT® nanoparticles were conjugated with TfR ligands for targeted drug delivery. Cylindrical poly(ethylene glycol)-based PRINT nanoparticles (diameter [d] = 200 nm, height [h] = 200 nm) labeled with transferrin receptor antibody (NP-OKT9) or human transferrin (NP-hTf), showed highly specific TfR-mediated uptake by all human tumor cell lines tested, relative to negative controls (IgG1 for OKT9 or bovine transferrin (bTf) for hTf). The targeting efficiency was dependent on particle concentration, ligand density, dosing time and cell surface receptor expression level. Interestingly, NP-OKT9 or NP-hTf showed little cytotoxicity on all solid tumor cell lines tested but were very toxic to Ramos B-cell lymphoma, whereas free OKT9 or hTf was not toxic. There was a strong correlation between TfR ligand density on particle surface and cell viability and particle uptake. NP-OKT9 and NP-hTf were internalized into acidic intracellular compartments but were not localized in EEA1 enriched early endosomes or lysosomes. Elevated caspase 3/7 activity indicates activation of apoptosis pathways upon particle treatment. Supplementation of iron suppressed the toxicity of NP-OKT9 but not NP-hTf, suggesting different mechanisms by which NP-hTf and NP-OKT9 exerts cytotoxicity on Ramos cells. Based on such an observation, the complex role of multivalency in nanoparticles is discussed. In addition, our data clearly reveal that one must be careful in making claims of "lack of toxicity" when a targeting molecule is used on nanoparticles and also raise concerns for unanticipated off-target effects when one is designing targeted chemotherapy nano-delivery agents. ‡ To whom correspondence should be addressed. desimone@unc.edu. * Present address: Department of Chemistry, University of North Texas, Denton, TX 76203 † J. W. and S. T. contributed equally to this work. Tables S1-S2. Transferrin receptor expression, the toxicity of free human transferrin and OKT9, the cell uptake and toxicity of 5 μm particles and the non-colocalization of PRINT nanoparticles with endosomes and lysosomes in Figures S1-S4. This material is available free of charge via the Internet at SUPPORTING INFORMATION AVAILABLE. Composition of PRINT particles and the IC 50 values of chemotherapeutics in

SUMMARYAbietane diterpenoids are major constituents of conifer resins that have important industrial and medicinal applications. However, their function in plants is poorly understood. Here we show that dehydroabietinal (DA), an abietane diterpenoid, is an activator of systemic acquired resistance (SAR), which is an inducible defense mechanism that is activated in the distal, non-colonized, organs of a plant that has experienced a local foliar infection. DA was purified as a SAR-activating factor from vascular sap of Arabidopsis thaliana leaves treated with a SAR-inducing microbe. Locally applied DA is translocated through the plant and systemically induces the accumulation of salicylic acid (SA), an important activator of defense, thus leading to enhanced resistance against subsequent infections. The NPR1 (NON-EXPRESSOR OF PR GENES1), FMO1 (FLAVIN-DEPENDENT MONOOXYGENASE1) and DIR1 (DEFECTIVE IN INDUCED RESISTANCE1) genes, which are critical for biologically induced SAR, are also required for the DA-induced SAR, which is further enhanced by azelaic acid, a defense priming molecule. In response to the biological induction of SAR, DA in vascular sap is redistributed into a SAR-inducing 'signaling DA' pool that is associated with a trypsin-sensitive high molecular weight fraction, a finding that suggests that DA-orchestrated SAR involves a vascular sap protein(s).