Design considerations for PAMAM dendrimer therapeutics.

We have previously shown that methotrexate (MTX) conjugated to a cancer-specific poly amido amine (PAMAM) dendrimer has a higher therapeutic index than MTX alone. Unfortunately, these therapeutics have been difficult to advance because of the complicated syntheses and an incomplete understanding of the dendrimer properties. We wished to address these obstacles by using copper-free click chemistry to functionalize the dendrimer scaffolds and to exploring the effects of two dendrimer properties (the targeting ligand and drug linkage) on cytotoxicity. We conjugated either ester or amide-linker modified MTX to dendrimer scaffolds with or without folic acid (FA). Because of multivalency, the FA and MTX functionalized dendrimers had similar capacities to target the folate receptor on cancer cells. Additionally, we found that the ester- and amide-linker modified MTX compounds had similar cytotoxicity but the dendrimer-ester MTX conjugates were much more cytotoxic than the dendrimer-amide MTX conjugates. These results clarify the impact of these properties on therapeutic efficacy and will allow us to design more effective polymer therapeutics.

Dendrimer-based multivalent vancomycin nanoplatform for targeting the drug-resistant bacterial surface.

Vancomycin represents the preferred ligand for bacteria-targeting nanosystems. However, it is inefficient for emerging vancomycin-resistant species because of its poor affinity to the reprogrammed cell wall structure. This study demonstrates the use of a multivalent strategy as an effective way for overcoming such an affinity limitation in bacteria targeting. We designed a series of fifth generation (G5) poly(amidoamine) (PAMAM) dendrimers tethered with vancomycin at the C-terminus at different valencies. We performed surface plasmon resonance (SPR) studies to determine their binding avidity to two cell wall models, each made with either a vancomycin-susceptible (D)-Ala-(D)-Ala or vancomycin-resistant (D)-Ala-(D)-Lac cell wall precursor. These conjugates showed remarkable enhancement in avidity in the cell wall models tested, including the vancomycin-resistant model, which had an increase in avidity of four to five orders of magnitude greater than free vancomycin. The tight adsorption of the conjugate to the model surface corresponded with its ability to bind vancomycin-susceptible Staphylococcus aureus bacterial cells in vitro as imaged by confocal fluorescent microscopy. This vancomycin platform was then used to fabricate the surface of iron oxide nanoparticles by coating them with the dendrimer conjugates, and the resulting dendrimer-covered magnetic nanoparticles were demonstrated to rapidly sequester bacterial cells. In summary, this article investigates the biophysical basis of the tight, multivalent association of dendrimer-based vancomycin conjugates to the bacterial cell wall, and proposes a potential new use of this nanoplatform in targeting Gram-positive bacteria.

Polyvalent dendrimer-methotrexate as a folate receptor-targeted cancer therapeutic.

Our previous studies have demonstrated that a generation 5 dendrimer (G5) conjugated with both folic acid (FA) and methotrexate (MTX) has a higher chemotherapeutic index than MTX alone. Despite this, batch-to-batch inconsistencies in the number of FA and MTX molecules linked to each dendrimer led to conjugate batches with varying biological activity, especially when scaleup synthesis was attempted. Since the MTX is conjugated through an ester linkage, there were concerns that biological inconsistency could also result from serum esterase activity and differential bioavailability of the targeted conjugate. In order to resolve these problems, we undertook a novel approach to synthesize a polyvalent G5-MTX(n) conjugate through click chemistry, attaching the MTX to the dendrimer through an esterase-stable amide linkage. Surface plasmon resonance binding studies show that a G5-MTX(10) conjugate synthesized in this manner binds to the FA receptor (FR) through polyvalent interaction showing 4300-fold higher affinity than free MTX. The conjugate inhibits dihydrofolate reductase, and induces cytotoxicity in FR-expressing KB cells through FR-specific cellular internalization. Thus, the polyvalent MTX on the dendrimer serves the dual role as a targeting molecule as well as a chemotherapeutic drug. The newly synthesized G5-MTX(n) conjugate may serve as a FR-targeted chemotherapeutic with potential for cancer therapy.

Bifunctional PAMAM dendrimer conjugates of folic acid and methotrexate with defined ratio.

Our group previously developed a multifunctional, targeted cancer therapeutic based on Generation 5 (G5) polyamidoamine (PAMAM) dendrimers. In those studies we conjugated the targeting molecule folic acid (FA) and the chemotherapeutic drug methotrexate (MTX) sequentially. This complex macromolecule was shown to selectively bind and kill KB tumor cells that overexpress folate receptor (FR) in vitro and in vivo. However, the multistep conjugation strategy employed in the synthesis of the molecule resulted in heterogeneous populations having differing numbers and ratios of the functionally antagonistic FA and MTX. This led to inconsistent and sometimes biologically inactive batches of molecules, especially during large-scale synthesis. We here resolved this issue by using a novel triazine scaffold approach that reduces the number of dendrimer conjugation steps required and allows for the synthesis of G5 conjugates with defined ratios of FA and MTX. Although an unoccupied γ-glutamyl carboxylate of FA has been previously suggested to be nonessential for FR binding, the functional requirement of an open α-carboxylate still remains unclear. In an attempt to also address this question, we have synthesized isomeric FA dendrimer conjugates (α-carboxyl or γ-carboxyl linked). Competitive binding studies revealed that both linkages have virtually identical affinity toward FR on KB cells. Our studies show that a novel bifunctional triazine-based conjugate G5-Triazine-γMTX-αFA with identical numbers of FA and MTX binds to FR through a polyvalent interaction and induces cytotoxicity in KB cells through FR-mediated cellular internalization, inducing higher toxicity as compared to conjugates synthesized by the multistep strategy. This work serves as a proof of concept for the development of bifunctional dendrimer conjugates that require a defined ratio of two functional molecules.

Acetonitrile shortage: use of isopropanol as an alternative elution system for ultra/high performance liquid chromatography.

Acetonitrile is a choice of solvent for almost all chromatographic separations. In recent years, researchers around the globe have faced an acetonitrile shortage that affected routine analytical operations. Researchers have tried to counter this shortage by applying many innovative solutions, including using ultra performance liquid chromatography (UPLC) columns that are shorter and smaller in diameter than traditional high performance liquid chromatography (HPLC) columns, thus significantly decreasing the volume of eluent required. Although utilizing UPLC in place of HPLC can alleviate the solvent demand to some extent, acetonitrile is generally thought of as the solvent of choice due to its versatility. In the following communication, we describe an alternative eluent system that uses isopropanol in place of acetonitrile as an organic modifier for routine chromatographic separations. We report here the development of an isopropanol based UPLC protocol for G5 PAMAM dendrimer based conjugates that was transferred to semi-preparative applications.

Polyvalent saccharide-functionalized generation 3 poly(amidoamine) dendrimer-methotrexate conjugate as a potential anticancer agent.

A saccharide-terminated generation 3 (G3) polyamidoamine (PAMAM) dendrimer was synthesized as a drug carrier. Utilizing this dendritic platform, we have successfully synthesized polyvalent conjugates (G3-MTX) containing the drug methotrexate (MTX). Surface Plasmon Resonance (SPR) results showed that G3-MTX presented three orders of magnitude enhancement in binding avidity to folate-binding protein (FBP) as compared to the free folic acid (FA). Flow cytometric and confocal microscopic analysis showed that conjugate (G3-MTX-FI) containing imaging agent fluorescein-5(6)-carboxamidohexanoic acid (FI) was internalized into folate receptor (FR)-expressing KB cells in dose-dependent and receptor-mediated fashion. The G3-MTX induced a dose-dependent cytotoxicity in the KB cells. Therefore, the polyvalent G3-MTX may have potential as an anticancer nanodevice for the specific targeting and killing of FR-expressing tumor cells.

Evaluation of a symmetry-based strategy for assembling protein complexes.

We evaluate a strategy for assembling proteins into large cage-like structures, based on the symmetry associated with the native protein's quaternary structure. Using a trimeric protein, KDPG aldolase, as a building block, two fusion proteins were designed that could assemble together upon mixing. The fusion proteins, designated A-(+) and A-(-), comprise the aldolase domain, a short, flexible spacer sequence, and a sequence designed to form a heterodimeric antiparallel coiled-coil between A-(+) and A-(-). The flexible spacer is included to minimize constraints on the ability of the fusion proteins to assemble into larger structures. On incubating together, A-(+) and A-(-) assembled into a mixture of complexes that were analyzed by size exclusion chromatography coupled to multi-angle laser light scattering, analytical ultracentrifugation, transmission electron microscopy and atomic force microscopy. Our analysis indicates that, despite the inherent flexibility of the assembly strategy, the proteins assemble into a limited number of globular structures. Dimeric and tetrameric complexes of A-(+) and A-(-) predominate, with some evidence for the formation of larger assemblies; e.g. octameric A-(+): A-(-) complexes.

RGD dendron bodies; synthetic avidity agents with defined and potentially interchangeable effector sites that can substitute for antibodies.

Poly(amidoamine) (PAMAM) dendrons were synthesized with c(RGDyK) peptide on the surface to create a scaffold for cellular targeting and multivalent binding. Binary dendron-RGD conjugates were synthesized with a single Alexa Fluor 488, biotin, methotrexate drug molecule, or additional functionalized dendron at the focal point. The targeted dendron platform was shown to specifically target αvß3 integrin expressing human umbilical vein endothelial cells (HUVEC) and human glioblastoma cells (U87MG) in Vitro via flow cytometry. Specific targeting of the dendron-RGD platform was further confirmed by confocal microscopy. Biological activity of the targeted drug conjugate was confirmed via XTT assay. The orthogonal reaction chemistry used at the dendron focal point gives a precise 1:1 ratio of the attachment of multiple functionalities to a small-molecular-weight, chemically stable, high avidity molecule. These studies serve as a framework to selectively combine biologically relevant functions with enhanced specific binding activity to substitute for antibodies in many diagnostic and therapeutic applications.

The implications of stochastic synthesis for the conjugation of functional groups to nanoparticles.

Stochastic synthesis of a ligand coupled to a nanoparticle results in a distribution of populations with different numbers of ligands per nanoparticle. This distribution was resolved and quantified using HPLC and is in excellent agreement with the ligand/nanoparticle average measured by 1H NMR, gel permeation chromatography (GPC), and potentiometric titration, and yet significantly more disperse than commonly held perceptions of monodispersity. Two statistical models were employed to confirm that the observed heterogeneity is consistent with theoretical expectations.

HER2 specific delivery of methotrexate by dendrimer conjugated anti-HER2 mAb.

Herceptin, a humanized monoclonal antibody that binds to human growth factor receptor-2 (HER2), was covalently attached to a fifth-generation (G5) polyamidoamine dendrimer containing the cytotoxic drug methotrexate. The specific binding and internalization of this conjugate labeled with FITC was clearly demonstrated in cell lines overexpressing HER2 by flow cytometry as well as confocal microscopic analysis. In addition, binding and uptake of antibody conjugated dendrimers was completely blocked by excess non-conjugated herceptin. The dendrimer conjugate was also shown to inhibit the dihydrofolate reductase with similar activity to methotrexate. Co-localization experiments with lysotracker red indicate that antibody conjugate, although internalized efficiently into cells, has an unusually long residence time in the lysosome. Somewhat lower cytotoxicity of the conjugate in comparison to free methotrexate was attributed to the slow release of methotrexate from the conjugate and its long retention in the lysosomal pocket.

HER2 specific tumor targeting with dendrimer conjugated anti-HER2 mAb.

In the present study, we report the synthesis and human growth factor receptor-2 (HER2) specific tumor targeting properties of a dendrimer conjugated to anti-HER2 mAb (monoclonal antibody) conjugate. The polyamidoamine (PAMAM) dendrimer generation five (G5) was labeled with alexaFluor 488 and conjugated to anti-HER2 mAb. The binding and internalization of the antibody-conjugated dendrimer to HER2-expressing cells was evaluated by flow cytometry and confocal microscopy. Uniquely, the conjugate demonstrated cellular uptake and internalization in HER2-expressing cells as compared to free antibody. The time course of internalization and blocking experiments with free antibody suggest that the rapid and efficient cellular internalization of the dendrimer-antibody conjugate was achieved without alterations in specificity of targeting. Animal studies demonstrated that the conjugate targets HER2-expressing tumors.