An in vitro assay based on surface plasmon resonance to predict the in vivo circulation kinetics of liposomes

Application and Method of Surface Plasmon Resonance Technology in the Preparation and Characterization of Biomedical Nanoparticle Materials

Surface Plasmon Resonance (SPR) technology, as a powerful analytical tool, plays a crucial role in the preparation, performance evaluation, and biomedical applications of nanoparticles due to its real-time, label-free, and highly sensitive detection capabilities. In the nanoparticle preparation process, SPR technology can monitor synthesis reactions and surface modifications in real-time, optimizing preparation techniques and conditions. SPR enables precise measurement of interactions between nanoparticles and biomolecules, including binding affinities and kinetic parameters, thereby assessing nanoparticle performance. In biomedical applications, SPR technology is extensively used in the study of drug delivery systems, biomarker detection for disease diagnosis, and nanoparticle-biomolecule interactions. This paper reviews the latest advancements in SPR technology for nanoparticle preparation, performance evaluation, and biomedical applications, discussing its advantages and challenges in biomedical applications, and forecasting future development directions.

Recent Advances on Surface-Modified GBM Targeted Nanoparticles: Targeting Strategies and Surface Characterization

Glioblastoma multiforme (GBM) is the most common malignant brain tumor, associated with low long-term survival. Nanoparticles (NPs) developed against GBM are a promising strategy to improve current therapies, by enhancing the brain delivery of active molecules and reducing off-target effects. In particular, NPs hold high potential for the targeted delivery of chemotherapeutics both across the blood-brain barrier (BBB) and specifically to GBM cell receptors, pathways, or the tumor microenvironment (TME). In this review, the most recent strategies to deliver drugs to GBM are explored. The main focus is on how surface functionalizations are essential for BBB crossing and for tumor specific targeting. We give a critical analysis of the various ligand-based approaches that have been used to target specific cancer cell receptors and the TME, or to interfere with the signaling pathways of GBM. Despite the increasing application of NPs in the clinical setting, new methods for ligand and surface characterization are needed to optimize the synthesis, as well as to predict their in vivo behavior. An expert opinion is given on the future of this research and what is still missing to create and characterize a functional NP system for improved GBM targeting.

Preparation and Evaluation of Paclitaxel-Loaded PEGylated Niosomes Composed of Sorbitan Esters

Niosomes are non-ionic surfactant (NIS)-based bilayer vesicles and, like liposomes, have great potential as drug-delivery systems. Our previous study revealed that polyethylene glycol (PEG) niosomes using different sorbitan ester (Span) surfactants (sorbitan monoester, Span 20, 40, 60, 80; sorbitan triester, Span 65) distributed within tumors similarly to PEG liposomes. The aim of this study was to encapsulate efficiently an anti-cancer drug, paclitaxel (PTX) into Span PEG niosomes, and evaluate PTX release profiles and anti-tumor efficacy of PTX-loaded Span PEG niosomes. Niosome sizes ranged between 100-150 nm, and the PTX encapsulation efficiency was more than 50%. All niosomes examined, in the presence of serum, yielded sustained PTX-release profiles. PTX release at 24 and 48 h from Span 80 PEG niosomes was significantly the highest among the other Span PEG niosomes examined. In C26 tumor-bearing mice, PTX-loaded Span 40 PEG niosomes (the lowest PTX release in vitro) suppressed tumor growth while PTX-loaded Span 80 PEG niosomes (the highest PTX release in vitro) did not. Thus, we succeeded in the control of PTX release from Span PEG niosomes by modifying the component of niosomes, and it influenced the effects of drugs loaded into niosomes. This demonstrates that the excellent NIS physicochemical properties of Spans make them an ideal candidate for anti-cancer drug-carrier niosomes.

Spotlight on the protein corona of liposomes

Although an established drug delivery platform, liposomes have not fulfilled their true potential. In the body, interactions of liposomes are mediated by the layer of plasma proteins adsorbed on the surface, the protein corona. The review aims to collect the data of the last decade on liposome protein corona, tracing the path from interactions of individual proteins to the effects mediated by the protein corona in vivo. It offers a classification of the approaches to exploitation of the protein corona-rather than elimination thereof-based on the bilayer composition-corona composition-molecular interactions-biological performance framework. The multitude of factors that affect each level of this relationship urge to the widest implementation of bioinformatics tools to predict the most effective liposome compositions relying on the data on protein corona. Supplementing the picture with new pieces of accurately reported experimental data will contribute to the accuracy and efficiency of the predictions. STATEMENT OF SIGNIFICANCE: The review focuses on liposomes as an established nanomedicine platform and analyzes the available data on how the protein corona formed on liposome surface in biological fluids affects performance of the liposomes. The review offers a rigorous account of existing literature and critical analysis of methodology currently applied to the assessment of liposome-plasma protein interactions. It introduces a classification of the approaches to exploitation of the protein corona and tailoring liposome carriers to advance the field of nanoparticulate drug delivery systems for the benefit of patients.

Surface Plasmon Resonance as a Characterization Tool for Lipid Nanoparticles Used in Drug Delivery

The development of drug carriers based in lipid nanoparticles (LNPs) aims toward the synthesis of non-toxic multifunctional nanovehicles that can bypass the immune system and allow specific site targeting, controlled release and complete degradation of the carrier components. Among label free techniques, Surface Plasmon Resonance (SPR) biosensing is a versatile tool to study LNPs in the field of nanotherapeutics research. SPR, widely used for the analysis of molecular interactions, is based on the immobilization of one of the interacting partners to the sensor surface, which can be easily achieved in the case of LNPs by hydrophobic attachment onto commercial lipid- capture sensor chips. In the last years SPR technology has emerged as an interesting strategy for studying molecular aspects of drug delivery that determines the efficacy of the nanotherapeutical such as LNPs' interactions with biological targets, with serum proteins and with tumor extracelullar matrix. Moreover, SPR has contributed to the obtention and characterization of LNPs, gathering information about the interplay between components of the formulations, their response to organic molecules and, more recently, the quantification and molecular characterization of exosomes. By the combination of available sensor platforms, assay quickness and straight forward platform adaptation for new carrier systems, SPR is becoming a high throughput technique for LNPs' characterization and analysis.

Evaluation of Nanotargeted 111In-Cyclic RGDfK-Liposome in a Human Melanoma Xenotransplantation Model

Nanotargeted liposomes may be modified with targeting peptide on the surface of a prepared liposome to endow specificity and elevate targeting efficiency. The aim of this study was to develop a radioactive targeted nanoparticle, the ¹¹¹In-cyclic RGDfK-liposome, and its advantage of recognizing the α_Vβ3 integrin was examined. The cyclic RGDfK modified liposomes were demonstrated the ability to bind the α_Vβ3 integrin expressed on the surface of human melanoma cell in vitro and in vivo. The effects of the cyclic RGDfK-liposome on the functioning of phagocytes was also examined, showing no considerable negative effects on the engulfment of bacteria and the generation of reactive oxygen species. Based upon these findings, the cyclic RGDfK- liposome is said to be a promising agent for tumor imaging.

Pharmacokinetics and Pharmacodynamics Modeling and Simulation Systems to Support the Development and Regulation of Liposomal Drugs

Liposomal formulations have been developed to improve the therapeutic index of encapsulated drugs by altering the balance of on- and off-targeted distribution. The improved therapeutic efficacy of liposomal drugs is primarily attributed to enhanced distribution at the sites of action. The targeted distribution of liposomal drugs depends not only on the physicochemical properties of the liposomes, but also on multiple components of the biological system. Pharmacokinetic⁻pharmacodynamic (PK⁻PD) modeling has recently emerged as a useful tool with which to assess the impact of formulation- and system-specific factors on the targeted disposition and therapeutic efficacy of liposomal drugs. The use of PK⁻PD modeling to facilitate the development and regulatory reviews of generic versions of liposomal drugs recently drew the attention of the U.S. Food and Drug Administration. The present review summarizes the physiological factors that affect the targeted delivery of liposomal drugs, challenges that influence the development and regulation of liposomal drugs, and the application of PK⁻PD modeling and simulation systems to address these challenges.

[In Vitro Release Method for Liposome Drug Products]

Over the past few decades, liposome drug delivery systems (liposome DDS) have attracted much attention as the most advanced DDS. Efficacy and toxicity profiles of liposomes are based on their characteristic pharmacokinetics, drug release, and disposition after administration. Many attempts have been made to develop these systems especially as liposomal anti-cancer drugs. In the development of liposome DDS, identification of critical quality attributes and establishment of a control strategy to ensure consistent drug product quality are crucial. Among the quality attributes, particle size, drug encapsulation, and drug release from liposomes would affect their in vivo pharmacokinetic and pharmacodynamic properties. Thus these features need to be evaluated with appropriate analytical methods to confirm the quality and performance of the drug products. This article focuses on drug release from liposomes and reviews the effects of physicochemical properties of loaded drugs on release, simulation of drug release from liposomes, and design of a simulated body fluid for drug release assay of drug products.

Cisplatin liposome and 6-amino nicotinamide combination to overcome drug resistance in ovarian cancer cells

Ovarian cancer is an aggressive and lethal cancer usually treated by cytoreductive surgery followed by chemotherapy. Unfortunately, after an initial response, many patients relapse owing mainly to the development of resistance against the standard chemotherapy regime, carboplatin/paclitaxel, which is also affected by heavy side effects. In view to addressing such issues here, an association of liposomal cisplatin with 6-amino nicotinamide is investigated. It is known that resistant cells increase their demand for glucose, which is partially redirected toward the pentose phosphate pathway (PPP). Interestingly, we have found that also a cisplatin-resistant subclone of the ovarian cancer cells IGROV1 switch their metabolism toward the glycolytic pathway and rely on PPP to elude cisplatin cytotoxicity. The drug 6-amino nicotinamide, an inhibitor of the enzyme glucose-6-phosphate dehydrogenase (the rate-limiting step of the PPP) can restore the sensitivity of resistant cells to cisplatin. Then, to reduce the toxicity of cisplatin and prolong its action, a lyophilized stealth liposomal formulation of cisplatin was developed. The combination treatment of liposomal cisplatin and 6-amino nicotinamide showed promising cytotoxic activities in drug-resistant cells and a prolonged pharmacokinetics in rats, thus opening the way for a new therapeutic option against ovarian cancer.

In Vitro Evaluation of Anti-Aggregation and Degradation Behavior of PEGylated Polymeric Nanogels under In Vivo Like Conditions

The in vivo stability and biodegradability of nanocarriers crucially determine therapeutic efficacy as well as safety when used for drug delivery. This study aims to evaluate optimized in vitro techniques predictive for in vivo nanocarrier behavior. Polymeric biodegradable nanogels based on hydroxyethyl methacrylamide-oligoglycolates-derivatized poly(hydroxyethyl methacrylamide-co-N-(2-azidoethyl)methacrylamide) and with various degrees of PEGylation and crosslinking densities are prepared. Three techniques are chosen and refined for specific in vitro evaluation of the nanocarrier performance: (1) fluorescence single particle tracking (fSPT) to study the stability of nanogels in human plasma, (2) tangential flow filtration (TFF) to study the degradation and filtration of nanogel degradation products, and (3) fluorescence fluctuation spectroscopy (FFS) to evaluate and compare the degradation behavior of nanogels in buffer and plasma. fSPT results demonstrate that nanogels with highest PEGylation content show the least aggregation. The TFF results reveal that nanogels with higher crosslink density have slower degradation and removal by filtration. FFS results indicate a similar degradation behavior in human plasma as compared to that in phosphate buffered saline. In conclusion, three methods can be used to compare and select the optimal nanogel composition, and these methods hold potential to predict the in vivo performance of nanocarriers.

Zebrafish as an early stage screening tool to study the systemic circulation of nanoparticulate drug delivery systems in vivo

Nanomedicines have gained much attention for the delivery of small molecules or nucleic acids as treatment options for many diseases. However, the transfer from experimental systems to in vivo applications remains a challenge since it is difficult to assess their circulation behavior in the body at an early stage of drug discovery. Thus, innovative and improved concepts are urgently needed to overcome this issue and to close the gap between empiric nanoparticle design, in vitro assessment, and first in vivo experiments using rodent animal models. This study was focused on the zebrafish as a vertebrate screening model to assess the circulation in blood and extravasation behavior of nanoparticulate drug delivery systems in vivo. To validate this novel approach, monodisperse preparations of fluorescently labeled liposomes with similar size and zeta potential were injected into transgenic zebrafish lines expressing green fluorescent protein in their vasculature. Phosphatidylcholine-based lipids differed by fatty acid chain length and saturation. Circulation behavior and vascular distribution pattern were evaluated qualitatively and semi-quantitatively using image analysis. Liposomes composed of lipids with lower transition temperature (<28°C) as well as PEGylated liposomes showed longer circulation times and extravasation. In contrast, liposomes composed of lipids with transition temperatures>28°C bound to venous parts of the vasculature. This circulation patterns in the zebrafish model did correlate with published and experimental pharmacokinetic data from mice and rats. Our findings indicate that the zebrafish model is a useful vertebrate screening tool for nanoparticulate drug delivery systems to predict their in vivo circulation behavior with respect to systemic circulation time and exposure.

Interaction between PEG lipid and DSPE/DSPC phospholipids: An insight of PEGylation degree and kinetics of de-PEGylation

The degree to which liposomes are PEGylated is the feature, which most influences the length of the presence of stealth liposomes in the bloodstream. In order to thoroughly investigate the maximum amount of DSPE-PEG₂₀₀₀ that can be used to stabilize stealth liposomes, these were synthesized at different concentrations of DSPE-PEG₂₀₀₀ and their physicochemical properties were investigated by using differential scanning calorimetry (DSC). The kinetics of PEGylation and de-PEGylation were performed by incubating non-stealth liposomes in a DSPE-PEG₂₀₀₀ suspension at different incubation times, and then analyzing the data using DSC and dynamic light scattering (DLS) techniques. The results demonstrated that DSPE-PEG₂₀₀₀ was self-assembled in the phospholipid bilayers, thus forming stealth liposomes. The different amounts of DSPE-PEG₂₀₀₀ in the bilayer triggered a de-PEGylation phenomenon, resulting in mixed nanoaggregates, which derived from the detergent-like properties of the PEGylated phospholipids.

Application of Microrheology in Food Science

Microrheology provides a technique to probe the local viscoelastic properties and dynamics of soft materials at the microscopic level by observing the motion of tracer particles embedded within them. It is divided into passive and active microrheology according to the force exerted on the embedded particles. Particles are driven by thermal fluctuations in passive microrheology, and the linear viscoelasticity of samples can be obtained on the basis of the generalized Stokes-Einstein equation. In active microrheology, tracer particles are controlled by external forces, and measurements can be extended to the nonlinear regime. Microrheology techniques have many advantages such as the need for only small sample amounts and a wider measurable frequency range. In particular, microrheology is able to examine the spatial heterogeneity of samples at the microlevel, which is not possible using traditional rheology. Therefore, microrheology has considerable potential for studying the local mechanical properties and dynamics of soft matter, particularly complex fluids, including solutions, dispersions, and other colloidal systems. Food products such as emulsions, foams, or gels are complex fluids with multiple ingredients and phases. Their macroscopic properties, such as stability and texture, are closely related to the structure and mechanical properties at the microlevel. In this article, the basic principles and methods of microrheology are reviewed, and the latest developments and achievements of microrheology in the field of food science are presented.

LHD-Modified Mechanism-Based Liposome Coencapsulation of Mitoxantrone and Prednisolone Using Novel Lipid Bilayer Fusion for Tissue-Specific Colocalization and Synergistic Antitumor Effects

Coencapsulation liposomes are of interest to researchers because they maximize the synergistic effect of loaded drugs. A combination regimen of mitoxantrone (MTO) and prednisolone (PLP) has been ideal for tumor therapy. MTO and PLP offer synergistic antitumor effects confirmed by several experiments in this research. The deduced synergistic mechanism is regulation of Akt signaling pathway including the targets of p-Akt, p-GSK-3β, p-s6 ribosomal protein, and p-AMPK by MTO reactivating PLP-induced apoptosis. The liposome fusion method is adopted to create coencapsulation liposomes (PLP-MTO-YM). Low molecular weight heparin-sodium deoxycholate conjugate (LHD) then is used as a targeting ligand to prove target binding and inhibition of angiogenesis. LHD-modified liposomes (PLP-MTO-HM) have a high entrapment efficiency around 95% for both MTO and PLP. DSC results indicate that both drugs interacted with liposomes to prevent drug leak during liposome fusion. DiD-C6-HM dyes colocalize well to tumor tissue, and coadministration of DiD-HM and C6-CM did not achieve dye colocalization until 24 h after administration. In both CT26 and B16F10 mouse model, PLP-MTO-HM shows a significantly higher tumor inhibition rate relative to the coadministration of MTO-HM and PLP-CM (p < 0.05 or p < 0.01). Thus, the coencapsulation system (PLP-MTO-HM) offers ideal antitumor effects relative to coadministration therapy due to enhanced synergistic effect, and this suggests a promising future for the tumor targeting vectors.

PEG stabilized DNA – poly(ferrocenylsilane) polyplexes for gene delivery

Polycationic poly(ferrocenylsilane)s (PFS) with tunable amounts of PEG side chains were used for the condensation of DNA into polyplexes of 110 nm in 5.0 mM HEPES.

In vitro-in vivo correlation for complex non-oral drug products: Where do we stand?

In vitro–in vivo correlation (IVIVC) is a predictive mathematical model describing the relationship between an in vitro property and a relevant in vivo response of drug products. Since the U.S. Food and Drug Administration (FDA) published a regulatory guidance on the development, evaluation, and applications of IVIVC for extended release (ER) oral dosage forms in 1997, IVIVC has been one of the most important issues in the field of pharmaceutics. However, even with the aid of the FDA IVIVC Guidance, only very limited Abbreviated New Drug Application (ANDA) submission for ER oral drug products included adequate IVIVC data to enable the completion of bioequivalence (BE) review within first review cycle. Establishing an IVIVC for non-oral dosage forms has remained extremely challenging due to their complex nature and the lack of in vitro release methods that are capable of mimicking in vivo drug release conditions. This review presents a general overview of recent advances in the development of IVIVC for complex non-oral dosage forms (such as parenteral polymeric microspheres/implants, and transdermal formulations), and briefly summarizes the knowledge gained over the past two decades. Lastly this review discusses possible directions for future development of IVIVC for complex non-oral dosage forms.

Product quality for nanomaterials: current U.S. experience and perspective

In recent years, there has been an increased focus on developing novel drug delivery systems and targeted therapies through the use of nanotechnology and nanomaterials. Such focus is translating to an increasing number of investigational new drug (IND) applications, new drug applications (NDAs), and abbreviated new drug applications (ANDAs) for drug products containing nanomaterials to the United States Food and Drug Administration (FDA). Although subject to the same rigorous regulatory standards and regulatory pathways as any drug product, unique properties that arise from the small size, large surface area, and polydispersity of nanomaterials may lead to additional scientific considerations when following current FDA guidelines and practices for drug evaluation. This review article will discuss these scientific considerations based on the experience with FDA-approved drug products containing nanomaterials.

Nanoparticle–blood interactions: the implications on solid tumour targeting

This review examines nanoparticle–blood interactions, their implications on solid tumour targeting, and provides an outlook to guide future nanoparticle design.

Hyaluronan polymer length, grafting density, and surface poly(ethylene glycol) coating influence in vivo circulation and tumor targeting of hyaluronan-grafted liposomes

Hyaluronan-grafted liposomes (HA-liposomes) preferentially target CD44-overexpressing tumor cells in vitro via receptor-mediated endocytosis. We investigated the pharmacokinetics and biodistribution of HA-liposomes with various sizes of HA (MW 5-8, 50-60, and 175-350 kDa) in mice. Incorporation of negatively charged HA on the liposome surface compromised its blood circulation time, which led to decreased tumor accumulation in CD44+ human breast cancer MDA-MB-231 xenografts compared to PEGylated liposomes (PEG-5000). Clearance of HA-liposomes was HA polymer length-dependent; high MW (175-350 kDa, highest ligand binding affinity) HA-liposomes displayed faster clearance compared to low MW (5-8, 50-60 kDa) HA-liposomes or PEGylated liposomes. Surface HA ligand density can also affect clearance of HA-liposomes. Thus, HA is not an effective stealth coating material. When dual coating of PEG and HA was used, the PEG-HA-liposomes displayed similar blood circulation time and tumor accumulation to that of the PEGylated liposomes; however, the PEG-HA-liposomes displayed better cellular internalization capability in vivo. Tumor histology showed that PEG-HA-liposomes had a more direct association with CD44+ cancer cells, while PEGylated liposomes located predominantly in the tumor periphery, with less association with CD44+ cells. Flow cytometry analysis of ex vivo tumor cells showed that PEG-HA-liposomes had significantly higher tumor cell internalization compared to PEGylated liposomes. This study demonstrates that a long blood circulation time is critical for active tumor targeting. Furthermore, the use of the tumor-targeting ligand HA does not increase total tumor accumulation of actively targeted liposomes in solid tumors; however, it can enhance intracellular delivery.

Protein corona fingerprinting predicts the cellular interaction of gold and silver nanoparticles

Using quantitative models to predict the biological interactions of nanoparticles will accelerate the translation of nanotechnology. Here, we characterized the serum protein corona 'fingerprint' formed around a library of 105 surface-modified gold nanoparticles. Applying a bioinformatics-inspired approach, we developed a multivariate model that uses the protein corona fingerprint to predict cell association 50% more accurately than a model that uses parameters describing nanoparticle size, aggregation state, and surface charge. Our model implicates a set of hyaluronan-binding proteins as mediators of nanoparticle-cell interactions. This study establishes a framework for developing a comprehensive database of protein corona fingerprints and biological responses for multiple nanoparticle types. Such a database can be used to develop quantitative relationships that predict the biological responses to nanoparticles and will aid in uncovering the fundamental mechanisms of nano-bio interactions.

Design, synthesis, and mechanistic investigations of bile acid-tamoxifen conjugates for breast cancer therapy

We have synthesized two series of bile acid tamoxifen conjugates using three bile acids lithocholic acid (LCA), deoxycholic acid (DCA), and cholic acid (CA). These bile acid-tamoxifen conjugates possess 1, 2, and 3 tamoxifen molecules attached to hydroxyl groups of bile acids having free acid and amine functionalities at the tail region of bile acids. The in vitro anticancer activities of these bile acid-tamoxifen conjugates show that the free amine headgroup based cholic acid-tamoxifen conjugate (CA-Tam3-Am) is the most potent anticancer conjugate as compared to the parent drug tamoxifen and other acid and amine headgroup based bile acid-tamoxifen conjugates. The cholic acid-tamoxifen conjugate (CA-Tam3-Am) bearing three tamoxifen molecules shows enhanced anticancer activities in both estrogen receptor +ve and estrogen receptor -ve breast cancer cell lines. The enhanced anticancer activity of CA-Tam3-Am is due to more favorable irreversible electrostatic interactions followed by intercalation of these conjugates in hydrophobic core of membrane lipids causing increase in membrane fluidity. Annexin-FITC based FACS analysis showed that cells undergo apoptosis, and cell cycle analysis showed the arrest of cells in sub G0 phase. ROS assays showed a high amount of generation of ROS independent of ER status of the cell line indicating changes in mitochondrial membrane fluidity upon the uptake of the conjugate that further leads to the release of cytochrome c, a direct and indirect regulator of ROS. The mechanistic studies for apoptosis using PCR and western analysis showed apoptotsis by intrinsic and extrinsic pathways in ER +ve MCF-7 cells and by only an intrinsic pathway in ER -ve cells. In vivo studies in the 4T1 tumor model showed that CA-Tam3-Am is more potent than tamoxifen. These studies showed that bile acids provide a new scaffold for high drug loading and that their anticancer activities strongly depend on charge and hydrophobicity of lipid-drug conjugates.

Trends in polymeric delivery of nucleic acids to tumors

Delivery of nucleic acids to tumors has received extensive attention in the past few decades since these molecules are capable of treating disease by modulating the source of abnormalities. Although high efficiency and low toxicity of numerous delivery systems for nucleic acids have been approved frequently with in vitro assays, contradictions have been observed in many cases between these results and what has occurred in the dynamic in vivo situation. Filling this gap seems to be crucial for further preclinical development of such systems. In this paper, we discuss various barriers which polymeric DNA or siRNA nanoparticles encounter upon systemic administration with an aim to assist in designing more relevant in vitro assays. Furthermore, individual considerations concerning delivery of DNA and siRNA have been addressed.

Nucleic acid delivery: the missing pieces of the puzzle?

The delivery of genes or RNA interference (RNAi) agents can increase or decrease the expression of virtually any protein in a cell, and this process opens the path for cures to most diseases that afflict humans. However, the high molecular weight, anionic nature, and instability of nucleic acids in the presence of enzymes pose major obstacles to their delivery and frustrates their use as human therapies. This Account describes current ideas about the mechanisms in nonviral nucleic acid delivery and how lipidic and polymeric carriers can overcome some of the critical barriers to delivery. Over the last 20 years, researchers have developed a multitude of polymeric and lipidic vectors, but only a small fraction of these have progressed into clinical trials. None of these vectors has received FDA approval, which indicates that the current vectors do not yet have suitable properties for effective in vivo nucleic acid delivery. Nucleic acid delivery is a multistep process and inefficiencies at any stage result in a dramatic decrease in gene delivery or gene silencing. However, the majority of studies investigating synthetic vectors focus solely on optimization of endosomal escape. A small number of studies address how to improve uptake via targeted delivery, and an even smaller fraction examine the intracellular fate of the delivery systems and nucleic acid cargo. The internalization of genes into the cell nucleus remains an inefficient and mysterious process. In the case of DNA delivery, strategies are needed to increase and accelerate the migration of DNA through the cytoplasm and transport it through the nuclear membrane. siRNA delivery involves fewer barriers. siRNA is more readily released from the carrier and more resistant to enzymatic degradation, and its target is in the cytoplasm; hence, siRNA delivery systems are becoming a clinical reality. With regard to siRNA therapy, the exact cytoplasmic location of RNA-induced silencing complex (RISC) formation and activity is unknown, which makes specific targeting of the RISC for more efficient delivery difficult. Furthermore, we would like to identify the factors that favor the binding of siRNA to Ago-2. If we could understand how the half-life of siRNA and Ago-2/siRNA complex in the cytoplasm can be modulated without interfering with RISC functions that are essential for normal cell activity, we could increase siRNA delivery efficiency. In this Account, we review the current synthetic vectors and propose alternative strategies in a few cases. We also suggest how certain cellular mechanisms might be exploited to improve gene transfection and silencing. Finally, we discuss whether some carriers that deliver the siRNA to cells could also repackage the siRNA into exosomes. The exosomes would then transport the siRNA into a subsequent population of cells that manifest the siRNA effect. This piggy-back mechanism may be responsible for reported deep tissue siRNA effects using certain carriers.