Chemical modification and formulation approaches to elevated drug transport across cell membranes

Biopolymers in Mucoadhesive Eye Drops for Treatment of Dry Eye and Allergic Conditions: Application and Perspectives

Dry eye syndrome and allergic conjunctivitis are the most common inflammatory disorders of the eye surface. Although eye drops are the most usual prescribed dosage form, they are characterized by low ocular availability due to numerous barrier mechanisms of the eye. The use of biopolymers in liquid ophthalmic preparations has numerous advantages, such as increasing the viscosity of the tear film, exhibiting bioadhesive properties, and resisting the drainage system, leading to prolonged retention of the preparation at the site of application, and improvement of the therapeutic effect. Some mucoadhesive polymers are multifunctional excipients, so they act by different mechanisms on increasing the permeability of the cornea. Additionally, many hydrophilic biopolymers can also represent the active substances in artificial tear preparations, due to their lubrication and moisturizing effect. With the modification of conventional ophthalmic preparations, there is a need for development of new methods for their characterization. Numerous methods for the assessment of mucoadhesiveness have been suggested by the literature. This review gives an overview related to the development of mucoadhesive liquid ophthalmic formulations for the treatment of dry eye and allergic conditions.

Formulation of olopatadine hydrochloride viscous eye drops - physicochemical, biopharmaceutical and efficacy assessment using in vitro and in vivo approaches

The aim of this work was the formulation and the comprehensive evaluation of the viscous eye drops using vehicles containing medium chain chitosan (0.5% w/v), hydroxypropyl guar gum (0.25% w/v) and their combination as carriers for olopatadine (0.1% w/v). Physicochemical properties (appearance, clarity, pH, osmolality, viscosity and drug content) of the tested formulations were within acceptable ranges for the ophthalmic preparations, while DSC and FT-IR techniques demonstrated the compatibility between olopatadine and polymers. The drug permeability was successfully estimated in vitro using both HCE-T cell-based models (Model I and Model II) and the parallel artificial membrane permeability assay (PAMPA), considering the impact of chitosan as a permeation enhancer. The MTT cytotoxicity assay demonstrates that the tested formulations (diluted 10-fold in HBSS pH 5.5) were non-toxic and well tolerated. An ocular itch test on mice was carried out with the formulation containing the combination of polymers comparable with a commercially available olopatadine eye drops without viscosity enhancers. The tested eye drops produced a slightly higher anti-pruritic/analgesic-like effect than the commercial preparation. It could be assumed that the use of this viscous ophthalmic vehicle due to its advanced mucoadhesive properties and good safety profile is a feasible strategy to improve the efficacy of olopatadine.

In vitro models replicating the human intestinal epithelium for absorption and metabolism studies: A systematic review

Absorption, distribution, metabolism and excretion (ADME) studies represent a fundamental step in the early stages of drug discovery. In particular, the absorption of orally administered drugs, which occurs at the intestinal level, has gained attention since poor oral bioavailability often led to failures for new drug approval. In this context, several in vitro preclinical models have been recently developed and optimized to better resemble human physiology in the lab and serve as an animal alternative to accomplish the 3Rs principles. However, numerous models are ineffective in recapitulating the key features of the human small intestine epithelium and lack of prediction potential for drug absorption and metabolism during the preclinical stage. In this review, we provide an overview of in vitro models aimed at mimicking the intestinal barrier for pharmaceutical screening. After briefly describing how the human small intestine works, we present i) conventional 2D synthetic and cell-based systems, ii) 3D models replicating the main features of the intestinal architecture, iii) micro-physiological systems (MPSs) reproducing the dynamic stimuli to which cells are exposed in the native microenvironment. In this review, we will highlight the benefits and drawbacks of the leading intestinal models used for drug absorption and metabolism studies.

Prodrugs and nanomicelles to overcome ocular barriers for drug penetration

INTRODUCTION

Ocular barriers hinder drug delivery and reduce drug bioavailability. This article focuses on enhancing drug absorption across the corneal and conjunctival epithelium. Both, transporter targeted prodrug formulations and nanomicellar strategy is proven to enhance the drug permeation of therapeutic agents across various ocular barriers. These strategies can increase aqueous drug solubility and stability of many hydrophobic drugs for topical ophthalmic formulations.

AREAS COVERED

The article discusses various ocular barriers, ocular influx, and efflux transporters. It elaborates various prodrug strategies used for enhancing drug absorption. Along with this, the article also describes nanomicellar formulation, its characteristic and advantages, and applications in for anterior and posterior segment drug delivery.

EXPERT OPINION

Prodrugs and nanomicellar formulations provide an effective strategy for improving drug absorption and drug bioavailability across various ocular barriers. It will be exciting to see the efficacy of nanomicelles for treating back of the eye disorders after their topical application. This is considered as a holy grail of ocular drug delivery due to the dynamic and static ocular barriers, restricting posterior entry of topically applied drug formulations.

Formulation of host-targeted therapeutics against bacterial infections

The global burden of bacterial infections is rising due to increasing resistance to the majority of first-line antibiotics, rendering these drugs ineffective against several clinically important pathogens. Limited transport of antibiotics into cells compounds this problem for gram-negative bacteria that exhibit prominent intracellular lifecycles. Furthermore, poor bioavailability of antibiotics in infected tissues necessitates higher doses and longer treatment regimens to treat resistant infections. Although emerging antibiotics can combat these problems, resistance still may develop over time. Expanding knowledge of host-pathogen interactions has inspired research and development of host-directed therapies (HDTs). HDTs target host-cell machinery critical for bacterial pathogenesis to treat bacterial infections alone or as adjunctive treatment with traditional antibiotics. Unlike traditional antibiotics that directly affect bacteria, a majority of HDTs function by boosting the endogenous antimicrobial activity of cells and are consequently less prone to bacterial tolerance induced by selection pressure. Therefore, HDTs can be quite effective against intracellular cytosolic or vacuolar bacteria, which a majority of traditional antibiotics are unable to eradicate. However, in vivo therapeutic efficacy of HDTs is reliant on adequate bioavailability. Particle-based formulations demonstrate the potential to enable targeted drug delivery, enhance cellular uptake, and increase drug concentration in the host cell of HDTs. This review selected HDTs for clinically important pathogens, identifies formulation strategies that can improve their therapeutic efficacy and offers insights toward further development of HDTs for bacterial infections.

Tumor homing peptide modified liposomes of capecitabine for improved apoptotic activity and HER2 targeted therapy in breast cancer: in vitro studies

In the present study, we have formulated a liposomal formulation of cytotoxic agent capecitabine (CAP) to overcome its bioavailability issues. Then we have surface modified CAP loaded liposomes (CAP-LPs) with a tumour homing peptide (THP-CAP-LPs) to achieve site specific delivery to breast cancer cells. We found a significant cellular internalization of THP-CAP-LPs when compared to unmodified CAP-LPs. The cytotoxic effect of CAP was also significantly improved with THP-CAP-LPs by downregulating anti-apoptotic proteins and upregulating pro-apoptotic proteins as observed by Western blot analysis. THP-CAP-LPs mediated delivery of CAP can be, therefore, a promising approach for improving antitumor activity and reducing off-target effects.

Influence of PLA-PEG nanoparticles manufacturing process on intestinal transporter PepT1 targeting and oxytocin transport

Oral administration of peptides still remains a challenging issue. We previously pointed out the possibility to target intestinal PepT1 transporter with functionalized PLA-PEG nanoparticles (NPs) formulated by nanoprecipitation, and to improve drug-loaded intestinal permeability. Nevertheless, alternative manufacturing processes exist and the impact on the intestinal transporter targeting could be interesting to study. Our objective is consequently to assess the ability of functionalized NPs to target PepT1 according to the manufacturing process, and the possibility to improve peptide absorption. PLA-PEG-Valine NPs were formulated by nanoprecipitation, double and simple emulsion with median particle size <200 nm. Using Caco-2 cells, the competition between PLA-PEG-Val NPs formulated by the different manufacturing processes, and [³H]Glycylsarcosine, a well-known substrate of PepT1, was observed to evaluate the impact of the process on the intestinal transporter PepT1 targeting. Simultaneously, PLA-PEG-Val NPs were labeled with fluorescein (FITC) to evaluate PepT1 targeting and to observe the behavior of the NPs close to the cell according to the manufacturing process by confocal imaging. Finally, oxytocin peptide (OXY) was encapsulated in Val-NPs according to the most relevant process and the transport of the drug was assessed in vitro and in vivo, and compared to free drug. It was possible to observe by TEM imaging a better organization and expression of the ligand at the surface for NPs formulated by emulsion processes. Furthermore, the competition between functionalized NPs and [³H]Glycylsarcosine revealed a better transport inhibition of [³H]Glycylsarcosine for NPs formulated by double emulsion (≈ 67%). These results were confirmed by fluorescence measurements, comparing the amount of fluorescence linked to the cells after incubation with fluorescent Val-NPs for the 3 processes (≈ 39% for double emulsion). Additionally, confocal microscopy confirmed the ability of Val-NPs prepared by double emulsion to target the cell membrane and even to reach the intracellular space. OXY was then encapsulated by double emulsion in Val-NPs with a drug load of ≈ 4%. It was thus shown in vitro that drug transport was doubled compared to free drug. In vivo, OXY plasma concentration after oral administration were significantly increased when encapsulated in Val-NPS obtained by double emulsion compared to free drug. These results demonstrated that NPs prepared by double emulsion allowed a better PepT1 targeting and is a promising approach for oral peptide delivery.

In-vitro and in-vivo cytotoxicity and efficacy evaluation of novel glycyl-glycine and alanyl-alanine conjugates of chitosan and trimethyl chitosan nano-particles as carriers for oral insulin delivery

PURPOSE

The aim of this research work was to explore the possibility of providing multifunctional oral insulin delivery system by conjugating several types of dipeptides on chitosan and trimethyl chitosan to be used as drug carriers.

METHOD

Conjugates of Glycyl-glycine and alanyl-alanine of chitosan and trimethyl chitosan (on primary alcohol group of polymer located on carbon 6) were synthesized and nanoparticles containing insulin were prepared for oral delivery. Preparation conditions of nanoparticles were optimized and their performance to enhance the permeability of insulin as well as cytotoxicity of nanoparticles in Caco-2 cell line was evaluated. To evaluate the efficacy of orally administered nanoparticles, nanoparticles with the most permeability enhancing ability were studied in male Wistar rats as animal model by measuring insulin and glucose Serum levels.

RESULT

Structural study of all the conjugates by infrared spectroscopy and nuclear magnetic resonance confirmed the successful formation of the conjugates with the desirable substitution degree. By optimizing preparation conditions, nanoparticles with expected size (157.3-197.7 nm), Zeta potential (24.35-34.37 mV), polydispersity index (0.365-0.512), entrapment efficiency (70.60-86.52%) and loading capacity (30.92-56.81%), proper morphology and desirable release pattern were obtained. Glycyl-glycine and alanyl-alanine conjugate nanoparticles of trimethyl chitosan showed 2.5-3.3 folds more effective insulin permeability in Caco-2 cell line than their chitosan counterparts. In animal model, oral administration of glycyl-glycine and alanyl-alanine conjugate nanoparticles of trimethyl chitosan demonstrated reasonable increase in Serum insulin level with relative bioavailability of 17.19% and 15.46% for glycyl-glycine and alanyl-alanine conjugate nanoparticles, respectively, and reduction in Serum glucose level compared with trimethyl chitosan nanoparticles (p < 0.05).

CONCLUSION

It seems that glycyl-glycine and alanyl-alanine conjugate nanoparticles of trimethyl chitosan have met the aim of this research work and have been able to orally deliver insulin with more than one mechanism in animal model. Hence, they are promising candidates for further research studies.

Characteristic Analysis of Intestinal Transport in Enterocyte-Like Cells Differentiated from Human Induced Pluripotent Stem Cells

We previously demonstrated that differentiated enterocytes from human induced pluripotent stem (iPS) cells exhibited drug-metabolizing activities and cytochrome P450 CYP3A4 inducibility. The aim of this study was to apply human iPS cell-derived enterocytes in pharmacokinetic studies by investigating the characteristics of drug transport into enterocyte-like cells. Human iPS cells cultured on feeder cells were differentiated into endodermal cells using activin A. These endodermal-like cells were then differentiated into intestinal stem cells by fibroblast growth factor 2. Finally, epidermal growth factor and small-molecule compounds induced the maturation of the intestinal stem cell-like cells. After differentiation, we performed transepithelial electrical resistance (TEER) measurements, immunofluorescence staining, and transport studies. TEER values increased in a time-dependent manner and reached approximately 100 Ω × cm(2) Efflux transport of Hoechst 33342, a substrate of breast cancer resistance protein (BCRP), was observed and inhibited by the BCRP inhibitor Ko143. The uptake of peptide transporter 1 substrate glycylsarcosine was also confirmed and suppressed when the temperature was lowered to 4°C. Using immunofluorescence staining, villin and Na(+)-K(+) ATPase were expressed. These results suggest that human iPS cell-derived enterocytes had loose tight junctions, polarity, as well as uptake and efflux transport functions. In addition, the rank order of apparent membrane permeability coefficient (Papp) values of these test compounds across the enterocyte-like cell membrane corresponded to the fraction absorbance (Fa) values. Therefore, differentiated enterocytes from human iPS cells may provide a useful comprehensive evaluation model of drug transport and metabolism in the small intestine.

Drug Transporters and Na+/H+ Exchange Regulatory Factor PSD-95/Drosophila Discs Large/ZO-1 Proteins

Drug transporters govern the absorption, distribution, and elimination of pharmacologically active compounds. Members of the solute carrier and ATP binding-cassette drug transporter family mediate cellular drug uptake and efflux processes, thereby coordinating the vectorial movement of drugs across epithelial barriers. To exert their physiologic and pharmacological function in polarized epithelia, drug transporters must be targeted and stabilized to appropriate regions of the cell membrane (i.e., apical versus basolateral). Despite the critical importance of drug transporter membrane targeting, the mechanisms that underlie these processes are largely unknown. Several clinically significant drug transporters possess a recognition sequence that binds to PSD-95/Drosophila discs large/ZO-1 (PDZ) proteins. PDZ proteins, such as the Na(+)/H(+) exchanger regulatory factor (NHERF) family, act to stabilize and organize membrane targeting of multiple transmembrane proteins, including many clinically relevant drug transporters. These PDZ proteins are normally abundant at apical membranes, where they tether membrane-delimited transporters. NHERF expression is particularly high at the apical membrane in polarized tissue such as intestinal, hepatic, and renal epithelia, tissues important to drug disposition. Several recent studies have highlighted NHERF proteins as determinants of drug transporter function secondary to their role in controlling membrane abundance and localization. Mounting evidence strongly suggests that NHERF proteins may have clinically significant roles in pharmacokinetics and pharmacodynamics of several pharmacologically active compounds and may affect drug action in cancer and chronic kidney disease. For these reasons, NHERF proteins represent a novel class of post-translational mediators of drug transport and novel targets for new drug development.

Drug transporters in the nasal epithelium: an overview of strategies in targeted drug delivery

In this article, we discussed the expression of some ABC (e.g., P-glycoprortein, MRP and CFTR) and SLC (e.g., POT, DAT, OAT, OATP, OCT, EAAT2/GLT1 and GLUT) amino acid, metal and nucleoside transporters in the nasal mucosa. The localization and therapeutic targeting of these transporters are explored in detail. The wide array of transporters discovered so far in the nasal mucosa implies that a plethora of compounds can be delivered by targeting these transporters. The article concludes with a discussion of the potential challenges and delivery options for transporter-mediated drug targeting via the nasal route.

Novel in situ gel systems based on P123/TPGS mixed micelles and gellan gum for ophthalmic delivery of curcumin

Curcumin, a natural polyphenol compound, has been widely reported for diverse pharmacological effects and already been investigated for eye diseases. However, the water-insolubility of curcumin and the inherent penetration barriers in cornea make it difficult for curcumin to enter eye. This work aimed to develop ion-sensitive curcumin-loaded Pluronic P123 (P123)/D-a-tocopheryl polyethylene glycolsuccinate (TPGS) mixed micelle in situ gels (CUR-MM-ISGs) to prolong ocular retention time and improve cornea permeability. Central composite design-response surface methodology was applied for the optimization of curcumin-loaded P123/TPGS mixed micelles (CUR-MMs). Characterization tests showed that CUR-MMs were in spherical shape with small size and low critical micelle concentration. After dispersing the micelles in gellan gum solution (0.2%, w/w) at the ratio of 3:1 and 1:1 (v/v), respectively, CUR-MM-ISGs were formed and presented transparent appearance. Sustained release profile was obtained in vitro for both CUR-MM-ISGs (3:1 or 1:1, v/v). The irritation test proved that CUR-MM-ISGs as ophthalmic formulations were gentle and biocompatible towards ocular tissues. In addition, the ex vivo corneal penetration study indicated that the cumulative drug permeation amount of CUR-MM-ISGs (3:1, v/v) was respectively 1.16-fold and 1.32-fold higher than CUR-MM-ISGs (1:1, v/v) and curcumin solution. It can be concluded from these results that the developed ion-sensitive mixed micelle in situ gel system is a potential ophthalmic delivery carrier for curcumin as a poorly soluble drug.

Nanosolvated microtubule-modulating chemotherapeutics: a case-to-case study

About 10% of the drugs in the preclinical stage are poorly soluble, 40% of the drugs in the pipeline have poor solubility, and even 60% of drugs coming directly from synthesis have aqueous solubility below 0.1 mg/ml. Out of the research around, 40% of lipophilic drug candidates fail to reach the market despite having potential pharmacodynamic activities. Microtubule-modulating chemotherapeutics is an important class of cancer chemotherapy. Most chemotherapeutics that belong to this category are plant-derived active constituents, such as vincristine, vinblastine, colchicine, docetaxel, paclitaxel, and noscapinoids. The pKa of a drug considerably affects its solubility in physiological fluids and consequently bioavailability. It usually ranges from 5 to 12 for microtubule-modulating drugs. Hence, the solubility of these drugs in physiological fluids is considerably affected by a change in pH. However, because of unpredictable parameters involved in poor solubility and the low oral bioavailability of these chemotherapeutics during the early phases of drug development, they often have an unusual pharmacokinetic profile. This makes the development process of novel chemotherapeutics slow, inefficient, patient-unfriendly, and very costly, emphasizing a need for more rational approaches on the basis of preclinical concepts. Nanosolvation is a process of increasing the polarity of a hydrophobic molecule either by solvation or cavitization in a hydrophilic macrocycle. The present review therefore focuses on the techniques applied in nanosolvation of microtubule-modulating chemotherapeutics to enhance solubility and bioavailability. The methodologies described will be highly beneficial for anticancer researchers to follow a trend of rational drug development.

Oral drug delivery systems using chemical conjugates or physical complexes

Oral delivery of therapeutics is extremely challenging. The digestive system is designed in a way that naturally allows the degradation of proteins or peptides into small molecules prior to absorption. For systemic absorption, the intact drug molecules must traverse the impending harsh gastrointestinal environment. Technologies, such as enteric coating, with oral dosage formulation strategies have successfully provided the protection of non-peptide based therapeutics against the harsh, acidic condition of the stomach. However, these technologies showed limited success on the protection of therapeutic proteins and peptides. Importantly, inherent permeability coefficient of the therapeutics is still a major problem that has remained unresolved for decades. Addressing this issue in the context, we summarize the strategies that are developed in enhancing the intestinal permeability of a drug molecule either by modifying the intestinal epithelium or by modifying the drug itself. These modifications have been pursued by using a group of molecules that can be conjugated to the drug molecule to alter the cell permeability of the drug or mixed with the drug molecule to alter the epithelial barrier function, in order to achieve the effective drug permeation. This article will address the current trends and future perspectives of the oral delivery strategies.

Natural borneol enhances geniposide ophthalmic absorption in rabbits

The purpose of this study was to investigate the effects of natural borneol (NB) on the pharmacokinetics and bioavailability of ophthalmic administered geniposide (Ge) in rabbits. In vitro permeability characteristics of Ge in excised rabbit corneas were evaluated using Franz-type cells. The effect of NB on Ge pharmacokinetic profiles in vivo was studied with microdialysis. Concentrations of Ge were determined with reversed-phase high performance liquid chromatography (HPLC) following ophthalmic administration of Ge alone or with NB (0.01%, 0.02%, and 0.04%) or 0.5% ethylendiaminetetraacetic acid (EDTA). Ocular irritation was evaluated using the Draize method and histological examination. Ge solution alone (control solution) had limited corneal permeability. The ratio of the apparent permeability coefficient (Papp) with respect to the control solution significantly increased by approximately 1.6-, 2.0-, and 2.4-fold at NB concentrations of 0.01, 0.02, and 0.04%, respectively. The Papp for Ge with 0.5% EDTA (positive control) was approximately 1.7-fold higher than that for control solution. Compared to control solution, Ge exhibited a 1.46-, 2.16-, and 2.47-fold greater AUC0-6h, and 2.0-, 3.5-, and 4.4-fold greater Cmax, with 0.01, 0.02, and 0.04% NB, respectively, while Tmax remained unchanged. In conclusion, the ocular bioavailability of Ge significantly increased in the presence of NB.

Novel strategies for anterior segment ocular drug delivery

Research advancements in pharmaceutical sciences have led to the development of new strategies in drug delivery to anterior segment. Designing a new delivery system that can efficiently target the diseased anterior ocular tissue, generate high drug levels, and maintain prolonged and effective concentrations with no or minimal side effects is the major focus of current research. Drug delivery by traditional method of administration via topical dosing is impeded by ocular static and dynamic barriers. Various products have been introduced into the market that prolong drug retention in the precorneal pocket and to improve bioavailability. However, there is a need of a delivery system that can provide controlled release to treat chronic ocular diseases with a reduced dosing frequency without causing any visual disturbances. This review provides an overview of anterior ocular barriers along with strategies to overcome these ocular barriers and deliver therapeutic agents to the affected anterior ocular tissue with a special emphasis on nanotechnology-based drug delivery approaches.

Recent overview of ocular patents

Ocular drug therapy has always been considered as a major challenge in the field of drug delivery. The presence of blood ocular barriers and efflux pumps has imposed a great concern as well. Various vision threatening disorders require a long term therapy of drug molecules, especially for the diseases that affect the posterior segment. Pharmaceutical companies and other research institutes have adopted a multidisciplinary approach to meet the current challenges which is evidenced by the trends seen in the published and filed U.S. patents. Various strategies have been employed to achieve long term sustained and targeted delivery for both the anterior and the posterior segments of the ocular diseases. These strategies include formulating drugs into implant, micro or nanoparticulate systems and hydrogel-based systems. Transporter targeted approach has also allowed scientists to deliver drugs to both the segments of the eye. Recent developments such as delivery of drugs utilizing ultrasound, iontophoresis and microneedle based devices have been promising. Genebased therapeutics has opened a new avenue for vision threatening disorders. In all, the current developments in the entire field have been very exciting for finding out new strategies to treat vision threatening disorders.

Diverse approaches for the enhancement of oral drug bioavailability

In conscious and co-operating patients, oral drug delivery remains the preferable route of drug administration. However, not all drugs possess the desirable physicochemical and pharmacokinetic properties which favor oral administration mainly due to poor bioavailability. This has in some cases led to the choice of other routes of administration, which may compromise the convenience and increase the risk of non-compliance. Poor bioavailability has necessitated the administration of higher than normally required oral doses which often leads to economic wastages, risk of toxicity, erratic and unpredictable responses. The challenge over the years has been to design techniques that will allow oral administration of most drugs, irrespective of their properties, to achieve a therapeutic systemic availability. This will be a worthy achievement since over 90% of therapeutic compounds are known to possess oral bioavailability limitations. In this review, an attempt has been made to explore various approaches that have been used in recent years to improve oral drug bioavailability, including physical and chemical means. This review strives to provide a comprehensive overview of advances made over the past 10 years (2000-2010) in the improvement of the oral bioavailability of drugs. Briefly, the design of prodrugs to bypass metabolism or to enhance solubility as well as modification of formulation techniques such as the use of additives, permeation enhancers, solubilizers, emulsifiers and non-aqueous vehicles have been discussed. Arising approaches, such as formulation modification techniques; novel drug delivery systems, which exploit the gastrointestinal regionality of drugs, and include the pharmaceutical application of nanotechnology as an emerging area in drug delivery; inhibition of efflux pumps; and inhibition of presystemic metabolism have been more extensively addressed. This critical review sought to assess each method aimed at enhancing the oral bioavailability of drugs in terms of the purpose, scientific basis, limitations, commercial application, as well as the areas in which current research efforts are being focused and should be focused in the future.

Multi-stage delivery nano-particle systems for therapeutic applications

BACKGROUND

The daunting task for drug molecules to reach pathological lesions has fueled rapid advances in Nanomedicine. The progressive evolution of nanovectors has led to the development of multi-stage delivery systems aimed at overcoming the numerous obstacles encountered by nanovectors on their journey to the target site.

SCOPE OF REVIEW

This review summarizes major findings with respect to silicon-based drug delivery vectors for cancer therapeutics and imaging. Based on rational design, well-established silicon technologies have been adapted for the fabrication of nanovectors with specific shapes, sizes, and porosities. These vectors are part of a multi-stage delivery system that contains multiple nano-components, each designed to achieve a specific task with the common goal of site-directed delivery of therapeutics.

MAJOR CONCLUSIONS

Quasi-hemispherical and discoidal silicon microparticles are superior to spherical particles with respect to margination in the blood, with particles of different shapes and sizes having unique distributions in vivo. Cellular adhesion and internalization of silicon microparticles is influenced by microparticle shape and surface charge, with the latter dictating binding of serum opsonins. Based on in vitro cell studies, the internalization of porous silicon microparticles by endothelial cells and macrophages is compatible with cellular morphology, intracellular trafficking, mitosis, cell cycle progression, cytokine release, and cell viability. In vivo studies support superior therapeutic efficacy of liposomal encapsulated siRNA when delivered in multi-stage systems compared to free nanoparticles. This article is part of a Special Issue entitled Nanotechnologies - Emerging Applications in Biomedicine.

Enhancement of intestinal absorption of 2-methyl cytidine prodrugs

The purpose of this study was to investigate the in vivo absorption enhancement of a nucleoside (phosphoramidate prodrug of 2'-methyl-cytidine) anti-viral agent of proven efficacy by means of intestinal permeation enhancers. Natural nucleosides are hydrophilic molecules that do not rapidly penetrate cell membranes by diffusion and their absorption relies on specialized transporters. Therefore, the oral absorption of nucleoside prodrugs and the target organ concentration of the biologically active nucleotide can be limited due to poor permeation across the intestinal epithelium. In the present study, the specificity, concentration dependence, and effect of four classes of absorption promoters, i.e. fatty acids, steroidal detergents, mucoadhesive polymers, and secretory transport inhibitors, were evaluated in a rat in vivo model. Sodium caprate and alpha-tocopheryl-polyethyleneglycol-1000-succinate (TPGS) showed a significant effect in increasing liver concentration of nucleotide (5-fold). These results suggested that both excipients might be suited in a controlled release matrix for the synchronous release of the drug and absorption promoter directly to the site of absorption and highlights that the effect is strictly dependent on the absorption promoter dose. The feasibility of such a formulation approach in humans was evaluated with the aim of developing a solid dosage form for the peroral delivery of nucleosides and showed that these excipients do provide a potential valuable tool in pre-clinical efficacy studies to drive discovery programs forward.

Frontiers in cancer nanomedicine: directing mass transport through biological barriers

The physics of mass transport within body compartments and across biological barriers differentiates cancers from healthy tissues. Variants of nanoparticles can be manufactured in combinatorially large sets, varying by only one transport-affecting design parameter at a time. Nanoparticles can also be used as building blocks for systems that perform sequences of coordinated actions, in accordance with a prescribed logic. We refer to these as Logic-Embedded Vectors (LEVs). Nanoparticles and LEVs are ideal probes for the determination of mass transport laws in tumors, acting as imaging contrast enhancers, and can be employed for lesion-selective delivery of therapy. Their size, shape, density and surface chemistry dominate convective transport in the bloodstream, margination, cell adhesion, selective cellular uptake, as well as sub-cellular trafficking and localization. As argued here, the understanding of transport differentials in cancer, termed 'transport oncophysics', reveals a promising new frontier in oncology: the development of lesion-specific delivery particulates that exploit mass transport differentials to deploy treatment of greater efficacy and reduced side effects.

Molecular design for enhancement of ocular penetration

Over the past two decades, many oral drugs have been designed in consideration of physicochemical properties to attain optimal pharmacokinetic properties. This strategy significantly reduced attrition in drug development owing to inadequate pharmacokinetics during the last decade. On the other hand, most ophthalmic drugs are generated from reformulation of other therapeutic dosage forms. Therefore, the modification of formulations has been used mainly as the approach to improve ocular pharmacokinetics. However, to maximize ocular pharmacokinetic properties, a specific molecular design for ocular drug is preferable. Passive diffusion of drugs across the cornea membranes requires appropriate lipophilicity and aqueous solubility. Improvement of such physicochemical properties has been achieved by structure optimization or prodrug approaches. This review discusses the current knowledge about ophthalmic drugs adapted from systemic drugs and molecular design for ocular drugs. I propose the approaches for molecular design to obtain the optimal ocular penetration into anterior segment based on published studies to date.

Polymeric Enhancers of Mucosal Epithelia Permeability: Synthesis, Transepithelial Penetration-Enhancing Properties, Mechanism of Action, Safety Issues

Transmucosal drug administration across nasal, buccal, and ocular mucosae is noninvasive, eliminates hepatic first-pass metabolism and harsh environmental conditions, allows rapid onset, and further, mucosal surfaces are readily accessible. Generally, however, hydrophilic drugs, such as peptides and proteins, are poorly permeable across the epithelium, which results in insufficient bioavailability. Therefore, reversible modifications of epithelial barrier structure by permeation enhancers are required. Low molecular weight enhancers generally have physicochemical characteristics favoring their own absorption, whereas polymeric enhancers are not absorbed, and this minimizes the risk of systemic toxicity. The above considerations have warranted the present survey of the studies on polymeric transmucosal penetration-enhancers that have appeared in the literature during the last decade. Studies on intestinal permeation enhancers are also reviewed as they give information on the mechanism of action and safety of polymers. The synthesis and characterization of polymers, their effectiveness in enhancing the absorption of different drugs across different epithelium types, their mechanism of action and structure-efficacy relationship, and the relevant safety issues are reviewed. The active polymers are classified into: polycations (chitosan and its quaternary ammonium derivatives, poly-L-arginine (poly-L-Arg), aminated gelatin), polyanions (N-carboxymethyl chitosan, poly(acrylic acid)), and thiolated polymers (carboxymethyl cellulose-cysteine, polycarbophil (PCP)-cysteine, chitosan-thiobutylamidine, chitosan-thioglycolic acid, chitosan-glutathione conjugates).