Improved paramagnetic liposomes for MRI visualization of pH triggered release

Derivatives of GdAAZTA Conjugated to Amino Acids: A Multinuclear and Multifrequency NMR Study

The GdAAZTA (AAZTA = 6-amino-6-methylperhydro-1,4-diazepinetetraacetic acid) complex represents a platform of great interest for the design of innovative MRI probes due to its remarkable magnetic properties, thermodynamic stability, kinetic inertness, and high chemical versatility. Here, we detail the synthesis and characterization of new derivatives functionalized with four amino acids with different molecular weights and charges: l-serine, l-cysteine, l-lysine, and l-glutamic acid. The main reason for conjugating these moieties to the ligand AAZTA is the in-depth study of the chemical properties in aqueous solution of model compounds that mimic complex structures based on polypeptide fragments used in molecular imaging applications. The analysis of the ¹H NMR spectra of the corresponding Eu(III)-complexes indicates the presence of a single isomeric species in solution, and measurements of the luminescence lifetimes show that functionalization with amino acid residues maintains the hydration state of the parent complex unaltered (q = 2). The relaxometric properties of the Gd(III) chelates were analyzed by multinuclear and multifrequency NMR techniques to evaluate the molecular parameters that determine their performance as MRI probes. The relaxivity values of all of the novel chelates are higher than that of GdAAZTA over the entire range of applied magnetic fields because of the slower rotational dynamics. Data obtained in reconstituted human serum indicate the occurrence of weak interactions with the proteins, which result in larger relaxivity values at the typical imaging fields. Finally, all of the new complexes are characterized by excellent chemical stability in biological matrices over time, by the absence of transmetallation processes, or the formation of ternary complexes with oxyanions of biological relevance. In particular, the kinetic stability of the new complexes, measured by monitoring the release of Gd³⁺ in the presence of a large excess of Zn²⁺, is ca. two orders of magnitude higher than that of the clinical MRI contrast agent GdDTPA.

Novel GdAAZTA derivatives conjugated to four amino acids were synthesized and characterized through a multi-technique approach. The complexes maintained the favorable thermodynamic and kinetic properties of the parent compound and showed higher relaxivity values in clinical fields. Therefore, they represent a useful model of more complex bio-conjugated structures used in molecular imaging applications.

Targeted Drug Delivery: Trends and Perspectives

BACKGROUND

Due to various limitations in conventional drug delivery system, it is important to focus on the target-specific drug delivery system where we can deliver the drug without any degradation. Among various challenges faced by a formulation scientist, delivering the drug to its right site, in its right dose, is also an important aim. A focused drug transport aims to extend, localize, target and have a safe drug interaction with the diseased tissue.

OBJECTIVE

The aim of targeted drug delivery is to make the required amount of the drug available at its desired site of action. Drug targeting can be accomplished in a number ways that include enzyme mediation, pH-dependent release, use of special vehicles, receptor targeting among other mechanisms. Intelligently designed targeted drug delivery systems also offer the advantages of a low dose of the drug along with reduced side effects which ultimately improves patient compliance. Incidences of dose dumping and dosage form failure are negligible. A focused drug transport aims to have a safe drug interaction with the diseased tissue.

CONCLUSION

This review focuses on the available targeting techniques for delivery to the colon, brain and other sites of interest. Overall, the article should make an excellent read for the researchers in this area. Newer drug targets may be identified and exploited for successful drug targeting.

Nanoplatforms for Targeted Stimuli-Responsive Drug Delivery: A Review of Platform Materials and Stimuli-Responsive Release and Targeting Mechanisms

To achieve the promise of stimuli-responsive drug delivery systems for the treatment of cancer, they should (1) avoid premature clearance; (2) accumulate in tumors and undergo endocytosis by cancer cells; and (3) exhibit appropriate stimuli-responsive release of the payload. It is challenging to address all of these requirements simultaneously. However, the numerous proof-of-concept studies addressing one or more of these requirements reported every year have dramatically expanded the toolbox available for the design of drug delivery systems. This review highlights recent advances in the targeting and stimuli-responsiveness of drug delivery systems. It begins with a discussion of nanocarrier types and an overview of the factors influencing nanocarrier biodistribution. On-demand release strategies and their application to each type of nanocarrier are reviewed, including both endogenous and exogenous stimuli. Recent developments in stimuli-responsive targeting strategies are also discussed. The remaining challenges and prospective solutions in the field are discussed throughout the review, which is intended to assist researchers in overcoming interdisciplinary knowledge barriers and increase the speed of development. This review presents a nanocarrier-based drug delivery systems toolbox that enables the application of techniques across platforms and inspires researchers with interdisciplinary information to boost the development of multifunctional therapeutic nanoplatforms for cancer therapy.

Harnessing Endogenous Stimuli for Responsive Materials in Theranostics

Materials that respond to endogenous stimuli are being leveraged to enhance spatiotemporal control in a range of biomedical applications from drug delivery to diagnostic tools. The design of materials that undergo morphological or chemical changes in response to specific biological cues or pathologies will be an important area of research for improving efficacies of existing therapies and imaging agents, while also being promising for developing personalized theranostic systems. Internal stimuli-responsive systems can be engineered across length scales from nanometers to macroscopic and can respond to endogenous signals such as enzymes, pH, glucose, ATP, hypoxia, redox signals, and nucleic acids by incorporating synthetic bio-inspired moieties or natural building blocks. This Review will summarize response mechanisms and fabrication strategies used in internal stimuli-responsive materials with a focus on drug delivery and imaging for a broad range of pathologies, including cancer, diabetes, vascular disorders, inflammation, and microbial infections. We will also discuss observed challenges, future research directions, and clinical translation aspects of these responsive materials.

Chemistry of MRI Contrast Agents: Current Challenges and New Frontiers

Tens of millions of contrast-enhanced magnetic resonance imaging (MRI) exams are performed annually around the world. The contrast agents, which improve diagnostic accuracy, are almost exclusively small, hydrophilic gadolinium(III) based chelates. In recent years concerns have arisen surrounding the long-term safety of these compounds, and this has spurred research into alternatives. There has also been a push to develop new molecularly targeted contrast agents or agents that can sense pathological changes in the local environment. This comprehensive review describes the state of the art of clinically approved contrast agents, their mechanism of action, and factors influencing their safety. From there we describe different mechanisms of generating MR image contrast such as relaxation, chemical exchange saturation transfer, and direct detection and the types of molecules that are effective for these purposes. Next we describe efforts to make safer contrast agents either by increasing relaxivity, increasing resistance to metal ion release, or by moving to gadolinium(III)-free alternatives. Finally we survey approaches to make contrast agents more specific for pathology either by direct biochemical targeting or by the design of responsive or activatable contrast agents.

Following nanomedicine activation with magnetic resonance imaging: why, how, and what's next?

Nanomedicines, such as liposomal formulations, play an important role in cancer therapy. To support their development, medical imaging modalities are employed for following the drug delivery. Encapsulation of MRI contrast agents, which change their relaxivity upon co-release with the drug, is a promising strategy for monitoring both the biodistribution and payload release from a nanocarrier. This approach is successfully applied in preclinical settings to image the activation of liposomes responsive to heat, pH changes or sonication. Recent advances include combination with different treatments and the implementation of chemical exchange saturation transfer imaging to gain spectral resolution over different contrast agents. However, this field still faces challenges, such as matching the pharmacokinetic profiles of the contrast agents and the liberated drugs.

Engineering of inorganic nanoparticles as magnetic resonance imaging contrast agents

Magnetic resonance imaging (MRI) is a highly valuable non-invasive imaging tool owing to its exquisite soft tissue contrast, high spatial resolution, lack of ionizing radiation, and wide clinical applicability. Contrast agents (CAs) can be used to further enhance the sensitivity of MRI to obtain information-rich images. Recently, extensive research efforts have been focused on the design and synthesis of high-performance inorganic nanoparticle-based CAs to improve the quality and specificity of MRI. Herein, the basic rules, including the choice of metal ions, effect of electron motion on water relaxation, and involved mechanisms, of CAs for MRI have been elucidated in detail. In particular, various design principles, including size control, surface modification (e.g. organic ligand, silica shell, and inorganic nanolayers), and shape regulation, to impact relaxation of water molecules have been discussed in detail. Comprehensive understanding of how these factors work can guide the engineering of future inorganic nanoparticles with high relaxivity. Finally, we have summarized the currently available strategies and their mechanism for obtaining high-performance CAs and discussed the challenges and future developments of nanoparticulate CAs for clinical translation in MRI.

Stimuli-responsive liposomes for drug delivery

The ultimate goal of drug delivery is to increase the bioavailability and reduce the toxic side effects of the active pharmaceutical ingredient (API) by releasing them at a specific site of action. In the case of antitumor therapy, association of the therapeutic agent with a carrier system can minimize damage to healthy, nontarget tissues, while limit systemic release and promoting long circulation to enhance uptake at the cancerous site due to the enhanced permeation and retention effect (EPR). Stimuli-responsive systems have become a promising way to deliver and release payloads in a site-selective manner. Potential carrier systems have been derived from a wide variety of materials, including inorganic nanoparticles, lipids, and polymers that have been imbued with stimuli-sensitive properties to accomplish triggered release based on an environmental cue. The unique features in the tumor microenvironment can serve as an endogenous stimulus (pH, redox potential, or unique enzymatic activity) or the locus of an applied external stimulus (heat or light) to trigger the controlled release of API. In liposomal carrier systems triggered release is generally based on the principle of membrane destabilization from local defects within bilayer membranes to effect release of liposome-entrapped drugs. This review focuses on the literature appearing between November 2008-February 2016 that reports new developments in stimuli-sensitive liposomal drug delivery strategies using pH change, enzyme transformation, redox reactions, and photochemical mechanisms of activation. WIREs Nanomed Nanobiotechnol 2017, 9:e1450. doi: 10.1002/wnan.1450 For further resources related to this article, please visit the WIREs website.

Recent progress on nanoparticle-based drug delivery systems for cancer therapy

The development of cancer nanotherapeutics has attracted great interest in the recent decade. Cancer nanotherapeutics have overcome several limitations of conventional therapies, such as nonspecific biodistribution, poor water solubility, and limited bioavailability. Nanoparticles with tuned size and surface characteristics are the key components of nanotherapeutics, and are designed to passively or actively deliver anti-cancer drugs to tumor cells. We provide an overview of nanoparticle-based drug delivery methods and cancer therapies based on tumor-targeting delivery strategies that have been developed in recent years.

Highlights in nanocarriers for the treatment against cervical cancer

Cervical cancer is the second most common malignant tumor in women worldwide and has a high mortality rate, especially when it is associated with human papillomavirus (HPV). In US, an estimated 12,820 cases of invasive cervical cancer and an estimated 4210 deaths from this cancer will occur in 2017. With rare and very aggressive conventional treatments, one sees in the real need of new alternatives of therapy as the delivery of chemotherapeutic agents by nanocarriers using nanotechnology. This review covers different drug delivery systems applied in the treatment of cervical cancer, such as solid lipid nanoparticles (SNLs), liposomes, nanoemulsions and polymeric nanoparticles (PNPs). The main advantages of drug delivery thus improving pharmacological activity, improving solubility, bioavailability to bioavailability reducing toxicity in the target tissue by targeting of ligands, thus facilitating new innovative therapeutic technologies in a too much needed area. Among the main disadvantage is the still high cost of production of these nanocarriers. Therefore, the aim this paper is review the nanotechnology based drug delivery systems in the treatment of cervical cancer.

Label-free CEST MRI Detection of Citicoline-Liposome Drug Delivery in Ischemic Stroke

Citicoline (CDPC) is a natural supplement with well-documented neuroprotective effects in the treatment of neurodegenerative diseases. In the present study, we sought to exploit citicoline as a theranostic agent with its inherent chemical exchange saturation transfer (CEST) MRI signal, which can be directly used as an MRI guidance in the citicoline drug delivery. Our in vitro CEST MRI results showed citicoline has two inherent CEST signals at +1 and +2 ppm, attributed to exchangeable hydroxyl and amine protons, respectively. To facilitate the targeted drug delivery of citicoline to ischemic regions, we prepared liposomes encapsulating citicoline (CDPC-lipo) and characterized the particle properties and CEST MRI properties. The in vivo CEST MRI detection of liposomal citicoline was then examined in a rat brain model of unilateral transient ischemia induced by a two-hour middle cerebral artery occlusion. The results showed that the delivery of CPDC-lipo to the brain ischemic areas could be monitored and quantified by CEST MRI. When administered intra-arterially, CDPC-lipo clearly demonstrated a detectable CEST MRI contrast at 2 ppm. CEST MRI revealed that liposomes preferentially accumulated in the areas of ischemia with a disrupted blood-brain-barrier. We furthermore used CEST MRI to detect the improvement in drug delivery using CDPC-lipo targeted against vascular cell adhesion molecule (VCAM)-1 in the same animal model. The MRI findings were validated using fluorescence microscopy. Hence, liposomal citicoline represents a prototype theranostic system, where the therapeutic agent can be detected directly by CEST MRI in a label-free fashion.

The evolution of gadolinium based contrast agents: from single-modality to multi-modality

Gadolinium-based contrast agents are extensively used as magnetic resonance imaging (MRI) contrast agents due to their outstanding signal enhancement and ease of chemical modification. However, it is increasingly recognized that information obtained from single modal molecular imaging cannot satisfy the higher requirements on the efficiency and accuracy for clinical diagnosis and medical research, due to its limitation and default rooted in single molecular imaging technique itself. To compensate for the deficiencies of single function magnetic resonance imaging contrast agents, the combination of multi-modality imaging has turned to be the research hotpot in recent years. This review presents an overview on the recent developments of the functionalization of gadolinium-based contrast agents, and their application in biomedicine applications.

The release of Doxorubicin from liposomes monitored by MRI and triggered by a combination of US stimuli led to a complete tumor regression in a breast cancer mouse model

The work aimed at developing a novel MRI-based theranostic protocol for improving the anticancer efficacy of a Doxil-like liposomal formulation. The goal was achieved stimulating the intratumor release of the drug from the nanocarrier and favoring its diffusion in the lesion by the sequential application of low-intensity pulsed ultrasound. The protocol was tested on mice bearing a syngeneic breast cancer model. The combination of acoustic waves with different characteristics allowed for: i) the release of the drug and the co-encapsulated MRI agent (Gadoteridol) from the liposomes in the vessels of the tumor region, and ii) the extravasation of the released material, as well as intact liposomes, in the tumor stroma. The MR-T1 contrast enhancement measured in the tumor reported on the delivery and US-triggered release of Doxorubicin. The developed protocol resulted in a marked increase in the intratumor drug concentration that, in turn, led to the complete regression of the lesion. The protocol has a good clinical translatability because all the components of the theranostic agent (Doxorubicin, liposomes, Gadoteridol) are approved for human use.

Small molecule therapeutic-loaded liposomes as therapeutic carriers: from development to clinical applications

In this review, various methods and mechanisms for encapsulation of small therapeutic molecules in liposomes for targeted delivery and triggered release, as well as their potential in the clinical uses, are discussed.

A review of responsive MRI contrast agents: 2005-2014

This review focuses on MRI contrast agents that are responsive to a change in a physiological biomarker. The response mechanisms are dependent on six physicochemical characteristics, including the accessibility of water to the agent, tumbling time, proton exchange rate, electron spin state, MR frequency or superparamagnetism of the agent. These characteristics can be affected by changes in concentrations or activities of enzymes, proteins, nucleic acids, metabolites, or metal ions, or changes in redox state, pH, temperature, or light. A total of 117 examples are presented, including ones that employ nuclei other than (1) H, which attests to the creativity of multidisciplinary research efforts to develop responsive MRI contrast agents.

Bioresponsive probes for molecular imaging: concepts and in vivo applications

Molecular imaging is a powerful tool to visualize and characterize biological processes at the cellular and molecular level in vivo. In most molecular imaging approaches, probes are used to bind to disease-specific biomarkers highlighting disease target sites. In recent years, a new subset of molecular imaging probes, known as bioresponsive molecular probes, has been developed. These probes generally benefit from signal enhancement at the site of interaction with its target. There are mainly two classes of bioresponsive imaging probes. The first class consists of probes that show direct activation of the imaging label (from "off" to "on" state) and have been applied in optical imaging and magnetic resonance imaging (MRI). The other class consists of probes that show specific retention of the imaging label at the site of target interaction and these probes have found application in all different imaging modalities, including photoacoustic imaging and nuclear imaging. In this review, we present a comprehensive overview of bioresponsive imaging probes in order to discuss the various molecular imaging strategies. The focus of the present article is the rationale behind the design of bioresponsive molecular imaging probes and their potential in vivo application for the detection of endogenous molecular targets in pathologies such as cancer and cardiovascular disease.

Functional liposomes in the cancer-targeted drug delivery

Cancer is considered as one of the most severe health problems and is currently the third most common cause of death in the world after heart and infectious diseases. Novel therapies are constantly being discovered, developed and trialed. Many of the current anticancer agents exhibit non-ideal pharmaceutical and pharmacological properties and are distributed non-specifically throughout the body. This results in death of the both normal healthy and malignant cells and substantially leads to accruing a variety of serious toxic side effects. Therefore, the efficient systemic therapy of cancer is almost impossible due to harmful side effects of anticancer agents to the healthy organs and tissues. Furthermore, several problems such as low bioavailability of the drugs, low drug concentrations at the site of action, lack of drug specificity and drug-resistance also cause many restrictions on clinical applications of these drugs in the tumor therapy. Different types of the liposomal formulations have been used in medicine due to their distinctive advantages associated with their structural flexibility in the encapsulation of various agents with different physicochemical properties. They can also mediate delivery of the cargo to the appropriate cell type and subcellular compartment, reducing the effective dosage and possible side effects which are related to high systemic concentrations. Therefore, these novel systems were found very promising and encouraging dosage forms for the treatment of different types of cancer by increasing efficiency and reducing the systemic toxicity due to the specific drug delivery and targeting.

Controlled Drug Release from Pharmaceutical Nanocarriers

Nanocarriers providing spatiotemporal control of drug release contribute to reducing toxicity and improving therapeutic efficacy of a drug. On the other hand, nanocarriers face unique challenges in controlling drug release kinetics, due to the large surface area per volume ratio and the short diffusion distance. To develop nanocarriers with desirable release kinetics for target applications, it is important to understand the mechanisms by which a carrier retains and releases a drug, the effects of composition and morphology of the carrier on the drug release kinetics, and current techniques for preparation and modification of nanocarriers. This review provides an overview of drug release mechanisms and various nanocarriers with a specific emphasis on approaches to control the drug release kinetics.

Sonosensitive theranostic liposomes for preclinical in vivo MRI-guided visualization of doxorubicin release stimulated by pulsed low intensity non-focused ultrasound

The main goal of this study was to assess the theranostic performance of a nanomedicine able to generate MRI contrast as a response to the release from liposomes of the antitumor drug Doxorubicin triggered by the local exposure to pulsed low intensity non focused ultrasounds (pLINFU). In vitro experiments showed that Gadoteridol was an excellent imaging agent for probing the release of Doxorubicin following pLINFU stimulation. On this basis, the theranostic system was investigated in vivo on a syngeneic murine model of TS/A breast cancer. MRI offered an excellent guidance for monitoring the pLINFU-stimulated release of the drug. Moreover, it provided: i) an in vivo proof of the effective release of the liposomal content, and ii) a confirmation of the therapeutic benefits of the overall protocol. Ex vivo fluorescence microscopy indicated that the good therapeutic outcome was originated from a better diffusion of the drug in the tumor following the pLINFU stimulus. Very interestingly, the broad diffusion of the drug in the tumor stroma appeared to be mediated by the presence of the liposomes themselves. The results of this study highlighted either the great potential of US-based stimuli to safely trigger the release of a drug from its nanocarrier or the associated significant therapeutic improvement. Finally, MRI demonstrated to be a valuable technique to support chemotherapy and monitoring the outcome. Furthermore, in this specific case, the theranostic agent developed has a high clinical translatability because the MRI agent utilized is already approved for human use.

Self-assembled polymeric nanoparticles as new, smart contrast agents for cancer early detection using magnetic resonance imaging

Early cancer detection is a major factor in the reduction of mortality and cancer management cost. Here we developed a smart and targeted micelle-based contrast agent for magnetic resonance imaging (MRI), able to turn on its imaging capability in the presence of acidic cancer tissues. This smart contrast agent consists of pH-sensitive polymeric micelles formed by self-assembly of a diblock copolymer (poly(ethyleneglycol-b-trimethylsilyl methacrylate)), loaded with a gadolinium hydrophobic complex (^tBuBipyGd) and exploits the acidic pH in cancer tissues. In vitro MRI experiments showed that ^tBuBipyGd-loaded micelles were pH-sensitive, as they turned on their imaging capability only in an acidic microenvironment. The micelle-targeting ability toward cancer cells was enhanced by conjugation with an antibody against the MUC1 protein. The ability of our antibody-decorated micelles to be switched on in acidic microenvironments and to target cancer cells expressing specific antigens, together with its high Gd(III) content and its small size (35–40 nm) reveals their potential use for early cancer detection by MRI.

Oxidation-responsive Eu(2+/3+)-liposomal contrast agent for dual-mode magnetic resonance imaging

An oxidation-responsive contrast agent for magnetic resonance imaging was synthesized using Eu(2+) and liposomes. Positive contrast enhancement was observed with Eu(2+), and chemical exchange saturation transfer was observed before and after oxidation of Eu(2+). Orthogonal detection modes render the concentration of Eu inconsequential to molecular information provided through imaging.

Effective nanoparticle-based gene delivery by a protease triggered charge switch

Gene carriers made from synthetic materials are of interest in relation to gene therapy but suffer from lack of transfection efficiency upon systemic delivery. To address this problem, a novel lipo-peptide-PEG conjugate constituted by a lipid-anchor, a peptide sensitive to proteases and a poly (ethylene glycol) (PEG) chain is investigated. Utilizing ethanol-mediated nucleic acid encapsulation to prepare lipo-nanoparticles (LNPs), LNPs that are stable in serum are obtained. The LNPs constitute a highly effective gene delivery systems in vitro and possess the right features for further investigation in vivo including a PEG layer and a net negative charge that should ensure long-circulating properties before being activated by proteases in diseased tissue. Protease activation leads to detachment of PEG and a charge switching where the LNPs become positive due to the presence of glutamates in the cleaved peptide moiety. The cationic lipid DOTAP is used mainly to complex DNA and proton titratable DODAP is used to increase endosomal escape and enhance transfection efficiency. The idea of using a mixture of permanently charged and titratable cationic lipids shielded by a protease sensitive negatively charged lipo-peptide-PEG coat appears to be a highly efficient solution for achieving effective non-viral gene delivery and the results warrant further investigations.

A review of mechanistic insight and application of pH-sensitive liposomes in drug delivery

The pH-sensitive liposomes have been extensively used as an alternative to conventional liposomes in effective intracellular delivery of therapeutics/antigen/DNA/diagnostics to various compartments of the target cell. Such liposomes are destabilized under acidic conditions of the endocytotic pathway as they usually contain pH-sensitive lipid components. Therefore, the encapsulated content is delivered into the intracellular bio-environment through destabilization or its fusion with the endosomal membrane. The therapeutic efficacy of pH-sensitive liposomes enables them as biomaterial with commercial utility especially in cancer treatment. In addition, targeting ligands including antibodies can be anchored on the surface of pH-sensitive liposomes to target specific cell surface receptors/antigen present on tumor cells. These vesicles have also been widely explored for antigen delivery and serve as immunological adjuvant to enhance the immune response to antigens. The present review deals with recent research updates on application of pH-sensitive liposomes in chemotherapy/diagnostics/antigen/gene delivery etc.

Diamagnetic chemical exchange saturation transfer (diaCEST) liposomes: physicochemical properties and imaging applications

Chemical exchange saturation transfer (CEST) is a new type of magnetic resonance imaging (MRI) contrast based on labile spins which rapidly exchange with solvent, resulting in an amplification of signal which allows detection of solute protons at millimolar to micromolar concentrations. An additional feature of these agents is that natural organic and biodegradable compounds can provide strong CEST contrast, allowing the development of diamagnetic CEST (diaCEST) MRI contrast agents. The sensitivity of the CEST approach per unit of agent increases further when diaCEST contrast agents are loaded into liposomes to become diaCEST liposomes. In this review, we will discuss the unique and favorable features of diaCEST liposomes which are well suited for in vivo imaging. diaCEST liposomes are nanocarriers which feature high concentrations of encapsulated contrast material, controlled release of payload, and an adjustable coating for passive or active tumor targeting. These liposomes have water permeable bilayers and both the interior and exterior can be fine-tuned for many biomedical applications. Furthermore, a number of liposome formulations are used in the clinic including Doxil™, which is an approved product for treating patients with cancer for decades, rapid translation of these materials can be envisaged. diaCEST liposomes have shown promise in imaging of cancer, and monitoring of chemotherapy and cell transplants. The unique features of diaCEST liposomes are discussed to provide an overview of the applications currently envisioned for this new technology and to provide an overall insight of their potential.

Paramagnetic liposomes for molecular MRI and MRI-guided drug delivery

Liposomes are a versatile class of nanoparticles with tunable properties, and multiple liposomal drug formulations have been clinically approved for cancer treatment. In recent years, an extensive library of gadolinium (Gd)-containing liposomal MRI contrast agents has been developed for molecular and cellular imaging of disease-specific markers and for image-guided drug delivery. This review discusses the advances in the development and novel applications of paramagnetic liposomes in molecular and cellular imaging, and in image-guided drug delivery. A high targeting specificity has been achieved in vitro using ligand-conjugated paramagnetic liposomes. On targeting of internalizing cell receptors, the effective longitudinal relaxivity r1 of paramagnetic liposomes is modulated by compartmentalization effects. This provides unique opportunities to monitor the biological fate of liposomes. In vivo contrast-enhanced MRI studies with nontargeted liposomes have shown the extravasation of liposomes in diseases associated with endothelial dysfunction, such as tumors and myocardial infarction. The in vivo use of targeted paramagnetic liposomes has facilitated the specific imaging of pathophysiological processes, such as angiogenesis and inflammation. Paramagnetic liposomes loaded with drugs have been utilized for therapeutic interventions. MR image-guided drug delivery using such liposomes allows the visualization and quantification of local drug delivery.

In vivo bio-safety evaluations and diagnostic/therapeutic applications of chemically designed mesoporous silica nanoparticles

The remarkable progress of nanotechnology and its application in biomedicine have greatly expanded the ranges and types of biomaterials from traditional organic material-based nanoparticles (NPs) to inorganic biomaterials or organic/inorganic hybrid nanocomposites due to the unprecedented advantages of the engineered inorganic material-based NPs. Colloidal mesoporous silica NPs (MSNs), one of the most representative and well-established inorganic materials, have been promoted into biology and medicine, and shifted from extensive in vitro research towards preliminary in vivo assays in small-animal disease models. In this comprehensive review, the recent progresses in chemical design and engineering of MSNs-based biomaterials for in vivo biomedical applications has been detailed and overviewed. Due to the intrinsic structural characteristics of elaborately designed MSNs such as large surface area, high pore volume and easy chemical functionalization, they have been extensively investigated for therapeutic, diagnostic and theranostic (concurrent diagnosis and therapy) purposes, especially in oncology. Systematic in vivo bio-safety evaluations of MSNs have revealed the evidences that the in vivo bio-behaviors of MSNs are strongly related to their preparation prodecures, particle sizes, geometries, surface chemistries, dosing parameters and even administration routes. In vivo pharmacokinetics and pharmacodynamics further demonstrated the effectiveness of MSNs as the passively and/or actively targeted drug delivery systems (DDSs) for cancer chemotherapy. Especially, the advance of nano-synthetic chemistry enables the production of composite MSNs for advanced in vivo therapeutic purposes such as gene delivery, stimuli-responsive drug release, photothermal therapy, photodynamic therapy, ultrasound therapy, or anti-bacteria in tissue engineering, or as the contrast agents for biological and diagnostic imaging. Additionally, the critical issues and potential challenges related to the chemical design/synthesis of MSNs-based "magic bullet" by advanced nano-synthetic chemistry and in vivo evaluation have been discussed to highlight the issues scientists face in promoting the translation of MSNs-based DDSs into clinical trials.

pH-sensitive Liposomes in Drug Delivery

The pH-sensitive liposomes have been extensively studied in recent years as an advantageous alternative to conventional liposomes in effective targeting and accumulation of anticancer drugs in tumors. pH-sensitive liposomes usually contain phosphatidylethanolamine and stabilizing amphiphiles and can destabilize under acidic conditions of the endocytotic pathway. The drug loaded is thought to be delivered into the cytoplasm, probably through destabilization of or fusion with the endosome membrane. This fusogenic property makes the pH-sensitive liposomes more efficient in delivering anticancer drugs than conventional liposomes. The intra-cellular release of drug/gene/diagnostic agents can be achieved without altering their therapeutic efficacy by means of the endosomal escape phenomenon. Cell surface targeting ligands, including antibodies, can be appended on the surface of pH-sensitive liposomes to target specific receptors on tumor cells. This chapter provides an introduction to pH-sensitive liposomes and examples of their therapeutic interest as smart drug-delivery systems.

Treatment of cerebral ischemia-reperfusion injury with PEGylated liposomes encapsulating FK506

FK506 (Tacrolimus) has the potential to decrease cerebral ischemia-reperfusion injury. However, the clinical trial of FK506 as a neuroprotectant failed due to adverse side effects. This present study aimed to conduct the selective delivery of FK506 to damaged regions, while at the same time reducing the dosage of FK506, by using a liposomal drug delivery system. First, the cytoprotective effect of polyethylene glycol-modified liposomes encapsulating FK506 (FK506-liposomes) on neuron-like pheochromocytoma PC12 cells was examined. FK506-liposomes protected these cells from H2O2-induced toxicity in a dose-dependent manner. Next, we investigated the usefulness of FK506-liposomes in transient middle cerebral artery occlusion (t-MCAO) rats. FK506-liposomes accumulated in the brain parenchyma by passing through the disrupted blood-brain barrier at an early stage after reperfusion had been initiated. Histological analysis showed that FK506-liposomes strongly suppressed neutrophil invasion and apoptotic cell death, events that lead to a poor stroke outcome. Corresponding to these results, a single injection of FK506-liposomes at a low dosage significantly reduced cerebral cell death and ameliorated motor function deficits in t-MCAO rats. These results suggest that liposomalization of FK506 could reduce the administration dose by enhancing the therapeutic efficacy; hence, FK506-liposomes should be a promising neuroprotectant after cerebral stroke.

Selective non-covalent triggered release from liposomes

A zinc(II)-dipicolylamine coordination complex selectively associates with anionic liposomes, including sterically protected PEGylated liposomes, and causes rapid leakage of encapsulated contents.

Ultrasound enhanced matrix metalloproteinase-9 triggered release of contents from echogenic liposomes

The extracellular enzyme matrix metalloproteinase-9 (MMP-9) is overexpressed in atherosclerotic plaques and in metastatic cancers. The enzyme is responsible for rupture of the plaques and for the invasion and metastasis of a large number of cancers. The ability of ultrasonic excitation to induce thermal and mechanical effects has been used to release drugs from different carriers. However, the majority of these studies were performed with low frequency ultrasound (LFUS) at kilohertz frequencies. Clinical usage of LFUS excitations will be limited due to harmful biological effects. Herein, we report our results on the release of encapsulated contents from substrate lipopeptide incorporated echogenic liposomes triggered by recombinant human MMP-9. The contents release was further enhanced by the application of diagnostic frequency (3 MHz) ultrasound. The echogenic liposomes were successfully imaged employing a medical ultrasound transducer (4-15 MHz). The conditioned cell culture media from cancer cells (secreting MMP-9) released the encapsulated dye from the liposomes (30-50%), and this release is also increased (50-80%) by applying diagnostic frequency ultrasound (3 MHz) for 3 min. With further developments, these liposomes have the potential to serve as multimodal carriers for triggered release and simultaneous ultrasound imaging.

Relaxometric investigations and MRI evaluation of a liposome-loaded pH-responsive gadolinium(III) complex

Accurate measurement of the tissue pH in vivo by MRI may be of clinical value for both diagnosis and selection/monitoring of therapy. To act as pH reporters, MRI contrast agents have to provide responsiveness to pH that does not require prior knowledge of the actual concentration of the contrast agent. This work deals with the use of a paramagnetic gadolinium(III) complex, loaded into liposomes, whose relaxometric properties are affected by the pH of the medium. In this system, the amphiphilic metal complex, which contains a moiety whose protonation changes the coordination properties of the metal chelate, experiences a different intraliposomial distribution depending on the pH conditions. The pH of the solution can be unambiguously identified by exploiting the peculiar characteristics of the resulting NMRD profiles, and a ratiometric pH-responsive method has been set up by comparing the relaxation enhancement at different magnetic field strengths.

The effect of tocopherol on the structure and permeability of phosphatidylcholine liposomes

There are numerous phospholipid formulations that incorporate α-tocopherol as a stabilizing agent but there are few studies of the effect of α-tocopherol on phospholipid structure and bilayer permeability. This study uses synchrotron X-ray powder diffraction methods to investigate how α-tocopherol changes the structure of distearoylphosphatidylcholines bilayers. Increasing proportions of α-tocopherol up to 20 mol% induces ripple structures in the bilayers. Two types of ripple structure are produced which are seen in electron micrographs of freeze-fracture replicas with periodicities of 16 and 12 nm, respectively. The stoichiometry of phospholipid: α-tocopherol in the ripple structures at 37 °C is 8:1. The presence of α-tocopherol tends to reduce the angle of tilt of the hydrocarbon chains of the phospholipid in the gel phase from about 34° to the bilayer normal at 20 °C into a more vertical orientation. Increasing proportions of α-tocopherol progressively decrease the temperature of the gel to liquid-crystal phase transition of the phospholipid. The presence of up to 20 mol% α-tocopherol in 1-palmitoyl-2-oleoyl-phosphocholine inhibits leakage of phenol red dye from liposomes. The effect of 7 mol% α-tocopherol on leakage was compared with phospholipid liposomes containing 50 mol% cholesterol. The cholesterol-containing liposomes inhibited leakage to a greater extent than the vesicles incorporating α-tocopherol but the effect of α-tocopherol at equivalent molar proportions was comparable to cholesterol.