A liposomal MRI contrast agent: phosphatidylethanolamine-DTPA

Relaxivity Enhancement of Hybrid Micelles via Modulation of Water Coordination Numbers for Magnetic Resonance Lymphography

Considerable efforts have been made to develop nanoparticle-based magnetic resonance contrast agents (CAs) with high relaxivity. The prolonged rotational correlation time (τR) induced relaxivity enhancement is commonly recognized, while the effect of the water coordination numbers (q) on the relaxivity of nanoparticle-based CAs gets less attention. Herein, we first investigated the relationship between T1 relaxivity (r1) and q in manganese-based hybrid micellar CAs and proposed a strategy to enhance the relaxivity by increasing q. Hybrid micelles with different ratios of amphiphilic manganese complex (MnL) and DSPE-PEG2000 were prepared, whose q values were evaluated by Oxygen-17-NMR spectroscopy. Micelles with lower manganese doping density exhibit increased q and enhanced relaxivity, corroborating the conception. In vivo sentinel lymph node (SLN) imaging demonstrates that DSPE-PEG/MnL micelles could differentiate metastatic SLN from inflammatory LN. Our strategy makes it feasible for relaxivity enhancement by modulating q, providing new approaches for the structural design of high-performance hybrid micellar CAs.

Diagnostic and therapeutic evaluation of folate-targeted paclitaxel and vinorelbine encapsulating theranostic liposomes for non-small cell lung cancer

NSCLC is the most common type of lung cancer. However, non-specific contrast agents, radiopharmaceuticals, and treatment methods are insufficient in early diagnosis and eradication of all tumor tissue. Therefore, the formulation of a novel, targeted, specific theranostic agents possess critical importance. In our previous study, paclitaxel and vinorelbine encapsulating, Tc-99m radiolabeled, folate targeted, nanosized liposomes were formulated and found promising due to characterization properties, high cellular uptake, and cytotoxicity. In this study, in vivo therapeutic and diagnostic efficacy of liposomal formulations were tested by biodistribution study, evaluation of tumor growth inhibition, and histopathologic examination after in vitro assays on LLC1 cells. Both actively and passively targeted liposomal formulations exhibited high cellular uptake, and co-drug encapsulating liposomes showed a greater cytotoxicity profiles than free drug combination in LLC1 cells. By the results of biodistribution studies performed in NSCLC tumor-bearing C57BL/6 mice, the uptake of radiolabeled, actively folate targeted, co-drug encapsulating liposomal formulation was found to be higher in tumor tissue when compared to non-actively targeted one. Also, more effective treatment was achieved by using folate-targeted, co-drug encapsulating liposomal formulation when compared to free drugs combination according to changes in tumor size of mice. Furthermore, liposomal formulations showed lower toxicity compared to free drug combinations in the toxicity study considering body weight. Moreover, according to the histopathological study, folate targeted, co-drug encapsulating liposomes not only inhibited the tumor growth effectively but also restricted the lung metastasis entirely.

The Formulation of Methylene Blue Encapsulated, Tc-99m Labeled Multifunctional Liposomes for Sentinel Lymph Node Imaging and Therapy

OBJECTIVES

Methylene blue (MB) is a commonly used dye that can be used for near-infrared (NIR) imaging and photodynamic therapy (PDT) by producing reactive oxygen species after light exposure, inducing apoptosis. The limiting factor of MB is its poor penetration through cell membranes. Its decreased cellular uptake can be prevented by encapsulation in drug delivery systems such as liposomes. Additionally, the enhanced permeability and retention effect of tumors enables enhanced accumulation of nanocarriers at the target site.

MATERIALS AND METHODS

Nanosized, MB encapsulated, Tc-99m radiolabeled Lipoid S PC:PEG2000-PE:Chol: DTPA-PE and DPPC:PEG2000-PE:Chol:DTPA-PE liposomes were formulated to design multifunctional theranostic nanocarriers for: 1) NIR imaging, 2) gamma probe detection of sentinel lymph nodes (SLNs), and 3) PDT, which can provide accurate imaging and therapy helping surgery with a single liposomal system. The characterization of liposomes was performed by measuring particle size, zeta potential, phospholipid content, and encapsulation efficiency. Additionally, the in vitro release profile of MB and physical stability were also evaluated over 6 months at determined time intervals by measuring the mean particle size, zeta potential, encapsulation efficiency, and phospholipid content of liposomes kept at room temperature (25°C) and 4°C.

RESULTS

Tc-99m radiolabeled, nanosized Lipoid S PC:PEG2000-PE:Chol:DTPA-PE and DPPC:PEG2000-PE:Chol:DTPA-PE liposomes showed suitable particle size (around 100 nm), zeta potential (-9 to -13 mV), encapsulation efficiency (around 10%), phospholipid efficiency (around 85-90%), and release profiles. Additionally, the liposomes found stable for 3 months especially when kept at 4°C.

CONCLUSION

MB encapsulated, Tc-99m radiolabeled, nanosized Lipoid S PC:PEG2000-PE:Chol:DTPA-PE and DPPC:PEG2000-PE:Chol:DTPA-PE liposomes were found to have potential for SLN imaging by gamma probe detection, NIR imaging, and PDT. In vitro and in vivo imaging and therapeutic efficiency should be definitely evaluated to enable a final decision and our studies on this research topic are continuing.

99mTc-radiolabeled Levofloxacin and micelles as infection and inflammation imaging agents

Easy and early detection of infection and inflammation is essential for early and effective treatment. In this study, PEGylated micelles were designed and both micelles and Levofloxacin were radiolabeled with 99mTcO4 - to develop potential radiotracers for detection of infection/inflammation. Radiolabeling efficiency, in vitro stability and bacterial binding of 99mTc-Levofloxacin and 99mTc-micelles were compared. The aim of this study is to formulate and compare 99mTc-Levofloxacin and 99mTc-micelles as infection and inflammation agents having different mechanisms for the accumulation at infection and inflammation site. PEGylated micelles were designed with a particle size of 80 ± 0.7 nm and proper characterization properties. High radiolabeling efficiency was achieved for 99mTc-Levofloxacin (96%) and 99mTc-micelles (87%). The radiolabeling efficiency was remained stable with some insignificant alterations for both radiotracers at 25 °C for 24 h. Although in vitro bacterial binding of 99mTc-levofloxacine was higher than 99mTc-micelles, 99mTc-micelles may also be evaluated potential agent due to long circulation and passive accumulation mechanisms at infection/inflammation site. Both radiopharmaceutical agents exhibit potential results in design, characterization, radiolabeling efficiency and in vitro bacterial binding point of view.

Advanced Contrast Agents for Multimodal Biomedical Imaging Based on Nanotechnology

Clinical imaging modalities have reached a prominent role in medical diagnosis and patient management in the last decades. Different image methodologies as Positron Emission Tomography, Single Photon Emission Tomography, X-Rays, or Magnetic Resonance Imaging are in continuous evolution to satisfy the increasing demands of current medical diagnosis. Progress in these methodologies has been favored by the parallel development of increasingly more powerful contrast agents. These are molecules that enhance the intrinsic contrast of the images in the tissues where they accumulate, revealing noninvasively the presence of characteristic molecular targets or differential physiopathological microenvironments. The contrast agent field is currently moving to improve the performance of these molecules by incorporating the advantages that modern nanotechnology offers. These include, mainly, the possibilities to combine imaging and therapeutic capabilities over the same theranostic platform or improve the targeting efficiency in vivo by molecular engineering of the nanostructures. In this review, we provide an introduction to multimodal imaging methods in biomedicine, the sub-nanometric imaging agents previously used and the development of advanced multimodal and theranostic imaging agents based in nanotechnology. We conclude providing some illustrative examples from our own laboratories, including recent progress in theranostic formulations of magnetoliposomes containing ω-3 poly-unsaturated fatty acids to treat inflammatory diseases, or the use of stealth liposomes engineered with a pH-sensitive nanovalve to release their cargo specifically in the acidic extracellular pH microenvironment of tumors.

Gadolinium-Functionalized Peptide Amphiphile Micelles for Multimodal Imaging of Atherosclerotic Lesions

The leading causes of morbidity and mortality globally are cardiovascular diseases, and nanomedicine can provide many improvements including disease-specific targeting, early detection, and local delivery of diagnostic agents. To this end, we designed fibrin-binding, peptide amphiphile micelles (PAMs), achieved by incorporating the targeting peptide cysteine-arginine-glutamic acid-lysine-alanine (CREKA), with two types of amphiphilic molecules containing the gadoliniuim (Gd) chelator diethylenetriaminepentaacetic acid (DTPA), DTPA-bis(stearylamide)(Gd), and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[(poly(ethylene glycol) (PEG))-2000]-DTPA(Gd) (DSPE-PEG2000-DTPA(Gd)). The material characteristics of the resulting nanoparticle diagnostic probes, clot-binding properties in vitro, and contrast enhancement and safety for dual, optical imaging-magnetic resonance imaging (MRI) were evaluated in the atherosclerotic mouse model. Transmission electron micrographs showed a homogenous population of spherical micelles for formulations containing DSPE-PEG2000-DTPA(Gd), whereas both spherical and cylindrical micelles were formed upon mixing DTPA-BSA(Gd) and CREKA amphiphiles. Clot-binding assays confirmed DSPE-PEG2000-DTPA(Gd)-based CREKA micelles targeted clots over 8-fold higher than nontargeting (NT) counterpart micelles, whereas no difference was found between CREKA and NT, DTPA-BSA(Gd) micelles. However, in vivo MRI and optical imaging studies of the aortas and hearts showed fibrin specificity was conferred by the peptide ligand without much difference between the nanoparticle formulations or shapes. Biodistribution studies confirmed that all micelles were cleared through both the reticuloendothelial system and renal clearance, and histology showed no signs of necrosis. In summary, these studies demonstrate the successful synthesis, and the molecular imaging capabilities of two types of CREKA-Gd PAMs for atherosclerosis. Moreover, we demonstrate the differences in micelle formulations and shapes and their outcomes in vitro versus in vivo for site-specific, diagnostic strategies, and provide the groundwork for the detection of thrombosis via contrast-enhancing agents and concurrent therapeutic delivery for theranostic applications.

Multifunctional, stimuli-sensitive nanoparticulate systems for drug delivery

The use of nanoparticulate pharmaceutical drug delivery systems (NDDSs) to enhance the in vivo effectiveness of drugs is now well established. The development of multifunctional and stimulus-sensitive NDDSs is an active area of current research. Such NDDSs can have long circulation times, target the site of the disease and enhance the intracellular delivery of a drug. This type of NDDS can also respond to local stimuli that are characteristic of the pathological site by, for example, releasing an entrapped drug or shedding a protective coating, thus facilitating the interaction between drug-loaded nanocarriers and target cells or tissues. In addition, imaging contrast moieties can be attached to these carriers to track their real-time biodistribution and accumulation in target cells or tissues. Here, I highlight recent developments with multifunctional and stimuli-sensitive NDDSs and their therapeutic potential for diseases including cancer, cardiovascular diseases and infectious diseases.

Formulation development of a novel targeted theranostic nanoemulsion of docetaxel to overcome multidrug resistance in ovarian cancer

OBJECTIVE

Ovarian cancer is a highly lethal disease in which the majority of patients eventually demonstrate multidrug resistance. Develop a novel active targeted theranostic nanomedicine designed to overcome drug efflux mechanisms, using a Generally Regarded As Safe (GRAS) grade nanoemulsion (NE) as a clinically relevant platform.

MATERIALS AND METHODS

The NEs surface-functionalized with folate and gadolinium, were made using GRAS grade excipients and a high-shear microfluidization process. Efficacy was evaluated in ovarian cancer cells, SKOV3 and SKOV3TR. The NE accumulation in tumors was evaluated in SKOV3 tumor-bearing mice by magnetic resonance imaging (MRI).

RESULTS AND DISCUSSION

The NE with particle size < 150 nm were stable in plasma and parenteral fluids for 24 h. Ovarian cancer cells in vitro efficiently took up the non-targeted and folate-targeted NEs; improved cytotoxicity was observed for the folate-targeted NEs showing a 270-fold drop in the IC50 in SKOV3TR cells as compared to docetaxel alone. The addition of gadolinium did not affect cell viability in vitro, but showed relaxation times comparable to Magnevist®. Folate-targeted NEs accumulated in tumors for prolonged period of time compared to Magnevist® and showed enhanced contrast compared to non-targeted NEs with MRI in SKOV3 tumor-bearing mice suggesting active targeting of NEs due to folate modification.

CONCLUSIONS

A folate-targeted, theranostic NE delivers docetaxel by receptor mediated endocytosis that shows enhanced cytotoxicity capable of overcoming ABC transporter mediated taxane resistance. The diagnostic capability of the targeted nanomedicine showed enhanced contrast in tumors compared to clinically relevant MRI contrast agent Magnevist®.

Development of EGFR-targeted nanoemulsion for imaging and novel platinum therapy of ovarian cancer

PURPOSE

Platinum-based chemotherapy is the treatment of choice for malignant epithelial ovarian cancers, but generalized toxicity and platinum resistance limits its use. Theranostic nanoemulsion with a novel platinum prodrug, myrisplatin, and the pro-apoptotic agent, C6-ceramide, were designed to overcome these limitations.

METHODS

The nanoemulsions, including ones with an EGFR binding peptide and gadolinium, were made using generally regarded as safe grade excipients and a high shear microfluidization process. Efficacy was evaluated in ovarian cancer cells, SKOV3, A2780 and A2780CP.

RESULTS

The nanoemulsion with particle size <150 nm were stable in plasma and parenteral fluids for 24 h. Ovarian cancer cells in vitro efficiently took up the non-targeted and EGFR-targeted nanoemulsions; improved cytotoxicity was observed for the these nanoemulsions with the latter showing a 50-fold drop in the IC50 in SKOV3 cells as compared to cisplatin alone. The addition of gadolinium did not affect cell viability in vitro, but showed relaxation times comparable to Magnevist(®).

CONCLUSION

The myrisplatin/C6-ceramide nanoemulsion synergistically enhanced in vitro cytotoxicity. An EGFR binding peptide addition further increased in vitro cytotoxicity in EGFR positive cancer cells. The diagnostic version showed MR imaging similar to the clinically relevant Magnevist® and may be suitable as a theranostic for ovarian cancer.

Synthesis of novel tetravalent galactosylated DTPA-DSPE and study on hepatocyte-targeting efficiency in vitro and in vivo

For the purposes of obtaining a hepatocyte-selective drug delivery system, a novel tetravalent galactosylated diethylenetriaminepentaacetic acid-distearoyl phosphatidylethanolamine (4Gal-DTPA-DSPE) was synthesized. The chemical structure of 4Gal-DTPA-DSPE was confirmed by proton nuclear magnetic resonance and mass spectrometry. The four galactose-modified liposomes (4Gal-liposomes) were prepared by thin-film hydration method, then doxorubicin (DOX) was encapsulated into liposomes using an ammonium sulfate gradient loading method. The liposomal formulations with 4Gal-DTPA-DSPE were characterized by laser confocal scanning microscopy and flow cytometry analysis, and the results demonstrated that the 4Gal-liposomes facilitated the intracellular uptake of DOX into HepG2 cells via asialoglycoprotein receptor-mediated endocytosis. Cytotoxicity assay showed that the cell proliferation inhibition effect of 4Gal-liposomes was higher than that of the conventional liposomes without the galactose. Additionally, pharmacokinetic experiments in rats revealed that the 4Gal-liposomes displayed slower clearance from the systemic circulation compared with conventional liposomes. The organ distributions in mice and the study on frozen sections of liver implied that the 4Gal-liposomes enhanced the intracellular uptake of DOX into hepatocytes and prolonged the circulation. Taken together, these results indicate that liposomes containing 4Gal-DTPA-DSPE have great potential as drug delivery carriers for hepatocyte-selective targeting.

Nanosized multifunctional liposomes for tumor diagnosis and molecular imaging by SPECT/CT

Among currently used cancer imaging methods, nuclear medicine modalities provide metabolic information, whereas modalities in radiology provide anatomical information. However, different modalities, having different acquisition times in separate machines, decrease the specificity and accuracy of images. To solve this problem, hybrid imaging modalities were developed as a new era, especially in the cancer imaging field. With widespread usage of hybrid imaging modalities, specific contrast agents are essentially needed to use in both modalities, such as single-photon emission computed tomography/computed tomography (SPECT/CT). Liposomes are one of the most desirable drug delivery systems, depending on their suitable properties. The aim of this study was to develop a liposomal contrast agent for the diagnosis and molecular imaging of tumor by SPECT/CT. Liposomes were prepared nanosized, coated with polyethylene glycol to obtain long blood circulation, and modified with monoclonal antibody 2C5 for specific tumor targeting. Although DTPA-PE and DTPA-PLL-NGPE (polychelating amphilic polymers; PAPs) were loaded onto liposomes for stable radiolabeling for SPECT imaging, iopromide was encapsulated into liposomes for CT imaging. Liposomes [(DPPC:PEG(2000)-PE:Chol:DTPA-PE), (PL 90G:PEG(2000)-PE:Chol:DTPA-PE), (DPPC:PEG(2000)-PE:Chol:PAPs), (PL 90G:PEG(2000)-PE:Chol:PAPs), (60:0.9:39:0.1% mol ratio)] were characterized in terms of entrapment efficiency, particle size, physical stability, and release kinetics. Additionally, in vitro cell-binding studies were carried out on two tumor cell lines (MCF-7 and EL 4) by counting radioactivity. Tumor-specific antibody-modified liposomes were found to be effective multimodal contrast agents by designating almost 3-8 fold more uptake than nonmodified ones in different tumor cell lines. These results could be considered as an important step in the development of tumor-targeted SPECT/CT contrast agents for cancer imaging.

Polymeric micelles and molecular modeling applied to the development of radiopharmaceuticals

Micelles composed of amphiphilic copolymers linked to a radioactive element are used in nuclear medicine predominantly as a diagnostic application. A relevant advantage of polymeric micelles in aqueous solution is their resulting particle size, which can vary from 10 to 100 nm in diameter. In this review, polymeric micelles labeled with radioisotopes including technetium (99mTc) and indium (111In), and their clinical applications for several diagnostic techniques, such as single photon emission computed tomography (SPECT), gamma-scintigraphy, and nuclear magnetic resonance (NMR), were discussed. Also, micelle use primarily for the diagnosis of lymphatic ducts and sentinel lymph nodes received special attention. Notably, the employment of these diagnostic techniques can be considered a significant tool for functionally exploring body systems as well as investigating molecular pathways involved in the disease process. The use of molecular modeling methodologies and computer-aided drug design strategies can also yield valuable information for the rational design and development of novel radiopharmaceuticals.

Naposomes: a new class of peptide-derivatized, target-selective multimodal nanoparticles for imaging and therapeutic applications

Modified supramolecular aggregates for selective delivery of contrast agents and/or drugs are examined with a focus on a new class of peptide-derivatized nanoparticles: naposomes. These nanoparticles are based on the co-aggregation of two different amphiphilic monomers that give aggregates of different shapes and sizes (micelles, vesicles and liposomes) with diameters ranging between 10 and 300 nm. Structural properties and in vitro and in vivo behaviors are discussed. For the high relaxitivity values (12–19 mM-1s-1) and to detect for the presence of a surface-exposed peptide, the new peptide-derived supramolecular aggregates are very promising candidates as target-selective MRI contrast agents. The efficiency of surface-exposed peptides in homing these nanovectors to a specific target introduces promising new opportunities for the development of diagnostic and therapeutic agents with high specificity toward the biological target and reduced toxic side effects on nontarget organs.

Magnetic field alignable domains in phospholipid vesicle membranes containing lanthanides

Magnetic fields were applied as a structuring force on phospholipid-based vesicular systems, using paramagnetic lanthanide ions as magnetic handles anchored to the vesicle membrane. Different vesicle formulations were investigated using small angle neutron scattering (SANS) in a magnetic field of up to 8 T, cryo-transmission electron microscopy (cryo-TEM), (31)P NMR spectroscopy, dynamic light scattering (DLS), and permeability measurements with a fluorescent water-soluble marker (calcein). The investigated vesicle formulations consisted usually of 80 mol % of the phospholipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and 20 mol % of a chelator lipid (DMPE-DTPA; 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine-diethylenetriaminepentaacetate) with complexed lanthanide ions (Tm(3+), Dy(3+), or La(3+)), and the total lipid concentration was 15 mM. Vesicles containing the paramagnetic lanthanide Tm(3+) or Dy(3+) exhibited a temperature-dependent response to magnetic fields, which can be explained by considering the formation of lipid domains, which upon reaching a critical size become alignable in a magnetic field. The features of this "magnetic field alignable domain model" are as follows: with decreasing temperature (from 30 to 2.5 degrees C) solid domains, consisting mainly of the higher melting phospholipid (DMPE-DTPA.lanthanide), begin to form and grow in size. The domains assemble the large magnetic moments conferred by the lanthanides and orient in magnetic fields. The direction of alignment depends on the type of lanthanide used. The domains orient with their normal parallel to the magnetic field with thulium (Tm(3+)) and perpendicular with dysprosium (Dy(3+)). No magnetic field alignable domains were observed if DMPE-DTPA is replaced either by POPE-DTPA (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine-diethylenetriamine-pentaacetate) or by DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine).

Triggered instability of liposomes bound to hydrophobically modified core-shell PNIPAM hydrogel beads

The ability to trigger a destabilization of the membrane integrity of liposomes bound to environmentally sensitive hydrophobically modified core-shell hydrogel beads is demonstrated. Hydrogel beads with a core composed of poly(N-isopropylacrylamide) lightly cross-linked with bisacrylamide (BA) (pNIPAM) and a shell composed of NIPAM highly cross-linked with BA and containing varying amounts of acrylic acid (AA) [p(NIPAM-co-AA)] undergo a volume phase transition (VPT) at approximately 32 degrees C, as determined from (1)H magic angle spinning (MAS) NMR, regardless of the AA content of the shell. When the shell was hydrophobically modified with either decylamine or tetradecylamine, binding of extruded large unilamellar vesicles (eLUVs) composed of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) was quantitative, as determined via fluorescence spectroscopy. Fluorescence microscopy showed that such bound eLUVs did not fuse. Hydrogel-bound eLUV membrane permeability was assessed using (31)P MAS NMR in the presence of the chemical shift agent praseodymium and demonstrated that only at lower degrees of hydrophobic modification of the core-shell hydrogels was eLUV membrane barrier integrity maintained when T < VPT. At a low degree of hydrophobic modification, cycling the temperature above the VPT even for short periods caused the eLUV membranes to become leaky. Hence, eLUV membrane permeability was coupled to the hydrogel VPT, a situation that would be useful in applications requiring triggered release of liposomal contents.

Liposome-hydrogel bead complexes prepared via biotin-avidin conjugation

Liposomes immobilized onto polymeric hydrogel microbeads have potential advantages both in tissue engineering applications and as drug delivery vehicles. Here we demonstrate, quantify, and optimize lipid vesicle binding to polymeric hydrogel microbeads via the avidin-biotin conjugation system and characterize the stability of the resulting microgel-bound liposomes. Microgels consisting of a copolymer of N-isopropylacrylamide (NIPAM) and acrylic acid (AA), cross-linked with bis-acrylamide, that is, p(NIPAM-co-AA), were biotinylated using aqueous carbodiimide chemistry. Extruded liposomes consisting of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) plus a small fraction of a biotin-derivatized phosphatidylethanolamine (B-PE) were saturated with avidin and allowed to bind to biotinylated hydrogel beads. Using a combination of fluorescence spectroscopy, quenching, and microscopy and 31P NMR static and magic angle spinning (MAS) spectroscopies, we demonstrate conditions for near-quantitative liposome binding to p(NIPAM-co-AA) microbeads and show that liposome fusion does not occur under such conditions, that the liposomes remain intact and impermeable when so bound, and that they can function as slow release vehicles for entrapped aqueous species.

Nanomedicine: perspective and promises with ligand-directed molecular imaging

Molecular imaging and targeted drug delivery play an important role toward personalized medicine, which is the future of patient management. Of late, nanoparticle-based molecular imaging has emerged as an interdisciplinary area, which shows promises to understand the components, processes, dynamics and therapies of a disease at a molecular level. The unprecedented potential of nanoplatforms for early detection, diagnosis and personalized treatment of diseases, have found application in every biomedical imaging modality. Biological and biophysical barriers are overcome by the integration of targeting ligands, imaging agents and therapeutics into the nanoplatform which allow for theranostic applications. In this article, we have discussed the opportunities and potential of targeted molecular imaging with various modalities putting a particular emphasis on perfluorocarbon nanoemulsion-based platform technology.

Multifunctional and stimuli-sensitive pharmaceutical nanocarriers

Currently used pharmaceutical nanocarriers, such as liposomes, micelles, and polymeric nanoparticles, demonstrate a broad variety of useful properties, such as longevity in the body; specific targeting to certain disease sites; enhanced intracellular penetration; contrast properties allowing for direct carrier visualization in vivo; stimuli-sensitivity, and others. Some of those pharmaceutical carriers have already made their way into clinic, while others are still under preclinical development. In certain cases, the pharmaceutical nanocarriers combine several of the listed properties. Long-circulating immunoliposomes capable of prolonged residence in the blood and specific target recognition represent one of the examples of this kind. The engineering of multifunctional pharmaceutical nanocarriers combining several useful properties in one particle can significantly enhance the efficacy of many therapeutic and diagnostic protocols. This paper considers the current status and possible future directions in the emerging area of multifunctional nanocarriers with primary attention on the combination of such properties as longevity, targetability, intracellular penetration, contrast loading, and stimuli-sensitivity.

Radionuclides delivery systems for nuclear imaging and radiotherapy of cancer

The recent developments of nuclear medicine in oncology have involved numerous investigations of novel specific tumor-targeting radiopharmaceuticals as a major area of interest for both cancer imaging and therapy. The current progress in pharmaceutical nanotechnology field has been exploited in the design of tumor-targeting nanoscale and microscale carriers being able to deliver radionuclides in a selective manner to improve the outcome of cancer diagnosis and treatment. These carriers include chiefly, among others, liposomes, microparticles, nanoparticles, micelles, dendrimers and hydrogels. Furthermore, combining the more recent nuclear imaging multimodalities which provide high sensitivity and anatomical resolution such as PET/CT (positron emission tomography/computed tomography) and SPECT/CT (combined single photon emission computed tomography/computed tomography system) with the use of these specific tumor-targeting carriers constitutes a promising rally which will, hopefully in the near future, allow for earlier tumor detection, better treatment planning and more powerful therapy. In this review, we highlight the use, limitations, advantages and possible improvements of different nano- and microcarriers as potential vehicles for radionuclides delivery in cancer nuclear imaging and radiotherapy.

Enhanced tumor MR imaging with gadolinium-loaded polychelating polymer-containing tumor-targeted liposomes

PURPOSE

To significantly enhance tumor MR imaging by using a contrast agent combining three components -- a long-circulating liposome, liposomal membrane-incorporated polychelating amphiphilic polymer heavily loaded with gadolinium, and cancer-specific monoclonal antibody 2C5 attached to the liposome surface.

MATERIALS AND METHODS

Tumor-bearing animals were imaged prior and 4, 24, and 48 hours after i.v. injection of 2C5-modified and unmodified Gd-PAP-containing PEGylated liposomes. The faster and more specific accumulation of the novel contrast nanoparticles in tumors was also confirmed by 3D angiograms and by direct visualization of Gd-immunoliposomes in tumor sections by confocal microscopy.

RESULTS

2C5-modified Gd-PAP-containing PEGylated liposomes allowed for fast and specific tumor imaging as early as 4 hours postinjection. T1 inversion recovery maps demonstrated a significant increase in tumor-associated R1 in animals injected with antibody-modified Gd-loaded liposomes 4 hours postinjection, followed by a gradual decrease consistent with clearance of the agent from the tumor region. In control animals injected with antibody-free liposomes the corresponding R1 values at all investigated timepoints were significantly smaller.

CONCLUSION

The results support the feasibility of using such multifunctional nanoparticular liposome-based agents simultaneously providing prolonged circulation, heavy Gd load, and specific cancer cell recognition as a superior contrast for MR tumor imaging.

Surface functionalization of magnetoliposomes in view of improving iron oxide-based magnetic resonance imaging contrast agents: Anchoring of gadolinium ions to a lipophilic chelate

Small unilamellar phospholipid vesicles containing the phosphatidylethanolamine-diethylene triamine pentaacetic acid (PE-DTPA) conjugate as one of the building stones were constructed. The ability of these colloids to complex gadolinium(III) ions at the surface of both the inner and outer bilayer shells was verified using a colorimetric method with Arsenazo III as a dye indicator. On incubation of these functionalized vesicles with magnetoliposomes (MLs, nanometer-sized magnetite cores encapsulated in a phospholipid bilayer), PE-DTPA percolates into the ML coat. The PE-DTPA content could be fine-tuned by varying the conjugate concentration in the donor vesicles. In the experimental conditions applied, up to 500 Gd(3+) ions were immobilized per ML colloid. The resulting ML-Gd(3+) complexes might have great potential, for example, as a novel magnetic resonance imaging contrast agent.

Targeted pharmaceutical nanocarriers for cancer therapy and imaging

The use of various pharmaceutical nanocarriers has become one of the most important areas of nanomedicine. Ideally, such carriers should be specifically delivered (targeted) to the pathological area to provide the maximum therapeutic efficacy. Among the many potential targets for such nanocarriers, tumors have been most often investigated. This review attempts to summarize currently available information regarding targeted pharmaceutical nanocarriers for cancer therapy and imaging. Certain issues related to some popular pharmaceutical nanocarriers, such as liposomes and polymeric micelles, are addressed, as are different ways to target tumors via specific ligands and via the stimuli sensitivity of the carriers. The importance of intracellular targeting of drug- and DNA-loaded pharmaceutical nanocarriers is specifically discussed, including intracellular delivery with cell-penetrating peptides.

Micellar nanocarriers: pharmaceutical perspectives

Micelles, self-assembling nanosized colloidal particles with a hydrophobic core and hydrophilic shell are currently successfully used as pharmaceutical carriers for water-insoluble drugs and demonstrate a series of attractive properties as drug carriers. Among the micelle-forming compounds, amphiphilic copolymers, i.e., polymers consisting of hydrophobic block and hydrophilic block, are gaining an increasing attention. Polymeric micelles possess high stability both in vitro and in vivo and good biocompatibility, and can solubilize a broad variety of poorly soluble pharmaceuticals many of these drug-loaded micelles are currently at different stages of preclinical and clinical trials. Among polymeric micelles, a special group is formed by lipid-core micelles, i.e., micelles formed by conjugates of soluble copolymers with lipids (such as polyethylene glycol-phosphatidyl ethanolamine conjugate, PEG-PE). Polymeric micelles, including lipid-core micelles, carrying various reporter (contrast) groups may become the imaging agents of choice in different imaging modalities. All these micelles can also be used as targeted drug delivery systems. The targeting can be achieved via the enhanced permeability and retention (EPR) effect (into the areas with the compromised vasculature), by making micelles of stimuli-responsive amphiphilic block-copolymers, or by attaching specific targeting ligand molecules to the micelle surface. Immunomicelles prepared by coupling monoclonal antibody molecules to p-nitrophenylcarbonyl groups on the water-exposed termini of the micelle corona-forming blocks demonstrate high binding specificity and targetability. This review will discuss some recent trends in using micelles as pharmaceutical carriers.

Multimodal Contrast Agent for Combined Computed Tomography and Magnetic Resonance Imaging Applications

OBJECTIVE

The objective of this study was to examine the feasibility of a multimodal system to effectively induce and maintain contrast enhancement in both computed tomography (CT) and magnetic resonance (MR) for radiation therapy applications.

MATERIALS AND METHODS

The physicochemical characteristics of a liposome-encapsulated iohexol and gadoteridol formulation were assessed in terms of agent loading efficiencies, size and morphology, in vitro stability, and release kinetics. The imaging properties of the liposome formulation were assessed based on T1 and T2 relaxivity measurements and in vitro CT and MR imaging in a phantom. A preliminary imaging-based evaluation of the in vivo stability of this multimodal contrast agent was also performed in a lupine model.

RESULTS

The average agent loading levels achieved were 26.5+/-3.8 mg/mL for iodine and 6.6+/- 1.5 mg/mL for gadolinium. These concentrations correspond to approximately 10% of that found in the commercially available preparations of each of these agents. However, this liposome-based formulation is expected to have a smaller volume of distribution and prolonged circulation lifetime in vivo. This multimodal system was found to have high agent retention in vitro, which translated into maintained contrast enhancement (up to 3 days) and stability in vivo.

CONCLUSIONS

This study demonstrated the feasibility of engineering a multimodal contrast agent with prolonged contrast enhancement in vivo for use in CT and MR. This contrast agent may serve as a valuable tool for cardiovascular imaging as well as image registration and guidance applications in radiation therapy.

Amplification strategies in MR imaging: Activation and accumulation of sensing contrast agents (SCAs)

We review new strategies for the development of Gd3+-based T1-relaxation agents and paramagnetic chemical exchange saturation transfer (PARACEST) "sensing" contrast agents (SCAs) designed specifically to detect small molecules or enzymatic activity in living systems. The first class of agents exhibits molecular "sensing" properties as a result of water coordination sphere effects, cleavage, or synthesis of reactive precursor compounds that recombine with macromolecules with the resultant formation of immobilized or rotationally constrained paramagnetic cations. This effect results in changes of water proton relaxation times. The second class (PARACEST) comprises a family of lanthanide-based paramagnetic compounds suitable for CEST imaging. The need for both types of MR agents is justified by efforts to utilize magnetic resonance imaging (MRI) to visualize fine structures in living tissue, and to increase the molecular specificity of MRI.

MR and fluorescent imaging of low-density lipoprotein receptors

RATIONALE AND OBJECTIVES

Over-expression of low-density lipoprotein receptors (LDLRs) occurs in many types of malignancies and is related to the requirement for lipids for rapid proliferation of the tumors. On the other hand, LDLRs that are unable to bind LDL are found on hepatocytes of patients with familial hypercholesterolemia (FH), a genetic disease that leads to premature atherosclerosis and death. The highly selective binding of LDL to LDLR makes these particles ideal carriers of therapeutic and diagnostic contrast agents into the targeted cells. The objectives of this paper are to examine whether a prototype contrast agent (PTIR267) with dual detection properties is suitable for labeling of LDL particles for in vivo detection of LDLR by magnetic resonance imaging (MRI) and for in vitro monitoring of cellular localization by confocal fluorescence microscopy.

MATERIALS AND METHODS

PTIR267 is a lipophilic GdDTPA derivative conjugated to a fluorescent dye. The conjugated dye molecule makes the probe sufficiently water soluble to allow labeling of LDL by a brief incubation of LDL with PTIR267 dissolved in PBS at 37 degrees C (mole ratio LDL: PTIR267 = 0.09:1). The molar relaxivity of PTIR267 in saline is 26 mM(-1)s(-1). Specific LDLR-mediated uptake of PTIR267-labeled LDL was demonstrated in vitro by confocal fluorescence imaging of B16 melanoma cells using confocal fluorescence imaging. In vivo uptake of PTIR267-labeled LDL by a subcutaneously implanted B16 melanoma in mice leads to 30% decrease in longitudinal relaxation time (T(1)) in the tumor. In vivo uptake of PTIR267-labeled LDL leads to 70% decrease in T(1) in a normal C57BL/6 mouse liver; however, in the liver of LDL receptor gene knockout (LDLr-/-) mice with C57BL/6 background, only 12% decrease in T(1) is observed.

CONCLUSIONS

The dual fluorescence and MR imaging properties of PTIR267, combined with the ease of LDL labeling, suggest that it will be a useful tool for optimization of LDLR-targeted cancer diagnosis or therapy and for monitoring the efficacy of gene therapy of FH.

Magnetic resonance molecular imaging with nanoparticles

Molecular imaging agents are extending the potential of noninvasive medical diagnosis from basic gross anatomic descriptions to complicated phenotypic characterizations based on the recognition of unique cell surface biochemical signatures. Although originally the purview of nuclear medicine, molecular imaging is now a prominent feature of most clinically relevant imaging modalities, in particular magnetic resonance (MR) imaging. MR nanoparticulate agents afford the opportunity not only for targeted diagnostic studies but also for image-monitored site-specific therapeutic delivery, much like the "magic bullet" envisioned by Paul Erhlich 100 years ago. Combining high-resolution MR molecular imaging with drug delivery will facilitate verification and quantification of treatment (ie, rational targeted therapy) and will offer new clinical approaches to many diseases.

Increased accumulation of PEG-PE micelles in the area of experimental myocardial infarction in rabbits

Micelles prepared from polyethyleneglycol/phosphatidyl-ethanolamine conjugates (PEG-PE) with a size of 7-20 nm and zeta-potential of approximately -18 mV were administered i.v. to rabbits with experimental myocardial infarctions. Micelles demonstrated a prolonged circulation in the blood (half-life of 2 h) and accumulated in the infarction zone with efficiency more than 8-fold higher as compared to a non-damaged part of the heart muscle. Obtained results suggest that the enhanced permeability and retention (EPR) effect is the primary mechanism of accumulation of microparticles in the infarct areas, and that drug carriers such as PEG-PE micelles can be used for the delivery of therapeutic or diagnostic agents to an area of myocardial infarction.

Improved molecular imaging contrast agent for detection of human thrombus

Molecular imaging of microthrombus within fissures of unstable atherosclerotic plaques requires sensitive detection with a thrombus-specific agent. Effective molecular imaging has been previously demonstrated with fibrin-targeted Gd-DTPA-bis-oleate (BOA) nanoparticles. In this study, the relaxivity of an improved fibrin-targeted paramagnetic formulation, Gd-DTPA-phosphatidylethanolamine (PE), was compared with Gd-DTPA-BOA at 0.05-4.7 T. Ion- and particle-based r(1) relaxivities (1.5 T) for Gd-DTPA-PE (33.7 (s*mM)(-1) and 2.48 x 10(6) (s*mM)(-1), respectively) were about twofold higher than for Gd-DTPA-BOA, perhaps due to faster water exchange with surface gadolinium. Gd-DTPA-PE nanoparticles bound to thrombus surfaces via anti-fibrin antibodies (1H10) induced 72% +/- 5% higher change in R(1) values at 1.5 T (deltaR(1) = 0.77 +/- 0.02 1/s) relative to Gd-DTPA-BOA (deltaR(1) = 0.45 +/- 0.02 1/s). These studies demonstrate marked improvement in a fibrin-specific molecular imaging agent that might allow sensitive, early detection of vascular microthrombi, the antecedent to stroke and heart attack.

Immunomicelles: targeted pharmaceutical carriers for poorly soluble drugs

To prepare immunomicelles, new targeted carriers for poorly soluble pharmaceuticals, a procedure has been developed to chemically attach mAbs to reactive groups incorporated into the corona of polymeric micelles made of polyethylene glycol-phosphatidylethanolamine conjugates. Micelle-attached antibodies retained their ability to specifically interact with their antigens. Immunomicelles with attached antitumor mAb 2C5 effectively recognized and bound various cancer cells in vitro and showed an increased accumulation in experimental tumors in mice when compared with nontargeted micelles. Intravenous administration of tumor-specific 2C5 immunomicelles loaded with a sparingly soluble anticancer agent, taxol, into experimental mice bearing Lewis lung carcinoma resulted in an increased accumulation of taxol in the tumor compared with free taxol or taxol in nontargeted micelles and in enhanced tumor growth inhibition. This family of pharmaceutical carriers can be used for the solubilization and enhanced delivery of poorly soluble drugs to various pathological sites in the body.

PEG-based micelles as carriers of contrast agents for different imaging modalities

The review deals with the fast growing field of diagnostic micelles. The need and requirements for microparticulate contrast agents are discussed. Brief analysis of the micellization process and micelle properties shows that micelles made of amphiphilic co-polymers seem to be the most attractive for practical application. These micelles can be prepared from the variety of co-polymers including hydrophilic polymers grafted on one terminus with lipid residues. Polymeric micelles are considered loaded or modified with various contrast reporter moieties for gamma-scintigraphy, magnetic resonance imaging (MRI), and computed tomography (CT). Their in vitro and in vivo properties are discussed and the results of the initial animal experiments are presented. Mixed micelles were prepared from diacyllipid-polyethylene glycol (PEG) conjugates and polymeric amphiphilic chelates, containing entrapped metals, such as 111-In or Gd, and used for the experimental gamma- and MR imaging of various components of lymphatic system in rabbits. The method is also described to prepare polymeric iodine-containing PEG-based micelles which may act as a long-circulating blood pool imaging agent for CT. Experimental CT-imaging performed in mice and rabbits demonstrated high potential of a micellar contrast agent.

Novel MRI contrast agent for molecular imaging of fibrin: implications for detecting vulnerable plaques

BACKGROUND

Molecular imaging of thrombus within fissures of vulnerable atherosclerotic plaques requires sensitive detection of a robust thrombus-specific contrast agent. In this study, we report the development and characterization of a novel ligand-targeted paramagnetic molecular imaging agent with high avidity for fibrin and the potential to sensitively detect active vulnerable plaques.

METHODS AND RESULTS

The nanoparticles were formulated with 2.5 to 50 mol% Gd-DTPA-BOA, which corresponds to >50 000 Gd(3+) atoms/particle. Paramagnetic nanoparticles were characterized in vitro and evaluated in vivo. In contradistinction to traditional blood-pool agents, T1 relaxation rate as a function of paramagnetic nanoparticle number was increased monotonically with Gd-DTPA concentration from 0.18 mL. s(-1). pmol(-1) (10% Gd-DTPA nanoparticles) to 0.54 mL. s(-1). pmol(-1) for the 40 mol% Gd-DTPA formulations. Fibrin clots targeted in vitro with paramagnetic nanoparticles presented a highly detectable, homogeneous T1-weighted contrast enhancement that improved with increasing gadolinium level (0, 2.5, and 20 mol% Gd). Higher-resolution scans and scanning electron microscopy revealed that the nanoparticles were present as a thin layer over the clot surface. In vivo contrast enhancement under open-circulation conditions was assessed in dogs. The contrast-to-noise ratio between the targeted clot (20 mol% Gd-DTPA nanoparticles) and blood was approximately 118+/-21, and that between the targeted clot and the control clot was 131+/-37.

CONCLUSIONS

These results suggest that molecular imaging of fibrin-targeted paramagnetic nanoparticles can provide sensitive detection and localization of fibrin and may allow early, direct identification of vulnerable plaques, leading to early therapeutic decisions.

Structure and design of polymeric surfactant-based drug delivery systems

The review concentrates on the use of polymeric micelles as pharmaceutical carriers. Micellization of biologically active substances is a general phenomenon that increases the bioavailability of lipophilic drugs and nutrients. Currently used low-molecular-weight pharmaceutical surfactants have low toxicity and high solubilization power towards poorly soluble pharmaceuticals. However, micelles made of such surfactants usually have relatively high critical micelle concentration (CMC) and are unstable upon strong dilution (for example, with the blood volume upon intravenous administration). On the other hand, amphiphilic block co-polymers are also known to form spherical micelles in solution. These micelles have very high solubilization capacity and rather low CMC value that makes them very stable in vivo. Amphiphilic block co-polymers suitable for micelle preparation are described and various types of polymeric micelles are considered as well as mechanisms of their formation, factors influencing their stability and disintegration, their loading capacity towards various poorly soluble pharmaceuticals, and their therapeutic potential. The basic mechanisms underlying micelle longevity and steric protection in vivo are considered with a special emphasis on long circulating drug delivery systems. Advantages and disadvantages of micelles when compared with other drug delivery systems are considered. New polymer-lipid amphiphilic compounds such as diacyillipid-polyethylene glycol, are described and discussed. These compounds are very attractive from a practical point of view, since they easily micellize yielding extremely stable micelles with very high loading capacity. Micelle passive accumulation in the areas with leaky vasculature (tumors, infarct zones) is discussed as an important physiology-based mechanism of drug delivery into certain target zones. Targeted polymeric micelles prepared by using thermo- or pH-sensitive components or by attaching specific targeted moieties (such as antibodies) to their outer surface are described as well as their preparation and some in vivo properties. The fast growing field of diagnostic micelles is analyzed. Polymeric micelles are considered loaded with various agents for gamma, magnetic resonance, and computed tomography imaging. Their in vitro and in vivo properties are discussed and the results of the initial animal experiments are presented.

Long-circulating gadolinium-loaded liposomes: potential use for magnetic resonance imaging of the blood pool

In our previous paper, we reported a method of liposome loading with Gadolinium (Gd) via so called polychelating amphiphilic polymer (PAP). A novel Gd-containing polymeric probe, suitable for the incorporation into the liposomal membrane, was prepared from a low-molecular-weight DTPA-polylysine by linking its N-terminus to a lipid anchor, NGPE-PE. When compared with known membranotropic MR probes, such as Gd-DTPA-SA and Gd-DTPA-PE, liposomes containing new membrane-bound polychelator possess enhanced relaxivity for water protons resulting in an increase of tissue signal intensity on MR images. In this study, we developed the optimized protocol to prepare a liposomal MR contrast agent with high relaxivity and narrow size distribution. Gd-containing liposomes were additionally modified with PEG to provide longevity in vivo. We also demonstrated that upon intravenous administration in rabbit and dog, the new preparation causes a prolonged decrease in the blood T(1) value (reflecting the proton relaxation rate in the blood) and may be considered as a potential contrast agent for MRI of the blood pool.