pH-sensitive paramagnetic liposomes as MRI contrast agents: in vitro feasibility studies

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.

Gadolinium complexes of diethylenetriamine-N-oxide pentaacetic acid-bisamide: a new class of highly stable MRI contrast agents with a hydration number of 3

Diethylenetriamine-N-oxide pentaacetic acid-bisamide-based Gd(iii) complexes with 3 coordinated water molecules have been synthesized to achieve high stability and over three times of the relaxivities of commercial MRI 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.

Gadolinium as an MRI contrast agent

MRI contrast is often enhanced using a contrast agent. Gd³⁺-complexes are the most widely used metallic MRI agents, and several types of Gd³⁺-based contrast agents (GBCAs) have been developed. Furthermore, recent advances in MRI technology have, in part, been driven by the development of new GBCAs. However, when designing new functional GBCAs in a small-molecular-weight or nanoparticle form for possible clinical applications, their functions are often compromised by poor pharmacokinetics and possible toxicity. Although great progress must be made in overcoming these limitations and many challenges remain, new functional GBCAs with either small-molecular-weight or nanoparticle forms offer an exciting opportunity for use in precision medicine.

Cross-Linked and Biodegradable Polymeric System as a Safe Magnetic Resonance Imaging Contrast Agent

Owing to the low efficacy of clinically used small-molecule gadolinium (Gd)-based magnetic resonance imaging (MRI) agents, we designed and explored biodegradable macromolecular conjugates as MRI contrast agents. The linear polymeric structure and core-cross-linked formulation possessed different characteristics and features, so we prepared and comparatively studied the two kinds of Gd-based N-(2-hydroxypropyl) methacrylamide (HPMA) polymeric systems (the core-cross-linked pHPMA-DOTA-Gd and the linear one) using the clinical agent diethylene-triamine pentaacetic acid-Gd(III) (DTPA-Gd) as a control. This study was aimed to find the optimal polymeric formulation as a biocompatible and efficient MRI contrast agent. The high molecular weight (MW, 181 kDa) and core-cross-linked copolymer was obtained via the cross-linked block linear copolymer and could be degraded to low-MW segments (29 kDa) in the presence of glutathione (GSH) and cleaned from the body. Both core-cross-linked and linear pHPMA-DOTA-Gd copolymers displayed 2-3-fold increased relaxivity (r₁ value) than that of DTPA-Gd. Animal studies demonstrated that two kinds of macromolecular systems led to much longer blood circulation time, higher tumor accumulation, and much higher signal intensity compared with the linear and clinical ones. Finally, in vivo and in vitro toxicity studies indicated that the two macromolecular agents had great biocompatibility. Therefore, we performed preliminary but important studies on the Gd-based HPMA polymeric systems as biocompatible and efficient MRI contrast agents and found that the biodegradable core-cross-linked pHPMA-DOTA-Gd copolymer might have greater benefits for the foreground.

Gadolinium-Loaded Polychelating Polymer-Containing Tumor-Targeted Liposomes

Magnetic resonance (MR) is one of the most widely used imaging modalities in contemporary medicine to obtain images of pathological areas. Still, there is a big effort to facilitate the accumulation of contrast in the required zone and further increase a local spatial concentration of a contrast agent for better imaging. Certain particulate carriers able to carry multiple contrast moieties can be used for an efficient delivery of contrast agents to areas of interest and enhancing a signal from these areas. Among those carriers, liposomes draw special attention because of their easily controlled properties and good pharmacological characteristics. To enhance the signal intensity from a given reporter metal in liposomes, one may attempt to increase the net quantity of carrier-associated reporter metal by using polylysine (PLL)-based polychelating amphiphilic polymers (PAP). In addition to heavy load of reporter metal onto the pharmaceutical nanocarrier (liposome), the accumulation of the contrast nanoparticles in organs and tissues of interest (such as tumors) can be significantly enhanced by targeting such particles both "passively," via the so-called enhanced permeability and retention (EPR) effect, or "actively," using various target-specific ligands, such as monoclonal antibodies. Combining three different properties-heavy load with gadolinium (Gd) via the liposome membrane-incorporated PAP and tumor specificity mediated by the liposome-attached mAb 2C5-in a single nanoparticle of long-circulating (PEGylated) liposomes could provide a new contrast agent for highly specific and efficient tumor MRI.

Smart thermosensitive liposomes for effective solid tumor therapy and in vivo imaging

In numerous studies, liposomes have been used to deliver anticancer drugs such as doxorubicin to local heat-triggered tumor. Here, we investigate: (i) the ability of thermosensitive liposomal nanoparticle (TSLnp) as a delivery system to deliver poorly membrane-permeable anticancer drug, gemcitabine (Gem) to solid pancreatic tumor with the aid of local mild hyperthermia and, (ii) the possibility of using gadolinium (Magnevist®) loaded-TSLnps (Gd-TSLnps) to increase magnetic resonance imaging (MRI) contrast in solid tumor. In this study, we developed and tested gemcitabine-loaded thermosensitive liposomal nanoparticles (Gem-TSLnps) and gadolinium-loaded thermosensitive liposomal nanoparticles (Gd-TSLnps) both in in-vitro and in-vivo. The TSLnps exhibited temperature-dependent release of Gem, at 40-42°C, 65% of Gem was released within 10 min, whereas < 23% Gem leakage occurred at 37°C after a period of 2 h. The pharmacokinetic parameters and tissue distribution of both Gem-TSLnps and Gd-TSLnps were significantly greater compared with free Gem and Gd, while Gem-TSLnps plasma clearance was reduced by 17-fold and that of Gd-TSLpns was decreased by 2-fold. Area under the plasma concentration time curve (AUC) of Gem-TSLnps (35.17± 0.04 μghr/mL) was significantly higher than that of free Gem (2.09 ± 0.01 μghr/mL) whereas, AUC of Gd-TSLnps was higher than free Gd by 3.9 fold high. TSLnps showed significant Gem accumulation in heated tumor relative to free Gem. Similar trend of increased Gd-TSLnps accumulation was observed in non-heated tumor compared to that of free Gd; however, no significant difference in MRI contrast enhancement between free Gd and Gd-TSLnps ex-vivo tumor images was observed. Despite Gem-TSLnps dose being half of free Gem dose, antitumor efficacy of Gem-TSLnps was comparable to that of free Gem(Gem-TSLnps 10 mg Gem/kg compared with free Gem 20 mg/kg). Overall, the findings suggest that TSLnps may be used to improve Gem delivery and enhance its antitumor activity. However, the formulation of Gd-TSLnp needs to be fully optimized to significantly enhance MRI contrast in tumor.

Octreotide Functionalized Nano-Contrast Agent for Targeted Magnetic Resonance Imaging

Reversible addition-fragmentation chain transfer (RAFT) polymerization has been employed to synthesize branched block copolymer nanoparticles possessing 1,4,7,10-tetraazacyclododecane-N,N,'N,″N,‴-tetraacetic acid (DO3A) macrocycles within their cores and octreotide (somatostatin mimic) cyclic peptides at their periphery. These polymeric nanoparticles have been chelated with Gd³⁺ and applied as magnetic resonance imaging (MRI) nanocontrast agents. This nanoparticle system has an r₁ relaxivity of 8.3 mM^-1 s^-1, which is 3 times the r₁ of commercial gadolinium-based contrast agents (GBCAs). The in vitro targeted binding efficiency of these nanoparticles shows 5 times greater affinity to somatostatin receptor type 2 (SSTR2) with K_i = 77 pM (compared to somatostatin with K_i = 0.385 nM). We have also evaluated the tumor targeting molecular imaging ability of these branched copolymer nanoparticle in vivo using nude/NCr mice bearing AR42J rat pancreatic tumor (SSTR2 positive) and A549 human lung carcinoma tumor (SSTR2 negative) xenografts.

MRI contrast agents: Classification and application (Review)

Magnetic resonance imaging (MRI) contrast agents are categorised according to the following specific features: chemical composition including the presence or absence of metal atoms, route of administration, magnetic properties, effect on the magnetic resonance image, biodistribution and imaging applications. The majority of these agents are either paramagnetic ion complexes or superparamagnetic magnetite particles and contain lanthanide elements such as gadolinium (Gd3+) or transition metal manganese (Mn2+). These elements shorten the T1 or T2 relaxation time, thereby causing increased signal intensity on T1-weighted images or reduced signal intensity on T2-weighted images. Most paramagnetic contrast agents are positive agents. These agents shorten the T1, so the enhanced parts appear bright on T1-weighted images. Dysprosium, superparamagnetic agents and ferromagnetic agents are negative contrast agents. The enhanced parts appear darker on T2-weighted images. MRI contrast agents incorporating chelating agents reduces storage in the human body, enhances excretion and reduces toxicity. MRI contrast agents may be administered orally or intravenously. According to biodistribution and applications, MRI contrast agents may be categorised into three types: extracellular fluid, blood pool and target/organ-specific agents. A number of contrast agents have been developed to selectively distinguish liver pathologies. Some agents are also capable of targeting other organs, inflammation as well as specific tumors.

Gadolinium(iii) based nanoparticles for T1-weighted magnetic resonance imaging probes

This review summarized the recent progress on Gd(iii)-based nanoparticles asT₁-weighted MRI contrast agents and multimodal contrast agents.

Synthesis and in vivo magnetic resonance imaging evaluation of biocompatible branched copolymer nanocontrast agents

Branched copolymer nanoparticles (D_h =20–35 nm) possessing 1,4,7, 10-tetraazacyclododecane-N,N′,N″,N‴-tetraacetic acid macrocycles within their cores have been synthesized and applied as magnetic resonance imaging (MRI) nanosized contrast agents in vivo. These nanoparticles have been generated from novel functional monomers via reversible addition–fragmentation chain transfer polymerization. The process is very robust and synthetically straightforward. Chelation with gadolinium and preliminary in vivo experiments have demonstrated promising characteristics as MRI contrast agents with prolonged blood retention time, good biocompatibility, and an intravascular distribution. The ability of these nanoparticles to perfuse and passively target tumor cells through the enhanced permeability and retention effect is also demonstrated. These novel highly functional nanoparticle platforms have succinimidyl ester-activated benzoate functionalities within their corona, which make them suitable for future peptide conjugation and subsequent active cell-targeted MRI or the conjugation of fluorophores for bimodal imaging. We have also demonstrated that these branched copolymer nanoparticles are able to noncovalently encapsulate hydrophobic guest molecules, which could allow simultaneous bioimaging and drug delivery.

Evaluating pH in the Extracellular Tumor Microenvironment Using CEST MRI and Other Imaging Methods

Tumor acidosis is a consequence of altered metabolism, which can lead to chemoresistance and can be a target of alkalinizing therapies. Noninvasive measurements of the extracellular pH (pHe) of the tumor microenvironment can improve diagnoses and treatment decisions. A variety of noninvasive imaging methods have been developed for measuring tumor pHe. This review provides a detailed description of the advantages and limitations of each method, providing many examples from previous research reports. A substantial emphasis is placed on methods that use MR spectroscopy and MR imaging, including recently developed methods that use chemical exchange saturation transfer MRI that combines some advantages of MR spectroscopy and imaging. Together, this review provides a comprehensive overview of methods for measuring tumor pHe, which may facilitate additional creative approaches in this research field.

Nanodevices for studying nano-pathophysiology

Nano-scaled devices are a promising platform for specific detection of pathological targets, facilitating the analysis of biological tissues in real-time, while improving the diagnostic approaches and the efficacy of therapies. Herein, we review nanodevice approaches, including liposomes, nanoparticles and polymeric nanoassemblies, such as polymeric micelles and vesicles, which can precisely control their structure and functions for specifically interacting with cells and tissues. These systems have been successfully used for the selective delivery of reporter and therapeutic agents to specific tissues with controlled cellular and subcellular targeting of biomolecules and programmed operation inside the body, suggesting a high potential for developing the analysis for nano-pathophysiology.

Computed tomography and magnetic resonance imaging

Imaging in Oncology is rapidly moving from the detection and size measurement of a lesion to the quantitative assessment of metabolic processes and cellular and molecular interactions. Increasing insights into cancer as a complex disease with involvement of the tumor stroma in tumor pathobiological processes have made it clear that for successful control of cancer, treatment strategies should not only be directed at the tumor cells but also targeted at the tumor microenvironment. This requires understanding of the complex molecular and cellular interactions in cancer tissue. Recent developments in imaging technology have increased the possibility to image various pathobiological processes in cancer development and response to treatment. For computed tomography (CT) and magnetic resonance imaging (MRI) various improvements in hardware, software, and imaging probes have lifted these modalities from classical anatomical imaging techniques to techniques suitable to image and quantify various physiological processes and molecular and cellular interactions. Next to a more general overview of possible imaging targets in oncology this chapter provides an overview of the various developments in CT and MRI technology and some specific applications.

Gd Complexes of DO3A-(Biphenyl-2,2'-bisamides) Conjugates as MRI Blood-Pool Contrast Agents

We report the synthesis of DO3A derivatives of 2,2'-diaminobiphenyl (1a,b) and their Gd complexes of the type [Gd(1)(H2O)]·xH2O (2a,b) for use as new MRI blood-pool contrast agents (BPCAs) that provide strong and prolonged vascular enhancement. Pharmacokinetic inertness of 2 compares well with that of structurally related Dotarem, a DOTA-based MRI CA currently in use. The R 1 relaxivity in water reaches 7.3 mM(-1) s(-1), which is approximately twice as high as that of Dotarem (R 1 = 3.9 mM(-1) s(-1)). They show interaction with HSA to give association constants (K a) in the order of two (∼10(2)), revealing the existence of the blood-pool effect. The in vivo MR images of mice obtained with 2 are coherent, showing strong signal enhancement in both heart, abdominal aorta, and small vessels. Furthermore, the brain tumor is vividly enhanced for an extended period of time.

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.

Improved paramagnetic liposomes for MRI visualization of pH triggered release

This work aims at assessing the in vitro potential of paramagnetic pH sensitive liposomes as imaging tools for visualizing drug-delivery and release processes by Magnetic Resonance Imaging (MRI). pH sensitive liposomes (pSLs) were formulated using the fusogenic phospholipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), the membrane stabilizer D-α-tocopherol-hemisuccinate (THS), and were loaded with several paramagnetic complexes including the clinically approved Gadoteridol (marketed as ProHance™). The proposed formulation allows the fast and full release of Gadoteridol at pH 5.5. The leakage of the imaging reporter from the vesicles was associated with a relaxivity enhancement that allowed its visualization by MRI. It was observed that the release mechanism implies the protonation of the THS basic sites that leads to vesicle aggregation, thus enabling the expression of the fusogenic property of POPE. Attempts for improving the MRI properties of pSLs were pursued through the encapsulation of imaging agents with higher relaxivity than Gadoteridol, but it was observed that the release kinetic can be significantly affected by the probe size. Aiming at preparing stealth pSLs, PEG chains were conjugated to the external surface of the vesicles via cleavable disulphide bridges. Such nanomedicines do not release their content at acidic pH as long as the coating polymer is not removed from the surface. The results obtained suggest that the liposomal formulation investigated in this work has the potential for visualizing drug-delivery and release processes by in vivo MRI preclinical studies.

Design and synthesis of novel functional lipid-based bioconjugates for drug delivery and other applications

The modification of biologicals such as proteins/peptides, small molecules, and other polymers with lipids provides an efficient method for mediating their insertion into liposomes and lipid-core micellar nanocarriers. In this chapter, we describe several representative protocols developed in our laboratory for the bioconjugation of liposomes and lipid-core micelles for drug/gene delivery and diagnostic imaging applications.

Gadolinium-loaded polychelating polymer-containing tumor-targeted liposomes

Magnetic resonance (MR) is one of the most widely used imaging modalities in contemporary medicine to obtain images of pathological areas. Still, there is a big effort to facilitate the accumulation of contrast in the required zone and further increase a local spatial concentration of a contrast agent for better imaging. Certain particulate carriers able to carry multiple contrast moieties can be used for an efficient delivery of contrast agents to areas of interest and enhancing a signal from these areas. Among those carriers, liposomes draw special attention because of their easily controlled properties and good pharmacological characteristics. To enhance the signal intensity from a given reporter metal in liposomes, one may attempt to increase the net quantity of carrier-associated reporter metal by using polylysine (PLL)-based polychelating amphiphilic polymers (PAP). In addition to heavy load of reporter metal onto the pharmaceutical nanocarrier (liposome), the accumulation of the contrast nanoparticles in organs and tissues of interest (such as tumors) can be significantly enhanced by targeting such particles both "passively," via the so-called enhanced permeability and retention (EPR) effect, or "actively," using various target-specific ligands, such as monoclonal antibodies. Combining three different properties--heavy load with Gd via the liposome membrane-incorporated PAP and tumor specificity mediated by the liposome-attached mAb 2C5--in a single nanoparticle of long-circulating (PEGylated) liposomes could provide a new contrast agent for highly specific and efficient tumor MRI.

Target molecular therapies: methods to enhance and monitor tumor drug delivery

Noninvasive monitoring/quantification of drug delivery to tumors is an ideal goal of chemotherapy treatment. However, the ability to overcome the barriers to the developing of targeted therapies-along with the physiological barriers that the tumor presents-is still needed. Recent advances demonstrate that targeted therapies can be used for diagnostic and therapeutic applications. The most mature of these technologies are liposomes that encapsulate a therapeutic drug in conjunction with a contrast agent. Through selective manipulation of the liposome composition, modification of the tumor microphysiology, and temporal sequencing of liposome administration with tumor microphysiology modification improvement in efficacy can be achieved. The future application of these targeted therapies will allow the radiologist to become a more central member of the cancer treatment team, further expanding the field and the radiologist's unique skills.

Novel nanomedicine-based MRI contrast agents for gynecological malignancies

Gynecological cancers result in significant morbidity and mortality in women despite advances in treatment and diagnosis. This is due to detection of the disease in the late stages following metastatic spread in which treatment options become limited and may not result in positive outcomes. In addition, traditional contrast agents are not very effective in detecting primary metastatic tumors and cells due to a lack of specificity and sensitivity of the diagnostic tools, which limits their effectiveness. Recently, the field of nanomedicine-based contrast agents offers a great opportunity to develop highly sophisticated devices that can overcome many traditional hurdles of contrast agents including solubility, cell-specific targeting, toxicities, and immunological responses. These nanomedicine-based contrast agents including liposomes, micelles, dendrimers, multifunctional magnetic polymeric nanohybrids, fullerenes, and nanotubes represent improvements over their traditional counterparts, which can significantly advance the field of molecular imaging.

Classification and basic properties of contrast agents for magnetic resonance imaging

A comprehensive classification of contrast agents currently used or under development for magnetic resonance imaging (MRI) is presented. Agents based on small chelates, macromolecular systems, iron oxides and other nanosystems, as well as responsive, chemical exchange saturation transfer (CEST) and hyperpolarization agents are covered in order to discuss the various possibilities of using MRI as a molecular imaging technique. The classification includes composition, magnetic properties, biodistribution and imaging applications. Chemical compositions of various classes of MRI contrast agents are tabulated, and their magnetic status including diamagnetic, paramagnetic and superparamagnetic are outlined. Classification according to biodistribution covers all types of MRI contrast agents including, among others, extracellular, blood pool, polymeric, particulate, responsive, oral, and organ specific (hepatobiliary, RES, lymph nodes, bone marrow and brain). Various targeting strategies of molecular, macromolecular and particulate carriers are also illustrated.

Environment-sensitive and enzyme-sensitive MR contrast agents

The majority of approved MR contrast agents belong to the class of paramagnetic chelates. These small molecules are uniquely suited to respond to changes in the microenvironment in vivo. These contrast agents can also function as substrates for several classes of enzymes. In both cases, the chelates can be designed in a way that the relaxivity - i.e., the ability of chelated paramagnetic metal cations to shorten the relaxation times of water - is directly affected by changes in the microenvironment. This chapter summarizes a variety of MR contrast agent designs that enable "sensing" of metal cations, pH and enzymatic activity.

Metabolic and molecular imaging in neuro-oncology

Techniques for human brain imaging have undergone rapid developments in recent years. Technological progress has enabled the assessment of many physiological parameters in vivo that are highly relevant for tumour grading, tissue characterisation, definition of the extent and infiltration of tumours, and planning and monitoring of therapy. In this review, we provide a brief overview of advanced MRI and molecular-tracer techniques that have many potential clinical uses. A broad range of techniques, including dynamic MRI, PET, and single photon emission computed tomography, provide measurements of various features of tumour blood flow and microvasculature. Using PET to measure glucose consumption enables visualisation of tumour metabolism, and magnetic resonance spectroscopy techniques provide complementary information on energy metabolism. Changes in protein and DNA synthesis can be assessed through uptake of labelled amino acids and nucleosides. Advanced imaging techniques can be used to assess tumour malignancy, extent, and infiltration, and might provide diagnostic clues to distinguish between lesion types and between recurrent tumour and necrosis. Stereotactic biopsies should be taken from the most malignant part of tumours, which can be identified by changes in microvascular structure and metabolic activity. Functional and metabolic imaging can improve the planning and monitoring of radiation and chemotherapy and contribute to the development of new therapies.

Mn(II)-monosubstituted polyoxometalates as candidates for contrast agents in magnetic resonance imaging

Two mono-substituted manganese polyoxometalates, K(6)MnSiW(11)O(39) (MnSiW(11)) and K(8)MnP(2)W(17)O(61) (MnP(2)W(17)), have been evaluated by in vivo and in vitro experiments as the candidates of potential tissue-specific contrast agents for magnetic resonance imaging (MRI). T1-relaxivities of 12.1mM(-1)s(-1) for MnSiW(11) and 4.7 mM(-1)s(-1) for MnP(2)W(17) (400 MHz, 25 degrees C) were higher than or similar to that of the commercial MRI contrast agent (GdDTPA). Their relaxivities in BSA and hTf solutions were also reported. After administration of MnSiW(11) and MnP(2)W(17) to Wistar rats, MR imaging showed longer and remarkable enhancement in rat liver and favorable renal excretion capability. The signal intensity increased by 74.0+/-4.9% for the liver during the whole imaging period (90 min) and by 67.2+/-5.3% for kidney within 20-70 min after injection at 40+/-3 micromol kg(-1) dose for MnSiW(11). MnP(2)W(17) induced 71.5+/-15.1% enhancement for the liver in 10-45 min range and 73.1+/-3.2% enhancement for kidney within 5-40 min after injection at 39+/-3 micromol kg(-1) dose. In vitro and in vivo study showed MnSiW(11) and MnP(2)W(17) being favorable candidates as the tissue-specific contrast agents for MRI.

The gadolinium complexes with polyoxometalates as potential MRI contrast agents

The two gadolinium (Gd) polyoxometalates, K(15)[Gd(BW(11)O(39))(2)] [Gd(BW(11))(2)] and K(17)[Gd(CuW(11)O(39))(2)] [Gd(CuW(11))(2)] have been evaluated by in vivo and in vitro experiments as the candidates of potential tissue-specific magnetic resonance imaging (MRI) contrast agents. T(1) relaxivities of 17.12 mM(-1) x s(-1) for Gd(BW(11))(2) and 19.95 mM(-1) x s(-1) for Gd(CuW(11))(2) (400 MHz, 25 degrees C) were much higher than that of the commercial MRI contrast agent (GdDTPA). Their relaxivities in bovine serum albumin and human serum transferrin solutions were also reported. After administration of Gd(BW(11))(2) and Gd(CuW(11))(2) to Wistar rats, MRI showed longer and remarkable enhancement in rat liver and favorable renal excretion capability. The signal intensity increased by 37.63+/-3.45% for the liver during the whole imaging period (100 min) and by 61.47+/-10.03% for kidney within 5-40 min after injection at 40+/-1-micromol x kg(-1) dose for Gd(CuW(11))(2), and Gd(BW(11))(2) induced 50.44+/-3.51% enhancement in the liver in 5-50-min range and 61.47+/-10.03% enhancement for kidney within 5-40 min after injection at 39+/-4 micromol x kg(-1) dose. In vitro and in vivo study showed that Gd(BW(11))(2) and Gd(CuW(11))(2) are favorable candidates as tissue-specific contrast agents for MRI.

Magnetic resonance imaging of temperature-sensitive liposome release: drug dose painting and antitumor effects

BACKGROUND

In preclinical studies, lysolipid-based temperature-sensitive liposomes (LTSLs) containing chemotherapy drugs administered in combination with local hyperthermia have been found to increase tumor drug concentrations and improve antitumor efficacy of the drugs. We used a novel magnetic resonance imaging (MRI) method to measure the temporal and spatial patterns of drug delivery in a rat fibrosarcoma model during treatment with LTSLs containing doxorubicin and an MRI contrast agent (manganese) (Dox/Mn-LTSLs) administered at different times with respect to hyperthermia.

METHODS

Rats bearing 10- to 12-mm fibrosarcomas (n = 6-7 per group) were treated with Dox/Mn-LTSLs (at a dose of 5 mg doxorubicin/kg body weight) before and/or during 60 minutes of local tumor hyperthermia administered via a catheter inserted at the center of the tumor. Drug distribution was monitored continuously via MRI. Magnetic resonance changes were used to calculate intratumoral doxorubicin concentrations throughout treatment. Tumors were monitored until they reached five times their volume on the day of treatment or 60 days. Doxorubicin concentrations and times for tumors to reach five times their volume on the day of treatment were analyzed using the Kruskal-Wallis test and the Kaplan-Meier product-limit method, respectively. All statistical tests were two-sided.

RESULTS

Administration of Dox/Mn-LTSLs before, during, and both before and during hyperthermia yielded central, peripheral, and uniform drug distributions, respectively. Doxorubicin accumulated more quickly and reached higher concentrations in the tumor when Dox/Mn-LTSLs were administered during hyperthermia than when administered before hyperthermia (rate: 9.8 versus 1.8 microg/min, difference = 8.0 microg/min, 95% confidence interval [CI] = 6.8 to 12.8 microg/min, P = .003; concentration: 15.1 versus 8.0 ng/mg, difference = 7.1 ng/mg, 95% CI = 3.6 to 10.6 ng/mg, P = .028). LTSL administered during hyperthermia also yielded the greatest antitumor effect, with a median time for tumors to reach five times their volume on the day of treatment of 34 days (95% CI = 30 days to infinity) compared with 18.5 days (95% CI = 16 to 23 days) for LTSL before hyperthermia and 22.5 days (95% CI = 15 to 25 days) for LTSL before and during hyperthermia.

CONCLUSIONS

In this rat fibrosarcoma model, LTSLs were most effective when delivered during hyperthermia, which resulted in a peripheral drug distribution.

Interfacial tension of phosphatidylcholine–phosphatidylserine system in bilayer lipid membrane

The effect of pH of electrolyte solution on the interfacial tension of lipid membrane formed of phosphatidylcholine (PC, lecithin)-phosphatidylserine (PS) system was studied. In this article, three models describing the H+ and OH- ions adsorption in the bilayer lipid surface are presented. In Model I and Model II, the surface is continuous with uniformly distributed functional groups constituting the centres of H+ and OH- ions adsorption while in the other the surface is built of lipid molecules, free or with attached H+ and OH- ions. In these models contribution of the individual lipid molecule forms to interfacial tension of the bilayer were assumed to be additive. In Model III the adsorption of the H+ and OH- ions at the PC-PS bilayer surface was described in terms of the Gibbs isotherm. Theoretical equations are derived to describe this dependence in the whole pH range.

Lipid-based nanoparticles for contrast-enhanced MRI and molecular imaging

In the field of MR imaging and especially in the emerging field of cellular and molecular MR imaging, flexible strategies to synthesize contrast agents that can be manipulated in terms of size and composition and that can be easily conjugated with targeting ligands are required. Furthermore, the relaxivity of the contrast agents, especially for molecular imaging applications, should be very high to deal with the low sensitivity of MRI. Lipid-based nanoparticles, such as liposomes or micelles, have been used extensively in recent decades as drug carrier vehicles. A relatively new and promising application of lipidic nanoparticles is their use as multimodal MR contrast agents. Lipids are amphiphilic molecules with both a hydrophobic and a hydrophilic part, which spontaneously assemble into aggregates in an aqueous environment. In these aggregates, the amphiphiles are arranged such that the hydrophobic parts cluster together and the hydrophilic parts face the water. In the low concentration regime, a wide variety of structures can be formed, ranging from spherical micelles to disks or liposomes. Furthermore, a monolayer of lipids can serve as a shell to enclose a hydrophobic core. Hydrophobic iron oxide particles, quantum dots or perfluorocarbon emulsions can be solubilized using this approach. MR-detectable and fluorescent amphiphilic molecules can easily be incorporated in lipidic nanoparticles. Furthermore, targeting ligands can be conjugated to lipidic particles by incorporating lipids with a functional moiety to allow a specific interaction with molecular markers and to achieve accumulation of the particles at disease sites. In this review, an overview of different lipidic nanoparticles for use in MRI is given, with the main emphasis on Gd-based contrast agents. The mechanisms of particle formation, conjugation strategies and applications in the field of contrast-enhanced, cellular and molecular MRI are discussed.

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.

Novel pH-sensitive paramagnetic liposomes with improved MR properties

The use of paramagnetic pH-sensitive liposomes was recently suggested as a new approach for monitoring pathologic changes in pH by MRI. Such liposomes must be stable in blood and selectively release the encapsulated paramagnetic agent when exposed to lower pH in the target tissue. In the present study, different liposomal systems were formulated and characterized by relaxometry, cryo-transmission electron microscopy (cryo-TEM), and MRI. The pH-sensitive system dipalmitoylphosphatidylethanolamine/palmitic acid (DPPE/PA) liposomal GdDTPA-BMA, which was previously shown to be unstable in blood, was modified to improve its stability. The incorporation of cholesterol into the DPPE/PA liposomes significantly increased their stability in blood, but the pH sensitivity was diminished. Polyethylene glycol (PEG)-modified DPPE/PA liposomes were pH-insensitive in buffer, and unstable in blood. However, exchanging PA with the double-chained amphiphile dipalmitoylglycerosuccinate (DPSG) yielded liposomes with improved properties. DPPE/DPSG liposomal GdDTPA-BMA was stable in blood at physiological pH, and displayed a marked pH sensitivity. The pH sensitivity was not diminished after preincubation in blood, contrary to what has been reported for analogues containing unsaturated lipids. The potential of this system for monitoring pH was demonstrated in an in vitro MRI phantom study.

Investigation of sandwiched gadolinium (III) complexes with tungstosilicates as potential MRI contrast agents

Two gadolinium-sandwiched complexes with tungstosilicates, K(13)[Gd(SiW(11)O(39))(2)] (Gd(SiW(11))(2)) and K(11)H(6)[Gd(3)O(3)(SiW(9)O(34))(2)] (Gd(3)(SiW(9))(2)), have been investigated by in vitro and in vivo experiments as potential contrast agents for magnetic resonance imaging (MRI). T(1)-relaxivity of Gd(SiW(11))(2)was 6.59 mM(-1).s(-1) in aqueous solution and 6.85 mM(-1).s(-1) in 0.725 mmol.L(-1) bovine serum albumin solution at 25 degrees C and 9.39 T, respectively. The corresponding T(1)-relaxivity of Gd(3)(SiW(9))(2) was 12.6 and 19.3 mM(-1).s(-1) per Gd, respectively. MRI for Sprague-Dawley rats showed longer and more remarkable enhancement in rat liver after i.v. injection of these two complexes: 39.4 +/- 3.9% and 57.4 +/- 11.6% within the first 30 min after injection, 31.2 +/- 2.6% and 39.9 +/- 7.6% in the next 60 min for Gd(SiW(11))(2) and Gd(3)(SiW(9))(2) at doses of 0.081 and 0.084 mmol Gd/kg, respectively. Our preliminary in vitro and in vivo study indicates that Gd(SiW(11))(2) and Gd(3)(SiW(9))(2) are favorable candidates for hepatic contrast agents for MRI. However, the two complexes exhibit higher acute toxicity and need to be modified and studied further before clinical use.