Mixed micelles containing lipophilic gadolinium complexes as MRA contrast agents

Recent Advances on Pt-Based Compounds for Theranostic Applications

Since the discovery of cisplatin's antitumoral activity and its approval as an anticancer drug, significant efforts have been made to enhance its physiological stability and anticancer efficacy and to reduce its side effects. With the rapid development of targeted and personalized therapies, and the promising theranostic approach, platinum drugs have found new opportunities in more sophisticated systems. Theranostic agents combine diagnostic and therapeutic moieties in one scaffold, enabling simultaneous disease monitoring, therapy delivery, response tracking, and treatment efficacy evaluation. In these systems, the platinum core serves as the therapeutic agent, while the functionalized ligand provides diagnostic tools using various imaging techniques. This review aims to highlight the significant role of platinum-based complexes in theranostic applications, and, to the best of our knowledge, this is the first focused contribution on this type of platinum compounds. This review presents a brief introduction to the development of platinum chemotherapeutic drugs, their limitations, and resistance mechanisms. It then describes recent advancements in integrating platinum complexes with diagnostic agents for both tumor treatment and monitoring. The main body is organized into three categories based on imaging techniques: fluorescence, positron emission tomography (PET), single-photon emission computed tomography (SPECT), and magnetic resonance imaging (MRI). Finally, this review outlines promising strategies and future perspectives in this evolving field.

RGD_PLGA Nanoparticles with Docetaxel: A Route for Improving Drug Efficiency and Reducing Toxicity in Breast Cancer Treatment

Breast cancer is the leading cause of cancer-related death in women. Although many therapeutic approaches are available, systemic chemotherapy remains the primary choice, especially for triple-negative and advanced breast cancers. Unfortunately, systemic chemotherapy causes serious side effects and requires high doses to achieve an effective concentration in the tumor. Thus, the use of nanosystems for drug delivery may overcome these limitations. Herein, we formulated Poly (lactic-co-glycolic acid) nanoparticles (PLGA-NPs) containing Docetaxel, a fluorescent probe, and a magnetic resonance imaging (MRI) probe. The cyclic RGD tripeptide was linked to the PLGA surface to actively target α_vβ₃ integrins, which are overexpressed in breast cancer. PLGA-NPs were characterized using dynamic light scattering, fast field-cycling ¹H-relaxometry, and ¹H-nuclear magnetic resonance. Their therapeutic effects were assessed both in vitro in triple-negative and HER2+ breast cancer cells, and in vivo in murine models. In vivo MRI and inductively coupled plasma mass spectrometry of excised tumors revealed a stronger accumulation of PLGA-NPs in the RGD_PLGA group. Targeted PLGAs have improved therapeutic efficacy and strongly reduced cardiac side effects compared to free Docetaxel. In conclusion, RGD-PLGA is a promising system for breast cancer treatment, with positive outcome in terms of therapeutic efficiency and reduction in side effects.

Detection of U-87 Tumor Cells by RGD-Functionalized/Gd-Containing Giant Unilamellar Vesicles in Magnetization Transfer Contrast Magnetic Resonance Images

OBJECTIVES

The targeting of tumor cells and their visualization with magnetic resonance imaging (MRI) is an important task in biomedicine. The low sensitivity of this technique is a significant drawback and one that may hamper the detection of the imaging reporters used.To overcome this sensitivity issue, this work explores the synergy between 2 strategies: (1) arginine, glycine, aspartic acid peptide (RGD)-functionalized giant unilamellar vesicles (GUVs) loaded with Gd complexes to accumulate large amounts of MRI contrast agent at the targeting site; and (2) the use of magnetization transfer contrast (MTC), which is a sensitive MRI technique for the detection of Gd complexes in the tumor region.

MATERIALS AND METHODS

Giant unilamellar vesicles were prepared using the gentle swelling method, and the cyclic RGD targeting moiety was introduced onto the external membrane. Paramagnetic Gd-containing complexes and the fluorescent probe rhodamine were both part of the vesicle membranes and Gd-complexes were also the payload within the inner aqueous cavity. Giant unilamellar vesicles that were loaded with the imaging reporters, but devoid of the RGD targeting moiety, were used as controls. U-87 MG human glioblastoma cells, which are known to overexpress the targets for RGD moieties, were used. In the in vivo experiments, U-87 MG cells were subcutaneously injected into nu/nu mice, and the generated tumors were imaged using MRI, 15 days after cell administration. Magnetic resonance imaging was carried out at 7 T, and T2W, T1W, and MTC/Z-spectra were acquired. Confocal microscopy images and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) were used for result validation.

RESULTS

In vitro results show that RGD GUVs specifically bind to U-87 MG cells. Microscopy demonstrates that (1) RGD GUVs were anchored onto the external surface of the tumor cells without any internalization; (2) a low number of GUVs per cell were clustered at specific regions; and (3) there is no evidence for macrophage uptake or cell toxicity. The MRI of cell pellets after incubation with RGD GUVs and untargeted ctrl-GUVs was performed. No difference in T1 signal was detected, whereas a 15% difference in MT contrast is present between the RGD GUV-treated cells and the ctrl-GUV-treated cells.Magnetic resonance imaging scans of tumor-bearing mice were acquired before and after (t = 0, 4 hours and 24 hours) the administration of RGD GUVs and ctrl-GUVs. A roughly 16% MTC difference between the 2 groups was observed after 4 hours. Immunofluorescence analyses and ICP-MS analyses (for Gd-detection) of the explanted tumors confirmed the specific accumulation of RGD GUVs in the tumor region.

CONCLUSIONS

RGD GUVs seem to be interesting carriers that can facilitate the specific accumulation of MRI contrast agents at the tumor region. However, the concentration achieved is still below the threshold needed for T1w-MRI visualization. Conversely, MTC proved to be sufficiently sensitive for the visualization of detectable contrast between pretargeting and posttargeting images.

Polymeric micelles for anticancer drug delivery

Polymeric micelles have gained interest as novel drug delivery systems for the treatment and diagnosis of cancer, as they offer several advantages over conventional drug therapies. This includes drug targeting to tumor tissue, in vivo biocompatibility and biodegradability, prolonged circulation time, enhanced accumulation, retention of the drug loaded micelle in the tumor and decreased side effects. This article provides an overview on the current state of micellar formulations as nanocarriers for anticancer drugs and their effectiveness in cancer therapeutics, including their clinical status. The type of copolymers used, their physicochemical properties and characterization as well as recent developments in the design of functional polymeric micelles are highlighted. The article also presents the design and outcomes of various types of stimuli-responsive polymeric micelles.

Polymeric micelles for anticancer drug delivery

Polymeric micelles have gained interest as novel drug delivery systems for the treatment and diagnosis of cancer, as they offer several advantages over conventional drug therapies. This includes drug targeting to tumor tissue, in vivo biocompatibility and biodegradability, prolonged circulation time, enhanced accumulation, retention of the drug loaded micelle in the tumor and decreased side effects. This article provides an overview on the current state of micellar formulations as nanocarriers for anticancer drugs and their effectiveness in cancer therapeutics, including their clinical status. The type of copolymers used, their physicochemical properties and characterization as well as recent developments in the design of functional polymeric micelles are highlighted. The article also presents the design and outcomes of various types of stimuli-responsive polymeric micelles.

Supramolecular assemblies based on amphiphilic Mn2+-complexes as high relaxivity MRI probes

The Mn²⁺ complexes of amphiphilic derivatives of EDTA and 1,4-DO2A ligands show a strong increase in relaxivity upon micellar aggregation and human serum albumin binding.

Theranostic Nanoparticles Loaded with Imaging Probes and Rubrocurcumin for Combined Cancer Therapy by Folate Receptor Targeting

The combination of different therapeutic modalities is a promising option to combat the recurrence of tumors. In this study, polylactic and polyglycolic acid nanoparticles were used for the simultaneous delivery of a boron-curcumin complex (RbCur) and an amphiphilic gadolinium complex into tumor cells with the aim of performing boron and gadolinium neutron capture therapy (NCT) in conjunction with the additional antiproliferative effects of curcumin. Furthermore, the use of Gd complexes allows magnetic resonance imaging (MRI) assessment of the amount of B and Gd internalized by tumor cells. Poly(lactic-co-glycolic acid) (PLGA) nanoparticles were targeted to ovarian cancer (IGROV-1) cells through folate receptors, by including in the formulation a PEGylated phospholipid functionalized with the folate moiety. NCT was performed on IGROV-1 cells internalizing 6.4 and 78.6 μg g^-1 of ¹⁰ B and ¹⁵⁷ Gd, respectively. The synergic action of neutron treatment and curcumin cytotoxicity was shown to result in a significant therapeutic improvement.

Polymeric mixed micelles as nanomedicines: Achievements and perspectives

During the past few decades, polymeric micelles have raised special attention as novel nano-sized drug delivery systems for optimizing the treatment and diagnosis of numerous diseases. These nanocarriers exhibit several in vitro and in vivo advantages as well as increased stability and solubility to hydrophobic drugs. An interesting approach for optimizing these properties and overcoming some of their disadvantages is the combination of two or more polymers in order to assemble polymeric mixed micelles. This review article gives an overview on the current state of the art of several mixed micellar formulations as nanocarriers for drugs and imaging probes, evaluating their ongoing status (preclinical or clinical stage), with special emphasis on type of copolymers, physicochemical properties, in vivo progress achieved so far and toxicity profiles. Besides, the present article presents relevant research outcomes about polymeric mixed micelles as better drug delivery systems, when compared to polymeric pristine micelles. The reported data clearly illustrates the promise of these nanovehicles reaching clinical stages in the near future.

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

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

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.

Gadolinium(III) Complexes with N-Alkyl-N-methylglucamine Surfactants Incorporated into Liposomes as Potential MRI Contrast Agents

Complexes of gadolinium(III) with N-octanoyl-N-methylglucamine (L8) and N-decanoyl-N-methylglucamine (L10) with 1 : 2 stoichiometry were synthesized and characterized by elemental analysis, electrospray ionization-tandem mass spectrometry (ESI-MS), infrared (IR) spectroscopy, and molar conductivity measurements. The transverse (r 2) and longitudinal (r 1) relaxivity protons were measured at 20 MHz and compared with those of the commercial contrasts. These complexes were incorporated in liposomes, resulting in the increase of the vesicle zeta potential. Both the free and liposome-incorporated gadolinium complexes showed high relaxation effectiveness, compared to commercial contrast agent gadopentetate dimeglumine (Magnevist). The high relaxivity of these complexes was attributed to the molecular rotation that occurs more slowly, because of the elevated molecular weight and incorporation in liposomes. The results establish that these paramagnetic complexes are highly potent contrast agents, making them excellent candidates for various applications in molecular MR imaging.

A theranostic approach based on the use of a dual boron/Gd agent to improve the efficacy of Boron Neutron Capture Therapy in the lung cancer treatment

This study aims at developing an innovative theranostic approach for lung tumor and metastases treatment, based on Boron Neutron Capture Therapy (BNCT). It relies on to the use of low density lipoproteins (LDL) as carriers able to maximize the selective uptake of boron atoms in tumor cells and, at the same time, to quantify the in vivo boron distribution by magnetic resonance imaging (MRI). Tumor cells uptake was initially assessed by ICP-MS and MRI on four types of tumor (TUBO, B16-F10, MCF-7, A549) and one healthy (N-MUG) cell lines. Lung metastases were generated by intravenous injection of a Her2+ breast cancer cell line (i.e. TUBO) in BALB/c mice and transgenic EML4-ALK mice were used as primary tumor model. After neutron irradiation, tumor growth was followed for 30-40 days by MRI. Tumor masses of boron treated mice increased markedly slowly than the control group. From the clinical editor: In this article, the authors described an improvement to existing boron neutron capture therapy. The dual MRI/BNCT agent, carried by LDLs, was able to maximize the selective uptake of boron in tumor cells, and, at the same time, quantify boron distribution in tumor and in other tissues using MRI. Subsequent in vitro and in vivo experiments showed tumor cell killing after neutron irradiation.

GdDOTAGA(C18)2: an efficient amphiphilic Gd(iii) chelate for the preparation of self-assembled high relaxivity MRI nanoprobes

MRI-nanoprobes endowed with improved relaxivity, incorporation stability and in vivo MRI efficiency were prepared using a newly synthesized amphiphilic GdDOTA-like complex.

MR imaging techniques for nano-pathophysiology and theranostics

The advent of nanoparticle DDSs (drug delivery systems, nano-DDSs) is opening new pathways to understanding physiology and pathophysiology at the nanometer scale. A nano-DDS can be used to deliver higher local concentrations of drugs to a target region and magnify therapeutic effects. However, interstitial cells or fibrosis in intractable tumors, as occurs in pancreatic or scirrhous stomach cancer, tend to impede nanoparticle delivery. Thus, it is critical to optimize the type and size of nanoparticles to reach the target. High-resolution 3D imaging provides a means of "seeing" the nanoparticle distribution and therapeutic effects. We introduce the concept of "nano-pathophysiological imaging" as a strategy for theranostics. The strategy consists of selecting an appropriate nano-DDS and rapidly evaluating drug effects in vivo to guide the next round of therapy. In this article we classify nano-DDSs by component carrier materials and present an overview of the significance of nano-pathophysiological MRI.

MRI tracking of macrophages labeled with glucan particles entrapping a water insoluble paramagnetic Gd-based agent

PURPOSE

This study is aimed at demonstrating the in vivo potential of Gd(III)-loaded glucan particles (Gd-GPs) as magnetic resonance imaging (MRI)-positive agents for labeling and tracking phagocytic cells.

PROCEDURE

GPs were obtained from Saccharomyces cerevisae and loaded with the water-insoluble complex Gd-DOTAMA(C18)2. The uptake kinetics of Gd-GPs by murine macrophages was studied in vitro and the internalization mechanism was assessed by competition assays. The in vivo performance of Gd-GPs was tested at 7.05 T on a mouse model of acute liver inflammation.

RESULTS

The minimum number of Gd-GPs-labeled J774.A1 macrophages detected in vitro by MRI was ca. 300 cells/μl of agar, which is the lowest number ever reported for cells labeled with a positive T1 agent. Intravenous injection of macrophages labeled with Gd-GPs in a mouse model of liver inflammation enabled the MRI visualization of the cellular infiltration in the diseased area.

CONCLUSIONS

Gd-GPs represent a promising platform for tracking macrophages by MRI as a T1 alternative to the golden standard T2-based iron oxide particles.

Supramolecular nanodevices: from design validation to theranostic nanomedicine

The increasing importance of nanotechnology in the biomedical field and the recent progress of nanomedicines into clinical testing have spurred the development of even more sophisticated nanoscale drug carriers. Current nanocarriers can successfully target cells, release their cargo in response to stimuli, and selectively deliver drugs. More sophisticated nanoscale carriers should evolve into fully integrated vehicles with more complex capabilities. First, they should be able to sense targets inside the body and adapt their functions based on these targets. Such devices will also have processing capabilities, modulating their properties and functions in response to internal or external stimuli. Finally, they will direct their function to the aimed site through both subcellular targeting and delivery of loaded drugs. These nanoscale, multifunctional drug carriers are defined here as nanodevices. Through the integration of various imaging elements into their design, the nanodevices can be made visible, which is an essential feature for the validation. The visualization of nanodevices also facilitates their use in the clinic: clinicians can observe the effectiveness of the devices and gain insights into both the disease progression and the therapeutic response. Nanodevices with this dual diagnostic and therapeutic function are called theranostic nanodevices. In this Account, we describe various challenges to be overcome in the development of smart nanodevices based on supramolecular assemblies of engineered block copolymers. In particular, we focus on polymeric micelles. Polymeric micelles have recently received considerable attention as a promising vehicle for drug delivery, and researchers are currently investigating several micellar formulations in preclinical and clinical studies. By engineering the constituent block copolymers to produce polymeric micelles that integrate multiple smart functionalities, we and other researchers are developing nanodevices with favorable clinical properties.

MRI evaluation of the antitumor activity of paramagnetic liposomes loaded with prednisolone phosphate

The design of long circulating liposomes co-loaded with the glucocorticoid prednisolone phosphate (PLP) and the amphiphilic paramagnetic contrast agent Gd-DOTAMA(C(18))(2) allowed the MRI-guided in vivo visualization of the delivery and biodistribution of PLP, as well as the monitoring of drug efficacy. The performance of this theranostic probe was investigated in a mouse model bearing a melanoma B16 syngeneic tumor. The release kinetics of the drug were evaluated in vitro where it displayed a peculiar behavior characterized by a fast process (completed in few hours) involving only a small portion (<5%) of the drug. Interestingly, the incorporation of the amphiphilic imaging reporter in the liposomal bilayer slightly increased the amount of the fast-release portion (<10%), thus suggesting that it could be attributed to a drug fraction embedded in the liposomal bilayer. In fact, the release of a hydrophilic imaging probe encapsulated in the inner core of the same long circulating liposomes formulated for carrying the drug, displayed different, single-step, kinetics. The in vivo monitoring of the antitumor activity of the nanomedicine revealed that the incorporation of the MRI probe into the liposome bilayer did not significantly affect the drug efficacy. The in vivo experiments also indicated a relevant and fast liposome uptake from macrophage-rich organs like spleen and liver, which reduced the tumor accumulation of the liposomes. The accumulation of the amphipatic MRI label caused the occurrence of a long-term residual T(1) contrast still detectable 1week after injection.

Strategies for the preparation of bifunctional gadolinium(III) chelators

The development of gadolinium chelators that can be easily and readily linked to various substrates is of primary importance for the development high relaxation efficiency and/or targeted magnetic resonance imaging (MRI) contrast agents. Over the last 25 years a large number of bifunctional chelators have been prepared. For the most part, these compounds are based on ligands that are already used in clinically approved contrast agents. More recently, new bifunctional chelators have been reported based on complexes that show a more potent relaxation effect, faster complexation kinetics and in some cases simpler synthetic procedures. This review provides an overview of the synthetic strategies used for the preparation of bifunctional chelators for MRI applications.

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.

Target-selective drug delivery through liposomes labeled with oligobranched neurotensin peptides

The structure and the in vitro behavior of liposomes filled with the cytotoxic drug doxorubicin (Doxo) and functionalized on the external surface with a branched moiety containing four copies of the 8-13 neurotensin (NT) peptide is reported. The new functionalized liposomes, DOPC-NT₄Lys(C₁₈)₂, are obtained by co-aggregation of the DOPC phospholipid with a new synthetic amphiphilic molecule, NT₄ Lys(C₁₈)₂, which contains a lysine scaffold derivatized with a lipophilic moiety and a tetrabranched hydrophilic peptide, NT8-13, a neurotensin peptide fragment well known for its ability to mimic the neurotensin peptide in receptor binding ability. Dynamic light scattering measurements indicate a value for the hydrodynamic radius (RH) of 88.3±4.4 nm. The selective internalization and cytotoxicity of DOPC-NT₄ Lys(C₁₈)₂ liposomes containing Doxo, as compared to pure DOPC liposomes, were tested in HT29 human colon adenocarcinoma and TE671 human rhabdomyosarcoma cells, both of which express neurotensin receptors. Peptide-functionalized liposomes show a clear advantage in comparison to pure DOPC liposomes with regard to drug internalization in both HT29 and TE671 tumor cells: FACS analysis indicates an increase in fluorescence signal of the NT₄-liposomes, compared to the DOPC pure analogues, in both cell lines; cytotoxicity of DOPC-NT₄ Lys(C₁₈)₂-Doxo liposomes is increased four-fold with respect to DOPC-Doxo liposomes in both HT29 and TE671 cell lines. These effects could to be ascribed to the higher rate of internalization for DOPC-NT₄ Lys(C₁₈)₂-Doxo liposomes, due to stronger binding driven by a lower dissociation constant of the NT₄-liposomes that bind the membrane onto a specific protein, in contrast to DOPC liposomes, which approach the plasma membrane unselectively.

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.

Paramagnetic and fluorescent liposomes for target-specific imaging and therapy of tumor angiogenesis

Angiogenesis is essential for tumor growth and metastatic potential and for that reason considered an important target for tumor treatment. Noninvasive imaging technologies, capable of visualizing tumor angiogenesis and evaluating the efficacy of angiostatic therapies, are therefore becoming increasingly important. Among the various imaging modalities, magnetic resonance imaging (MRI) is characterized by a superb spatial resolution and anatomical soft-tissue contrast. Revolutionary advances in contrast agent chemistry have delivered versatile angiogenesis-specific molecular MRI contrast agents. In this paper, we review recent advances in the preclinical application of paramagnetic and fluorescent liposomes for noninvasive visualization of the molecular processes involved in tumor angiogenesis. This liposomal contrast agent platform can be prepared with a high payload of contrast generating material, thereby facilitating its detection, and is equipped with one or more types of targeting ligands for binding to specific molecules expressed at the angiogenic site. Multimodal liposomes endowed with contrast material for complementary imaging technologies, e.g., MRI and optical, can be exploited to gain important preclinical insights into the mechanisms of binding and accumulation at angiogenic vascular endothelium and to corroborate the in vivo findings. Interestingly, liposomes can be designed to contain angiostatic therapeutics, allowing for image-supervised drug delivery and subsequent monitoring of therapeutic efficacy.

Biocompatible Polyhydroxyethylaspartamide-based Micelles with Gadolinium for MRI Contrast Agents

Biocompatible poly-[N-(2-hydroxyethyl)-d,l-aspartamide]-methoxypoly(ethyleneglycol)-hexadecylamine (PHEA-mPEG-C(16)) conjugated with 1,4,7,10-tetraazacyclododecan-1,4,7,10-tetraacetic acid-gadolinium (DOTA-Gd) via ethylenediamine (ED) was synthesized as a magnetic resonance imaging (MRI) contrast agent. Amphiphilic PHEA-mPEG-C(16)-ED-DOTA-Gd forms micelle in aqueous solution. All the synthesized materials were characterized by proton nuclear magnetic resonance ((1)H NMR). Micelle size and shape were examined by dynamic light scattering (DLS) and atomic force microscopy (AFM). Micelles with PHEA-mPEG-C(16)-ED-DOTA-Gd showed higher relaxivities than the commercially available gadolinium contrast agent. Moreover, the signal intensity of a rabbit liver was effectively increased after intravenous injection of PHEA-mPEG-C(16)-ED-DOTA-Gd.

Combined delivery and magnetic resonance imaging of neural cell adhesion molecule-targeted doxorubicin-containing liposomes in experimentally induced Kaposi's sarcoma

Specific targeting of tumors by combined delivery of drugs and of imaging agents represents an attractive strategy for treatment of cancer. The aim of the present study was to investigate whether neural cell adhesion molecule (NCAM)-targeted liposomes may enhance drug delivery and allow magnetic resonance imaging (MRI) in a severe combined immunodeficient mouse model of NCAM-positive Kaposi's sarcoma. NCAM-binding peptide-coated liposomes loaded with both doxorubicin and a lipophilic gadolinium (Gd) derivative were generated. NCAM-targeted liposomes induced an enhanced in vitro doxorubicin internalization within Kaposi's cells as detected by MRI with respect to untargeted polyethylene glycol liposomes. Internalization resulted in enhanced apoptosis. In vivo weekly administration of NCAM-targeted liposomes containing 5 mg/kg doxorubicin for 4 consecutive weeks induced a significant reduction of tumor mass and vascularization and enhanced cell necrosis and apoptosis with respect to untargeted liposomes. These effects were associated with an enhanced concentration of doxorubicin within the tumor and a reduced systemic toxicity of doxorubicin. By electron microscopy, NCAM-targeted liposomes were detected mainly within tumor cells whereas the untargeted liposomes were mainly accumulated in the extracellular space. Gd-labeled liposomes allowed the MRI visualization of drug delivery in the tumor region. The intensity of MRI signal was partially hampered by the "quenching" of the attainable relaxation enhancement on endosomal entrapment of the Gd-labeled liposomes. In conclusion, targeting NCAM may be a suitable strategy for specific drug delivery and imaging by liposomes in NCAM-expressing tumors. Moreover, treatment with NCAM-targeted liposomes showed enhanced therapeutic effect and reduced toxicity with respect to untargeted liposomes.

Chapter 10 - Lanthanide-loaded paramagnetic liposomes as switchable magnetically oriented nanovesicles

Magnetically oriented liposomes can be prepared by exposing unilamellar spherical systems loaded with paramagnetic lanthanide(III) complexes to hyperosmotic stress. The resulting nonspherical, lens-shaped, nanoparticles can orient within a static magnetic field, depending on the magnetic properties of their membrane bilayer. The orientation of the vesicles can be easily determined by measuring the paramagnetic contribution to the (1)H chemical shift of the intraliposomal water proton resonance. As the latter shift is dominated by the bulk magnetic susceptibility contribution, its sign (negative or positive) reports about the preferred orientation adopted by the nanovesicles. The alignment within the field depends upon the magnetic susceptibility anisotropy of the liposome membrane, Delta(chi)(LIPO). When Delta(chi)(LIPO) has a negative value (e.g., for nonspherical liposomes made of conventional phospholipids), the nanoparticles align with their long axis parallel to the field, whereas when Delta(chi)(LIPO)>0 the vesicles flip by 90 degrees . The sign of the chemical shift of the intraliposomal water resonance is positive in the former case and negative in the latter, respectively. The liposome orientation can be switched by incorporating in the liposome bilayer suitable amphiphilic paramagnetic lanthanide(III) complexes. The sign of Delta(chi)(LIPO), and consequently the magnetic alignment, will correspond to the sign of the magnetic susceptibility anisotropy of the metal complex. The magnetic susceptibility anisotropy is dependent on both the electronic configuration of the lanthanide ion and the structural characteristics of the amphiphilic complex incorporated in the liposome membrane. The magnetic orientation of such vesicles is maintained in vivo, thus opening promising perspectives for the application of nonspherical liposomes in medical imaging.

Evidence for in vivo macrophage mediated tumor uptake of paramagnetic/fluorescent liposomes

Dysprosium (Dy)-loaded liposomes act as excellent T(2)-susceptibility agents at high magnetic field strength. The R(2)-enhancement increases with the size of the liposomes and the concentration of entrapped paramagnetic metal complexes. Neuro-2a tumor cells are readily labeled when Dy-loaded liposomes, suitably functionalized with glutamine residues (Gln), are added to the culture medium as glutamine receptors are highly expressed in such proliferating tumor cells. By using fluorescent liposomes doped with fluorescent dyes (either incorporated in the membrane or included in the inner cavity), confocal microscopy experiments showed that targeted liposomes are taken up much more avidly than non-targeted vesicles. In vivo studies showed that glutamine-functionalized and non-functionalized liposomes accumulate in the tumor region to a similar extent. Confocal images of the excised tumor showed extensive co-localization of liposomes and macrophages in both cases. It is suggested that the loss of tumor specificity, shown by Gln-functionalized liposomes in vivo, has to be associated with the efficient removal of liposomes operated by the RES (reticulo endoplasmatic system) or tumor associated macrophages.

High-relaxivity gadolinium-modified high-density lipoproteins as magnetic resonance imaging contrast agents

There is an ongoing desire to produce high-relaxivity, Gd-based magnetic resonance imaging (MRI) contrast agents. These may allow for lower doses to be used, which is especially important in view of the current safety concerns surrounding Gd in patients. Here we report the synthesis of a high-relaxivity MRI contrast agent, by incorporating Gd-chelating lipids that coordinate two water molecules into high-density lipoprotein (q = 2 HDL). We compared the properties of q = 2 HDL with those of an analogous HDL particle labeled with Gd-chelating lipids that coordinate only one water molecule (q = 1 HDL). We found that the q = 2 HDL possessed an elevated r(1) of 41 mM(-1) s(-1) compared to 9 mM(-1) s(-1) for q = 1 HDL at 20 MHz, but the q = 2 HDL exhibited high R(2)* values at high fields, precluding imaging above 128 MHz. While carrying out this investigation we observed that enlarged, disrupted particles were formed when the synthesis was carried out above the lipid critical micelle concentration (cmc), indicating the importance of synthesis below the cmc when modifying lipoproteins in this manner. The high relaxivity of q = 2 HDL means it will be an efficacious contrast agent for future MR imaging studies.

Osmotically shrunken LIPOCEST agents: an innovative class of magnetic resonance imaging contrast media based on chemical exchange saturation transfer

The peculiar properties of osmotically shrunken liposomes acting as magnetic resonance imaging-chemical exchange saturation transfer (MRI-CEST) contrast agents have been investigated. Attention has been primarily devoted to assessing the contribution arising from encapsulated and incorporated paramagnetic lanthanide(III)-based shift reagents in determining the chemical shift of the intraliposomal water protons, which is a relevant factor for generating the CEST contrast. It is demonstrated that a highly shifted resonance for the encapsulated water can be attained by increasing the percentage of the amphiphilic shift reagent incorporated in the liposome bilayer. It is also demonstrated that the shift contribution arising from the bulk magnetic susceptibility can be optimized through the modulation of the osmotic shrinkage. In terms of sensitivity, it is shown that the saturation transfer efficiency can be significantly improved by increasing the size of the vesicle, thus allowing a high number of exchangeable protons to be saturated. In addition, the role played by the intensity of the saturating radiofrequency field has also been highlighted.

Structural and relaxometric characterization of peptide aggregates containing gadolinium complexes as potential selective contrast agents in MRI

The structural and relaxometric characterization of a novel class of supramolecular aggregates, as potential tumor-specific contrast agents in magnetic resonance imaging (MRI), is reported. The aggregates are based on a new monomer with an upsilon shape (MonY) that contains, in the same molecule, all three fundamental tasks that are required: 1) a hydrophobic moiety that allows the formation of supramolecular aggregates; 2) the bioactive CCK8 peptide for target recognition; and 3) a chelating agent able to give stable gadolinium complexes. As indicated by dynamic light scattering and small-angle neutron scattering (SANS) measurements, MonY and its gadolinium complex MonY(Gd) aggregate in aqueous solution to give ellipsoidal micelles with a ratio between the micellar axes of approximately 1.7 and an aggregation number N(agg) of approximately 30. There are no differences in the aggregation behavior of MonY and MonY(Gd), which indicates that the presence of metal ions, and therefore the reduction of the net charge, does not influence the aggregation behavior. When MonY or MonY(Gd) are blended with dioleoyl phosphatidylcholine (DOPC), the aggregation behavior is dictated by the tendency of DOPC to give liposomes. Only when the amount of MonY or MonY(Gd) is higher than 20 % is the coexistence of liposomes and micelles observed. The thickness d of the bilayer is estimated by SANS to be approximately 35-40 A, whereas cryogenic transmission electron microscopy images show that the diameter of the liposomes ranges from approximately 50 to 150 nm. Self-assembling micelles of MonY(Gd) present high relaxivity values (r(1p)=15.03 mM(-1) s(-1)) for each gadolinium complex in the aggregate. Liposomes containing MonY(Gd) inserted in the DOPC bilayer at a molar ratio of 20:80 present slightly lower relaxivity values (r(1p)=12.7 mM(-1) s(-1)), independently of their internal or external position in the liposome.

Contrast agents: magnetic resonance

Even though the intrinsic magnetic resonance imaging (MRI) contrast is much more flexible than in other clinical imaging techniques, the diagnosis of several pathologies requires the involvement of contrast agents (CAs) that can enhance the difference between normal and diseased tissues by modifying their intrinsic parameters. MR CAs are indirect agents because they do not become visible by themselves as opposed to other imaging modalities. The signal enhancement produced by MRI CAs (i.e., the efficiency of the CAs) depends on their longitudinal (r1) and transverse (r2) relaxivity (expressed in s(-1) mmol(-1) 1), which is defined as the increase of the nuclear relaxation rate (the reciprocal of the relaxation time) of water protons produced by 1 mmol per liter of CA. Paramagnetic CAs (most of them complexes of gadolinium) are frequently used in clinics as extracellular, hepatobiliary or blood pool agents. Low molecular weight paramagnetic CAs have similar effects on R1 and R2, but the predominant effect at low doses is that of T1 shortening (and R1 enhancement). Thus, organs taking up such agents will become bright in a T1-weighted MRI sequence; these CAs are thus called positive contrast media. The CAs known as negative agents influence signal intensity mainly by shortening T2* and T2, which produces the darkening of the contrast-enhanced tissue. These CAs are generally composed of superparamagnetic nanoparticles, consisting of iron oxides (magnetite, Fe3O4, maghemite, gammaFe2O3, or other ferrites). Iron oxide nanoparticles are taken up by the monocyte-macrophage system, which explains their potential application as MRI markers of inflammatory and degenerative disorders. Most of the contemporary MRI CAs approved for clinical applications are non-specific for a particular pathology and report exclusively on the anatomy and the physiological status of various organs. A new generation of MRI CAs is progressively emerging in the current context of molecular imaging, agents that are designed to detect with a high specificity the cellular and molecular hallmarks of various pathologies.

Peptide derivatized lamellar aggregates as target-specific MRI contrast agents

The relaxivity behaviour and the structural characterization of new supramolecular aggregates (bilayer structures and micelles) obtained by combining two different amphiphilic monomers are reported. One monomer, (C18)(2)DTPAGlu-Gd, contains a very stable gadolinium complex, and the other, DSPE-PEG(2000)-CCK8, contains the bioactive CCK8 peptide. Samples that contained only DSPE-PEG(2000)-CCK8, or up to 50 % (C18)(2)DTPAGlu-Gd, aggregated as double-layer structures (lamellar aggregates) with a thickness of approximately 80-100 A, as evaluated by SANS measurement and Cryo-TEM imaging. A transition to micelle formation was observed when the amount of (C18)(2)DTPAGlu-Gd in the aggregate was increased. These were rod-like micelles approximately 40 A in radius and >200 A in length. The proton relaxivities for both lamellar aggregates and rod-like micelles were the same (17.2 mM(-1) s(-1)), although the values were the results of different combinations of local and global contributions. The in vitro target selectivity of aggregates that contained the CCK-8 peptide was demonstrated by using nuclear medicine techniques.

Gadolinium mixed-micelles: effect of the amphiphile on in vitro and in vivo efficacy in apolipoprotein E knockout mouse models of atherosclerosis

Gadolinium (Gd) micelles are nanoparticles that incorporate phospholipids, surfactants, and lipophilic Gd complexes. Preliminary studies have shown that lipid-based nanoparticles may penetrate atherosclerotic plaque. The aim of the current study was to prepare, characterize, and evaluate in vivo the efficacy of two Gd micelle formulations using apolipoprotein E knockout (ApoE(-/-)) mouse models of atherosclerosis. Gd micelles were prepared using two different amphiphiles but similar GdDTPA lipids, surfactants, and fluorescent labels. The results indicate that the choice of amphiphile may affect the particle size, relaxivity, and blood clearance in wild-type mice (WT). However, the in vivo MR efficacy, with respect to uptake in the vessel wall of ApoE(-/-) mice, was not affected by the amphiphile used. Significant wall enhancement of ApoE(-/-) mice was observed following administration of 0.015 and 0.038 mmol Gd/kg of both micelle formulations. No significant enhancement of the vessel wall of WT mice was observed for any of the dosages or formulations tested. Additionally, liver uptake 24 hr post-injection (p.i.) was not influenced by the choice of amphiphile. The results of this study strongly suggest that liver uptake and wall enhancement may be regulated by the surface properties of the micelle and not by other factors, such as micelle size.

Magnetic resonance imaging of vulnerable atherosclerotic plaques: Current imaging strategies and molecular imaging probes

The vulnerability or destabilization of atherosclerotic plaques has been directly linked to plaque composition. Imaging modalities, such as magnetic resonance (MR) imaging, that allow for evaluation of plaque composition at a cellular and molecular level, could further improve the detection of vulnerable plaque and may allow for monitoring the efficacy of antiatherosclerotic therapies. In this review we focus on MR imaging strategies for the detection and evaluation of atherosclerotic plaques and their composition. We highlight recent advancements in the development of MR pulse sequences, computer image analysis, and the use of commercially available MR contrast agents, such as gadopentic acid (Gd-DTPA), for plaque characterization. We also discuss molecular imaging strategies that are currently being used to design specific imaging probes targeted to biochemical and cellular markers of atherosclerotic plaque vulnerability.