Synthesis and Characterization of a Doxorubicin−Gd(III) Contrast Agent Conjugate: A New Approach toward Prodrug−Procontrast Complexes

Cyclic Peptide-Gadolinium Nanocomplexes as siRNA Delivery Tools

We have recently reported that a cyclic peptide containing five tryptophan, five arginine, and one cysteine amino acids [(WR)₅C], was able to produce peptide-capped gadolinium nanoparticles, [(WR)₅C]-GdNPs, in the range of 240 to 260 nm upon mixing with an aqueous solution of GdCl₃. Herein, we report [(WR)₅C]-GdNPs as an efficient siRNA delivery system. The peptide-based gadolinium nanoparticles (50 µM) did not exhibit significant cytotoxicity (~93% cell viability at 50 µM) in human leukemia T lymphoblast cells (CCRF-CEM) and triple-negative breast cancer cells (MDA-MB-231) after 48 h. Fluorescence-activated cell sorting (FACS) analysis indicated that the cellular uptakes of Alexa-488-labeled siRNA were found to be enhanced by more than 10 folds in the presence of [(WR)₅C]-GdNPs compared with siRNA alone in CCRF-CEM and MDA-MB-231 cells after 6 h of incubation at 37 °C. The gene silencing efficacy of the nanoparticles was determined via the western blot technique using an over-expressed gene, STAT-3 protein, in MDA-MB-231 cells. The results showed ~62% reduction of STAT-3 was observed in MDA-MB-231 with [(WR)₅C]-GdNPs at N/P 40. The integrity of the cellular membrane of CCRF-CEM cells was found to be intact when incubated with [(WR)₅C]-Gd nanoparticles (50 µM) for 2 h. Confocal microscopy reveals higher internalization of siRNA in MDA-MB-231 cells using [(WR)₅C]-GdNPs at N/P 40. These results provided insight about the use of the [(WR)₅C]-GdNPs complex as a potent intracellular siRNA transporter that could be a nontoxic choice to be used as a transfection agent for nucleic-acid-based therapeutics.

Recent Advancements in Stimuli Responsive Drug Delivery Platforms for Active and Passive Cancer Targeting

The tumor-specific targeting of chemotherapeutic agents for specific necrosis of cancer cells without affecting the normal cells poses a great challenge for researchers and scientists. Though extensive research has been carried out to investigate chemotherapy-based targeted drug delivery, the identification of the most promising strategy capable of bypassing non-specific cytotoxicity is still a major concern. Recent advancements in the arena of onco-targeted therapies have enabled safe and effective tumor-specific localization through stimuli-responsive drug delivery systems. Owing to their promising characteristic features, stimuli-responsive drug delivery platforms have revolutionized the chemotherapy-based treatments with added benefits of enhanced bioavailability and selective cytotoxicity of cancer cells compared to the conventional modalities. The insensitivity of stimuli-responsive drug delivery platforms when exposed to normal cells prevents the release of cytotoxic drugs into the normal cells and therefore alleviates the off-target events associated with chemotherapy. Contrastingly, they showed amplified sensitivity and triggered release of chemotherapeutic payload when internalized into the tumor microenvironment causing maximum cytotoxic responses and the induction of cancer cell necrosis. This review focuses on the physical stimuli-responsive drug delivery systems and chemical stimuli-responsive drug delivery systems for triggered cancer chemotherapy through active and/or passive targeting. Moreover, the review also provided a brief insight into the molecular dynamic simulations associated with stimuli-based tumor targeting.

Cyclic Peptide-Gadolinium Nanoparticles for Enhanced Intracellular Delivery

A cyclic peptide containing one cysteine and five alternating tryptophan and arginine amino acids [(WR)₅C] was synthesized using Fmoc/tBu solid-phase methodology. The ability of the synthesized cyclic peptide to produce gadolinium nanoparticles through an in situ one-pot mixing of an aqueous solution of GdCl₃ with [(WR)₅C] peptide solution was evaluated. Transmission electron microscopy showed the formed peptide-Gd nanoparticles in star-shape morphology with a size of ~250 nm. Flow cytometry investigation showed that the cellular uptake of a cell-impermeable fluorescence-labeled phosphopeptide (F′-GpYEEI, where F′ = fluorescein) was approximately six times higher in the presence of [(WR)₅C]-Gd nanoparticles than those of F′-GpYEEI alone in human leukemia adenocarcinoma (CCRF-CEM) cells after 2 h incubation. The antiproliferative activities of cisplatin and carboplatin (5 µM) were increased in the presence of [(WR)₅C]-GdNPs (50 μM) by 41% and 18%, respectively, after 72-h incubation in CCRF-CEM cells. The intracellular release of epirubicin, an anticancer drug, from the complex showed that 15% and 60% of the drug was released intracellularly within 12 and 48 h, respectively. This report provides insight about using a non-toxic MRI agent, gadolinium nanoparticles, for the delivery of various types of molecular cargos.

Z-Group ketone chain transfer agents for RAFT polymer nanoparticle modification via hydrazone conjugation

A ketal-containing trithiocarbonyl compound has been synthesized and characterized as a chain transfer agent (CTA) in Reversible Addition Fragmentation Transfer (RAFT) polymerization. The ketal functionality does not interfere with RAFT polymerization of acrylate monomers, which proceeds as previously reported to yield macro-CTA polymers and block co-polymers. Post-polymerization ketal cleavage revealed ketone functionality at the polar terminus of an amphiphilic block co-polymer. Hydrazone-formation was facile in both organic solution as well as in aqueous buffer where polymer nanoparticle assemblies were formed, indicating a conjugation/end-functionalization yield of 40-50%. Conjugation was verified with fluorescein, biotin and Gd-DOTA derivatives, and though the trithiocarbonate linkage is hydrolytically labile, we observed stable conjugation for several days at pH 7.4. and 37°C. As expected, streptavidin binding to biotinylated polymer micelles was observed, and size-change based relaxivity increases were observed when Gd-DOTA hydrazide was conjugated to polymer micelles. Cell-uptake of fluorescently labeled polymer micelles was also readily tracked by FACS and fluorescence microscopy. These polymer derivatives demonstrate a range of potential theranostic/biotechnological applications for this conveniently accessible keto-CTA, which include ligand-based nanoparticle targeting and fluorescent/MR nanoparticle contrast agents.

Gadolinium-encapsulating iron oxide nanoprobe as activatable NMR/MRI contrast agent

Herein we report a novel gadolinium-encapsulating iron oxide nanoparticle-based activatable NMR/MRI nanoprobe. In our design, Gd-DTPA is encapsulated within the poly(acrylic acid) (PAA) polymer coating of a superparamagnetic iron oxide nanoparticle (IO-PAA), yielding a composite magnetic nanoprobe (IO-PAA-Gd-DTPA) with quenched longitudinal spin-lattice magnetic relaxation (T(1)). Upon release of the Gd-DTPA complex from the nanoprobe's polymeric coating in acidic media, an increase in the T(1) relaxation rate (1/T(1)) of the composite magnetic nanoprobe was observed, indicating a dequenching of the nanoprobe with a corresponding increase in the T(1)-weighted MRI signal. When a folate-conjugated nanoprobe was incubated in HeLa cells, a cancer cell line overexpressing folate receptors, an increase in the 1/T(1) signal was observed. This result suggests that, upon receptor-mediated internalization, the composite magnetic nanoprobe degraded within the cell's lysosome acidic (pH 5.0) environment, resulting in an intracellular release of Gd-DTPA complex with subsequent T(1) activation. In addition, when an anticancer drug (Taxol) was coencapsulated with the Gd-DTPA within the folate receptor targeting composite magnetic nanoprobe, the T(1) activation of the probe coincided with the rate of drug release and corresponding cytotoxic effect in cell culture studies. Taken together, these results suggest that our activatable T(1) nanoagent could be of great importance for the detection of acidic tumors and assessment of drug targeting and release by MRI.

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.

Superparamagnetic colloidal nanocrystal clusters coated with polyethylene glycol fumarate: a possible novel theranostic agent

We report cell endocytosis, drug release, NMR relaxometry and in vitro MRI studies on a novel class of superparamagnetic colloidal nanocrystal clusters (CNCs) with various biocompatible coatings. It is shown that the transverse relaxivity r2, the parameter representing the MRI efficiency in negative contrast agents, for the PVA-coated, PEGF-coated, and crosslinked PEGF-coated CNCs, is high enough to contrast suitably the magnetic resonance images. The same samples have shown a good ability also in drug releasing (particularly the crosslinked PEGF-coated compound), thus finally allowing us to propose this class of compounds for future applications in theranostics.

Bioresponsive, cell-penetrating, and multimeric MR contrast agents

Magnetic resonance imaging (MRI) has become increasingly popular in experimental molecular imaging and clinical radiology because it is non-invasive and capable of producing three-dimensional representations of opaque organisms with high spatial and temporal resolution. Approximately 35% of all clinical MR scans utilize contrast media, however a primary limitation of MR imaging is the sensitivity of contrast agents that require high concentrations (0.1-0.6 mM). A number of strategies have been employed to amplify the observed in vivo signal of MR contrast agents. Approaches include attachment of Gd(III) chelates to polymers, proteins and particles, encapsulation into micelles and caged structures, and targeting to receptors. While each of these approaches has yielded significant increases in the relaxivity of MR contrast agents (and therefore sensitivity), all of these classes of complexes possess intrinsic background signal and function solely as anatomical reporters. In order to reduce the background signal and simultaneously create probes that are modulated by biochemical processes, caged complexes were designed to coordinatively saturate the paramagnetic ion. Coupled with amplification strategies, these agents represent a means to selectively modulate the observed MR signal and function as in vivo biochemical reporters. For example, to create an in vivo MR assay of enzymatic activities and secondary messengers, agents have been designed and synthesized with removable protection groups that largely prevent access of water to a paramagnetic center. By limiting the access of bulk water (q-modulation) the unprocessed agent is designed to be an ineffective contrast agent, and hence serves as a reliable marker for regions of enzyme activity or the presence of secondary messengers. Further, we have focused on designing multimodal contrast agents that are simultaneously detectable by more than one imaging technique. For example, attaching an optical probe to a MR contrast agent provides the means to detect the probe in a whole animal and subsequently validate the results by histological methods. Finally, we describe strategies for signal amplification, and cell delivery vehicles attached to imaging probes for in vivo long-term fate mapping experiments.

Imaging matrix metalloproteinases in cancer

Early detection of both primary tumors and metastatic disease remains a major challenge in the diagnosis and staging of cancer. The recognition of the role of MMPs in both the growth and metastasis of tumors has guided the development not only of therapeutic strategies utilizing synthetic, small-molecule MMP inhibitors (MMPIs), but has also catalyzed methods to detect and image tumors in vivo by means of tumor-associated proteolytic activity. These imaging approaches target MMPs involved in cancer progression via contrast agents linked to MMPIs or to MMP selective and specific substrates with sensitivity enhanced by amplification during enzymatic processing. This review draws attention to a variety of strategies utilized to image MMP activity in vivo.

In vivo imaging of molecular targets and their function in endocrinology

Imaging is one of the fastest growing fields of study. New technologies and multimodal approaches are increasing the application of imaging to determine molecular targets and functional processes in vivo. The identification of a specific target, transporter, or biological process using imaging has introduced major breakthroughs to the field of endocrinology primarily utilizing computed tomography, magnetic resonance imaging, ultrasonography, positron emission tomography, single-photon emission computed tomography, and optical imaging. This review provides a general background to the specific developments in imaging that pertains to in vivo function and target identification in endocrine-based diseases.

MRI detection of thrombin with aptamer functionalized superparamagnetic iron oxide nanoparticles

Design of smart MRI contrast agent based on superparamagnetic iron oxide nanoparticles and aptamers has been described for the detection of human alpha-thrombin protein. The contrast agent is based on the assembly of the aptamer functionalized nanoparticles in the presence of thrombin. A detectable change in MRI signal is observed with 25 nM thrombin in human serum. Changes were neither observed with control analytes, streptavidin, or bovine serum albumin, nor with inactive aptamer functionalized nanoparticles.