Pramod K. Avti

Superparamagnetic iron oxide based nanoprobes for imaging and theranostics

The need to target, deliver and subsequently evaluate the efficacy of therapeutics in the treatment of a disease has provided added impetus in developing novel and highly efficient contrast agents. Superparamagnetic iron oxide nanoparticles (SPIONs) have offered tremendous potential in designing advanced magnetic resonance imaging (MRI) diagnostic agents, due to their unique physicochemical properties. There has been tremendous effort devoted in the recent past in developing synthetic methodologies through which their size, hydrodynamic radii, chemical composition and morphologies could be tailored at the nanoscale. This enables one to fine tune their magnetic behavior, and thus their MRI response. While novel synthetic strategies are being assembled for directing SPIONs to the diseased site as well as imparting them stealth and biocompatibility, it is also essential to evaluate their biological toxicological profiles. This review highlights recent advances that have been made in the synthesis of SPIONs, subsequent functionalization with desired entities, and a discussion on their use as MRI contrast agents in cardiovascular research.

Alkyne-Azide “Click” Chemistry in Designing Nanocarriers for Applications in Biology

The alkyne-azide cycloaddition, popularly known as the “click” reaction, has been extensively exploited in molecule/macromolecule build-up, and has offered tremendous potential in the design of nanomaterials for applications in a diverse range of disciplines, including biology. Some advantageous characteristics of this coupling include high efficiency, and adaptability to the environment in which the desired covalent linking of the alkyne and azide terminated moieties needs to be carried out. The efficient delivery of active pharmaceutical agents to specific organelles, employing nanocarriers developed through the use of “click” chemistry, constitutes a continuing topical area of research. In this review, we highlight important contributions click chemistry has made in the design of macromolecule-based nanomaterials for therapeutic intervention in mitochondria and lipid droplets.

Dendrimers as anti-inflammatory agents

Dendrimers constitute an intriguing class of macromolecules which find applications in a variety of areas including biology. These hyperbranched macromolecules with tailored backbone and surface groups have been extensively investigated as nanocarriers for gene and drug delivery, by molecular encapsulation or covalent conjugation. Dendrimers have provided an excellent platform to develop multivalent and multifunctional nanoconjugates incorporating a variety of functional groups including drugs which are known to be anti-inflammatory agents. Recently, dendrimers have been shown to possess anti-inflammatory properties themselves. This unexpected and intriguing discovery has provided an additional impetus in designing novel active pharmaceutical agents. In this review, we highlight some of the recent developments in the field of dendrimers as nanoscale anti-inflammatory agents.

Fabricating Water Dispersible Superparamagnetic Iron Oxide Nanoparticles for Biomedical Applications through Ligand Exchange and Direct Conjugation

Stable superparamagnetic iron oxide nanoparticles (SPIONs), which can be easily dispersed in an aqueous medium and exhibit high magnetic relaxivities, are ideal candidates for biomedical applications including contrast agents for magnetic resonance imaging. We describe a versatile methodology to render water dispersibility to SPIONs using tetraethylene glycol (TEG)-based phosphonate ligands, which are easily introduced onto SPIONs by either a ligand exchange process of surface-anchored oleic-acid (OA) molecules or via direct conjugation. Both protocols confer good colloidal stability to SPIONs at different NaCl concentrations. A detailed characterization of functionalized SPIONs suggests that the ligand exchange method leads to nanoparticles with better magnetic properties but higher toxicity and cell death, than the direct conjugation methodology.

Magnetic resonance imaging/fluorescence dual modality protocol using designed phosphonate ligands coupled to superparamagnetic iron oxide nanoparticles

A simple and versatile methodology to tailor the surface of superparamagnetic iron oxide nanoparticles (SPIONs), and render additional fluorescence capability to these contrast agents, is reported. The dual modality imaging protocol was developed by designing multi-functional scaffolds with a combination of orthogonal moieties for aqueous dispersion and stealth, to covalently link them to SPIONs, and carry out post-functionalization of nanoparticles. SPIONs stabilized with ligands incorporating surface-anchoring phosphonate groups, ethylene glycol backbone for aqueous dispersion, and free surface exposed OH moieties were coupled to near-infrared dye Cy5.5A. Our results demonstrate that design of multi-tasking ligands with desired combination and spatial distribution of functions provides an ideal platform to construct highly efficient dual imaging probes with balanced magnetic, optical and cell viability properties.

Conjugation of multivalent ligands to gold nanoshells and designing a dual modality imaging probe

Design and synthesis of branched tetraethylene glycol (TEG) based ligands for subsequent conjugation to gold nanoshells are reported. TEG enhances the aqueous solubility of hollow gold nanoshells (HAuNShs), and the branched architecture provides stability. An examination of the supernatant of the surface displacement reaction shows that the structure of the ligand plays an important role in the functionalization of HAuNShs. The binding of multivalent ligands leads to rupturing of the gold nanoshell architecture; most probably due to the large dendron not compensating the replacement of small citrate capping agents. The construction of a probe with dual imaging capabilities is demonstrated by covalent linking of a dendron containing Cy5.5A dye to gold nanoshells. It leads to fluorescence quenching of Cy5.5A by the gold nanoshells, as evidenced in solution and in cellular internalization studies with J774 and bEnd.3 cells.

Magnetic resonance fingerprinting based on realistic vasculature in mice

ABSTRACT Magnetic resonance fingerprinting (MRF) was recently proposed as a novel strategy for MR data acquisition and analysis. A variant of MRF called vascular MRF (vMRF) followed, that extracted maps of three parameters of physiological importance: cerebral oxygen saturation (SatO2), mean vessel radius and cerebral blood volume (CBV). However, this estimation was based on idealized 2‐dimensional simulations of vascular networks using random cylinders and the empirical Bloch equations convolved with a diffusion kernel. Here we focus on studying the vascular MR fingerprint using real mouse angiograms and physiological values as the substrate for the MR simulations. The MR signal is calculated ab initio with a Monte Carlo approximation, by tracking the accumulated phase from a large number of protons diffusing within the angiogram. We first study the identifiability of parameters in simulations, showing that parameters are fully estimable at realistically high signal‐to‐noise ratios (SNR) when the same angiogram is used for dictionary generation and parameter estimation, but that large biases in the estimates persist when the angiograms are different. Despite these biases, simulations show that differences in parameters remain estimable. We then applied this methodology to data acquired using the GESFIDE sequence with SPIONs injected into 9 young wild type and 9 old atherosclerotic mice. Both the pre injection signal and the ratio of post‐to‐pre injection signals were modeled, using 5‐dimensional dictionaries. The vMRF methodology extracted significant differences in SatO2, mean vessel radius and CBV between the two groups, consistent across brain regions and dictionaries. Further validation work is essential before vMRF can gain wider application. HighlightsModeling GESFIDE fingerprinting from realistic angiograms of microvasculature.Validation with in vivo acquisition in wild‐type and atherosclerotic mice.Evidence of bias in the vMRF extracted parameters, using simulations.However, differences in vMRF extracted parameters are more robust to bias.Group differences observed in cerebral oxygen saturation, mean vessel radius and CBV.

Investigating the correlation between white matter and microvasculature changes in aging using large scale optical coherence tomography and confocal fluorescence imaging combined with tissue sectioning

Here, we present a serial OCT/confocal scanner for histological study of the mouse brain. Three axis linear stages combined with a sectioning vibratome allows to cut thru the entire biological tissue and to image every section at a microscopic resolution. After acquisition, each OCT volume and confocal image is re-stitched with adjacent acquisitions to obtain a reconstructed, digital volume of the imaged tissue. This imaging platform was used to investigate correlations between white matter and microvasculature changes in aging mice. Three age groups were used in this study (4, 12, 24 months). At sacrifice, mice were transcardially perfused with a FITC containing gel. The dual imaging capability of the system allowed to reveal different contrast information: OCT imaging reveals changes in refractive indices giving contrast between white and grey matter in the mouse brain, while transcardial perfusion of a FITC shows microsvasculature in the brain with confocal imaging.

Effects of anesthesia on the cerebral capillary blood flow in young and old mice

Despite recent findings on the possible role of age-related cerebral microvasculature changes in cognition decline, previous studies of capillary blood flow in aging (using animal models) are scarce and limited to anesthetized conditions. Since anesthesia can have different effects in young and old animals, it may introduce a confounding effect in aging studies. The present study aimed to eliminate the potential confound introduced by anesthesia by measuring capillary blood flow parameters in both awake conditions and under isoflurane anesthesia. We used 2-photon laser scanning fluorescence microscopy to measure capillary diameter, red blood cell velocity and flux, hematocrit and capillary volumetric flow in individual capillaries in the barrel cortex of 6- and 24-month old C57Bl/6 mice. It was observed that microvascular properties are significantly affected by anesthesia leading to different trends in capillary blood flow parameters with aging when measured under awake or anesthetized conditions. The findings in this study suggest taking extra care in interpreting aging studies from anesthetized animals.

Miktoarm star conjugated multifunctional gold nanoshells: synthesis and an evaluation of biocompatibility and cellular uptake

A simple and highly versatile click chemistry based synthetic strategy to develop an ABC type miktoarm star ligand that is conjugated to gold nanoshells (GNS) is reported. The surface functionalized multifunctional GNS contain lipoic acid (LA) as a model therapeutic agent, poly(ethylene glycol) (PEG350) as a solubilizing and stealth agent, and tetraethylene glycol (TEG) with a terminally conjugated thiol moiety. These GNS have an average size of 40 nm, a shell thickness of 6 nm, a well-defined crystal structure lattice (111), and a surface absorption plasmon band in the near infrared (NIR) region. The miktoarm star and GNS functionalized with this ligand are non-cytotoxic for up to 5 μg mL-1 concentrations, and human umbilical vein endothelial cells internalize more than 85% of these GNS at 5 μg mL-1. Our results establish that the biocompatible miktoarm star ligand provides a useful platform to synthetically articulate the introduction of multiple functions onto GNS, and enhance their scope by combining their inherent imaging capabilities with efficient delivery and accumulation of active therapeutic agents.