Mikhail I. Papisov

Long-circulating iron oxides for MR imaging

Abstract A variety of iron oxide preparations with long blood half-lives have been synthesized for MR imaging. Potential applications of these agents include MR angiography, RES imaging, target specific imaging and neuronal transport imaging. This chapter reviews types of iron oxide and mechanisms that are responsible for a prolonged plasma half-life.

Colloidal magnetic resonance contrast agents: effect of particle surface on biodistribution

Abstract Surface structure of dextran-coated superparamagnetic particles is studied as a biodistribution-defining factor. Colloidal particle and polymer molecule of the same size and similar outer structure are shown to possess similar biokinetics and biodistribution. A dense dextran layer on the particle surface is identified as a structure responsible for particle accumulation in the lymph nodes.

CNS penetration of intrathecal-lumbar idursulfase in the monkey, dog and mouse: implications for neurological outcomes of lysosomal storage disorder.

A major challenge for the treatment of many central nervous system (CNS) disorders is the lack of convenient and effective methods for delivering biological agents to the brain. Mucopolysaccharidosis II (Hunter syndrome) is a rare inherited lysosomal storage disorder resulting from a deficiency of iduronate-2-sulfatase (I2S). I2S is a large, highly glycosylated enzyme. Intravenous administration is not likely to be an effective therapy for disease-related neurological outcomes that require enzyme access to the brain cells, in particular neurons and oligodendrocytes. We demonstrate that intracerebroventricular and lumbar intrathecal administration of recombinant I2S in dogs and nonhuman primates resulted in widespread enzyme distribution in the brain parenchyma, including remarkable deposition in the lysosomes of both neurons and oligodendrocytes. Lumbar intrathecal administration also resulted in enzyme delivery to the spinal cord, whereas little enzyme was detected there after intraventricular administration. Mucopolysaccharidosis II model is available in mice. Lumbar administration of recombinant I2S to enzyme deficient animals reduced the storage of glycosaminoglycans in both superficial and deep brain tissues, with concurrent morphological improvements. The observed patterns of enzyme transport from cerebrospinal fluid to the CNS tissues and the resultant biological activity (a) warrant further investigation of intrathecal delivery of I2S via lumbar catheter as an experimental treatment for the neurological symptoms of Hunter syndrome and (b) may have broader implications for CNS treatment with biopharmaceuticals.

Mion-ASF: Biokinetics of an MR receptor agent

Receptor-directed MR contrast agents are currently being designed to improve sensitivity and specificity of MR imaging and to provide for functional MR imaging. In the current study we have synthesized a conjugate of asialofetuin (ASF), a bovine plasma protein with a known, high affinity for the hepatic asialoglycoprotein receptor, and a well defined, single crystal superparamagnetic label (monocrystalline iron oxide nanoparticle, MION). MION-ASF is cleared from the circulation more than 300 times faster than MION, has a 3.7 times higher hepatic accumulation, increases liver R2 relaxivity 2.8-fold compared to MION, and accumulates in hepatocytes unlike MION, which accumulates only in macrophages. Competition assays indicate that receptor-mediated hepatocyte uptake can be competitively blocked and that this effect can be demonstrated by imaging. These studies indicate that sensitive iron oxide based probes can be developed for functional MR imaging.

Targeted delivery of diagnostic agents by surface-modified liposomes

Surface-modified LS have been used for the specific delivery of heavy metal-based imaging agents. The liposome surface was modified with PEG, AMmAb, Dext-SA, chelating agent DTPA-PE, and with NGPE-modified chelating polymer: DTPA-NPLL-NGPE. The hypothesis is suggested attempting to explain the phenomenon of long circulation of PEG-coated LS from the point of view of statistical properties of flexible polymer molecule in solution. Direct experiments using fluorescent labels were performed to prove the hypothesis. The calculations performed on the basis of the hypothesis were designed to find the optimal concentration of PEG on the LS surface, and suggested that it not only provides a protective effect but also does not create steric hindrances for the surface-immobilized mAb. As a result, long circulating targeted ILS have been prepared. Intravenously administered 111In-labelled PEG-AMmAb-LS were targeted to the area of experimental myocardial infarction in rabbits under γ-scintigraphic control. Infarct-to-normal ratios of 111In radioactivity of about 20 were achieved. PEG-and Dext-modified liposomes with surface-incorporated Gd-labelled DTPA-PE or DTPA-NPLLNGPE were used for the subcutaneous administration and subsequent NMR-imaging of lymph nodes in rabbits. Two mechanisms of MR-signal enhancement were found for the surface-modified Gd-containing LS: the increase in signal intensity due to the increase in water quantity in the vicinity of Gd atoms because of PEG-associated water; and better lymph node accumulation of Dext-LS via receptormediated endocytosis. Surface modification of LS opens the possibilitiy for targeted delivery of heavy metal-based imaging agents.

MR Imaging of Slow Axonal Transport in Vivo

Three magnetopharmaceuticals based on a monocrystalline iron oxide nanocompound (MION) are evaluated as potential contrast agents for demonstrating axonal transport in vivo by magnetic resonance (MR) imaging. One agent has a strong positive charge, one has a strong negative charge, and the third is covalently linked to wheat germ agglutinin, a plant lectin with a high affinity for axon terminals. All three agents were tagged with rhodamine, and fluorescence microscopy was used to determine their fate after administration and to validate the imaging results. Following injection into or near various neural structures in the motor and visual systems of rats, MR images were obtained at multiple times up to 11 days later, and the imaged tissues were processed for subsequent histological examination. Similar results were obtained with all three agents. Axonal transport was not seen by MR imaging or fluorescence microscopy when the agents were injected into the calf muscles, the vitreous of the eye, or the superior colliculus. However, bidirectional axonal transport was shown unequivocally by both methods after injection directly into the site of a focal crush injury to the sciatic nerve. The nerve, which otherwise is isointense with surrounding tissues on MR images, appeared as a uniformly hypointense structure having a length approximately in proportion to the time from injection to imaging. By 11 days, the course of the nerve was traceable from its component roots in the cauda equina to its bifurcation into the tibial and common peroneal nerves in the leg. A transport rate of about 5 mm/day was calculated, which is consistent with the mechanism of slow transport. MION-based magnetopharmaceuticals thus can be used to demonstrate slow axonal transport, and thereby visualize peripheral nerves, in vivo by MR imaging.

Iodine-124 as a label for pharmacological PET imaging

With the growing number of biotechnology products and drug delivery systems entering preclinical and clinical studies, pharmacological imaging studies with PET play an increasingly significant role. Such studies often require investigation of slow and complex pharmacokinetics (PK). This suggests labeling of the drug candidate with radionuclides that have long physical half-lives. Among the currently available PET positron emitters, ¹²⁴I has the longest physical half-life (4.2 days). This, combined with the well-investigated behavior of iodine in vivo, makes ¹²⁴I very attractive for pharmacological studies. However, the high energy of the positrons emitted by ¹²⁴I and the presence of single photons in the ¹²⁴I emission can potentially introduce limitations in the quantitative analysis of the images. The objective of this research was to determine whether the use of ¹²⁴I as a PET label provides data quality suitable for PK studies. The study was carried out using MicroPET P4 scanner (Siemens/Concorde Microsystems). Spatial resolution, count-rate performance, sensitivity and scatter fraction were measured using a line source and a cylindrical phantom. Model animal studies in rats and cynomolgus monkeys were carried out using human recombinant proteins. The proteins were labeled with ¹²⁴I, up to 185 MBq/mg. The transaxial and axial spatial resolutions in the center of the camera were satisfactory and higher for OSEM3D/MAP than FORE-2DFBP (FWHM 2.52 vs 3.31 mm, and 3.10 vs 3.69 mm). Linearity of the true coincidence count-rate was observed up to 44 MBq. Animal studies demonstrated excellent delineation and resolution of even very small organs. At optimal doses, 2-10 MBq per animal for rodents and 4-10 MBq per kg of body weight for larger animals, the quality of numerical data was appropriate for PK analysis in all experimental timeframes from minutes (dynamic studies) to 10 days. Overall, the data suggest that ¹²⁴I is an excellent label for quantitative pharmacological PET imaging studies.

Modeling in vivo transfer of long-circulating polymers (two classes of long circulating polymers and factors affecting their transfer in vivo)

Abstract The relationships between polymer structure and circulation in vivo are discussed on the basis of a mathematical model of polymer transfer. Significant differences in polymer distribution in liquid compartments allow to divide long-circulating polymers and particulates into two groups, large (non-extravasating) and small (extravasating) polymers. Transfer processes that members of these two groups undergo are distinctively different and can be described by different idealized models. Although polymers of both classes may consist of the same constituents, their in vivo localization in liquid compartments and capability of cooperative interactions with components of biological systems may be essentially different. Therefore, relative impact of polymer structure on biokinetics of large and small polymers may differ. Minimization of polymer clearance due to interactions with biological systems and renal filtration is important in the development of long-circulating polymers. Hydrophilic interface brushes assembled of non-reactive polymer chains proved to be effective in prolongation of circulation of both extravasating and non-extravasating polymers, although prolongation mechanisms may differ.

Magnetically driven thrombolytic preparation containing immobilized streptokinase-targeted transport and action

Streptokinase was immobilized on the magnetic carrier at a concentration of 2,000-2,700 IU/ml of preparation. Thrombosis was induced in both canine carotid arteries by means of the vascular wall flap inverting into its lumen. The red, completely occluding thrombus was formed inside the vessel 4-5 h later. A samarium-cobalt magnet was secured externally to one of the arteries. After the clot formation the magnetically immobilized streptokinase was injected intravenously. The preparation concentrated in the region of the magnetic field action caused complete thrombus degradation and normal blood flow restoration; no effect on the clot in the control artery was observed.

Skin Rejuvenation with Non-Invasive Pulsed Electric Fields

Degenerative skin diseases affect one third of individuals over the age of sixty. Current therapies use various physical and chemical methods to rejuvenate skin; but since the therapies affect many tissue components including cells and extracellular matrix, they may also induce significant side effects, such as scarring. Here we report on a new, non-invasive, non-thermal technique to rejuvenate skin with pulsed electric fields. The fields destroy cells while simultaneously completely preserving the extracellular matrix architecture and releasing multiple growth factors locally that induce new cells and tissue growth. We have identified the specific pulsed electric field parameters in rats that lead to prominent proliferation of the epidermis, formation of microvasculature, and secretion of new collagen at treated areas without scarring. Our results suggest that pulsed electric fields can improve skin function and thus can potentially serve as a novel non-invasive skin therapy for multiple degenerative skin diseases.