Vladimir S. Trubetskoy

Which polymers can make nanoparticulate drug carriers long-circulating?

Abstract The protective effect of poly(ethylene glycol) and some other polymers on nanoparticulate carriers including liposomes is considered in terms of statistical behavior of macromolecules in solution, when polymer flexibility plays a key role. According to the mechanism proposed, surface-grafted chains of flexible and hydrophilic polymers form dense “conformational clouds” preventing other macromolecules from the interaction with the surface even at low concentration of protecting polymer. Using liposomes as an example, experimental evidence is presented of the importance of protecting polymer flexibility in liposome steric protection. Further possible applications of the suggested model are discussed. The possibility of using protecting polymers other than poly(ethylene glycol) is analyzed, and examples of such polymers are given based on polymer-coated liposome biodistribution data. General requirements for protecting polymers are formulated, and differences in steric protection of liposomes and particles are discussed. The scale of protective effect is interpreted as the balance between the energy of hydrophobic anchor interaction with the liposome membrane core or with the particle surface and the energy of polymer chain free motion in solution.

Amphiphilic vinyl polymers effectively prolong liposome circulation time in vivo

Newly synthesized amphiphilic polyacrylamide and poly(vinyl pyrrolidone), single terminus-modified with long-chain fatty acyl groups, are able to incorporate into the liposomal membrane, and similar to poly(ethylene glycol) prolong liposome circulation in vivo and decrease liposome accumulation in the liver. Protective efficacy of modified polymers increases with the increase in the length of acyl moiety and decreases for higher molecular weight polymers. The data on amphiphilic polymer-modified liposome biodistribution are presented.

Delayed Treatment with Intravenous Basic Fibroblast Growth Factor Reduces Infarct Size following Permanent Focal Cerebral Ischemia in Rats

Basic fibroblast growth factor (bFGF) is a polypeptide that supports the survival of brain cells (including neurons, glia, and endothelia) and protects neurons against a number of toxins and insults in vitro. This factor is also a potent dilator of cerebral pial arterioles in vivo. In previous studies, we found that intraventricularly administered bFGF reduced infarct volume in a model of focal cerebral ischemia in rats. In the current study, bFGF (45 μg/kg/h) in vehicle, or vehicle alone, was infused intravenously for 3 h, beginning at 30 min after permanent middle cerebral artery occlusion by intraluminal suture in mature Sprague–Dawley rats. After 24 h, neurological deficit (as assessed by a 0- to 5-point scale, with 5 = most severe) was 2.6 ± 1.0 in vehicle-treated and 1.5 ± 1.3 in bFGF-treated rats (mean ± SD; TV = 12 vs. 11; p = 0.009). Infarct volume was 297 ± 65 mm3 in vehicle- and 143 ± 135 mm3 in bFGF-treated animals (p = 0.002). During infusion, there was a modest decrease in mean arterial blood pressure but no changes in arterial blood gases or core or brain temperature in bFGF-treated rats. Autoradiography following intravenous administration of 111In-labeled bFGF showed that labeled bFGF crossed the damaged blood–brain barrier to enter the ischemic (but not the nonischemic) hemisphere. Whether the infarct-reducing effects of bFGF depend on intraparenchymal or intravascular mechanisms requires further study.

Use of polyoxyethylene-lipid conjugates as long-circulating carriers for delivery of therapeutic and diagnostic agents

Abstract The utilization of micelle forming block-copolymers as long-circulating drug carriers for sparingly soluble or amphiphilic drugs or diagnostic agents is reviewed with the emphasis on the use of phosphatidylethanolamine-polyethyleneoxide (PE-PEO) conjugates as structural amphiphilic polymers for formulation of polymeric micelles. PE-PEO conjugates spontaneously form particles with average diameter 10–30 nm depending on the molecular weight of PEO block and which are stable upon dilution at ambient temperature. Topoisomerase II inhibitor ellipticine was shown to be successfully incorporated into PE-PEO (5 kDa) conjugate micelles in vitro. Percutaneous lymphatic delivery of amphiphilic diagnostic agents incorporated into PE-PEO micelles was demonstrated using magnetic resonance imaging and gamma-scintigraphy. For water-soluble low molecular weight drugs one can expect hydrophobic prodrug approach to be most suitable for use with this drug delivery system.

Basic fibroblast growth factor protects against excitotoxicity and chemical hypoxia in both neonatal and adult rats

Basic fibroblast growth factor (bFGF) is a polypeptide growth factor that promotes neuronal survival. We recently found that systemic administration of bFGF protects against both excitotoxicity and hypoxia–ischemia in neonatal animals. In the present study, we examined whether systemically administered bFGF could prevent neuronal death induced by intrastriatal injection of N-methyl-d-aspartate (NMDA) or chemical hypoxia induced by intrastriatal injection of malonate in adult rats and 1-methyl-4-phenylpyridinium (MPP+) in neonatal rats. Systemic administration of bFGF (100 μg/kg) for three doses both before and after intrastriatal injection of either NMDA or malonate in adult rats produced a significant neuroprotective effect. In neonatal rats, bFGF produced dose-dependent significant neuroprotective effects against MPP+ neurotoxicity, with a maximal protection of ∼50% seen with either a single dose of bFGF of 300 μg/kg or three doses of 100 μg/kg. These results show that systemic administration of bFGF is effective in preventing neuronal injury under circumstances in which the blood–brain barrier may be compromised, raising the possibility that this strategy could be effective in stroke.

Polymeric micelles as carriers of diagnostic agents

This review deals with diagnostic applications of polymeric micelles composed of amphiphilic block-copolymers. In aqueous solutions these polymers spontaneously form particles with diameter 20-100 nm. A variety of diagnostic moieties can be incorporated covalently or non-covalently into the particulates with high loads. Resulting particles can be used as particulate agents for diagnostic imaging using three major imaging modalities: gamma-scintigraphy, magnetic resonance imaging and computed tomography. The use of polyethyleneoxide-diacyllipid micelles loaded with chelated (111)In/Gd(3+) as well as iodine-containing amphiphilic copolymer in percutaneous lymphography and blood pool/liver imaging are discussed as specific examples.

Controlled delivery of Gd-containing liposomes to lymph nodes: surface modification may enhance MRI contrast properties

Surface modification of liposomes containing Gd-diethylenetriaminepentaacetyl-phosphatidylethanolamine (Gd-DTPA-PE) as an amphiphilic paramagnetic label influences their contrast properties as MRI agents. Liposomes with the mean diameter ca. 220 nm and 10% mol content of Gd-DTPA-PE were modified with dextran (molecular weight 6 kDa) or polyethyleneglycol (PEG, molecular weight 5 kDa). The preparations obtained were used for MR visualization of axillary and subscapular lymph nodes after SC injection into rabbits paw. Dextran-modified liposomes were found to have limited or no effect on lymph node/muscle MR signal intensity ratio during the first 2 h after the administration when compared to nonmodified plain liposomes, whereas the liposomes modified with PEG demonstrated a 3-3.5-fold enhancement of lymph node signal. However, the partial biodistribution studies with the 111In-labeled liposomes revealed that PEG-modified vesicles accumulated in the axillary lymph node at half the amount comparing with nonmodified and dextran-modified liposomes. The phenomenon found might be explained by noticeably increased relaxivity of PEG-modified Gd-liposomes.

Block-copolymer of Polyethylene Glycol and Polylysine as a Carrier of Organic Iodine: Design of Long-circulating Particulate Contrast Medium for X-ray Computed Tomography

In order to obtain small, polymer-stabilized particulate carriers for organic iodine to serve as a contrast agent for X-ray computed tomography (CT) an attempt was made to design a carrier based on polymeric micelles. Here we describe the synthesis of an iodine-containing amphiphilic block-copolymer which can micellize in aqueous solutions. The two blocks of the copolymer consisted of methoxypoly(ethyleneglycol) and poly[epsilon,N-(triiodobenzoyl)-L-lysine]. Upon dispersion in water, the block copolymer formed particles with average diameter 80 nm and iodine content up to 44.7%. The particles start to dissociate to the individual polymeric chains in the concentration range of 0.05-0.5 microM in water at 23 degrees C. Upon intravenous injection at 250 mg of iodine/kg (570 mg of the agent/kg) in rabbits the medium demonstrated exceptional 24 hr half-life in the blood substantiating corona/core structure of the particles with PEG chains protecting the iodine-containing core. The possible use of these particulates as contrast medium for X-ray computed tomography is discussed.

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.

Stable polymeric micelles: Lymphangiographic contrast media for gamma scintigraphy and magnetic resonance imaging

RATIONALE AND OBJECTIVES Amphiphilic biocompatible polyoxyethylene (PEO)-based polymers form particles (micelles) that are 10-50 nm in diameter. In the current research, we successfully incorporated amphiphilic indium-111 (111In) and gadolinium chelates into these particles and used them as particulate contrast media in percutaneous lymphography. METHODS Micelles of amphiphilic PEO-lipid conjugates were loaded with 111In and gadolinium diethylenetriamine pentaacetic acid-phosphatidylethanolamine (Gd-DTPA-PE) and were injected subcutaneously into the rabbits paw. Corresponding images of local lymphatics were acquired using a gamma camera and a magnetic resonance (MR) imager. RESULTS The entire lymphatic chain from the paw to the thoracic duct could be visualized using 111In micelles after injection site massage. T1-weighted MR images of the primary lymph node and collecting vessels were obtained within 4 min after administration of gadolinium micelles and massage. CONCLUSION Polymeric PEO-containing micelles can be loaded with diagnostic metals and, on subcutaneous injection, can visualize elements of lymphatic system. The major fraction of injected micelles stays within the lymph fluid, thus serving as lymphangiographic agents for indirect MR or gamma lymphography.