The present studies were designed to determine if the endogenous cationic protein, e.g., histone, is capable of penetrating the blood-brain barrier (BBB) in vivo. Calf thymus histone was iodinated with [125I]iodine and was found to be taken up rapidly by isolated bovine brain capillaries used as an in vitro model system of the BBB via a time- and temperature-dependent mechanism. The binding was saturable and a Scatchard plot of the binding data was linear, yielding a KD = 15.2 +/- 2.8 microM and a maximal binding = 7.7 +/- 1.0 nmol/mg of protein. Other polycations such as protamine or polylysine markedly inhibited uptake of [125I] histone, but cationized albumin demonstrated minimal inhibition and cationized immunoglobulin caused no inhibition of bovine brain capillary uptake of [125I]histone. The in vivo brain VD of [125I] histone reached 159 +/- 70 microliters/g by 10 min of carotid arterial perfusion as compared to the 10-min VD for [3H]albumin, 17 +/- 7 microliter/g. Most of this uptake represented sequestration by the vasculature, but approximately 8% of the total histone taken up by brain was found to be transported unmetabolized (based on trichloroacetic acid precipitability of brain supernatant [( 125I]) into brain interstitium. These studies demonstrate that histone is transported through the BBB in vivo via absorptive-mediated transport. Thus, histone is an endogenous protein that is capable of transport through the BBB and may be a potential vector for pharmaceutical delivery through the BBB.
The brain amyloid of Alzheimer disease (AD) may potentially be imaged in patients with AD by using neuroimaging technology and a radiolabeled form of the 40-residue beta-amyloid peptide A beta 1-40 that is enabled to undergo transport through the brain capillary endothelial wall, which makes up the blood-brain barrier (BBB) in vivo. Transport of 125I-labeled A beta 1-40 (125I-A beta 1-40) through the BBB was found to be negligible by experiments with both an intravenous injection technique and an internal carotid artery perfusion method in anesthetized rats. In addition, 125I-A beta 1-40 was rapidly metabolized after either intravenous injection or internal carotid artery perfusion. BBB transport was increased and peripheral metabolism was decreased by conjugation of monobiotinylated 125I-A beta 1-40 to a vector-mediated drug delivery system, which consisted of a conjugate of streptavidin (SA) and the OX26 monoclonal antibody to the rat transferrin receptor, which undergoes receptor-mediated transcytosis through the BBB. The brain uptake, expressed as percent of injected dose delivered per gram of brain, of the 125I,bio-A beta 1-40/SA-OX26 conjugate was 0.15 +/- 0.01, a level that is 2-fold greater than the brain uptake of morphine. The binding of the 125I,bio-A beta 1-40/SA-OX26 conjugate to the amyloid of AD brain was demonstrated by both film and emulsion autoradiography performed on frozen sections of AD brain. Binding of the 125I,bio-A beta 1-40/SA-OX26 conjugate to the amyloid of AD brain was completely inhibited by high concentrations of unlabeled A beta 1-40. In conclusion, these studies show that BBB transport and access to amyloid within brain may be achieved by conjugation of A beta 1-40 to a vector-mediated BBB drug delivery system.
Abstract Antigen presentation within the human central nervous system by the class II histocompatibility DR‐antigen may take place at either the brain capillary–endothelial interface or at perivascular cells, such as smooth muscle or pericytes. The present studies employ a new sensitive immunoperoxidase technique, a mouse monoclonal antibody to the human DR‐antigen, and microvessels isolated from autopsied human brain. The experiments show that the DR‐antigen is readily detectable in human microvasculature of normal brain and is found in the smooth muscle pre‐capillary arterioles and capillary pericytes with minimal, if any, staining of capillary endothelium. These results are consistent with the hypothesis that antigen presentation in the CNS occurs primarily at a site immediately distal to the blood–endothelial interface.
Transmembrane transport and endocytosis of antibodies is facilitated by cationization, when the isoelectric point of the antibody is raised to the cationic range. The present studies describe the preparation of affinity-purified polyclonal antibodies directed against a 16-amino acid synthetic peptide corresponding to amino acids 35-50 of the 116-amino acid rev protein of human immunodeficiency virus type 1 (HIV-1). The concentration of cationized anti-rev35-50 antibody that results in 50% binding to the rev epitope, based on results with an immunoradiometric assay, also results in a statistically significant 37% inhibition of HIV-1 replication in human peripheral blood lymphocytes. The cationized antibodies caused no measurable toxicity to the cells, on the basis of [3H]thymidine incorporation. These studies demonstrate that cationization results in enhanced endocytosis of the antibody and enhanced inhibition of HIV-1 replication, consistent with intracellular immunization of the rev protein.
Abstract: Recent studies indicate that circulating peptides or plasma proteins, such as insulin or transferrin, or modified proteins, such as cationized albumin, undergo receptor‐mediated or absorptive‐mediated transport through the brain capillary wall, i.e., the blood‐brain barrier (BBB). Although morphologic studies such as autoradiography or immunoperoxidase labeling can demonstrate transport of blood‐borne protein into brain, there is a need for a rapid, sensitive, and quantifiable physiology‐based technique for comparing the relative rates of transport of several different blood‐borne peptides or proteins into brain. Therefore, the present investigations describe a carotid arterial infusion technique coupled with a capillary depletion method for quantifying transport of blood‐borne cationized albumin, cationized IgG, and acetylated low‐density lipoprotein (LDL). Because differentiation of true transcytosis into the postcapillary compartment of brain parenchyma from binding and/or endocytosis to the brain microvasculature is important, the present studies use a dextran density centrifugation step to deplete brain homogenate of the vasculature. In addition, 3 H‐labeled native albumin is used as a vascular space marker to account for release of capillary contents into the postcapillary supernatant following homogenization of brain. This study demonstrates rapid transport of cationized IgG or cationized albumin into brain, as these compounds achieve a volume of distribution of 20–30 μl/g within 10 min of arterial perfusion. Conversely, acetylated LDL, although rapidly bound by cerebral microvasculature, is shown not to undergo transport into the postcapillary compartment of brain parenchyma. These studies provide the basis for a sensitive, quantifiable technique for studying transport of radiolabeled blood‐borne peptides and proteins across the BBB of anesthetized animals.
IgG molecules are potential neuropharmaceuticals that may be used for therapeutic or diagnostic purposes. However, IgG molecules are excluded from entering brain, owing to a lack of transport of these plasma proteins through the brain capillary wall, or blood-brain barrier (BBB). The possibility of enhanced IgG delivery through the BBB by cationization of the proteins was explored in the present studies. Native bovine IgG molecules were cationized by covalent coupling of hexamethylenediamine and the isoelectric point was raised to greater than 10.7 based on isoelectric focusing studies. Native and cationized IgG molecules were radiolabeled with 125I and chloramine T. Cationized IgG, but not native IgG, was rapidly taken up by isolated bovine brain microvessels, which were used as an in vitro model system of the BBB. Cationized IgG binding was time and temperature dependent and was saturated by increasing concentrations of unlabeled cationized IgG (dissociation constant of the high-affinity binding site, 0.90 +/- 0.37 microM; Bmax, 1.4 +/- 0.4 nmol per mg of protein). In vivo studies documented enhanced brain uptake of 125I-labeled cationized IgG relative to [3H]albumin, and complete transcytosis of the 125I-labeled cationized IgG molecule through the BBB and into brain parenchyma was demonstrated by thaw-mount autoradiography of frozen sections of rat brain obtained after carotid arterial infusions of 125I-labeled cationized IgG. These studies demonstrate that cationization of IgG molecules greatly facilitates the transport of these plasma proteins through the BBB in vivo, and this process may provide a new strategy for IgG delivery through the BBB.
For monoclonal antibody therapeutics to access target antigen in extravascular compartments, an antibody drug delivery technology is required that has the dual properties of 1) transendothelial migration of the antibody and 2) endocytosis of the antibody into the target cell. These two objectives may be achieved with antibody cationization, and the present studies examine the feasibility of cationizing the humanized 4D5 monoclonal antibody directed against the p185HER2 oncogenic protein. The cationized antibody binds to the p185HER2 extracellular domain with an ED50 of 35 micrograms/ml and inhibits SK-BR3 cell proliferation similar to the native antibody. Confocal microscopy showed that although there was binding of the native 4D5 antibody to the plasma membrane of SK-BR3 cells, this antibody was confined to the periplasma membrane space with minimal endocytosis into the cell. In contrast, robust internalization of the cationized 4D5 antibody by the SK-BR3 cells was demonstrated by confocal microscopy. The systemic volume of distribution of the cationized 4D5 antibody was 11-fold greater than that of the native antibody. In summary, these studies show that a humanized monoclonal antibody may be cationized with retention of antibody affinity for the target antigen and biological activity, yet with a marked alteration in the cellular distribution and pharmacokinetics in vivo.
The present investigations evaluated the effects in rats of repetitive administration of cationized rat albumin over an 8-week period with the future aim of using this modified protein as a vector to transport drugs across the brain capillary endothelial wall, i.e., the blood-brain barrier. Rat albumin was cationized at pH = 7.8 with hexamethylenediamine, and the isoelectric point of the protein was raised from 5.5 to approximately 8. The cationized protein was monomeric based on mobility during sodium dodecylsulfate polyacrylamide gel electrophoresis. After radiolabeling, the cationized rat serum albumin (RSA) was taken up by isolated rat or bovine brain microvessels, whereas radio-labeled native RSA was not taken up by the capillaries in vitro. The brain volume of distribution of the 3H-cationized RSA increased linearly over a 5-hr period after an intravenous injection of the isotope and reached a value of 46 +/- 3 microliter/g (mean +/- S.E.) by 5 hr, whereas the brain volume of distribution of the 125I-native RSA was constant during the 5-hr time period (9.3 +/- 0.7 microliter/g, which is equal to the brain blood volume). The cationized and native RSAs were administered daily (Monday through Friday) at a dose of 1 mg/kg subcutaneously to groups of rats for 4- and 8-week periods. This dosage regimen resulted in no discernible toxicity, based on the findings of normal weight gain, normal tissue histology and normal serum chemistry. Therefore, cationized rat albumin may be used in future studies that use the repetitive administration of cationized rat albumin chimeric peptides for the evaluation of the transport of these substances through the blood-brain barrier in vivo.
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