PROCESS FOR THE PREPARATION OF TRANSFERRIN RECEPTOR SPECIFIC ANTIBODY-NEUROPHARMACEUTICAL OR DIAGNOSTIC AGENT CONJUGATES

Abstract: The present invention pertains to a method for delivering a neuropharmaceutical or diagnostic agent across the blood brain barrier to the brain of a host. The method comprises administering to the host a therapeutically effective amount of an antibody-neuropharmaceutical or diagnostic agent conjugate wherein the antibody is reactive with a transferrin receptor and the antibody is a chimera between the variable region from one animal source and the constant region from a different animal source. Other aspects of this invention include a delivery system comprising an antibody reactive with a transferrin receptor linked to a neuropharmaceutical or diganostic agent and methods for treating hosts afflicted with a disease associated with a neurological disorder. In embodiments of the present invention, the antibody that is reactive with a tranferrin receptor is a chimeric antibody. This antibody is composed of a variable region, immunologically reactive with the transferrin receptor, that is delivered from one animal source and a constant region that is derived from an animal source other than the one which provided the variable region. The chimeric antibodies of this invention can exist either as isolated entities or as conjugates with a neuropharmaceutical agent for transferal across the blood brain barrier. In the latter mode, the chimeric antibody-neuropharmaceutical agent conjugate forms a delivery system for delivering the neuropharmaceutical agent across the blood brain barrier.

Description: P Process for the preparation of transferrin receptor specific antibody-neuro ¬ pharmaceutical or diagnostic agent conjugates. Description Background The capillaries that supply blood to the tissues of the brain constitute the blood brain barrier (Goldstein et al. (1986) Scientific American 255:74-83; Pardridge, W.M. (1986) Endocrin. Rev. 7:314-330). The endothelial cells which form the brain capillaries are different from those found in other tissues in the body. Brain capillary endo¬ thelial cells are joined together by tight inter¬ cellular junctions which form a continuous wall against the passive movement of substances from the blood to the brain. These cells are also different in that they have few pinocytic vesicles which in other tissues allow somewhat unselective transport across the capillary wall. Also lacking are con¬ tinuous gaps or channels running through the cells which would allow unrestricted passage. The blood-brain barrier functions to ensure that the environment of the brain is constantly controlled. The levels of various substances in the blood, such as hormones, amino acids and ions, undergo frequent small fluctuations which can be brought about by activities such as eating and exercise (Goldstein et al_, cited supra) . If the brain were not protected by the blood brain barrier from these variations in serum composition, the result could be uncontrolled neural activity. The isolation of the brain from the bloodstream is not complete. If this were the case, the brain would be unable to function properly due to a lack of nutrients and because of the need to exchange chemicals with the rest of the body. The presence of specific transport systems within the capillary endothelial cells assures that the brain receives, in a controlled manner, all of the compounds required for normal growth and function. In many instances, these transport systems consist of membrane-associated receptors which, upon binding of their respective ligand, are internalized by the cell (Pardridge, .M. , cited supra) . Vesicles containing the receptor-ligand complex then migrate to the abluminal surface of the endothelial cell where the ligand is released. The problem posed by the blood-brain barrier is that, in the process of protecting the brain, it excludes many potentially useful therapeutic agents. Presently, only substances which are sufficiently lipophilic can penetrate the blood-brain barrier < (Goldstein et al, cited supra; Pardridge, W.M. , cited • s. 4 » supra) . Some drugs can be modified to make them more 5 " li.pophi.li.c and thereby i.ncrease thei.r abi.li.ty to h cross the blood brain barrier. However, each modification has to be tested individually on each drug and the modification can alter the activity of the drug. The modification can also have a very general effect in that it will increase the ability of the compound to cross all cellular membranes, not only those of brain capillary endothelial cells. Summary of the Invention The present invention pertains to a method for delivering a neuropharmaceutical or diagnostic agent across the blood brain barrier to the brain of a hpst. The method comprises administering to the host a therapeutically effective amount of an antibody- neuropharmaceutical or diagnostic agent conjugate wherein the antibody is reactive with a transferrin receptor and the antibody is a chimera between the variable region from one animal source and the constant region from a different animal source. The conjugate is administered under conditions whereby binding of the antibody to a transferrin receptor on a brain capillary endothelial cell occurs and the neuropharmaceutical agent is transferred across the blood brain barrier in a pharmaceutically active form. Other aspects of this invention include a delivery system comprising an antibody reactive with a transferrin receptor linked to a * neuropharmaceutical agent and methods for treating hosts afflicted with a disease associated with a neurological disorder. In embodiments of the present invention, the antibody that is reactive with a transferrin receptor is a chimeric antibody. This antibody is composed of a variable region, immunologically reactive with the transferrin receptor, that is derived from one animal source and a constant region that is derived from an animal source other than the one which provided the variable region. The chimeric antibodies of this invention can exist either as isolated entities or as conjugates with a neuropharmaceutical agent for transferal across the blood brain barrier. In the latter mode, the chimeric antibody-neuropharmaceut¬ ical agent conjugate forms a delivery system for delivering the neuropharmaceutical agent across the blood brain barrier. Presently available means for delivering therapeutic agents to the brain are limited in that they are invasive. The delivery system of the present invention is non-invasive and can utilize readily available antibodies reactive with a trans¬ ferrin receptor as carriers for neuropharmaceutical agents. The delivery system is advantageous in that the antibodies are capable of transporting neuropharmaceutical agents across the blood brain barrier without being susceptible to premature release of the neuropharmaceutical agent prior to reaching the brain-side of the blood brain barrier. Further, if the therapeutic activity of the agent to be delivered to the brain is not altered by the addition of a linker, a noncleavable linker can be used to link the neuropharmaceutical agent to the antibody. Description of the Drawings Figure 1 is a graphic representation of rat brain uptake of 14C-labelled murine monoclonal antibody (OX-26) to rat transferrin receptor in rats where the percent injected dose of radiolabelled antibody per brain and per 55 μl of blood is plotted versus time post-injection. Figure 2 is a histogram illustrating time dependent changes in the disposition of radiolabelled OX-26 between brain parenchyma and vasculature. Figure 3 is a histogram illustrating the enhanced delivery of methotrexate across the blood-brain barrier when, administered as a conjugate with OX-26. Figure 4 illustrates in three histograms (A,B and C) the distribution in the brain of both the antibody and the AZT components of an OX-26-AZT conjugate. Figure 5 is a histogram illustrating the experimental results of delivery of a protein, horseradish peroxidase, across the blood-brain barrier in rat brains in the form of a conjugate with OX-26. Figure 6 is a histogram illustrating the experimental results of delivering soluble CD4 to rat brain parenchyma using CD4 in the form of a conjugate with OX-26. Figure 7 is a histogram illustrating the biodistribution of antibody 128.1 and control IgG in a cynomolgous monkey. Figure 8 is a flow diagram of the general strategy for the expression of immunoglobulin variable region genes obtained by PCR. Figure 9 illustrates the primers used for variable region amplification, both for first cloning and sequencing the V region and then for cloning into the final expression vector. Figure 10 illustrates the cloning of the 128.1 heavy chain variable region. Figure 11 is the antibody coding sequence of heavy chain expression vector pAH4602 containing the 7 -1 isotype constant region. Figure 12 illustrates the cloning of the 128.1 light chain variable region. Figure 13 is the antibody coding sequence of light chain expression vector pAG4611. Figure 14 illustrates the plasmid map of the heavy chain expression vector pAH4625 containing the 7 -2 isotype. Figure 15 illustrates the plasmid map of the heavy chain expression vector pAH4807 containing the 7 -3 isotype. Figure 16 illustrates the plasmid map of the heavy chain expression vector pAH4808 containing the 7 -4 isotype. Figure 17 is the antibody coding sequence of heavy chain expression vector pAH4625 containing the 7 -2 isotype constant region. Figure 18 is the antibody coding sequence of heavy chain expression vector pAH4807 containing the 7 -3 isotype constant region. Figure 19 is the antibody coding sequence of heavy chain expression vector pAH4808 containing the 7 -4 isotype constant region. Detailed Description The method for delivering a neuropharmaceutical agent across the blood brain barrier to the brain of a host comprises administering to the host a therapeutically effective amount of an antibody- neuropharmaceutical agent conjugate wherein the antibody is reactive with a transferrin receptor present on a brain capillary endothelial cell. The method is conducted under conditions whereby the antibody binds to the transferrin receptor on the brain capillary endothelial cell and the neuropharm¬ aceutical agent is transferred across the blood brain barrier in a pharmaceutically active form. The host can be an animal susceptible to a neurological disorder (i.e., an animal having a brain) . Examples of hosts include mammals such as humans, domestic animals (e.g., dog, cat, cow or horse) , mice and rats. The neuropharmaceutical agent can be an agent having a therapeutic or prophylactic effect on a neurological disorder or any condition which affects biological functioning of the central nervous system. Examples of neurological disorders include cancer (e.g. brain tumors) , Autoimmune Deficiency Syndrome (AIDS) , stroke, epilepsy, Parkinson's disease, multiple sclerosis, neurodegenerative disease, trauma, depression, Alzheimer's disease, migraine, pain, or a seizure disorder. Classes of neuropharmaceutical agents which can be used in this invention include proteins, antibiotics, adrenergic agents, anticonvulsants, small molecules, nucleotide analogs, che otherapeutic agents, anti-trauma agents, peptides and other classes of agents used to treat or prevent a neurological disorder. Examples of proteins include CD4 (including soluble portions thereof) , growth factors (e.g. nerve growth factor and interferon) , dopamine decarboxylase and tricosanthin. Examples of antibiotics include amphotericin B, gentamycin sulfate, and pyrimethamine. Examples of adrenergic agents (including blockers) include dopamine and atenolol. Examples of chemotherapeutic agents include adriamycin, methotrexate, cyclophosphamide, etoposide, and carboplatin. An example of an anticonvulsant which can be used is valproate and an anti-trauma agent which can be used is superoxide dismutase. Examples of peptides would be somatostatin analogues and enkephalinase inhibitors. Nucleotide analogs which can be used include azido thymidine (hereinafter AZT) , dideoxy Inosine (ddl) and dideoxy cytodine (ddc) . The antibody, which is reactive with a transferrin receptor present on a brain capillary endothelial cell, may also be conjugated to a diagnostic agent. In this method and delivery system, the neuropharmaceutical agent of the neuropharmaceutical agent - anti-transferrin receptor conjugate has been replaced with a diagnostic agent. The diagnostic agent is then delivered across the blood brain barrier to the brain of the host. The diagnostic agent is then detected as indicative of the presence of a physiological condition for which the diagnostic agent is intended. For example, the diagnostic agent may be an antibody to amyloid plaques. When conjugated to an antibody reactive with a transferrin receptor present on a brain capillary endothelial cell, this diagnostic agent antibody can be transferred across the blood brain barrier and can then subsequently immunoreact with amyloid plaques. Such an immunoreaction is indicative of Alzheimer's Disease. Serum transferrin is a monomeric glycoprotein with a molecular weight of 80,000 daltons that binds iron in the circulation and transports it to the various tissues(Aisen et al. (1980) Ann. Rev. Biochem. 49:357-393; MacGillivray et al. (1981) J. Biol. Chem. 258:3543-3553). The uptake of iron by individual cells is mediated by the transferrin receptor, an integral membrane glycoprotein consisting of two identical 95,000 dalton subunits that are linked by a disulfide bond. The number of receptors on the surface of a cell appears to correlate with cellular proliferation, with the highest number being on actively growing cells and the lowest being on resting and terminally differentiated cells. Jeffries et al (Nature Vol. 312 (November 1984) pp. 167-168) used monoclonal antibodies to show that brain capillary endothelial cells have a high density of transferrin receptors on their cell surface. Antibodies which can be used within this invention are reactive with a transferrin receptor. The term antibody is intended to encompass both polyclonal and monoclonal antibodies. The preferred antibody is a monoclonal antibody reactive with a transferrin receptor. The term antibody is also intended to encompass mixtures of more than one antibody reactive with a transferrin receptor (e.g., a cocktail of different types of monoclonal antibodies reactive with a transferrin receptor) . The term antibody is further intended to encompass whole antibodies, biologically functional fragments thereof, and chimeric antibodies comprising portions from more than one species, bifunctional antibodies, etc. Biologically functional antibody fragments which can be used are those fragments sufficient for binding of the antibody fragment to the transferrin receptor to occur. The antibodies, chimeric or otherwise, are not to be considered as being restricted to a specific isotype. Any of the antibody isotypes are within the present invention. For example, antibodies with identical light chains but different heavy chains are intended. In addition, mutations of certain regions of the antibodies, e.g., in the chains, are also intended. These mutations, particularly point mutations, may occur anywhere provided functionality of the antibodies as reactive with a transferrin receptor is still maintained. The chimeric antibodies can comprise portions derived from two different species (e.g., human constant region and murine variable or binding region) . The portions derived from two different species can be joined together chemically by conventional techniques or can be prepared as single contiguous proteins using genetic engineering techniques. DNA encoding the proteins of both the light chain and heavy chain portions of the chimeric antibody can be expressed as contiguous proteins. One genetic engineering approach that can be used to produce or clone chimeric antibodies reactive with a transferrin receptor is to prime the DNAs encoding the variable region of functional antibodies for amplification by PCR using specific oligonucleotides. The variable region of functional antibodies is that portion of the antibody that immunologically reacts with the transferrin receptor antigen. Both the heavy chain and light chain of antibodies contribute to the variable region. Thus, the DNA encoding the variable region has two portions: a polynucleotide sequence encoding the variable region heavy chain and a polynucleotide sequence encoding the variable region light chain. The primed variable regions can then be cloned into vectors which contain the DNA encoding the constant region of antibodies. A particularly useful vector is one which contains DNA encoding the constant region of human antibodies that has been designed to also express immunoglobulin variable regions from other sources. The DNA encoding the constant region is usually from a separate source than the one whose DNA encodes the variable region. Although different animals from the same species may be the sources of the DNA encoding the variable region and the constant region, the usual situation is where the animal species are different (e.g., human constant region and murine variable region) . Following the cloning of the primed variable regions into vectors containing the constant region, chimeric antibodies can be expressed from such vectors. A general strategy that can be used to amplify immunoglobulin variable regions has been previously described (Orlandi et al. , Proc. Natl. Acad. Sci., 86: 3833-3837 (1989) ; Larrick et al>, Bio/techno1ogy, 2: 934-938 (1989) ; Gavilondo et al. , Hybridoma, 9(5) : 407-417 (1990)) . Two approaches have been used in the general strategy. In one approach, 5' primers are designed to prime the first framework region of the variable region. The 3' primers are designed to prime either the J region or the constant region. Priming in the frameworks (Orlandi) takes advantage of the conserved nature of these sequences. This makes it feasible to use relatively few degenerate primers to clone the majority of the variable regions. The disadvantage of this approach is that it may introduce amino acid substitutions into the framework regions which affect antibody affinity. In the second approach (Larrick, Gavilondo) , 5' primers are designed to prime some portion of the leader sequence. The 3' primers are designed to prime either the J region or the constant region, as in the first approach. The second approach takes advantage of the relatively conserved nature of the leader sequences and uses a set of redundant oligonucleotides to prime this site. Priming in the leader sequences is generally the more powerful approach since this (leader) peptide is removed from the mature antibody molecule and variations in its sequence will have no effect on antibody affinity. Many different leader peptide sequences are effective in targeting the immature antibody molecule to the endoplasmic reticulum. This second approach is the preferred embodiment in this disclosure. The term transferrin receptor is intended to encompass the entire receptor or portions thereof. Portions of the transferrin receptor include those portions sufficient for binding of the receptor to an anti-transferrin receptor antibody to occur. Monoclonal antibodies reactive with at least a portion of the transferrin receptor can be obtained (e.g., OX-26, B3/25 (Omary et al. (1980) Nature 28i6,888-891) , T56/14 (Gatter et al. (1983) J. Clin. Path. 36 539-545; Jefferies et al. Immunology (1985) 54:333-341), OKT-9 (Sutherland et al. (1981) Proc. Natl. Acad. Sci. USA 78:4515-4519) , L5.1 (Rovera, C. (1982) Blood 59:671-678) , 5E-9 (Haynes et al. (1981) J. Immunol. 127:347-351), RI7 217 (Trowbridge et al. Proc. Natl. Acad. Sci. USA 78:3039 (1981) and T58/30 (Omary et al. cited supra)or can be produced using conventional somatic cell hybridization techniques (Kohler and Milstein (1975) Nature 256, 495-497). A crude or purified protein or peptide comprising at least a portion of the transferrin receptor can be used as the immunogen. An animal is vaccinated with the immunogen to obtain an anti-transferrin receptor antibody-producing spleen cells. The species of animal immunized will vary depending on the species of monoclonal antibody desired. The antibody produc¬ ing cell is fused with an immortalizing cell (e.g. myeloma cell) to create a hybridoma capable of secreting anti-transferrin receptor antibodies. The unfused residual antibody-producing cells and immortalizing cells are eliminated. Hybridomas producing the anti-transferrin receptor antibodies are selected using conventional techniques and the selected anti-tranferrin receptor antibody producing hybridomas are cloned and cultured. Polyclonal antibodies can be prepared by immunizing an animal with a crude or purified protein or peptide comprising at least a portion of a transferrin receptor. The animal is maintained under conditions whereby antibodies reactive with a transferrin receptor are produced. Blood is collected from the animal upon reaching a desired titer of antibodies. The serum containing the polyclonal antibodies (antisera) is separated from the other blood components. The polyclonal antibody-containing serum can optionally be further separated into fractions of particular types of antibodies (e.g. IgG, IgM) . The neuropharmaceutical agent can be linked to the antibody using standard chemical conjugation techniques. Generally, the link is made via an amine or a sulfhydryl group. The link can be a cleavable link or non-cleavable link depending upon whether the neuropharmaceutical agent is more effective when released in its native form or whether the pharmaceutical activity of the agent can be maintained while linked to the antibody. The determination of whether to use a cleavable or non-cleavable linker can be made without undue experimentation by measuring the activity of the drug in both native and linked forms or for some drugs can be determined based on known activities of the drug in both the native and linked form. For some cases involving the delivery of proteins or peptides to the brain, release of the free protein or peptide may not be necessary if the biologically active portion of the protein or peptide is uneffected by the link. As a result, antibody-protein or antibody peptide conjugates can be constructed using noncleavable linkers. Examples of such proteins or peptides include CD4, superoxide dismutase, interferon, nerve growth factor, tricosanthin, dopamine decarboxylase, somatostatin analogues and enkephalinase inhibitors. Terms such as "CD4" are used herein to include modified versions of the natural molecule, such as soluble CD4, truncated CD4, etc. Examples of non-cleavable linker systems which can be used in this invention include the carbodiimide (EDC) , the sulfhydryl-maleimide, the N-succinimidyl-3-(2-pyridyldithio) propionate (SPDP; Pharmacia) , and the periodate systems. In the carbodiimide system, a water soluble carbodiimide reacts with carboxylic acid groups on proteins and activates the carboxyl group. The carboxyl group is coupled to an amino group of the second protein. The result of this reaction is a noncleavable amide bond between two proteins. In the sulfhydryl-maleimide system, a sulfhydryl group is introduced onto an amine group of one of the proteins using a compound such as Traut's reagent. The other protein is reacted with an NHS ester (such as gamma-malei idobutyric acid NHS ester (GMBS)) to form a maleimide derivative that is reactive with sulfhydryl groups. The two modified proteins are then reacted to form a covalent linkage that is noncleavable. SPDP is a heterobifunctional crosslinking reagent that introduces thiol-reactive groups into either the monoclonal antibody or the neuropharma¬ ceutical agent. The thiol-reactive group reacts with a free sulfhydryl group forming a disulfide bond. Periodate coupling requires the presence of oligosaccharide groups on either the carrier or the protein to be delivered. If these groups are available on the protein to be delivered (as in the case of horseradish peroxidase (HRP) ) , an active aldehyde is formed on the protein to be delivered which can react with an amino group on the carrier. It is also possible to form active aldehyde groups from the carbohydrate groups present on antibody molecules. These groups can then be reacted with amino groups on the protein to be delivered generat¬ ing a stable conjugate. Alternatively, the periodate oxidized antibody can be reacted with a hydrazide derivative of a protein to be delivered which will also yield a stable conjugate. Cleavable linkers can be used to link neuro¬ pharmaceutical agents which are to be deposited in the brain or when a non-cleavable linker alters the activity of a neuropharmaceutical agent. Examples of cleavable linkers include the acid labile linkers described in copending patent application Serial No. 07/308,960 filed February 6, 1989, the contents of which are hereby incorporated by reference. Acid labile linkers include cis-aconitic acid, cis-carboxylic alkadienes, cis-carboxylic alkatrienes, and poly-maleic anhydrides. Other cleavable linkers are linkers capable of attaching to primary alcohol groups. Examples of neuropharmaceutical agents which can be linked via a cleavable link include AZT, ddl, ddc, adriamycin, amphotericin B, pyrimethamine, valproate, inethotrexate, cyclophosphamide, carboplatin and superoxide dimutase. The noncleavable linkers used generally to link proteins to the antibody can also be used to link other neuropharmaceutical agents to the antibody. The antibody-neuropharmaceutical agent conjugates can be administered orally, by subcutaneous or other injection, intravenously, intramuscularly, parenternally, transdermally, nasally or rectally. The form in which the conjugate is administered (e.g., capsule, tablet, solution, emulsion) will depend at least in part on the route by which it is administered. A therapeutically effective amount of an antibody-neuropharmaceutical agent conjugate is that amount necessary to significantly reduce or eliminate symptoms associated with a particular neurological disorder. The therapeutically effective amount will be determined on an individual basis ' and will be based, at least in part, on consideration of the individuals's size, the severity of symptoms to be treated, the result sought, the specific antibody, etc. Thus, the therapeutically effective amount can be determined by one of ordinary skill in the art employing such factors and using no more than routine experimentation. Although the description above focuses on antibodies, any protein which interacts with the extracellular domain of the transferrin receptor, including the ligand binding site, could potentially serve as a vehicle for the delivery of drugs across the blood-brain barrier. In addition to anti-transferrin receptor antibodies, this would include transferrin, the ligand which binds to the receptor, and any transferrin derivatives which retain receptor-binding activity. In fact, any ligand which binds to the transferrin receptors could potentially be employed. A procedure for producing chimeric antibodies reactive with a transferrin receptor may be performed as follows: cDNA is synthesized from mRNA purified from a small number of cells producing the antibody of interest. A PCR reaction is performed in order to obtain the antibody heavy and light chain variable regions which are then cloned and sequenced. After a second PCR reaction to modify the ends of these regions to make them compatible with the expression cassettes, they are cloned into novel expression vectors which contain human constant regions, immunoglobulin promoter and enhancers, and selection markers. In these vectors, a murine heavy chain promoter has been provided with restriction sites so that the leader sequences primed and expanded can be directly cloned into a functional promoter. Restriction sites have also been provided for the direct cloning of the 3' end of the variable region into a constant region. In the heavy chain vector, a novel restriction site has been engineered into the CHI domain of the human I heavy chain gene. VH can then be joined at this site to provide a complete heavy chain protein. For VL, a restriction site has been engineered just 3' of the splice site so that the cloned VL will then splice the kappa to produce a complete K light chain protein. The final constructs are then transfected into non-producer hybridoma cell lines as SP2/0 or P3.X63.Ag8653 and the supernatants tested for antibody production (Figure 8) . Further procedures and materials, such as expression cassettes, for producing chimeric antibodies reactive with a transferrin receptor can be found in the patent application: Attorney Docket No. 38996, filed on the same date as the present application. Such teachings of this co-filed application are herein incorporated by reference. The present invention will be illustrated by the following examples. EXAMPLE 1- In Vitro Binding of Murine Monoclonal Antibodies to Human Brain Endothelial Cells Two murine monoclonal antibodies, B3/25 and T58/30, described by Trowbridge (U.S. Patent 4,434,156 issued February 28, 1984, and Nature Vol. 294, pp. 171-173 (1981)), the contents of both are hereby incorporated by reference, which recognize the human transferrin receptor were tested for their ability to bind to human brain capillary endothelial cells. Hybridoma cell lines which produce B3/25 and T58/30 antibodies were obtained from the American Type Culture Collection (ATTC) in Rockville, Maryland, and grown in DMEM medium supplemented with 2.0 mM glutamine, 10.0 mM HEPES (pH 7.2), 100 μ non- essential amino acids and 10% heat-inactivated fetal calf serum. The hybridoma cultures were scaled-up in 225 cm 2 T-flasks for the producti.on of mi.lli.gram quantities of IgG antibody. The hybridoma super- natants were concentrated 50x using vacuum dialysis and applied to a protein-A sepharose column using the BioRad MAPS buffer system. Purified antibody was eluted from the column, dialyzed against 0.1 M sodium phosphate (pH 8.0), concentrated and stored in .aliquots at -20°C. Primary cultures of human brain endothelial cells were grown in flat-bottom 96-well plates until five days post-confluency. The cells were then fixed using 3.0% buffered formalin and the plate blocked with 1.0% bovine serum albumin (BSA) in Dulbecco's phosphate buffered saline (DPBS) . Aliquots (100 μl) of the B3/25 or T58/30 antibodies, either in the form of culture supernatants or purified protein, were then added to the wells (antibody concentrations were in the range of 1-50 μg/ml) . Antibody which had specifically bound to the fixed cells was detected using a biotin-labeled polyclonal goat-anti-mouse IgG antisera followed by a biotinylated horseradish peroxidase (HRP) /avidin mixture (Avidin Biotin Complex technique) . Positive wells were determined using a Titertek Multiscan Enzyme Linked Immunosorbent Assay (ELISA) plate reader. The results showed that both antibodies bind to human brain capillary endothelial cells with the T58/30 antibody exhibiting a higher level of binding. These same antibodies were also tested for binding to human brain capillaries using sections of human brain tissue that were fresh frozen (without fixation) , sectioned on a cryostat (section thickness was 5-20 μm) , placed on glass slides and fixed in acetone (10 minutes at room temperature) . These sections were then stored at -20°C prior to use. The slides containing the human brain sections were allowed to come to room temperature prior to use. The sections were then rehydrated in DPBS and incubated in methanol containing 0.3% H»0 to block endogenous peroxidate activity. The sections were blocked for fifteen minutes in a solution containing 0.2% non-fat dry milk and 0.2% methylmannopyrano- side. B3/25 and T58/30 antibodies, purified as discussed above, were applied to the sections at a concentration of 5-50 μg/ml and incubated at room temperature for one to two hours. Antibody that specifically bound to the tissue was detected using the Avidin-Biotin Complex (ABC) technique as described above for the ELISA assay. Staining of capillaries in the human brain sections was observed with both the B3/25 and T58/30 antibodies. The T58/30 antibody also displayed some binding to the white matter of the brain cortex. EXAMPLE 2- In-Vitro Binding of Murine Monoclonal Antibody OX-26 to Rat Transferrin Receptor The OX-26 murine antibody, which recognizes the rat transferrin receptor, has been shown in vivo to bind to brain capillary endothelial cells (Jeffries et al. , cited supra) . The murine hybridoma line which produces the OX-26 murine antibody was obtained and the hybridoma cell line was grown in RPMI 1640 medium supplemented with 2.0 mM glutamine and 10% heat-inactivated fetal calf serum. The OX-26 antibody was purified using the affinity chroma¬ tography technique described in Example 1. The purified antibody was tested i i vitro as described for the anti-human transferrin receptor antibodies in Example 1 to determine whether it would bind to brain capillaries in fresh frozen, acetone-fixed rat brain sections. The results showed that the OX-26 anti-transferrin receptor antibody did bind to capillaries in rat brain sections In vitro. EXAMPLE 3- In-Vivo Binding of OX-26 Murine Monoclonal Antibody to Rat Transferrin Receptor Dose Range The rat anti-transferrin receptor antibody OX-26 was tested jLn vivo by injecting purified antibody (purification as described in Example 1) into female Sprague-Dawley rats (100-150 g ) via the tail vein. Prior to injection, the rats were anesthetized with halothane. The samples, ranging from 2.0 mg to 0.05 mg of antibody/rat were injected into the tail vein in 400 μl aliquots. All doses were tested in duplicate animals. One hour post-injection, the animals were sacrificed and perfused through the heart with DPBS to clear the blood from the organs. Immediately after the perfusion was completed, the brain was removed and quick frozen in liquid nitrogen. The frozen brain was then sectioned (30-50 μm) on a cryostat and the sections placed on glass microscope slides. The brain sections were air dried at room temperature one to two hours before fixation in acetone (10 minutes at room temperature) . After this treatment the sections could be stored at -20°C. The OX-26 antibody was localized in the brain sections using immunohistochemistry as-described above for the in vitro experiments in Example 1. The addition of the primary antibody was unnecessary in that it is present in the brain sections. The results indicated that the OX-26 antibody binds to rat brain capillary endothelial cells and that doses of as little as 50 μg result in detectable levels of antibody in the brain using the methods described herein. Doses above 0.5 mg did not appear to show significantly more antibody binding to the endothelial cells, suggesting that the sites for antibody binding may be saturated. No specific binding to capillary endothelium was detected in the liver, kidney, heart, spleen or lung. A non-specific antibody of the same subclass as OX-26 (IgG 2a) was also tested in vivo to show that the binding of OX-26 to rat brain endothelial cells that has been observed is specific to the OX-26 antibody. 0.5 mg of the control antibody was injected per rat as described above. The results indicate that the staining pattern observed with the OX-26 antibody is specific to that antibody. Time Course After establishing that the OX-26 antibody is detectable in the rat brain capillaries after _in vivo administration, the time frame in which this binding occurred was determined. Using 0.5 mg of purified OX-26 antibody as the standard dose, brain sections taken from animals sacrificed 5 minutes, 15 minutes, 1 hour, 2 hours, 4 hours, 8 hours and 24 hours post-injection were examined for the presence of OX-26 antibody. All doses were administered in 400 μl aliquots and each time point was tested in duplicate animals. Samples were injected and the rats were processed at the various times post- injection as described above in the dose range section. The results showed that the OX-26 antibody can be detected in or on the rat brain capillary endothelial cells as early as five minutes and as late as 24 hours post-injection. At 4 and 8 hours post-injection, the staining pattern of the antibody is very punctate suggesting that the antibody has accumulated in vesicular compartments either in endothelial or perivascular cells. EXAMPLE 4- The Use of a Conjugate of OX-26 Murine Monoclonal Antibody for Tranferring Horseradish Peroxidase Across the Blood Brain Barrier Horseradish Peroxidase (HRP; 40 kD) was chosen as a compound to be delivered to the brain because it is similar in size to several therapeutic agents and it can be easily detected in the brain using an enzymatic assay. HRP was conjugated to the OX-26 antibody using a non-cleavable periodate linkage and the ability of the antibody to function as a carrier of compounds to the brain was examined. The antibody conjugate was tested _in vivo to determine if the antibody could deliver HRP to the brain. The antibody (10 mg) was first dialyzed overnight against 0.01 M sodium bicarbonate (pH 9.0). The HRP (10 mg) was dissolved in 2.5 ml deionized water, 0.1 M sodium periodate (160 μl) was added and the mixture was incubated for five minutes at room temperature. Ethylene glycol (250 μl) was added to the HRP solution followed by an additional five minute incubation. This solution was then dialyzed overnight against 1.0 mM sodium acetate buffer (pH 4.4). To the dialyzed OX-26 antibody (2.0 ml, 5.08 mg/ml) was added 200 μl of 1.0 M sodium bicarbonate buffer, pH 9.5 and 1.25 ml of the dialyzed HRP solution. This mixture was incubated in the dark for two hours followed by the addition of 100 μl of 10 mg/ml sodium borohydride. The resulting mixture was incubated two additional hours in the dark at 4°C. The protein was precipitated from the solution by the addition of an equal volume of saturated ammonium sulfate and resuspended in a minimal volume of water. Free antibody was removed from the mixture by chromatography on a concanavalin A-sepharose column (a column which binds HRP and the HRP-antibody conjugate and allows the free antibody to pass through) . The free HRP was removed by chromatography on a protein A-sepharose column which retains the antibody-HRP conjugate. The final product had an HRP/antibody ratio of 4/1. A time course experiment identical to that described in Example 3 was performed using the antibody-HRP conjugate. The antibody-HRP conjugate (0.5 mg) was injected in a 400 μl aliquot/rat. The animals were sacrificed at the various times post-injection and the brains processed as described above in Example 3. The antibody HRP conjugate was localized in the brain either by staining for antibody immunohistochemically as described in Example 1 or by directly staining the brain sections for the presence of HRP. To detect HRP, the slides were first allowed to come to room temperature before incubating in methanol for thirty minutes. The brain sections were then washed in DPBS and reacted with 3,3'-diamino benzidine (DAB), the substrate for HRP. The results showed that the OX-26 antibody HRP conjugate binds to rat brain capillary endothelial cells in a manner identical to that of the unconjugated antibody. The punctate staining 4-8 hours after injection which was seen with the antibody alone is also seen with the antibody conjugate, suggesting that the conjugate can also be going into the pericytes on the abluminal side of the blood brain barrier. Taken together, these results indicate that the OX-26 antibody can deliver a protein molecule of at least 40 KD to the brain. EXAMPLE 5- The In-Vivo Delivery of Adria ycin to the Brain by Murine Monoclonal Antibody OX-26 A non-cleavable linker system similar to that used in Example 4 , was used to couple the chemotherapeutic drug adriamycin to the OX-26 antibody. The availability of antibodies that can detect adriamycin as well as the system previously described in Example 1 for detecting the antibody carrier allowed the use of immunohistochemical tech¬ niques for monitoring the localization of the antibody carrier as well as the delivery of adriamycin to the brain. To conjugate adriamycin to the antibody, the drug (10 mg in 0.5 ml DPBS) was oxidized by the addition of 200 μl of 0.1 M sodium periodate. This mixture was incubated for one hour at room temperature in the dark. The reaction was quenched by the addition of 200 μl of ethylene glycol followed by a five minute incubation. The OX-26 antibody (5.0 mg in 0.5 ml of carbonate buffer (pH 9.5)) was added to the oxidized adriamycin and incubated at room temperature for one hour. Sodium borohydride (100 μl of 10 mg/ml) was added and the mixture was incubated for an additional two hours at room temperature. The free adriamycin was separated from the OX-26 antibody-adriamycin conjugate by chromatography on a PD-10 column. The adriamycin/OX-26 antibody ratio within the conjugate was 2/1. for this particular batch of conjugate. The effectiveness of the OX-26 antibody as a carrier for delivering adriamycin to the brain was determined by administering 0.5 mg of the antibody-adriamycin conjugate in a 400 μl aliquot per rat by injection via the tail vein. One hour post-injection, the rat was sacrificed and the brain processed as described in Example 1. All injections were performed in duplicate. As a control, 400 μg of free adriamycin in a 400 μl aliquot was also injected into a rat. Immunohistochemistry was used to detect both the carrier OX-26 antibody and the adriamycin in the rat brain sections. In the case of adriamycin, polyclonal rabbit anti-adria ycin antisera was applied to the sections followed by a biotinylated goat anti-rabbit IgG antisera. This was then followed by the addition of a biotinylated HRP/avidin mixture and enzymatic detection of HRP. The results indicate that both the OX-26 antibody and the conjugated adriamycin localized to the rat brain capillary endothelial cells after in vivo administration. There is no evidence that free adriamycin binds to brain capillary endothelial cells or enters the brain. An adriamycin-OX-26 conjugate coupled via a carbodiimide linkage was also synthesized (drug/ antibody ratio of 10/1) and tested _in vivo. The results of this experiment were essentially identical to that obtained with the periodate-linked antibody-drug conjugate. In both cases, staining for the antibody carrier was quite strong and was visualized in the capillaries in all areas of the brain. This staining was evenly distributed along the capillaries. Staining for adriamycin was less intense but again was seen in capillaries throughout the brain. Some punctate staining was observed which suggests accumulation in pericytes which lie on the brain side of the blood-brain barrier. EXAMPLE 6- ^n Vivo Delivery of Methotrexate to the Brain by Murine Monoclonal Antibody OX-26. A noncleavable carbodiimide linkage was used to couple methotrexate to the OX-26 murine monoclonal antibody. A system analogous to that described in Example 5 was used to monitor the delivery of both the methotrexate and the carrier antibody to the brain capillary endothelial cells. Methotrexate was coupled to murine monoclonal antibody OX-26 via its active ester. Briefly, 81 mg (0.178 mM) of methotrexate (Aldrich) was stirred with 21 mg (0.182 mM) of N-hydroxysuccinimide (Aldrich) in 3 ml of dimethylformamide (DMF) at 4°C. Ethyl-3-dimethylaminopropyl-carbodiimide (180 mg;EDC;0.52mM) was added to this solution and the reaction mixture was stirred overnight. The crude ester was purified from the reaction by-products by flash chromatography over silica gel.60 (Merck) using a solution of 10% methanol in chloroform as an eluant. The purified active ester fractions were pooled and concentrated to dryness. The ester was dissolved in 1 ml of DMF and stored at -20 C until use. 50 mg (50%) of active ester was recovered as determined by A__ (e _ =7200) . A solution of OX-26 containing 2.1 mg (14 nmoles) of antibody in 0.9 ml of 0.1 M phosphate (pH 8.0) was thawed to 4 C. To this stirred antibody solution was added 1.4 μL (140 nmoles) of the active ester prepared as described above. After 16 hours at 4°C, the mixture was chromatographed over Sephadex PD-10 column (Pharmacia) using phosphate buffered saline (PBS) to separate conjugate from free drug. The fractions containing the antibody-methotrexate conjugate were pooled. Antibody and drug concentration were determined spectrophotometrically as described by Endo et al. (Cancer Research (1988) 48:3330-3335) . The final conjugate contained 7 methotrexates/antibody. The ability of the OX-26 monoclonal antibody to deliver methotrexate to the rat brain capillary endothelial cells was tested _in vivo by injecting 0.2 mg of conjugate (in 400 μl) into each of two rats via the tail vein. The animals were sacrificed one hour post-injection and the brains processed for immunohistochemistry as described in Example 1. To detect methotrexate in the brain, a rabbit antisera raised against methotrexate was used as the primary antibody. A biotinylated goat-anti-rabbit antisera in conjunction with a mixture of biotinylated HRP and avidin was then used to visualize methotrexate in the rat brain. The carrier antibody was detected as described previously. The results of these experiments indicate that methotrexate in the form of a conjugate with OX-26 does accumulate along or in the capillary endothelial cells of the brain. The staining observed for methotrexate is comparable in intensity to that seen for the carrier. The staining appears to be in all areas of the brain and is evenly distributed along the capillaries. EXAMPLE 7- Antibody Derivatives The Fc portion of the OX-26 murine monoclonal antibody was removed to determine whether this would alter its localization to or uptake by the rat brain capillary endothelial cells. F(ab) fragments of OX-26 were produced from intact IgG's via digestion with pepsin. A kit available from Pierce Chemical Co. contains the reagents and protocols for cleaving the antibody to obtain the fragments . The F(ab')„ fragment (0.2 mg doses) in 400 μl aliquots were injected into rats via the tail vein. A time course experiment identical to that done with the intact antibody (Example 2) was then performed. F(ab')_ fragment was detected immunohistochemically using a goat anti-mouse F(ab')_ antisera followed by a biotinylated rabbit anti-goat IgG antisera. A biotinylated HRP/avidin mixture was added and the antibody complex was visualized using an HRP enzymatic assay. The results indicate that the F(ab) fragment of the OX-26 antibody binds to the capillary endothelial cells of the rat brain. EXAMPLE 8 - Measurement of OX-26 in Brain Tissue To quantitate the amount of OX-26 which accumulates in the brain, radioactively-labelled antibody was injected into rats via the tail vein. Antibodies were labelled with either 14C-acetic 3 anhydride or H-succinimidyl proprionate essentially as described in Kummer, U. , Methods in Enzymology, 121: 670-678 (1986), Mondelaro, R.C., and Ruec ert, R.R. , J. of Biological Chemistry, 250: 1413-1421 (1975) , hereby incorporated by reference. For all experiments, the radiolabelled compounds were injected as a 400 μl bolus into the tail vein of female Sprague-Dawley rats (100-125 gms) under Halothane anesthesia and the animals were sacrificed at the appropriate time post-injection using a lethal dose of anesthetic. A 3H-labelled IgC2a control antibody was co- jected with the 14C-labelled OX-26 to serve as a control for non-specific radioactivity in the brain due to residual blood. At the appropriate time post-injection, animals were sacrificed and the brains were removed immedicately and homogenized in 5 ml of 0.5% sodium dodecysulfate using an Omni-mixer. An aliquot of the homogenate was incubated overnight with 2 ml of Soluene 350 tissue solubilizer prior to liquid scintillation counting. All data were collected as disintegrations per minute (dpm) . Blood samples were centrifuged to pellet red blood cells (which do not display significant binding of radiolabelled materials) and the radioactivity in an aliquot of serum determined using liquid scintillation counting. The amount of antibody associated with the brain was determined at various times post-injection to examine the pharmacokinetics of brain uptake. In addition, the amount of labelled antibody in the blood was measured so that the rate of clearance from the bloodstream could be determined. This information was also used to calculate the amount of radioactivity in the brain due to blood contamination, which was then subtracted from the total to give the amount of antibody that is specifically associated with the brain. A peak level of 14C-labelled OX-26 corresponding to approximately 0.9% of the injected dose was reached in the brain between 1 and 4 hours post-injection as illustrated in Figure 1 (with the values shown as means plus or minus standard error of measurement (SEM) and N=3 rats per time point) . The amount of radioactivity associated with the brain decreased steadily from 4 to 48 hours post-injection, at which point it leveled off at approximately 0.3% of the injected dose. The accumulation of OX-26 in the brain was significantly reduced by the addition of unlabelled monoclonal antibody (0.5 or 2.0 mg in the bolus injection) . As an additional control, a 3 H-IgG2a .control anti.body was co-mj.ected wi.th the 14 C-OX-26. The control antibody did not accumulate in the brain and represented the blood contamination of the brain. In contrast to the levels in the brain, the blood level of OX-26 dropped quite dramatically immediately after injection such that by 1 hour post—injection, the percent of injected dose in 55 μl of blood (the volume of blood associated with the brain) was approximately 0.16% as illustrated in Figure 1. This corresponds to a value of approximately 20% of the injected dose in the total blood volume of the rat. Extraction of total IgG from serum followed by polyacrylamide gel electro- phoresis (PAGE) and autoradiography did not reveal detectable levels of OX-26 degradation indicating that the antibody remains intact in the blood as long as 48 hours after injection. EXAMPLE 9 - Distribution of OX-26 in Brain Parenchyma and Capillaries To demonstrate that anti-transferrin receptor antibody accumulates in the brain parenchyma, homogenates of brains taken from animals injected with labelled OX-26 were depleted of capillaries by centrifugation through dextran to yield a brain tissue supernatant and a capillary pellet. Capillary depletion experiments followed the procedure of Triguero, et al. , J. of Neurochemistry, 54 : 1882-1888 (1990), hereby incorporated by reference. As for the brain uptake experiments of Example 8, the radiolabelled compounds were injected as a 400 μl bolus into the tail vein of femals Sprague-Dawley rats (100-125 gm) under Halothane anesthesia and the animals were sacrificed at the appropriate time post-injection using a lethal dose of anesthetic. A 3H-labelled IgG 2a control antibody was co-injected with the 14C-labelled OX-26 to serve as a control for non-specific radioactivity in the brain due to residual blood. After sacrifice, the brains were removed and kept on ice. After an initial mincing, the brains were homogenized by hand (8-10 strokes) in 3.5 ml of ice cold physiologic buffer (100 mM NaCl, 4.7 mM KCl, 2.5 M CaCl , 1.2 mM KH PO , 1.2 mM MgS0 4 , 14.5 mM HEPES, 10 mM D-glucose, pH 7.4) . Four ml of 26% dextran solution in buffer was added and homogenization was continued (3 strokes) . After removing an aliquot of the homogenate, the remainder was spun at 7200 rpm in a swinging bucket rotor. The resulting supernatant was carefully removed- from the capillary pellet. The entire capillary pellet and aliquots of of the homogenate and supernatant were incubated overnight with 2 ml of Soluene 350 prior to liquid scintillation counting. This method removes greater than 90% of the vasculature from the brain homogenate (Triguero et al. , cited supra) . A comparison of the relative amounts of radioactivity in the different brain fractions as a function of time indicates whether transcytosis of the labelled antibody has occurred. The amount of 0X-26 in total brain homogenate, the brain parenchyma fraction and the brain capillary fraction at an early time (30 minutes) and a later time (24 hours) post-injection is illustrated in Figure 2. The values in Figure 2 are shown as means±SEM with N=3 rats per time point. At the 30 minute time point, more of the radiolabelled antibody is associated with the capillary fraction than with the brain parenchyma fraction (0.36% of the injected dose (%ID) and 0.23% ID, respectively) . By 24 hours post-injection, the distribution is reversed and the majority of the radioactivity (0.36% ID) is in the parenchymal fraction as compared to the capillary fraction (0.12% ID) . The redistribution of the radiolabelled OX-26 from the capillary fraction to the parenchyma fraction is consistent with the time dependent migration of the anti-transferrin receptor antibody across the blood-brain barrier. EXAMPLE 10 - Distribution of an OX-26-methotrexate Conjugate in Brain Parenchyma and Capillaries Capillary depletion studies following the procedures described in Example 9 were performed with an OX-26-methotrexate (MTX) conjugate linked via a gamma-hydrazid as described in Kralovec, et al. , J. of Medicinal Chem. , 32 : 2426-2431 (1989) , hereby incorporated by reference, in which the MTX moiety 3 was labelled with H. As with unconjugated antibody, the amount of label in the capillary fraction at 30 minutes post-injection is greater than the parenchyma fraction (approximately 2-fold as illustrated in Figure 3, with the data expressed as eans±SEM and N=3 rats per time point) . This distribution changes over time such that by 24 hours post-injection, approximately 4.5-fold more of the labelled MTX is in the brain parenchyma than in the capillaries. These results are consistent to those obtained with unconjugated antibody and, again, suggest that these compounds cross the blood-brain barrier. To ensure that these results were not due to contaminating amounts of free 3H-MTX or 3H-MTX that had been cleaved from the conjugate after injection, a co-mix of labelled drug and antibody was injected into rats and a capillary depletion experiment performed. The amount of 3H-MTX m the different brain fraction is significantly lower for the co-mix as compared to the conjugate (as much as 47 fold in the case of the capillary fraction at 30 minutes post—injection as illustrated in Figure 3) . The 3 H-MTX and the co-mix also does not show the change in distribution of the label between the different brain fractions over time as was seen with the antibody-MTX conjugate or antibody alone. These 3 results demonstrate that delivery of H-MTX across the blood-brain barrier to the brain parenchyma is greatly enhanced by the conjugation of the drug to the anti-transferrin receptor antibody OX-26. EXAMPLE 11 - Distribution of 0X-26-AZT in Brain Parenchyma and Capillaries Capillary depletion studies following the procedures of Example 9 were performed with an 0X-26-AZT conjugate using a pH-sensitive succinate linker. These studies employed a dual-labelled conjugate m which the AZT was 14C-labelled and the 3 antibody carrier was H-labelled. The use of such a conjugate allowed independent monitoring of the disposition of both the antibody and AZT within the brain. The linker was synthesized as follows. Succinic anhydride was used to acylate the AZT by reacting equimolar amounts of these two compounds for 3 hours at room temperature under argon in the presence of dimethylaminopyridine and sodium bisulfate in freshly distilled pyridine. The product was isolated by chromatography on a DEAE sephadex A50 column run with a triethylammonium bicarbonate buffer. The succinate derivative of AZT was activated at the carboxyl group as the NHS ester by reaction with equimolar amounts of N-hydroxysuccinimide and dicyclohexylcarbodiimide (DCC) in freshly distilled THF at 4°C for 2 hours. The product was purified by flash charomatography on silica gel. The resulting NHS-ester of AZT-succinate was used to acylate amine groups on OX-26, resulting in an AZT-OX-26 conjugate. A 15-fold molar excess of AZT-NHS ester was reacted with OX-26 in HEPES buffer overnight at 4°C. The antibody-drug conjugate was isolated from free drug on a PD-10 column. The molar ratio of drug to antibody was 7:1. These studies employed a dual-labelled conjugate in which the AZT ww;as 14C-labelled and the antibody carrier was 3, H-labelled. Similar levels of OX-26 and AZT are seen in the capillary fraction of the brain and these levels decrease with time, suggesting that the materials are not being retained by the capillary endothelial cells as illustrated in Figure 4c. As the levels of OX-26 in the capillary fraction decrease, the levels in the parenchyma fraction increase, indicating that the antibody is migrating from the capillaries to the parenchyma in a time-dependent manner as illustrated in Figure 4b. In contrast, the levels of AZT in the brain parenchyma do not rise significantly, suggesting that the majority of the drug is released in the endothelial cells and is not transported across the blood-brain barrier. The levels of OX-26 and AZT remained similar in unfractionated homogenates over time as illustrated in Figure 4a. The data in Figure 4 are expressed as means±SEM with N=3 rats per time point. These results indicate that the linker is cleaved within the endothelial cells and may represent a method for delivering compounds to those cells. EXAMPLE 12 - Distribution of OX-26-Horseradish Peroxidase (HRP) in Brain Perenchyma and Capillaries Capillary depletion studies following the procedures described for OX-26 in Example 9 were 3 performed with a H-labelled OX-26-HRP conjugate that was prepared using a non-cleavable periodate linkage as described in Example 4. The tritium label was distributed between the antibody and-the HRP portion of the conjugate. At 1 hour post-injection, the majority of the radioactivity associated with the brain is in the capillary fraction as illustrated in Figure 5. The data in Figure 5 are expressed as means±SEM with N=3 rats per time point. By 4 hours post-injection, the distribution of radioactivity associated with the brain changed such that the majority is in the fraction which represents the brain parenchyma. At 24 hours post-injection, essentially all of the 3 H-labelled OX-26-HRP conjugate is in the parenchyma fraction of the brain indicating that the material has crossed the blood-brain barrier. Similar results were obtained in experiments in which only the HRP portion of the conjugate was radiolabelled. The percent of injected dose of the OX-26-HRP conjugate that reaches the brain is somewhat lower than that for antibody alone or the OX-26-HRP conjugate. This is most likely due to the presence of 2 to 3 40 kD HRP molecules attached to each carrier and that these "passenger" molecules are randomly attached to the carrier. Due to this, many of the HRP passengers may be attached to the antibody in such a way as to interfere with antigen recognition. This problem can be alleviated by directing the attachment of the passenger to regions of the carrier removed from critical functional domains. EXAMPLE 13 - Distribution of OX-26-CD4 in Brain Parenchyma and Capillaries A soluble form of CD4, consisting of amino acids 1-368, was conjugated to OX-26 using a linkage that directed the attachment of the CD4 to the carbohydrate groups located in the Fc portion of the antibody. By directing the site of attachment in this way, the chance that the passenger molecules will interfere with antibody-antigen recongition is lessened. The linkage between the proteins was achieved by first introducing a sulfhydryl group onto CD4 using SATA (N-Succinimidyl S-acetylthioacetate) , a commerically available compound. A hydrizid derivative of SDPD, another commercial cross-linking agent, was attached to OX-26 via carbohydrate groups on the antibody. Reaction of the two modified proteins gives rise to a disulfide-linked conjugate. More specificallyk the linkage between the proteins was achieved by first introducing a sulfhydryl group onto CD4 using N-succinimidyl S-acetylthioacetate (SATA) , a commercially available compound. A 4-fold molar excess of SATA was added to 5 mg of CD4 in 0.1 M sodium phosphate buffer containing 3 mM EDTA (pH 7.5) . This mixture was reacted at room temperature in the dark for 30 minutes. Unreacted starting materials were removed by passage over a PD-10 column. A hydrizid derivative of SPDP, another commercially available cross-linking agent, was attached to OX-26 via carbohydrate groups on the antibody. Ten milligrams of OX-26 in 2.0 ml of 0.1 M sodium acetate, 0.15 M sodium chloride (pH 5.0) was reacted with a 1000-fold molar excess of sodium periodate for 1 hour at 4°C in the dark. Unreacted starting materials were removed by passage over a PD-10 column. The oxidized antibody was reacted with a 30-fold molar excess of hydrazido-SPDP overnight at 4°C with stirring. Reaction of the two modified proteins gives rise to a disulfide-linked conjugate. One tenth volume of 0.5 M hydroxylamine was added to the thioacetylated CD4 (CD4-DATA) and derivatized antibody was then added such that the ratio of CD4 to antibody was 7.5:1. This mixture was reacted at room temperature in the dark for 2 hours. Conjugate was purified by running the reaction mixture over a protein A column followed by a CD4 affinity column. Capillary depletion experiments following the procedures described in Example 9 with OX-26 were performed with an OX-26-CD4 conjugate in which only 3 the CD4 portion was H-labelled. Time dependent changes in the distribution of the labelled conjugate between the capillary and parenchyma fractions of the brain which are consistent with transcytosis across the blood-brain barrier were observed as illustrated in Figure 6. The data in Figure 6 are expressed as means±SEM with N=3 rats per time point. EXAMPLE 14 - Biodistribution and Brain Uptake of Anti-Human Transferrin Receptor Antibodies in Cynomolgous Monkeys A collection of 32 murine monoclonal antibodies which recognize various epitopes on the human transferrin receptor were examined for reactivity with brain capillary endothelial cells in sections from human, monkey (cynomolgous) , rat and rabbit brain samples by the immunohistochemical methods described in Example 1. These antibodies were obtained from Dr. Ian Trowbridge of the Salk Institute, LaJolla, CA. All 32 antibodies displayed some reactivity with human brain endothelial cells. Two antibodies reacted very weakly with rabbit brain capillaries and none reacted with rat. While 21 of the antibodies reacted with monkey brain capillaries, only 2 displayed strong reactivity comparable to that seen with human brain capillaries. These 2 antibodies are herewithin referred to as 128.1 and Z35.2. These antibodies were used to determine the tissue distribution and blood clearance of the 14 C-labelled anti-human transferrin receptor antibodies 128.1 and Z35.2 in 2 male cynomolgous monkeys. 128.1 or Z35.2 was administered 3 concurrently with a H-labelled control IgG to one of the monkeys with an intravenous catheter. During the course of the study, blood samples were collected to determine the clearance of the antibodies from the circulation. At 24 hours post-injection, the animals were euthanized and selected organs and representa¬ tive tissues were collected for the determination of isotope distribution and clearance by combustion. In addition,, samples from different regions of the brain were processed as described for the capillary deple¬ tion experiments in Example 9 to determine whether the antibodies had crossed the blood-brain barrier. The results of the capillary depletion experiments were performed on samples from the cortex, frontal cortex, cerebellum and striatum. All samples had greater than 90% of the 128.1 or Z35.2 in the brain parenchyma, suggesting that the antibodies crossed the blood-brain barrier. The levels of the control antibody in the same samples were from 5 to 10-fold lower. Using the average brain homogenate value for dpm/G tissue, the percent injected dose of 128.1 in the whole brain is approximately 0.2-0.3%. This compares to a value of 0.3-0.5% for OX-26 in the rat at 24 hours post-injection. A comparison of the ratios of 128.1 to the control antibody for various organs is illustrated in Figure 7. Similar results were obtained for Z35.2. These results suggest that 128.1 is preferentially taken up by the brain as compared to control antibody. For the majority of organs and tissues tested, the ratio of 128.1 to control is less than 2. EXAMPLE 15 - Cloning and Expressing of ALK 128.1: An Anti-Human Transferrin Receptor Chimeric Antibody RNA EXTRACTION: RNA was extracted following the single step guanidinium/phenol method (P.Chomczynski and S. Sacchi. 1987, Anal. Bioch. 162:156-259) . All the instruments and containers used were previously autoclaved and rinsed with diethyl pyrocarbonate (depc) treated water to avoid degradation due to RNAases. Several samples each contai .ni.ng 5x105 cells from the 128.1 hybridoma which secretes a murine anti human transferrin receptor monoclonal antibody, were washed twice with PBS. The pellets were quick frozen in liquid nitrogen and either kept at -70°C for later use or extracted immediately. For the extraction, in a RNase free microfuge tube, 1/2 ml of solution D (Solution D:36 μl 2-mercaptoethanol per 5.ml of IX GITC [lx GITC: 250 g guanidinium thiocyanate, 17.6 ml 0.75 M Na citrate pH7, 26.4 ml 10% sarcosyl, 293 ml dH20]) , 50 μl of 2M Na acetate pH 4, 0.5 ml phenol (dH20 equilibrated) and 100 μl of chloroform:isoamylalcohol (49:1) were •added to the cell pellet mixing by inversion after each addition. The extraction was left on ice for 15 minutes and centrifuged at 13000g for 20 min at 4°C. The upper aqueous phase containing the RNA was removed to a new tube and precipitated with 2 volumes of cold absolute ethanol for 2 hr. at -70°C. After two 70% depc-ethanol washes the RNA pellet was dried briefly and resuspended in dH20 0.5% SDS. FIRST STRAND cDNA SYNTHESIS 5 Total RNA from 5x10 cells was resuspended in 18 μ.1 of 0.5% SDS. 9μl of RNA were annealed with 2μ.l of 3' primer (lmg/ml) at 60°C for 10 minutes. For light chain V region amplifications, an oligo dT primer was used, whereas for the amplification of heavy chain V regions a 7 CHI antisense primer, containing an Xbal site (underlined in Table 1) , with degeneracies introduced so that it will prime all isotypes of murine heavy chains except 7 3 was used (Table 1) . After annealing, the samples were cooled on ice, 4 μl of first strand cDNA buffer (50 mM Tris pH 8.3, 50 mM KCl, 10 mM MgCl 2 , 1 mM DTT, 1 M EDTA, 0.5 mM spermidine) , 1 μl of RNAse inhibitor (Promega) , 2 μl of 10 mM dNTP's and 2 μl of prediluted 1:10 Promega AMV Reverse Transcriptase were added and the reaction incubated for 1 hour at 42°C. The cDNA was kept at -20°C until used for PCR. Table 1: PRIMERS FOR cDNA SYNTHESIS PRIMER FOR SYNTHESIS OF LIGHT CHAIN V REGION cDNA OLIGO dT.Rl.XBA.H3 5 • GCCGGAATTCTAGAAGC( ) 17 PRIMER FOR SYNTHESIS OF HEAVY CHAIN V REGION cDNA MγC.CHI AS (Degeneracies at a single position are shown in parenthesis. ) 5 « AGG TCTAGA A(CT)C TCC ACA CAC AGG (AG) (AG)C CAG TGG ATA GAC -48- PRIMERS AND PCR REACTION: A first PCR reaction was performed in order to amplify the variable regions and determine their sequence. To achieve this the PCR primers were designed to hybridize to the leader sequence (5' primer) and to the constant region immediately downstream of the V-J region (3' primer). The oligonucleotides were synthesized in an Applied Biosystem 391 DNA Synthesizer, eluted without purification, diluted to 20 μM and kept at 4°C. All primers were designed with a restriction site with three additional bases upstream to protect the site and facilitate enzyme digestion. The sites were chosen to make possible the cloning of the PCR product into a subcloning vector and into the final expression cassett vectors. For the leader region, the primers contain a ribosome recognition site (Kozak's sequence CACC; Kozak M. 1981, Nucl. Acid. Res., 9:20, 5233-5252) 5' of the start codon, and an EcoR V site (underlined in Tables 2 and 3) protected by three 5' G's. A set of 4 universal 5' sense primers was used simultaneously in the light variable region amplification, and a set of 3 universal 5' sense primers in the case of heavy variable regions (Coloma et al. 1991, Biotechniques 11,2,152-156). An equimolar amount of each primer was used in the PCR reaction. These primers contain degeneracies in order to hybridize with all the families of murine leader sequences reported in Rabat's database. (Kabat E. 1987, Sequences of Proteins of Immunological Interest, NIH) . The 3' primers were designed in the constant region 20 bases downstream of the V-J region and contain an Xbal site (underlined in Tables 2 and 3) for subcloning purposes (Tables 2 and 3) . Table 2: PRIMERS FOR MURINE HEAVY CHAIN VARIABLE REGION AMPLIFICATION. (Degeneracies at a single position are shown in parenthesis.) LEADER REGION PRIMERS (5 « SENSE) MHALTI.RV #085 Leader Murine Heavy IgV 5' GGG GATATC CACC ATG G(AG)A TG(CG) AGC TG(TG) GT(CA) AT(CG) CTC TT MHALT2.RV #086 Leader Murine Heavy IgV 5' GGG GATATC CACC ATG (AG)AC TTC GGG (TC)TG AGC T(TG)G GTT TT MHALT3-RV #087 Leader Murine Heavy IgV 5 1 GGG GATATC CACC ATG GCT GTC TTG GGG CTG CTC TTC T CONSTANT REGION PRIMER (3•ANTISENSE) Primer designed to hybridize at aminoacids 130-120 in CHI of Igγ. This primer is identical to the primer used for heavy chain first strand cDNA synthesis. MCγ CH1AS.XBA #097 CHI antisense primer for murine Igy, except Igγ3 5' AGG TCTAGA A(CT)C TCC ACA CAC AGG (AG) (AG)C CAG TGG ATA GAC Table 3: PRIMERS FOR MURINE LIGHT CHAIN VARIABLE REGION AMPLIFICATION. (Degeneracies at a single position are shown in parenthesis.) LEADER REGION PRIMERS (5*SENSE) MLALT1.RV #088 Leader Murine Light IgV 5 1 GGG GATATC CACC ATG GAG ACA GAC ACA CTC CTG CTA T MLALT2.RV #089 Leader Murine Light IgV 5' GGG GATATC CACC ATG GAT TTT CAA GTG CAG ATT TTC AG MLALT3.RV #090 Leader Murine Light IgV 5' GGG GATATC CACC ATG GAG (TA)CA CA(GT) (TA)CT CAG GTC TTT (GA)TA MLALT4.RV #091 Leader Murine Light IgV 5' GGG GATATC CACC ATG (GT)CC CC(AT) (GA)CT CAG (CT)T(CT) CT(TG) GT CONSTANT REGION PRIMER (3'ANTISENSE) Primer designed to hybridize to amino acids 122-116 of kappa constant region. MCK AS.XBA #096 Constant Murine Light 5 1 GCG TCTAGA ACT GGA TGG TGG GAA GAT GGA The primers for the second PCR reaction (Table 4) have the actual sequence of the V-J regions, determined by sequencing of the subcloned products (Figure 9) . These primers have a Nhe I site in the case of the VH primer and Sal I for the VL primer, which permits the cloning into the expression vectors. (The restriction enzyme sites are underlined in Table 4) . The Nhe I site in the 3' primer for the VH allows the direct ligation of the VH-J region to the first two amino acids of the CHI of the I constant region. The VL 3' primer has a donor splice sequence before its Sal I site which is necessary to splice the VL to C K in the expression vector. Table 4: PRIMERS FOR 128.1 V-J REGION MODIFICATION BY SECOND PCR PRIOR TO THE CLONING INTO EXPRESSION VECTORS HEAVY CHAIN PRIMER (3'ANTISENSE) : Primer designed to hybridize to amino acids 111-113 in J4 region of 128.1 heavy chain V region. It includes a Nhe I site for cloning into the expression vector (links J4 to CHI) and Sal I for subcloning (upstream Nhe I) . ALKJ4 AS.NHE.SAL1 #098 Antisense of VHJ4 + γl CHI 5' TGG GTCGAC AGA TGG GGG TGT TGT GCTAGC TGA GGA GAC LIGHT CHAIN PRIMER (3'ANTISENSE) : Primer designed to hybridize to amino acids 101-107 in J4 region of 128.1 light chain V region. It includes a donor splicing sequence which is highlighted. ALKK-J4AS.SAL1 #101 Antisense of VL J4 + splicing donor 5 1 AGC GTCGAC TTACG TCT GAT TTC CAG CCT GGT CCCT PCR reactions were performed in a volume of 100 μl with the following final conditions: 2μl of cDNA, 0.5 μl Taq polymerase (Cetus Corporation) , IX buffer (10 mM Tris pH8, 1.5 mM MgCl , 50 mM KCl, 100 μg BSA) , 200μM each dNTP, lμM of each primer and 50 μl of mineral oil. PCR was carried out for 30 cycles in a PTC 100 Thermal Controller (M.J. Research Inc.) with 1 min. denaturing (94°C) , l min. annealing (55°C) , 1.5 min. extension (72°C) , and a final extension of 10 min. The size of the PCR products was verified by agarose gel electrophoresis in a 2% TAE gel stained with ethidium bromide. The correct products were approximately 380 base pairs for the light chain and 420 base pairs for the heavy chain variable region. SUBCLONING AND SEQUENCING: After the PCR reaction the oil was removed by chloroform extraction and the samples kept at 4°C. For subcloning, the products were either directly cloned into Bluescript KS T-A (blunt ended by digestion at EcoR V site and tailed with dideoxythymidine triphosphate using terminal transferase) prepared following the procedure by Holton (T.A. Holton and M.W. Graham. 1990 Nucl. Acid. Res., 19:5, 1156), or gel isolated, cut with the appropriate restriction enzymes (EcoR V and Sal I) and cloned into Bluescript KS previously cut with the same enzymes. For TA cloning 3 μl of the PCR product was directly ligated with 50 ng of T-A vector in a 15 μl reaction for 4-12 hours at 16°C. For sticky end ligations 200 ng of cut Bluescript was ligated with 200-400 ng of cut product in 20 μl ligation reactions. 5 μl of the ligation was used for transformation of E. Coli. XLl-blue (Stratagene) competent cells prepared by calcium chloride treatment. White colonies, containing inserts were picked above a blue colony background. Miniprep DNA was restriction digested, analyzed and the apparently correct clones sequenced. Dideoxynucleotide chain termination sequencing was carried out using T7 DNA polymerase (Pharmacia, Uppsala, Sweden or Sequenase, US Biochemical Corp., Cleveland, Ohio) according to the manufacturer's protocol. Four independent clones from different PCR reactions were sequenced in both directions, to obtain the concensus sequence. The obtained sequences were compared against other murine sequences in Genbank and aligned with reported V regions in Kabat's database to identify their family and conserved amino acids. (See Tables 5 and 6. ) Ta b le 5 : CO M P LETE SEQUENCE OF CHIMERIC 128.1 (Anti-Human Transferrin Receptor) LIGHT CHAIN VARIABLE REGION, MOU S E KAPPA SUBGROUP VI -22 LEADER ATG GAT TTT CAA GTG CAG ATT Met Asp Phe Gin Val Gin He TTC AGC TTC CTG CTA ATC AGT GCC TCA GTC ATA CTG TCC AGA Phe Ser Phe Leu Leu He Ser Ala Ser Val He Leu Ser Arg -1 1 FR1 GGA CAA ATT GTT CTC ACC CAG TCT CCA GCA ATC ATG TCT Gly Gin He VAL LEU Thr GLN 8ER PRO ALA ILE Met Ser FR1 24 CDR1 GCA TCT CCA GGG GAG AAG GTC ACC ATG ACC TGC AGT GCC AGC ALA SER Pro GLY Glu LYS VAL THR Met THR CYS Ser ALA SER 27-29 * CDR1 35 FR2 TCA AGT ATA GAT TAC ATT CAC TGG TAC CAG CAG AAG TCA GGC SER SER He Asp TYR He His TRP Tyr GLN GLN LYS Ser Gly FR2 50 CDR2 ACC TCC CCC AAA AGA TGG ATT TAT GAC ACA TCC AAA CTG GCT Thr SER PRO LYS Arg Trp ILE TYR Asp Thr SER Lys LEU Ala 57 PR3 TCT GGA GTC CCT GCT CGC TTC AGT GGC AGT GGG TCT GGG ACC SER GLY VAL FRO Ala ARG PHE SER GLY 8ER GLY 8ER GLY Thr FR3 TCT TAT TCT CTC ACA ATC AGC AGC ATG GAG CCT GAA GAT GCT Ser Tyr Ser LEU Thr ILE Ser Ser Met GLU Pro GLU ASP Ala 89 CDR3 97 GCC ACT TAT TAC TGC CAT CAG CGG AAT AGT TAC CCA TGG ACG ALA THR TYR TYR CYS His GLN Arg Lys Ser Tyr Pro Trp THR 98 FR4 * 107 CONST. TTC GGT GGA GGG ACC AGG CTG GAA ATC AGA —> CGG GCT PHE GLY GLY GLY THR Arg LEU GLU He ARG —> ARG ALA j* Conserved amino acids are capitalized and bold. * NOTE: Amino acid # 30 is a conserved Val and amino acid # 103 and #107 a conserved Lys in 98% of the sequences reported in Kabat's database for this family. Table 6: COMPLETE SEQUENCE OF CHIMERIC 128.1 (Anti-Human Transferrin Receptor) HEAVY CHAIN VARIABLE REGION. MOUSE GAMMA SUBGROUP IIB. -19 LEADER ATG GAA TGG AGC TGG GTA Met Glu Trp Ser Trp Val LEADER -1 ATG CTC TTC CTC CTG TCA GGA ACT GCA GGT GTC CGC TCT — Met Leu Phe LEU Leu Ser Gly Thr Ala Gly Val Arg Ser 1 FR1 GAG GTC CAG CTG CAA CAG TCT GGA CCT GAA CTG GTG AAG CCT Glu VAL GLN LEU Gin GLN Ser GLY Pro Glu LEU VAL Lys PRO *18 FR1 GGA GCT TCA ATG AAG ATT TCC TGC AAG GCT TCT GGT TAC TCA GLY Ala SER Met LYS He SER CYS LYS ALA SER GLY TYR Ser 31 CDR1 36 FR2 TTC ACT GGC TAC ACC ATG AAC TGG GTG AAG CAG AGC CAT GGA Phe Thr Gly Tyr Thr Met Asn TRP VAL Lys GLN Ser His Gly FR2 50 52—a- 53 CDR2 GAG AAC CTT GAG TGG ATT GGA CGT ATT AAT CCT CAC AAT GGT Glu Asn Leu Glu Trp He Gly Arg He Asn PRO His Asn Gly CDR2 66 *68 GGT ACT GAC TAC AAC CAG AAG TTC AAG GAC AAG GCC CCT TTA Gly Thr Asp TYR Asn Gin LYS PHE Lys Asp LYS Ala Pro LEU FR3 82—a- ACT GTA GAC AAG TCA TCC AAC ACA GCC TAC ATG GAG CTC CTC THR Val Asp Lys SER Ser Asn THR Ala TYR Met Glu LEU Leu 82b-C- 83 FR3 AGT CTG ACA TCT GAG GAC TCT GCA GTC TAT TAC TGT GCA AGA Ser Leu THR 8ER GLU ASP Ser ALA Val TYR Tyr CYS Ala Arg 95 CDR3 100—a- 103 FR4 GGC TAC TAT TAC TAT TCT TTG GAC TAC TGG GGT CAA GGA ACC Gly Tyr Tyr Tyr Tyr Ser Leu Asp Tyr TRP GLY Gin GLY THR FR4 113 CHI TCA GTC ACC GTC TCC TCA —> GCC AAA Ser Val THR VAL SER Ser —> Ala Lys J4 — Conserved amino acids are capitalized and bold. Amino acid # 18 is a conserved Val and amino acid # 68 a conserved Thr in 98% of the sequences reported in Kabat's database for this family. The final clones were named pBKS4600 for the VH region and pBKS4601 for the VL region. CLONING INTO EXPRESSION VECTORS: Plasmid pAH4274 is the vector for expression of heavy chain variable regions obtained by PCR with leader/J region priming. V region cloning into this cassette is performed by a complete digestion of vector and product with EcoR V and Nhe I. This vector has a human 7 I constant region whose CHI is directly linked with the 3' end of the VH-J region by means of a Nhe I site. This 11 kb vector contains an ampicillin resistance gene for procaryotic selection, a heavy chain immunoglobulin enhancer and a histidine (histidinol) selection marker for selection of transfectants (Hartman, S., R. Mulligan, Proc. Natl. Acad. Sci. 85, 8047-8051) ; transcription is from the VH promoter of the murine 27.44 gene. The 400 bp. EcoR V-Nhe I fragment (VH of 128.1) from pBKS4600 was used to replace the EcoR V-Nhe I fragment in plasmid pAH 4274. HB101 competent cells were transformed and plated on LB plates with 50 μg/ml of ampicillin. Colonies were screened by colony hybridization with a 32P end labelled leader region oligonucleotide. Positive clones were restriction mapped and maxi plasmid preps prepared using the QIAGEN maxi prep kit (QIAGEN Inc., Studio City, California) . The final expression vector with the VH of 128.1 joined to human 7 I constant region was named pAH4602 (Figure 10) . The coding sequence for this expression vector is given in Figure 11. Plasmid pAG4270 is the expression vector for light chain variable regions obtained by PCR with leader/J region priming. The 14 kb vector has an ampicillin resistance gene, a gpt (mycophenolic acid resistance) selected marker, an immunoglobulin H enhancer and an introl for V-Constant region splicing; transcription is from the murine VH promoter from the 27.44 gene. Due to the presence of an EcoR V within the gpt gene in the vector, the cloning of the anti-transferrin receptor VL was performed in two steps to avoid inefficient partial digestions. The 380 bp EcoR V - Sal I fragment (VL) from pBKS4601 was cloned into pBR460x (6.9 kb) , a subcloning vector with the VH promoter, previously cut with the same enzymes. The resulting construct (pBR4608) was then cut with Pvu I - Sal I and the 4 kb fragment containing the promoter, the V region and part of the ampicillin resistance gene was ligated to the 9.7 kb Pvu I - Sal I fragment of pSV4271 an intermediate vector which lacks the promoter. HB101 competent cells were transformed and positives screened by colony hybridization and restriction digestion. Maxipreps were prepared as described above. The final expression vector was named pAG4611 (Figure 12) . The coding sequence of this expression vector is shown in Figure 13. TRANSFECTION AND SELECTION: Ten μg of maxiprep DNA from each final expression vector was linearized by BSPC1 (Stratagene, Pvu I isochizomer) digestion and 1x10 7 SP2/0 cells were cotransfected by electroporation. Prior to transfection the cells were washed with cold PBS, then resuspended in 0.9 ml of the same cold buffer and placed in a 0.4 cm electrode gap electroporation cuvette (Bio-Rad) with the DNA. For the electrical pulse, the Gene Pulser from Bio-Rad (Bio-Rad, Richmond, California) was set at a capacitance of 960 μF and 200 V. After the pulse the cells were incubated on ice for 10 minutes then washed once in IMDM with 10% calf serum and resuspended in IMDM with 10% calf serum at a 5 concentration of 10 cells/ml. The transfected cells were plated into five 96 4 well plates at a concentration of 10 cells/well. Selection was started after 48 hours. Two plates were selected with 5 mM histidinol (heavy chain selection) , 2 plates were selected with 1 μl/ml mycophenolic acid (light chain selection) and 1 plate was selected with histidinol and mycophenolic acid (heavy and light chain selection) . Twelve days post selection supernatants were screened by ELISA to test for the secretion of both chains. Immulon II 96 well plates were coated with 5 μg/ml of goat anti human γl in carbonate buffer at pH9.6, and blocked with 3% BSA. Supernatants from the transfectants were added and the plates were incubated overnight at 4°C. After washing, plates were developed with goat anti-human k conjugated with alkaline phosphatase and wells secreting H and L chains identified (Table 7). Table 7: RESULTS OF TRANSFECTIONS Results of cotransfection with vectors pAH4602 and pAG4611 in SP2/0 cells. 2 plates were selected with 5mM histidinol (HIS) , 2 plates with lμg/ml mycophenolic acid (HXM) and 1 plate selected with both (HIS+HXM) . Wells containing clones were analyzed by ELISA to determine those containing secreted antibody (# positive wells) . SELECTION High producers were expanded for further analysis; selected transfectants were subcloned. ANTIBODY ANALYSIS: To determine the nature of the protein being produced, transfectants were biosynthetically 35 labelled with S methionine, cytoplasmic and secreted antibodies immunoprecipitated with rabbit anti-human Ig and protein-A and the immunoprecipi- tates fractioned on SDS polyacrylamide gels. Clones with the highest production identified by ELISA were expanded to 5 ml petri dishes and removed from selection. 1x10 cells were pelleted at 220xg for 5 minutes at 4°C and washed twice with labelling medium (high glucose DME deficient in methionine: GIBCO) . Cells were finally resuspended in 1 ml . . 35 labeling medium containing 25 μCi S-Methionine (Amersham Corp.) and allowed to incorporate label for 3 hours at 37°C under tissue culture atmospheric conditions. Cells were pelleted and supernatants drawn off for immunoprecipitation of secreted IgG. Cell pellets were lysed in NDET (1% NP-40, 0.4% deoxycholate, 66 mM EDTA, 10 mM Tris, pH 7.4), centrifuged, and the supernatants removed and incubated 1 hour at 4°C with rabbit anti-human IgG Fc polyclonal antiserum (5 μl/ml) . To the labelled supernatants, 100 μl/ml of protein A (10% in NDET, IgG Sorb) was added and mixed by rotation at 4°C for 15 minutes. Protein-A bound IgG was washed by centrifuging through 1 ml 30% sucrose in 100 μl NDET + 0.3% SDS. The protein A pellet was then resuspended in 100 μl NDET/3% SDS, transferred to a 1.5 ml polypropylene tube with 100 μl of the same buffer, and the previous tube rinsed with 100 μl. The 300 μl suspension was centrifuged and washed with deionized water. Finally, the protein A pellet was resuspended in 50 μl of loading buffer (25 mMTris pH 6.7, 0.2% SDS, 10% glycerol, 8% μg/100 ml bromophenol blue) and boiled for two minutes prior to gel loading. Antibodies were analyzed by SDS-PAGE (5% acrylamide gels, 0.1% sodium phosphate buffered) to confirm proper assembly of H and L chains. In addition, a portion of the labelled sample was reduced by treatment with 0.15 M 2-mercaptoethanol, 37°C for 1 hour and analyzed on 12% acrylamide gels to confirm the size of the unassembled H and L chains. The gels were stained, dried and exposed for autoradiograms. The resultant autoradiograms revealed the expected patterns for fully functional antibodies. The secreted antibodies that were in the cell supernatant exhibited the expected molecular weight pattern of free light chain, light chain dimer and the tetramer formed from two light chains and two heavy chains for fully expressed and assembled functional antibodies. The pattern for antibody parts in the cell cytoplasm was also as expected for fully expressed antibody constitutents. EXAMPLE 16 - Further Mouse/Human Chimeras of the Anti-Human Transferrin Receptor Antibody 128.1. As described in Example 15, the initial cloning of the gene encoding the heavy chain of the murine monoclonal antibody 128.1, which binds the human transferrin receptor, involved placing the sequences encoding the variable region of the heavy chain into an expression vector containing the human 71 constant region framework. This created a mouse/human chimera in which the sequences encoding the variable region of the antibody heavy chain (VH) were derived from a murine source and the sequences encoding CH , CH and CH_ were derived from a human source. Because the different human gamma isotypes ( 7 -1, -2, -3 and -4) have different biological properties, it was necessary to create chimeric antibodies with constant region sequences from each isotype in order to obtain mouse/human chimeras for each of these isotypes. The production of these chimeras was accomplished by cloning the 400 bp Eco RV-Nhe l fragment containing the VH region of antibody 128.1 from plasmid pBSK4600 into expression vectors containing the 7 -2, 7 -3 and 7 -4 constant regions in a fashion similar to that previously described in Example 15 for the cloning of the VH region of antibody 128.1 into the expression vector containing the 7 -1 constant region. These clonings with the 7 -2, 7 -3 and 7 -4 constant regions resulted in respective plasmids pAH4625, pAH4807 and pAH4808 whose plasmid maps are shown in Figure 14, Figure 15 and Figure 16, respectively. The antibody coding sequences of the heavy chain expression vectors pAH4625, pAH4807 and pAH4808 are shown in Figure 17, Figure 18 and Figure 19, respectively. These vectors, in combination with the chimeric light chain vector pAG4611, were transfected into SP2/0 cells and clones selected as described in Example 15. Initial antibody analysis using biosynthetically labeled proteins, immunoprecipi- tation and SDS-PAGE as previously described gave rise to the appropriate bands for the heavy and light chains as well as the assembled antibody for the 7 -3 and 7-4 chimeras. No detectable protein was made by the 7-2 transfectants. EXAMPLE 17 - Antibody Production by Transfectants Antibody production by selected transfectants was assessed by ELISA. Cells were diluted in fresh medium to a density of 10 cells/ml and 1 ml was aliquoted into each of 3 wells on a 24-well culture plate. The plates were then incubated for 24 hours at 37°C with 5% CO . The media was then collected from the wells and the cells and debris were spun down to give a clarified supernatant. For the ELISA, a 96-well microtiter dish was coated with a goat antisera against human IgG. After blocking with 3% BSA, the plate was washed and a series of dilutions of both the cell supernatants and human IgG standard of known concentration were applied to the plate and incubated for 1 hour at room temperature. The plate was then washed and biotinylated goat antisera against human IgG was added, followed by a mixture of avidin and biotinylated horseradish peroxidase (HRP) . The amount of antibody present in the samples was then determined, based on the amount of substrate converted by the HRP. Three clones resulting from the 7 -1 chimera transfection were tested for antibody production. The average values from three experiments were 39, 21 and 24 μg/ml IgG/10 cells/24 hours, respectively, for the different clones. One 7 -3 clone has been tested and it was found to produce approximately 1 μg/ml IgG/10 cells/24 hours. Two different clones of the 7 -4 chimera have been tested and were found to produce 2.8 and 0.2 ng/ml IgG/10 cells/24 hours, respectively. Equivalents Those skilled in the art will know, or be able to ascertain using no more than routine experimenta¬ tion, many equivalents to the specific embodiments expressly described herein. These are intended to be within the scope of the invention as described by the claims herein.

Claims: CLAIMS 1. An antibody-neuropharmaceutical or diagnostic agent conjugate for the manufacture of a medicament for delivering the neuropharmaceutical or diagnostic agent across the blood-brain barrier to the brain of a host whereby the antibody binds to a transferrin receptor present on brain capillary endothelial cells and the neuropharmaceutical or diagnostic agent is transferred across the blood-brain barrier in a pharmaceutically active form, wherein the antibody is a chimeric antibody that is reactive with said transferrin receptor. 2. An antibody-neuropharmaceutical or diagnostic agent conjugate according to Claim 1 wherein the chimeric antibody is a chimera between the variable region of a murine antibody and the constant region of a separate antibody. 3. An antibody-neuropharmaceutical or diagnostic agent conjugate according to Claim 2 wherein the constant region is of an animal source other than murine. 4. An antibody-neuropharmaceutical or diagnostic agent conjugate according to Claim 3 wherein the animal source is human. 102 -68- 5. An antibody-neuropharmaceutical or diagnostic agent conjugate according to Claim 2 wherein the variable region is from a monoclonal antibody produced by the 128.1 hybridoma. 6. An antibody-neuropharmaceutical or diagnostic agent conjugate according to Claim 4 wherein the variable region is from a monoclonal antibody produced by the 128.1 hybridoma. 7. An antibody-neuropharmaceutical or diagnostic agent conjugate according to Claim 6 wherein the constant region is from plasmids pAH4274 and PAG4270. 8.. An antibody-neuropharmaceutical. or diagnostic agent conjugate according to Claim 6 wherein the constant region is from plasmids pAH4625 and pAG4270. 9. An antibody-neuropharmaceutical or diagnostic agent conjugate according to Claim 6 wherein the constant region is from plasmids pAH4807 and pAG4270. 10. An antibody-neuropharmaceutical or diagnostic agent conjugate according to Claim 6 wherein the constant region is from plasmids pAH4808 and pAG4270. 11. A delivery system for delivering a neuropharma¬ ceutical or diagnostic agent across the blood brain barrier comprising a chimeric antibody, reactive with a transferrin receptor present on brain capillary endothelial cells, linked to a neuropharmaceutical or diagnostic agent, whereby the delivery system transports the neuropharma¬ ceutical or diagnostic agent across the blood brain barrier when administered in vivo. 12. A delivery system according to Claim 11 wherein the chimeric antibody is a chimera between the variable region of a murine antibody and the constant region of a separate antibody. 13. A delivery system according to Claim 12 wherein the constant region is of an animal source other than murine. 14. A delivery system according to Claim 13 wherein the animal source is human. 15. A delivery system according to Claim 12 wherein the variable region is from a monoclonal antibody produced by the 128.1 hybridoma. 16. A delivery system according to Claim 14 wherein the variable region is from a monoclonal antibody produced by the 128.1 hybridoma. -70- 17. A delivery system according to Claim 16 wherein the constant region is from plasmids pAH4274 and pAG4270. 18. A delivery system according to Claim 16 wherein the constant region is from plasmids pAH4625 and pAG4270. 19. A delivery system according to Claim 16 wherein the constant region is from plasmids pAH4807 and pAG4270. 20. A delivery system according to Claim 16 wherein the constant region is from plasmids pAH4808 and pAG4270. 21. A chimeric antibody comprising a variable region reactive with a transferrin receptor present on brain capillary endothelial cells and a constant region of a separate antibody. 22. A chimeric antibody of Claim 21 wherein the variable region is of murine origin. 23. A chimeric antibody of Claim 22 wherein the constant region is of an animal source other than murine. 24. A chimeric antibody of Claim 23 wherein the animal source is human.

Inventors: FRIDEN PHILLIP M, FRIDEN, PHILLIP, M.
Applicants: ALKERMES INC, ALKERMES, INC.
Application year: 1992
Main classification: A61K47
IPC codes: A61K47, C07K16