The functional effects of biotinylation of anti-angiotensin-converting enzyme monoclonal antibody in terms of targeting in vivo

Pharmacokinetic and Pharmacodynamic Properties of Drug Delivery Systems

The use of drug delivery systems (DDS) is an attractive approach to facilitate uptake of therapeutic agents at the desired site of action, particularly when free drug has poor pharmacokinetics/biodistribution (PK/BD) or significant off-site toxicities. Successful translation of DDS into the clinic is dependent on a thorough understanding of the in vivo behavior of the carrier, which has, for the most part, been an elusive goal. This is, at least in part, due to significant differences in the mechanisms controlling pharmacokinetics for classic drugs and DDSs. In this review, we summarize the key physiologic mechanisms controlling the in vivo behavior of DDS, compare and contrast this with classic drugs, and describe engineering strategies designed to improve DDS PK/BD. In addition, we describe quantitative approaches that could be useful for describing PK/BD of DDS, as well as critical steps between tissue uptake and pharmacologic effect.

Vascular targeting of nanocarriers: perplexing aspects of the seemingly straightforward paradigm

Targeted nanomedicine holds promise to find clinical use in many medical areas. Endothelial cells that line the luminal surface of blood vessels represent a key target for treatment of inflammation, ischemia, thrombosis, stroke, and other neurological, cardiovascular, pulmonary, and oncological conditions. In other cases, the endothelium is a barrier for tissue penetration or a victim of adverse effects. Several endothelial surface markers including peptidases (e.g., ACE, APP, and APN) and adhesion molecules (e.g., ICAM-1 and PECAM) have been identified as key targets. Binding of nanocarriers to these molecules enables drug targeting and subsequent penetration into or across the endothelium, offering therapeutic effects that are unattainable by their nontargeted counterparts. We analyze diverse aspects of endothelial nanomedicine including (i) circulation and targeting of carriers with diverse geometries, (ii) multivalent interactions of carrier with endothelium, (iii) anchoring to multiple determinants, (iv) accessibility of binding sites and cellular response to their engagement, (v) role of cell phenotype and microenvironment in targeting, (vi) optimization of targeting by lowering carrier avidity, (vii) endocytosis of multivalent carriers via molecules not implicated in internalization of their ligands, and (viii) modulation of cellular uptake and trafficking by selection of specific epitopes on the target determinant, carrier geometry, and hydrodynamic factors. Refinement of these aspects and improving our understanding of vascular biology and pathology is likely to enable the clinical translation of vascular endothelial targeting of nanocarriers.

Characterization of neutralizing affinity-matured human respiratory syncytial virus F binding antibodies in the sub-picomolar affinity range

In the human adaptation and optimization of a mouse anti-human respiratory syncytial virus neutralizing antibody, affinity assessment was crucial to distinguish among potential candidates and to evaluate whether this correlated with function in vitro and in vivo. This affinity assessment was complicated by the trimeric nature of the antigen target, respiratory syncytial virus F (RSV-F) glycoprotein. In the initial affinity screen, surface plasmon resonance was used to determine the intrinsic binding affinities of anti-RSV-F Fab and immunoglobulin G (IgG) to the extracellular domain of RSV-F. This assessment required minimal biotinylation of the RSV-F protein and design of a capture strategy to minimize avidity effects. Approximately 30 Fabs were selected from three optimization phage display libraries on the basis of an initial ELISA screen. Surface plasmon resonance analysis demonstrated the success of optimization with some candidates from the screened libraries having low picomolar dissociation constants, more than 700-fold tighter than the parental monoclonal antibody (B21M). The affinities of these antibodies were further evaluated by a kinetic exclusion assay, a solution binding technology. One IgG (monoclonal antibody 029) displayed a low picomolar K(D) comparable with that of motavizumab, an RSV antibody in clinical study. Kinetic exclusion assay showed that two other of the matured IgGs (011 and 019) had sub-picomolar dissociation constants that could not be resolved further. We discuss the relevance of these interaction analysis results in the light of recently published data on the mechanism of F-driven viral fusion during paramyxoviral infection and 101F epitope conservation revealed from the recent crystal structure of RSV-F in the post-fusion state.

Biotinylation of heat shock protein 70 induces RANTES production in HEK293 cells in a CD40-independent pathway

Biotinylated proteins and peptides have been used as popular ligands for characterization of cell surface receptors by a variety of methods including flow cytometry. The number and the location of biotin moieties incorporated could alter the structural and physicochemical properties of ligands, although biotin is thought to be such a small molecule (244Da) that it is capable of being conjugated to most proteins without affecting their activity. Here, we demonstrate that the biotinylated HSP70 molecule via primary amines bound to epithelium-like HEK 293 cells in a saturable manner whereas the unlabeled counterparts of HSP70 other than mouse Hsp72 do not. This binding was not competed by either HSP70 or the biotin entity itself. Interestingly, the biotinylated HSP70 also elicited the production of CC-chemokine RANTES independent of CD40 signaling. This response occurred regardless of sequence diversity of HSP70 derived from different species, and neither the biotinylated ovalbumin nor the unlabeled HSP70 cross-linked with a biotinylated protein stimulated a significant level of RANTES production which was induced by biotinylated HSP70 itself. Our findings suggest that modification of HSP70 such as biotinylation may function as a biological alarm signal in the innate immune system.

Pharmacokinetic characteristics and biodistribution of radioiodinated chimeric TNT-1, -2, and -3 monoclonal antibodies after chemical modification with biotin

To improve the clinical potential of monoclonal antibodies (MAbs), new methods are required to augment antibody uptake in the tumor while minimizing binding in normal tissues. Our laboratory has pioneered the use of chemical modification to accomplish this goal. Using three chimeric MAbs, chTNT-1, chTNT-2, and chTNT-3, which target solid tumors by binding to common antigens found in the central necrotic core, we now demonstrate the potential of chemical modification to improve the pharmacokinetic characteristics of these unique MAbs. To identify optimal modification conditions, TNT MAbs were reacted with biotin at various ratios and tested by clearance and biodistribution analyses. The biodistribution results revealed that the numbers of biotin molecules per MAb yielding optimal tumor uptake were 3:1 for chTNT-1, 5:1 for chTNT-2, and 8:1 for chTNT-3. Biotinylated MAbs were found to have faster whole body clearance times and better biodistribution profiles compared to unmodified antibodies. Although chTNT-2 showed only a modest improvement after biotinylation, biodistribution results indicated that this MAb had the highest uptake in tumor. By reducing the charge of the antibody molecule, chemical modification appears to be a useful method for improving the pharmacokinetics and biodistribution of TNT antibodies directed to the necrotic region of solid tumors.

Quality control of coated antibodies: new, rapid determination of binding affinity

A procedure is described for the determination of the affinity constant between a fluid-phase biotinylated antigen and a solid-phase monoclonal antibody. This procedure allows evaluation of the efficiency of an antibody as a coated tool for an immunoassay. For this purpose, the biotinylation of the antigen and its further quantitative measurement by streptavidin-peroxidase led to a single reversible interaction, the binding affinity of which greatly determines the quality of the assay. The free and bound fractions of the biotinylated antigen were obtained in wells coated with a low level of immobilized antibodies. At the equilibrium state, the free antigen present in the supernatant of these wells was further transferred to high level antibody coated wells which captured all the free antigen molecules. These molecules were quantified using a standard curve established with known concentrations of biotinylated antigen, also incubated in wells coated with the high level of antibody.

Immunotargeting of glucose oxidase: intracellular production of H(2)O(2) and endothelial oxidative stress

Extracellular and intracellular reactive oxygen species attack different targets and may, therefore, result in different forms of oxidative stress. To specifically study an oxidative stress induced by a regulated intracellular flux of a defined reactive oxygen species in endothelium, we used immunotargeting of the H(2)O(2)-generating enzyme glucose oxidase (GOX) conjugated with an antibody to platelet-endothelial cell adhesion molecule (PECAM)-1, an endothelial surface antigen. Anti-PECAM-(125)I-GOX conjugates specifically bind to both endothelial and PECAM-transfected cells. Approximately 70% of cell-bound anti-PECAM-(125)I-GOX was internalized. The cell-bound conjugate was enzymatically active and generated H(2)O(2) from glucose. Use of the fluorescent dye dihydrorhodamine 123 revealed that 70% of H(2)O(2) was generated intracellularly, whereas 30% of H(2)O(2) was detected in the cell medium. Catalase added to the cells eliminated H(2)O(2) in the medium but had little effect on the intracellular generation of H(2)O(2) by anti-PECAM-GOX. Both H(2)O(2) added exogenously to the cell medium (extracellular H(2)O(2)) and that generated by anti-PECAM-GOX caused oxidative stress manifested by time- and dose-dependent irreversible plasma membrane damage. Inactivation of cellular catalase by aminotriazole treatment augmented damage caused by either extracellular H(2)O(2) or anti-PECAM-GOX. Catalase added to the medium protected either normal or aminotriazole-treated cells against extracellular H(2)O(2), yet failed to protect cells against injury induced by anti-PECAM-GOX. Therefore, treatment of PECAM-positive cells with anti-PECAM-GOX leads to conjugate internalization, predominantly intracellular H(2)O(2) generation and intracellular oxidative stress. These results indicate that anti-PECAM-GOX 1) provides cell-specific intracellular delivery of an active enzyme and 2) causes intracellular oxidative stress in PECAM-positive cells.

Streptavidin facilitates internalization and pulmonary targeting of an anti-endothelial cell antibody (platelet-endothelial cell adhesion molecule 1): a strategy for vascular immunotargeting of drugs

Conjugation of drugs with antibodies to surface endothelial antigens is a potential strategy for drug delivery to endothelium. We studied antibodies to platelet-endothelial adhesion molecule 1 (PECAM-1, a stably expressed endothelial antigen) as carriers for vascular immunotargeting. Although 125I-labeled anti-PECAM bound to endothelial cells in culture, the antibody was poorly internalized by the cells and accumulated poorly after intravenous administration in mice and rats. However, conjugation of biotinylated anti-PECAM (b-anti-PECAM) with streptavidin (SA) markedly stimulated uptake and internalization of anti-PECAM by endothelial cells and by cells expressing PECAM. In addition, conjugation with streptavidin markedly stimulated uptake of 125I-labeled b-anti-PECAM in perfused rat lungs and in the lungs of intact animals after either intravenous or intraarterial injection. The antioxidant enzyme catalase conjugated with b-anti-PECAM/SA bound to endothelial cells in culture, entered the cells, escaped intracellular degradation, and protected the cells against H2O2-induced injury. Anti-PECAM/SA/125I-catalase accumulated in the lungs after intravenous injection or in the perfused rat lungs and protected these lungs against H2O2-induced injury. Thus, modification of a poor carrier antibody with biotin and SA provides an approach for facilitation of antibody-mediated drug targeting. Anti-PECAM/SA is a promising candidate for vascular immunotargeting of bioactive drugs.

Albumin-binding surfaces: synthesis and characterization

The nature of the proteinaceous film deposited on a biomaterial surface following implantation is a key determinant of the subsequent biological response. To achieve selectivity in the formation of this film, monoclonal antibodies have been coupled to a range of solid substrates using avidin-biotin technology. Antibody clones varied in their antigen-binding activity following insertion of biotin groups into lysine residues. Biotinylated antibodies coupled to solid substrates via an immobilized avidin bridge retained their biological activity. During immobilization of avidin a significant proportion of the protein molecules were passively adsorbed rather than covalently attached to the surface. This loosely bound material could be removed by stringent elution procedures which resulted in a surface density of 5.4 pmol avidin cm(-2). Although these conditions would be harsh enough to denature monoclonal antibodies, they did not destroy the biotin-binding activity of the residual surface-coupled avidin, enabling the subsequent immobilization of biotinylated antibodies. The two-step immobilization technique allowed the use of gentle protein modification procedures, reduced the risk of surface-induced denaturation and removed loosely bound material from the surface. The versatility of the technique encourages its application to a wide range of immobilization systems where retention of biological activity is a key requirement.

Improving tumour targeting and decreasing normal tissue uptake by optimizing the stoichiometry of a two-step biotinylated-antibody/streptavidin-based targeting strategy: studies in a nude mouse xenograft model

We aimed to study in detail the in vivo stoichiometry of the individual elements of the 2-step streptavidin based approach to tumour targeting, in a nude mouse xenograft model, by the administration of a first step consisting of biotinylated anti-tumour specific antibody and a second step consisting of streptavidin. This process was undertaken to identify the optimum conditions for radiotherapeutic tumour targeting using this approach. Antibody was biotinylated to various degrees (1-25 biotins per antibody). Protein stoichiometry of the 2 steps was studied over a range of 2 logs. Both steps, i.e., the biotinylated-antibody (1st step) and streptavidin (2nd step) were radiolabelled (125I and 131I, respectively). A 24-hr interval between 1st and 2nd step was studied, animals being killed 24 hr after the 2nd step. Streptavidin excess led to a decrease in levels of monobiotinylated-antibody in the circulation and in the tumour. Biotinylated-antibody excess led to an increase in circulating levels of streptavidin, a decrease in renal uptake of streptavidin and increased targeting of streptavidin to tumour. At a constant protein molar ratio of biotinylated antibody to streptavidin of 10:1, increasing biotinylation density resulted in an increase in circulating levels, increase in tumour uptake, decrease in renal uptake and increase in liver uptake of streptavidin. As early as 24 hr, the tumour-to-blood ratios of streptavidin already exceeded 1 (max 1.27). Compared with antibody tumour-to-blood ratios, they were better by a factor of between 2 and 3. Tumour-to-normal tissue ratios of radiolabelled streptavidin (with the exception of liver) were also significantly improved when polybiotinylated-antibody was administered first. We have thus shown that the 2-step biotinylated antibody/streptavidin approach can lead to a significant increase in absolute amounts of activity in the tumour under appropriate stoichiometric conditions. This was accompanied by high levels of circulating streptavidin and relatively favourable tumour-to-blood and normal tissue ratios of streptavidin.

Immunotargeting of catalase to ACE or ICAM-1 protects perfused rat lungs against oxidative stress

The pulmonary endothelium is susceptible to oxidative insults. Catalase conjugated with monoclonal antibodies (MAbs) against endothelial surface antigens, angiotensin-converting enzyme (MAb 9B9) or intercellular adhesion molecule-1 (MAb 1A29), accumulates in the lungs after systemic injection in rats (V. Muzykantov, E. Atochina, H. Ischiropoulos, S. Danilov, and A. Fisher. Proc. Natl. Acad. Sci. USA 93: 5213-5218, 1996). The present study characterizes the augmentation of antioxidant defense by these antibody-catalase conjugates in isolated rat lungs perfused for 1 h with catalase conjugated with either MAb 9B9, MAb 1A29, or control mouse IgG. Approximately 20% of the injected dose of Ab-125I-catalase accumulated in the perfused rat lungs (vs. <5% for IgG-125I-catalase). After elimination of nonbound material, the lungs were perfused further for 1 h with 5 mM hydrogen peroxide (H2O2). H2O2 induced an elevation in tracheal and pulmonary arterial pressures (126 +/- 7 and 132 +/- 5%, respectively, of the control level), lung wet-to-dry weight ratio (7.1 +/- 0.4 vs. 6.0 +/- 0.01 in the control lungs), and ACE release into the perfusate (436 +/- 20 vs. 75 +/- 7 mU in the control perfusates). Both MAb 9B9-catalase and MAb 1A29-catalase significantly attenuated the H2O2-induced elevation in 1) angiotensin-converting enzyme release to the perfusate (215 +/- 14 and 217 +/- 38 mU, respectively), 2) lung wet-to-dry ratio (6.25 +/- 0.1 and 6.3 +/- 0.3, respectively), 3) tracheal pressure (94 +/- 4 and 101 +/- 4%, respectively, of the control level), and 4) pulmonary arterial pressure (103 +/- 3 and 104 +/- 7%, respectively, of the control level). Nonconjugated catalase, nonconjugated antibodies, nonspecific IgG, and IgG-catalase conjugate had no protective effect, thus confirming the specificity of the effect of MAb-catalase. These results support a strategy of catalase immunotargeting for protection against pulmonary oxidative injury.

Biotinylation of proteins via amino groups can induce binding to U937 cells, HL-60 cells, monocytes and granulocytes

The use of biotinylated ligands for the flow cytometric detection of cell surface receptors has become a popular alternative to radioreceptor assays. Although the biotinylation of a protein is a relatively mild chemical reaction several reports have mentioned the fact that the number and location of biotin moieties coupled to amino groups of a protein can alter its physicochemical properties and impair biological activity. In the present study we show for a variety of biotinylated functionally unaltered ligands that biotinylation by N-hydroxysuccinimide (NHS) esters of biotin can induce a binding to cell surfaces, which is not specific for the respective unlabelled ligand. C1q, C1 inhibitor (C1-INH), alpha 1-antitrypsin (AT), ovalbumin (OV), transferrin and soybean trypsin inhibitor (STI) were labelled with S-NHS-LC-biotin and activated C1s (C1s) with NHS-biotin. Biotinylation of C1q, C1s and C1-INH exerted negligible effects on biological function, antigenicity or electrophoretic mobility but when labelled and unlabelled proteins were assayed for binding to monocytic U937 cells, promyelocytic HL-60 cells, monocytes and granulocytes, a remarkable binding was observed for biotinylated C1q, C1-INH and C1s. In contrast, no binding was observed when we used unlabelled C1q, C1s and C1-INH and employed specific antibodies, alpha-mouse-FITC or alpha-rabbit-FITC for detection. Increasing molar ratios of biotin-to-protein (B : P) for biotinylated AT, OV and STI evoked increased fluorescence intensities of the cells. Most importantly the unlabelled ligands did not compete for cell binding with their biotinylated derivatives, with the exception of transferrin. Preincubation of the cells with an excess of free d-biotin did not reduce binding of biotinylated proteins, thus excluding a potential involvement of biotin receptors. Hydrophobic interaction chromatography revealed a remarkable increase in hydrophobicity of the biotinylated proteins compared to their unlabelled counterparts, suggesting that the biotinylation-induced binding is due to increased hydrophobicity. Our findings indicate that biotinylation by the common amino acid esterification method may be critical for proteins if they are to be used as ligands for receptor binding studies.

Immunotargeting of antioxidant enzyme to the pulmonary endothelium

Oxidative injury to the pulmonary endothelium has pathological significance for a spectrum of diseases. Administration of antioxidant enzymes, superoxide dismutase (SOD) and catalase (Cat), has been proposed as a method to protect endothelium. However, neither these enzymes nor their derivatives possess specific affinity to endothelium and do not accumulate in the lung. Previously we have described a monoclonal antibody to angiotensin-converting enzyme (ACE) that accumulates selectively in the lung after systemic injection in rats, hamsters, cats, monkeys, and humans. In the present work we describe a system for selective intrapulmonary delivery of CuZn-SOD and Cat conjugated with biotinylated anti-ACE antibody mAb 9B9 (b-mAb 9B9) by a streptavidin (SA)-biotin bridge. Both enzymes biotinylated with biotin ester at biotin/enzyme ratio 20 retain enzymatic activity and bind SA without loss of activity. We have constructed tri-molecular heteropolymer complexes consisting of b-mAb 9B9, SA, and biotinylated SOD or biotinylated Cat and have studied biodistribution and pulmonary uptake of these complexes in the rat after i.v. injection. Biodistribution of biotinylated enzymes was similar to that of nonmodified enzymes. Binding of SA markedly prolonged lifetime of biotinylated enzymes in the circulation. In contrast to enzymes conjugated with nonspecific IgG, other enzyme derivatives, and nonmodified enzymes, biotinylated enzymes conjugated with b-mAb 9B9 accumulated specifically in the rat lung (9% of injected SOD/g of lung tissue and 7.5% of injected Cat/g of lung tissue). Pulmonary uptake of nonmodified enzymes or derivatives with nonspecific IgG did not exceed 0.5% of injected dose/g. Both SOD and Cat conjugated with b-mAb 9B9 were retained in the rat lung for at least several hours. Trichloracetic acid-precipitable radiolabeled Cat was associated with microsomal and plasma membrane fractions of the lung tissue homogenate. Thus, modification of antioxidant enzymes with biotin and SA-mediated conjugation with b-mAb 9B9 prolongs the circulation of enzymes resulting in selective accumulation in the lung and intracellular delivery of enzymes to the pulmonary endothelium. These results provide the background for an approach to provide protection of pulmonary endothelium against oxidative insults.