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Originally published In Press as doi:10.1074/jbc.M709179200 on January 22, 2008

J. Biol. Chem., Vol. 283, Issue 12, 7804-7812, March 21, 2008
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N-Glycosylation as Novel Strategy to Improve Pharmacokinetic Properties of Bispecific Single-chain Diabodies*

Roland Stork{ddagger}, Kirstin A. Zettlitz{ddagger}1, Dafne Müller{ddagger}, Miriam Rether{ddagger}, Franz-Georg Hanisch§, and Roland E. Kontermann{ddagger}2

From the {ddagger}Institut für Zellbiologie und Immunologie, Universität Stuttgart, Allmandring 31, 70569 Stuttgart, Germany and the §Institut für Biochemie II, Medizinische Fakultät, Zentrum für Molekulare Medizin Köln, Universität Köln, Joseph-Stelzmann-Strasse 52, 50931 Köln, Germany

Received for publication, November 8, 2007 , and in revised form, January 9, 2008.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 REFERENCES
 
The therapeutic efficacy of recombinant antibodies such as single-chain Fv fragments and small bispecific or bifunctional molecules is often limited by rapid elimination from the circulation because of their small size. Here, we have investigated the effects of N-glycosylation on the activity and pharmacokinetics of a small bispecific single-chain diabody (scDb CEACD3) developed for the retargeting of cytotoxic T cells to CEA-expressing tumor cells. We could show that the introduction of N-glycosylation sequons into the flanking linker and a C-terminal extension results in the production of N-glycosylated molecules after expression in transfected HEK293 cells. N-Glycosylated scDb variants possessing 3, 6, or 9 N-glycosylation sites, respectively, retained antigen binding activity and bispecificity for target and effector cells as shown in a target cell-dependent IL-2 release assay, although activity was reduced ~3–5-fold compared with the unmodified scDb. All N-glycosylated scDb variants exhibited a prolonged circulation time compared with scDb, leading to a 2–3-fold increase of the area under curve (AUC). In comparison, conjugation of a branched 40-kDa PEG chain increased AUC by a factor of 10.6, while a chimeric anti-CEA IgG1 molecule had the longest circulation time with a 17-fold increase in AUC. Thus, N-glycosylation complements the repertoire of strategies to modulate pharmacokinetics of small recombinant antibody molecules by an approach that moderately prolongs circulation time.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
 REFERENCES
 
Whole antibodies, especially chimeric, humanized or fully human IgG molecules, exhibit a long circulation time in the human body that can reach a half-life of 27 days (1, 2). In contrast, antibody fragments, e.g. Fab fragments, or recombinant formats, such as single-chain Fv fragments (scFv)3 or bispecific derivatives thereof (tandem scFv, diabodies, single-chain diabodies), are rapidly cleared from circulation (2–4). This is mainly due to the small size leading to rapid renal clearance and the lack of recycling processes mediated by the neonatal Fc receptor (FcRn). Thus, repeated injections or infusions are required to maintain a therapeutically effective dose in the body (5).

Several strategies to improve pharmacokinetic properties and thus dosing and therapeutic efficacy of recombinant antibodies have been developed in recent years. These strategies can be divided into: 1) those based on reducing renal clearance by increasing the apparent size of the therapeutic molecule, and 2) those that in addition implement FcRn-mediated recycling processes (6–8).

PEGylation of proteins is a well-established strategy to improve pharmacokinetic properties by increasing the molecular mass and the hydrodynamic radius (9–11). Several PEGylated proteins are in clinical use, e.g. PEGylated interferon {alpha}-2a (PegIntron, PEGASYS) and PEGylated granulocyte-colony stimulating factor (Neulasta), or are under clinical development, e.g. a PEGylated anti-TNF Fab fragment (certolizumab pegol) (9, 12) Furthermore, direct fusion to albumin or an albumin binding moiety, such as albumin-binding peptides or bacterial albumin-binding domains, was applied to improve pharmacokinetics of therapeutic proteins including interferon-{alpha}, interleukin-2, insulin, Fab fragments, and recombinant bispecific antibody molecules (4, 8, 13–15). This strategy is based on the observation that albumin has a similar half-life as IgG and also utilizes FcRn-mediated recycling processes. One albumin fusion protein, albumin-interferon-{alpha} (Albuferon {alpha}), is currently in clinical development (16).

The introduction of N-glycosylation sites is another approach successfully applied to improve pharmacokinetic and pharmacodynamic properties of a few therapeutic proteins (17, 18). Darbepoetin alpha (Aranesp) is a recombinant form of human erythropoietin possessing 2 additional N-glycosylation sites resulting in a hyperglycosylated protein (5 N-glycosylation sites in total) with a ~2-fold prolonged half-life and an improved in vivo activity (19). In other studies, glycosylated analogs of follicle stimulating hormone (FSH) with prolonged circulation time and increased activity have been generated by adding N-glycans through an N-terminal extension of the {alpha} subunit (20) or through a peptide linker joining the β- and the {alpha}-subunit (21), demonstrating the feasibility of this approach.

As yet, no studies have been described that successfully applied N-glycosylation to prolong the circulation time of small recombinant antibodies. Here, we performed a comparative analysis of N-glycosylated bispecific single-chain diabody (scDb) molecules with other modifications including PEGylation. N-Glycosylation at 3, 6, or 9 Asn-X-Thr sites introduced at the flanking linker sequences and C-terminal extensions prolonged half-life ~2-fold leading to a 2–3-fold increase of the AUC. In contrast, site-directed chemical coupling of a branched PEG molecule of 40 kDa to a cysteine residue introduced in one of the linker sequences prolonged circulation time ~5-fold resulting in a 11-fold increase of the AUC, similar to fusion of the scDb molecule with human serum albumin or a albumin binding domain of streptococcal protein G.


   EXPERIMENTAL PROCEDURES
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
RESULTS
 DISCUSSION
 REFERENCES
 
Materials—Horseradish peroxidase-conjugated anti-His tag antibody was purchased from Santa Cruz Biotechnology, unconjugated anti-His tag antibody from Dianova (Hamburg, Germany) and anti-mouse IgG-FITC or PE-conjugated antibody as well as goat anti-rabbit IgG-FITC-conjugated antibody from Sigma (Taufkirchen, Germany). Rabbit antiserum recognizing PEGylated scDb was produced by Pineda Antikörper-Service (Berlin, Germany). Horseradish peroxidase-conjugated anti-rabbit IgG was purchased from Roche Applied Sciences (Mannheim, Germany). Carcinoembryonic antigen was obtained from Europa Bioproducts (Cambridge, UK). Branched mPEG2-Mal was purchased from Nectar Therapeutics (Birmingham, AL). The human colon adenocarcinoma cell line LS174T was purchased from ECACC (Wiltshire, UK) and cultured in RPMI 1640 supplemented with 5% FBS. HEK293 were cultured in RPMI 1640, 5% FBS. Buffy coat from a healthy human donor was kindly provided by Prof. G. Multhoff (München, Germany). IL-2 was purchased from Immunotools (Friesoythe, Germany) and phytohemagglutinin-L (PHA-L) from Roche Applied Sciences (Germany). CD1 mice were purchased from Elevage Janvier (Le Genest St. Isle, France).

Construction of scDb Molecules—scDbCEACD3 (4) was used as starting material for the construction of scDb variants. Cysteine residues were introduced into linker A or the C terminus by PCR (scDb-A', scDb-C'). Similarly, N-glycosylation sites were introduced into the linkers A and B as well as at the C terminus (scDbABC1, scDbABC4, scDbABC7) (see also Fig. 1). The N-glycosylation sites for scDbABC4 and scDbABC7 were first optimized using the NetNglyc-Software,4 which allowed an in silico prediction of N-glycosylation efficiency. All constructs were cloned into pSecTagA (Invitrogen, Karlsruhe, Germany) via SfiI/EcoRI for eukaryotic expression.

Protein Expression and Purification—Plasmid DNA encoding the respective recombinant antibody was transfected with LipofectamineTM 2000 (Invitrogen) into HEK293 cells. Stable transfectants were generated by selection with zeocin (300 µg/ml). For protein production, cells were first expanded and grown in RPMI 1640, 5% FBS to 90% confluence and subsequently cultured in Opti-MEM®I (Invitrogen) replacing media every 3 days 3–4 times. Supernatants were pooled, and proteins were concentrated by ammonium sulfate precipitation (60% saturation), before loading onto a Ni-nitrilotriacetic acid column (Qiagen, Hilden, Germany). Purification by immobilized metal ion affinity chromatography (IMAC) was performed as described elsewhere (4).

PEGylation of scDb Molecules—scDb-A' and scDb-C' were reduced with 5 mM Tris(2-carboxyethyl)-phosphine (TCEP) (Pierce) for 2 h at room temperature. TCEP was removed by dialyzing the solution overnight in an airtight bottle with oxygen-free phosphate buffer (10 mM Na2HPO4/NaH2PO4, 0.2 mM EDTA, 30 mM NaCl, pH 6.7). Subsequently, a 10-fold molar excess of mPEG2-Mal (40 kDa) was added and incubated overnight at 4 °C under non-oxygen conditions. The unbound mPEG2-Mal was removed by IMAC purification. PEGylation was visualized by barium iodide staining of SDS-polyacrylamide gels as described (23).

Chimeric Anti-CEA IgG1—The regions encoding the human {gamma}1 heavy chain constant region and the human C{kappa} domain were amplified from cDNA generated from RNA isolated from human PBMCs and combined with DNA encoding the VH and VL domain of anti-CEA antibody MFE23 (24) cloned previously into vector pSecTagA. DNA encoding the complete heavy and light chain was then cloned into pEE6.4 or pEE12.4, respectively (Lonza Biologics, Berkshire, UK). The two plasmids were then combined according to the manufacturer's protocol resulting in a bicistronic expression plasmid (pEE12.4-IgG1-CEA). The linearized plasmid was transfected into CHO-K1 cells and stably transfected clones were selected in glutamine-free RPMI 1640 medium (Invitrogen) in the presence of 25 µM methionine sulfoximine (MSX). IgG1 was purified from cell culture supernatant by protein A chromatography. In brief, proteins were precipitated from supernatant with 60% saturated ammonium sulfate, resuspended in PBS, pH 8 and loaded onto a protein A-Sepharose CL-4B column (Sigma). Bound protein was eluted with 100 mM glycine, pH 3.0 and protein-containing fractions were dialyzed against PBS.

ELISA—Carcinoembryonic antigen (CEA) (300 ng/well) was coated overnight at 4 °C. After 2 h blocking with 2% (w/v) dry milk/PBS, recombinant antibody fragments were titrated in duplicates and incubated for 1 h at room temperature. Detection was performed either with mouse horseradish peroxidase-conjugated anti-His tag antibody or rabbit antiserum and horseradish peroxidase-conjugated goat anti-rabbit antibody using TMB substrate (1 mg/ml TMB, sodium acetate buffer, pH 6.0, 0.006% H2O2). The reaction was stopped with 50 µl of 1 M H2SO4. Absorbance was measured at 450 nm in an ELISA reader.

Flow Cytometry—5 x 105 cells/well were incubated with recombinant antibodies (10 µg/ml) for 2 h at 4 °C. After washing, cells were incubated for 1 h at 4 °C with mouse anti-His tag antibody or rabbit antiserum followed by washing and 30 min of incubation with PE-labeled anti-mouse IgG or FITC-conjugated goat anti-rabbit antibody. Wash cycles and incubation steps were performed with PBS, 2% FBS, 0.02% azide. Finally, cells were analyzed by flow cytometry using an EPICS XL-MCL (Beckman Coulter, Krefeld, Germany).


Figure 1
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  		FIGURE 1.
Composition of N-glycosylated scDbs and PEGylated scDbs. Schematic arrangement of variable domains in a single-chain diabody as well as sequences of linkers A and B and the C-terminal extensions of the modified scDb molecules are shown (A, linker A; M, linker M; B, linker B; L, leader peptide; His6, hexahistidyl-tag).

 
Size Exclusion Chromatography—Apparent molecular masses (Mr) of the native proteins were determined by HPLC size exclusion chromatography using a BioSep-Sec-3000 or BioSep-Sec-2000 column (Phenomenex, Torrance) and PBS as mobile phase at a flow rate of 0.5 ml/min. The following standard proteins were used: thyroglobulin (669 kDa, RS 8.5 nm), apoferritin (443 kDa, RS 6.1 nm), β-amylase (200 kDa, RS 5.4 nm), bovine serum albumin (67 kDa, RS 3.55 nm), carbonic anhydrase (29 kDa, RS 2.35 nm), cytochrome c (12.4 kDa, RS 1.77 nm).

Carbohydrate Analysis—Glycoproteins (10–30 µg) were denatured in the presence of SDS and 2-mercaptoethanol prior to addition of Nonidet P-40 and 100 units of PNGaseF (NEB Biolabs) according to the supplier's instructions. After incubation for 18 h the sample was desalted on graphitized carbon columns (Carbograph, Alltech) (25). The samples were dried by vacuum centrifugation and in a desiccator in the presence of P2O5/KOH. To the dry sample, 50 µl of base (NaOH/Me2SO) was added and incubated for 30 min at room temperature with occasional shaking. Finally, an aliquot of 25 µl of methyl iodide was added to the reaction mixture followed by incubation for another 30 min at room temperature. After neutralization with dilute acetic acid the methylated glycans were extracted with chloroform-water. The chloroform phase was dried under nitrogen and the glycans solubilized in methanol. Matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) was performed on a Bruker Reflex IV instrument (Bruker Daltonics, Bremen, Germany). The methylated glycan samples (~500 pmol per µl) in methanol were applied to the stainless steel target by mixing a 0.5-µl aliquot with 1.0 µl of matrix (saturated solution of 2,5-dihydroxybenzoic acid in acetonitrile/0.1% trifluoroacetic acid, 1:2). Analyses were performed by positive ion detection in the reflectron mode as described previously (26).

IL-2 Release Assay—PBMCs were prepared from a leukapheresis as previously described (4). 1 x 105 LS174T or HT1080 13.8 cells/100 µl/well were seeded in 96-well plates. The next day, the supernatant was removed, and 100 µl of recombinant antibody in RPMI, 10% FBS added. After 1 h of preincubation at room temperature, 2 x 105 PBMC/100 µl/well were added. PBMCs had been thawed the day before and seeded in RPMI, 10% FBS on a culture dish, to remove monocytes by attachment to the plastic surface. Only cells that remained in suspension were used for the assay. After addition of PBMCs, the 96-well-plate was incubated for 24 h at 37 °C, 5% CO2. Plates were centrifuged, and the cell-free supernatant collected. The concentration of human IL-2 in the supernatant after T-cell retargeting was determined by an IL-2-Sandwich-ELISA. Antihuman IL-2 antibodies as well as the standard of recombinant human IL-2 was provided by DuoSet IL-2 ELISA Development System kit (R&D Systems, Nordenstadt, Germany), and the assay was performed following the manufacturer's protocol.

Pharmacokinetics—Animal care and all experiments performed were in accordance with federal guidelines and have been approved by university and state authorities. CD1 mice (female, 18–30 weeks, weight between 31–44 g, 3–6 mice/group) received an i.v. injection of 25 µg of recombinant antibody in a total volume of 200 µl. In time intervals of 3, 10, 30, 60, 120, 360 min and 24-h blood samples (100 µl) were taken from the tail and incubated on ice. Clotted blood was centrifuged at 10,000 x g for 10 min, 4 °C and serum samples stored at -20 °C. Serum concentrations of CEA-binding recombinant antibodies were determined by ELISA (as described above), interpolating the corresponding calibration curves. For comparison, the first value (3 min) was set to 100%. Pharmacokinetic parameters AUC, t1/2{alpha} and t1/2β were calculated with Excel using the first 3 times points to calculate t1/2{alpha} and the last three time points to calculate t1/2β. For statistics, the Student's t test was applied.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
DISCUSSION
 REFERENCES
 
N-Glycosylated scDb Molecules—To generate N-glycosylated scDb molecules 2 N-glycosylation sequons (NGT) were introduced in the flanking linkers (A, B) and 1 to 7 sequons at C-terminal extensions (Fig. 1). Thus, scDb-ABC1 contains 3 sequons, scDb-ABC4 6 sequons, and scDb-ABC7 9 sequons. These modified scDb molecules were expressed in stably transfected HEK293 cells and purified by IMAC with yields of 8–17 mg/liter supernatant. SDS-PAGE analysis revealed reduced mobility of these three molecules compared with unmodified scDb depending on the number of sequons (Fig. 2a and Table 1). N-Glycosylation of these proteins was confirmed by enzymatic deglycosylation with PNGase F, which reduced the apparent molecular mass to that calculated for the unmodified protein (Fig. 2a). Some heterogeneity in the degree of N-glycosylation was observed. In size exclusion chromatography (SEC), proteins eluted with a major peak corresponding in size to apparent molecular masses of 36 kDa for unmodified scDb, 38 kDa for scDb-ABC1, 65 kDa for scDb-ABC4, and 78 kDa for scDb-ABC7 (Fig. 2b and Table 1). As expected from SDS-PAGE analysis scDb-ABC4 and scDb-ABC7 eluted over a broad range, further indicating size heterogeneity of these proteins. Binding to CEA was not or only marginally impaired by N-glycosylation as shown in ELISA with immobilized CEA and flow cytometry with CEA-expressing cell line LS174T as well as CD3-positive PBMCs (Fig. 2, c and d). MALDI-MS further revealed the structural heterogeneity of attached N-glycans, which were of the bi-, tri-, and tetra-antennary complex type (Table 2). No high-mannose or hybrid N-glycans were detected. The degree of sialylation of the N-glycans was found to be 34% for scDb-ABC1, 23% for scDb-ABC4, and 22% for scDb-ABC7 as deduced from a semi-quantitative analysis of the MALDI-MS data.


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  		TABLE 1
scDb constructs

 


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  		TABLE 2
N-Glycans of scDbs variants expressed in HEK293 cells

 


Figure 2
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  		FIGURE 2.
N-glycosylated scDbs. a, SDS-PAGE analysis of N-glycosylated scDb molecules (1 + 2 scDb-ABC7;3 + 4, scDb-ABC4;5 + 6, scDb-ABC1; 7, scDb) before (lanes 1, 3, 5, 7) and after (lanes 2, 4, 6) treatment with PNGase F. 3 µg of purified protein was loaded per lane, and the gel was stained with Coomassie Blue. b, size exclusion chromatography of unmodified (gray line) and N-glycosylated scDb molecules (black lines). c, ELISA of binding of unmodified and N-glycosylated scDb variants to immobilized CEA (n = 2). d, flow cytometry analysis of binding of unmodified and N-glycosylated scDb variants to CEA-expressing tumor cells (LS174T), CD3-positive PBMCs, or HEK293 cells included as negative control. Gray-filled, cells incubated with secondary antibody alone; black line, scDb-ABC1; black dotted line, scDb-ABC4; gray line, scDb-ABC7.

 


Figure 3
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  		FIGURE 3.
PEGylated scDbs. a, SDS-PAGE analysis of purified scDb-A' (lanes 1, 3) and scDb-C' (lanes 2, 4) under reducing (lanes 1, 2) or non-reducing (lanes 3, 4) conditions. 3 µg of purified protein was loaded per lane, and the gel was stained with Coomassie Blue. b, SDS-PAGE analysis of PEGylated scDb-A'-PEG40k (lanes 1, 3, 5) and scDb-C'-PEG40k (lanes 2, 4, 6). Gels were stained with Coomassie Blue (lanes 1, 2), barium iodide (lanes 3, 4), or immunoblotted with an anti-His tag antibody (lanes 5, 6). c, size exclusion chromatography of scDb-A'-PEG40k and scDb-C'-PEG40k. d, ELISA of binding of unmodified and PEGylated scDb molecules to immobilized CEA (n = 2). e, flow cytometry analysis of binding of unmodified and PEGylated scDb molecules to CEA-expressing tumor cells (LS174T), CD3-positive PBMCs, or HEK293 cells included as negative control. Gray-filled, cells incubated with immune serum and secondary antibody alone; black line, scDb; black dotted line, scDb-A'-PEG40k; gray line, scDb-C'-PEG40k.

 
PEGylated scDb Molecules—For site-directed PEGylation we generated 2 scDb variants having an additional cysteine residue either in linker A (scDb-A') or at the C terminus (scDb-C') (Fig. 1). Both variants were expressed in stably transfected HEK293 cells and purified by IMAC with yields of 8 and 10 mg/liters supernatant, respectively. SDS-PAGE analysis showed under reducing conditions a single band with an apparent molecular mass of 59 kDa and under non-reducing conditions two bands corresponding to monomeric protein (apparent molecular mass 57 kDa) and disulfide-linked proteins (apparent molecular mass of 170 kDa for scDb-A' and 112 kDa for scDb-C') (Fig. 3a). Both proteins were PEGylated by chemical conjugation of a branched 40-kDa mPEG chain (mPEG2-Mal) to reduced scDb-A' and scDb-C' (Fig. 3b). PEGylation resulted in an increase of the molecular mass to ~230 kDa as shown by SDS-PAGE analysis. Coupling efficiency were 94% for scDb-A' and 79% for scDb-C' as determined by SEC (Fig. 3c). Both proteins eluted with a major peak corresponding to an apparent molecular mass of ~650 kDa and a hydrodynamic radius of 7.9 nm (Table 1). Interestingly, PEGylated scDb-A' (scDb-A'-PEG40k) and scDb-C' (scDb-C'-PEG40k) were not or only weakly recognized by an anti-His tag antibody (Fig. 3b). Thus, for further analysis a polyclonal rabbit antiserum was produced by immunization with scDb-A'-PEG40k, which recognized both unPEGylated and PEGylated scDb variants in ELISA (not shown). Using this antiserum similar binding of scDb and scDb-A'-PEG40k to immobilized CEA was found (Fig. 3d). Furthermore, PEGylation did not influence binding to CEA-expressing cell line LS174T or CD3-positive PBMCs as shown by flow cytometry (Fig. 4e). Because of the higher PEGylation efficiency all further studies were performed with scDb-A'-PEG40k.

Chimeric Anti-CEA IgG1—A chimeric anti-CEA IgG1 was generated by fusing murine anti-CEA VH and VL (24) to human constant regions (C{gamma}1, Ck). The antibody was purified from the supernatant of stably transfected CHO-K1 cells with yields of 3.7 mg/liter supernatant. In SDS-PAGE analysis, a single band with an apparent molecular mass of >250 kDa was observed under non-reducing conditions (Fig. 4a). After reduction, two bands with apparent molecular masses of 56 kDa and 29 kDa were observed. By size exclusion chromatography an apparent molecular mass of 280 kDa and a hydrodynamic radius of 5.7 nm was determined (Fig. 4b) (Table 1). The discrepancy between calculated and apparent molecular mass of the intact IgG molecules was also observed for other recombinant and natural whole antibodies using an established set of standard proteins (see "Experimental Procedures") as molecular weight markers. This discrepancy is best explained by the Y-shaped structure of IgG molecules, which discerns it from other globular proteins. The purified antibody recognized CEA in ELISA (Fig. 4c) and on CEA-expressing tumor cell line LS174T in flow cytometry (Fig. 4d).


Figure 4
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  		FIGURE 4.
Chimeric anti-CEA IgG1. a, SDS-PAGE analysis of purified anti-CEA IgG1 under non-reducing (lane 1) or reducing (lane 2) conditions. 5 µg of purified protein was loaded per lane, and the gel was stained with Coomassie Blue. b, size exclusion chromatography. c, ELISA of binding of anti-CEA IgG1 to immobilized CEA (n = 2). d, flow cytometry analysis of binding of anti-CEA IgG1 to CEA-expressing tumor cells (LS174T) or HEK293 cells included as negative control. Gray-filled, cells incubated with secondary antibody alone; black line, anti-CEA IgG1.

 
Target Cell-dependent Effector Cell Activation—The potential of the glycosylated and PEGylated bispecific anti-CEACD3 scDb to activate effector cells (PBMCs) in a target cell-specific manner was determined by an IL-2 release assay. In this assay, CEA-expressing cells are incubated with PBMCs in the presence of increasing concentrations of bispecific antibody, and stimulation of PBMCs is determined by measuring IL-2 release after 24 h. This assay showed that glycosylated scDb (determined for scDb-ABC1 and scDb-ABC7) as well as scDb-A'-PEG40k are able to stimulate PBMCs. However, both modifications reduced stimulatory efficacy of the scDb (Fig. 5a). Unmodified scDb mediated a maximal release of ~1.2 ng/ml IL-2 with an EC50 of 0.14 nM and an optimum between 0.3 and 3 nM. ScDb-A'-PEG40k reached a similar maximal release although with an EC50 of 1.3 nM. ScDb-ABC1 and scDb-ABC7 reached only a maximal IL-2 release of ~0.8 ng/ml with an EC50 of 0.4 and 0.9 nM, respectively. Selectivity of target cell-dependent stimulation was shown with control experiments using CEA-negative HT1080 cells stably transfected to express fibroblast activation protein (FAP). All scDb CEACD3 molecules (at a concentration of 25 nM) mediated no or only marginal stimulation of PBMCs, while a control scDb33CD3 directed against FAP and CD3 showed strong stimulation of PBMCs (Fig. 5b).


Figure 5
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  		FIGURE 5.
IL-2 release assays. a, CEA-expressing LS174T were preincubated for 1 h with anti-CEA x anti-CD3 specific scDb, scDb-ABC1, scDb-ABC7 or scDb-A'-PEG40k at varying concentrations before adding human PBMCs. IL-2 release was measured after 24 h by ELISA (n = 2). b, PBMCs were incubated in presence of LS174T or HT1080-FAP together with 25 nM bispecific anti-CEA x anti-CD3 antibodies scDb, scDb-ABC1, scDb-ABC7 and scDb-A'-PEG40k or a control scDb (scDb33CD3) directed against fibroblast activation protein and CD3. IL-2 release was measured after 24 h by ELISA.

 
Pharmacokinetic Properties—Pharmacokinetics was determined after a single dose (25 µg) i.v. injection into CD1 mice (Fig. 6 and Table 3). All constructs showed a biphasic elimination from circulation. The glycosylated scDb molecules were all cleared similarly from circulation with an ~2-fold increased initial half-life (t1/2{alpha}) compared with unmodified scDb, while terminal half-life (t1/2β) was increased by a factor 1.1–1.6. Compared with scDb, the area under the curve (AUC0–24 h) was increased 2.3-fold for scDb-ABC1 and scDb-ABC4, and 2.9-fold for scDb-ABC7 (Table 2). ScDb-A'-PEG40k showed markedly improved pharmacokinetics. Compared with unmodified scDb, half-lives were increased 2.3–5-fold and AUC increased by a factor of 10.6. Chimeric anti-CEA IgG1 showed the strongest improvement of pharmacokinetics with 7-fold increase of the terminal half-life and a 17-fold increase of the AUC (Table 3).


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  		TABLE 3
Pharmacokinetic properties of antibody constructs

 


Figure 6
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  		FIGURE 6.
Pharmacokinetics. 25 µg of antibody molecules were injected i.v. into CD1 mice. Serum concentrations of the antibody molecules were determined at different time points by ELISA. Data were normalized considering maximal concentration at the first time point (3 min).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
REFERENCES
 
We have generated N-glycosylated scDb by genetically introducing several N-glycosylation sites into linker sequences connecting the variable domains as well as a C-terminal extension of varying length. All scDb variants containing 3, 6, or 9 sequons were expressed in glycosylated form in HEK293 cells. Yields were similar to that obtained for the unmodified scDb indicating that the modifications do not interfere with translation and secretion into the cell culture supernatant. SDS-PAGE analysis revealed heterogeneity in the degree of N-glycosylation. This finding indicates that not all sites are glycosylated with the same efficiency, especially in the scDb-ABC7 construct containing 7 sequons in the C-terminal extension in close proximity to each other.

All N-glycosylated constructs were biologically active and recognized CEA and CD3, similar to unmodified scDb as shown by ELISA and flow cytometry analysis. Furthermore, a target cell-dependent IL-2 release assay demonstrated the capability to simultaneously bind both antigens on cells and thus to stimulate effector cells. However, compared with unmodified scDb the N-glycosylated scDb variants showed an ~3–5-fold reduced stimulatory activity. Similar observations were made with a PEGylated scDb as well as for fusion proteins of scDb and human serum albumin or an albumin-binding domain of streptococcal protein G (4, 15). These findings indicate that modifications of the scDb structure and size have a negative effect on stimulation of PBMCs, which might be caused by interfering with the formation of close contacts between target and effector cells. To investigate if this is a general phenomenon we have initiated studies investigating other scDb molecules with different target cell specificity.

Glycosylation of scDb led to a prolonged circulation with a 2.3–2.9-fold increase of the AUC. Interestingly, pharmacokinetic properties of the three N-glycosylated scDb variants did not differ significantly. This finding indicates that the addition of 3 N-glycosylation sites is sufficient to prolong circulation time. These three sites increase the molecular mass by ~6 kDa as determined by SDS-PAGE analysis leading to a small increase of the Stokes radius from 2.7 to 2.9 nm. For hyperglycosylated erythropoietin (darbepoetin alpha) with a molecular mass of 37 kDa an ~2-fold increase of the terminal half-life was observed after i.v. injection into rats and a 3-fold increase of the terminal half-life from 8.5 h for normally glycosylated epoetin alpha to 26.3 h for darbepoetin alpha in dialysis patients (19). Glycosylated FSH analogs containing 2 or 4 N-glycosylation sites and a molecular mass of ~55 kDa exhibited a 2–4-fold increase of the AUC after i.v. injection into rats (20, 21). Thus, for hyperglycosylated erythropoietin and glycosylated FSH a similar increase in half-lives was observed as for our glycosylated scDb variants. These findings indicate that the addition of a few N-glycans improves the pharmacokinetic properties of a protein leading to a moderate increase in circulation time by a factor of 2–4.

Comparison of an anti-CEA Fab' molecule with a natural N-glycosylation site in the VL domain obtained from hybridoma-produced IgG (thus being glycosylated) and a bacterially expressed non-glycosylated Fab' revealed faster elimination from circulation and reduced accumulation in the tumor of the non-glycosylated Fab' (27). Accelerated clearance was also described for the non-glycosylated forms of various other glycoproteins, including antithrombin, lymphotoxin, interferon-{gamma}, and granulocyte macrophage colony-stimulating factor (GM-CSF), further underlying the importance of N-glycosylation for pharmacokinetics (28–31).

The type of N-glycosylation plays an important role in determining the circulation time of glycoproteins. Thus, an engineered N-glycosylated scFv containing high-mannose carbohydrate chains at a C-terminal extension was more rapidly cleared from circulation than the unmodified scFv (32). Similar observations were made for interferon-{gamma} (29). We found that the three N-glycosylated scDb variants all contained N-glycans of the complex type (bi-, tri-, and tetra-antennary). No hybrid or high-mannose N-glycans could be detected. These complex type N-glycans were composed of a mixture of sialylated and non-sialylated chains. The predominant N-glycans found in all three N-glycosylated scDb variants are FH4N5, F2H4N5, and FH5N5, which account for ~40–50% of the N-glycans. These N-glycans differ from those found in the Fc-regions of antibodies, which bear mainly FH3N4 (G0F) oligosaccharides (33). The N-glycosylation pattern of the scDb variants is, however, similar to that found in other recombinant glycoproteins expressed in HEK293 cells (34). Glycosylation of recombinant antibodies and other proteins is influenced by the cell line and conditions used for expression (18, 35). Thus, it was found that expression of glycoproteins in CHO, NSO, and Sp2/0 cells can produce abnormally glycosylated antibodies that lack potency and are potentially immunogenic (36). For example, murine NSO cells express more heterogeneous oligosaccharide patterns, e.g. than CHO cells, with a tendency for {alpha}-linked galactose residues and N-glycolylneuraminic acid absent in glycoproteins from primate cell lines (37). Furthermore, experimental data indicate that the N-glycosylation pattern are also protein-dependent, e.g. as shown by studies of the glycosylation pattern of recombinant osteonectin, TSC-36, and bone sialoprotein expressed in HEK293 cells (34, 38, 39).

In vivo experiments have shown that the extent and type of sialylation influences serum half-lives of recombinant proteins (40). Hence, the pharmacokinetic properties of our N-glycosylated scDb molecules might be further improved using expression systems leading to increased production of fully sialylated N-glycans with {alpha}-2,6-linked terminal sialic acid residues (41).

PEGylation of scDb strongly improved pharmacokinetics of scDb, similarly to fusion of scDb to serum albumin or an albumin-binding domain as shown in previous studies (see also Tables 1 and 3) (4, 15). We found that conjugation of a 40-kDa branched mPEG chain strongly increased the hydrodynamic radius of the protein from 2.7 to 7.9 nm as determined by SEC. Similar observations were made for other PEGylated recombinant antibodies (42). Thus, the increased circulation time is likely caused by a reduced renal clearance. The introduction of a cysteine residue into one of the flanking linkers or at the C terminus allowed for a site-directed and efficient coupling of a single PEG molecule. Several studies showed that random PEGylation of whole antibodies or Fab fragments can reduce antigen-binding activity depending on number and length of conjugated PEG chains (summarized in Ref. 10). However, reduced antigen binding was also described for PEGylated antibody molecules generated by site-directed PEGylation mainly caused by reduced target association rates (43). We could not observe a reduced binding of the modified scDb molecules to each antigen separately as shown in ELISA and flow cytometry experiments, although for all modified scDb reduced activity was observed in IL-2 release assays where simultaneous binding is required.

None of the applied strategies reached the pharmacokinetics observed for a chimeric IgG1, which exhibited a 17-fold higher AUC than scDb. Similar to the albumin-based strategies the long circulation time of IgG is based on a reduced renal clearance and on FcRn-mediated recycling processes (44). However, currently we do not know to which extent the latter contributes to pharmacokinetics of IgG and the albumin-utilizing modifications. A comparison of the pharmacokinetics in wild-type and FcRn knock-out mice revealed a reduction of terminal half-life of albumin (from 39 to 24 h) and IgG (from 95 to 19 h), while the half-life of IgA was not affected (45). This finding indicates that FcRn-mediated recycling contributes to different extents to pharmacokinetics of albumin and IgG in mice.

In summary, we have completed our comparative analysis of various strategies to prolong circulation time of a bispecific single-chain diabody including fusion to albumin (4), fusion to an albumin-binding domain (ABD) of streptococcal protein G (15), PEGylation and N-glycosylation. These studies established that in mice longest circulation time is obtained by fusion of scDb to an ABD (14-fold increase of AUC), followed by a scDb-HSA fusion protein and PEGylated scDb (~10-fold increase of AUC), while N-glycosylation in HEK293 cells resulted only in a moderate increase in circulation time (2–3-fold increase of AUC) (summarized in Table 3). Further studies have now to demonstrate how these increases in circulation time translate into improved efficacy in vivo.


   FOOTNOTES
 
* This work was supported by a grant from the Deutsche Forschungsgemeinschaft (Ko1461/2). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Back

1 Present address: Multimmune GmbH, c/o Klinikum Rechts der Isar der TU München, Ismaninger Strasse 22, 81675 München, Germany. Back

2 To whom correspondence should be addressed: Institut für Zellbiologie und Immunologie, Universität Stuttgart, Allmandring 31, 70569 Stuttgart, Germany. Fax: 49-711-685-67484; E-mail: roland.kontermann{at}izi.uni-stuttgart.de.

3 The abbreviations used are: scFv, single-chain Fv fragments; FcRn, neonatal Fc receptor; FBS, fetal bovine serum; PBS, phosphate-buffered saline; ELISA, enzyme-linked immunosorbent assay; IL, interleukin; FITC, fluorescein isothiocyanate; HSA, human serum albumin; ABD, albumin-binding domain; PEG, polyethylene glycol; AUC, area under curve; scDb, single-chain diabody; PBMC, peripheral blood mononuclear cells; IMAC, immobilized metal ion affinity chromatography; CEA, carcinoembryonic antigen; MALDI, matrix-assisted laser desorption. Back

4 R. Gupta, E. Jung, and S. Brunak, personal communication. Back


   ACKNOWLEDGMENTS
 
We thank Sabine Münkel (IZI, Stuttgart) for help with HPLC analysis and Prof. Arnd G. Heyer (University of Stuttgart) for initial help in glycoanalysis.



   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 EXPERIMENTAL PROCEDURES
 RESULTS
 DISCUSSION
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