Adapting Ferritin, a Naturally Occurring Protein Cage, to Modulate Intrinsic Agonism of OX40.

Ferritin is a multivalent, self-assembling protein scaffold found in most human cell types, in addition to being present in invertebrates, higher plants, fungi, and bacteria, that offers an attractive alternative to polymer-based drug delivery systems (DDS). In this study, the utility of the ferritin cage as a DDS was demonstrated within the context of T cell agonism for tumor killing. Members of the tumor necrosis factor receptor superfamily (TNFRSF) are attractive targets for the development of anticancer therapeutics. These receptors are endogenously activated by trimeric ligands that occur in transmembrane or soluble forms, and oligomerization and cell-surface anchoring have been shown to be essential aspects of the targeted agonism of this receptor class. Here, we demonstrated that the ferritin cage could be easily tailored for multivalent display of anti-OX40 antibody fragments on its surface and determined that these arrays are capable of pathway activation through cell-surface clustering. Together, these results confirm the utility, versatility, and developability of ferritin as a DDS.

Ubiquitin chain editing revealed by polyubiquitin linkage-specific antibodies.

Posttranslational modification of proteins with polyubiquitin occurs in diverse signaling pathways and is tightly regulated to ensure cellular homeostasis. Studies employing ubiquitin mutants suggest that the fate of polyubiquitinated proteins is determined by which lysine within ubiquitin is linked to the C terminus of an adjacent ubiquitin. We have developed linkage-specific antibodies that recognize polyubiquitin chains joined through lysine 63 (K63) or 48 (K48). A cocrystal structure of an anti-K63 linkage Fab bound to K63-linked diubiquitin provides insight into the molecular basis for specificity. We use these antibodies to demonstrate that RIP1, which is essential for tumor necrosis factor-induced NF-kappaB activation, and IRAK1, which participates in signaling by interleukin-1beta and Toll-like receptors, both undergo polyubiquitin editing in stimulated cells. Both kinase adaptors initially acquire K63-linked polyubiquitin, while at later times K48-linked polyubiquitin targets them for proteasomal degradation. Polyubiquitin editing may therefore be a general mechanism for attenuating innate immune signaling.

Generation of a Porcine Antibody Fab Fragment Using Protein Engineering to Facilitate the Evaluation of Ocular Sustained Delivery Technology.

Treatment of age-related macular degeneration (AMD) with anti-vascular endothelial growth factor (VEGF) biologic agents has been shown to restore and maintain visual acuity for many patients afflicted with wet AMD. These agents are usually administered via intravitreal injection at a dosing interval of 4-8 weeks. Employment of long-acting delivery (LAD) technologies could improve the therapeutic outcome, ensure timely treatment, and reduce burden on patients, caregivers, and the health care system. Development of LAD approaches requires thorough testing in pre-clinical species; however, therapeutic proteins of human origin may not be well tolerated during testing in non-human species due to immunogenicity. Here, we have engineered a surrogate porcine antibody Fab fragment (pigG6.31) from a human antibody for testing ocular LAD technologies in a porcine model. The engineered Fab retains the VEGF-A-binding and inhibition properties of the parental human Fab and has stability properties suitable for LAD evaluation. Upon intravitreal injection in minipigs, pigG6.31 showed first-order clearance from the ocular compartments with vitreal elimination rates consistent with other molecules of this size. Application of the surrogate molecule in an in vivo evaluation in minipigs of a prototype of the port delivery (PD) platform indicated continuous ocular delivery from the implant, with release kinetics consistent with both the results from in vitro release studies and the efficacy observed in human clinical studies of the PD system with ranibizumab (PDS). Anti-drug antibodies in the serum against pigG6.31 were not detected over exposure durations up to 16 weeks, suggesting that this molecule has low porcine immunogenicity.

Retinal and Lens Degeneration in New Zealand White Rabbits Administered Intravitreal TSG-6 Link Domain-Rabbit FAb Fusion Proteins.

Fusion of biologic therapeutics to hyaluronic acid binding proteins, such as the link domain (LD) of Tumor necrosis factor (TNF)-Stimulated Gene-6 (TSG-6), is expected to increase vitreous residence time following intravitreal injection and provide for long-acting delivery. The toxicity of a single intravitreal dose of free TSG-6-LD and fusion proteins of TSG-6-LD and a nonbinding rabbit antibody fragment (RabFab) were assessed in New Zealand White rabbits. Animals administered free TSG-6-LD exhibited extensive lens opacities and variable retinal vascular attenuation, correlated with microscopic findings of lens and retinal degeneration. Similar but less severe findings were present in animals dosed with the RabFab-TSG-6-LD fusion proteins. In-life ocular inflammation was noted in all animals from 7-days postdose and was associated with high anti-RabFab antibody titers in animals administered fusion proteins. Inflammation and retinal degeneration were multifocally associated with evidence of retinal detachment, and hypertrophy and migration of vimentin, glial fibrillary acidic protein, and glutamine synthetase positive Müller cells to the outer nuclear layer. Further assessment of alternative hyaluronic acid binding protein fusions should consider the potential for retinal degeneration and enhanced immune responses early in development.

Tailoring translational strength using Kozak sequence variants improves bispecific antibody assembly and reduces product-related impurities in CHO cells.

Optimal production of bispecific antibodies (bsAb) requires efficient and tailored co-expression and assembly of two distinct heavy and two distinct light chains. Here, we describe a novel technology to modulate the translational strength of antibody chains via Kozak sequence variants to produce bsAb in a single cell line. In this study, we designed and screened a large Kozak sequence library to identify 10 independent variants that can modulate protein expression levels from approximately 0.2 to 1.3-fold compared with the wild-type sequence in transient transfection. We used a combination of several of these variants, covering a wide range of translational strength, to develop stable single cell Chinese hamster ovary bispecific cell lines and compared the results with those obtained from the wild-type sequence. A significant increase in bispecific antibody assembly with a concomitant reduction in the level of product-related impurities was observed. Our findings suggest that for production of bsAb it can be advantageous to modify translational strength for selected protein chains to improve overall yield and product quality. By extension, tuning of translational strength can also be applied to improving the production of a wide variety of heterologous proteins.

In Vivo Stability Profiles of Anti-factor D Molecules Support Long-Acting Delivery Approaches.

The collection of aqueous humor (phase 1 b/2 Mahalo study) from patients dosed intravitreally with anti-factor D (AFD; FCFD4514S, lampalizumab), a humanized antibody fragment previously under investigation to treat geographic atrophy (GA) secondary to age-related macular degeneration, presented a unique opportunity to examine AFD properties in clinical samples. We investigated AFD stability and target-binding characteristics to set up strategies for engineering and evaluating optimized molecules that enable less frequent dosing. Two variants, AFD.v8 and AFD.v14, were evaluated as alternatives to AFD for longer-acting treatments. Mass spectrometry, surface plasmon resonance, and immunoassay were used to assess AFD stability and binding activity in aqueous humor samples from Mahalo patients. In vitro stability and binding activity of AFD, AFD.v8, and AFD.v14 were assessed in human vitreous humor versus buffer at 37 °C over 16 weeks and in vivo in rabbits over 28 days along with pharmacokinetic determinations. In human aqueous humor, AFD specific binding was >85% through 30 days, and deamidation was <3% through 60 days, consistent with the AFD stability and binding activity in vitreous humor from humans in vitro and rabbits in vivo. Target binding, stability, and rabbit pharmacokinetic parameters of AFD.v8 and AFD.v14 were similar to those of AFD. Physiological stability and activity of AFD translated across in vitro and in vivo studies in humans and rabbits. The two variants AFD.v8 and AFD.v14 demonstrated comparable potency and pharmacokinetics. These findings, along with previously demonstrated improved solubility of AFD.v8 and AFD.v14, provide proof-of-concept for developing other similar long-acting therapeutic variants.

K11-linked polyubiquitination in cell cycle control revealed by a K11 linkage-specific antibody.

Polyubiquitination is a posttranslational modification where ubiquitin chains containing isopeptide bonds linking one of seven ubiquitin lysines with the C terminus of an adjoining ubiquitin are covalently attached to proteins. While functions of K48- and K63-linked polyubiquitin are understood, the role(s) of noncanonical K11-linked chains is less clear. A crystal structure of K11-linked diubiquitin demonstrates a distinct conformation from K48- or K63-linked diubiquitin. We engineered a K11 linkage-specific antibody and use it to demonstrate that K11 chains are highly upregulated in mitotic human cells precisely when substrates of the ubiquitin ligase anaphase-promoting complex (APC/C) are degraded. These chains increased with proteasomal inhibition, suggesting they act as degradation signals in vivo. Inhibition of the APC/C strongly impeded the formation of K11-linked chains, suggesting that a single ubiquitin ligase is the major source of mitotic K11-linked chains. Our results underscore the importance of K11-linked ubiquitin chains as critical regulators of mitotic protein degradation.

Enhancing the antitumor efficacy of a cell-surface death ligand by covalent membrane display.

TNF superfamily death ligands are expressed on the surface of immune cells and can trigger apoptosis in susceptible cancer cells by engaging cognate death receptors. A recombinant soluble protein comprising the ectodomain of Apo2 ligand/TNF-related apoptosis-inducing ligand (Apo2L/TRAIL) has shown remarkable preclinical anticancer activity but lacked broad efficacy in patients, possibly owing to insufficient exposure or potency. We observed that antibody cross-linking substantially enhanced cytotoxicity of soluble Apo2L/TRAIL against diverse cancer cell lines. Presentation of the ligand on glass-supported lipid bilayers enhanced its ability to drive receptor microclustering and apoptotic signaling. Furthermore, covalent surface attachment of Apo2L/TRAIL onto liposomes--synthetic lipid-bilayer nanospheres--similarly augmented activity. In vivo, liposome-displayed Apo2L/TRAIL achieved markedly better exposure and antitumor activity. Thus, covalent synthetic-membrane attachment of a cell-surface ligand enhances efficacy, increasing therapeutic potential. These findings have translational implications for liposomal approaches as well as for Apo2L/TRAIL and other clinically relevant TNF ligands.

In silico selection of therapeutic antibodies for development: viscosity, clearance, and chemical stability.

For mAbs to be viable therapeutics, they must be formulated to have low viscosity, be chemically stable, and have normal in vivo clearance rates. We explored these properties by observing correlations of up to 60 different antibodies of the IgG1 isotype. Unexpectedly, we observe significant correlations with simple physical properties obtainable from antibody sequences and by molecular dynamics simulations of individual antibody molecules. mAbs viscosities increase strongly with hydrophobicity and charge dipole distribution and decrease with net charge. Fast clearance correlates with high hydrophobicities of certain complementarity determining regions and with high positive or high negative net charge. Chemical degradation from tryptophan oxidation correlates with the average solvent exposure time of tryptophan residues. Aspartic acid isomerization rates can be predicted from solvent exposure and flexibility as determined by molecular dynamics simulations. These studies should aid in more rapid screening and selection of mAb candidates during early discovery.

Evaluating the Use of Antibody Variable Region (Fv) Charge as a Risk Assessment Tool for Predicting Typical Cynomolgus Monkey Pharmacokinetics.

The pharmacokinetic (PK) behavior of monoclonal antibodies in cynomolgus monkeys (cynos) is generally translatable to that in humans. Unfortunately, about 39% of the antibodies evaluated for PKs in cynos have fast nonspecific (or non-target-mediated) clearance (in-house data). An empirical model relating variable region (Fv) charge and hydrophobicity to cyno nonspecific clearance was developed to gauge the risk an antibody would have for fast nonspecific clearance in the monkey. The purpose of this study was to evaluate the predictability of this empirical model on cyno nonspecific clearance with antibodies specifically engineered to have either high or low Fv charge. These amino acid changes were made in the Fv region of two test antibodies, humAb4D5-8 and anti-lymphotoxin α. The humAb4D5-8 has a typical nonspecific clearance in cynos, and by making it more positively charged, the antibody acquires fast nonspecific clearance, and making it less positively charged did not impact its clearance. Anti-lymphotoxin α has fast nonspecific clearance in cynos, and making it more positively charged caused it to clear even faster, whereas making it less positively charged caused it to clear slower and within the typical range. These trends in clearance were also observed in two other preclinical species, mice and rats. The effect of modifying Fv charge on subcutaneous bioavailability was also examined, and in general bioavailability was inversely related to the direction of the Fv charge change. Thus, modifying Fv charge appears to impact antibody PKs, and the changes tended to correlate with those predicted by the empirical model.

Contribution of Antibody Hydrodynamic Size to Vitreal Clearance Revealed through Rabbit Studies Using a Species-Matched Fab.

We have developed a tool Fab fragment of a rabbit monoclonal antibody that is useful for early evaluation in rabbit models of technologies for long acting delivery (LAD) of proteins to the eye. Using this Fab we show that vitreal clearance can be slowed through increased hydrodynamic size. Fab (G10rabFab) and Fab' (G10rabFab') fragments of a rabbit monoclonal antibody (G10rabIgG) were expressed in Chinese hamster ovary (CHO) cells and purified using antigen-based affinity chromatography. G10rabFab retains antigen-binding upon thermal stress (37 °C) for 8 weeks in phosphate-buffered saline (PBS) and can be detected in rabbit tissues using an antigen-based ELISA. Hydrodynamic radius, measured using quasi-elastic light scattering (QELS), was increased through site-specific modification of the G10rabFab' free cysteine with linear methoxy-polyethylene glycol(PEG)-maleimide of 20000 or 40000 molecular weight. Pharmacokinetic studies upon intravitreal dosing in New Zealand white rabbits were conducted on the G10rabFab and PEGylated G10rabFab'. Results of single and multidose pharmacokinetic experiments yield reproducible results and a vitreal half-life for G10rabFab of 3.2 days. Clearance from the eye is slowed through increased hydrodynamic size, with vitreal half-life showing a linear dependence on hydrodynamic radius (RH). A linear dependence of vitreal half-life on RH suggests that molecule diffusivity makes an important contribution to vitreal clearance. A method for prediction of vitreal half-life from RH measurements is proposed.

Solubility Challenges in High Concentration Monoclonal Antibody Formulations: Relationship with Amino Acid Sequence and Intermolecular Interactions.

The purpose of this work was to elucidate the molecular interactions leading to monoclonal antibody self-association and precipitation and utilize biophysical measurements to predict solubility behavior at high protein concentration. Two monoclonal antibodies (mAb-G and mAb-R) binding to overlapping epitopes were investigated. Precipitation of mAb-G solutions was most prominent at high ionic strength conditions and demonstrated strong dependence on ionic strength, as well as slight dependence on solution pH. At similar conditions no precipitation was observed for mAb-R solutions. Intermolecular interactions (interaction parameter, kD) related well with high concentration solubility behavior of both antibodies. Upon increasing buffer ionic strength, interactions of mAb-R tended to weaken, while those of mAb-G became more attractive. To investigate the role of amino acid sequence on precipitation behavior, mutants were designed by substituting the CDR of mAb-R into the mAb-G framework (GM-1) or deleting two hydrophobic residues in the CDR of mAb-G (GM-2). No precipitation was observed at high ionic strength for either mutant. The molecular interactions of mutants were similar in magnitude to those of mAb-R. The results suggest that presence of hydrophobic groups in the CDR of mAb-G may be responsible for compromising its solubility at high ionic strength conditions since deleting these residues mitigated the solubility issue.

Stability Comparison Between Microglassification and Lyophilization Using a Monoclonal Antibody.

Producing solid-state formulations of biologics remains a daunting task despite the prevalent use of lyophilization and spray drying technologies in the biopharmaceutical industry. The challenges include protein stability (temperature stresses), high capital costs, particle design/controllability, shortened processing times and manufacturing considerations (scalability, yield improvements, aseptic operation, etc.). Thus, scientists/engineers are constantly working to improve existing methodologies and exploring novel dehydration/powder-forming technologies. Microglassification™ is a dehydration technology that uses solvent extraction to rapidly dehydrate protein formulations at ambient temperatures, eliminating the temperature stress experienced by biologics in traditional lyophilization and spray drying methods. The process results in microparticles that are spherical, dense, and chemically stable. In this study, we compared the molecular stability of a monoclonal antibody formulation processed by lyophilization to the same formulation processed using Microglassification™. Both powders were placed on stability for 3 months at 40 °C and 6 months at 25 °C. Both dehydration methods showed similar chemical stability, including percent monomer, charge variants, and antigen binding. These results show that Microglassification™ is viable for the production of stable solid-state monoclonal antibody formulations.

Inhibiting alternative pathway complement activation by targeting the factor D exosite.

By virtue of its amplifying property, the alternative complement pathway has been implicated in a number of inflammatory diseases and constitutes an attractive therapeutic target. An anti-factor D Fab fragment (AFD) was generated to inhibit the alternative complement pathway in advanced dry age-related macular degeneration. AFD potently prevented factor D (FD)-mediated proteolytic activation of its macromolecular substrate C3bB, but not proteolysis of a small synthetic substrate, indicating that AFD did not block access of the substrate to the catalytic site. The crystal structures of AFD in complex with human and cynomolgus FD (at 2.4 and 2.3 Å, respectively) revealed the molecular details of the inhibitory mechanism. The structures show that the AFD-binding site includes surface loops of FD that form part of the FD exosite. Thus, AFD inhibits FD proteolytic function by interfering with macromolecular substrate access rather than by inhibiting FD catalysis, providing the molecular basis of AFD-mediated inhibition of a rate-limiting step in the alternative complement pathway.

Improved quantitative mass spectrometry methods for characterizing complex ubiquitin signals.

Ubiquitinated substrates can be recruited to macromolecular complexes through interactions between their covalently bound ubiquitin (Ub) signals and Ub receptor proteins. To develop a functional understanding of the Ub system in vivo, methods are needed to determine the composition of Ub signals on individual substrates and in protein mixtures. Mass spectrometry has emerged as an important tool for characterizing the various forms of Ub. In the Ubiquitin-AQUA approach, synthetic isotopically labeled internal standard peptides are used to quantify unbranched peptides and the branched -GG signature peptides generated by trypsin digestion of Ub signals. Here we have built upon existing methods and established a comprehensive platform for the characterization of Ub signals. Digested peptides and isotopically labeled standards are analyzed either by selected reaction monitoring on a QTRAP mass spectrometer or by narrow window extracted ion chromatograms on a high resolution LTQ-Orbitrap. Additional peptides are now monitored to account for the N terminus of ubiquitin, linear polyUb chains, the peptides surrounding K33 and K48, and incomplete digestion products. Using this expanded battery of peptides, the total amount of Ub in a sample can be determined from multiple loci within the protein, minimizing possible confounding effects of complex Ub signals, digestion abnormalities, or use of mutant Ub in experiments. These methods have been useful for the characterization of in vitro, multistage ubiquitination and have now been extended to reactions catalyzed by multiple E2 enzymes. One question arising from in vitro studies is whether individual protein substrates in cells may be modified by multiple forms of polyUb. Here we have taken advantage of recently developed polyubiquitin linkage-specific antibodies recognizing K48- and K63-linked polyUb chains, coupled with these mass spectrometry methods, to further evaluate the abundance of mixed linkage Ub substrates in cultured mammalian cells. By combining these two powerful tools, we show that polyubiquitinated substrates purified from cells can be modified by mixtures of K48, K63, and K11 linkages.

Insights into ultra-low affinity lipase-antibody noncovalent complex binding mechanisms.

Detection of host cell protein (HCP) impurities is critical to ensuring that recombinant drug products, including monoclonal antibodies (mAbs), are safe. Mechanistic characterization as to how HCPs persist in drug products is important to refining downstream processing. It has been hypothesized that weak lipase-mAb interactions enable HCP lipases to evade drug purification processes. Here, we apply state-of-the-art methods to establish lipase-mAb binding mechanisms. First, the mass spectrometry (MS) approach of fast photochemical oxidation of proteins was used to elucidate putative binding regions. The CH1 domain was identified as a conserved interaction site for IgG1 and IgG4 mAbs against the HCPs phospholipase B-like protein (PLBL2) and lysosomal phospholipase A2 (LPLA2). Rationally designed mutations in the CH1 domain of the IgG4 mAb caused a 3- to 70-fold KD reduction against PLBL2 by surface plasmon resonance (SPR). LPLA2-IgG4 mutant complexes, undetected by SPR and studied using native MS collisional dissociation experiments, also showed significant complex disruption, from 16% to 100%. Native MS and ion mobility (IM) determined complex stoichiometries for four lipase-IgG4 complexes and directly interrogated the enrichment of specific lipase glycoforms. Confirmed with time-course and exoglycosidase experiments, deglycosylated lipases prevented binding, and low-molecular-weight glycoforms promoted binding, to mAbs. This work demonstrates the value of integrated biophysical approaches to characterize micromolar affinity complexes. It is the first in-depth structural report of lipase-mAb binding, finding roles for the CH1 domain and lipase glycosylation in mediating binding. The structural insights gained offer new approaches for the bioengineering of cells or mAbs to reduce HCP impurity levels.Abbreviations: CAN, Acetonitrile; AMAC, Ammonium acetate; BFGS, Broyden-Fletcher-Goldfarb-Shanno; CHO, Chinese Hamster Ovary; KD, Dissociation constant; DTT, Dithiothreitol; ELISA, Enzyme-linked immunosorbent assay; FPOP, Fast photochemical oxidation of proteins; FA, Formic acid; F(ab'), Fragment antibodies; HCP, Host cell protein; IgG, Immunoglobulin; IM, Ion mobility; LOD, Lower limit of detection; LPLA2, Lysosomal phospholipase A2; Man, Mannose; MS, Mass spectrometry; MeOH, Methanol; MST, Microscale thermophoresis; mAbs, Monoclonal antibodies; PPT1, Palmitoyl protein thioesterase; ppm, Parts per million; PLBL2, Phospholipase B-like protein; PLD3, Phospholipase D3; PS-20, Polysorbate-20; SP, Sphingomyelin phosphodiesterase; SPR, Surface plasmon resonance; TFA, Trifluoroacetic acid.

An Approach to Bioactivity Assessment for Critical Quality Attribute Identification Based on Antibody-Antigen Complex Structure.

Identification of critical quality attributes (CQAs) is an important step for development of biopharmaceuticals with intended performance. An accurate CQA assessment is needed to ensure product quality and focusing on development efforts where control is needed. The assignment of criticality is based on safety and efficacy. Efficacy is related to PK and bioactivity. Here, we developed a novel approach based on antibody-antigen complex structure and modeling as a complementary method for bioactivity assessment. To validate this approach, common product related quality attributes and mutagenesis data from several IgGs were assessed using available antibody-antigen complex structures, and results were compared with experimental data from bioactivity or binding affinity measurements. A stepwise evaluation scheme for structural based analysis is proposed; based on systematic assessment following the scheme, good correlation has been observed between structural analysis and experimental data. This demonstrates that such an approach can be applied as a complementary tool for bioactivity assessment. Main applications are 1) To decouple multiple attributes to achieve amino acid resolution for bioactivity assessment, 2) To assess bioactivity of attributes that cannot be experimentally generated, 3) To provide molecular mechanism for experimental observation and understand structure function relationship. Examples are provided to illustrate these applications.

Antibody Format and Serum Disposition Govern Ocular Pharmacokinetics of Intravenously Administered Protein Therapeutics.

Protein therapeutics have witnessed tremendous use and application in recent years in treatment of various diseases. Predicting efficacy and safety during drug discovery and translational development is a key factor for successful clinical development of these therapies. In general, drug related toxicities are predominantly driven by pharmacokinetic (PK) exposure at off-target sites. This work explores the ocular PK of intravenously administered protein therapeutics to understand impact of antibody format on off-site exposure. Species matched non-binding rabbit antibody proteins (rabFab and rabIgG) were intravenously administered to male New Zealand White rabbits at a single 1 mg bolus dose and exposure was measured up to 3 weeks. As anticipated based on absence of FcRn recycling, rabFab has relatively fast systemic PK (CL-943 mL/day and t1/2-1.93 days) compared to rabIgG (CL-18.5 mL/day and t1/2-8.93 days). Similarly, rabFab has lower absolute ocular exposure in ocular compartments (e.g., vitreous and aqueous humor) compared to rabIgG, despite higher relative exposures (measured as percent tissue partition in ocular tissues relative to serum, based on Cmax and AUC). In general, percent tissue partition based on AUC (in aqueous and vitreous humor) relative to serum exposure were 10.4 and 8.62 for rabFab respectively and 1.11 and 0.64 for rabIgG respectively. This work emphasizes size and format based ocular exposure of intravenously administered protein therapeutics. Findings from this work enable prediction of format based ocular exposure for systemically administered antibody based therapeutics and aid in selection of molecule format for clinical candidate to minimize ocular exposure.

Improving therapeutic efficacy of a complement receptor by structure-based affinity maturation.

CRIg is a recently discovered complement C3 receptor expressed on a subpopulation of tissue-resident macrophages. The extracellular IgV domain of CRIg (CRIg-ECD) holds considerable promise as a potential therapeutic because it selectively inhibits the alternative pathway of complement by binding to C3b and inhibiting proteolytic activation of C3 and C5. However, CRIg binds weakly to the convertase subunit C3b (K(D) = 1.1 microm), and thus a relatively high concentration of protein is required to reach nearly complete complement inhibition. To improve therapeutic efficacy while minimizing risk of immunogenicity, we devised a phage display strategy to evolve a high affinity CRIg-ECD variant with a minimal number of mutations. Using the crystal structure of CRIg in complex with C3b as a guide for library design, we isolated a CRIg-ECD double mutant (Q64R/M86Y, CRIg-v27) that showed increased binding affinity and improved complement inhibitory activity relative to CRIg-ECD. In a mouse model of arthritis, treatment with a Fc fusion of CRIg-v27 resulted in a significant reduction in clinical scores compared with treatment with an Fc fusion of CRIg-ECD. This study clearly illustrates how phage display technology and structural information can be combined to generate proteins with nearly natural sequences that act as potent complement inhibitors with greatly improved therapeutic efficacy.

Protein conjugates and fusion proteins as ocular therapeutics.

Long-acting delivery (LAD) of ocular therapeutics has potential to improve the standard of care for ocular diseases, such as age-related macular degeneration (AMD), by increasing patient compliance and reducing overall treatment burden on patients and healthcare providers. Although relatively few ocular LAD technologies are currently on the market, a variety of emergent and novel protein engineering-based technologies are being investigated in both the laboratory and clinical settings. Here, we review some of the key indications and treatments that would benefit from the development of LAD for the treatment of ocular diseases and examine the current state of LAD technologies that leverage protein-engineering approaches as well as nascent technologies with potential for future impact.