An erythropoietin fusion protein comprised of identical repeating domains exhibits enhanced biological properties

Instability Challenges and Stabilization Strategies of Pharmaceutical Proteins

Maintaining the structure of protein and peptide drugs has become one of the most important goals of scientists in recent decades. Cold and thermal denaturation conditions, lyophilization and freeze drying, different pH conditions, concentrations, ionic strength, environmental agitation, the interaction between the surface of liquid and air as well as liquid and solid, and even the architectural structure of storage containers are among the factors that affect the stability of these therapeutic biomacromolecules. The use of genetic engineering, side-directed mutagenesis, fusion strategies, solvent engineering, the addition of various preservatives, surfactants, and additives are some of the solutions to overcome these problems. This article will discuss the types of stress that lead to instabilities of different proteins used in pharmaceutics including regulatory proteins, antibodies, and antibody-drug conjugates, and then all the methods for fighting these stresses will be reviewed. New and existing analytical methods that are used to detect the instabilities, mainly changes in their primary and higher order structures, are briefly summarized.

Probing protease sensitivity of recombinant human erythropoietin reveals α3-α4 inter-helical loop as a stability determinant

Although unglycosylated HuEpo is fully functional, it has very short serum half-life. However, the mechanism of in vivo clearance of human Epo (HuEpo) remains largely unknown. In this study, the relative importance of protease-sensitive sites of recombinant HuEpo (rHuEpo) has been investigated by analysis of structural data coupled with in vivo half-life measurements. Our results identify α3-α4 inter-helical loop region as a target site of lysosomal protease Cathepsin L. Consistent with previously-reported lysosomal degradation of HuEpo, these results for the first time identify cleavage sites of rHuEpo by specific lysosomal proteases. Furthermore, in agreement with the lowered exposure of the peptide backbone around the cleavage site, remarkably substitutions of residues with bulkier amino acids result in significantly improved in vivo stability. Together, these results have implications for the mechanism of in vivo clearance of the protein in humans.

Pharmacokinetics and pharmacodynamics of recombinant human EPO-Fc fusion protein in vivo

In this study, the in vivo pharmacokinetics and pharmacodynamics of a novel recombinant human erythropoietin (rhEPO) Fc fusion protein, rhEPO-Fc, were studied in both rodents and rhesus monkeys. Animal models of anemia induced by irradiation, cyclophosphamide and partial renal ablation were used to evaluate therapeutic effects of rhEPO-Fc. We have demonstrated that serum half-life of rhEPO-Fc was 29.5 to 38.9 h at doses of 8, 25, 80 µg/kg in rhesus monkeys and 35.5 to 43.5 h at doses of 16, 50, 160 µg/kg in rats. In anemia animal models, rhEPO-Fc dose-dependently (7.5–30.0 µg/kg in mice, 5.4–21.4 µg/kg in rats and 5.0–10.0 µg/kg in rhesus monkeys) increased reticulocyte level, followed by an increase of RBC count, hemoglobin and hematocrit levels. At reduced intervention frequency of weekly treatments, rhEPO-Fc showed similar hematopoietic effects as compared with rhEPO given three times a week. These results indicated that rhEPO-Fc could potentially be used in treatment of anemia and warrants future clinical trials.

Erythropoiesis stimulating agents: approaches to modulate activity

Recombinant human erythropoietin (rHuEPO), such as the approved agents epoetin alfa and epoetin beta, has been used successfully for over 20 years to treat anemia in millions of patients. However, due to the relatively short half-life of the molecule (approximately 8 hours), frequent dosing may be required to achieve required hemoglobin levels. Therefore, a need was identified in some anemic patient populations for erythropoiesis stimulating agents with longer half-lives that required less frequent dosing. This need led to the development of second generation molecules which are modified versions of rHuEPO with improved pharma-cokinetic and pharmacodynamic properties such as darbepoetin alfa, a hyperglycosylated analog of rHuEPO, and pegzyrepoetin, a pegylated rHuEPO. Third generation molecules, such as peginesatide, which are peptide mimetics that have no sequence homology to rHuEPO have also recently been developed. The various molecular, pharmacokinetic, and pharmacodynamic properties of these and other erythropoiesis stimulating agents will be discussed in this review.

Erythropoietin and erythropoiesis stimulating agents

Erythropoietin (EPO) is the main hormonal regulator of red blood cell production. Recombinant EPO has become the leading drug for treatment of anaemia from a variety of causes; however, it is sometimes misused in sport with the aim of improving performance and endurance. This paper presents an introductory overview of EPO, its receptor, and a variety of recombinant human EPOs/erythropoiesis stimulating agents (ESAs) available on the market (e.g. epoetins and their long acting analogs--darbepoetin alfa and continuous erythropoiesis receptor activator). Recent efforts to improve on EPO's pharmaceutical properties and to develop novel replacement products are also presented. In most cases, these efforts have emphasized a reduction in frequency of injections or complete elimination of intravenous or subcutaneous injections of the hormone (biosimilars, EPO mimetic peptides, fusion proteins, endogenous EPO gene activators and gene doping). Isoelectric focusing (IEF) combined with double immunoblotting can detect the subtle differences in glycosylation/sialylation, enabling differentiation among endogenous and recombinant EPO analogues. This method, using the highly sensitive anti-EPO monoclonal antibody AE7A5, has been accepted internationally as one of the methods for detecting misuse of ESAs in sport.

Aberrant phenotypes of transgenic mice expressing dimeric human erythropoietin

BACKGROUND

Dimeric human erythropoietin (dHuEPO) peptides are reported to exhibit significantly higher biological activity than the monomeric form of recombinant EPO. The objective of this study was to produce transgenic (tg) mice expressing dHuEPO and to investigate the characteristics of these mice.

METHODS

A dHuEPO-expressing vector under the control of the goat beta-casein promoter, which produced a dimer of human EPO molecules linked by a 2-amino acid peptide linker (Asp-Ile), was constructed and injected into 1-cell fertilized embryos by microinjection. Mice were screened using genomic DNA samples obtained from tail biopsies. Blood samples were obtained by heart puncture using heparinized tubes, and hematologic parameters were assessed. Using the microarray analysis tool, we analyzed differences in gene expression in the spleens of tg and control mice.

RESULTS

A high rate of spontaneous abortion or death of the offspring was observed in the recipients of dHuEPO embryos. We obtained 3 founder lines (#4, #11, and #47) of tg mice expressing the dHuEPO gene. However, only one founder line showed stable germline integration and transmission, subsequently establishing the only transgenic line (#11). We obtained 2 F1 mice and 3 F2 mice from line #11. The dHuEPO protein could not be obtained because of repeated spontaneous abortions in the tg mice. Tg mice exhibited symptoms such as short lifespan and abnormal blood composition. The red blood cell count, white blood cell count, and hematocrit levels in the tg mice were remarkably higher than those in the control mice. The spleens of the tg mice (F1 and F2 females) were 11- and -21-fold larger than those of the control mice. Microarray analysis revealed 2,672 spleen-derived candidate genes; more genes were downregulated than upregulated (849/764). Reverse transcriptase-polymerase chain reaction (RT-PCR) and quantitative real-time PCR (qRT-PCR) were used for validating the results of the microarray analysis of mRNA expression.

CONCLUSIONS

In conclusion, dHuEPO tg mice caused excessive erythrocytosis that led to abnormal blood composition, short lifespan, and abnormal splenomegaly. Further, we identified 2,672 genes associated with splenomegaly by microarray analysis. These results could be useful in the development of dHuEPO-producing tg animals.

Natural form of noncytolytic flexible human Fc as a long-acting carrier of agonistic ligand, erythropoietin

Human IgG1 Fc has been widely used as a bioconjugate, but exhibits shortcomings, such as antibody- and complement-mediated cytotoxicity as well as decreased bioactivity, when applied to agonistic proteins. Here, we constructed a nonimmunogenic, noncytolytic and flexible hybrid Fc (hyFc) consisting of IgD and IgG4, and tested its function using erythropoietin (EPO) conjugate, EPO-hyFc. Despite low amino acid homology (20.5%) between IgD Fc and IgG4 Fc, EPO-hyFc retained “Y-shaped” structure and repeated intravenous administrations of EPO-hyFc into monkeys did not generate EPO-hyFc-specific antibody responses. Furthermore, EPO-hyFc could not bind to FcγR I and C1q in contrast to EPO-IgG1 Fc. In addition, EPO-hyFc exhibited better in vitro bioactivity and in vivo bioactivity in rats than EPO-IgG1 Fc, presumably due to the high flexibility of IgD. Moreover, the mean serum half-life of EPO-hyFc(H), a high sialic acid content form of EPO-hyFc, was approximately 2-fold longer than that of the heavily glycosylated EPO, darbepoetin alfa, in rats. More importantly, subcutaneous injection of EPO-hyFc(H) not only induced a significantly greater elevation of serum hemoglobin levels than darbepoetin alfa in both normal rats and cisplatin-induced anemic rats, but also displayed a delayed time to maximal serum level and twice final area-under-the-curve (AUC_last). Taken together, hyFc might be a more attractive Fc conjugate for agonistic proteins/peptides than IgG1 Fc due to its capability to elongate their half-lives without inducing host effector functions and hindering bioactivity of fused molecules. Additionally, a head-to-head comparison demonstrated that hyFc-fusion strategy more effectively improved the in vivo bioactivity of EPO than the hyperglycosylation approach.

A diversity of antibody epitopes can induce signaling through the erythropoietin receptor

Stimulation of red cell production through agonism of the erythropoietin receptor (EpoR) has historically been accomplished through administration of erythropoietin (EPO), the native ligand. The short half-life of EPO has led to the development of a variety of other agonists, including antibodies. It is of considerable interest to understand how these agents might activate the EpoR and whether or not it is important to bind in a manner similar to the native ligand. The binding epitopes of a panel of eight agonistic, single-chain antibody (scFv-Fc) constructs were determined through scanning alanine mutagenesis as well as more limited arginine mutagenesis of the receptor. It was found that while some of these constructs bound to receptor epitopes shared by the ligand, others bound in completely unique ways. The use of a panel of agonists and scanning mutagenesis can define the critical binding regions for signaling; in the case of the EpoR, these regions were remarkably broad.

Drug targeting of erythropoietin across the primate blood-brain barrier with an IgG molecular Trojan horse

Erythropoietin (EPO) is a neurotrophic factor that could be developed as a new drug for brain disorders. However, EPO does not cross the blood-brain barrier (BBB). In the present study, human EPO was re-engineered by fusion to the carboxyl terminus of the heavy chain of a chimeric monoclonal antibody (MAb) to the human insulin receptor (HIR). The HIRMAb acts as a molecular Trojan horse to ferry the EPO into the brain via receptor-mediated transport on the endogenous BBB insulin receptor. The HIRMAb-EPO fusion protein was immunoreactive with antibodies to both human IgG and EPO. The HIRMAb-EPO fusion protein bound with high affinity to the extracellular domain of both the HIR (ED(50) = 0.21 +/- 0.05 nM) and the EPO receptor (ED(50) = 0.30 +/- 0.01 nM) and activated thymidine incorporation into human TF-1 cells with an ED(50) of 0.1 nM. Differentially radiolabeled EPO and the HIRMAb-EPO fusion protein were injected intravenously into adult rhesus monkeys. Whereas EPO did not cross the primate BBB, the HIRMAb-EPO fusion protein was rapidly transported into brain, at levels that produce pharmacologic elevations in brain EPO at small systemic doses. The HIRMAb fusion protein selectively targeted the brain relative to peripheral organs. In conclusion, a novel IgG-EPO fusion protein has been engineered, expressed, and shown to be bifunctional with retention of high-affinity binding to both the insulin and EPO receptors. The IgG-EPO fusion protein represents a new class of EPO neurotherapeutics that has been specifically re-engineered to penetrate the human BBB.

The role of asymmetric binding in ligand-receptor systems with 1:2 interaction ratio

Dynamical models for cellular ligand-receptor systems are among the most successful examples of mathematical approaches in systems biology. Here we present a general kinetic and mechanistic model for systems with asymmetric 1:2 ligand-receptor interaction ratio, such as erythropoietin and growth hormone systems. In these systems, the ligand presents two very different binding affinities to its receptor, and the weak interaction being often neglected for modeling purposes. Here, we demonstrate that the weak binding is the one tightly regulating the signaling, while the strong binding sets the threshold for the auto-inhibition effect characteristic of 1:2 asymmetric ligand-receptor systems. The model constitutes an improved mathematical framework for erythropoietin activation and equivalent biological processes, which are, due to their widespread use and relevance, on the forefront of pharmacological systems biology.

Epo delivery by genetically engineered C2C12 myoblasts immobilized in microcapsules

ver the last half century, the use of erythropoietin (Epo) in the management of malignancies has been extensively studied. Originally viewed as the renal hormone responsible for red blood cell production, many recent in vivo and clinical approaches demonstrate that various tissues locally produce Epo in response to physical or metabolic stress. Thus, not only its circulating erythrocyte mass regulator activity but also the recently discovered nonhematological actions are being thoroughly investigated in order to fulfill the specific Epo delivery requirements for each therapeutic approach.

Use of agents stimulating erythropoiesis in digestive diseases

Anemia is the most common complication of inflammatory bowel disease (IBD). Control and inadequate treatment leads to a worse quality of life and increased morbidity and hospitalization. Blood loss, and to a lesser extent, malabsorption of iron are the main causes of iron deficiency in IBD. There is also a variable component of anemia related to chronic inflammation. The anemia of chronic renal failure has been treated for many years with recombinant human erythropoietin (rHuEPO), which significantly improves quality of life and survival. Subsequently, rHuEPO has been used progressively in other conditions that occur with anemia of chronic processes such as cancer, rheumatoid arthritis or IBD, and anemia associated with the treatment of hepatitis C virus. Erythropoietic agents complete the range of available therapeutic options for treatment of anemia associated with IBD, which begins by treating the basis of the inflammatory disease, along with intravenous iron therapy as first choice. In cases of resistance to treatment with iron, combined therapy with erythropoietic agents aims to achieve near-normal levels of hemoglobin/hematocrit (11-12 g/dL). New formulations of intravenous iron (iron carboxymaltose) and the new generation of erythropoietic agents (darbepoetin and continuous erythropoietin receptor activator) will allow better dosing with the same efficacy and safety.

CNTO 530 functions as a potent EPO mimetic via unique sustained effects on bone marrow proerythroblast pools

Anemia as associated with numerous clinical conditions can be debilitating, but frequently can be treated via administration of epoetin-alfa, darbepoietin-alfa, or methoxy-PEG epoetin-beta. Despite the complexity of EPO-EPO receptor interactions, the development of interesting EPO mimetic peptides (EMPs) also has been possible. CNTO 530 is one such novel MIMETIBODY Fc-domain dimeric EMP fusion protein. In a mouse model, single-dose CNTO 530 (unlike epoetin-alfa or darbepoietin-alfa) bolstered red cell production for up to 1 month. In 5-fluorouracil and carboplatin-paclitaxel models, CNTO 530 also protected against anemia with unique efficiency. These actions were not fully accounted for by half-life estimates, and CNTO 530 signaling events therefore were studied. Within primary bone marrow erythroblasts, kinetics of STAT5, ERK, and AKT activation were similar for CNTO 530 and epoetin-alfa. p70S6K activation by CNTO 530, however, was selectively sustained. In vivo, CNTO 530 uniquely stimulated the enhanced formation of PODXL(high)CD71(high) (pro)erythroblasts at frequencies multifold above epoetin-alfa or darbepoietin-alfa. CNTO 530 moreover supported the sustained expansion of a bone marrow-resident Kit(neg)CD71(high)Ter119(neg) progenitor pool. Based on these distinct erythropoietic and EPOR signaling properties, CNTO 530 holds excellent promise as a new EPO mimetic.

Erythropoiesis-stimulating protein delivery in providing erythropoiesis and neuroprotection

Erythropoietin (EPO), a glycoprotein, plays an important role in erythropoiesis and neuroprotection. EPO therapies for anemia or neurodegenerative diseases require frequent injections or high-dose systemic administration which may cause unwanted side effects. Various strategies for EPO delivery have been investigated for increasing EPO bioavailability and decreasing side effects, including nano/micro particles, PEGylation of EPO and transport-mediated delivery systems. Nano/micro particles provide EPO with long-term effect and protect EPO against proteolytic cleavage. PEGylated EPO prolong circulating time and reduce injection frequency of anemia treatment. A transport-mediated delivery system enables protein to cross biological barriers. Presently, there is no report about an effective delivery system of EPO for neuroprotection. This review focuses on EPO delivery systems for erythropoiesis or neuroprotection with prolonged duration and enhanced bioavailability.

Hematopoietic growth factor mimetics: from concept to clinic

Hematopoietic growth factor (HGF) mimetics offer a number of attractive advantages as therapeutic agents. Small chemical compounds, in particular, provide reduced cost and oral availability. As many of these mimetics are unrelated in structure to the normal cytokine the immunogenic response is not a significant issue. Isolation of small peptide agonists for erythropoietin (EPO) and thrombopoietin (TPO) receptors has been associated with significant translational challenges and here we summarize approaches used to achieve the potency and stability required for clinical utility. We also compare and contrast the initial screening approaches, and the translational and clinical issues associated with two recently approved TPO mimetics, romiplostim and the orally available eltrombopag. Finally we summarize the development and clinical findings for the EPO mimetic, Hematide, consider alternative approaches, and discuss the future potential for isolation of growth factor (GF) mimetics.

Survival and proliferative roles of erythropoietin beyond the erythroid lineage

Since the isolation and purification of erythropoietin (EPO) in 1977, the essential role of EPO for mature red blood cell production has been well established. The cloning of the EPO gene and production of recombinant human EPO led to the widespread use of EPO in treating patients with anaemia. However, the biological activity of EPO is not restricted to regulation of erythropoiesis. EPO receptor (EPOR) expression is also found in endothelial, brain, cardiovascular and other tissues, although at levels considerably lower than that of erythroid progenitor cells. This review discusses the survival and proliferative activity of EPO that extends beyond erythroid progenitor cells. Loss of EpoR expression in mouse models provides evidence for the role of endogenous EPO signalling in nonhaematopoietic tissue during development or for tissue maintenance and/or repair. Determining the extent and distribution of receptor expression provides insights into the potential protective activity of EPO in brain, heart and other nonhaematopoietic tissues.

Erythropoietin: physiology and molecular mechanisms

Erythropoietin, the primary regulator of erythropoiesis, is produced by the kidney and levels vary inversely with oxygen availability. Hypoxia-inducible factor-1 (HIF-1), a major transcriptional regulator of several hypoxia-sensitive genes, including erythropoietin, is functionally deactivated by oxygen in a reaction catalyzed by prolyl hydroxylase. Erythropoietin acts by binding to a specific trans-membrane dimeric receptor which has been found in erythroid and non-erythroid cell types. The interaction between erythropoietin and its receptor ultimately leads to conformational change and phosphorylation of the receptor and expression of genes coding for proteins that are anti-apoptotic. Development of erythropoietin stimulating agents is an area of active research. To date, research has focused on activating the erythropoietin receptor, prevention of HIF-1 inactivation, and gene therapy. Even with biologically effective therapies, defining appropriate hemoglobin targets remains challenging. For example, despite decades of clinical trials, target hemoglobin levels in chronic kidney disease remain uncertain, as hemoglobin targets above 13 g/dl have been associated with both benefit (quality of life) and harm (cardiovascular events).

CNTO 530: molecular pharmacology in human UT-7EPO cells and pharmacokinetics and pharmacodynamics in mice

CNTO 530 is a 58 kD antibody Fc domain fusion protein, created using Centocor's MIMETIBODY platform, that contains two EMP1 sequences as a pharmacophore. CNTO 530 has no sequence homology with EPO but acts as a novel erythropoietin receptor agonist. In UT-7(EPO) cells, CNTO 530 caused protein phosporylation of the erythropoietin receptor associated signaling pathway (Jak2, STAT5, AKT and ERK1/2). CNTO 530 also rescued these cells from apoptosis and mediated proliferation. In mice, pharmacokinetic analysis showed that CNTO 530 was slowly cleared from circulation with a t(1/2) approximately 40 h. Pharmacodynamic analysis in mice showed that a single sc dose of CNTO 530 caused a long-lived stimulation of erythropoiesis that translated into increases in red blood cell counts and hemoglobin values that were maintained for at least 28 d. In conclusion, CNTO 530 is a long-lived EPO-R agonist that stimulates erythropoiesis in a manner similar to epoetin-alpha. These data suggest that CNTO 530 may be an effective treatment of anemia in humans.

Stimulating erythropoiesis in inflammatory bowel disease associated anemia

Anemia is a frequent complication in patients with inflammatory bowel disease (IBD), and is associated with decreased quality of life and increased rate of hospitalization. The primary therapeutic targets of IBD-associated anemia are iron deficiency and anemia of chronic disease. An important prognostic parameter of the success or failure of therapy is the outcome of the underlying disease. Iron deficiency should be appropriately managed with iron supplementation. However, the use of oral iron therapy is limited by several problems, the most important being gastrointestinal side effects leading occasionally to disease relapse and poor iron absorption. Intravenous iron preparations are more reliable, with iron sucrose demonstrating the best efficacy and tolerability. Treatment with erythropoietin or darbepoetin has been proven to be effective in patients with anemia, who fail to respond to intravenous iron. Patients with ongoing inflammation have anemia of chronic disease and may require combination therapy comprising of intravenous iron sucrose and erythropoietin. After initiating treatment, careful monitoring of hemoglobin levels and iron parameters is needed in order to avoid recurrence of anemia. In conclusion, anemia in the setting of IBD should be aggressively diagnosed, investigated, and treated. Future studies should define the optimal dose and schedule of intravenous iron supplementation and appropriate erythropoietin therapy in these patients.

Erythropoietin after a century of research: younger than ever

In the light of the enthusiasm regarding the use of recombinant human erythropoietin (Epo) and its analogues for treatment of the anaemias of chronic renal failure and malignancies it is worth remembering that today's success has been based on a century of laborious research. The concept of the humoral regulation of haematopoiesis was first formulated in 1906. The term 'erythropoietin' for the erythropoiesis-stimulating hormone was introduced in 1948. Native human Epo was isolated in 1977 and its gene cloned in 1985. During the last 15 yr, major progress has been made in identifying the molecules controlling Epo gene expression, primarily the hypoxia-inducible transcription factors (HIF) that are regulated by specific O2 and oxoglutarate requiring Fe2+-containing dioxygenases. With respect to the action of Epo, its dimeric receptor (Epo-R) has been characterised and shown to signal through protein kinases, anti-apoptotic proteins and transcription factors. The demonstration of Epo-R in non-haematopoietic tissues indicates that Epo is a pleiotropic viability and growth factor. The neuroprotective and cardioprotective potentials of Epo are reviewed with a focus on clinical research. In addition, studies utilising the Epo derivatives with prolonged half-life, peptidic and non-peptidic Epo mimetics, orally active drugs stimulating endogenous Epo production and Epo gene transfer are reviewed.

Recent advances in erythropoietic agents in renal anemia

Red cell production in chronic kidney disease is usually too low to maintain a normal haemoglobin, and thus anaemia develops in a large proportion of patients. The ability to stimulate erythropoiesis in the bone marrow by the use of therapeutic agents has only been possible in the last 20 years, initially with recombinant human erythropoietin (epoetin), and later darbepoetin alfa. Many new agents are, however, in clinical development, and these include CERA, Hematide, and HIF stabilisers, in addition to the imminent launch of biosimilar epoetins. The main issue with biosimilars is the unknown risk of immunogenicity. CERA is a large molecule, approximately twice the size of epoetin, which was created by integrating a single polymer chain into the erythropoietin molecule. CERA has a much prolonged half-life, and Phase II and III clinical trials have investigated administration of CERA every 3 or 4 weeks. Hematide is derived from original research on the erythropoietin-mimetic peptides, and is in Phase II of its clinical trial programme. Again, this compound is being investigated as a once-monthly administration. The HIF stabilizers are orally-active inhibitors of the enzyme that degrades hypoxia-inducible factor (prolyl hydroxylase), and this leads to upregulation of erythropoietin gene expression. Other strategies for stimulating erythropoiesis, briefly described in this review, are at an earlier stage of development. This is an exciting and rapidly developing area of scientific and translational research.

Multivalency—a way to enhance binding avidities and bioactivity—preliminary applications to EPO

Multivalency has advantages over monovalency for binding interactions and even for activity. In particular, avidity is higher since the off-rate of a multivalent species is much slower than that of a monomer. This is particularly profitable for ligand-binding receptors that require dimerization for activity, such as the receptor of erythropoietin (EPOR). Peptides that mimic the action of erythropoietin (EPO) have been described with no sequence similarity with the human hormone: erythropoietin mimetic peptide (EMP) and EPO receptor peptide (ERP). These two peptides have similar activity but interact through different sites on the EPOR. Here, we describe the construction of several new synthetic homo- and hetero-dimers based on EMP-ERP sequences. To link the monomeric molecules together, several monodisperse polyamide linkers of different lengths were synthesized with dialdehyde functionalities. Chemoselective oxime chemistry was used to obtain homogeneous constructs. Certain chemical incompatibilities were dealt with via a protection approach. The oximes are stable under normal conditions and so lend themselves to biological testing.

New agents that stimulate erythropoiesis

Recombinant human erythropoietin (rhEpo) has proven to be remarkably safe and effective for treatment of anemias, primarily those secondary to renal disease and malignancy. Despite the worldwide use of rhEpo, concerns about its cost, the need for frequent parenteral administration, and the development of anti-Epo antibodies have prompted development of improved agents to stimulate erythropoiesis. Three strategies appear to be particularly promising. The half-life of Epo in the circulation can be prolonged by the addition of N-linked carbohydrate groups, by formation of adducts with polyethylene glycol, and by preparation of Epo multimers. Second, mimetic peptides can effectively trigger signal transduction at the Epo receptor, thereby boosting red-cell production. Finally, the hypoxia inducible transcription factor (HIF) can be pharmacologically induced by oral agents, resulting in enhanced expression not only of endogenous Epo but also of other genes important in the regulation of erythropoiesis.

Novel strategies for stimulating erythropoiesis and potential new treatments for anaemia

As with many other therapeutic areas in modern-day medicine, scientific advances in drug development (using such techniques as recombinant DNA technology, site-directed mutagenesis, pegylation of molecules, peptide library screening, and gene transfer) have resulted in the development of potential new agents and strategies for stimulating erythropoiesis. These advances are of possible benefit in treating anaemia due to various causes, including chronic renal failure. Several new treatments will soon become clinically available, while others are at present at an early stage of development but are nevertheless of scientific interest. We review these new therapeutic strategies, and discuss at what stage some of the newer products are in relation to their clinical development programme.

The prolonged half-lives of new erythropoietin derivatives via peptide addition

Erythropoietin, or Epo, is a hematopoietic cytokine that promotes erythropoiesis, and recombinant human Epo has been used in the treatment of anemia in various chronic diseases. Here, we have constructed novel Epo derivatives with prolonged half-lives by adding peptides to the carboxy terminus of Epo without using linkers. The fused peptides were selected from the carboxy terminal region of human chorionic gonadotropin (hCG) or human thrombopoietin (hTpo), which promote the proper folding, secretion, and stabilization of bioactive glycoproteins. Addition of these peptides did not interfere with secretion or receptor binding, and significantly increased the in vivo half-life of human Epo, as measured by intravenous administration in rats. The plasma half-life of the Epo constructs was longest when the carboxy terminal 28 aa of the beta subunit of hCG was added (Epo-CGC), a half-life that was slightly longer than NESP (Aranesp), which is the most effective Epo product in current clinical use. The transformation of four Ser glycosylation sites to Ala on the CGC sequence also lengthened the plasma half-life of Epo, indicating that the in vivo stabilizing effect of the hCG peptide was due to both structures within the peptide itself and its O-glycosylations. The application of the carboxy terminal half of hTpo also resulted in remarkably reduced elimination of the Epo chimera (Epo-TpC), possibly due to protection by the TpC sequence. The in vivo hematopoietic activity of Epo derivatives in mice was consistent with their pharmacokinetic profiles. Therefore, these derivatives with prolonged half-lives may provide opportunities for developing new Epo therapeutics with less frequent administration.

Two wrongs can make a right: dimers of prolactin and growth hormone receptor antagonists behave as agonists

Prolactin (PRL) and GH have two distinct binding sites (site 1 with high affinity; site 2 with low affinity) that each interact with a PRL receptor (PRLR) to form a functional receptor dimer that activates signal transduction. The G129R mutation in PRL and the G120R mutation in GH disrupt the structural integrity of site 2 such that the ligands retain the ability to bind to the first receptor with high affinity, but act as receptor antagonists. In this study, we examined the ability of monomeric and dimeric forms of these ligands, human (h) PRL and hGH, and their antagonists (hPRL-G129R and hGH-G120R) to 1) bind to PRLRs; 2) induce conformational changes in PRLRs; 3) activate signaling pathways associated with the PRLR; and 4) mediate cell proliferation in vitro. In contrast to monomeric hPRL-G129R, homodimeric hPRL-G129R induced PRLR dimerization; activated Janus family of tyrosine kinases 2/signal transducer and activator of transcription 5, Ras/Raf/MAPK kinase/Erk, and phosphatidylinositol 3-kinase/Akt signaling; and stimulated Nb2 cell proliferation. Similarly, homodimeric hGH-G120R was able to mediate signaling via the PRLR and to stimulate Nb2 cell proliferation. These experiments demonstrate that a ligand must have two functional binding sites, but that these may be site 1 plus site 2 or two site 1's, to elicit receptor-mediated signal transduction. The size of the ligand plays less of a role in receptor activation, suggesting that the extracellular portion of the PRLR (and possibly the GH receptor) is rather flexible and can accommodate larger ligands. These findings may have implications for designing multifunctional therapeutics that target this class of cytokine receptors.

Metabolic Functionalization of Recombinant Glycoproteins

Glycoproteins are essential for cellular communication and are the most rapidly growing class of therapeutic agents. Chemical modification of glycoproteins has been employed to improve their in vivo efficacy or to label them for detection. Methods for the controlled derivatization of glycoproteins are presently limited by the repertoire of natural amino acid side chain and carbohydrate functionalities. Here, we use metabolic oligosaccharide engineering to introduce a bioorthogonal functional group, the azide, into cellular and recombinant glycoproteins for subsequent chemical elaboration via Staudinger ligation. As most therapeutic glycoproteins are sialylated and require this saccharide for optimal pharmacokinetics, we targeted sialic acid as a host for azides using N-azidoacetylmannosamine (ManNAz) as a biosynthetic precursor. Metabolic conversion of ManNAz to N-azidoacetylsialic acid (SiaNAz) within membrane-bound and secreted glycoproteins was quantified in a variety of cell types. SiaNAz was found to comprise between 4% and 41% of total sialosides, depending on the system. Metabolic labeling of recombinant interferon-beta and GlyCAM-Ig was achieved, demonstrating the utility of the method for functionalizing N-linked and O-linked glycoproteins of therapeutic interest. More generally, the generation of recombinant glycoproteins containing chemical handles within their glycans provides a means for studying their behavior and for improving their in vivo efficacy.

Enhancement of therapeutic protein in vivo activities through glycoengineering

Delivery of protein therapeutics often requires frequent injections because of low activity or rapid clearance, thereby placing a burden on patients and caregivers. Using glycoengineering, we have increased and prolonged the activity of proteins, thus allowing reduced frequency of administration. Glycosylation analogs with new N-linked glycosylation consensus sequences introduced into the protein were screened for the presence of additional N-linked carbohydrates and retention of in vitro activity. Suitable consensus sequences were combined in one molecule, resulting in glycosylation analogs of rHuEPO, leptin, and Mpl ligand. All three molecules had substantially increased in vivo activity and prolonged duration of action. Because these proteins were of three different classes (rHuEPO is an N-linked glycoprotein, Mpl ligand an O-linked glycoprotein, and leptin contains no carbohydrate), glycoengineering may be generally applicable as a strategy for increasing the in vivo activity and duration of action of proteins. This strategy has been validated clinically for glycoengineered rHuEPO (darbopoetin alfa).

Rational design and engineering of therapeutic proteins

An increasing number of engineered protein therapeutics are currently being developed, tested in clinical trials and marketed for use. Many of these proteins arose out of hit-and-miss efforts to discover specific mutations, fusion partners or chemical modifications that confer desired properties. Through these efforts, several useful strategies have emerged for rational optimization of therapeutic candidates. The controlled manipulation of the physical, chemical and biological properties of proteins enabled by structure-based simulation is now being used to refine established rational engineering approaches and to advance new strategies. These methods provide clear, hypothesis-driven routes to solve problems that plague many proteins and to create novel mechanisms of action. We anticipate that rational protein engineering will shape the field of protein therapeutics dramatically by improving existing products and enabling the development of novel therapeutic agents.

The enigma of the metabolic fate of circulating erythropoietin (Epo) in view of the pharmacokinetics of the recombinant drugs rhEpo and NESP

Recombinant human erythropoietin (rhEpo) is a mainstay in the treatment of anaemia, primarily in renal failure. Because the half-life of circulating rhEpo is relatively short (4-8 h), the drug is usually administered 2-3 times weekly. Recently, a novel erythropoiesis-stimulating protein (NESP) with a longer half-life (24-26 h) has been approved. NESP possesses two additional N-glycans compared to endogenous Epo or rhEpo. The pharmacokinetics of rhEpo and NESP in humans have been investigated in detail. The composition of the N-glycans is clearly important in determining the biological activity and the velocity of the degradation of Epo and its analogues. However, due to the lack of knowledge of the main site and mechanism of the removal of Epo from circulation, the difference in survival of rhEpo and NESP has remained phenomenological. Investigators have implicated the liver, kidneys, and bone marrow as possible sites of the catabolism of Epo. However, while hepatocytes take up desialylated Epo, the liver does not appear to play a major role in the degradation of intact Epo. Likewise, renal Epo clearance is apparently of secondary importance. Studies showing non-linear pharmacokinetics of Epo suggest that Epo is eliminated by saturable mechanisms. The hormone, as well as the recombinant drugs, can be incorporated by erythrocytic progenitors and other tissues expressing the Epo receptor. The affinity of the Epo receptor for rhEpo is 4.3-fold higher than for NESP. Taken together, it seems most likely that native Epo, rhEpo and NESP are degraded following Epo receptor-mediated uptake, mainly in bone marrow.

Dimeric erythropoietin fusion protein with enhanced erythropoietic activity in vitro and in vivo

High doses of recombinant human erythropoietin (rhEpo) are required for the treatment of chronic anemia. Thus, it is clear that therapy for chronic anemia would greatly benefit from an erythropoietin derivative with increased erythropoietic activity rather than the native endogenous hormone. In this report, the activity of a human Epo-Epo dimer protein, obtained by recombinant technology, is described and compared with its Epo monomer counterpart produced under identical conditions. Although monomer Epo and dimer Epo-Epo had similar pharmacokinetics in normal mice, the increase in hematocrit value was greater with the dimer than with the monomer. Moreover, in clonogenic assays using CD34(+) human hematopoietic cells, the human dimer induced a 3- to 4-fold-greater proliferation of erythroid cells than the monomer. Controlled secretion of dimeric erythropoietin was achieved in beta-thalassemic mice by in vivo intramuscular electrotransfer of a mouse Epo-Epo plasmid containing the tetO element and of a plasmid encoding the tetracycline controlled transactivator tTA. Administration of tetracycline completely inhibited the expression of the mEpo dimer. On tetracycline withdrawal, expression of the Epo-Epo dimer resumed, thereby resulting in a large and sustained hematocrit increase in beta-thalassemic mice. No immunologic response against the dimer was apparent in mice because the duration of the hematocrit increase was similar to that observed with the monomeric form of mouse erythropoietin. (Blood. 2001;97:3776-3782)