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Summary Antibodies to first‐generation recombinant thrombopoietin (TPO) neutralized endogenous TPO and caused thrombocytopenia in some healthy subjects and chemotherapy patients. The second‐generation TPO receptor agonist romiplostim, having no sequence homology to TPO, was developed to avoid immunogenicity. This analysis examined development of binding and neutralising antibodies to romiplostim or TPO among adults with immune thrombocytopenia (ITP) in 13 clinical trials and a global postmarketing registry. 60/961 (6·2%) patients from clinical trials developed anti‐romiplostim‐binding antibodies post‐baseline. The first positive binding antibody was detected 14 weeks (median) after starting romiplostim, at median romiplostim dose of 2 µg/kg and median platelet count of 29.5 × 109/l; most subjects had ≥98·5% of platelet assessments showing response. Neutralising antibodies to romiplostim developed in 0·4% of patients, but were unrelated to romiplostim dose and did not affect platelet count. Thirty‐three patients (3·4%) developed anti‐TPO‐binding antibodies; none developed anti‐TPO‐neutralising antibodies. In the global postmarketing registry, 9/184 (4·9%) patients with spontaneously submitted samples had binding antibodies. One patient with loss of response had anti‐romiplostim‐neutralising antibodies (negative at follow‐up). Collectively, anti‐romiplostim‐binding antibodies developed infrequently. In the few patients who developed neutralising antibodies to romiplostim, there was no cross‐reactivity with TPO and no associated loss of platelet response.

A panel of 9 fully human monoclonal antibodies against human erythropoietin (EPO) with defined characteristics (non-neutralizing, neutralizing, various isotypes, affinities) representative of those evident in antibody-mediated pure red cell aplasia (PRCA) and non-PRCA patients were formulated and lyophilized. The panel was evaluated in a multi-centre international collaborative study comprising eighteen different laboratories using different assay platforms including those in routine use. These included binding assays, some based on use of novel technologies and neutralization assays predominantly employing EPO responsive cell-lines. Results showed that detection and titre varied depending on antibody characteristics and the method used. Only selective assay platforms were capable of detecting the diverse repertoire of EPO antibodies in the panel indicating that some clinically relevant antibodies are likely to be missed in some assays. Importantly, the clinical samples from PRCA patients were distinguished as antibody-positive and the healthy donor serum as antibody negative across all different platforms tested. For neutralization, data was generally consistent across the assays for the different samples regardless of the cell-line and the assay conditions. The heterogeneity in data from the study clearly indicated the need for reference standards for consistency in detecting and measuring EPO antibodies across different assay platforms for monitoring the safety and efficacy of erythropoiesis stimulating agents. Therefore, the WHO ECBS at its meeting in October'15 established the EPO antibody panel, available from NIBSC, to facilitate decision-making on assay selection for testing antibodies against human EPO, for evaluating assay performance of antibody assays for clinical use, for assay validation and for standardization.

We previously reported an increased incidence of thrombotic toxicities in Sprague-Dawley rats administered the highest dose level of a hyperglycosylated analog of recombinant human erythropoietin (AMG 114) for 1 month as not solely dependent on high hematocrit (HCT). Thereafter, we identified increased erythropoiesis as a prothrombotic risk factor increased in the AMG 114 high-dose group with thrombotic toxicities, compared to a low-dose group with no toxicities but similar HCT. Here, we identified pleiotropic cytokines as prothrombotic factors associated with AMG 114 dose level. Before a high HCT was achieved, rats in the AMG 114 high, but not the low-dose group, had imbalanced hemostasis (increased von Willebrand factor and prothrombin time, decreased antithrombin III) coexistent with cytokines implicated in thrombosis: monocyte chemotactic protein 1 (MCP-1), MCP-3, tissue inhibitor of metalloproteinases 1, macrophage inhibitory protein-2, oncostatin M, T-cell-specific protein, stem cell factor, vascular endothelial growth factor, and interleukin-11. While no unique pathway to erythropoiesis stimulating agent-related thrombosis was identified, cytokines associated with increased erythropoiesis contributed to a prothrombotic intravascular environment in the AMG 114 high-dose group, but not in lower dose groups with a similar high HCT.

Background. The antibody characteristics in erythropoiesis-stimulating agent (ESA)-treated patients who develop antibody-mediated pure red cell aplasia (PRCA; amPRCA) can be described as high-affinity, neutralizing anti-ESA antibodies with a mixed immunoglobulin G (IgG) subclass. The characteristics of an early-onset anti-ESA antibody response are not well documented, especially in the months prior to the development of amPRCA. Therefore, a detailed characterization of antiESA antibodies was performed in patients in both clinical studies and in a post-market setting. Both baseline and post-dose samples were tested and antibody-positive samples were characterized. Antibody characteristics such as concentration, isotype and specificity were evaluated in subjects with non-neutralizing anti-ESA antibodies and subjects that developed neutralizing anti-ESA antibodies associated with amPRCA. Methods. Serum samples were analyzed for the presence of anti-ESA antibodies, using a validated surface plasmon resonance (SPR)-based immunoassay or SPRIA. Results. Among the clinical studies, pre-existing nonneutralizing anti-ESA antibodies were found in 6% of the subjects from clinical studies in nephrology, oncology and congestive heart failure (CHF). After ESA treatment, 2.3% of the subjects developed binding, non-neutralizing antibodies with 0.1% confirmed as having an IgG isotype and were specific to the ESA protein. IgM antibodies were detected at baseline and post-ESA treatment and reported to be specific to the glycosylation of the ESA. No clinical study subjects progressed to amPRCA. In contrast, anti-ESA antibody-positive subjects from the postmarket setting with a confirmed IgG subclass were specific to the ESA protein. Subjects that had progressed to amPRCA were noted to have high antibody concentrations with neutralizing activity and a diverse IgG subtype. Conclusions. A low prevalence of non-neutralizing antiESA IgM specific to glycosylation on the ESA and IgG1 antibodies specific to the ESA protein was detected across all clinical patient populations. Patients with amPRCA were noted to have high IgG antibody concentrations, neutralizing antibodies and the presence of anti-ESA IgG4 antibodies.

Recombinant human erythropoietin (EPO) has been used therapeutically for more than two decades in the treatment of anemia. Although EPO is generally well tolerated, in rare cases, patients have developed anti-EPO antibodies that can negatively impact safety and efficacy. Therefore, the detection of antibodies against EPO is a regulatory requirement during clinical development and post-approval. Although it is a rare phenomenon, antibody-mediated pure red cell aplasia (PRCA) is a serious complication than can result from antibodies that develop and neutralize EPO as well as endogenous erythropoietin. Currently, there are no universally accepted analytical methods to detect the full repertoire of binding and neutralizing anti-EPO antibodies. A number of different methods that differ in terms of antibodies detected and assay sensitivities are used by different manufacturers. There is also a lack of antibody reference reagents, and therefore no consistent basis for detecting and measuring anti-EPO antibodies. Reference reagents, with established ranges, are essential to monitor the safety and efficacy of all erythropoiesis-stimulating agents (ESAs) structurally related to human erythropoietin. This is the first report of the development and characterization of a panel of fully human antibodies against EPO suitable as reference reagents. The characteristics of antibodies within the panel were selected based on the prevalence of non-neutralizing IgG and IgM antibodies in non-PRCA patients and neutralizing IgG antibodies, including IgG1 and IgG4, in antibody-mediated PRCA subjects. The reference panel includes antibodies of high- and low-affinity with binding specificity to neutralizing and non-neutralizing erythropoietin epitopes. The subclass of human antibodies in this reference panel includes an IgG1, IgG2, and IgG4, as well as an IgM isotype. This antibody panel could help select appropriate immunogenicity assays, guide validation, and monitor assay performance. Further, this human anti-ESA antibody panel may help set the limits of each assay platform in terms of the full repertoire of the anti-ESA antibodies, and may facilitate standardization of ESA immunogenicity reporting across assay platforms.

Functional cell-based assays are the preferred method to test for the presence of anti-rHuEPO neutralizing antibodies (NAbs). However, due to the unpredictable nature of test serum matrix effects on cell-based assays, confirmatory assays are essential for verifying NAb positive results observed during the course of sample testing. The cell-based assay used for the detection of NAbs described by Wei et al. [1] used 32D-EPOR cells, a murine myeloid cell line transfected with the human EPO receptor (EPOR). The 32D-EPOR cell line responded to either rHuEPO or murine interleukin 3 (mIL-3) with proliferation. NAbs were expected to only inhibit rHuEPO-induced cell proliferation and not mIL-3 induced proliferation. Due to reliance on proliferation, the results from this cell-based assay can be confounded by the presence of non-antibody inhibitory serum factors. This paper describes a strategy for confirming that the inhibition of rHuEPO-induced proliferation in a cell-based assay is only attributable to NAbs. The strategy of antibody depletion uses a resin mixture composed of Protein G Sepharose and Protein L Sepharose (Protein G/L resin) to significantly reduce the concentration of immunoglobulins of IgG, IgM and IgA isotypes from human serum prior to testing in the cell-based assay. If the reduction in immunoglobulins in a serum sample corresponds to a reduction in inhibition of EPO-induced proliferation, it would infer that EPO neutralizing activity is antibody-mediated and not due to non-antibody inhibitory serum factors.

Background. The immunological methods for detecting antibodies to erythropoiesis-stimulating agents (ESAs) differ in assay sensitivity. However, this parameter, routinely determined in clinical assays using a high-affinity nonhuman polyclonal antibody, gives a one-dimensional assessment of antibody detection. We compare three widely used immunological methods and evaluate the ability of each to detect mature human antibodies and human antibodies characteristic of an early immune response. Methods. The detection of anti-ESA antibodies was compared between a radioimmunoprecipitation (RIP) assay, an electrochemiluminescence (ECL) bridging enzyme-linked immunosorbent assay and a surface plasmon resonance (SPR)-based immunoassay. All three methods were validated for sensitivity, specificity and precision. Specimens from clinical studies or post market testing were categorized as pure red cell aplasia (PRCA) or non-PRCA and then analyzed in each method. Results. Among the antibody-mediated PRCA samples, which contain high affinity neutralizing antibodies, there was strong correlation between all methods. The results from non-PRCA sample analysis show high correlation between RIP and ECL methods; however, differences between the SPR immunoassay and the ECL and RIP were demonstrated. The samples that scored positive in the SPR immunoassay and negative by RIP and ECL were characterized to be of low antibody concentration, contained a high percentage of rapidly dissociating antibodies, or were antibodies of the IgM isotype. Conclusions. All three immunological methods are appropriate for detection of antibodies associated with antibodymediated PRCA. However, the SPR immunoassay platform detected an early, low affinity IgG and IgM antibody response as well as detected and characterized a pathogenic antibody response associated with antibody-mediated PRCA.

Background: Romiplostim (Nplate®) is a thrombopoietin (TPO) receptor agonist approved for the treatment of adult chronic immune thrombocytopenia (ITP). The formation of antibodies (Abs) against romiplostim may lead to a loss of response, and a theoretical risk exists that an immune response against romiplostim might lead to the formation of Abs that bind both romiplostim and native TPO. We therefore examined Abs against romiplostim and TPO in adult patients (pts) enrolled in clinical trials and in a global postmarketing registry based on spontaneously submitted requests for Ab testing. Methods: The clinical trial population included adult pts (age ≥18 years at screening) with ITP who received ≥1 dose of romiplostim in any of 13 completed romiplostim clinical trials, including phase 1 (n=1), phase 2 (n=3), phase 1/2 (n=1), phase 3 (n=5), and open-label extension (n=3) studies. Duration of romiplostim treatment varied among the trials. The postmarketing global registry population included romiplostim-treated adult pts with ITP who had blood samples sent for Ab testing. Serum samples from the clinical trials and the registry were assessed for romiplostim and TPO binding Abs in a surface plasmon resonance (SPR)-based immunoassay, and samples positive for binding were assessed for neutralizing activity in a cell-based bioassay, as previously described (Jawa et al. Ann Hematol 2010). Pt characteristics were summarized for those who did and did not develop Abs in the clinical trials. Pts in the registry found to have romiplostim neutralizing Abs were to be followed every 3 months for up to 12 months. No formal statistical analyses were conducted. Results: A total of 1046 romiplostim-treated pts from clinical trials were available for analysis. At baseline, 958 pts had romiplostim Ab results: 35 pts (3.7%) were positive for binding Abs and 1 pt (0.1%) was positive for neutralizing Abs. Post-baseline, 961 pts had romiplostim Ab results: 80 pts (8.3%) were positive for binding Abs and 4 pts (0.4%) were positive for neutralizing Abs, independent of the baseline result. Sixty (6.2%) pts negative for anti-drug Abs at baseline developed romiplostim binding Abs during the clinical trials. Characteristics of the pts who did and did not develop romiplostim binding Abs are presented in Table 1. Development of romiplostim binding Abs was more frequent in pts with ITP duration >3 years at baseline (70.0% vs 57.4%), prior splenectomy (48.3% vs 37.5%), and a history of allergies (21.7% vs 8.4%). Pts who developed Abs also had lower baseline TPO levels (84.8 pg/mL vs 104.6 pg/mL), lower baseline platelet counts (12.5× 109/L vs 20.5 × 109/L), and a higher number of previous ITP treatments (median 3 vs 2). Four of the 60 pts with romiplostim binding Abs developed romiplostim neutralizing Abs after treatment (0.38% of 1046 pts overall; Table 2). The emergence of neutralizing Abs did not appear to be related to romiplostim dose or platelet count. The neutralizing Abs were directed against the peptide component of romiplostim and did not bind native TPO. Pts in the clinical trials were also tested for TPO Abs. At baseline, 956 pts had TPO Ab results: 31 pts (3.2%) were positive for TPO binding Abs and 1 pt (0.1%) was positive for neutralizing Abs. Post-baseline, 960 pts had TPO Ab results: 33 pts (3.4%) were positive for TPO binding Abs and no pts were positive for neutralizing Abs. Of the 184 adult pts in the registry, 9 (4.9%) were positive for binding Abs: 5 were positive for romiplostim binding, 2 for TPO binding, and 2 for romiplostim and TPO binding. No predose Ab results were available for these pts. One pt received romiplostim for 11 months at a dose of 2 µg/kg and then experienced an abrupt fall in platelet count even as romiplostim dose was increased to 10 µg/kg. The pt tested positive for romiplostim binding and neutralizing Abs. Romiplostim was discontinued, and the pt was switched to alternative therapy. Conclusions: An analysis of pts in 13 clinical trials shows that pts with indicators of more severe disease (longer duration of ITP, prior splenectomy, and a higher number of previous ITP treatments) may be at increased risk of developing romiplostim binding Abs. Development of romiplostim neutralizing Abs was uncommon in the clinical trials, and these Abs did not bind native TPO. Data from a postmarketing registry showed that the overall risk of clinically significant immunogenicity following exposure to romiplostim is low. Disclosures Barger: Amgen Inc.: Employment, Equity Ownership. Boshier:Sanofi: Other: rents a room to a Sanofi employee; Amgen Inc.: Employment, Equity Ownership. Kim:Amgen Inc.: Employment, Equity Ownership. Mytych:Amgen Inc.: Employment, Equity Ownership. Park:Amgen Inc.: Employment, Equity Ownership. Kuter:Amgen Inc.: Consultancy; Argenx: Consultancy; Rigel: Consultancy, Research Funding; Dova Pharmaceuticals: Consultancy, Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy; Principia: Research Funding; Protalex: Research Funding; Pfizer: Consultancy; Bioverativ: Consultancy, Research Funding; ONO: Consultancy; Syntimmune: Consultancy; BMS: Research Funding.

Anti-drug neutralizing antibodies (NAbs) formed due to unwanted immunogenicity of a therapeutic protein point towards a mature immune response. NAb detection is important in interpreting the therapeutic's efficacy and safety in vivo. In vitro cell-based NAb assays provide a physiological system for NAb detection, however are complex assays. Non-cell-based competitive ligand binding (CLB) approaches are also employed for NAb detection. Instead of cells, CLB assays use soluble receptor and conjugated reagents and are easier to perform, however have reduced physiological relevance. The aim of this study was to compare the performance of CLB assays to established cell-based assays to determine the former's ability to detect clinically relevant NAbs towards therapeutics that (i) acted as an agonist or (ii) acted as antagonists by binding to a target receptor. We performed a head-to-head comparison of the performance of cell-based and CLB NAb assays for erythropoietin (EPO) and two anti-receptor monoclonal antibodies (AMG-X and AMG 317). Clinically relevant NAb-positive samples identified previously by a cell-based assay were assessed in the corresponding CLB format(s). A panel of 12 engineered fully human anti-EPO monoclonal antibodies (MAbs) was tested in both EPO NAb assay formats. Our results showed that the CLB format was (i) capable of detecting human anti-EPO MAbs of differing neutralizing capabilities and affinities and (ii) provided similar results as the cell-based assay for detecting NAbs in patient samples. The cell-based and CLB assays also behaved comparably in detecting NAbs in clinical samples for AMG-X. In the case of anti-AMG 317 NAbs, the CLB format failed to detect NAbs in more than 50% of the tested samples. We conclude that assay sensitivity, drug tolerance and the selected assay matrix played an important role in the inability of AMG 317 CLB assays to detect clinically relevant NAbs.

We recently reported results that erythropoiesis-stimulating agent (ESA)–related thrombotic toxicities in preclinical species were not solely dependent on a high hematocrit (HCT) but also associated with increased ESA dose level, dose frequency, and dosing duration. In this article, we conclude that sequelae of an increased magnitude of ESA-stimulated erythropoiesis potentially contributed to thrombosis in the highest ESA dose groups. The results were obtained from two investigative studies we conducted in Sprague-Dawley rats administered a low (no thrombotic toxicities) or high (with thrombotic toxicities) dose level of a hyperglycosylated analog of recombinant human erythropoietin (AMG 114), 3 times weekly for up to 9 days or for 1 month. Despite similarly increased HCT at both dose levels, animals in the high-dose group had an increased magnitude of erythropoiesis measured by spleen weights, splenic erythropoiesis, and circulating reticulocytes. Resulting prothrombotic risk factors identified predominantly or uniquely in the high-dose group were higher numbers of immature reticulocytes and nucleated red blood cells in circulation, severe functional iron deficiency, and increased intravascular destruction of iron-deficient reticulocyte/red blood cells. No thrombotic events were detected in rats dosed up to 9 days suggesting a sustained high HCT is a requisite cofactor for development of ESA-related thrombotic toxicities.

A surface plasmon resonance (SPR)-based biosensor immunoassay was developed and validated using the Biacore 3000 instrument to detect, semi-quantitate, and characterize serum antibodies against darbepoetin alfa (Aranesp) and epoetin alfa (EPOGEN). In this sensitive, dual-flow cell assay, epoetin alfa and darbepoetin alfa are covalently immobilized onto consecutive flow cells of a carboxymethyl dextran-coated sensor chip. Diluted human serum samples are injected sequentially over both surfaces. The binding of serum antibodies to the immobilized proteins are detected and recorded in real time based on the principles of SPR. Furthermore, antibody binding is confirmed with a secondary anti-human immunoglobulin antibody. Positive samples are further characterized to determine the relative concentration of the antibodies using an affinity-purified, rabbit anti-epoetin alfa antibody as a reference control. The assay can detect 80ng/ml and 100ng/ml of antibody to epoetin alfa and darbepoetin alfa, respectively. The dynamic range of the assay is from 0.078microg/ml to 10microg/ml using a rabbit antibody with demonstrated accuracy and intra- and inter-assay precision. Approximately 80 serum samples can be analyzed on each sensor chip while maintaining a stable baseline and consistent immunological reactivity. The analysis of serum samples from subjects administered with epoetin alfa or darbepoetin alfa provided evidence that the assay can detect varying concentrations of antibodies of different off rates, isotypes, and IgG subclasses.