U.S. regulations require biosimilars to be highly similar to their reference product and demonstrate no clinically meaningful differences in safety, purity, or potency. For bio­similars of erythropoiesis-stimulating agents (ESAs), this standard is challenging because structural differences are likely, and their effect on safety and efficacy cannot be predicted from analytical studies. Thus, clinical trials should com­ pare hemoglobin, dose, and immunogenicity endpoints.


The Biologics Price Competition and Innovation Act of 2009 created the biosimilars approval pathway in the United States.1 The introduction ofbiosimilars is intended to foster price competition and improve patient access to biologic therapies.12 A biosimilar approved in the United States must be highly similar to its reference product in terms of analytical attributes and have no clinically meaningful differences from it in terms of safety, purity, and potency.1 As of 2014, at least five biosimilar applications were submitted for review by the U.S. Food and Drug Administration, including the first application for a bio­ similar of epoetin alfa. 3,6

To address the scientific and regulatory challenges of demonstrating biosimilarity, in 2012 the FDA published draft guidance for biosimilar manufacturers (i.e., sponsors), which were finalized in April 2015.7 The guidance describes requirements for the stepwise development exercise, including comparative analytical, nonclinical, clinical pharmacology, and immunogenicity studies and additional clinical studies as needed to demonstrate biosimilarity. For biosimilars of recombinant ESAs, achieving the necessary degree of similarity relies first and foremost on analytic comparability. Demonstration of biosimilarity can then be challenging because glycosylation profiles are likely to differ from the reference product’s profile.8, 9

It should be noted that potency and safety of ESAs can be exquisitely sensitive to small differences in structure.10, 11 In particular, the specific potency and pharmacokinetics (PK)

of ESAs are sensitive to changes in posttranslational modifications, particularly glycosylation; therefore, small differences in these attributes can affect the dose required to correct or maintain hemoglobin in patients with chronic kidney disease who have anemia. 2 ,13 The evidence submitted for biosimilar approval should therefore include key data and sensitive studies that can discriminate differences in purity, potency, and safety between the candidate and the reference biologic.

Also, post-approval epidemiologic studies have suggested that subtle differences in drug product, including impurity profiles, can affect immunogenicity of ESAs.14

Consistent with FDA guidance, residual uncertainties about the similarity of a biosimilar candidate should be addressed in part through preapproval clinical efficacy, safety, and immunogenicity studies using populations and endpoints that are most sensitive to potential differences in potency and immunogenicity, as well as robust post­ approval surveillance.7 The historical experience with epo­ etin biosimilar development in the European Union (EU), as well as industry experience with post-approval manu­ facturing changes, can inform the designs of sensitive clinical studies to demonstrate that there are no clinically meaningful differences in safety, purity, or potency.


Sensitivity to safety, purity, and potency

Biosimilar versions of ESAs have previously been approved in the EU and other jurisdictions. In particular, the EU has approved two independent versions of biosimilars to Eprex/Erypo (epoetin alfa; marketing approval since

1988), and the European Medicines Agency (EMA) has published a class-specific guideline addressing nonclinical and clinical considerations for development of biosimilar ESAs.15, 17 The EU experience with biosimilar ESAs over the last eight years may be relevant to the implementation of the FDA’s draft guidance for development of biosimilar ESAs in the United States.

A key concept in EMA and FDA biosimilars guidance is that head-to-head clinical efficacy studies in sensitive patient populations are typically required to confirm similar efficacy between a biosimilar candidate and a reference product.7,17 Such clinical studies are especially relevant when clinical pharmacology studies (i.e., PK and pharmacodynamics studies) may not be sufficiently sensitive to detect clinically relevant differences in efficacy.7,17

In the case of ESAs, the steady-state hemoglobin dose response in patients with CKD is relatively sensitive to small but relevant differences in the biologics potency. Accordingly, the EMA guidelines for biosimilar ESAs recommend that similar efficacy should be confirmed using a comparative hemoglobin correction or maintenance study in patients with CKDP For the maintenance study, patients stable on the reference ESA should be randomized to the reference or the ESA biosimilar candidate at their pre­viously established maintenance dose; then the population hemoglobin and ESA dose characteristics should be compared over a suitable evaluation period.17



Competition associated with devel­opment and market entry of biosimilars may bring benefits to patients in terms of cost and access. Different classes of innovator medicines will necessarily incorporate different study design considerations in generating evidence that the proposed biosimilar does not demonstrate clinically mean­ingful differences in purity, potency, and safety compared with the reference product. Our recommendations to ensure equivalent clinical potency include the following:

  • In vitro and in vivo functional assays and single-dose clinical PK studies are not sufficient to confirm equivalent clinical potency of ESAs, especially when there are observed differences in posttranslational modifications. A discriminating clinical comparison therefore includes a sensitive hemo­globin maintenance study in patients with CKD-including titration to equivalent hemoglobin and typically at least 20 weeks of dose titration­ allowing stabilization of dose, before measurement and assessment of the average maintenance dose during the subsequent four to eight weeks.
  • Some published biosimilar clinical trial protocols have listed mean hemoglobin change as the primary efficacy endpoint and the maintenance dose as a secondary endpoint. 15,16,18,19 In particular, it is noteworthy that the published clinical trial protocols for the first U.S. ESA biosimilar candidate include two separate hemoglobin maintenance studies in patients with chronic renal failure with study durations of 16 or 24 weeks for the subcutaneous (SC) and intravenous routes of administration, respectively.18, 19 The primary endpoints for these studies assess the hemoglobin levels during the last four weeks of the maintenance period, but the secondary endpoints both compare mean weekly dosage of ESA averaged across their respective double-blind maintenance periods.18, 19
  • In patients with CKD, particularly in the context of hemodialysis, hemoglobin is quite variable, and ESAs are routinely titrated over a broad range of doses to achieve the desired hemoglobin concentration. ESAs of widely differing potencies can be titrated to achieve the same hemoglobin con­ centrations. Thus, similar hemoglobin concentrations alone, although a precondition to assessing equivalent potency, are not a sufficient or meaningful sole primary endpoint in deter­ mining biosimilarity; dose equivalence must be established by demonstrating highly similar doses over an evaluation period after stabilizing the study population at highly similar hemoglo­bin concentration.1 Along with equivalent hemoglobin, the maintenance ESA dose after stabilization (and not over the entire study period) should be the co-primary endpoint for confirma­tory biosimilar clinical efficacy studies, consistent with the current EMA guideline on erythropoietins.
  • Real-world data for the average maintenance ESA dose at the population level may not be required from a regulatory perspective but may inform payer, formulary, and patient care practices. Therefore, it is likely that providers will wish to assess evidence from preapproval efficacy studies and also collect and compare real-world data on maintenance doses among approved ESAs.
  • Experience with originator and biosimilar ESAs clearly demonstrates that patients are more likely to develop antidrug antibodies following ESA administration via the SC route;17, 20 the median time to develop clinical manifestations of neutralizing antibodies is after nine months of treatment.21 Because of their low incidence, neutralizing antibodies are very unlikely to occur during a preapproval study, but they have been detected in at least one small biosimilar clinical safety study and, in that case, represent an increase in immunogenicity risk by several orders of magnitude relative to existing products.17, 23 Therefore, to minimize risk to patients, preapproval studies should always include com­ prehensive immunogenicity comparisons in immunocompetent patients receiving multiple SC doses, preferably using the SC route if SC administration will be included in the proposed indications or is expected to be widely used in practice.17, 23 Preapproval safety evaluations can reduce the uncertain­ ty of differences in relatively common events; however, it can only rule out hundreds-fold increases in rare events such as pure red cell aplasia (PRCA). For this reason, detection of uncommon product-specific safety events, including PRCA, requires robust risk­ management measures, including active pharmaco-vigilance, and pro­ vides the earliest possible detection of disproportionate risk relative to other ESAs. Such vigilance requires a bio­ logic naming approach that facilitates accurate tracing of adverse events to the correct product and consistent use of one product for a prolonged period. 24,25 It should be noted that, in addition to ESAs, the need for post­ approval safety monitoring to detect rare immunogenicity-related adverse events, as well as the need for clinical comparability studies to use sensitive study designs and endpoints, is appli­cable to development of other classes of biosimilars.


The authors acknowledge James Balwit, MS (Complete Healthcare Communications, LLC, Chadds Ford, Pa.), whose work was funded by Amgen Inc., for assistance in writing this manuscript.


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