Designating interchangeability between a biosimilar and its reference drug allows a pharmacist to substitute the biosimilar product for the reference product without involving the prescribing physician. Interchangeability can therefore promote strong price competition, as can be seen by competition between brand-name and generic small-molecule drug products.

The US Food and Drug Administration (FDA) released draft guidance on the interchangeability of biosimilars with their reference biologicals in January 2017 [2]. The comment period on the guidance ended on 19 May 2017 [3]. However, according to Professor Frank, this guidance has ‘created confusion in the industry’.

A demonstration of biosimilarity requires demonstrating that there are no clinical differences in outcome. But to demonstrate interchangeability, FDA requires that the biosimilar ‘can be expected to produce the same clinical result as the reference product in any given patient’ and ‘the risk in terms of safety or diminished efficacy of alternating or switching between use of the biological product and the reference product is not greater than the risk of using the reference product without such alternation or switch’ [4].

The guidance does not go into details about whether this requirement involves meeting an additional standard or producing more data. Conducting additional studies in specific patient populations would be very expensive for biosimilars makers.

FDA is concerned about immune system responses to substitution of biosimilars for reference products. However, regulators in Europe have noted that such risks are very small and in fact Austria is therefore narrowing the distinction between biosimilarity and interchangeability [5].

To date, there are no interchangeable biosimilars on the market in the US. This, according to Professor Frank, is due to the slow development of clear US guidance on interchangeability, which in turn is limiting price competition.

Conflict of interest
Disclosure forms provided by the author [1] are available at NEJM.org.

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References
1.  Frank RG. Friction in the path to use of biosimilar drugs. N Engl J Med. 2018;378(9):791-3.
2.  GaBI Online - Generics and Biosimilars Initiative. FDA issues draft guidance on biosimilar interchangeability [www.gabionline.net]. Mol, Belgium: Pro Pharma Communications International; [cited 2018 Apr 27]. Available from: www.gabionline.net/Guidelines/FDA-issues-draft-guidance-on-biosimilar-interchangeability
3.  GaBI Online - Generics and Biosimilars Initiative. FDA extends comment period for interchangeability guidance [www.gabionline.net]. Mol, Belgium: Pro Pharma Communications International; [cited 2018 Apr 27]. Available from: www.gabionline.net/Guidelines/FDA-extends-comment-period-for-interchangeability-guidance
4.  GaBI Online - Generics and Biosimilars Initiative. Establishing interchangeability for biosimilars in the US [www.gabionline.net]. Mol, Belgium: Pro Pharma Communications International; [cited 2018 Apr 27]. Available from: www.gabionline.net/Reports/Establishing-interchangeability-for-biosimilars-in-the-US
5.  Baumgärtel C. Austrian medicines authority positive towards biosimilar interchangeability. Generics and Biosimilars Initiative Journal (GaBI Journal). 2017;6(1):41. doi:10.5639/gabij.2017.0601.009

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