Biosimilar

The Biologics Price Competition and Innovation Act of 2009 created section 351(k), an abbreviated route for biologics. An applicant must show its product is highly similar to the reference product notwithstanding minor differences in clinically inactive components, and that there are no clinically meaningful differences in safety, purity and potency. The evidence is built from the bottom up: extensive analytical comparison first, then pharmacokinetic and immunogenicity comparison, then clinical study only where residual uncertainty remains.

Interchangeability is a separate and additional designation, and it is the one that permits pharmacy-level substitution without the prescriber, subject to state law. The first US biosimilar was filgrastim-sndz in 2015, and insulin glargine-yfgn became the first interchangeable biosimilar in 2021. FDA has been moving away from requiring dedicated switching studies to support interchangeability, and legislative changes to the distinction have been proposed, so the current requirement should be read from FDA's live guidance rather than assumed from an older account.

The point that matters clinically is that biosimilar was never intended to mean identical. Batch-to-batch variability exists within the reference product too, and the question the pathway answers is whether the differences fall inside that band. Anti-drug antibody data is the part of a biosimilar dossier worth reading closely, because immunogenicity is where a small structural difference would surface first.

Two errors recur. One is calling a copy of a peptide a biosimilar when the molecule is 40 residues or fewer and therefore a drug eligible for an abbreviated application; a generic semaglutide would be a generic. The other is describing an unlicensed foreign or compounded copy as a biosimilar, when the word denotes a regulatory status rather than a resemblance.