Immgolis and Immgolis Intri: What the New Golimumab Biosimilars Tell Writers About Totality of Evidence

The Approval

In May 2026, FDA approved Immgolis (golimumab-sldi) as an interchangeable biosimilar to Simponi and Immgolis Intri (golimumab-sldi) as an interchangeable biosimilar to Simponi Aria. FDA described them as the first biosimilars approved for those respective reference products. Immgolis is administered subcutaneously and is approved for adult rheumatoid arthritis in combination with methotrexate and adult ulcerative colitis. Immgolis Intri is administered intravenously and is approved for adult rheumatoid arthritis in combination with methotrexate.

For regulatory writers, the useful signal is not just that another biosimilar entered the US market. It is the evidence hierarchy FDA chose to emphasize in the approval announcement: extensive analytical comparison across structural and functional quality attributes, followed by human pharmacokinetic similarity and immunogenicity results. That is the totality-of-evidence narrative in miniature.

The approval brings the US biosimilar count into the mid-80s, while the number of unique reference products with at least one approved biosimilar remains much smaller. Each new approval inside the corpus is precedent the next 351(k) writer can study — though, as we will return to, "precedent" and "comparator" are not the same thing.

The Evidence Pyramid Inverted

The traditional originator BLA or NDA narrative often begins with clinical efficacy and then supports that story with nonclinical, CMC, and clinical pharmacology evidence. A 351(k) biosimilar narrative works differently. The foundation is analytical similarity. Clinical pharmacology and immunogenicity then help address residual uncertainty. Comparative clinical efficacy data, where included, usually support the overall biosimilarity conclusion rather than serve as the primary evidence base.

FDA's Immgolis announcement reflects that order. The agency highlighted structural and functional product-quality comparisons first, including attributes known to affect safety and efficacy. It then described a human pharmacokinetic similarity study with comparable exposure and immunogenicity results. Clinical efficacy was not the center of the public rationale.

For the 351(k) writer, that order matters. The comparative analytical assessment is not a technical appendix to the clinical story. It is the spine of the application. The rest of the submission should explain how PK, immunogenicity, safety, and labeling support the residual uncertainty left after the analytical comparison.

This framing is not new — FDA has emphasized the totality-of-evidence approach in biosimilar guidance for years. What is new is how visible the hierarchy is becoming in the approval announcements themselves. Writers who default to leading the cover narrative with clinical efficacy, because that is how they were trained in originator filings, are misreading what the agency is actually putting at the top of its public reasoning.

What Interchangeability Adds

Interchangeability adds a substitution question to the biosimilar framework. A biosimilar may be prescribed in place of the reference product. An interchangeable biosimilar may be substituted at the pharmacy level, subject to state pharmacy substitution laws. The writing task is therefore not only to show biosimilarity, but to explain why substitution is not expected to increase risk or reduce effectiveness.

Historically, many interchangeability packages have relied on dedicated switching study designs. FDA's current thinking is evolving on whether such studies are always required, with the agency moving to streamline biosimilar development where scientific justification supports it. So the writing burden should be framed more broadly than "describe the switching study." The sponsor has to justify, using the totality of evidence, why patients can move between the reference product and the biosimilar without clinically meaningful differences in safety or effectiveness.

Three sections of an interchangeable 351(k) submission carry disproportionate weight under this framing.

The immunogenicity narrative. Whether the supporting evidence is a switching arm in the PK study, an immunogenicity sub-study, or a totality-of-evidence argument drawing on multiple sources, the writing has to make explicit what was measured, when it was measured, and what would have constituted a signal. Reviewers reading this section need to find the absence-of-signal conclusion stated against pre-specified thresholds, not in qualitative terms.

The substitution-risk narrative. The submission needs to articulate, in prose, why the reviewer should accept that the pharmacokinetic and pharmacodynamic similarity demonstrated in the studied population will hold in a substitution context. This is a writing problem more than a science problem. The science is identical to the non-interchangeable case; the framing has to do more work.

The labeling and patient-information narrative. The label and patient-facing materials still need to support safe use in a substitution context, but the core labeling is largely anchored to the reference product's approved conditions of use and safety profile. The writing has to align the labeling with the reference product's approved uses while accommodating the substitution context that interchangeability creates.

Interchangeability also operates inside a layered legal context. Pharmacy-level substitution without prescriber intervention depends on state law, payer policy, and dispensing practice — not on the interchangeable designation alone. Writers drafting the substitution-risk narrative should be careful not to overstate what the designation operationally enables.

The Growing Corpus, Carefully Read

The US biosimilar corpus is growing year over year, but the way that corpus is useful to the next sponsor is often misunderstood.

A biosimilar application is anchored to the reference product. Prior biosimilars do not become formal primary comparators in the analytical similarity package. The reference product remains the comparator of record. What prior approvals provide is precedent: how FDA has described analytical similarity for that class of molecule, how residual uncertainty was characterized, how interchangeability was framed, and how labeling was handled.

For the next golimumab biosimilar developer, Immgolis and Immgolis Intri do not replace Simponi and Simponi Aria as comparators. But they do become part of the public precedent environment. A strong writer will understand both layers: the formal statutory comparison to the reference product, and the practical precedent created by earlier approvals in the same molecule class.

As the corpus grows, the writing burden becomes more precedent-aware rather than more comparator-heavy. The writer still anchors the application to the reference product, but now has a larger public record to consider when framing analytical similarity, residual uncertainty, interchangeability, and labeling.

What This Means for the 351(k) Writer

Four practical takeaways from the Immgolis approval.

Lead the cover narrative with analytical similarity, not clinical efficacy. This is counterintuitive for writers trained on originator submissions but it is what the agency is reading first. The 351(k) cover letter and Module 2 summary should foreground the comparative analytical assessment.

Treat clinical pharmacology data as a bridge that addresses residual uncertainty. Where comparative clinical data are included, the writing function is to demonstrate that the analytical similarity translates to in-human comparability and that residual uncertainty is acceptably bounded. This is a bridging argument, not a treatment-effect argument, and the prose should reflect that posture.

Plan the interchangeability story even if not pursuing the designation at first filing. The agency may grant interchangeability on supplements rather than at original BLA. Writing the original submission with the substitution-risk framing in mind makes the supplement materially cheaper to file later.

Anchor every claim to a specific structural or functional similarity result, assay, or study finding. Reviewers in the biosimilar division work from the analytical similarity tables outward. Claims in the narrative that do not trace back to a specific assay, table, or study result are obvious candidates for reviewer questions.

The Broader Trajectory

The Immgolis approval is a useful reminder that biosimilar writing is not a clinical-efficacy storytelling exercise. It is a totality-of-evidence exercise. The strongest 351(k) narratives start with analytical similarity, use PK and immunogenicity to resolve residual uncertainty, and frame interchangeability as a substitution-risk question rather than a marketing claim.

As the US biosimilar corpus grows, writers will have more precedent to draw from. But the discipline remains the same: anchor the application to the reference product, make every claim traceable to a specific assay or study result, and ensure the narrative follows the evidence hierarchy FDA actually reads.