Originally published on Peptide Briefs

Last week we covered the basics: a DOJ letter had tipped off a court case that could decide whether retatrutide counts as a peptide drug or a biologic, with real stakes for compounding and RUO sourcing either way. Oral arguments were scheduled for Thursday, September 24, before the Seventh Circuit.

That hearing happened. Here's what we know now, and why it turns out this case is about something bigger than amino acid counts.

What happened Wednesday

Oral arguments were held as scheduled. There's no ruling yet, and there won't be one from the bench, appellate courts don't decide cases live in the room. Written opinions typically take weeks to months after argument, so this is the beginning of a waiting period, not the end of the story.

But the filings leading up to the hearing revealed something that changes how to read this case.

What Lilly is actually asking for

The surface-level dispute is still the amino acid question: FDA counts alpha amino acids only, and says retatrutide falls short of the 40-amino-acid threshold that would make it a biologic. Lilly's own complaint says the count is 41 once you include a non-alpha residue. That fight is real, and it's still unresolved.

But Lilly isn't just asking the Seventh Circuit to referee that count. Here's the actual sequence: back in September 2025, a district court ruled that FDA's "analogous to a protein" reasoning was legally deficient, essentially, FDA hadn't explained why "analogous to a protein" should mean the same thing as "protein." That sent the analogous question back to FDA to redo.

Lilly is now asking the Seventh Circuit to skip that redo entirely. Instead of letting FDA take another pass at the "analogous" question, Lilly wants the appellate court to rule, as a matter of law, that retatrutide must be classified as a biological product, full stop. That's a meaningfully bigger ask than "make FDA explain itself better." It's "don't even let FDA try again, just decide it our way."

The DOJ's last-minute citation

Three days before oral argument, on September 21, the Department of Justice filed something called a Rule 28(j) letter, a short notice pointing the judges to a relevant new ruling. The case they cited: Franco v. Chobani.

Yes, the yogurt case. Stay with this, because the legal principle matters even though the product doesn't.

Franco v. Chobani was about whether Chobani's "Zero Sugar" yogurt could be sued over under state consumer protection law, despite containing four grams of allulose per serving. Chobani's defense leaned on an FDA guidance document from 2020 that said the agency wouldn't enforce sugar-labeling rules against allulose while it studied the issue further. The Seventh Circuit ruled in July 2026 that this didn't matter: FDA's guidance said the agency wouldn't enforce a rule, but it never said the underlying labeling was actually legal. Informal agency signals, the court said, aren't the same as a formal, binding decision. The lawsuit was allowed to proceed.

DOJ is borrowing that exact logic here. The likely concern: if Lilly argues that FDA's earlier informal handling of retatrutide's classification should carry legal weight, or should let the court shortcut FDA's own process, Franco is precedent for saying no, informal agency positions don't bind that way, and courts shouldn't treat them as if they do. It's a defensive citation, aimed at blocking exactly the kind of shortcut Lilly is asking for.

Why this matters beyond one compound

If the Seventh Circuit sides with Lilly's narrower ask, sending the analogous question back to FDA with clearer instructions, this stays a relatively contained dispute about one compound's classification.

If the court sides with Lilly's bigger ask, and rules directly that retatrutide is a biologic without letting FDA finish its own process, that's a different kind of precedent. It would say something about how much power an appellate court has to override an agency's own administrative process rather than sending questions back to the agency that's actually supposed to answer them. That precedent wouldn't stay contained to retatrutide. It would matter for every future peptide sitting near that same 40-amino-acid line, and arguably for agency processes well beyond peptide classification.

That's the real reason this case is worth tracking closely, not because of what it says about one molecule, but because of what it could say about who gets to make classification calls at all.

Where things stand

No ruling yet. Lilly's Q1 2027 Biologics License Application timeline proceeds regardless of how this resolves, they've said they're moving forward either way and will file under whichever pathway is operative when the time comes.

We'll cover the ruling itself when it lands, whenever that turns out to be.