Peptide Research Week in Review: 22-28 September 2026
TL;DR: This week: 7th Circuit heard Lilly's retatrutide appeal, FDA's Empower letter landed, and EASD 2026 opened in Milan.
What Happened in Peptide Research This Week (22-28 Sep 2026)?
The date our last roundup previewed arrived on 24 September, when the US Seventh Circuit Court of Appeals heard oral arguments in Eli Lilly and Company v. Kennedy, the appeal over whether retatrutide, Lilly's investigational triple agonist, is legally a biologic.[1] The rest of the week was regulatory: the FDA's 18 September warning letter to a large compounding pharmacy reached wider coverage,[3] public comment closed on 28 September for FDA's revised generic-peptide guidances,[4] and the European Association for the Study of Diabetes (EASD) annual meeting opened in Milan.[5]
There was no new clinical readout for retatrutide this week. Its most recent trial reporting remains Lilly's summer Phase 3 announcements, with the next scheduled presentation on 30 September. Everything below is third-party reporting on pharmaceutical and legal developments. It is not a description of, or endorsement of, any use of research reagents outside the laboratory.
What Happened at the Seventh Circuit Retatrutide Hearing?
Background: In September 2025 a federal district court upheld the FDA's conclusion that retatrutide does not meet the regulatory definition of a protein, but rejected the agency's reasoning on whether it could be “analogous” to a protein and sent that question back to the FDA. Lilly appealed, asking the Seventh Circuit to go further and direct the classification itself.[2]
Where it stands: Arguments were heard on 24 September and the panel has not ruled. Three days earlier, on 21 September, the Department of Justice filed a Rule 28(j) letter drawing the panel's attention to Franco v. Chobani, a new Seventh Circuit decision about how much weight courts give the FDA when it interprets a technical term in its own regulations.[1]
Full breakdown: Retatrutide's Seventh Circuit Hearing Was Held — What Happens Next · the DOJ Franco v. Chobani letter
Why it matters to the wider field: biologic status would change which exclusivity and compounding rules apply to retatrutide once Lilly files with the FDA, which the company has said it plans to do in the first quarter of 2027.[5] The case is a question of administrative law, not of clinical science.
What Did the FDA's Empower Pharmacy Warning Letter Say?
The FDA's warning letter, dated 18 September 2026, was addressed to Empower Clinic Services, LLC (dba Empower Pharmacy). It states that the pharmacy's tirzepatide/niacinamide and semaglutide/cyanocobalamin products appear to be essentially copies of FDA-approved drugs, that they were compounded in inordinate amounts, and that prescriptions lacked a genuine prescriber determination of “significant difference”. The letter also cites sterile-manufacturing deficiencies, including smoke studies, media fills and environmental monitoring.[3]
The regulatory lesson is narrow but useful: adding a vitamin to an approved drug does not, in the FDA's view, create a genuinely different product. Legal commentary published through the week debated how far the reasoning extends. We summarised the letter in our own explainer. It concerns a US pharmacy and approved medicines; it does not concern research reagents.
What Closed on 28 September for FDA's Generic-Peptide Guidance?
The public comment window on the FDA's revised draft product-specific guidances for certain generic peptide products, announced in the Federal Register on 29 July 2026, closed on 28 September.[4] The 17 peptides include tirzepatide and semaglutide. These guidances tell generic developers what bioequivalence evidence the agency expects when they seek approval to copy an approved peptide drug. That is a licensed-pharmaceutical pathway and is separate from the research-use status of compounds sold for laboratory work. See our full breakdown.
What Is on the EASD 2026 Agenda for Retatrutide?
EASD's 62nd annual meeting runs 28 September to 2 October 2026 in Milan. In its 15 September announcement, Lilly listed retatrutide among the compounds it will present, with a dedicated retatrutide symposium on Wednesday 30 September.[5] Trade press previewing the meeting said the retatrutide session would cover Phase 3 diabetes-population data.[6] We will report on the data only once Lilly publishes it. Earlier trial coverage is collected in our EASD 2026 preview and the main retatrutide guide.
What Should Researchers Watch Next?
| Track | Next milestone | Expected timing |
|---|---|---|
| Retatrutide (Lilly) | EASD symposium presentation | 30 September 2026, Milan |
| Eli Lilly v. Kennedy | Seventh Circuit decision | No date set; could take months |
| FDA generic-peptide guidances | Agency review of submitted comments | No date announced |
| Retatrutide (Lilly) | FDA submission | Q1 2027 (company guidance) |
Does This Week's News Change UK Research-Reagent Supply?
No. A US appellate hearing, a US warning letter and US generic-drug guidance do not alter the Human Medicines Regulations 2012 or MHRA guidance in Great Britain and Northern Ireland. The constant across this week's stories is that regulatory status attaches to a pharmaceutical product, whereas identity and purity attach to a specific batch. For the batch-level side, see our guides on reading a certificate of analysis and whether research peptides are legal in the UK.
Retatrutide is supplied as a research reagent only. It is not a medicine and has not been evaluated by the MHRA, HPRA or FDA. Not for human or veterinary consumption. See our Research Use Policy and MHRA Statement.
References
- Velox Peptides. Retatrutide's Seventh Circuit Hearing Was Held (25 September 2026), citing oral argument on 24 September 2026 in Eli Lilly and Company v. Kennedy. veloxpeps.com
- Reason. Can a new weight loss treatment escape the legal and regulatory thicket? 10 September 2026. reason.com
- US FDA. Warning letter to Empower Clinic Services, LLC dba Empower Pharmacy, 18 September 2026. fda.gov
- Federal Register. Product-Specific Guidances; Revised Draft Guidances for Industry; Availability, 29 July 2026. federalregister.gov. FDA notice: fda.gov
- Eli Lilly and Company. Lilly to present new data on Foundayo, retatrutide, and eloraTZP at EASD 2026. Press release, 15 September 2026. investor.lilly.com
- Medical Daily. Retatrutide Diabetes Results, Mounjaro Heart Data, and Wegovy Pill Switching Study Lead EASD 2026 Meeting Opening in Milan. medicaldaily.com
- Velox Peptides. Peptide Research Week in Review: 22-28 September 2026, for the preceding week. veloxpeps.com
Frequently Asked Questions
What's the biggest peptide research story this week (22-28 September 2026)?
The Seventh Circuit heard oral arguments on 24 September in Eli Lilly and Company v. Kennedy, the appeal over whether Lilly's investigational retatrutide is a biologic. No ruling has been issued; the panel's decision could take months. Around it, the FDA's 18 September warning letter to Empower Pharmacy, the 28 September close of comments on FDA's revised guidance for 17 generic peptides, and the opening of EASD 2026 in Milan filled the week.
Has the Seventh Circuit ruled on retatrutide's classification?
Not as of 28 September 2026. Oral arguments were held on 24 September and the appeal is under consideration. Appellate rulings commonly take weeks or months, and the remand question that started the dispute remains unresolved until the court decides.
What did the FDA's Empower Pharmacy warning letter say?
The FDA's letter dated 18 September 2026 to Empower Clinic Services, LLC (dba Empower Pharmacy) says its tirzepatide/niacinamide and semaglutide/cyanocobalamin products appear to be essentially copies of FDA-approved drugs, and cites sterile-manufacturing deficiencies. It is a US compounding-law action and does not involve research reagents.
When is retatrutide data being presented at EASD 2026?
Eli Lilly's 15 September announcement lists a retatrutide symposium on Wednesday 30 September 2026 at EASD in Milan (28 September to 2 October). This roundup reports the scheduling only; results will be covered once Lilly publishes them.
Does this week's news change UK research-reagent supply?
No. The court case, the FDA letter and the FDA guidance concern US regulation of pharmaceutical products. None of it changes the Human Medicines Regulations 2012 or MHRA guidance in the UK. Velox Peptides supplies retatrutide strictly as an HPLC-verified in vitro research reagent, not a medicine.