Insulin efsitora alfa (basal Fc)

Insulin efsitora alfa (formerly "basal insulin Fc" / BIF / LY3209590) is an investigational once-weekly basal insulin from Eli Lilly. It is built by fusing a single-chain insulin variant — engineered with reduced insulin-receptor affinity — to a human IgG2 Fc domain.¹ The Fc portion lets the molecule ride the body's neonatal Fc receptor (FcRn) recycling pathway, which rescues it from degradation and stretches its action out to a week.² Like all basals, its job is steady background coverage; it does not replace mealtime (bolus) insulin, which people with type 1 diabetes still inject at meals.³

PK/PD — the numbers. Efsitora's defining feature is an extraordinarily long, flat profile. Its half-life is roughly 17 days after a single dose and about 19 days at steady state.⁴⁵ Glucose-lowering effect peaks gently around days 4-5 after a dose, and the profile is remarkably even: at steady state the drug-level peak-to-trough ratio is just 1.16, and the glucose-lowering peak-to-trough ratio only 1.07 — essentially a flat line across the week.⁵ Because of that long half-life, reaching steady state would otherwise take 8-10 weeks, so Lilly uses a one-time loading dose to bring levels up quickly; steady state is then reached between the 4th and 6th weekly dose.⁵ Within-week exposure varies little, and immunogenicity is very low (treatment-emergent anti-drug antibodies in ~0.6% of type 1 participants), with no measurable effect on clearance, efficacy or safety.⁶

Absorption variability & exercise. The flat depot means little day-to-day swing in background insulin — good for predictability.⁴ The trade-off is rigidity: a fixed weekly dose cannot be reduced for a planned active day, illness, or hormonal change. In the type 1 trial, excess lows were driven by non-nocturnal episodes, and 64% of severe hypoglycemia events occurred during the titration period — pointing to dosing, not the molecule, as the challenge.⁷⁸

Delivery. Efsitora is given subcutaneously once a week. Lilly has tested fixed doses in a single-use autoinjector pen, with a multi-dose KwikPen for people needing higher doses.⁹

Evidence in type 1 diabetes. In the Phase 3 QWINT-5 trial (NCT05463744; 692 adults, 52 weeks), weekly efsitora plus mealtime lispro was non-inferior to daily degludec: HbA1c fell 0.51% vs 0.56% (treatment difference 0.052%).⁷ But combined level 2/3 hypoglycemia was higher with efsitora (14.03 vs 11.59 events per patient-year; rate ratio 1.21), and severe hypoglycemia hit 10% of efsitora users vs 3% on degludec, concentrated in the first 12 weeks.⁷ An earlier Phase 2 trial showed similar HbA1c but higher fasting glucose and slightly lower time-in-range (56.1% vs 58.9%).¹⁰

The meta-analyses now disagree — and we won't paper over it. A 2025 type-1-specific systematic review found comparable HbA1c but a roughly 2.5-fold higher rate of severe hypoglycemia with weekly basals in type 1 diabetes.¹¹ A June 2026 meta-analysis of five efsitora RCTs (2,562 participants, all comparing against degludec) found no statistically significant difference in hypoglycemia — overall, level 2/3, or nocturnal — in either diabetes type, with an overall HbA1c treatment difference of just 0.08% (95% CI -0.01 to 0.17).¹² The catch is that the newer analysis pools type 1 and type 2 together, and its type 1 subgroup is small and wide-intervalled (HbA1c treatment difference 0.10%, 95% CI -0.64 to 0.85) — underpowered for exactly the question a person with type 1 is asking. The hard number that has not moved is from QWINT-5 itself: severe hypoglycemia in 10% of efsitora users versus 3% on degludec, concentrated in the first 12 weeks.⁷ Until a properly powered type 1 trial with a refined titration protocol reports, we treat weekly basal insulin in type 1 as an unresolved safety question, not a solved one.

Approvals, access & cost. Efsitora is not yet approved anywhere, but the first regulatory step landed in June 2026: at its 22-25 June meeting the EMA's CHMP adopted a positive opinion recommending EU marketing authorisation for Onswik (efsitora's EU brand name) in adults with type 2 diabetes, with a European Commission decision expected to follow.¹³ A positive opinion is a recommendation, not an approval. A US FDA decision — also type 2 only — is due in Q3 2026.¹⁴ Crucially for readers here: neither filing covers type 1 diabetes. The EU opinion rests on the QWINT 1-4 type 2 trials; QWINT-5, the type 1 study, is not part of it, and the EMA notes that the most common side effect of Onswik is hypoglycaemia.¹³ Nothing about this changes type 1 access. There is still no price and no biosimilar. Context matters, too: the FDA's advisory committee voted against the rival weekly insulin icodec in type 1 diabetes over hypoglycemia, so the type 1 pathway for weekly basals faces real scrutiny.¹⁵

What's coming. The near-term opportunity is type 2 diabetes, where QWINT-1/3/4 showed clean non-inferiority with low severe-hypoglycemia rates — and where the first regulatory green light has now arrived in the form of the CHMP's positive opinion.⁷¹⁶¹³ That the first recommendation for a once-weekly basal insulin explicitly excludes type 1 diabetes is itself the story: type 1 use will hinge on refined titration protocols to tame the early-weeks hypoglycemia signal.⁷¹⁶ If it is eventually approved for type 1, efsitora would cut basal injections from 365 to 52 per year — a meaningful adherence lever, especially for people who struggle with daily dosing.³

References

1. Heise T, Chien J, Beals JM, et al. Pharmacokinetic and pharmacodynamic properties of the novel basal insulin Fc (insulin efsitora alfa). Diabetes Obes Metab (2023). https://pubmed.ncbi.nlm.nih.gov/36541037/ ↩

2. Rosenstock J, Juneja R, Beals JM, et al. The Basis for Weekly Insulin Therapy: Evolving Evidence With Insulin Icodec and Insulin Efsitora Alfa. Endocr Rev (2024). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11091825/ ↩

3. Denimal D. Emerging perspectives on once-weekly insulins in type 1 and type 2 diabetes: a mini-review. Front Endocrinol (2025). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12420296/ ↩ ↩²

4. Heise T, Chien J, Beals JM, et al. PK/PD properties of basal insulin Fc (insulin efsitora alfa). Diabetes Obes Metab (2023); DOI 10.1111/dom.14956. https://doi.org/10.1111/dom.14956 ↩ ↩²

5. Leohr J, Klein O, Heise T, et al. Characterisation of steady-state pharmacokinetics and glucodynamics of once-weekly insulin efsitora alfa in type 2 diabetes. Diabetes Obes Metab (2026). https://pubmed.ncbi.nlm.nih.gov/41725424/ ↩ ↩² ↩³

6. Wang Y, Wang S, Wang W, et al. Low immunogenicity of insulin efsitora alfa in type 1 and type 2 diabetes. Diabetes Obes Metab (2025). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12803677/ ↩

7. Bergenstal RM, Weinstock RS, Mathieu C, et al. Once-weekly insulin efsitora alfa versus once-daily insulin degludec in adults with type 1 diabetes (QWINT-5): a phase 3 randomised non-inferiority trial. Lancet (2024);404:1132-1142. https://pubmed.ncbi.nlm.nih.gov/39270686/ ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

8. Once-weekly basal insulin therapy in type 1 diabetes: a paradigm shift or a work in progress? Am J Health Syst Pharm (2025); DOI 10.1093/ajhp/zxaf169. https://doi.org/10.1093/ajhp/zxaf169 ↩

9. Eli Lilly. In a first-of-its-kind fixed dose study, once weekly insulin efsitora alfa leads to A1C reduction similar to daily insulin (QWINT-1). PR Newswire (2024). https://www.prnewswire.com/news-releases/in-a-first-of-its-kind-fixed-dose-study-once-weekly-insulin-efsitora-alfa-leads-to-a1c-reduction-similar-to-daily-insulin-302238497.html ↩

10. Kazda CM, Bue-Valleskey JM, Chien J, et al. Novel Once-Weekly Basal Insulin Fc Achieved Similar Glycemic Control With a Safety Profile Comparable to Insulin Degludec in Patients With Type 1 Diabetes. Diabetes Care (2023);46:1052-1059. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10154655/ ↩

11. Di Gioia L, Di Molfetta S, Caruso I, et al. Efficacy and safety of once-weekly basal insulin therapy in people with type 1 diabetes: a systematic review and meta-analysis. Diabetes Obes Metab (2025). https://pubmed.ncbi.nlm.nih.gov/41048193/ ↩

12. Copur S, Mizrak B, Ozbek L, et al. Once-Weekly Subcutaneous Insulin Efsitora for the Management of Diabetes Mellitus: A Systematic Review and Meta-Analysis Study. Medeniyet Medical Journal (2026);41(2):203-211; DOI 10.4274/MMJ.galenos.2026.35858. https://doi.org/10.4274/MMJ.galenos.2026.35858 ↩

13. European Medicines Agency. Meeting highlights from the Committee for Medicinal Products for Human Use (CHMP) 22-25 June 2026 — positive opinion for Onswik (insulin efsitora alfa) in type 2 diabetes (2026). https://www.ema.europa.eu/en/news/meeting-highlights-committee-medicinal-products-human-use-chmp-22-25-june-2026 ↩ ↩² ↩³

14. Prime Therapeutics. FDA Decisions Expected: July 2026 (insulin efsitora alfa, type 2 diabetes) (2026). https://www.primetherapeutics.com/fda-decisions-expected-july-2026 ↩

15. DeLuca A, Schultz A, Ofori H, et al. The Safety, Efficacy, and Clinical Use of Novel Once-Weekly Insulins in the Management of Diabetes. Expert Opin Drug Saf (2025); DOI 10.1080/14740338.2025.2593372. https://doi.org/10.1080/14740338.2025.2593372 ↩

16. Ramachandran A, Batra J, Desouza C. Evaluating once-weekly insulin efsitora alfa for adults with type 2 diabetes. Expert Opin Pharmacother (2026); DOI 10.1080/14656566.2026.2667324. https://doi.org/10.1080/14656566.2026.2667324 ↩ ↩²