Thymosin alpha-1 vs LL-37

Thymosin alpha-1 and LL-37 both participate in immune biology, but the selected clinical studies test different interventions. Thymosin alpha-1 was injected as an adjunct to sepsis care; synthetic LL-37 was applied to chronic leg ulcers. Larger trials did not establish the hoped-for overall mortality or ulcer-closure benefits, while earlier favorable findings and biological activity remain important to understand.

Research reviewed 2026-10-15. Educational information only.

Immune-cell regulation and a barrier-defense peptide begin with different molecules

Thymosin alpha-1 is an N-acetylated 28-amino-acid peptide. Synthetic thymalfasin has the same amino-acid sequence as the naturally occurring peptide; acetate is a salt form that requires separate material characterization. Thymalin is a complex thymus preparation, and its extract studies are not studies of the defined 28-residue molecule.

LL-37 is a 37-residue fragment released from the larger human precursor protein hCAP18. Its role in innate defense helped motivate research on inflammation and tissue repair. Human skin-wound studies found more hCAP18/LL-37 staining in healing acute wounds than in chronic ulcer-edge tissue. In separate cultured human skin wounds, antibodies directed against LL-37 inhibited rebuilding of the surface layer. Finding an endogenous peptide in repair and disrupting it experimentally supports a biological role; administering a synthetic preparation to patients is a separate test. Heilborn and colleagues (2003).

The main differences

Immune regulation includes restraint as well as activation

Experiments with thymosin alpha-1 examined dendritic cells, which help direct other immune cells toward a response. In human and mouse cell preparations, the peptide was linked to the enzyme indoleamine 2,3-dioxygenase, or IDO, which breaks down tryptophan. In the mouse cells, this response involved TLR9, an immune-sensing receptor, and type I interferon signaling and was associated with production of IL-10, a signal that restrains inflammation, and regulatory T cells, which help limit immune responses. This provides a concrete reason to describe regulation: the experiments included mechanisms that can restrain inflammation as well as support immune responses. Romani and colleagues (2006).

The LL-37 skin experiments examined a barrier surface and its regrowth. The subsequent ulcer trials tested a topical synthetic formulation alongside wound care. Neither the dendritic-cell work nor the skin-blocking experiment establishes that one peptide broadly strengthens immunity more than the other. Their clinical effects need to be judged in the populations where treatment was actually tested.

Thymosin alpha-1: an earlier survival signal was not confirmed by the larger trial

Wu and colleagues randomized 367 adults with severe sepsis in ETASS. Six consent withdrawals left 361 participants in the analysis of thymosin alpha-1 plus conventional care versus control care. Death within 28 days occurred in 26.0% of 181 peptide recipients and 35.0% of 180 controls. The risk ratio was 0.74, with a 95% confidence interval of 0.54–1.02; that comparison did not establish a mortality reduction (P=0.062). A time-to-event analysis gave a borderline result (log-rank P=0.049). The proportion of monocytes displaying HLA-DR, a cell-surface protein involved in presenting material to other immune cells, improved more in the treatment group. A change in that immune marker did not resolve the uncertain mortality comparison. Wu and colleagues (2013).

The later TESTS trial was double-blind and placebo-controlled at 22 centers in China. It randomized 1,106 adults with sepsis; 1,089 entered the modified intention-to-treat analysis after receiving study treatment. At 28 days, 127/542 participants given thymosin alpha-1 died (23.4%), compared with 132/547 given placebo (24.1%). The corrected hazard ratio was 0.97 (95% CI 0.76–1.24; P=0.82). Deaths were numerically similar, and the uncertainty still allows clinically relevant benefit or harm; the result does not establish equivalence or a mortality reduction. Wu and colleagues (2025).

The publisher corrected survival data and some immune measurements in May 2025. The corrected estimate above replaces the earlier hazard ratio, rather than creating another trial. Differences in blinding, size and populations matter when weighing the earlier favorable observation against this larger result.

LL-37: faster early shrinkage did not become established overall closure benefit

The first topical LL-37 trial enrolled 34 people with difficult-to-heal venous leg ulcers. During four weeks of randomized blinded treatment, the lowest concentration produced a modeled wound-shrinkage rate about six times the placebo value (P=0.003). The middle concentration produced an approximately threefold estimate, but that comparison remained uncertain (P=0.088); the highest concentration did not establish faster shrinkage. A sixfold modeled rate does not mean six times as many ulcers healed completely. Grönberg and colleagues (2014).

The larger phase IIb trial randomized 149 people; 148 received treatment and 144 entered the main efficacy analysis. All groups received compression and dressings. The synthetic LL-37 acetate was applied in polyvinyl alcohol; the comparator contained the same polyvinyl alcohol without peptide. Confirmed closure meant that skin covered the ulcer without drainage or dressing requirements and remained closed at the confirmation visit. Mahlapuu and colleagues (2021).

At 13 weeks, estimated confirmed closure was 26.5% (90% CI 17.1–38.7) and 24.7% (15.8–36.4) with the two LL-37 concentrations, versus 25.3% (16.4–36.9) with the preparation without peptide. Neither comparison established a benefit (one-sided P=0.4453 and 0.5274). These are model-estimated proportions rather than raw fractions. An analysis conducted after the main analysis examined ulcers at least 10 cm². It suggested a possible benefit, including estimated closure of 28.1% versus 8.1% for the lower concentration and control, but this post hoc subgroup needs a study designed to confirm it.

Blood elimination and local wound stability do not create a shared half-life

Nine healthy volunteers underwent a crossover study of three subcutaneous thymosin alpha-1 formulations. Serum concentrations peaked after one to two hours, and elimination half-life was under three hours. Timosina produced higher exposure than Zadaxin and thymosin alpha-1-HLR, illustrating why a shared sequence name does not guarantee identical drug concentrations. Rost and colleagues (1999).

The LL-37 ulcer trials concern a local topical preparation and do not supply a comparable human systemic concentration-time study. Their treatment periods also cannot establish how long intact peptide persists in blood. With different routes, products and measured compartments, these sources do not support a numerical human exposure comparison or an injected-LL-37 prediction from topical tolerability.

Clinical tolerability must stay connected to the tested setting

In TESTS, at least one adverse event within 90 days occurred in 360/542 thymosin alpha-1 recipients and 370/547 placebo recipients; serious events occurred in 145/542 and 160/547, respectively. No unexpected serious event was reported as related to thymosin alpha-1. These are observations in critically ill adults, whose illness and background care contribute many events. They do not establish long-term safety in healthy people or the quality of another preparation. Wu and colleagues (2025).

The larger LL-37 topical trial described the preparations as well tolerated. At least one event occurred in 20/48 and 24/49 participants in the two active safety groups, versus 20/51 controls. Investigators judged none of the serious events related to study treatment. Those safety denominators differ from the 144-person efficacy population, and the route was topical wound treatment. Mahlapuu and colleagues (2021).

FDA identifies potential immune responses and peptide-impurity concerns for compounded thymosin alpha-1 and LL-37, together with inadequate safety information for the proposed products. The agency also cites nonclinical LL-37 findings suggesting harmful effects on male reproduction and tumor-promoting activity in some tissues. Its table does not specify the underlying models, preparations or routes; these concerns therefore cannot be translated into a human incidence estimate. The 2024 thymosin alpha-1 assessment states that neither free peptide nor acetate is a component of an FDA-approved drug. Neither a compounding assessment nor an orphan designation is a treatment approval.

A common immune label does not supply a common clinical benefit

The strongest selected clinical tests asked whether fewer adults died from sepsis and whether more chronic ulcers closed. The larger trials did not establish those overall benefits, while leaving uncertainty about effect sizes and particular subgroups. Earlier favorable findings deserve explanation alongside the later results. They do not establish that either peptide is generally ineffective, nor do they show that one is a better immune treatment than the other. No selected head-to-head study connects these different diseases, routes and outcomes.

Frequently asked questions

Profiles and related reading

References

1. U.S. Food and Drug Administration. Thymosin Alpha-1-related bulk drug substances evaluation. Briefing document for the December4,2024 Pharmacy Compounding Advisory Committee meeting.

   The official assessment identifies N-acetylated 28-residue thymosin alpha-1, distinguishes free peptide from acetate and states that neither is a component of an FDA-approved drug. Orphan designation is different from marketing approval.

2. Lezhava T, Khavison V, Monaselidze J, Jokhadze T, Dvalishvili N, Bablishvili N, Barbakadze S. Bioregulator Vilon-induced reactivation of chromatin in cultured lymphocytes from old people. Biogerontology. 2004;5(2):73–79. doi:10.1023/B:BGEN.0000025070.90330.7f. PMID:15105581.

   In paired cultures from older donors, Vilon loosened some DNA packaging and increased silver-stained regions associated with ribosome production. Structural chromosome regions were unchanged; chromosome-aberration comparisons were too uncertain to establish a difference.

3. Romani L, Bistoni F, Perruccio K, Montagnoli C, Gaziano R, Bozza S, Bonifazi P, Bistoni G, Rasi G, Velardi A, Fallarino F, Garaci E, Puccetti P. Thymosin alpha1 activates dendritic cell tryptophan catabolism and establishes a regulatory environment for balance of inflammation and tolerance. Blood. 2006;108(7):2265–2274. DOI: 10.1182/blood-2006-02-004762.

   Human and mouse dendritic-cell experiments connected thymosin alpha-1 to tryptophan breakdown and regulatory immune responses, explaining why immune regulation can include restraint of inflammation.

4. Heilborn JD, Frohm Nilsson M, Kratz G, Weber G, Sørensen O, Borregaard N, Ståhle-Bäckdahl M. The cathelicidin anti-microbial peptide LL-37 is involved in re-epithelialization of human skin wounds and is lacking in chronic ulcer epithelium. Journal of Investigative Dermatology. 2003;120(3):379–389. doi:10.1046/j.1523-1747.2003.12069.x.

   Human wound samples and separate cultured-skin experiments linked the endogenous LL-37 system to skin resurfacing. Antibodies against LL-37 inhibited regrowth in the cultured wounds.

5. Wu J, Zhou L, Liu J, Ma G, Kou Q, He Z, Chen J, Ou-Yang B, Chen M, Li Y, Wu X, Gu B, Chen L, Zou Z, Qiang X, Chen Y, Lin A, Zhang G, Guan X. The efficacy of thymosin alpha 1 for severe sepsis (ETASS): a multicenter, single-blind, randomized and controlled trial. Critical care (London, England). 2013;17(1):R8. doi:10.1186/cc11932. PMID:23327199.

   The earlier 361-person sepsis trial observed 28-day mortality of 26.0% with thymosin alpha-1 and 35.0% with control. The risk-ratio interval included no difference, while time-to-event testing gave a borderline result.

6. Wu J, Pei F, Zhou L, Li W, Sun R, Li Y, Wang Z, He Z, Zhang X, Jin X, Long Y, Cui W, Wang C, Chen E, Zeng J, Yan J, Lin Q, Zhou F, Huang L, Shang Y, Duan M, Zheng W, Zhu D, Kou Q, Zhang S, Liu Y, Yao C, Shang M, Peng S, Zhou Q, Cheng KK, Guan X, TESTS study collaborator group. The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial. BMJ (Clinical research ed.). 2025;388:e082583. doi:10.1136/bmj-2024-082583. PMID:39814420.

   The larger blinded TESTS sepsis trial observed 127/542 deaths with thymosin alpha-1 and 132/547 with placebo at 28 days. The corrected hazard ratio 0.97 (95% CI 0.76–1.24) did not establish a mortality reduction.

7. BMJ. Correction: The efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase3 trial. BMJ.2025;389:r1098. doi:10.1136/bmj.r1098.

   The correction updates survival data for nine participants and immune data for 35 participants. The primary corrected hazard ratio is 0.97 (0.76–1.24); this is a correction to the same TESTS trial.

8. Grönberg A, Mahlapuu M, Ståhle M, Whately-Smith C, Rollman O. Treatment with LL-37 is safe and effective in enhancing healing of hard-to-heal venous leg ulcers: a randomized, placebo-controlled clinical trial. Wound Repair and Regeneration. 2014;22(5):613–621. doi:10.1111/wrr.12211.

   A 34-person LL-37 ulcer trial found faster modeled wound shrinkage at the lowest topical concentration. The middle estimate was uncertain and the highest concentration did not establish benefit.

9. Mahlapuu M, Sidorowicz A, Mikosinski J, Krzyżanowski M, Orleanski J, Twardowska-Saucha K, Nykaza A, Dyaczynski M, Belz-Lagoda B, Dziwiszek G, Kujawiak M, Karczewski M, Sjöberg F, Grzela T, Wegrzynowski A, Thunarf F, Björk J, Ekblom J, Jawien A, Apelqvist J. Evaluation of LL-37 in healing of hard-to-heal venous leg ulcers: A multicentric prospective randomized placebo-controlled clinical trial. Wound Repair and Regeneration. 2021;29(6):938–950. doi:10.1111/wrr.12977.

   In the larger topical LL-37 trial, confirmed venous-ulcer closure estimates were similar with both concentrations and the preparation without peptide. A favorable large-ulcer subgroup was examined after the main analysis.

10. Rost KL, Wierich W, Masayuki F, Tuthill CW, Horwitz DL, Herrmann WM. Pharmacokinetics of thymosin alpha1 after subcutaneous injection of three different formulations in healthy volunteers. International journal of clinical pharmacology and therapeutics. 1999;37(1):51–57. PMID:10027483.

    A nine-person crossover study measured serum thymosin alpha-1 after three injected formulations. Concentrations peaked in one to two hours and elimination half-life was under three hours, with formulation-dependent exposure.

11. U.S. Food and Drug Administration. Certain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks. Current agency webpage accessed October15,2026.

    FDA identifies potential immune responses and impurity concerns for compounded thymosin alpha-1 and LL-37. It also cites nonclinical LL-37 findings concerning male reproduction and tumor promotion, without giving the underlying models or human risk estimates.