Immune & host-defense peptides

Both peptides occur naturally in the body. Thymosin alpha-1 is a 28-residue thymic peptide studied as an immune modulator11PubMedThe efficacy and safety of thymosin alpha-1 in Japanese patients with chronic hepatitis B; results from a randomized clinical trial. Iino S et al. J Viral Hepat. 2005.; its largest trial, TESTS, randomized 1,106 adults with sepsis and found no difference in 28-day mortality22PubMedThe efficacy and safety of thymosin α1 for sepsis (TESTS): multicentre, double blinded, randomised, placebo controlled, phase 3 trial. Wu J et al. BMJ. 2025.. LL-37 is the human cathelicidin, an antimicrobial peptide33PubMedLL-37 but Not 25-Hydroxy-Vitamin D Serum Level Correlates with Healing of Venous Leg Ulcers. Krejner A et al. Arch Immunol Ther Exp (Warsz). 2017.; topical LL-37 helped venous leg ulcers heal in a 34-patient trial44PubMedTreatment with LL-37 is safe and effective in enhancing healing of hard-to-heal venous leg ulcers: a randomized, placebo-controlled clinical trial. Grönberg A et al. Wound Repair Regen. 2014. but not in a 148-patient phase IIb trial55PubMedEvaluation of LL-37 in healing of hard-to-heal venous leg ulcers: A multicentric prospective randomized placebo-controlled clinical trial. Mahlapuu M et al. Wound Repair Regen. 2021..

Neither has an FDA-approved application. FDA lists both among withdrawn compounding nominations, and for LL-37 it records nonclinical findings of harm to male reproduction66FDACertain Bulk Drug Substances for Use in Compounding that May Present Significant Safety Risks. U.S. FDA, content current as of 04/22/2026..