Thymogen

LL-37 is the only cathelicidin antimicrobial peptide humans make, a 37-amino-acid, positively charged, helical fragment cut from a precursor protein called hCAP-18. It is a frontline player in the innate immune system, part of the body's chemical defense against bacteria, viruses, and fungi. This is mainstream, heavily studied human biology, not a fringe research peptide, though LL-37 itself is not an approved drug.

Thymosin alpha-1 (sold as Zadaxin, generic name thymalfasin) is a 28-amino-acid peptide originally isolated from the thymus gland, the organ that trains your immune system. Unlike most peptides in this space, it is a real, approved drug in over 35 countries for chronic hepatitis B and as an immune booster, though it has never been approved by the FDA in the United States. It has one of the larger human evidence bases of any peptide here, with trials in tens of thousands of patients.

KPV is a tiny tripeptide, just three amino acids (lysine, proline, valine), that forms the tail end of the natural hormone alpha-MSH. It is studied almost entirely as an anti-inflammatory agent, particularly for gut and skin inflammation. There are no registered human clinical trials proving its benefits in people; the evidence base is cell-culture and animal studies, so anything you read about it treating disease is preliminary.

VIP (vasoactive intestinal peptide) is a 28-amino-acid signaling peptide your own gut, nerves, and immune cells make. It is a natural anti-inflammatory and a potent vasodilator, and a synthetic version called aviptadil has been tested in humans for COVID-19 respiratory failure and pulmonary conditions. No VIP product is FDA-approved for the wellness or anti-aging uses it gets marketed for, and most of that human data is in lung disease, not in healthy people.

Enfuvirtide (brand name Fuzeon, originally T-20) is a 36-amino-acid synthetic peptide and the first HIV fusion inhibitor, FDA-approved in March 2003. It is a genuine prescription antiretroviral, not a research-only compound, and it is given as a twice-daily subcutaneous injection. Its job is narrow but important: it blocks HIV from entering a host cell in the first place, and it is reserved for people whose virus has stopped responding to other drugs.

PNC-27 is a 32-amino-acid lab-designed peptide that fuses a fragment of the tumor-suppressor protein p53 (residues 12 to 26) to a membrane-penetrating leader sequence. The interesting claim is that it kills cancer cells while leaving normal cells alone, by punching holes in the cancer cell membrane. It is a research compound only, with no approval and no human clinical trials.