Tetrapeptide-30 is a label name that the research literature almost never uses. That single fact decides how the ingredient looks to anyone checking it: under its INCI name it appears untested, and under its sequence name it has one of the better-designed small files in brightening peptides.
The molecule, under both names
PubChem CID 25001762 resolves both "tetrapeptide-30" and "Pro-Lys-Glu-Lys" to the same record: formula C22H40N6O7, 500.6 g/mol. In one-letter code the four residues spell PKEK, which is the name every clinical paper uses.
That weight is worth pausing on. The 500-dalton heuristic puts the practical ceiling for passive movement through intact stratum corneum at roughly 500 Da. Tetrapeptide-30 sits on the line. Nonapeptide-1 is 1,206.5 and decapeptide-12 is 1,395.5 — both well beyond it. Among the brightening peptides sold today, this is the one whose size is not an argument against it.
The search that returns nothing, and the one that does
On 2026-09-21 PubMed returned 0 records for tetrapeptide-30, searched both in titles and abstracts and across all fields. Before writing that zero down we ran a control in the same session — a retinol randomized-trial query that returned 915 — so the zero is the literature's answer, not a broken search.
Searched as PKEK, the same database returns 10 records. Four of them are other uses of the same four letters: three corneal-transplant papers in which PK and EK abbreviate two types of keratoplasty, and a bacterial shuttle-vector paper. Six are about the peptide: the two clinical papers below, three skin-delivery studies and one review.
| Search term (title/abstract) | Records, 2026-09-21 | About the peptide |
|---|---|---|
tetrapeptide-30 |
0 | 0 |
PKEK |
10 | 6 |
The practical consequence is not academic. A reader, a journalist or an assistant checking this ingredient by its label name will report that it has no published evidence. It has two human papers, and one of them is designed better than most of what this category offers.
The trial that separated the peptide from vitamin C
Marini and colleagues, Experimental Dermatology 2012, is one paper containing one cell study and four double-blinded, vehicle-controlled human studies. Its first author is at a Düsseldorf environmental-medicine institute; its second author, Mike Farwick, is the corresponding author of the 2011 paper below, where he gives an Evonik address — Evonik supplies the ingredient.
- In keratinocytes, PKEK reduced UVB-stimulated expression of IL-6, IL-8, TNF-α and — the one the paper leads with — POMC, the gene for the precursor of α-MSH, the hormone that tells pigment cells to darken.
- In 10 volunteers pretreated once daily for four weeks, PKEK blunted the UVB-induced rise in those genes and in tyrosinase.
- In 39 women, facial pigment spots faded significantly after six weeks when PKEK was combined with sodium ascorbyl phosphate, a vitamin C derivative. PKEK alone, and the vitamin C derivative alone, produced less pronounced fading.
- On the backs of the hands of 19 people, adding PKEK enhanced a vitamin C preparation over eight weeks.
- In 27 Japanese women, twice daily for eight weeks, PKEK plus the vitamin C derivative reduced pigmentation by 26% and by 18% on the SCINEXA score, against the vitamin C derivative alone.
The 39-woman study is the part worth dwelling on. It has an arm for each ingredient and an arm for both. That is the design almost every combination claim in skin care lacks: most "peptide plus vitamin C" products are supported, if at all, by a trial of the finished product against nothing. Here the peptide's own contribution was at least put to the test. We cover what the rest of that literature looks like on our page on peptides and vitamin C.
What the abstract does not give is the effect size for each single-ingredient arm, and "less pronounced" is not a number. The factorial result is real and it is also one study, with the supplier's scientist among its authors.
The trial in darker skin
Farwick and colleagues, Journal of Cosmetic Dermatology 2011, tested PKEK against its vehicle, double-blind, in people with skin types V–VI in South Africa who had facial melasma and post-inflammatory hyperpigmentation. Expert grading of digital images found the peptide formulation significantly superior to vehicle at 12 weeks on overall appearance (P<0.05) and evenness of skin tone (P<0.01).
Two things bound it. The abstract gives no participant number. And the corresponding author lists an Evonik Goldschmidt address. It is a supplier study, which does not make it wrong — it makes it unreplicated. Its population is also the one in which pigmentation treatment is hardest and irritation matters most, and brightening peptides are most often sold into it on the promise of gentleness.
Where the peptide ends up
Neubert and colleagues, European Journal of Pharmaceutics and Biopharmaceutics 2018, with Evonik co-authors, applied PKEK to excised human skin. From a standard cream, 40 to 58% of it entered the skin — a high figure for a peptide — but most of it stayed in the stratum corneum and did not reach the living layers where pigment signalling happens. A microemulsion drove far more through, most of it straight out the other side into the receiving chamber, so the amount sitting in the viable layers was about the same either way.
A 2022 review of anti-wrinkle peptide penetration in the International Journal of Cosmetic Science (PMID 35302659) lists PKEK among the few peptides whose penetration has been measured at all, and concludes that most such peptides do not reach their targets at useful concentrations without an enhancement strategy.
What this supports saying
- Tetrapeptide-30 and PKEK are the same molecule, 500.6 g/mol, on the penetration line rather than beyond it.
- Its literature is invisible under its label name and present under its sequence name.
- Two clinical papers report brightening against vehicle; the supplier's scientist is an author on both, and neither has an independent replication in the indexed literature.
- One of them contains the rarest thing in combination skin care: an arm for the peptide, an arm for the vitamin C derivative and an arm for both, with the combination ahead.
- Most of the peptide that enters skin from a cream stays in the outer layer.
Where to go next
- Nonapeptide-1 — the other brightening peptide on this site, and a far thinner file.
- Decapeptide-12 — the tyrosinase-inhibitor peptide, and whose trials tested what.
- Copper-peptide serum formulas — Biossance's list declares tetrapeptide-30; that page's molecular-weight table puts it six-tenths of a dalton over the line.
- Peptides for skin: the whole category, counted — the census and the molecular-weight table this entry sits in.
Sources
- PubChem CID 25001762 — tetrapeptide-30 / Pro-Lys-Glu-Lys, read 2026-09-21.
- Marini A, Farwick M, Grether-Beck S, et al. Modulation of skin pigmentation by the tetrapeptide PKEK: in vitro and in vivo evidence for skin whitening effects. Exp Dermatol 2012;21(2):140-6.
- Farwick M, Maczkiewitz U, Lersch P, Summers B, Rawlings AV. Facial skin-lightening benefits of the tetrapeptide Pro-Lys-Glu-Lys on subjects with skin types V-VI living in South Africa. J Cosmet Dermatol 2011;10(3):217-23.
- Neubert RHH, Sommer E, Schölzel M, et al. Dermal peptide delivery using enhancer molecules and colloidal carrier systems. Part II: Tetrapeptide PKEK. Eur J Pharm Biopharm 2018;124:28-33.
- Mortazavi SM, Moghimi HR. Skin permeability, a dismissed necessity for anti-wrinkle peptide performance. Int J Cosmet Sci 2022;44(2):232-248.
- Bos JD, Meinardi MMHM. The 500 Dalton rule. Exp Dermatol 2000.
- PubMed counts run 2026-09-21 via NCBI E-utilities, validated against a control query (retinol, randomized controlled trial publication type: 915).