P21 (P021) Research: What Published Studies Have Investigated

What is P21 (P021)?

What is P021? It is a small synthetic peptidergic compound derived from ciliary neurotrophic factor (CNTF). Li and colleagues reported its design in FEBS Letters in 2010 (PMID 20600002), describing the compound Ac-DGGLAG-NH₂ — an acetylated, C-terminally amidated hexapeptide — which they called P21. Papers from the same group thereafter use the designation P021 for the same sequence; Khatoon and colleagues (2015, PMID 26401692) write of “a neurotrophic peptidergic compound, DGGLAG named P021”. Both designations appear in the literature and refer to the same DGGLAG core.

It is not the p21 protein

This needs saying at the top because the name collision is severe and consequential. p21 — p21^Cip1/Waf1, the product of the CDKN1A gene — is a cyclin-dependent kinase inhibitor and one of the standard markers of cellular senescence, appearing in thousands of papers including, incidentally, the senescence literature covered on this site’s FOXO4-DRI page. It is a nuclear protein of 164 residues with no relationship whatever to the CNTF-derived hexapeptide described here. A literature search for “p21” returns overwhelmingly the former; anyone researching the latter should search for P021 or for “neurotrophic peptidergic compound”.

Discovery and origin in the literature

Why cut CNTF down at all

Ciliary neurotrophic factor has neuroprotective properties in preclinical models but its clinical use, as Blanchard and colleagues put it in 2010, has not materialised because of side effects including anorexia, skeletal muscle loss, hyperalgesia, cramps and muscle pain. The strategy pursued by the Iqbal group at the New York State Institute for Basic Research in Developmental Disabilities was to identify short sequences from within the protein that retained the properties of interest without the whole molecule.

The lineage, in order

• Peptide 6 — Chohan and colleagues (2011, Neurobiol Aging, PMID 19767127) reported an 11-mer designed from a biologically active region of CNTF. They reported it induced proliferation and increased survival and maturation of neural progenitor cells into neurons in the dentate gyrus of normal adult mice, increased MAP2 and synaptophysin immunoreactivity there, and improved reference memory in the Morris water maze after thirty days of a slow-release subcutaneous implant. They also reported a plasma half-life of over six hours, blood-brain-barrier permeability, and a mechanism of competitive inhibition of leukemia inhibitory factor signalling.
• Peptide 6c — Blanchard and colleagues (2010, J Alzheimers Dis, PMID 20952820) reported a tetrapeptide, GDDL, corresponding to CNTF residues 147–150, with dentate-gyrus neurogenesis and spatial-memory results in mice and, they reported, without weight loss or other apparent side effects.
• P21 / P021 — Li and colleagues (2010, FEBS Lett, PMID 20600002) reported a study of neurotrophic peptides incorporating adamantane and, within it, the design of Ac-DGGLAG-NH₂, called P21, which they reported enhanced learning and both short-term and spatial reference memory in normal adult C57Bl6 mice given peripherally, with enhanced neurogenesis and maturation of newly born neurons in the dentate gyrus granular cell layer and subgranular zone.

Note the point of confusion that follows from this history: the phrase “CNTF tetrapeptide” belongs to Peptide 6c, not to P021. Sources describing P021 itself as a tetrapeptide are conflating two members of the same lineage. The compound the later literature carries forward is the DGGLAG hexapeptide.

Reference data

The table below is deliberately partial. No molecular formula, molecular weight or CAS number is given because we could not source one from the primary literature or from a chemical database; a PubChem lookup by name returns no compound entry. Estimating those fields would be worse than omitting them.

No reconstitution procedure, quantity or route is given or implied on this page.

Mechanisms researchers have examined

• Leukemia inhibitory factor signalling — the one explicit molecular mechanism in this lineage, reported by Chohan and colleagues (2011) for Peptide 6 as competitive inhibition of LIF signalling. LIF and CNTF share receptor components, which is the structural logic behind a CNTF fragment acting on that pathway.
• Dentate-gyrus neurogenesis — the most consistently measured endpoint across the whole series, assessed by markers of neuronal commitment, differentiation and survival of newborn progenitor cells.
• Synaptic and dendritic markers — MAP2, synaptophysin and PSD-95 immunoreactivity, and CREB activity, recur as readouts across the rodent papers.
• BDNF expression — Bolognin and colleagues (2014, Neurobiol Aging, PMID 24702821) reported increased brain-derived neurotrophic factor expression in aged rats in their study.
• Tau and amyloid pathology markers — in the Alzheimer’s-model work, hyperphosphorylated tau at defined sites, total tau in brain and cerebrospinal fluid, GSK3β activity and amyloid plaque load are the measured pathological endpoints.

Each point names what was measured in a defined animal or cell system, not an effect in a reader.

Research areas in the literature

Cognitive aging in rats

Bolognin and colleagues (2014) reported that chronic oral administration of P021 to 22-to-24-month-old Fisher rats reduced the age-dependent decline in learning and memory measures, inhibited the neurogenesis deficit, increased BDNF expression, and restored synaptic deficits in cortex and hippocampus. They also used in-vivo magnetic resonance spectroscopy and reported a reduction in hippocampal myoinositol, which had been elevated in aged relative to young animals.

Alzheimer’s and Down-syndrome mouse models

Kazim and colleagues (2014, Neurobiol Dis, PMID 25046994) reported chronic oral treatment in the 3xTg-AD triple transgenic mouse. Kazim and colleagues (2017, Sci Rep, PMID 28368015) reported early postnatal treatment in the Ts65Dn Down-syndrome model, with results on developmental delay and hippocampus-dependent memory measures in adult life, alongside decreased GSK3β activity and increased synaptic plasticity markers. Wei and colleagues (2020, Alzheimers Res Ther, PMID 32854771) reported prenatal-to-early-postnatal dietary treatment in 3xTg-AD mice with cognitive measures rescued at four months and reduced tau hyperphosphorylation and plaque load assessed at twenty-two months.

Cerebrospinal-fluid tau as a translational marker

Khatoon and colleagues (2015, J Alzheimers Dis, PMID 26401692) is the paper most explicitly aimed at translation: it reported that P021 is blood-brain-barrier permeable, that chronic oral treatment reduced brain total tau in aged rats, and that CSF tau was reduced toward young-adult levels. The authors also reported that P021 did not induce any detectable immune reaction in rats — a relevant question for any peptide intended for chronic administration.

Research models and evidence status

One laboratory, fifteen years

Iqbal, Grundke-Iqbal, Blanchard, Li, Chohan, Kazim, Bolognin, Khatoon, Baazaoui and Wei are the recurring author set across effectively the entire literature above, working from the same institute and, in the later papers, its collaborators. The 2022 review by Baazaoui and Iqbal (PMID 36291618) is candid about this: its own stated limitation is that “it focuses only on P021 and the relevant literature”. Kazim and colleagues (2017) carry a conflict-of-interest statement noting that K.I. holds US patents on CNTF small-molecule peptide mimetics for the treatment of Alzheimer’s disease and related conditions. None of this makes the work wrong. It does mean the findings have not been through the process that most reliably surfaces error, which is independent replication by an unaffiliated group.

What is not established

There is no human data of any kind. A ClinicalTrials.gov search returns no registered trial of P021, so there is no published human pharmacokinetic, safety, tolerability or efficacy information. No independent laboratory replication of the principal rodent findings could be located. The mechanism attributed to the lineage — competitive inhibition of LIF signalling — was reported for Peptide 6 and is not, as far as the published record goes, separately established for P021 itself. The compound has no chemical registry entry we could source, which is a practical obstacle to verifying identity independently. And the naming inconsistency across the group’s own papers (P21 versus P021, and the “tetrapeptide” label belonging to a sibling compound) is a real hazard for anyone reading secondary sources.

Nothing in this literature establishes any property of the supplied research material in a person. This page makes no efficacy, therapeutic, neurotrophic, neurogenic, cognitive or other use claim of any kind, and the material is supplied for laboratory research use only — not for human or veterinary use.

How to verify this compound yourself

The naming problems described above make analytical verification unusually important for this particular compound:

• Mass-spec identity, against a stated sequence — with no registry number to cross-check, the mass spectrum is the primary independent evidence of what is in the vial. The published sequence is a six-residue chain with an acetylated N-terminus and a C-terminal amide; both modifications shift the mass in defined, calculable ways, and a supplier’s stated structure should be consistent with the measured value.
• Which member of the lineage — Peptide 6 (11 residues), Peptide 6c (4 residues) and P021 (6 residues) are separate molecules with substantially different masses, easily distinguished by mass spectrometry and not at all by a label. Given how often secondary sources conflate them, this is worth checking rather than assuming.
• HPLC purity — a six-residue synthesis is short and should be clean; the purity percentage summarises separation of full-length product from deletion sequences.
• Counter-ion documentation — as with any synthetic peptide, commonly trifluoroacetate, affecting net peptide content per unit mass.

See how to read a COA for what each certificate line means, and how to verify peptide purity for how the methods fit together. The exact batch received can be checked on the self-serve verify tool.

Research-grade sourcing and verification

P021 is not held in stock. It is listed on our sourcing catalogue as available to order — our supplier lists it, we have not bought it, and material of this kind typically takes about two to three weeks to reach us. For laboratory research use only, it is supplied with a per-batch Certificate of Analysis reporting HPLC purity (%) and mass-spec identity confirmation, verifiable at the lot level. For adjacent CNS research compounds documented here, see Dihexa research and Cerebrolysin research — different molecules, different literatures, handed off rather than merged with this one. This is sourcing and quality-assurance framing only.

Frequently asked questions

Literature cited

1. Chohan MO, Li B, Blanchard J, et al. “Enhancement of dentate gyrus neurogenesis, dendritic and synaptic plasticity and memory by a neurotrophic peptide.” Neurobiol Aging. 2011;32(8):1420–1434. PMID 19767127. pubmed.ncbi.nlm.nih.gov/19767127. Peptide 6; LIF-signalling mechanism.
2. Blanchard J, Chohan MO, Li B, Liu F, Iqbal K, Grundke-Iqbal I. “Beneficial effect of a CNTF tetrapeptide on adult hippocampal neurogenesis, neuronal plasticity, and spatial memory in mice.” J Alzheimers Dis. 2010;21(4):1185–1195. PMID 20952820. pubmed.ncbi.nlm.nih.gov/20952820. Peptide 6c (GDDL, CNTF 147–150).
3. Li B, Wanka L, Blanchard J, Liu F, Chohan MO, Iqbal K, Grundke-Iqbal I. “Neurotrophic peptides incorporating adamantane improve learning and memory, promote neurogenesis and synaptic plasticity in mice.” FEBS Lett. 2010;584(15):3359–3365. PMID 20600002. pubmed.ncbi.nlm.nih.gov/20600002. Design of Ac-DGGLAG-NH₂, called P21.
4. Bolognin S, Buffelli M, Puoliväli J, Iqbal K. “Rescue of cognitive-aging by administration of a neurogenic and/or neurotrophic compound.” Neurobiol Aging. 2014;35(9):2134–2146. PMID 24702821. pubmed.ncbi.nlm.nih.gov/24702821.
5. Kazim SF, Blanchard J, Dai CL, et al. “Disease modifying effect of chronic oral treatment with a neurotrophic peptidergic compound in a triple transgenic mouse model of Alzheimer’s disease.” Neurobiol Dis. 2014;71:110–130. PMID 25046994. pubmed.ncbi.nlm.nih.gov/25046994.
6. Khatoon S, Chalbot S, Bolognin S, et al. “Elevated Tau Level in Aged Rat Cerebrospinal Fluid Reduced by Treatment with a Neurotrophic Compound.” J Alzheimers Dis. 2015;47(3):557–564. PMID 26401692. pubmed.ncbi.nlm.nih.gov/26401692. “DGGLAG named P021”; BBB permeability.
7. Kazim SF, Blanchard J, Bianchi R, Iqbal K. “Early neurotrophic pharmacotherapy rescues developmental delay and Alzheimer’s-like memory deficits in the Ts65Dn mouse model of Down syndrome.” Sci Rep. 2017;7:45561. PMID 28368015. pubmed.ncbi.nlm.nih.gov/28368015. Includes the authors’ patent disclosure.
8. Wei W, Wang Y, Liu Y, et al. “Prenatal to early postnatal neurotrophic treatment prevents Alzheimer-like behavior and pathology in mice.” Alzheimers Res Ther. 2020;12(1):102. PMID 32854771. pubmed.ncbi.nlm.nih.gov/32854771.
9. Baazaoui N, Iqbal K. “Alzheimer’s Disease: Challenges and a Therapeutic Opportunity to Treat It with a Neurotrophic Compound.” Biomolecules. 2022;12(10):1409. PMID 36291618. pubmed.ncbi.nlm.nih.gov/36291618. Review; states its own single-compound scope as a limitation.