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Peptide Atlas
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P21

Tier 3 · Reported use
Also known as P021 peptide · Ac-DGGLAG-NH2 · CNTF-derived peptide

Strongest evidence is Tier 2: preclinical (animal, in-vitro, and mechanistic) data, chiefly rodent Alzheimer's/aging/Down-syndrome models plus in-vitro stability and pharmacokinetic work. There are no completed or registered human clinical trials, no regulatory approvals, and no human safety or efficacy data. Nearly all efficacy findings originate from a single laboratory (Khalid Iqbal group) without independent replication, and human dosing/benefit reports are anecdotal community/vendor sources (Tier 3). Note also that many secondary sources conflate this CNTF-derived peptide with the unrelated cell-cycle protein p21 (CDKN1A/WAF1/Cip1); those claims were excluded.

Half-life
Not recorded
Routes
Oral (used in rodent studies; reported orally bioavailable) · Subcutaneous injection (anecdotal/community) · Intranasal (anecdotal/community)
Goals
Cognitive enhancement / memory · Neuroprotection · Neurodegenerative disease research (Alzheimer's, Down syndrome, TBI) · Healthy aging / age-related cognitive decline
Cost / mg
Not recorded

How it works

P21 is a tiny, chemically stabilized fragment of ciliary neurotrophic factor (CNTF) designed to mimic CNTF's brain-protective effects while being small enough to cross into the brain and stable enough to survive oral or injected delivery. In animal studies it boosts the brain's own production of BDNF (a key growth factor), encourages the birth and maturation of new hippocampal neurons, strengthens synaptic connections, and reduces the abnormal 'tangling' (hyperphosphorylation) of the tau protein linked to Alzheimer's disease. Unlike full CNTF, it does not appear to suppress appetite or cause weight loss in animals. All of these findings are preclinical or anecdotal — there is no human trial data.

Overview

Overview

P21 (formal designation P021; aliases Ac-DGGLAG-NH2, CNTF-derived peptide) is a small, chemically modified peptide derived from the biologically active region of ciliary neurotrophic factor (CNTF). It was engineered to mimic CNTF's neurotrophic properties while overcoming the limitations of the full-length protein — namely poor blood-brain-barrier (BBB) penetration, a very short half-life, and the need for parenteral administration.

Full-length CNTF is roughly 23,000 daltons, far too large to cross the BBB when given peripherally; P21 is approximately forty times smaller. The peptide corresponds to CNTF residues 148-151 (one source instead cites 147-150) and carries an N-terminal acetylation plus a C-terminal modification (amidation and/or an adamantylated glycine). The adamantane group — a highly lipophilic tricyclic alkane — increases lipid solubility to aid BBB penetration, while the terminal caps resist exopeptidase/carboxypeptidase degradation, improving metabolic stability and oral bioavailability.

Proposed Mechanism

In preclinical work, P21 stimulates adult hippocampal neurogenesis, primarily by:

  • Upregulating BDNF transcription and secretion from hippocampal neurons (with elevated phospho-CREB)
  • Competitively inhibiting LIF signaling (acting as a partial LIF antagonist), relieving suppression of neural progenitor proliferation

BDNF acts through the TrkB receptor to activate PI3K/Akt, promoting neuronal survival, synaptic plasticity, and long-term potentiation. Akt in turn phosphorylates and inactivates GSK-3beta, the primary kinase driving tau hyperphosphorylation, thereby reducing tau pathology. P21 has also been proposed to engage the tripartite CNTF receptor complex (CNTFR-alpha/LIFR/gp130) and activate JAK/STAT (STAT3) and possibly RAS/MAPK.

Preclinical Findings

Across rodent studies, P21 has been reported to:

  • Increase newly born mature neurons (BrdU/NeuN double-positive cells) in the dentate gyrus by 30-50%
  • Enhance dendritic arborization, spine density, and synaptic markers (synaptophysin, PSD-95, MAP-2)
  • Prevent and rescue cognitive deficits, reduce tau pathology, and promote neurogenesis in transgenic Alzheimer's models
  • Improve hippocampus-dependent, spatial (Morris water maze), and recognition memory
  • Reduce amyloid-beta and phospho-tau and decrease mortality in 3xTg-AD mice
  • Reverse age-related memory decline in aged rats and lower elevated tau in CSF of aged rats

Models tested include 3xTg-AD, APP/PS1, and Tg2576 Alzheimer's mice, Ts65Dn Down syndrome mice, normal aging rodents, and traumatic brain injury models; one report describes prevention of age-related macular degeneration pathology in rodents.

Development & Regulatory Status

P21 was developed by the Khalid Iqbal laboratory at the New York State Institute for Basic Research in Developmental Disabilities. It is not FDA approved, is not on the 503A bulks list, and remains a research chemical under 'research use only' labeling. No human clinical trials have been published or registered on ClinicalTrials.gov; the corporate developer (Phanes Biotech) targeted a Phase 1 IND for 2025-2026, but no IND had been filed or trial announced as of early-to-mid 2026.

Important Caveats

  • Structural disagreement: Sources conflict on P21's sequence, length, and molecular weight (variously a 4-mer ~470-592 Da, a 5-6-mer Ac-DGGLAG-NH2 ~578 Da, an 8-mer, 11-mer, or a 9-mer at ~985 Da). This reflects inconsistent secondary reporting rather than an agreed structure.
  • Single-lab data: Essentially all efficacy findings come from one laboratory with no independent replication.
  • Name confusion: Many online sources describe an entirely unrelated molecule — the cyclin-dependent kinase inhibitor protein p21 (CDKN1A/WAF1/Cip1) — which has nothing to do with this CNTF-derived peptide.
  • Contested premise: Adult human hippocampal neurogenesis, central to P21's proposed benefit, is itself debated in neuroscience, qualifying how rodent findings might translate.

What the research shows

289 findings extracted from the 30 sources cited below, strongest evidence first within each group. Every one links to the source it came from.

What human studies found

Based on 32 animal findings, 2 expert opinion findings, 6 anecdotal findings and 6 theoretical findings.

  • animalChronic treatment with P021 restores cognitive function, increases neurogenesis and synaptic markers, and reduces Aβ and tau in 3xTg-AD mice1

  • animalP021 decreases mortality in a mouse model of Alzheimer's disease1

  • animalP21 promotes adult hippocampal neurogenesis in aged and diseased animal models2

  • animalP21 has shown cognitive-enhancing effects in Alzheimer's disease animal models3

  • animalMultiple Iqbal-lab mouse studies in AD and tauopathy models show improved cognition, reduced tau hyperphosphorylation, and increased dentate gyrus neurogenesis3

  • animalP021 consistently improved hippocampus-dependent memory in 3xTg-AD mice, aged Fisher rats, and Ts65Dn Down syndrome mice4

  • animalA 2010 FEBS Letters paper reported improved learning and memory, increased neurogenesis and enhanced synaptic plasticity in mice given P215

  • animalA 2014 Neurobiology of Disease study reported that chronic oral P21 reduced abnormal tau hyperphosphorylation, preserved synaptic markers and better cognitive performance in triple-transgenic Alzheimer's-model mice5

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  • animalA 2017 paper reported that P21 rescued dendritic and synaptic deficits and reversed cognitive impairment in transgenic AD mice5

  • animalA 2015 study reported that P21 reduced elevated tau in the cerebrospinal fluid of aged rats5

  • animalIn transgenic Alzheimer's mice (3xTg-AD), P21 increases hippocampal neurogenesis6

  • animalIn transgenic Alzheimer's mice, P21 improves spatial memory on the Morris water maze6

  • animalIn aged rats, P21 reverses age-related memory decline6

  • animalP21 improves spatial memory in Morris water maze in Alzheimer's mouse models7

  • animalP21 improves recognition memory in Alzheimer's mouse models7

  • animalP21 sparks new-neuron growth in mouse brains7

  • animalP21 reduces tau hyperphosphorylation in rodent Alzheimer's models7

  • animalP21 increases synaptic density markers (synaptophysin, PSD-95, MAP-2)7

  • animalIn transgenic mouse models of Alzheimer's disease, oral P21 treatment demonstrates efficacy in preventing and rescuing cognitive deficits, reducing tau pathology, and promoting neurogenesis8

  • animalP21 treatment increases the number of BrdU/NeuN double-positive cells (newly born mature neurons) in the dentate gyrus by 30-50%8

  • animalP21 enhances dendritic arborization of newborn neurons and promotes synaptic integration8

  • animalP21 enhanced neurogenesis, dendritic and synaptic plasticity, with increased spine density and expression of synaptic markers8

  • animalLoss of p21 increased trabecular bone formation9

  • animalLoss of E2f1 increased cortical bone formation9

  • animalLoss of E2f1 led to poorer bone repair overall9

  • animalSirt1 protects cardiac function post-myocardial infarction15

  • animalOverexpression of p21 results in G1 arrest and has been shown to suppress effectively tumor growth in vitro and in vivo17

  • animalIn preclinical studies on Alzheimer disease mouse models, P21 reduced tau pathology, cleared amyloid-beta accumulation, rescued synaptic plasticity, and reversed cognitive impairment20

  • animalIn preclinical models it has been associated with hippocampal neurogenesis21

  • animalAll evidence is preclinical — five rodent studies from a single lab group21

  • animalThe findings are internally consistent: increased hippocampal neurogenesis, BDNF pathway activation, improved cognitive performance in Alzheimer's disease models21

  • animalIn Alzheimer's model mice, P21 rescued cognitive deficits, increased neurogenesis, and reduced tau pathology24

  • expert opinionNo human has ever received P21 in a clinical trial6

  • expert opinionNo human clinical trials registered21

  • anecdotalResearch suggests improvements in memory, focus, and cognitive processing19

  • anecdotalSome studies report anxiolytic and mood-stabilizing effects19

  • anecdotalSubtle mood and focus improvements may begin in Week 1–219

  • anecdotalSleep quality often improves early19

  • anecdotalIncreased mental clarity, word recall, and reduced brain fog reported in Week 2–419

  • anecdotalBetter stress response and working memory in Week 4–819

  • theoreticalNo studies have tested P021 in humans yet1

  • theoreticalP21 has never been tested in humans in a published clinical trial of any phase3

  • theoreticalNo human clinical trials of P021 have been published, and no ClinicalTrials.gov registrations exist as of 2026-06-214

  • theoreticalP21 is extensively studied for its ability to shield neurons from damage in models of neurodegenerative disorders such as Alzheimer's and Parkinson's22

  • theoreticalNo human clinical trials have been completed24

  • theoreticalIn 1994, p21 was found as a tumor suppressor in brain, lung and colon cancer by targeting p53 and was associated with tumorigenesis and metastasis26

How it works

Based on 37 animal findings, 36 in vitro findings, 9 expert opinion findings and 82 theoretical findings.

  • animalP021 increases neurogenesis, BDNF, and synaptic markers in mouse model of Alzheimer's disease1

  • animalP021 reduces phosphorylated tau and soluble Aβ in mouse model of Alzheimer's disease1

  • animalP21 enhances synaptic density and glutamate receptor expression critical for memory formation2

  • animalP21 increases BDNF expression and reduces tau hyperphosphorylation in AD models3

  • animalP21 does not activate STAT3 signaling in the hypothalamus, avoiding appetite suppression3

  • animalP021 signals downstream through the TrkB / PI3K / Akt / phospho-GSK3β axis to increase dendritic spine density, reduce tau hyperphosphorylation, and restore hippocampal neurogenesis in rodent Alzheimer's models4

  • animalP21 drives new neuron formation in the dentate gyrus of the hippocampus and supports dendritic and synaptic maintenance5

  • animalIn transgenic Alzheimer's mice, P21 reduces tau hyperphosphorylation via GSK-3β inhibition6

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  • animalP21 raises BDNF in the memory region7

  • animalP21 blocks the enzymes that tangle proteins in Alzheimer's7

  • animalP21 stimulates hippocampal neurogenesis in the adult brain through upregulation of brain-derived neurotrophic factor (BDNF) and competitive inhibition of leukemia inhibitory factor (LIF) signaling8

  • animalP21 increases BDNF expression and secretion from hippocampal neurons8

  • animalBDNF acts through TrkB receptor to activate the PI3K/Akt signaling pathway, which promotes neuronal survival, synaptic plasticity, and long-term potentiation8

  • animalAkt phosphorylates and inactivates glycogen synthase kinase 3beta (GSK3beta), the primary kinase responsible for tau hyperphosphorylation in Alzheimer's disease8

  • animalP21 competitively inhibits LIF binding to its receptor, relieving suppressive signal and allowing increased proliferation of neural progenitor cells in the hippocampal dentate gyrus subgranular zone8

  • animalLoss of p21 leads to increased bone formation post-injury9

  • animalE2f1 is downstream of p21 and as a transcription factor can act directly on gene expression9

  • animalWhen E2f1 was absent, there was a dramatic decrease of the number of osteoblasts, osteoclasts, and chondrocytes at the site of injury compared to p21 and C57BL/6 mice9

  • animalE2f1 regulates the cell populations required for bone repair and has a distinct role in bone formation/repair compared to p219

  • animalResidual p21 levels in the S phase control DNA replication speed and origin firing to preserve genomic stability14

  • animalp21 inhibits cardiomyocyte proliferation and cardiac regeneration15

  • animalDeacetylation of p21 by Sirt1 deacetylase may reduce p21 abundance and remove p21-induced cell cycle arrest15

  • animalAcetylation of p21 induces cardiomyocyte proliferation arrest15

  • animalBlocking the acetylation of p21 increases cardiomyocyte proliferation15

  • animalP21 can be acetylated by Sirt115

  • animalSirt1 activates p21 ubiquitination through deacetylation15

  • animalOverexpression of Sirt1 induces EdU-, pH3-, and Aurora B-positive cardiomyocytes in neonatal and adult mice15

  • animalDepletion of Sirt1 reduces cardiomyocyte proliferation15

  • animalSirt1 reduces cardiac remodeling post-myocardial infarction15

  • animalSirt1 inhibits cardiomyocyte apoptosis post-myocardial infarction15

  • animalSirt1 attenuates cardiomyocyte hypertrophy post-myocardial infarction15

  • animalp21 can induce G1 arrest and block entry into S phase by inactivating Cdks or by inhibiting activity of proliferating cell nuclear antigen (PCNA)17

  • animalIn normal cells, p21 exists in quaternary complexes with cyclin, Cdk, and PCNA17

  • animalTranscription of the p21 gene is activated by p53-dependent and -independent mechanisms17

  • animalp21 function has been demonstrated to be necessary for p53-mediated G1 arrest following irradiation of p21-deficient mouse embryonic fibroblasts17

  • animalThe function of p21 under normal circumstances appears to be redundant17

  • animalp21 is expressed in terminally differentiating cells of a variety of tissues in a p53-independent manner17

  • in vitroThe published mechanism of P21 involves competitive inhibition of leukemia inhibitory factor (LIF) signaling, increased transcription of brain-derived neurotrophic factor (BDNF), elevated phosphorylated CREB, and reduced activity of GSK-3 beta5

  • in vitrolincRNA-p21 is proposed to act in cis to promote p53-dependent expression of the neighboring cell cycle gene, Cdkn1a/p2110

  • in vitrofull-length transcription, splicing, and accumulation of lincRNA-p21 are dispensable for the chromatin organization of the locus and for cis-regulation10

  • in vitroproduction of lincRNA-p21 through conserved regions in exon 1 of lincRNA-p21 promotes cis-activation10

  • in vitroactivation of nascent transcription from the lncRNA locus, but not the generation or accumulation of a mature lncRNA transcript, is necessary to enact local gene expression control10

  • in vitrop53/p21 interaction influences the transcriptional activity of p5311

  • in vitroExpression of p53 promoter-based reporter gene is dependent on p21 levels11

  • in vitrop53 promoter activity requires p53/p21 interaction11

  • in vitroPuma expression is dependent on p53 regardless of γ-irradiation11

  • in vitrop53 mediates expression of Mdm2 and Gadd45a only in irradiated cells11

  • in vitrop53/p21 interaction is required for p53-dependent expression of Puma and not for Mdm2 and Gadd45a in non-irradiated cells11

  • in vitrop21 is required for binding of p53 to promoters of Puma, Mdm2, and Gadd45a11

  • in vitrop53/p21 complex is involved in regulating p53-dependent gene expression11

  • in vitrop21 places cells under immunosurveillance to establish a biological timer mechanism that controls cell fate13

  • in vitrop21 activates retinoblastoma protein (Rb)–dependent transcription at select gene promoters to generate a complex bioactive secretome, termed p21-activated secretory phenotype (PASP)13

  • in vitroPASP includes the chemokine CXCL14, which attracts macrophages13

  • in vitroMacrophages disengage if cells normalize p21 within 4 days13

  • in vitroIf p21 induction persists, macrophages polarize toward an M1 phenotype and lymphocytes mount a cytotoxic T cell response to eliminate target cells, including preneoplastic cells13

  • in vitrop21 concurrently induces proliferative arrest and immunosurveillance of cells under duress13

  • in vitrop21 is a small unstructured protein that binds and inactivates cyclin-dependent kinases (CDKs)14

  • in vitrop21 levels increase following the activation of the p53 tumor suppressor14

  • in vitroCDK inhibition by p21 triggers cell-cycle arrest in the G1 and G2 phases of the cell cycle14

  • in vitroS-phase function of p21 depends fully on its ability to displace partners from chromatin-bound proliferating cell nuclear antigen (PCNA)14

  • in vitroPCNA prevents p21 upregulation in the S phase, even in the context of robust p21 induction by irradiation14

  • in vitroThe cyclin-dependent kinase inhibitor p21WAF1/CIP1 is a major player in cell cycle control and it is mainly regulated at the transcriptional level16

  • in vitroInduction of p21 predominantly leads to cell cycle arrest16

  • in vitroRepression of p21 may have a variety of outcomes depending on the context16

  • in vitroThe major mode of p21 transcriptional repression by negative regulators is the interference with positive transcription factors without direct binding to the p21 promoter16

  • in vitroNegative factors may either inhibit binding of positive regulators to the promoter or hinder their transcriptional activity16

  • in vitroThe ability of p21 to inhibit proliferation may contribute to its tumor suppressor function16

  • in vitroA number of oncogenes repress p21 to promote cell growth and tumorigenesis16

  • in vitrop21 is an inhibitor of apoptosis16

  • in vitrop21 repression may have an anticancer effect16

  • in vitroc-Myc and chemical p21 inhibitors, which repress p21, sensitize tumor cells to apoptosis by anticancer drugs16

  • in vitroCombination treatment with bortezomib and ELP-bound p21 Cip1/Waf1 protein leads to increased cell cycle arrest23

  • in vitroCombination treatment with bortezomib and ELP-bound p21 Cip1/Waf1 protein leads to apoptosis23

  • expert opinionCNTF is approximately 23,000 daltons, far too large to cross the blood-brain barrier when delivered peripherally6

  • expert opinionAdult human hippocampal neurogenesis remains a contested concept in neuroscience6

  • expert opinionp21 was initially identified as a widespread inhibitor of cyclin-dependent kinases, transcriptionally modulated by p53 and a marker of cellular senescence12

  • expert opinionp21 acts as a tumour suppressor mainly to restrain cell cycle progression, thereby resulting in growth suppression12

  • expert opinionp21 regulates radiation responses via participating in multiple cellular processes, including cell cycle arrest, apoptosis, DNA repair, senescence and autophagy12

  • expert opinionP21 is a synthetic tetrapeptide derived from the most active region of ciliary neurotrophic factor20

  • expert opinionFull-length CNTF is too large to cross the blood-brain barrier and has poor plasma stability20

  • expert opinionP21 consists of four amino acid residues, positions 148 through 151 of CNTF20

  • expert opinionAn adamantane moiety was added at the C-terminal end of P21 to increase lipophilicity for better blood-brain barrier penetration and protect the peptide from degradation by exopeptidases20

  • theoreticalP021 is a tetra-peptide derived from the biologically active region of human ciliary neurotrophic factor (CNTF; amino acid residues 148–151)1

  • theoreticalP021 competitively inhibits leukemia inhibitory factor (LIF) signaling to promote formation of neural progenitor cells1

  • theoreticalP021 increases expression of neurotrophic factor BDNF, promoting neurogenesis1

  • theoreticalP21 is a synthetically engineered tetrapeptide mimetic of ciliary neurotrophic factor (CNTF)2

  • theoreticalP21 consists of a tetrapeptide core sequence (DGGL) derived from amino acid residues 147-150 of ciliary neurotrophic factor (CNTF)2

  • theoreticalThe complete structure is Ac-DGGLAG-NH₂2

  • theoreticalThe adamantane group is a tricyclic alkane with exceptional lipophilicity2

  • theoreticalN-terminal acetylation and C-terminal amidation provide enzymatic stability by protecting against aminopeptidases and carboxypeptidases2

  • theoreticalP21 is an 11-mer peptide derived from ciliary neurotrophic factor (CNTF)3

  • theoreticalP21 has been modified to be small enough to cross the blood-brain barrier3

  • theoreticalP21 is derived from the active region of CNTF (residues 148-151) with adamantylated glycine modifications for BBB penetration3

  • theoreticalP21 competitively inhibits leukemia inhibitory factor (LIF) signaling, which normally suppresses neurogenesis3

  • theoreticalP021 acts as a partial leukemia inhibitory factor (LIF) antagonist and a robust upregulator of brain-derived neurotrophic factor (BDNF)4

  • theoreticalThe structure is Ac-DGGL(A)G-NH2, where Ac- is an N-terminal acetyl cap, D-G-G-L are the four CNTF-derived residues (Asp-Gly-Gly-Leu, corresponding to human ciliary neurotrophic factor residues 148-151)4

  • theoretical(A)G is a glycine residue whose alpha-carbon side chain has been replaced by a 3-aminoadamantane-1-carboxamide moiety to increase blood-brain-barrier penetration and resist carboxypeptidase digestion4

  • theoretical-NH2 at the C-terminus is amidation, a further block against carboxypeptidase digestion4

  • theoreticalP21 is a small peptide engineered to imitate part of ciliary neurotrophic factor (CNTF), a naturally occurring protein that supports neuron survival and plasticity5

  • theoreticalP21 was designed to be small enough to cross the blood-brain barrier and survive digestion5

  • theoreticalP21 was engineered from CNTF residues around positions 148 to 151 by epitope mapping, with an adamantylated glycine added for stability and brain penetration5

  • theoreticalP21 is a four-amino-acid tetrapeptide designed to mimic CNTF (ciliary neurotrophic factor), a brain growth factor that cannot cross the blood-brain barrier6

  • theoreticalP21 is approximately forty times smaller than CNTF6

  • theoreticalP21 activates JAK/STAT and PI3K/Akt pathways leading to hippocampal neurogenesis and GSK-3β inhibition6

  • theoreticalP21 is an eight-amino-acid peptide fragment mapped to CNTF peptide 6 with an adamantane group conjugated via a flexible linker7

  • theoreticalThe adamantyl modification confers blood-brain barrier penetration, resistance to peptidase degradation, and oral bioavailability7

  • theoreticalP21 is hypothesized to activate the tripartite CNTF receptor (CNTFR-α / LIFR / gp130) or a subset of its downstream signaling cascade, engaging JAK/STAT3 and RAS/MAPK pathways7

  • theoreticalP21 is a small, modified tetrapeptide derived from the biologically active region of ciliary neurotrophic factor (CNTF)8

  • theoreticalP21 was designed to capture the neurotrophic properties of CNTF while overcoming limitations of the full-length protein including poor blood-brain barrier penetration, short half-life, and need for parenteral administration8

  • theoreticalP21 corresponds to residues 148-151 of CNTF8

  • theoreticalP21 has N-terminal acetylation and C-terminal adamantylation modifications to enhance metabolic stability and lipophilicity for oral bioavailability and CNS penetration8

  • theoreticalMay protect neurons from oxidative stress and inflammatory damage19

  • theoreticalP21 crosses the blood-brain barrier and boosts brain-derived neurotrophic factor expression20

  • theoreticalP21 promotes the growth of entirely new neurons in the hippocampus20

  • theoreticalP21 is a synthetic CNTF (ciliary neurotrophic factor) mimetic studied for its effect on BDNF — the primary growth factor for new neuron formation21

  • theoreticalP21 is a synthetic nine-amino-acid peptide derived from ciliary neurotrophic factor (CNTF)21

  • theoreticalP21's proposed mechanism -- generating and integrating new neurons -- unfolds over weeks21

  • theoreticalP21 sequence is Asp-Gly-Gly-Leu-Phe-Glu-Lys-Lys-Leu (DGGLFEKKL)21

  • theoreticalMolecular weight of 985.11 g/mol21

  • theoreticalP21 offers potential therapeutic value in conditions that affect brain function and cognition by supporting neuronal survival and neuroplasticity22

  • theoreticalP21 promotes neuronal survival and function, making it a candidate for exploring treatments aimed at slowing disease progression22

  • theoreticalThe peptide demonstrates anti-inflammatory effects in preclinical models, particularly in reducing neuroinflammation22

  • theoreticalP21 works by copying and boosting the helpful effects of a natural protein called CNTF (ciliary neurotrophic factor)22

  • theoreticalP21 blocks the activity of certain proteins called CDKs, especially one called CDK5 when it pairs with another protein called p2522

  • theoreticalP21 sticks to the CDK5/p25 pair, calms it down, and helps keep brain cells alive and healthy22

  • theoreticalP21 helps brain cells grow and form new connections by influencing the signals that guide brain cell repair and development22

  • theoreticalP21 slightly interacts with the same receptors that CNTF uses, turning on helpful cell pathways that support the growth and survival of brain cells22

  • theoreticalP21 encourages special tooth-forming cells called odontoblasts to develop by boosting markers linked to tooth repair22

  • theoreticalP21 helps lower brain inflammation by calming down overactive immune cells in the brain22

  • theoreticalP21 mainly acts by blocking a protein pair called CDK5/p2522

  • theoreticalP21 lightly activates the JAK/STAT pathway, which helps brain cells grow and survive22

  • theoreticalP21 is a small peptide derived from ciliary neurotrophic factor (CNTF)24

  • theoreticalP21 promotes neurogenesis (new brain cell creation) in the hippocampus24

  • theoreticalP21 inhibits tau hyperphosphorylation that forms neurofibrillary tangles in Alzheimer's disease24

  • theoreticalP21 was designed to be small enough to cross the blood-brain barrier24

  • theoreticalP21 upregulates BDNF expression in the hippocampal dentate gyrus24

  • theoreticalP21 inhibits GSK-3beta, the primary kinase responsible for pathological tau phosphorylation24

  • theoreticalp21, encoded by gene, also named p21, was first identified as a cyclin-dependent kinase regulator that suppresses cell cycle G1/S phase and retinoblastoma protein phosphorylation26

  • theoreticalp21 acts as the downstream target gene of p53, and its expression is induced by wild-type p53 and it is not associated with mutant p5326

  • theoreticalp21 has been characterized as a vital regulator that involves multiple cell functions, including G1/S cell cycle progression, cell growth, DNA damage, and cell stemness26

  • theoreticalp21 plays a significant role in tumor development through p53-dependent and p53-independent pathways26

  • theoreticalexpression of p21 is closely related to the resting state or terminal differentiation of cells26

  • theoreticalp21 is associated with cancer stem cells and acts as a biomarker for such cells26

  • theoreticalp21 functions as a cell cycle inhibitor and anti-proliferative effector in normal cells27

  • theoreticalp21 is dysregulated in some cancers27

  • theoreticalp21 plays a role in cell cycle arrest under p53 transcription factor activity27

  • theoreticalp21 has tumor-suppressor function in cancer27

  • theoreticalp21 has a role in phenotypic plasticity27

  • theoreticalp21 has oncogenic/anti-apoptotic function depending on p21 subcellular localization and p53 status27

  • theoreticalPrecision gene editing can be used to manipulate p21 and proteins that interact with it for therapeutic approaches in cancer27

  • theoreticalCyclin-dependent kinase (CDK) inhibitor p21 (also known as p21(WAF1/Cip1)) promotes cell cycle arrest in response to a variety of stimuli28

  • theoreticalThe inhibitory effect of P21 on cell cycle progression correlates with its nuclear localization28

  • theoreticalP21 can be induced by both p53-dependent and p53-independent mechanisms28

  • theoreticalP21 has functions in transcriptional regulation, modulation or inhibition of apoptosis28

  • theoreticalP21 functions are largely dependent on direct p21/protein interactions and on p21 subcellular localizations28

  • theoreticalP21 can play a role in DNA repair by interacting with proliferating cell nuclear antigen (PCNA)28

  • theoreticalp21(Cip1) is a multifunctional protein and a key player in regulating different cellular processes29

  • theoreticalThe transcription of p21 is regulated by p53-dependent and -independent pathways29

  • theoreticalThe expression of p21 is increased in response to various cellular stresses to arrest the cell cycle and ensure genomic stability29

  • theoreticalp21 binds to and inhibits the activity of Cdk1/cyclin B129

  • theoreticalp21 is important for a fine-tuned mitotic progression29

  • theoreticalLoss of p21 prolongs the duration of mitosis and results in severe mitotic defects like chromosome segregation and cytokinesis failures promoting consequently genomic instability29

  • theoreticalp21 is dramatically stabilized in mitotic tumor cells upon treatment with mitotic agents like paclitaxel or mitotic kinase inhibitors29

  • theoreticalIncreased p21 is mainly localized in the cytoplasm and associates with cell survival indicating a crucial role of p21 in susceptibility to mitotic agents in tumor cells29

Dosing

Based on 3 expert opinion findings, 2 anecdotal findings and 1 theoretical finding.

  • expert opinionThere is no validated human dose for P215

  • expert opinionUse bacteriostatic (BAC) water only. Avoid saline — may cause precipitation19

  • expert opinionGently swirl in circular motions — DO NOT shake vigorously as this degrades the peptide19

  • anecdotalReported dosing centers on 100-300 mcg/day SubQ, cycled 1-4 weeks on with extended off-periods21

  • anecdotalUser-reported dosing is 1-2 mg intranasal or subcutaneous daily24

  • theoreticalAdministration is intranasal or subcutaneous injection24

How the body handles it

Based on 2 animal findings, 2 in vitro findings, 1 expert opinion finding and 9 theoretical findings.

  • animalP21 demonstrates blood-brain barrier penetration via adamantane modification2

  • animalPharmacokinetic studies have been published in mice and cynomolgus monkeys7

  • in vitroP21 shows greater than 95 percent stability in simulated gastric fluid and plasma stability exceeding three hours5

  • in vitroP21 is reported to be orally bioavailable and metabolically stable5

  • expert opinionReconstituted solution should be used within 28 days19

  • theoreticalP021 has adamantylated glycine added at C-terminal to increase lipophilicity and blood-brain permeability while decreasing degradation by exopeptidases1

  • theoreticalThe adamantane modification serves enhanced penetration across the blood-brain barrier through increased lipid solubility2

  • theoreticalP21's molecular weight of 578.3 Da positions it at the upper boundary of the typical blood-brain barrier (BBB) exclusion threshold for peptides (400-600 Da)2

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  • theoreticalThere is no human pharmacokinetic, dose-response, or efficacy data3

  • theoreticalP21 sequence is Ac-DGGL-NH₂ with an adamantane group for metabolic stability and BBB penetration6

  • theoreticalP21 crosses the blood-brain barrier and doesn't trigger the immune suppression associated with the parent molecule21

  • theoreticalP21 can cross the blood-brain barrier22

  • theoreticalThe adamantane helps protect P21 from being quickly broken down by enzymes, and the acetyl group helps it pass into the brain more easily22

  • theoreticalHalf-life is 4-6 hours with limited pharmacokinetic data24

Safety and side effects

Based on 1 human study finding, 6 animal findings, 8 expert opinion findings, 4 anecdotal findings and 5 theoretical findings.

  • human studyClinical trials using full-length CNTF in ALS patients showed no benefit and produced serious side effects including severe weight loss, anorexia, cramps, and muscle pain20

  • animalP021 does not have anorectic effects in rodents1

  • animalP021 may increase body weight in rodents1

  • animalP021 treatment for up to 18 months in rodents does not result in weight loss, tumors, or signs of pain1

  • animalP21 circumvents severe adverse effects of full-length CNTF including weight loss and immunogenicity2

  • animalP21 exhibits favorable safety profile in preclinical studies up to 18 months duration2

  • animalNo rodent toxicity signature even in 18-month chronic dosing4

  • expert opinionThere are no completed human clinical trials for P215

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  • expert opinionThere are no regulatory approvals for P215

  • expert opinionThere is no human safety data for P21; all toxicology and tolerability information is preclinical5

  • expert opinionP21 peptide administered subcutaneously is reported safe in recommended dosages18

  • expert opinionRedness and pain at the site of injection may occur with P21 injection18

  • expert opinionCaution with Stimulants (Modafinil, Adderall) and Racetams19

  • expert opinionMonitor when combining with MAOIs / SSRIs19

  • expert opinionResearch compound only, not FDA-approved for any indication21

  • anecdotalSide effects include headache, mild anxiety or restlessness, nasal irritation, sleep disturbance, and rare mood changes19

  • anecdotalVivid dreams and mild emotional sensitivity reported during use21

  • anecdotalHair thinning reported anecdotally with heavy dosing21

  • anecdotalNo psychostimulant effects, no dependency, no crash21

  • theoreticalSafety in humans has not been assessed1

  • theoreticalP21 lacks the appetite-suppressing side effects of full-length CNTF3

  • theoreticalP21 has never entered human clinical trials as of April 20267

  • theoreticalCommon side effects include headache, nasal irritation (intranasal route), and mild fatigue24

  • theoreticalVery limited human safety data and no long-term data on effects on brain tissue24

What people use it for

Based on 7 animal findings, 5 expert opinion findings, 1 anecdotal finding and 10 theoretical findings.

  • animalP021 appears to prevent pathologies of age-related macular degeneration in rodents1

  • animalP21 shows efficacy in traumatic brain injury and Alzheimer's disease models2

  • animalP21 promotes neurogenesis (new neuron formation) in the hippocampus3

  • animalP21 has been tested in 3xTg-AD, APP/PS1, and Tg2576 Alzheimer's mouse models7

  • animalP21 has been tested in Ts65Dn Down syndrome cognitive impairment models7

  • animalP21 has been tested in normal aging rodent models7

  • animalMice deficient in p21 exhibit no apparent phenotype17

  • expert opinionp21 influences the effect of cancer radiotherapy via involving in multiple signaling pathways12

Show the remaining 15

  • expert opinionp21 is a promising therapeutic target for cancer radiotherapy12

  • expert opinionCDK-independent functions of p21 may have potential for the improvement of cancer treatments14

  • expert opinionp21 may have potential gene therapy applications17

  • expert opinionExperimental neurocognitive peptide candidate with limited human evidence19

  • anecdotalCommunity reports describe improved memory consolidation, learning speed, and cognitive clarity21

  • theoreticalp21/E2f1 could be potential druggable targets that could be leveraged in clinical therapies to improve bone healing in pathologies such as osteoporosis9

  • theoreticalSirt1-induced p21 deacetylation could be a novel therapeutic strategy for myocardial infarction15

  • theoreticalP21 is a synthetic peptide derived from the neurotrophic factor cerebrolysin, which has shown potential in improving cognitive function and promoting neural regeneration22

  • theoreticalP21 is primarily studied for its neuroprotective and cognitive-enhancing properties, with promising applications in treating neurodegenerative diseases such as Alzheimer's disease, traumatic brain injury (TBI), and cognitive decline22

  • theoreticalAs a nootropic, P21 is investigated for its potential to boost cognitive functions like memory, learning, and attention22

  • theoreticalP21 shows promise in promoting dentinogenesis, the process of dentin formation in teeth and stimulates odontoblast differentiation, aiding in dental tissue repair and regeneration22

  • theoreticalP21 is being studied for its impact on glucose metabolism and insulin sensitivity22

  • theoreticalNot FDA approved and is a research compound24

  • theoreticalp21 is implicated in response to many cancer treatments and p21 promotes the effect of oncolytic virotherapy26

  • theoreticalp21 has been shown to be a tumor suppressor and an oncogene as well29

Other findings

Based on 5 expert opinion findings and 7 theoretical findings.

  • expert opinionThe corporate developer (Phanes Biotech) has targeted a Phase 1 IND for 2025-2026 but has not announced a trial start4

  • expert opinionAll P21 data is from one research group (Khalid Iqbal's group at New York State Institute for Basic Research in Developmental Disabilities)6

  • expert opinionNo independent lab has replicated P21 findings6

  • expert opinionCompatible with NAD+ and BPC-15719

  • expert opinionAfter age 25, the average adult loses roughly 0.5% of hippocampal volume per year20

  • theoreticalP21 (formal designation P021) is a synthetic 5-residue, CNTF-derived research peptide developed by the Khalid Iqbal laboratory at the New York State Institute for Basic Research4

  • theoreticalP021 is not FDA approved, is not on the 503A bulks list, and remains a research chemical under 'research use only' labeling4

  • theoreticalP21 has the sequence Ac-DGGL(A)G-NH25

Show the remaining 4

  • theoreticalP21 has accumulated 15+ papers of preclinical evidence from 2010-2023 from the Iqbal lab7

  • theoreticalNo IND filed for P21 as of April 20267

  • theoreticalP21 sequence is Ac-DGGL (Acetyl-Asp-Gly-Gly-Leu) with an adamantylated C-terminus8

  • theoreticalP21 is a man-made short protein made of four amino acids, based on CNTF, including aspartic acid, glycine, glycine, and leucine, with an acetyl group at the start and an adamantane group attached to glycine22

Points of contention

Where the evidence is unsettled, thin, or says less than the popular claim — worth knowing before you draw conclusions.

Other

Many cited sources describe a completely different molecule that shares the name 'p21'.

Sources Thermally targeted p21 peptide enhances bortezomib ... - PMC through Multifaceted p21 in carcinogenesis, stemness of tumor and tumor therapy. (and parts of P21 - Exploring Peptides) discuss the cyclin-dependent kinase inhibitor protein p21 (also called p21WAF1/Cip1/CDKN1A), a tumor-suppressor/cell-cycle regulator that is entirely unrelated to the CNTF-derived research peptide P021. P21 - Exploring Peptides also incorrectly states P21 is derived from cerebrolysin and acts by blocking CDK5/p25. None of these claims apply to the CNTF-derived peptide and were excluded from the peptide claims above.

Contested

Reported half-life values conflict widely.

Plasma half-life is reported as over 3 hours in mice (1 Last updated on May 12, 2025 Cognitive Vitality Reports ...), greater than 6 hours in rodents (P21 — Molecular Specifications & Research Monograph | Peptide Biologix), 4-6 hours (P21 (P021) — Dosage, Half-Life & Research | Peptide Reference), and 'typically less than one hour' (P21 - Exploring Peptides). These cannot all be reconciled, and low-tier sources contribute to the spread.

Limited evidence

No human data exists; efficacy and safety are entirely preclinical or anecdotal.

There are no completed or registered human clinical trials, no human pharmacokinetic/dose-response/efficacy data, and no human safety data. Human dosing, side-effect, and benefit reports come only from low-to-mid-tier community/vendor sources (P21 (P021) — Dosage, Half-Life & Research | Peptide Reference, P21 Peptide Research, P21 (Nootropic Peptide Candidate) — Dosing, Side Effects, FDA Status & Research | PeptIQ | PeptIQ, P21 - The International Peptide Society).

Contested

The core premise (adult human hippocampal neurogenesis) is itself debated.

P21 Peptide: Research, Evidence, Dosing & Safety | Peptidings notes that adult human hippocampal neurogenesis—central to P21's proposed benefit—remains a contested concept in neuroscience, which qualifies how directly rodent neurogenesis findings would translate to humans.

Using it with other compounds

Check a stack
  • SelankSame downstream effect

    No documented conflict

    Selank supports BDNF (and NGF) signaling alongside its anxiolytic/nootropic actions, overlapping with the BDNF/TrkB pathway P21 activates. Coming from different upstream mechanisms and converging on BDNF-mediated plasticity, they are plausibly complementary for cognition and mood support.

    Tier 4Theoretical — not established

    What the research doesn't fully establish

    Both peptides' mechanisms explicitly include BDNF signaling as a pathway. P21 directly targets TrkB receptor via induced BDNF and lists BDNF/TrkB as a core pathway with approved tag BDNF_signaling. Selank lists BDNF signaling as an explicit pathway and carries the same BDNF_signaling approved tag. The proposed relationship correctly identifies that despite different upstream mechanisms (P21 via CNTF-mimicry and TrkB activation; Selank via GABAergic, serotonergic, and enkephalinase pathways), both converge on BDNF-mediated signaling. The explanation accurately characterizes this as downstream convergence on a shared dimension (BDNF signaling) that supports neuroplasticity-related effects. The mechanism descriptions fully justify the same_downstream relationship type and the BDNF_signaling shared dimension claim.

    Shares BDNF signaling

  • SemaxSame downstream effect

    May be complementary

    P21 raises BDNF that then activates TrkB; Semax also upregulates BDNF/NGF and signals through TrkB and CREB. Both end up driving the same BDNF/TrkB neurotrophic and neurogenic output through different starting points, so they are potentially additive for memory and neuroprotection rather than redundant. Monitor for over-stimulation, since both also touch CREB and plasticity pathways.

    Tier 4Theoretical — not established

    What the research doesn't fully establish

    Both peptides' mechanisms clearly converge on BDNF/TrkB signaling and neurogenesis. P21 upregulates BDNF expression which activates TrkB, and Semax directly upregulates BDNF/NGF and signals through TrkB. Both activate CREB and drive neurogenesis through overlapping downstream pathways (PI3K/Akt and BDNF/TrkB for P21; BDNF/NGF-TrkB and CREB for Semax). The proposed relationship accurately identifies that despite different mechanisms of action (P21 as CNTF mimetic, Semax as ACTH-derived peptide), they converge on the same neurogenic and BDNF-signaling outputs, making them potentially additive rather than redundant. The shared dimensions of neurogenesis and BDNF_signaling are explicitly supported by both mechanisms.

    Shares neurogenesis · BDNF signaling

  • PinealonComplementary

    Worth caution

    Both report enhanced dendritic arborization, synaptic plasticity and neurogenesis. P21 drives BDNF/TrkB and PI3K/Akt–GSK-3β signaling, while Pinealon is proposed to reach similar endpoints via gene-expression/ERK modulation. The distinct mechanisms pointing at the same plasticity outcome make them complementary in principle, with only preclinical/anecdotal support for combining.

    Tier 4Theoretical — not established

    What the research doesn't fully establish

    Both peptides' mechanisms clearly support the claimed shared dimension of neurogenesis and dendritic arborization. Pinealon reports dendritic arborization/neuroplasticity (src-26, src-47) and MAPK/ERK modulation (src-18, src-8, src-22, src-32, src-47). P21 reports enhanced dendritic arborization and dendritic spine density, stimulation of adult hippocampal neurogenesis, and BDNF/TrkB and PI3K/Akt signaling. Both mechanisms converge on neuroplasticity outcomes through distinct pathways (ERK-based vs. BDNF/TrkB/PI3K-Akt-GSK-3β). The 'complementary' relationship type is justified: they target overlapping functional endpoints (neurogenesis, dendritic plasticity, synaptic enhancement) via mechanistically distinct routes, which is the definition of complementarity. The explanation accurately reflects the mechanism material provided—different signaling cascades achieving similar neuroprotective/neuroplastic effects. The caveat about preclinical/anecdotal support is appropriate given the evidence levels cited.

    Shares neurogenesis

  • HumaninSame mechanism

    Worth caution

    This is a real overlap worth understanding: humanin's reported neuroprotection depends on the CNTFR-alpha/WSX-1/gp130 cytokine receptor complex acting through JAK2/STAT3 and PI3K/Akt, and P21 is a CNTF-fragment that engages the same CNTF/LIFR/gp130 receptor family and STAT3/Akt signaling. Because they converge on the same gp130/JAK-STAT machinery, combining them is more likely to be redundant than additive at that receptor, though their broader effects differ.

    Tier 4Theoretical — not established

    What the research doesn't fully establish

    Both peptides' mechanisms clearly establish convergence on shared signaling pathways. Humanin targets the CNTFR-alpha/WSX-1/gp130 heterotrimeric complex and activates JAK2/STAT3 and PI3K/Akt pathways. P21, as a CNTF fragment, targets the CNTF receptor complex (CNTFR-alpha/LIFR/gp130) and activates JAK/STAT (STAT3) and PI3K/Akt pathways. Both mechanisms explicitly document gp130 engagement and both activate PI3K/Akt signaling. The proposed shared dimension (mTOR_PI3K) is directly supported by both peptides' documented PI3K/Akt pathway activation. The explanation correctly identifies the convergence on gp130/JAK-STAT machinery as a basis for potential redundancy. This is a mechanistically justified relationship based on the provided descriptions.

    Shares mTOR PI3K

  • CerebrolysinComplementary

    May be complementary

    Cerebrolysin stimulates BDNF and signals through TrkB and PI3K/Akt survival pathways, while P21 (a CNTF fragment) drives BDNF expression that also feeds TrkB and neurogenesis. They reach the same BDNF/TrkB neuroplasticity endpoint from different starting points, so they can reinforce each other's neurotrophic and pro-neurogenic effects. Evidence for the specific combination is anecdotal.

    Tier 4Theoretical — not established

    What the research doesn't fully establish

    Both peptides' mechanisms clearly establish the three claimed shared dimensions. Cerebrolysin targets TrkB, upregulates BDNF/VEGF/IGF-1, activates PI3K/AKT survival signaling, and promotes neurogenesis (approved tags: BDNF_signaling, mTOR_PI3K, neurogenesis). P21 induces BDNF expression, signals via TrkB, activates PI3K/Akt and Akt–GSK-3beta pathways, and stimulates adult hippocampal neurogenesis (approved tags: BDNF_signaling, mTOR_PI3K, neurogenesis). The explanation correctly identifies that both converge on BDNF/TrkB and PI3K/Akt pathways despite different mechanisms of action (Cerebrolysin as a multi-target neuroprotective mixture; P21 as a CNTF-mimetic fragment). The 'complementary' relationship type is justified: they operate through overlapping downstream pathways (BDNF/TrkB, PI3K/Akt, neurogenesis) but via distinct upstream mechanisms, supporting potential synergistic or reinforcing effects. The caveat about anecdotal evidence for the specific combination does not undermine the mechanistic justification for the relationship itself.

    Shares neurogenesis · mTOR PI3K · BDNF signaling

  • NoopeptSame downstream effect

    May be complementary

    Both peptides ultimately drive the BDNF/TrkB neurotrophin cascade and promote synaptic plasticity and memory — P21 as a CNTF-derived fragment that upregulates BDNF, Noopept via its metabolite acting on TrkB and AMPA. They reach the same downstream output (enhanced BDNF signaling and neuroprotection) from different upstream starting points, which is generally synergistic but worth noting as overlapping rather than independent.

    Tier 4Theoretical — not established

    What the research doesn't fully establish

    Both peptides' mechanisms clearly establish BDNF/TrkB pathway engagement as a shared downstream output. Noopept's mechanism explicitly lists 'BDNF / NGF neurotrophin signaling' as a pathway and 'BDNF_signaling' as an approved tag, with TrkB receptor activation documented. P21's mechanism explicitly states 'Upregulation of BDNF expression and secretion' and 'BDNF/TrkB' as a primary pathway with 'BDNF_signaling' as an approved tag. Both mechanisms converge on enhanced synaptic plasticity and neuroprotection through this shared cascade, despite different upstream mechanisms (Noopept via metabolite/TrkB/AMPA; P21 via CNTF-derived BDNF upregulation). The 'same_downstream' relationship type and BDNF_signaling shared dimension are directly supported by the provided mechanism material.

    Shares BDNF signaling

  • DavunetideSame downstream effect

    May be complementary

    Both peptides converge on reducing tau hyperphosphorylation, but by different routes: davunetide stabilizes microtubules and dampens GSK-3 activity directly, while P21 raises BDNF and inactivates GSK-3beta via the Akt pathway. They arrive at the same anti-tau, pro-synaptic endpoint from different starting points, which is why they are complementary rather than redundant.

    Tier 4Theoretical — not established

    What the research doesn't fully establish

    Both peptides' mechanisms clearly converge on the mTOR_PI3K pathway and share the downstream effect of reducing tau hyperphosphorylation through GSK-3 inhibition. Davunetide directly reduces tau phosphorylation and inhibits GSK-3 (asserted), while P21 inactivates GSK-3beta via the Akt–GSK-3beta axis within the PI3K/Akt pathway. Both peptides are tagged with mTOR_PI3K, and both achieve tau reduction and synaptic/neuroprotective effects through distinct upstream mechanisms converging on shared downstream pathways. The explanation accurately characterizes their complementary routes to the same endpoint.

    Shares mTOR PI3K

  • CortexinComplementary

    May be complementary

    P21 is a CNTF fragment that strongly upregulates BDNF and hippocampal neurogenesis while reducing tau phosphorylation, whereas Cortexin provides broad neurotrophic and anti-excitotoxic protection. Different upstream mechanisms both feeding neuroplasticity and neuronal survival, so the two are complementary.

    Tier 4Theoretical — not established

    What the research doesn't fully establish

    Both peptides' mechanisms clearly support the claimed shared dimensions. Cortexin explicitly targets BDNF-like/NGF-like neurotrophic signaling and has approved tags for neurogenesis and BDNF_signaling. P21 directly upregulates BDNF expression via TrkB and stimulates adult hippocampal neurogenesis with approved tags matching these dimensions. The proposed relationship as 'complementary' is justified: P21 acts through BDNF/TrkB and GSK-3beta pathways to enhance neurogenesis and synaptic plasticity, while Cortexin provides broader neuroprotection through multiple mechanisms (glutamatergic modulation, caspase-8 inhibition, antioxidant effects, and neurotrophic support). These represent distinct upstream mechanisms converging on shared neuroplasticity and neuronal survival outcomes, fitting the complementary characterization.

    Shares neurogenesis · BDNF signaling

Safety and side effects

Safety Overview

There are no completed human clinical trials of P21, no regulatory approvals, no validated human dose, and no human safety data. All toxicology and tolerability information is preclinical or anecdotal.

Preclinical (Animal) Safety

  • In rodents, P21 does not have anorectic effects and may actually increase body weight.
  • Treatment for up to 18 months produced no weight loss, no tumors, and no signs of pain.
  • Unlike full-length CNTF, P21 reportedly lacks appetite-suppressing side effects — attributed to its not activating STAT3 signaling in the hypothalamus — and appears to circumvent CNTF adverse effects such as weight loss and immunogenicity.

Contrast with Full-Length CNTF

Clinical trials using full-length CNTF in ALS patients showed no benefit and produced serious side effects including severe weight loss, anorexia, cramps, and muscle pain. P21 was specifically engineered to avoid these liabilities, but this advantage has only been demonstrated in animals.

Anecdotal (Community) Reports

These come from low-to-mid-tier community/vendor sources and should be treated cautiously:

  • Reported side effects: headache, nasal irritation (intranasal route), mild fatigue, mild anxiety/restlessness, sleep disturbance, vivid dreams, rare mood changes, and hair thinning with heavy dosing.
  • Users report no psychostimulant effects, dependency, or crash.
  • One source reports subcutaneous P21 as safe at recommended dosages, with possible redness and pain at the injection site.

Reported Combination Cautions (Vendor Source)

One vendor source suggests P21 is compatible with NAD+ and BPC-157, advises caution when combining with stimulants (modafinil, Adderall) and racetams, and recommends monitoring when combined with MAOIs/SSRIs. These are not clinically validated recommendations.

Bottom line: Human safety is entirely unestablished. Any use is experimental, and reported human tolerability rests on anecdote alone.

Reconstitution and handling

Reconstitution & Preparation

No dose has been established for this compound. No regulatory label exists for it, so the figures below are what sources report — not guidance.

Note: No human dose has been clinically validated. The guidance below reflects reported preclinical dosing and anecdotal community/vendor practices — not established protocols.

For reconstitution, expert/vendor sources advise:

  • Use bacteriostatic water — saline may cause precipitation.
  • Use the reconstituted solution within 28 days.
  • Avoid vigorous shaking; swirl gently.

Administration Routes

  • Oral — used in rodent studies; P21 is reported to be orally bioavailable and metabolically stable (>95% stability in simulated gastric fluid, plasma stability >3 hours).
  • Subcutaneous injection — anecdotal/community route.
  • Intranasal — anecdotal/community route.

Reported Dosing

Preclinical (animal):

  • Mouse studies have used 60 nM per gram of feed.

Anecdotal (community — wide variation, not validated):

  • 1-2 mg intranasal or subcutaneous daily
  • 100-300 mcg/day subcutaneous
  • 200-400 mcg cited as a median dose
  • Typically cycled (e.g., 4-8 weeks)

Anecdotal users describe subjective onset of effects (memory consolidation, learning speed, cognitive clarity, mood/focus, sleep quality) over 1-4 weeks.

Pharmacokinetic Context

Pharmacokinetic studies have been published in mice and cynomolgus monkeys, with plasma half-life reportedly extended from minutes (full-length CNTF ~2.9 min) to hours, though reported values conflict widely across sources (see half-life notes). P21's molecular weight (~578 Da) sits at the upper boundary of the typical BBB peptide exclusion threshold (400-600 Da), with the adamantane modification reported to enable CNS penetration.

Sources

Ordered by evidence quality — the strongest first.

  1. 1 Last updated on May 12, 2025 Cognitive Vitality Reports ...(opens in a new tab)
    Tier 2Web · alzdiscovery.org
  2. P21 — Molecular Specifications & Research Monograph | Peptide Biologix(opens in a new tab)
    Tier 2Web · peptidebiologix.com
  3. P21 (P021, P21 Adamantane, CNTF-derived peptide) | Peptide List(opens in a new tab)
    Tier 2Web · peptidelist.org
  4. P21 Peptide Guide 2026: BDNF Mimetic Neuropeptide Review(opens in a new tab)
    Tier 2Web · thepeptidetoolkit.com · 2026
  5. P21 Peptide (P021): CNTF Mimetic Research Guide | PeptideStat(opens in a new tab)
    Tier 2Web · peptidestat.com · 2026
  6. P21 Peptide: Research, Evidence, Dosing & Safety | Peptidings(opens in a new tab)
    Tier 2Web · peptidings.com · 2026
  7. p21: The CNTF-Derived Neurotrophic Peptide | Kalios(opens in a new tab)
    Tier 2Web · kalios.health · 2026
  8. P21 (CNTF-Derived Peptide): Research Evidence & Safety Profile | PeptideInsight(opens in a new tab)
    Tier 2Web · peptideinsight.com · 2026
  9. Absence of E2f1 Negates Pro-osteogenic Impacts of p21 Absence.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 2024
  10. Functional elements of the cis-regulatory lincRNA-p21.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 2022
  11. Involvement of the p53/p21 complex in p53-dependent gene expression.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 2022
  12. Multiple functions of p21 in cancer radiotherapy.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 2021
  13. p21 produces a bioactive secretome that places stressed cells under immunosurveillance.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 2021
  14. CDK-Independent and PCNA-Dependent Functions of p21 in DNA Replication.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 2020
  15. Sirt1-inducible deacetylation of p21 promotes cardiomyocyte proliferation.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 2019
  16. Lost in transcription: p21 repression, mechanisms, and consequences.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 2005
  17. p21--negative regulator of the cell cycle.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 1996
  18. P21 - The International Peptide Society(opens in a new tab)
    Tier 3Web · peptidesociety.org
  19. P21 (Nootropic Peptide Candidate) — Dosing, Side Effects, FDA Status & Research | PeptIQ | PeptIQ(opens in a new tab)
    Tier 3Web · peptiq.io
  20. P21 Peptide Benefits: the complete guide to this CNTF-derived nootropic - SeekPeptides(opens in a new tab)
    Tier 3Web · seekpeptides.com
  21. P21 Peptide Research(opens in a new tab)
    Tier 3Web · peptidefox.com
  22. P21 - Exploring Peptides(opens in a new tab)
    Tier 4Web · exploring-peptides.com
  23. Thermally targeted p21 peptide enhances bortezomib ... - PMC(opens in a new tab)
    Tier 4Web · pmc.ncbi.nlm.nih.gov
  24. P21 (P021) — Dosage, Half-Life & Research | Peptide Reference(opens in a new tab)
    Tier 4Web · peptide-reference.com · 2026
  25. An inducible-Cre mouse model to monitor and manipulate-highly-expressing senescent cells.(opens in a new tab)
    Tier 4PubMed · pubmed.ncbi.nlm.nih.gov · 2021
  26. Multifaceted p21 in carcinogenesis, stemness of tumor and tumor therapy.(opens in a new tab)
    Tier 4PubMed · pubmed.ncbi.nlm.nih.gov · 2020
  27. p21 in Cancer Research.(opens in a new tab)
    Tier 4PubMed · pubmed.ncbi.nlm.nih.gov · 2019
  28. Multiple functions of p21 in cell cycle, apoptosis and transcriptional regulation after DNA damage.(opens in a new tab)
    Tier 4PubMed · pubmed.ncbi.nlm.nih.gov · 2016
  29. Less understood issues: p21(Cip1) in mitosis and its therapeutic potential.(opens in a new tab)
    Tier 4PubMed · pubmed.ncbi.nlm.nih.gov · 2015
  30. LincRNA-p21 suppresses target mRNA translation.(opens in a new tab)
    Tier 4PubMed · pubmed.ncbi.nlm.nih.gov · 2012