P21

This peptide corresponds to the N-terminal 21–amino-acid segment of the full p21 protein, retaining the cyclin-binding motif that interacts with CDK2/Cyclin E. It contains only natural L-amino acids with free N- and C-termini. Research-grade P21 peptide is synthesized via solid-phase methods and purified by HPLC to ≥95 % purity. Mass spectrometry confirms its sequence and integrity.

P21 binds directly to cyclin–CDK complexes, preventing phosphorylation of retinoblastoma protein and halting progression from G₁ to S phase. It also interacts with proliferating-cell nuclear antigen (PCNA), slowing DNA synthesis during repair. By enforcing a cell-cycle checkpoint, P21 allows time for DNA damage repair or triggers senescence if damage is irreparable. Its activity is observed within hours of cellular stress induction.

The peptide is studied for its ability to induce cell-cycle arrest and promote DNA-repair mechanisms. In human cell-culture models, P21 treatment reduces proliferation of cancer-derived cell lines and enhances sensitivity to DNA-damaging agents. It is used to explore anti-proliferative strategies in oncology and to investigate aging-related pathways. Further work aims to assess its potential in combination with chemotherapy.

In cell-based systems, high concentrations of P21 can trigger irreversible senescence or apoptotic pathways if DNA damage is severe. No systemic or clinical human safety data exist, as P21 is not used therapeutically. In research settings, cytotoxicity assays guide dosing to avoid off-target cell death. Monitoring of cell-viability and apoptosis markers is standard practice.

P21 peptide is synthesized by Fmoc solid-phase peptide synthesis, assembling the 21 residues sequentially on a resin support. After cleavage and deprotection, it undergoes preparative HPLC purification to ≥95 % purity. Analytical HPLC and mass spectrometry verify correct sequence and purity. Manufacturing follows laboratory-grade peptide standards.

P21 is classified for laboratory research use only and is not approved by any regulatory agency for therapeutic application. It cannot be prescribed or sold for human treatment. Laboratories obtain it under research-reagent regulations. No commercial clinical formulations exist.

In cell-culture experiments, P21 is typically applied at 1–10 μM for 24–72 hours. Concentrations and exposure times vary by cell type and experimental aim. No in vivo dosing guidelines exist. All use should follow institutional biosafety and research-protocol approvals.

Do optimize concentration and exposure time to balance arrest versus toxicity.Do include appropriate cell-viability and cell-cycle assays.Don’t extrapolate cell-culture findings directly to clinical settings.Don’t combine with other CDK inhibitors without mechanistic rationale.

Q: Can P21 reverse cancer progression?A: In vitro it halts cell proliferation, but in vivo efficacy and delivery remain untested.Q: Does it affect normal cells?A: Yes; P21 arrests the cell cycle in any proliferating cell type.Q: Is it stable in culture media?A: It is stable for 24–72 hours but may require serum-free conditions to prevent degradation.