P21 (P021)

P21 (often written as P021 in the literature) is an 11-amino-acid neurogenic peptide derived from the ciliary neurotrophic factor (CNTF) molecule, designed by Khalid Iqbal's group at the New York State Institute for Basic Research in Developmental Disabilities. The parent CNTF molecule is a potent neurotrophic factor that has been investigated clinically (including unsuccessful trials in amyotrophic lateral sclerosis), but its large size, instability, and significant side-effect profile limited its therapeutic utility. P21 was engineered as a small, blood-brain-barrier-permeable peptide that retains the neurogenic and synaptogenic activity of CNTF.

In preclinical studies, P21 promotes adult hippocampal neurogenesis, increases dendritic spine density, reduces tau hyperphosphorylation, and improves cognitive performance in mouse models of Alzheimer's disease and other tauopathies. The proposed mechanism involves rescue of leptin/CNTF receptor signaling, downregulation of glycogen synthase kinase-3β (GSK-3β, the kinase responsible for tau hyperphosphorylation), and stabilization of dendritic architecture.

P21 has been the subject of sustained academic research from the Iqbal group across more than a decade. It has not advanced to clinical trials, however, and remains a research compound. It is sold by some research-peptide vendors. Human safety and efficacy data are absent.

P21 (P021) was developed in the late 2000s by Khalid Iqbal's laboratory as part of a research program seeking small-molecule peptide neurogenic factors for Alzheimer's disease and tauopathies. The compound was specifically engineered for blood-brain barrier permeability (enabling peripheral administration to reach the CNS) and to retain the neurogenic activity of the parent CNTF molecule without its side effects. Subsequent studies in transgenic mouse models of Alzheimer's disease, frontotemporal dementia, and Down syndrome have shown consistent neuroprotective and cognition-restoring effects.