P21

P21, sometimes written P021, strings together four amino acids taken from a small active piece of a natural nerve-cell survival protein called ciliary neurotrophic factor, or CNTF, and made in a lab. Chemists attached a small extra chemical group to help it survive and reach the brain. All of the published work on it comes from mouse studies, not people, looking at whether it can support the growth of new brain cells and protect memory after injury or in models of Alzheimer's disease.

What it is

P21 is built from just four amino acids, taken from the part of CNTF, a protein the nervous system uses to help neurons survive, that appears responsible for its activity. A chemical tag added during manufacturing, based on a compound called adamantane, is reported to help the peptide resist breakdown and cross into brain tissue.

Researchers describe P21 as promoting neurogenesis, the growth of new nerve cells, mainly in a brain region called the hippocampus that is central to memory. It has been studied given into the body generally, and separately as a nasal spray, a route that can reach the brain more directly through nerves near the nose.

How it is studied

Every citation reviewed for this entry is a mouse study. Researchers gave P21 (or a closely related parent peptide) by injection or in feed and then measured learning and memory in behavioral tests, along with laboratory markers of new nerve-cell growth and brain proteins tied to nerve health. No human clinical trial of P21 has been published in the sources reviewed here.

What studies report

In healthy adult mice, P21 improved performance on short-term and spatial memory tests and increased new neuron growth, along with two proteins involved in the connections between nerve cells, in the hippocampus. [1]
In mice bred to develop Alzheimer's-like changes, long-term treatment reduced an abnormal buildup of a protein called tau, raised a nerve-growth protein called BDNF, and improved memory-related behavior. [2]
In mice given a controlled brain injury, treatment with the parent peptide increased the number of new neurons in the hippocampus by about 80 percent and reversed some of the loss of nerve-cell connections caused by the injury. [3]

What is not known

P21 has not been tested in any published human trial, so its safety, the right amount to use, and whether the mouse findings on memory and nerve-cell growth would carry over to people are all unknown. Reports of effects in people that circulate outside the peer-reviewed literature are not backed by the primary sources reviewed for this entry and should be read as unverified.

Reported adverse findings

The primary mouse studies reviewed here focused on memory and nerve-growth outcomes and did not report a formal safety or toxicity profile. [1][2][3]

Regulatory status

No regulatory body identified in the sources reviewed, including the FDA, has cleared P21 for use, and it has not entered human clinical trials.