P21 is a peptide that was developed by reverse engineering Cerebrolysin’s effect using epitope mapping. Researchers found that the main neurogenic and memory enhancing effects from Cerebrolysin were due to its ability to react to human CNTF. So the researchers narrowed their search, and found that an 11-mer peptide, labeled peptide 6 (Ac-VGDGGLFEKKL-NH2), enhanced hippocampus-dependent learning and memory, increased neurogenesis, and neuronal plasticity. They then determined the most active sub-sequence in the peptide called peptide 6C (Ac-DGGL-NH2). However, this peptide sequence showed to have poor blood brain barrier permeability, and was easily broken down by plasma enzymes. To combat this, they added an adamantane moiety to the N-terminus of the peptide sequence. This highly lipophilic tricyclic alkane greatly increased the BBB permeability, and protected the peptide from enzymatic degredation. This made the new peptide able to be active when taken via subcutaneous administration. That is how P21 (Ac-DGGL^A-NH2) was born