Delivery of peptide nucleic acid (PNA) oligonucleotides to targeted cells is becoming a viable therapeutic strategy by redirecting RNA splicing for conditions like Duchenne muscular dystrophy (DMD). The use of novel cell-penetrating peptides (CPP) has helped create targeted, and increased activity of the PNAs delivered.

PNA internalization peptides (Pip)  is a specifically designed CPP for conjugation to PNAs. Pips were designed to improve PNA activity observed in HeLa cells determined by splicing redirection assays and increased serum stability determined by mass spectrometry. Of the Pip series, Pip1 enters the cell in an energy-dependent manner, primarily via clathrin-dependent endocytosis. Pip1 conjugated to a PNA was determined to be well transported into the cell but is not the most stable against proteolysis. However, this may benefit some cargo depending on the intracellular destination. As a CPP for DMD, the proteolysis of Pip1 was deemed an issue but showed promise for other conjugates in HeLa cells.

Pip1 is labelled at the N-terminus with an alkyne attachment for ease of reaction with an opposite Click reactive partner (azide). Azide-alkyne cycloaddition has become the most popular Click reaction. Alkyne-Pip1 allows various applications, particularly for protein conjugation, modification, and drug delivery.