Polypropyleneimine

Polypropyleneimine (PPI) is a cationic polymer that belongs to the class of dendrimer-like macromolecules, commonly referred to in the literature as polypropyleneimine dendrimers. Its backbone consists of a series of 2,3‑epoxypropylamine units that are connected via nitrogen atoms, forming highly branched, globular structures. The first generation of PPI dendrimers contains 8 surface amine groups, while higher generations possess 2^n surface functionalities, where n is the generation number. The synthesis is typically carried out by the successive addition of epichlorohydrin and a primary amine to a surface amino group, followed by dehydrohalogenation and amine conversion steps that build the dendrimer shell in a stepwise manner. The process allows precise control over the number of terminal amines and the overall molecular weight.

PPI dendrimers possess a rigid, globular architecture that distinguishes them from linear polyethylenimine (PEI). Their compact shape leads to a very high charge density and a well-defined size distribution, which in turn influences their interaction with biological membranes. The surface amines can be protonated under physiological conditions, conferring a strong positive charge. This property makes PPIs efficient carriers for negatively charged biomolecules such as DNA, RNA, and small interfering RNA (siRNA). In gene delivery studies, PPI dendrimers demonstrate high transfection efficiency and comparatively low cytotoxicity relative to linear PEI, especially at lower generation numbers (generations 2–3). Modifications of terminal groups, such as polyethylene glycol (PEG)ylation or attachment of targeting ligands, further improve biocompatibility and tissue specificity.

Besides gene therapy, PPI dendrimers have found roles in vaccine development, where they function as fluxers to present antigens, and in imaging, where they can be conjugated with radiolabels or fluorophores. Their robust chemical scaffold allows for multivalent functionalization, enabling co-delivery of drugs, toxins, or imaging agents within a single construct. However, higher-generation PPI dendrimers may exhibit increased cytotoxicity due to stronger membrane disruption, necessitating careful optimization of dosage and surface chemistry for therapeutic applications.