Functional Polymers for Gene Delivery - Functional Polymers for Nanomedicine - page 66

d n 4 y 3 n g | 3

Figure 4.3

Chemical structures of linear PEI (lPEI) and branched PEI (bPEI).

have strong connections with the cytotoxicity or even exacerbate it, including
molecular weight, molecular structure, degree of branching, cationic charge
density,

buffer capacity, polyplex particle size, polyplex zeta potential,

polyplex concentration, the transfection time, etc.
High molecular weight (HMW) PEI, such as 25 kDa PEI (both branched
and linear structures; Figure 4.3), shows high transfection efficiency. None-
theless, the HMW PEI is nonbiodegradable and thus cannot be cleared from
the circulation, which leads to accumulative toxicity inside the body and other
side effects. In contrast, low molecular weight (LMW) PEI (less than 2 kDa)
has demonstrated low cytotoxicity but cannot be used as a gene vector due
to the poor transfection efficiency. Therefore, various methods have been
adopted to overcome PEI's drawbacks, and extensive work was focused on
modification of LMW PEI. Because the advantages of HMW PEI and LMW
PEI are well complementary to each other, we believe that appropriate
combinations may generate advanced gene delivery systems retaining the
advantages while avoiding the shortcomings of each.

4.2.1 Low-Toxicity Polyethylenimine
Currently, in order to reduce the toxicity and enhance the in vivo gene delivery
efficiency of PEI-based delivery systems, the design of biocompatible and
biodegradable PEI vectors presents two main trends: (1) surface decoration of
PEIs with hydrophilic polymers and biodegradable polymers by covalent or
noncovalent attachment in order to shield positive charges and enhance serum
stability; (2) synthesis of HMW crosslinked PEI compounds by LMW PEIs via
the incorporation of reducible disulfide linkages or ester conjugation, and these
crosslinked PEIs could be biodegraded into LMW PEIs again in the

physiological environment (Figure 4.4).
Various biocompatible polymers have been employed to modify PEI
molecules, such as poly(ethylene glycol) (PEG), natural glucose polymers,
proteins, peptides, etc. The most common method is conjugating PEG to PEI
molecules, as PEGylation is a well-established technique that can mask the
complex from the host's immune system and prolong the circulation time of
complexes in the bloodstream. PEGylated PEIs have been studied widely as
potential gene delivery systems. Kissel's group have focused on synthesizing
PEG-g-PEI conjugates by grafting linear PEG (550 Da, 2 kDa, 5 kDa,