Non-viral Gene Therapy - Intracellular Delivery: Fundamentals and Applications - page 618

(25 kDa), these disulfide-containing PEIs displayed a lower cytotoxicity and
higher gene transfection efficiency. Fore these types of polycations, their cytotox-
icity and transfection efficacy are dependent on both disulfide density and MW
of raw PEIs (Peng et al. 2009 ). Among the polymers derived from PEIs of
800 Da, 1,800 Da, and 25 kDa, the cross-linked polymer based on 800 Da-PEI
gave the best results.
The biodegradable derivatives of 14-30 kDa were synthesized by cross-linking
800 Da bPEI using 1,3-butanediol diacrylate or 1,6-hexanediol diacrylate (Forrest
et al. 2003 ). These polymers exhibited low cytotoxicity, and they can mediate the
luciferase gene expression more than one order of magnitude more efficiently than
commercially available PEI25, according to the cell culture experiments using
human breast carcinoma cells (MDAMB-231), and murine myoblasts (C2C12).
Kim and colleagues synthesized HMW-PEI (13-23.7 kDa) using glutadialdehyde
as a cross-linker (Kim et al. 2005b ). Due to the presence of Schiff base units, these
synthesized PEIs are acid-labile, which can be rapidly degraded into nontoxic
LMW-PEI in acidic endosome. Compared with their precursor PEI (1,800 Da), the
acid-labile PEIs exhibited dramatically higher transfection efficiency to 293
Tcells and A7R5 cells. In addition, their cytotoxicity was greatly lower than that
of PEI25, despite of the slightly lower efficacy in comparison to the latter.
CDs are a family of naturally occurring oligosaccharides constructed from
a(1→4)-linked glucose units, which have been widely used for drug formulations
(Davis and Brewster 2004 ). b-CD has been used recently to prepare polycation via
the cross-linking of LMW-PEI (600 Da). This new polymer with improved biocom-
patibility could mediate effective gene transfection in cultured neurons and in the
central nervous system (Tang et al. 2006 ). Biocompatible derivatives of cyclodex-
trins, i.e. (2-hydroxypropyl)-b-cyclodextrin (HP-b-CD) and (2-hydroxypropyl)-g-
cyclodextrin (HP-g-CD) were also employed to prepare cross-linked PEI, and a
600 Da PEI served as the raw material (Huang et al. 2006 ). Cell culture experiments
using SKOV-3 cells indicated that the new polymers showed significantly lower
cytotoxicity and 1.5-1.7 fold higher transfection efficiency than that of PEI25.
Most recently, HP-g-CD cross-linked 600 Da PEI coupled with MC-10 oligopep-
tide containing a sequence of Met-Ala-Arg-Ala-Lys-Glu that can target to HER2
(human epidermal growth factor receptor 2, which is often over expressed in many
breast and ovary cancers) was synthesized (Huang et al. 2010 ). This vector showed
very low cytotoxicity, strong targeting specificity to HER2 receptor, and high effi-
ciency for delivering reporter genes to target cells in vitro and in vivo . Moreover,
the new polycation could more efficiently deliver a therapeutic IFN-a gene to
tumor-bearing nude mice, implementing a significantly enhanced anti-tumor effect,
in comparison to PEI25.
On the other hand, LMW-PEI has also been grafted onto polymers such as
poly(aspartic acid), poly(glutamic acid), dextran and chitosan, to achieve effective
gene delivery (Table 4 ). These anionic or neutral polymers may be regarded as
pseudo-macromolecular cross-linkers. By the ring opening reaction of poly(L-
succinimide) (PSI) with LMW-PEI (600 and 1,200 Da), pseudo-branched PEIs
(PSI-PEIs) were synthesized (Yu et al. 2009a ). The cytotoxicity against 293 T,