Non-viral Gene Therapy - Intracellular Delivery: Fundamentals and Applications - page 663

producing LDH materials were also developed, for instance, from metal oxides
and/or hydroxides (Whilton et al. 1997 ; Choy et al. 2001 ; Gardner et al. 2001 ;
Xu et al. 2006, 2007 ), precipitation via aluminate (Zikmund et al. 1996 ), by sol-gel
techniques (Lopez et al. 1996 ).
LDHs have a high potential to exchange intercalated anions which allows for the
direct loading of a variety of negatively charged drugs/biomolecules (such as vita-
mins and nucleic acids) into their interlayer galleries (Kwak et al. 2004 ; Zhang
et al. 2006a ; Xu et al. 2008 ; Ladewig et al. 2009 ). Although this intercalation
decreases their positive surface charge, LDHs remain sufficiently positively charged
to facilitate cellular uptake. The high anion-exchange capability of LDHs has
attracted particular attention in the field of bio-hybrid nanomaterials owing to their
good biocompatibility, excellent chemical stability as well as the controlled release
character. Organic molecules can be released from LDHs at a rate that related to the
pH value and the ionic strength of the surrounding medium (Zhang et al. 2006a ).
With respect to the use of LDH nanoparticles as gene vectors, the size of nucleic
acids to be delivered should be concerned. It seems that larger and sterically hin-
dered biomolecules such as pDNA, are not completely accessible to intercalation in
the interlayer galleries by means of anion exchange, because of their supercoiled
structure in solution. Accordingly, most of the reports have focused on the delivery
of smaller nucleic acids like antisense ODNs, siRNA, sheared genomic DNA, or
PCR fragments. Choy et al. intercalated sheared genomic DNA (500-1,000 bp) in
the interlayer galleries of LDHs and used these hybrids as non-viral vectors (Choy
et al. 2000 ). Because of its negative charges, DNA could be strongly incorporated
into such a layered double hydroxide, while the positive surface charges of the
nanoparticles attracted them to the cell surface by electrostatic interaction. The
LDH-bio-nanohybrids were then taken up by receptor-mediated endocytosis and
escaped into the cytoplasm by endosome rupture. Similar phenomena were
observed by Tyner et al. using PCR fragments of 800 bp length (Tyner et al. 2004 ).
In view of their good biocompatibility and wide availability, LDHs deserve further
in vivo studies to justify their usefulness.

6 Multifunctional Nano-Systems for Gene Delivery

Multifunctionalized nanocarriers integrated with multiple functionalities such as
longevity, targetability, intracellular penetration, contrast loading, multi-therapeutics
payload, and stimuli-sensitivity, have attracted great attention in recent years, owing
to their great potentials in molecular pharmaceutics, medical imaging, biomedical
engineering, and gene therapy (Torchilin 2006, 2009 ). For instance, drug delivery,
cell targeting and/or multi-modal imaging can be simultaneously achieved by multi-
functional inorganic nanoparticles or nanocrystals, inorganic-organic hybrid nano-
particles and polymeric assemblies (Kim et al. 2006b, 2007b ; Huang et al. 2007 ;
Li et al. 2008 ; Liong et al. 2008 ; Kim et al. 2007c, 2008 ; Rieter et al. 2007 ). Studies
based on these versatile pharmaceutical nanocarriers have provided profound insights