Calcium Phosphate and Calcium Phosphosilicate Mediated Drug Delivery and Imaging - Intracellular Delivery: Fundamentals and Applications - page 731

of the particles has been demonstrated under in vitro (Kester 2008 ; Morgan 2008 )
and in vivo (Barth 2010 ; Altınoğlu 2008, 2009, 2010b ) conditions.

4 Applications of Calcium Phosphate Delivery Systems

4.1 Nucleic Acid Delivery Using Calcium Phosphate Systems

RNA interference (RNAi) is an exploratory treatment option that is of great interest
in the development of personalized treatment options particularly for cancer treat-
ment. RNAi involves the delivery of small interfering RNA (siRNA) to target cells
with the intention of down-regulating (or up-regulating in some instances) gene
activity and hence protein synthesis. Although promising as a treatment option, the
primary limitation of this technique is the susceptibility of the siRNA to degrade by
nucleases present in the blood and tissue in systemic delivery (Nguyen 2008 ).
While viral vectors (primarily the adeno-associated viruses (AAVs) and adenovi-
ruses) have been used experimentally as a delivery vehicle, severe toxicity (hepatox-
icity and immunogenicity) associated with these agents have raised safety concerns
and hence prevented their use in humans (Grimm 2006 ).
The earliest studies on calcium phosphate for the transfection of DNA date back
to 1973 (Graham 1973 ). However, though promising, this initial approach had
problems with dispersion, size control and low transfection efficiency. Most cal-
cium phosphate delivery systems being developed today have taken into consider-
ation the importance of nucleic acid encapsulation, delivery vehicle colloidal
stability and size, and address it in their synthetic schemes.
The biocompatibility and bioresorbability of calcium phosphates make this
material an advantageous gene delivery vehicle. Syntheses of nucleic acid-calcium
phosphate complexes take advantage of the affinity of the positively charged cal-
cium ions for the negatively charged phosphate backbone of RNA and DNA. The
electrostatic interaction of the RNA/DNA with the calcium ion stabilizes the
nucleic acid/CP complex which can consequently be carried across the cell mem-
brane via ion-channel mediated endocytosis (Truong-Le 1999 ; Roy 2003 ).
The calcium-nucleic acid complex is a non-toxic alternative to the viral delivery
systems (Kulkarni 2006 ). Investigators that pursue calcium phosphate as a gene
delivery agent have employed synthetic methods such as reverse microemulsions
(Liu 2005 ; Roy 2003 ; Bisht 2005 ; Li 2010 ), liposomal coated calcium phosphate
nanoparticles by the ethanol injection method (Zhou 2010 ), and the coprecipitation
of calcium and phosphate solutions with oligonucleotides for single shell and multi-
shell calcium phosphate nanoparticles (Zhang 2010 ; Sokolova 2006a,b, 2007b ).
Since particle size and stability as well as nucleic acid stability are important
factors affecting intracellular delivery, investigators are addressing these issues
when designing delivery systems for nucleic acids. In an effort to protect the genetic
material from in vivo degradation by nucleases, the coprecipitation of calcium and
phosphate with the negatively charged nucleic acid was investigated (Welzel 2004 ).