Calcium Phosphate and Calcium Phosphosilicate Mediated Drug Delivery and Imaging - Intracellular Delivery: Fundamentals and Applications - page 730

single-shell, DNA stabilized nanoparticles. The problem with the double-shell
approach is the lack of colloidal stability due to the absence of electrostatic or
steric stabilizers (Sokolova 2006a ). The triple-shell architecture consists of a layer
of DNA or oligonucleotide on the double-shell nanoparticle. The additional shell
is an improvement to the original single-shell and double-shell method. The chal-
lenge associated with the single shell method is the lack of protection of the DNA
or oligonucleotides from enzymatic attack, resulting in low transfection and
knock-down efficiency while the double-shell lacks colloidal stability (Sokolova
2006a ). By creating another shell of calcium phosphate and DNA, the interior
DNA is now encapsulated and protected from the outside environment, and thus
available for transfection. The outer layer of DNA stabilizes the particle but at the
expense of increased size.
A potential problem that is yet to be addressed in the layer by layer system is the
lack of colloidal stability of the particles in the presence of serum proteins under
in vivo conditions. Initial results indicate the instability of the DNA stabilized par-
ticles in the presence of serum containing media is due to the adsorption of proteins
on the surface which cause agglomeration (Sokolova 2006b, 2007a, b, 2010 ).

3.5 Amorphous Calcium Phosphate

Amorphous calcium phosphate (ACP) is another class of calcium phosphates with
use as a nanocarrier. ACP is the phase of calcium phosphate that first precipitates
from aqueous solution. Unless stabilized by dopants or stored under dry conditions,
transformation to the thermodynamically stable hydroxyapatite occurs (Dorozhkin
2010 ). The biological relevance of amorphous calcium phosphates is addressed in
detail by a recent review published by Dorozhkin (Dorozhkin 2010 ).
The amorphous phase of calcium phosphate is selected as the material system of
choice by a handful of investigators. Cross et al . used an amorphous calcium fluo-
ride phosphate stabilized with casein phosphopeptides (CPP) as calcium, phosphate
and fluoride ion carriers for the remineralization of teeth (Cross 2004 ). The amor-
phous particles were synthesized by the coprecipitation of calcium, phosphate and
fluoride solutions with CPP (the stabilization agent) with the resulting nanoparticle
diameters on the order of 4-5 nm (Cross 2004 ).
Another promising amorphous calcium phosphate system developed by Adair
and coworkers includes the synthesis of calcium phosphosilicate nanoparticles
(CPSNPs) in double reverse microemulsions (Altınoğlu 2008 ; Morgan 2008 ; Kester
2008 ; Muddana 2009 ; Russin 2010 ; Barth 2010 ). The investigators utilize silicate
to stabilize the amorphous phase. Citrate is employed as a dispersant during the
synthetic process. The core CPSNPs are 20 nm in diameter but can range in hydro-
dynamic diameter between 20 and 200 nm depending on the surface functionaliza-
tion scheme (Barth 2010 ). The CPSNPs encapsulate therapeutics (Kester 2008 ) and
a wide variety of fluorescent dyes (Altınoğlu 2008 ; Morgan 2008 ) for imaging and
photodynamic therapy (Altınoğlu 2010 b; Barth 2010 ). Additionally, the efficacy