Gene Delivery Using Chemical Methods - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 145

using -galactosidase reinforced that SP1017 appreciably enhanced distribution of
transgene expression through the tissue [110] .
4.2.2.2 Poly( d , l -Lactide- co -Glycolide)
Poly(lactide- co -glycolic acid) (PLGA) and polylactic acid (PLA) are the most stud-
ied biodegradable and biocompatible polymers used for formulating DNA matrix
particles ( Fig. 4.7 ) [111] . PLGA is approved for human use by the FDA. The long
history of safe use in drug delivery of protein and peptides in medical applications
such as resorbable sutures, screws, and implants makes PLGA a promising candi-
date for use in DNA delivery. pDNA entrapped in the polymeric matrix not only pro-
tects DNA from nucleases but also allows modulating DNA release from micro- or
nanoparticles. Thereby, slow release of DNA from the micro- or nanoparticles facili-
tates sustained levels of gene expression. PLGA degrades through a process of hydro-
lysis by breakage of the chemical bond between glycolide and lactide monomers to
yield individual monomers of lactic acid and glycolic acid, which are removed from
the body by the citric acid cycle. Because the rate of degradation is slow, degraded
products do not affect the normal functioning of the cell.
There is a high probability that DNA in endocytosed microparticles becomes
finally delivered in lysosomal compartments. Several efforts are made to develop
particulate systems that escape in time from the endosomes and deliver their DNA
load in the cytosol. The vector system of DNA particles should fulfill a few basic
requirements [112] :
1. The particles should be able to carry efficient and high amounts of intact DNA.
2. Detrimental conditions during particle formation should be avoided as they may degrade
the DNA.
3. The polymer should encapsulate DNA sufficiently to provide protection against enzymatic
degradation by nuclease. pDNA breaks down rapidly when exposed to serum.
4. The degradation of the matrix particles should not generate any unfavorable (such as
acidic) internal environment.
5. The particles should b e relatively stable at neutral pH but hydrolyze rapidly around pH 5,
which is the pH inside the phagosomal compartment of the macrophages.
The characteristics possessed by PLGA in the context of the above ideal require-
ments are as follows:
1. Encapsulation of large-size hydrophilic pDNA in hydrophobic PLGA particles is a
challenge.
2. Exposing the particle to an unfavorable condition during preparation of PLGA particles,
for example, sonication to form the emulsion, polymerization reactions involving free radi-
cals, and heat drying of the formulation, should be avoided.
3. A PLGA particle protects DNA adequately.
4. Hydrolysis of PLGA may substantially decrease the pH in PLGA particles, poten-
tially resulting in DNA degradation [113] . Addition of basic additives like Mg(OH) 2 and
Ca(OH) 2 encapsulated in the PLGA particles may solve the problem [114] .
5. PLGA particles are stable at neutral pH and degrade in acidic conditions. This prevents
early release in blood and facilitates DNA release inside the cells.