Atelocollagen
Although siRNA target molecules are overexpressed in
cancer cells, most of them are essential to maintain
homeostasis of physiological functions in humans.
Therefore, siRNAs must be delivered selectively into
cancer cells. Moreover, naked siRNAs are degraded by
endogenous nucleases when administered in vivo, so that
delivery methods that protect siRNAs from such degradation
are essential. For these reasons, safer and more
effective DDSs must be developed. DDSs are divided into
two categories: viral vector based carriers, and non-viral
based carriers. Viral vectors are highly efficient delivery
systems and they are the most powerful tools for transfection
so far. However, viral vectors have several critical
problems in in vivo application. Especially, retroviral and
lentiviral vectors have major concerns of insertional
mutagenesis [59,60]. Consequently, non-viral DDSs have
been strenuously developed [11-13].
Atelocollagen, one of powerful non-viral DDSs, is type
I collagen obtained from calf dermis [61]. The molecular
weight of atelocollagen is approximately 300,000 and the
length is 300 nm. It forms a helix of 3 polypeptide
chains. Amino acid sequences at the N- and C-termini
of the collagen molecules are called telopeptide, and
they have antigenecity of collagen molecules. As the telopeptide
is removed from collagen molecules by pepsin
treatment, atelocollagen shows low immunogenicity.
Therefore, atelocollagen has been shown to be a suitable
biomaterial with an excellent safety profile and it is used
clinically for a wide range of purposes. Atelocollagen is
positively charged, which enable binding to negatively
charged nucleic acid molecules, and bind to cell membranes.
Moreover, at low temperature atelocollagen
exists in liquid form, which facilitates easy mixing with
nucleic acid solutions. The size of the atelocollagennucleic
acid complex can be varied by altering the ratio
of siRNA to atelocollagen. Because atelocollagen naturally
forms a fiber-like structure under physiological
conditions, particles formed a high concentration of atelocollagen
persist for an extended period of time at the
site of introduction, which is advantageous to achieve a
sustained release of the associated nucleic acid. Atelocollagen
is eliminated through a process of degradation
and absorption similar to the metabolism of endogenous
collagen [61]. Alternatively, particles formed under conditions
of low atelocollagen concentrations result in
siRNA/atelocollagen complexes approximately 100-300
nm in size that are suitable for systemic delivery by
intravenous administration. Atelocollagen complexes
protect siRNA from degradation by nucleases and are
transduced efficiently into cells, resulting in long-term
gene silencing. For instance, Takeshita et al. demonstrated
that the systemic siRNA delivery with atelocollagen
existed intact for at least 3 days in tumor tissues
using a mouse model [62].
AtelocollagenAlthough siRNA target molecules are overexpressed incancer cells, most of them are essential to maintainhomeostasis of physiological functions in humans.Therefore, siRNAs must be delivered selectively intocancer cells. Moreover, naked siRNAs are degraded byendogenous nucleases when administered in vivo, so thatdelivery methods that protect siRNAs from such degradationare essential. For these reasons, safer and moreeffective DDSs must be developed. DDSs are divided intotwo categories: viral vector based carriers, and non-viralbased carriers. Viral vectors are highly efficient deliverysystems and they are the most powerful tools for transfectionso far. However, viral vectors have several criticalproblems in in vivo application. Especially, retroviral andlentiviral vectors have major concerns of insertionalmutagenesis [59,60]. Consequently, non-viral DDSs havebeen strenuously developed [11-13].Atelocollagen, one of powerful non-viral DDSs, is typeI collagen obtained from calf dermis [61]. The molecularweight of atelocollagen is approximately 300,000 and thelength is 300 nm. It forms a helix of 3 polypeptidechains. Amino acid sequences at the N- and C-terminiof the collagen molecules are called telopeptide, andthey have antigenecity of collagen molecules. As the telopeptideis removed from collagen molecules by pepsintreatment, atelocollagen shows low immunogenicity.Therefore, atelocollagen has been shown to be a suitablebiomaterial with an excellent safety profile and it is usedclinically for a wide range of purposes. Atelocollagen ispositively charged, which enable binding to negativelycharged nucleic acid molecules, and bind to cell membranes.Moreover, at low temperature atelocollagenexists in liquid form, which facilitates easy mixing withnucleic acid solutions. The size of the atelocollagennucleicacid complex can be varied by altering the ratioof siRNA to atelocollagen. Because atelocollagen naturallyforms a fiber-like structure under physiologicalconditions, particles formed a high concentration of atelocollagenpersist for an extended period of time at thesite of introduction, which is advantageous to achieve asustained release of the associated nucleic acid. Atelocollagenis eliminated through a process of degradationand absorption similar to the metabolism of endogenouscollagen [61]. Alternatively, particles formed under conditionsof low atelocollagen concentrations result insiRNA/atelocollagen complexes approximately 100-300nm in size that are suitable for systemic delivery byintravenous administration. Atelocollagen complexesprotect siRNA from degradation by nucleases and aretransduced efficiently into cells, resulting in long-termgene silencing. For instance, Takeshita et al. demonstratedthat the systemic siRNA delivery with atelocollagenexisted intact for at least 3 days in tumor tissuesusing a mouse model [62].
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