Mass Transport via Cellular Barriers and Endocytosis - Intracellular Delivery: Fundamentals and Applications - page 33

other material, such a silica (Slowing et al. 2006 ). Yet, while it has been shown to be
effective, it has been postulated that the rupturing of too many lysosomes is cyto-
toxic to the cell; hence the application of too many of these particles to the cell can
be cytotoxic (Xia et al. 2008 ; Terman et al. 2006 ; Reiners et al. 2002 ).
In 2003, Wakefield et al. introduced another mechanism for endosomal escape
employing the use of a dynamic polyconjugate known as EMMA (Endosomolysis
by Masking of a Membrane-Active Agent). This macromolecule is designed to
transition from a nonreactive entity at normal pH to a reactive membrane-active
agent at an acidic pH (Fig. 3 -II). The membrane agent, known as melittin, contains
a reactive region that is masked using a mealic anhydride. Once protonated it
unmasks itself, exposing the membrane active domain of the molecule (Wakefield
et al. 2005 ). It has been suggested that the design of EMMA could be improved by
increasing the number of hydrophobic regions on the peptide, thereby increasing its
transfection capability (Rozema et al. 2003 ).
The last of the four nanoparticle escape mechanisms utilizes a viral model.
Beginning in 2004, Harashima et al. has been working on transferrin modified
liposomes. To improve the nanoparticle's ability to escape the late-endosome or
lysosome, a pH-sensitive fusogenic peptide was added to the particle. This
30-amino acid fusogenic peptide, composed of the nonpolar tryptophan, alanine,
and leucine and the polar acidic and basic glutamate and histidine, respectively, is
known to make a conformational change from a random coil at a pH of 7.4 to a
more hydrophobic alpha-helix at a pH of 5.5 (Simoes et al. 1998 ). This change
increases its interaction with the endosomal membrane. A cholesterol moiety and a
PEGylated peptide improve the interaction and penetration of the fusogenic peptide
with the endosomal membrane (Kakudo et al. 2004 ). These actions then induce
fusion of the liposomal and endosomal membrane releasing the nanoparticle's
contents into the cytoplasm (Fig. 3 -I) (Sasaki et al. 2008 ).

4.4 Endoplasmic Reticulum Targeting

Targeting the endoplasmic reticulum (ER) is of great importance because there are
several degenerative processes which involve the ER, such as degenerative diseases
(e.g. cystic fibrosis and cholera) and the drug-efflux mechanism in multi-drug resis-
tance. Some drugs aggravate the ER stress response to induce apoptosis in cancer
cells. However, since the ER is a complex organelle, consisting in a diffuse network
of vesicles, and it is involved in multiple cellular functions, it cannot be directly
targeted by certain drugs, and therefore treatments have not been developed for
these diseases (Lai et al. 2008 ). Furthermore, intracellular trafficking dynamics are not
entirely understood. Among the most complicated variables to control are the
variations in trafficking dynamics which can be modified by attaching ligands to
the nanoparticles. Understanding how to favor one transport pathway or how to optimize
the efficiency of transport from one structure to another (e.g. endosome to lysosome
or endosome to Golgi) is essential for successful intracellular targeting.