Stimuli-Responsive Polymersomes - Intracellular Delivery: Fundamentals and Applications - page 305

Fig. 8 Schematic representation of the self-assembly of vesicles from the diblock copolymer,
PGlu 15 - b -PLys 15 (PGlu is noted as PGA in the sketch). (Reproduced from ref. Rodriguez-Hernandez
and Lecommandoux 2005 )
schizophrenic vesicles (see Fig. 8 ) (Rodriguez-Hernandez and Lecommandoux
2005 ). Using this polyacid-b-polybase, the vesicles can be reversibly produced as a
function of pH. At low pH, the poly(L-glutamic acid) with helical structure consti-
tutes the hydrophobic part of the membrane. At high pH, this hydrophobic part is
destabilized and becomes hydrophilic because of its transition to the charged coil
conformation; instead, the deprotonated poly(L-lysine) takes its place to form the
hydrophobic part with its rodlike a-helical structure.
These last two copolypeptide systems are very promising candidates for macro-
molecular nanobiotechnologies. For drug delivery in vivo , the pH of transition
(around 3) for lysine is not optimal but other aminoacids (for example, histidine,
pK a = 6.0) could be substituted to the lysine to adjust the pH range.
The pH-dependent permeability and reversible structural transition of polyion
complex vesicles (PICsomes) in aqueous media was also recently reported by
Kataoka and coworkers (Kishimura et al. 2009 ). At first, the aqueous solution prop-
erties of PEG 45 -P(Asp-AP) 75 and PEG 45 -PAsp 75 , where, P(Asp-AP) stands for poly-
[(5-aminopentyl)-a,b-aspartamide] and PAsp for poly(a,b-aspartic acid), were
analyzed by potentiometric titration. The pKa values of PEG-P(Asp-AP) and PEG-
PAsp were calculated to be 10.47 and 4.88, respectively. These titration results
revealed that both block copolymers are equally charged at around physiological
pH (pH 7.8, ionization degree = 96%). After mixing the two copolymers, PICsomes
form with a mean diameter of 2 mm, these vesicle structures maintain their structure
at pH 7.4 for more than 48 h and only dissociate into small particles upon lowering
the pH to 5.7. Interestingly, guest molecules can be trapped through this process
which suggests that PICsomes can deliver, release and also trap their cargoes by
sensing acidic conditions of the intracellular endosomal compartments.