Design of functional dendritic polymers for application as drug and ...

Sideratou et al: Dendritic polymers for application as drug and gene delivery systems
range of the sodium cation present in extracellular fluids,
i.e. 0.142 M, (Guyton and Hall, 2000) the betamethasone
valerate was released in relatively small quantities. The
gradually released betamethasone valerate from the
multifunctional dendrimer formed crystallites in the
aqueous medium as determined by light scattering. The
precipitated material was analyzed with 1 H NMR and its
spectrum corresponded to that of betamethasone valerate.
This finding should be considered when PEGylated
dendrimers are used as drug delivery systems in
experiments in vitro and in vivo, since sodium chloride in
extracellular fluids and potassium chloride in intracellular
environment can be complexed with PEG chains (Wang et
al, 2000; Bogan and Agnes, 2002) affecting the overall
release profile of the drug. Thus, the possibility of
triggering drug release in the extracellular fluid, i.e. before
endocytosis to the target-cells, should be taken into
account when designing a targeted PEGylated drug
delivery system.
The drug delivery effectiveness of analogous
multifunctional dendrimers was modeled by investigating
their interaction with multilamellar liposomes consisting
of phosphatidylcholine/cholesterol/dihexadecyl phosphate
(19:9.5:1) and dispersed in aqueous or phosphate buffer
solutions (Pantos et al, 2005). The multilamellar liposomes
bear the phosphate moiety as recognizable group.
They were used as simple models before one resorts
to the use of cells; after all liposomes are considered as the
closest analogues to cells. On the other hand,
poly(propylene imine) fourth generation dendrimers were
functionalized with 6 (DAB-G6 ) or 12 (DAB-G12)
guanidinium groups as targeting ligands, while the
remaining toxic, external primary amino groups of the
dendrimers were allowed to interact with propylene oxide
affording the corresponding hydroxylated derivatives. The
scheme of the reactions modifying the dendrimeric surface
[BTV] x10 -4 M
1.6
1.4
1.2
1.0
0.8
0.6
0.4
is shown in Figure 15. DAB-G0 dendrimer, which does
not contain any guanidinium group was used as a
reference compound. The so-prepared dendrimers were
loaded with corticosteroid drugs, i.e. betamethasone
dipropionate and betamethasone valerate for investigating
their transfer to liposomes.
Microscopic, ζ-potential, and Dynamic Light
Scattering (DLS) techniques have shown that liposomesdendrimers
molecular recognition occurs leading to the
formation of large aggregates at dendrimer/dihexadecyl
phosphate molar ratios higher than 1:30, as visually
observed with phase contrast optical microscopy. Calcein
liposomal entrapment experiments demonstrate a limited
leakage, i.e. less than 13%, following liposomes
interaction with the modified dendrimers at 1:25
dendrimer/ dihexadecyl phosphate molar ratio. This
indicates that the membrane of the liposomes remains
almost intact during their molecular recognition with these
dendrimers. Isothermal Titration Calorimety (ITC)
indicates that the enthalpy of the interaction is dependent
on the number of the guanidinium groups present at the
dendrimeric surface. Furthermore, the process is reversible
and redispersion of the aggregates occurs by adding
concentrated phosphate buffer.
The interaction between these drug-loaded
dendrimers and multilamellar liposomes results in the
transport of drugs from the dendrimeric derivatives to the
‘empty’ liposomes as summarized in Table 3. The
experiments demonstrate that about 25% of BD or BV is
present in the precipitated aggregates when DAB-G0 was
used. When the guanidinylated dendrimers DAB-G6 and
DAB-G12 were used, the amount of drugs in the
precipitate increases substantially becoming about 60%
and 80%, respectively.
0.2
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
[NaCl] / M
Figure 14. Plot of the concentration of betamethasone valerate in a 2.50 x 10 -5 M dendrimeric solution as a function of added NaCl.
Reproduced from Paleos et al, 2004 with kind permission from American Chemical Society.
80