Biomedical Engineering Reference
In-Depth Information
the initial dose retained in the plasma over the same time frame.
Also, the volumes of distribution were quite high (four times the
total blood volume of the mouse). This indicates a broad tissue
distribution likely resulting from the amphiphilic nature of the PEO-
PCL copolymer. Thus, the optimization of the block lengths of PEO
for stability
in vitro
translated over to stability
in vivo
based on the
pharmacokinetics and thermodynamic stability.
3.3.3.2 Stability of PEO-PCL micelles
in vivo
Another approach to evaluating
in vivo
stability utilizes conjugated
fluorescent dyes to PEO
45
-PCL
21
copolymers [82]. A derivative of
5-fluorescein cadaverine (F-5-CADA) was conjugated to the terminal
end of the PCL chain to investigate the stability of the PEO
45
-PCL
21
micelles in PBS with 5% fetal bovine serum (FBS). F-5-CADA is
not fluorescent, but becomes fluorescent upon hydrolytic cleavage
in water. Through 48 h only 2% of the dye was fluorescent from
the micelle solution. Thus these micelles were stable, because the
intact micelles displayed virtually no fluorescence and the free dye
emitted 33% fluorescence through 48 h and at the same conditions.
The reason the free dye emitted only 33% is due to the slow partial
hydrolytic activation in PBS. In RPMI-1640 cell culture medium
with and without FBS, the micelles were less stable. Through 48 h
in RPMI without serum, the PEO-PCL micelles had 13% activation,
compared with 83% for free dye. The presence of serum drastically
reduced the stability of the micelles as RPMI with 5% FBS, and FBS
without RPMI activated 64% and 74% respectively. The free dye was
fully activated in FBS without RPMI, while 83% of the free dye was
activated in RPMI with 5% FBS. These results demonstrate that FBS
is an excellent screening agent for the stability of micelles since the
equilibrium between micelles and unimers is media dependent. The
stability of the micelles in FBS at 10 and 100 times the CMC depicted
modest increases in stability (10-12%) with similar activations of
F-5-CADA to those already mentioned. When these micelles were
incubated with T24 human bladder carcinoma cells, an increase in
fluorescence due to disruption of PEO
45
-PCL
21
-F-5-CADA micelles
was seen with time in lysed cells and was substantial at 18 h (approx
70% of maximum). Intravenous bolus injections of PEO
45
-PCL
21
-F-5-
CADA micelles showed accumulation in the bladder of hairless SKH-1
mice. This implies that the filtration in the kidneys disrupted these
micelles. Both subcutaneous and intramuscular injections showed
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