We recently demonstrated
the far-red light-activatable prodrug
of paclitaxel (PTX), Pc-(L-PTX)<sub>2</sub>. Upon illumination with
a 690 nm laser, Pc-(L-PTX)<sub>2</sub> showed combinational cell killing
from rapid photodynamic therapy damage by singlet oxygen, followed
by sustained chemotherapy effects from locally released PTX. However,
its high lipophilicity (log <i>D</i><sub>7.4</sub> >
3.1)
caused aggregation in aqueous solutions and has nonselectivity toward
cancer cells. To solve these important problems, we prepared folic
acid (FA)-conjugated and photoactivatable prodrugs of PTX with a polyethylene
glycol (PEG) spacer of various chain lengths: FA-PEG<sub><i>n</i></sub>-Pc-L-PTX [<i>n</i> = 0 (0k, <b>5</b>), ∼23
(1k, <b>7a</b>), ∼45 (2k, <b>7b</b>), ∼80
(3.5k, <b>7c</b>), or ∼114 (5k, <b>7d</b>)]. The
PEGylated prodrugs <b>7a–d</b> had a much improved hydrophilicity
compared with the non-PEGylated prodrug, Pc-(L-PTX)<sub>2</sub>. As
the PEG length increased, the hydrophilicity of the prodrug increased
(log <i>D</i><sub>7.4</sub> values: 1.28, 0.09, −0.24,
and −0.59 for 1k, 2k, 3.5k, and 5k PEG prodrugs, respectively).
Fluorescence spectral data suggested that the PEGylated prodrugs had
good solubility in the culture medium at lower concentrations (<1–2
μM), but showed fluorescence quenching due to limited solubility
at higher concentrations (>2 μM). Dynamic light scattering
indicated
that all of the prodrugs formed nanosized particles in both phosphate-buffered
saline and culture medium at a concentration of 5 μM. The PEG
length affected both nonspecific and folate receptor (FR)-mediated
uptake of the prodrugs. The enhanced cellular uptake was observed
for the prodrugs with medium-sized PEGs (1k, 2k, or 3.5k) in FR-positive
SKOV-3 cells, but not for the prodrugs with no PEG or with the longest
PEG (5k), which suggests the optimal range of PEG length around 1k–3.5k
for effective uptake of our prodrug system. Consistent with the cellular
uptake pattern, medium-sized PEGylated prodrugs showed more potent
phototoxic activity (IC<sub>50s</sub>, ∼130 nM) than prodrugs
with no PEG or the longest PEG (IC<sub>50</sub>, ∼400 nM).
In conclusion, we have developed far-red light-activatable prodrugs
with improved water solubility and FR-targeting properties compared
with the nontargeted prodrug