[CANCER RESEARCH 63, 1025–1033, March 1, 2003]
Photodynamic Therapy with Verteporfin in the Radiation-induced Fibrosarcoma-1
Tumor Causes Enhanced Radiation Sensitivity1
Brian W. Pogue,2 Julia A. O’Hara, Eugene Demidenko, Carmen M. Wilmot, Isak A. Goodwin, Bin Chen,
Harold M. Swartz, and Tayyaba Hasan
Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire 03755 [B. W. P., I. A. G., B. C.]; Department of Diagnostic Radiology, Dartmouth Medical School,
Hanover, New Hampshire 03755 [J. A. O., C. M. W., H. M. S.]; Biostatistics, Norris Cotton Cancer Center, Dartmouth-Hitchcock Medical Center, Lebanon, New Hampshire
03756 [E. D.]; and Wellman Laboratories of Photomedicine, Massachusetts General Hospital, Department of Dermatology, Harvard Medical School, Boston, Massachusetts
02114 [B. W. P., T. H.]
ABSTRACT ultimately causing necrosis or rapid apoptosis in the parenchyma. The
mechanism for cellular killing is thought to be different from that of
Photodynamic therapy (PDT) with verteporfin (lipid form of benzopor-
radiation treatment, where unrepaired DNA strand breaks provide the
phyrin derivative, benzoporphyrin derivative monoacid ring A) was used
destructive effect. Several studies have investigated the combination
to treat radiation-induced fibrosarcoma tumors before X-ray treatment.
When verteporfin was injected 3 h before light irradiation, the tumor effect of PDT and -radiation, and many have concluded that the
partial pressure of oxygen (pO2) rose from a pretreatment value of 2.8 1 combined treatment provides an additive, but not synergistic, effect
to 15.2 6.9 mm Hg immediately after light application was complete (5, 6). In this study, we examine a non-vascular-targeting type of PDT
(P 0.048). When the optical irradiation was given 15 min after verte- treatment with verteporfin photosensitizer (7), which induces in-
porfin injection, the tumor pO2 decreased slightly after treatment [i.e., creased oxygen tension in the tumor (8) and, when combined with
6.8 1.6 mm Hg (pretreatment) versus 4.1 0.3 mm Hg (posttreatment)], radiation treatment, induces a greater than additive effect in tumor
whereas control tumor pO2 did not change significantly. In vitro study of killing.
the cellular oxygen consumption rate before and after PDT treatment Previous studies have examined specific interactions between PDT
indicated that the consumption rate decreased linearly with delivered
and radiation therapy to determine what the effect of combined
optical dose and quantitatively matched the loss of cell viability as meas-
ured by a mitochondrial tetrazolium assay. Doppler measurements show
therapy would be. Several early studies have examined whether the
that red cell flux is still patent immediately after treatment, indicating that photosensitizer molecule itself acts as a radiosensitizer, and whereas
oxygen should still be delivered to the tumor. Computational simulations some have shown effects with hematoporphyrin derivative and ion-
of the oxygen supply from the vessels and the consumption from mito- izing radiation (9), the majority of studies have concluded that there
chondrial activity confirmed that if oxygen consumption is decreased in was no radiation sensitivity enhancement due to the photosensitizer
the presence of unhindered blood flow, the tumor oxygenation should rise, (10 –12). In comparison, many more studies have examined the utility
and the hypoxic fraction of the tumor should decrease. Combination of combined PDT treatment with radiation treatment, but most have
treatments with PDT delivered (100 J/cm2 optical dose, with 1 mg/kg been carried out in monolayer cell culture (5, 13–15), largely with the
benzoporphyrin derivative monoacid ring A injected 3 h before treat- conclusion that synergistic effects are not observed. However, studies
ment) after radiation treatment (10 Gy from 300 keV source) were com-
with both aluminum phthalocyanine and aminolevulinic acid have
pared with PDT delivered simultaneously with radiation. Tumor regrowth
assay showed that the delays to reach double the tumor volume for PDT indicated that timing may be a key factor in determining whether
alone and radiation alone were 2.7 1.6 and 3.2 1.7 days, respectively. synergistic effects are observed (13, 16) because synergistic effects
When radiation was given before PDT, the delay was 5.4 1.4 days, and are observed at longer temporal separations between the radiation and
when PDT was given at the same time as radiation, the delay was 8.1 1.5 PDT treatments. In addition, any tissue response characteristics that
days. This observation indicates that the combined effect in the latter case occur would not be present with in vitro assays. In vivo studies have
was greater than additive (P 0.049). demonstrated mixed effects: Winter et al. (17) demonstrated no syn-
ergistic effect of PDT and radiation in tumors; and Benstead and
INTRODUCTION Moore (18) indicated that combined treatment of meta-tetra(hydroxy-
phenyl) chlorin (mTHPC-PDT) with X-rays increased the normal
PDT3 is a treatment modality for dysplastic tissues, which uses a
tissue damage but that the effect of time sequence of the two treat-
light-activated drug to kill targeted regions of tissue (1– 4). A photo-
ments upon outcome was not significant. To date, none of these
sensitizer with delivery vehicle is applied to the tissue topically or
studies have examined specifically whether PDT could be delivered in
systemically, and moderate intensity light is used to selectively excite
a manner that changes the tissue metabolism such that, when used in
these molecules to their first excited triplet state, which is then
conjunction with radiation therapy, it provides a superadditive or
efficiently quenched by molecular oxygen to produce singlet state
oxygen. The cellular response to high doses of singlet oxygen is
Our studies have shown that mitochondria function can be acutely
complex but generally causes phospholipid peroxidation leading to
impaired when verteporfin treatment is applied directly to the cells
cellular membrane damage and vessel occlusion-mediated ischemia,
and that this effect is observed in vivo as well, as measured by a loss
of NADH fluorescence (19). This effect is likely a direct result of the
Received 7/10/02; accepted 1/6/03.
The costs of publication of this article were defrayed in part by the payment of page fact that BPD-MA (the photoactive molecule in verteporfin) predom-
charges. This article must therefore be hereby marked advertisement in accordance with inantly localizes on the mitochondria membranes, and therefore PDT
18 U.S.C. Section 1734 solely to indicate this fact. action would likely cause widespread mitochondria damage (20, 21).
Supported by National Cancer Institute Grants RO1CA78734 and PO1CA84203 and
by the Electron Paramagnetic Resonance Center for the Study of Viable Systems at If PDT can be used to halt cellular metabolism shortly after treatment,
Dartmouth Medical School (supported by the National Center for Research Resources, then there should be a net beneficial effect by increasing the oxygen
NIH Grant P41 RR11602).
To whom requests for reprints should be addressed, at Thayer School of Engineering,
available to redistribute to previously hypoxic areas (8). The hypoth-
8000 Cummings Hall, Dartmouth College, Hanover NH 03755. E-mail: pogue@ esis of this work was that if a PDT treatment could be applied that
dartmouth.edu. preserved blood flow immediately after treatment yet reduced the
The abbreviations used are: PDT, photodynamic therapy; RIF-1, radiation-induced
fibrosarcoma; BPD-MA, benzoporphyrin derivative monoacid ring A; EPR, electron oxygen consumption by the parenchyma cells, then the tumor tissue
paramagnetic resonance; pO2, partial pressure of oxygen; LiPc, lithium phthalocyanine. oxygenation would increase. This reoxygenation phenomenon would
NONVASCULAR PDT ENHANCES RADIATION SENSITIVITY
be most dominantly observed where preexisting hypoxia is present reduction in cellular respiration. The continuation of blood flow
due to the larger diffusion distances from capillaries (22). In our coupled with acute reduction in cellular respiration could increase
studies, we have chosen to follow this effect in the RIF-1 tumor model or redistribute oxygen during PDT. This effect, combined with
(23), which is known to have large regions of hypoxic cells (22, 24, reduced killing in regions that were hypoxic before PDT, could
25) and has also been well studied in our previous studies (22, 26 –29) lead to reoxygenation of chronically hypoxic areas, as has been
as well as in PDT studies of several other groups (25, 30 –35). observed in our earlier study with both verteporfin and amino
pO2 changes that occur in vivo during PDT are complex and have levulinic acid-protoporphyrin IX treatment in the RIF-1 tumor (8).
been shown to vary with many factors (36). The necessity for oxygen In this study, changes in tumor pO2 were quantified both during and
to be present for tissue killing by PDT has been shown both in vitro after treatment with non-vascular-targeting verteporfin-PDT. The cel-
and in vivo (31), and the consumption of oxygen in this process can lular oxygen consumption rate was determined for RIF-1 cells in vitro.
be readily observed as an acute decrease of tissue pO2 (37–39). These Computer simulations are used to interpret the observations and
observations led to a suggestion that high optical dose rates could lead examine what the change in hypoxic fraction is expected to be. This
to less tumor killing due to the transient depletion of oxygen (39). This treatment is then combined with X-ray irradiation to look for benefi-
concept has been supported by the observation that the effectiveness cial combinatorial effects of the dual therapy. Preliminary evidence
of treatment decreases in some tumor models when high optical dose suggests that non-vascular-targeting PDT could be a good mechanism
rates are used for treatment (35, 40 – 42). In addition to this transient to decrease the hypoxic fraction of tumors, thereby increasing the
decrease in tumor oxygenation, it has been observed that rapid and efficacy of radiation therapy.
permanent reduction in blood flow and tumor oxygenation (43– 45)
can occur due to massive vascular occlusion during or soon after PDT MATERIALS AND METHODS
when a large fraction of the exogenous photosensitizer is present in
the vasculature (46). Whereas this vascular-targeting approach to Cells and Tumor Model. RIF-1 cells were used for in vitro and in vivo
delivering PDT has been a dominant line of study, it is also interesting studies. RIF-1 cells were originally supplied by James B. Mitchell (National
to consider the longer time regime, where the photosensitizer has Cancer Institute), and they were grown in vitro in RPMI 1640 supplemented
largely leaked out of the vasculature or been cleared from the blood. with 10% fetal bovine serum, glutamine, and antibiotics and grown with no
more than four passes from the original stock. In vitro studies were carried out
Iinuma et al. (46) have shown that verteporfin is initially confined to
with the cells in 96-well plates, given 3 days between plating and use for
the vasculature at 5 min postinjection but that at 1 h postinjection, it
treatment. For in vivo studies, cells were grown in large culture flasks,
appears to be predominantly perivascular in the orthotopic rat prostate removed, resuspended in medium without fetal bovine serum, and injected at
NBT-II tumor. Fingar et al. (45) showed that the effect of vascular 4 105 cells/mouse in 50 l volume. Intradermal injection on the upper right
stasis appears acutely in chondrosarcoma and muscle when light is leg was used in female C3H/HeJ (5– 6 weeks old) mice (The Jackson Labo-
delivered 15 min after injection with verteporfin-based therapy but ratory, Bar Harbor, ME), approximately 11–14 days before the anticipated
that it was inhibited or delayed by several hours when light was given treatment time.
3 h after injection. Furthermore, Major et al. (47) have shown that Photosensitizer. The photosensitizer, verteporfin, was obtained from QLT
blood vessels can dilate after verteporfin treatment, potentially in- Phototherapeutics Inc. (Vancouver, British Columbia, Canada) for this exper-
creasing the flow after verteporfin-based PDT. Our own studies have imental study. Verteporfin is the active ingredient in Verteporfin For Injection
(VFI), the lipid formulated drug that is marketed as Visudyne for the treatment
indicated that even when verteporfin-based PDT is given in a vascu-
of ocular disease. The photoactive molecule present in this preparation is
lar-targeting approach, the response is heterogeneous at the micro- BPD-MA (57). For in vivo use, the drug as supplied was reconstituted in PBS
scopic scale, indicating that individual vessels vary in their response at 0.2 mg BPD-MA/ml and then injected into the mice through the lateral tail
to the treatment. Our studies in the RIF-1 tumor model with verte- vein to give a dose of 1 mg BPD-MA/kg body weight. For in vitro use,
porfin treatment have shown that blood flow is still patent immedi- BPD-MA was reconstituted in PBS at 1 mg BPD-MA/ml solution in PBS and
ately after treatment and decreases steadily during the hours following diluted in medium to expose cells to 1 g/ml BPD-MA. For both in vivo and
therapy but that if an incubation time of 3 h is used, then the decrease in vitro work, an incubation period of 3 h was used in these experiments,
in blood flow is only to 55% of the pretreatment value.4 Thus, current except where otherwise specified. The drug was kept in the dark and frozen
evidence would indicate that when verteporfin-based PDT with opti- between experiments.
Light Delivery. A diode laser system up to 200 mW average power was
cal irradiation is given several hours after injection, the blood vessel
used throughout these studies (Applied Optronics, South Plainfield, CT), with
occlusion does not predominantly occur immediately during treat- a wavelength of 690 nm. The beam was delivered to the animals through a 140
ment, preserving blood flow during and after therapy. m fiberoptic and expanded onto the tumor or cell plate in a circular top-hat
In contrast to blood flow, the metabolic consumption rate of the beam, using a fiberoptic collimator (Thor Labs, North Newton, NJ). The beam
tumor parenchyma cells has not been adequately examined in PDT. diameter for in vivo work was 1.1 cm, using an irradiance of 133 mW/cm2, and
Whereas this parameter is very challenging to measure directly in for in vitro work, it was a 2-cm-diameter beam for an irradiance of 63.7
vivo (48 –51), it could be significantly altered by PDT for photo- mW/cm2. In vivo light treatment was given transcutaneously, with the animals
sensitizers that primarily target the mitochondria. The rate of cleanly shaven before treatment.
oxygen consumption would be inhibited and the mechanism would In Vitro PDT Treatment. Cells were plated in black plastic 96-well plates
with a transparent bottom (Fisher Scientific, Springfield, NJ) at a density of
depend upon the light fluence, irradiance, and localization of the
5000 cells/200 l medium/well. The cells were used for PDT treatment after
photosensitizer. It has been observed that one of the earliest events 3 days of growth in 95% oxygen with 5% CO2, at 37°C in a humidified
in PDT damage in vitro is the loss of mitochondrial membrane incubator. On the day of treatment, the medium was replaced with 100 l/well
integrity, which would lead to a loss of cellular respiration and, medium with 1 g/ml BPD-MA in the verteporfin preparation. After 3 h in the
ultimately, apoptosis (52–56). In necrotic cell death, loss of res- incubator, the BPD-MA was decanted off, cells were rinsed once with HBSS,
piration would be instantaneous, but it is also likely that sublethal and then 100 l of medium were added to each well. Cells were illuminated
damage to the mitochondria induces a transient or permanent in groups of four wells, with blank wells between the treated groups to ensure
that each group of wells received the correct dose. For each 96-well plate,
4 squares of four were treated with increasing total light dose with 8 groups/
B. Chen, B. W. Pogue, I. A. Goodwin, C. M. Wilmot, J. O’Hara, and P. J. Hoopes.
Blood flow dynamics following PDT with verteporfin in the RIF-1 tumor, submitted for plate, including the control with no light. Cells were then assayed for cell
publication. viability or cellular oxygen consumption.
NONVASCULAR PDT ENHANCES RADIATION SENSITIVITY
Cell Viability Assay (MTS). Cell viability was assayed 24 h after treat- linewidth. The LiPc was precalibrated to determine the ambient pO2 from the
ment, using the MTS assay kit (CellTiter 96AQ; Promega, Madison, WI), linewidth, such that the data could be used to calculate effective tissue pO2
which is used to measure the reduction of a tetrazolium compound by the during the experiment.
cellular mitochondria, producing an optically active soluble formazan. After Laser Doppler Flow Measurement. Laser Doppler measurements of RBC
PDT treatment, the medium from the 96-well plate was removed, and the MTS flux were taken on a section of animals given PDT treatment, using an
tetrazolium reagent was added. After a 1-h incubation at 37°C, the absorbance implantable fiberoptic probes (Moor Instruments, Wilmington, DE). The meas-
of each well was measured in an automated plate reader (Thermo Max; urement of red cell flux can be used to monitor microcirculatory function and
Molecular Devices, Menlo Park, CA) at the 490 nm absorption of the MTS provides a proportionate measurement of blood flow, assuming that hematocrit
soluble formazan product. For data analysis, the absorbance values from levels are constant throughout a given measurement. Two fiber probes had
groups of wells that received the same light dose were averaged, and the data external diameters of 200 m and were implanted in the tumor tissue through
were normalized by the average value from the control (no light) wells. holes made with a 26-gauge needle immediately before insertion. Both probes
Cellular pO2 Measurement. Immediately after PDT, cells were trypso- were implanted within the light field, and automated data collection was done
nized and suspended in medium, and cell viability was determined by trypan with the supplied software on a computer. Blood flow was sampled at 40 Hz,
blue exclusion assay before use in the oxygen consumption measurement. and data points were averaged together over 5-min and 30-min periods after
Cells were resuspended at 2 107 viable cells/ml in complete RPMI 1640. the experiment. All flow values during the treatment time were standardized to
Just before use, the cell suspension was diluted to 107 cells/ml with a solution the average value during the first 2 min after probe implantation. Animals were
of 10% dextran in complete medium and used for oxygen consumption assay. divided into three control groups: (a) no light, no drug; (b) drug, no light; and
The rate of oxygen consumption was measured from the cells using EPR. (c) light, no drug. Two treatment groups were used including a 15-min as well
Cells suspensions with dextran were extracted in 0.100 ml volumes and mixed as one treatment group using a 3-h interval between verteporfin injection and
with a neutral nitroxide, 15N PDT (4-oxo-2,2,6,6-tetramethylpiperidine-d16- light irradiation. Control animals were further divided into three subgroups: (a)
N-1-oxyl) at 0.2 mM (Cambridge Isotope Laboratories, Quebec, Canada) in no verteporfin, no light; (b) verteporfin, no light; and (c) light, no verteporfin.
a 4- l aliquot. This mixture was then drawn into glass capillary tubes, which Verteporfin was injected i.v. at 1 mg/kg BPD-MA concentration 3 h before
were then sealed with Critoseal (Sherwood Medical, St. Louis, MO). The animal treatment. In the treatment group, blood flow was monitored for 2 min,
sealed tubes were placed into quartz ESR tubes, and samples were maintained followed by 12 min of light treatment, followed by 1 h of blood flow
at 37°C by a heated flow of gas through the resonator. All spectra were measurement. Blood flow during laser treatment was meaningless because
recorded on a Bruker EMX EPR spectrometer operating at 9 GHz. Because the during this time, the irradiation light saturated the Doppler probe. All values
resulting linewidth was proportional to oxygen concentration, oxygen con- were normalized to their pretreatment value before averaging together the
sumption rates were obtained by measuring the concentration repeatedly over numbers from each animal in each treatment group. The animals in the PDT
10 –20 min, and the slope of the resulting data was determined by linear treatment group (as opposed to controls) were not sacrificed after the 30-min
regression. Three repeated trials were completed for each sample of cells, and time point but were later reanesthetized, and their tumors were reimplanted
the slope values were averaged. with blood flow probes to measure flow at 6 h posttreatment.
Tumor pO2 during in Vivo PDT. Animals were anesthetized with inha- Combined PDT and Radiation Therapy Studies. Experimental studies
lation of isoflurane at 1.5% mixed with 26% oxygen in a continuous flow, examining the combination effect of PDT and radiation therapy were com-
delivered by a nose cone to loosely cover the head. The animal was maintained pleted in this tumor model. The injection of BPD-MA or saline (for control)
at 37°C on a heated water pad, and warm air flowed over the animal. The was given 3 h before treatment in all cases. Light at 690 nm wavelength was
animals were used for continuous monitoring of tumor pO2 before, during, and delivered to the PDT-treated animals while the animal was positioned in the
after the treatment using EPR oximetry as described below. In the first irradiator cone. An optical irradiance of 133 mW/cm2 was used over 12 min,
treatment group, tumors were treated with light 3 h after the verteporfin for a total optical dose of 100 J/cm2. Radiation was a single dose of 10 Gy (300
injection. In the second group, the photosensitizer was injected just 15 min keV, 10 mA, half value layer (HVL) 2.33 Gy/min). The treatment groups
before optical irradiation, and these animals were followed for tumor pO2 were as follows: (a) sham-irradiated controls; (b) radiation treatment alone; (c)
before, during, and after treatment. This type of short-term treatment is thought PDT alone; (d) radiation treatment followed by PDT; and (e) PDT with
to elicit a strong vascular occlusion response and thus provides a contrast to the radiation treatment given simultaneously. In the fourth treatment group, X-ray
effect from a cellular targeting treatment, where a 3-h delay is given between radiation was given for the full 3-min treatment time after the photosensitizer
injection and irradiation. The third group was control animals that were treated was injected 3 h after photosensitive injection, and then optical irradiation was
with light alone to simulate the effects of light absorption in the tissue without given for the 12-min treatment. In the fifth treatment group, the optical
photosensitizer. irradiation was given 3 h after the drug injection, and during the last 3 min of
Tumor pO2 was monitored with an oxygen-sensitive EPR probe material, the optical irradiation, X-ray irradiation was also given to the tumor at the same
synthetic LiPc, which can be implanted into the tissue and provide stable time. This latter timing would result in the tumor being exposed to X-rays
measurements of tissue pO2 over several weeks (27, 58 – 61). Initial experi- during the maximal reoxygenation time of the tumor tissue.
ments demonstrated that pO2 could be monitored with EPR during the PDT Irradiation of the 10-Gy dose using approximately 300 keV was delivered in
treatment without any interaction between the probe and the photosensitizer or 3 min from a Pantak Therapax 300 Orthovoltage irradiator, using a 2-cm
light (8, 29). Small particles (approximately between 50 and 200 m) of LiPc collimation cone. This cone was placed in intimate contact with the mouse leg,
were implanted in the animal tumors at least 24 h before PDT treatment to surrounding the tumor. The end of the collimation cone was transparent plastic,
allow for resolution of any acute effects due to the injection. A 23-gauge allowing radiation and optical treatment to occur at the same time, as required
syringe needle was used to implant the autoclaved, dry material at a depth of in the last group, where PDT and radiation were simultaneously applied. All
1–3 mm within the tumor. Only one implantation of oxygen-sensitive material animals were treated with the cone in place, even those treated with sham
was necessary, and pO2 could be monitored as often as desired during the irradiation and PDT alone, to ensure that equal doses of light, including
experiments. changes in the beam from reflection and refraction through the irradiator cone,
The animals were placed in an L-band (1.2 GHz) EPR spectrometer with a were given to all groups.
microwave bridge custom-built for measurements of tissue pO2 in small Tumor Regrowth Assay. Mice were assigned to the different treatment
animals. The external loop resonator was positioned over the tumor to record groups in the combined radiation and PDT study in a manner that produced
the EPR signal of the LiPc within the tissue. Typical settings for the spec- matching initial tumor volumes at the time of treatment to minimize any
trometer were incident microwave power of 50 mW, magnetic field of 400 G, systematic errors associated with groups having different initial volumes at the
scan range of 0.5 G, modulation frequency of 27 kHz, modulation amplitude time of treatment. The resulting treatment effect was assayed by calculation of
of 15 mG, and scan time of 30 s. The settings were not significantly changed the tumor volume, as determined by measurement of the three major axis
between animals in this study nor during an acquisition of an individual dimensions (volume length width height/2). The time for a tumor to
animal. After accumulation of the linewidths as a function of time before, reach double its volume on the day of treatment was calculated for each animal
during, and after treatment, the data were fit with a custom-written software separately by estimating the data by a linear fit to the logarithmic growth curve
program to match the Voigt line shape of the EPR spectrum and extract the versus time and estimating when it had reached twice the tumor volume as on
NONVASCULAR PDT ENHANCES RADIATION SENSITIVITY
the day of treatment. The average values from these times to double in volume treatment, and 1, 4, and 24 h posttreatment. The pO2 values for each
were calculated for each treatment group. group of animals were averaged together, and the SDs were calcu-
To analyze the tumor regrowth data and obtain estimates of the mean lated. These values are shown in Fig. 1.
doubling time and its SE, individual growth curve data were combined for each Control animals were given the laser light but no photosensitizer
group and fitted using a mixed effects model (28, 62). It was assumed that after injection. The control animals all had pO2 values initially at 3.6 1.1
day 3, the tumor regrows exponentially, which corresponds to a linear function
mm Hg, indicating that the tumors have a high hypoxic fraction, as
when plotted on a semi-log graph. To address mice heterogeneity, a mixed
effects modeling technique was applied, which has been previously applied in
has been shown in previous studies (22, 61). Throughout the laser
O’Hara et al. (28) and described by Demidenko (62). The time for a tumor to treatment and afterward, the control animals maintained hypoxic pO2
reach double its volume on the day of treatment was calculated for each group values averaging 3.7 1.2 mm Hg. In the treatment group, the initial
along with the respective SE using the delta-method (63). These SEs give rise pO2 was 2.8 1.0 mm Hg. Initial changes in pO2 had a high variance
to the group comparison using Z-test to test for significant differences between between animals, with some increasing in pO2 immediately, some
the mean values of the difference treatment groups. staying constant, and some fluctuating. By the end of the treatment
Oxygen Distribution Simulation Studies in RIF-1 Tumors. A previously period, all five of the treated tumors had risen significantly in their
reported simulation study was used to evaluate the effect of changes in pO2 (8). The final value immediately after treatment was 15.2 6.9
metabolic consumption on the resulting tumor tissue pO2 (22). This study used mm Hg, which is significantly different from the control value, as
a finite element solution to the steady-state diffusion equation to solve for the
calculated by a paired Student’s t test with P 0.048. At 1 h
oxygen concentration everywhere within an arbitrary volume of tissue. The
differential diffusion equation is given in steady state by
posttreatment, the pO2 was back to within control or pretreatment
values. A group was also included with the photosensitizer injected
D CO2 r kmet r, O2 SO2 r 0 (1) just 15 min before treatment, which allows observation of the “vas-
cular-targeting” type of therapy. In this case, the average tumor pO2
where CO2(r) is the oxygen concentration at position r, D is the diffusion
decreased slightly during treatment while staying in the hypoxic range
coefficient for oxygen in tissue (which is generally taken to be spatially
(i.e., 6.8 1.6 mm Hg before light treatment and 4.1 0.3 mm Hg
independent), kmet(r,O2) is the metabolic oxygen consumption rate, and SO2(r)
is the supply of oxygen by the capillaries at each point r. immediately after treatment).
The capillary geometries for the simulation were derived from eight sepa- In Vitro Cellular Oxygen Consumption and Viability. Measure-
rate H&E-stained sections of RIF-1 tumor tissue, digitized, and manually ments of oxygen in suspensions of RIF-1 cells show that the oxygen
thresholded. The capillary oxygen supply rates were estimated based on fitting consumption rate of untreated cells is higher than that of cells treated
to boundary information, which was given by pimonidazole staining of adja- with BPD-MA-based PDT, as shown in Fig. 2. The average oxygen
cent sections of the tissue. This staining provides demarcation of regions of consumption rate in the untreated cells was 1.84 0.14 nmol/min/
hypoxia, thereby allowing estimation of the neighboring capillary oxygen million cells. When treated with light alone, there are small increases
supply rates, assuming that the oxygen diffusion coefficient is known in this oxygen consumption rate on the order of 0.2 nmol/min/106
(D 2 10 5 cm2/s). Our earlier study indicated that a metabolic oxygen
cells, and when treated with the photosensitizer and light, there is a
consumption rate of kmet 10 M/s was appropriate to simulate an oxygen
monotonic decrease in oxygen consumption correlated to the deliv-
histogram distribution that was quantitatively similar to experimental meas-
urements with Eppendorf electrode measurements taken from a set of RIF-1 ered light dose. At the highest light dose used here (16 J/cm2), the
tumors (22). oxygen consumption rate fell to 0.37 0.04 nmol/min/106 cells. The
In the present study, the same eight sections of tumor tissue were used to data for these measurements are plotted in Fig. 2, shown by open
simulate the oxygen distribution within the tumor tissue. The oxygen supply symbols (squares for treated cells and circles for control), with the
rates of the capillaries were given by the previous fitting to the pimonidazole absolute units of oxygen consumption plotted on the right-hand
data, and then the value of kmet was varied to appreciate the change anticipated vertical axis. The control group received light doses without incuba-
in the tumor under normal conditions (i.e., kmet 10 M/s) and when tion with photosensitizer.
metabolic consumption is halted (i.e., kmet 0 M/s). The calculated oxygen The cell viability as measured by MTS assay is plotted on the same
distributions were then integrated into a histogram so that the data could be
presented in the format typically obtained with an Eppendorf electrode.
Whereas all calculations were completed in micromolar concentration units,
the final data are presented in units of pO2 (mm Hg), by multiplication by the
solubility of oxygen in water, assuming that the solubility of the tumor tissue
would be similar.
In Vivo Tumor pO2. Measurements of tumor pO2 were taken in
control and treated tumor-bearing animals. Mean SD tumor volume
for all groups at the time of treatment was 203 23 mm3 (range,
110 –300 mm ). There was no significant difference between the
tumor volumes of the groups at the time of treatment. The EPR probe
was placed within the top 2–3 mm of tumor tissue to ensure that it was
reporting from a region that had a full effect of the PDT. This
measurement is localized and was repeated at several different times
before, during, and after the treatment in each animal. Typically, the
pO2 was recorded for 10 min before light irradiation, then recorded Fig. 1. Tumor pO2 measurements in RIF-1 tumors in mice treated with verteporfin-
based PDT and control animals with light alone but no photosensitizer (n 5). There were
every minute throughout the treatment, and recorded for 10 –15 min two treatment groups, each of which received 1 mg/kg BPD-MA in the verteporfin
after the treatment. The mice were then reanesthetized at 1, 4, and 24 h preparation (n 5 each) either 15 min or 3 h before light treatment. In both cases, the light
posttreatment for pO2 measurement, requiring approximately 10 –15 was delivered with a total dose of 144 J/cm2 at 690 nm wavelength. The error bars in the
figure represent the SD, and the values for the 3-h-treated group immediately after
min. The average values at each time point were calculated separately treatment (labeled after Tx on the graph) are significantly different from control values
for each animal for pretreatment, during treatment, immediately after (P 0.048).
NONVASCULAR PDT ENHANCES RADIATION SENSITIVITY
graph to illustrate the similarity between the dose responses, with
survival plotted on the left-hand vertical axis, normalized to 1.0 for
the untreated cell results. The closed symbols in Fig. 2 show these
data, and the line on the graph shows a best fit to the survival data.
The dose response is quantitatively identical in this study between cell
survival and oxygen consumption change. At the maximal light dose,
oxygen consumption is 21 4% of the control oxygen consumption
rate, and correspondingly, the MTS assay for the same light dose
indicated a decrease to 20 5% cell survival.
Laser Doppler Flow Measurements. Doppler RBC flux measure-
ments were taken on six animals using the same treatment protocol
and are shown summarized in Fig. 3. The flux immediately after
treatment is equivalent to the flux in the control groups of no drug and
no light, as well as drug only. Interestingly, the light alone control
group shows a modest increase in blood cell flux immediately after
the light delivery. The treatment group then has a reduction in blood
cell flux over a longer time scale, with a reduction to 70% at 30 min
after PDT and to 55% at 6 h after PDT.
Calculation of Changes in pO2 Based on Changes in Metabolic
Consumption Rate. Using the capillary distributions and values ob-
tained from RIF-1 tumor sections in our previous study (22), oxygen
histograms were simulated for the two conditions of kmet 10 M/s
Fig. 4. Calculated oxygen histograms from oxygen diffusion calculations where the
metabolic oxygen consumption term (kmet in Eq. 1) was varied. In a, kmet 10 M/s,
whereas in b, kmet was 0 M/s. In both cases, the simulations were carried out on the same
capillary distribution patterns with D 2 10 5 cm2/s. In a, the median pO2 is 2 mm
Hg, and in b, the median pO2 is 7 mm Hg.
and kmet 0 M/s. The values of pO2 were summed from the images
used, and the data are presented in a histogram format. These are
shown in Fig. 4, a and b, respectively. The median pO2 values for
these two distributions are 2 and 7 mm Hg, respectively, indicating
that when the metabolic oxygen consumption rate of the cells is
minimal, the pO2 of the tumor tissue could increase by a median of 5
mm Hg. These data predict that the hypoxic fraction (the fraction of
Fig. 2. Cellular viability (closed symbols) and oxygen consumption rate (open symbols)
the tumor less than 10 mm Hg pO2) is initially 98%, whereas after the
as a function of the delivered light dose to RIF-1 cells incubated for 3 h with 1 g/ml change in metabolic consumption, the hypoxic fraction is reduced to
BPD-MA. Cell viability was determined using the MTS assay. Oxygen consumption rate 65%. This simulation represents the extreme case of zero oxygen
was measured from cells in suspension using EPR oximetry. In both cases, the data points
represent the average of three successive trials, and the error bars are the SD. consumption after treatment, which is not the case from the measure-
ments. Nonetheless, the increase in oxygenation is monotonically
correlated with the oxygen consumption rate of the cellular regions,
and so the two extremes simulated here present a simulated maximal
range for the effect observed.
PDT Combined with Radiation Treatment. Measurements of
tumor volumes after treatment with radiation, PDT, or both are shown
in Fig. 5. In this experiment, the five treatment groups were as
follows: (a) control with saline injection alone; (b) radiation therapy
alone with BPD-MA injection but no light; (c) PDT alone with sham
X-ray irradiation; (d) X-ray irradiation followed by PDT; and (e) PDT
and radiation treatment given together. The data in this graph show
that the tumor regrowth rate in all groups is slightly lower than that of
the control group but that these slopes are similar between all treated
groups. The slopes of the natural logarithm of tumor volume versus
Fig. 3. Measured average normalized blood flow from Doppler flow probes implanted
days were 0.20, 0.11, 0.13, 0.13, and 0.11, respectively, for the five
in the tumor. Each bar represents the average of six animals, where the flow value was treatment groups. PDT alone (group 3) and radiation alone (group 2)
normalized by the pretreatment flow measured over 2 min, and the error bars are the SE induce the same approximate time for the tumor to reach twice the
between animals. Three control groups (no drug and no light, verteporfin alone, and light
alone) are shown as an average measurement over 30 min. The PDT-treated animals are volume of the treatment size (i.e., doubling times were 8.3 and 8.9
shown both immediately after light treatment as well as 30 min and 6 h posttreatment. days, from Table 1). In comparison, group 4 and group 5 indicate
NONVASCULAR PDT ENHANCES RADIATION SENSITIVITY
they essentially both measure mitochondria activity, but because the
MTS assay has been shown to be proportional to long-term cell
survival in other studies, it is likely that the loss of oxygen consump-
tion rate indicates acute damage to the mitochondria that can ulti-
mately cause cell death. This change in oxygen consumption is also
directly correlated to the optical dose delivered, so that the effect is
likely due to singlet oxygen-mediated cellular death. The fact that
these changes are observed immediately after the light treatment is an
indicator that the predominant effect causing cell death is necrosis or
some acute damage mechanism.
Tumor pO2 Increases Immediately after PDT. In vivo tumor
pO2 measurements indicated that the tissue oxygen increased acutely
in response to the treatment (Fig. 1). Our studies show that blood flow
is not significantly reduced immediately after this type of treatment in
the RIF tumor. Whereas this flow eventually degraded over time for
all treatment groups, this indicates that there is a time period after
Fig. 5. Average tumor volume is plotted relative to the number of days after treatment
(days posttreatment, X axis) for the five treatment groups [control (n 6 mice), radiation
PDT in this treatment regime where oxygen is still being delivered to
only (n 5), PDT alone (n 6), radiation followed by PDT (n 7), and PDT together the tissue. Because we observe that oxygen consumption is acutely
with radiation (n 7)]. The points represent the average values of each group, and the reduced by the PDT treatment, we conclude that the tissue pO2 could
error bars are the SE. The tumor volumes on the day of treatment and for doubling the
treatment volume are denoted by horizontal dotted lines. rise in response to therapy in which the blood vessels are not acutely
occluded. This observation also agrees with the results we have
reported for treatment of the RIF-1 tumor with aminolevulinic acid,
increasingly better treatment effects, with doubling times of 11.0 and where the rise in tumor oxygenation was also observed (8), and the
13.7 days, respectively. decay in the oxygen level after treatment was observed to decay
over the time course of over 1 h, indicating that this time period
DISCUSSION was probably when blood was flowing. Further study of the micro-
regional kinetics after PDT would be useful to help characterize this
In this study, verteporfin-based PDT has been explored as an phenomenon.
adjuvant to radiation therapy, using a 3-h interval between injection Simulations of oxygen distribution within the RIF-1 tumor tissue
and irradiation because this time point allows continuous blood flow were carried out, and the histogram calculations shown in Fig. 4
in tissue after treatment, as well as increased tumor pO2 (8). This work present representative values that indicate the relative effect that
began under the hypothesis that non-vascular targeting of PDT could changes in cellular oxygen consumption rate would have. In Fig. 4a,
be achieved using a drug interval of several hours between injection the histogram is very similar to that typically observed in the RIF-1
of the photosensitizer and the time of optical irradiation. Studies have tumor model, on average, showing a dominant hypoxic fraction and a
shown that the verteporfin photosensitizer leaks into the surrounding large fraction less than 5 mm Hg (22). By changing the oxygen
parenchyma on time scales that are longer than a few hours, whereas consumption rate in these simulations to zero, the oxygen distribution
intravascular concentrations have a metabolic lifetime of about 6 h in the tumor is simply given by the distribution of capillary pO2 values
(46). Thus, a treatment protocol that allows several hours of latency that supply the region. In this case, the median pO2 significantly
provides preferential targeting to the parenchyma while minimizing increases from 2 to 7 mm Hg. This model approach assumes that the
the effect on the vasculature. It is important to note that whereas this blood flow is not altered at all during therapy, in agreement with our
photosensitizer has been most used clinically for vascular targeting in measurements here. These simulations illustrate that changes in met-
age-related macular degeneration treatment (64), it is not limited to abolic consumption rate can have a measurable effect on the tumor
this type of therapy in principle. A longer drug-light interval is key to pO2 value, decreasing the hypoxic fraction from 98% initially to 65%
minimizing the vascular occlusion within the tumor4 and maximizing after loss of oxygen consumption. Alterations in blood flow would
the direct cell damage, which causes acute mitochondrial damage. Our change the supply rate of oxygen and further perturb the measured
blood flow studies with verteporfin treatment in the RIF-1 tumor show pO2, yet our studies did not demonstrate significant changes in flow.
results similar to the observations of Fingar et al. (45) in the chon- The limitation of this calculation is that it is for the specific tumors
drosarcoma tumor. When the 3-h drug-light interval is used, then the studied in our earlier paper (22) and that because this tumor is highly
flow decreases after treatment, but at a slower rate than when 15 min heterogeneous, it is likely that the effect will be higher in some
is used. When the 15-min drug-light interval is used, the flow de- regions and lower in other regions.
creases steadily, decaying to 50% of initial values at 1 h after treat- These observations have implications for PDT, suggesting that
ment and down to zero at 6 h after treatment.4 The potential applica- PDT-induced cellular damage may be used to increase oxygen in
tions for vascular-targeting versus parenchyma-targeting therapies are tumor tissue, including regions that were in a chronically hypoxic
quite different, and yet little study has been devoted to contrasting state. In our case, the pO2 of the RIF-1 tumor increased from an
these two potential dosimetry regimes (3, 21, 65– 67). This study
focuses on the effect of PDT in cellular respiration and pO2 changes
that occur in this parenchyma-targeting regime. Table 1 Tumor doubling time and regrowth delay
Mitochondrial Function Is Inhibited after Verteporfin PDT. In Time for tumor to Regrowth delay in
our experiments, cells that have been treated in vitro show a reduction Group Treatment double initial volume doubling time
in oxygen consumption rate (shown in Fig. 2). These data indicate that 1 Control 5.6 0.4
2 Radiation only 8.3 1.7 2.7 1.6
the change in oxygen consumption is directly proportional to the loss 3 PDT only 8.9 1.7 3.2 1.7
of cell mitochondrial function, as measured by the MTS assay. It is 4 Radiation then PDT 11.0 1.5 5.4 1.4
not surprising that these two assays should be proportional because 5 PDT and Radiation 13.7 1.6 8.1 1.5
NONVASCULAR PDT ENHANCES RADIATION SENSITIVITY
Table 2 Student’s t test comparisons a pretreatment, it should enhance the radiation effect. It is also
Difference test P possible that the blood flow shunting that could occur as some
Groups 2 vs. 3 0.77 microvessels are occluded may also cause oxygen enhancement in
Groups 2 vs. 5 0.12 areas of continued flow, and pathological analysis and Doppler blood
Groups 3 vs. 5 0.0013a
Groups 4 vs. 5 0.049a
flow measurements are ongoing to examine this as a potential alter-
a native hypothesis. Independent of the mechanism, combining PDT in
Significant difference (P 0.05).
a non-vascular-targeting regime (i.e., where there is a long latency
between drug injection and the time of optical treatment) may be used
average of 3.6 mm Hg (control) to 15.6 mm Hg immediately after effectively to enhance the efficacy of radiation therapy.
treatment. This would be sufficient to raise the average tumor oxy- Experiments of combined therapies can often have potentially
genation out of the region that is considered radiobiologically hypoxic confounding causes, and it is important to not attribute the supra-
and allow it to be effectively targeted by conventional radiation additive effect observed here to a mechanism that cannot be directly
therapy. Whereas microscopic regions of hypoxia could still exist, the proven. We hypothesize that a major mechanism causing this effect is
overall average oxygenation is sufficiently high that the beneficial the shift in tumor oxygenation toward higher pO2 and possibly spa-
effect of the oxygen enhancement ratio should be apparent. Thus, tially to reduce the preexisting hypoxic fraction within the tumor. One
PDT-induced tumor reoxygenation could be used as a pretreatment to potentially confounding issue, which needs to be addressed, is the
radiation therapy, thereby enhancing the radiation killing effect. These potential for hyperthermia induced by the high fluence rate of 133
effects are quite time dependent, and therefore measurements of the mW/cm2. Whereas some authors have indicated that this can induce
tissue pO2 could be a useful guide for timing subsequent therapy with hyperthermia when delivered at 633 nm, our previous measurements
ionizing radiation. Ultimately, the tumor oxygenation falls back to (16) have shown that the temperature rise with a higher fluence rate of
hypoxic levels, indicative of something else happening at longer 200 mW/cm2 at 690 nm is 3– 4 degrees C maximally and thus does not
times, and this would correlate with the reduction in blood flow correspond to a hyperthermic rise, which requires 8 –10 degrees C
observed in our study at longer times after irradiation. Additional increase for a measurable effect on the tissue (51, 70). Nevertheless,
studies are needed to explore this effect and the potential for syner- it is possible that localized microscopic regions of the tumor experi-
gistic interaction with radiation therapy. ence hyperthermic rises in temperature. Nonetheless, there is a clear
Combined PDT and Radiation Therapy Induce a Greater than observation that combining PDT with radiation therapy simulta-
Additive Killing Effect. After the observation of increased tumor neously induces a greater than additive effect of tumor killing, and the
pO2 in response to PDT, it was concluded that there would likely be mechanism remains to be conclusively delineated.
a significant benefit of PDT and radiation therapy if used together in In summary, the cellular respiration of in vitro RIF-1 tumor cells
this model. The tumor regrowth delay data shown in Fig. 5 and can be significantly reduced in response to PDT treatment under the
analyzed in Tables 1 and 2 support this hypothesis. Whereas the tumor conditions examined here, and the dose response of this effect is
doubling times increased from 5.6 0.4 days in the control group to identical to the results of a cell survival (MTS) assay. These results in
8.3 1.7 and 8.9 1.7 days, respectively, for the radiation and PDT vitro support the conclusion that this loss in cell metabolism is directly
alone groups, the combined treatment times illustrated that the order proportional to the loss of cell viability. The decrease in metabolism
of the radiation versus PDT delivery matters significantly. A doubling likely leads to a greater diffusion of oxygen throughout the remaining
time of 11.0 1.5 days was observed for the delivery of radiation tissue in vivo, when the blood flow remains patent immediately after
first, followed by PDT, and an increase to 13.7 1.6 days was treatment. Experiments confirm that an increase in tumor tissue pO2
observed when PDT was used as the pretreatment adjuvant for the occurs after treatment and that the tumor tissue actually can rise above
radiation. Using the tumor regrowth delay (last column of Table 1) as the radiobiologically hypoxic level. These observations, taken to-
the measure of damage, this indicates that group 4 treatment (when gether, indicate a new direction and potentially a new set of applica-
radiation is given before PDT) induces an additive effect of tions for PDT in modulating the oxygenation of tissue.
the radiation alone (i.e., group 2, delay 2.7 days) and PDT alone Combining this type of PDT treatment with radiation treatment
(group 3, delay 3.2 days) where given separately, because using a single 10-Gy dose of radiation demonstrated a greater than
2.7 3.2 5.9 1.9 days. This additive delay is in good agreement additive effect of tumor killing, as measured by tumor volume re-
with the observed effect of group 4 treatment at 5.4 1.4 days. In the growth delay assay for the tumor doubling time. The mechanism for
last group (group 5), when PDT and radiation were delivered together, this synergistic effect may be due to the enhanced oxygen available
the regrowth delay was 8.1 1.5 days, indicating an effect that is during the treatment, effectively reducing the fraction of the tumor
more than additive (68, 69). Student’s t test comparisons were com- that is chronically hypoxic. Calculations of the oxygen change are in
pleted between groups 2 and 3 and between group 5 and all others to reasonable agreement with experimental observations. Additional ex-
determine whether there were significantly differences (P 0.05), periments are required to help verify that this change in hypoxic
with the results shown in Table 2. The significant difference between fraction is the cause of the enhanced killing effect.
groups 4 and 5 indicates that the mechanism underlying the effect may
be different or that some feature of the treatment was more dominant
in the latter case.
These results indicate that there is a significant effect of vertepor- B. W. P. acknowledges thoughtful discussions and use of resources with
fin-based PDT when used as a pretreatment adjuvant to radiation Brian C. Wilson (University of Toronto) and P. Jack Hoopes (Dartmouth
therapy when given as a single 10-Gy dose to the RIF-1 tumor. The Medical School) in the course of this work. Verteporfin was supplied by QLT
tumor regrowth delay is increased by a factor that is more than Phototherapeutics Inc.
additive, indicating a potential benefit for PDT when used as an
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