The parameters which limit supply of photosensitizer to the cancer cells in a solid tumor were systematically analyzed
using microvascular transport modeling and histology data from frozen sections. In particular the vascular permeability
transport coefficient and the effective interstitial diffusion coefficient were quantified for verteporfin-for-injection
delivery of benzoporphyrin derivative (BPD). Orthotopic tumors had a higher permeability and diffusion coefficients (P<sub>d</sub>= 0.036 &mgr;m/s and D = 1.6 &mgr;m<sup>2</sup>/s, respectively) as compared to subcutaneously grown tumors (P<sub>d</sub> = 0.025 &mgr;m/s and D = 0.9 &mgr;m<sup>2</sup>/s, respectively), likely due to the fact that the vessel patterns are more homogeneous orthotopically. In general,
large inter-subject and intra-tumor variability exist in the verteporfin concentration, in the range of 25% in plasma
concentration and in the range of 20% for tissue concentrations, predominantly due to these micro-regional variations in
transport. However, the average individual uptake of photosensitizer in tumor tissue was only correlated to the total
vascular area within the tumor (R2 = 64.1%, p < 0.001). The data is consistent with a view that micro-regional variation
in the vascular permeability, interstitial diffusion rate, all contributes the spatial heterogeneity observed in verteporfin
uptake, but that average supply to the tissue is limited by the total area of perfused blood vessels. This study presents a
method to systematically analyze microheterogeneity as well as possible methods to increase delivery and homogeneity
of photosensitizer within tumor tissue.
Effective Photodynamic therapy (PDT) treatment depends on the amount of active photosensitizer and the delivered light in the targeting tissue. For the same PDT treatment protocol, variation in photosensitizer uptake between animals induces variation in the treatment response between animals. This variation can be
compensated via control of delivered light dose through photodynamic dose escalation based on online dosimetry of photosensitizer in the animal. The subcutaneous MAT-LyLu Dunning prostate tumor model was used in this study. Photosensitizer BPD-MA uptake was quantified by multiple fluorescence micro-probe measurements at 3 hours after verteporfin administration. PDT irradiation was carried out after photosensitizer uptake measurement with a total light dose of 75 J/cm2 and a light dose rate of 50 mW/cm2. Therapeutic response of PDT treatments was evaluated by the tumor regrowth assay. Verteporfin uptake varied considerably among tumors (inter-tumor
variation 56% standard deviation) and within a tumor (largest intra-tumor variation 64%). An inverse correlation was found between mean photosensitizer intensity and PDT treatment effectiveness (R2 = 37.3%, p < 0.005). In order to compensate individual PDT treatments, photodynamic doses were calculated on an individual animal basis, by matching the light delivered to provide an equal photosensitizer dose multiplied by light dose. This was completed for the lower-quartile, mean and upper-quartile of the photosensitizer distribution. The coefficient of variance in the surviving fraction decreased from 24.9% in non-compensated PDT (NC-PDT) treatments to
16.0%, 14.0% and 15.9% in groups compensated to the lower-quartile (CL-PDT), the median (CM-PDT) and the upper-quartile (CU-PDT), respectively. In terms of treatment efficacy, the CL-PDT group was significantly less effective compared with NC-PDT, CM-PDT and CU-PDT treatments (p < 0.005). No significant difference in effectiveness was observed between NC-PDT, CM-PDT and CU-PDT. The results indicate that by measuring the mean photosensitizer concentration prior to light treatment, and then adjusting the light dose appropriately,
a more uniform treatment can be applied to different animals thereby reducing the inter-individual variation in the treatment outcome.
Protoporphyrin IX (PpIX) is produced via the heme synthesis pathway by the cell following administration of aminolevulinic acid (ALA). ALA synthase, the enzyme that produces ALA in the cell from glycine and succinyl-coenzyme A, is inhibited in a feedback mechanism by heme and thus is the rate limiting enzyme in the heme synthesis pathway. Since ALA is administered systemically, the rate limiting step that naturally exists in the cells is bypassed, however it is currently unclear why cells have different rate limiting steps in the ALA-PpIX synthesis pathway, and more specifically which types of cancer cells are most productive. It has been determined that when the same amount of ALA is administered to a wide panel of cancer cells in vitro that vastly differing amounts of PpIX are produced. The steps for the ALA-PpIX pathway occur in and around the mitochondria of the cell, but interestingly no correlation is seen between PpIX production and mitochondrial content of the cell, following ALA administration. However, total cell area shows positive correlation with PpIX production. Administration of the iron chelator, 1,2-dimethyl-3-hydroxy-4-pyridone (L1) in combination with ALA allows the final step in the heme synthesis pathway, conversion of PpIX to heme, to be delayed and thus increases the detectable amount of PpIX in each cell line. The cell lines that have the lowest PpIX production following administration of ALA alone show the largest increase in production following the combined administration of ALA and L1. PpIX fluorescence is thought to be a measure of cellular activity and the goal of the current study was to determine which cell lines would be the most promising targets for fluorescence detection or monitoring response to therapy. The results indicate that the cells with larger size and larger numbers of mitochondria may be good potential targets for this therapy. While this conclusion may appear obvious, it is not universally true, and cellular specific variations exist which are still not fully understood.
The effect of sampling region size and tissue heterogeneity is examined using fluorescence histogram assessment in a rat prostate tumor model with benzoporphyrin derivative fluorophore. Spatial heterogeneity in the fluorescence signal occurs on both macroscopic and microscopic scales. The periphery of the tumor is more fluorescent than the center. Fluorescence is also highest nearest the blood vessels immediately after injection, but over time this fluorescence becomes uniform through the tumor tissue. Using microscopy analysis, the fluorescence intensity histogram distributions follow a normal distribution, yet as the sampling area is increased from the micron scale to the millimeter scale, the variance of the distribution decreases. The mean fluorescence intensity is accurately measured with a millimeter size scale, but this cannot provide accurate measurements of the microscopic variance of drug in tissue. Fiber probe measurements taken in vivo are used to confirm that the variance observed is smaller than would be expected with microscopic sampling, but that the average fluorescence can be measured with fibers. Sampling tissue with fibers smaller than the intercapillary spacing could provide a way to estimate the spatial variance more accurately. In summary, sampling fiber size affects the fluorescence intensities detected and use of multiple region microscopic sampling could provide better information about the distribution of values that occur.
The success of photodynamic therapy with verteporfin is partially determined by the pharmacokinetic distribution of the sensitizer at the time of treatment. In this study tumor blood flow changes in the RIF-1 murine tumor model and tumor resopnse using the regrowth assay were measured, comparing two different intervals between drug and light administration. Blood flow measurements were taken with a laser Doppler system monitoring continuously over 1 hour and periodically up to 6 hours after treatment. Treatment after the longer interval caused significantly less flow decrease, to only 50% of the initial flow in 6 h. Hoechst staining of functional tumor vasculature confirmed the primary vascular damage and the decrease in tumor perfusion. The regrowth rate of tumors after the longer time interval, the regrowth rate was not signifincalty different from that of the control, indicating that only the 15-min interval group caused serious damage to the vascular bed of the tumor. These studies support the hypothesis that temporal pharmacokinetic changes in the photosensitizer distribution between the tumor parenchyma and blood vessels can significantly alter the mechanism of tumor targeting during therapy.
The combination of verteporfin-based photodynamic therapy (PDT) wiht radiaiton therapy from an orthovoltage device has been examiend in the radiation induced fibrosarcoma tumor model. PDT with verteporfin using a 3 hour delay between injection and the time of optical irradiation has been shown to cause a significant rise in overlal tumor oxygenation. It was huypothesized that this mechanism arises from the reduced oxygen consumption from cells where the PDT has targeted the mitochondria and shut down cellular respiration. Tumor blood flow was measured and found to be still be patent immediately following therapy. This increasing oxygenation was thought to provide an opportunity to increase the radiation sensitivity of the tumor immediately following PDT. When this type of treatment was combined with radiation therapy, a delay in the tumor regrowth time demonstrated that the combined effect was greater than additive. Further study of this phenomenon will provide a more complete mechanistic understanding of the effect and possibly provide a viable pre-treatment for radiation therapy of tumore that increases the therapeutic ratio. This effect could be used to either increase the radiaton dose without increasing the side effects or decrease the dose needed for the same effect on the tumor.