Showing papers by "Thomas H. Foster published in 1994"

Oxygen diffusion and reaction kinetics in the photodynamic therapy of multicell tumour spheroids

Abstract: Effects of oxygen diffusion and reaction kinetics in photodynamic therapy are considered in the context of a multicell tumour spheroid model. Steady-state measurements of oxygen made with a Clark-style microelectrode (4 mu m diameter tip) enable the authors to determine the rate of metabolic oxygen consumption and the oxygen diffusion coefficient in 500 mu m diameter EMT6/Ro spheroids. These values are 5.77 mu mol 1-1 s-1 and 1460 mu m2 s-1, respectively. Time-dependent electrode measurements of oxygen concentration during laser irradiation of individual Photofrin-sensitized spheroids are fitted to numerical solutions of a pair of diffusion-with-reaction equations. The analysis yields the rate of photodynamic oxygen consumption and a parameter that governs the oxygen sensitivity of photodynamic therapy. These experimentally derived quantities are used to calculate the temporal and spatial distributions of oxygen and the rate of oxygen consumption in a spheroid during irradiation at several fluence rates. The spatial distribution of photodynamic oxygen consumption is strongly fluence rate dependent. Using the experimental and theoretical results developed in this report, previously published survival data are analysed. The analysis indicates that the threshold dose of reacting singlet oxygen in the EMT6/Ro spheroid is 323+or-38 mu mol 1-1 (mean+or-SEM).

Effects of photodynamic therapy on xenografts of human mesothelioma and rat mammary carcinoma in nude mice

Abstract: We have examined the effectiveness of photodynamic therapy against R3230AC rat mammary adenocarcinoma and human mesothelioma as xenografts in the same host. The results demonstrate that the xenografted human tumour is significantly more responsive to photodynamic treatment than the rodent mammary tumour. Studies also showed that the mesothelioma xenograft was fluence rate- and fluence-dependent while the rat tumour exposed to the same conditions demonstrated neither of these dependencies. This disparity in response was not attributable to a difference in either whole-tumour uptake or subcellular distribution of the porphyrin photosensitiser. Analysis of the effects of visible irradiation on cytochrome c oxidase activity, measured in mitochondria prepared from tumours borne on hosts injected with photosensitiser, demonstrated that photoradiation-induced enzyme inhibition was significantly greater in mesothelioma than in R3230AC mammary tumour preparations. However, in parallel studies conducted in vitro, when photosensitiser and light were delivered to previously unperturbed mitochondria, rates of enzyme inhibition were not significantly different. Both tumours were established in long-term cell culture. While the uptake of porphyrin photosensitiser was equivalent in both cell lines, the R3230AC cells displayed a significantly greater photosensitivity than the mesothelioma cells. The data presented here demonstrate that the mechanisms that govern response to photodynamic therapy are complex, but in the case of these two xenografted tumours host response to therapy is not likely to play a significant role.

Quantitative MRI of Gd-DTPA uptake in tumors: response to photodynamic therapy.

Abstract: A partial saturation method is described for obtaining rapid images of tissue 1H spin-lattice relaxation rates following administration of the paramagnetic contrast agent gadolinium-diethylenetriaminepentaacetate. The paramagnetic contribution to the relaxation rates is proportional to the concentration of contrast agent, making possible quantitative studies of paramagnetic contrast agent uptake or vessel leakage. Snapshot imaging capabilities are not required. Maps of contrast agent uptake rates are made in rat borne tumors before and following photodynamic therapy, which is known to cause vascular damage. Uptake efficiency is spatially heterogeneous before and after therapy. Decreases in uptake rate are observed after two photo-irradiation protocols, which differ by a factor of four in fluence rate but deliver the same total fluence. There is no apparent fluence rate dependence for changes in the uptake rates within 5 h after therapy. Whole tumor measurements of nucleotide triphosphates, inorganic phosphate, pH, and lactate made with NMR spectroscopy indicate that, while net ATP production is inhibited, lactate concentrations are not strongly affected by photodynamic therapy. The ratio of nucleotide triphosphates to inorganic phosphate falls to 0.21 +/- 0.02 of initial values 5 h after tumors are treated with the lower fluence rate protocol, and falls to 0.40 +/- 0.06 in tumors treated with the higher fluence rate.

Microelectrode measurements of photochemical oxygen depletion in multicell tumor spheroids during photodynamic therapy

Abstract: We have previously reported results of experiments in which EMT6/Ro spheroids were subjected to Photofrin®-PDT consisting of a fixed incident fluence (60 J-cm2) delivered at 200, 50, and 25 mW-cm2. Surviving fractions from treated spheroids decreased as the incidentfluence rate was lowered over this range. We have interpreted these data using a model whereinthe cells comprising the surviving fractions are assumed to originate from within a therapy-induced anoxic volume resulting from Type-il photochemistry. In this paper, we demonstratedirect measurements of the phenomenon in individual photosensitized EMT6 spheroids. Steady-state measurements of O2 gradients in and around metabolizing spheroids allow determination ofthe O2 diffusion constant and the rate of metabolic O2 consumption within a spheroid. Time-dependent measurements obtained at a single spatial location during laser irradiation are fit tonumerical solutions of a pair of time-dependent diffusion with consumption equations. Fits allowa determination of the rate of PDT-induced O2 consumption. Based on these fits, it is possibleto calculate the spatial and temporal distributions of oxygen within a spheroid undergoing PDT.2. INTRODUCTIONFor several years, we and our collaborators have been seeking strategies to improvephotodynamic therapy (PDT) through modifications in the irradiation protocol. In particular, wehave explored the effects of irradiation fluence rate and of various fractionated irradiationschemes. Two transplantable rodent tumors, the R3230AC mammary carcinoma in the femaleFischer rat and the human mesothelioma xenograft in the nude mouse, show significantlydelayed post-treatment regrowth when a fixed dose of PDT is administered at a relativelyreduced incident fluence rate"2. Our interpretation of these data has been developed in terms ofa model of therapy-induced oxygen depletion. Model calculations show that photochemicaloxygen consumption during PDT can create regions of transient anoxia within which tumor cellswould be spared from singlet oxygen mediated damage. This can occur in tissue which, prior tothe onset of irradiation, was sufficiently well oxygenated to support formation of singlet oxygen.A summary of the experimental results and of the essential features of the theoretical model waspresented as part of these proceedings in 1992. In that report, we stated that, if our basicassumptions regarding oxygen consumption and diffusion were valid, it should be possible toreproduce a fluence-rate-dependent response to PDT in multicell tumor spheroids. Since then,we have in fact demonstrated that surviving fractions from EMT6 spheroids treated with