Physical and theoretical models of anatomical and physiological systems are being used in our laboratory to study a variety of phenomena such as the transport of drugs into the eye and the thermal ablation of tumors. The following three projects are reported for this year: ? ? (1) Magnetic Resonance Imaging (MRI) of Drug Movement in the Eye: A number of inflammatory and neoplastic diseases of the eye are currently treated by repeated intravitreal drug injection. We are developing sustained drug release devices for both intravitreal and subconjunctival implantation that could release drugs for periods as long as months. We are also testing the use of a thermosetting polymer, ReGel, into the subconjunctival space of rats and rabbits as an injectable agent for long term release of drugs. These devices would eliminate the need for frequent invasive intervention. A number of different drugs and device configurations are being evaluated in vitro and in vivo. Understanding the mechanism of drug transport within the eye is crucial to optimal delivery of agents with the implant devices. In one project, we are using MRI as a non-invasive means to track the movement of several MRI-enhancers, such as Gd-DPTA and Gd-albumin. The delivery of these agents is by slow continuous infusion either into the subconjunctiva or intra-scleral space of rabbits. Finite element mathematical models, which will incorporate the physico-chemical properties of the drug, and the physiology of the eye will be investigated. These models are useful in guiding the design of the drug release devices for optimal therapy.? ? (2) To study the feasibility of using an injectable thermosetting gel that may release drugs into the eye over sustained periods of days or weeks, we are using florescence imaging to map and to quantitate the concentration of marker agents, like Alexaflor dye and fluorescently labeled ovalbumin, from microtomed rat eye tissue sections. Rabbits are injected with the dye and sacrificed at serial time points for tissue sampling. Image processing tools are employed to quantitate image intensity with marker concentration.? ? (3) Radio-frequency Ablation of Solid Tumors: We have developed an in vitro method of simulating thermal ablation of tumor tissue using chicken egg whites whose protein coagulates at approximately 60 degrees C. Our initial experiments used video recording of the egg protein coagulation and demonstrated the temporal and spatial pattern of tissue 'death' which coincides with the protein coagulation. Several clinical thermal ablation probes were studied. These experimental ablation patterns have been corroborated by heat transfer mathematical models. We have extended these studies by using polyacrylimide gels as tissue simulants and we have measured temperature profiles during RF ablation by means of thermocouples and infra-red thermography. Our experiments help us to gain a better understanding of the effect of cooling by adjacent blood vessels on the thermal burn patterns and to predict temperature profiles from a variety of commercially available thermal probes. In addition to the RF ablation, in vitro studies are underway that investigate the use of high frequency focused ultrasound (HIFU) as a means of thermal killing of tumors and also for enhancing the uptake of chemotherapeutic drugs selectively into tumor cells.
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