Doxorubicin (Dox) is one of the most widely used chemotherapeutic agents, but its effectiveness is severely limited by cumulative, irreversible cardiac toxicity. The mechanism by which this drug damages cardiac tissue is unknown, but is thought to involve generation of oxygen free radicals. The goal of this project is to explore the potential of gene transfer to modify cardiac resistance to Dox. First, the gene encoding multiple drug resistance (mdr1), a protein known to confer high resistance to Dox, will be directed to express in hearts of transgenic mice and rats by linkage to the promoter/enhancer complexes of rat alpha or beta cardiac myosin. If, as expected, expression occurs in heart, the tolerance of the cardiac muscle to Dox will be determined. In related experiments, we will introduce the murine Cu/Zn superoxide dismutase (SOD) gene into transgenic mice and rats. This enzyme plays a key role in oxygen free radical scavenging reduces sensitivity to Dox. This experiment should allow us to determine if Dox toxicity involves free radical damage. In another approach to the same problem, we will transfer the antisense gene for SOD into animals in order to reduce free radical scavenging and determine if such reductions increase Dox toxicity. The myosin promoters will first be tested for cardiac expression in mice using the reporter gene chloramphenicol acetyl transferase (CAT), and then the genes for mdr1 and SOD. When constructs with high and specific cardiac expression are identified, transgenic rats will be produced. We have chosen to produce rats because the battery of tests for cardiac function available in this species are far more informative than those applicable to the smaller mouse heart. Because of the high resistance to Dox conferred by mdr1, expression of this protein specifically in cardiac tissue would constitute a model in which cardiac toxicity of Dox was eliminated and thus, in which unlimited courses of this drug could be used to treat malignancies in other sites. These animals would not only allow completely novel Dox therapy protocols, but they could possibly be used to reveal irreversible toxicity of Dox in other organs. Studies with SOD complement those with mdr1. These experiments provide an alternative strategy to engineering of cardiac resistance to Dox, while at the same time allowing an exploration of the mechanism of Dox toxicity and the role of SOD in ameliorating such toxicity.
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