Contrary to popular belief, tumors are not merely a collection of cancerous cells they are a whole system of malignant cells, vasculature, and interstitial space. Chemical engineer Rakesh Jain has discovered three barriers to the delivery of therapeutic agents to their targets. First, the blood supply is nonuniform. Some cells have adequate blood supply while others do not. Second, the pressure of the interstitial space is elevated throughout the tumor, causing drugs to be pushed back into the blood vessels or washed out of the space. And third, because of the disruption of circulation, genetically engineered macromolecules must often travel large distances, taking days, weeks, or even months to reach isolated sections of the tumor.

Initially, Jain used two animal tumor models for his work. One model is an isolated tumor, pioneered by then-NIH pathologist Pietro Gullino. In this model, the tumor has a single artery and a single vein....

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