Others have attempted to grow functioning liver tissues on scaffolds. But, says Khetani, this approach lets the cells do their own organizing, so the architecture of the resulting models is different every time. Bhatia and Khetani, by contrast, precisely specify the organization of the cells in their model, giving them tighter control over functionality.
To verify that their micropatterned liver cells actually behave like hepatocytes in the human body, Bhatia and Khetani put them through a series of rigorous tests. They analyzed the cells’ gene-expression profile and measured the amount of drug-metabolizing enzymes they produced. They exposed the cells to a battery of substances known to be either benign or toxic to the human liver, from caffeine to cadmium. To test the toxicity of a drug, Khetani creates a solution of the desired concentration and pipettes it into a set of wells, where it’s incubated with the liver tissues. Then he looks for changes in hepatocyte function or cell death.
Drug companies could, says Bhatia, use this assay to compare several chemically similar compounds and eliminate toxic ones early in the drug development process. “If I were at a drug company,”she asks, “and my medicinal chemists gave me four compounds, could I have picked which one would have been the most toxic using my assay?”The answer seems to be yes. She and Khetani have compared chemically similar drugs known to be benign or toxic to the liver and confirmed that the new assay can measure differences in toxicity.
Strengths and Limitations
Bhatia’s assay is good at detecting drugs toxic to the general population, but it may not uncover drugs with adverse effects on only a small number of people. It might not, that is, have detected the trouble with Rezulin, a diabetes drug that caused liver damage in many patients and which the U.S. Food and Drug Administration ordered off the market in 2000. The liver cells in the assays do not represent a wide enough sample of the population to predict such effects, though it’s theoretically possible to test a drug on cells from thousands of different livers.
The assay is unique in being able to test drugs for chronic toxicity, which is caused by low-level repeat exposure, “which is actually the way we take our drugs clinically,” says Bhatia – one pill a day. Bhatia’s model could be used to test the effects of a drug over four to six weeks. Existing models simply cannot detect chronic effects because liver cells die so quickly in culture. Bhatia says that pharmaceutical companies know that potential drugs that become toxic only over time are slipping through the cracks, but the FDA does not require chronic toxicity tests. “We’re in this kind of funny position where we’ve developed a really powerful tool and have to convince people to use it.”
The miniature tissues can also be used to detect acute toxicity, which has much more immediate effects. Acute toxicity can be studied using an existing method, with simple cultures of hepatocytes that die within a week. But Bhatia believes that her assay will be more efficient: because it uses wells, it requires a lower volume of drugs, and the micropatterning means fewer hepatocytes are required.
Bhatia is developing her test for commercialization, and several pharmaceutical companies are interested. She and Khetani are also looking into other uses for the assay – for example, studying interactions between drugs and how liver cells transport drugs. “My hope is that the assay would make drugs safer, better labeled, and would help ensure that toxic drugs never reach patients,”says Bhatia.