Micro-Livers Could Help in Testing for New Drugs

Researchers have designed a unique way to create tiny colonies of living human liver cells that model the full-sized organ. This research could allow better screening of new drugs that are potentially harmful to the liver and reduce the costs associated with their development.

Liver toxicity is one of the main reasons pharmaceutical companies take drugs off the market. These dangerous drugs slip through approval processes due in part to the inadequacies of liver toxicity tests. Existing tests rely on liver cells from laboratory rats, which do not always respond to toxins the way human cells do. Or they rely on dying human cells that survive for only a few days in the laboratory.

The new technology arranges human liver cells into tiny colonies only 500 micrometers (millionths of a meter) in diameter that act much like a real liver and survive for up to six weeks. Dr. Sangeeta Bhatia, associate professor in the Harvard University (Cambridge, MA, USA)-Massachusetts Institute of Technology (MIT; Cambridge, MA, USA) Division of Health Sciences and Technology (HST) and MIT's department of electrical engineering and computer science, and HST postdoctoral associate Dr. Salman Khetani described their model liver tissue and its behavior in the November 18, 2007, online issue of the journal Nature Biotechnology.

To construct these model livers, Dr. Khetani uses micropatterning technology--the same technology used to position little copper wires on computer chips--to precisely arrange human liver cells and other supporting cells on a plate. Dr. Khetani modified this method from Dr. Bhatia's previous work as an HST graduate student building micropatterned co-cultures of rat liver cells and supporting cells.

Such precisely arranged cells results in what Dr. Bhatia calls a "high-fidelity tissue model” because it so closely mimics the behavior of a human liver. For example, each model "organ” secretes the blood protein albumin, synthesizes urea, and produces the enzymes necessary to break down drugs and toxins.

To predict how close their model tissue is to real liver tissue, which has over 500 different functions, they also assessed its gene expression profiles, measures of the levels of gene activation in the tissues. They found that these profiles are very similar to those of fresh liver cells, "giving us confidence that other [liver] functions are preserved,” said Dr. Khetani. For drug testing purposes, this similarity to the human liver allows each colony to provide a window into the human liver's response to a drug without having to expose human patients to the drug in a clinical trial, according to Dr. Bhatia.

Furthermore, because the engineered tissue lives for so long, it has the potential to make new types of toxicity tests possible. For instance, it provides a way to evaluate the effects of long-term drug use similar to taking one pill a day over multiple weeks. It also will allow more extensive testing of drug-drug interactions. In addition to being a good biologic model, the engineered tissue is designed to be integrated into an industrial pharmaceutical science environment.

To mass-produce plates of the miniature liver models, Dr. Khetani used a technique called soft lithography. This technique constructs a reusable micropatterned rubber stencil from a silicon master. Each stencil contains an array of 24 wells, and each well contains a matrix of 37 tiny holes. Dr. Khetani "peels and sticks” the stencil onto plates and places the liver cells into the holes, patterning over 888 miniature model livers across the microwells in a matter of minutes.

In tests of drugs with a range of well-known toxicity levels, assays on the miniature liver models showed the expected levels of toxicity. "Our platform was able to predict the relative toxicity of these drugs as seen in the clinic,” stated Dr. Khetani. For instance, troglitazone, a drug withdrawn from the market by the U.S. Food and Drug Demonstration (FDA) due to liver toxicity, demonstrated toxicity levels much higher than its FDA-approved analogues, Rosiglitazone and Pioglitazone.

The model utilizes a portion of the expensive human liver cells used in other test platforms and can be assembled using frozen cells. Moreover, the expanded toxicity testing capabilities have the potential to allow drug developers to identify toxicity earlier in the development process, thereby avoiding the expense of investing in formulas that are bound to fail. A startup company called Hepregen has licensed the technology and is working to introduce it into the pharmaceutical marketplace.

"My hope is that this new model will make drugs safer, cheaper, and better labeled,” concluded Dr. Bhatia.


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