In today’s drug development process, hepatocytes or liver cells are used in toxicity screening and other tests. The supply of primary human is limited, and thus such cells are expensive. Moreover, the drug-metabolizing activity of the liver varies greatly by ethnicity and individual. We are working to develop a method of inducing differentiation of human iPS (induced pluripotent stem) cell-derived hepatocytes that are functionally equivalent to primary human hepatocytes, aimed at the actualization of drug development and medicinal treatment optimized to each ethnicity and individual.
The liver is the principal organ in charge of metabolizing chemicals. Hepatic toxicity is one of the major reasons for suspending development or withdrawing a drug from the market. Therefore, many pharmaceutical companies try to identify potentially hepatotoxic drug candidates by conducting toxicity screening using primary human hepatocytes. However, primary human hepatocytes have low proliferation potency, differ by lot (and it is difficult to obtain the same lot), and are very expensive. It is thus difficult to use primary human hepatocytes for large-scale toxicity screening which requires a large amount of uniform cells. We believe that it would be a very significant tool if we could prepare pluripotent, proliferating hepatocytes that have similar functions to primary human hepatocytes from human iPS cells.
We have conducted two studies with the aim of preparing human iPS cell-derived hepatocytes to contribute to acceleration of the drug development process. The first aimed to establish an efficient method of inducing differentiation into hepatocytes that express equal levels of drug-metabolizing enzymes as primary human hepatocytes. The technology available when we started the study allowed only 10 to 30% of human iPS cells to be differentiated into hepatocytes. The level of drug-metabolizing enzyme expression of the induced hepatocytes was also low. We developed a novel method of inducing that involves introducing FOXA2 and HNF1α genes, which play important roles in liver development, into cells under differentiation and culturing the cells in three-dimensional conditions using equipment called a Nanopillar plate. This resulted in a differentiation efficiency of at least 80%; furthermore, the induced hepatocytes had a level of drug-metabolizing enzyme expression equivalent to primary human hepatocytes. The second study aimed to establish a mass cultivation method of cells to secure the necessary number of cells for toxicity screening. Like primary human hepatocytes, terminally differentiated human iPS cell-derived hepatocytes have low proliferation potency. Therefore, we decided to develop a technology for reproducing progenitor cells (hepatic progenitor cells) from iPS cell-derived hepatocytes with a high proliferation potency and can differentiate into hepatocytes in a short period of time. By culturing progenitor cells on laminin 111, an extracellular matrix protein, we successfully reproduced the cells while maintaining the capabilities of hepatic progenitor cells. With these two technologies, we have established the technical basis for supplying a large amount of iPS cell-derived hepatocytes at a low cost.
On the other hand, there are still issues to be solved. The prepared iPS cell-derived hepatocytes have phenotypes similar to fetal hepatocytes rather than adult hepatocytes. Additionally, the induced cell population contains cells other than hepatocytes, and is thus heterogeneous. Although the non-hepatocyte cells are few in number, improving the purity is another important goal . We are planning to raise liver maturity by culturing human iPS cell-derived hepatocytes together with various kinds of cells and to develop an advanced method for purifying human iPS cell-derived hepatocytes by using genome editing and other methods.
We also believe that we can actualize personalized medicine by using human iPS cell-derived hepatocytes. In conventional studies that used primary human hepatocytes, it was difficult to develop a drug or perform pharmacological treatment optimized for each individual or group (optimal selection of drug and dosage). However, toxicity testing and metabolic testing using human iPS cell-derived hepatocytes established from diverse individuals and groups are expected to lead to safer and more efficient medicinal treatment than today’s treatments. Actually, we have prepared iPS cell-derived hepatocytes from 12 separate donors and confirmed that the cells strongly reflect the characteristics of the liver cells of the donor, an important preliminary step towards actualizing personalized medicine.
I am also considering human iPS cell-derived hepatocytes as a supply source for liver and hepatocyte transplantation for patients with chronic hepatic insufficiency, acute hepatic insufficiency, or hereditary hepatopathy. I hope to efficiently and speedily accelerate studies to improve the drug development process and pharmacological treatments with these cells and make it possible to heal severe hepatic disorders currently requiring transplantation.