Although medicines that inhibit the growth of cancer cells are available, the prognosis for patients with solid tumours is generally still rather poor. The reason for this is that tumour cells develop resistance to therapy during treatment, and the tumour can continue to grow. However, tumours are vulnerable – if only more robust medicines were available. Prof. Dr. Lars Zender, senior physician and head of the Division of Translational Gastrointestinal Oncology at Tübingen University Hospital, and his team are working on the development of such drugs. The effect of one of the compounds identified for the treatment of hepatocellular carcinoma has already been proven in a preclinical trial involving mice, and is close to entering the clinical phase of development.
Prof. Lars Zender, physician and cancer specialist, is specifically interested in finding the Achilles' heel of tumours and eventually provoking their demise. Together with his team of 20 scientists and doctors in the Division of Translational Gastrointestinal Oncology at Tübingen University Hospital, the oncologist has been working on innovative, more robust therapeutic targets in tumour cells since 2012. The scientists are investigating a broad range of tumours, but are mainly interested in cancers of the liver cells, bile ducts and pancreas. Prof. Zender has been awarded numerous prizes for his achievements. The German Research Foundation recently awarded him the Leibniz Award worth 2.5 million euros, the most prestigious research prize in Germany.
In their search for new generation cancer drugs, Zender and his team attach great importance to tumour mouse models. Such models can realistically mimic the behaviour of human cancer cells and are used to perform functional genetic screens in vivo, i.e. in a three-dimensional environment. Professor Zender has long been involved in the development of such chimeras – mice with human cells. He developed the first models, which were the forerunner of the mouse models that are now developed and used in Tübingen, while he was a postdoctoral fellow at the Cold Spring Harbor Laboratory in New York. When he returned to Germany and became head of a junior research group at the Helmholtz Centre in Braunschweig and at the Hanover Medical School, he continued improving the model. Many years of research culminated in a transposon-based second-generation mouse model, which enables cells to be manipulated without having to remove them from their three-dimensional context of the living organism. "Our models are completely different from the artificial xenograft models that are widely used and that cannot provide reliable information about the effect a compound has in humans," Zender explains.
The researchers from Tübingen begin their search for more robust tumour cell targets with genetic screens using RNA interference (RNAi) technology, which makes it possible to switch off specific genes, either directly or in the living organism. The researchers use RNAi screens to look for genes that enable cancer cells to evade the initially successful effect of anti-cancer drugs such as sorafenib. Zender's team generates the ribonucleic acid molecules used for the screens as RNAi and shRNA libraries. "We have numerous themed libraries based on human data," said Zender. Gene candidates that are identified will be tested for their potential pharmaceutical effect in preclinical mouse models. The mouse models in Zender's laboratory are rather special: they are realistic models that can provide reliable information about the effect a compound has in humans. Explaining why realistic mouse models and in vivo tests are so important, Zender says: "95% of all new cancer therapies that enter the clinical stage of development fail due to the lack of more stringent success criteria during the non-clinical stages of the drug development process."
Zender, a senior physician who spends every day working in hospitals, is frustrated by the situation that has led to his research. "Unfortunately, the current situation is such that over the past 50 years we have not managed to significantly reduce cancer mortality. Although the prognosis is relatively good for some tumours, for example certain types of blood or lymph node cancer, the chance of survival is rather bad for patients with solid tumours. Patients who are diagnosed at an advanced stage of cancer cannot usually be cured, but are offered treatments that relieve symptoms and prolong life," says Zender. The oncologists from Tübingen therefore decided to pursue a completely new approach that does not involve looking for compounds that only target one specific signalling pathway in tumour cells. It is known that the effectiveness of drugs that block such pathways is relatively good, at least at the outset of treatment. However, tumour cells eventually find a detour in their search for signals that they need for growth. Resistance to the active substance develops and the drug loses its effect. "Based on this knowledge, we decided to use RNAi screens to look for central switches in the tumour that are prone to vulnerability. We know that tumours have such vulnerable sites, but we have not yet managed to identify the Achilles' heel of tumour cells," says Zender.
The researchers have now found what they were looking for. They have been able to show that the effect of sorafenib could be increased and extended when they blocked the gene that codes for the so-called p38α protein. Sorafenib is currently the only systemic medication approved for the treatment of hepatocellular carcinoma. At the same time, the researchers showed that this particular signalling pathway played a role in the development of sorafenib resistance, both in human tumour samples and in preclinical mouse models*. The research also showed that the effect of sorafenib, and with it the survival of the mice, was significantly extended if the newly discovered p38α gene and its gene product were inhibited. The same effect was also observed in hepatocellular carcinoma cell cultures.
"The target structure has been found, but our job is far from over," says Zender. "We want to test the new concept as quickly as possible in a clinical trial," says Zender. The researchers were in the fortunate situation to have access to a potential pharmaceutical compound as Tübingen is home to what is probably the largest academic compound library of protein kinase inhibitors. Prof. Dr. Stefan Laufer from the Institute of Pharmaceutical Sciences at the University of Tübingen has been working on this compound class for some years, and was in fact the researcher who established the protein kinase inhibitor collection. Fortunately, the library also contained the p38α inhibitor that the researchers plan to test in clinical studies in around a year's time at most. In addition, Tübingen has something else of great importance for drug development. "As soon as we find targets, we can use our own drug development pipeline with which we are able to quickly develop appropriate inhibitors. This work is done in cooperation with structural biologists, structure based drug design specialists and medical chemists," says the oncologist. "With all this knowledge available, we are able to cover the entire drug development process from validation of the targets to execution of clinical trials.
Before entering the clinical stage of drug development, the p38α inhibitor will have to undergo a few more tests and the compound needs to be produced under GMP quality standards. The researchers hope to be able to start the clinical trial in around 12 months' time at the most. Zender points out that the drug has already been shown to be well tolerated by patients. This information comes from a clinical phase 2 trial undertaken by Pfizer, as the protein has also been identified as a target structure in COPD. The researchers will of course continue looking for new targets for more robust cancer medications. And Zender hopes that he will also be able to expand the drug discovery pipeline of the Comprehensive Cancer Centre, which was established in Tübingen in 2007.
* Original publication:
In vivo RNAi screening identifies a mechanism of sorafenib resistance in liver cancer. Ramona Rudalska et al. Advance Online Publication (AOP) on Nature Medicine's website on 14 September doi:10.1038/nm.3679.