Scientists at Cold Spring Harbor Laboratory (CSHL) have discovered that many anti-cancer drugs do not work the way we think they work. The study, led by Jason M. Sheltzer, evaluated 10 anti-cancer drugs in the pre-clinical or clinical trials. Findings of this research may help in establishing more stringent tests for drugs before trying them on humans.
Sheltzer’s earlier investigations of a protein, MELK, paved way to this study. Several reports had suggested that MELK was essential for the survival of cancer cells. But Sheltzer found otherwise – cancer cells survived even in the absence of MELK. They also found that an anti-cancer drug (OTS167), which the clinicians claimed would target MELK, killed cancer cells that were depleted of MELK. Clearly, OTS167 did not target MELK to kill the cancer cells.
Sheltzer adopted this strategy to study more anti-cancer drugs. The team shortlisted 10 anti-cancer drugs that were in pre-clinical or clinical testing. They selected drugs that clinicians claimed would stop the proliferation of cancer cells by targeting a single protein. The researchers then deleted the gene that coded for the claimed target protein using the CRISPR-Cas9 technology and checked if the cancer cells survived. But cancer cells were killed. This suggested that the drugs were not targeting these proteins.
The researchers particularly studied a drug (OTS964) targeting the protein PBK in greater detail. They gave enough time for cancer cells to accumulate mutations and develop resistance to OTS964 to find what protein the drug targeted. Cancer cells are genetically unstable and accumulate mutations over time. These mutations, resulting in changes in the protein they code for, restrain the drug from binding to the target protein. Knowing the genes that are mutated help us understand the actual target proteins. On analysis, they found that the gene coding for CDK11 had several mutations in it. This indicated that CDK11 was the actual target of this drug.
This may be one reason why 97 per cent of cancer drugs tested in clinical trials do not go on to receive US FDA approval. Understanding the accurate ‘mode of action’ of a drug increases the chances of its success. It is possible that the earlier experimental methods of inhibiting a gene or protein – RNAi and small molecule inhibitors – could be blocking some other protein in the cell. This may well the reason why drugs work differently than how we thought they would.
With advanced technologies like CRISPR-Cas9 technology, we can be more accurate. As the current study showed, this can be used as a method to test the claimed “mode of action” of the drug before they are tested on humans. This study also gives us a method to understand what proteins or genes are essential for the survival of cancer cells.
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