The large majority of compounds tested for anti-cancer medicine wash out in development.  This is one reason the pharmaceutical industry spends so much on finding new drugs and one reason new medicines cost too much.

Reasons for failure

When people speculate about why there aren’t more chemotherapy drugs, some ideas that come up are

1. Biology is complex, especially in mammals.  Every intervention (including pharmaceutical ones) has multiple effects.  Drugs that kill cancer cells may also kill healthy cells.
2. It is hard to slow or stop cancer growth, even if you think you have identified the biochemical pathway that drives the cancer, you may not be able to plug that pathway.  And even if you can plug the pathway, there are often other pathways the cancer can take that you didn’t know about.
3. The results published in scientific papers are not accurate.  This is not a problem unique to biochemistry or cancer biology.  Many results of scientific experiments are irreproducible, leading to any conclusions being suspect.  Sometimes other scientists show the results are wrong or the authors or publication issue a withdrawal.  This confusion makes it difficult to avoid going down the wrong path when looking for a new medicine.

Dead ends

Antisense drugs generated quite a bit of excitement in the early 21st Century, but that excitement has faded. A targeted therapy, antisense agents switch off specific genes. The idea is that by inserting an oligonucleotide that essentially blocks the gene transcription process, the disease cannot proceed. There was hope they would be used for genetic disorders, including cancer. And although some antisense therapies are used in treatment of some genetic diseases, none have been approved for cancer yet.

Stem cells are an area of research in rehabilitative therapy. Although they have generated excitement among scientists, few practical solutions have come out of the lab. A type of cell called Cancer Stem Cells (CSC) might be important in growth and metastasis of some types of cancers, but this idea has not been proven yet.

Some of the clinical trials have not focused on the disease of cancer but on the effect of new drugs on CSCs. Stem cells are a difficult enemy; scientists suspect combination therapy might be required in order to prevent crosstalk among signaling pathways.  Cancer stem cells use one of three biochemical pathways when they divide: the Notch pathway, the Hedgehog pathway, and the Wnt/beta-catenin pathway.  For drug developers, one approach to anti-neoplastic medicines is to focus on these pathways and find ways to disrupt them.