Few months back for the first time in medical history, in a small cohort trial consisting of rectal cancer patients a relatively new cancer drug dostarlimab showed positive results with 100% clinical response in all participants (cancer disappeared), giving hope to various cancer patients. Dostarlimab is an anti-PD-1 humanized monoclonal antibody drug, unheard till recently has become a household name over the past few months. It is an immunotherapy drug which belongs to a class of drugs called checkpoint inhibitors. Our body immunity helps us protect from various diseases. The main type of immune cell that does this job is the ‘T’ cell. These ‘T’ cells have proteins in them. One such protein function is to activate the immune response and another protein function is to deactivate the immune response when necessary. In cancer, cancer cells manufacture the proteins which deactivate the ‘T’ cells in high abundance. So, these proteins switch off the ‘T’ cells by pushing a stop button on the immune system. This allows the cancer cells to grow. Few drugs act to antagonise such proteins which inactivate the immune system. This activates the ‘T’ cells to find and attack the cancer cells and kill them. Such drugs are called immunotherapy drugs. Dostarlimab is an immunotherapy drug and a recent study by the Cancer Centre, New York, has shown its effectiveness against rectal cancer. This clinical trial was done in a small subgroup of rectal cancer patients who were mismatch repair deficient (MMR). Normally, the body has a variety of mechanisms to prevent a normal cell from becoming a cancer cell. When a cell divides, the DNA of the old cell and the new cell must be the same (identical). During cell division (DNA synthesis), there is a process by which the cell self-checks the DNA called proofreading. In this process, if there is any mismatch of the replicated DNA, the repair process begins and if the repair cannot be done the cell is killed – self-destruction. When this property is lost, they become mismatched repair-deficient cells. Such cells grow into cancer cells. This deficit can be identified by few chemical tests. It is in these types of MMR deficit cancers that this wonder drug is effective. It is not effective in all types of cancers. The ‘one-size-fits-all’ does not apply to this drug. I have many patients and attendants who walk into my out-patient and ask about this drug and if it can be used in them.
The interesting fact is that one of the patients enrolled for the study was of Indian origin. Not only that, in this study none of the participants reported any major side effects which was the most remarkable aspect to most of the medical professionals. Currently, these rectal tumours are treated with a combination of surgery, chemotherapy, radiation and immunotherapy. These treatments can be used alone or in combinations. Dostarlimab is not a new drug and was tried in treating uterine cancer patients before and was found to be effective. While the results of this study are truly remarkable but premature. It is important to note that this was a very small trial in a relatively rare type of rectal cancer.
In the recent time, immunotherapies have become important in cancer care. Many drugs like dostarlimab exist in the market. Although these are promising drugs, they have very specific indications. The cost of therapy is a limiting factor usually around 5 to 8 lakhs per cycle. This cost might limit access for a considerable number of patients. And an Indian study showed that only about 2 to 3 % of cancer patients in India can afford this kind of treatment.
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