Explainer | Dostarlimab for cancer cure: Hope or hype?

Cancer cells move and divide rapidly. (Image by the US National Cancer Institute via Unsplash)

Dostarlimab, an immunotherapy drug from GlaxoSmithKline, is dominating headlines internationally for making rectal cancer ‘vanish’ in a small group of patients in New York and is being touted as a ‘miracle therapy’ for cancer.

The drug was given to 12 patients with rectal cancer, all of whom went into remission, in a small study at the Memorial Sloan Kettering Cancer Center in New York, as part of a phase 2 clinical trial.

Nine doses of the drug, required over a six-month period, cost a whopping $99,000 (approximately Rs 77 lakh).

Reports say that after the course of treatment, cancer was undetectable on physical examination, endoscopy, positron-emission-tomography (PET) and MRI scans of every patient and the participants did not need any other treatment for up to a year, on average.

Also, there were no side effects severe enough to curtail day-to-day activities, according to the researchers, some of whom claimed that such results had not been seen in the history of cancer treatment ever.

How does the drug work?

Dostarlimab, marketed by GlaxoSmithKline, is a type of monoclonal antibody that blocks proteins called checkpoints which are made up of immune system cells, such as T cells, and some cancer cells.

These checkpoints help keep immune responses from acting too strong and may prevent T cells from killing cancer cells. When these checkpoints are blocked, T cells are free to kill cancer cells more efficiently.

Examples of checkpoint proteins found on T cells or cancer cells include PD-1, PD-L1, CTLA-4 and B7-1. Some immune checkpoint inhibitors, called PD-1 inhibitors, are already used to treat various types of cancers.

Will the drug work against all cancers?

That's where the catch lies. According to oncologist and cancer researcher Dr Aju Mathew, who is associated with the Ernakulam Cancer Centre in Kerala, drugs based around the fundamentals of PD-1 inhibitors, like dostarlimab, can be used only in patients with the genetic property of mismatch repair (MMR) deficiency.

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These patients are identified based on genetic or cytology tests, and have cells with mutations or changes in certain genes involved in correcting mistakes made when DNA is copied in a cell.

MMR deficient cells normally have many DNA mutations, which may lead to cancer but estimates indicate that less than 10 percent of colon cancer patients may have MMR deficiency.

But while MMR deficiency is most common in colorectal cancer, other types of gastrointestinal cancer, and endometrial cancer, it may also be seen in patients with breast, thyroid, prostate and bladder cancer.

Why are the findings from the dostarlimab trial remarkable?

The findings of the small clinical trial, published in the New England Journal of Medicine and shared in a representation during a recent meeting of the American Society of Clinical Oncology, said that the drug worked like a wonder in mismatch repair-deficient stage II or III rectal cancer patients without metastasis.

Dr Nitesh Rohatgi, senior director, medical oncology, Fortis Memorial Research Institute in Gurugram, pointed out that this was a theory put to the test where patients who were yet to undergo surgery for colon cancer and had something called microsatellite instability or the condition of genetic predisposition to mutation that results from impaired DNA MMR.

During this research, dostarlimab was administered every three weeks for six months in patients and was planned to be followed by standard chemoradiotherapy and surgery.

But the 12 patients who completed the course did not need any follow-up treatment during the study period (further follow-up is still on).

Dr Parveen Jain, head of the oncology department at Aakash Healthcare in Delhi, said that immunotherapy agents, such as dostarlimab, are better tolerated than conventional chemotherapy.

“This is the first time an immunotherapy agent — and in fact any anti-cancer treatment — has achieved a 100 percent complete remission rate,” he said.

Is it really the miracle cancer drug everyone has been hoping for?

Mathew says that a number of his patients have flooded him with queries over the last few days asking if the drug, which is being projected as a wonder cure for cancer, is going to be available in India soon and may be suitable for them.

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“I am happy about the promising results from a research trial but we need to understand that it is a very small trial and has worked in patients with a very specific type of cancer,” he told Moneycontrol.

Both Mathew and Rohatgi said that they have been using immunotherapy to treat many patients but in specified types of cancer.

Dr CS Pramesh, director of the Tata Memorial Centre in Mumbai, pointed out on Twitter that to call dostarlimab a ‘miracle cure’ that would impact cancer treatment globally would be “premature and fanciful”.

While the study findings may be exciting, he wrote, a much higher number of patients need to be treated, followed up and the results evaluated from a large phase 3 randomised trial before the drug could be called “practice-changing”.

Rohtagi said that for now, the only implication of the new research findings may be that patients with colon cancer and microsatellite instability can undergo immunotherapy before surgery.

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Dr Sewanti Limaye, director, precision and medical oncology at Sir HN Reliance Foundation hospital in Mumbai, pointed out that the message from the trial may be to carry out genomic analysis, understand the biology of the cancer and figure out specific treatment for a patient’s specific problem.

“Precision is the way forward,” she said.

Jain noted that while the new findings may be thought-provoking, the trial needed to be replicated on a larger scale.