Last year, Mr Tan Teng Keong was on the losing end of a battle with cancer.

Just two months after Chinese New Year, after failing five different treatments for a rare form of lymphoma called natural killer T-cell lymphoma, his doctor said there was nothing else that could be done. Mr Tan was told he had just weeks left to live.

“I told my wife that it’s better to stop,” said the 52-year-old of his multiple treatments. “Although it wasn’t painful, it felt like you were trying for nothing.”

It was his wife, Mrs Tracey Tan, who persuaded him and his doctor to give it one final shot in May.

This last-ditch attempt, involving a drug not designed to fight this cancer, has kept Mr Tan alive for eight months and counting.

It is possibly the first time that the drug, known as pembrolizumab, has been used to treat this specific variant of blood cancer or lymphoma, said Dr Daryl Tan, who is Mr Tan’s doctor.

Pembrolizumab is typically used to treat lung cancer and a type of skin cancer. It is not, however, approved for use against the cancer that Mr Tan had.

“There was no evidence (that it would work), not even an anecdotal report,” recalled Dr Tan, who is a haematology specialist at Raffles Cancer Centre. “I told them that there could be hope, but to prepare for the worst anyway.”

Although lymphoma as a whole is among the top 10 most common cancers for both men and women here, the natural killer T-cell variant makes up only 5 per cent of all cases. If discovered early, its cure rate is between 30 and 40 per cent.

If detected late, as in Mr Tan’s case, it is nearly zero.

Dr Tan, who is also research director at Raffles Hospital, said he chose the drug because his understanding of the biology behind it led him to believe it might work.

Clinical trials ongoing at the time showed encouraging results when the drug was used against Hodgkin’s lymphoma–another variant of lymphoma that seemed to attack the body in similar ways.

At the point of administering the medication, Mr Tan was bedridden. “His immune system had started to attack his body,” said Dr Tan. “He was in very bad shape. He was going into multi-organ failure and the disease had spread to his bone marrow, liver and spleen.”

With nothing to lose, Mr Tan was given the medication – a single injection – before he was sent home to wait for death. But a week and a half later, he was able to walk. Two weeks later, a scan showed that all the cancer was gone.

“I was quite sceptical that it would work because of my previous experience with relapses,” Mr Tan recalled. “I thought it was just transient.”

Mr Tan now has to go for repeat injections every three weeks, each of which costs more than $10,000 and is covered by his insurance.

Dr Tan has worked with his counterparts in HongKong and South Korea to treat patients like Mr Tan with the drug. Eight people – including two more from Singapore – have been successfully treated so far.

The findings will be published in prestigious international journal Blood next month.

Dr Tan is also working with other specialists at healthcare institutions here – such as the National Cancer Centre Singapore (NCCS) – to dig deeper into the genetics behind this particular cancer.

Professor Lim Soon Thye, who heads the medical oncology division at NCCS, said that the type of lymphoma that Mr Tan had caused very high levels of a certain protein to be produced in the body.

“We want to understand, at the fundamental level, the processes that lead to an increase in this protein,” Prof Lim said. “And in the process, we could predict who might respond best to which drugs.”

Mr Tan, who could barely walk eight months ago, is now in remission and back to full-time work as an assistant general manager with an electronics company.

He told The Straits Times that he has always lived an active life and plans to return to windsurfing. “It seems like I’m quite back to my normal self,” Mr Tan said.