Police are reinvestigating the death of an Oxford graduate who collapsed after being injected by her sister with an experimental drug that they hoped would reverse ageing.

Yolanda Cox, 22, suffered a massive allergic reaction after being given three times the normal dose as part of a test of the unlicensed drug invented by their mother. Mrs Cox had been married for just nine months when she agreed to be a guinea pig for the drug, which was administered by her elder sister, Dr Yvonne Pambakian, a GP.

Both sisters worked for the company set up by their mother, Dr Apri Matossian-Rogers, which has spent more than £3 million developing the drug, known as B71. Details of the extraordinary background to the tragedy were revealed during an inquest into Mrs Cox’s death this week.

St Pancras Coroner’s Court heard that Dr Pambakian, 38, had been arrested on suspicion of manslaughter and that detectives were reviewing the investigation into her sister’s death. Mrs Cox collapsed after receiving an injection of the drug in the living room of the family home in Hampstead, North London, in June 2007.

Hospital doctors and paramedics described feeling uneasy after her family resisted requests to give them information about the drug even when she was in a critical condition in intensive care, the inquest heard. Ambulance staff were the first to contact police with their concerns.

Dr Alexander Mackay, of the Royal Free Hospital, said: “The family were extremely reluctant to go into detail about the drug. I got the impression it was an experimental drug.

“They wouldn’t say what was in it, and said I didn’t need to know anything and the drug was extremely safe. I said Yolanda is extremely unwell and I asked for a sample of the drug. Some time later the family brought in paper information in two files.”

Mrs Cox’s husband, Patrick, 24, said his wife had initially complained about itchiness in her arm after the injection.

“Two minutes later she was inside sitting on the sofa and she was struggling for breath,” said Mr Cox, who has a masters degree in biochemistry from Oxford University, “we called the ambulance. She was treated with oxygen and taken to hospital. I think her heart stopped beating for a long time.” Mr Cox described his wife as “a very faithful woman, who knew the gods believe in her”.

Hospital tests carried out against her relatives’ wishes revealed that Mrs Cox’s brain was irreversibly damaged and four days later her life support machine was turned off, despite threats of legal action from her family.

Mrs Cox’s mother, who qualified as a pathologist, had spent more than a decade attempting to perfect the drug, which was being developed by the family’s pharmaceutical company, Amro Biotech.

She had developed the drug during her research into insulin.

Dr Pambakian told the inquest that there was “always the element of the unknown” when giving a patient a drug, as “you don’t know how they will respond”.

The GP said she had previously injected herself and her mother. The inquest heard that the deeply religious family would pray before each injection and that Dr Pambakian did not know the exact dose because they were not measured.

“Yolanda knew all about this product,” said Dr Pambakian. “She had grown up with it. She was working for the company. She asked for the drug. Believe it or not, we do not like taking medicines so we really believed this to be safe.”

She said that the high dose was given to achieve better results and rejected the suggestion that they were all taking part in a drugs trial.

A terminal cancer patient had also been given the drug, the inquest heard. Catherine Clayton, who had been given eight weeks to live, survived for a year but died after being prevented from taking the drug after Mrs Cox’s death.

After her sister’s death, Dr Pambakian was suspended from practising for more than a year by the General Medical Council and is currently banned from prescribing drugs while an investigation is completed into her conduct.

The drug was manufactured in Switzerland and sent to Britain as a freeze-dried compound, which Dr Matossian-Rogers mixed up in the kitchen with water and aluminium hydroxide.

The Medicines and Healthcare products Regulatory Authority ordered the quarantining of all supplies of the drug. A spokeswoman said: “We are looking further into this matter and seeking legal advice.”

Dr Andrew Reid, the Coroner, recorded a verdict of misadventure after accepting that the death was an unintentional result of the drug being administered.

He ruled out unlawful killing because the standard of proof required — that it was beyond reasonable doubt — could not be met, but suggested that the verdict could be found in a civil case.

Detective Sergeant Mark Sayer, who led the investigation into Mrs Cox’s death, said after the inquest: “Based on the finding of the coroner, I’ll be taking further advice and the matter will be further reviewed with the Crown Prosecution Service.”

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