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Try, try and try again: some personal reflections on the development of the anaesthetic propofol
Dr John (Iain) Glen BVMS PhD DVA FRCA MRCVS
Fellowship Day 2019
Report of presentation
The highs and lows of drug development were the theme of the keynote address at RCVS Fellowship Day. Speaker Dr Iain Glen qualified as a vet in 1963 and spent some years in academia before moving into industry in 1972, when he joined what was then ICI Pharmaceuticals and later became AstraZeneca. While at AstraZeneca, Dr Glen was intimately involved in the discovery and development of the anaesthetic propofol. In his keynote address, he described the process of drug development and the successful – and not so successful – steps that had ultimately resulted in the registration of what is now one of the most widely used anaesthetics in the world.
Having grown up on a farm on the Isle of Arran, Dr Glen said he had studied veterinary medicine at Glasgow envisaging that he would spend his career “driving round the countryside doing large animal practice”. However, after graduation, he had decided that he would like to remain in an academic environment. Unfortunately, the position he wanted – that of house surgeon in the surgery department – had already been filled. Instead, in 1963, he travelled to Kenya, where a team from Glasgow was setting up a degree conversion course for East African diplomates at the University of Nairobi.
In Kenya, he had been involved in an ambulatory clinic in the local Kikuyu Reserve, identifying suitable animals for the degree students to study. “Many of the cases were cattle, and the three common things were anaplasmosis, which was treatable; three-day sickness, which got better on its own – although they [the animal] would be very ill; and East Coast fever, which was invariably fatal.” He said that the animal owners were all very polite and recalled receiving a letter from the owner of a cow with East Coast fever, which read: “Dear Sir, I have the honour to inform you that the cow you treated yesterday has died.”
After a year in Kenya, the role of house surgeon at Glasgow became available, and he returned to the surgery department there. He developed an interest in sublingual mucocele in dogs which, at that time, required major surgery to treat. Using silography of the sublingual and mandibular salivary glands and ducts, he was able to show that simply excising the glands was effective. He registered to do a PhD on salivary gland conditions in the dog; many years later, after switching to anaesthesia and moving to ICI, he received a letter from the faculty in Glasgow, noting that he had not submitted his PhD and suggesting that, possibly, they could consider a change of title. “I was delighted that they were able to change it to ‘Studies of intravenous anaesthetics’,” he said.
While at Glasgow, he was one of the first candidates to achieve the RCVS Diploma in Veterinary Anaesthesia, and undertook all the anaesthesia teaching and research at the veterinary school. He developed an interest in some of the new drugs that were available for anaesthesia and in measuring and predicting various parameters, primarily in dogs, but also in horses.
“It was this interest in the research side looking at these new drugs that stimulated me to respond to an advertisement from ICI Pharmaceuticals looking for someone to come and do research in anaesthesia,” he said.
Moving to Alderley Park in Cheshire in 1972, he joined the anaesthetic project team to search for new anaesthetic agents and to evaluate new methods for studying anaesthetic drugs. ICI had a history in this area, having discovered halothane, and, initially, the team had looked for potential new inhalational agents. They did not find anything interesting enough to take forward, although Dr Glen admitted that he had examined sevoflurane as a possible “licensing-in compound”. While he could see that it was a very good compound in terms of its anaesthetic properties, he was concerned about its stability and the potential nephrotoxicity of its breakdown products. “The Americans came to the same conclusion,” he said, “but Maruishi in Japan took a gamble and got away with it!”
The focus had then turned to intravenous agents. Thiopentone had been the gold standard intravenous agent since the 1930s, primarily because it allowed a rapid, smooth onset of anaesthesia and lacked excitatory side effects. However, giving patients repeated injections over any length of time resulted in a prolonged recovery period, with rapid initial recovery being followed by postoperative drowsiness.
Any new drug would not only have to match the qualities of thiopentone but also add something extra. “The target that we were looking for was a drug that would mimic the quality of anaesthesia that thiopentone produced but would provide the opportunity for use by continuous infusion for maintenance of anaesthesia or sedation because it would be much more rapidly metabolised,” Dr Glen said.
He explained that a ‘Catch-22’ situation exists in developing new anaesthetics – for a drug to be able to cross the blood-brain barrier, it has to be lipophilic, which results in poor water solubility. But, for intravenous injection, aqueous solutions are needed. This meant that many compounds that may have had potential as anaesthetics were not tested because they could not be obtained in solution.
In 1966, the drug propanidid had been developed; while propanidid is insoluble in water, it had been formulated with the surfactant Cremphor EL to promote water solubility. The availability of Cremophor as a vehicle allowed Dr Glen’s team to consider drugs that were already in ICI’s compound collection but which had not previously been tested for their anaesthetic properties.
The team selected a number of lipophilic agents and dissolved them in Cremophor. Initially, the agents were evaluated for their median hypnotic dose (the dose that would produce loss of the righting reflex for a 30-second period in 50% of the mice on which the agent was tested) and for their median lethal dose: the ratio between these two doses gave an indication of the therapeutic ratio, or safety margin. “We wanted the widest margin between the two as possible,” Dr Glen said.
The first lead compound identified was 2,6 diethylphenol. A lead compound is one that fulfils many of the requirements but not all, and while 2,6 diethylphenol had anaesthetic properties, it was not very potent or particularly rapid in its action. However, it indicated a possible direction for further study and the team subsequently examined related compounds in ICI’s collection and synthesised new compounds themselves. “I think we had looked at 300 related alkylphenols by the end of this part of the process,” Dr Glen said.
Showing a photograph of a laboratory record from 23 May 1973, he explained that this was the first time the anaesthetic effects of 2,6 diisopropylphenol – now known as propofol – had been recorded. Of all the phenols the team had examined, 2,6 diisopropylphenol had the most optimal balance of qualities.
The potential of 2,6 diisopropylphenol was confirmed in rabbits as well as mice. Dr Glen explained that rabbits are remarkably sensitive to compounds that produce central excitation and many compounds that showed promise in mice would fail when tested in rabbits.
The team refined its methodology to ensure that equipotent doses of different compounds were used, allowing comparisons to be made. Comparison of propofol against thiopentone showed that mice given propofol recovered more quickly and exhibited less of a ‘hangover effect’ than those given thiopentone. The team also demonstrated that while thiopentone accumulated in the body (and sleeping time increased) with repeated dosing, propofol did not.
“The lack of accumulation potentially suggested that the drug would be suitable for administration to maintain anaesthesia by repeated injection or by infusion,” he said.
In the next phase of development, the effect of propofol on blood pressure was evaluated in pigs – the effects were similar to those of thiopentone and were judged to be acceptable. The safety of propofol was assessed in rats and pigs and, eventually, a package of animal safety, pharmaceutical and pharmacological data was submitted to the relevant authorities before human clinical trials could begin.
The opportunity then arose to carry out some clinical testing in Belgium. Propofol was evaluated in 22 patients; this confirmed its anaesthetic potential, but suggested that propofol was more potent than thought. Also, some patients experienced pain on injection, something that had not been seen in the animal testing. A struggling response had been recorded in rats, but it had been thought this was due to the restraint required for injection; however, later studies showed that giving fentanyl before propofol abolished the struggling response.
“Rat and man seem to have something in common,” Dr Glen said. “I believe they have some kinin precursor that is triggered by propofol and the cascade goes down and eventually produces a painful kinin that produces pain and seems to be very much species-specific.”
As a result of the findings in Belgium, Dr Glen’s team reduced the propofol concentration to 1 mg/kg and removed the alcohol component of the formulation. Studies continued and propofol was tested against althesin in a double-blind study. This indicated that the 1 mg/kg dose did not result in anaesthesia, so the dose-finding work was repeated, and concluded that the effective dose of propofol was close to 2 mg/kg.
It was only many years later that Dr Glen discovered the reason for the discrepancy in the Belgian study – the first patient in which propofol had been evaluated had been premedicated with another drug before being given propofol.
Once propofol had been accepted as a candidate drug, a team was formed to decide whether ICI should continue to invest in its development. There were a number of possible negatives identified, including a potential link between Cremophor EL and anaphylactoid reactions, but after a close vote, the decision was made to go ahead.
Because of the concerns about Cremophor, the search began for an alternative surfactant that was tolerated at the same sort of doses as Cremophor. A synperonic compound was identified and all the required pharmacology and toxicology studies were repeated and the formulation appeared to be well tolerated; however, histological changes were identified in the liver of animals in which the formulation was tested. As a result, ICI pathologists vetoed moving this formulation into clinical trials.
Against Dr Glen’s advice, the decision was made to launch the original Cremophor-containing formulation. Within a few months, there had been four anaphylactoid responses reported, so this formulation was withdrawn.
Dr Glen’s team returned to looking at emulsion formulations – emulsion technology had improved since their earlier evaluations and it now was possible to get much smaller particle sizes and more stable formulations. They eventually developed a soya bean oil emulsion formulation of propofol.
This formulation had to be tested in the same way as the earlier formulations, so all the studies and trials were repeated. The soya bean oil emulsion formulation was approved for the induction and short-term maintenance of anaesthesia in 1986. In total, it had taken 13 years, three different formulations, three sets of toxicology and pharmacology studies and two sets of clinical trials to bring propofol to market. “We did have to try, try and try again,” he said.
Following the approval of propofol, the clinical trial base was expanded to look at other indications for the drug, such as long-term maintenance of anaesthesia, conscious sedation for investigative procedures and surgery with local regional anaesthesia, use in children and the sedation of ventilated intensive care unit patients.
Dr Glen also examined the practicality of delivering propofol for intravenous maintenance of anaesthesia. He approached the manufacturers of syringe pumps to give them the opportunity to produce a pump that would facilitate the use of propofol for this purpose. Pumps were subsequently developed that could deliver larger volumes of drugs, at variable infusion rates or as bolus infusions. It became possible to set a target blood level of anaesthetic and titrate the depth of anaesthesia up and down as required.
Summing up, Dr Glen said that propofol offered a number of benefits: it provided smooth, rapid induction of anaesthesia, it facilitated the introduction of a laryngeal mask and it was easily titrated for maintenance. Patients appreciated the swift, clear-headed recovery and the minimal nausea and vomiting associated with its use, he said.
“These, I believe, are the main features that have made propofol as popular as it is today. A recent survey in the UK said that it was used in about 2 million patients in a year and used in about 90 percent of general anaesthetics in the UK. It is certainly well established and, I think, quite well used in veterinary anaesthesia as well.”
Taking questions from the audience after his presentation, Dr Glen was asked to speculate on why the injection of propofol caused pain in some patients but not others. He replied that he believed there were differences between patients; in his view, some patients had the kinin precursor and some did not. He also believed that the delay between injection and the pain response was due to the triggering of a cascade reaction leading to kinin activation. He pointed out that the pain could be easily controlled either by mixing lignocaine with the propofol or giving lignocaine separately.