London
What do painful injections and solar-powered refrigerators in the middle of Africa have in common? Well, neither might be so crucial in the battle to save lives if a novel way of preserving drugs takes off. Drugs and vaccines lose their potency during dehydration and at high temperatures because the molecules lose their structure and degrade. But biochemists and pharmacologists are using solutions of simple sugars to keep drugs and vaccines in a state of suspended animation, and to develop easier, pain-free ways of taking them.
The inspiration for this approach comes from nature. During drought some desert plants and organisms can lose up to 99 per cent of their water and still keep their cells intact thanks to a sugar called trehalose. This forms a glassy matrix when the plant dries out, preserving internal structures until the rains arrive, when the plant miraculously springs back to life.
Glassy matrixes could be put to good use in treating people with diabetes who must have regular shots of insulin. Californian company Inhale Therapeutic Systems is testing an inhaler system for delivering insulin through the lungs. The drug is ferried to the bloodstream in little particles made of a glass-like sugar matrix. The company is also working on similar treatments for osteoporosis, cystic fibrosis and hepatitis B and C.
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Sugar glasses could also be used in preserving drugs and vaccines distributed by the WHO which normally have to be refrigerated for the journey to remote areas of Africa, Asia and South America. Despite huge efforts to create a 鈥渃old chain鈥� of refrigerators across the developing world, doctors urgently need a better way of storing vaccines and preventing them from dying.
This life-preserving vitrification technique has already been copied to keep food fresh (鈥淎 sweeter way to fresher food鈥�, 快猫短视频, 15 May 1993, p 24) and is now set to improve the preservation of vital drugs and biomedical products. 鈥淭he physical stability of a glass gives the drug its chemical stability,鈥� says Felix Franks, a pioneer of drug stabilisation using sugars. Franks, director of British company Pafra BioPreseveration in Cambridge, has spent many years developing methods of preserving delicate molecules in their active forms.
Many drugs are made in water, and have to be given in an aqueous solution. So why not just store them in water? 鈥淲ater is a problem,鈥� says Franks. Not only is it highly reactive itself, but it may contain contaminants from the drug manufacturing process including enzymes such as proteases. 鈥淚n aqueous solution, these will [attack] the drug and reduce its activity,鈥� he says.
Freeze-drying is normally used to prevent reactions and preserve drugs. The technique involves freezing a solution of the drug, and removing the ice crystals that form by converting them straight into water vapour under vacuum-a process known as sublimation. But this is as damaging as dehydration for delicate drug molecules and biological species such as restriction enzymes.
However, if sugar molecules are present, they can protect the drug molecules by 鈥減ropping up鈥� the active structure, preventing it from denaturing when the water molecules are removed. Unlike pure water, a sugar solution stays liquid well below its expected freezing point. The water in the solution then begins to form tiny ice crystals, increasing the proportion of sugar to water left in the liquid. Further cooling creates a thick gooey syrup in which molecular motion is slowed. Eventually, the syrup forms a hard, amorphous glass-like overcooked treacle toffee-which keeps drugs in a state of suspended animation.
This glass behaves exactly like glass in windows, says Tony Auffret, head of Pafra BioPreservation鈥檚 research into glass-based stabilisation. Suspending drugs in a sugar glass locks them up in a slow-moving casing. 鈥淎ll molecular motions and chemical reactions are frozen out and an amorphous glass should preserve proteins for years,鈥� says Auffret. In vitrified form, a drug that would have needed storing at -40 掳C will stay stable at room temperature. Factor 8, the blood clotting agent given to haemophiliacs, may be similarly stabilised.
Given the right conditions, most sugars will turn to glass as they cool, but not all are suitable for preservation purposes. If most sugar glasses are kept in a humid atmosphere they absorb water, and can turn back into a syrup, giving poor preservation. Trehalose glass is different. It 鈥渓ocks-up鈥� excess moisture, and stays as a glass at levels of humidity which would leave glasses made from other sugars runny and useless. But trehalose is expensive compared to other sugars, says Franks. So one important strand of Pafra BioPreservation鈥檚 research is investigating which other sugars make the most stable glasses. 鈥淲e鈥檝e used sucrose, lactose and mannitol, but organic salts such as citrates also work,鈥� says Franks. All have shown potential for preserving drugs.
Take a deep breath
Such sugar glasses could soon revolutionise the way millions of diabetics receive their daily dose of insulin. Researchers at Inhale Therapeutics, based in Palo Alto, California, have developed a new delivery method that relies on tiny glassy particles to carry insulin molecules into the body via the lungs. So instead of an injection every day, diabetics will simply have to take a dose of insulin from an inhaler.
Diabetes sufferers don鈥檛 have much choice about how they take their medicine at the moment, although many routes are being explored. If we swallow insulin, our bodies break most of it down in the gut before it reaches the bloodstream. And the molecules are too big to pass through our skin without the help of a 鈥減enetration enhancer鈥�. The lungs, however, make an ideal route of entry. The alveoli-tiny sacs found at the base of the lungs-maximise absorption into the bloodstream.
But to get drugs down to the alveoli, the size of the aerosol particles must be carefully controlled-too big and they will stick in the mouth or throat on the way down, too small and they will be breathed out having not been absorbed in the lungs. The challenge is to make stable particles of exactly the right size-which is where previous aerosols have come unstuck.
Inhale Therapeutics reckons it鈥檚 cracked the problem thanks to sugar glass technology. 鈥淭here鈥檚 a real need for long term stability,鈥� says Chris Searcy, head researcher at the company, which is licensed by Pafra BioPreservation to develop new ways of administering drugs using glasses. 鈥淭he inhaler must create the same aerosol in two years鈥� time that it does today.鈥�
Searcy and his team manage to do this by creating a fine powder of glassy spheres containing drug molecules from a solution containing sugar and insulin. The exact details of the sugar or sugars used by Inhale Therapeutics is a closely guarded secret, but trehalose is an unlikely candidate, having only recently been approved by the US Food and Drug Administration.
The solution is sprayed as a mist into a stream of hot, dry air, which quickly dries the mist to a powder-a process known as spray drying. The transformation from liquid to glassy powder is rapid, preventing denaturing of the insulin, and the molecules do not suffer in the heat, which is carried away by the evaporating water.
The company鈥檚 process produces tiny glass particles just one to three micrometres in diameter-less than a tenth the size of powders made in standard industrial spray dryers. Fine powders cause many problems on an industrial scale-the tiny spheres clump together, and refuse to flow like a powder should. The researchers claim to have designed their own powder production and handling system from scratch to solve this problem. This is now being used to make and package individual doses for phase IIb clinical trials, which are being conducted with Pfizer in the US.
Just as important, Searcy鈥檚 team had to find the best way to get the aerosol into the patient. Their device works in the same way as an air gun-the user 鈥渃harges鈥� and 鈥渃ocks鈥� the system with compressed air using a hand pump, and presses a button to 鈥渇ire鈥�. The pressurised air pierces a blister containing a standard dose of the powdered insulin and the drug-containing particles are released to form the aerosol.
But the inhaler doesn鈥檛 pump the drug directly into the throat like many standard inhalers. It produces an aerosol cloud, which stays in a holding chamber for 10 seconds. The patient can then inhale the drug into their lungs with a single, deep breath. This avoids the co-ordination problem some people have with traditional inhalers which must be pressed at the moment the patient inhales. Another problem with traditional devices is that because they create an aerosol containing over- or under-sized particles, a lot of the drug in each dose is wasted. Searcy claims that by allowing the user to slowly suck in their medicine as a correctly-sized aerosol, his system delivers all of the particles to the sites of absorption in the lungs.
Rapid breakdown
But George Alberti, director of the WHO Collaborating Centre for Diabetes at Newcastle University in Britain still needs convincing. 鈥淭heoretically, the lungs are an ideal route,鈥� he says, 鈥渂ut you need to control the dose precisely-to within a few per cent.鈥� Clinical tests show this is possible, says Searcy, who hopes the phase IIb and subsequent worldwide phase III trials with Pfizer will prove the efficiency of his system.
Alberti also thinks that the speed with which the drug is taken up by the body through the lungs may be a problem. Once the sugar glass particles start breaking down in the lung lining, the insulin released can reach the main bloodstream in less than half the time that an injected dose takes. This may be a problem where a low level of insulin is required in the body over a long period of time. 鈥淵ou need both fast-acting and slow-acting forms,鈥� says Alberti.
Julian Blair and his team at Quadrant Healthcare, a British company based in Cambridge, may give Alberti what he wants. They are trying to give a sustained release of insulin over many hours by chemically modifying the sugars used to form glasses so that they will dissolve more slowly in the bloodstream.
Prescription of vitrified medicine in aerosol form may not be restricted to insulin. Inhale Therapeutics has at least 11 projects at various stages of development in partnerships with other companies. Eli Lilly is hoping to use the company鈥檚 inhaler technology to deliver an osteoporosis drug, and an undisclosed manufacturer is working on the delivery of calcitonin to treat the same disease.
Pre-clinical testing is being carried out with Centeon into using alpha-1 antitrypsin to treat emphysema and cystic fibrosis, and research with Immunex into delivering interleukin-1 receptor to combat asthma is at the same stage. Still at the formulation stage are projects to deliver beta and alpha interferons for treating hepatitis B and C, and the blood clotting agent heparin. Genzyme is also working with Inhale Therapeutics on the delivery of stabilised gene vectors.
Back in Britain, Pafra BioPreservation is developing vitrified blood plasma which could be easily stored and rapidly rehydrated for emergency use. Having large stockpiles of such a product on hand may help save lives at scenes of accidents and on the battlefield. Not surprisingly, says Franks, the military have realised the value of such a product. Without the need to store the plasma in freezers, aircraft could carry 12 times as much as is possible at present.
In fact, Franks reckons that sugar glass is ideal for stabilising almost any delicate molecules. The technique could even work for human growth factor and other molecules used in the biotechnology industry. But some bioactive molecules are more difficult to preserve than others. 鈥淟ive vaccines are harder to preserve in a glass because they are more complex,鈥� he says, 鈥渂ut it is possible.鈥�
As the WHO knows only too well, most vaccines are unstable, and die if exposed to extreme temperatures. But Pafra BioPreservation and Quadrant Healthcare have already had some success in laboratory tests into stabilising vaccines for cholera and polio, and researchers think any number of vaccines could be preserved in this way, including those for tetanus and measles. The snag at the moment is that not enough active vaccine is yielded after it is reconstituted in water.
Like many drugs, vaccines are often reconstituted with a buffer solution just before they are given to the patients. It is at this stage that they become especially vulnerable to degradation. 鈥淭here is a definite need to develop a vaccine which will survive for a week at 40 掳C,鈥� says Michael Zaffron, of the WHO鈥檚 Global Programme for Vaccination and Immunisation in Geneva. But the WHO has another headache-vaccines such as those against hepatitis B and tetanus are as easily damaged by cold as they are by heat. So just sticking them in a freezer isn鈥檛 the answer. The temperature range within which these vaccines remain stable is very narrow.
Sealing vaccines inside a sugar glass could prove to be the answer. The sugar glass effectively widens the temperature range over which the vaccines can survive unharmed. John Clements, a medical officer at the WHO in Geneva is excited by the prospect of this new technology. 鈥淥ne of our next priorities is the worldwide eradication of measles. Stabilising the vaccine will make our job much easier, especially if we can work without relying on the cold chain.鈥�
So improved stability should not only benefit health, but reduce waste and cut costs in the long term. 鈥淭hese developments are probably where the future lies,鈥� suggests Zaffron, 鈥渂ut it would be tragic if only the wealthy nations could afford the technology.鈥� Franks doesn鈥檛 think that vitrified drugs are only for the rich. 鈥淪pray drying apparatus is considerably cheaper than freeze-dryers, and you won鈥檛 need cold storage,鈥� he says.
Vitrification based on this cheaper technology could find a whole range of uses outside the field of healthcare, Franks reckons. The brewing industry might see sugar glasses as an ideal way to preserve batches of yeast. And other applications in the food industry are possible- storing enzymes for food ripening, for example. But the most unappealing research is still to be done-into glassifying blobs of excreta. Sugar glasses could be used to store 鈥渟emi-solid waste鈥� from astronauts until they return from space-a truly down-to-Earth application.
