Using nature's building blocks

Warwick Effect Polymers has gone right back to basics and is creating polymers with highly tailored functionality. Hilary Ayshford looks at the potential for drug development.

It is not often that the personal care sector steals a technological march on the pharmaceutical industry, but when it comes to using polymer chemistry to achieve continuous improvement in performance and functionality, pharma companies are trailing well behind.Whereas a new polymer is introduced into personal care products every few months, the pharma industry is sticking firmly to its small range of tried and tested polymers, which are the same as those used in hair products some 40 years ago.

Polymers have been used in healthcare for years in a variety of ways, including bone cement, contact lenses, sutures, tablet coatings and slow release anticancer therapies, where the drug is embedded in a matrix of biodegradable polymer. But growing interest in large molecule drugs has led the industry to look increasingly to polymers as a way of solving the inherent delivery and pharmacokinetic problems of macromolecules.

Until now, PEGylation - attaching the active molecule to strands of polyethylene glycol - has been the chosen method for slowing a drug's progress through the body and protecting it from digestive enzymes in the GI tract.

But according to David Haddleton, Professor of Chemistry at Warwick University in the UK and founder and chief technology officer of spin-out company Warwick Effect Polymers (WEP), things are starting to change: pharma companies are waking up to the potential of polymers to create drugs with better performance and greater functionality.

Living polymerisation technology has been around since 1956 but was never commercially viable as the process needed very low temperatures and very high purity solvents. But WEP has succeeded in growing complex polymer chains outside these harsh laboratory conditions. Furthermore, the kinetics of the technology scale up very well, and the company is now able to manufacture its products in quantities of up to 150kg in association with a local partner company.

The company's core technology is living radical polymerisation (LRP), which allows the 500 or so commercially available monomers to be put together in a way that achieves a level of control unparalleled in polymer synthesis. Control over molecular weight produces narrow polydispersity - relative uniformity in the molecular weight of a material - and enables the formation of well-defined polymer structures including block, graft and hyperbranched copolymers as well as controlling alpha terminal functionality. This allows the building in of specific properties, such as hydrophobicity or hydrophilicity - or even both properties at different points on the polymer.

Based on this process technology, WEP has developed POLY PEG, a new generation of pegylating agents for conjugation to proteins, peptides, biomolecules and other molecules. POLY PEGs are comb-shaped polymers with PEG teeth on a methacrylic back bone, and are available in a variety of molecular weights, PEG chain lengths and with different conjugating end-groups.

The structure can be varied in three ways:

1. The active end-group, which determines the method of conjugation between the POLY PEG and the desired biomolecule;

2. The PEG chain length, which determines the quantity of PEG on each 'tooth' of the 'comb';

3. The methacrylic backbone, which determines the length of the 'comb'.

POLY PEG conjugates to biomolecules using similar chemistry to that of existing PEGylation technology to give the desired adduct. Conjugation can be achieved through a number of mechanisms by selecting the POLY PEG with the appropriate active end-group. Currently these include succinimidyl ester, aldehyde, aniline and maleimide, which react with, among other things, lysine, N-termini, tyrosine and cysteine units respectively. The POLY PEG range has also been successfully conjugated to a range of model proteins, including insulin, lysozyme, haemoglobin and calcitonin.

'Attaching polymers to protein-based drugs is useful for a number of reasons,' Haddleton explains. 'Protein-based drugs are generally administered intravenously because the pH and the enzymes in the stomach would generally digest the protein. But you can use a polymer to protect the protein from chemical or enzymatic degradation.

'But more importantly you attach a polymer to increase the size of the drug because the body will generally clear out a molecule less than approximately 12,000 molecular weight in 4-6 hours through renal excretion. PEGylated interferon, for example, will stay in the body for weeks rather than hours, so injections can be given once every few weeks instead of on an hourly timetable.'

However, conventional PEGylation technology is not without problems of its own, which can be eliminated using WEP's POLY PEG technology, Haddleton says. The first of these is multisite attachment. As lysine residues are normally used to attach the polymer to the protein, the conjugate is likely to consist of a mixture of drug molecules, some with two, three, four or even more polymers attached. In addition, purity of starting polymers results in only 80% pure drug, with 10% difunctional and 10% non-functional.

'For a single site attachment, rather than using a lysine, it is possible to attach it through a cysteine, of which there is generally only one, or a tyrosine or just to the terminal amine,' Haddleton points out. 'Our chemistry is perfect for that because you just use an initiator.' The initiator forms the alpha terminus of the polymer, so by incorporating a desired functionality in the initiator every polymer chain will have the desired alpha functionality.

'If you have a functional group on your protein that you want to attach to, you just tell us what the complementary functional group is and we will grow a polymer from it. If you have one functional group in your active, we generally have a way of attaching it to the polymer chain,' he adds.

A second problem with conventional PEGylation is what happens to the polymer chain once the protein has become detached. To build a molecule of sufficient weight to avoid elimination by the kidneys requires a long polymer chain, but once the protein has done its job the remaining polymer is still too big to be excreted and will therefore build up in the blood - especially if the drug is being used to treat a chronic condition.

'Polymers dissolved in any solvent, even water, create viscous solutions, so if you are administering this drug over 30 years, you will get a build up of polymer in your blood and your blood will get more viscous,' warns Haddleton.

POLY PEG molecules, on the other hand, are designed in such a way that enzyme action will cause them to fall apart but over a period of weeks rather than hours. Once it becomes detached from the protein, the polymer will itself decompose into chunks of PEG with a molecular weight of 1,000 or 2,000, which is already FDA approved, and that will get cleared by the body through renal excretion in the same way as normal small molecules.

Another way that POLY PEG technology can be used to increase the retention time of drugs in the body is to create a molecule with mucoadhesive properties. Mucoadhesives enable delivery of drugs such as morphine or calcitonin to the nasal passage or can make oral therapies adhere to the wall of the stomach or gut. Chitosan has been typically used as a mucoadhesive, but it has poor solubility. WEP developed a polymer - originally used as a hair gel ingredient - that was just as mucoadhesive as chitosan but with a number of additional benefits.

Unlike chitosan, which is bioadhesive and will stick to any biological surface, WEP's polymers are preferentially adhesive and will adhere only to the mucosal membrane. Furthermore, some fluorescence was incorporated into the polymer making it possible at the experimental stage to verify exactly where the polymer was adhering. And unlike chitosan, it is possible to chemically modify the WEP polymer and covalently attach the active ingredient.

But perhaps the most exciting - albeit unexpected - result came from cytotoxicity tests. Not only did the polymer not kill any cells, it was actually found to have a cytoprotective effect, opening the way to its use as an antiviral treatment.

Haddleton's group at Warwick University is working closely with David Brayden at University College Dublin, who has recently been awarded a grant by the Science Foundation of Ireland to investigate POLY PEGylation of osteoporosis drug salmon calcitonin. The objective is to create an orally administered formulation that will be protected from gastric digestion.

Haddleton has been working in the area of controlled polymerisation since 1996, and WEP was set up in 2001 to explore the growing commercial opportunities arising from the technology. The company's IP is protected by worldwide patents, and besides its own range of standard polymers, WEP also undertakes contract research to design polymers to meet specific functionality requirements. The r&d is generally funded by the client and once the product is commercialised WEP will receive licensing and royalty payments.

'The area of controlled polymerisation is growing very rapidly and there is very diverse market potential,' ceo Fergal O'Brien points out. 'In particular on the pharmaceuticals side there is an opportunity to grow the capital value of the company.'

An initial £3,360,000 tranche of venture capital was followed by an additional £470,000 in November 2003 to take the company through the next stage of its development. Recently it has secured a further tranche of £1.5m, which will be used to expand further its scientific capabilities and to support the commercialisation of its standard polymer range and its POLY PEG range of bioconjugates.

At present, the pharma sector accounts for about 50% of the business but represents the vast majority of the research work. There are many products on the market that are not PEGylated, but which would benefit from the technology, offering a great opportunity, O'Brien believes.

'Working with pharma and healthcare people is relatively new for us - development timescales are much longer, costs are higher and traditional manufacturers can be resistant to change,' says Haddleton. 'Pharmaceutical companies do not have a great deal of expertise in polymers. So what we can offer is the chance to find out if polymers can be useful to them. 'If you are prepared to talk in a true multidisciplinary environment and avoid blinding each other with technical jargon, that is where the best progress is made.'