It’s a risky business being a peptide drug in the gastrointestinal tract. The first digestive stop is the stomach, where the acidic environment can cause damage and pepsin enzymes do their best to break them down into smaller fragments. Protease enzymes, for example, don’t discriminate between the amino acid chains in food, and those in peptide and protein medicines.
In the unlikely event the peptide makes it through the stomach intact, further perils await. The next stop, the intestine, contains a whole range of different enzymes secreted by the pancreas, which continue the food digestion process. If enzyme degradation can’t be prevented in both stomach and intestine, the result is zero bioavailability for the drug – and thus no activity.
This isn’t the only problem. Even if degradation is prevented, peptides and proteins are large molecules that struggle to reach the bloodstream – they need to be transported across epithelial cells, or squeeze through the gaps between them. ‘Aspirin, for example, has a molecular weight of 180, whereas that of salmon calcitonin [a polypeptide hormone], which comprises 32 amino acids, is nearly 3500 – almost 20 times as big,’ explains James Gilligan , chief scientific officer at Tarsa Therapeutics in the US, one of the companies looking to deliver peptide drugs orally. ‘Small molecules and tiny fragments of, say, two amino acids, can get through the walls of the intestines, but the physical size of longer chains and larger molecules prevents them being efficiently absorbed across the intestines into the blood.’
There’s a third problem for peptide drugs that needs to be solved. Most drugs have to be lipophilic to be absorbed through the intestines, but proteins and peptides have positive and negative charges on them. This can drastically reduce their bioavailability. Alendronate, for example, a bisphosphate drug used for several bone diseases, including osteoporosis, has a bioavailability below 1%, Gilligan says, because of the charge, making it difficult for it to get across the intestines. ‘To design a formulation, you have to be cognisant of the fact that those three hurdles – at least – have to be overcome in order to succeed.’
A coated pill
Tarsa is using technology developed by drug delivery company Unigene – now Enteris BioPharma – in its formulations. The basis of the delivery form is an ‘enteric’ coating to protect the drug in the stomach, and promoters to encourage it to pass through the intestines into the bloodstream. ‘The enteric coat is acid-stable, and therefore the tablet does not dissolve in the acid environment of the stomach,’ explains Enteris chief scientific officer Nozer Mehta. ‘Our technology is also less susceptible to food effects or dilution with large volumes of liquids ingested at the same time as the tablet.’
Once the intact tablet reaches the intestine, the pH rises and the enteric coat dissolves, releasing the contents of the tablet into the intestine. ‘It is specifically designed to release all of the contents as a bolus in a very small localised environment in the intestine,’ Mehta explains. ‘This is achieved by putting a subcoat under the enteric coat, which keeps the contents of the tablet intact until the enteric coat has completely dissolved.’
The tablets also contain excipients, including an organic acid (usually citric) to create a local acidic microenvironment of around pH 3.5–4, in contrast to the general pH in the intestine which is typically 6 or higher. This is to counteract proteolytic enzymes, which have a neutral to alkaline pH optimum. ‘Transiently, and just in that localised area, their activity is inhibited and the peptide remains intact,’ Mehta says.
A second essential excipient is typically lauryl carnitine, which acts as an absorption enhancer. This works by helping to open tight junctions between cells at high concentrations to allow the large peptide molecules to pass through – another reason why the dose needs to be released all at the same time. But it has other beneficial properties, Mehta says. ‘It’s a surfactant, and thus can improve the solubility of molecules, helping them get through the mucus layer that lines the intestine,’ he explains.
To date, Enteris BioPharma has carried out several clinical studies on peptide drugs, such as parathyroid hormone (PTH) and calcitonin, and there are various others in development, both in-house and with partners.
The most advanced of these is oral salmon calcitonin for treating osteoporosis and this drug is currently being developed by its spin-out company Tarsa. The 32 amino acid peptide is typically administered via a nasal spray, which was introduced in 1995 by Novartis as a replacement for injectable doses. Applying the coating technology has allowed Tara to create an oral tablet, with the additional advantage of being stable at room temperature.
Phase 3 trials of this oral product in women with established osteoporosis were successful, as was a Phase 2 study in women with osteopoenia, often a precursor to osteoporosis. Plans for filing the tablet with the regulators were well advanced when, in mid-2012, an analysis revealed a potential relationship between calcitonin treatment and cancer. As a consequence, the nasal spray was withdrawn in Europe, and the duration of use of other calcitonin products limited. ‘This forced us to re-evaluate the situation, and we brought in a number of independent statisticians and luminaries in the field to do an analysis from both a biologic and an oncological standpoint,’ Gilligan says.
The results from the independents were presented at the American Society for Bone and Mineral Research’s annual meeting in October 2013. ‘They concluded there was no causal relationship, and there was no basis that the molecule causes cancer.’ Tara is now awaiting the decisions of the regulators about calcitonin in both the US and Europe, and in the meantime has been looking at its potential in vertebral compression fractures. A proof-of-concept Phase 2 study is being planned to find out whether, in addition to increasing bone density, the polypeptide has an analgesic effect at a higher dose.
Other commercial interest
Other US-based drug delivery companies are showing interest and experimenting with the coating technology. Catalent is looking into it to achieve oral peptide delivery. As project manager Ted Andrew explains, initially the company focused on a low molecular weight oligosaccharide product. ‘We looked at chemical modification of the API to create a pro-drug, and although this did show some enhancement, it was more effective to provide an enteric coat to deliver the oligosaccharide right to the site of absorption so that it is not sitting in the gastric environment or the small intestine,’ he says. ‘We then used a mix and screen process on permeation enhancers that would deliver the oligosaccharide across the lumen at a therapeutic dose in an animal model.’
These investigations are still under way, but in the meantime the company is also investigating the potential of the coating to deliver peptides. Andrew explains that a ‘permeation enhancer’ has to achieve a delicate balance – the ‘gate’ has to be opened for long enough for the peptide to pass through, but not so long that leakage is created, or the therapeutic profile altered for any other medications the patient is taking. ‘The trick is to open it up long enough and specifically enough for the peptide, and just allow that to reach the bloodstream,’ he says.
Other factors that need to be considered include size. According to Andrew, as a general rule, the maximum is around 10kDa. The geometry and rigidity of the secondary structure are also important as are its charge, molecular weight and its hydrodynamic radius, which is an indicator of how it interacts with water. ‘Often people will simply think that because a peptide has a molecular weight of, say, 6000, they should be able to do something with it,’ he says. ‘But all of these other factors must also be considered in order to determine the best route of action, and the best way to achieve permeation enhancement.’
The chemists use a screening sequence to find out how the peptide is digested, and how much is digested. Once the optimum API formulation has been determined, they investigate the effects of different components for the coating, such as medium-chain triglyceride concentration, additional surfactants and hydrophilic solvents. The best candidates are then tested to check for uptake in an in vitro permeation screen in the absence of digestion conditions, followed by bioavailability testing in a rodent model.
With a bioavailable product, the next step is to look at how the coating performs in real-life digestive conditions. ‘We frequently find that an enteric coating is required, and again we use a series of screens to look at enteric coating combinations,’ he says. ‘A typical [enteric] coating formulation will include a methacrylic acid, glycerol and purified water – a plasticiser, a lubricant and a solvent – not only to protect the peptide from the conditions in the stomach, but also to deliver it to the exact right place.’
Another important factor for an enteric coating is its homogeneity around the pill, Andrew says. ‘If you have a highly variable coating around the outside of the capsule, premature release of the API is likely, so it will not get to the right place,’ he says. ‘We run a series of tests to ensure the coating procedure works well.’ Initial studies gave highly variable results, and electron micrograph studies show the homogeneity of the coating. Oval shaped capsules, for example, tend to be difficult at the two ends. We then go through a disintegration test with the enteric coating to make sure it complies with specifications.’ Animal studies with the oligosaccharide product showed that without the enteric coating, the capsules were retained in the stomach and there was very limited absorption; with the coating, when the capsules were able to reach the duodenum, therapeutic levels were achieved.
A different approach
Meanwhile, Emisphere is taking a different approach. Rather than protecting the peptide inside a coated tablet and promoting absorption after release, its Eligen technology uses carrier molecules to transport the peptides, without altering their form or pharmacological properties. These carriers have no pharmacological activity at the quantities used in the formulations, and thus are considered excipients. Instead of the peptides being squeezed through the gaps between cells, passive transcellular transport allows the peptides to cross cell membranes. The peptide and the carrier dissociate once they have entered the intracellular space from the gastrointestinal lumen, enabling the drug to pass into the bloodstream to exert its effect, while the carrier is excreted. There is no negative effect on the integrity of the cells.
The idea is not new – Emisphere has been working on the technology for more than two decades, and following a restructuring in 2013 is now focusing on what chief executive Alan Rubino describes as ‘scientific selling’ of its carriers. Its main partner companies are Novo Nordisk, where current collaborations include GLP-1 analogues in diabetes, and Novartis, but it has also worked with many others over the years, and Rubino says the carriers have always worked. ‘The secret is in matching the right API and the right carrier,’ he says.
The carriers are small molecules that interact with both the epithelial membrane in the gastrointestinal tract and the drugs themselves, explains lead chemist David Gschneidner. ‘They make the membranes more permeable, and also make the drugs more lipophilic, and thus more likely to pass through the membrane,’ he says. ‘The weak interaction between the drug and the carrier is temporary, which are at high concentrations in the dosage form. When it hits the bloodstream, there is a massive dilution, which separates the drug from the carrier.’
Libraries of carrier molecules can be screened to identify the best one, and over the years they have got more of a ‘feel’ for what properties are required in a carrier molecule to achieve efficient delivery. ‘Almost as important is to know which drugs will be best served by the technology, to avoid wasting time trying to deliver molecules that are not really compatible with the delivery system.’
Avoiding the attentions of the digestive system is a huge challenge for the growing numbers of peptide- and protein-based drugs reaching the market but progress is being made and patient-friendly oral formulations are now closer to becoming a reality.
Sarah Houlton is a freelance science writer based in Boston, US