Four half-day sessions at the Conference were devoted to discussion on aspects
of drug delivery. The sessions were organised in association with the UKI Controlled
Release Society
The biochemical barriers that can hinder effective drug delivery, and possible
ways of overcoming them, were discussed by several Conference speakers.
Professor LESLIE BENET (University of California, San Francisco) explained how
oral drug delivery was affected.
He said that, until relatively recently, poor oral bioavailability of a drug
was thought to be related to physico-chemical problems or hepatic first pass
metabolism. It was now clear that there was another barrier to absorption: the
potential for significant metabolism in the intestine itself. This was related
to the presence of cytochrome P450 drug metabolising enzymes and an efflux pump.
The enzymes were low in concentration (compared with the liver) but they were
strategically located and any drug molecule being absorbed had to go through
this enzyme barrier. Almost all the gut P450 enzymes were CYP3A, the major drug
metabolising enzymes (70 per cent were CYP3A4, and the rest CYP3A5 and CPY3A23).
The second part of the barrier was the multidrug efflux pump, P-glycoprotein
(P-gp). This was present naturally throughout the body. In the gut, it pumped
absorbed drug back into the intestinal lumen.
Inhibition or induction of the CYP3A and P-gp proteins could affect drug bioavailability.
With ciclosporin, for example, the poor oral bioavailability of the Sandimmun
brand was not, as originally thought, related to poor absorption, but to significant
gut metabolism. Bioavailability increased from 22 to 56 per cent after concurrent
administration of ketoconazole, an inhibitor of both CYP3A and P-gp. Similar
increases had been seen with tacrolimus. With sirolimus, new data showed a tremendous
11-fold increase in the area under the curve (AUC) if ketoconazole was given
at the same time.
Conversely, inducing the enzyme and efflux systems (eg, with rifampicin) could
greatly decrease bioavailability of the immunosuppressants. Professor Benet
said that CYP3A and P-gp appeared to act in concert in the gut. “The drug
molecule is absorbed, and it may or may not be metabolised by the enzyme. What
the transporter does is keep pumping the drug molecule back, so the enzyme continually
has a chance to metabolise the drug. The transporter controls access to the
enzyme.”
The CYP3A and P-gp proteins were induced or inhibited by many of the same compounds
but some drugs were substrates for one and not the other. Fexofenadine, for
example, was a substrate for P-gp but not CYP3A. If given with erythromycin
or ketoconazole, blood levels of the antihistamine were raised. Midazolam was
a substrate for CYP3A but not P-gp. If given with clarithromycin (an inhibitor
of CYP3A and, though not relevant here, P-gp), there was an increase in bioavailability,
primarily by an effect on gut metabolism. Paclitaxel had poor bioavailability
because of P-gp effects and, if given with an inhibitor of P-gp, blood levels
increased. “So there are examples of inhibiting the enzyme, inhibiting
the transporter, or inhibiting both, having a clinically important effect in
terms of relationship between drug interactions and our ability to increase
bioavailability,” said Professor Benet.
CYP3A and P-gp were also thought to work together in the liver, Professor Benet
said, adding that he had new data suggesting that premenopausal women had lower
P-gp transport in the liver than postmenopausal women and men, indicating that
a drug would stay longer in the liver and be metabolised more efficiently in
younger women.
Polymer P-gp inhibitors
Professor RUTH DUNCAN (school of pharmacy, University of London) has been working
on the development of polymeric excipients to overcome P-gp in the gut, with
a view to improving oral delivery of anticancer agents. She said that the research
was proving successful in that several polymer anticancer conjugates were now
in early clinical trials.
There was an evolving literature on the properties of the intestinal P-gp pump.
It was a complicated situation and there were many different mechanisms of interaction
with the pump. “There is an important role for P-gp in the clinical situation.
We have to take it seriously,” she said. A large number of low molecular
weight compounds that inhibited P-gp had been identified (eg, verapamil, PSC833)
but these tended to be toxic for routine combination with anticancer drugs.
Her work involved the development of non-toxic polymeric inhibitors that would
be suitable for incorporation into commercial formulations to enhance oral bioavailability
of anticancer agents. “We are looking to interact transiently with P-gp
to let drug in,” she said.
Professor Duncan described how her research group used a rat everted gut sac
as an in vitro screening model to select polymeric excipients that affected
P-gp transport. Polymer candidates included xanthan gum, polidocanol, alginates
and some newer compounds. They had then moved to in vivo studies and found that,
compared with doxorubicin on its own, a doxorubicin polymer formulation was
associated with increased survival of mice with cancer. The effect of the polymer
conjugate was similar to that seen when doxorubicin was given with verapamil.
Professor Duncan was optimistic that it would be possible to improve and standardise
the bioavailability of many compounds with poor or irreproducible bioavailability.
Dr DAVID BEGLEY (King’s College London) discussed how the blood brain barrier
could impede delivery of drugs. Treatment of many central nervous system diseases,
including Alzheimer’s disease, multiple sclerosis and Parkinson’s
disease, was frustrated by the difficulty in delivering therapeutic quantities
of effective drugs, he said.
The blood brain barrier was a biochemical barrier as well as an anatomical barrier.
CNS uptake was influenced by lipid solubility, ionisation, binding to blood
proteins and also by influx mechanisms (transporters) taking things into the
brain and efflux systems (P-gp and multidrug resistance proteins [MRP]) pushing
things out. These efflux mechanisms might have neuroprotective functions but
they also had the effect of excluding many drugs from the brain. Cerebral capillaries
and brain tissue also contained a large number of drug metabolising enzymes,
which could prevent a drug from reaching its target.
Strategies to increase transport into the CNS included chemical modification
to enhance passive permeation by increasing lipophilicity — but this did
not always work as some drugs, such as vincristine and bleomycin, were lipid
soluble but were also substrates for the efflux transporters and so were rapidly
pumped out.
Another possibility was to design drugs as substrates for the endogenous inwardly
directed transporters which might be used to carry otherwise hydrophilic drugs
into the CNS. Dr Begley said that researchers were beginning to learn the structural
characteristics needed to get drugs in to the brain, or keep them out. For example,
levodopa, melphalan and gabapentin were now known to get into the CNS by carrier
mediated delivery via system L transporter.
Techniques for minimising the effect of efflux transporters involved use of
P-gp inhibitors in combination with the therapeutic drug, or chemical modification
of the drug so that it was no longer recognised as a substrate for the transporter.
Nanoparticle technology could be used to avoid P-gp. Loperamide and doxorubicin,
both of which were substrates for P-gp, could get into the brain bound to nanoparticles.
Professor VINCE LEE (University of Southern California) discussed the challenges
of delivering drugs topically to the deep tissues of the eye, a target for treating
conditions such as macular degeneration for which companies were designing anti-
angiogenic drugs. There were three requirements: a mucoadhesive delivery system
to anchor the drug to the front of the eye, a permeable conjunctiva and, most
difficult of all, minimal drug clearance by the vasculature.
The bulbar conjunctiva, covering the sclera, was the best site for getting drug
to the back of eye. Rabbit experiments had shown that the conjunctiva was not
as permeable as the gut but it had all the gut transport mechanisms. Professor
Lee said that he was working on the development of polymers that could upregulate
a specific transport protein (PEPT1) in conjunctival epithelial cells. He believed
that it would be possible to design polymers selective for specific transport
pathways.
In a presentation on new vaccine adjuvants, Dr DEREK O’HAGAN (Chiron Corp)
described his company’s development of MF59, an adjuvant that was now used
in various vaccines, including the company’s Fluad influenza vaccine, to
enhance immune response. MF59 was an oil in water emulsion, based on squalane.
It was arguably the first proved vaccine adjuvant for over 50 years (since alum).
Chiron was also looking at intranasal vaccines for producing mucosal immunity,
working with heat labile enterotoxin from E coli. This was a potent adjuvant
in animal studies but it was toxic, producing copious diarrhoea from minute
doses. They had manipulated the molecule (by site directed mutagenesis) so that
it was no longer enzymatically active and no longer toxic. Influenza vaccine
given intranasally with this adjuvant (called LTK 63), in a bioadhesive system,
was as effective as traditional intramuscular vaccination. Studies in pigs had
also shown that LTK 63 could be given repeatedly, for different vaccines, without
inhibition of immunological response.
For DNA vaccines, Chiron was developing cationic polymer microparticles with
DNA adsorbed on to the surface. The idea was that the particles would target
the DNA to the antigen presenting cells and so enhance vaccine potency. The
company was working on an HIV vaccine containing HIV1 p55 gag DNA adsorbed on
to microparticles of poly(lactide-co-glycolide). In experiments in guinea pigs,
the response to this vaccine was stronger than that to naked DNA, Dr O’Hagan
said.
Pulmonary delivery
“The inhaled route for systemic drug delivery is no longer a fantasy but
is a reality,” said Dr STEPHEN FARR (Aradigm Corp). There was interest
in this route for both small and large molecules, he said.
Why give small molecules this way? The reason was that quite dramatic changes
in pharmacokinetic properties could be produced. For example, pulmonary morphine,
being developed for treating breakthrough pain, produced “IV-like”
pharmacokinetics after inhaled delivery.
However, most interest was in systemic delivery of biopharmaceutical agents,
such as leuprolide, insulin and growth hormone. Current metered dose inhalers,
as used in asthma treatment, would not be sufficient for delivery to the deep
lung (the alveoli) — which was needed for systemic delivery — and
they also gave inter-patient variability in dosing which would not be appropriate
for high potency compounds.
Dr Farrr described the AERx system designed by Aradigm for insulin delivery.
It was an electronic system that created a fine aerosol from an aqueous solution.
To control lung deposition, the system actuated aerosol delivery at a preprogrammed
time, which was defined by both flow rate and cumulative inspired volume. “If
the patient is not breathing correctly, the device will not fire,” Dr Farr
said. Early work had shown that absorption was faster than from subcutaneous
injection. The system, which was intended for giving insulin immediately before
eating, was now going into phase 2 clinical studies.
Further discussion on the pulmonary route for systemic delivery was given by
Dr JOHN PATTON (Inhale Therapeutics). He said that inhaled delivery would be
important for macromolecules. “People say that orally active protein-mimetics
are around the corner. This is not so. It is a daunting task.”
Inhale was developing dry powder systems. Insulin phase 3 trials were underway,
while beta-interferon and alpha-1 antitrypsin were in phase 1.
The device being developed for insulin was called Inhance. It was designed to
give “meal time” insulin and phase 1 data showed that it produced
a “physiological peak”. Over 1,000 patients to date had received treatment
with no serious adverse events.
Dr Patton said that the technical limitations to inhalation were dose (probably
10 to 20mg deposited per puff), molecular weight (compounds with molecular weight
over 100,000 might not have good lung bioavailability) and the cost of sophisticated
dose forms. It was also possible that some molecules would be absorbed too fast
from the lungs and might need to be slowed down, eg, by PEGylation to decrease
water solubility.
Also discussing insulin, Dr NICHOLAS PEPPAS (University of Purdue, US) said
that recent work on new mucoadhesive drug delivery systems led him to be “cautiously
optimistic” that a commercial product for oral insulin delivery would be
produced.
Mucoadhesive delivery systems had many potential applications, he said. As well
as oral drug delivery, they might, for example, be used in nasal, buccal and
bladder delivery. They were based on the principle that interaction between
polymer and the mucus lining of tissue could keep a controlled release device
within the tissue for the desired time.
Transfer through the skin
Iontophoresis — the principle of driving drugs through the skin by application
of an electric current — had yet to have a significant impact but certain
applications had now reached phase 3 testing, said Professor RICHARD GUY (University
of Geneva). These included iontophoretic systems containing lidocaine for rapid
local anaesthesia and fentanyl for analgesia.
Many of the problems with iontophoresis might have been predicted. Effort had
been wasted trying to give insulin in this way: it was too big, too negative
and was “going against the iontophoretic flow”. However, relatively
large cationic peptides, including somatostatin analogues and calcitonin, could
be delivered.
Closest to market was a system of reverse iontophoresis — using the technique
to extract molecules from the skin as against driving drug molecules into the
skin. This had obvious potential for non-invasive clinical testing and therapeutic
drug monitoring. A system for glucose testing (Glucowatch) was expected to be
launched in the UK later this year. After one calibration at the beginning of
the day with a finger prick sample, the machine measured iontophoretically extracted
blood glucose every 20 minutes. It looked like a diver’s watch and had
an autosensor in contact with the skin. Glucose, being uncharged, was extracted
by electro-osmosis. The results were similar to those from finger prick testing.
Some skin irritation occurred but this was not a problem in comparison with
finger pricking.
Intransal delivery
Describing research in nasal drug delivery, Professor LISBETH ILLUM (West Pharmaceutical
Services) explained that while this route was already being exploited for delivery
of small polar molecules and proteins, bioavailabilities were generally low.
Methods were needed to improve absorption by improving transepithelial transport
and reducing mucociliary clearance.
Various absorption enhancers had been tested but many were toxic. Professor
Illum favoured use of bioadhesive delivery systems based on chitosan. This was
a cationic polysaccharide derived from natural chitin by deacetylation. It was
water soluble and could be formulated into solutions or as microsphere powders.
To date, it had produced no toxicity. Chitosan enhanced drug transport across
mucosal membranes by increasing the contact time between drug and membrane (the
droplets were deposited anteriorly, where there were few cilia and so clearance
was slower) and it also had an effect on paracellular transport by transiently
opening the tight junctions between cells.
Nasal administration of morphine formulated with chitosan produced a dose response
curve similar to intravenous infusion. Further clinical testing with this formulation
was under way, including a phase 2 study in cancer patients with breakthrough
pain. For protein delivery, salmon calcitonin absorption from the nose was better
when formulated with chitosan than when given alone. Leuprolide was another
candidate drug.
Professor Illum believed that there would be an increasing number of intransal
products for “crisis treatment” (eg, sleep disturbance, pain) as well
as products for long-term treatment (eg, oestradiol for HRT, parathyroid hormone
for osteoporosis) and nasal vaccines. Another possibility was whether, with
a suitable delivery system, the nasal route could be used to deliver drugs that
did not pass the blood brain barrier to the central nervous system.
Dr Jorge Heller (Advanced Polymer Systems, California) discussed the use of
synthetic bioerodible polymers, specifically poly(ortho esters), for drug delivery.
Four families of these esters had been synthesised. The latest, POE IV, was
under development by his company for a series of commercial applications. It
was based on the reaction between a diketene acetal and a diol. Drug release
and polymer erosion occurred concomitantly. Potential applications for POE IV,
which could be formulated as a semi-solid injectable, included periodontal disease
and pain control. For dental use, the polymer was injected into the periodontal
pocket and released tetracycline for seven to 10 days. A clinical trial was
currently under way in Geneva.
Another study, in rats, involved placing polymer containing bupivacaine close
to the sciatic nerve, with the aim of releasing analgesic over four to five
days to block pain without high blood levels and systemic side effects.
Powder injection
Dr PASCAL HICKEY (Powderject Pharmaceuticals) gave an update on his company’s
system of powder injection, a drug delivery method that involves use of compressed
gas to accelerate particles to a velocity sufficient to physically penetrate
the stratum corneum.
He said that the advantages included simplicity, no needle stick issues (eg,
pain, disposal) and avoidance of the traditional barriers to transdermal delivery.
Drugs delivered this way could act locally or diffuse into the bloodstream.
Lidocaine was an example of small molecule transfer by powder injection. The
target was to deliver drug to the epidermis for local anaesthesia. In volunteer
studies, powder injection produced anaesthesia within three minutes as against
60 minutes with Emla cream. The powder anaesthesia lasted around 20 minutes.
Cacitonin was an example of use of powder injection for delivering biopharmaceuticals
to the systemic circulation. Studies had shown “reasonably similar”
plasma levels to subcutaneous calcitonin.
Dr Hickey’s final example was use of powder injection for DNA vaccines,
“perhaps the most exciting area”. Powderject was using gold microspheres
designed to deliver DNA intracellularly to antigen presenting cells in the epidermis.
In 12 hepatitis naïve volunteers, humoral and cellular responses had been
seen in all subjects injected with DNA coding for hepatitis B surface antigen.
Dr Hickey emphasised the unique challenges to formulation scientists in producing
particles that achieved the required depth of penetration.
Dr GEROLD MOSHER (Cydex) described the development of a drug formulation system
based on cyclodextrin. He said that the first product using this system would
be Pfizer’s intramuscular antipsychotic ziprasidone, which had just received
its first marketing approval.
Cydex’s product (Captisol) was a chemically modified cyclodextrin (sulfobutyl
ether beta cyclodextrin), which formed a ring of carbon chains with a negative
charge. Cyclodextrin drug complexes were able to increase a drug’s water
solubility and aqueous stability and were formulated to release drug immediately
on administration.
As well as parenteral use, cyclodextrin could increase a drug’s oral bioavailability
if solubility and dissolution were limiting factors. Cyclodextrin was not a
penetration enhancer and did not cross biological membranes, Dr Mosher emphasised.
Intravaginal ring
An intravaginal ring system for systemic drug delivery has been developed, Professor
DAVID WOOLFSON (Queen’s University of Belfast) reported. He said that the
ring was designed for delivery of oestradiol for treatment of menopausal symptoms.
It was developed in a research project between the university and Galen Holdings.
Professor Woolfson said that they originally tested oestradiol itself but could
not deliver the required amount of drug. Oestradiol 3-acetate had higher solubility
in silicone and higher water solubility, making it a better choice. “We
need to balance hydrophobicity through silicone with water solubility to allow
absorption,” he said. Drug was delivered from the ring over a three-month
period.
Speaking more generally about vaginal rings, Professor Woolfson said that they
were usually made from room temperature vulcanised silicone rubber, were around
55 mm diameter, permeable, biocompatible, non-toxic, elastomeric and hydrophobic.
There were two main designs: a matrix system, with drug distributed throughout
the matrix, and a core system which had a drug reservoir and allowed zero order
controlled delivery. The ring was positioned high in the vagina, and was held
in place by its elastomeric nature. It was designed to be left in place for
weeks or months. To date, the main use of these rings had been for contraceptive
steroids and for local delivery of oestradiol for treating vaginal atrophy.
To his knowledge, the oestradiol ring was the first for systemic drug delivery.
Gene therapy
In a presentation on gene delivery, Dr TONY PHILLIPS (Glaxo Wellcome) said that,
in the 10 years since the first patient was given gene therapy, 3,500 patients
had been given this therapy and, notwithstanding the hype, there were some encouraging
data. Among the current protocols for DNA transfer, treatment of cancer accounted
for the largest number, followed by monogenic disorders (such as cystic fibrosis)
and then infectious disease. There was also work under way in cardiovascular
disease and rheumatoid arthritis.
Viral vectors (particularly retroviruses and adenoviruses) were most commonly
used and were able to give significantly greater transfection than non-viral
vectors. However, there were manufacturing and quality control difficulties
and these vectors had given rise to severe adverse events in clinical trials
because of immunogenicity.
The development of non-viral vectors was probably where pharmacists could have
most impact. Most of these were lipid complexes, essentially of cationic liposome
and DNA. Professor Phillips believed that the non-viral approach to gene delivery
would probably be best, if transfection efficiency could be increased. Non-viral
vectors had the benefit of comparative ease of large-scale manufacture and control,
they were not infectious and had low immunogenicity. It might be that the vectors
of the future would be combinations of cationic lipids/polymers with incorporation
of specific viral components to facilitate transfection and duration of expression.
Among examples of gene transfer by viral vector, Dr Phillips mentioned recent
work in coronary artery disease where an adenoviral vector had been used to
transfer the gene for vascular endothelial growth factor (VEGF), the aim being
to encourage new blood vessel formation around a thrombus. Eight patients had
now been treated successfully with this “bio-bypass.” Peripheral vascular
disease could be another candidate for this type of therapy.
Would gene therapy fulfil its promise in the next 10 years? “I believe
it will for some diseases, but possibly not for cancer,” Professor Phillips
said.
Professor Patrick Stayton (University of Washington) discussed how efficiency
of intracellular delivery of DNA and proteins might be enhanced by use of pH-sensitive
synthetic polymers with membrane destabilising properties. Many naturally occurring
viruses and toxins had evolved membrane active proteins that enhanced transport
of DNA or proteins out of the endosome and into the cytosol as the pH dropped
during endosomal development. His polymer-based approach to manipulating intracellular
trafficking was inspired by biological polymers. “Nature has worked it
out for viruses. We are trying to design elements from nature to put into polymers,”
he said.
He had found that transfection efficiency could be enhanced with the use of
cationic lipid/poly(propyl acrylic acid)/DNA complexes.