Effects of the Mucoadhesive Polymer Polycarbophil on the Intestinal Absorption of a Peptide Drug in the Rat

The absorption across rat intestinal tissue of the model peptide drug 9-desglycinamide, 8-arginine vasopressin from bioadhesive formulations was studied in-vitro, in a chronically isolated internal loop in-situ and after intraduodenal administration in-vivo. A controlled-release bioadhesive drug delivery system was tested, consisting of microspheres of poly(2-hydroxyethyl methacrylate) with a mucoadhesive Polycarbophil-coating, as well as a fast-release formulation consisting of an aqueous solution of the peptide in a suspension of Polycarbophil particles. Using the controlled-release system, a slight improvement of peptide absorption was found in-vitro in comparison with a non-adhesive control system, but not in-situ or in-vivo. In contrast, bioavailability was significantly increased in all three models from the Polycarbophil suspension in comparison with a solution of the drug in saline. The effect appeared to be dose-dependent, indicative of intrinsic penetration-enhancing properties of the mucoadhesive polymer. A prolongation of the absorption phase in-vitro and in the chronically isolated loop in-situ suggested that the polymer was able to protect the peptide from proteolytic degradation. This could be confirmed by degradation studies in-vitro. The duration of the penetration enhancing/enzyme inhibiting effect was diminished with increasing complexity of the test model, in the same way as was previously found for the bioadhesive effect. This interrelationship suggests that the observed improvement in peptide absorption and the mucoadhesive properties of this polymer are associated. The development of a fast-release oral dosage form for peptide drugs on the basis of Polycarbophil appears to be possible.

Population pharmacokinetic modeling of blood-brain barrier transport of synthetic adenosine A1 receptor agonists

A population pharmacokinetic model is proposed for estimation of the brain distribution clearance of synthetic A1 receptor agonists in vivo. Rats with permanent venous and arterial cannulas in combination with a microdialysis probe in the striatum received intravenous infusions of 8-methylamino-N6-cyclopentyladenosine (MCPA) and 2'-deoxyribose-N6-cyclopentyladenosine (2'-dCPA) (10 mg kg(-1)). The clearance for transport from blood to the brain was estimated by simultaneous analysis of the blood and extracellular fluid concentrations using a compartmental pharmacokinetic model. The proposed pharmacokinetic model consists of three compartments describing the time course of the concentration in blood in combination with three compartments for the brain extracellular fluid concentrations. The blood clearance was 7.4 +/- 0.5 for MCPA and 7.2 +/- 1.4 ml min(-1) for 2'-dCPA. The in vivo microdialysis recoveries determined by the dynamic-no-net-flux method were independent of time with values of 0.21 +/- 0.02 and 0.22 +/- 0.01 for MCPA and 2'-dCPA, respectively. The values of the intercompartmental clearance for the distribution from blood to brain were 1.9 +/- 0.4 versus 1.6 +/- 0.3 mul min(-1) for MCPA and 2'-dCPA, respectively. It is concluded that on basis of the novel six-compartment model precise estimates of the rate of brain distribution are obtained that are independent of eventual differences in systemic exposure. The low brain distribution rates of MCPA and 2'-dCPA were consistent with in vitro tests. Furthermore, a slow elimination from the brain compartment was observed, indicating that the duration of central nervous system effects may be much longer than expected on the basis of the terminal half-life in blood.

The role of the efflux transporter P-glycoprotein in brain penetration of prednisolone

In the present study, we have investigated the role of the multidrug resistance (mdr) P-glycoprotein (Pgp) at the blood-brain barrier in hampering the access of the synthetic glucocorticoid, prednisolone. In vivo, a tracer dose of [(3)H]prednisolone poorly penetrated the brain of adrenalectomised wild-type mice, but the uptake was more than threefold enhanced in the absence of Pgp expression in mdr1a (-/-) mice. In vitro, in stably transfected LLC-PK1 monolayers the human MDR1 P-glycoprotein was able to transport prednisolone present at a micromolar concentration. A specific Pgp blocker, LY 335979, could block this polar transport of [(3)H]prednisolone. Human Pgp does not transport all steroids, as cortexolone was not transported at all and aldosterone was only weakly transported. The ability of Pgp to export the synthetic glucocorticoid, prednisolone, suggests that uptake of prednisolone in the human brain is impaired, leading to a discrepancy between central and peripheral actions. Furthermore, the ensuing imbalance in activation of the two types of brain corticosteroid receptors may have consequences for cognitive performance and mood.

Quality of life in long-term survivors after curative transhiatal oesophagectomy for oesophageal carcinoma

BACKGROUND

Transhiatal resection for oesophageal cancer is a major operation with potentially severe physical, emotional and social consequences. The aim of this study was to assess various aspects of quality of life in long-term survivors following oesophageal resection for cancer.

METHODS

Between January 1993 and May 1996, 100 consecutive patients with cancer of the oesophagus or oesophagogastric junction underwent a potentially curative transhiatal oesophagectomy. Patients with a minimum follow-up of 2 years and with no tumour recurrence (n = 35) were mailed questionnaires which consisted of: (a) the Short Form-36 Health Survey to assess general quality of life, (b) an adapted Rotterdam Symptom Checklist to assess disease-specific quality of life, and (c) additional questions about other effects of the operation.

RESULTS

All patients returned the questionnaire. General quality of life was comparable with reference values for the same age group. However, more than half of the patients still experienced at least some early satiety, fatigue, dysphagia, heartburn and/or psychological irritability. Nine of 13 patients who worked in paid employment before operation continued to do so.

CONCLUSION

Patients who survive 2 years or more after transhiatal oesophageal resection for cancer can lead satisfactory lives. Although some residual symptoms may persist, their general quality of life is similar to that of healthy individuals of the same age.

Dopamine release in the prefrontal cortex in response to memantine following sub-chronic NMDA receptor blockade with memantine: a microdialysis study in rats

Memantine is an uncompetitive N-methyl-D-aspartate receptor antagonist which blocks the NMDA receptor with moderate-affinity in a use- and voltage dependent manner. In clinical practice it is used chronically in the treatment of dementia and does not induce psychotomimetic effects as, high affinity, uncompetitive antagonists. Thus, it was of interest to determine dopamine (DA) and metabolite (DOPAC - dihydroxyphenylacetic acid and HVA - homovanillic acid) concentrations in the prefrontal cortex (PFC) in response to 14 days administration of memantine (20 mg/kg/day). It was previously determined that in rats this treatment induces sensitization to the locomotor effect and tolerance to the learning impairing properties of high doses of memantine. Acute administration of memantine (20 mg/kg, i.p.) did not affect dopamine levels in the PFC. It did however increase DA metabolite (DOPAC and HVA) concentrations. Administration of memantine (20 mg/kg/day) for 14 days before the acute challenge only slightly changed memantine's effect on PFC neurochemistry even though pharmacokinetic tolerance was observed. When memantine was administered to the sham group, which had been repeatedly treated with Hypnorm (including neuroleptic), an increase in PFC dopamine and metabolite content was seen. In accordance with the fact that memantine does not possess psychotomimetic activity at therapeutically relevant doses, these experiments showed that it does not affect the prefrontal cortex dopamine levels.

Adaptations of NMDA and dopamine D2, but not of muscarinic receptors following 14 days administration of uncompetitive NMDA receptor antagonists

Behavioral changes have previously been reported following administrations of uncompetitive NMDA receptor antagonists memantine, amantadine and MK-801 for 14 days, at the doses that produce plasma levels comparable to those seen in patients (20, 100 and 0.31 mg/kg/day respectively). Using the same doses, the effect on receptor binding (autoradiography) was studied in rats. [3H]MK-801 binding was increased in the dentate gyrus and CA3 region of the hippocampus (35.2 and 24.3% respectively) following 3 days S.C. infusion of memantine by ALZET minipumps. One daily injection of memantine for 14 days, increased [3H]MK-801 binding in the frontal cortex by 40.3%. The same treatment with amantadine did increase [3H]raclopride binding to dopamine D2 receptors by 13.5%. None of these treatments changed the expression of muscarinic receptors. It is concluded that subchronic blockade of the NMDA receptor by uncompetitive antagonists at moderate (therapeutically-relevant) doses induced only minor changes in NMDA and dopamine D2 receptor expression.

The role of probenecid-sensitive organic acid transport in the pharmacokinetics of N-methyl-D-aspartate receptor antagonists acting at the glycine(B)-site: microdialysis and maximum electroshock seizures studies

The purpose of the present study was to determine whether the probenecid-sensitive organic acid transporter is responsible for the short duration of action of a new group of N-methyl-D-aspartate receptor glycine(B)-site antagonists, MRZ 2/570, 2/571, and 2/576. A prolongation of their anticonvulsant activity from 60 to 180 to 240 min, was found in mice after pretreatment with probenecid (200 mg/kg i.p.). Microdialysis studies in rats showed that this is likely due to a change in central nervous system concentrations of these drugs because cotreatment with probenecid caused an increase in the brain extracellular fluid half-life (0.5- to 4-fold) and the brain area under the curve (1.8- to 3.6-fold). In serum the half-life of MRZ 2/576 (30 mg/kg) was also increased by coadministration of probenecid from 15.6 +/- 1.3 to 40.6 +/- 6.0 min. At steady state (MRZ 2/576, 20 mg/kg/h i.v.), brain extracellular fluid concentration was elevated 2.5-fold by concomitant administration of probenecid. These results clearly show that these glycine(B)-site antagonists are rapidly cleared from the systemic circulation and the central nervous system by the probenecid-sensitive organic acid transport system. Moreover, the present data show that MRZ 2/570, 2/571, and 2/576 reach the brain in concentrations (1.34-2.32 microM) above the range of their in vitro potencies at the glycine site of the N-methyl-D-aspartate receptor (0.1-1.0 microM).

Modifications of the behavioral profile of non-competitive NMDA receptor antagonists, memantine, amantadine and (+)MK-801 after chronic administration

Non-competitive N-methyl-D-aspartate (NMDA) receptor antagonists show antiparkinsonian-like activity in animal models, and possess neuroprotective properties. However they also induce a number of behavioral side effects in rodents at higher doses; these include learning impairment, hyperlocomotion, and ataxia. The present study focused on the possible development of tolerance, or sensitization, to any of these effects after sustained administration, either by repeated injection or continuous infusion. When memantine or (+)MK-801 (20 and 0.31 mg/kg/day respectively) were either infused or repeatedly injected for 14 days, tolerance was observed to their learning impairing effect at high doses, in a passive avoidance test. Tolerance to their ataxic effect developed after repeated administration ((+)MK-801 and memantine), or after infusion (memantine). Sensitization to the locomotor stimulation was seen following repetitive injections of memantine for 14 days, but not seen with (+)MK-801. In animals with an unilateral 6-OHDA lesion of the nigrostriatal system, acute administration of memantine caused ipsilateral rotations, which were augmented following 14 days of infusion. The potency of amantadine to antagonize neuroleptic-induced catalepsy was unchanged following either infusion or repeated injections. The various acute effects of non-competitive NMDA receptor antagonists were modified differently by sustained treatment (i.e. tolerance to learning impairment and ataxia; sensitization to memantine's locomotor stimulation). The anti-cataleptic activity of amantadine remained unaltered. However, differences between drugs and the two treatment regimens (i.e. repetitive versus continuous treatments) were apparent.

Effect of the Mdr1a P-glycoprotein gene disruption on the tissue distribution of SDZ PSC 833, a multidrug resistance-reversing agent, in mice

The involvement of mdr1a P-glycoprotein (P-gP) on the tissue distribution of the multidrug resistance-reversing agent SDZ PSC 833 was assessed by use of mdr1a (-/-) mice. The mdr1a (-/-) and wild-type mdr1a (+/+) mice received a 4-h constantrate i.v. infusion (2 micrograms/min) of [14C]SDZ PSC 833. Mice were sacrificed at 0, 0.5, 1, 2 and 4 h during infusion and at 0.5, 1, 3, 8 and 24 h after stopping the infusion. Blood and tissues were analyzed on total (14C) and parental SDZ PSC 833 concentrations. Mdr1a (-/-) mice exhibited increased SDZ PSC 833 accumulation in cerebrum, cerebellum and somewhat in testes and small intestine compared with the wild-type mice. The difference between mdr1a (-/-) and (+/+) brain (cerebrum and cerebellum) penetration depended on SDZ PSC 833 blood concentrations, because this cyclosporin analog apparently governs its own brain penetration by inhibiting the P-glycoprotein pump in mdr1a (+/+) mice. Thus the mdr1a (-/-)/(+/+) ratio of brain concentrations tended to decrease and increase at high and low blood concentrations, respectively. These findings clearly demonstrate the interaction of SDZ PSC 833 with the P-glycoprotein present at the blood-brain barrier. The SDZ PSC 833 distribution in other mdr1a P-glycoprotein-expressed tissues, as well as its metabolism and elimination, was not affected by the mdr1a gene disruption. This suggests that factors other than mdr1a P-gP are involved in the disposition of this multidrug resistance-reversing agent.

Interleukin-1 receptors on rat brain endothelial cells: a role in neuroimmune interaction?

Inflammation following an infection induces a range of nonspecific symptoms of sickness in animals and humans. The cytokine interleukin-1 (IL-1) mediates many of the brain-mediated symptoms of sickness. Binding sites for IL-1 have been found in mouse brain, but not in the brains of rats. This raises questions as to the involvement of these neuronally localized IL-1 binding sites in the induction of sickness symptoms. Based on observations of IL-1 receptor mRNA in close vicinity to the vasculature in the mouse and rat brain, we studied the possibility that endothelial cells in the rat brain exhibit IL-1 receptors to transduce information to the brain. Ligand binding studies reveal that cultured endothelial cells of adult rat brain exhibit specific binding sites for rat IL-1beta. Polymerase chain reaction experiments demonstrated that mRNA of the type I but not that of the type II IL-1 receptor is present in rat brain endothelial cells. Incubation of these endothelial cells with recombinant rat IL-1beta showed a dose-dependent increase in interleukin-6, prostaglandin E(2), and prostacyclin secretion. Intravenous administration of rat IL-1beta to adult rats enhanced prostaglandin E(2) immunoreactivity in endothelial cells of the brain microvasculature. These results indicate that functional type I IL-1 receptors are present on endothelial cells of adult rat brain. We postulate that circulating IL-1 can be translated by brain endothelial cells into other signals such as interleukin-6 or prostaglandins that have access to the brain and induce sickness symptoms.

Absorption enhancement of a hydrophilic model compound by verapamil after rectal administration to rats

The use of verapamil as an absorption enhancer for the paracellular route in-vivo was studied using FITC-labelled dextran (molecular weight 4000) (FD-4) as a hydrophilic model compound for transport enhancement. The kinetics of FD-4 after intravenous doses of 1 or 10 mg could be described by a two-compartment model with a systemic clearance of approximately 2 mL min-1 and a terminal plasma half-life of approximately 36 min. Rectal administration to rats, performed as a rectal infusion of 10 mg FD-4 together with 7 mM verapamil, resulted in a 10-fold increase in the percentage of the dose absorbed over a 5-h period compared with the control and a 6-fold increase compared with a bolus administration, although the total amount absorbed remained relatively low (approx. 3% maximum). Large inter-animal variation in effect values were noted. The data indicate that although verapamil is able to enhance the absorption of hydrophilic compounds in-vivo, practical application of verapamil for this purpose doses not seem feasible.

Quantification and visualization of the transport of octreotide, a somatostatin analogue, across monolayers of cerebrovascular endothelial cells

Confocal laser scanning microscopy (CLSM) was used to quantify and visualize the transport of the octapeptide and somatostatin analogue, octreotide (SMS 201-995, Sandostatin), across monolayers of bovine cerebrovascular endothelial cells, an in vitro model of the blood-brain barrier. The concentrations of octreotide and its conjugates in the cell culture medium were determined by radioimmunoassay (RIA). Two fluorescent conjugates of octreotide (FITC- and NBD-octreotide) were used to obtain CLSM images. The peptides did not undergo significant degradation in the presence of brain endothelial cell monolayers. The transport rate of octreotide expressed as clearance (Cl) and endothelial permeability (Pe) did not depend on either the initial concentration (between 10 nM and 1 microM) or the site of administration (luminal or abluminal side of the monolayer), indicating the absence of saturable and/or asymmetrical transport mechanisms. The Pe of octreotide and that of the paracellular permeability marker fluorescein correlated well. Although the conjugates are more lipophilic than octreotide itself, they exhibited lower Cl and Pe, values probably because of their larger molecular size. On the CLSM images, FITC-octreotide was present only in the intercellular space, while the cells did not exhibit detectable fluorescence. Transport studies and CLSM images suggest that octreotide passes the endothelial monolayer primarily via the paracellular route without significant contribution of carrier-mediated transport.

The blood-brain barrier: clinical implications for drug delivery to the brain

The blood-brain barrier (BBB) determines whether or not a given drug can reach the central nervous system (CNS), either by passive diffusion or through carrier or receptor systems. Initial work focused on the structural and physico-chemical requirements favouring transport across the BBB as related to anatomical and physiological features. Such studies have had a significant effect on the design of CNS-active drugs with improved permeability across the BBB. Progress in pharmacology and neurosciences resulted in greater knowledge of CNS diseases and of potential therapies, but also created the need to develop new strategies to improve drug delivery to the brain. For a long time the BBB was considered to be a physical barrier, mainly represented by the cerebrovascular endothelium; however, transport of drugs to the brain may be limited by the metabolic activity of the BBB. The BBB should be regarded as a dynamic rather than a rigid barrier; it can be influenced by astrocytes and probably also by neuronal and hormonal stimuli, and its properties are also affected by diseases of the CNS. This may offer new strategies for targeting drugs to the brain.

Drug transport across the blood-brain barrier. III. Mechanisms and methods to improve drug delivery to the central nervous system

This is the third part of a review on the transport of drugs across the blood-brain barrier. In the first two parts, the anatomical and physiological aspects and the various techniques that can be used to study blood-brain transport have been discussed and reviewed. This third part focuses specifically on the mechanisms that are involved in drug transport across the blood-brain barrier. In addition, the opportunities to improve drug transport into the brain will be reviewed. Emphasis is on the transport of peptides.

Drug transport across the blood-brain barrier. II. Experimental techniques to study drug transport

This is part II of a review on the transport of drugs across the blood-brain barrier. In this part, the emphasis is on the various experimental techniques that can be used to characterize the blood-brain barrier transport of drugs. Generally speaking, three approaches can be distinguished: in vitro techniques using isolated brain capillaries, cerebrovascular endothelial cells in primary culture or endothelium-derived cell lines; in vivo techniques (both single-passage and multi-passage techniques) and in situ perfusion techniques. Each of these techniques has specific advantages and disadvantages associated with it. Therefore, in many instances, a combination of different approaches is needed to study the fundamental aspects of drug transport across the blood-brain barrier.

Drug transport across the blood--brain barrier. I. Anatomical and physiological aspects

This review describes various aspects of the transport of drugs across the blood-brain barrier and comprises three parts. In this first part, the anatomical and physiological aspects of blood-brain transport are discussed. It appears that the blood-brain barrier has an anatomical basis at the endothelium of the capillary wall. This endothelium is characterized by the presence of very tight junctions. As a result, the transport by passive diffusion of drugs with a low lipophilicity, is restricted. For certain classes of closely related relatively hydrophilic compounds, however, the presence of specialized carrier systems has been demonstrated which may facilitate transport. Also evidence is presently available, that the permeability of the blood-brain barrier may be under active regulatory control. It is expected that improved knowledge of the anatomical and physiological aspects of the blood-brain barrier and its regulation will provide a scientific basis for the development of strategies to improve the transport of drugs into the central nervous system.

Modeling of the effect site equilibration kinetics and pharmacodynamics of racemic baclofen and its enantiomers using quantitative EEG effect measures

The plasma to effect site equilibration kinetics and the steady-state plasma concentration-EEG effect relationship of baclofen were characterized after separate administration of racemic baclofen and its two enantiomers. Male Wistar-derived rats received an i.v. infusion of 1.25 mg of racemic, 0.63 mg of R- or 1.25 mg of S-baclofen in 10 min. Frequent arterial blood samples were obtained and the EEG was continuously quantified using aperiodic analysis. The baclofen-induced decrease in amplitudes in the 11.5 to 30 Hz frequency band was used as an EEG effect measure. The delay between plasma concentrations and EEG effect measure was best modeled by a monoexponential conductance function. No differences were observed with respect to the rate of equilibration between plasma and effect for racemic, R- and S-baclofen, with values of the time to reach 50% equilibrium (BET50ss) of (mean +/- S.E.) 23 +/- 4 min, 35 +/- 6 min and 32 +/- 7 min, respectively. The steady-state plasma concentration-EEG effect relationships of racemic and R-baclofen were characterized by the sigmoidal Emax model. The EEG effects of the S-isomer were small and best described by a linear pharmacodynamic model. The pharmacodynamic parameters of the R-isomer were: Emax = -20 +/- 2 microV/sec, EC50 = 0.61 +/- 0.11 mg/l and N = 3.7 +/- 0.7, and not significantly different from the parameters of the racemate when only the "active" R-isomer was taken into account. This indicates that no pharmacodynamic interactions between the two enantiomers occur.(ABSTRACT TRUNCATED AT 250 WORDS)

Rate-controlled rectal absorption enhancement of cefoxitin by co-administration of sodium salicylate or sodium octanoate in healthy volunteers

1. The effects of sodium octanoate and sodium salicylate on the rectal absorption of cefoxitin were investigated in healthy volunteers. Drug solutions were given either as a bolus or as a zero-order infusion. 2. On rectal infusion sodium octanoate and sodium salicylate both enhanced mean cefoxitin bioavailability (+/- s.d.) from 5.0 +/- 1.2% to 9.1 +/- 1.3% and 9.2 +/- 1.5%, respectively. After rectal bolus delivery octanoate increased the mean cefoxitin bioavailability from 7 +/- 3% to 17 +/- 3%, whereas bolus salicylate did not produce a statistically significant effect. All formulations were well tolerated by the volunteers. 3. It is concluded that both octanoate and salicylate are capable of enhancing rectal cefoxitin absorption in man; rate of delivery seems to be an important factor.

The influence of components on the rectal absorption of cefazolin in rats

A study has been made to identify the component(s) responsible for the absorption-promoting effect of MGK medium chain glyceride preparation commercially available as a mixture of glyceryl-1-monooctanoate, glyceryl-1,3-dioctanoate, glyceryl-1,2-dioctanoate, glyceryl trioctanoate, octanoic acid and glycerol. The action of the individual constituents has been evaluated on the rectal absorption of cefazolin in conscious rats. The results indicate that the action of MGK can be completely explained by the effect of glyceryl-1-monooctanoate, which both enhanced the extent and rate of cefazolin uptake.

Rectal absorption enhancement of rate-controlled delivered ampicillin sodium by sodium decanoate in conscious rats

Because of the relatively poor intestinal absorption of ampicillin sodium, efforts have been made to enhance ampicillin absorption by co-administration of absorption promoters. In the present study the enhancing effect of sodium decanoate on rate and extent of rectal ampicillin absorption in rats has been evaluated after rate-controlled and site-controlled delivery of aqueous solutions. Rectal absorption without enhancer was extremely low (8 +/- 7%), and the addition of 0.032 M sodium decanoate gave comparable values. However, administration in 0.16 M decanoate considerably increased ampicillin bioavailability, to 79 +/- 30%, whereas the absorption rate was not significantly affected.

Kinetics of rate controlled rectally administered ranitidine to male volunteers

Ranitidine hydrochloride solution was given rectally to six healthy volunteers by means of an osmotic pump (Osmet) at a zero-order rate for 8 h. Quite constant steady-state drug concentrations were achieved in the range of 64-123 ng/ml plasma (mean Css: 100.6 ng/ml). The mean absorption time was 0.45 h (range 0-I.02 h), indicating that absorption was not always instantaneous. It is concluded that the Osmet system together with the rectal route offer the possibility for achieving steady-state concentrations of ranitidine, enabling the determination of pharmacodynamic effects. In addition, the rectal route can be considered as an alternative to oral and intravenous administration.

Influence of salicylate and anaesthesia on the rectal absorption of theophylline in rats

The effect of sodium salicylate and anaesthesia on the rectal absorption of theophylline was evaluated in rats. Theophylline proved to be slowly but completely absorbed on rectal infusion in conscious rats, compared with intravenous infusion. Pentobarbital anaesthesia did not influence absorption. In contrast to literature data, the results with salicylate showed that rectal absorption of theophylline was not enhanced.

Synthetic hydrogels as drug delivery systems

Hydrogels are widely studied materials for the preparation of sustained release drug dosage forms. Their soft, tissue-like consistency and their high biocompatibility in a number of applications make them promising candidates for this purpose. The water and the polymer in the gel form intricate structures and much research has been devoted to the elucidation of these structures, and of the interactions involved in their formation. Simple, drug-loaded hydrogels normally give a matrix-type delivery profile, in which the release rate is proportional to the square root of time; a number of approaches has been used to change this profile to other types of delivery, for instance to zero-order release. A number of in vivo tests using hydrogel delivery systems has given favourable results.