Fate of orally ingested enzymes in pancreatic insufficiency: comparison of two pancreatic enzyme preparations

The effect on steatorrhoea of a pH-sensitive enteric-coated pancreatic preparation (Eurobiol 25,000) was compared with a conventional pancreatic enzyme preparation (Eurobiol) in six adult patients with exocrine pancreatic insufficiency. In addition, the fate of orally ingested pancreatic enzymes in the upper digestive tract was evaluated by measuring gastric and duodenal pH, amount of enzymes in the stomach, duodenal enzyme output, and fat absorption at the angle of Treitz for the 4 hours following a standard meal. When compared with placebo, Eurobiol and Eurobiol 25,000 reduced daily faecal fat excretion by 24% (not significant) and 43% (P less than 0.05), respectively. With the conventional preparation, enzyme output and fat absorption at the duodeno-jejunal flexure were significantly improved (P less than 0.05). Marked inter-individual differences in duodenal enzyme recovery (lipase 3% to 80%; chymotrypsin 26% to 100%) and, consequently, in the reduction of steatorrhoea (0% to 67%) were observed, with the gastric emptying rate emerging as a key determinant factor. With the enteric-coated preparation, enzyme output and fat absorption at the duodenojejunal flexure were not significantly improved. Discrepancy between the marked reduction of faecal fat excretion and the low duodenal enzyme recovery could indicate that enzyme delivery from microtablets occurs further down in the small bowel. Efficacy of enteric-coated preparations could be enhanced by adding unprotected enzymes, especially in patients with rapid gastric emptying.     

In vitro dissolution profiles of enteric-coated microsphere/microtablet pancreatin preparations at different pH values

Objectives : Enteric-coated microsphere/microtablet pancreatin should stay intact in the stomach and dissolve promptly on entering the duodenum. Post-prandial intraluminal pH in the distal duodenum is 5.75 and is lower in exocrine pancreatic insufficiency. The aim of the study was to measure in vitro dissolution times in buffer solutions with pH 4.0–6.0 for five currently available enteric-coated microsphere/microtablet pancreatin preparations.
Methods : The following preparations were tested: Creon, Creon Forte, Pancrease, Pancrease HL and Panzytrat. Two capsules were placed in the buffer solution at 37 °C in a USP dissolution testing apparatus. Buffer solutions with pH between 4.0 and 6.0 were used. Solutions were stirred at 125 r.p.m. and the rate of dissolution was monitored by taking 2-mL samples at regular intervals and measuring extinction at 280 nm. Measurements were repeated six times.
Results : All preparations failed to dissolve at pH 4.0. At pH 5.0 Pancrease HL showed 43% dissolution within 30 min, all other preparations 15% or less. Panzytrat and Pancrease HL showed more than 50% dissolution within 30 min at pH 5.2. Panzytrat, Pancrease HL and Creon Forte had more than 90% dissolution within 30 min at pH 5.6, and all preparations more than 90% dissolution within 30 min at pH 5.8 and higher.
Conclusions : For the treatment of exocrine panceatic insufficiency conventional strength enteric-coated microsphere/microtablet pancreatin preparations do not have an optimal dissolution profile. The newer, high lipase preparations such as Pancrease HL perform better, although still not optimally, at pH 5.4 and lower.     

Effects of product formulation on in vitro activity of pancreatic enzymes

The lipase activity of two enteric-coated and two uncoated pancreatic enzyme formulations was evaluated in vitro at different pH values and compared with postprandial duodenal pH data in cystic fibrosis patients. Lipase activity was measured over a pH range of 4-8 in four formulations: Viokase tablets and Viokase powder (Viobin), Cotazym-S (Organon), and Pancrease enteric-coated spherules (McNeil). A pH-stat technique was used in which the amount of hydroxyl ion that must be added to maintain a preset pH value is measured to determine the amount of lipase substrate (tributyrin) that is split into butyric acid. At least three determinations of activity were made at each pH. Six capsules of each enteric-coated formulation were subjected to disintegration testing at various pH values. These data were then compared with data available for postprandial duodenal pH in patients with cystic fibrosis. The lipase activity of all formulations studied decreased when pH decreased, especially when the pH was below 5.75. At a pH value between 5 and 5.5, which represents the postprandial duodenal pH in cystic fibrosis patients, activity was reduced 50% or more for all formulations tested. The two enteric-coated products displayed no activity at pH 5.5 and below because the coating did not dissolve in this pH range. Lipase activity in enteric-coated pancreatic enzyme preparations is limited because the enteric coating of these products dissolves slowly in the duodenal pH range (5-5.75) that is found in patients with cystic fibrosis.     

Pancreatin preparations used in the treatment of cystic fibrosis-- lipase content and in vitro release.

BACKGROUND: Pancreatic extracts are essential in the treatment of the majority of cystic fibrosis patients. The clinical response to different preparations is often unpredictable and at present there is no sure method of determining the best preparation for a particular patient. METHODS: Creon, Nutrizym GR, Pancrease and the high-lipase versions, Creon 25,000, Nutrizym 22 and Pancrease HL, were investigated for lipase content and resistance to simulated gastric conditions. The rates of lipase release in response to pH change, bile salts and duodenal solids were investigated. The stability of lipase and its binding to duodenal solids were also investigated. RESULTS: Declared values for lipase content were exceeded in all preparations. All preparations were acid resistant. The release of lipase in response to pH change showed notable differences in release rates. After 20 min at pH 5.5, Creon released three times the amount of lipase compared with Pancrease, the other preparations coming within the range. Above pH 5.75, the release rates were comparable amongst the preparations. Bile salts influenced release variably whilst release in a solid-rich duodenal fluid was much slower than in buffers. The released lipase was susceptible to proteolysis and pH-dependent binding to duodenal solids; these effects may compromise lipolysis. CONCLUSIONS: These results show some factors contributing to variable clinical responses to pancreatic supplements. Improvements may result if a patient is assessed on different preparations.     

Effect of oral pancreatic enzyme administration on digestive function in healthy subjects: comparison between two enzyme preparations

Background : Intraduodenal proteases exert a negative feedback on pancreatic secretion.
Aim : To investigate the effect of two pancreatic enzyme preparations (enteric-coated tablets, and capsules with enteric-coated microtablets) on postprandial pancreatic and bile acid secretion, gastroduodenal motility and release of gastrin and pancreatic polypeptide in healthy humans.
Methods : Twenty healthy males were studied on two different days one week apart. After an overnight fast a nine-lumen motility tube was positioned with the distal tip at the Treitz angle. On each study day, 30 min after an interdigestive migrating motor complex-phase III, a semi-liquid test meal was given either alone ( n =20) or with enzymes (3 tablets ( n =10) or 2 capsules with microtablets ( n =10); 40000 U lipase and 2000 proteases) in a randomized order, and the study continued over 2 h. Motility was continuously recorded with four ports in the antrum and three in the duodenum, using a low-compliance pneumohydraulic perfusion system. Secretion of human-specific pancreatic elastase and bile acids was measured by a standard duodenal intubation perfusion technique. Plasma concentrations of gastrin and pancreatic polypeptide were measured by specific radioimmunoassays.
Results : Postprandial pancreatic secretion was significantly reduced by administration of microtablets (median 82 mg/2 h vs. 70 mg/2 h, P <0.02) but not by tablets (median 59 mg/2 h vs. 58 mg/2 h, N.S.). No changes were observed in bile acid secretion, antroduodenal motility or release of gastrin and pancreatic polypeptide.
Conclusions : Oral administration of pancreatic enzymes at normal therapeutic doses significantly inhibits postprandial pancreatic secretion in healthy humans, when capsules with enteric-coated microtablets are given. Exogenous pancreatic enzymes have no significant effect on bile acid secretion, gastroduodenal motility and hormone release.     

A 2-year post-authorization safety study of high-strength pancreatic enzyme replacement therapy (pancreatin 40,000) in cystic fibrosis

Objective: At the request of the Medicines and Healthcare Regulatory Agency and in agreement with the appropriate authorities, an observational, multi-center, non-interventional, post-authorization safety study of high-strength pancreatic enzymes was conducted.

Research design and methods: Patients with exocrine pancreatic insufficiency due to cystic fibrosis (CF) who had previously taken high doses of pancreatic enzymes received pancreatin 40,000 capsules (Creon 40,000 Minimicrospheres, Abbott GmbH, Hanover, Germany) as part of their normal treatment for up to 2 years. Initial doses were calculated to match previous established doses in lipase units, with adjustment if required.

Main outcome measures: Safety focused on serious suspected adverse drug reactions. Maldigestion symptoms and body weight were also monitored. Patients were managed according to general guidelines common to all major CF units in the UK, although minor variations were expected. The coefficient of fat absorption was not assessed as this was a safety rather than an efficacy study.

Results: Sixty-four patients were enrolled at nine UK centers. Two deaths occurred during the study, which were considered unrelated to therapy by investigators. There were no further serious suspected adverse drug reactions related to pancreatin 40,000 and no cases of fibrosing colonopathy. Daily lipase doses were reduced by 11% after switching to pancreatin 40,000. Maldigestion symptoms improved and mean body weight increased from baseline to last observation (mean + 6.1 kg in patients < 18 years old).

Conclusions: No safety concerns were identified with pancreatin 40,000 therapy for up to 2 years. Daily lipase doses were not increased when switching to pancreatin 40,000.     

H2-receptor antagonists and antacids have an aggravating effect on Helicobacter pylori gastritis in duodenal ulcer patients

Background : Antacids, such as aluminium–magnesium hydroxide (AlMg(OH)₃), or H₂-receptor antagonists, such as ranitidine, are common drugs used for treating peptic ulcer disease and acid-related symptoms.
Methods : In a prospective double-blind controlled study, 174 patients were randomized to a 4-week course of treatment with either AlMg(OH)₃ (acid-binding capacity: 280 mval/day) or ranitidine 300 mg for active Helicobacter pylori -associated duodenal ulcers (as determined by histology and the urease test). Before and after treatment, two biopsy specimens each were obtained from the antrum and corpus, and the grade and activity of gastritis, as well as H. pylori density, were determined using a score ranging from 0 = none to 4 = severe.
Results : Pre- and post-treatment histology were available for 138 patients (AlMg(OH)₃: 67, ranitidine: 71). Treatment with AlMg(OH)₃ significantly increased the activity of corpus gastritis (Wilcoxon signed-rank: P  = 0.0014), while ranitidine treatment significantly increased both the grade and activity of corpus gastritis ( P  = 0.0002 and P  = 0.0001 respectively). In the antrum, both regimens provoked a significant increase in the frequency of intestinal metaplasia, but this may be a consequence of sampling error.
Conclusions : Ranitidine and AlMg(OH)₃ have an aggravating effect on H. pylori gastritis in duodenal ulcer patients. This should be considered a side-effect of the respective drugs and is more pronounced with ranitidine.     

A comparison of two pancreatin microsphere preparations in cystic fibrosis.

OBJECTIVES: to compare the efficacy and tolerance of Creon and Pancrease in children and young adults with cystic fibrosis. METHODS: a double blind, crossover study of two pH sensitive microsphere preparations of pancreatin (Creon, Pancrease), given in equivalent lipase dosage to 27 children with cystic fibrosis, was conducted. RESULTS: at similar lipase activity no significant difference was found in the following: coefficient of fat absorption (CFA), coefficient of nitrogen absorption (CNA), weight gain, mean adequate daily intake for energy, and subjective symptoms. Three children who had a CFA less than 70% while receiving Pancrease all improved on Creon. No children had a CFA less than 70% while receiving Creon. A significant reduction in the number of capsules required daily to achieve similar control was possible when changing from Pancrease (mean 25/day) to Creon (mean 15/day). Seventy percent of patients preferred Creon and this was likely to be related to a perceived reduction in abdominal pain and stool frequency, and need for less capsules per day. CONCLUSION: Creon and Pancrease are equally effective at doses providing equal lipase activity, however, the reduced number of capsules, fewer symptoms, and possible improvement of more severe steatorrhoea result in an increased patient preference for Creon.     

Release of digestive enzymes from isolated rat pancreatic acini following muscarinic stimulation: a comparative study with enzyme release and receptor binding.

Effect of cholinergic stimulation on the release of digestive enzymes from isolated rat pancreatic acini prepared by collagenase digestion was investigated. The release of enzymes (amylase, chymotrypsinogen, lipase) increased linearly with the time of incubation in the absence of secretagogues. Carbachol, a muscarinic agonist, induced a remarkable increase in the release of these enzymes. This carbachol-stimulated amylase release showed a biphasic curve, and its maximal response was observed at 10(-5) M. The release patterns of chymotrypsinogen and lipase were similar to that of amylase. This carbachol-stimulated amylase release was inhibited by atropine in a dose-dependent manner, with an ED50 value of 1.17 X 10(-8) M. Other cholinergic agonists such as methacholine also stimulated the release of these enzymes, and these increases in the release were inhibited by atropine. Scatchard analysis for [3H]quinuclidinyl benzilate ([3H]QNB) binding to isolated pancreatic acini revealed that the binding site of [3H]QNB was a single component with a Kd value of 0.09 nM and a Bmax value of 89.3 fmol/mg protein, respectively. The effect of other cholinergic antagonists, pirenzepine and AF-DX 116 (11-[[2-[(diethylamino) methyl]1-piperidinyl]acetyl]-5,11-dihydro- 6H-pyrido[2,3-b][1,4]-benzodiazepine-6-one), on pancreatic acini was also investigated. Based on these results, it has been concluded that isolated rat pancreatic acini have muscarinic receptors and are useful for analyzing the mechanism of pancreatic enzyme secretion.     

[13C]-pantoprazole breath test to predict CYP2C19 phenotype and efficacy of a proton pump inhibitor, lansoprazole

Background  ¹³CO₂ is produced on metabolism of ¹³C-labelled-pantoprazole ([¹³C]-pantoprazole) by CYP2C19.
Aim  To investigate whether the [¹³C]-pantoprazole breath test can predict CYP2C19 status and efficacy of proton pump inhibitors (PPIs) in Japanese.
Methods  We classified 110 healthy volunteers as rapid metabolizers (RM), intermediate metabolizers (IM) or poor metabolizers (PM) of CYP2C19 by genotyping. Breath samples were collected at 10-min intervals for 60 min after dosing with 100 mg [¹³C]-pantoprazole. Changes in the carbon isotope ratios (¹³CO₂/¹²CO₂) in carbon dioxide in breath samples were measured and expressed as a delta-over-baseline (DOB) ratio (‰). Of the 110 subjects, twenty-two randomly selected subjects underwent intragastric pH monitoring on day 7 of dosing with 30 mg of lansoprazole.
Results  The DOB values of RMs were the highest and those of PMs the lowest of the three groups. Statistically significant differences were observed in the area-under-the-curve ( AUC )_20–60 min of DOB among the three groups. The mean 24-h intragastric pHs attained by lansoprazole 30 mg for 7 days were inversely correlated with the AUC _20–60 min of DOB.
Conclusions  [¹³C]-pantoprazole breath test can easily estimate the individual activity of CYP2C19 and predict the efficacy of a PPI (i.e. lansoprazole). This test would be useful for individualized medicine with a PPI.     

Metabolic interactions of selected antimalarial and non-antimalarial drugs with the major pathway (3-hydroxylation) of quinine in human liver microsomes

Aims Nine antimalarial (plus two metabolites of proguanil) and twelve non-antimalarial drugs were tested for their possible interaction with CYP3A4-catalysed 3-hydroxylation of quinine by human liver microsomes in vitro .
Methods 3-Hydroxyquinine was assayed in the incubation mixture by an h.p.l.c. method using fluorometric detection. The respective I C ₅₀ values were estimated for the twenty-one drugs and two metabolites of proguanil tested herein.
Results Thirteen drugs exhibited an inhibitory effect on the 3-hydroxylation of quinine. According to the respective mean I C ₅₀ values, the inhibitory rank order of the drugs was: ketoconazole>troleandomycin (TAO, with preincubation)> doxycycline>omeprazole>primaquine>tetracycline=TAO (without preincubation)>nifedipine>erythromycin>verapamil>cimetidine>diltiazem>oleandomycin>hydralazine. Other drugs or metabolites showed little or no inhibition of quinine metabolism (mean I C ₅₀>200 or 500  &mgr;m ). Among the antimalarial drugs, doxycycline showed relatively potent inhibition of quinine 3-hydroxylation with a mean I C ₅₀ value of 17  &mgr;m, followed by primaquine and tetracycline, with mean I C ₅₀ values of 20 and 29  &mgr;m, respectively.
Conclusions When the plasma/serum concentrations possibly attained after their usual therapeutic doses were taken into account, tetracycline, doxycycline, omeprazole, ketoconazole, nifedipine, TAO and erythromycin are likely to be inhibitors of quinine metabolism in patients when the drugs are co-administrated with quinine.     

Pharmacodynamics of oxypurinol after administration of allopurinol to healthy subjects

1   Eight healthy subjects received 50, 100, 300, 600 and 900  mg allopurinol daily for 1 week each, in random order with 1 week separating each treatment period. The pre-dose plasma concentration of oxypurinol, the extent of inhibition of xanthine oxidase, plasma urate concentration and urine urate excretion rate were assessed on the last 2 days of each treatment week.
2   The ratio of 1-methyluric acid (1MU) over 1-methylxanthine (1MX) in the urine, following a dose of 50  mg 1MX infused intravenously over 20  min, was used to measure the inhibition of xanthine oxidase.
3   The steady-state plasma concentration of oxypurinol increased linearly with increasing dose of allopurinol between 50  mg to 600  mg day⁻¹, with a weak indication of saturation at the higher 900  mg day⁻¹ dose rate.
4   The relationships between plasma oxypurinol concentration and xanthine oxidase inhibition (1MU/1MX ratio), plasma urate concentration and urine urate excretion rate were fitted to an inhibition sigmoid E_max model and the C ₅₀ values for oxypurinol were 26.38±4.87, (mean±s.d.) 36.58±8.36 and 24.61±9.08  μm, respectively.
5   1MU/1MX ratio appeared to be a reliable index of xanthine oxidase activity in vivo as the C ₅₀ for oxypurinol observed for 1MU/1MX ratio, plasma urate concentration and urine urate excretion rate were similar.
6   The concentration of oxypurinol required for inhibition of xanthine oxidase, as indicated by C ₅₀, was lower than those often observed in clinical practice.     

Nitric oxide: an important role in the maintenance of systemic and pulmonary vascular tone in man

Aims The aim of this study was to examine whether nitric oxide (NO) has an important role in maintaining basal vascular tone in normal man by examining the effects of nitric oxide inhibition using N ^G-monomethyl-l-arginine (l-NMMA) on systemic and pulmonary haemodynamics.
Methods Ten normal male volunteers 26±1.6 years were studied on two separate occasions in a double-blind, placebo controlled crossover study. They were randomised to receive either a continuous infusion of l-NMMA (4  mg  kg⁻¹  h⁻¹ ) with a front loaded bolus (4  mg  kg⁻¹ ) or volume matched placebo. Pulsed wave Doppler echocardiography was used to measure cardiac output (CO), mean pulmonary artery pressure (MPAP) and hence systemic vascular resistance (SVR) and total pulmonary vascular resistance (TPR). Measurements were made prior to infusion ( t ₀ ) and after 4, 8, and 12  min ( t ₁, t ₂ and t ₃ ).
Results Infusion of l-NMMA significantly increased mean arterial blood pressure (MAP), SVR and TPR and significantly reduced heart rate (HR), stroke volume (SV) and CO compared to placebo. These effects were observed at t ₁ and persisted during the entire infusion period.
Conclusions These results are consistent with a role for basal nitric oxide generation in the maintenance of basal systemic and pulmonary vascular tone in normal man.     

Impact of ribavirin plasma level on sustained virological response in patients treated with pegylated interferon and ribavirin for chronic hepatitis C

Background  The main goal of therapy in hepatitis C virus (HCV) infection is to achieve a sustained virological response (SVR). However, the impact of the pharmacological properties of ribavirin on the SVR has not been fully investigated.
Aim  To evaluate, through a prospective study, the association between ribavirin plasma level and SVR response in HCV patients treated with pegylated interferon (PEG-IFN) and ribavirin.
Patients and methods  Patients treated with PEG-IFN and ribavirin had plasmatic ribavirin dosage at weeks 4 and 12. SVR was evaluated 6 months after the end of treatment.
Results  At week 4, a strong correlation was found between HCV-RNA and C _min of ribavirin plasma level ( r  = −0.376, P  = 0.002) and AUC _0→12h of ribavirin plasma level ( r  = −0.277, P  = 0.018). At week 12, a strong correlation was found between HCV-RNA and C _min of ribavirin plasma level ( r  = −0.384, P  < 0.0001) and AUC _0→12h of ribavirin plasma level ( r  = −0.257, P  = 0.002). In genotype 1 patients, AUC _0→12h ribavirin and C _min were significantly correlated with negative HCV-RNA at week 12 and SVR. In the multiple logistic regression model, the only factor independently associated with SVR in genotype 1 patients was negative HCV-RNA at week 12.
Conclusion  C_min of ribavirin at weeks 4 and 12 was significantly higher in sustained virological responders compared with relapser or nonresponder patients. However, in genotype 1 patients, plasma ribavirin level at weeks 4 and 2 was not associated with SVR.     

Benazepril, an angiotensin converting enzyme inhibitor: drug interaction with salbutamol and bronchial response to histamine in normal subjects

Aims To investigate the effect of the angiotensin converting enzyme inhibitor, benazepril, on pulmonary function.
Methods We investigated the influence of benazepril, on lung function and the interaction with inhaled salbutamol (0.1 to 6.6  mg) and histamine (0.03 to 30.69  g  l⁻¹ ) in normal subjects. Benazepril 20  mg, salbutamol 8  mg, propranolol 160  mg, and placebo were given orally once daily over 10 days.
Results On day 8, there was no difference in the area under the salbutamol dose-response curves between benazepril, placebo and oral salbutamol ( P >0.05), propranolol shifted the curves to the right ( P <0.05). On day 10, histamine challenge resulted in following P D ₃₅sGaw values (geometric mean and 95% CI): with placebo 1.02 (0.95–1.09)  g  l⁻¹ , benazepril 1.04 (0.99–1.08), salbutamol 1.19 (1.13–1.25), propranolol 0.57 (0.50–0.65).
Conclusions Benazepril had no influence on baseline lung function, caused no interaction with inhaled salbutamol and the bronchial response to histamine was similar to placebo. However, our findings in normal subjects cannot be extrapolated automatically to asthmatics.     

Inhibitory effects of antiarrhythmic drugs on phenacetin O-deethylation catalysed by human CYP1A2

Aims The aim of the study was to clarify whether the pharmacokinetic interaction between theophylline and mexiletine is mediated by inhibition of CYP1A2 and to assess the possible interaction potential of other antiarrhythmic drugs with drugs metabolized by CYP1A2.
Methods The inhibitory effects of mexiletine and 10 antiarrhythmic drugs on phenacetin O -deethylation, a marker reaction of CYP1A2, were studied using human liver microsomes and cDNA-expressed CYP1A2.
Results Propafenone and mexiletine inhibited phenacetin O -deethylation with I C ₅₀ values of 29 and 37  μm, respectively. Disopyramide, procainamide and pilsicainide produced negligible inhibition of phenacetin O -deethylation (I C ₅₀>1  mm ). Amiodarone, bepridil, aprindine, lignocaine, flecainide and quinidine inhibited phenacetin O -deethylation in a concentration-dependent manner, although the inhibitory effects were relatively weak with I C ₅₀ values ranging from 86 to 704  μm. Propafenone and mexiletine selectively abolished the high-affinity component of phenacetin O -deethylation in human liver microsomes. In addition, propafenone and mexiletine inhibited phenacetin O -deethylation catalysed by cDNA-expressed CYP1A2.
Conclusions These data suggest that, among the antiarrhythmic drugs studied, propafenone and mexiletine are relatively potent inhibitors of CYP1A2, which may cause a drug-drug interaction with drugs metabolized by CYP1A2.     

Diazepam–omeprazole inhibition interaction: an in vitro investigation using human liver microsomes

1 The metabolism of diazepam to its primary metabolites 3-hydroxydiazepam (3HDZ) and nordiazepam (NDZ) was evaluated in human liver microsomes. The 3HDZ pathway was the major route of metabolism representing 90% of total metabolism with a V _max /K _m ratio of 0.50–7.26  μl  min⁻¹  mg ⁻¹ protein.
2 Inhibition of the two metabolic pathways of diazepam by omeprazole was investigated. The NDZ pathway was not affected by omeprazole whilst a K _i of 201±89  μm was obtained for the 3HDZ pathway ( K _m /K _i ratio of 3.0±0.9).
3 Inhibitory effects of omeprazole sulphone on the 3HDZ and NDZ pathways were also investigated. Omeprazole sulphone inhibited both pathways with similar K_is of 121±45 and 188±73  μm respectively ( K _m /K _i ratios of 5.2±2.3 and 3.3±1.5 respectively).
4 These in vitro data provide direct evidence for cytochrome P450 inhibition as the mechanism for the well documented diazepam-omeprazole clinical interaction and indicate that omeprazole sulphone, as well as the parent drug, contribute to the inhibition effect.     

1-Methylxanthine derived from caffeine as a pharmacodynamic probe of oxypurinol effect

Aims In the present study we have investigated the use of caffeine, administered in the form of instant coffee, as a prodrug for 1MX to validate the use of the 1MU:1MX ratio following caffeine administration as a pharmacodynamic measure of oxypurinol effect on xanthine oxidase.
Methods Five healthy volunteers took caffeine 75  mg 8 hourly administered as instant coffee over a 7 day period. They were given allopurinol 600  mg on day 4. Urine was collected in 8  h aliquots from day 1–day 7. The ratio of 1-methyluric acid (1MU) to 1-methylxanthuric (1MX) was determined.
Results The relationship between the plasma oxypurinol (the active metabolite of allopurinol) concentration at the midpoint of each caffeine dosage interval and the decrement in the urinary 1MX to 1MU ratio fitted well by a sigmoid E_max model. Mean (±s.d.) values of the oxypurinol E C ₅₀(3.9±1.4  mg  l^{− 1} ), E C ₉₀(8.7±1.8  mg  l^{− 1} ) and the exponent, n (3.0±1.2) were similar to those obtained previously following either the direct administration of 1MX or the use of theophylline as a prodrug for 1MX.
Conclusions These data indicate that the use of caffeine as a source of 1MX could provide a simple and ethically acceptable method for monitoring oxypurinol effect in patients taking allopurinol for the treatment of gout.     

The risk of chikungunya fever in a dengue-endemic area

Background.  Chikungunya, an alphavirus of the Togaviridae family, causes a febrile disease transmitted to humans by the bite of infected Aedes mosquitoes. This infection is reaching endemic levels in many Southeast Asian countries. Symptoms include sudden onset of fever, chills, headache, nausea, vomiting, joint pain with or without swelling, low back pain, and rash. According to the World Health Organization, there are 2 billion people living in Aedes -infested areas. In addition, traveling to these areas is popular, making the potential risk of infections transmitted by the bite of infected Aedes mosquitoes very high.
Methods.  We proposed a mathematical model to estimate the risk of acquiring chikungunya fever in an Aedes -infested area by taking the prevalence of dengue fever into account. The basic reproduction number for chikungunya fever R _0chik can be written as a function of the basic reproduction number of dengue R _0dengue by calculating the ratio R _0chik/ R _0dengue. From R _0chik, we estimated the force of infection and the risk of acquiring the disease both for local residents of a dengue-endemic area and for travelers to this area.
Results.  We calculated that R _0chik is 64.4% that of R _0dengue. The model was applied to a hypothetical situation, namely, estimating the individual risk of acquiring chikungunya fever in a dengue-endemic area, both for local inhabitants (22% in steady state) and for visiting travelers (from 0.31% to 1.23% depending on the time spent in the area).
Conclusions.  The method proposed based on the output of a dynamical model is innovative and provided an estimation of the risk of infection, both for local inhabitants and for visiting travelers.     

(−)-Timolol is a more potent antagonist of the positive inotropic effects of (−)-adrenaline than of those of (−)-noradrenaline in human atrium

1 The affinity of (−)-timolol for β₁- and β₂-adrenoceptors was determined on isolated atrial preparations from patients undergoing open heart surgery. The times for onset and offset of antagonism of the positive inotropic effects of (−)-adrenaline and (−)-noradrenaline by (−)-timolol were measured.
2 The antagonism of the positive inotropic effects of (−)-adrenaline and (−)-noradrenaline by (−)-timolol (0.1–100 nm) was simple competitive in human atrium tissue. The slope of Schild-plots was not significantly different from 1.0 [0.93±0.09 for (−)-adrenaline, 0.97±0.09 for (−)-noradrenaline].
3 The inotropic effects of (−)-adrenaline were antagonized significantly more by each concentration of (−)-timolol than those of (−)-noradrenaline. K _B-values (-log m) were 10.10±0.09 against (−)-adrenaline and 9.43±0.07 against (−)-noradrenaline ( P <0.001).
4 Blocking kinetics of (−)-timolol for the β-adrenoceptor were relatively slow. Half-times for the onset of blockade by 10 times K _B of (−)-timolol were approximately 30  min for both (−)-adrenaline and (−)-noradrenaline; offset times were similar.
5 It is concluded that (−)-timolol has a higher affinity for the β₂-adrenoceptor than for the β₁-adrenoceptor in human atrium. This property may be beneficial clinically in protecting against the β₂-adrenoceptor hypersensitivity induced by cardiac β₁-adrenoceptor blockade, but also explain why severe asthma can occur after administration of very low intra-ocular doses of the drug.