Capsaicin and neurokinin A-induced bronchoconstriction in the anaesthetised guinea-pig: evidence for a direct action of menthol on isolated bronchial smooth muscle.

1. For many years menthol has been used in the treatment of respiratory disorders although, a bronchodilator effect of menthol has yet to be described. Using the bronchoconstrictors capsaicin (acting via stimulating the release of neuropeptides from sensory afferents) and neurokinin A (NKA) we have raised airways resistance in the guinea-pig (GP) and studied the effect of menthol on both capsaicin and NKA-induced bronchoconstriction in vivo. In vitro the effect of menthol on acetylcholine (ACh) and KCl precontracted GP bronchi was also studied. 2. GP (n = 13) were anaesthetized (urethane 1.5 g kg-1, i.p.) and a bolus injection of capsaicin (7.5 micrograms ml-1, i.v.) or infusion of NKA (1 microgram min-1, i.v.) was given either in the presence of air (0.81 min-1) or air impregnated with menthol vapour (7.5 micrograms l-1) freely breathed from a tracheal cannula via a T-piece. Airways resistance (Raw) and ventilation were measured throughout. Bronchi of mean internal diameter (1029 + 73.6 microns; n = 24) were removed from GP (n = 16) and mounted in the Cambustion myograph. Bronchial rings were maximally precontracted with 80 mM KCl or 2 mM ACh. Relaxation due to a cumulative dose of menthol (1- 3000 microM) was measured. 3. Menthol produced a significant (P < 0.05) 51.3% reversal of the capsaicin-induced increase in Raw, and also inhibited the significant (P < 0.05) reduction in minute ventilation (Ve) associated with the capsaicin-induced increased in Raw. Menthol also caused a significant (P < 0.05) 41% reversal of the NKA-induced increase in Raw. The NKA-induced decrease in Ve was again significantly (P < 0.05) reversed with menthol inhalation. Menthol caused a significant (P < 0.001) dose-dependent relaxation of KCl and ACh precontracted bronchi. 4. We have shown that menthol attenuates both capsaicin and NKA-induced bronchoconstriction in vivo and relaxes KCl and ACh preconstricted bronchi in vitro. Menthol inhibition of NKA and capsaicin-induced bronchoconstriction could be, in part, explained by a direct action of menthol on bronchial smooth muscle.

Identification of the L-menthol binding site in guinea-pig lung membranes.

1. L-Menthol inhibits both neurokinin A and capsaicin-induced bronchoconstriction in the guinea-pig and relaxes pre-constricted guinea-pig isolated bronchi. Structure-activity relationships have been defined for the action of (-)-menthol and related compounds on cold receptors, suggesting an action of L-menthol at a pharmacological receptor. We have performed radioligand binding studies to characterize the binding sites for [3H]-L-menthol in whole cell membranes prepared from guinea-pig lung tissue. 2. In kinetic studies. [3H]-L-menthol was found to bind rapidly and reversibly. Binding of [3H]-L-menthol to lung membranes was found to be time-dependent becoming fully associated to its site within 40 min, and half-maximum association occurred within 8 min (t1/2=8 min). [3H]-L-menthol was fully dissociated from its binding site within 8 min, (t1/2=2 min). 3. Inhibition studies presented a pharmacological profile of the 'L-menthol site'. Capsaicin, capsazepine, D-menthol, eugenol, SCH23390 and camphor were all found to displace [3H]-L-menthol binding. In contrast WS3, noradrenaline, 5-hydroxytryptamine, spiperone, flunarazine, bepridil and nicardipine were without effect. 4. We have identified a L-menthol binding site in the guinea-pig, which may represent a site common to a variety of compounds.

A pharmacokinetic study investigating the rate of absorption of a 500 mg dose of a rapidly absorbed paracetamol tablet and a standard paracetamol tablet.

OBJECTIVE: A rapidly absorbed tablet formulation of paracetamol containing sodium bicarbonate (PS) has been previously shown to be absorbed at least twice as fast as a standard paracetamol tablet (P) at a 1 g dose. In South America and Asia it is customary for patients to take a 500 mg dose of analgesic. The objective of this pharmacokinetic study was to compare the rate of absorption of PS versus P at a 500 mg dose. RESEARCH DESIGN AND METHODS: An open, randomized, single dose, cross-over study. Thirty Hispanic healthy volunteers randomly received a 500 mg dose taken orally with 50 mL of water 2 h after a standard breakfast. Blood samples were taken up to 10 h post-dose. Plasma concentrations of paracetamol were determined by HPLC with UV detection. MAIN OUTCOME MEASURES: AUC(0-30 min), C(plasma 30 min) and T(max) were analyzed non-parametrically by the Wilcoxon's rank sum test. A linear mixed effects model was used to analyze the logarithmically transformed AUC(0-alpha) and C(max). Bioequivalence was accepted if the 90% confidence intervals (CI) for the ratio of the means of the primary pharmacokinetic variable AUC(0-alpha) lay completely within the range 0.80-1.25. RESULTS: AUC(0-30 min) and C(plasma 30 min) were significantly greater and T(max) was significantly shorter (all p < 0.0001) for PS versus P. The formulations were bioequivalent for AUC(0-alpha) (90% CI 0.99:1.05) and no statistical difference was seen for C(max) (95% CI 0.91:1.14). CONCLUSIONS: Paracetamol was absorbed at least twice as fast from PS compared to P at a 500 mg dose. The extent of absorption was equivalent for both formulations.

The antitussive effects of menthol, camphor and cineole in conscious guinea-pigs.

Menthol and other aromatic vapours have been widely used in the symptomatic treatment of upper respiratory tract infections, although there is little objective evidence as to their benefit. We have investigated the action of aromatic vapours on the cough reflex in conscious guinea-pigs. Animals (n = 13) were pretreated with air or test vapours for 5 min at a rate of 1 l/min. One minute later the animal was challenged with aerosolized citric acid for 2 min. Control responses to air pretreatment were not significantly different throughout the procedures. Three concentrations of each aromatic vapour were used (3, 10 and 30 micrograms/l menthol, 50, 133 and 500 micrograms/l camphor and 0.8, 2.7 and 8 mg/l cineole). Menthol proved the most effective antitussive--10 and 30 micrograms/l produced a significant 28 and 56% reduction in cough frequency--500 micrograms/l camphor gave a significant 33% reduction, while cineole, at the concentrations used, had no significant effect. An increase in cough latency coincided with a reduction in cough frequency. These results demonstrate the efficacy of aromatic vapours as antitussives in chemically induced cough.

Effect of inhaled menthol on citric acid induced cough in normal subjects.

BACKGROUND: Menthol is a commonly used ingredient in many over the counter cough remedies, but there is little objective evidence as to its efficacy. METHODS: Twenty healthy subjects received a cough challenge consisting of five inhalations of 33 mumol citric acid from an air driven dosimeter. The challenge was repeated at hourly intervals for five hours. Five minutes before each challenge subjects inhaled, in a randomised design, either menthol 75% in eucalyptus oil or one of two placebos (pine oil or air). RESULTS: Menthol inhalation caused a reduction in evoked cough when compared with either placebo. CONCLUSIONS: Menthol is an effective antitussive agent in an evoked cough model.

The effect of inhaled and oral dextromethorphan on citric acid induced cough in man.

1. Dextromethorphan is a widely used antitussive agent which is a non-narcotic codeine analogue. We have investigated whether inhaled administration of dextromethorphan provides antitussive activity in a citric acid induced cough model. 2. Twenty normal subjects underwent repeated cough challenge with 5% citric acid. Subjects were studied on six occasions. Study medication consisted of oral dextromethorphan 30 mg or oral matched placebo or 1, 3 and 30 mg inhaled dextromethorphan or matched inhaled placebo. Cough challenge was administered 10 min after study medication and hourly thereafter up to 250 min. 3. No significant differences were seen between baseline cough responses. Oral dextromethorphan (30 mg) produced a mean percentage reduction in cough of 38% (P < 0.002), which remained significant at 250 min. Inhaled dextromethorphan had no clinically significant effect although activity at later time points was not excluded. The antitussive effect of oral dextromethorphan is confirmed with prolonged inhibition of induced cough. It is possible that dextromethorphan or its active metabolites act centrally to inhibit the cough reflex.

The development of a modified dissolution method suitable for investigating powder mixtures.

A novel dissolution method was developed, suitable for powder mixtures, based on the USP basket apparatus. The baskets were modified such that the powder mixtures were retained within the baskets and not dispersed, a potential difficulty that may arise when using conventional USP basket and paddle apparatus. The advantages of this method were that the components of the mixtures were maintained in close proximity, maximizing any drug: excipient interaction and leading to more linear dissolution profiles. Two weakly acidic model drugs, ibuprofen and acetaminophen, and a selection of pharmaceutical excipients, including potential dissolution-enhancing alkalizing agents, were chosen for investigation. Dissolution profiles were obtained for simple physical mixtures. The f1 fit factor values, calculated using pure drug as the reference material, demonstrated a trend in line with expectations, with several dissolution enhancers apparent for both drugs. Also, the dissolution rates were linear over substantial parts of the profiles. For both drugs, a rank order comparison between the f1 fit factor and calculated dissolution rate, obtained from the linear section of the dissolution profile, demonstrated a correlation using a significance level of P = 0.05. The method was proven to be suitable for discriminating between the effects of excipients on the dissolution of the model drugs. The method design produced dissolution profiles where the dissolution rate was linear for a substantial time, allowing determination of the dissolution rate without mathematical transformation of the data. This method may be suitable as a preliminary excipient-screening tool in the drug formulation development process.

A new laser pain threshold model detects a faster onset of action from a liquid formulation of 1 g paracetamol than an equivalent tablet formulation.

AIMS: To discover whether a new infra-red laser method could detect a change in pain threshold after as mild an analgesic as paracetamol and whether an effervescent liquid formulation produced a faster onset of action than tablets. METHODS: This double-blind, placebo controlled randomized study used a portable, infra-red laser to measure "first pain" thresholds on the nondominant forearm in 12 normal volunteers before and after 1 g of paracetamol or placebo. The mean of six recordings was determined three times before dosing, the first being used as a familiarization procedure, and 14 times after dosing. RESULTS: We detected a small (2%), statistically significant difference in pain threshold between a liquid formulation of paracetamol and placebo at 30 and 60 min (P = 0.004 and P = 0.001), but not between tablets and placebo. Liquid also increased the threshold significantly compared with tablets at 60 min (P = 0.01). CONCLUSIONS: To detect such a small increase in pain threshold requires a highly consistent measure and the coefficient of variation was 2% for the study overall, surprisingly low for a subjective phenomenon. The reasons for this include minimizing reflectance by blacking the skin, using a nonhairy site, averaging six data points at each sample time and controlling closely the ambient conditions and the subjects' preparation for studies.

A new rapidly absorbed paracetamol tablet containing sodium bicarbonate. I. A four-way crossover study to compare the concentration-time profile of paracetamol from the new paracetamol/sodium bicarbonate tablet and a conventional paracetamol tablet in fed and fasted volunteers.

The primary objective of this four-way crossover study was to compare the concentration-time profile of paracetamol from a new rapidly absorbed paracetamol tablet containing sodium bicarbonate (PS) with a conventional paracetamol tablet (P), in a panel of 28 fed and fasted healthy volunteers. The results demonstrated that paracetamol was absorbed more rapidly from tablets containing sodium bicarbonate compared to conventional tablets, as indicated by a shorter tmax in both the fed and fasted state and a higher Cmax in the fasted state. The two formulations were bioequivalent with respect to area under curve (AUC). Food did not affect the extent of absorption from either formulation, as indicated by AUC, however, food did reduce the rate of absorption from both formulations, as indicated by a longer tmax and a lower Cmax. Metabolic activation of paracetamol to its oxidation metabolites, as assessed by combined partial clearances to subsequent secondary metabolites cysteine and mercapturic acid conjugates, indicated that the two formulations were bioequivalent in this respect.

A new rapidly absorbed paracetamol tablet containing sodium bicarbonate. II. Dissolution studies and in vitro/in vivo correlation.

The objective of this study was to compare the in vitro dissolution profile of a new rapidly absorbed paracetamol tablet containing sodium bicarbonate (PS) with that of a conventional paracetamol tablet (P), and to relate these by deconvolution and mapping to in vivo release. The dissolution methods used include the standard procedure described in the USP monograph for paracetamol tablets, employing buffer at pH 5.8 or 0.05 M HCl at stirrer speeds between 10 and 50 rpm. The mapping process was developed and implemented in Microsoft Excel worksheets that iteratively calculated the optimal values of scale and shape factors which linked in vivo time to in vitro time. The in vitro-in vivo correlation (IVIVC) was carried out simultaneously for both formulations to produce common mapping factors. The USP method, using buffer at pH 5.8, demonstrated no difference between the two products. However, using an acidic medium the rate of dissolution of P but not of PS decreased with decreasing stirrer speed. A significant correlation (r = 0.773; p < .00001) was established between in vivo release and in vitro dissolution using the profiles obtained with 0.05 M HCl and a stirrer speed of 30 rpm. The scale factor for optimal simultaneous IVIVC in the fasting state was 2.54 and the shape factor was 0.16; corresponding values for mapping in the fed state were 3.37 and 0.13 (implying a larger in vitro-in vivo time difference but reduced shape difference in the fed state). The current IVIVC explains, in part, the observed in vivo variability of the two products. The approach to mapping may also be extended to different batches of these products, to predict the impact of any changes of in vitro dissolution on in vivo release and plasma drug concentration-time profiles.