The effect of selected water-soluble excipients on the dissolution of paracetamol and Ibuprofen.

The purpose of this investigation was to study the dissolution behavior of paracetamol and ibuprofen in the presence of a range of selected potential excipients. First, a pH-solubility profile was generated for both drugs, and the effect of changing hydrodynamic conditions on the intrinsic dissolution rate was investigated. It was established that both drugs dissolved according to the diffusion-layer model. Paracetamol solubility (approximately 20.3 mg mL(-1)) did not vary from pH 1.2-8.0, corresponding to the in vivo range in the gastrointestinal tract. Ibuprofen had an intrinsic solubility of approximately 0.06 mg mL(-1), and pK(a) was calculated as 4.4. Second, the effects of selected potential excipients (lactose, potassium bicarbonate, sodium bicarbonate, sodium chloride, and tartaric acid) were evaluated by measuring the effect of the inclusion of each additive in the dissolution medium on drug solubility, drug intrinsic dissolution rate, and solution viscosity. The results were evaluated using the diffusion-layer model, and it was determined that for paracetamol, the collected data fitted the model for all the excipients studied. For ibuprofen, it was found that there were differences between the excipients that raised the solution pH above the pK(a) to those that did not. For the excipients raising the pH above the pK(a), the effect on intrinsic dissolution rate was not as high as that expected from the change in drug solubility. It was postulated that this might be due to lack of penetration of the excipient into the drug boundary layer microenvironment. Formulators may calculate the effect of adding an excipient based on solubility increases but may not find the dissolution rate improvement expected.

The influence of excipients on the diffusion of ibuprofen and paracetamol in gastric mucus.

The aim of this study was to examine the diffusion of commonly administered analgesics, ibuprofen and paracetamol, through gastric mucus. As ibuprofen and paracetamol are often formulated with alkalising excipients, or are commonly co-administered with antacids that have been demonstrated to alter their absorption, diffusion was also studied in the presence of a range of soluble and insoluble antacids or buffering agents. The effect of pH, which has been demonstrated to modify the properties of mucus, was also studied. Mucus was a significant barrier to diffusion for both drugs, compared to an unstirred aqueous layer with diffusion rates significantly lower in the presence of a mucus barrier for both drugs; ibuprofen diffusion also demonstrated a significant increase in the lag time. Paracetamol diffusion was not significantly affected by addition of any antacid, whereas ibuprofen rates were affected and the diffusion lag time for ibuprofen was significantly reduced in all cases. Isolated increases in pH increased the rate and reduced the lag time for ibuprofen diffusion. It was shown that mucus acts as a passive barrier in the case of paracetamol diffusion, and an interactive barrier to ibuprofen diffusion. Changes in mucus viscosity at different pH values may be responsible for the observed changes in ibuprofen diffusion rate.

Comparison of the rates of disintegration, gastric emptying, and drug absorption following administration of a new and a conventional paracetamol formulation, using gamma scintigraphy.

PURPOSE: To investigate the hypothesis that faster drug absorption from a new paracetamol formulation containing sodium bicarbonate compared to that from a conventional formulation results from a combination of enhanced gastric emptying and disintegration/dissolution. METHODS: Each formulation was administered in both fasted and fed states to 12 healthy volunteers. Gastric emptying and disintegration times were assessed by gamma scintigraphy, and serum paracetamol concentrations were determined by HPLC. RESULTS: The mean time to complete disintegration of the new tablets was faster than that for conventional tablets in both fasted (10.2 min vs. 22.5 min) and fed (14.3 min vs. 46.4 min) states, although this difference was statistically significant in the fed state only (p = 0.0053). Mean gastric emptying times for the new tablets, as measured by t50 and t90, were also faster than those for conventional tablets in both fasted (t50 = 22.4 min vs. 47.5 min, t90 = 30.9 min vs. 64.1 min) and fed (t50 = 76.9 min vs. 106.4 min, t90 = 152.7 min vs. 155.5 min) states, although these differences were not statistically significant. Two subjects showed dramatically retarded gastric emptying of the new tablets in the fasted state: if these atypical data are excluded, the differences in both t50 and t90 in the fasted state are significant (p = 0.0110 and 0.0035, respectively). Rate of paracetamol absorption reflected the gastric emptying profiles as shown by significant correlation of emptying times with partial AUC. CONCLUSIONS: It would seem that a combination of faster disintegration and gastric emptying of the new tablets is responsible for the faster rate of absorption of paracetamol from PA compared to P observed in both this study and in previous studies. The differences in gastric emptying are more pronounced in the fasted state, and the differences in disintegration are more pronounced in the fed state.