Evaluation of the absorption from some commercial enteric-release theophylline products

Bioequivalence revisited

The FDA permits marketing of a generic formulation of a drug G for the same indications as a standard preparation S if one can show that G is bioequivalent to S. Present implementation requires convincing evidence that the population mean difference in bioavailability (drug exposure) between the two preparations lies within specified bounds. The basis for this standard does not appear to involve a comprehensive model for the dose-response relationship, or consideration of clinical issues, notably (i) whether a patient is to commence on the drug or to switch from an established regimen to a new one; or (ii) that the risk of inequivalence relates to uncertainty of outcome. In this paper, I propose a comprehensive model for dose response and a tentative model for risk that addresses these issues. Specifically, I propose two new measures of bioequivalence which are based on these models, which differ in the two clinical circumstances above, and which respond to both bias and variance of outcome. I present two examples, and some simulations of the application of the new measures.

Radiotelemetric method for evaluating enteric coatings in vivo

A radiotelemetric method for the in vivo evaluation of enteric coating performance is described, and its advantages and disadvantages are compared with those of other available methods. Hydroxypropyl methylcellulose phthalate was used as the test enteric coating. Four dogs were administered several batches of enteric-coated tablets containing buffers. Tablet disintegration was determined by radiotelemetric detection of the pH drop in the upper intestine due to release of the buffer. Premature rupture of the coating in the stomach was detected by a rise and then a fall in gastric pH prior to gastric emptying. The average gastric emptying time was 80 +/- 18 min (SEM), while the average time for a tablet to disintegrate in the upper intestine was 14.2 +/- 2 min. The average disintegration time was not affected by a change in the batch (for a given tablet core pH) or the dog used, suggesting that the method yielded readily reproducible results. Although there was little correlation with in vitro disintegration times, the method gave results similar to those reported in the literature for the same enteric coating in a human study. Of the formulations tested, it was concluded that buffering the core to pH 4 was most suitable for studying enteric coating performance.

A clinical and pharmacokinetic basis for the selection and use of slow release theophylline products

In order to achieve the greatest chance for maximum benefit from theophylline in the management of chronic asthma, the serum concentration should be maintained in the therapeutic range of 10 to 20 micrograms/ml. Conventional rapid release formulations produce excessive fluctuations in serum concentrations that can result in variability in clinical response between doses. In contrast, slow release formulations have the potential to achieve relatively constant serum concentrations with 12-hour dosing intervals, thus providing around-the-clock stabilisation of the hyper-reactive airways that characterise chronic asthma. Furthermore, the decreased frequency of dosing with these formulations can improve patient compliance. However, significant differences in rate and extent of absorption exist between the available formulations. Single-dose bioavailability studies comparing a slow release product with an oral solution or plain uncoated tablet in a crossover design permit examination of the rate and extent of absorption. Comparison of a slow release product with an oral reference following multiple doses at steady-state permits examination of the extent but generally not rate of absorption. The mean fraction absorbed-time profile, calculated from a modification of the Wagner-Nelson equation, is a process-independent method of comparing rates of absorption of different products after single doses. A prospective study in 14 children with chronic asthma has demonstrated that this modified equation, when rearranged to iteratively solve for serum concentrations, can accurately predict steady-state serum concentrations for different dosing intervals in patient populations with different rates of elimination. When slow release products are compared in this manner at 8- or 12-hour dosing intervals for patients with slow elimination, clinically relevant differences between brands are not apparent. However, in patients with rapid elimination, i.e. children, cigarette smokers, and 25% of non-smoking adults, application of this method shows that only some formulations (i.e. 'Slo-Bid Gyrocaps' and 'Theo-Dur', which is also marketed under different brand names names such as 'Sustaire', 'Pulmi-Dur' and 'Theolin Retard') can maintain serum concentrations within the therapeutic range for an entire 12-hour dosing interval. More rapidly absorbed slow release products must be administered at 8-hour dosing intervals in patients with rapid elimination, despite promotional claims to the contrary. Current products promoted for once-a-day administration are clinically inadequate because of incomplete and erratic absorption, and/or excessive serum concentration fluctuations.(ABSTRACT TRUNCATED AT 400 WORDS)

Absorption of theophylline from enteric coated and sustained release formulations in fasted and non-fasted subjects

The influence of prior food ingestion, and also of varying fluid volumes, on plasma theophylline levels was examined following single oral doses of two sustained-release formulations, Theobid (260 mg) and Theo-Dur (200 mg) and one partially enteric-coated formulation, Choledyl (128 mg), to 9 healthy volunteers. Prior food ingestion tended to delay the absorption of theophylline from all formulations to a small extent. This effect was observed only at early sampling times, and plasma drug profiles were similar for all treatments within a particular formulation. Theobid and Theo-Dur gave rise to plasma profiles that were characteristic of sustained-release formulations, with mean Cmax values of 5.5-5.7 micrograms ml-1 (Theobid) and 2.8-3.2 micrograms ml-1 (Theo-Dur) occurring at 5.8-9.1 h after dosing. Choledyl gave rise to a longer absorption lag time than the other formulations but was subsequently absorbed at a faster rate yielding mean Cmax values of 3.2-3.5 micrograms ml-1 at 2.8-4.1 h. The intersubject variability in theophylline plasma levels, and also in most pharmacokinetic parameter values, was generally less following Theo-Dur compared to the other formulations.

Theophylline. A "state of the art" review

Theophylline is a bronchodilator and respiratory stimulant that is effective in the treatment of acute and chronic asthma, Cheyne-Stokes respirations, and apnea/bradycardia episodes in newborns. It is also used as an adjunct in the treatment of congestive heart failure and acute pulmonary edema, but it has no established efficacy in patients with chronic irreversible airways obstruction. Benefits and risks from theophylline relate directly to serum concentration, which is a function of both dose and elimination characteristics of the drug in an individual patient. When used to treat acute symptoms, an initial loading dose based on a mean volume of distribution is required to rapidly obtain maximum bronchodilator effect. Because of large interpatient differences in elimination, constant intravenous infusion rates for continued therapy must be guided by monitoring serum theophylline concentration at intervals until a steady-state serum concentration is reached within the 10-20 micrograms/ml therapeutic range. Intravenous, oral or rectal solutions and plain uncoated tablets are appropriate for acute therapy, while reliably absorbed slow-release formulations offer therapeutic advantages for the management of chronic asthma, particularly in patients with rapid elimination. Dosage for long-term therapy is determined by starting with low doses that allow virtually complete acceptance of the medication followed by gradual increases, if tolerated, at three day intervals until mean age-specific doses are reached. Subsequent adjustment in dosage regimens are then based upon serum concentration measurements. Most clinical laboratories now measure theophylline, and newer systems have been developed to provide emergency results within minutes at a reasonable cost. In cases of theophylline poisoning, the drug must be rapidly removed to prevent life-threatening toxicity. When serum concentrations are in excess of 60 micrograms/ml charcoal hemoperfusion dialysis may be indicated, even in the absence of obvious signs of toxicity.

Evaluation of the absorption from 15 commercial theophylline products indicating deficiencies in currently applied bioavailability criteria

The bioavailability of theophylline from alcoholic and aqueous oral solutions was compared to that from an intravenous dose in 12 normal adults. The alcoholic elixir surprisingly gave rise to a significantly greater (114 +/- 14%, mean +/- SD) amount absorbed than did the intravenous dose. The aqueous solution (99 +/- 8%) and intravenous dose were statisticlly indistinguishable in this respect, and, furthermore, the extent of absorption from a 300-mg dose of the aqueous solution was 99 +/- 10% of that from a 500-mg dose, and not statistically different. The aqueous solution was thus employed in three subsequent studies as a standard with which to compare 13 different types of theophylline tablets, all marketed in the United States. Of the 13 tablets, eight showed bioavailability statistically distinguishable from that of the standard. Nevertheless, for only two tablets could it be claimed with 95% confidence level that the bioavailability was less than 95%. For none can it be stated at this confidence level that the bioavailability is less than 90%. Bioavailability studies should include criteria of clinical significance in addition to criteria of statistical significance. Contrary to the usual rationale behind choice of a bioavailability standard, nine of the 12 uncoated tablets appeared to allow more rapid absorption of theophylline than did the standard oral solution, an aqueous syrup. Increasing the dose of syrup decreased the rate of theophylline absorption. Orally administered drug solutions may have properties more absorption rate limiting than the disintegration of many brands of tablet.

Evaluation of the absorption from some commercial sustained-release theophylline products

Absorption of theophylline from three commerical products labeled as sustained release was compared to the absorption from a standard uncoated tablet (Searle 200-mg aminophylline tablet) in a single-dose study. Aminodur tablets (Cooper) and Slophyllin Gyrocap capsules (Dooner) had bioavailability (100.2% +/- 19.8% and 98.5% +/- 13.8%) statistically indistinguishable from that of the standard but showed significantly slower absorption (peak times of 10.4 +/- 2.8 and 4.36 +/- 1.35 hr) and lower peak plasma concentrations (13.9 +/- 4.5 and 22.6 +/- 3.5 micrograms/ml/g dose) than the standard (tpeak, 1.52 +/- 0.45 hr; Cpeak, 28.1 +/- 6.2 micrograms/ml/g dose). The time of the plasma concentration peak (2.47 +/- 1.38 hr) after a dose of Tedral S.A. (Warner/Chilcott) was not statistically different from that after the standard, but both the peak concentration (16.0 +/- 3.9 micrograms/ml/g dose) and availability (76.0 +/- 18.4%) were. Multiple-dose projections from single-dose data indicate that of the three test products only Aminodur maintains reasonably constant interdose plasma levels during 12 hoursly dosing. With an 8 hourly dosing schedule Gyrocaps also might be satisfactory. Reasonable predictions of the plasma concentrations arising from Aminodur doses have been made using a single-compartment body model and assuming input from an outer followed by an inner layer of the tablet.