Intravenous aminophylline therapy for asthma. A comparison of two methods of administration in children

Using pharmacokinetic modelling to improve prescribing practices of intravenous aminophylline in childhood asthma exacerbations

OBJECTIVE

To evaluate physiologically based pharmacokinetic modelling (PBPK) software in paediatric asthma patients using intravenous aminophylline.

METHODS

Prospective clinical audit of children receiving iv aminophylline (July 2014 to June 2016), and in-silico modelling using Simcyp software.

RESULTS

Thirty-eight admissions (25 children) were included. Children with aminophylline levels ≥10 mg/l had equivalent clinical outcomes compared to those <10 mg/L, and adverse effects occurred in 57%. Therapeutic drug monitoring (TDM) data correlated well with PBPK model. PBPK modelling of a 5 mg/kg iv loading dose (≤18yr) shows a mean C_max of 8.99 mg/L (5th-95th centiles 5.5-13.7 mg/L), with 70.3% of subjects <10 mg/L, 29.4% achieving 10-20 mg/L, and 0.1% > 20 mg/L. For an aminophylline infusion (0-12 y) of 1.0  mg/kg/h, the mean steady state infusion concentration was 16.4 mg/L, (5th-95th centiles 5.3-32 mg/L), with 26.8% having a serum concentration >20 mg/L. For 12-18yr receiving 0.5  mg/kg/h infusion, the mean steady state infusion concentration was 9.37 mg/L (5th-95th centiles 3.4-18 mg/L), with 59.8% having a serum concentration <10 mg/L.

CONCLUSION

PBPK software modelling correlates well with clinical data. Current aminophylline iv loading dosage recommendations achieve levels <10 mg/l in 70% of children. Routine TDM may need altering as low risk of toxicity (>20 mg/l).

Comparison of intravenously administered doxofylline and placebo for the treatment of severe acute airways obstruction

This double-blind, randomized, placebo-controlled study investigated the therapeutic effects of a single dose of doxofylline, a methylxanthine derivative, in 10 patients aged 26-79 years. All patients had acute exacerbation of chronic obstructive airways disease partially reversible with salbutamol inhaler. Doxofylline was administered intravenously at a dose of 200 mg over 15 min on two different occasions separated by at least 24 h. Doxofylline increased forced expiratory volume in the first second of expiration compared with baseline as follows: +20% after 2 h (P less than 0.01); +31% after 4 h (P less than 0.01); and +13% after 6 h (NS). Changes produced by placebo at these times were -4.4%, -14% and -5% (all NS). The average differences between the groups were significant at all observation times. At the end of the observation period eight out of 10 patients given doxofylline and one out of 10 patients given placebo had improved clinically according to the patients' own opinion. Clinical tolerability of doxofylline proved to be good since no signs of local or general side-effects were observed in any of the patients treated.

Management of acute severe asthma

Discussed in this article are the pathophysiology of acute asthma and its differential diagnosis and management, placing special emphasis on rapid assessment, prompt and decisive therapeutic intervention, and on recognition of the severity of the disease.

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)

The effects of erythromycin on theophylline elimination in normal males

The effects of three erythromycin preparations on theophylline elimination kinetics were examined in 23 male subjects. Subjects received 6 mg/kg of the theophylline elixir orally and elimination kinetics were determined. The population was then randomized to receive either a lactose placebo, erythromycin base, erythromycin stearate, or erythromycin ethylsuccinate. Each 250-mg preparation was given four times a day for six days. On day seven, a repeat kinetic study was performed. The mean theophylline half-life of controls was 7.8 +/- 2.6 hours. The half-life increased significantly in all erythromycin treatment groups. The increase for the base, stearate, and ethylsuccinate groups was 51.7, 21.3, and 60.3 per cent, respectively. The total body theophylline clearance decreased significantly in all treatment groups. This was not associated with biochemical evidence of hepatitis. Three of four smokers who received erythromycin manifested no increase in theophylline half-life, in contrast to one of 13 nonsmokers. There was no difference in the percentage theophylline bound to serum protein for any of the erythromycin treatment groups or controls as determined by ultracentrifugation.