Pharmacokinetic Optimisation of Asthma Treatment

Abstract

Asthma is generally managed with bronchodilator therapy and/or anti-inflammatory drugs. Guidelines now advocate selection of drugs and pharmaceutical formulations (long-acting vs short-acting, inhaled vs systemic) on the basis of disease severity. Theophylline has a narrow therapeutic margin. Clearance is highly variable and plasma concentrations should be monitored to avoid the occurrence of plasma concentration-related adverse effects. The rate of absorption of theophylline differs depending on the sustained release formulation administered. Some products do not provide sufficient plasma drug concentrations for therapeutic efficacy over a 12-hour period, particularly in patients with high clearance rates (e.g. children and patients who smoke). Administration of drugs via inhalation offers several advantages over systemic routes of administration (e.g. adverse effects are decreased). Inhalation is now advocated as first-line therapy. Aerosol medications available for the treatment of asthma are beta 2-agonist (including the newer long-acting agents such as salmeterol), corticosteroids, anticholinergic drugs, sodium cromoglycate (cromolyn sodium) and nedocromil. To reach the airways, aerosolised particles should be 1 to 5 microns in diameter. Particles of this size can be produced by nebuliser for continuous administration or by metered-dose inhaler and drug powder inhaler for unit dose medication. For efficient use of the metered-dose inhaler, slow inhalation and actuation must be coordinated. However, efficacy and convenience can be improved when spacer devices are used. Furthermore, spacer devices lessen the oropharyngeal adverse effects of inhaled corticosteroids. Dry powder inhalers are more easily used by children and elderly patients than metered-dose inhalers. Regardless of the device used, a maximum of 10% of the inhaled dose reaches the airways. The rest of the dose is swallowed and absorbed through the gastrointestinal tract. Most inhaled drugs have low oral bioavailability, either because of a high first-pass metabolism (beta 2-agonists and glucocorticoids) or because of lack of absorption (sodium cromoglycate). Sulphation of beta 2-agonists occurs in the wall of the gastrointestinal tract and extensive metabolism of inhaled corticosteroids occurs in the liver. Low bioavailability of the swallowed fraction contributes to reduced adverse effects. The pharmacokinetic properties of an inhaled drug are of interest. The fraction of the dose absorbed through the lung has the same disposition characteristics as an intravenous dose, and the swallowed fraction has the same disposition as an orally administered dose. However, for many drugs, pharmacokinetic data after inhalation are limited and cannot be used as a criteria for selection of therapy.(ABSTRACT TRUNCATED AT 400 WORDS)

Aerosol therapy in childhood asthma

“…Aerossóis são suspensões de diminutas partículas (lí-quidas ou sólidas) em um gás ou em uma mistura de gases, como o ar atmosférico 1 . Na natureza, temos vários exemplos de aerossóis, como as nuvens de chuva, que são aerossóis de partículas líquidas; as nuvens de poeira em uma estrada de barro; ou as nuvens de ácaros ao se sacudirem lençóis, travesseiros, ou colchões, que são aerossóis de partículas sólidas.…”

“…A estabilidade dos aerossóis é transitória: as partícu-las tendem a se depositar por gravidade (sedimentação) ou por impactação a outras estruturas próximas 1 ; as partículas também aderem umas às outras, formando partículas maiores 1 ; algumas partículas têm característi-cas higroscópicas e aumentam progressivamente de tamanho pela incorporação de moléculas de água. O tamanho das partículas determina a velocidade de sedimentação; a movimentação das partículas no aerossol (a sua energia cinética) influencia a impactação.…”

“…Para que um aerossol possa ser inalado para as vias aéreas inferiores é preciso que suas partículas tenham diâmetros inferiores a 5 micra [1][2][3][4] . Partículas desse tamanho são chamadas de partículas respiráveis.…”

“…Partículas com diâmetros inferiores a 1 micron chegam até os alvéolos e voltam com o ar expirado, sem que se depositem. Portanto, para que haja deposição nas vias aéreas inferiores, as partículas devem ter diâmetro entre 1 e 5 micra [1][2][3] . A eficiência da via inalatória está condicionada ao uso de aerossóis contendo uma grande proporção de partículas respiráveis.…”

“…O tamanho das partículas é determinado pelos aparelhos geradores de aerossol 1 . Estes deverão conter especificações, a fim de orientar o usuário, sobre a proporção de partículas respiráveis que são capazes de gerar.…”