Pharmacokinetic interactions between zileuton and prednisone

Benefit-risk assessment of antileukotrienes in the management of asthma

Antileukotrienes are a relatively new class of anti-asthma drugs that either block leukotriene synthesis (5-lipoxygenase inhibitors) like zileuton, or antagonise the most relevant of their receptors (the cysteinyl leukotriene 1 receptor [CysLT1]) like montelukast, zafirlukast or pranlukast. Hence, their major effect is an anti-inflammatory one. With the exception of pranlukast, the other antileukotrienes have been studied and marketed in the US and Europe for long enough to establish that they are useful drugs in the management of asthma. Their effects, significantly better than placebo, seem more pronounced in subjective measurements (i.e. symptoms scores or quality-of-life tests) than in objective parameters (i.e. forced expiratory volume in 1 second or peak expiratory flow rate). Also, there is some evidence that these drugs work better in some subsets of patients with certain genetic polymorphisms - probably related to their leukotriene metabolism - or patients with certain asthma characteristics. There are a small number of comparative studies only, and with regard to long-term asthma control differences between the agents have not been evaluated. Nevertheless, their overall effect appears comparable with sodium cromoglycate (cromolyn sodium) or theophylline, but significantly less than low-dose inhaled corticosteroids. Antileukotrienes have been shown to have a degree of corticosteroid-sparing effect, but salmeterol appears to perform better as an add-on drug. Montelukast is probably the most useful antileukotriene for continuous treatment of exercise-induced asthma, performing as well as salmeterol without inducing any tolerance. All antileukotrienes are taken orally; their frequency of administration is quite different ranging from four times daily (zileuton) to once daily (montelukast). Antileukotrienes are well tolerated drugs, even though zileuton intake has been related to transitional liver enzyme elevations in some cases. Also Churg-Strauss syndrome (a systemic vasculitis), has been described in small numbers of patients taking CysLT1 antagonists. It is quite probable that this disease appears as a consequence of an 'unmasking' effect when corticosteroid dosages are reduced in patients with severe asthma once CysLT1 antagonists are introduced, but more data are needed to definitely establish the mechanism behind this effect. Overall, however, the benefits of antileukotrienes in the treatment of asthma greatly outweigh their risks.

New perspectives for asthma treatment: anti-leukotriene drugs

Leukotrienes are synthesized by different cells, including eosinophils, neutrophils, basophils, lymphocytes, macrophages, and mast cells. Cysteinyl leukotrienes (LTC4, LTD4, and LTE4) are the most important leukotrienes in the pathogenesis of asthma. Pharmacologically, there are two different ways of blocking the action of leukotrienes: inhibiting their production by blocking 5-lipoxygenase or its activating protein, 5-lipoxygenase-activating protein (FLAP), (inhibitors) or by blocking their receptors (antagonists). The available antagonists are, for the moment, directed against the one receptor demonstrated to play a role in asthma symptoms, CysLT1, and they act in a competitive way. The only marketed 5-lipoxygenase inhibitor is zileuton (Zyflo). CysLT1 antagonists, currently on the pharmaceutical market in some countries, are zafirlukast (Accolate), pranlukast (Ultair, Onon), and montelukast (Singulair). Undoubtedly, drugs acting on leukotrienes constitute a new pharmacologic class in the therapeutic armamentarium for the management of asthma. From the pediatric point of view, montelukast is currently the most interesting drug of the group to date because of published trials in patients as young as 6 yr of age. At present, zafirlukast is only approved for use in patients 12 yr of age and older, although we understand that applications are likely to extend the age range into childhood shortly. However, more experience is necessary to establish a definite place for both leukotrienes in the step-by-step asthma treatment. New comparative studies (with sodium cromoglycate and inhaled steroids), which will probably be published in the near future, as well as studies on the use of montelukast in the treatment of children under 6 yr of age will add crucial information to our knowledge, and help to identify an appropriate use in the therapeutic algorithm. Montelukast will not be a substitute for inhaled corticosteroids or beta-agonists, although it may act as a 'sparing drug' (which might help tapering of steroids in some instances). Its role in exercise-induced asthma seems promising. Although its more widespread use could highlight low-frequency adverse effects, its apparent excellent tolerability is an additional advantage for the drug. Patients' preference for a twice-daily dosage over the inhaled medication, resulting in a better compliance, is relatively well established, at least for zafirlukast in adults, and these findings can probably be extended to montelukast, which only requires once-daily dosing in children.

The prospects for long-term intervention in asthma with antileukotrienes

The antileukotriene drugs are the first new therapeutic agents approved for the treatment of asthma in more than 20 years. The currently available compounds are orally active and either prevent the cysteinyl leukotrienes from binding to and activating the cysLT-1 receptor in the lung (leukotriene receptor antagonists, LTRAs) or inhibit leukotriene synthesis (leukotriene synthesis inhibitors, LTSIs). The antileukotrienes acutely bronchodilate the airways of patients with baseline bronchoconstriction, although usually not as well as beta-agonists. When used regularly they produce rapid improvements of pulmonary function and symptoms in patients with mild-to-moderate asthma, and probably of those with more severe asthma as well, which persists for the duration of treatment. Yet, individual responses to the antileukotrienes are variable and not predictable based on clinical criteria. Recent studies suggest they reduce asthmatic airway inflammation. The few comparator studies with other asthma medications indicate they are equal or more effective than cromolyn but equal or less effective than low-to-moderate doses of inhaled corticosteroids. Initial experience with the antileukotrienes reveals limited toxicity and a favourable therapeutic-to-toxic ratio. The exact role of the antileukotrienes in the treatment of asthma remains to be determined, as does the relative potency of the various agents.

A risk-benefit assessment of antileukotrienes in asthma

The antileukotriene drugs are the first new therapeutic agents approved for the treatment of asthma in more than 20 years. The currently available compounds are orally active and either prevent the cysteinyl leukotrienes from binding to and activating the cysLT-1 receptor in the lung (leukotriene receptor antagonists) or inhibit leukotriene synthesis (leukotriene synthesis inhibitors). Studies performed in individuals without asthma and patients with asthma reveal that antileukotrienes prevent the bronchoconstriction produced by exercise, cold-air, allergen, aspirin (acetylsalicylic acid) and sulphur dioxide. Except for the setting of aspirin sensitivity where the antileukotrienes are nearly uniformly effective, individual responses to them are variable with complete protection in some, no protection in others and a modest degree of protection in the majority. The antileukotrienes bronchodilate the airways of patients with baseline bronchoconstriction, although usually not as well as beta-agonists. When given for weeks to months they rapidly improve pulmonary function and symptoms in patients with mild-to-moderate asthma, and probably in patients with more severe asthma as well, and these improvements persist for the duration of treatment. Here too, their beneficial effects are variable and not predictable based on clinical criteria. Recent studies suggest they can reduce asthma-induced airway inflammation and are equal or more effective than sodium cromoglycate, but equal or less effective than low-to-moderate dosages of inhaled corticosteroids. Initial experience with the antileukotrienes reveals limited toxicity and what appears to be a favourable therapeutic-to-toxic ratio. However, exposure of more patients with differing characteristics for longer periods of time is needed to substantiate this initial impression. The exact role of the antileukotrienes in the treatment of asthma remains to be determined, as does the relative potency of the various agents.