Prostaglandins and leukotrienes: advances in eicosanoid biology

Prostaglandins and leukotrienes are potent eicosanoid lipid mediators derived from phospholipase-released arachidonic acid that are involved in numerous homeostatic biological functions and inflammation. They are generated by cyclooxygenase isozymes and 5-lipoxygenase, respectively, and their biosynthesis and actions are blocked by clinically relevant nonsteroidal anti-inflammatory drugs, the newer generation coxibs (selective inhibitors of cyclooxygenase-2), and leukotriene modifiers. The prime mode of prostaglandin and leukotriene action is through specific G protein-coupled receptors, many of which have been cloned recently, thus enabling specific receptor agonist and antagonist development. Important insights into the mechanisms of inflammatory responses, pain, and fever have been gleaned from our current understanding of eicosanoid biology.

International Union of Basic and Clinical Pharmacology. LXXXIV: leukotriene receptor nomenclature, distribution, and pathophysiological functions

The seven-transmembrane G protein-coupled receptors activated by leukotrienes are divided into two subclasses based on their ligand specificity for either leukotriene B(4) or the cysteinyl leukotrienes (LTC(4), LTD(4), and LTE(4)). These receptors have been designated BLT and CysLT receptors, respectively, and a subdivision into BLT(1) and BLT(2) receptors and CysLT(1) and CysLT(2) receptors has been established. However, recent findings have also indicated the existence of putative additional leukotriene receptor subtypes. Furthermore, other ligands interact with the leukotriene receptors. Finally, leukotrienes may also activate other receptor classes, such as purinergic receptors. The aim of this review is to provide an update on the pharmacology, expression patterns, and pathophysiological roles of the leukotriene receptors as well as the therapeutic developments in this area of research.

Reevaluation of the microsomal metabolism of montelukast: major contribution by CYP2C8 at clinically relevant concentrations

According to published in vitro studies, cytochrome P450 3A4 catalyzes montelukast 21-hydroxylation (M5 formation), whereas CYP2C9 catalyzes 36-hydroxylation (M6), the primary step in the main metabolic pathway of montelukast. However, montelukast is a selective competitive CYP2C8 inhibitor, and our recent in vivo studies suggest that CYP2C8 is involved in its metabolism. We therefore reevaluated the contributions of different cytochrome P450 (P450) enzymes, particularly that of CYP2C8, to the hepatic microsomal metabolism of montelukast using clinically relevant substrate concentrations in vitro. The effects of P450 isoform inhibitors on montelukast metabolism were examined using pooled human liver microsomes, and montelukast oxidations by human recombinant CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, and CYP3A5 were investigated. The results verified the central role of CYP3A4 in M5 formation. The CYP2C8 inhibitors gemfibrozil 1-O-β glucuronide and trimethoprim inhibited the depletion of 0.02 μM montelukast and formation of M6 from 0.05 μM montelukast more potently than did the CYP2C9 inhibitor sulfaphenazole. Likewise, recombinant CYP2C8 catalyzed montelukast depletion and M6 formation at a 6 times higher intrinsic clearance than did CYP2C9, whereas other P450 isoforms produced no M6. On the basis of depletion of 0.02 μM montelukast, CYP2C8 was estimated to account for 72% of the oxidative metabolism of montelukast in vivo, with a 16% contribution for CYP3A4 and 12% for CYP2C9. Moreover, CYP2C8 catalyzed the further metabolism of M6 more actively than did any other P450. In conclusion, CYP2C8 plays a major role in the main metabolic pathway of montelukast at clinically relevant montelukast concentrations in vitro.

Heterogeneity in response to asthma medications

PURPOSE OF REVIEW

Evidence for the heterogeneity of response to asthma medications including inhaled corticosteroids and leukotriene receptor antagonists is mounting. beta2-Adrenoceptor gene polymorphisms may contribute to asthma responsiveness to short- and long-acting beta2-agonists. This review examines recent articles describing variability in response to inhaled corticosteroids, leukotriene receptor antagonists and short-acting beta2-agonists specifically in pediatric persistent asthmatics.

RECENT FINDINGS

In the late 1990's, differences in the response to a leukotriene receptor antagonist and an inhaled corticosteroid in adults with moderate persistent asthma were first described. Subsequently, similar findings have recently been elucidated in children with mild to moderate persistent asthma. The variability in response to these two classes of control medicines now appears to encompass all ages with persistent asthma. In general, despite the variability in response to these medications, both resulted in improved clinical and physiologic control measures.

SUMMARY

Childhood asthma is a complex disease with numerous clinical phenotypes that contribute to response variability to asthma medications.