Mechanism of action

Corticosteroids diffuse readily across plasma membranes to enter cells where they encounter the glucocorticoid receptor, a cytosolic protein. When the steroid binds the receptor, the receptor undergoes a conformational change.⁸ Chaperone proteins dissociate from the receptor, and nuclear receptor localization regions on the receptor are exposed. The receptor-steroid complex dimerizes and is transported to the nucleus, where it binds to DNA in promoter sequences termed glucocorticoid response elements (GREs). Repression of genes activated by inflammatory diseases is the predominate effect of corticosteroids. These genes encode cytokines; chemokines; adhesion molecules; and inflammatory enzymes, proteins, and receptors. Repression of these genes by corticosteroids occurs at concentrations easily achieved clinical practice. There are no recognizable GRE sites within these genes, however. This response seems to be a result of corticosteroid effects on chromatin remodeling.⁶

Chronic inflammation involves activation of the proinflammatory transcription cofactors nuclear factor-κB (NF-κB) and activating protein-1 (AP-1). NF-κB and AP-1 bind to large coactivator molecules, such as cyclic adenosine 3′,5′-monophosphate (cAMP) response element–binding protein, which possess intrinsic histone acetyltransferase activity. Acetylation of core histones on specific lysine residues reduces their charge and changes the electrostatic attraction between the histone protein and DNA; this allows the histone-DNA complex to undergo a conformational change from the closed structure to the activated open form. DNA unwinds and is accessible to RNA polymerases, which initiate transcription of inflammatory genes. Histone deacetylases (HDACs) act as corepressors along with other proteins, such as nuclear receptor corepressor, to silence gene expression. Deacetylation of histone proteins on lysine residues returns the DNA to the closed basal state, countering the activity of the histone acetyltransferases.¹

Corticosteroids have multiple effects on chromatin remodeling proteins. Anti-inflammatory genes are activated by selective acetylation of histone H₄ by coactivator molecules, such as steroid receptor coactivator-1 (SRC-1) and glucocorticoid receptor interacting protein-1 (GRIP-1). Corticosteroids also repress gene activity by reversal of histone acetylation of proinflammatory sequences; this can occur in two pathways. First, activated glucocorticoid receptor can bind directly to cAMP response element–binding protein and other coactivators and inhibit their histone acetyltransferase activity. Second, and more important, activated glucocorticoid receptor can recruit HDACs to the transcriptional complex, which results in deacetylation of histones associated with inflammatory gene expression.²

Given the effects that corticosteroids have on chromatin remodeling pathways, it is now apparent why low doses are effective in the treatment of asthma. The pathophysiology of asthma involves the increased expression of multiple inflammatory genes activated by the proinflammatory transcription factors AP-1 and NF-κB. In bronchial biopsy specimens from patients with asthma, there is an increase of histone acetyltransferase and a small decrease of HDAC activity compared with airways of patients without asthma, indicating an increase of inflammatory gene expression.

Steroids can also reduce chronic inflammation by transactivation.⁸ Transactivation requires higher concentrations of steroids, however, than those concentrations required for chromatin remodeling. Transactivation involves the monomeric glucocorticoid receptor complex, which can interact directly with AP-1 and NF-κB through protein-protein interactions. This interaction decreases their transcription factor–mediated proinflammatory actions. Corticosteroids also increase the expression of two proteins that affect these inflammatory signal transduction pathways: glucocorticoid-induced leucine zipper protein (GILZ) and mitogen-activated protein kinase phosphatase-1 (MKP-1). GILZ inhibits AP-1 and NF-κB. MKP-1 inhibits p38 mitogen-activated protein kinase, which is activated by AP-1.

Pharmacologic effects

The pharmacologic effects of steroids on inflammatory lung diseases indicate that they are very effective at controlling symptoms of the disease, but they do not cure the disease. When the steroid dose is reduced, or the steroid is discontinued, the symptoms eventually return. In asthma, corticosteroids reduce airway inflammation and hyperresponsiveness, improve lung function, and decrease the incidence and severity of acute asthma exacerbations. Treatment of COPD with corticosteroids is controversial.⁴² Experimental data indicate that the inflammatory processes in COPD may be resistant to the effects of corticosteroids because of oxidation and nitrosylation of proteins involved in chromatin remodeling by the oxidants found in cigarette smoke. Clinical data show inhaled corticosteroids improve the health of patients with COPD, however.⁵⁰ The pharmacologic effects seen with corticosteroids in COPD are largely the same as those seen with asthma, but they also reduce systemic inflammation and reduce the rate of decline of health in patients with COPD.

Efficacy and safety

The optimal route of administration of corticosteroids for inflammatory lung disease is inhalation. Inhalation delivers the drug directly to the lungs. The corticosteroid acts locally, which minimizes the systemic adverse effects encountered with oral or parenteral administration. Receptor-binding affinity is the only pharmacodynamic parameter that differs among the corticosteroids that are available for inhalation.²⁴ High binding affinity can be considered a desirable trait providing for better efficacy in the lung. High binding affinity can also be detrimental because it can lead to increased systemic adverse effects.

The differences between the inhaled products depend on factors such as formulation, pulmonary and systemic bioavailability, and excretion.²³ Characteristics that enhance the efficacy of inhaled corticosteroids include small particle size and long pulmonary residence time. Solution aerosols formulated with hydrofluoroalkane, such as ciclesonide and beclomethasone dipropionate, have a small mass median aerodynamic diameter (MMAD) <2 µm. A small MMAD can have effects on efficacy because the smallest airways have an internal perimeter ≤2 µm. MMAD can also have an impact on local adverse effects because larger particles are more likely to be deposited in the oropharyngeal cavity.

Pulmonary residence time depends on two factors, lipophilicity and lipid conjugation. Lipophilic side chains on the D-ring of the steroid portion of the drug (see Figure 35-2) slow the dissolution in aqueous bronchial fluid. Lipophilic groups also aid the passage of the drug through the phospholipid bilayer of cell membranes to reach the receptors in the cell. Lipid conjugation to fatty acids in the pulmonary cells occurs via a reversible ester bond. Esterification provides a slow-release reservoir of the corticosteroid, prolonging residence time. This prolonged residence time allows a once-a-day dosing schedule for budesonide and ciclesonide.

Adverse effects

The most common adverse effects of inhaled corticosteroids are oropharyngeal candidiasis and dysphonia.³⁶ Although they can be easily managed, these localized adverse effects diminish compliance leading to uncontrolled disease and reduced quality of life. Less common adverse effects of inhaled corticosteroids are systemic in nature, including adrenal suppression, bone loss, skin thinning, metabolic changes, behavioral abnormalities, weight gain, and decreased linear growth in children. The concentration of the corticosteroid in the blood is a combination of the portion that is swallowed and the portion that is delivered to and absorbed from the lungs.

The safety of inhaled corticosteroids depends on factors such as activation in the lungs, low oral bioavailability, high protein binding, and rapid systemic clearance. Although most corticosteroids are inhaled in their pharmacologically active form, beclomethasone dipropionate undergoes activation in the human lung.³⁰ Beclomethasone dipropionate is metabolized to beclomethasone 17-monopropionate, which has a relative receptor affinity about 30 times greater than the parent compound. Metabolism in human plasma is quite different from metabolism in human lung. Human lung metabolism involves a simple hydrolysis reaction to the active form. Metabolism in human plasma involves hydrolysis, transesterification, and loss of hydrogen chloride. Metabolism in human plasma results in beclomethasone 21-monopropionate, which has no binding affinity for the glucocorticoid receptor. This differential metabolism in lung and plasma leads to fewer systemic effects of beclomethasone.

Ciclesonide is metabolized to desisobutyryl-ciclesonide by esterases located in human lung. This metabolite has a relative receptor binding affinity 120 times that of ciclesonide. Metabolism of ciclesonide in the oropharynx region is very low, resulting in low amounts of active drug entering the systemic circulation by swallowing. Activation in the lung can improve the safety profile of an inhaled corticosteroid. The degree of protein binding can also affect the safety of an inhaled corticosteroid. Protein binding to albumin in the blood controls the systemic unbound concentrations and limits adverse effects.

Mechanism of action

The β₂-adrenergic receptor is a member of the seven-transmembrane receptor superfamily of G protein–coupled receptors.² The ligand-binding pocket open to the extracellular space is formed when the seven α helices of the receptor cluster together in a ring. When a receptor is moved to its active state by binding of ligand, its associated G protein dissociates into a G_α subunit and a β/γ dimer. As illustrated in Figure 5-7, the G_αs subunit activates adenylyl cyclase, which increases the concentration of cAMP in the cell. cAMP activates protein kinase A. Protein kinase A phosphorylates protein substrates that control Ca⁺⁺ availability in the cell. With decreased Ca⁺⁺ availability, the myosin light chain is ineffective in sustaining active tone in airway smooth muscle. The tissue relaxes passively.

The mechanism previously described is the traditional cAMP/cAMP-dependent protein kinase A cascade. Studies since the 1990s suggest that there are other mechanisms involved with β-adrenergic signaling in the airways.³¹ In addition to G_αs, the β₂-adrenergic receptor can also couple to G_αi and inhibit adenylyl cyclase. β₂ Receptors are found on proinflammatory and immune cells, including mast cells, macrophages, neutrophils, lymphocytes, eosinophils, epithelial and endothelial cells, and types 1 and 2 alveolar cells. Inhibition of adenylyl cyclase may be involved in decreasing the reactivity of these proinflammatory cells.

Membrane hyperpolarization is another significant effect of β₂ agonists. Activation of K⁺ channels by β₂-adrenergic receptor agonists in the plasma membrane decreases electrical excitation. This decrease in electrical excitation inhibits extracellular Ca⁺⁺ from entering the cell and decreases the available Ca⁺⁺ for smooth muscle contraction. The effect of β₂ agonists on K⁺ channels is thought to occur by direct coupling of the big-conductance K⁺ channel and the G_αs subunit of the receptor.

Pharmacologic effects

Bronchodilation is not the sole pharmacologic effect of β₂ receptor activation. Other effects include reduction of inflammatory cytokine production, suppression of plasma exudation secondary to receptor presence on postcapillary venules, regulation of fluid balance in alveolar epithelial cells, and reduction of cholinergic neurotransmission. In asthma and COPD, β₂-adrenergic agonists not only cause bronchodilation but also offer bronchoprotection, or reduced responsiveness, to noxious stimuli.

Adverse effects

The most serious adverse effect of β₂ agonist use is an increase in sensitivity of the smooth muscle in the lungs to noxious stimuli. Paradoxically, an increased incidence of asthma exacerbations and morbidity and mortality is seen after long-term use of β₂ agonists⁴⁴; this is attributable to the loss of bronchoprotection and bronchodilation effects of the drugs. This effect is called tolerance, a state of refractoriness that occurs after prolonged exposure to an agonist. The mechanistic basis for tolerance involving β-adrenergic receptors is currently unclear. Factors that can contribute to tolerance include receptor downregulation, due to receptor internalization, and receptor desensitization (see Figure 1-12) which can be due in part to receptor uncoupling from adenylyl cyclase.¹⁵ It is not the use of intermittent short-acting β₂ agonists, such as albuterol, that leads to tolerance, but rather the continued use of long-acting β₂ agonists.

The U.S. Food and Drug Administration (FDA) has required manufacturers of long-acting β₂ agonists to place a “black box” warning on the physician information for salmeterol and formoterol. Data for the advisory labeling came from the Salmeterol Multi-Center Asthma Research Trial (SMART),¹⁸ which examined the safety of salmeterol when added to usual asthma therapy. Study subjects who were on salmeterol showed a small but significant increase in asthma-related death compared with subjects receiving placebo.

Upregulation of phosphodiesterase-4, the enzyme responsible for degradation of cAMP, is also thought to play a role in the loss of effect of β₂ agonists. This hypothesis led to the development of phosphodiesterase-4 inhibitors, roflumilast and cilomilast. Currently available in Europe, phosphodiesterase-4 inhibitors seem to improve FEV₁ in COPD patients.

With the exception of levalbuterol, all β₂ agonists are racemic mixtures of R-enantiomers and S-enantiomers.³⁹ Although the R-enantiomer of albuterol, levalbuterol, is 100 times more potent at the receptor, the S-enantiomer of albuterol is metabolized much more slowly and is retained in the lung. Preclinical data indicate that S-albuterol is associated with proinflammatory activity that can lead to bronchial hyperresponsiveness. These activities include an increase in intracellular Ca⁺⁺, an increase in inflammatory stimuli, an increase in eosinophil activation and recruitment, and an increase in mucin production. These proinflammatory activities have led to speculation that the S-enantiomer of salmeterol is responsible for the loss of effect seen with long-term use of this drug. Formoterol has two chiral centers, and there is evidence to suggest that S-formoterol accumulates in bronchial tissue relative to the R-enantiomer because of different rates of metabolism. Many new long-acting β₂ agonists in clinical development are pure active enantiomers.

Clinical trial retrospective analyses of genetic polymorphisms of the β₂ receptor in individual patients seem to suggest that individuals who are homozygous for arginine at position 16 on the β₂ receptor (about 15% of the population) show greater rates of asthma exacerbations than patients who have glycine at this position, or patients who are heterozygous. It is unclear what role the arginine substitution at position 16 on the β₂ receptor plays in the mechanism of β₂ agonists. More data are needed to determine if genotype is another factor contributing to receptor desensitization.

Specific agents

Interaction between corticosteroids and β₂-adrenergic receptor agonists