For good reasons, glucocorticoids (GLs) have been the cornerstone of immunosuppressive therapy in humans and animals.
For good reasons, glucocorticoids (GLs) have been the cornerstone of immunosuppressive therapy in humans and animals. Their impact on immunomodulation reflects inhibition at every stage of the immune response, including both innate and acquired, and cell- mediated responses; the humoral response, however, is minimally directly affected.
Molecular Mechanism of Action : The effects of GLs are generally recognized to be dose dependent. GLs bind to receptors that are complexed with heat shock proteins in the cytoplasm of the target cell (Hsp 70 and 90). Binding causes the receptor to dissociate from the heat shock protein; the GL and receptor then move into the nucleus where the GL binds to specific DNA sequences or receptors called GL responsive elements (GRE). Once activated, the activated ligand-receptor influences gene expression by binding to GRE in the promoter regions of GL-regulated genes (positive GRE), or by repressing genes through negative GRE (nGRE). The GL receptor (GR) generally has a lower affinity for nGRE compared to pGRE. Two types of interaction occur between the GL and either GRE: cis or trans, and for each, either activation or repression. Cis actions involve binding to specific DNA recognition (direct transcription) sites whereas trans actions involve protein-protein coupling that modulates other transcription factors (eg, NFk-B, AP-1, STAT-5or NF-AT) such that their activity is modulated (indirect, or transactivation). The largely undesirable metabolic effects associated with long-term GLC therapy appear to be mediated by activation (particularly cis?), whereas desirable anti-inflammatory and immunomodulatory effects appear to reflect repression (eg, particularly trans?, et nGREs). Trans nGRE interface also appear to occur at lower concentrations compared to cis effects, thus offering a mechanism of minimizing side effects by using lower doses. Alternative pathways increasing can be expected to be specifically targeted by "designer or dissociative GLC" that are more potent for trans repression rather than other actions. Dexamethasone and prednisolone are examples of a "symmetrical" GLC, characterized by equal binding affinity for both cis and trans actions (or trans repression and activation). In contrast, medroxyprogesterone acetate is predominately transrepressive.
Glucocorticoid Resistance : Human patients may fail to respond to GLs. Causes include poor compliance and poor bioavailability as well as GL resistance (both familial and iatrogenic). Resistance may reflect decreased receptor number (eg, down regulation) or affinity. Reversible down-regulation isa documented sequela of GL treatment (demonstrated in T-lymphocytes of humans receiving GL to treat host-versus graft rejection). This type of resistance might be avoided by higher doses of GL, including pulse dosing. A relative imbalance of GL receptor isoforms may also be responsible: the α isoform binds to GLs, DNA and transcription factors, thus modulating transcription, whereas the β isoform binds to DNA, but not other ligands and fails to activate transcription, thus potentially interfering α isoform actions. Some human patients with severe IBD that fail to respond to high doses of GLs have poor antiproliferative response by blood T-lymphocytes whereas responder completely inhibit. A similar situation has been demonstrated for other chronic allergy-based diseases, such as asthma or rheumatoid arthritis, and renal allograft rejection. Other factors that may contribute to poor response to GLC include overexpression of the multidrug resistance gene (MDR1 polymorphism)which might be reversible by co administration of cyclosporine, an inhibitor of P-glycoprotein.
Selected pharmacodymic effects. Intermediary metabolism: The natural function of GLs is to protect glucose-dependent cerebral functions by stimulating the formation of glucose by the liver, decreasing its peripheral utilization and promoting its storage as glycogen, thus protecting glucose-dependent tissues, the brain and heart. The hyperglycemic effect of GLs, reflects increased gluconeogenesis and insulin antagonism. Increased breakdown of proteins, particularly skeletal muscle and collagen, provides gluconeogenic precursors (e.g., amino acids and glycerol). Inflammation and immunomodulation: For lymphocytes, GLs cause up or down regulation of up to 2000 genes involved in the regulation of the immune response such that both. early and late phases of the inflammation.
1. reduce circulating lymphocytes;
2. alter lymphocyte response to mitogens and antigens (T lymphocytes are inhibited to a greater degree than B lymphocytes);
3. alter white blood cell function;
4. inhibit edema, fibrin deposition, leukocyte migration, phagocytic activity, collagen deposition, and capillary and fibroblast proliferation (generally through inhibition of lymphokines and other soluble mediators of inflammation);
5. induce annexin I, which inhibits phospholipase 2, thus blocking the release of arachidonic acid and its subsequent conversion to eicosanoids (i.e., prostaglandins, thromboxanes, prostacyclins, and leukotrienes);
6. preferentially inhibit transcription of cyclooxygenase 2, the inducible form of cyclooxygenase, thus decreasing the risk of toxicity.
7. induce protein MAPK phosphatase 1, which, through various actions inactivates a number of proteins important in the signaling of cytokines;
8. inhibit transcription of NF-k-B. Effect on immune cells;
9. inhibit release of tumor necrosis factor and interleukin-2 (IL-2) from activated macrophages.
10. Inhibit release of platelet-activating factor from leukocytes and mast cells;
11. inhibit macrophage migration-inhibition factor, (macrophages migrate away from the affected area);
12. Block IFN-γ released from activated T cells (needed to facilitate antigen processing by macrophages).
13. inhibit synthesis and release of IL-1 by macrophages thus, suppressing activation of T cells, and IL-2 synthesis by activated T cells.
14. Inhibit bactericidal and fungicidal actions of macrophages.
15. Altered synthesis of and biologic response to collagenase, lipase, and plasminogen activator.
16. Inhibition of the inducible form of nitric oxide synthase (iNOS).
Interestingly, despite their effective immunosuppressant effects, gluocorticoids have been associated with allergic reactions, including type I acute anaphylaxis. Cardiovascular: GLs (and mineralocorticoids) impact the maintenance of extracellular fluid volume and enhance vascular reactivity to other vasoactive substances (e.g., norepinephrine, angiotensin II). Patients with insufficient concentrations of GLs subsequently endure increased capillary permeability, decreased cardiac output, and inadequate vasomotor response of the smaller blood vessels to catecholamines. Bone and healing: inhibition collagen synthesis by fibroblasts, depress chondrocyte metabolism, and decrease the proteoglycan content of cartilage, resulting in morphologic changes in articular cartilage.
1. Maintain adequate plasma concentrations of glucose for cerebral functions, maintain cerebral blood flow, and influence electrolyte balance;
2. Decrease formation of cerebrospinal fluid;
3. influence mood (including "euphoria"), behavior, and brain excitability.
4. regulate neuronal excitation;
5. induce glutamine synthetase in both the central and peripheral nervous systems. Increased glutamate has been associated with CNS pathology. Respiratory: GLs impart "permissive" effects on β2-receptors, promoting bronchodilation.
Gastrointestinal GLs: Decrease the absorption of calcium and iron and increase the absorption of fats. Secretion of gastric acid, pepsin, and trypsin are increased by GLs. Gastric mucosal cell growth and renewal are reduced by GLs, and mucus production is decreased, resulting in compromise of the protective barrier of the gastric mucosa. Collectively these effects contribute to increased susceptibility to gastric ulceration. A retrospective study in humans found 5% developed gastric mucosal lesions while receiving GLs, particularly patients with rheumatoid arthritis and collagenosis.
Glucocorticoid Preparations. Close to 50 different generic corticosteroid products are approved for human use and several for (small) animal use. They differ in their routes of delivery, but also in their duration of action, mineralocorticoid activity, and anti-inflammatory potency; As the anti-inflammatory potency of a particular agent increases, its biologic half-life and duration of action also increase. For example, dexamethasone is 30 X and prednisolone 4 X as potent as hydrocortisone in impairing glucose metabolism (da Silva 2005). With current drugs, anti-inflammatory properties parallel the effects on carbohydrate and protein metabolism, but mineralocorticoid effects can be altered independently by changing the molecular structure of the steroid. The 4,5 double bond and the 3-ketone are necessary for mineralocorticoid and GL effects. Synthetic modifications of cortisol increase the anti-inflammatory activity, decreased protein binding, and decreases hepatic metabolism, thus prolonging activity. First generation glucocortiocoids were formed with the addition of a 1,2 double bond increased the ratio of GL to mineralocorticoid effects ( prednisolone, prednisone and methylprednisone).The second generation steroids were fluorinated at the C-9 position, increasing potency. Methylation at the C-16 position eliminates mineralocorticoid activity (dexamethasone, betamethasone and triamcinolone). Third generation glucocortiocids include _______-(Ventura 2005).
Clinical Pharmacology: Several products are well absorbed orally. For intramuscular or subcutaneous administration, the duration and onset of action of a particular GL can be altered by the addition of an ester, usually bound to C-21. The GL esters must be hydrolyzed to release the active, free form of the drug. The sodium phosphate and sodium succinate esters are water soluble, can be administered intravenously, and are rapidly hydrolyzed. These characteristics make them ideal for treatment of acute conditions. The acetate, acetonide, valerate, and dipropionate esters are water insoluble and release the active steroid very slowly, providing GL activity for days to weeks (i.e., repositol or "depo" products). The major advantage of these esters is convenience of administration. Administration at 2- to 6-week intervals, depending on the preparation used and disease being treated, has been recommended. Disadvantages include unpredictability of blood concentrations, chronic suppression of the hypothalamic-pituitary-adrenal axis (up to 12 weeks or more following administration of a single dose), possible induction of steroid resistance (mediated by receptor down-regulation), and the fact that the drug cannot be withdrawn should adverse reactions develop. For these reasons, the authors recommend the use of short-acting to intermediate-acting preparations administered daily or on alternate days over repositol steroid preparations.
There are many forms of GLs available for topical use. Note that absorption of the drug through the skin and its extensions can be sufficient to suppress the hypothalamic-pituitary-adrenal axis. Once absorbed through the skin, topical corticosteroids are handled by the body in the same capacity as systemically administered GLs. The extent of percutaneous absorption of topical GLs depends on factors such as the vehicle, the ester form of the steroid (greater lipid solubility enhances percutaneous absorption), duration of exposure, surface area, and the integrity of the epidermal barrier. Ointment bases are occlusive and are therefore more likely to increase percutaneous absorption of the same GL in a cream base. Highly potent preparations in any form should not be used on abraded skin. Steroidal hormones tend to be eliminated by oxidation or reduction followed by conjugation (generally glucuronide or sulfate) and excretion (principally renal). Metabolism occurs at both hepatic and extrahepatic (including the kidney) sites.
Drugs: Prednisolone versus Prednisone: Prednisone is rapidly metabolized by the liver to prednisolone (C-11 ketol reduction). Prednisone and prednisolone generally are (inappropriately) considered equivalent in terms of therapeutic use in veterinary medicine; veterinary dosing formularies generally make no distraction between the two. Yet, in the cat, the AUC for prednisolone was 3230.55 ng/mL/h and Cmax of 1400.81 ng/mL with a half -life for excretion of 1 h. This compares to a prednisolone AUC of 672.63 ng/mL/h and Cmax of 122.18 ng/mL following oral administration of prednisone; interestingly, a portion of the AUC reflected a half-life which was much longer at 2.46 h. In cats, a 3 to 5 fold dose should be given; in dogs, a 2 fold increase should be sufficient. Methylprednisolone has greater antioxidant activity that has been shown to be beneficial in the treatment of experimental spinal cord trauma in cats and experimentally induced E. coli bacteremia. "Soft" glucocorticoids are applied topically in the respiratory or gastrointestinal tract (with local effects) in order to avoid side effects associated with consumption. Soft GLs are potent for GR but also rapidly metabolized by the liver (ie, undergo 1st pass metabolism) should the drug be absorbed into systemic circulation. Examples include beclamethasone, budesonide and fluticasone proprionate, steroids designed specifically for use in inhalant metered doses. Their inhalant potency varies, with fluticasone propionate being most potent and budesonide and beclomethasone dipropionate approximately equipotent. Time of onset in humans to budesonide is approximately 10 hrs based on evidence of clinical improvement at that time. Improvement can be expected over the next 1-2 days, with maximum effects potentially not being evident until 2 weeks after therapy has begun.
In humans, budesonide is rapidly metabolized in the liver by CYP3A4 with affinity of metabolites of the GCR being less than 1% of the parent. Yet, 100% of of topically (inhalant ) administered drug in humans appears as metabolites in the urine, indicating that systemic absorption of the drug does occur, with as much as 25% circumventing hepatic metabolism before entering systemic circulation. Budesonide is particularly interesting because it binds locally to the receptor in the lungs, more so than receptors in peripheral tissues. Inside the cell, the drug is esterified and is released only with de-esterification. As such, the drug is stored locally and slowly released. The ability to esterify varies among tissues, with pulmonary tissue apparently having a much higher capacity compared to other tissues, leading to greater storage in airways compared to peripheral tissues. Drugs which impaire CYP3A4 may increase the plasma drug concentration of budesonide over 7 fold. In the gastrointestinal tract, budesonide appears to be sufficiently orally absorbed in dogs that the hypothalamic-pituitary –adrenal – axis will be suppressed. The extent that budesonide will be removed by first pass metabolism following oral administration in cats is not known, but it is possible metabolism will be incomplete.
Therapeutic Considerations: Unless one is administering GLs for replacement therapy in a deficiency state (i.e., hypoadrenocorticism), GL therapy is not directed at the inciting agent. GL therapy is intended to reduce the physiologic processes that are activated in response to the disease. Despite the adverse events associated with their use, GCL continue to be heavily used in veterinary medicine, and potentially at doses that exceed that recommended. Indeed, in human medicine, the use of GLC clearly exceeds that recommended in text books and review papers. The advantages of low versus high doses have been previously discussed and are addressed again in Adverse Reactions. In general, an anti-inflammatory dose is considered to be 10 times the "physiologic" dose, and immunosuppressive doses are twice the anti-inflammatory dose. Shock doses of GLs have been reported at 5 to 10 times the immunosuppressive dose; however, the disadvantages of this high dose and the advantages of low dose therapy in shock patients are discussed below. When treating a patient for an immediately life-threatening condition such as immune-mediated hemolytic anemia, therapy should be aggressive, with a minimum effective dose determined after response has been achieved. Because high doses of GLs are often required to adequately treat immune-mediated diseases, adverse effects are likely to occur and should be anticipated. Tapering of doses not only helps avoid side effects associated with long term therapy but may also avoid antibody rebound that has been associated with abrupt withdrawal of gluccorticoids in human patients treated for prevention of graft versus host transplant rejection (REF). Dose reduction in patients with autoimmune diseases should be conducted gradually. The reduced dose should be continued for at least 2 weeks before the next attempted dose reduction, and the actual dose should be decreased by no more than half.
It is essential to assess the patient's status frequently for recurrence of clinical signs. Concurrent administration of additional immunosuppressive (azathioprine, cyclophosphamide, chlorambucil) or anti-inflammatory drugs antihistamines, omega fatty acids, pentoxyfylline, leukotriene receptor antagonists) may allow the GL dose to be decreased ("dose sparing" effect). High-dose pulse therapy has been reported in human patients with acute relapse of chronic graft-versus-host disease. Using an open design, patients either receiving no immunosuppressive therapy or patients which failed (a median of 2 failures) current therapy (mean prednisolone dose of 0.2 mg/kg/day, range of 0-2.5 mg/kg/day) were treated with methyprednisolone at 10 mg/kg IV or PO for 4 days. The rationale behind the high dose is based on the lympholytic properties of this dose, thus causing destruction of lymphcytes that otherwise would cause irreversible organ damage. The high dose is assumed to target the (non-genomic) metabolic processes necessary for sustained activity of lymphocytes, as opposed to the low (genomic) doses which target lympyocyte replication. Additionally, the high dose is considered to overcome GL receptor saturation associated with GL therapy, causing significant GL down regulation. Induction of T- lymphocyte apoptosis may also occur. Antiviral and antimicrobial therapy (sulfanomides) accompanied high dose GL therapy. During a 2 year follow-up, patients tolerated the therapy well, with no major life-threatening effects occurring in the first 3 post treatment months. However, three patients developed infections after completion of the therapy, suggesting profound immunosuppression. Yet, the median time to progression of disease was 2 years after treatment, leading the authors to conclude that high-dose pulse steroid therapy is an effective and well tolerated treatment for progressive graft-versu-host disease. Side effects of GLs can occur if withdrawal of a GL occurs too rapidly. In human patients receiving GLs, the most frequent problem encountered with rapid withdrawals is recrudescence of the underlying condition for which the GL was indicated.
Pathophysiology: Although not the sole diseases associated with inflammation in the respiratory tract, asthma has served as a model for research. Asthma is a complex chronic inflammatory disease characterized by airway narrowing associated with contraction and hypertrophy of bronchial smooth muscle, swelling of mucous membranes, and excessive production of mucous. Mediators released during inflammation are the major contributors to the pathogenesis of pulmonary disease, and particularly feline asthma. Mediators important in the pathogenesis of chronic allergic disease include preformed mediators histamine and serotonin (particularly in cat lungss), mediators formed in situ, including prostaglandins, leukotrienes (1000 fold more potent than histamine, particularly in the lungs), and platelet activating factor, and oxygen reactive species. The role of the T cells (particularly TH2) has been well described. These cells produce a number of interleukins which contribute to the inflammatory process by enhancing: IL-4: IgE synthesis; IL-5: eosinophil growth and differentiation; IL-9: mast cell differentiation; IL-13: mucus production, airway hyperactivity. Previously, TH1cells were thought to exert beneficial effects through down modulation of TH2 cells. However, more recent evidence suggests that TH1 are, in fact, proinflammatory, and along with interferon (IFN) γ, contributes to the inflammatory process.
Treatment of human chronic allergic disease increasingly appears to be focusing on systemic rather than simply a local allergic disease. In particular, the bone marrow response to allergens, and subsequent release of eosinophils, is recognized to be an important systemic process in allergic inflammation. A central role of eotaxin and IL-5 has been suggested. Sources of IL-5 include TH2 lymphocytes, mast cells, eosinophils and bone marrow stroma. Among its actions are promotion of differentiation and maturation of progenitor cells, release of mature eosinophils, promotion of survival and inhibition of apotosis. The role of LTs in asthma also is not clear, although cysteinyl LTs are expressed on a number of bone marrow progenitor cells and appear to be involved (based on effects of antagonists) in eosinophil/ basophil progenitor differentiation. Leukotrienes are very potent (being 1000X fold more so than histamine), causing marked edema, inflammation, and bronchoconstriction.
Respiratory Inflammatory Disease: Glucocorticoids have "permissive" effects on β2-receptors, promoting bronchodilation as well as inhibit basically all aspects of early and late phases of the inflammation including many activities directly oriented toward the lymphocytic and macrophage contributions to the immune response. In 1997, the National Heart Lung Blood Institute Expert Panel Guidelines recommended control of mild persistent asthma in people with a single, long-term control medication with anti-inflammatory properties. Because glucocorticoid efficacy is depends on therapeutic concentrations both in and below the diseased epithelium of all airways, systemic therapy provides the most consistent exposure to diseased airways although the risk of side effects is increased. In human asthmatics, deposition studies reveal that the majority of drug is deposited on central airways. However, reduced airway caliber will further decrease efficacy by reducing drug delivery to the peripheral airways. Thus, while aerosolized glucocorticoids decreases the side effects associated with systemic glucocorticoids use in humans, penetration of small airways and the epithelium. Additionally, the peak effect of inhaled glucocorticoids may not occur for 1 to 2 weeks after therapy has begun. Recent studies of the efficacy of inhaled beclomethasone diproprionate in humans found improvements to be short-lived, probably because the inhaled drug does not control inflammation well. Thus, short courses (5 to 7 days) of high doses of oral glucocorticoids tend to be used to treat acute exacerbations. The addition of inhaled glucocorticoids has increased asthmatic control in humans. Beclomethasone was among the first aerosol glucocorticoid developed for inhalant therapy. Systemic side effects associated with ingestion of drug deposited on the pharynx and central airways led to the inclusion of "spacers" that removed larger particles before they penetrated the pharynx. Additionally, administration of glucocorticoids removed by first pass metabolism (eg, budesonide or fluticasone [preferred]) decreased the risk of systemic exposure to swallowed drug. However, poor compliance of inhaled glucocorticoids in human patients led to the development of combinations of steroids with long-acting β2 agonists (eg, salmeterol/fluticasone or formoterol/budesonide). Anti-inflammatory therapy should target both large and small airways. Thus, systemic therapy should be considered (either as sole therapy or in addition to systemic therapy) in animals with moderate to severe disease.
Inflammatory bowel disease: The role of GLCs for treatment of IBD is well documented. Use should be reserved for animals in whom biopsy has confirmed a diagnosis of IBD. Glucocorticoids are indicated in dogs and cats with lymphocytic-plasmacytic IBD. Prednisolone (prednisolone 2–4 mg/kg PO) should result in clinical response within 1 to 2 weeks (see above regarding prednisone inc cats). Therapy should continue at the same rate for another 2 weeks (beyond clinical response) and then slowly be tapered. More severe cases of IBD or cases that do not initially respond to prednisolone may respond to dexamethasone (0.22 mg/kg/day orally). The development of multidrug resistance has been implicated as a cause of therapeutic failure with GLCs in some human patients with IBD; expression of mucosal multidrug resistance may ultimately be used to determine response of IBD patients to therapy. Equally well known are the side effects of glucocorcorticoids. In human medicine, glucocorticoids that under go extensive first pass metabolism (ie, budesonide) or with marked topical anti-inflammatory activity (beclomethasone) have been developed in order to minimize side effects while maintaining efficacy. Budesonide has been recommended in dogs (1– 3 mg daily depending on dog size) although no scientific or anecdotal information has been published regarding its efficacy in animals. Means to reduce these side effects include combination drug therapies. Xats (especially those with lymphocytic-plasmacytic infiltrates) frequently respond to prednisolone at 2.2 mg/kg per day. For more severe cases, dexamethasone at 0.22 mg/kg per day plus metronidazole may be effective. In either case, the initial dose should be maintained for 2 weeks beyond the time that the cat's clinical signs begin to resolve. Cats that respond immediately should receive the initial dose for at least 4 weeks before dose reduction is attempted.