Non-glucocorticoid immunomodulation in small animals (Proceedings)

Despite their marked efficacy in suppressing the immune system, alternatives to glucocorticoids must often be considered when treatment immune-mediated or chronic allergic diseases in dogs and cats. Intolerance to glucocorticoid side effect, contra-indicating diseases (eg, diabetes mellitus, renal disease) and insufficient response (ie, combination therapy with glucocorticoids) are potential indications. This discussion will be limited to systemic immunomodulation.

Cyclosporine i s a T-cell specific immunomodulator approved for use in humans for immune suppression of graft vs host transplant rejection, autoimmune disorders, red cell aplasias and, increasing IBD. The standard preparation is very lipophilic and as such must be prepared in oil and oral absorption is bile acid dependent; oral bioavailability ranges from 20 to 50%. Newer microemulsion preparations (Atopica® or Neoral®) is not as dependent on bile acids for oral absorption and is more bioavailable.

Bioavailability of orally administered cyclosporine approximates 25-30% following multiple oral administration in cats (n=6), Food may impair the absorption of CsA. The drug is not absorbed topically following transdermal administration. Cyclosporine is widely distributed, characterized by a large volume of distribution (13 L/kg in humans). The drug accumulates in erythrocytes (accounting for 50% or more of the drug in humans), and leukocytes (accounting for 10 to 20% of circulating drug in humans). As such, monitoring of whole blood rather than plasma or serum is recommended. Remaining circulating drug is bound to plasma lipoproteins. Further, concentrations in skin are up to 10 fold higher than blood, although this is based on homogenate data. In contrast, largely due to the influence of ATP-binding transporters P-glycoproteins, especially the B1 cassette (PgP, MDR1 gene product), little CsA crosses the blood brain barrier. The drug is metabolized by the liver to a large number of metabolites, which are likely to vary among animals. The drug is metabolized predominantly by the liver, although sufficient metabolism occurs by intestinal enterocytes that oral bioavailability is impacted. Liver disease will decrease clearance of CsA substantially. However, for the non-microemulsion form, decreased oral absorption in the face of decreased bile may balance decreased clearance. Monitoring is recommended in the presence of hepatic or gastrointestinal disease. Cytochrome 3A4 (commonly associated with P-glycoprotein or other efflux pumps) plays a major role in metabolism; many of the drug interactions involving CsA also involve CYP3A4 or associated glycoprotein. Over 25 to 30 metabolites have been documented in humans.

In cats, peak concentration (Cmax) at 3 mg/kg orally (Neoral®) in one study was 480.0 ng/mL (AUC =6243 ng*hr/mL versus 7413 ng*hr/ml following IV [Sandimmune®] administration of 1 mg/kg). Volume of distribution was 1.7 L/kg. Variability in plasma drug concentrations among cats and across time underlines the importance of monitoring. Following multiple (7 to 14 day) day oral administration, mean (peak) plasma concentrations at 2 hrs were 740 ± 326 (7 d) and 655 ± 285 ng/mL (14 d) whereas trough concentrations were 332 ± 237 and 301.5 ± 250 ng/ml, respectively. Mehl and coworkers also investigated the relationship between peak and trough plasma drug concentrations and area under the curve. Following 14 days of dosing, both peak and trough concentrations correlated well (0.89 and 0.98, respectively) with AUC at 12 hrs, suggesting that either time point will serve as a monitoring point. Ocular administration might serve as an alternative route for systemic delivery of CsA in cats. Following administration of either an oral preparation or an ocular preparation of CsA in olive oil, peak concentrations ranged from 450 to 1033 ng/ml and 288 to 648 ng/ml, respectively, with an absorption lag time ranging from 0 to 1.34 hr for the oral solution, and 0.27 to 1.2 hr for ocular preparation. The elimination half-life ranged from 2.41 to 10.04 hr, and 3.09 to 15.75 hr, respectively. Blood concentrations were sufficient to inhibit lymphocyte activity as measured in vitro, leading the authors to conclude that ocular administration of CsA in an olive oil vehicle might be a reasonable alternative to oral, or even IV, administration in cats intolerate to the latter.

Side Effects: Cyclosporine is characterized by a narrow therapeutic index in human patients, with renal toxicity the primary adverse effect. Renal tubular cells develop hyperuricemia (worsened by diuretics) and hyperkalemia (a renal tubular and erythrocyte ion channel effect). Hepatic injury also occurs. Although less common than renal dysfunction, the risk of severe hepatic damage is markedly increased when cyclosporine is used in combination with cytotoxic drugs. Cyclosporine also increases the incidence of gallstones in human patients. The risk of renal and hepatic toxicities does not appear to be as great in small animals. The risk of renal damage is, however, likely to be enhanced with concurrent administration of several other drugs, including other nephrotoxic drugs, and may be of greater risk in animals undergoing renal transplantation. Hyperlipidemia also has been reported in humans, particularly in patients receiving glucocorticoids. Other side effects reported in humans include hepatotoxicity, neurotoxicity, gastrointestinal upset, and hypertension. Gingival hyperplasia increasingly is being reported as a side effect of chronic therapy; mucosal hypertrophy in the sinuses of cats has been anecdotally reported. Development of B-cell lymphoma also has been reported. Side effects in dogs reflect gastrointestinal upset and dermatologic or mucosal abnormalities. Vomiting may occur in put to 40% of dogs, although it may be intermittent and short I duration. Diarrhea occurs less commonly (16-18%). Cyclosporine apparently inhibits insulin secretion in a dose-dependent manner. Rare neurotoxicity, characterized by reversible cortical blindness has been reported in human patients receiving CsA following bone marrow transplantation.

Monitoring: Cyclosporine concentrations should be monitored as a guide to effective yet safe dosing regimens. Guidelines offered in human medicine have served as targets for veterinary patients. although the appropriateness of extrapolation has not been addressed. Plasma or blood concentration of cyclosporine is the predominant determinant of drug effect,, although an active metabolite may contribute up to 10% of activity in humans. Note that the dosing regimen recommended for treatment of atopy should not be assumed to be effective for treatment of immune mediated diseases. In humans, cyclosporine is usually administered by mouth every 12 h. Estimates of drug exposure using the area under the time concentration curve (AUC) appears to be the most accurate pharmacokinetic measure upon which to based dosing in human patients subject to acute rejection episodes. However, the description of AUC requires multiple sample collection, which is not only inconvenient, but costly. Two established therapeutic drug monitoring protocols have been used in humans: the traditional approach, based on trough blood concentrations, or the newer strategy based on a sample 2 h after oral administration (C2). Because the greatest contributor to inter-individual variability in CsA AUC in humans is absorption, monitoring that focuses on peak concentrations is evolving as more predictive for CsA. The risk of graft versus host rejection was markedly reduced when CsA dosing was based on peak rather than trough concentrations in humans. However, the use of this parameter as a predicator of CsA exposure is based on a 12 hour dosing interval, regardless of the method of CsA detection. Several methods are available for measuring cyclosporine concentrations, including high-performance liquid chromatography (HPLC), which generally detects only the parent compound, , and immunoassays based on radioimmunoassays (RIA) and polarized immunofluorescence, which detect parent drug and metabolites. Immunoassays that are monoclonal (eg, FIPA by Abbott TdX) are less likely to detect metabolites compared to polyclonal based assays. In dogs, blood concentrations measured by TDx assay are 1.8 times higher than those measured with HPLC assay. Each assay appears to be clinically useful, although the target concentrations will vary with the methodology, the sample collected (eg, whole blood, plasma or serum) and the timing of the sample (eg, peak or trough). For example, because much of CsA in whole blood is located in red blood cells, concentrations based on whole blood (WB) will be higher than that measured in serum or plasma or serum (PS) as will concentrations of parent and metabolite (antibody based assays, particularly polyclonal) compared to parent compound only (HPLC). In humans, recommended trough (before next dose) CsA concentrations (ng/ml) are: for HPLC, 100 to 300 (WB); RIA, monoclonal antibody methodology: 150 to 400 (WB) or 50 to 125 (PS); RIA, polyclonal antibody methodology: 200 to 800; fluorescent polarized immunoassay: 250 to 1000 (400-800 ng/ml). For 2 hr peak concentrations, we recommend targeting 800-1300 ng/ml. For IBD, concentrations of 250 ng/ml are recommended whereas concentrations of 100-300 ng/ml are recommended for perianal fistulas in dogs. With the advent of renal transplantation availability at several facilities, CsA is increasingly being used to prevent graft-host rejections. Cats respond better than dogs, with renal allografts being maintained at 7.5 mg/kg every 12 hours coupled with prednisolone (0.125 to 0.25 mg/kg every 12 hours). Whole blood CsA should be maintained at 500 ng/mL (HPLC) the first month after transplantation but can be reduced to 250 ng/mL thereafter. Other potential indications for cyclosporine warrant further investigation.

Note that often a more cost effective approach for reaching target concentrations is decreasing the interval rather than increasing the dose. Doubling concentrations will require doubling the dose, but tripling concentrations will require a 4 fold dose increase, and quadrupling the drug concentrations will require an 8 fold dose increase. The use of ketoconazole (KCZ) may prolong elimination and may be a cost effective approach to increasing CsA concentrations. Note, however, that we have documented a change CsA in half-life to 150 hr in a dog receiving KCZ; for such patients, steady-state requires 18-30 days. Monitoring a peak and trough sample is recommended in patients receiving KCZ such that half-life can be determined.

Mycophenolate mofetil Mycophenolate mofetil (MMF) is the morpholinoethyl ester pro-drug of mycophenolic acid (MPA) is a product of several Penicillium species that possesses antibacterial, antifungal, antiviral, antitumor, and immunosuppressive properties. MPA is a potent, selective, non-competitive yet reversible inhibitor of inosine monophosphate dehydrogenase (IMPDH). This enzyme is critical for the production of the guanine triphosphate precursor guanine monophosphate, which in turn is necessary for the de novo synthesis of purine nucleotides. As such, the drug is an antimetabolite. Two isoforms of human inosine monophosphate dehydrogenase (IMPDH) exist. Type I is expressed constitutively expressed in normal, non-replicating cells, and while type II is up-regulated in replicating (including neoplastic) cells to the point that it is the predominating isoform. As IMPDH is inhibited, GTP is depleted; failure to make mRNA precludes synthesis of proteins, including cytokines, necessary for cell proliferation. Unlike other anticancer antimetabolites, MMF and its active metabolite MPA are relatively selective for lymphocytes because they are solely dependent on de novo synthesis of purines for DNA synthesis (Langman et al., 1996; Pirsch and Sollinger, 1996). Mycophenolate mofetil is the first drug since cyclosporine to be approved in the United States for prevention of renal allograft rejection; it has proven useful for treatment of steroid—resistant acute liver rejection. In addition to its antiproliferative properties towards lymphocytes, MPA also impairs proliferation in non-immune cells, including. smooth muscle cells, renal tubular cells and mesangial cells, and dermal fibroblasts. As such, MMF might be indicated for non immune mediated diseases associated with fibrosis. The primary side effects in human patients receiving the drug for allograft rejection are leukopenia, gastrointestinal upset, and cytomegalovirus disease. The incidence is, however, small. Gastrointestinal upset is characterized by nausea, diarrhea, vomiting, and abdominal cramping. Gastrointestinal toxicity (and bone marrow suppression) may reflect the need for IMDPH II because of the normally replicating nature of these tissues. Oral bioavailability of MMF is poor, but extensive first pass metabolism to MPA yields a reasonably orally bioactive compound in humans and dogs. Thus far, no information is available for mycophenolate in cats. Because it is a prodrug, and will not be effective unless activated, this drug should be used cautiously if at all in cats.

Other Steroids: Danazol (5 mg/kg 2-3 times daily orally) is classified as an androgen but is characterized by weak androgenic activity. In human patients, danazol has proved effective in stimulating increased concentrations of complement inhibitor and thus is indicated for treatment of angioneurotic edema (Wilson, 1995). Danazol also has immunomodulatory effects that are of benefit in type II immune complex diseases. Danazol apparently decreases the expression of or blocks Fc receptors on macrophages; initial displacement of glucocorticoids due to competition of binding sites is less likely to be effective as glucocorticoid clearance increases. The drug has proved useful in a number of type II immune-mediated diseases, including immune-mediated hemolytic anemia (autoimmune hemolytic anemia) and immune-mediated thrombocytopenia. Medroxyprogesterone acetate has been used for treatment of selected immune-mediated diseases in lieu of glucocorticoids, or in patients that have not responded to glucocorticoids. Medroxyprogesterone exhibits glucocorticoid-like effects, which reflect, in part, actions at the level of DNA-transcription. In some studies, medroxyprogesterone is characterized by a preponderance of transrepression activity rather than transactivation, indicating is it less associated with undesirable side effects. However, clinically, this does not appear to be true in dogs or cats. Chlorambucil: Chlorambucil is an alkylating anticancer drug used for immunosuppression in diseases associated with marked lymphocytic infiltration. It often is used (0.1 mg/kg orally every 48 hours) used in lieu of cyclophosphamide. As with cyclophosphamide, leukopenia and thrombocytopenia are potential side effects of this drug. Pemphigus foliaceus in cats can be treated with chlorambucil as the first choice.. Daily therapy (0.1 to 0.2 mg/kg/day) should be continued until lesions have markedly reduced, which may take 4 to 8 weeks. Alternate-day therapy should be implemented when approximately 75% improvement occurs and continued for several weeks. Complete blood counts should be monitored every 2 weeks of chlorambucil therapy.

Leukotriene receptor antagonists: Leukotrienes are very potent causes of marked edema, inflammation, and bronchoconstriction. Their inhibition by glucocorticoids may be limited, leaving a void in therapy . The approval of drugs that specifically inhibit the formation of LTs or their actions have offered a new avenue of control of respiratory inflammatory disease (e.g., asthma) in human medicine. Two classes of drugs have focused on their impact on leukotrienes: leukotriene synthesis (5-lipoxygenase) inhibitors (zileuton) and leukotriene-receptor antagonists (LRA). The latter class has proven to be more effective. Currently, two LRA are approved in the United States : zafirulokast, administered twice daily, and montelukast, administered once daily. The disposition of neither drug has been studied in dogs or cats; in humans, the dosing intervals reflect a half-life of 5 and 10 hr respectively, for the two drugs.

In human clinical trials, LRA inhibit early and late phase bronchoconstriction and increased bronchial hyperresponsiveness in response to allergens, accumulation of inflammatory cells and mediators in bronchial lavage fluid, and acute bronchospasms stimulated by exercise, cold air and aspirin. Their bronchodilatory effects are less than that of long acting β-adrenergics whereas the anti-inflammatory effects are less than that of glucocorticoids. However, response does occur to single doses. In contrast to β-adrenergics, tolerance does not develop toward LAR effects. Their use has been associated with improvement of asthma either as sole therapy (in lieu of low dose inhaled glucocorticoids) in mild to moderate asthma, or as add-on therapy regardless of diseases severity in glucocorticoids non-responders. The LAR are well tolerated, particularly when compared to glucocorticoids. Recommendations regarding the role of LAR in treatment of human asthma are dynamic. Currently, because they are less effective than glucocorticoids, their role as monotherapy should be avoided, even with mild disease. In contrast, LRA have proven effective as monotherapy for treatment of allergic rhinitis in humans. For asthma, because of their unique mechanism of action, LAR have been combined with a number of other drugs used to treat asthma. For example, Glucocorticoids minimally impact cys-LT; as such, combination with LAR would be expected to have an additive effect as has been demonstrated when combined with inhaled glucocorticoids. The combination of LAR and glucocorticoids generally is as effective as glucocorticoids combined with β-adrenergic agonists. The use of LRA as part of triple therapy, ie, with corticosteroids, β-adrenergics is a reasonable, albeit, understudied therapeutic approach in patients not responding to dual therapy. Finally, LRA also have been recommended in the treatment of aspirin-sensitive asthma (see below).

The role of LAR in the treatment of feline asthma has had little scientific support, although this should not preclude their use, particularly in animals that have not sufficiently responded to, or can not tolerate (eg, diabetics) corticosteroids. Receptors for leukotrienes have yet to be identified in the smooth muscle of airways in cats; further, LTs have not been identified in the urine or plasma of cats with experimentally – induced asthma. However, LTs were associated with experimentally-induced heartworm disease in cats (author). The recent approach to asthma as a systemic response mediated at the level of the bone marrow, warrants consideration of the use of LRA for treatment of feline asthma, particularly in non-responders or patients that can not tolerate glucocorticoids. Other potential indications would include control of inflammation in dogs in which glucocorticoids are contraindicated, or pulmonary diseases characterized by eosinophilic infiltrate. Anecdotally, LAR appear to be safe in both dogs and cats, although no study has validated safety.

Antihistamines: Second generation antihistamines reasonably should be more effective as anti-inflammatory drugs compared to their first generation counterparts. They block both constitutive and histamine-induced H-1 receptors, hence are referred to as "inverse agonists" or "double-antagonists". Because they block histamine release, they are more likely to control inflammation. However, efficacy may still be limited by the competitive nature of histamine receptor blockade which can be overwhelmed by high concentrations of histamine. The newer antihistamines (H1) do not interact with muscarinic receptors as effectively as did the first generation drugs, thus avoiding many side effects (eg, xerostomia). Sedation associated with antihistamines is affected by pH, lipophilicity, and activity of transport proteins such as p-glycoprotein. Lack of sedation of the newer drugs has been attributed to transport proteins which mediate active efflux from the CNS and thus limit CNS distribution. Collies and related breeds deficient in p-glycoprotein might be expected to manifest more CNS reactions with newer drugs compared to other dog breeds. Two early second-generation H1 antihistamines, astemizole and terfenadine, were associated with cardiotoxicity (prolongation of the QT interval: torsade de pointes) as a result of blockade of the rapid component of the delayed rectifier potassium current. More recent drugs are not associated with this effect, including loratadine, desloratadine (a metabolite of loratadine), cetirizine, and fexofenadine (a metabolite of terfenadine). Cyproheptadine, is an antiserotinergic, antihistaminergic drug. Because feline airways are exquisitely sensitive to the constrictor effects of serotonin, this drug may prove particularly useful in cats either alone or as an adjunct to bronchodilators or glucocorticoids. The disposition of cyproheptadine has been described in cats (n=6). It is characterized by 100% oral bioavailability, a volume of distribution of 1 ± 0.5 l/kg and an elimination half-life of 12.8± 9.9 hrs, supporting a once to twice daily dosing interval. A single IV dose of 2 mg was well tolerated. An oral dose of 8 mg yielded a Cmax of 419± 99 ng/ml. Both the IV and oral dose were well tolerated. Studies following transdermal administration are pending.

Pentoxifylline is a methylxanthine derivative (similar to theophylline) with minimal respiratory or cardiac effects. Its ability to change rheological properties (perhaps by changing red blood cell deformability, or blood viscosity) (Amrus 1990) and potential anti-inflammatory effects (including potential antiplatelet aggregation) has led to its use in dermatomyositis, atopy, and pruritus associated with atopy in dogs. It decreases synthesis of mRNA for TNF α, which may be its primary mechanism of action.

It also has been used for treatment of immune-mediated diseases of the skin. Metabolites of the compound contribute a large component of pharmacologic activity. It has been studied in dogs, but not cats, but appears to be very safe. 10-15 mg/kg orally twice daily, for 2 to 6 weeks).

Tepoxalin (Zubrin) is a dual inhibitor NSAID, meaning it targets both cyclo-oxygenase and lipoxygenases, thus inhibiting the formation of both prostaglandins and leukotrienes. Dual inhibitors might be expected to be effective in the treatment of chronic allergic diseases because of their effect on leukotrienes. Approved for use in dogs, cats also appear to tolerate tepoxalin well although saturation kinetics occur at 60 mg/kg. A dose of 3 to 10 mg/kg per day might be considered in cats.

Miscellaneous: Several antibiotics are used to treat CIBD, presumably for their immunomodulatory effects. These include metronidazole and potentially fluorinated quinolones, with the latter being potentially effective in treating histiocytic IBD in a series of clinical cases. A number of monoclonal antibodies have been developed in humans to target specific cytokines that signal the inflammatory/immune response of immune-mediated disease. Target proteins include TNF-α and selected interleukins with indications including rheumatoid arthritis or IBD. Most are recombinant products and as such, are associated with the risk of acute or chronic immune reactions. No apparent information appears to be available regarding their use in dogs or cats.