Background and perspectives
The majority of retrospective studies evaluating immunosuppressive treatment protocols for canine idiopathic ME include very few cases with confirmed histopathological diagnoses. The various combination protocols published in small cases series have been applied predominantly to cases of MUE (Zarfoss and others 2006, Adamo and others 2007, Coates and others 2007, Menaut and others 2008). Because MUE represents a broad spectrum of disease, it is unlikely that a “gold standard” therapy will be identified. Further complicating the interpretation of published cases is a paucity of prospective data on steroid monotherapy for MUE. As such, the utility of secondary immunomodulation is difficult to evaluate objectively. Standardised corticosteroid protocols also are lacking within and among published reports. Occasionally, standardised dosing and intervals for the adjunctive immunotherapy under investigation are lacking, and “exit strategies” have not been designed for patients that are non-responsive to treatment. In summary, the true efficacy of the various immunosuppressive agents for confirmed GME and NE is presently unknown.
At present, immunosuppression is the mainstay of therapy for MUE. Most clinicians treat MUE with corticosteroids (prednisone or dexamethasone). Depending on the severity of signs and the index of suspicion for infectious disease, some specialists will initiate therapy with anti-inflammatory steroids (0·5 to 1·0 mg/kg prednisone) and await serology and PCR results for regional infectious diseases. If the index of suspicion is extremely high for idiopathic inflammatory disease (for example, pug with MRI lesions consistent with NME), the authors directly initiate immunosuppressive therapy. Response to corticosteroids is variable and may be temporary, but dogs often have a favourable, initial response to steroid monotherapy. Additional immunosuppression is considered on a case by case basis, but the authors typically utilise secondary immunomodulatory agents upon review of negative serology and PCR results. At such time, we utilise the below prednisone protocol (based on clinical experience and not published data), often in combination with one or more of the immunomodulatory drugs. Cytosine arabinoside (CA), procarbazine, cyclosporine, lomustine (CCNU), leflunomide, mycophenolate mofetil (MMF) and azathioprine (Tipold and Schatzberg, in press) have been reported as adjunctive therapies.
•1·5 mg/kg twice a day for three weeks
•1·0 mg/kg twice a day for six weeks
•0·5 mg/kg twice a day for three weeks
•0·5 mg/kg once a day for three weeks
•0·5 mg/kg every other day indefinitely (may reduce to 0·25 mg/kg every other day)
As mentioned earlier, steroid monotherapy has not been investigated prospectively as a treatment for MUE. With disseminated GME, 23 cases were reported retrospectively with a survival range of 8-41 days (Munana and Luttgen 1998). In a clinical setting, steroid monotherapy may resolve signs associated with MUE in some dogs, but insufficiently or only transiently provides resolution in others. Moreover, long-term, high-dose corticosteroid therapy often causes adverse effects including polyuria-polydipsia, polyphagia, weight gain, hepatotoxicity, gastrointestinal ulceration, pancreatitis and iatrogenic hyperadrenocorticism. These combined factors have led to a recent focus on complementary immunomodulatory drugs to treat MUE.
Cytosine arabinoside is a chemotherapeutic agent used to treat several neoplastic conditions in both human and veterinary medicine. Over the past several years, CA has been utilised for its immunosuppressive properties as an adjunctive therapy for MUE ( Zarfoss and others 2006, Menaut and others 2008). Cytosine arabinoside is a synthetic nucleoside analogue, which crosses the BBB in dogs, undergoes enzymatic activation, competes for incorporation into nucleic acids and competitively inhibits DNA polymerase in mitotically active cells (Scott-Moncrieff and others 1991). Cytosine arabinoside also causes topoisomerase dysfunction, prevents DNA repair, and inhibits ribonucleotide reductase and glycoprotein synthesis ( Griffin and others 1982, Garcia-Carbonero and others 2001). Cytosine arabinoside is metabolised via deamination in the liver, plasma, granulocytes and gastrointestinal tract. Side effects are dose dependent and include myelosuppression, vomiting, diarrhoea and hairloss (Scott-Moncrieff and others 1991).
Cytosine arabinoside typically is administered as a subcutaneous injection at a dose of 50 mg/m2 every 12 hours for two consecutive days and repeated every three to six weeks, indefinitely (Zarfoss and others 2006). Previous reports of CA treatment regimes for MUE showed survival ranges of 46 to 1025 days (Zarfoss and others 2006) and 78 to 603 days (Menaut and others 2008). The authors commonly use CA as an adjunctive therapy for MUE in combination with prednisone as described earlier. Typically, a CBC is performed 10 to 14 days after the first course of CA therapy and then periodically throughout the course of treatment. In our experience, side effects have been minimal and dogs with MUE have a fair long-term prognosis with combined CA/prednisone therapy.
With combined CA/prednisone therapy, the CA dosing interval is gradually increased over several months and the steroids are tapered to the lowest dose possible, which often ameliorates clinical signs and minimises systemic side effects (Zarfoss and others 2006). Recurrence of clinical signs with steroid dose reductions may occur, so the authors gradually taper steroids as per the above section. After four months of a steroid taper, we typically maintain dogs indefinitely on 0·5 mg/kg prednisone po once daily or every other day depending on the resolution of neurological signs. Relapses are treated aggressively as they may be refractory to treatment. Recently, intravenous (iv) rescue CA protocols (iv constant rate infusion of CA at 200 mg/m2 over 48 hours) have been described for the initial treatment of severe MUE, which at UGA-CVM also has proven useful for severe relapses (de Stefani and others 2007). With relapses, some dogs also may require tertiary immunomodulatory drugs for the control of clinical signs.
Procarbazine is an antineoplastic, alkylating agent with multiple sites of action, and also has been used extensively to treat MUE. It is lipid soluble, crosses the BBB, and alkylates DNA at the O6 position of guanine, inhibiting insertion of essential DNA precursors. Procarbazine also disrupts ribonucleic acid (RNA) and protein synthesis. For treatment of MUE, procarbazine is given po at a dose of 25 to 50 mg/m2/day (Cuddon and Coates 2002). Side effects include myelosuppression, nausea, vomiting, hepatic dysfunction and neurotoxicity.
Procarbazine has been used as an adjunctive therapy with corticosteroids and as a sole immunomodulatory agent for MUE. The use of procarbazine and prednisone as combination therapy was investigated in 20 dogs with MUE and compared with an untreated group of 11 dogs with confirmed GME (Coates and others 2007). The prednisone dose was reduced or discontinued in 17 dogs and median survival time was 15 months. The authors recommended monitoring a CBC once weekly for the first month of therapy and monthly thereafter. If improvement was noted after the first month, the procarbazine dose was reduced to every other day, provided that relapses were not observed.
Cyclosporine is an imm-unosuppressive agent that can be used as a monotherapy but more typically is combined with prednisone and/or ketoconazole to achieve remission in cases of MUE (Adamo and others 2007). Cyclosporine acts by directly suppressing T lymphocyte activation and proliferation (Bennett and Norman 1986). In addition, cyclosporine prevents synthesis of several cytokines including interleukin-2, which indirectly inhibits T-cell proliferation. The rationale for its use in the treatment of MUE/presumptive GME is based on the suggestion that GME is T-cell-mediated delayed-type hypersensitivity (Kipar and others 1998). Although cyclosporine has poor BBB permeability, ME may allow the drug access to the CNS compartment. Moreover, cyclosporine likely concentrates effectively in the cerebral endothelial cells and choroid plexus (Begley and others 1990). Lesions associated with GME and NE are primarily perivascular; therefore, a therapeutic concentration of cyclosporine likely reaches the intracellular compartments of the lymphocytes and macrophages in affected areas of the CNS in these disorders (Adamo and O’Brien 2004).
Cyclosporine works rapidly and reaches effective steady state blood levels within 24 to 48 hours of initiation of therapy (Adamo and O’Brien 2004). When used as the sole therapeutic agent for MUE, a starting dose of 6 mg/kg po every 12 hours of cyclosporine has been recommended to achieve therapeutic serum concentrations (Adamo and others 2007). The microemulsified form (Neoral®), or its generic equivalent (cyclosporine modified), is recommended, as a uniform blood level is attained at lower doses compared to Cyclosporine USP, Sandimmune® (Novartis Pharmaceuticals Corporation, East Hanover, NJ 07936) (Gregory 2000). The most common adverse effects include diarrhoea, anorexia and vomiting, all of which typically subside when the dose is divided more evenly throughout the day. Occasionally, gingival hyperplasia, papillomatosis, hirsutism, excessive shedding and insulin resistance may occur, requiring discontinuation of therapy (Robson 2003). Rare side effects include nephrotoxicity and/or hepatotoxicity.
Cyclosporine is metabolised by cytochrome P-450; thus, phenobarbital will decrease cyclosporine blood levels as it induces the P-450 enzyme (Robson 2003). If the use of cyclosporine is cost prohibitive, it may be combined with ketoconazole. Ketoconazole significantly lowers the dose of cyclosporine needed to achieve effective blood levels by inhibiting the cytochrome P-450 enzymes and decreasing the systemic clearance of the drug. The recommended combined doses for combination therapy are 5 mg/kg po once a day cyclosporine and 8 mg/kg po once a day ketoconazole (Adamo and others 2007) Side effects associated with ketoconazole include anorexia, vomiting and diarrhoea. Hepatotoxicity has been reported rarely, and it is noteworthy that ketoconazole is teratogenic.
In 2007, Adamo and others retrospectively evaluated the utility of cyclosporine for the treatment of MUE. Ten cases of MUE were evaluated including dogs treated with cyclosporin monotherapy and cyclosporin in combination with corticosteroids and/or ketoconazole. The overall median survival time for all dogs in the study was 930 days (range, 60 to more than 1290 days). Side effects were minimal and included excessive shedding, gingival hyperplasia and hypertrichosis.
Lomustine (CCNU) is an antineoplastic agent with potent immunosuppressive properties that relate to its toxic effect on lymphocytes. Lomustine is a highly lipid soluble, nitrosourea compound. It readily crosses the BBB and alkylates both DNA and RNA. Bone marrow suppression (leukopenia and delayed thrombocytopenia) and gastrointestinal upset (vomiting and diarrhoea) are the most common side effects. Hepatotoxicity also has been reported in dogs when used at very high doses (90 mg/m2 every three to four weeks concurrently with other hepatotoxic drugs) (Kristal and others 2004). Serum chemistry monitoring is recommended after the first treatment, then every three months thereafter. Although the use of lomustine for MUE is common, and anecdotally effective (Dr Allen Sisson, personal communication), there are no peer reviewed manuscripts that have evaluated its utility in this application.
In 2007, investigators on two separate abstracts reported that when combined with low-dose prednisone to treat MUE (23 cases total), lomustine resulted in longer survival times compared to prednisone alone (Flegel and others 2007, Uriarte and others 2007). Flegel and others reported a dose of 60 mg/m2po every six weeks to be effective, with minimal side effects (Flegel and others 2007). Further evaluation of lomustine as an adjunctive therapy for MUE is needed.
Mycophenolate mofetil is a lymphocyte specific immunomodulatory drug that decreases the recruitment of inflammatory cells and has been preliminarily reported in five dogs as an adjunctive therapy for MUE (Feliu-Pascual and others 2007). An initial dose of 20 mg/kg po twice a day was recommended; after one month of treatment, the dose was decreased to 10 mg/kg twice a day. Side effects included haemorrhagic diarrhoea, which subsided with dose reduction and/or discontinuation of the drug. Neither bone marrow suppression nor hepatoxicity were reported in the limited dogs treated. Although initial responses are encouraging, the authors concluded that larger, prospective studies are needed to evaluate the efficacy of MMF in the treatment of MUE.
Leflunomide is an immunomodulatory drug that has efficacy in experimental models of autoimmune diseases (Gregory and others 1998). The active metabolite of this drug, teriflunomide (A77), inhibits T and B-cell proliferation, suppresses immunoglobulin production and interferes with cell adhesion. In addition to its immunosuppressive effects, leflunomide has both in vitro and in vivo antiviral properties (Chong and others 2006). The recommended dose range of leflunomide is 1·5 to 4 mg/kg po once a day; however, this dose may be adjusted, based on A77 blood level measured 24 hours after administration (Gregory and others 1998). In human beings, A77 reaches peak blood levels in 6 to 12 hours, has a long half life of approximately two weeks, and can take up to two months to reach steady state. Dose adjustments should be made to keep blood levels in a safe, therapeutic range (20 to 40μg/ml). Adverse side effects are seemingly rare in dogs; however, they may include thrombocytopenia and haemorrhagic colitis.
In 1998, Gregory and others reported on five dogs with MUE that were treated with leflunomide due to a poor response or side effects associated with prednisolone therapy (Gregory and others 1998). All dogs, treated over 4 to 11 months, had good to excellent improvement in their neurologic status with no reported side effects. Post-treatment MRI of two dogs showed partial to marked resolution of cortical lesions. Further studies are needed to critically evaluate the potentially useful role of leflunomide in MUE.