Mycophenolate Mofetil Therapy for Acquired Myasthenia Gravis in Dogs: A Comparative Retrospective Study (1999-2007)
Acquired myasthenia gravis (MG) is a common autoimmune neuromuscular disorder of dogs, the effective treatment of which can be extremely challenging. Mycophenolate mofetil (MMF) is a lymphocyte-specific immunosuppressive drug that has been used successfully in human autoimmune disorders and transplant recipients. There is anecdotal evidence of MMF efficacy in dogs with various autoimmune diseases. The purpose of this investigation was to retrospectively evaluate the outcome of dogs with serologically confirmed MG that were treated with MMF and compare these MG patients to MG dogs treated with pyridostigmine (PYR) alone.
Myasthenic dogs, 14 treated with MMF (mean dose--18.6 mg/kg BID) and 8 treated with PYR alone (mean dose--1.16 mg/kg BID-TID), were identified. All but 2 MMF dogs also received PYR. Dogs in both groups were of several breeds of both sexes, and all clinical forms (focal, generalized, acute fulminating and paraneoplastic) of MG were represented. Megaesophagus was present in 12/14 (86%) dogs in the MMF group and 7/8 (88%) dogs in the PYR group. At initial diagnosis, acetylcholine receptor (ACh R) antibody titers ranged from 0.96 to 14.4 nM/L (mean--3.30 nM/L) in the MMF group, and from 1.21 to 7.13 nM/L (mean--3.97 nM/L) in the PYR group. Thymoma or lymphoma was identified in 1 MMF dog and 2 PYR dogs. 8 of 14 dogs (57%) treated with MMF achieved clinical remission of MG; 7 of these dogs had megaesophagus, 5 had pneumonia. 3 of these 8 dogs (38%) came out of remission during MMF weaning. 2 of 8 PYR dogs (25%) achieved clinical and immune remission; 1 of these 2 dogs had megaesophagus and pneumonia. The 1-yr mortality rate (due to MG) was 37.5% for PYR dogs, 28.6 % for MMF dogs. Based on a Fisher's exact test (p<0.05), the differences between groups in remission rates and mortality rates were not significant.
Results of this study suggest that clinical remission of MG in dogs is more likely to be achieved with MMF than with sole PYR therapy; however, this was not statistically significant. Also, dogs attaining clinical remission with MMF should be monitored closely for disease recurrence during MMF weaning (i.e., sustained immune remission less likely). Further investigation of MMF as a therapy for canine acquired MG with a larger group of dogs is warranted.