Pharmacologic Interventions to Immunologic and Immune-Mediated Conditions in Horses

Immunomodulators can stimulate, suppress, or regulate one or many aspects of the immune response. Use of a variety of immunostimulants, immunosuppressors, and anti-inflammatory drugs are described in horses, but the evidence supporting their efficacy is variable. Corticosteroids and nonsteroidal anti-inflammatory drugs are the best characterized immunomodulators in horses, but further study is needed to fully define their ideal dosing protocols and indications and to characterize the efficacy of other immunomodulators in equine medicine.

Autoimmune Dermatopathies of Horses

Autoimmune dermatopathies are not common in horses. These autoimmune diseases can be idiopathic or triggered by an antigen such as drugs, vaccines, or neoplasia. The most common one is pemphigus foliaceus, which manifests as a pustular, crusting eruption. Other more common pustular diseases should be ruled out before considering pemphigus. Vasculitis is relatively common in horses and can be triggered by a variety of antigenic stimulations. Systemic lupus and true idiopathic autoimmune vasculitis are very rare in horses. Every effort should be made to reach a final diagnosis, as the prognosis for true idiopathic autoimmune skin diseases is poor.

Noncompartmental pharmacokinetics of three intravenous mycophenolate mofetil concentrations in healthy Standardbred mares

BACKGROUND

Mycophenolate mofetil (MMF) is the prodrug of mycophenolic acid (MPA) which acts as an immunosuppressive agent. During the biotransformation of MMF to MPA, additional metabolites including MPA phenol glucuronide (MPAG), MPA acyl glucuronide (AcMPAG) and MPA phenol glucoside (MPG) are formed.

OBJECTIVE

To define the noncompartmental pharmacokinetic (PK) parameters of three single doses of intravenous (i.v.) MMF and its downstream metabolites in healthy horses.

ANIMALS

Six healthy Standardbred mares.

MATERIALS AND METHODS

Generic MMF (Par Pharmaceuticals; Chestnut Ridge, NY, USA) was reconstituted and administered as a single i.v. bolus at 1.0 mg/kg, 5.0 mg/kg and 10.0 mg/kg with an eight day washout between treatments. Blood samples were collected immediately before MMF administration and over 24 h. A liquid chromatography-tandem mass spectrometry assay was developed following FDA guidance to determine plasma MMF, MPA, MPAG, AcMPAG and MPG concentrations. Plasma concentrations were analysed independently, followed by calculation of geometric mean and coefficient of variation.

RESULTS

Noncompartmental PK parameters were determined for MMF and all metabolites at all doses. MMF was rapidly converted to MPA in all horses. Each incremental dose of MMF resulted in increases in C_max and AUC_inf _obs for MPA and the three additional metabolites. Within the 10-fold dose range, the increase in C_max and AUC_inf _obs for MMF and its metabolites was nonlinear.

CONCLUSIONS AND CLINICAL RELEVANCE

Horses biotransform MMF into MPA, MPAG, AcMPAG and MPG via the glucuronidation and glucosidation clearance pathways. Equine reference PK profiles for MPA and the metabolites, MPAG, AcMPAG and MPG were established.

Pharmacokinetics of mycophenolate mofetil following single-dose intravenous and single- and multiple-dose oral administration and clinicopathologic effects of mycophenolate mofetil following long-term oral administration in healthy horses

OBJECTIVE

To characterize the pharmacokinetics of mycophenolate mofetil (MMF) following single-dose IV or PO administration, characterize the pharmacokinetics of MMF following long-term PO administration, and describe the clinicopathologic effects of long-term MMF administration in horses.

ANIMALS

12 healthy adult horses.

PROCEDURES

In phase 1, 6 horses received a single IV (2.5 mg/kg) or PO (5 mg/kg) dose of MMF in a randomized balanced crossover assessment (≥ 2-week interval between administrations). In phase 2, 6 other horses received MMF for 60 days (5 mg/kg, PO, q 24 h for 30 days and then 5 mg/kg, PO, q 48 h for an additional 30 days).

RESULTS

Following IV (single-dose) or PO (single- or multiple-dose) administration, MMF was rapidly converted to mycophenolic acid. For single-dose PO administration, mean ± SD maximum plasma mycophenolic acid concentration was 1,778.3 ± 441.5 ng/mL at 0.71 ± 0.29 hours. For single-dose IV administration, mean systemic clearance and volume of distribution at steady state were 0.689 ± 0.194 L/h/kg and 1.57 ± 0.626 L/kg, respectively. Following single doses, mean terminal half-life was 3.99 ± 0.865 hours for IV administration and 4.02 ± 1.01 hours for PO administration. The accumulation index following long-term PO administration was 1.0 ± 0.002, and the terminal half-life was 4.59 ± 1.25 hours following the final dose on day 60. None of the horses developed abnormal clinical signs or had any consistently abnormal clinicopathologic findings.

CONCLUSIONS AND CLINICAL RELEVANCE

Further investigation of the clinical efficacy of long-term MMF treatment of horses with autoimmune diseases is warranted.

Deep pemphigus (pemphigus vulgaris, pemphigus vegetans and paraneoplastic pemphigus) in dogs, cats and horses: a comprehensive review

Pemphigus is the term used to describe a group of rare mucocutaneous autoimmune bullous diseases characterized by flaccid blisters and erosions of the mucous membranes and/or skin. When the autoantibodies target desmosomes in the deep layers of the epidermis, deep pemphigus variants such as pemphigus vulgaris, pemphigus vegetans and paraneoplastic pemphigus develop. In this article, we will review the signalment, clinical signs, histopathology and treatment outcome of pemphigus vulgaris, pemphigus vegetans and paraneoplastic pemphigus in dogs, cats and horses; where pertinent, we compare the animal diseases to their human homologue. Canine, feline and equine pemphigus vulgaris, pemphigus vegetans and paraneoplastic pemphigus have many features similar to the human counterpart. These chronic and often relapsing autoimmune dermatoses require aggressive immunosuppressive therapy. In animals, the partial-to-complete remission of pemphigus vulgaris and pemphigus vegetans has been achieved with high dose glucocorticoid therapy, with or without adjunct immunosuppressants; the prognosis is grave for paraneoplastic pemphigus.

Corticosteroids and Immune Suppressive Therapies in Horses

Immune suppressive therapies target exaggerated and deleterious responses of the immune system. Triggered by exogenous or endogenous factors, these improper responses can lead to immune or inflammatory manifestations, such as urticaria, equine asthma, or autoimmune and immune-mediated diseases. Glucocorticoids are the most commonly used immune suppressive drugs and the only ones supported by robust evidence of clinical efficacy in equine medicine. In some conditions, combining glucocorticoids with other pharmacologic and nonpharmacologic treatments, such as azathioprine, antihistamine, bronchodilators, environmental management, or desensitization, can help to decrease dosages and associated side effects.

Does oral prednisolone treatment increase the incidence of acute laminitis?

REASONS FOR PERFORMING STUDY

It is accepted among equine practitioners that glucocorticoid treatment is a risk factor for the development of laminitis. However, there is little published evidence of a link between glucocorticoids and laminitis.

OBJECTIVES

To determine whether horses receiving oral prednisolone are at increased risk of laminitis.

STUDY DESIGN

Retrospective case-control study.

METHODS

Clinical records of horses registered with the ambulatory service at Liphook Equine Hospital between January 2001 and November 2014 were reviewed retrospectively to identify horses that had received treatment with oral prednisolone. For each treated horse, 2 time-matched controls that received veterinary attention but were not treated with prednisolone were selected. Incidence of laminitis was compared between the 2 groups and factors associated with laminitis were assessed using Cox regression analysis.

RESULTS

Of the 416 horses treated with prednisolone, 16 (3.8%) were diagnosed with laminitis subsequent to the initiation of prednisolone treatment with an overall incidence of 2.60 (95% CI 1.49-4.22) cases per 100 horse-years at risk. A total of 7 horses (1.7%) developed laminitis during the course of their treatment and 3 (0.7%) of the horses treated with prednisolone were ultimately subjected to euthanasia as a result of laminitis. A total of 46 (5.7%), of the 814 time-matched control horses were diagnosed with laminitis during the study period with an overall incidence of 3.46 (95% CI 2.54-4.62) cases per 100 horse-years at risk. Of these, 12 (1.5%) were subjected to euthanasia as a result of laminitis. There were no significant differences in the overall laminitis incidence rate (P = 0.8), incidence rate during prednisolone treatment (P = 0.09), or probability of laminitis (P = 0.3) between the 2 groups. Mean survival time was greater in the prednisolone than the control group. Equine metabolic syndrome and increasing age were associated with increased risk of laminitis.

CONCLUSIONS

Administration of oral prednisolone did not increase the risk of laminitis.

Immune-mediated dermatoses

Pemphigus foliaceus is the most common autoimmune skin disease in horses and is associated with the production of autoantibodies directed against surface proteins of the keratinocyte. Pemphigus vulgaris is a rare autoimmune skin disease in horses. Systemic lupus erythematosus and cutaneous lupus erythematosus are recognized in horses and both are rare. Bullous pemphigoid is a rare autoimmune disease in horses caused by immunologic attack of the basement membrane zone by autoantibodies. Erythema muliforme is an immunologic reaction in the skin in which keratinocyte cell death is the prominent change seen on biopsy. Purpura hemorrhagica is thought analogous to nonthrombocytopenic purpura in humans.