Randomized Controlled Trial of Silymarin in Asthma

The purpose of this study is to determine if intake of the antioxidant enzyme inducer, silymarin, will improve lung function and symptom scores in participants with asthma.

SPECIFIC AIMS Dietary intake of exogenous antioxidants has been shown to have only a modest impact on asthma inception and control. Our work has demonstrated that endogenous antioxidant enzyme concentrations are a far stronger predictor of asthma inception compared with dietary antioxidant intake and/or serum antioxidant concentrations. This is likely because we regulate oxidative stress to a greater extent, not by exogenous intake, but by antioxidant enzymes. The milk thistle plant extract, silymarin, has been shown to be an inducer of the endogenous antioxidant enzymes, superoxide dismutase and catalase. As reactive oxygen species have been implicated in the pathogenesis of asthma and, in atopic asthmatics, endogenous superoxide dismutase (SOD) enzyme levels are known to decrease, this supports that increasing SOD levels, either by induction of endogenous SOD or replacing a failed endogenous SOD enzyme system with a mimetic of the endogenous enzyme, would be beneficial and protective. We hypothesize that administration of silymarin, an inducer of antioxidant enzymes, to subjects with atopic asthma will increase antioxidant enzyme concentrations, decrease markers of oxidative stress, decrease indirect measures of airway inflammation that have been correlated with clinical outcomes (exhaled nitric oxide, eNO), and thus improve lung function and symptom scores in participants with asthma. To test this hypothesis we will conduct a randomized, double-masked, placebo-controlled cross-over pilot investigation of an inducer of endogenous antioxidant enzymes, silymarin, in asthma. No clinical trial has tested either silymarin, or any inducer of antioxidant enzymes in patients with asthma. We therefore aim to determine whether this novel treatment is effective, and if both inflammatory and clinical endpoints are improved with treatment. Specific questions related to modification of endogenous antioxidant enzymes in prevalent asthma that this investigation aims to address are whether induction of antioxidant enzymes can alter inflammatory markers in asthma that are known to be linked with clinical endpoints, lung function and oxidant stress. Measurements of antioxidant enzymes will be made in blood samples prior to and following administration of silymarin; measures of systemic oxidative stress will be made in urine samples using an assay for isoprostanes, considered the most accurate marker of oxidative stress currently available; measures of airway inflammation will be measured using exhaled NO, and lung function testing and symptom control will be assessed as clinical measures of disease control. Aim #1: To confirm that oral administration of silymarin in subjects with atopic asthma increases endogenous antioxidant enzymes. We hypothesize that silymarin will increase levels of antioxidant enzymes, superoxide dismutase activity, catalase activity, and glutathione peroxidase activity in subjects with atopic asthma. To test this hypothesis, we will first conduct a dose escalation study to determine optimal dosing, dose effect, and washout on airway inflammation, followed by a randomized, double-masked, placebo-controlled cross-over pilot investigation of supplementation with silymarin in subjects with atopic asthma with measurements of antioxidant enzyme activity pre-, during and at the completion of both active supplementation and placebo arms of the study. Aim #2: To determine whether oral administration of silymarin to subjects with atopic asthma alters indirect measures of airway inflammation and systemic oxidative stress. We hypothesize that silymarin will decrease airway inflammation and measures of systemic oxidative stress. To test this we will assess indirect measures of airway inflammation, including exhaled nitric oxide and nitric oxide related products, and measures of systemic oxidant stress, urinary isoprostanes pre-, during and at the end of supplementation in the study subjects enrolled in this clinical trial. Aim #3: To determine whether oral administration of silymarin to subjects with atopic asthma improves asthma morbidity, daily symptoms, disease control, disease exacerbations, and spirometry. We hypothesize that silymarin will improve asthma control and decrease morbidity. Although not powered for these outcomes, to test this hypothesis we will assess and evaluate the trends and magnitude of the effect of silymarin on asthma control, spirometry and disease exacerbations in the above study subjects. Aim #4: To determine feasibility, acceptance of randomization, adherence to therapy, acceptance of drug delivery and dosing, ability to maintain blinding, and clinical effect size. The intent of this aim is to optimize the overall protocol design and procedures for a future larger study.