Tetrahydrobiopterin in Patients With Chronic Kidney Disease (CKD) and Albuminuria

Patients with chronic kidney disease and albuminuria are at increased risk of developing cardiovascular disease which is often associated with hypertension, left ventricular hypertrophy, endothelial dysfunction and increased generation of reactive oxygen species (ROS). These patients also manifest a decrease in nitric oxide availability which is thought to play an important role in their progressive vascular disease.

Tetrahydrobiopterin (BH4), an essential cofactor for endothelial nitric oxide synthase(eNOS), an important regulator of nitric oxide (NO) and that is a key mediator of endothelial dysfunction. Changes in nitric oxide availability are believed to contribute to endothelial dysfunction seen in chronic kidney disease and common cardiovascular disease states. 6R-tetrahydrobiopterin (6R-BH4 or sapropterin dihydrochloride) is an investigational oral drug that is being evaluated to determine whether it will restore NO availability, leading to beneficial effects on vascular function and ultimately positive clinical outcomes in patients with chronic kidney disease. The primary endpoint in this study is the level of albuminuria, an easily measured marker that has served as a predictor of kidney disease progression. If 6R-BH4 reduces albuminuria in patients with kidney disease, it may have implications to slow the disease progression as well as decreased risk of cardiovascular disease.

ABSTRACT Background: Chronic kidney disease (CKD) is characterized by a high propensity to cardiovascular disease (CVD); therefore treatments that impact both CKD and CVD are needed. CKD is accompanied by endothelial dysfunction and nitric oxide (NO) deficiency. Tetrahydrobiopterin (BH4), an important co-factor for endothelial NO synthase (eNOS) increases the availability of NO. Administration of BH4 has the potential to improve endothelial function and thereby reduce albuminuria in CKD.

Patients and Methods: This Phase 2 open-label study is designed to assess the efficacy and safety of twice daily oral dosing of 6R-BH4 in 30 subjects with CKD (eGFR ≥40ml/min/1.73m2). .

Trial Design: Subjects will receive 6R-BH4 400mg bid for 6 weeks, sequentially followed by 6R-BH4 plus Vitamin C 500mg bid for another 6 weeks. Patients will have scheduled visits at Weeks 0,3,6,9 and 12, with an exit-visit at week 16. Albuminuria will be assessed in 24-hour urine collections as well as early morning spot urine samples for albumin:creatinine ratio. Blood and urine will be tested for routine clinical laboratory tests, blood NO, and also archived for later assays for special biomarkers. The primary outcome will be level of albuminuria as measured in a 24-hour urine collection at 6 and 12 weeks of therapy. Secondary outcomes will include urine albumin/creatinine ratio, eGFR, and blood pressure. Adverse events will be monitored closely.

Data analysis: For all patients combined and for each of the above outcomes, we will sequentially compare each time point to the baseline level using paired t-tests. For the comparison of 6R-BH4 versus 6R-BH4+vitamin C, we will compare albuminuria at 6 and 12-weeks, adjusted for baseline values, using regression analysis. We will also use regression to test for an interaction between baseline value and treatment group.

Anticipated results: We postulate that 6R-BH4 alone or in conjunction with high dose vitamin C will reduce albuminuria in patients with CKD by improvement in endothelial function that is integral to glomerular filtration.

Future Implications: Reduction in albuminuria if demonstrable, will have implications for simultaneous renal and cardiovascular protection. This will need to be confirmed in a larger randomized controlled clinical trial in subjects with CKD.

• Kidney Disease
• Albuminuria

• Drug: Tetrahydrobiopterin (6R BH4)
  400 mg 6R BH4 oral BID for 6 weeks then 400 mg of 6R BH4 for another 6 weeks
• Dietary Supplement: Vitamin C
  500 mg Vitamin C oral BID for another 6 weeks

Inclusion Criteria:

• Patients with controlled hypertension (BP less than 150/90 mmHg) using standard antihypertensive medications.
• Stable CKD (eGFR 40-90 ml/min/173m2 by the abbreviated (4-variable) MDRD equation and with a rate of decline of eGFR no greater than 1ml/min/1.73m2 per month over the prior 3 months with albuminuria (urine albumin excretion in the 24-hr urine sample of between 300-3000mg).

• No concomitant use with:

  • Vitamin C supplements
  • Multivitamins containing vitamin C
  • Any other dietary supplements, nutraceuticals, or other over-the-counter products containing vitamin C
  • Vitamin E containing supplements
• Concurrently taking study approved antihypertensive medications at a stable dose for at least 3 months prior to screening.
• Sexually active subjects must be willing and able to use an acceptable method of contraception
• Females of childbearing potential must have a negative pregnancy test at screening. Females considered not of childbearing potential include those who have been in menopause at least 2 years, or had tubal ligation at least 1 year prior to screening, or who have had total hysterectomy.

Exclusion Criteria:

• Uncontrolled hypertension with BP greater than 150/90 or with frequent changes to antihypertensive regimen during the last 3 months
• Concurrent disease or condition that would interfere with study participation or safety, such as bleeding disorders, history of syncope or vertigo; severe gastroesophageal reflux disease (GERD) or gastric ulcers; heart failure; symptomatic coronary or peripheral vascular disease; arrhythmia; serious neurologic disorders, including seizures; or organ transplant.
• Diabetics that are uncontrolled, unstable, newly diagnosed, or have undergone major changes in therapy in the last three months or HbA1C consistently greater than 9.0.
• Any severe comorbid condition that would limit life expectancy to less than 6 months.
• Advanced stage III CKD or worse , i.e. estimated GFR less than 40 ml/min/1.73m2 (by abbreviated MDRD formula)
• History of nephrolithiasis
• Patients with albuminuria due to causes other than hypertension and /or diabetes (e.g., SLE)
• Hepatic enzyme concentrations greater than 2 times the upper limit of normal.
• HIV infection, hepatic cirrhosis, other preexisting liver disease, or positive HIV, Hepatitis B or C test at screening.
• Concomitant treatment with drugs known to inhibit folate metabolism, Levodopa, phosphodiesterase (PDE) 5 inhibitors or PDE 3 inhibitors.
• Myocardial infarction, stroke, or surgery within the last 60 days prior to screening.
• History of alcohol and/or drug abuse.
• Pregnant or breastfeeding at screening, or planning to become pregnant (subject or partner) at any time during the study.
• Previous treatment with 6R-BH4
• Has known hypersensitivity to 6R-BH4 or its excipients.
• Any condition that, in the view of the PI, places the subject at high risk of poor treatment compliance or of not completing the study.