Sitaxsentan in Proteinuric Chronic Kidney Disease

Inclusion Criteria: 1. Has Stage 1-5 chronic kidney disease (CKD) as defined by the Kidney Disease Outcomes Quality Initiative (using the Cockcroft and Gault equation for calculation of glomerular filtration rate) with proteinuria, including any of the following aetiologies: immunoglobulin A (IgA) nephropathy, polycystic kidney disease (PCKD), congenital abnormalities, reflux nephropathy, focal segmental glomerulosclerosis, minimal change nephropathy, and membranous nephropathy. 2. Is between 18 and 70 years of age, inclusive. 3. Has a body mass index (BMI) between 18 and 35 kg/m2, inclusive. 4. Is willing and able to adhere to the protocol requirements. 5. Provides written informed consent before any study procedure is performed. Exclusion Criteria: 1. Requires peritoneal dialysis or haemodialysis. 2. Has kidney disease due to diabetes mellitus, vasculitis, systemic lupus erythematosus, or known renovascular disease; antiglomerular basement membrane disease; or is on immunosuppressive medication. 3. Has a serum albumin in the nephrotic range (\< 30 g/L) during Screening. 4. Has a sustained sitting systolic blood pressure (BP) \> 160 mmHg or sustained sitting diastolic BP \> 100 mmHg during Screening. 5. Has postural hypotension during Screening, which is defined as a decrease in systolic BP ≥ 20 mmHg and/or a decrease in diastolic BP ≥ 10 mmHg, comparing sitting and standing measurements. 6. Has a history and/or evidence of ischaemic heart disease. 7. Has or had a malignancy, with the exception of adequately-treated basal cell or squamous cell carcinoma of the skin, that required significant medical intervention within the past 3 months and/or is likely to result in death within the next 2 years. 8. Has a history of allergies or hypersensitivity to sitaxsentan or nifedipine or the excipients of either drug. 9. Has a clinically significant psychiatric, addictive, neurological disease or any other condition that, in the Investigator's opinion, would compromise his/her ability to give informed consent, participate fully in this study, prevent adherence to the requirements of the study protocol, or would compromise the interpretation of the data obtained from this study. 10. Uses a prohibited medication or plans to use a prohibited medication during the study. * Prohibited medications include cyclosporine A, alternative endothelin (ET) receptor antagonists, phosphodiesterase inhibitors, and/or vitamin K antagonists (e.g., warfarin). The intermittent use of phosphodiesterase inhibitors (e.g., sildenafil) "as needed" for erectile dysfunction is acceptable, however, as long as the subject is not dosed within 24 hours of an efficacy assessment. 11. Received treatment with an investigational drug or device within 30 days prior to study entry. 12. Has a history of organ transplantation. 13. Has atrial fibrillation requiring anticoagulation or a history (in the preceding 6 months) of any intermittent cardiac dysrhythmia that may require anticoagulation therapy. 14. Has an alanine aminotransferase (ALT) and/or aspartate aminotransferase (AST) level \> 1.5 × the upper limit of the normal range (ULN) at Screening and/or serum total bilirubin \> ULN. 15. Has a haemoglobin concentration \< 8.0 mg/dL at Screening. 16. Has positive serological results for hepatitis B and/or hepatitis C. 17. Is a woman of childbearing potential who is unwilling to use 2 forms of contraceptive therapy, including at least 1 barrier method, throughout the study. (Women who are surgically sterile or who are post-menopausal for at least 2 years are not considered to be of childbearing potential.) 18. Is pregnant, lactating, or breastfeeding. 19. Has, in the opinion of the Investigator, a dependence on alcohol. 20. Has, in the opinion of the Investigator, a dependence on illicit drugs.