The aim of this study is to investigate effects of sevelamer carbonate on reducing TMAO in stage 3b-4 CKD (pre-dialysis) patients. The study will also investigate the safety and tolerability of sevelamer carbonate in study population and the effects of sevelamer carbonate on serum p-cresyl sulfate, indoxyl sulfate, LDL-C and uric acid.

Inclusion Criteria:

• 1. Men or women, aged from 18 to 75 years old;
• 2. Provide informed consent prior to enrolling in the study;
• 3. Estimated glomerular filtration rate (eGFR) between 15-45 ml/min/1.73 m2 (calculated by CKD-EPI equation)

Exclusion Criteria:

• 1. Documented poorly controlled diabetes mellitus, poorly controlled hypertension, malignant tumour, or any clinically significant unstable medical condition;
• 2. Active dysphagia or swallowing disorder; or a predisposition to or current bowel obstruction, ileus, or severe gastrointestinal motility disorders including severe constipation;
• 3. Known hypersensitivity to sevelamer or any constituents of the study drug;
• 4. Unable to comply with the requirements of the study;
• 5. Hypophosphatemia (serum phosphorus level <0.87mmol/L);
• 6. Women who have a positive pregnancy test at enrollment or women who are breast-feeding;
• 7. Have been enrolled in other interventional study;
• 8. Received sevelamer or other intestinal adsorbents, or broad-spectrum antibiotic within one month prior to the screening period.