Chronic kidney disease (CKD) is a major health concern both in the general and Veteran populations. Indeed, the prevalence of CKD in a large Veteran population is 20%. Cardiovascular disease (CVD) is significantly increased in CKD and is an important cause of morbidity and mortality. As much as 80% of all CVD is associated with vascular dysfunction, particularly impaired endothelium-dependent dilation (EDD), measured by brachial artery flow-mediate dilation (FMD), and stiffening of the large elastic arteries, measured by aortic pulse-wave velocity (aPWV). Not surprisingly, patients with CKD demonstrate these dysfunctional vascular phenotypes. Even in early stages of CKD, there is an increase in oxidative stress resulting in structural and functional vascular changes, which, in turn, contributes to vascular dysfunction (impaired EDD and large elastic artery stiffening). In CKD, phosphorus remains within the normal range (2.5-4.5 mg/dL) until late in the disease. However, elevated serum phosphorus, even within the normal range, is associated with impaired EDD and with indirect measures of arterial stiffness. Whether lowering serum phosphorus in patients with CKD will improve EDD and arterial stiffness is unknown. This study is a randomized-controlled trial of lanthanum carbonate, a non-calcium based phosphate binder, to treat vascular dysfunction. The efficacy of phosphate binding with lanthanum carbonate for treating vascular endothelial dysfunction and large elastic artery stiffness in patients with stage IIIb and IV CKD (estimated glomerular filtration rate 15-45 mL/min/1.73m2) with baseline serum phosphorus of 2.8-5.5 mg/dL will be assessed. The study will also determine if lowering serum phosphorus with lanthanum carbonate also reduces circulating and endothelial cell markers of oxidative stress. This study could shift clinical practice guidelines by establishing a novel therapy for reducing CVD risk in CKD patients not requiring chronic hemodialysis. \[COMIRB 13-0328\]

Little is known about phosphorus balance in CKD. It is assumed that CKD patients remain in neutral phosphorus balance despite decreases in kidney function. Serum phosphorus remains in the normal range until late in CKD thus making it difficult to recognize perturbations in phosphorus balance. Indeed, among CKD patients treated with the non-calcium containing phosphate binder, sevelamer, serum phosphorus did not change after six weeks of treatment but urinary phosphate excretion, parathyroid hormone, and fibroblast growth factor-23 changed significantly, suggesting a shift in phosphorus homeostasis. However, two other studies found that patients with CKD III-IV treated with calcium-containing phosphate binders remained in neutral phosphorus balance. There are no studies evaluating the effects of non-calcium based phosphate binders on phosphorus balance among patients with CKD nor other studies examining the effect of changing phosphorus balance on vascular function.

An extension of the above-described 12-week prospective randomized, placebo-controlled double-blind trial (COMIRB 13-0328) will be conducted in a subset of subjects. A total of 24 subjects from COMIRB 13-0328 will be recruited to participate in the Phosphorus Balance sub-investigation (12 subjects treated with lanthanum carbonate and 12 subjects treated with placebo). \[15-0384\] They will consume a diet with a fixed phosphorus content (1000 +/- 50 mg) for seven days. They will then be admitted to the inpatient Center for Translational Clinical Research at the University of Colorado Denver for 48 hours to accurately collect urine and stool samples. The goal of the Phosphorus Balance sub-investigation (COMIRB 15-0384) is to determine whether lowering serum phosphorus, accomplished during the parent phosphorus lowering randomized-controlled trial (COMIRB 13-0328), affects phosphorus balance compared to those subjects treated with placebo. A key secondary goal is to determine if differences in phosphorus balance affect vascular function as measured by FMD. \[15-0384\]

To ensure adequate enrollment in the Phosphorus Balance Study (COMIRB 15-0384), an amendment was approved to recruit patients with stage IIIb and IV CKD with normal or modestly elevated serum phosphorus (2.8-5.5 mg/dL) who are not currently participating in the parent Phosphorus Lowering RCT (COMIRB 13-0328). Similar to the Phosphorus Lowering RCT, these patients will follow a low phosphorus diet and will be randomized to lanthanum carbonate or placebo for 12 weeks (run-in period) prior to beginning the current Phosphorus Balance protocol.