CKD is a serious condition characterized by the gradual loss of essential kidney functions over time. In patients with CKD, normal fluid and electrolyte balance can no longer be maintained, and the excretion of metabolic end products, toxins and drugs is impaired. Furthermore, production and secretion of certain enzymes and hormones are disturbed.
According to the Centers for Disease Control, or CDC, more than 30 million people in the United States are afflicted with CKD, representing an overall prevalence in the adult population of approximately 15%. The incidence of CKD is primarily driven by the increasing prevalence of diabetes and hypertension. CKD represents the ninth leading cause of death in the United States. The annual Medicare expense for CKD including end-stage renal disease, or ESRD, exceeds $98 billion.
To help improve the diagnosis and management of kidney disease, the National Kidney Foundation, or NKF, has divided CKD into five stages. The severity of CKD at each stage is identified by the estimated glomerular filtration rate of the kidney, or eGFR. Treatment during the first four stages of CKD focuses on ways to preserve kidney function for as long as possible. ESRD is the fifth stage of CKD in which the patient typically requires renal replacement therapy, i.e., dialysis or a kidney transplant, for survival.
The prevalence and severity of metabolic acidosis increases from CKD Stage 3 to Stage 5. We estimate the prevalence of metabolic acidosis to be 9% of the estimated 13.4 million Stage 3a CKD patients, 18% of the estimated 5.7 million Stage 3b CKD patients and over 30% of the estimated 2.5 million Stage 4 and Stage 5 CKD patients resulting in a total prevalence of approximately 3 million CKD patients with metabolic acidosis in the United States.
Diabetes, hypertension and age have long been recognized as primary risk factors for the progression of CKD. More recently, metabolic acidosis, a serious condition in which the body has accumulated too much acid, has also been identified as a key risk factor in the progression of CKD.
The maintenance of a delicate acid-base balance in the body is essential for the health and proper functioning of organs, tissues and cells. Blood pH is kept within a narrow range of 7.36 to 7.44 by buffering systems in the body (primarily bicarbonate (HCO3-)), and excretion mechanisms to remove excess volatile acid (carbon dioxide (CO2)) through the lungs and non-volatile acids through the kidneys. Blood pH is defined by the ratio of serum bicarbonate (regulated by the kidneys) and CO2 (regulated by the lungs). Patients’ acid-base status can be diagnosed by measuring the level of bicarbonate in the blood, which is part of a standard metabolic panel. Properly functioning kidneys will maintain a blood bicarbonate level of between 22 to 29 milliequivalents per liter (mEq/L). A persistent blood bicarbonate level below 22 mEq/L indicates metabolic acidosis. Blood bicarbonate concentrations less than 22 mEq/L are associated with increased risk of CKD progression and increased risk of muscle wasting, loss of bone density and death.
In general, individuals produce between 50 and 100 mEq of acid each day through metabolic processes and the digestion of food (especially protein). This acid must first be buffered, and then excreted, by the kidneys. Patients with CKD continue to generate acid from their diet and metabolism but have a reduced ability to excrete acid via the kidneys. As a result, metabolic acidosis, characterized by a reduced blood bicarbonate concentration (i.e., below 22 mEq/L), can develop in CKD patients.
As currently understood, the mechanism that links metabolic acidosis to the progression of kidney disease involves a cascade of events whereby patients with compromised kidneys do not excrete adequate amounts of acid to maintain acid-base balance. This imbalance and acid load accumulation leads to increases in the production of select peptides and hormones, including endothelin-1, aldosterone and angiotensin II, that increase the secretion of acid through the proximal and distal renal tubules of the remaining healthy nephrons. As currently understood, this sustained over-production of hormones exacerbates the damage in the diseased kidneys, resulting in long-term consequences, including renal fibrosis, proteinuria and inflammation, as well as sodium and water retention, which are hallmarks of the progression of CKD.
In multiple prospective clinical studies ranging from 1 to 5 years in duration, treating metabolic acidosis (i.e., increasing blood bicarbonate) resulted in improved renal outcomes in CKD patients (de Brito-Ashurst, 2009; Garneata, 2016; Goraya, 2013; Goraya, 2014; Mahajan, 2010; Phisitkul, 2010). In these studies, attenuation of CKD progression was observed when metabolic acidosis was treated with alkali therapy or very low protein diets. Clinical observations included fewer cases of rapid kidney function decline and fewer patients entering ESRD requiring dialysis. Furthermore, in several large retrospective cohort studies (Shah 2009; Dobre 2013; Raphael 2011; Tangri 2011; Kovesdy 2009; Navaneethan 2011; Raphael 2016), an association between low serum bicarbonate levels and the progression of renal disease was observed, independent of baseline eGFR and other factors.
There are no FDA approved therapies for the chronic treatment of metabolic acidosis.
However, the National Kidney Foundation’s Kidney Disease Outcomes Quality Initiative, or KDOQI, guidelines and the International Society of Nephology’s Kidney Disease: Improving Global Outcomes, or KDIGO, guidelines recommend maintaining blood bicarbonate at levels above 22 mEq/L (the normal range of blood bicarbonate is 22 to 29 mEq/L.)
Unapproved methods to increase blood bicarbonate include oral alkali supplements, such as sodium bicarbonate, which introduce significant amounts of sodium to patients. Yet approximately 85% to 95% of later-stage CKD patients suffer from sodium sensitive comorbid conditions, such as hypertension, cardiovascular disease, heart failure or edema and require a sodium-restricted diet. As such, the use of sodium-based supplements can lead to worsening blood pressure control and volume overload in this population. Given the significant limitations on the use of sodium-based supplements, there exists a significant unmet medical need for a chronic therapy for CKD patients with metabolic acidosis. To chronically treat the broad population of CKD patients with metabolic acidosis, physicians need an FDA-approved treatment that is proven to be safe, effective and easy to comply with.