Impact of the SGLT2 Inhibitor Empagliflozin on Urinary Supersaturations in Kidney Stone Formers

Kidney stones - a global epidemic associated with obesity and diabetes

Kidney stones are a worldwide healthcare problem with a current lifetime risk of ~18.8 % in men and ~9.4 % in women in Western civilizations. Recurrence rates are high, up to 40 % and 75 % at 5 and 10 years, respectively. Hospitalizations, surgery and lost work time associated with kidney stones cause enormous healthcare-related expenditures. Although kidney stone disease is traditionally considered an isolated renal disorder, there is overwhelming evidence that it is in fact a systemic disease. Arterial hypertension, obesity, diabetes mellitus, gouty diathesis, dyslipidemia, cardiovascular disease, chronic kidney disease and low bone mass are much more prevalent in kidney stone formers than in non-stone formers. It is currently unknown if stone disease is a cause of this co-morbidity per se or if it is a consequence of the same underlying conditions that lead to these disorders and kidney stones. Clearly, however, these co-morbidities contribute significantly to stone-related morbidity and mortality.

The strong, positive and independent association of Body Mass Index (BMI) with kidney stone disease is well established. The magnitude of the increased risk is larger in women than in men. In addition to BMI, significant weight gain is also associated with a greater risk of developing symptomatic kidney stone disease in the future. In recent decades, the prevalence of both kidney stone disease and obesity increased significantly, irrespective of age, sex and race. In large cross-sectional analyses, the prevalence of both symptomatic and asymptomatic kidney stones increased proportionally with the number of metabolic syndrome features present. Both calcium and uric acid stones are associated with obesity, but the ratio of calcium stones to uric acid stones is lower in obese compared to non-obese stone formers, suggesting a disproportionate increase of uric acid stone disease in obesity. Absolute urinary excretion rates of stone formation promoters (calcium, phosphate, oxalate and uric acid) as well as urinary supersaturation ratios for calcium oxalate and uric acid are increased in obese individuals. In addition, there is a well documented negative association of BMI with urinary pH in both stone formers and non-stone formers, and low urinary pH is the main driver of uric acid stone formation. The latter may be explained by insulin resistance which affects the generation of renal ammonium by direct and indirect mechanisms. In contrast to urinary pH, urinary calcium excretion (the main driver for calcium stone formation) is not independently associated with BMI but rather due to other factors known to affect urinary calcium excretion (e.g. secondary to increased sodium and animal protein intake).

Supersaturation - driver of kidney stone formation

Supersaturation, the presence of a material in solution at a concentration above its own solubility, is the driving force for crystallization and therefore kidney stone formation. Relevant supersaturations for kidney stones disease in humans include calcium oxalate, brushite (calcium phosphate) and uric acid. At a supersaturation <1, crystals dissolve, at a supersaturation >1, crystals form. Urinary supersaturations calculated from ambulatory 24 h urine collections accurately reflect the long-term average supersaturation values in the urine and are highly correlated with the kidney stone composition encountered in the individual kidney stone former. Treatments that reduced stone events in Randomized Controlled Trials (RCTs) are highly correlated with reductions in urinary supersaturations. A recent analysis of a large 5-year kidney stone RCT revealed that as early as 1 week after randomization, every 10 % reduction of urinary calcium oxalate supersaturation from baseline was associated with an 8 % reduction in the risk of stone recurrence during follow-up.

SGLT2 inhibitors - a promising new drug class for kidney stone formers

Inhibitors of the sodium/glucose co-transporter isoform 2 (SLC5A2 or SGLT2) belong to a new class of oral hypoglycemic drugs. SGLT2 resides in the brush border membrane of proximal tubular cells in the kidney and reabsorbs ~90 % of glucose filtered at the glomerulus. SGLT2 inhibitors block the physiological glucose reabsorption in the proximal tubule from the glomerular filtrate, thereby inducing significant glucosuria accompanied by a reduction of blood glucose levels. Empagliflozin, the clinically best characterized SGLT2 inhibitor to date, decreases cardiovascular mortality, death from any cause, hospitalizations for heart failure, decline of GFR and need for renal replacement therapy in patients with type 2 diabetes. Some of these findings were also observed with two other SGLT2 inhibitors canagliflozin and dapagliflozin in large outcome trials. Due to their unique mode of action, SGLT2 inhibitors induce weight loss, decrease blood pressure and increase urinary volume, the latter being a very effective measure to reduce stone recurrence. Driven by promising results in RCTs, SGLT2 inhibitors are currently widely tested in non-diabetics e.g. for the treatment of heart failure, non-diabetic kidney disease, arterial hypertension or obesity (www.clinicaltrials.gov). The pleiotropic beneficial effects make SGLT2 inhibitors also a very attractive class of drugs for kidney stone formers, which often suffer concomitantly from arterial hypertension, CKD, obesity and diabetes.

Effect of SGLT2 inhibitors on bone fractures, mineral metabolism and kidney stone events

Therapy with SGLT2 inhibitors as monotherapy or add-on therapy to various glucose-lowering agents is generally well tolerated. An increased incidence of genital infections and (although rare) euglycemic ketoacidosis are known side effects of this new class of medications. The latter is mainly observed in type I diabetics and less frequently in type II diabetics. To the best of our knowledge, no cases of euglycemic ketoacidosis in individuals without diabetes treated with SGLT2 inhibitors have been reported.

In the large canagliflozin outcome study CANVAS, an increased incidence of lower extremity amputations was noted. This adverse effect has not been reported with other SGLT2 inhibitors. In addition, both canagliflozin and dapagliflozin have been associated with an increased risk of bone fractures compared to placebo. In a short term study in healthy volunteers, canagliflozin increased serum phosphate, plasma fibroblast growth factor 23 (FGF23) and plasma parathyroid hormone (PTH) and decreased the level of 1,25-dihydroxyvitamin D. Similar results were obtained in individuals with diabetes treated with dapagliflozin. In contrast, pooled analyses of phase I, II and III trials of patients with type 2 diabetes treated with empagliflozin encompassing > 15'000 patient-years of exposure did not reveal an increased rate of bone fractures, alterations of blood electrolytes, PTH, 25-dihydroxyvitamin D or bone turnover markers.

While blood electrolyte and mineral metabolism parameters in patients treated with SGLT2 inhibitors have been well studied in healthy volunteers and patients with diabetes, there is a lack of data on the impact of SGLT2 inhibition on urinary parameters, especially on parameters that influence the kidney stone formation rate. Also, to our knowledge, no studies have been conducted thus far with SGLT2 inhibitors in kidney stone formers.

Interestingly, in pooled analyses of phase I, II and III trials, the rate of kidney stone events tended to be 30-50 % lower in patients treated with 10 or 25 mg empagliflozin versus placebo. However, detailed analyses for kidney stone events in empagliflozin outcome trials have not been reported. Reported stone event rates in these pooled empagliflozin trials (0.5 - 1/100 person years) are similar to what has been observed in individuals with diabetes in three large prospective US cohorts (Nurses' Health Study I, the Nurses' Health Study II and the Health Professionals Follow-up Study. In contrast, stone event rates are considerably (10 - 100-fold) higher in patients with a history of kidney stone disease. RCTs testing dietary or pharmacologic measures for recurrence prevention typically included patients with stone event rates between 20 and 200 events/100 person-years.

In summary, SGLT2 inhibitors represent a promising new drug class for kidney stone formers. Kidney stone formers are likely to profit from the metabolic and cardiovascular effects of SGLT2 inhibition. In addition, SGLT2 inhibitors may decrease the stone formation rate. Based on the overall experience thus far, empagliflozin seems to have by far the most favorable side effect profile. Clearly, there is a dire need for clinical studies with SGLT2 inhibitors in kidney stone formers.