Two pharmacological principles, one at least 80 years old and another over 60 years old, have recently come to fruition. The first, phlorizin, was thought to cause diabetes, but then a closer look revealed a renal mechanism that was independent of insulin or insulin deficiency. Research into its action facilitated the discovery of sodium-dependent glucose transporters. The second compound came close on the heels of the discovery that the mineralocorticoid, aldosterone, can be released through the action of angiotensin II. The blocker eventually developed as spironolactone gave insights into mineralocorticoid receptor functions in many tissues. The drug was therapeutically largely ignored for decades, but then roared back relatively recently. Novel follow-up compounds are even more effective. These lines of research uncover not only expected mechanisms but also a panoply of unexpected metabolic pathways. Most important are the much-appreciated improvements in treatment options for our patients.

1 SGLT

Sodium-dependent glucose cotransporters (SGLT) belong to the mammalian solute carrier family SLC5.¹ They are present in the intestinal mucosa (mostly SGLT1) and the proximal tubule of the nephron (mostly SGLT2). In the kidneys, 100% of the filtered glucose in the glomerulus must be reabsorbed along the nephron (98% proximal convoluted tubule via SGLT2). In hyperglycemia, glucose is excreted in the urine (glycosuria) when SGLTs become saturated wiht the filtered glucose.²

Phlorizin is a glucoside of phloretin, dihydrochalcone, belonging to the family of bicyclic flavonoids, which, in turn, is a subgroup in the diverse phenylpropanoid synthesis pathway in plants.³ We are now aware that phlorizin is a competitive inhibitor of SGLT1 and SGLT2. The compound competes with D-glucose for binding to the carrier, a state-of-affairs that reduce renal glucose transport, lowering the amount of glucose in the blood. Phlorizin was of interest as a potential pharmaceutical treatment for type 2 diabetes and even restores insulin sensitivity by correcting hyperglycemia.⁴ However, phlorizin has since been superseded by more selective and more promising synthetic analogues, such as dapagliflozin, empagliflozin, canagliflozin, and sotagliflozin.⁵

Early research into phlorizin’s action was conducted by Walter Kempner, a Jewish-German physician trained in internal medicine and in biochemistry.⁶ His research fellowship was conducted in the Warburg laboratory and as a result, he was an expert in the best biochemistry of his time. However, before joining Warburg, he completed his doctoral thesis on phlorizin.⁷ Kempner performed 250 glucose measurements on six rabbits and six dogs, one of which was outfitted with an Eck fistula to bypass the liver. The animals had consistently normal glucose plasma values but developed glycosuria even in the face of hypoglycemia. Treating the animals with insulin caused further hypoglycemia and seizures, while glycosuria was only slightly reduced. Kempner concluded that phlorizin caused glycosuria and could also alter glucose metabolism and utilization of sugar, as underscored by experiments by others, 50 years thereafter.⁴

After Kempner mastered the Warburg apparatus to investigate metabolism, his studies became more sophisticated. Kempner was aware that the kidney presents a massive oxygen gradient from the cortex to medulla. His renal-slice/Warburg-apparatus experiments led him to conclude that the acidosis accompanying uremia was largely due to inhibitory effects of low oxygen tension on the deamination of amino acids. Kempner was forced to emigrate and found a new home at Duke University in Durham, NC, USA. In a remarkable 1939 presentation at the Cold Spring Harbour Laboratory Conference, he crystallized his ideas that I believe are relevant to the discussions of today. His studies underscored a relationship between the kidney and liver with regard to the deamination of amino acids. Any disturbance in renal metabolism must lead to a decrease in ammonia formation in the kidney and a rise of urea formation in the liver. If the disturbance of cellular metabolism is potentially reversible, the pathological manifestations will disappear when normal conditions are reestablished or if the metabolic workload is ameliorated.⁸ This line of thinking, coupled with findings indicating low oxygen tension within the kidneys caused Kempner to develop a dietary intervention for malignant hypertension, with and without diabetes mellitus. The dietary treatment produced dramatic results remarkable for the time.⁶

2 SGLT2 actions

SGLT2 inhibitors currently available are dapagliflozin, empagliflozin, canagliflozin, and sotagliflozin. SGLT2 is a low affinity, high capacity luminal transporter in the S1 and S2 segments of the proximal tubule where about 97% of filtered glucose is reabsorbed. The inhibitors reduce body weight 1-3 kg not only by the loss of water, but also by body fat reduction. Glycosuria causes a negative caloric balance and some glucose metabolism is switched to fatty acids and ketones. SGLT2 inhibitors reduce blood pressure both daytime and nighttime even in patients with reduced renal function from chronic kidney disease (CKD). Some of this decrease may be related to weight loss. SGLT2 inhibitors cause a modest diuresis that persists in CKD patients. Proximal tubular reabsorption largely involves the sodium-hydrogen exchanger-3 (NHE3). By administrating SGLT2 inhibitors, NHE3 activity is reduced. The mechanism is more than indirect and appears to involve membrane-associated protein 17 (MAP17) and the post-synaptic density protein/tight-junction protein 1 (PDZK1). The sodium delivery to Henle’s loop activates tubuloglomerular feedback to correct glomerular hyperfiltration (Figure 1A).⁹

In addition, SGLT2 inhibitors could exhibit effects on metabolic energy and water conservation, actions that would surely have interested Kempner. Although the drugs elevate solute excretion, long-term osmotic diuresis does not occur. Marton et al suggest that under SGLT2 inhibition, urea-mediated renal water conservation becomes increasingly important.¹⁰ Treatment of diabetic rats with dapagliflozin induced increased the expression of the urea transporter UT-A1 in the renal medulla. The drug also increases vasopressin release that also facilitates urea transport. The increased urea requirement has overall metabolic implications involving liver and skeletal muscle. The alanine cycle is the biochemical pathway for shuttling amino groups from muscle to liver. Marton et al suggest that SGLT2 inhibitor treatment brings into play generalized metabolic effects that could improve cellular lifespans and not solely in patients with diabetes mellitus (Figure 1B).¹⁰ These notions are in accord with Kempner’s Cold-Springs Harbour report.⁸

3 Diabetes mellitus type-2

The Germans have a saying, “Lügen aus der Tasche” (lying out of your pocket), which basically means to lie, even when one knows that the position one defends is nonsense. Thus, it has been with the treatment of diabetes mellitus type-2 (DMT2). The University Group Diabetes Programme (UGDP) was an early randomized controlled trial, to test the efficacy of DMT2 treatments. The experience underscores the argument. As a result, tolbutamide (sulfonyl urea) and metformin were terminated early because efficacy in terms of prolonging life was lacking. Two insulin treatments were continued further, but were not judged to be any more effective than placebo in prolonging life or delaying vascular complications. The responses of the food and drug administration (FDA) and ensuing court battles make for interesting reading.^{11, 12} The practice of physicians was only minimally altered by UGDP. The United Kingdom Prospective Diabetes Study (UKPDS) provided some confirmatory findings; however, the results were commonly presented in an obfuscatory fashion (as shown earlier, with treatment the tested population invariably got fatter). Shaughnessy and Slawson underscored that patient-oriented evidence is what matters.¹³ They scrutinized the findings reported in the reviews of UKPDS and found the rarely mentioned fact that blood glucose control had no effect on diabetes-related or overall mortality. Solely metformin treatment was associated with decreased mortality as opposed to any of the other treatments. Diabetic patients with hypertension benefited more from good blood pressure control than from good blood glucose control. No UKPDS study review pointed out that the treatment of overweight patients with DMT2 with insulin or sulfonylurea drugs had no effect on microvascular or macrovascular outcomes. Then came the rosiglitazone debacle.¹⁴ The safety and efficacy of thioglitazones came into question. Rosiglitazone, an FDA approved DMT2 drug, was inspected in a meta-analysis of available data and was found to increase myocardial-infarction risk by 43% and cardiovascular death by 64%. The experience at least moved the FDA to become more targeted in its adjudication regarding the cardiovascular safety of new diabetes drugs. Regardless of the presence or absence of preclinical or clinical signals, the safety guidance principle has been applied broadly to all new diabetes drugs, creating substantial challenges in the drug development, and approval process. Thus, in a short period of time, the picture has improved dramatically.

Cardiovascular outcome trials in patients with DMT2 at high cardiovascular risk have led to remarkable advances in our understanding of the effectiveness of glucagon-like peptide-1 (GLP-1) receptor agonists (welcome news but not covered in this commentary) and SGLT2 inhibitors to reduce cardiorenal events. In 2019, the American Diabetes Association (ADA), European Association for the Study of Diabetes (EASD), and European Society of Cardiology (ESC) published updated recommendations for the management of diabetic patients. What were the studies that so much improved an earlier dismal picture?

4 SGLT2 inhibitor efficacy

Striking were the results in patients with heart failure from reduced ejection fraction from two landmark trials.¹⁵ Amongst 8474 patients combined from both trials, the estimated treatment effect was a 13% reduction in all-cause death and 14% reduction in cardiovascular death. SGLT2 inhibition was accompanied by a 26% relative reduction in the combined risk of cardiovascular death or first hospitalization for heart failure, and by a 25% decrease in the composite of recurrent hospitalizations for heart failure or cardiovascular death. Equally surprising was the reduction of a composite renal endpoint. These benefits were achieved in patients already receiving guideline accepted treatments and were consistent in subgroups based on age, sex, diabetes, and baseline estimated glomerular filtration rate (eGFR).^{16, 17} Equally impressive were the results of dapagliflozin in patients with chronic kidney disease in a parallel trial.¹⁸ In this study, 4304 participants with an eGFR of 25 to 75 mL per minute per 1.73 m² of the body surface area and a urinary albumin-to-creatinine ratio of 200 to 5000 to receive dapagliflozin (10 mg once daily) or placebo. The primary outcome was a composite of a sustained decline in the estimated GFR of at least 50%, end-stage kidney disease, or death from renal or cardiovascular causes. Regardless of the presence or absence of diabetes, all these endpoints were significantly lower with dapagliflozin than with placebo.

The SGLT2 studies indicate a class effect rather than specific actions of any one substance. In patients with type 2 diabetes and kidney disease, the risk of kidney failure and cardiovascular events was lower in those given canagliflozin compared to placebo.¹⁹ A subgroup analysis of that study suggested that decreased progression of renal disease extended to those with eGFR <30 mL/min. Sotagliflozin resulted in a significantly lower total number of deaths from cardiovascular causes, hospitalizations, and urgent visits for heart failure than placebo in patients with diabetes and heart failure.²⁰

5 Mineralocorticoid receptors

The Thornton laboratory resurrected an ancestral protein and showed it to be a 450 million-year-old precursor of glucocorticoid and mineralocorticoid receptors (MCR).²¹ The group demonstrated that specific historical mutations recapitulate receptor evolution from an MCR-like ancestor. The substitutions repositioned crucial residues to create new receptor-ligand and intra-protein contacts. Thus, the fact that MCR may mediate much in the way of signaling aside from sodium reabsorption and potassium excretion should not be surprising. The physiological MCR ligand is aldosterone. Kagawa et al reported on new steroids effective in blocking the effects of aldosterone and deoxycorticosterone in 1957.²² Therapeutic application for hepatic cirrhosis followed shortly thereafter.²³ Side effects largely through inappropriate dosage caused the compound to be largely forgotten until two clinical conditions resurrected spironolactone, namely resistant and refractory hypertension²⁴ and refractory heart failure.²⁵ The success of MCR blockade appears far removed from merely increased solute excretion or potassium retention.

Angiotensin (Ang) II, corticotropin, and potassium are the main stimulators of aldosterone release, while nitric oxide, endothelin, and various pituitary and adipose tissue-related factors can contribute to aldosterone synthesis. In addition to mediating sodium retention and facilitating potassium excretion, aldosterone bound to the MCR functions as a transcription factor that targets many genes. As a result, vascular and cardiac cells are greatly affected, as are nontubular cells within the kidney.²⁶

The role of mineralocorticoid receptor antagonists in cardiovascular disease has recently been reviewed.²⁷ Ferreira et al have underscored how aldosterone interacts with the immune system, induces inflammation by activating dendritic cells, monocytes, macrophages, and T lymphocytes.²⁸ These activities directly contribute to hypertension. Thus, the MCR blockade now represents a highly desirable target. Actually, spironolactone is quite well tolerated when the dosage is appropriate. The drug currently costs next to nothing and can cause hyperkalemia, a dreaded but perhaps overdrawn complication (Figure 2).

6 Novel MRC blocker

Finerenone is a nonsteroidal (dihydropyridine-derivative) antimineralocorticoid that has a less relative affinity for other steroid hormone receptors, notably the estrogen receptor, compared to spironolactone or eplerenone.²⁹ Thus, common male side effects such as low libido, impotence, and gynaecomastia are reduced. Finerenone is said to have fewer rates of hyperkalemia compared to spironolactone. Thus, finerenone could be suitable for patients with substantially reduced renal function. The notion receives support since finerenone binds to MR helix 12 of the MRC, which effectively disables MR activation upon ligand binding. Amazit et al recently characterized the molecular mechanisms in detail.³⁰ They used a modeling and mutagenesis approach and identified Ser-810 and Ala-773 as key residues for the high selectivity of finerenone. Aldosterone-dependent phosphorylation and MRC degradation were inhibited by both spironolactone and finerenone. However, only finerenone delayed aldosterone-induced nuclear accumulation. Chromatin immunoprecipitation assays revealed that basal MRC and steroid receptor coactivator-1 recruitment were remarkably reduced. Furthermore, polymerase II binding at the regulatory sequence of the SCNN1A gene encoding the epithelial sodium channel (ENaC) was diminished. Finerenone reduced albuminuria in patients with diabetic nephropathy (eGFR<60 mL/min).³¹ Finerenone has been compared head-to-head to spironolactone in a randomized trial. Pitt et al assigned 65 patients with heart failure and reduced ejection fraction to finerenone at three doses and compared the results to 393 similar patients assigned to spironolactone 25 or 50 mg/day.³² Plasma potassium values were slightly lower in the finerenone patients and hyperkalemia occurred in 5.3% as opposed to 12.7% with spironolactone.

Two large phase III placebo-controlled trials of finerenone were launched. In the “finerenone in reducing cardiovascular mortality and morbidity in diabetic kidney disease” (FIGARO-DKD) trial >7000 patients with DMTA with proteinuria and eGFR >25 mL/min were assigned to finerenone or placebo. Tested was the notion that treatment reduces the composite of time to the first occurrence of cardiovascular death, nonfatal myocardial infarction, nonfatal stroke, or hospitalization for heart failure.³³ In the “finerenone in reducing kidney failure and disease progression in diabetic kidney disease” (FIDELIO-DKD) trial, >5000 DMT2 patients were randomized. The eGFR values ranged between 75 and 25 mL/min. The focus of this study was directed towards renal failure, namely a sustained decrease in eGFR or renal death.³⁴

Just in time for Beethoven’s 250th birthday, FIDELIO-DKD has seen the light of day. Filippatos et al report that amongst DMT2 patients with CKD, finerenone reduced the incidence of a composite cardiovascular outcome, with no evidence of differences in treatment effect based on pre-existing cardiovascular disease status.³⁵ Ingelfinger and Rosen³⁶ reviewed the now published report by Bakris et al, who found a benefit of finerenone as compared with placebo with respect to CDK progression amongst patients with relatively advanced CKD with DMT2.³⁷ A cardiovascular benefit was observed early in the latter study, although the results were not as impressive as in the canagliflozin and renal events in diabetes with established nephropathy clinical evaluation (CREDENCE) trial reported earlier.¹⁹ However, some patients in FIDELIO-DKD were already receiving SGLT2 inhibition, while the CREDENCE patients were not allowed MRC blockade.

In conclusion, exciting is to observe two solute excreting strategies making such a dent in the course of cardiovascular and renal disease in patients with or without DMT2. Classifying the drugs as diuretics is technically correct. However, additional actions addressed by both these drug classes give us a broad spectrum of future investigations. The best news is the fact that physicians now do not have to “lie out of their pockets” when they tell their DMT2 patients, “this drug will improve your outcomes and make you better.” An alternative, or effective add on, might be Kempner’s diet.