Diabetic kidney disease (DKD), characterized by reduced whole-kidney glomerular filtration rate (GFR) and/or urinary protein leakage, is a feared complication of type 2 diabetes (T2DM). With severe consequences such as endstage kidney disease (ESKD) and renal death, and strongly linked to cardiovascular (CV) morbidity and mortality, optimal treatment of DKD is vital. Still, even with multifactorial treatment of renal risk factors, including hyperglycemia, hypertension, obesity, dyslipidemia and albuminuria, residual risk remains high worldwide. Since the introduction of blockers of the renin-angiotensin-aldosterone system (RAS), no other renoprotective drug for T2D has been successfully developed, highlighting the need for novel strategies or new therapeutic drugs to improve renal outcome in T2DM.

In this regard, the introduction of the sodium glucose cotransporter (SGLT)2 inhibitors has been met with great enthusiasm. Designed to inhibit glucose reabsorption in the proximal tubule they induce glycosuria which indeed reduces hyperglycemia. More importantly, these drugs have shown remarkable benefits on CV disease and renal outcome in large CV safety trials in T2DM patients with high risk of or established atherosclerotic cardiovascular disease (CVD) as well as in patients with DKD. The first of these trials, the EMPAgliflozin cardiovascular outcome event trial in type 2 diabetes mellitus patients-Removing Excess Glucose (EMPA-REG OUTCOME), was reported in 2015 and demonstrated, next to risk reductions in CV outcomes, impressive reductions in the prespecified secondary renal outcome. In two subsequently reported CV safety trials conducted with canagliflozin (CANVAS-Program) and dapagliflozin (DECLARE-TIMI 58), these promising results indicating renal benefit were further strengthened. Recently, the results of a dedicated placebo-controlled trial with canagliflozin (CREDENCE) in DKD patients were reported. The study was terminated early due to overwhelming beneficial effects. Yet, at this point in time, the renoprotective mechanisms involved with SGLT2 inhibition still remain speculative, though a consistent finding is that SGLT2 inhibitors reduce estimated GFR after first dosing, which is reversible after treatment cessation. This "dip" indicates a renal hemodynamic phenomenon reminiscent of the RAS blockers and is thought to reflect a reduction in intraglomerular pressure. From studies in rodent models of type 1 diabetes (T1DM) and humans with type 1 diabetes it is hypothesized that SGLT2 inhibition leads to urinary sodium excretion by inhibiting in the proximal tubule, which influences renal hemodynamics through a mechanism known as tubuloglomerular feedback. In short, reduced sodium reabsorption at the level of the proximal tubule leads to increased sodium chloride delivery at the downstream located macula densa, which in turn increases afferent arteriolar resistance and reduces glomerular (hyper)filtration and hydrostatic pressure. In the recent RED trial (NCT02682563) the investigators assessed whether this is also true in T2DM patients. Suprisingly, this study showed that the renohemodynamic actions of SGLT2 inhibition in T2DM are not due to afferent vasoconstriction but rather efferent vasodilation. This is also the proposed working mechanism of inhibitors of the RAS system in T2DM, although dedicated studies in humans are scarcely done. Indeed, people with T2DM that do not respond to RAS blockers in terms of albuminuria reduction, also do not respond to SGLT2 inhibitor treatment.

Consequently, several questions remain regarding the combination of SGLT2 and RAS inhibitors. Especially with the recent results of CREDENCE, it is very likely that the combination of these agents will become standard of care in patients with T2DM and DKD. Both agents dilate the postglomerular arteriole, which might lead to relevant interactions or even synergistic effects. Since the majority of the population in the cardiovascular outcome trials used RAS inhibition, it is known that the renoprotective effect of SGLT2 inhibition is present with concurrent RAS inhibition. However, to what extend these agents interact and which of the various complex pathways involved in blood pressure and plasma volume control are affected by mono or combination therapy with these agents is unknown. It is important to emphasize that in the large trials, RAS blockade was not randomized and that the participants not on RAS blockade were small in numbers, making additional analyses on this topic difficult.

In conclusion: Despite multifactorial treatment approaches, residual risk for the development and progression of DKD remains high, and novel therapies or strategies to halt renal burden in T2DM are urgently needed. SGLT2 inhibitors and RAS inhibitors both induce glucose-independent renoprotective effects and improve renal outcome, seemingly via an at least partly equal mechanism, the dilation of the efferent glomerular arteriole resulting in an eGFR dip. The use of combination therapy with these agents could lead to an additive or even synergistic renoprotective effect in T2DM. As such, combined use of an SGLT2 inhibitor and RAS inhibitor may enhance individual benefits (e.g. reduction of glomerular pressure, activation of tubuloglomerular feedback, proximal and distal natriuresis, plasma volume contraction and reduction of blood pressure).