The same authors, along with some additional associates, also published a second letter that noted a further unexpected twist with the renal outcome: “In prior large-scale clinical trials, the effect of SGLT2 inhibitors on heart failure and renal outcomes had consistently tracked together,” they noted, but in this case it didn’t, a discordance they said was “extraordinarily puzzling”.
This led the study’s leaders to reanalyze the renal outcomes using a different definition, one that Milton Packer, MD, who helped design the trial and oversaw several of its analyses, called “a more conventional definition of renal events,” during his presentation of these findings at the congress. The researchers swapped out a 40% drop from baseline eGFR as an event and replaced it with a 50% decline, a change designed to screen out less severe, and often transient, reductions in kidney function that have less lasting impact on health. They also added an additional component to the composite endpoint, renal death. A revised analysis using this new renal composite outcome appeared in the European Journal of Heart Failure letter.
This change cut the total number of renal events tallied in the trial nearly in half, down to 112, and showed a more robust decline in renal events with empagliflozin treatment compared with the initial analysis, although the drop remained nonsignificant. The revised analysis also showed that the overall, nonsignificant 22% relative reduction in renal events in patients on empagliflozin, compared with placebo, dwindled down to completely nonexistent in the tertile of patients with a left ventricular ejection fraction of 60% or greater. In this tertile the hazard ratio actually showed a nonsignificant point estimate of a 24% increased rate of renal events on empagliflozin, with the caveat that this subgroup now included a total of just 40 total events between the two treatment arms. (Each of the two other tertiles also had roughly the same number of total events.)
The biggest effect on renal-event reduction was in the tertile of patients with an ejection fraction of 41%-49%, in which empagliflozin treatment was linked with a significant 59% cut in renal events, compared with placebo. The analysis also showed significant heterogeneity in thus outcome between this subgroup and the other two tertiles that had higher ejection fractions and showed reduced rates of protection by empagliflozin against renal events.
This apparent blunting of a renal effect despite preservation of renal function seemed to mimic the blunting of the primary cardiovascular outcome effect that also appeared in patients with ejection fractions in the 60%-65% range or above.
“If we knew what blunted the effect of empagliflozin on heart failure outcomes at higher ejection fraction levels, we think the same explanation may also apply to the blunting of effect on renal outcomes, but right now we do not know the answer to either question,” Dr. Packer said in an interview. He’s suggested that one possibility is that many of the enrolled patients identified as having HFpEF, but with these high ejection fractions may have not actually had HFpEF, and their signs and symptoms may have instead resulted from atrial fibrillation.
“Many patients with an ejection fraction of 60%-65% and above had atrial fibrillation,” he noted, with a prevalence at enrollment in this subgroup of about 50%. Atrial fibrillation can cause dyspnea, a hallmark symptom leading to diagnosis of heart failure, and it also increases levels of N-terminal of the prohormone brain natriuretic peptide, a metric that served as a gatekeeper for entry into the trial. “Essentially, we are saying that many of the criteria that we specified to ensure that patients had heart failure probably did not work very well in patients with an ejection fraction of 65% or greater,” said Dr. Packer, a cardiologist at Baylor University Medical Center in Dallas. “We need to figure out who these patients are.”
Some experts not involved with the study voiced skepticism that the renal findings reflected a real issue.
“I’m quite optimistic that in the long-term the effect on eGFR will translate into renal protection,” said Rudolf A. de Boer, MD, PhD, a professor of translational cardiology at University Medical Center Groningen (the Netherlands), and designated discussant at the congress for the presentation by Dr. Packer.
John J.V. McMurray, MD, a professor of cardiology and a heart failure specialist at Glasgow University, speculated that the unexpected renal outcomes data may relate to the initial decline in renal function produced by treatment with SGLT2 inhibitors despite their longer-term enhancement of renal protection.
“If you use a treatment that protects the kidneys in the long-term but causes an initial dip in eGFR, more patients receiving that treatment will have an early ‘event,’ ” he noted in an interview. He also cautioned about the dangers of subgroup analyses that dice the study population into small cohorts.
“Trials are powered to look at the effect of treatment in the overall population. Everything else is exploratory, underpowered, and subject to the play of chance,” Dr. McMurray stressed.