Joshua J. Neumiller, PharmD, CDCES; Radica Z. Alicic, MD, MSc
Initially approved by the US Food and Drug Administration (FDA) in 1994, metformin has been the preferred first-line glucose-lowering agent for patients with type 2 diabetes (T2D) owing to its effectiveness, low hypoglycemia risk, weight neutrality, long clinical track record of safety, and affordability. However, the advent of newer glucose-lowering agents with evidence-based cardiovascular (CV) and renal benefits calls into question whether metformin should continue to be the initial pharmacotherapy for all patients with T2D. To help determine whether metformin has been “dethroned” as first-line treatment for T2D, here is a brief review of recent evidence and current guideline recommendations.
Cardiovascular Outcome Trials Transform Standard of Care
In 2008, the FDA issued guidance to industry to ensure that CV risk is more thoroughly addressed during development of T2D therapies. This guidance document required dedicated trials to establish CV safety of new glucose-lowering therapies. Findings from subsequent cardiovascular outcome trials (CVOTs) and subsequent large renal and heart failure (HF) outcome trials have since prompted frequent and substantial updates to major guidelines. On the basis of recent evidence from CVOT and renal trials, contemporary clinical practice guidelines have transitioned from a traditional glucocentric treatment approach to a holistic management approach that emphasizes organ protection through heart-kidney-metabolic risk reduction.
Per the 2008 FDA guidance, dipeptidyl peptidase-4 (DPP-4) inhibitors, glucagonlike peptide-1 (GLP-1) receptor agonists, and sodium-glucose cotransporter-2 (SGLT2) inhibitors, were evaluated in large dedicated CVOTs. Findings from several CVOTs established GLP-1 receptor agonist and SGLT2 inhibitor CV safety, and unexpectedly demonstrated reduced rates of major adverse cardiovascular events (MACE) relative to placebo. The LEADER and EMPA-REG OUTCOME trials were the first CVOTs to report cardioprotective benefits of the GLP-1 receptor agonist liraglutide and the SGLT2 inhibitor empagliflozin, respectively. The LEADER trial reported a 13% significant relative risk reduction for its primary composite MACE outcome, and the EMPA-REG OUTCOME trial similarly reported a 14% relative risk reduction for MACE. After CVOTs on other GLP-1 receptor agonists and SGLT2 inhibitors reported CV benefit, clinical practice guidelines began to recommend use of these agents in at-risk patients to mitigate CV risk.
During the period when most CVOTs were designed and conducted, a majority of trial participants were receiving metformin at baseline. Inclusion of a small subset of metformin-naive participants in these trials allowed for several post hoc– and meta-analyses investigating the impact of background metformin use on the overall CV benefits reported. Depending on the trial, baseline metformin use in large GLP-1 receptor agonist CVOTs ranged from 66% to 81%. For instance, 76% of participants in the LEADER trial were receiving metformin at baseline, but a post hoc analysis found no heterogeneity for the observed CV benefit based on background metformin use. Similarly, a subgroup analysis of pooled data from the SUSTAIN-6 and PIONEER 6 trials of injectable and oral formulations of semaglutide, respectively, reported similar CV outcomes for participants, regardless of concomitant metformin use. When looking at the GLP-1 receptor agonist class overall, a meta-analysis of seven CVOTs, which included participants with established atherosclerotic cardiovascular disease (ASCVD) and those with multiple ASCVD risk factors, concluded that GLP-1 receptor agonist therapy reduced the overall incidence of MACE in participants not receiving concomitant metformin at baseline.
Similar analyses have examined the impact of background metformin use on CV outcomes with SGLT2 inhibitors. An analysis of EMPA-REG OUTCOME found that empagliflozin improved CV outcomes and reduced mortality irrespective of background metformin, sulfonylurea, or insulin use. Of note, this analysis suggested a greater risk reduction for incident or worsening nephropathy in patients not on concomitant metformin (hazard ratio [HR], 0.47; 95% CI, 0.37-0.59; P = .01) when compared with those taking metformin at baseline (HR, 0.68; 95% CI, 0.58-0.79; P = .01). In addition, a meta-analysis of six large outcome trials found consistent benefits of SGLT2 inhibition on CV, kidney, and mortality outcomes regardless of background metformin treatment. Therefore, although CVOTs on GLP-1 receptor agonists and SGLT2 inhibitors were not designed to assess the impact of background metformin use on CV outcomes, available evidence supports the CV benefits of these agents independent of metformin use.
Individualizing Care to Attain Cardiorenal-Metabolic Goals
Three dedicated SGLT2 inhibitor renal outcome trials have been published to date: CREDENCE, DAPA-CKD, and EMPA-KIDNEY. All three studies confirmed the positive secondary renal outcomes observed in SGLT2 inhibitor CVOTs: reduced progression of kidney disease, HF-associated hospital admissions, and CV-related death. The observed renal and CV benefits from the CREDENCE trial were consistent across different levels of kidney function. Similarly, a meta-analysis of five SGLT2 inhibitor trials of patients with HF demonstrated a decreased risk for CV-related death and admission for HF, irrespective of baseline heart function. The ongoing FLOW is the first dedicated kidney-outcome trial to evaluate the effectiveness of a GLP-1 receptor agonist (semaglutide) in slowing the progression and worsening of chronic kidney disease (CKD) in patients with T2D.
As previously noted, findings from the LEADER and EMPA-REG OUTCOME trials demonstrated the beneficial effects of GLP-1 receptor agonists and SGLT2 inhibitors not only on MACE but also on secondary HF and kidney disease outcomes. These findings have supported a series of dedicated HF and kidney outcome trials further informing the standard of care for patients with these key comorbidities. Indeed, the American Diabetes Association’s (ADA) 2023 Standards of Care in Diabetes updated its recommendations and algorithm for the use of glucose-lowering medications in the management of T2D. The current ADA recommendations stress cardiorenal risk reduction while concurrently achieving and maintaining glycemic and weight management goals. On the basis of evolving outcome trial data, GLP-1 receptor agonists and SGLT2 inhibitors with evidence of benefit are recommended for patients with established or at high risk for ASCVD. Further, the Standards preferentially recommend SGLT2 inhibitors for patients with HF and/or CKD. Because evidence suggests no heterogeneity of benefit based on A1c for MACE outcomes with GLP-1 receptor agonists and no heterogeneity of benefit for HF or CKD benefits with SGLT2 inhibitors, these agents are recommended for cardiorenal risk reduction regardless of the need to lower glucose.
The 2023 update to the American Association of Clinical Endocrinology (AACE) Consensus Statement: Type 2 Diabetes Management Algorithm similarly recommends the use of GLP-1 receptor agonists and SGLT2 inhibitors to improve cardiorenal outcomes. To further emphasize the importance of prescribing agents with proven organ-protective benefits, the AACE consensus statement provides a complications-centric algorithm to guide therapeutic decisions for risk reduction in patients with key comorbidities (eg, ASCVD, HF, CKD) and a separate glucocentric algorithm to guide selection and intensification of glucose-lowering agents in patients without key comorbidities to meet individualized glycemic targets. Within the complications-centric algorithm, AACE recommends GLP-1 receptor agonists and SGLT2 inhibitors as first-line treatment for cardiorenal risk reduction regardless of background metformin use or A1c level.
In addition to the emphasis on the use of GLP-1 receptor agonists and SGLT2 inhibitors for organ protection, guidelines now recommend SGLT2 inhibitors as the standard-of-care therapy in patients with T2D and CKD with an estimated glomerular filtration rate ≥ 20 mL/min/1.73 m2, and irrespective of ejection fraction or a diagnosis of diabetes in the setting of HF. Overall, a common thread within current guidelines is the importance of individualized therapy based on patient- and medication-specific factors.
Optimizing Guideline-Directed Medical Therapy
Results from the DISCOVER trial found that GLP-1 receptor agonist and SGLT2 inhibitor use was less likely in the key patient subgroups most likely to benefit from therapy, including patients with peripheral artery disease and CKD. Factors contributing to underutilization of newer cardiorenal protective glucose-lowering therapies range from cost and access barriers to clinician-level barriers (eg, lack of knowledge on CKD, lack of familiarity with CKD practice guidelines). Addressing these issues and helping patients work through financial and other access barriers is essential to optimize the utilization of these therapies and improve cardiorenal and metabolic outcomes.
So, has metformin been “dethroned” as a first-line therapy for T2D? As is often the case in medicine, the answer depends on the individual patient and clinical situation. Metformin remains an important first-line treatment in combination with lifestyle interventions to help patients with T2D without key cardiorenal comorbidities achieve individualized glycemic targets. However, based on evidence demonstrating cardiorenal protective benefits and improved glycemia and weight loss, GLP-1 agonists and SGLT2 inhibitors may be considered as first-line treatment for patients with T2D with or at high risk for ASCVD, HF, or CKD, regardless of the need for additional glucose-lowering agents and independent of background metformin. Ultimately, the choice of first-line therapy for patients with T2D should be informed by individualized treatment goals, preferences, and cost-related access. Continued efforts to increase patient access to GLP-1 receptor agonists and SGLT2 inhibitors as first-line treatment when indicated are essential to ensure optimal treatment and outcomes.
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