The prevalence of type 2 diabetes is becoming epidemic and is expected to increase in the next decades (1). Compelling prevention strategies must, therefore, be implemented. Metformin, one of the possible choices for first-line glucose-lowering therapy for hyperglycemia in type 2 diabetes (2), is also effective in preventing the disease (3). However, as with all drugs, there are patients who do not respond well to metformin (4,5) and/or who report side effects, including gastrointestinal distress (6) and reduced vitamin B12 serum concentration (7). In an ideal world, clinicians would have models that predict how each patient reacts to metformin (or any drug) so that they primarily treat the best responders (i.e., maximize treatment efficacy) and exclude patients at high risk of side effects (i.e., minimize personal costs). Such a precision therapeutics approach would also reduce the economic burden of any treatment.
A step toward precision therapeutics has been made by the genome-wide association study of people with prediabetes (from the Diabetes Prevention Program [DPP]) for metformin response, published in this issue of Diabetes (8). The incidence of diabetes and several 1-year changes in quantitative traits, including HbA1c and body weight, were the outcomes investigated. A very conservative experiment-wide statistical significance threshold (P < 9.0 × 10−9) was used, taking into account both the genome-wide design and several comparisons. Importantly, the gene–treatment interaction was also tested to identify possible associations due specifically to metformin. Unfortunately, no significant findings were observed concerning the incidence of diabetes. Conversely, two variants were significantly associated in the metformin arm, but not the placebo arm, with quantitative traits: rs144322333 was associated with changes in HbA1c (this single nucleotide polymorphism [SNP] is near ENOSF1 in chromosome 19), and rs145591055 was associated with changes in body weight (this SNP is near OMSR in chromosome 5). In addition, in the interaction model, two additional SNPs reached the experiment-wide P value, namely, rs6838493 (near LINC01093 in chromosome 4) and rs148219263 (near CDH20 in chromosome 18), with differential changes in HbA1c and body weight, respectively. None of the genes closest to the four SNPs has a biological function easily correlated with metformin efficacy. Unfortunately, despite several efforts, no replication of the association with HbA1c change was obtained, and no attempt was made with body weight reduction.
The design of the DPP uniquely addresses the pharmacogenetics of metformin efficacy in diabetes prevention and represents one of the strengths of this genome-wide association study. However, such uniqueness also represents a limitation, as attempting to replicate the associations in a second independent diabetes prediction study is not actionable. To try to overcome this obstacle, the authors looked for replication in people with type 2 diabetes, but this makes it possible to hypothesize that the failure to replicate results is due to the different phenotype used rather than pointing to a false-positive result.
As usually happens in pharmacogenomics (9), the sample size is also a limitation of this study, which can lead to false-negative findings, especially when testing gene–treatment interaction, an indispensable approach to identify individual responses to drugs (10). Therefore, we agree with the authors when they indicate the need for large pharmacological studies based on omics data in different populations.
A further limitation concerns the analysis of the DPP sample as a whole, even though data were derived from individuals of multiple ancestries. While this approach was somewhat unavoidable, given the small number of individuals of each ethnicity, it is biased toward variants with concordant effects across different ancestries, thus reducing the likelihood of identifying genetic markers associated with the effect of metformin in particular ethnic groups. Indeed, the findings reported do not appear to be ancestry specific, and some are found only in certain ethnicities as a likely sole consequence of the higher minor allele frequencies of the associated SNPs.
Studies like this are important because they discover markers that could unravel new pathways of a given drug's mechanism of action. Indeed, new biomarkers have recently emerged through omics investigation that are associated with several metformin effects (both efficacy and side effects) in different populations. Among these are genetic variants that are clearly advantageous because they are stable and constant throughout life (11), lipid metabolites (12), molecules from gut microbiota (13), and blood-based epigenetic markers (14). All these discoveries may open new avenues in the understanding of metformin’s mechanisms of action. Epidemiologically, the small individual effects that these or similar markers usually have on treatment response can be exploited to create subgroups of different average responses (e.g., low vs. high), but these effects cannot help to make accurate predictions at the individual level, thus providing little progress in precision therapeutics (15). Indeed, models are needed to implement a precision treatment approach, possibly based on common and readily available clinical variables that predict the benefits or harms of a given treatment in each patient. This effect must be examined objectively by specific measures (16) related to the discriminatory capacity of the different individual effects. In recent years, much attention has been paid to the methodology that aims to provide prediction models that take advantage of patient demographic, clinical, and laboratory characteristics, as well as their interactions, while limiting overly optimistic results and obtaining parsimonious algorithms (17). Methodological guidance for modeling treatment effect heterogeneity was recently published (18). Eventually, omics markers should be used to improve preexisting clinical risk models (19). The crucial difference between identifying subgroups at different average levels of risk and predicting risk at the individual level is detailed in Fig. 1.
Unfortunately, in the field of type 2 diabetes, there are very few validated models to predict the efficacy of some second- and third-line therapies (20,21), so much so that today most treatments for hyperglycemia, including first-line treatments, are still recommended by expert panels based on clinical considerations (2). Indeed, a couple of models have been proposed to predict the efficacy of metformin in people with diabetes, but these are either simple and straightforward preliminary pilot attempts (22) or are based on complex equations with nonlinear terms that are not easy to interpret and implement clinically (23). More importantly, both models lack validation in independent samples, an inescapable prerequisite for implementing prediction medicine approaches (24).
In conclusion, to make the dream of precision therapeutics in the field of diabetes come true, we need a deeper understanding of how each of the molecules that are currently used works. To this aim, comprehensive omics measurements in large pharmacological studies with samples from different ethnicities will play a role. Concurrently, but ideally before omics studies are completed, we need to develop models that can predict the benefits or side effects of a given drug at the individual level. Because the majority of people with type 2 diabetes live in resource-limited countries, these models should be easy to use and based on inexpensive common variables. New markers emerging from omics (or even non-omics) studies will then be added to these models, with the aim of further improving their predictive ability so that a precision therapeutic approach in diabetes can finally be implemented.
See accompanying article, p. 1161.
Article Information
Funding. This study was supported by the Italian Ministry of University and Research, “Progetti di Ricerca di Interesse Nazionale” 2015 and PON_2017_BIO-D ARS01_00876 (V.T.), Italian Ministry of Health, Ricerca Corrente 2021–2022 (V.T. and C.M.) and RF-2013-02356459 (C.M.), and European Foundation for the Study of Diabetes/Sanofi grant 2017 (C.M.).
Duality of Interest. No potential conflicts of interest relevant to this article were reported.