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. 2018 Aug 28;9(5):1995–2014. doi: 10.1007/s13300-018-0493-2

Efficacy and Safety of Initial Combination Therapy in Treatment-Naïve Type 2 Diabetes Patients: A Systematic Review and Meta-analysis

Xiaoling Cai 1, Xueying Gao 1, Wenjia Yang 1, Xueyao Han 1, Linong Ji 1,✉
PMCID: PMC6167297  PMID: 30155646

Abstract

Introduction

The aim of this study was to evaluate the efficacy and safety of initial combination therapy compared with monotherapy in drug-naïve type 2 diabetes patients.

Methods

MEDLINE, Embase and the Cochrane Central Register of Controlled Trials were searched for randomized clinical trials of initial combination therapy with hypoglycemic agents compared with monotherapy. Those which satisfied the search criteria were included in the meta-analysis. Weighted mean difference and relative risks were calculated.

Results

A total of 36 studies were included in the meta-analysis. Compared with metformin monotherapy, initial combination therapy with metformin plus another anti-diabetes drug exhibited significant reductions in glycated hemoglobin (HbA1c) (p < 0.001). Most of the combination therapies had a similar risk of hypoglycemia (p > 0.05), with the exception of combinations of sulfonylurea/glinide and metformin or combinations of thiazolidinedione and metformin. Compared with dipeptidyl peptidase-4 (DPP-4) inhibitor monotherapy, initial combination therapy with DPP-4 inhibitor plus another anti-diabetes drug showed a significant decrease in HbA1c (p < 0.001) and a similar risk of hypoglycemia (p > 0.05). Compared with monotherapy with other anti-diabetes drugs, initial combination therapies also resulted in significant HbA1c reductions, a similar risk of hypoglycemia and similar risks of other adverse events.

Conclusion

Compared with monotherapy, all initial combination therapies resulted in significant HbA1c reductions. Compared with metformin monotherapy, initial combination therapies with DPP-4 inhibitors plus metformin, sodium/glucose cotransporter 2 inhibitors and metformin, respectively, were associated with similar risks of hypoglycemia, but initial combination therapies with sulfonylurea plus metformin, thiazolidinedione and metformin, respectively, were associated with higher risks of hypoglycemia.

Funding

AstraZeneca Ltd. (China).

Trial registration

Registration number CRD42017060717 in PROSPERO.

Electronic supplementary material

The online version of this article (10.1007/s13300-018-0493-2) contains supplementary material, which is available to authorized users.

Keywords: DPP-4 inhibitors, Drug-naïve, HbA1c, Hypoglycemia, Initial combination, Metformin, Sulfonylurea, Thiazolidinedione, Type 2 diabetes

Introduction

Initial hypoglycemic monotherapy is usually used in newly diagnosed type 2 diabetes patients, as currently recommended by the guidelines of the American Diabetes Association (ADA) and the European Association for the Study of Diabetes (EASD) [1, 2]. However, initial monotherapy is frequently insufficient to enable patients to achieve or sustain glycemic targets [3, 4]. Thus, initial combination therapy has emerged as an alternative approach. The latest position statement from the ADA/EASD [2] called for an initial combination of two non-insulin agents in patients with a high baseline glycated hemoglobin (HbA1c) level (≥ 9.0%). Additionally, the latest American Association of Clinical Endocrinologists (AACE) treatment algorithm [5] recommended that patients with a HbA1c level of > 7.5% should receive combination therapy with metformin plus an additional drug.

However, we asked the question of whether initial combination therapy is actually more efficacious than monotherapy in terms of glucose control and confirmed safety. To search for the answer, we identified two published systematic reviews and meta-analyses. In one meta-analysis [6] that included 15 randomized controlled trials (RCTs), the authors found that compared to metformin alone, combination therapy with metformin plus another anti-diabetes drug provided statistically significant reductions of 0.43% in HbA1c level and of 14.30 mg/dl in fasting plasma glucose (FPG) level. In another meta-analysis [7] that included eight RCTs, the authors reported that compared with metformin monotherapy, initial combination therapy with dipeptidyl peptidase-4 (DPP-4) inhibitors plus metformin was associated with a higher reduction of 0.49% in HbA1c level, a higher reduction of 0.80 mmol/l in FPG level and a lower weight loss of 0.44 kg. However, the authors of both of these meta-analyses did not present any further analysis with regard to the different types of hypoglycemic agent tested. Therefore, the aims of this study reported here were to comprehensively evaluate the efficacy and safety of initial combination therapies versus monotherapy using updated trial data in type 2 diabetes patients.

Methods

Literature Search

According to recommendations from the Cochrane Handbook for Systematic Reviews for meta-analysis, two independent investigators (XYG and WJY) conducted systematic searches of MEDLINE, Embase and the Cochrane Central Register of Controlled Trials (CENTRAL) for studies published between the date of inception and April 2017. The search terms were: “type 2 diabetes,” “initial combination therapy,” “early combination therapy,” “treatment-naïve,” “drug-naïve,” “newly diagnosed diabetes” and “randomized controlled trials.” Treatment-naïve or drug-naïve patients were defined as those patients diagnosed with type 2 diabetes who have not received treatment with any hypoglycemic agent. “Newly diagnosed diabetes patients” were defined as those patients diagnosed with type 2 diabetes for the first time and who had not received treatment. “Early combination studies” referred to the initial combination therapy for type 2 diabetes patients. This meta-analysis is registered as CRD42017060717 in PROSPERO (International Prospective Register of Systematic Reviews).

Study Selection and Data Extraction

The inclusion criteria for this meta-analysis were: (1) studies of initial combination therapy with hypoglycemic agents compared with monotherapy; (2) efficacy of glucose control was the primary outcome of the study; (3) double-blind RCTs; (4) studies conducted with treatment-naïve type 2 diabetes patients. The exclusion criteria were: (1) studies conducted in type 1 diabetes patients; (2) the study was an extension study and not the original one; (3) study duration of < 12 weeks.

Using the above inclusion and exclusion criteria, XYG and WJY independently evaluated the eligibility of all the studies identified in their search MEDLINE, Embase and CENTRAL. The Cochrane Collaboration tool [8] was used to rate each RCT as having a low, high or unclear risk of bias from the following aspects: sequence generation, allocation concealment, blinding of participants and personnel, blinding of outcome assessors, incomplete outcome data, selective outcome reporting, as well as other sources of bias (Electronic Supplementary Material [ESM] Table S1 and Fig. S1). WJY and XYG then extracted details from each article, including the publication data, study design, baseline characteristics, treatment arms, study duration, changes in glucose and weight control and the hypoglycemic rate. If several doses were used in one trial, the standard doses recommended and approved in the clinical practice were documented (ESM Table S2). The definition of drug-naïve patients and the percentage of drug-naïve patients in each treatment arm were also documented (ESM Table S3).

Statistical Analyses

The primary endpoint of this meta-analysis was the change in HbA1c level from baseline to the study endpoint in patients who received initial combination therapies compared with those receiving monotherapy. The secondary endpoints included changes in FPG, postprandial glucose (PPG) and body weight and the risk of hypoglycemia in patients who received initial combination therapies compared with those receiving monotherapy. Continuous outcomes were evaluated by computing the weighted mean differences (WMDs) and the 95% confidence intervals (CIs). Categorical outcomes were evaluated by computing the relative risks (RRs) and accompanying 95% CIs. Due to between-study heterogeneity, Higgins I2 statistics were used to evaluate the percentage of variance. Heterogeneity can be quantified as low, moderate and high, with upper limits of 25, 50 and 75% for I2, respectively [9–11]. The 95% CIs of I2 were also calculated [11]. Publication bias was assessed using a funnel plot (ESM Fig. S2).

Meta-regression analysis was performed to evaluate whether the pre-specified covariates of baseline age, gender, HbA1c level and baseline body mass index (BMI) were associated with HbA1c changes from baseline corrected by monotherapy. Differences were considered to be statistically significant as p < 0.05.

Statistical analyses were primarily performed using the Review Manager statistical software package (version 5.2; Nordic Cochrane Centre, Copenhagen, Denmark). Analyses were conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines for conducting and reporting meta-analyses of RCTs [12]. Meta-regression analyses were performed using the STATA statistical software package (version 11.0; StataCorp, College Station, TX, USA).

This article does not contain any studies with human participants or animals performed by any of the authors.

Results

Characteristics and Methodological Quality of Included Studies

A total of 36 studies were included in the meta-analysis (Fig. 1; Table 1). Of these, 12 were studies [13–24] with initial combination therapies of DPP-4 inhibitors plus metformin, three were studies [25–27] in which the initial combination therapy was sulfonylurea (SU) or glinide plus metformin, four were studies [28–31] in which the initial combination therapy was thiazolidinedione (TZD) plus metformin, three were studies [32–34] in which the initial combination therapy was sodium glucose cotransporter 2 (SGLT2) inhibitor plus metformin and six studies [35–40] utilized an initial combination therapy of DPP-4 inhibitor plus TZD. There were also eight trials with other initial combination therapies [41–48].

Fig. 1.

Fig. 1

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Flow chart of included studies. DDT-4 Dipeptidyl peptidase-4, SGLT2 sodium glucose cotransporter 2, SU sulfonylurea, TZD thiazolidinedione

Table 1.

Characteristics of randomized controlled trials in initial combination therapy in type 2 diabetes

First author, year Study duration Treatment groups No. of patients Age (years) Male (%) Body mass index (kg/m2) Duration of diabetes mellitus (years) Baseline glycated hemoglobin (HbA1c) (%) Baseline weight (kg)
DPP-4 inhibitors + metformin initial combination therapy vs. metformin monotherapy
 Goldstein, 2007 [13] 24 weeks Sitagliptin 50 mg + metformin 1000 mg bid 182 53.3 ± 9.6 42.3 32.4 ± 6.6 4.4 ± 4.2 8.7 ± 0.9 –
Metformin 1000 mg bid 182 53.2 ± 9.6 45.1 32.2 ± 7.1 4.4 ± 4.4 8.7 ± 0.9 –
 Goldstein, 2007-2 [13] 24 weeks Sitagliptin 50 mg + metformin 500 mg bid 190 54.1 ± 10.0 55.3 32.1 ± 6.7 4.5 ± 4.7 8.8 ± 1.0 –
Metformmin 500 mg bid 182 53.2 ± 10.2 48.9 32.1 ± 6.8 4.5 ± 3.9 8.9 ± 1.0 –
 Bosi, 2009 [14] 24 weeks Vildagliptin 50 mg + metformin 1000 mg bid 295 52.8 ± 10.64 58 31.37 ± 4.75 1.87 ± 2.60 8.70 ± 1.03 89.79 ± 18.87
Metformin 1000 mg bid 294 52.4 ± 10.71 58.2 31.31 ± 4.58 2.19 ± 3.33 8.62 ± 0.93 88.43 ± 17.39
 Jadzinsky, 2009 [15] 24 weeks Saxagliptin 10 mg + metformin 323 52.1 ± 11.6 45.2 30.3 ± 5.0 1.4 ± 2.5 9.5 ± 1.2 82.5 ± 16.9
Metformin 328 51.8 ± 10.7 49.7 30.2 ± 4.9 1.7 ± 3.1 9.4 ± 1.3 82.8 ± 17.5
 Reasner, 2011 [16] 18 weeks Sitagliptin/metformin FDC 625 49.4 ± 10.5 56 32.9 ± 7.2 3.5 ± 4.5 9.9 ± 1.8 94.7 ± 23.4
Metformmin 621 50.0 ± 10.5 57 33.7 ± 7.8 3.2 ± 4.3 9.8 ± 1.8 97.2 ± 25.5
 Haak, 2012 [17] 24 weeks Linagliptin 2.5 mg + metformin 1000 mg bid 143 56.4 ± 10.7 53.8 28.6 ± 4.8 – 8.7 ± 1.0 76.7 ± 16.0
Metformin 1000 mg bid 147 55.2 ± 10.6 53.1 29.5 ± 5.3 – 8.5 ± 0.9 80.0 ± 18.5
 Haak, 2012-2 [17] 24 weeks Linagliptin 2.5 mg + metformin 500 mg bid 143 55.6 ± 11.2 51.0 29.7 ± 5.3 – 8.7 ± 1.0 80.8 ± 19.0
Metformin 500 mg bid 144 52.9 ± 10.4 56.9 28.9 ± 4.8 – 8.7 ± 0.9 79.9 ± 18.4
 Pratley, 2014 [18] 26 weeks Alogliptin/metformin 12.5/1000 mg bid 114 54.6 ± 10.42 54.4 31.0 ± 5.38 4.2 ± 4.97 – –
Metformin 1000 mg bid 111 52.6 ± 11.30 45.9 30.5 ± 5.0 4.1 ± 4.59 – –
 Pratley, 2014-2 [18] 26 weeks Alogliptin/metformin 12.5/500 mg bid 111 53.7 ± 11.59 43.2 30.9 ± 5.35 4.1 ± 4.78 – –
Metformin 500 mg bid 114 54.6 ± 10.20 41.2 30.2 ± 4.84 3.8 ± 3.90 – –
 Ji, 2015 [19] 24 weeks Linagliptin 5 mg + metformin 1000 mg 344 53.1 ± 10.7 49.1 29 ± 5.7 – 8 ± 1.0 76.7 ± 18.8
Metformin 2000 mg 345 52.9 ± 10.7 45.8 29 ± 5.6 – 8 ± 0.8 76.0 ± 18.8
 Ji, 2016 [20] 24 weeks Sitagliptin 50 mg/metformin 850 mg bid 125 52.4 ± 9.3 53.6 25.4 ± 3.1 1.1 ± 0.3 8.6 ± 0.9 69.4 ± 10.8
Metformin 850 mg bid 124 53.0 ± 10.3 60.5 25.8 ± 3.5 1.1 ± 0.2 8.7 ± 1.1 71.1 ± 11.8
Ji, 2016-2 [20] 24 weeks Sitagliptin 50 mg/metformin 500 mg bid 122 52.6 ± 11.3 69.7 26.1 ± 3.4 1.1 ± 0.3 8.5 ± 1.0 72.4 ± 12.1
Metformin 500 mg bid 126 52.6 ± 9.5 54.8 26.0 ± 3.7 1.0 ± 0.2 8.7 ± 1.0 71.1 ± 13.7
 Mu, 2016 [21] 24 weeks Linagliptin 2.5 mg/metformin 1000 mg bid 147 50.7 ± 9.4 59.2 26.0 ± 3.7 – 8.7 ± 1.0 70.5 ± 12
Metformin 1000 mg bid 144 51.4 ± 10.4 63.2 26.1 ± 3.3 – 8.6 ± 1.0 71.0 ± 12
 Mu, 2016-2 [21] 24 weeks Linagliptin 2.5 mg/metformin 500 mg bid 147 51.4 ± 10.2 62.6 26.0 ± 3.6 – 8.7 ± 0.9 70.8 ± 12
Metformin 500 mg bid 145 52.1 ± 9.6 62.8 25.8 ± 3.3 – 8.7 ± 1.1 69.1 ± 10.7
 Dou, 2017 [22] 24 weeks Saxagliptin 5 mg + metformin 500 mg 210 50.8 ± 10.4 64.8 26.7 ± 3.7 0.97 ± 2.1 9.4 ± 1.1 –
Metformin 500 mg + placebo 207 50.1 ± 11.0 63.8 26.5 ± 3.6 0.72 ± 2.1 9.5 ± 1.0 –
 JI, 2017 [23] 26 weeks Alogliptin 12.5 mg + metformin 500 mg FDC bid 159 53.4 ± 10.46 57.2 26.16 ± 3.51 – 8.39 ± 0.81 –
Metformin 500 mg bid 162 53.6 ± 9.91 50.6 26.30 ± 3.57 – 8.40 ± 0.78 –
DPP-4 inhibitors + metformin initial combination therapy vs. DPP-4 inhibitor monotherapy
 Goldstein, 2007 [13] 24 weeks Sitagliptin 50 mg + metformin 1000 mg bid 182 53.3 ± 9.6 42.3 32.4 ± 6.6 4.4 ± 4.2 8.7 ± 0.9 –
Sitagliptin 100 mg qd 179 53.3 ± 10.2 52.0 31.2 ± 5.9 4.4 ± 4.6 8.9 ± 1.0 –
 Bosi, 2009 [14] 24 weeks Vildagliptin 50 mg + metformin 1000 mg bid 295 52.8 ± 10.64 58 31.37 ± 4.75 1.87 ± 2.60 8.70 ± 1.03 89.79 ± 18.87
Vildagliptin 50 mg bid 300 53.5 ± 10.95 60 31.26 ± 4.82 2.12 ± 3.32 8.68 ± 1.02 87.84 ± 17.93
 Jadzinsky, 2009 [15] 24 weeks Saxagliptin 10 mg + metformin 323 52.1 ± 11.6 45.2 30.3 ± 5.0 1.4 ± 2.5 9.5 ± 1.2 82.5 ± 16.9
Saxagliptin 10 mg 335 52.1 ± 10.2 50.4 30.2 ± 4.9 1.7 ± 2.8 9.6 ± 1.3 83.1 ± 16.9
 Haak, 2012 [17] 24 weeks Linagliptin 2.5 mg + metformin 1000 mg bid 143 56.4 ± 10.7 53.8 28.6 ± 4.8 – 8.7 ± 1.0 76.7 ± 16.0
Linagliptin 5 mg qd 142 56.2 ± 10.8 56.3 29.0 ± 4.7 – 8.7 ± 1.0 79.1 ± 17.3
 Pratley, 2014 [18] 26 weeks Alogliptin/metformin 12.5/1000 mg bid 114 54.6 ± 10.42 54.4 31.0 ± 5.38 4.2 ± 4.97 – –
Alogliptin 25 mg qd 112 52.6 ± 9.38 42.9 30.8 ± 5.22 3.6 ± 4.12 – –
 Ross, 2015 [24] 24 weeks Linagliptin 5 mg + metformin 159 49 ± 10.9 43.4 29.84 ± 5.82 – 9.79 ± 1.19 –
Linagliptin 5 mg 157 48.6 ± 11.2 49 29.63 ± 5.43 – 9.88 ± 1.10 –
 Ji, 2016 [20] 24 weeks Sitagliptin 50 mg/metformin 850 mg bid 125 52.4 ± 9.3 53.6 25.4 ± 3.1 1.1 ± 0.3 8.6 ± 0.9 69.4 ± 10.8
Sitagliptin 50 mg bid 120 51.7 ± 10.2 61.7 26.0 ± 3.5 1.1 ± 0.2 8.7 ± 1.1 71.8 ± 12.1
 Mu, 2016 [21] 24 weeks Linagliptin 2.5 mg/metformin 1000 mg bid 147 50.7 ± 9.4 59.2 26.0 ± 3.7 – 8.7 ± 1.0 70.5 ± 12
Linagliptin 5 mg qd 147 50.8 ± 10.5 51.7 26.2 ± 3.9 – 8.7 ± 0.9 70.2 ± 13.5
 Dou, 2017 [22] 24 weeks Saxagliptin 5 mg + metformin 500 mg 210 50.8 ± 10.4 64.8 26.7 ± 3.7 0.97 ± 2.1 9.4 ± 1.1 –
Saxagliptin 5 mg + placebo 213 49.5 ± 10.9 70.9 26.5 ± 3.2 0.73 ± 1.6 9.4 ± 1.0 –
 JI, 2017 [23] 26 weeks Alogliptin 12.5 mg + metformin 500 mg FDC bid 159 53.4 ± 10.46 57.2 26.16 ± 3.51 – 8.39 ± 0.81 –
Alogliptin 12.5 mg bid 163 55.4 ± 9.62 60.1 26.16 ± 3.92 – 8.48 ± 0.71 –
SU + metformin initial combination therapy vs. metformin monotherapy
 Garber, 2002 [25] 20 weeks Glyburide/metformin 2.5/500 mg 165 58.1 ± 9.8 58.2 29.6 ± 4.5 3.30 ± 3.18 8.18 ± 1.14 86.7 ± 17.5
Metformin 500 mg 161 56.0 ± 11.0 57.8 30.4 ± 4.3 2.98 ± 2.74 8.26 ± 1.08 88.6 ± 14.9
 Garber, 2003 [26] 16 weeks Glyburide/metformin 1.25/500 mg 171 55.6 ± 11.2 44.4 31.4 ± 4.6 3.0 ± 3.0 8.8 ± 1.5 91.9 ± 17.4
Metformin 500 mg 164 54.7 ± 11.8 43.3 31.4 ± 4.0 2.6 ± 2.3 8.5 ± 1.4 92.8 ± 15.6
 Horton, 2004 [27] 24 weeks Nateglinide 120 mg + metformin 500 mg tid 89 57.7 ± 1.2 65.2 30.6 ± 0.4 3.4 ± 0.4 8.2 ± 0.1 –
Metformin 500 mg tid 104 55.4 ± 1.1 67.3 29.9 ± 0.4 3.7 ± 0.4 8.3 ± 0.1 –
SU/glinide + metformin initial combination therapy vs. SU/glinide monotherapy
 Garber, 2002 [25] 20 weeks Glyburide/metformin 2.5/500 mg 165 58.1 ± 9.8 58.2 29.6 ± 4.5 3.30 ± 3.18 8.18 ± 1.14 86.7 ± 17.5
Glyburide 2.5 mg 161 56.5 ± 10.5 50.9 30.3 ± 3.9 2.81 ± 3.14 8.21 ± 1.09 87.2 ± 15.3
 Garber, 2003 [26] 16 weeks Glyburide/metformin 1.25/500 mg 171 55.6 ± 11.2 44.4 31.4 ± 4.6 3.0 ± 3.0 8.8 ± 1.5 91.9 ± 17.4
Glyburide 2.5 mg 151 55.3 ± 12.2 43.7 31.1 ± 4.3 3.0 ± 2.6 8.7 ± 1.4 91.0 ± 16.0
 Horton, 2004 [27] 24 weeks Nateglinide 120 mg +Metformin 500 mg tid 89 57.7 ± 1.2 65.2 30.6 ± 0.4 3.4 ± 0.4 8.2 ± 0.1 –
Nateglinide 120 mg 104 57.9 ± 1.0 56.7 29.9 ± 0.4 4.7 ± 0.6 8.1 ± 0.1 –
TZD + metformin initial combination therapy vs. metformin monotherapy
 Rosenstock, 2006 [28] 32 weeks Rosiglitazone/Metformmin 155 50.1 ± 10.7 57 33.2 ± 7.7 2.3 ± 2.7 8.9 ± 1.1 –
Metformin 154 51.5 ± 10.4 56 32.5 ± 7.0 2.9 ± 3.7 8.8 ± 1.0 –
 Stewart, 2006 [29] 32 weeks Roziglitazone + metformin 254 58.9 ± 8.4 55 30.9 ± 5.4 3.7 ± 3.6 7.2 ± 0.6 88.1 ± 16.3
metformin 272 59.0 ± 7.9 56 30.6 ± 5.5 3.7 ± 3.6 7.2 ± 0.6 87.2 ± 16.5
 Perez, 2009 [30] 24 weeks Pioglitazone 15 mg + metformin 850 mg bid 201 54.7 ± 12.2 44.8 30.8 ± 5.7 – 8.89 ± 0.07 –
Metformin 850 mg bid 210 53.7 ± 12.0 46.7 30.8 ± 5.7 – 8.65 ± 0.07 –
 Borges, 2011 [31] 80 weeks Rosiglitazone/metformin 344 51.5 ± 10.5 53 32.2 ± 6.8 2.3 ± 3.1 8.6 ± 0.9 87.1 ± 21.3
Metformin 334 50.7 ± 10.5 53 33.1 ± 7.1 2.6 ± 3.3 8.6 ± 0.9 90.6 ± 22.8
TZD + metformin initial combination therapy vs. TZD monotherapy
 Rosenstock, 2006 [28] 32 weeks Rosiglitazone/Metformmin 155 50.1 ± 10.7 57 33.2 ± 7.7 2.3 ± 2.7 8.9 ± 1.1 –
Rosiglitazone 159 50.6 ± 10.2 58 32.8 ± 7.1 2.7 ± 3.0 8.8 ± 1.0 –
 Perez, 2009 [30] 24 weeks Pioglitazone 15 mg + metformin 850 mg bid 201 54.7 ± 12.2 44.8 30.8 ± 5.7 – 8.89 ± 0.07 –
Pioglitazone 15 mg bid 189 54.0 ± 12.1 34.9 31.2 ± 5.5 – 8.69 ± 0.07 –
SGLT2 inhibitors + metformin initial combination therapy vs. metformin monotherapy
 Henry, 2012–2 [32] 24 weeks Dapagliflozin 10 mg + metformin 2000 mg 211 51.0 ± 10.1 50.2 – 2.2 ± 3.3 9.1 ± 1.3 88.4 ± 19.7
Metformin 2000 mg + placebo 208 52.7 ± 10.4 46.6 – 1.9 ± 4.0 9.1 ± 1.3 87.2 ± 19.4
 Hadjadj, 2016 [33] 24 weeks Empagliflozin 25 mg + metformin 2000 mg 169 53.6 ± 10.7 52.1 30.4 ± 5.3 – 8.66 ± 1.14 83.8 ± 19.8
Metformin 2000 mg 164 51.6 ± 10.8 56.1 30.5 ± 5.9 – 8.58 ± 1.13 83.7 ± 20.1
 Rosenstock, 2016 [34] 26 weeks Canagliflozin 300/Metformin 2000 mg 237 55.4 ± 9.8 48.5 32.8 ± 6.5 3.3 ± 3.9 8.9 ± 1.2 91.4 ± 21.4
Metformin 2000 mg 237 55.2 ± 9.8 48.9 33.0 ± 6.0 3.3 ± 4.5 8.8 ± 1.2 92.1 ± 20.1
SGLT2 inhibitors + metformin initial combination therapy vs. SGLT2 inhibitor monotherapy
 Henry, 2012–1 [32] 24 weeks Dapagliflozin 5 mg + metformin 194 51.7 ± 9.3 40.2 – 1.6 ± 2.4 9.2 ± 1.3 84.1 ± 19.5
Dapagliflozin 5 mg + placebo 203 52.3 ± 10.2 45.3 – 1.6 ± 3.1 9.1 ± 1.4 86.2 ± 21.1
 Henry, 2012–2 [32] 24 weeks Dapagliflozin 10 mg + metformin 211 51.0 ± 10.1 50.2 – 2.2 ± 3.3 9.1 ± 1.3 88.4 ± 19.7
Dapaglifozin 10 mg + placebo 219 51.1 ± 11.5 47.9 – 2.1 ± 3.8 9.1 ± 1.3 88.5 ± 19.3
 Hadjadj, 2016 [33] 24 weeks Empagliflozin 25 mg + metformin 2000 mg 169 53.6 ± 10.7 52.1 30.4 ± 5.3 – 8.66 ± 1.14 83.8 ± 19.8
Empagliflozin 25 mg 164 53.3 ± 10.7 50.6 30.6 ± 5.9 – 8.86 ± 1.29 83.1 ± 20.3
 Hadjadj, 2016-2 [33] 24 weeks Empagliflozin 10 mg + metformin 2000 mg 167 52.3 ± 11.3 59.3 30.5 ± 5.0 – 8.65 ± 1.23 83.0 ± 19.1
Empagliflozin 10 mg 169 53.1 ± 10.7 57.4 30.3 ± 5.2 – 8.62 ± 1.24 83.8 ± 19.8
 Rosenstock, 2016 [34] 26 weeks Canagliflozin 100 mg/Metformin 237 54.2 ± 9.6 45.6 31.9 ± 5.3 2.9 ± 3.3 8.8 ± 1.1 88.3 ± 17.6
Canagliflozin 100 mg 237 54.0 ± 10.7 44.3 32.4 ± 5.4 3.5 ± 4.4 8.8 ± 1.2 90.2 ± 18.6
 Rosenstock, 2016-2 [34] 26 weeks Canagliflozin 300/Metformin 237 55.4 ± 9.8 48.5 32.8 ± 6.5 3.3 ± 3.9 8.9 ± 1.2 91.4 ± 21.4
Canagliflozin 300 mg 238 55.8 ± 9.6 52.5 32.6 ± 5.8 3.3 ± 4.4 8.8 ± 1.2 93.0 ± 19.9
DPP-4 inhibitors + TZD initial combination therapy vs. TZD monotherapy
 Rosenstock, 2007 [35] 24 weeks Vildagliptin + piogglitazone 100/30 mg qd 148 51.0 ± 11.3 58.1 29.6 ± 5.8 2.0 ± 3.1 8.8 ± 1.1 –
Piogglitazone 30 mg qd 161 52.4 ± 10.3 64.0 28.9 ± 5.5 2.2 ± 3.3 8.7 ± 1.0 –
 Rosenstock, 2010 [36] 26 weeks Alogliptin 25 mg + piogglitazone 30 mg 164 – – – – 8.80 ± 0.962 –
Pioglitazone 30 mg 163 – – – – 8.76 ± 1.005 –
 Yoon, 2011 [37] 24 weeks Sitagliptin 100 mg + piogglitazone 30 mg 261 50.2 ± 10.2 52.5 29.7 ± 5.1 2.6 ± 4.3 9.5 ± 1.2 80.1 ± 17.4
Piogglitazone 30 mg 259 51.7 ± 11.2 56.0 29.6 ± 5.2 2.1 ± 3.9 9.5 ± 1.2 80.4 ± 17.8
 Yoon, 2012 [38] 54 weeks Sitagliptin 100 mg + piogglitazone 45 mg 164 51.4 ± 10.0 52.4 29.7 ± 4.8 2.6 ± 4.0 9.4 ± 1.1 81.6 ± 17.4
Piogglitazone 45 mg 153 52.3 ± 11.5 58.8 29.9 ± 5.3 1.6 ± 3.7 9.4 ± 1.4 81.9 ± 18.4
 Gomis, 2011 [39] 24 weeks Linagliptin 5 mg + pioglitazone 30 mg 259 57.7 ± 9.6 58.7 28.7 ± 4.8 – 8.60 ± 0.79 78.3 ± 15.6
Pioglitazone 30 mg + placebo 130 57.1 ± 10.1 65.4 29.7 ± 4.8 – 8.58 ± 0.87 82.7 ± 15.8
 Henry, 2014 [40] 54 weeks Sitagliptin 100 mg + pioglitazone 15 mg 193 52.6 50.8 30.7 ± 5.4 4.1 ± 4 8.9 ± 1.2 –
Pioglitazone 15 mg 183 50.3 65 30.7 ± 5.2 3.7 ± 4.2 8.9 ± 1.0 –
 Henry, 2014–2 [40] 54 weeks Sitagliptin 100 mg + pioglitazone 30 mg 190 51.1 58.9 31.1 ± 5.8 3.8 ± 3.8 8.7 ± 1.1 –
Pioglitazone 30 mg 194 51.8 54.1 30.9 ± 5.6 3.9 ± 4.0 8.9 ± 1.1 –
 Henry, 2014–3 [40] 54 weeks Sitagliptin 100 mg + pioglitazone 45 mg 198 53.5 59.6 30.5 ± 4.9 4.0 ± 4.5 8.9 ± 1.1 –
Pioglitazone 45 mg 188 52.5 50.5 31.2 ± 5.1 3.7 ± 4.0 8.8 ± 1.1 –
DPP-4 inhibitors + TZD initial combination therapy vs. DPP-4 inhibitor monotherapy
 Henry, 2014 [40] 54 weeks Sitagliptin 100 mg + pio 30 mg 190 51.1 58.9 31.1 ± 5.8 3.8 ± 3.8 8.7 ± 1.1 –
Sitagliptin 100 mg 186 51 60.2 31.4 ± 5.7 4.5 ± 6.8 8.7 ± 1.2 –
 Rosenstock, 2007 [35] 24 weeks Vildagliptin + piog 100/30 mg qd 148 51.0 ± 11.3 58.1 29.6 ± 5.8 2.0 ± 3.1 8.8 ± 1.1 –
Vildagliptin 100 mg qd 154 51.4 ± 10.8 63.6 29.4 ± 5.8 1.9 ± 3.1 8.6 ± 1.0 –
 Rosenstock, 2010 [36] 26 weeks Alogliptin 25 mg + piogglitazone 30 mg 164 – – – – 8.80 ± 0.962 –
Alogliptin 25 mg 164 – – – – 8.80 ± 0.988 –
SU/glinide + AGI initial combination therapy vs. AGI monotherapy
 Tatsumi, 2013 [41] 12 weeks Miglitol + mitiglinide 21 63.4 ± 8.9 47.6 24.8 ± 0.9 7.6 ± 5.5 7.19 ± 0.50 62.2 ± 2.9
Miglitol 22 62.9 ± 11.4 68.2 24.9 ± 1.2 7.3 ± 9.3 7.09 ± 0.82 67.7 ± 3.4
SU/glinide + AGI initial combination therapy vs. SU/glinide monotherapy
 Tatsumi, 2013 [41] 12 weeks Miglitol + mitiglinide 21 63.4 ± 8.9 47.6 24.8 ± 0.9 7.6 ± 5.5 7.19 ± 0.50 62.2 ± 2.9
Mitiglinide 21 65.4 ± 10.4 42.9 25.2 ± 0.8 6.1 ± 6.2 7.10 ± 0.48 62.7 ± 2.5
SU/glinide + TZD initial combination therapy vs. TZD monotherapy
 Chou, 2008 [42] 28 weeks Rosiglitazone + glimepiride (8 mg/4 mg) 218 54.9 ± 11.6 59.6 31.8 ± 6.2 2.0 ± 0.30 9.2 ± 1.3 90.2 ± 19.7
Rosiglitazone 230 53.6 ± 10.7 60.0 31.3 ± 5.8 2.0 ± 0.21 9.1 ± 1.3 88.9 ± 19.8
SU/glinide + TZD initial combination therapy vs. SU/glinide monotherapy
 Chou, 2008 [42] 28 weeks Rosiglitazone + glimepiride (8 mg/4 mg) 218 54.9 ± 11.6 59.6 31.8 ± 6.2 2.0 ± 0.30 9.2 ± 1.3 90.2 ± 19.7
Glimepiride 222 53.0 ± 11.0 57.7 31.8 ± 7.2 1.0 ± 0.18 9.0 ± 1.3 91.6 ± 23.6
DPP-4 inhibitors + metformin initial combination therapy vs. TZD monotherapy
 Wainstein, 2012 [43] 32 weeks Sitagliptin + metformin 261 52.4 ± 10.7 54.8 30.0 ± 6.1 3.2 ± 4.0 9.0 ± 1.3 82.8 ± 21.1
Pioglitazone 256 52.2 ± 11.0 52.3 29.6 ± 5.5 3.3 ± 3.5 8.9 ± 1.3 81.4 ± 19.9
DPP-4 inhibitors + metformin initial combination therapy vs SU monotherapy
 Amblee, 2016 [44] 12 weeks Saxagliptin + metformin FDC 50 45.6 ± 7.3 80 34.3 ± 11.3 – 10.9 ± 1.4 –
Glipizide 50 43.2 ± 10.6 82 34.3 ± 5.8 – 11.1 ± 1.39 –
Colesvelam + metformin initial combination therapy vs. metformin monotherapy
 Rosenstock, 2010 [45] 16 weeks Colesvelam + metformin 145 52.7 ± 11.5 48 30.6 ± 4.7 – 7.8 ± 1.0 80.8 ± 15.5
Metformin 141 53.9 ± 10.1 40 29.8 ± 4.4 – 7.5 ± 0.9 77.3 ± 16.2
DPP-4 inhibitors + AGI initial combination therapy vs AGI monotherapy
 Mikada, 2014 [46] 24 weeks Miglitol + sitagliptin 13 60.5 ± 11.5 53.8 28.3 ± 2.5 7.4 ± 3.1 7.14 ± 0.76 73.8 ± 10.2
Miglitol 14 58.7 ± 7.0 78.6 29.5 ± 5.5 9.3 ± 5.8 6.90 ± 0.51 81.4 ± 11.2
DPP-4 inhibitors + AGI initial combination therapy vs. DPP-4 inhibitor monotherapy
 Mikada, 2014 [46] 24 weeks Miglitol + sitagliptin 13 60.5 ± 11.5 53.8 28.3 ± 2.5 7.4 ± 3.1 7.14 ± 0.76 73.8 ± 10.2
Sitagliptin 14 59.2 ± 11.8 78.6 28.8 ± 2.5 7.6 ± 8.0 7.45 ± 0.93 76.8 ± 11.4
SGLT2 inhibitors + DPP-4 inhibitors initial combination therapy vs SGLT2 inhibitor monotherapy
 Lewin 2015 [47] 24 weeks Empagliflozin 25 mg + linagliptin 5 mg 134 54.2 ± 10.0 52.2 31.8 ± 5.3 – 7.99 ± 0.95 87.9 ± 18.2
Empagliflozin 25 mg 133 56.0 ± 9.3 57.9 31.2 ± 5.7 – 7.99 ± 0.97 86.7 ± 19.7
SGLT2 inhibitors + DPP-4 inhibitors initial combination therapy vs. DPP-4 inhibitor monotherapy
 Lewin 2015 [47] 24 weeks Empagliflozin 25 mg + linagliptin 5 mg 134 54.2 ± 10.0 52.2 31.8 ± 5.3 – 7.99 ± 0.95 87.9 ± 18.2
Linagliptin 5 mg 133 53.8 ± 11.5 56.4 31.9 ± 5.9 – 8.05 ± 0.89 89.5 ± 20.1
Triple initial combination therapy vs. conventional therapy
 Abdul-Ghani, 2015 [48] 24 months Metformin + pioglitazone + exenatide 79 47 ± 1 55 36.4 ± 1 0.42 ± 0.06 8.6 ± 0.2 101.6 ± 2.3
Conventional therapy 91 46 ± 1 62 36.6 ± 1 0.42 ± 0.05 8.6 ± 0.2 101.0 ± 3.4
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AGI Alpha-glucosidase inhibitor, bid twice daily, DDT-4 Dipeptidyl peptidase-4, FDC fixed-dose combination, qd once daily, SGLT2 sodium glucose cotransporter 2, SU sulfonylurea, tid three times daily TZD thiazolidinedione

Our meta-analysis included studies that were randomized, placebo-controlled, with double-blind treatment. The eligibility criteria were clearly reported in all of the trials. Most studies reported baseline age, BMI, HbA1c level and duration of diabetes between the comparison groups. The risk of bias as evaluated by the Cochrane instrument was low (ESM Fig. S1). The visual inspection of the funnel plots indicated low risks of publication bias (ESM Fig. S2). For some treatment groups included only one trial, no further meta-analysis was done in each group [41–48]. Those extension studies were excluded from this meta-analysis.

Efficacy of Initial Combination Therapy

Compared with metformin monotherapy, initial combinations of DPP-4 inhibitors and metformin exhibited significant decreases in HbA1c (WMD, − 0.44%, p < 0.001), FPG (WMD, − 0.77 mmol/l, p < 0.001) and PPG (WMD, − 1.65 mmol/l, p < 0.001), but increased body weight significantly (WMD, 0.38 kg, p < 0.001). Compared with DPP-4 inhibitors monotherapy, initial combinations of DPP-4 inhibitors and metformin caused significant decreases in HbA1c (WMD, − 0.88%, p < 0.001), FPG (WMD, − 1.61 mmol/l, p < 0.001), PPG (WMD, − 2.69 mmol/l, p < 0.001) and body weight (WMD, − 1.00 kg, p < 0.001) (Table 2; Figure S3).

Table 2.

Comparisons of initial combination therapy versus monotherapy in terms of glycemic control and change in body weight

Comparison group Included studies No. of patients WMD 95% CI p value I2 (%) 95% CI of I2
DPP-4 inhibitors +  metformin vs. DPP-4 inhibitors
 HbA1c (%) 10 1967/1951 − 0.88 − 0.99, − 0.78 < 0.001 100 0.76, 1.24
 FPG (mmol/l) 9 1824/1823 − 1.61 − 1.84, − 1.37 < 0.001 100 0.75, 1.25
 PPG (mmol/l) 6 1065/1020 − 2.69 − 3.27, − 2.12 < 0.001 100 0.65, 1.35
 Weight (kg) 8 1627/1624 − 1.00 − 1.28, − 0.77 < 0.001 100 0.73, 1.27
DPP-4 inhibitors + metformin vs. metformin
 HbA1c (%) 11 3379/3375 − 0.44 − 0.57, − 0.31 < 0.001 100 0.81, 1.19
 FPG (mmol/l) 10 3085/3086 − 0.77 − 1.02, − 0.51 < 0.001 100 0.80, 1.20
 PPG (mmol/l) 5 1377/1374 − 1.65 − 2.09, − 1.21 < 0.001 99 0.70, 1.28
 Weight (kg) 8 2505/2505 0.38 0.22, 0.54 < 0.001 99 0.77, 1.21
SU/glinide + metformin vs. metformin
 HbA1c (%) 3 425/429 − 0.68 − 0.86, − 0.50 < 0.001 100 0.32, 1.68
 FPG (mmol/l) 3 425/429 − 0.87 − 1.38, − 0.36 < 0.001 100 0.32, 1.68
 PPG (mmol/l) 3 425/429 − 0.70 − 1.02, − 0.38 < 0.001 99 0.31, 1.67
 Weight (kg) 2 336/325 2.60 2.40, 2.80 < 0.001 95 –
SU/glinide + metformin vs. SU/glinide
 HbA1c (%) 3 425/416 − 0.49 − 0.77, − 0.20 < 0.001 100 0.32, 1.68
 FPG (mmol/l) 3 425/416 − 0.66 − 1.12, − 0.20 0.005 100 0.32, 1.68
 PPG (mmol/l) 3 425/416 − 0.87 − 1.29, − 0.46 < 0.001 100 0.32, 1.68
 Weight (kg) 2 336/312 − 0.10 − 0.69, 0.49 0.74 99 –
TZD + metformin vs. metformin
 HbA1c (%) 4 954/970 − 0.44 − 0.68, − 0.19 < 0.001 99 0.50, 1.48
 FPG (mmol/l) 4 954/970 − 0.88 − 1.20, − 0.55 < 0.001 100 0.51, 1.49
 PPG (mmol/l) – – – – – – –
 Weight (kg) 4 954/970 1.93 1.88, 1.97 < 0.001 40 − 0.09, 0.89
TZD + metformin vs. TZD
 HbA1c (%) 2 356/348 − 0.83 − 0.97, − 0.68 < 0.001 41 –
 FPG (mmol/l) 2 356/348 − 1.25 − 1.75, − 0.75 < 0.001 99 –
 PPG (mmol/l) – – – – – – –
 Weight (kg) 2 356/348 − 1.22 − 1.89, − 0.55 < 0.001 76 –
SGLT2 inhibitors +  metformin vs. metformin
 HbA1c (%) 3 978/974 − 0.47 − 0.58, − 0.37 < 0.001 98 0.30, 1.66
 FPG (mmol/l) 2 642/646 − 1.38 − 1.60, − 1.17 < 0.001 99 –
 PPG (mmol/l) – – – – – – –
 Weight (kg) 3 978/974 − 2.00 − 2.29, − 1.71 < 0.001 98 0.30, 1.66
SGLT2 inhibitors + metformin vs, SGLT2 inhibitors
 HbA1c (%) 3 978/989 − 0.64 − 0.84, − 0.43 < 0.001 100 0.32, 1.68
 FPG (mmol/l) 2 642/646 − 0.83 − 1.05, − 0.61 < 0.001 99 –
 PPG (mmol/l) – – – – – – –
 Weight (kg) 3 978/989 − 0.66 − 1.06, − 0.27 < 0.001 99 0.31, 1.67
DPP-4 inhibitors + TZD vs. TZD
 HbA1c (%) 6 1577/1431 − 0.54 − 0.65, − 0.44 < 0.001 99 0.70, 1.28
 FPG (mmol/l) 6 1577/1431 − 0.89 − 1.01, − 0.76 < 0.001 97 0.68, 1.26
 PPG (mmol/l) 4 842/824 − 1.97 − 2.37, − 1.58 < 0.001 97 0.48, 1.46
 Weight (kg) 6 1577/1431 0.96 0.79, 1.14 < 0.001 96 0.67, 1.25
DPP-4 inhibitors + TZD vs. DPP-4 inhibitors
 HbA1c (%) 3 502/504 − 0.62 − 0.75, − 0.48 < 0.001 99 0.31, 1.67
 I2 (mmol/l) 3 502/504 − 1.41 − 1.50, − 1.31 < 0.001 90 0.22, 1.58
 PPG (mmol/l) – – – – – – –
 Weight (kg) 3 502/504 3.51 2.13, 4.88 < 0.001 100 0.32, 1.68
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CI Confidence interval, FPG fasting plasma glucose, HbA1c glycated hemoglobin, PPG postprandial plasma glucose, I2 Higgins I2 statistics, WMD weighted mean difference

Compared with metformin monotherapy, initial treatment combinations of SU/glinides plus metformin resulted in significant decreases in the levels of HbA1c (WMD − 0.68%; p < 0.001), FPG (WMD,− 0.87 mmol/l; p < 0.001) and PPG (WMD − 0.70 mmol/l; p < 0.001), but significant increases in body weight (WMD 2.60 kg; p < 0.001). Compared with SU/glinide monotherapy, initial combinations of SU/glinides plus metformin exhibited significant decreases in the levels of HbA1c (WMD − 0.49%; p < 0.001), FPG (WMD − 0.66 mmol/l; p = 0.005) and PPG (WMD − 0.87 mmol/l; p < 0.001) and similar changes in weight (WMD − 0.10 kg; p = 0.74) (Table 2; ESM Fig. S3).

Compared with metformin monotherapy, initial combinations of TZDs plus metformin led to significant decreases in HbA1c (WMD − 0.44%; p < 0.001) and FPG levels (WMD, − 0.88 mmol/l; p < 0.001) but increased body weight significantly (WMD 1.93 kg; p < 0.001). Compared with TZD monotherapy, initial combinations of TZDs plus metformin led to significant decreases in the levels of HbA1c (WMD − 0.83%; p < 0.001) and FPG (WMD − 1.25 mmol/l; p < 0.001) and body weight (WMD − 1.22 kg; p < 0.001) (Table 2; ESM Fig. S3).

Initial combinations of SGLT2 inhibitors plus metformin led to significant decreases in HbA1c (WMD, − 0.47%, p < 0.001), FPG (WMD, − 1.38 mmol/l, p < 0.001) and body weight (WMD, − 2.00 kg, p < 0.001) when compared with metformin monotherapy. Initial combinations of SGLT2 inhibitors plus metformin also led to significant decreases in HbA1c (WMD − 0.64%; p < 0.001) and FPG (WMD − 0.83 mmol/l; p < 0.001) levels and body weight (WMD − 0.66 kg; p < 0.001) when compared to SGLT2 inhibitor monotherapy (Table 2; ESM Fig. S3).

Compared with TZD monotherapy, initial combinations of DPP-4 inhibitors plus TZD exhibited significant decreases in the levels of HbA1c (WMD − 0.54%; p < 0.001), FPG (WMD − 0.89 mmol/l; p < 0.001) and PPG (WMD − 1.97 mmol/l; p < 0.001) but increased body weight significantly (WMD 0.96 kg; p < 0.001). Compared with DPP-4 inhibitor monotherapy, initial combinations of DPP-4 inhibitors plus TZD resulted in significant decreases in HbA1c (WMD − 0.62%; p < 0.001) and FPG (WMD − 1.41 mmol/l; p < 0.001) levels but significant increases in body weight (WMD 3.51 kg; p < 0.001) (Table 2; ESM Fig. S3).

Meta-regression analysis indicated that compared with monotherapy, the decrease in HbA1c level from baseline at initial combination therapy in each treatment group was not associated with the baseline HbA1c level adjusted by age, gender, and baseline BMI. However, when all data were pooled together, adjusted by age, gender and baseline BMI, HbA1c changes from baseline in the total combination therapy corrected by monotherapy was associated with baseline HbA1c level (coefficient − 2.98, 95% CI − 5.32 to − 0.63; p = 0.014) (ESM Table S4).

Adverse Effects of Initial Combination Therapy

Compared with metformin monotherapy, initial combinations of DPP-4 inhibitors plus metformin did not increase the risks of hypoglycemia, serious adverse effects (SAEs) or gastrointestinal (GI) side effects or the risk of discontinuation due to adverse effects (AEs) or drug-related AEs. When compared with DPP-4 inhibitor monotherapy, initial combinations of DPP-4 inhibitors plus metformin significantly increased the risks of hypoglycemia (RR 1.84; p = 0.007) and GI side effects (RR 2.19; p < 0.001) and the risk of drug-related AEs (RR, 1.73, p < 0.001).

Compared with metformin monotherapy, initial combinations of SU/glinides plus metformin significantly increased the risk of hypoglycemia (RR 8.91; p = 0.02). Compared with SU/glinide monotherapy, initial combinations of SU/glinides plus metformin significantly decreased the risk of hypoglycemia (RR 0.63; p < 0.001) but increased the risk of GI side effects (RR 1.42; p = 0.01).

Compared with metformin monotherapy, initial combinations of TZDs and metformin significantly increased the risk of hypoglycemia (RR 1.60; p = 0.03). Compared with TZD monotherapy, initial combinations of TZDs plus metformin did not increase the risks of any AEs.

Compared with metformin monotherapy, initial combinations of SGLT2 inhibitors and metformin significantly increased the risk of drug-related AEs (RR 1.45; p = 0.004). Compared with SGLT2 inhibitor monotherapy, initial combinations of SGLT2 inhibitors plus metformin significantly increased the risks of hypoglycemia (RR 2.23; p = 0.02) and GI side effects (RR 1.99; p = 0.002).

Compared with DPP-4 inhibitor monotherapy or TZD monotherapy, initial combinations of DPP-4 inhibitors plus TZD did not increase any risk of AEs (Table 3).

Table 3.

Comparisons of initial combination therapy versus monotherapy in terms of the risks of hypoglycemia and other adverse effects

Comparison group No. of patients Relative risk 95% CI p value I2 (%) 95% CI of I2
DPP-4 inhibitors + metformin vs. DPP-4 inhibitors
 AE 1967/1951 1.07 0.94, 1.22 0.29 0 − 0.24, 0.24
 Drug-related AE 1514/1489 1.73 1.39, 2.16 < 0.001 2 − 0.25, 0.29
 Hypoglycemia 1824/1823 1.84 1.19, 2.85 0.007 27 0.02, 0.52
 GI adverse effects 1584/1591 2.19 1.48, 3.23 < 0.001 62 0.33, 0.91
 SAE 1742/1746 0.70 0.45, 1.08 0.11 42 0.15, 0.69
 Discontinuation due to AE 1584/1591 0.77 0.48, 1.24 0.29 12 − 0.17, 0.41
DPP-4 inhibitors + metformin vs. metformin
 AE 3379/3375 0.92 0.83, 1.01 0.09 0 − 0.19, 0.19
 Drug-related AE 2926/2920 0.97 0.84, 1.11 0.63 0 − 0.20, 0.20
 Hypoglycemia 3379/3375 1.15 0.84, 1.55 0.38 17 − 0.02, 0.36
 GI adverse effects 2996/2989 0.91 0.80, 1.04 0.17 0 − 0.21, 0.21
 SAE 3154/3150 0.71 0.50, 1.01 0.05 0 − 0.20, 0.20
 Discontinuation due to AE 2996/2989 0.88 0.63, 1.22 0.44 0 − 0.21, 0.21
SU/glinide + metformin vs.metformin
 AE 425/429 1.26 0.90, 1.76 0.17 0 − 0.68, 0.68
 Hypoglycemia 425/429 8.91 1.46, 54.34 0.02 76 0.08, 1.44
 GI adverse effects 425/429 0.70 0.48, 1.01 0.06 65 − 0.03, 1.33
 SAE – – – – – –
 Discontinuation due to AE – – – – – –
SU/glinide + metformin vs. SU/glinide
 AE 425/416 0.98 0.70, 1.37 0.92 0 − 0.68, 0.68
 Hypoglycemia 425/416 0.63 0.48, 0.82 < 0.001 93 0.25, 1.61
 GI adverse effects 425/416 1.42 1.08,1.88 0.01 25 − 0.43, 0.93
 SAE – – – – – –
 Discontinuation due to AE – – – – – –
TZD +  metformin vs.metformin
 AE 954/970 1.06 0.88, 1.28 0.55 0 − 0.49, 0.49
 Hypoglycemia 954/970 1.60 1.05, 2.46 0.03 0 − 0.49, 0.49
 GI adverse effects 954/970 0.87 0.75, 1.01 0.07 0 − 0.49, 0.49
 SAE 954/970 0.98 0.65, 1.47 0.91 0 − 0.49, 0.49
 Discontinuation due to AE 954/970 1.06 0.72, 1.56 0.76 0 − 0.49, 0.49
TZD + metformin vs. TZD
 AE 356/348 1.31 0.97, 1.76 0.08 84 –
 Hypoglycemia 356/348 1.53 0.80, 2.91 0.20 0 –
 GI adverse effects – – – – – –
 SAE 356/348 0.87 0.32, 2.37 0.79 0 –
 Discontinuation due to AE – – – – – –
SGLT2 inhibitors + metformin vs. metformin
 AE 978/974 1.19 0.99, 1.43 0.06 3 − 0.37, 0.43
 Drug-related AE 978/974 1.45 1.12, 1.87 0.004 0 − 0.40, 0.40
 Hypoglycemia 642/646 1.37 0.64, 2.92 0.42 17 − 0.51, 0.85
 GI adverse effects 978/974 0.72 0.40, 1.07 0.25 73 0.33, 1.13
 SAE 978/974 0.84 0.43, 1.65 0.62 0 − 0.49, 0.49
 Discontinuation due to AE 978/974 0.82 0.47, 1.41 0.46 0 − 0.40, 0.40
SGLT2 inhibitors + metformin vs. SGLT2 inhibitors
 AE 1220/1236 1.16 0.99, 1.37 0.07 52 0.12, 0.92
 Drug-related AE 1220/1236 1.13 0.90, 1.42 0.31 68 0.28, 1.08
 Hypoglycemia 642/646 2.23 1.13, 4.41 0.02 27 − 0.41, 0.95
 GI adverse effects 978/989 1.99 1.39, 2.86 0.002 0 − 0.40, 0.40
 SAE 978/989 0.62 0.33, 1.16 0.13 0 − 0.40, 0.40
 Discontinuation due to AE 978/989 0.83 0.48, 1.43 0.50 0 − 0.40, 0.40
DPP-4 inhibitors + TZD vs. TZD
 AE 1154/1138 0.94 0.80, 1.12 0.50 0 − 0.35, 0.35
 Drug-related AE 1265/1107 1.06 0.79, 1.41 0.70 0 − 0.35, 0.35
 Hypoglycemia 1413/1268 1.08 0.77, 1.53 0.65 0 − 0.31, 0.31
 GI adverse effects 1265/1107 0.86 0.56, 1.33 0.50 25 − 0.10, 0.60
 SAE 1170/1140 1.31 0.85, .2.01 0.22 0 − 0.35, 0.35
 Discontinuation due to AE 1006/977 0.80 0.47, 1.38 0.42 3 − 0.37, 0.43
DPP-4 inhibitors + TZD vs. DPP-4 inhibitors
 AE 502/504 1.09 0.85, 1.40 0.50 45 − 0.68, 0.68
 Drug-related AE 350/354 1.40 0.92, 2.15 0.12 17 –
 Hypoglycemia 350/354 0.84 0.46, 1.53 0.57 0 –
 GI adverse effects – – – – – –
 SAE 350/354 1.31 0.66, 2.59 0.44 78 –
 Discontinuation due to AE – – – – – –
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AE Adverse effect, GI gastrointestinal, SAE serious adverse effect

Subgroup Analysis and Sensitivity Analysis

The data were further analyzed by stratification by the study time periods. Since most studies were conducted with a 24-week follow-up, therefore, subgroup analyses were made in those studies which reported on a 24-week period of outcomes. These studies showed similar comparison results between initial combination therapy and monotherapy (ESM Table S5). We also included and excluded the study with the longest study duration of 80 weeks [31] for sensitivity analysis and found the results were all similar with the total ones. Moreover, there were several studies including both drug-naïve patients and patients previously on anti-hyperglycemia agents [13, 17, 20, 27, 29, 39, 40], in which the percentage of drug-naïve patients ranged from 50 to 90% (ESM Table S3). We also conducted a sensitivity analysis and found similar results as those for the efficacy and safety evaluations.

Discussion

Montherapy is unlike to achieve glycemic targets in patients with a high baseline HbA1c level (≥ 9%) [2], and in such cases the guidelines of the ADA/EASD recommend that the patient receive initial combination therapy [2]. In terms of “high” baseline HbA1c level, the AACE recommends initial pharmacologic combination treatment in patients with a HbA1c level of > 7.5% [5], and the Canadian Diabetes Association recommends initial combination therapy in patients with a HbA1c level of > 8.5% [49]. Among all sets of guidelines, the justification for initiating combination therapy is that patient would be unlikely to reach the glycemic target with monotherapy. The results of our meta-analysis supports that rationale, with most initial combination therapies—compared with monotherapy—showing superior glucose control in type 2 diabetes patients with an initial HbA1c level of > 7.5% at a similar risk of hypoglycemia.

As previously indicated [50, 51], there are a number of rationales for initial combination therapy in patients with type 2 diabetes. First, such therapy may lead to early robust lowering of HbA1c levels; as demonstrated by our meta-analysis, most initial combination therapies showed superior glucose control compared to monotherapy. Second, initial combination therapy may avoid the clinical inertia associated with a stepwise approach to therapy. The authors of one study suggested that the time to receive additional anti-hyperglycemic medication exceeded 1 year for patients who failed metformin monotherapy and that this delay was associated with clinical inertia [52]. Consequently, initial combination therapy may one of the best options to directly address the causes of clinical inertia [52]. Third, initial combination therapy may improve ß-cell function [50, 51]. However, this finding was not clearly evident in our meta-analysis due to the lack of data. Fourth, the complementary mechanisms of action provided by initial combination therapy may require comparatively lower doses of individual agents and therefore may cause fewer AEs. This benefit was indicated by the results of our meta-analysis which showed that most initial combination therapies exhibited better glucose control with comparable risks of hypoglycemia, SAEs, discontinuation due to AEs and GI side effects. Fifth, initial combination therapy may avoid the long-term consequences of metabolic memory, as the initial use of combination therapy could lead to greater HbA1c reduction, enabling more individuals to achieve their glycemic goals while avoiding AEs stemming from multiple metabolic defects [51, 53, 54]. However, this latter potential benefit may not be concluded from the present meta-analysis because most of the studies included were of short-term duration.

The evidence is compelling that type 2 diabetes is a progressive, physiologically and genetically complex heterogeneous disease. Achieving glycemic control is necessary to prevent or delay the progression of vascular complications. As current treatment approaches do not adequately acknowledge the complexity of diabetes, a compelling case may be made for combination treatment [51]. Initial combination therapy may be required to address the complex pathophysiology of type 2 diabetes, which includes improving insulin secretion and insulin sensitivity, inhibiting hepatic glucose production and addressing delayed gastric emptying or glucose absorption, while focusing on satiety and renal glucosuria. Among the mechanisms of hypoglycemic agents [55], metformin inhibits hepatic gluconeogenesis and improves peripheral insulin sensitivity, SUs/glinides stimulate insulin secretion by β-cells, DPP-4 inhibitors stimulate insulin secretion and suppress glucagon secretion, SGLT2 inhibitors reduce renal glucose reabsorption and induce urinary glucose excretion, TZDs activate peroxisome proliferator-activated receptor gamma (PPAR-γ) and increase insulin sensitivity. Therefore, choices for initial combinations of the above agents should also be supported by the pathophysiology of type 2 diabetes.

However, a number of unresolved issues associated with initial combination therapy in type 2 diabetes patients remain. One of these is whether initial combination therapy improve adherence. To date, there is no evidence suggesting that initial combination therapy versus monotherapy or sequential titration therapy would result in a greater adherence of patients to the therapeutic regimen. However, published studies do show that the more complex the drug regimen, the lower the adherence to that regimen [56]. In our meta-analysis, we did not collect any data on a possible improvement in adherence. Another issue is cost; is initial combination therapy less costly? The relatively high cost of including novel agents, such as DPP-4 inhibitors or SGLT2 inhibitors, in an initial combination with metformin remains a significant barrier to their use in many regions of the world [51]. Several studies have estimated the cost-effectiveness associated with monotherapy compared to combination therapy with oral anti-diabetes agents, but a number of these these were derived from non-RCT data and had multiple confounders [57, 58]. Moreover, the authors of another study indicated that it was difficult to quantify the cost-effectiveness of softer outcomes such as fewer hypoglycemic events or improved quality of life [59]. We did not collect any data on the costs of initial combination therapy in our meta-analysis, but there are other economic models which could be used to answer this question. Moreover, the association between initial combination therapy and cardiovascular risk has not been fully examined in the literature. Gaps still exist in the evidence on treatment paradigms utilizing sequential versus initial combination therapy. Therefore, carefully designed, pragmatic, prospective real-world studies to assess the clinical effectiveness of initial combinations versus sequential treatment in patients with newly diagnosed or poorly controlled type 2 diabetes should be performed to provide more evidence.

There were several limitations to our meta-analysis. First, data from the separate studies covered different durations of the study. As previously indicated, RRs are sensitive to the length of the follow-up; consequently, the pooling of results from studies with different durations of follow-up might lead to an artificial heterogeneity and discrepancy in the meta-analyses [60]. We therefore explored the outcomes in subgroup analyses by pooling all of the studies with a study period of 24 weeks to conduct a sensitivity analysis, which showed similar results with the total results. Second, the definitions of treatment-naïve patients varied depending on the protocols of the trials included in our meta-analysis, and these differences may also be associated with the high heterogeneity of this study and also lower the ability of the authors of this study to propose solid conclusions. Therefore, we also conducted a sensitivity analysis to minimize the bias and found the similar results to the efficacy and safety evaluations. The large differences in the number of studies for several combinations is another limitation. For those treatment groups with only one trial included [41–48], no further meta-analysis was done for evaluation purposes. Another problem may be the variations in dosages used in the different studies. Therefore, the standard doses recommended and approved in the clinical practice were used in this meta-analysis to minimize the bias. Since baseline characteristics were variable across studies, we used the random-effects model for analysis when the level of heterogeneity was high. Given these factors, we suggest that our results be interpreted cautiously.

Conclusions

In conclusion, compared with monotherapy, all initial combination therapies resulted in significantly reduced HbA1c levels in treatment-naïve type 2 diabetes patients. Compared with metformin monotherapy, the initial combination therapies of DPP-4 inhibitors plus metformin and SGLT2 inhibitors plus metformin exhibited similar risks of hypoglycemia, but the initial combination therapies of SU plus metformin and TZD plus metformin exhibited higher risks of hypoglycemia.

Electronic supplementary material

Below is the link to the electronic supplementary material.

Supplementary material 1 (PDF 439 kb) (439.5KB, pdf)

Acknowledgements

Funding

This meta-analysis was financially supported by AstraZeneca Ltd. (China) and partially supported by the National Key R&D Program of China (2016YFC1304901). The funding agencies played no role in the study design, data collection or analysis, decision to publish or preparation of the manuscript. All authors had full access to all of the data in this study and take complete responsibility for the integrity of the data and accuracy of the data analysis. The article processing charges were funded by the authors.

Medical Writing, Editorial, and Other Assistance

The authors express their gratitude to X. Zhang, M. Wang and Z. Ye at AstraZeneca Ltd. China for assisting in the literature search during the preparation of this article. This part of work was supported by AstraZeneca Ltd. (China).

Authorship

All named authors meet the International Committee of Medical Journal Editors (ICMJE) criteria for authorship for this article, take responsibility for the integrity of the work as a whole, and have given their approval for this version to be published.

Authorship Contributions

Linong Ji and Xiaoling Cai designed the manuscript. Xiaoling Cai, Xueying Gao, Wenjia Yang and Xueyao Han were responsible for the study selection and data extraction of the meta-analysis. Xiaoling Cai and Xueyao Han were responsible for the statistical analyses. Linong Ji and Xiaoling Cai were responsible for the manuscript writing.

Disclosures

Linong Ji has received fees for lecture presentations from AstraZeneca, Merck, Novartis, Lilly, Roche, Sanofi-Aventis and Takeda, consulting fees from companies including AstraZeneca, Merck, Novartis, Lilly, Roche, Sanofi-Aventis and Takeda and grants/research support from AstraZeneca, Bristol-Myers Squibb, Merck, Novartis and Sanofi-Aventis. Xiaoling Cai, Xueying Gao, Wenjia Yang and Xueyao Han have nothing to disclose.

Compliance with Ethics Guidelines

This article does not contain any studies with human participants or animals performed by any of the authors.

Data Availability

Data sharing is not applicable to this article as this study was based on published trials which were all included in the supplementary files and no datasets were generated during the current study.

Open Access

This article is distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International License (http://creativecommons.org/licenses/by-nc/4.0/), which permits any noncommercial use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Footnotes

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supplementary material 1 (PDF 439 kb) (439.5KB, pdf)

Data Availability Statement

Data sharing is not applicable to this article as this study was based on published trials which were all included in the supplementary files and no datasets were generated during the current study.

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