FormalPara Key Summary Points

Limited data compare effects of switching to pioglitazone/metformin fixed-dose combination (FDC) versus uptitrated metformin after inadequate response to metformin monotherapy.

Pioglitazone/metformin FDC should be superior to uptitrated metformin in improving glucose metabolism, blood lipids, body weight, inflammatory factors.

Pioglitazone/metformin FDC enhanced proportions of patients with an glycated hemoglobin A1c (HbA1c) ≤ 6.5% or ≤ 7% compared with uptitrated metformin at week 16.

Pioglitazone/metformin FDC led to a significant decrease in changes in fasting blood glucose, homeostatic model assessment of insulin resistance (HOMA-IR), diastolic blood pressure, and high-sensitivity C-reactive protein (Hs-CRP), with a remarkable rise in high-density lipoprotein cholesterol (HDL-C). However, it elevated weight and body mass index (BMI).

No significant differences in safety were observed.

Pioglitazone/metformin FDC was superior to uptitrated metformin among patients with type 2 diabetes mellitus (T2DM) without adequate glycemic control.

Introduction

Type 2 diabetes mellitus (T2DM) is a group of metabolic disorders characterized by long-term high blood glucose, which is caused by defects in insulin secretion and/or action [1]. It has become the third most serious chronic non-communicable disease threatening human health, following cardiovascular diseases and malignant tumors [2]. Diabetes results in chronic damage of multiple organs, including the eyes, heart, and kidneys, posing a great threat to patients’ lives and quality of life, and bringing heavy economic burden to both families and individuals [3, 4]. Oral antidiabetic drugs are convenient to administer, promoting strong adherence for patients with T2DM.

The management of T2DM involves a multifaceted approach, including lifestyle changes, pharmacotherapy, and monitoring of blood glucose levels [5, 6]. Metformin is a biguanide that is used as an oral hypoglycemic agent and has been considered as a first-line treatment of T2DM for over 50 years. However, after years of disease regression, inadequate dosing, poor adherence, or the presence of comorbidities that affect glucose metabolism, metformin used alone usually cannot achieve satisfactory serum glucose levels [7]. In such cases, clinicians intensify treatment by uptitrating the metformin dose or adding another T2DM agent to metformin to achieve glycemic control. Pioglitazone/metformin fixed-dose combination (FDC) combines two insulin sensitizers – metformin and pioglitazone. Pioglitazone is a kind of thiazolidinedione that improves insulin sensitivity by interacting with the peroxisome proliferator-activated receptor gamma (PPARγ) [8]. Despite its benefits, the application of pioglitazone has been scrutinized due to concerns over side effects such as weight gain and fluid retention [9]. However, recent reviews have continued to recognize its role in managing T2DM, especially in patients with atherosclerotic cardiovascular disease (CVD) or cerebrovascular disease, even beyond diabetes such as non-alcoholic steatohepatitis, and polycystic ovary disease, which are associated with insulin resistance [8, 10]. Pioglitazone has even been reported to improve neuroinflammation and ferroptosis in preclinical models of Alzheimer’s disease [11]. Furthermore, pioglitazone has also been shown to improve hepatic fibrosis, serum transaminase levels, and lipid profiles [12]. The pleiotropic effects of pioglitazone are prompting a reevaluation.

The combination of pioglitazone and metformin has been proposed as a potential alternative to uptitrated metformin in patients with T2DM without adequate glycemic control [13]. The rationale for combining these agents is that pioglitazone and metformin have complementary mechanisms of action. The combination of pioglitazone and metformin improves glycemic control by reducing insulin resistance and improving insulin sensitivity, without causing hypoglycemia. Moreover, it was observed that the increase in the medications taken by patients makes it more challenging for individual patients to adhere to their drug regimens. By contrast, the use of a compound tablet containing two different fixed-dose drugs with distinct mechanisms could decrease the number of medications required while also ensuring stricter glycemic control and enhancing overall patient compliance. Several clinical studies have evaluated the efficacy and safety of pioglitazone/metformin combination therapy.

In the past two decades, sodium-glucose cotransporter-2 inhibitors (SGLT-2is) and glucagon-like peptide-1 receptor agonists (GLP-1RAs) have become a focus of interest. SGLT-2is promote glucosuria, leading to glucose lowering, weight loss, and blood pressure reduction, especially in patients with CVD and chronic kidney disease (CKD), yet with a risk of genitourinary infections and diabetic ketoacidosis [14, 15]. GLP-1RAs mimic incretins, enhancing insulin secretion and satiety, which aids in weight management but might cause gastrointestinal upsets, cancer, systemic complications, and hypoglycemia when used in combination therapy, and require injections mostly. The FDA announced that GLP-1RAs on the market had shown potential safety concerns [16, 17]. The cost-effectiveness of the three regimens (SGLT-2is, GLP-1Ras, and pioglitazone + metformin) has been demonstrated in different clinical settings [18,19,20]. However, considering the findings on the loose-dose combination of pioglitazone and metformin, FDC should appear to enhance adherence and be particularly advantageous for overweight patients with T2DM, which better fits with our trial design of exploring effects on this population [20,21,22]. However, there are limited documents available that compare the efficacy and safety of switching to pioglitazone/metformin FDC and uptitrated metformin following an inadequate response to metformin monotherapy.

Methods

Study Design

This study was a prospective, open-label, multicenter, randomized clinical trial with a 16-week follow-up. Participants were recruited at 15 secondary or tertiary hospitals in China (Beijing Hospital; Yinchuan First People's Hospital; Beijing Aerospace General Hospital; Peking University Shougang Hospital; Haidian Hospital of Beijing, Beijing Boai Hospital; Emergency General Hospital; Zhejiang Provincial People's Hospital; The First Affiliated Hospital of Zhejiang University School of Medicine, Beijing Hepingli Hospital, General Hospital of Civil Aviation, Anyang District Hospital of Puyang City, Zhumadian Central Hospital, Hebi Coal General Hospital, and The First People's Hospital of Qujing City). The study protocol was reviewed and approved by the local ethics committee. All procedures followed the Helsinki Declaration of 1964, as revised in 2013. Comprehensive information is provided to female participants of reproductive age during the screening phase. Participants who express reluctance to adhere to contraceptive requirements are excluded from the study. Throughout the follow-up period, clinical doctors advise female participants of reproductive age and assess their willingness to continue participating in the study. Informed, written consent was obtained from all participants in the study. This trial is registered with the Chinese Clinical Trial Registry (ChiCTR1900028606).

Participants

Between May 6, 2020 and March 1, 2022, 304 of 335 participants were enrolled in this trial. Inclusion criteria encompassed the following: (1) individuals with T2DM aged between 18 and 65 years; (2) body mass index (BMI) ranging from 19 to 45 kg/m2; (3) undergoing monotherapy with metformin of 750–1500 mg with poor glycemic control (7.5 ≤ glycated hemoglobin A1c [HbA1c] ≤ 10%, or fasting blood glucose [FBG] of ≥ 7 and ≤ 13.9 mmol/l). Exclusion criteria included: (1) intolerance of a daily 1500 mg of metformin; (2) a history of hypertension: Systolic blood pressure (SBP) > 180 mmHg and/or Diastolic blood pressure (DBP) > 110 mmHg; (3) a history of severe cardiovascular disease, renal insufficiency, pregnancy, chronic pulmonary disease, or chronic gastrointestinal disease; (4) pregnant women. The flow of screening and recruitment of study subjects is presented in Fig. 1. Patients were 1:1 assigned to two groups stratified in blocks by the baseline HbA1c (< or ≥ 8.5%). Randomization was performed centrally with an interactive web response system. All patients were fully aware of their physical condition, disease diagnosis, treatment protocol, and potential risks and complications throughout the study.

Fig. 1
figure 1

Flowchart of randomization and enrollment. Poor compliance refers to a participant's failure to consistently adhere to the prescribed investigational drug regimen as indicated by a compliance rate falling below 80% or exceeding 120%. Compliance is calculated by comparing the actual amount of the drug taken by the participant to the prescribed drug amount, expressed as a percentage

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Treatment Protocol

Participants in the control group received a 16-week treatment of 2500 mg/day metformin (Merck & Co Inc), or a maximum-tolerated dose (2000–2500 mg/day) with gradual increments within 0–2 weeks. Participants in the test group received a 16-week treatment of 15/500 mg pioglitazone/metformin (Hangzhou Zhongmei Huadong Pharma Ceutical Co., Ltd) with meals three times daily (twice a day if not well tolerated). During the screening visit, all concomitant medications (baseline recording) were recorded in detail. An explicit record of antidiabetic medication was maintained from the beginning of the randomization until the end of the study. Medications that affect glucose metabolism were not allowed to be used during the observation period, except for the investigational drugs Patients not adhering to combination therapy were excluded from the PPS group. To check for medications that needed to be continued for comorbidities and concomitant medication, doctors requested that the subjects bring all medications they were taking to the follow-up visits. Any other treatments were recorded throughout the study, including the name (or other therapy name), dosage, frequency of use, and time of use.

Study Endpoints and Outcome Measurements

The primary endpoint of the current study was the proportions of patients with HbA1c ≤ 6.5% or ≤ 7% at week 16. The secondary endpoints included bodyweight, BMI, FBG, HbA1c, fasting insulin (FI), 2-h postprandial blood glucose (2 h PPBG), fasting capillary blood glucose (FCBG), homeostatic model assessment of insulin resistance (HOMA-IR), SBP and DBP, total cholesterol (TC), triglycerides (TG), free fatty acids (FFA), high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), alanine aminotransferase (ALT), aspartate aminotransferase (AST), C-reactive protein (CRP), high-sensitivity C-reactive protein (Hs-CRP), and tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6) at week 16. Adverse events (AEs) were also assessed. Homeostatic model assessment of insulin resistance (HOMA-IR) = (FBG × FI)/22.5, where FI is measured in μU/ml and FBG in mmol/l. HbA1c, FI, CRP, hsCRP, TNF-α, and IL-6 were measured in the central laboratory in Shanghai (Shanghai Jince Medical Laboratory Co., Ltd.). TNF-α and IL-6 were measured by enzyme-linked immunosorbent assay (ELISA). The following baseline laboratory variables were analyzed at a local laboratory: clinical chemistry (serum or plasma), S/P-creatine, hematology (whole blood), B-hemoglobin, S/P-LDL-C, S/P-triglycerides, B-platelet count, B-glycated hemoglobin, and urinalysis.

Laboratory safety assessments were not routinely conducted during follow-up visits, but if there were serious AEs or potential endpoint events, the investigator may have decided to conduct different evaluations, including local laboratory evaluations. Follow-up checks for abnormal laboratory results were performed under local practices.

Follow-Up

Before enrollment and at week 16, the participants underwent FBG, complete blood count, urinalysis, and electrocardiography. FCBG levels were measured at week 0, 4, 8, and 12. HbA1c, FCBG, 2 h PPBG, and HOMA-IR were assessed at week 0 and 16. FI levels were measured at week 0, 12, and 16. In addition, laboratory tests were conducted for ALT, AST, TC, TG, FFA, HDL-C, LDL-C, CRP, Hs-CRP, TNF-α, and IL-6 at baseline, week 12 and 16.

Statistical Analysis

The sample size was calculated using an estimated primary outcome, with an efficacy in the control group of 46.88%, and 66.03% in the test group (HbA1c ≤ 6.5%). In total, 136 participants in each group were required to achieve 90% power and an α level of 0.05 (two-tailed t test). A total of 152 participants were planned to be assigned to both groups to allow for 10% dropouts. The mean and 95% Confidence Interval (CI) were reported for the primary and secondary outcomes, which had an approximately normal distribution. The median and interquartile range (IQR) were reported for outcomes with skewed distribution. Student’s t test and nonparametric tests were used for comparison of means. Chi-square test and Fisher’s exact test were used for contingency table comparison. Both full analysis set (FAS), where data from all participants randomized and at least one time treated were included, and per-protocol set (PPS) analyses, where data exclusively from patients who completed the treatment as initially assigned were incorporated, were performed for sensitivity analysis. The FAS analysis reflects real-world conditions but might introduce bias. In contrast, the PPS analysis offers a more precise evaluation of drug efficacy but forfeits the advantages of randomization. The purpose of employing both analyses is to gain a more comprehensive understanding of the efficacy of drugs in real-world settings. The last observation carried forward method was used for missing data input. PPS analysis was performed on those participants with a > 80–120% compliance rate and an HbA1c value recorded at week 16. Statistical analyses were performed using STATA (Ver. 16.0) and SAS (Ver. 9.4). All statistical tests used in the study were two-sided and considered to be significant if P < 0.05.

Results

Baseline Characteristics

A total of 293 cases were included in FAS and 265 cases in PPS. The mean age of participants was 49.81 (48.35, 51.28) and 51.00 (49.66, 52.34) for the test and control groups in FAS, with 99 (66.89%) and 76 (52.41%) male patients, respectively. Specifically, the mean HbA1c was 7.88% in the test group, with 7.79% in the control group in FAS. Except for the percentage of patients gender in PPS (P = 0.003), no significant differences were found between the two groups (Table 1).

Table 1 Patient demographics and disease characteristics at baseline
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Primary Outcome

The proportions of patients with an HbA1c ≤ 7% in the test group was significantly higher than in the control group (FAS: 69.59 vs. 54.48%; PPS: 72.06 vs. 55.81%, both P < 0.05); and a greater proportion of patients with an HbA1c ≤ 6.5% was also found in the test group than in the control group (FAS: 45.95 vs. 33.79%; PPS: 47.06 vs. 33.33%, both P < 0.05) after 16 weeks of treatments (Fig. 2, Table 2).

Fig. 2
figure 2

Proportions of patients with an HbA1c ≤ 6.5% or ≤ 7% at week 16. glycated hemoglobin A1c, HbA1c

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Table 2 Proportions of patients with an HbA1c ≤ 6.5% or ≤ 7% at week 16
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Secondary Endpoints

A greater reduction in changes in the HbA1c from baseline was observed in patients with pioglitazone/metformin FDC treatment at week 16 (FAS: − 1.17 (− 1.37, − 0.98) versus − 0.77 (− 0.97, − 0.56), P = 0.005; PPS: − 1.12 (− 1.32, − 0.93) versus − 0.71 (− 0.92, − 0.50), P = 0.005). Nevertheless, the test group gained weight along with an increased BMI, whereas those reduced in the control group (all P < 0.001). DBP presented a more profound reduction in the test group in both sets (both P < 0.05). In addition, the test group had greater decrease in TG in both sets (both P < 0.01), with HDL-C remarkable increased (both P < 0.001). ALT and AST showed a marked decrease in both sets (all P < 0.05). Hs-CRP was significantly lower in the test group in both sets (both P < 0.05) (Table 3).

Table 3 Changes in bodyweight, BMI, blood pressure, HbA1c level, blood glucose, liver function, and inflammatory markers from baseline
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Adverse Events

The most common AE in the test group was diarrhea (3/146), followed by fatigue (2/146). An episode of CVD was documented in a patient who had a heart attack and underwent stent implantation. However, it was not considered to be associated with the FDC. In the control group, diarrhea was similarly the predominant AE (8/147), followed by nausea (4/147), and no Grade 4/5 AEs were reported. These AEs either resolved spontaneously or were resolved after symptomatic treatment (Table 4). Adverse events occurred in similar proportions in the two groups.

Table 4 Summary of adverse events
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Discussion

This study showed that in patients with T2DM without adequate glycemic control, pioglitazone/metformin FDC was superior to uptitrated metformin in proportions of patients with an HbA1c ≤ 6.5% or ≤ 7% at week 16. In both sets, the test group exhibited a significant decrease in HbA1c, DP, FBG, TG, ALT, AST, and Hs-CRP, with a greater rise in weight, BMI, and HDL-C. The results of this study did not show any significant differences in safety measures between the two groups.

HbA1c is a measure of average blood glucose levels over the previous 2–3 months. Currently, the alteration in HbA1c levels from the baseline remains a widely accepted surrogate endpoint in clinical trials investigating hypoglycemic medications. Moreover, it serves as the primary efficacy endpoint in pivotal studies aimed at assessing the hypoglycemic impact of drugs as recommended by the Center for Drug Evaluation [23]. It is commonly used as a marker of long-term glycemic control in those with diabetes. The American Diabetes Association recommends a target HbA1c level of < 7% for most non-pregnant adults with diabetes, while the American Association of Clinical Endocrinologists recommends a target of < 6.5%. This is based on the findings of several large clinical trials, which have shown that maintaining HbA1c levels below 7% or 6.5% significantly reduces the risk of developing long-term complications of diabetes, including retinopathy, nephropathy, neuropathy, and cardiovascular disease. Our results showed that the test group demonstrated larger proportions of patients with an HbA1c ≤ 6.5% or ≤ 7% and a greater reduction in FBG at week 16. Both metformin and pioglitazone are insulin sensitizers. Metformin improves insulin sensitivity by activating AMP-activated protein kinase, which plays a crucial role in regulating energy metabolism in cells and increases glucose uptake in muscles and the liver, leading to improved insulin sensitivity [24]. While pioglitazone promotes glucose utilization by activating PPARγ signaling in target organs, such as the liver and muscle [8]. A combination of these two drugs produces a complementary mechanism of action, which enhances glycemic regulation and improves insulin sensitivity compared with either drug alone [25, 26]. It was reported that a 6-month treatment with pioglitazone/metformin FDC led to a 1.83% reduction in hemoglobin A1c (HbA1c), compared with pioglitazone (− 0.96%) and metformin (− 0.99%) monotherapy, with 63.8% of FDC patients achieving HbA1c ≤ 7% vs. 46.9% of pioglitazone- and 38.9% of metformin-treated patients [27]. Additionally, pioglitazone/metformin FDC was shown to have a better tolerability profile compared with uptitrated metformin, with a lower incidence of gastrointestinal adverse events [28,29,30]. The findings in our clinical trial are consistent with these studies, suggesting again the value of FDC in comparison to metformin monotherapy.

Several studies have reported that pioglitazone caused weight gain [31, 32]. One proposed mechanism for this effect is that pioglitazone promotes preadipocyte differentiation into mature adipocytes, leading to an increase in adipose tissue mass. Another proposed mechanism is that pioglitazone increases insulin sensitivity in adipose tissue, leading to increased glucose uptake and storage as fat [33]. Metformin might mitigate the weight gain caused by pioglitazone, yet the ultimate outcomes are not uniform for their combination [34,35,36,37]. Despite an adverse tendency to weight gain, pioglitazone significantly decreased both intrahepatic and visceral fat in patients with T2DM and also improved metabolic activity, which might have an independent mechanism from its hypoglycemic effects [37]. Additionally, some studies have shown that the greater the weight gain after glitazone treatment, the better the actual glycemic control, with more significant improvements in HbA1c reduction, insulin sensitivity, beta-cell function, and cardiovascular risk factors [38]. The results of HbA1c, FBG, and insulin resistance in the FDC group in this study also support this view.

Therapeutic interventions with pioglitazone, metformin, or pioglitazone/metformin FDC are likely associated with a reduction in DBP [39,40,41], which would bring about a decrease in the incidence of cardiovascular events [40]. Pioglitazone improves atherosclerosis in patients with T2DM, possibly related to the rise in Serum adiponectin, a widely recognized sign of insulin resistance [41]. Similarly, the mechanisms by which metformin lowers DBP might correlate with the amelioration of insulin resistance [42]. Thus, we observed a reasonable decrease in diastolic blood pressure. Pioglitazone has been shown to positively influence the lipid profile in patients with T2DM, with increased HDL-C, decreased TG, as well as uncertain LDL alteration [32, 43,44,45]. The mechanisms behind are believed to be PPARγ regulating the expression of genes involved in lipid metabolism such as influx gene CD36 and efflux genes Liver X Receptor a (LXRa) [46]. Thereby, pioglitazone augments HDL by enhancing the synthesis of apoA-I [47], promotes fatty acid uptake, and facilitates TG synthesis and storage in lipid droplets [48]. Metformin generally has a positive effect on LDL-C, TG, and TC in patients with T2DM, with probable improvement in HDL-C [49, 50]. Some molecular pathways are believed to be involved in this process, for example, Sirtuin 1 (SIRT1)/Sterol regulatory element-binding protein (SREBP) [51]. FDC or a loose-dose combination of pioglitazone and metformin might improve HDL-C and TG, with uncertain changes in LDL-C [52, 53]. The findings revealed a significant change in HDL-C and TG, which are generally consistent with previous studies. However, whether the change in LDL-C is associated with an increase in larger-sized particles has not been confirmed in this study, after all, small-sized LDL particles are widely acknowledged as promoting atherosclerosis [54]. Metformin also mitigates chronic inflammation in the body, which helps to improve insulin sensitivity, glucose uptake, and lipid metabolism [55, 56]. The results indicate that the pioglitazone/metformin FDC is superior to uptitrated metformin in improving lipid profiles and inflammation. The study also confirmed the expected effects of FDC on reducing liver enzymes and improving liver function as previous findings [57], suggesting that the combination have demonstrated advantages in controlling blood glucose and improving liver-related diseases. Given the complementary or synergistic effects of metformin and pioglitazone, FDC might give play to their strengths whilst circumvent weaknesses, deserving more attention from clinicians, especially when patients treated with metformin alone are in case of poor glucose control.

Previous studies have indicated that glitazones cause edema [58], which is related not only to fluid retention but also to factors such as vascular permeability [59]. In a retrospective study, 13 patients with idiopathic edema benefited from long-term tolerance to metformin after receiving metformin treatment, demonstrating the effectiveness of metformin in reducing blood sugar and capillary permeability [60]. Among the 146 patients in the test group, one case reported mild edema. Therefore, the low incidence of edema in the FDC group might be related to the combination of pioglitazone and metformin. Although one case reported an episode of CVD, the patient had already been diagnosed with CVD prior to enrollment but not met grade III of NYHA classification of cardiac function, thus enrolled to the clinical trial. However, the condition of the patient remained stable and was not exacerbated by FDC treatment. Therefore, the AE was not considered to be related to the FDC.

Due to the differing appearances of the investigational drugs, we did not employ blinding. Although an open-label design introduces bias, it enhances transparency and aligns more closely with real-world conditions. Secondly, the dispute likely arises regarding the cause of a coronary heart disease-related AE. A more accurate assessment of enrolled patients is needed in future studies to mitigate the impact of unexpected. Additionally, considering the short duration of the trial, the current results do not necessarily reflect long-term outcomes. Although comprehensive information and instructions for female participants who were of child-bearing age were provided, due to patient privacy, we could not ensure that all female participants complied with the contraceptive requirements.

Conclusions

Our study shows that patients with T2DM without adequate glycemic control with metformin are more likely to achieve a satisfactory blood glucose level when they change to pioglitazone/metformin FDC, without being accompanied by an increase in adverse events.