Type 2 diabetes mellitus (T2DM) patients often require a multi-drug regimen to achieve adequate glycaemic control. Janumet^® is a combination of two active substances, sitagliptin and metformin hydrochloride. Sitagliptin is a selective inhibitor of dipeptidylpeptidase-4, thus slowing the rapid inactivation of incretins, which helps lowering postprandial glucose (Lyseng-Williamson, 2007). Metformin is a biguanide compound used as the first-line drug in T2DM treatment for its well-established safety and effectiveness in lowering blood glucose (Flory and Lipska, 2019). The use of sitagliptin in combination with metformin consistently demonstrated a clinically significant reduction in average glycosylated hemoglobin than metformin monotherapy in inadequately controlled T2DM patients (Charbonnel et al., 2006; Goldstein et al., 2007; Nauck et al., 2007; Scheen et al., 2010; Arechavaleta et al., 2011; Schernthaner et al., 2013). In addition, the bioequivalence of Janumet^® versus equivalent doses of sitagliptin and metformin as individual tablets was demonstrated in healthy subjects (Janumet, 2022). Janumet^® was approved by the United States of America (US) Food and Drug Administration (FDA) in 2007 as an adjunct to diet and exercise to improve glycemic control in T2DM patients who were not adequately controlled on metformin or sitagliptin alone or in patients already being treated with both drugs (Janumet, 2022).

Janumet^® was supposed to be given twice daily. To bring the convenience of once-daily dosing, FDA approved Janumet^® XR designed for a fixed dosing regimen of sitagliptin and extended-release metformin in February 2012. It was available as 100 mg/1000 mg, 50 mg/1000 mg, and 50 mg/500 mg. Compared with the immediate release form, the extended form of metformin exhibits steadier glycemic control, better gastrointestinal tolerance, and improves patient adherence. Nevertheless, the method of developing extended-release metformin core tablet and synergistic coating of sitagliptin immediate-release (IR) formulation requires a more sophisticated formula and preparation technique (Ahmed et al., 2022a). Up to now, no generic products of Janumet^® XR have been authorized by FDA or European Medicines and Healthcare Products Regulatory Agency (MHRA).

FDA released a draft guidance on Metformin Hydrochloride; Sitagliptin Phosphate (Tablet, Extended Release) in July 2014 (US Food and Drug Administration, 2014). Both fasting and fed studies are suggested. Single-dose, two-way crossover design is recommended. The drug should be administered with 240 ml of a 20% glucose solution in water, followed by 60 ml of the same solution administered every 15 min for up to 4 h after dosing. This dosing method was recommended mainly to avoid hypoglycemia. The drug strength studied was 1000 mg/EQ 100 mg base. Other strengths can gain waivers of in vivo testing under three conditions, including bioequivalence of 1000 mg/EQ 100 mg base strength, acceptable in-vitro dissolution testing of all strengths, and proportional similarity of the formulations across all strengths.

The current study was conducted under the FDA guidance to compare the bioequivalence (BE) of sitagliptin 100 mg and metformin 1000 mg produced by Nanjing Chia-Tai Tianqing Pharmaceutical Company to Janumet^® XR. This was the first reported bioequivalent study of Janumet^® XR, and the study may shed light on the clinical study design and conduction of Janumet^® XR BE.

This study aimed to compare the pharmacokinetic profiles and safety of sitagliptin and metformin HCl extended-release tablets with JANUMET^® XR in healthy volunteers. The results supported the bioequivalence of the T drug to the R drug in terms of both rate (C_max) and extent (AUC_0−t) of absorption under fasting and fed conditions. The PK parameters were similar to the published FDA review data (FDA, 2005; FDA, 2006; FDA, 2012) under the fed state, with a slightly lower AUC_0−∞ and C_max for metformin in our study and a slightly higher C_max and lower AUC_0−∞ for sitagliptin results. Under the fasting state, the C_max of sitagliptin was consistent with the FDA review data, while AUC_0−∞ was slightly lower in our study. The PK parameters of metformin after taking JANUMET^® XR under the fasting state were not available in the FDA review data. Recently, BE studies of sitagliptin (100 mg) and fixed-dose combination (FDC) of sitagliptin–metformin (50/1000 mg) IR have been reported (Schnaars et al., 2023). The PK parameters C_max, AUC_0−t, AUC_0−∞ of sitagliptin (100 mg) under the fasting condition were approximately 14–32% higher than our data. The results indicated a combination formulation may decrease the absorption rate and extent of sitagliptin. Metformin (1000 mg, IR) exhibited a much higher C_max (1592.89 ng/L), shorter median T_max (3.5 h), and similar AUC results compared to our XR products under the fed condition. The results showed metformin (XR) extended the releasing process and exhibited steadier blood concentration than metformin (IR).

The absorption of sitagliptin in terms of C_max, AUC_0−t and AUC_0−∞ was slightly higher under the fed state than the fasting state, which was opposite to the FDA review data that the AUC_0−∞ and C_max for sitagliptin were slightly decreased under the fed state. On the contrary, the high-fat diet slightly decreased the absorption rate of metformin with a lower C_max and longer T_max while increasing the AUC_0−t by approximately 35% and AUC_0−∞ by approximately 32% compared to the fasting state. It was generally consistent with the JANUMET^® XR FDA review data (FDA, 2012) that a high-fat breakfast decreased the C_max for metformin by approximately 9% and increased AUC_0−∞ for metformin by approximately 62%. A BE study of Janumet reported a slightly higher C_max of sitagliptin under the fed state, and the AUC_0−t and AUC_0−∞ were comparable between the fasting and fed condition (Shi et al., 2022). The results were consistent with ours. In summary, the intake of a meal within 30 min of dosing can limitedly affect the absorption extent of sitagliptin while increasing the absorption extent of metformin.

The general study design followed the FDA guidance as single-dose, two-way crossover. Two types including fasting and fed were conducted. Healthy men and non-pregnant women were included. Our study provided some details regarding the design of sampling point and hypoglycemia prevention. The T_max of sitagliptin was reported between 1 and 4 h, and t_1/2 was around 12 h. The fed state has a limited effect on its PK profiles. The T_max of metformin was reported between 7 and 10 h, and t_1/2 was around 12 h. The state of fasting or fed was not revealed (FDA, 2005; FDA, 2006; FDA, 2012). The sampling point was designed to be quite dense within 12 h after dosing accordingly to depict accurate PK profiles of both drugs.

According to the JANUMET^® XR label (Janumet, 2012), hypoglycemia was one of the most frequently observed AEs in clinical trials. The FDA guidance suggested 60 mL of a 20% glucose solution in water administered every 15 min for up to 4 h after dosing in the in vivo study in case of hypoglycemia. Considering the risk of hypoglycemia in healthy subjects after taking one pill is relatively low, we did not follow the suggestion. In order to prevent the risk of hypoglycemia, we arranged blood glucose testing 2 and 8 h after dosing. The time point 2 h is close to the T_max of sitagliptin, and 8 h is around 4 h after lunch, both are the time points indicating a high possibility of hypoglycemia. We have also made a hypoglycemia emergency plan and lay in a supply of glucose water in case of hypoglycemia. In the fed study, none of the subjects experienced hypoglycemia. In the fasting study, one subject had transient hypoglycemia and recovered after taking 60 ml of a 20% glucose solution in water. The results demonstrated that our consideration on hypoglycemia prevention was practicable.

Most of the drug-related AEs reported in our study were known to date. According to JANUMET^® XR label (Janumet, 2012), a slight increase in the white blood cell count of patients treated with sitagliptin and metformin immediate-release compared to patients treated with placebo and metformin was observed. However, we observed one case of decreased white blood cell count in the fed and fasting study after the R drug administration, respectively. As the sample size is relatively small, we cannot conclude that the decrease in leukocyte count is a newly identified adverse reaction of JANUMET^® XR. More post-marketing experience is in need. In summary, the combination dose of sitagliptin and metformin were well tolerated in healthy subjects, the safety results were in line with previous reports (Shi et al., 2022; Schnaars et al., 2023).

As a selective DPPIV inhibitor, sitagliptin stabilizes and increases active incretins, namely glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) (Miller and St Onge, 2006), and metformin administration resulted in an increased total GLP-1 level (Mannucci et al., 2001). The effect of metformin on active GLP-1was complementary to those of sitagliptin. Given its limited hepatic metabolism and low protein binding, sitagliptin has a low potential for drug-drug interactions. No clinically significant drug-drug interactions were found between sitagliptin and metformin (Januvia, 2006; European Medicines Agency, 2021). The above facts indicated them as prime candidates for combination therapy.

Sitagliptin provided better efficacy than a placebo add-on to metformin and was non-inferior to glimepiride, glipizide, saxagliptin, and empaglifozin in improvements in HbA_1c levels (Charbonnel et al., 2006; Nauck et al., 2007; Scheen et al., 2010; Arechavaleta et al., 2011; Ahmed et al., 2022b). In addition, sitagliptin is generally well tolerated with a low risk of hypoglycaemia (Hanefeld et al., 2007; Janumet, 2022) and a neutral effect on bodyweight (Umezawa et al., 2015), thus lowering the risk of cardiovascular disease. On the contrary, sulfonylureas, meglitinides, thiazolidinediones, and insulin therapies all increase the risk of hypoglycaemia and bodyweight gain (Garber et al., 2015). GLP-1 RAs are more effective in terms of HbA_1c and body weight control than sitagliptin (Li et al., 2017). However, they require subcutaneous administration, usually cause severe gastrointestinal side effects, and are more expensive than sitagliptin (Nauck, 2016). It is noteworthy that a recent case-control cohort study found metformin plus sitagliptin might exert a protection against COVID-19 in T2DM patients, as evidenced by the mitigation of oxidative stress, CT scan score, and clinical outcomes (Al-Kuraishy et al., 2022). To sum up, with its convenient once-daily oral regimen, low potential for pharmacokinetic drug-drug interactions, and good efficacy and safety profiles, sitagliptin remains a critical add-on therapy to metformin in the management of T2DM patients (Karki et al., 2022). The fixed formulation of sitagliptin and metformin (extended-release) made a convenient once-daily oral regimen for T2DM patients. Sitagliptin 50 mg BID was equally effective to 100 mg QD in glycemic control. Lower doses were less effective, and doses up to 200 mg did not show an efficacy advantage (Aschner et al., 2006; Raz et al., 2006; Scott et al., 2007). Thus the 100 mg/1000 mg strength selected in our studies was prompt to be a popular dose regime.

Though patients can benefit from JANUMET^® XR, a relatively higher cost may render the prescribing decisions. It has been reported that sitagliptin was less cost-effective compared to empagliflozin as a second-line treatment to metformin (Reifsnider et al., 2021). Up to now, no generic form of JANUMET^® XR has been approved by FDA nor MHRA. The sitagliptin 100 mg and metformin 1000 mg produced by Nanjing Chia-Tai Tianqing Pharmaceutical Company was the first generic form of JANUMET^® XR approved by the National Medical Products Administration (NMPA) of China, which can make the medications more affordable to those patients who cannot afford brand formulations.

In summary, the present study was the first to study the bioequivalence of sitagliptin 100 mg and metformin 1000 mg produced by Nanjing Chia-Tai Tianqing Pharmaceutical Company to Janumet^® XR. The protocol design followed the FDA guidance with small modifications on the glucose water supplement. The results demonstrate both bio-equivalence and satisfactory safety of the T and R drugs. Our study may make a contribution to the drug accessibility of T2DM patients.