Comparative evaluation of quality attributes among six different brands of metformin hydrochloride tablets marketed in Bahir Dar city, Ethiopia

Abstract

The prevalence of chronic disease such as diabetes is increasing in developing countries like Ethiopia. Metformin has been the most widely used first-line agent for the treatment of type II diabetes for more than 10 decades. Generic substitution is a common practice in community pharmacies. Therefore, ongoing bioequivalence evaluations are needed to ensure therapeutic equivalence among the brands. The main objective of this study is to evaluate and compare the common physicochemical properties and drug release profiles of six brands of metformin HCL 500 mg tablets available in community drug retailers in Bahir Dar city, Ethiopia. To achieve this objective, official and non-official compendial tests were conducted among the sampled brands. The results of the analysis demonstrated that all tested products complied with the specifications of the main test parameters as stipulated in the pharmacopoeias. Hence, the available brands can be regarded as acceptable substitutes for each other. However, an in vivo bioequivalence study may be needed for a final assessment of the similarity in efficacy of the generic formulations. Strict abidance to regulatory recommendations and progressive post-market surveillance are still required to promote the rational use of branded drugs and ensure safe and effective pharmacotherapy.

Introduction

Diabetes mellitus (DM) is a commonly known non-transmissible chronic disease that causes significant public health concern for both developed and developing nations around the world. The disease significantly affects insulin release and sensitivity^[1^-3^]. There are two main types of diabetes: type 1 DM and type 2 DM. Type 2 DM is the most common form of diabetes and a rapidly growing health concern in both developed and developing nations. It accounts for 90–95% of all people with diabetes and is expected to increase to 439 million by 2030^[3^]. The World Health Organization (WHO) has reported that diabetes was responsible for over 6.7 million deaths in 2021, highlighting the severity of the disease’s impact on global health. Notably, type 2 diabetes comprises over 90% of all diabetes cases, driven largely by lifestyle factors such as unhealthy diets, physical inactivity, and obesity. The burden of the disease is particularly acute in low- and middle-income countries, where health care systems often struggle to manage the growing numbers of patients, and the African continent is expected to experience one of the most dramatic increases in prevalence. These alarming statistics underscore the urgent need for comprehensive strategies in diabetes prevention, early diagnosis, and effective management on a global scale^[4^] The higher prevalence of diabetes may be associated with obesity, age-related problems, life style and lack of physical exercise. Currently, it has become a major cause of morbidity and mortality worldwide due to associated medical complications such as cardiovascular problems, kidney disease, and eye- and nerve-related diseases^[5,6^].

HIGHLIGHTS

• Conducted a detailed evaluation of six brands of metformin hydrochloride tablets marketed in Bahir Dar city.

• Physicochemical properties, such as weight variation, hardness, friability, disintegration, and dissolution, were tested.

• All brands were assessed for compliance with pharmacopoeial standards (British Pharmacopoeia/United States Pharmacopoeia).

• Significant variation was observed in the dissolution profiles; however, all brands released the required percentage of the drug within the acceptable time frame.

• Suggested the need for better regulatory oversight and periodic quality assurance to ensure the consistent therapeutic efficacy of marketed brands.

Nowadays, several drugs are available on the market to be administered through different routes to control DM. In this regard, the oral route is found to be the preferred option due to its convenience and easiness^[7^]. Metformin is one of the oral antidiabetic medications used to treat type II (non-insulin-dependent) DM. It improves insulin sensitivity, decreases fasting and random blood glucose levels, minimizes glucose production in the liver, and reduces its absorption in the gastrointestinal tract^[8^]. Metformin does not cause hypoglycemia or hyperinsulinemia. It belongs to the biguanide group of drugs and is chemically known as 3-(diaminomethylidene)-1,1-dimethylguanidine. It is soluble in water and practically insoluble in acetone. Under fasting conditions, the bio availability of an orally administered 500 mg metformin tablet reaches around 50–60%. It exhibits insignificant plasma binding properties, has a half-life of 2.5 hours, and a volume of distribution of 63–276 L^[9,10^].

The Food and Drug Authority (FDA) revealed that a drug product is considered pharmaceutically equivalent when it contains the same active ingredient(s), the same dosage form, the same route of administration, and identical strength or concentration. Pharmaceutical products are considered to be therapeutically equivalent only when they are pharmaceutically equivalent. As is well known, safety and efficacy are not separable concepts from the quality of pharmaceutical products^[2^]. Many of the quality control tests investigate the quality of medicines during production and in the final product batches. Generic drugs contain similar chemical entities but differ in color, shape, excipients, and manufacturing process^[11^]. Dissolution testing plays a key role as a quality control parameter in assessing batch-to-batch consistency of drug release from a dosage form. It is also used as a qualitative and quantitative tool, which can provide important information about the bioavailability of a drug^[9,12^]. According to a study conducted in Jimma Town, Ethiopia, on available metformin brands in the market using in vitro testing approaches, all brands showed no signs of counterfeiting or substandard products and can be utilized interchangeably. However, only the Insumet brand failed to fulfill the United States Pharmacopoeia (USP) specification for the weight variation test^[3^].

The availability of multisource generic medicines may pose a problem for health care providers in selecting equivalent products. While it is helpful in increasing the accessibility of some medicines and reducing costs, it also addresses affordability issues for some important pharmaceutical products. However, despite these positive impacts, it contributes to the production of fake, substandard, and counterfeit pharmaceutical products^[13^]. Though the impact of counterfeit medication is significant, it is poorly reported and predominantly affects developing countries, which could depreciate the reliability and accountability of pharmaceutical products and the health care system. According to reports from the WHO, nearly 30% of medication consumed in Africa and parts of Asia consists of counterfeit or substandard products^[14,15^]. Both generic and brand-name pharmaceutical products are susceptible to counterfeiting, which may involve genuine or incorrect active ingredients, insufficient active ingredients, or false packaging^[12^]. Under quality control testing, these counterfeit products fail to meet pharmacopoeial specifications and the quality standards claimed by manufacturers. Numerous studies have indicated that substandard or counterfeit products result in treatment failures for many infectious diseases^[14^]. The aim of this investigation is to assess various brands of metformin hydrochloride tablets available in the Bahir Dar, Ethiopian market. Their physical, chemical, and pharmaceutical quality standards were checked and compared with official specifications.

Experimental

Materials

Chemicals, reagents, and solvents

Metformin HCl, with a labeled strength of 500 mg tablets from six different brands (coded A to F), was purchased from registered community drug retailers in Bahir Dar city, Ethiopia. The Metformin reference standard was obtained from the Ethiopian Food and Drug Authority (EFDA). Chemicals such as NaOH pellets (Guangdong Guanghua Sci-Tech Co., Ltd, China) and potassium dihydrogen orthophosphate (Guangdong Guanghua Sci-Tech Co., Ltd, China) were purchased from the market. Freshly prepared distilled water was used throughout the work. The reagents used were analytical grades, and the study was performed within the products’ expiration dates.

Instruments, apparatus, and equipment

The instruments used during the study include a disintegration apparatus (ERWEKA, Germany), a dissolution testing apparatus (ERWEKA, Germany), a UV–Vis spectrophotometer (ERWEKA, T80+ Spectrophotometry, Double Beam, Ver 3.3, PG Instruments Ltd), Whatman filtration equipment (0.45 μm PVDF W/GMF), a disintegration apparatus (ERWEKA, Germany), a hardness tester (Sotax HT, Model: HT 1500 N, Switzerland), a thickness tester (Sotax HT, Model: HT 1500 N, Switzerland), a friability tester (FTA-023, Single Drum, India), weighing balances, a pH meter, beakers, filter paper, a water bath, ovens, cuvettes, flasks, pipettes, measuring cylinders, and a mortar and pestle.

Method

Study area and period

This study was conducted in community drug retail outlets (drug stores and pharmacies) in Bahir Dar city, the capital city of Amhara Regional State, Ethiopia, which is located 565 km northwest of the capital city, Addis Ababa. The study setting included all registered community drug retailers in the city. There are 6 sub-cities, and within these sub-cities, there are 3 governmental hospitals (2 tertiary-level and 1 primary), 10 governmental health centers, more than 4 private hospitals (primary and general), more than 22 private clinics (medium and specialized), more than 28 pharmaceutical wholesalers, and over 140 community drug retail outlets (75 pharmacies and 65 drug stores). The city has an estimated projected population of 455 901 people, of whom 227 712 are females, as of 2022^[16,17^]. A systematic review and meta-analysis on the prevalence and risk factors of type 2 DM in Ethiopia demonstrated that the pooled prevalence of DM in Ethiopia is 6.5%^[18^]. Recent study reports indicated that the prevalence of DM in Bahir Dar city is estimated to be 7.5% for diagnosed cases and 10.2% for undiagnosed adult cases^[19,20^]. A laboratory-based cross-sectional study was conducted from March to June 2022. The reason for selecting this study area was the high population density, the higher prevalence of diagnosed and undiagnosed diabetic cases, and the availability of a large number of community pharmacies, drug stores, wholesalers, and private health organizations (hospitals and clinics). Hence, many clients have access to both prescribed and non-prescribed (over-the-counter) drugs in the study area. Most of the governmental and community pharmacies in the Amhara region purchase pharmaceuticals from this city, as it is the center and capital of the second most densely populated region in Ethiopia.

Sampling and sample size determination

Sampling design, sampling, and sample size determination were adapted with some modifications from a previous study conducted by Abebe et al^[17^]. The number of available metformin HCl tablet brands in Bahir Dar city during the study period was determined after conducting surveillance in community pharmacies prior to the actual data collection. Six brands (four imported and two manufactured in Ethiopia) were found to be available in the market during the surveillance. These six brands of the drug were selected based on their widespread availability in the market, high prescription rates, and frequent use by patients. This selection ensured a representative assessment of commonly used formulations. Subsequently, all the registered community drug shops were listed alphabetically, numbered, and coded. To avoid repetition of drugs to be sampled, 15 community pharmacies (10 pharmacies and 5 drug stores) were randomly selected using a lottery method. Pharmacy personnel, who were trained and acted as simulated caregivers, visited each of the selected community pharmacies with a prescription for metformin tablets. For each brand, a total of 120 tablets were purchased from the selected community pharmacies. After collection, all samples were subjected to visual inspection, and the collected samples were then randomly coded from “A” to “F” and immediately registered with their expiration dates, batch numbers, manufacturers, and countries of origin. The samples were kept in their original packaging, transported to Bahir Dar, EFDA laboratory of drug quality, and stored under the storage conditions specified on the label of each product until analysis. Detailed information about the samples is presented in Table 1. The experimental work was conducted at the laboratories of EFDA, Bahir Dar office, and the pharmaceutical laboratory rooms of GAMBY Medical and Business College, Bahir Dar, North-West Ethiopia. To ensure accuracy and reproducibility, all analytical instruments were calibrated before use following standard protocols. The validation of test methods was conducted in accordance with pharmacopoeial guidelines, including assessments of linearity, precision, accuracy, and robustness.

Table 1.

The coding and details of products used for the study

Product code	Country	Batch number	Expiration date
A	Cyprus	A5k111	October 2026
B	India	BNT0122078	December 2024
C	Germany	AES	September 2024
D	Cyprus	93627	March 2026
E	Ethiopia	D22018T218	August 2025
F	Ethiopia	01823120010	December 2026

In vitro quality control test methods

The collected brands of metformin HCl 500 mg tablets were tested for uniformity of dosage units, friability, hardness, thickness, disintegration time, dissolution, and assay according to procedures described in the USP^[17^]. The similarity in the % dissolution among the different brands of the formulations was also computed.

Thickness

Ten tablets were taken from each batch to assess the thickness of metformin tablets using a tablet caliper slide scale (Sotax HT, Model: HT 1500 N, Switzerland). The results from the measurements were presented as the mean and standard deviation^[21^].

Hardness

The tablet hardness was examined by randomly taking 10 tablets from each brand using a hardness tester (Sotax HT, Model: HT 1500 N, Switzerland). Each tablet was placed between the anvils, and the force required to break the tablet was measured. The results of the crushing force required were presented as the mean and standard deviation^[17,22^].

Friability

Initially, the weight of twenty metformin HCl tablets was measured using an electronic balance after de-dusting them, and they were placed into a friability tester (FTA-023, Single Drum, India) set to 100 revolutions in 4 minutes. After removing the tablets from the friability drum, the tablets were de-dusted, and the percentage weight loss was evaluated using the equation (Eq. 1) depicted below^[23^].

$$\mathrm{\%}\mathrm{F}\mathrm{r}\mathrm{i}\mathrm{a}\mathrm{b}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{y}=\frac{\mathrm{I}\mathrm{n}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}-\mathrm{F}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l}\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}{\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}}\times 100$$ (1)

Weight variation test

The weight variation test was conducted by taking twenty tablets from each batch and weighing them individually using a digital analytical electronic balance. The results for the individual tablets were presented as the mean weight and standard deviation^[21^].

Disintegration test

The disintegration test was conducted using beakers containing 900 ml of dissolution medium (distilled water) at 37 ± 0.5°C. Afterward, the tablets were placed in the basket rack and set into the disintegration apparatus (ERWEKA, Germany). The time required for disintegration was then measured and presented as the mean value and standard deviation^[5,24^].

Construction of a calibration curve for metformin hydrochloride

The calibration curve was constructed using a metformin HCl reference standard solution with various concentrations derived from the stock solution (2, 3, 4, 5, 6, and 7 μg/ml) in phosphate buffer as the medium (pH = 6.8). The absorbance was evaluated at the λ max of 233 nm using a UV-Visible spectrometer (ERWEKA, T80+ Spectrophotometry, Double Beam, Ver 3.3, PG Instruments Ltd).

In vitro drug release study

The dissolution test for metformin tablets was evaluated using USP apparatus II (paddle method). Six tablets were taken from each batch, and the dissolution medium (phosphate buffer) was used in a beaker containing 900 ml at pH 6.8 and 37 ± 0.5℃. The paddle method was employed, with the rotation set at 50 rpm. The sample was filtered using Whatman filtration equipment (0.45 μm PVDF W/GMF) after taking sample solutions (5 ml) at 10, 20, 30, 45, 60, 75, and 90 minutes. Proper dilution was then made, and the absorbance was recorded using a UV/Vis spectrophotometer at 233 nm. The dissolution medium was utilized as a blank. The amount of drug released at each time point was assessed using a calibration curve, and the drug release profile was statistically analyzed^[6^].

Similarity (f2) factor

The similarity factor (f2) is a logarithmic reciprocal square root transformation of the sum of squared errors and is a measurement of the similarity in the % dissolution between two curves. The following equation (Eq. 2) was used to calculate f2 from the results of in vitro drug release.

$$f2=50\log ((1+1/n{\displaystyle \sum _{i=1}^n{(rt-Tt)}^2){)}^{-0.5}}\times 100)$$ (2)

where Rt and Tt are the cumulative percentages of the dissolved drug for the reference and test formulations at time t, respectively, and n is the number of time points.

Assay of metformin HCl tablets

The assay test was conducted as per USP guidelines to determine the actual amount of active ingredient present in the tablets of each brand compared with the labeled amount. First, a 10 μg/ml concentration of metformin HCl reference standard in water was prepared as a standard solution from the prepared stock solution (500 μg/ml). Then, a sample solution was prepared by taking samples from each brand. Twenty tablets of each brand of metformin hydrochloride tablets were weighed and finely powdered. A quantity of the powder equivalent to 100 mg of metformin HCl was weighed and transferred into a 100-ml volumetric flask and dissolved in about 70 ml of water. After mechanically shaking for 15 minutes, it was diluted with water to volume and filtered using Whatman filter paper. Ten milliliter of the filtrate was diluted with water to 100 ml after discarding the first 20 ml of the filtrate. Finally, 10 ml of the resulting solution was further diluted with water to 100 ml to obtain the nominal concentration of a 10 μg/ml solution. The absorbances of the standard and sample solutions were determined using a UV/Visible spectrophotometer at 232 nm, with water as a blank. The percentage of the labeled amount of metformin HCl in each brand was calculated using the following equation (Eq. 3):

$$Assay(\mathrm{\%})=\left(\frac{A_u}{A_s}\right)\times \left(\frac{C_s}{C_u}\right)\times 100$$ (3)

Data quality control

Data were checked for completeness and consistency. The quality of experimental results was assured by performing system suitability tests and by strictly applying the procedures described in the specified monographs of the pharmacopoeia. The work has been reported in line with the STROCSS criteria^[25^].

Statistical analysis

The overall results of the experiment were statistically analyzed using origin lab software (Origin Lab Corporation, MA, USA). All the results data were evaluated three times and presented as mean ± standard deviation.

Results and discussion

Tablet mechanical strength

Tablets require appropriate strength to resist damage during storage, transportation, and handling, but the hardness of tablets should not interfere with proper disintegration after swallowing. Additionally, the dissolution rate of tablets is influenced by their strength or hardness. Friability, disintegration, and dissolution of tablets are affected by hardness or crushing strength, which is an important parameter. Tablet machine force control during tablet compression is important to limit tablet hardness. Excessive tablet hardness can result in failure to disintegrate within the required period in a suitable medium, while very soft tablets may lose their shape due to mechanical forces during manufacturing processes, such as coating, packaging, and transportation operations^[24^]. Hardness is one of the characteristics of a tablet that is examined to assess its ability to maintain its form under normal condition^[25^]. Tablet hardness, and tablet disintegration and dissolution have a direct relationship. Excessive hardness of tablets influences dissolution and absorption rates, bioavailability, and the onset of action of the drug by prolonging disintegration time. Conversely, tablets with poor strength are unable to resist forces during distribution, storage, and usage, compromising their integrity. Therefore, during tablet compression, it is important to fix and manage tablet hardness within an acceptable range^[26^]. Many factors influence tablet hardness, such as the amount and type of binder used and the compression force applied during production. The friability test is one of the methods used to assess tablet hardness and is designed to evaluate the tablet's ability to withstand changes during manufacturing processes, such as coating, packaging, handling, and distribution. To achieve high consumer acceptance, tablets should have suitable hardness as well as reasonable friability^[21^].

The mean values of the strength-related properties and the thicknesses of the different metformin HCl tablet brands are depicted in Table 2. The lowest mean tablet hardness (105.8 N) was observed for brand A, and the highest value (146.2 N) was obtained for brand F. Most of the time, tablets having hardness strength greater than 40 N are believed to meet the standard^[26^]. All of the examined tablets showed a crushing strength higher than 40 N. Based on the USP Convention, the friability value of tablets should be less than 1%^[27^] and the results showed that all the brands of metformin HCl tablets under this investigation had friability values ranging from 0.11% to 0.26%. Thus, all brands passed the strength test based on percentage friability. The nature and amount of binder used, as well as the compression force, are factors that affect the friability of tablets^[28^]. Nevertheless, while thickness and diameter are not pharmacopoeial requirements, they do have an effect on packaging and should be considered in the calculation of the tensile strength of tablets. Therefore, these factors should be kept within the minimum range of their mean values ^[25^].

Table 2.

Thickness, hardness, friability, and disintegration profile of metformin HCl brands

Product	Thickness (mm)	Hardness (N)	Friability	Disintegration time (min)
Code	(mean ± SD)	(mean ± SD)	%	(mean ± SD)
A	5.64 ± 0.21	105.8 ± 9.81	0.26	9.15 ± 0.21
B	5.38 ± 0.18	116.3 ± 7.34	0.12	10.38 ± 0.35
C	5.85 ± 0.35	127.4 ± 4.92	0.15	9.85 ± 0.18
D	6.01 ± 0.26	118.5 ± 8.81	0.22	10.04 ± 0.30
E	5.75 ± 0.25	133.7 ± 9.16	0.26	9.95 ± 0.28
F	5.90 ± 0.37	146.2 ± 6.77	0.11	11.25 ± 0.14

N, Newton; SD, Standard Deviation.

Tablet disintegration property

The tablet disintegration test is used to determine the time required for a tablet to entirely break down in the gastrointestinal tract. The quality of tablets depends on disintegration time, which influences the drug release rate^[29^]. Tablet disintegration is a precondition to dissolution, which subsequently leads to the absorption of a drug from the gastrointestinal tract. The rate of disintegration is directly proportional to the rate of dissolution. Disintegration rate is affected by the amount and time of water inflow into the tablets. When a tablet disintegrates, the active pharmaceutical ingredient (API) is released promptly because the rate of disintegration affects the dissolution rate and, consequently, the therapeutic effects of a drug^[21,25^]. Tablet disintegration is influenced by the type of excipients and processes used by different producers during formulation, which subsequently impacts the bioavailability of the drug. Therefore, tablet disintegration is one of the factors that affect the rate of drug absorption^[12^]. The British Pharmacopoeia (BP) stipulates that the disintegration time for uncoated tablets should not exceed 15 minutes, while film-coated tablets should disintegrate within 30 minutes^[30^]. The disintegration time data from the experiment showed that all the brands comply with the pharmacopoeial limits, falling within the range of 9.15–11.25 minutes. Low values of disintegration time are most often attributed to the presence of large amounts of disintegrants or lower binder concentrations, in addition to the manufacturing process. Additionally, tablets with good mechanical strength are expected to have relatively higher disintegration time profiles. In this study, it was observed that brand F, characterized by higher crushing strength and lower friability, exhibited the highest disintegration time.

Weight variation

Weight variation is appropriate for the test of Uniformity of Dosage Form if uncoated or film-coated tablets contain 25 mg or more of the drug substance, comprising 25% of each tablet’s weight^[8^]. Deviation from the weight variation limit may result in the absence of a sufficient API crucial for the therapeutic effect of the medicine, which might lead to a decrease in the therapeutic activity of the medicine and cause a reduced treatment outcome. The difference in types of excipients used, i.e., diluents, disintegrants, lubricants, and glidants, during the manufacturing process might lead to weight variation. This variation might also be associated with differences in the manufacturing procedure^[3^]. According to BP, a deviation of ±5% from the mean tablet weight for uncoated or film-coated tablets weighing 250 mg and above is acceptable under the uniformity of weight test. Furthermore, for the batch to pass the weight test, at least 18 of the 20 tablets must be within the ±5% range, and all tablets must be within ±10%^[30^].

The mean weight of the tablets of the six brands ranged from 559.04 to 627.97 mg. All brands, except brand E, showed acceptable weight uniformity. Compliance with the weight uniformity test is one pre-request to avoid variation in content uniformity and, ultimately, the probability of a sub-therapeutic dose or over dose^[2^]. Brand E deviated from the standard because one of the tablets in the sample deviated by more than 5% of the mean weight, as stated in the BP^[29^]. This could be attributed to poor flow properties of the granules, deprived feeding of granules into the die, and poor control of some other process parameters during manufacturing^[28^]. The details of the weight uniformity evaluation results are presented in Table 3.

Table 3.

Weight uniformity evaluation results of metformin HCl tablet brands

Product code	Weight (mg) Mean ± SD	Minimum % weight variation	Maximum % weight variation	Number of tablets out of specification
A	585 ± 5.05	579.95	590.05	None
B	564 ± 4.96	559.04	568.96	None
C	602 ± 8.84	610.84	593.16	None
D	625 ± 6.22	618.78	631.22	None
E	641 ± 13.03	627.97	654.03	1
F	605 ± 8.25	596.75	613.25	None

In vitro drug release

Dissolution testing is used to predict drug characteristics in vivo and to determine the effects of binders, mixing, granulation procedures, and excipient types on tablets. It is an important control test for assuring product consistency and batch-to-batch equivalence of products. Dissolution testing is applied as an in vitro bioequivalence test to identify the similarity of solid dosage forms^[3,8,24^]. Dissolution testing provides information about the drug dissolution rate, which determines how a drug is made available in the body to produce an optimum therapeutic effect^[26^]. Evaluating the therapeutic effects of two or more formulations containing the same API is a critical factor in using the innovator and other similarly formulated drug products interchangeably^[31^]. To ensure product similarity in terms of drug release and post-approval changes, sufficient evidence about drug dissolution data is required. Products with different formulations, APIs, and product designs may have different dissolution characteristics, which may subsequently lead to different bioavailability^[32^]. In vitro dissolution testing is more useful for indicating the bioequivalence of drugs than in vivo pharmacokinetic studies. Additionally, in vitro dissolution studies provide insights into noninvasiveness, lower costs, better assessment of product performance, and improved ethical considerations for the product^[24^].

Factors that affect drug dissolution include the physicochemical properties of the drug, drug design, mechanical strength of the tablet, manufacturing procedure, and testing conditions (i.e., apparatus, agitation, medium, etc.). Studies have reported that different excipients were tested in metformin tablets manufactured via wet granulation and determined that the amount and type of excipients used in the metformin products modified their dissolution behavior ^[32,33^]. A higher disintegration rate indicates faster break down of a tablet and consequently enhances the dissolution of the API into the bloodstream, thus increasing bioavailability. A good in vitro–in vivo association can allow the use of in vitro dissolution testing to estimate a product's in vivo performance, and therefore, in vitro dissolution profiles can be employed as a substitute for in vivo bioavailability studies^[21^]. Specifically, in vitro dissolution studies can replace expensive and time-consuming in vivo dissolution studies to identify the theoretical consistency of generic formulations^[1^]. According to USP recommendations, the quantity of metformin hydrochloride released within 30 minutes from the tablet should be ≥ 80% of the amount stated on the label claim^[28^].

As shown in Fig. 1, the results from the calibration curve revealed the presence of a very strong positive relationship (r2 = 0.9993) between the concentration and absorbance over the concentration range of 2–6 μg/ml. All the generic products released about 80% of metformin hydrochloride within 30 minutes, hence complying with the USP dissolution tolerance limit with almost similar drug release properties.

Figure 1.

(a) Calibration curve; (b) dissolution profile of six metformin brands.

Similarity factor (f2)

To substitute one product with another that contains identical APIs, equivalence in therapeutic effect should be examined and approved. According to the FDA of the US guidelines, the profiles of testing products and comparators are evaluated using the implemented similarity factor (f2), and when 2 profiles are identical, f2 equals 100. The similarity factor is an independent parameter used to evaluate the dissolution rate of samples from different brands^[16^]. The FDA, European Medicines Agency, and WHO have suggested the use of f2 to identify similarity between the dissolution data of drug products^[32^]. To conclude that products are bioequivalent based on dissolution profiles, the similarity factor (f2) should be within the range of 50 to 100^[20^]. Currently, the similarity factor is a major concern in many regulatory policies^[24^]. The similarity factor (f2) values of the studied generic products are presented in Table 4. The f2 values for the generic products were found to be in the range of 50–71. Significant differences were not observed in both parameters and this confirmed similarity between all brand formulations. Thus, all brands can be considered equivalent in dissolution profiles, as their f2 values were greater than or equal to 50. Among all generic products, brand A showed a higher f2 value (71), suggesting it is a more potential substitute for the innovator product than other products. This is because as the f2 value becomes higher and closer to 100, the tested generic product exhibits greater similarity to the comparator with respect to the dissolution profile^[21^].

Table 4.

The similarity factor of different metformin HCl tablet brands

Brands	A	B	C	D	E	F
f2 value	71	58	51	53	59	50

Assay of metformin HCl tablet

The amount of API found in a product influences the quality and efficacy of its therapeutic effect. Products with inappropriate levels of API produce unnecessary therapeutic outcomes, i.e., treatment failure, increased patient morbidity, and mortality. An assay test is a critical method used to measure the amount of API present in a drug and determine the quality of products to optimize the release of the drug from the product. Poor quality and adverse outcomes of treatment occurred due to substandard APIs found in the product^[3^]. The presence of metformin hydrochloride outside the standard range has severe impacts on patient health care. Amounts of metformin hydrochloride below the limit result in glucose accumulation, leading to treatment failure, resistance, and severe complications, which may ultimately worsen the patient’s health status. Conversely, metformin hydrochloride above the limit results in severe adverse effects, i.e., hypoglycemia, organ failure, coma, or death^[26^]. According to the BP, metformin hydrochloride tablets should contain 95–105% of the API as indicated on the label claim upon assay^[30^]. Assay non-compliance has been a principal problem with metformin hydrochloride tablets in many countries^[34^]. Under this study, all the tested brands complied with this pharmacopoeial specification. The lowest and highest mean percentage drug contents in the brands under investigation were 100.35% (for brand B) and 104.63% (for brand F), respectively. Almost all of the tablets in the brands showed drug contents higher than 100%, which may be caused by inaccuracies in weighing the API, poor mixing during granulation, and the use of excess amounts of API during tablet formulation. This particular problem has also been reported in similar studies ^[14,28^]. The content assay of the formulation is depicted in Table 5.

Table 5.

The content assay results of different metformin HCl tablet brands

Brands	A	B	C	D	E	F
Assay (%)	102.20 ± 1.64	100.35 ± 0.95	104.44 ± 1.48	103.27 ± 1.92	101.75 ± 2.05	104.63 ± 2.41

In general, this study demonstrated that the six brands available in the pharmaceutical market of Bahir Dar city passed almost all of the tests conducted for their equivalence evaluation, except for the deviation of one brand (E) in the weight uniformity test. This is a very commendable result, as compliance with all official and non-official pharmacopoeial tests is required for acceptable quality, safety, and efficacy of pharmaceuticals. This finding aligns with study reports from Addis Ababa and Jimma cities in Ethiopia, where all brands of metformin hydrochloride tablets met pharmacopoeial specifications for the tested parameters of physicochemical properties, such as weight variation, tablet hardness, disintegration time, drug release studies, and assay^[3,21,30^]. Similar results were obtained from China^[1^], Albania^[7^], India^[8,10^], and Uttarakhand^[35^]. Even though the sampled metformin hydrochloride tablets in the current study area showed acceptable evaluation results, various studies in Ethiopia and abroad have reported non-compliance with one or more of the tests. In two different studies conducted in Tigray (Ethiopia), results indicated differences in f2^[31^] and dissolution profiles among some generic brands^[22^]. Similar non-compliant results in one or more test parameters were also reported in studies from developing and developed nations, including Iran^[9^], Saudi Arabia^[23^], Ghana^[28^], Canada^[32^], Kenya^[33^], Egypt^[34^], and Libya^[36^]. As in vitro–in vivo bioequivalence correlation remains a subject of debate for metformin hydrochloride tablets, further in vivo bioequivalence (BE) studies should be conducted in accordance with the ICH guidelines on Good Clinical Practice and the Guidance for Industry on Bioavailability and Bioequivalence Studies. These studies can help confirm the bioequivalence of the brands with supportive evidence derived from pharmacokinetic parameter findings after in vivo investigations. It must be demonstrated that the safety and efficacy of the generic drugs are comparable to those of the innovator drugs^[37^].

Conclusion

From this study, it can be concluded that the six metformin HCl brand tablets available in Bahir Dar city have comparable physicochemical and drug release properties based on pharmacopoeial test parameters. Hence, they can be considered bioequivalent and interchangeable, provided their safety and other therapeutic parameters not covered in this investigation remain similar without significant deviations for each brand. This study had a limitation in that the in-vitro dissolution test was extrapolated to gain insight into in-vivo bioavailability instead of using actual in-vivo bioavailability data. However, an in-vivo bioequivalence study is recommended to confirm the therapeutic similarity between drugs over time. Positive results from such studies should not negate the need for constant post-marketing monitoring of the marketed products to ensure bioequivalence and compliance with pharmacopoeial standards. Further ongoing post-marketing surveillance is highly recommended for other generic brands available in the market to ensure the safety of generics and their interchangeability with both other generics and the innovator product.

Ethical approval and consent to participate

Ethical approval was obtained from the Ethical Review Committee of GAMBY Medical and Business College with the reference number ጋኮ432/2015 in May 2023. The study included community pharmacies that provided their informed consent to participate and supply different brands of metformin 500 mg tablets after receiving a detailed explanation of the study's aim and objectives. Subsequently, the data was collected following consent and in accordance with the relevant guidelines.

Consent for publication

Not applicable.

Sources of funding

No Funding Available.

Author contributions

E.A.S. works on conceptualization, data curation, formal analysis, investigation, methodology, project administration, supervision, validation, visualization, and writing – original draft preparation. A.T.Y., A.S., A.T.T., C.T., T.A.T., S.Z., B.M., S.Y., and Z.D.A. contributed to conceptualization, formal analysis, methodology, validation, writing – review and editing. All authors contributed to data analysis, drafting or revising the article, agreed on the journal to which the article will be submitted, gave final approval of the version to be published, and agreed to be accountable for all aspects of the work.

Conflicts of interest disclosure

The authors declare that they have no competing interests.

Research registration unique identifying number (UIN)

Not applicable. Since the study is experimental and does not involve human participants.

Guarantor

The corresponding author will serve as the guarantor.

Provenance and peer review

Not commissioned and externally peer-reviewed.

Data availability statement

All data are included within the article.

Acknowledgements

GAMBY Medical and Business College is gratefully acknowledged by the authors for providing the available materials, equipment, and reagents to conduct the studies. EFDA is also acknowledged for providing laboratory equipment and metformin HCl standards.