Drugs and chemicals
All single-drug tablet formulations of 50 mg diclofenac sodium that were commercially available in retail pharmacies in Riyadh, Saudi Arabia during the period December 2019/January 2020 and were within 25%–75% of their manufacture-expiration period were included in the study. Label information of the included formulations (a reference and seven generic (G1 to G7) formulations) is presented in (Additional file 1: Table S1, Label information). Six other generic formulations were listed on the Saudi Formulary  but were not commercially available and were not included in the study.
Diclofenac sodium standard was purchased from Sigma-Aldrich (St Louis, MO, USA), HPLC grade acetonitrile and methanol from Fisher Scientific Co. (Loughborough, UK), disodium hydrogen phosphate from Fluka (Buchs, Switzerland), potassium phosphate monobasic and glacial acetic acid from Fisher Chemical (Fair Lawn, New Jersey, USA), and hydrochloric acid (HCl) from Merck (Darmstadt, F.R. Germany).
HPLC-dissolution system (Waters Associates, Inc. Milford, MA. USA) consisted of Waters 2690D Separation Module with eight-needle dissolution dispenser, Waters Transfer Module with eight syringes, one dissolution test bath (Hanson Research SR8-Plus, USP dissolution apparatus II (paddle)), eight Uni-Probes, and Waters 996 Photodiode array detector set at 276 nm. Other instruments used included an electronic balance (Model AG 204, Mettler Toledo, Greifensee, Switzerland), as well as Microprocessor Disintegration Test Apparatus (Model SSE-731), Microprocessor Friability Apparatus (Model SSE-710), and Digital Tablet Hardness Tester (Model SSE-DIGIT AB-SPV), all from Sunshine Scientific Equipments, Delhi, India.
Sample preparation and HPLC assay
A stock solution of diclofenac sodium (1000 µg/ml) was prepared in methanol and stored at − 20º C. It was diluted in a phosphate buffer (pH 6.8 ± 0.05) composed of 0.05 M disodium hydrogen phosphate and 0.05 M potassium dihydrogen phosphate (50:50, v:v) to produce standard curve (0.1, 0.5, 1.0, 5.0, 10.0, 20.0, 40.0, 60.0 and 80.0 µg/ml) and quality control (1.5, 7.5, 15, and 50 μg/ml) samples. This phosphate buffer was also used for disintegration and dissolution tests and for determining active substance content (ASC). A standard curve and three sets of quality control samples were used in each run. The HPLC assay uses liquid–liquid extraction and naproxen as an internal standard, and is linear (R2 ≥ 0.998) in the range 0.1–80.0 µg/ml . It was used to determine ASC in tablets and the dissolution profile. No interference from tablet’s excipients was observed.
Quality control tests and calculations
Weight variation test: 20 randomly-selected unites of each formulation were examined. Mean (SD) was calculated and% deviation of individual unit weight from mean weight of the formulation was determined.
Friability test: 20 randomly-selected unites of each formulation were examined. They were weighted, placed in the friabilator (25 revolutions/minute for 4 min), de-dusted and weighted again, and friability was determined as% weight loss.
Hardness test: 10 randomly-selected unites of each formulation were examined. Mean (SD) required pressure to break diametrically placed tablets was determined.
ASC test: 20 randomly-selected unites of each formulation were examined. Tablets were individually crushed using morter, dissolved in 100 ml methanol, filtered with a syringe using 0.2 µm filter, diluted with 9.0 ml phosphate buffer, and 100 µl was injected in the HPLC system. Mean (SD) content in mg and percent deviation of individual unites from label were determined.
Disintegration test: 6 randomly-selected unites of each formulation were examined using 0.1 N HCL for 2 h followed by phosphate buffer (pH 6.8) as disintegration medium. The basket rack was placed in a 1000 ml vessel containing 900 ml disintegration medium maintained at 37 ± 2 °C with the test unit remaining 1.5 cm below the surface of the liquid on their upward movement and above 2.5 cm from the bottom of the beaker in their downward movement. The basket rack moved up and down (5–6 cm) at a frequency of 31 cycles per minute. Range of disintegration time (time to no particle on the basket) was determined.
Dissolution test: 8 randomly-selected unites of each formulation were examined using 0.1 N HCL for 2 h followed by phosphate buffer (pH 6.8) as dissolution medium (900 ml), one unite in each vessel, a stirring rate of 50 ± 1 rpm, and a temperature of 37 ± 0.5 °C. The test ended with a stirring rate of 250 rpm for 15 min (infinity). A sample of 1.0 ml was withdrawn from a zone midway between the surface of the dissolution medium and the top of the rotating blade (not less than 1 cm from the vessel wall) and was immediately replaced with an identical volume of fresh medium. Samples were withdrawn at 60 and 120 min in 0.1 N HCl and at 10, 15, 20, 30, 45, 60, 90 and 105 min in phosphate buffer. 100 µl of the 1 ml samples were injected into the HPLC system. The vessel was kept covered for the duration of the test, the temperature of the mixture was verified at suitable times, and the behavior of the unit was observed throughout the dissolution testing. Mean (SD) amount released and% of label ASC released at each time point was determined. Time to release 50% of label ASC was also determined.