## 1 Introduction  
Metformin (a biguainide) and sulfonylurea (such as glipizide and gliclazide) were clinically proven as efficient therapeutics for diabetes decades ago [1]. The literature reveals that these agents have a good safety profile and could be repurposed for various pharmacological applications beyond glycaemic control [2; 3; 4]. Glipizide was recently reported to ameliorate some neurodegenerative disorders [5]. Suppression of tumour growth and inhibition of angiogenesis were also reported with glipizide [6], which has been investigated in breast cancer [7], lung cancer [8] and prostate cancer [3]. Gliclazide was reported to have a unique antioxidant profile [9; 10], with some studies highlighting its vascular benefits [11; 12] and neuroprotective effect in peripheral neuropathy [10] and diabetic atherosclerosis [2]. Recent studies revealed the potential therapeutic role of gliclazide in lung adenocarcinoma [13] and colorectal cancer [14]. Metformin, on the other hand, is a pleotropic agent that plays a multifaceted beneficial role in aging, cancer, neuroprotection, inflammation and polycystic ovary syndrome, and ameliorating the oxidative stress [4]. Co-administration of biguaniides and sulfonylureas in diabetes management is frequently reported. The new pharmacological potentials of these therapeutics necessitate the availability of a quick, reliable, sensitive and easily accessible analytical tool for the analysis or the therapeutic monitoring of these agents. Several HPLC methods were reported for the analysis of these analytes, either individually [15; 16; 17] or with other therapeutics [18; 19; 20; 21; 22; 23]. Some of these reported methods are complicated [24; 25], or are time consuming [26; 27], or require special columns [28] or exhibit unsatisfactory detection sensitivity for the analyte(s) of interest [29]. Unlike gas chromatography and mass spectroscopy, reversed-phase HPLC methods are still the preferred tool for quality control and clinical laboratories for routine analysis and therapeutic drug monitoring of various pharmacological agents, as they do not require special instrumentation, and the analysis could be achieved in a reasonable time [29; 30]. The most common HPLC approaches in speeding up the analysis run are either increasing the flow rate of the mobile phase or shortening the stationary phase length (chromatographic columns) [31]. In fact, these approaches could partially enhance the throughput of individual analytes without improving the analytical run time for samples of mixed agents [31]. Conventional reversed-phase HPLC methods on C8 and C18 columns can be satisfactorily used in the analysis of semi-polar and ionic apolar agents such as sulfonylureas or even non-ionic substrates [32; 33]. However, retaining metformin on reversed-phase chromatographic columns is quite challenging due to its high polarity and small molecular size (minimal interaction/partitioning with the non-polar free silanol groups of the stationary phase) [29; 33]. The reversed-phase ion-pairing HPLC technique is an alternative approach that has versatile applications in the separation of neutral analytes and inorganic, organic or zwitterionic substrates, and was historically known as soap chromatography and often called ion-interaction chromatography or ion-pair adsorption [34; 35; 36]. The technique primarily relies on the adsorption of hydrophobic surfactant ions known as ion-pairing solute (added to the mobile phase) to dynamically modify the surface of the stationary phase so that they can interact/couple with analyte ions of opposite charge to improve their retention on C18 columns [34; 35; 36]. on the other hand, the choice of the organic modifier (methanol or acetonitrile) in the mobile phase helps to predict and customise the elution pattern of analytes of same charge by improving their solubility as well as the wettability of the stationary phase [34; 35; 36].