## 1 Background  
Antibiotics, especially aminoglycos, have been notably linked to the development of nephrotoxicity that may end in acute kidney injury (AKI). Gentamicin (GM), a major aminoglycosic family member with low bacterial resistance rates (when compared with the older aminoglycosic members, such as streptomycin [1]). Moreover, the prevalence of aminoglycosic resistance remained relatively low when compared with that for cephalosporporins or fluoroquinolones [2]. Although, GM is commonly used in the treatment of life threatening conditions caused mainly by Gram-negative bacteria, its clinical uses are greatly limited because of its nephrotoxicity [3].  
Although GM nephrotoxicity has been attributed mainly to tubular cytotoxicity that ranges from loss of brush border of epithelial cells to tubular necrosis [4], the exact main site of tubular damage is still under investigation. Very early, in 1980, [PERSON]' et al., proposed a central mechanism for the gentamicin-induced AKI that involved the accumulation of the drug in the lysosomes of the cells of the proximal tubule with subsequent inhibition of specific enzymes activities. In addition, lysosomal accumulation of the drug led to altered properties of the lysosomal membrane permeability [5]. More recently, gentamicin-induced AKI had been attributed mainly to apoptosis of the proximal tubule and collecting cells [6]. Thus, the precise mechanism of GM-induced nephrotoxicity is still not fully clarified, although the principle pathophysiology involves the generation of reactive oxygen species (ROS), apoptosis, increased endothelin-1 levels, and increased cellular infiltration [7]. [PERSON] et al. showed that increased ROS generation resulting from GM-induced nephrotoxicity results in the inactivation of many enzymes involved in oxidative stress [8]. It had been reported that GM promotes AKI through the generation of ROS, such as superoxide anions [9], hydroxyl radicals, hydrogen peroxide [10], and reactive nitrogen species (RNS) in the kidney [11]. Excess generation of ROS is responsible for the pathogenesis of acute tubular necrosis (ATN) and a reduced glomerular filtration rate (GFR), which are hallmarks of drug-induced nephrotoxicity [12]. Therefore, studies aiming to minimise GM-induced nephrotoxicity should be based on the concurrent use of reno-protective agents, including ROS scavengers and antioxidant agents. At the histopathological level, the previous studies revealed that gentamicin-induced nephrotoxicity involved both glomerular and tubular changes in the form of tubular necrosis, apoptosis of glomerular mesangial cells [13], tubular degeneration associated with swelling, cytosis, and tubular irregularities [14].  
TQ, the main volatile product of _Nigella sativa_, and MF (a well-known oral hypoglycaemic drug) both have known antioxidant effects, and as such could potentially help alleviate kidney damage in patients undergoing treatment with GM. In 2011, [PERSON] and [PERSON] reported that TQ has antioxidant effects and possesses a notable capability to prevent beta-cell damage in diabetic rats [15]. In addition, TQ has been extensively investigated as an anti-inflammatory [16], gastoprotective [17], and tumour anti-proliferative therapeutic agent [18]. MF, on the other hand, has been reported to have novel antioxidant and anti-inflammatory effects in type 2 diabetic patients [19].  
Based on the findings that TQ and MF have antioxidant properties, the present study aimed to evaluate the efficacy of both TQ and metformin as possible reno-protective agents in GM-induced nephropathy in the same model (rats) at the same time. In addition, we aimed to further understand the effects underlying GM-induced nephrotoxicity and whether or not the antioxidant effect alone could protect against this nephrotoxicity.