The molecular mechanism of contrast-induced nephropathy (CIN) and its link to in vitro studies on iodinated contrast media (CM)

Iodinated contrast media (iodinated CM) have increased ability to absorb x-rays and to visualize structures that normally are impossible to observe in a radiological examination. The use of iodinated CM may destory renal function, commonly known as contrast-induced nephropathy (CIN), which can result in acute renal failure (ARF). This review article mainly focuses on the following areas: (1) classifications of iodinated CM: ionic or non-ionic, high-osmolarity contrast media (HOCM), low-osmolarity contrast media (LOCM) and iso-osmolarity contrast media (IOCM); (2) an introduction to the physical and chemical properties of the non-ionic iodinated CM; (3) the management of anaphylactic reaction by iodinated CM; (4) a suggested single injection of adult doses and maximum dose for non-ionic iodinated CM; (5) the molecular mechanism of contrast-induced nephropathy (CIN); (6) In vitro studies on iodinated CM. Based on above information, this review article provide an insight for understanding the drug safety of iodinated CM.


Introduction
Iodinated contrast media (iodinated CM) absorb x-rays and visualize structures that are normally hard to observe in a radiological examination [1][2][3][4]. It has been used widely for many years. Contrast media provide an ability to enhance normal structures or pathological lesions, which makes these places look different † These authors contributed equally to this work. with high risk of adverse drug reactions (ADR) and renal toxicity. Since the late 1960s, the nonionic low-osmolar contrast media (LOCM) have been developed to better safety and replace ionic iodinated CM for clinical uses. In 1996, the US Food and Drug Administration (FDA) approved the iso-osmolar contrast media (IOCM), iodixanol (Visipaque ® ), to have a better safety profile [14]. Furthermore, discomfort such as pain and heat associated with the injection site was found to be lower when using iso-osmolarity contrast media (IOCM) than low osmolar contrast media (LOCM) [14]. It is low neuro-toxicity and low osmolality that are important to intrathecal route injected contrast media, such as Iopamidol (Iopamiro ® ) 300 and Iohexol (Omnipaque ® ) 300 [16,17]. Table 1 shows the biologic adverse drug reaction (ADR) difference between ionic iodinated CM and non-ionic iodinated CM. Currently used non-ionic iodinated CM in Taiwan and their chemical properties are summarized in Table 2. The chemical structures of non-ionic iodinated CM are shown in Fig. 2 [3,11,[18][19][20][21][22][23][24][25][26][27][28][29][30][31]. In Table 3, we summarized the suggested single injection of adult doses and maximum dose for non-ionic iodinated CM by intra-arterial route. In Table 4, we summarized the suggested single injection of adult doses and maximum dose for non-ionic [1, [5][6][7].
Based on the solubility, iodinated CM are divided into three groups: oily iodinated CM, water-soluble iodinated CM and water-insoluble iodinated CM [8][9][10]. Iodinated CM are usually classified into ionic iodinated CM and non-ionic iodinated CM [10,11]. Generally, ionic contrast media have higher osmolality, higher toxicity and higher anaphylactic reaction. Non-ionic contrast media possess lower osmolality, lower toxicity and lower anaphylactic reaction [12,13]. Based on the structure, iodinated CM are divided into four groups: ionic monomer, ionic dimer, nonionic monomer and nonionic dimer (Fig. 1). Based on the osmolality, iodinated CM are classified into high-osmolar contrast media (HOCM), low-osmolar contrast media (LOCM) and isoosmolar contrast media (IOCM). High-osmolar contrast media (HOCM) is characterized by osmolarity of above 1500 mOsm/ kg H 2 O. Low osmolar contrast media (LOCM) is characterized by osmolarities within a relatively wide range of 300-900 mOsm/ kg H 2 O. The iso-osmolar contrast media (IOCM) is characterized by osmolarity level similar to that of blood (290 mOsm/kg H 2 O) [14,15]. The osmolarity of high-osmolar contrast media (HOCM) is up to 7 or 8 fold greater than blood and has been associated  iodinated CM by intravenous route. In Table 5, we summarized the suggested single injection of adult doses and maximum dose for non-ionic iodinated CM by intrathecal route.

The adverse drug reaction (ADR) of iodinated contrast media and management
ADR caused by iodinated CM includes chemical and constitutional effects. Chemical effects are mainly referred as contrast-induced nephropathy (CIN) and will be discussed later. Anaphylactic reaction is the most common situation in constitutional effect and may cause mild symptom such as nausea and vomiting, dizziness, rash and itch, or chest discomfort, shock in more severe situation, or even death [21,23,28,29,32]. Iodinated contrast media cause little allergic reactions, especially for low-osmolar contrast media (LOCM). The incidence of adverse effect to LOCM is 2 to 7/1000, that of severe allergic reaction to LOCM is lower 1 to 4/100,000, and that of lethal rate to LOCM is around 2-9/1000,000 [33,34]. We should recognize adverse effects and receive early intervene to reverse bad situation. The management and treatment of adverse effects on anaphylactic reaction by Advanced Cardiovascular Life Support (ACLS) guideline is shown in Fig. 3. The Fig. 4 shows that management and treatment of anaphylactic reaction by iodinated CM is proposed in 2017 RSROC Contrast Media Manual [33]. There are several affecting factors for anaphylactic reaction by iodinated CM such as particularly allergy (arising from consuming sea foods or drugs), previous adverse reactions, history of asthma or bronchospasm, history of allergy, cardiac disease, dehydration, haematological and metabolic conditions (sickle cell anaemia, patients with thrombotic tendency), renal disease, neonates, old patients, anxiety and apprehension medications (β-blockers, interleukin-2 (IL-2), aspirin, NSAIDs) [33]. In addition, IOCM (ie, iodixanol (Visipaque ® )) are associated with the highest risk of causing a delayed hypersensitivity reactions. The incidence of delayed hypersensitivity reactions to IOCM is 10.9% and 5%-6% for LOCM [33,35,36]. Lasser et al. suggested that two doses of corticosteroid prophylaxis (32 mg of methyl prednisolone, orally 12 and 24 h before iodinated CM injection signification reduced the iodinated CM-induce anaphylactic reaction [13,34].

Molecular mechanism of contrast-induced nephropathy (CIN)
CIN is one of chemical adverse effects of iodinated CM. The pathophysiology of CIN is related to hemodynamic changes caused by vasoconstriction which makes a decrease of glomerular filtration rate (GFR) and a renal ischemia. Direct cytotoxic-  ity to renal tubular cell is another pathway leading to kidney damage [37][38][39][40][41][42][43][44][45][46][47][48][49]. Norbert H. et al. medullary ischemia and direct cytotoxicity to renal tubular cell are two main mechanism to result in CIN. Medullary ischemia is a complex result of vasoconstriction, lower oxygen delivery and higher oxygen demand. In Fig. 5, there are three factors such as increasing oxidative stress, enhancing renal vasoconstriction and inducing tubular cell damage responsible for CIN [50,51]. Several factors including renal ischemia, particularly in the medulla, reactive oxygen species (ROS) formation, reduction of nitric oxide production, tubular epithelial and vascular endothelial injury may be implicated in CIN. Many studies demonstrated that iodinated CM exert cytotoxic effects and renal tubular epithelial cells present severe cell death by autophagy and/or apoptosis [6]. Iodinated CM induces renal vasoconstriction by increase of adenosine and endothelin, and changes the blood flow from the medulla to the cortex and GFR are reduced. Reduction in renal blood flow can increase ROS release by oxidative stress. In tubular cells, iodinated CM directly caused osmotic necrosis or vacuolization leading to acute tubular cell death [15,[37][38][39]. Several antioxidant compounds have been demonstrated prevention effects by CIN, including sodium bicarbonate, N-acetylcysteine (NAC), ascorbic acid, statins, and recently, phosphodiesterase type 5 inhibitors [4][5][6][7]. The detailed molecular mechanisms of CIN are described in Fig. 6.

In-vitro studies on contrast-induced nephropathy (CIN) by iodinated CM.
In 2017 year, Charalampos Mamoulakis et al. summarize recent in vivo studies on oxidative stress related to CIN in animal mod-els and humans [6]. Hereby, we summarize recent in vitro studies on the mechanisms in contrast-induced nephropathy (CIN). Direct damage, a risk factor of CIN, induces cell death to renal tubular cells caused by iodinated CM. Table 6 is a summary of the manifestation of CIN which is collected from in vitro studies. Inhibiting cell proliferation and inducing cell death are found in renal cell lines including KRK52-E, LLC-PK1, HKCS, HK-2 at the concentration higher than 75 mgI/ml. Importantly, iodinated CM induced cell death no matter whether in LOCM or IOCM. Apoptosis and/or autophagy are two cell types in cell death [52][53][54][55][56][57][58]. Readers refer to our previous article for detailed molecular mechanisms of apoptosis and autophagy [59].

Conclusion
Autophagy and apoptosis were associated with the pathophysiology of CIN in in vitro reports. In conclusion, in vitro studies showed that increased cell death by apoptosis and/or autophagy was demonstrated in the kidney cell lines after the administration of iodinated CM. Inhibition of autophagy induced cell apoptosis suggested the protective role of autophagy in CIN. In the future, studies about how to reduce cellular stress and cell death by new methods or new compounds and understanding the details molecular mechanisms may be helpful for the development of new therapeutic strategies for the treatment of CIN.

Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.