1. O3 Therapy Protects the Kidney against Ischemic Damage
Ischemic damage in renal tissue occurs when kidneys experiment periods of diminished or restricted blood supply. In contrast, oxidative damage occurs when tissue is re-oxygenated, which might happen during experimental procedures in rats, such as clamping and unclamping renal pedicle, or during renal transplantation
[1]. This kind of damage is proposedly produced through xanthine oxidase (XO). This enzyme degrades nucleotides upon cell ischemia. However, after O
2 reperfusion, XO forms uric acid and high quantities of superoxide radical, which further produces oxidative stress
[2]. This explains why treatment with XO inhibitors, such as tungsten
[2], allopurinol
[3], or even XO knockout models
[4], ameliorates ischemia-reperfusion injury (IRI) and oxidative stress after short periods of ischemia. Finding auxiliary treatments for oxidative damage is clinically important since ischemic-producing scenarios are highly prevalent. Just in 2010, for instance, more than 2 million patients received renal transplants
[5].
O
3 therapy has previously been used before IRI (preconditioning)
[6][7][8][9][10][11] or after IRI (postconditioning)
[12][13][14][15][16] and has been described as a potential treatment (
Table 1). O
3 therapy is demonstrated to act with similar efficacy, but not synergic, to that achieved when IRI preconditioning is made with other protective strategies, such as inducing short, repeated periods of ischemia before the main IRI. This prepares the renal tissue against the IRI via similar controlled mechanisms as that of the O
3 and is called ischemic (pre)conditioning
[17]. Interestingly, when administered after the main IRI, ischemic postconditioning in conjunction with O
3 therapy upregulate beneficial effects and even diminishes cell death
[18]. After transplantation, rats also show a protective effect against the oxidative state when treated with O
3 [19][20]. Antioxidant enzymes are also upregulated in cultured kidney cells after they were submitted to hypoxia and reoxygenation
[21].
Table 1. Ozone (O3) effects on ischemic damage models.
Nitric oxide (NO) and NO synthase (endothelial, eNOS, and inducible, iNOS) have been proposed as oxidants that damage renal tubules through highly reactive peroxynitrite
[22]. However, NO was found to be a protective mechanism favored by O
3 therapy against IRI inflammation and vasoconstriction caused by Endothelin-1
[7][8]. In fact, nitrate-derived NO, when applied topically, is an effective therapy against IRI damage
[23].
In summary, O3 therapy, either before or after IRI, improves kidney damage by decreasing markers of kidney damage, inflammation, and fibrosis. Therefore, it is a good treatment for ischemic injuries such as kidney transplantation, iatrogenic trauma, partial nephrectomy, heart failure, and hypovolemia, among other prevalent clinical conditions that reduce renal blood flow, such as those that produce AKI.
2. O3 Therapy Protects the Kidney against Xenobiotic-Induced Damage
Xenobiotics are exogenous chemicals not synthesized by a certain organism; therefore, they are not essential for its physiological functions and processes. That way, synthetical drugs, metals, and environmental factors, amongst others, are considered as such
[24]. In this section, the mechanisms through which some of these xenobiotics cause nephrotoxicity will be discussed; along with the described protective effects of O
3 therapy against it, looking forward to discovering the usage of new therapeutic alternatives against damaging products we are constantly in contact with (
Table 2).
Acetaminophen (APAP), a common anti-inflammatory drug, has been demonstrated to produce severe nephrotoxicity
[25]. Proposed mechanisms include APAP’s hepatic degradation and further enzymatic formation of a highly toxic and reactive metabolite, N-acetyl-p-benzoquinone (NAPQI), which glutathione (GSH) normally neutralizes. However, in APAP overdose, NAPQI is formed in major quantities, proving uncontainable by antioxidant enzymes, and therefore producing oxidative damage, especially in proximal tubules
[26]. O
3 therapy has proven to be an effective antioxidant therapy by enhancing antioxidant enzymes and diminishing oxidation
[25]. Interestingly, the administration of O
3 therapy in APAP induced nephrotoxicity, when combined with another antioxidant therapy, N-acetylcysteine (NAC), produced no significant changes in the kidney’s function (creatinine, urea) and inflammation (IL-6, IL-10) but did produce significant changes against oxidative stress, showing lower levels of MDA, as well as a reduction of histopathologic glomerular, tubular, and interstitial damage
[27].
Cadmium (Cd) is a heavy non-essential metal that is accumulated in body tissues progressively
[28] and to which humans are exposed through air particles
[29], occupational exposure
[30] and seafood such as mollusks, crustaceans, or fish
[31]. Cd can produce nephrotoxicity by many mechanisms, including DNA damage, altered gene expression, and, most importantly, oxidative damage by depleting cells’ antioxidant defenses, such as selenium, which binds to Cd to neutralize it
[32]. Other proteins, e.g., metallothionein (MT), bind Cd in others to diminish its toxicity in organs such as kidneys and testis
[33][34]. O
3 therapy can diminish Cd accumulation, augment MT levels, and reduce morphologic damage, serving as an effective protective mechanism against Cd
2⁺ renal damage
[33]. It also reduces N-acetyl-β-D-glucosaminidase (NAG)
[35], a lysosomal enzyme found mainly in proximal convoluted tubules, its function is the digestion of cell’s glycoconjugates
[36]. The NAG increase is mediated by loss of the tubular brush border, thus liberating the enzyme into the urine
[37]; such an increase is associated with pathologic processes such as Cd intoxication and malignancies of the kidney, liver, pancreas, lung, and breast, amongst many others
[35][38], as well as an increased risk of requiring dialysis treatment and lethality in hospitalized patients
[37]. Even when stimulating lipid peroxidation, as a result, O
3 was also demonstrated to induce antioxidant enzymes in Cd-treated rats
[39].
Some antineoplastics are proven to cause nephrotoxicity. For instance, doxorubicin, often known as Adriamycin, binds to cell membranes and inhibits nucleotide replication. However, it can be oxidized into forming reactive species like hydroxyl radicals
[40]. It is demonstrated to cause severe progressive damage, fibrosis, and proteinuria
[41]. O
3 therapy, in certain doses, has proven to mediate protective effects against this morphologic damage, and arterial pressure, as well as proteinuria, have been ameliorated in rats receiving this treatment
[42].
Another example is cisplatin (CDDP), an FDA (American Food and Drug Administration) approved treatment for advanced solid cancers such as that of the testis, ovary, and bladder
[43]. CDDP is a molecule composed of a single platinum atom bound to chloride and ammonium; due to its small size, it filtrates freely into the glomerular barrier without tubular reabsorption
[44]. It then enters tubular cells and dissociates into its toxic components, which damage DNA, membrane transporters, and mitochondrial function, thus producing oxidative stress, inflammation, and apoptosis
[44][45]. O
3 has been used as a therapy against CDDP induced damage, improving function and augmenting antioxidant defenses. Thiobarbituric acid reactive substances (TBARS, an assay used to measure lipid peroxidation;
[46]), as well as NAG and morphologic damage, displayed decreased values when treated with O
3 [47][48][49]. Protective effects, however, varied according to the administered O
3 concentration, given that the administration of 0.36 mg/kg might be therapeutic
[34] or might not
[49]. On the other hand, 1.1 mg/kg always shows protective tendencies in CDDP-induced damage
[47][48][49]. Higher concentrations, e.g., 1.8 mg/kg, might be protective
[36]. However, due to the high formation rate of hydrogen peroxide and oxidative stress mediated by O
3, toxic effects might be produced
[47]. Very similar protective morphologic, anti-inflammatory, and antioxidant effects have been found against the damage induced by methotrexate, another cancer drug, in the kidneys, as well as the intestines and liver
[50].
Radiographic contrast media (CM) is constantly used in clinical procedures which require the observation of vascular compartments. Mechanisms through which CM might cause renal dysfunction include direct oxygen-free radical damage, modified hemodynamics, and hypoxic renal medullary injury mediated by shortness of blood flow and an increase in tubular O
2 supply. Therefore, the employment of CM produces high toxicity
[51], which can be treated with O
3. Neutrophil gelatinase-associated lipocalin (NGAL) is a damage marker observed in contrast-induced nephropathy (CIN) which augmented its expression when treated with O
3; no further discussion was provided, although the initial oxidation by O
3 might have produced it
[52][53].
In the medical field, the use of xenobiotics as drugs to treat and diagnose diseases is an irreplaceable factor. However, during their metabolism and excretion, some might become nephrotoxic by accumulation, directing damage, the formation of free radicals, and depletion of antioxidant substances. This represents a risk for patients with neoplasia or other conditions which require constant chemical induction or those in contact with environmental components such as Cd, which is also demonstrated to cause similar renal damage. However, O3 is an effective treatment against this damage, at least experimentally, and thus the importance of further research in clinical environments.
Table 2. Ozone (O3) effects on chemical-induced damage models.
3. O3 Therapy Protects the Kidney against CKD
CKD is a major global health issue due to its high worldwide prevalence. In 2010, an analysis showed that about 500 million adults over 20 years old suffered from this disease
[54]. As its name suggests, CKD is a progressive condition in which kidney function diminishes progressively, as indicated by a lesser glomerular filtration rate (GFR) (<60 mL/min per 1.73 m
2) or the presence of pathologic markers, such as albuminuria, hematuria, glucosuria, or other abnormalities detected by imaging, for at least three months
[55]. Many factors are involved in its development, such as hypertension, pollution, glomerulonephritis, and, most importantly, type 2 diabetes mellitus
[56]. In this section, the effects of O
3 therapy against CKD will be discussed, hoping to decipher the use of new therapeutic alternatives to delay or prevent this pathology (
Table 3).
Several procedures are induced in rats to simulate CKD, such as subtotal (5/6) nephrectomy, which exposes remaining renal tissue to high pressure and perfusion, eventually diminishing renal function and hence great inflammation. O
3 can ameliorate this condition, enhancing kidney function and antioxidant status. TBARS showed higher levels, possibly due to O
3 mediated oxidative stress
[57][58]. Adenine administration also simulates CKD through its enzymatic degradation by xanthine dehydrogenase and further accumulation of the product 2,8-dihydroxyadenine (DHA) in the renal tubules, leading to inflammation and oxidative stress
[59]. O
3 ameliorated this damaging condition mainly by stimulating the expression of antioxidant enzymes and reducing inflammation
[60][61].
Diabetic kidney disease (DKD) is the main cause of CKD. It is a chronic condition caused by diabetes (whether type 1 or 2) via apoptosis, formation of free radicals, advanced glycation end-products (AGES), inflammatory cytokines, and other growth molecules.
[62]. Diagnosis is made essentially through diminished GFR and proteinuria in humans. Risk factors include smoking habits and high arterial pressure. The discussion of this disease becomes important since its prevalence, and therefore that of CKD, is augmenting
[63]. In experimental DKD studies that use streptozotocin (STZ) as a toxic component to β-cells, O
3 has shown beneficial anti-apoptotic and antioxidative effects in response
[64][65].
Table 3. Ozone (O3) effects on chronic kidney damage models.
CKD usually reaches an advanced terminal stage, which require therapy for replacing renal function, or dialysis, as the indicated treatment
[66]. O
3 has been shown as a coadjutant therapy to dialysis, as demonstrated by case reports in which conventional treatment did not work. For example, Biedunkiewicz and collaborators
[67] described the case of a dialyzed patient with calciphylaxis-induced ulcerations who did not respond to antibiotics and surgical treatment. Ozonated autohemotherapy in concentrations of 50 µg/mL, as well as O
3 topic administration, allowed a successful skin transplant. Authors propose that effects are mediated through O
3 induced the synthesis of platelet-derived growth factor (PDGF), TGF-β1, and IL-8. Paolo and collaborators
[68] described the case of a hemodialyzed patient who presented necrotizing fasciitis and a fatal prognosis. However, after extracorporeal blood oxygenation and ozonization (EBOO) and O
3 topic administration, drastic wellness, including diminished hyperpyrexia and restoration of skin lesions, was reported. EBOO might be a more comfortable and practical alternative to O
3 administration to patients over i.p. or rectal insufflation pathways, and its safety has also been proven experimentally
[69]. A clinical trial in hemodialyzed patients conducted by Tylicki and collaborators
[57] showed diminished GSH levels after nine weeks of O
3 treatment, possibly caused by an augmented antioxidant system that consumes GSH. The same authors found no difference in NK cell activity after O₃ therapy, indicating it as a safe treatment in hemodialyzed patients
[70]. Interestingly, another case report concluded that this therapy might cause heart failure in complex patients, such as those with CKD, diabetes, and hypertension. This association resulted from the speculation that O
3 therapy augmented K
+ serum levels, which, along the diminished excretion, produced sinus arrest
[71]. Contrasting effects were found by Gu and collaborators
[72], who treated patients suffering from chronic hepatitis with O
3 and, while measuring kidney function, found diminished renal damage, augmented renal blood flow, and even a significant association with lesser fatalities.
To sum up, CKD is usually caused by diabetes. Both are highly prevalent, and dialysis is the standard treatment in advanced stages. O3 treatment is useful against these chronic diseases by reducing inflammation and oxidative stress. On top of that, O3 works as a coadjutant therapy for dialyzed patients to ameliorate not only kidney function, but aggravated topical microbial infections, which are common. Figure 1 shows the effects of ozone on ischemia/reperfusion, renal damage by xenobiotics, and chronic kidney disease.
Figure 1. Effects of ozone therapy (O
3) against xenobiotics, ischemia-reperfusion (IRI) and chronic kidney disease (CKD). O
3 inhibits inflammation and ROS production by increasing the expression of antioxidant enzymes in all models. Additionally, during IRI, xanthine oxidase (XO) degrades nucleotides and forms uric acid, generating large amounts of reactive oxygen species (ROS) and inflammation. Endothelin-1 (ET-1) causes vasoconstriction and exacerbates inflammation leading to fibrosis. O
3 therapy increases nitric oxide (NO), which inhibits vasoconstriction. While O
3, by inhibiting ROS, causes a decrease in advanced glycation end products (AGES) and apoptosis, preventing CKD. H
2O: water, H
2O
2: hydrogen peroxide, O
2: oxygen molecule, NAG: N-acetyl-β-D-glucosaminidase. Created with
Biorender.com, accessed on 10 February 2023.
4. Otherapeutic Uses of O3 in Kidney
Extracorporeal shock wave lithotripsy is the first-line treatment for patients with renal calculi of under 2.0 cm; therapy fragments such stones and is highly efficient. Nevertheless, adverse effects such as hematuria might be present after the procedure
[73]. Experimentally, O
3 treatment has been proven as effective against the morphological and oxidative damage caused by shock wave therapy
[74]. The novel therapy, due to its antimicrobial capacity, has also ameliorated oxidative damage caused by microorganisms in kidney infection (pyelonephritis)
[63] and septic shock in kidneys
[75], as well as in other organs
[76].