Oxidative stress biomarkers in treatment-responsive and treatment-resistant schizophrenia patients

Abstract Introduction Schizophrenia is a complex psychiatric disorder that affects approximately twenty million people worldwide. Various factors have been associated with the physiopathology of this disease such as oxidative stress, which is an imbalance between pro-oxidant and antioxidant molecules. Objective This study evaluated the association between biomarkers of oxidative stress and response to pharmacological treatment among patients with schizophrenia in the context of their clinical information, demographic data, and lifestyle. Methods A total of 89 subjects were included, 26 of whom were treatment-responsive schizophrenia patients (Group 1), 27 treatment-resistant schizophrenia patients (Group 2), and 36 healthy controls (Group 3). All of the subjects completed a questionnaire to provide clinical and demographic data, and all provided peripheral blood samples. The oxidative stress markers analyzed using spectrophotometry were catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GPx), total glutathione (GSH-t), malondialdehyde (MDA), and Trolox-equivalent antioxidant capacity (TEAC; p < 0.05). Results When all schizophrenia patients (G1 + G2) were compared to the control group, SOD levels were found to be lower among schizophrenia patients (p < 0.0001), while MDA and CAT levels were higher (p < 0.0001 and p = 0.0191, respectively). GPx, GSH-t, and TEAC levels were similar in all three groups (p > 0.05). Conclusion Lower SOD levels and higher MDA and CAT levels indicate oxidative damage in schizophrenia patients, regardless of their response to pharmacological treatment. Smoking is associated with oxidative stress, in addition, a family history of the disease was also found to be correlated with cases of schizophrenia, which reflects the relevance of genetics in disease development.


Introduction
Schizophrenia is a complex, chronic, and progressive psychiatric disorder that affects approximately twenty million people worldwide, thus generating important economic impacts on society. Its onset typically occurs in late adolescence or early adulthood, a period which represents the start of working age. [1][2][3] Patients may exhibit symptoms classified as positive (delirium, hallucinations, and disorganized behavior) or negative (a lack of movement, anhedonia, and flat affect); these symptoms can be evaluated using the Positive and Negative Syndrome Scale (PANSS). [4][5][6][7][8] Schizophrenia treatment requires a broad-scale approach with a multidisciplinary team; psychotic symptoms are treated using antipsychotics, but, despite their efficacy, one third of schizophrenia patients fail to respond to pharmacological treatment and are therefore classified as treatment-resistant. [9][10][11] Various factors may influence manifestation of symptoms and patient response to pharmacological treatment, including environmental, genetic, and biochemical conditions. 12 Oxidative stress, which is understood as an imbalance between pro-oxidant and antioxidant molecules, is associated with schizophrenia progression, as well as with the development of other neurodegenerative diseases such as Alzheimer's and Parkinson's diseases. 11,[13][14][15] In this case, progression is stimulated by an increase in reactive oxygen species (ROS) production, decreased antioxidant defense, or a combination of the two. 13 The increase in ROS production may occur in the brain as a consequence of high oxygen saturation resulting from intense neuronal metabolic activity that damages the tissues, lipid membranes, proteins, enzymes, and DNA. [16][17][18][19] In this case, lipid peroxidation may generate a bioproduct such as malondialdehyde (MDA), which can be used as an oxidative biomarker. [16][17][18]20,21 In addition, Trolox-equivalent antioxidant capacity (TEAC) evaluates total (enzymatic and nonenzymatic) antioxidant efficiency. 17,21,22 Antioxidants inhibit the damaging reactions caused by ROS and may be represented by catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase (GPx), or total glutathione (GSH-t), the last of which consists of glutathione in its reduced (GSH) and oxidized (GSSG) states. These antioxidants protect the body from free radicals and maintain redox balance inside cells. 14,23,24 In this context, oxidative stress may be involved predominant symptomology. 28 The control group (G3) was recruited from voluntary blood donors at the study center's blood bank. These Patients who failed to provide consent or to comply with treatment were also excluded. during GSSG reduction to GSH. 31 5-mercapto-2nitrobenzoic acid (Nbs), a derivative produced by the oxidation of GSH, was measured to determine GSH-t levels (GSH + GSSG). The final product was analyzed in a spectrophotometer at 412 nm. In this process, GSSG was recycled into GSH by GR activity. 32,33 TEAC levels were determined using plasma spectrophotometry and an adapted protocol. 22 The TEAC assay depends on plasma antioxidant capacity to inhibit 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) oxidation as measured by absorbance at 734

All of the study subjects completed a clinical
nm. An adapted protocol was used for the plasma MDA analysis. 18 The high-performance liquid chromatography reading was performed at 533 nm. The significance level was set at 0.05.

Results
The patient groups (G1 and G2) were similar in terms of age, gender, and PANSS scores (p > 0.05; Table 1). Relative to the controls, the patient groups

Discussion
This study evaluated oxidative stress markers in treatment-resistant and treatment-responsive schizophrenia patients and found the disease to be associated with lower SOD levels and higher MDA levels, regardless of antipsychotic treatment. These results are consistent with those of other studies performed on both treated and chronic schizophrenia patients, 18 as well as in studies performed on treatment-naive schizophrenia patients. 34 It is important to emphasize the efficacy of follow-up and of antipsychotic treatment that rendered the patient groups homogenous. A prior study has reported that these drugs contribute to improved cognitive behavior. 35 It is also important to note the similar PANSS symptomology between the treatment-resistant and treatment-responsive patients studied herein.
The patients also exhibited a greater rate of smoking relative to the controls, as has been reported in other studies. 36 Some cigarette ingredients generate the superoxide radical, which may inactivate or reduce SOD levels. 37 A similar condition may have been present in the patients studied herein and may have contributed to the reduction in SOD levels [38][39][40] and the consequently higher oxidative damage. This hypothesis may be supported by the high levels of lipid peroxidation represented by MDA levels in schizophrenia patients. 16,18 Antioxidant levels may vary depending on the severity and on the oxidative process of schizophrenia itself. 38,41 Thus, when CAT levels were compared between all of the schizophrenia patients treated as a single group (G1 + G2) and the healthy controls (G3), the enzyme was found to be higher among the patients. There are conflicting reports on CAT activity in schizophrenia patients and these reports include reduced CAT levels, 42 increased CAT levels, and unchanged CAT levels. 38 We obtained no significant differences in GPx or GSH-t between patients and controls. Similarly, other studies have reported consistent GPx levels between schizophrenia patients and controls. 42 However, lower GPx levels, 43,44 GSH-t levels, 42 and TEAC levels (particularly in patients using atypical antipsychotics) 22 have been reported by other researchers.
It is important to note the major role that CAT, SOD, GSH, and GPx play in protecting the brain against ROS toxicity. 34 Analyses were not performed to determine any correlations between the oxidative stress biomarkers and the antipsychotics used by the patients included herein.
It is known that typical and atypical antipsychotics may influence oxidative stress in different ways. 43,49,50 Information on the criteria used to define patients as treatment-resistant were obtained during the early stages of the study; this information included medical records, as well as patients', family members', and caregivers' reports on patients' use of and compliance with antipsychotics. Stricter criteria, including the use of long-acting injectable antipsychotics and tests of antipsychotic serum levels, have been suggested as ways to more accurately establish cases of treatmentresistant schizophrenia. 51 Methods such as these help prevent issues such as failed compliance with treatment, poor medication absorption, and pharmacological interactions, any of which may produce inadequate treatment or prevent correct classification of these patients.
In conclusion, increased MDA and CAT levels and decreased SOD levels indicate that schizophrenia patients are exposed to oxidative stress. Furthermore, the prevalence of a family history of the disease in the patient groups supports the involvement of genetic factors in the development of the disease, regardless of treatment resistance. Therefore, the role of the oxidative pathway as either a cause or consequence of schizophrenia still requires clarification. This crosssectional study's findings on the association between oxidative stress biomarkers and schizophrenia may contribute to future longitudinal studies that seek to prove causal relationships between neurodegenerative processes, the severity of psychotic symptoms, and resistance to treatment.