Abstract
Treatment-resistant schizophrenia (TRS) is associated with great disability, functional impairment, and substantial socioeconomic costs. While clozapine is indicated in patients with TRS, its use is restricted to patients who have not responded to at least 2 other antipsychotics, thus implying a series of empirical trials of different drugs before receiving effective treatment. In this scenario, the identification of reliable biological markers to predict the risk for TRS before starting pharmacological treatments might significantly improve the management of TRS in its early stages. We conducted a systematic review on PubMed, Scopus and Web of Science to identify studies investigating peripheral biological markers of TRS. A total of 75 articles were included. These studies mostly investigated the association between TRS and genetic markers (n = 42, of which 16 with a genome-wide and 25 with a candidate-gene design) and protein/metabolite markers (n = 23), while only a minority of studies investigated RNA markers (n = 5), methylation levels (n = 4), gut microbiota profiles (n = 1), or more than one type of marker (n = 3). The elucidation of peripheral biomarkers of TRS is challenging due to the large heterogeneity across studies in terms of clinical definition of TRS, the relatively small sample size of many studies, as well as the lack of powered studies integrating data at a multi-omic level. Nonetheless, available studies suggest TRS to be a trait with a significant heritability and point to inflammation and cytokine imbalance as the most promising pathways involved in this complex phenotype.