Rac1/pSTAT3 expression: A pharmacodynamic marker panel as a first step toward optimization of thiopurine therapy in inflammatory bowel disease patients

Abstract

Thiopurine derivatives, such as azathioprine and mercaptopurine, are standard conventional treatment options in inflammatory bowel disease (IBD). Unfortunately, approximately half of patients discontinue thiopurine therapy within 2 years. To improve the prediction of clinical effectiveness, thiopurine therapy is currently optimized using therapeutic drug monitoring. Ras-related C3 botulinum toxin substrate 1 (Rac1) has been suggested as a potential pharmacodynamic marker of the thiopurine effect in lymphocytes. The active thiopurine metabolite 6-thioguanine triphosphate (6-Thio-GTP) causes T cell apoptosis via Rac1 and the downstream transcription factor signal transducer and activator of transcription 3 (STAT3). The aim of this study was to develop and validate a functional pharmacodynamic multiparameter flow cytometric assay to determine Rac1/pSTAT3 expression in the various leukocyte subpopulations in peripheral blood in order to predict therapeutic response in IBD patients in the future. Peripheral blood samples of healthy subjects (no fever or clinical complaints of active disease, C-reactive protein < 10 mg/L) were used for immunocytochemical labeling, applying an optimized fixation and permeabilization strategy. A gating procedure was performed to separate all leukocyte subpopulations. Quantitative data were obtained by measuring presence and median fluorescent intensity. In vitro, Rac1 presence and expression were detectable in all leukocyte subpopulations. After IL-6 stimulation, used as proxy for inflammation, a distinct pSTAT3 signal could be detected in T lymphocytes of healthy subjects. In vivo, an upregulated pSTAT3 signal was detected in nearly all IBD patients with active disease and differed substantially from the signal found in IBD patients in remission on thiopurines and healthy subjects. We developed and validated a functional flow cytometric assay to assess Rac1 and pSTAT3 presence and expression. This opens a venue for a pharmacodynamic assay to predict thiopurine effectiveness in IBD patients.

A report on the potential of Rac1/pSTAT3 protein levels in T lymphocytes to assess the pharmacodynamic effect of thiopurine therapy in Inflammatory Bowel Disease patients

“…Data from healthy subjects were obtained from the previous laboratory validation study and were used as reference values 24 . Healthy subjects were defined by having no known history of auto-immune diseases, no fever or other clinical complaints of active disease, no use of immune suppressive medication and a C-Reactive Protein (CRP) concentration below 10 mg/L.…”

“…Rac1/pSTAT3 measurement. Peripheral ethylenediamine tetraacetic acid (EDTA) blood samples were used for immunocytochemical labelling, as recently described 24 . In brief, after blood collection, blood cells were washed and prepared for analysis with flowcytometry.…”

“…In brief, after blood collection, blood cells were washed and prepared for analysis with flowcytometry. All analyses were performed within one hour after venepuncture, since stability of Rac1 protein levels this period has been demonstrated before 24 . First, the cells in suspension were incubated with fluorochrome-labelled antibodies targeting extracellular antigens, followed by addition of Lyse and Fix buffer for erythrocyte lysing and fixation of the antibodies to the membrane-bound antigens.…”

“…Recently, we developed and validated a functional pharmacodynamic multiparameter flow cytometric assay to determine the protein levels of both Rac1 and phosphorylated (i.e. activated) STAT3 (pSTAT3) in various leukocyte subpopulations in peripheral blood of healthy subjects and IBD patients 24 .…”

Reply to: Therapeutic Drug Monitoring for IBD Children on Thioguanine

“…The active thiopurine metabolite 6‐thioguanine triphosphate causes T cell apoptosis via Rac1 and the downstream transcription factor signal transducer and activator of transcription 3 (STAT3) (10). A recent study found a reduced pSTAT3 signaling in healthy participants and thiopurine‐treated IBD patients in remission compared to patients with active disease (11). More research is needed to establish the potential role of Rac1/STAT3 determination in thiopurine‐treated IBD patients.…”