ObjectiveMycophenolate mofetil (MMF) is an established therapy for systemic sclerosis (SSc), but its pharmacokinetics in this disease remains unexplored. We have investigated drug exposure in MMF-treated patients with SSc in relation to clinical features of the disease and common concomitant drugs.MethodsThis study was predefined to include 35 MMF-treated SSc patients who were using MMF at a fixed dose of 0.5, 1.0 or 1.5 g twice daily since at least 3 months. The 12-h drug exposure of the active MMF metabolite mycophenolic acid (MPA) was estimated by repeated analysis of plasma MPA over a 6-h period. This 12-h drug exposure was dose normalised to a daily intake of 3 g MMF (MPA_AUC3g) in order to compare subjects using MMF at different doses. Drug exposure was analysed in reference to the clinical characteristics including body weight, renal function, autoantibodies, intestinal dysbiosis, intestinal inflammation assessed by faecal (F)-calprotectin, intestinal symptoms assessed by the University of California Los Angeles Scleroderma Trial Consortium Gastrointestinal Tract Instrument 2.0 and concomitant drug usage including proton-pump inhibitors (PPI).ResultsThirty-four out of 35 study participants completed the study. The mean daily MMF dose was 2.1 g. Drug exposure expressed as MPA_AUC3g varied up to 8-fold between patients (median 115, range 27–226 mg h/L).MPA_AUC3g was inversely related to body weight (rs = − 0.58, p < 0.001) and renal function (rs = − 0.34, p = 0.054). Anti-topoisomerase-1 antibodies and male sex were associated with lower MPA_AUC3g (87 vs 123 and 71 vs 141; p = 0.008 and p = 0.015, respectively). MPA_AUC3g was inversely related to the intestinal abundance of lactobacilli and to F-calprotectin (rs = − 0.54, p = 0.004; rs = − 0.36, p = 0.034), but not to gastrointestinal symptoms. MPA_AUC3g was inversely related to PPI usage (rs = − 0.45, p = 0.007). We found no association between MPA_AUC3g and disease subtype, disease duration or disease activity.ConclusionMMF-treated SSc patients exhibit considerable inter-individual variation in drug exposure, and lower MPA levels were primarily found in PPI users with poor prognostic factors. Body weight, renal function, sex, serology, gastrointestinal manifestations and/or measuring individual MPA exposure should be considered when using MMF for SSc.