Rhein-13C4 is intended for use as an internal standard for the quantification of rhein by GC- or LC-MS. Rhein is an anti-inflammatory anthraquinone found in rhubarb and is the bioactive derivative of its prodrug diacerein . At 10 μM, rhein inhibits IL-1β signaling, suppressing signaling through NF-κB, and reduces the expression of the matrix metalloproteases MMP-1 and MMP-13.1 It inhibits IKKβ (IC50 = 11.8 μM), decreasing iNOS and IL-6 expression in LPS-stimulated macrophages but paradoxically increasing TNF-α, IL-1β, and HMBG1 expression.2 Rhein shows efficacy against pancreatic fibrosis, chronic pancreatitis, and hyperglycemia-induced pancreatic β-cell apoptosis.3,4 It also inhibits angiogenesis of breast cancer cells under normoxic and hypoxic conditions.5