DPP4 and rheumatoid arthritis: Compared to patients with osteoarthritis (OA), rheumatoid arthritis (RA) patients recruited in the active phase of their disease exhibited significantly lower blood plasma DPP-IV-like enzymatic activity (median ± SD 220.15 ± 83.6 pkat/mL in RA vs. 376.9 ± 144.9 pkat/mL in OA, p < 0.001) and blood plasma DPP-IV concentration (median ± SD 465.1 ± 215.6 ng/mL in RA vs. 953.3 ± 368.4 ng/mL in OA, p < 0.001) (Fig. 1).