Serum activity of prolyl endopeptidase, but not of dipeptidyl peptidase IV, is decreased by immunotherapy with IFN-α in high-risk melanoma patients

Immunotherapy with interferon-α (IFN-α) induces neuropsychiatric side effects, most notably depression. In hepatitis patients treated with IFN-α, severity of depression correlates with a decrease in serum activity of dipeptidyl peptidase IV (DPP-IV, EC 3.4.14.5), a membrane-bound protease involved in the cleavage of cytoldnes and neuroactive peptides. Abnormal serum activity of the cytosolic peptidase prolyl endopeptidase (PEP, EC 3.4.21.26, postprolyl cleaving enzyme, prolyl oligopeptidase) has been documented in patients with a variety of psychiatric disorders, most consistently in mood disorders. The serum activity of PEP and DPP-IV was measured before and after 4 weeks of high-dose induction treatment with IFN-α in 18 patients with high-risk melanoma. In this exploratory study, we show a clear decrease in the serum activity of PEP after 4 weeks of treatment with IFN-α. This decrease was not related to changes in hematologic parameters. In contrast, serum activity of DPP-IV did not change. Further studies focusing on a possible role of PEP in the pathophysiology of IFN-α-induced depression are warranted.