Effect of minocycline in experimental autoimmune encephalomyelitis

With interest we read the report by Popovic and colleagues showing the effect of minocycline in experimental autoimmune encephalomyelitis (EAE). Minocycline is a secondgeneration tetracycline, which recently was shown by us and others to have antiinflammatory as well as neuroprotective properties. The authors demonstrated that minocycline suppressed disease activity and inflammation in the central nervous system in association with immune deviation in the periphery. In their experiments, minocycline was dissolved in distilled water or saline and injected daily intraperitoneally (45mg/kg). We recently performed similar experiments in female SJL mice using an adoptive transfer EAE model. Mice were immunized with proteolipid protein 139-151 (200 g) plus adjuvant. After 10 days, draining lymph node cells were prepared, cultured for 4 days with antigen (10 g/ml), and injected intravenously (5 10) into naı̈ve mice. In contrast with the study by Popovic and colleagues, we had an oral treatment group in addition to an intraperitoneal treatment group. Intraperitoneal treatment led to a marked suppression of EAE, whereas, in orally treated animals, the effect of minocycline was markedly reduced (Fig). Also, we found that intraperitoneal application of minocycline may cause painful peritoneal irritation despite rigorous pH adjustment. In in vitro experiments on encephalitogenic lymph node cells and T-cell clones, we found that up to 10 M minocycline did not suppress proliferation or production of tumor necrosis factor– , interferon, or interleukin-10 (not shown). Minocycline is a compelling new therapeutic approach for multiple sclerosis, and our data support the findings by Popovic and colleagues that intraperitoneal minocycline suppresses EAE. However, our observation that oral dosing is less effective is remarkable and raised our concerns about the mechanisms involved and the impact on a potential application in humans. Tetracyclines distribute widely throughout the body as a function of their lipophilicity. Oral resorption of minocycline is approximately 95 to 100% with a half-life of 12 to 18 hours. Plasma levels are similar after oral or parenteral application. A concern relates to the observation that minocycline solutions are toxic and painful after injection mainly because of a rather low pH and also, though less so, after pH adjustment. We hypothesize that stress-induced immunosuppression, an established phenomenon in EAE, may have contributed to the effect of intraperitoneal minocycline. In conclusion, we advise that the mechanisms of EAE suppression by minocycline are investigated in more detail and intraperitoneal experiments are interpreted with caution before a potential (oral) application in humans.