Use of IMiD3, a thalidomide analog, as an adjunct to therapy for experimental tuberculous meningitis.

Tuberculous meningitis (TBM), the most severe form of Mycobacterium tuberculosis infection, continues to be associated with significant morbidity and mortality, particularly in children (12, 19, 21, 30, 32, 35). Unfortunately, bacteriologic cure of TBM with antituberculous chemotherapy does not necessarily prevent the severe neurologic sequelae of the infection. This is due to the inflammatory response to the mycobacteria, which causes vasculitis and infarcts, resulting in brain edema and increased intracranial pressure, leading to irreversible tissue damage in the central nervous system (CNS) (8, 12, 13, 23, 24). The inflammatory response is driven by cytokines, including tumor necrosis factor alpha (TNF-α), produced by mononuclear phagocytes in response to infection with M. tuberculosis (8, 13, 23, 24). Immunomodulatory drugs, such as corticosteroids, that reduce cytokine production, thereby dampening the inflammatory response, are often used in addition to the conventional antibiotic treatment (1, 4, 7, 22). A rabbit model of acute TBM that can be used to study the course of the disease and to design immunomodulatory therapies has been described previously (34). It was observed that a combination of antituberculous drugs plus thalidomide as an adjunct immunomodulator dramatically improved the outcome and survival of rabbits with acute CNS mycobacterial infection. In that study, thalidomide treatment was associated with inhibition of TNF production and downregulation of the mononuclear leukocyte infiltrate into the brain. However, the model used in those studies of mycobacterial meningitis was acute and more rapidly progressive than the naturally occurring M. tuberculosis infection in humans. Also in those studies, the therapeutic interventions used were initiated prior to or simultaneously with experimental infection. Therefore, to more closely mimic the course of human TBM, we have developed a subacute model of mycobacterial CNS infection with a delayed onset of disease. This model enables us to initiate interventions after the expected occurrence of clinical signs and to evaluate the effect of different regimens of antituberculosis and immunomodulatory treatments, such as thalidomide, on the course of the disease. Thalidomide is a drug with pleiotropic effects. The immunomodulatory effects of thalidomide are at least in part mediated through its ability to downregulate the pathogenic overproduction of TNF-α (5, 6). In vitro studies of TNF-α inhibition in human peripheral blood mononuclear cells have shown that thalidomide has a 50% inhibitory concentration of ∼194 μM (50.2 μg/ml). Preclinical animal studies have shown that 90 days of treatment with thalidomide result in no observed adverse event level (NOAEL) for mice at a dose of 3,000 mg/kg of body weight, whereas for rats, the NOAEL dose is 3,000 mg/kg for females and 30 mg/kg for males (31). New analogs of thalidomide are being produced that are more potent immunomodulators than the parent drug. One group of such analogs was shown to be more potent inhibitors of TNF-α, interleukin 1 beta (IL-1β), IL-6, and IL-12 secretion by monocytes (6, 9). Immunomodulatory drug 3 (IMiD3) is an analog of thalidomide that is under development for potential use as an immunomodulator for the treatment of inflammatory conditions in humans. In in vitro studies of TNF-α inhibition in human peripheral blood mononuclear cells, IMiD3 has a 50% inhibitory concentration of ∼100 nM (25.9 ng/ml) (5, 6). Repeated oral administration of IMiD3 to rats for 28 days shows an NOAEL dose of 300 mg/kg. Both thalidomide and IMiD3 have been tested for teratogenicity in the pregnant rabbit model. When administered to pregnant rabbits, thalidomide causes postimplantation loss and external and visceral fetal defects in the majority of fetuses. In comparison, IMiD3 at either 5, 15, or 25 mg/kg/day shows no evidence of teratogenicity at any dose level (D. Stirling, unpublished observations). Here we report studies on the effect of combining antituberculous therapy either with thalidomide or with the thalidomide analog IMiD3 on disease manifestation in rabbits infected intrathecally with Mycobacterium bovis Ravenel. We examined the bacillary load in the CNS and other tissues, the inflammatory response in the CNS, the severity of the disease, and the survival of rabbits with subacute mycobacterial meningitis treated with these drug combinations.