Background: Thromboxane B2 (TXB2) and superoxide anion (O2-) are neuroinflammatory mediators that appear to be involved in the pathogenesis of several neurodegenerative diseases. Because activated-microglia are the main source of TXB2 and O2- in these disorders, modulation of their synthesis has been hypothesized as a potential therapeutic approach for neuroinflammatory disorders. Marine natural products have become a source of novel agents that modulate eicosanoids and O2- generation from activated murine and human leukocytes. With the exception of manzamine C, all other manzamines tested are characterized by a complex pentacyclic diamine linked to C-1 of the beta-carboline moiety. These marine-derived alkaloids have been reported to possess a diverse range of bioactivities including anticancer, immunostimulatory, insecticidal, antibacterial, antimalarial and antituberculosis activities. The purpose of this investigation was to conduct a structure-activity relationship study with manzamines (MZ) A, B, C, D, E and F on agonist-stimulated release of TXB2 and O2- from E. coli LPS-activated rat neonatal microglia in vitro.
Results: The manzamines differentially attenuated PMA (phorbol 12-myristate 13-acetate)-stimulated TXB2 generation in the following order of decreasing potency: MZA (IC50 < 0.016 microM) > MZD (IC50 = 0.23 microM) > MZB (IC50 = 1.6 microM) > MZC (IC50 = 2.98 microM) > MZE and F (IC50 > 10 microM). In contrast, there was less effect on OPZ (opsonized zymosan)-stimulated TXB2 generation: MZB (IC50 = 1.44 microM) > MZA (IC50 = 3.16 microM) > MZC (IC50 = 3.34 microM) > MZD, MZE and MZF (IC50 > 10 microM). Similarly, PMA-stimulated O2- generation was affected differentially as follows: MZD (apparent IC50 < 0.1 microM) > MZA (IC50 = 0.1 microM) > MZB (IC50 = 3.16 microM) > MZC (IC50 = 3.43 microM) > MZE and MZF (IC50 > 10 microM). In contrast, OPZ-stimulated O2- generation was minimally affected: MZB (IC50 = 4.17 microM) > MZC (IC50 = 9.3 microM) > MZA, MZD, MZE and MZF (IC50 > 10 microM). From the structure-activity relationship perspective, contributing factors to the observed differential bioactivity on TXB2 and O2- generation are the solubility or ionic forms of MZA and D as well as changes such as saturation or oxidation of the beta carboline or 8-membered amine ring. In contrast, the fused 13-membered macrocyclic and isoquinoline ring system, and any substitutions in these rings would not appear to be factors contributing to bioactivity.
Conclusion: To our knowledge, this is the first experimental study that demonstrates that MZA, at in vitro concentrations that are non toxic to E. coli LPS-activated rat neonatal microglia, potently modulates PMA-stimulated TXB2 and O2- generation. MZA may thus be a lead candidate for the development of novel therapeutic agents for the modulation of TXB2 and O2- release in neuroinflammatory diseases. Marine natural products provide a novel and rich source of chemical diversity that can contribute to the design and development of new and potentially useful anti-inflammatory agents to treat neurodegenerative diseases.