Suppression of Neuroinflammation as a Treatment for Neurodegenerative Disease
There is a growing focus on inflammation in the brain as an important factor in the progression of neurodegenerative disease. One result is greater thought given to therapeutic strategies involving the suppression of inflammatory signaling, akin to the approaches used to control inflammatory autoimmune conditions such as rheumatoid arthritis. I would wager that this is probably not as good a strategy as removing senescent glial cells in the brain, and thus removing their sizable contribution to inflammatory signaling, given the animal data in support of that approach, but it will certainly be attempted in the years ahead.
Inflammation is initiated as the body's immune cells activate inflammatory cascades to prevent tissue damage from injury or infiltrating antigens. Within the central nervous system, microglia, known as 'the brain's immune cells,' interact with astrocytes and neurons by assuming phagocytic phenotypes and releasing inflammatory cytokines. This can cause neurodegeneration, phagocytosis of synapses, diminished neural function, microglial activation, inflammatory cytokine release, and further microglial activation until threat to the neural environment abates. Activation of astrocytes, termed astrogliosis, also occurs as part of the inflammatory process.
When acute, this neuroinflammatory response is necessary and even beneficial to the neural environment in eliminating pathogens or aiding brain repair. However, when extreme threats to the neural environment such as protein aggregates (i.e., lewy bodies, neurofibrillary tangles) accumulate in the brain and protractedly sustain inflammation, continuous gliosis and apoptosis can occur as a result of unregulated inflammatory cytokine release. Continuity of this activated state results in chronic inflammation, which is implicated in virtually all neurological disorders, including Alzheimer's disease, Parkinson's disease, and ALS.
Overexpression of tumor necrosis factor-α (TNF-α), a proinflammatory cytokine with a central role in microglial activation, has been associated with neuronal excitotoxicity, synapse loss, and propagation of the inflammatory state. Thalidomide and its derivatives, termed immunomodulatory imide drugs (IMiDs), are a class of drugs that inhibit TNF-α production. Due to their multi-potent effects, several IMiDs, including thalidomide, lenalidomide, and pomalidomide, have been repurposed as drug treatments for diseases such as multiple myeloma and psoriatic arthritis. Preclinical studies of currently marketed IMiDs, as well as novel IMiDs, support the development of IMiDs as therapeutics for neurological disease. IMiDs have a competitive edge compared to similar anti-inflammatory drugs due to their blood-brain barrier permeability and high bioavailability, with the potential to alleviate symptoms of neurodegenerative disease and slow disease progression.