Phosphodiesterase type 4 inhibitors: potential in the treatment of multiple sclerosis?

Phosphodiesterases (PDEs) are involved in the regulation of intracellular levels of the second messengers cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). These enzymes hydrolyse the cyclic nucleotides to the corresponding nucleoside 5′-monophosphates. Nine PDE subtypes have been identified; these differ in their substrate specificity and mode of activation. The type 4 PDE (PDE4) hydrolyses cAMP, is activated by elevated levels of cAMP, and is inhibited by rolipram. Inhibition of enzyme activity has been shown to modulate the activity of cells of the immune system. The production of tumour necrosis factor (TNF)α by activated monocytes and macrophages is inhibited, and cytokine secretion and proliferation of type 1 T helper cells are suppressed. Both immune cell activation and their concomitant induction of cytokine secretion are implicated in multiple sclerosis (MS), which is the major demyelinating disease of the central nervous system. Studies with the selective PDE4 inhibitor rolipram in experimental autoimmune encephalomyelitis (an animal model of MS) in mice, rats and nonhuman primates have demonstrated the efficacy of the compound in this disease model, suggesting that PDE4 inhibitors could ameliorate the clinical course of MS. Unfortunately, clinical trials with PDE4 inhibitors revealed the major adverse effects of these drugs, namely nausea and vomiting. However, novel PDE4 inhibitors, which target only a subpopulation of PDE4 enzymes, may provoke fewer adverse effects. The efficacy of a PDE4 inhibitor in MS still needs to be demonstrated in a well designed clinical trial.