Fumaric acid esters (FAEs) such as dimethylfumarate (DMF) are used for the treatment of adults with moderate-to-severe psoriasis. The mode of action of FAEs is complex. Here, we provide a comprehensive review of the literature to describe the molecular mechanisms by which DMF and its active metabolite monomethylfumarate (MMF) exert their anti-inflammatory and immune modulatory effects. MMF can bind to the hydroxy-carboxylic acid receptor 2 (HCA2) on the cell surface and both DMF and MMF react with intracellular glutathione following cell penetration. DMF and to some extent also MMF modulate the activity of certain cellular signalling proteins such as the nuclear factor (erythroid-derived 2)-like 2 (Nrf2), nuclear factor kappa B (Nf-κB) and the cellular concentration of cyclic adenosine monophosphate. Some studies show that DMF can also affect the hypoxia-inducible factor 1-alpha (HIF-1α). These actions seem to be responsible for i) the downregulation of inflammatory cytokines and ii) an overall shift from a proinflammatory Th1/Th17 response to an anti-inflammatory/regulatory Th2 response. Both steps are necessary for the amelioration of psoriatic inflammation, although additional mechanisms have been proposed. There is a growing body of evidence to support the notion that DMF/MMF may also exert effects on granulocytes and non-immune cell lineages including keratinocytes and endothelial cells. A better understanding of the multiple molecular mechanisms involved in the cellular action of FAEs will help to adapt and further improve the use of such small molecules for the treatment of psoriasis and other chronic inflammatory diseases.

Fumarates (fumaric acid esters, FAEs) are orally administered systemic agents used for the treatment of psoriasis and multiple sclerosis. In 1994, a proprietary combination of FAEs was licensed for psoriasis by the German Drug Administration for use within Germany. Since then, fumarates have been established as one of the most commonly used treatments for moderate-to-severe psoriasis in Germany and other countries. The licensed FAE formulation contains dimethyl fumarate (DMF), as well as calcium, zinc, and magnesium salts of monoethyl fumarate (MEF). While the clinical efficacy of this FAE mixture is well established, the combination of esters on which it is based, and its dosing regimen, was determined empirically. Since the mid-1990s, the modes of action and contribution of the different FAEs to their overall therapeutic effect in psoriasis, as well as their adverse event profile, have been investigated in more detail. In this article, the available clinical data for DMF are reviewed and compared with data for the other FAEs. The current evidence substantiates that DMF is the main active compound, via its metabolic transformation to monomethyl fumarate (MMF). A recent phase III randomized and placebo-controlled trial including more than 700 patients demonstrated therapeutic equivalence when comparing the licensed FAE combination with DMF alone, in terms of psoriasis clearance according to the Psoriasis Area and Severity Index (PASI) and Physician’s Global Assessment (PGA). Thus, DMF as monotherapy for the treatment of psoriasis is as efficacious as in combination with MEF, making the addition of such fumarate derivatives unnecessary.

A mixture of fumaric acid esters (FAEs) is approved for the oral therapy of psoriasis. However, for a long time the active ingredient of this mixture was unknown. We reviewed the in vitro data available for the different FAEs present in the multi compound drug and elaborate how they may contribute to possible clinical effects. Although helpful overall, many in vitro data must be viewed critically because the concentrations used in the experiments exceed the plasma levels reached in patients. The data suggest that dimethylfumarate (DMF) is the most active compound, mediating the major therapeutic effect after metabolization into monomethylfumarate (MMF) via an according receptor expressed on target cells. Identifying the active pharmaceutical ingredient within a mixture of compounds helps to subsequently eliminate unnecessary, potentially harmful compounds. This provides a promising example for an alternative precision medicine approach in clinical practice.