This enzyme is located in the mitochondria and catalyzes the conversion of dihydroorotate (“DHO”) to orotate as the fourth step in the de novo synthesis of pyrimidines that are ultimately used in the production of nucleotides.
Nucleotides are required for cell growth, function and replication. Nucleotides are the activated precursors of nucleic acids and are necessary for the replication of the genome and the transcription of the DNA into RNA. Nucleotides also serve as an energy source for a more select group of biological processes (ATP and GTP), as well as play a role in the formation of glycogen, involved in signal-transduction pathways, and act as components of co-enzymes (NAD and FAD). An ample supply of nucleotides in the cell is essential for all cellular processes.
There are two pathways for the biosynthesis of nucleotides: recycling and de novo. In the recycling pathway, the bases are recovered (salvaged) from existing RNA and DNA degradation. In the de novo pathway, the bases are assembled from simple precursor molecules (i.e., made from scratch).
One critical requirement of fast-growing or proliferating cells, such as the expansion of activated B and T cells that occurs in many inflammatory processes, is the requirement of a stockpile of nucleotide bases. These metabolic activities will predominately utilize the de novo pathway for nucleotide biosynthesis, of which DHODH plays a critical role. A key advantage of DHODH inhibition is the selectivity towards metabolically activated cells (with a high need for RNA and DNA production), which should mitigate any negative impact on normal cells. Depletion of cellular pyrimidine pools through the selective inhibition of DHODH has been shown to be a successful approach to therapeutic development.
Currently, two first-generation DHODH inhibitors have been approved in the U.S. and abroad; they are marketed by Sanofi as leflunomide (Arava®) and the active metabolite teriflunomide (Aubagio®). These oral tablets are approved for the treatment of the autoimmune diseases rheumatoid arthritis and multiple sclerosis (“MS”), respectively. Whilst both therapeutics are effective, they carry the risk of hepatotoxicity, which is reported as a major side effect and carries a black box warning from the FDA. It is hypothesized this side effect is due to either the off-target effects of the drugs or the large amount required to cause inhibition.
KIO-101 was identified as a promising novel third-generation DHODH inhibitor, with a half-maximal inhibitory concentration IC50-value of 0.3nM, which is more than 1,000-fold more potent than teriflunomide (IC50 DHODH 415 nM). Furthermore, KIO-101 suppresses the expression of key pro-inflammatory cytokines such as IL-17, IFN-γ and others. IL-17 and IFN-γ are the hallmark cytokines expressed by Th1 and Th17 T-cells, respectively, and play a crucial role in initiating the inflammatory processes in several diseases, including dry eye disease. KIO-101 is structurally and mechanistically different from Arava® and does not carry the same off-target effects. Thus, severe side effects (including hepatotoxicity) are not expected, and have not been observed to date in animal and human studies after KIO-101 administration.