PP-001 is a third-generation small molecule DHODH inhibitor. DHODH is extensively exploited as potential drug targets for immunological disorders, oncology, and infectious diseases. DHODH is a key enzyme in the de novo pyrimidine synthesis pathway. 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 and replication. Nucleotides are the activated precursors of nucleic acids and are necessary for the replication of the genome and the transcription of the genetic information into RNA. Nucleotides also serve as an energy source for a more select group of biological processes (ATP and GTP). They also play a role in the formation of glycogen, signal-transduction pathways, and 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: salvage and de novo. The main difference is where the nucleotide bases come from. In the salvage pathway, the bases are recovered (salvaged) from RNA and DNA degradation. In the de novo pathway, the bases are assembled from simple precursor molecules (made from scratch).
One critical requirement of fast-growing or proliferating cells, such as the expansion of activated B and T-cells, cancer cells, and pathogen infected host cells, is the requirement of an abundance of nucleotide bases. These metabolic activities will predominately utilize the de novo pathway for nucleotide biosynthesis. 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 for therapeutic development.
Currently, two first generation DHODH inhibitors have been approved in the U.S. and abroad and are marketed by Sanofi as leflunomide (Arava®) and the active metabolite teriflunomide (Aubagio®). These oral tablets are approved for the treatment of rheumatoid and psoriatic arthritis and multiple sclerosis (“MS”), respectively. Both diseases are autoimmune disorders. One potential explanation for the therapeutic effects of Arava® in arthritis is the reduction in the numbers or reactivity of activated T-cells, which are involved in the pathogenesis of arthritis. The generally accepted view of human MS pathogenesis implicates peripheral activation of myelin-specific autoreactive T-cells that lead to inflammatory disease in the central nervous system (“CNS”). By blocking the de novo pyrimidine synthesis pathway via DHODH inhibition, it is suggested that Aubagio® reduces T-cell proliferation in the periphery. Arava® and Aubagio® are formulated as oral drugs and it is established that leflunomide will be metabolized in the liver to the active metabolite teriflunomide. Hepatotoxicity was reported as a major side effect after oral administration, possibly as a result of extensive liver metabolization. Moreover, it was shown that apart from DHOHD, a series of protein kinases are inhibited by Arava® and Aubagio®.
PP-001 was identified as a promising novel third generation DHODH inhibitor, with a half-maximal inhibitory concentration IC50-value of 0.3nM. Based on internal work completed, we believe that this is more than 1,000-fold more potent than teriflunomide (IC50 DHODH 415 nM). Furthermore, PP-001 represses the expression of key pro-inflammatory cytokines such as IL-17, IFN-g, VEGF and others, potentially as a consequence of inhibiting DHODH. IL-17 and IFN-g are the hallmark cytokines expressed by Th1 and Th17 T-cells, respectively, and play a crucial role in initiating the inflammatory processes in several ocular diseases, including non-noninfectious uveitis and dry eye disease. PP-001 is structurally and mechanistically different from Arava®. The IC50 of PP-001 on selected tyrosine kinases, such as PI3K, AKT and JAK, is more than 10,000-fold above the IC50 of PP-001 for DHODH. In general, side effects are not expected and have not been observed to date in animal and human studies afterPP-001 administration.
Despite the fact that the DHODH protein is ubiquitously expressed in most cells, malignant cells seem to be more metabolically dependent on de novo pyrimidine production, forming the potential basis of a therapeutic window. Inhibiting DHODH alone or in combination with standard-of-care has been shown to be very active in a series of different in vivo cancer models for AML, breast, lung, and others.
Additionally, viral replication and viral cell metabolism is dependent on a large nucleotide pool. Therefore, PP-001 demonstrates antiviral efficacy, which is likely due to pyrimidine depletion caused by DHODH inhibition. The postulated DHODH directed mode of action of PP-001 is underlined by reversibility of the antiviral activity by co-application of uridine or other pyrimidine precursors.