Rescuing a Troubled Tolcapone with PEGylated PLGA Nanoparticles: Design, Characterization, and Hepatotoxicity Evaluation

Tolcapone is an orally active catechol-O-methyltransferase (COMT) inhibitor used as adjuvant therapy in Parkinson's disease. However, it has a highly hepatotoxic profile, as recognized by the U.S. Food and Drug Administration. As a possible solution, nanoscience brought us several tools in the development of new functional nanomaterials with tunable physicochemical properties, which can be part of a solution to solve several drawbacks, including drug's short half-life and toxicity. This work aims to use PEGylated poly(lactic-co-glycolic acid) (PLGA) nanoparticles as a stable carrier with lower hydrodynamic size and polydispersity to encapsulate tolcapone in order to overcome its therapeutic drawbacks. Using the nanoprecipitation method, tolcapone-loaded nanoparticles with a DLC% of 5.7% were obtained (EE% of 47.0%) and subjected to a lyophilization optimization process to obtain a final shelf-stable formulation. Six different cryoprotectants in concentrations up to 10% (w/v) were tested. A formulation of PLGA nanoparticles with 3% hydroxypropyl-β-cyclodextrin (HPβCD) as a cryoprotectant (PLGA-HP@Tolc), presenting sub-200 nm sizes and low polydispersity (PdI < 0.200) was selected. Cytotoxicity assays, namely, MTT and SRB, were used to study the metabolic activity and cell density of tolcapone and PLGA-HP@Tolc-treated cells. In both assays, a hepatocarcinoma cell line (HepG2) growing in glucose or glucose-free media (galactose-supplemented medium) was used. The results demonstrated that the treatment with the PLGA-HP@Tolc formulation led to a decrease in cytotoxicity in comparison to free tolcapone-treated cells in both media tested. Moreover, the elected formulation also counteracted ATP-depletion and excessive ROS production induced by tolcapone. The results suggest that HPβCD might have a dual function in the formulation: cryoprotectant and anticytotoxic agent, protecting cells from tolcapone-induced damage. Using an in vitro COMT inhibition assay, the PLGA-HP@Tolc formulation demonstrated to inhibit COMT as efficiently as free tolcapone. Overall, the results suggest that tolcapone-loaded PLGA NPs could be an interesting alternative to free tolcapone, demonstrating the same in vitro efficacy in inhibiting COMT but with a safer cytotoxic profile.

Nordihydroguaiaretic Acid as a Novel Substrate and Inhibitor of Catechol O-Methyltransferase Modulates 4-Hydroxyestradiol-Induced Cyto- and Genotoxicity in MCF-7 Cells

Nordihydroguaiaretic acid (NDGA) is a major lignan metabolite found in Larrea spp., which are widely used in South America to treat various diseases. In breast tissue, estradiol is metabolized to the catechol estrogens such as 4-hydroxyestradiol (4-OHE₂), which have been proposed to be cancer initiators potentially involved in mammary carcinogenesis. Catechol-O-methyltransferase (COMT) catalyzes the O-methylation of catechol estrogens to their less toxic methoxy derivatives, such as 4-O-methylestradiol (4-MeOE₂). The present study investigated the novel biological activities of NDGA in relation to COMT and the effects of COMT inhibition by NDGA on 4-OHE₂-induced cyto- and genotoxicity in MCF-7 human breast cancer cells. Two methoxylated metabolites of NDGA, 3-O-methylNDGA (3-MNDGA) and 4-O-methyl NDGA (4-MNDGA), were identified in the reaction mixture containing human recombinant COMT, NDGA, and cofactors. K_m values for the COMT-catalyzed metabolism of NDGA were 2.6 µM and 2.2 µM for 3-MNDGA and 4-MNDGA, respectively. The COMT-catalyzed methylation of 4-OHE₂ was inhibited by NDGA at an IC₅₀ of 22.4 µM in a mixed-type mode of inhibition by double reciprocal plot analysis. Molecular docking studies predicted that NDGA would adopt a stable conformation at the COMT active site, mainly owing to the hydrogen bond network. NDGA is likely both a substrate for and an inhibitor of COMT. Comet and apurinic/apyrimidinic site quantitation assays, cell death, and apoptosis in MCF-7 cells showed that NDGA decreased COMT-mediated formation of 4-MeOE₂ and increased 4-OHE₂-induced DNA damage and cytotoxicity. Thus, NDGA has the potential to reduce COMT activity in mammary tissues and prevent the inactivation of mutagenic estradiol metabolites, thereby increasing catechol estrogen-induced genotoxicities.

Inhibitors of catechol-O-methyltransferase in the treatment of neurological disorders

Catechol-O-methyltransferase (COMT) is the enzyme which catalyzes the transfer of a methyl group from S-adenosylmethionine to catechols and catecholamines, like the neurotransmitters dopamine, epinephrine and norepinephrine. COMT has implications in many neurological and psychiatric disorders like schizophrenia, Parkinson's disease (PD), bipolar disorders, etc. and therefore, it serves as an important drug target. Since its characterization in 1957, many inhibitors were designed where the first generation inhibitors were found to be highly toxic, short acting and had poor bioavailability. Currently, two of the second generation inhibitors, tolcapone and entacapone have been used for treatment of PD but are associated with various dopaminergic and gastro-intestinal side-effects. There have been several approaches for the design of novel COMT inhibitors with a good and safe therapeutic profile. The focus of this article is to review the current knowledge on COMT and the role of COMT inhibitors in the treatment of neurological disorders. The inhibitors have been classified into six different classes based on the structural framework. A historical overview of the discovery and development of COMT inhibitors is presented with a special emphasis on new generation of inhibitors till date.