Tolcapone-loaded nanostructured lipid carriers for improved oral delivery

Abstract

Tolcapone is a nitrocatechol-based drug used as adjuvant therapy in Parkinson’s Disease (PD). Despite being effective in managing PD-associated symptoms, its use in the clinics is limited by its highly hepatotoxic profile. Due to its low serum half-life and moderate oral bioavailability, frequent and high daily doses are necessary to obtain a significant therapeutic effect, which might enhance the risk of hepatotoxic events in patients. Hence, our study focuses on the development, characterization and early in vitro characterization of tolcapone-loaded nanostructured lipid carriers (NLCs) to improve tolcapone therapy. The newly designed NLCs, with sizes up to 120 nm, were prepared using the emulsion-solvent evaporation method with an entrapment efficiency (EE%) over 98%. Plus, NLCs were shown to be stable at different pHs and to have mucoadhesive properties. Cytotoxicity assays in HepG2 cells revealed that NLCs showed decreased cell viability, likely as a result of excessive intracellular lipid accumulation. Using Caco-2 and Caco-2/HT29-MTX co-culture models for permeability assays, the nanoformulations were capable of significantly transporting more tolcapone across the cell monolayers in comparison to free tolcapone without compromising cell monolayer integrity. Plus, the nanoformulations efficiently inhibited COMT in a cellular-based COMT inhibition assay using HepG2 cells. Early in vivo toxicological testing using Caenorhabditis elegans (C. elegans) demonstrated survival percentages > 88% after acute exposure to the nanoformulations in concentrations up to 100 µM. Together, the results obtained so far demonstrate that tolcapone-loaded NLCs have the potential to be a valid therapeutic option for the treatment of PD in the future.

Introduction

Parkinson’s disease (PD) is the second most prevalent neurodegenerative disorder, affecting millions of people worldwide and with prospects of increasing incidence in the near future. This represents not only a heavy burden for the health authorities but also has impacts on society and the economy, as the increasing incidence will demand more caring needs from families/communities to give some life quality to Parkinsonian patients (Hou et al., 2019). For this reason, the World Health Organization (WHO) recognized neurodegenerative disorders in general as a public health priority and presented a plan to mitigate the impact of such disorders on society, with emphasis on the development of more effective diagnosis and treatment tools (WHO, 2017). So far, PD treatment relies on palliative therapies focused on the management of the symptoms. Due to its multifactorial nature, involving protein aggregation, mitochondrial dysfunction, and oxidative stress, it has been a struggle to develop effective therapies capable of stopping the neurodegenerative progression, and no such therapy has reached the market yet (Fu et al., 2018, Sheikh et al., 2013, Udayar et al., 2022).

From the pharmacological toolbox available for the treatment of PD, the COMT inhibitor tolcapone is probably the most effective clinically (Connolly and Lang, 2014). COMT inhibitors are used as adjuvant therapy to avoid premature metabolization of the dopamine precursor and drug L-dopa, hence restoring the dopaminergic pool and correcting the associated dopaminergic-depletion symptoms such as bradykinesia, rigidity, tremors and postural instability (Stefano et al., 2012). Despite being effective, COMT inhibitors are generally used in PD later stages as last-resort drugs due to their side effects (Kaakkola, 2000). Tolcapone itself has a “black-box” warning by the FDA, meaning it can only be used in patients who are not responding satisfactorily to other adjunctive therapies, and the hepatic function has to be properly monitored along with the treatment due to the risk of fulminant hepatic failure (Olanow, 2000). Two other COMT inhibitors have been approved for PD treatment: entacapone and opicapone. However, tolcapone remains the most clinically effective drug (Jenner et al., 2021, Song et al., 2021).

Tolcapone is a nitrocatechol-based COMT inhibitor with high lipophilicity (logP = 3.3) and acid dissociation constant (pKa1 = 4.64; pKa2 = 10.20) (Kiss et al., 2010, Novaroli et al., 2006). Part of its hepatotoxic profile seems linked to its physicochemical properties, as it behaves as a mitochondrial uncoupler, leading to mitochondrial dysfunction and cell apoptosis in a dose-dependent manner, although another mechanism of toxicity involving highly reactive metabolites has been pointed out (Assal et al., 1998, Demine et al., 2019, Grunig et al., 2017, Smith et al., 2003). After oral administration, tolcapone is rapidly absorbed and biotransformed by first-pass metabolism in the liver, resulting in an overall bioavailability of 60–65% and a serum half-life of around 2 h (Jorga et al., 1998, Keating and Lyseng-Williamson, 2005). For that reason, tolcapone is recommended to be administered 3 times per day in 200 mg tablets. We hypothesize that the development of a drug delivery system with controlled tolcapone release properties would benefit tolcapone therapy and patient’s commodity by enhancing its oral bioavailability, serum half-life and reducing the risk of hepatotoxic events.

Since the mid-1900 s, colloidal nanosized drug delivery systems have been studied to improve drug drawbacks, thus improving their therapeutic efficacy (Kuentz, 2012, Nguyen et al., 2022). Lipid-based nanoparticles (LBNs) became widely studied drug delivery systems for oral drug delivery due to their low toxicity profile, good bioavailability, high drug loading content and relatively simple production processes that allow their large-scale production (Garcia-Pinel et al., 2019, Muller et al., 2000). From a therapeutic point of view, LBNs improve drug therapeutic outcomes by increasing gastrointestinal absorption and, consequently, drugs’ overall bioavailability (Dhiman et al., 2021, Nguyen et al., 2022). Plus, it offers controlled drug release properties, avoiding the rapid metabolization and elimination of drugs, expanding their serum half-life, and prolonging their effect (Li and Huang, 2008). This allows not only a reduction in the number of dosages over time, but also allows the administration of smaller doses of the drug, which decreases the risk of side effects.

Nanostructured lipid carriers (NLCs) were introduced in the early 2000 s to solve other LBNs drawbacks, such as the low entrapment efficiency and drug loading content, poor shelf stability and issues regarding controlled drug release properties (Muller et al., 2011, Nguyen et al., 2022). By combining solid lipids and liquid lipids in their formulation, NLCs have a less ordered structure in comparison to solid-lipid nanoparticles, allowing higher incorporation of drugs and improved stability (Iqbal et al., 2012). The use of NLCs to improve the oral delivery of drugs has been extensively reviewed, and there is consensus that NLCs improve the oral bioavailability and therapeutic outcomes of drugs, thus making NLCs one of the most desirable drug delivery systems in the field (Haider et al., 2020, Plaza-Oliver et al., 2021, Poonia et al., 2016). Plus, several studies demonstrated that TPGS, a non-ionic surfactant derivative of vitamin E, has been shown to improve NLCs capacity to overcome biological barriers and improve drug uptake (Mehata et al., 2023, Zhao et al., 2018).

In this work, we propose the development, characterization, and biological evaluation of tolcapone-loaded NLCs, plain and TPGS-modified, to overcome tolcapone oral bioavailability, improve its serum half-life, and ameliorate its hepatotoxic profile.

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Materials

Tolcapone was acquired from Amadis Chemicals (Zhejiang, China). Glycerol monostearate 40–55 was purchased from Roig Farma S.A. (Barcelona, Spain). Soy lecithin was acquired from Acofarma® (Barcelona, Spain). Miglyol® 812 N was kindly donated by IOI Oleochemical GmbH (Witten, Germany). Capryol® 90, Labrafil® M 1944, Labrasol®, Transcutol® HP, Plurol® Oleique CC 497 and Peceol® were generously presented by Gattefosse (Paris, France).

Miguel Pinto, Cláudia Sofia Machado, Sofia Costa, Victoria Díaz-Tomé, Bruno Sarmento, Francisco J. Otero-Espinar, Fernando Remião, Vinicius de Monte-Vidal, Selene Cuello-Rodríguez, Fernanda Borges, Carlos Fernandes, Tolcapone-loaded nanostructured lipid carriers for improved oral delivery, International Journal of Pharmaceutics, Volume 699, 2026, 126969, ISSN 0378-5173, https://doi.org/10.1016/j.ijpharm.2026.126969.