The synthesis and property evaluation of novel L -dopa prodrugs for the treatment of Parkinson’s disease
5.1. Introduction
L-Dopa (3,4-dihydroxy-L-phenylalanine) is a naturally occurring amino acid first isolated from the bean of Vicia faba in 1910–1911 (Fig. 5.1) (Hornykiewicz, 2010). In the human, dietary and clinically administered L-dopa is absorbed from the gastrointestinal tract via the amino acid transport machinery. L-Dopa also gains access to the central nervous system via amino acid transporters at the blood-brain barrier and thus dietary amino acids are avoided to decrease competition for transport in intestine and at the blood-brain barrier (Camargo et al., 2014). L-Dopa is extensively metabolised with approximately 70% of an oral dose undergoing pre-systemic decarboxylation to DA by the enzyme, aromatic L-amino acid decarboxylase (AADC), present in the stomach, lumen of the intestine, kidney and liver (Khor & Hsu, 2007;
Contin & Martinelli, 2010). Another prominent metabolic pathway for L-dopa is 3-O-methylation by hepatic catechol-O-methyltransferase (COMT) to yield 3-O-methyldopa (Nutt & Fellman, 1984). L-Dopa thus has a short half-life of approximately 0.7 to 1.4 h (Contin et al., 1990). In spite of these and other shortcomings, L-dopa is used as DA replacement therapy in PD and since its first use in the 1960s, remains the most effective treatment (Freitas et al., 2016;
Poewe & Antonini, 2015). To enhance bioavailability and limit peripheral metabolism, L-dopa is co-administered with inhibitors of AADC such as carbidopa or benserazide (Seeberger &
Hauser, 2015). This greatly enhances the systemic bioavailability of an oral L-dopa dose. The metabolism of L-dopa may be further reduced and efficacy enhanced by administering COMT inhibitors such as entacapone (Nutt, 2000; Learmonth et al., 2004; Nissinen et al., 1992). Of great clinical significance is the observation that DA itself cannot be used in the treatment of PD because of its inability to penetrate the blood-brain barrier. DA generated in the periphery from L-dopa, thus does not have access to the brain, and enhances peripheral dopaminergic side effects (Hornykiewicz, 2010). L-Dopa thus is a prodrug which enters the brain and is decarboxylated to yield DA.
Figure 5.1: The structures of L-dopa and other compounds discussed in the text.
Recently, much effort has been devoted to improving the pharmacokinetic profile of L-dopa by novel formulations to improve absorption, exploring nonoral routes of administration and reducing peripheral metabolism (Freitas et al., 2016; Poewe & Antonini, 2015). For example, IPX066 is a novel extended-release oral formulation of L-dopa/carbidopa that combines immediate-release with extended-release (Freitas et al., 2016; Hauser et al., 2013). This formulation has recently been approved in the USA and the EU. XP21279 is an orally active prodrug of L-dopa that is absorbed from the small and large intestine by high-capacity nutrient transporters (Lewitt et al., 2012). Unfortunately the development of this prodrug has been discontinued. ODM-101, an oral formulation of L-dopa/carbidopa/entacapone, delivers a higher dose of carbidopa. AP09004 is an extended release ‘accordion pill’ formulation of L- dopa/carbidopa with gastroretentive properties (Freitas et al., 2016). DM-1992 is a bilayer formulation and consists of immediate-release and extended-release layers of L- dopa/carbidopa (Verhagen Metman et al., 2015). An intestinal gel, which is infused directly into the proximal jejunum, contains a suspension of L-dopa/carbidopa in carboxymethyl- cellulose and represents an approved therapy (Olanow et al., 2014). ND0612 is a liquid formulation of L-dopa/carbidopa for subcutaneous administration by a patch-pump device, while CVT-301 is a L-dopa inhalation powder with rapid onset of action (Freitas et al., 2016;
LeWitt et al., 2016).
A number of experimental prodrugs of L-dopa have also been designed and evaluated (Di Stefano et al., 2011). For example, a prodrug (1) in which L-dopa is linked via a biodegradable carbamate to entacapone has been reported (Fig. 5.2) (Savolainen et al., 2000; Leppänen et
al., 2002). In this respect, prodrugs (2–3) with benserazide linked to L-dopa have also been designed (Di Stefano et al., 2006). Peptidyl prodrugs such as the tripeptide mimetic prodrug, in which D-p-hydroxyphenylglycine and L-proline is linked to L-dopa (4), was designed as a delivery system for improved L-dopa oral absorption (Wang et al., 1995). L-Dopa has also been linked via an amide bond with glutathione (5), in an attempt to reduce oxidative stress, a process that has been linked to neurodegeneration in PD (Pinnen et al., 2007). Glycosyl prodrugs such as 6, in turn, have been designed for active transport across the blood-brain barrier (Di Stefano et al., 2008). Cyclic prodrugs of L-dopa such as 7 may possess enhanced absorption and metabolic resistance toward AADC (Cingolani et al., 2000). The general approach to designing L-dopa prodrugs is attaching appropriate carriers at the aminium, carboxylate or phenolic hydroxyl groups. These points of attachment should be biodegradable to allow for the release of L-dopa and the carrier. In this respect, carriers are most frequently linked to L-dopa via the ester, amide (peptide) and carbamate functions (Di Stefano et al., 2011).
Figure 5.2: The structures of selected experimental prodrugs of L-dopa (Di Stefano et al., 2011).
Based on the interest and therapeutic potential ofL-dopa prodrugs, the present study synthesises four carrier-linked prodrugs (8–11) of L-dopa in which 4-pyridylmethylamine, 2- (4-pyridyl)ethylamine, 2-(2-pyridyl)ethylamine and 3-phenyl-1-propylamine are linked to the carboxylate of L-dopa (Fig. 5.3). The key physicochemical and biochemical parameters of the prodrugs were subsequently evaluated in an attempt to assess the potential of these prodrugs as vehicles to enhance the absorption and central delivery of L-dopa. These selected carriers were linked to L-dopa at the carboxylate with the primary aminyl functional group. This would protect the carboxylic acid of L-dopa against peripheral decarboxylation and possibly enhance passive diffusion permeability by elimination of the carboxylate charge. Furthermore, unlike L-dopa, the prodrugs do not contain the carboxylate group, which is known to reduce membrane permeation of small organic compounds (Gleeson, 2008; Manallack et al., 2013). A further consideration is that the relative stability of the amide link may allow the prodrug more time to diffuse into the brain prior to activation, thus
effectively delivering L-dopa in the brain. Since the prodrugs are expected to be more lipophilic than L-dopa, absorption from the gastrointestinal tract by passive diffusion and enhanced penetration of the blood-brain barrier are probable. The selection of the pyridine- containing carriers in this study was based on the high Caco-2 permeability of pyridine (Chen et al., 2006). Although chemical substitution of pyridine reduces permeability, the overall effect of the addition of the pyridine moiety to the prodrug would be an enhancement of permeability. The 3-phenyl-1-propylamine carrier was included as a comparator for the prodrugs incorporating the pyridine function.
Figure 5.3: The structures of the L-dopa prodrugs (8–11) examined in this study.