Chemical and enzymatic stability as well as transport properties of a Leu-enkephalin analogue and ester prodrugs thereof

Abstract

The Leu-enkephalin analogue (Tyr-d-Ala-Gly-Phe-Leu-NH₂) was synthesized together with three esters prodrugs hereof. The prodrugs synthesized were the O-acetyl, O-propionyl and O-pivaloyl esters of the tyrosine phenolic group. The compounds were isolated in good purity (HPLC purity >99%) and in good yields (60–75%). The chemical and enzymatic stability of the prodrugs has been investigated in detail. The prodrugs studied are quite chemically stable and the degradation of the prodrugs follows the pattern previously shown for similar esters (U-shaped pH-profile; maximal stability at pH 4–5). The prodrugs are degraded quantitatively in plasma to the parent peptide with half-lives in the range 2.9 min–2.6 h. Type B esterases were shown to be involved in the degradation as the half-lives increased in the presence of paraoxon. No significant stabilization was seen in 10% porcine gut homogenate. Half-lives in the same order were seen for the analogue and the prodrugs in pure Leucine aminopeptidase solution. The analogue was stable in Carboxypeptidase A solution whereas a faster degradation of the prodrugs was seen in this media. Furthermore the transport properties of the compounds has been studied. A P_app value of 0.284×10⁻⁶ cm/s for the analogue was obtained for the transport across Caco-2 cell monolayers in the BL-AP direction. The P_app values were increased by a factor of 2, 7 and 18 for the acetyl-, propionyl- and pivaloylprodrug. The increase could be explained by higher lipofilicities of the prodrugs compared to the analogue.

Introduction

Application of peptides as clinically useful drugs represents a major challenge. This is due to their poor delivery characteristics caused by their metabolic instability and in general a non-lipophilic character resulting in poor biomembrane passage. This typically leads to bioavailabilities less than 1–2%. Once within the systemic circulation short biological half-lives are seen due to rapid metabolism and clearance from the body [1], [2], [3]. Clinical development of drug candidates that are peptide based (e.g. the opioid peptides [Leu⁵]-enkephalin and [Met⁵]-enkephalin) has therefore been restricted.

Leu- and Met-enkephalin (Tyr-Gly-Gly-Phe-Leu/Met) were first isolated from whole pig brains and their structure determined in 1975 [4]. It was shown that the enkephalins induced analgesia upon central administration in rats [5]. However, the metabolic degradation of enkephalins at all absorptive mucosae and within the body present a significant barrier to the use of these peptides as drugs. The most important degradation pathway for enkephalins in the intestinal brush-border appears to be cleavage of the Tyr¹–Gly² bond by aminopeptidases [6], [7], [8], [9], [10], [11]. Of minor importance at this site is Endopeptidase 24.11 cleaving the Gly³–Phe⁴ bond [11]. The only luminal enzyme capable of cleaving enkephalins at a significant rate appear to be Carboxypeptidase A [12]. It has been estimated that the degradation of Leu-enkephalin in the intestine was 10% by luminal enzymes whereas the contribution from brush-border enzymes was 90% [7]. In plasma as well as at the blood–brain barrier the major degradation route is cleavage at the Tyr¹–Gly² mediated by aminopeptidases [12], [13], [14], [15]. Another drawback is that the enkephalins are too hydrophilic to be absorbed by the transcellular route resulting in poor transport properties [14], [16], [17].

In recent years the use of the prodrug principle has been investigated in order to improve the delivery characteristics of peptides. The approach has been used on model peptides as well as on bioactive peptides. It has been shown that it’s possible to improve the transport across biomembranes and/or the stability against degradation by enzymes (for reviews see [18], [19], [20]).

Various prodrugs have been made of enkephalins and enkephalin analogues. 4-imidazolidinones, a prodrug derivative of the α-aminoamide moiety found at the N-terminal end of enkephalins, have been made of both Leu-enkephalin and Met-enkephalin. The prodrug formation resulted in stabilization against degradation by Aminopeptidase N, Leucine aminopeptidase, Peptidyl peptidase A (angiotensin converting enzyme-ACE) and Carboxypeptidase A as well as an improved delivery across bovine BMEC monolayers [12], [17], [21]. Another approach has been cyclization of the peptide backbone from the N-terminal to the C-terminal using different chemical linkers [22]. Cyclization enhances the extent of intramolecular hydrogen bonding, it reduces the potential for intermolecular hydrogen binding to aqueous solvents, it blocks the N- and C-terminal of the peptide affording protection against degradation by amino- and carboxypeptidases and a possible reduction of the overall charge and a reduction of size may all together improve the delivery of the derivatized peptide [19], [23]. A coumaric acid-based, a phenylpropionic acid-based and an acyloxyalkoxy-based cyclic prodrug has been prepared of Leu-enkephalin and the metabolically stable analogue DADLE (Tyr-d-Ala-Gly-Phe-d-Leu) [24], [25], [26]. The coumaric acid-based and the phenylpropionic acid-based prodrugs showed an enhanced transport across Caco-2 cell monolayers [24], [26]. In contrast the acyloxyalkoxy-based prodrug showed a much higher transport from the basolateral to the apical side than from the apical to the basolateral side in the Caco-2 model suggesting that this prodrug type is substrate for an apically polarized efflux system [24], [25]. In another study prodrugs of cyclic conformational restricted Met-enkephalin analogues have been synthesized by adding amino acids to either the C-terminal or the N-terminal. The prodrugs synthesized showed improved permeability across bovine BMEC monolayers [27].

In this study we have synthesized the Leu-enkephalin analogue Tyr-d-Ala-Gly-Phe-Leu-NH₂. Substitution of Gly² with d-Ala² significantly decreases the enzymatic hydrolysis of the Tyr¹–Gly² bond [7], [9]. Amidation of the C-terminal offers protection against degradation by carboxypeptidases [28]. To further investigate the use of the prodrug principle on opioid peptides three different ester prodrugs of the tyrosine phenolic group has been synthesized. The chemical and enzymatic stability as well as the transport properties have been investigated. The general structure of the parent compound and the prodrugs is shown in Fig. 1. An outline of the synthesis procedure is shown in Fig. 2.