Peptidomimetics in Drug Design

Athanassios Giannis, *; Frank Rübsam    Institut für Organische Chemie und Biochemie der Universität Bonn, D–53121 Bonn, Germany

1 Introduction

A great diversity of peptides acting as neurotransmitters, neuromodulators, and hormones has been discovered and characterized during the last 30 years (Krieger, 1983; Schmidt, 1986; Schwarz, 1991). Many of these have been found in both neuronal and in non-neuronal tissues. After binding to their membrane-bound receptors, which belong mainly to the category of G protein-coupled receptors, they influence cell–cell communication and adhesion, and control a series of vital functions such as cell proliferation, tissue development, metabolism, immune defence, digestion, perception of pain, reproduction, behaviour, and blood pressure (Savarese and Fraser, 1992; Berridge, 1993). Peptides are also involved in the pathogenesis and/or maintenance of several diseases. For these reasons, selective agonists and particularly antagonists are indispensable for the investigation of peptidergic systems and can also be potential therapeutic agents.

As a result of major advances in organic chemistry and in molecular biology (Jung and Beck-Sickinger, 1992) most bioactive peptides have been prepared in larger quantities and made available for pharmacological and clinical experiments. However, the use of peptides as drugs is limited by the following factors: (a) their low metabolic stability towards proteolysis in the gastrointestinal tract and in serum; (b) their poor transport from the gastrointestinal tract to the blood and also their poor penetration into the central nervous system, in particular, due to their relatively high molecular mass or the lack of specific transport systems, or both; (c) their rapid excretion through liver and kidneys; and (d) their side-effects caused by interaction of the conformationally flexible peptides with distinct receptors. In addition, a bioactive peptide can cause effects on several types of cells and organ systems, since peptide receptors and/or isoreceptors can be widely distributed in an organism. In recent years intensive efforts have been made to develop peptidomimetics (Veber and Freidinger, 1985; Rose et al., 1985; De Grado, 1988; Freidinger, 1989; Morgan and Gainor, 1989; Hruby et al., 1990; Griffith, 1991; Hirschmann, 1991; Hölzemann, 1991; Rizo and Gierasch, 1992; Giannis and Kolter, 1993; Gante, 1994; Liskamp, 1994) which display more favourable pharmacological properties than their endogenous prototypes. For the purpose of this review we define a peptidomimetic as a compound that, as the ligand of a receptor, can mimic or block the biological effects of a peptide (Morgan and Gainor, 1989; Veber, 1992). As the ligand of an enzyme it can serve as substrate or as inhibitor. Enzyme ligands will be considered here only in special cases. In contrast, we will discuss basic principles of peptidomimetic design, presenting selected examples of ligands developed for several G-protein-coupled receptors as well as ligands for proteins involved in cell adhesion. Emphasis will be given to rational approaches to small nonpeptide ligands. Physiological functions and mode of action of the endogenous peptides as well as possible uses of peptidomimetics in therapy will also be discussed. An exhaustive treatment is beyond the scope of this review.

2 Design of Peptidomimetics

2.1 INTRODUCTION

As for any drug, a peptidomimetic must fulfil the following requirements: (a) metabolic stability, (b) good bioavailability, (c) high receptor affinity and receptor selectivity, and (d) minimal side-effects. For the rational design of such compounds knowledge of the biosynthesis, transport, release, and inactivation of the peptide are extremely useful. Regardless of their mode of action (as neurotransmitters, neuromodulators or hormones), peptides show some common characteristics (van Nispen and Pinder, 1986; Brownstein, 1989). First, they are synthesized in the ribosomes as higher molecular forms (prepro forms). Thereafter the pro forms are formed by cleavage of an N-terminal signal peptide by peptidases. During the subsequent vesicular transport to the Golgi apparatus they are processed further to their active forms and finally transported to the intracellular storage pools. They are released from the latter after stimulation. The inactivation of the peptides and thus termination of their biological action result mainly from the action of proteolytic enzymes (endo- and exo-peptidases).

2.2 THE BIOACTIVE CONFORMATION

Of crucial importance for the rational design of peptidomimetics are insights into the three-dimensional structure of the peptide–receptor complex, and also the subsequent signal transduction and the coordination and interaction with other signal transduction systems and integration in the organism. Our understanding of the mechanisms of signal transduction of peptide receptors has increased greatly in recent years (Berridge, 1993). However, due to the hydrophobic nature and size of the G-protein-coupled receptors the detailed determination of their three-dimensional structure has not yet been possible. In the study of peptide–receptor interactions impressive progress has been made through site-directed mutagenesis, molecular modelling and analysis of structure–activity relationships.

For the development of peptidomimetics having a peptidic nature the endogenous ligand generally serves as lead structure. Peptides of small to medium size (< 30–50 amino acid units) generally exist in dilute aqueous solution in a multitude of conformations in dynamic equilibrium (Fig. 1). If the ligand has the biologically active conformation (receptor-bound conformation), then an increased receptor affinity is expected, since the decrease in entropy on binding is less than that on the binding of a flexible ligand. In solution and in the absence of the receptor, the biologically active conformation may be poorly populated and is frequently quite different from the conformation obtained by, for example, X-ray or nuclear magnetic resonance (NMR) methods (Kessler, 1982; Fesik, 1991; Jorgensen, 1991; Wüthrich et al., 1991).

2.3 CONFORMATIONAL RESTRICTION

A successful method for the development of peptidomimetics involves synthesis of conformationally restricted compounds, i.e. locally or globally constrained peptide analogues that imitate the receptor-bound conformation of the endogenous ligands as closely as possible (Veber and Freidinger, 1985; Burt and Greer, 1988; Rizo and Gierasch, 1992). Investigations of these analogues show them to have increased metabolic stability, as well as increased receptor selectivity (Veber and Freidinger, 1985). The fact that, frequently, only a small number of three to eight amino acid side-chains in the peptide are responsible for the biological activity (‘message’) proves favourable for this approach (Kessler, 1982; Freidinger, 1989). In such cases the rest of the molecular framework may serve to present the pharmacophore in a specific conformation. In addition, in the part of the peptide not containing the message, additional binding affinity to various receptor types (an ‘address’) or to an interface (the plasma membrane) can be localized.

The metabolic stability of the peptide ligand can be controlled by the existence of cleavage sites for peptidases in this region of the molecule. Simplification of the structure of the ligand by removal or modification of this part leads to a simultaneous increase of the metabolic stability. Alternatively, the resistance toward amino- and carboxy-peptidases can be increased by acylation of the N-terminal group or by amidation of the C-terminus.

For the development of conformationally restricted peptide ligands the identification of the amino acids necessary for receptor recognition and activation is essential. For this reason shorter analogues are synthesized to identify the minimal sequence required for receptor binding and biological activity. Thereafter, the importance of parameters such as stereochemistry, charge, lipophilicity, and peptide backbone are examined by systematic changes in the individual amino acids. Compounds with rigid conformations are then produced, and the most active structures are selected by studying...