Inhibitors of dipeptidyl peptidase 4

Dipeptidyl peptidase 4 (EC 3.4.14.5, DPP-IV, DPP4, CD26) is a ubiquitous serine protease that modulates the biological activities of numerous peptides, including glucagon-like peptide-1 (GLP-1). GLP-1 plays an important role in the control of post-prandial glucose levels by potentiating glucose-stimulated insulin release and inhibiting the release of glucagon. Other actions of GLP-1 include delaying gastric emptying, inducing satiety and increasing beta cell mass. GLP-1 has shown efficacy in diabetics, but suffers from a very short physiological half-life (t1/2 ∼2 min) due to DPP4-mediated cleavage of the active peptide (7-36 amide or 7-37) to an inactive form (9-36 amide or 9-37). Intense research in the pharmaceutical industry aims to discover and develop stable GLP-1 analogs, exogenous agonists of the GLP-1 receptor or small-molecule inhibitors of DPP4. This research has been buoyed recently by positive clinical trial data on GLP-1 analogs and DPP4 inhibitors. The field of DPP4 inhibition has been reviewed extensively [1-12]. This review attempts to provide an update to the previous ARMC article on DPP4 inhibitors [13] covering the primary literature from 2001 through the end of March 2005. It is not the intent of the authors to provide another review of the pharmacology of DPP4, but to concentrate on the medicinal chemistry in the field. Function of DPP4: DPP4 functions as a serine protease and cleaves the amino-terminal dipeptide from oligopeptides with a proline or alanine at the penultimate position. Peptides with residues other than Pro or Ala at the penultimate position may also be low-affinity substrates for DPP4. In contrast, DPP4 is not selective with respect to the N-terminal residue [14] and shows little discrimination of various prime-side residues [15,16]. A number of biologically important peptides are substrates for DPP4 in vitro [17,18]. Structure of DPP4: DPP4 is a 110-kDa glycoprotein expressed on the cell surface and widely distributed throughout the body. Cleavage of the extracellular portion of DPP4 from the 22-residue transmembrane section results in a soluble, circulating form of approximately 100 kDa. Functional DPP4 is a homodimer, although an active heterodimer with fibroblast activation protein has been observed [19]. The consensus sequence for DPP4 is G-W-S-Y-G and the catalytic triad is made up of Ser630, Asp708 and His740. It has been shown that the glycosylation state of the enzyme is not important for enzyme activity, dimerization, and adenosine deaminase binding [20]. Several groups have reported crystal structures of human DPP4 [15,21-24], and one group has reported the structure of porcine DPP4 [25]. These structures show the dimeric nature of the enzyme and reveal that the catalytic site is located in a cavity between the α/β hydrolase domain and an eight-bladed propeller domain. Also revealed is the oxyanion hole, which is composed of the backbone NH of Tyr631 and the OH of Tyr47. A co-complex of DPP4 and the inhibitor Val-pyrrolidide demonstrates that two glutamates in the active site play an important role in substrate binding by forming a salt bridge with the N-terminus of a peptide substrate. The pyrrolidine of the inhibitor effectively fills a hydrophobic pocket that will only accommodate small residues. This pocket engenders DPP4's selectivity for proline at P1. This work also revealed that two openings in the enzyme may provide access to and egress from the catalytic site for some substrates and products [21]. The importance of Tyr547 in the stabilization of the intermediate oxyanion was confirmed through site-directed mutagenesis [26]. Most authors agree that peptides enter the larger side opening to access the active site [15]. It has been postulated that the dipeptide product is expelled through the narrow β-propeller opening [21,24]. The co-complex of DPP4 and a compound related to NVP-DPP728 [23] confirms that cyanopyrrolidine inhibitors form an imidate with the active site serine, consistent with a model proposed earlier [27]. Two groups have observed the trapping of tetrahedral intermediates in co-complexes of peptides with DPP4 [15,24]. Therapeutic significance: Relative to wild-type controls, DPP4-deficient mice are resistant to the development of obesity and hyperinsulinemia when fed a high-fat diet [28]. DPP4 knockout mice also show elevated GLP-1 levels and improved metabolic control. Relative to DPP4 positive controls, DPP4-deficient Fischer rats show improved glucose tolerance following an oral glucose challenge due to enhanced insulin release mediated by high levels of active GLP-1 [29,30]. In these studies, the authors note that fasting and post-challenge glucose levels in both strains are similar, supporting previous assertions that hypoglycemia is unlikely during treatment with DPP4 inhibitors. The use of GLP-1 and its analogs in the treatment of diabetes has been reviewed recently [31,32]. It has been shown that DPP4 inhibition prevents the degradation of endogenous GLP-1 and glucose-dependent insulinotropic polypeptide (GIP) in dogs, thereby preserving the insulinotropic effects of these peptides [33]. In the same study, it was noted that total incretin secretion was reduced, suggesting that feedback mechanisms restrict the secretion of incretins when levels of active peptide are elevated. It has been demonstrated that agonism of the GLP-1 receptor results in growth and differentiation of pancreatic islet beta cells [34-36]. If realized in humans, such an effect may result in preservation or restoration of β-cell function in diabetics. In human clinical trials, infusion of GLP-1 led to such beneficial effects as decreases in post-prandial glucose excursions, increases in post-prandial insulin, reductions in HbA1c, weight loss, enhanced insulin sensitivity and improved β-cell function [37,38]. Administration of the GLP-1 analogs exendin-4, CJC-1131 and NN2211 resulted in similar beneficial effects [31,32]. Notably, DPP4 inhibition has been shown to augment the insulin secretion effects of not only GLP-1 and GIP, but also pituitary adenylate cyclase-activating polypeptide (PACAP) and gastrin-releasing peptide (GRP) [39]. © 2005 Elsevier Inc. All rights reserved.