Generation and Characterization of a Human Bradykinin Receptor B1 Transgenic Rat as a Pharmacodynamic Model

Novel kinin B1 receptor agonists with improved pharmacological profiles

There is some evidence to suggest that inducible kinin B1 receptors (B1R) may play beneficial and protecting roles in cardiovascular-related pathologies such as hypertension, diabetes, and ischemic organ diseases. Peptide B1R agonists bearing optimized pharmacological features (high potency, selectivity and stability toward proteolysis) hold promise as valuable therapeutic agents in the treatment of these diseases. In the present study, we used solid-phase methodology to synthesize a series of novel peptide analogues based on the sequence of Sar[dPhe(8)]desArg(9)-bradykinin, a relatively stable peptide agonist with moderate affinity for the human B1R. We evaluated the pharmacological properties of these peptides using (1) in vitro competitive binding experiments on recombinant human B1R and B2R (for index of selectivity determination) in transiently transfected human embryonic kidney 293 cells (HEK-293T cells), (2) ex vivo vasomotor assays on isolated human umbilical veins expressing endogenous human B1R, and (3) in vivo blood pressure tests using anesthetized lipopolysaccharide-immunostimulated rabbits. Key chemical modifications at the N-terminus, the positions 3 and 5 on Sar[dPhe(8)]desArg(9)-bradykinin led to potent analogues. For example, peptides 18 (SarLys[Hyp(3),Cha(5), dPhe(8)]desArg(9)-bradykinin) and 20 (SarLys[Hyp(3),Igl(5), dPhe(8)]desArg(9)-bradykinin) outperformed the parental molecule in terms of affinity, functional potency and duration of action in vitro and in vivo. These selective agonists should be valuable in future animal and human studies to investigate the potential benefits of B1R activation.

A new class of bradykinin B1 receptor antagonists with high oral bioavailability and minimal PXR activity

The design and synthesis of a novel class of human bradykinin B1 antagonists featuring difluoroethyl ether and isoxazole carboxamide moieties are disclosed. Compound 7g displayed excellent pharmacokinetic properties, efficient ex vivo receptor occupancy, and low potential for P450 induction via PXR activation.

Potent bradykinin B1 receptor antagonists: 4-Substituted phenyl cyclohexanes

Selective bradykinin (BK) B(1) receptor antagonists have been shown to be antinociceptive in animal models and could be novel therapeutic agents for the treatment of pain and inflammation. Elucidation of the structure-activity relationships of the biphenyl moiety of the lead compound 1 provided a potent new structural class of BK B(1) receptor antagonists.

Neuropeptide and kinin antagonists

Neuropeptides and kinins are important messengers in the nervous system and--on the basis of their anatomical localisation and the effects produced when the substances themselves are administered, to animals or to human subjects-a significant number of them have been suggested to have a role in pain and inflammation. Experiments in gene deletion (knock-out or null mutant) mice and parallel experiments with pharmacological receptor antagonists in a variety of species have strengthened the evidence that a number of peptides, notably substance P and calcitonin gene-related peptide (CGRP), and the kinins have a pathophysiological role in nociception. Clinical studies with non-peptide pharmacological antagonists are now in progress to determine if blocking the action of these peptides might have utility in the treatment of pain.

Non-peptide antagonists for kinin B1 receptors: new insights into their therapeutic potential for the management of inflammation and pain

Kinin B1 and B2 receptors are central to the aetiology of pain and inflammation. Constitutive B2 receptors are commonly associated with the acute phase of inflammation and nociception, whereas the inducible B1 receptors are mostly linked to the chronic or persistent phase (or both). Therefore, selective, orally active kinin B1 receptor antagonists could be potentially therapeutic. B1 receptor antagonists have long been exclusively peptides, but recently a few non-peptide representatives have been identified. The clinical potential of these non-peptide molecules has not yet been evaluated, but they might have a role in treating persistent inflammation and pain, especially when no satisfactory therapy is available. This review summarizes recent advances in the identification and the potential therapeutic properties of these molecules.

Genetically altered animal models in the kallikrein-kinin system

Transgenic and gene-targeting technologies allowing the generation of genetically altered animal models have greatly advanced our understanding of the function of specific genes. This is also true for the kallikrein-kinin system (KKS), in which some, but not yet all, components have been functionally characterized using such techniques. The first genetically altered animal model for a KKS component was supplied by nature, the brown Norway rat carrying an inactivating mutation in the kininogen gene. Mice deficient in tissue kallikrein, B1 and B2 receptors, some kinin-degrading enzymes, and factor XII followed, together with transgenic rat and mouse strains overexpressing tissue kallikrein, B1 and B2 receptors, and degrading enzymes. There are still no animal models with genetic alterations in plasma kallikrein, kininases I and some other degrading enzymes. The models have confirmed an important role of the KKS in cardiovascular pathology, inflammation, and pain, and have partially elucidated the distinct function of the two receptors. This created the basis for rational decisions concerning the putative use of kinin receptor agonists and antagonists in therapeutic applications. However, a more thorough analysis of the existing models and the generation of new, more sophisticated transgenic models will be necessary to clarify the still elusive issue as to where and by which mechanisms the kinins exert their actions.

Transgenic modifications of the rat genome

The laboratory rat (R. norvegicus) is a very important experimental animal in several fields of biomedical research. This review describes the various techniques that have been used to generate transgenic rats: classical DNA microinjection and more recently described techniques such as lentiviral vector-mediated DNA transfer into early embryos, sperm-mediated transgenesis, embryo cloning by nuclear transfer and germline mutagenesis. It will also cover techniques associated to transgenesis such as sperm cryopreservation, embryo freezing and determination of zygosity. The availability of several technologies allowing genetic manipulation in the rat coupled to genomic data will allow biomedical research to fully benefit from the rat as an experimental animal.

Generation and characterization of a humanized bradykinin B1 receptor mouse

Antagonists of the B1 bradykinin receptor (B1R), encoded by the BDKRB1 gene, offer the promise of novel therapeutic agents for inflammatory and neuropathic pain. However, the in vivo characterization of the pharmacodynamics of B1R antagonists is hindered by the low level of B1R expression in healthy tissue and the profound species selectivity exhibited by many compounds for the B1R. To circumvent these issues we generated two genetically engineered rodent models. The first is a transgenic rat over-expressing the human B1R under the control of the neuronal-specific enolase promoter; we previously reported the utility of this model in assessing human B1R receptor occupancy in the central nervous system of the rat. The second model, reported here, utilized gene-targeting by homologous recombination to replace the genomic coding sequence for the endogenous mouse B1R with that of the human B1R. The mRNA expression profile of the humanized Bdkrb1 (hBkdrb1) allele is similar to that of the mouse Bdkrb1 (mBkdrb1) in the wild-type animal. Furthermore, in vitro assays indicate that tissues isolated from the humanized mouse possess pharmacological properties characteristic of the human B1R. Therefore, we have generated a humanized B1R mouse model that is suitable for testing the efficacy of human B1R-selective compounds.

Humanizing mice: catching up with elusive B1 receptors

Bradykinin receptor activation plays an important role in pain arising following tissue inflammation, and recent studies have suggested that bradykinin B1 receptors in particular may be important in chronic pain related to arthritis and various neuropathies. The investigation of the function of the B1 receptors in vivo has been hampered by the lack of nonpeptide antagonists, and the development of such compounds made more difficult by the considerable species variation between human and rodent B1 receptors. In this issue, Fox and co-workers report the creation of a mouse that has had the human B1 gene inserted into the corresponding mouse locus, and they exploit this animal to study the effects of a novel, nonpeptide B1 receptor antagonist on measures of acute nociception and nociception following inflammation. By creating a platform that allows the study of human B1 receptors in vivo, these investigators have provided a tool to significantly advance the understanding of the kallikrein-kinin system in physiological and pathophysiological states.

Antihyperalgesic activity of a novel nonpeptide bradykinin B1 receptor antagonist in transgenic mice expressing the human B1 receptor

We describe the properties of a novel nonpeptide kinin B1 receptor antagonist, NVP-SAA164, and demonstrate its in vivo activity in models of inflammatory pain in transgenic mice expressing the human B1 receptor. NVP-SAA164 showed high affinity for the human B1 receptor expressed in HEK293 cells (K(i) 8 nM), and inhibited increases in intracellular calcium induced by desArg10kallidin (desArg10KD) (IC50 33 nM). While a similar high affinity was observed in monkey fibroblasts (K(i) 7.7 nM), NVP-SAA164 showed no affinity for the rat B1 receptor expressed in Cos-7 cells. In transgenic mice in which the native B1 receptor was deleted and the gene encoding the human B1 receptor was inserted (hB1 knockin, hB1-KI), hB1 receptor mRNA was induced in tissues following LPS treatment. No mRNA encoding the mouse or human B1 receptor was detected in mouse B1 receptor knockout (mB1-KO) mice following LPS treatment. Freund's complete adjuvant-induced mechanical hyperalgesia was similar in wild-type and hB(1)-KI mice, but was significantly reduced in mB1-KO animals. Mechanical hyperalgesia induced by injection of the B1 agonist desArg10KD into the contralateral paw 24 h following FCA injection was similar in wild-type and hB1-KI mice, but was absent in mB1-KO animals. Oral administration of NVP-SAA164 produced a dose-related reversal of FCA-induced mechanical hyperalgesia and desArg10KD-induced hyperalgesia in hB1-KI mice, but was inactive against inflammatory pain in wild-type mice. These data demonstrate the use of transgenic technology to investigate the in vivo efficacy of species selective agents and show that NVP-SAA164 is a novel orally active B1 receptor antagonist, providing further support for the utility of B1 receptor antagonists in inflammatory pain conditions in man.

Development of an efficient and selective radioligand for bradykinin B1 receptor occupancy studies

We have developed an efficient and selective radioligand, the [35S]-radiolabeled dihydroquinoxalinone derivative, 4, for an ex vivo receptor occupancy assay in transgenic rats over-expressing the human bradykinin B1 receptor.

Bradykinin receptor ligands: therapeutic perspectives

Kinins, which are produced by the action of kallikrein enzymes, are blood-derived local-acting peptides that have broad effects mediated by two related G-protein-coupled receptors termed the bradykinin receptors. The endogenous kallikrein-kinin system controls blood circulation and kidney function, and promotes inflammation and pain in pathological conditions, which has led to interest in developing modulators of bradykinin receptors as potential therapeutics. This review discusses recent progress in our understanding of the genetics, molecular biology and pathophysiology of kinins and their receptors, as well as developments in medicinal chemistry, which have brought us closer to therapeutic applications of kinin receptor ligands in various indications. The potential of kinin receptor antagonists as novel analgesic agents that do not result in tolerance or have a liability for abuse has attracted particular interest.

Pharmacological characterization and radioligand binding properties of a high-affinity, nonpeptide, bradykinin B1 receptor antagonist

Compound A (N-[2-[4-(4,5-dihydro-1H-imidazol-2-yl)phenyl]ethyl]-2-[(2R)-1-(2-napthylsulfonyl)-3-oxo-1,2,3,4-tetrahydroquinoxalin-2-yl]acetamide) is a member of a new class of aryl sulfonamide dihydroquinoxalinone bradykinin B1 receptor antagonists that should be useful pharmacological tools. Here we report on some of the pharmacological properties of compound A as well as the characterization of [35S]compound A as the first nonpeptide bradykinin B1 receptor radioligand. Compound A inhibited tritiated peptide ligand binding to the cloned human, rabbit, dog, and rat bradykinin B1 receptors expressed in CHO cells with Ki values of 0.016, 0.050, 0.56, and 29 nM, respectively. It was inactive at 10 microM in binding assays with the cloned human bradykinin B2 receptor. In functional antagonist assays with the cloned bradykinin B1 receptors, compound A inhibited agonist-induced signaling with activities consistent with the competition binding results, but had no antagonist activity at the bradykinin B2 receptor. Compound A was also found to be a potent antagonist in a rabbit aorta tissue bath preparation and to effectively block des-Arg9 bradykinin depressor responses in lipopolysaccharide-treated rabbit following intravenous administration. The binding of [35S]compound A was evaluated with the cloned bradykinin B1 receptors. In assays with human, rabbit, and dog receptors, [35S]compound A labeled a single site with Kd values of 0.012, 0.064, and 0.37 nM, respectively, and with binding site densities equivalent to those obtained using the conventional tritiated peptide ligands. Binding assays with the cloned rat bradykinin B1 receptor were not successful, presumably due to the low affinity of the ligand for this species receptor. There was no specific binding of the ligand detected in CHO cells expressing the human bradykinin B2 receptor. In assays with the cloned human bradykinin B1 receptor, the pharmacologies of the binding of [35S]compound A and [3H][Leu9]des-Arg10-kallidin were the same. The high signal-to-noise ratio obtained with [35S]compound A will allow this ligand to be a very useful tool for future investigations of the bradykinin B1 receptor.