Building bridges for highly selective, potent and stable oxytocin and vasopressin analogs

Oxytocin and vasopressin signaling in health and disease

Neurohypophysial peptides are ancient and evolutionarily highly conserved neuropeptides that regulate many crucial physiological functions in vertebrates and invertebrates. The human neurohypophysial oxytocin/vasopressin (OT/VP) signaling system with its four receptors has become an attractive drug target for a variety of diseases, including cancer, pain, cardiovascular indications, and neurological disorders. Despite its promise, drug development faces hurdles, including signaling complexity, selectivity and off-target concerns, translational interspecies differences, and inefficient drug delivery. In this review we dive into the complexity of the OT/VP signaling system in health and disease, provide an overview of relevant pharmacological probes, and discuss the latest trends in therapeutic lead discovery and drug development.

Targeting the Oxytocin Receptor for Breast Cancer Management: A Niche for Peptide Tracers

Breast cancer is a leading cause of death in women, and its management highly depends on early disease diagnosis and monitoring. This remains challenging due to breast cancer's heterogeneity and a scarcity of specific biomarkers that could predict responses to therapy and enable personalized treatment. This Perspective describes the diagnostic landscape for breast cancer management, molecular strategies targeting receptors overexpressed in tumors, the theranostic potential of the oxytocin receptor (OTR) as an emerging breast cancer target, and the development of OTR-specific optical and nuclear tracers to study, visualize, and treat tumors. A special focus is on the chemistry and pharmacology underpinning OTR tracer development, preclinical in vitro and in vivo studies, challenges, and future directions. The use of peptide-based tracers targeting upregulated receptors in cancer is a highly promising strategy complementing current diagnostics and therapies and providing new opportunities to improve cancer management and patient survival.

The Relationship Between Oxytocin and Alcohol Dependence

The hypothalamic neuropeptide oxytocin (OT) is well known for its prosocial, anxiolytic, and ameliorating effects on various psychiatric conditions, including alcohol use disorder (AUD). In this chapter, we will first introduce the basic neurophysiology of the OT system and its interaction with other neuromodulatory and neurotransmitter systems in the brain. Next, we provide an overview over the current state of research examining the effects of acute and chronic alcohol exposure on the OT system as well as the effects of OT system manipulation on alcohol-related behaviors in rodents and humans. In rodent models of AUD, OT has been repeatedly shown to reduce ethanol consumption, particularly in models of acute alcohol exposure. In humans however, the results of OT administration on alcohol-related behaviors are promising but not yet conclusive. Therefore, we further discuss several physiological and methodological limitations to the effective application of OT in the clinic and how they may be mitigated by the application of synthetic OT receptor (OTR) agonists. Finally, we discuss the potential efficacy of cutting-edge pharmacology and gene therapies designed to specifically enhance endogenous OT release and thereby rescue deficient expression of OT in the brains of patients with severe forms of AUD and other incurable mental disorders.

Oxytocin measurements in saliva: an analytical perspective

Oxytocin has traditionally been known for its physiological effects on muscle contraction associated with birth and lactation, but in the last years is widely used as a biomarker of "positive experiences" in psychology and behavior. Different types of samples have been used for oxytocin measurements with saliva samples having the particular advantage of an easy and non-stressful collection. However, the low concentration of oxytocin in saliva can represent a limitation for its use. For this reason, sensitive assays and even a previous sample treatment in some cases are required for saliva oxytocin quantification. In addition, the lack of standardized and generally agreed-upon approach to peripheral oxytocin measurement leads to large discrepancies between different laboratories, that use different sample treatment protocols and different assays. The main objectives of this review are to describe the current status of the use of saliva for oxytocin measurement, provide details of the different sample processing techniques that can be applied and inform about the analytical techniques and assays available in different animal species, and also in humans for comparative purposes. It is expected that this information can contribute to an increase in the knowledge about the measurements of oxytocin in saliva and to its wider use in the future.

Designing Formulation Strategies for Enhanced Stability of Therapeutic Peptides in Aqueous Solutions: A Review

Over the past few decades, there has been a tremendous increase in the utilization of therapeutic peptides. Therapeutic peptides are usually administered via the parenteral route, requiring an aqueous formulation. Unfortunately, peptides are often unstable in aqueous solutions, affecting stability and bioactivity. Although a stable and dry formulation for reconstitution might be designed, from a pharmaco-economic and practical convenience point of view, a peptide formulation in an aqueous liquid form is preferred. Designing formulation strategies that optimize peptide stability may improve bioavailability and increase therapeutic efficacy. This literature review provides an overview of various degradation pathways and formulation strategies to stabilize therapeutic peptides in aqueous solutions. First, we introduce the major peptide stability issues in liquid formulations and the degradation mechanisms. Then, we present a variety of known strategies to inhibit or slow down peptide degradation. Overall, the most practical approaches to peptide stabilization are pH optimization and selecting the appropriate type of buffer. Other practical strategies to reduce peptide degradation rates in solution are the application of co-solvency, air exclusion, viscosity enhancement, PEGylation, and using polyol excipients.

Translational aspect in peptide drug discovery and development: An emerging therapeutic candidate

In the last two decades, protein-protein interactions (PPIs) have been used as the main target for drug development. However, with larger or superficial binding sites, it has been extremely difficult to disrupt PPIs with small molecules. On the other hand, intracellular PPIs cannot be targeted by antibodies that cannot penetrate the cell membrane. Peptides that have a combination of conformational rigidity and flexibility can be used to target difficult binding interfaces with appropriate binding affinity and specificity. Since the introduction of insulin nearly a century ago, more than 80 peptide drugs have been approved to treat a variety of diseases. These include deadly diseases such as cancer and human immunodeficiency virus infection. It is also useful against diabetes, chronic pain, and osteoporosis. Today, more research is being done on these drugs as lessons learned from earlier approaches, which are still valid today, complement newer approaches such as peptide display libraries. At the same time, integrated genomics and peptide display libraries are new strategies that open new avenues for peptide drug discovery. The purpose of this review is to examine the problems in elucidating the peptide-protein recognition mechanism. This is important to develop peptide-based interventions that interfere with endogenous protein interactions. New approaches are being developed to improve the binding affinity and specificity of existing approaches and to develop peptide agents as potentially useful drugs. We also highlight the key challenges that must be overcome in peptide drug development to realize their potential and provide an overview of recent trends in peptide drug development. In addition, we take an in-depth look at early efforts in human hormone discovery, smart medicinal chemistry and design, natural peptide drugs, and breakthrough advances in molecular biology and peptide chemistry.

The Current Status of Drug Discovery for the Oxytocin Receptor

The oxytocin receptor plays a significant role in peripheral regulation of parturition and lactation. Given this important role, multiple drug discovery programs have been conducted to develop agonists and antagonists for peripheral activity. The role of the oxytocin receptor in the central nervous system is also significant, promoting social interaction, trust, and empathy in humans. As such, molecules that can access the central nervous system and target the oxytocin receptor are of significant interest. Due to the role of the oxytocin receptor in regulating social function and psychological well-being, agonists of this receptor have considerable promise for the treatment of numerous neuropsychiatric conditions. The poor pharmacokinetic properties and blood-brain barrier penetration of peptide-based molecules means nonpeptide compounds have more commonly been the focus for central nervous system activity. This chapter aims to summarize the current standing of peptide and nonpeptide drug discovery for antagonists and agonists of the oxytocin receptor and focusses on centrally active nonpeptidic agonists.

Oxytocin signaling in the treatment of drug addiction: Therapeutic opportunities and challenges

Drug addiction is one of the leading causes of mortality worldwide. Despite great advances were achieved in understanding the neurobiology of drug addiction, the therapeutic options are severely limited, with poor effectiveness and serious side effects. The neuropeptide oxytocin (OXT) is well known for its effects on uterine contraction, sexual/maternal behaviors, social affiliation, stress and learning/memory by interacting with the OXT receptor and other neuromodulators. Emerging evidence suggests that the acute or chronic exposure to drugs can affect the OXT system. Additionally, OXT administration can ameliorate a wide range of abused drug-induced neurobehavioral changes. Overall, OXT not only suppresses drug reward in the binge stage of drug addiction, but also reduces stress responses and social impairments during the withdrawal stage and, finally, prevents drug/cue/stress-induced reinstatement. More importantly, clinical studies have also shown that OXT can exert beneficial effects on reducing substance use disorders of a series of drugs, such as heroin, cocaine, alcohol, cannabis and nicotine. Thus, the present review focuses on the role of OXT in treating drug addiction, including the preclinical and clinical therapeutic potential of OXT and its analogs on the neurobiological perspectives of drugs, to provide a better insight of the efficacy of OXT as a clinical addiction therapeutic agent.

Oxytocin in the Male Reproductive Tract; The Therapeutic Potential of Oxytocin-Agonists and-Antagonists

In this review, the role of oxytocin and oxytocin-like agents (acting via the oxytocin receptor and belonging to the oxytocin-family) in the male reproductive tract is considered. Previous research (dating back over 60 years) is revised and connected with recently found aspects of the role oxytocin plays in male reproductive health. The local expression of oxytocin and its receptor in the male reproductive tract of different species is summarized. Colocalization and possible crosstalk to other agents and receptors and their resulting effects are discussed. The role of the newly reported oxytocin focused signaling pathways in the male reproductive tract, other than mediating contractility, is critically examined. The structure and effect of the most promising oxytocin-agonists and -antagonists are reviewed for their potential in treating male disorders with origins in the male reproductive tract such as prostate diseases and ejaculatory disorders.

On-Resin Preparation of Allenamidyl Peptides: A Versatile Chemoselective Conjugation and Intramolecular Cyclisation Tool

The ability to modify peptides and proteins chemoselectively is of continued interest in medicinal chemistry, with peptide conjugation, lipidation, stapling, and disulfide engineering at the forefront of modern peptide chemistry. Herein we report a robust method for the on-resin preparation of allenamide-modified peptides, an unexplored functionality for peptides that provides a versatile chemical tool for chemoselective inter- or intramolecular bridging reactions with thiols. The bridging reaction is biocompatible, occurring spontaneously at pH 7.4 in catalyst-free aqueous media. By this "click" approach, a model peptide was successfully modified with a diverse range of alkyl and aryl thiols. Furthermore, this technique was demonstrated as a valuable tool to induce spontaneous intramolecular cyclisation by preparation of an oxytocin analogue, in which the native disulfide bridge was replaced with a vinyl sulfide moiety formed by thia-Michael addition of a cysteine thiol to the allenamide handle.

Improved oxytocin analysis from human serum and urine by orbitrap ESI-LC-HRAM-MS

Native circulating oxytocin (OT) levels in non-pregnant/non-lactating/non-medicated humans are very low (≤ 8 pg/mL). The lower limit of detection (LLOD) of our previous liquid chromatography mass spectrometry (LC-MS) method (10-25 pg/mL) precluded their quantification in serum and urine. Thus, we sought to improve the LC-MS sensitivity of OT measurements in these matrices by hydrophobic tagging and solid phase extraction (SPE). In the former approach, OT was reduced then alkylated with N-alkyl acetamide (C12, C14, C16, and C18) tags or derivatized using sulfonyl chloride-based reagents. In the latter approach, native OT in serum and urine was concentrated by offline SPE using gradient acetonitrile washings after first crashing with acetonitrile. Peak urinary eluate fractions were further concentrated online then analyzed by orbitrap-based LC-MS with electrospray ionization. All hydrophobic OT derivatives had lower sensitivity than native OT. Washing with a water-acetonitrile gradient during SPE improved the LLOD of OT in spiked serum to 2.5 pg/mL, while adding a subsequent online-concentration step improved the LLOD in spiked urine to 1-5 pg/mL and allowed us to detect OT in urine from lactating women. We were unable to improve the sensitivity of OT measurements by hydrophobic tagging or by derivatization using sulfonyl chloride-based reagents. However, we were successful in improving the sensitivity of native OT measurements in serum and urine 2- and 5-fold, respectively, from our previous orbitrap-based LC-MS method. Offline SPE was mandatory for both matrices and a subsequent online-concentration step was required for urine.

Comparison of the pharmacological profiles of arginine vasopressin and oxytocin analogs at marmoset, macaque, and human vasopressin 1a receptor

Arginine vasopressin (AVP) and oxytocin (OT) are nonapeptides that bind to G-protein coupled receptors and influence social behaviors. Consensus mammalian AVP and OT (Leu⁸-OT) sequences are highly conserved. In marmosets, an amino acid change in the 8th position of the peptide (Pro⁸-OT) exhibits unique structural and functional properties. There is ∼85 % structural homology between the OT receptor (OTR) and vasopressin 1a receptor (V1aR) resulting in significant cross-reactivity between the ligands and receptors. Chinese hamster ovary (CHO) cells expressing marmoset (mV1aR), macaque (qV1aR), or human vasopressin receptor 1a (hV1aR) were used to assess AVP, Leu⁸-OT and Pro⁸-OT pharmacological profiles. To assess activation of Gq, functional assays were performed using Fluo-3 to measure ligand-induced Ca²⁺ mobilization. In all three V1aR-expressing cell lines, AVP was more potent than the OT ligands. To assess ligand-induced hyperpolarization, FLIPR Membrane Potential (FMP) assays were performed. In all three V1aR lines, AVP was more potent than the OT analogs. The distinctive U-shaped concentration-response curve displayed by AVP may reflect enhanced desensitization of the mV1aR and hV1aR, which is not observed with qV1aR. Evaluation of Ca²⁺-activated potassium (K⁺) channels using the inhibitors apamin, paxilline, and TRAM-34 demonstrated that both intermediate and large conductance Ca²⁺-activated K⁺ channels contributed to membrane hyperpolarization, with different pharmacological profiles identified for distinct ligand-receptor combinations. Taken together, these data suggest differences in ligand-receptor signaling that may underlie differences in social behavior. Integrative studies of behavior, genetics and ligand-receptor interaction will help elucidate the connection between receptor pharmacology and social behaviors.

Comparison of the pharmacologic profiles of arginine vasopressin and oxytocin analogs at marmoset, titi monkey, macaque, and human oxytocin receptors

The oxytocin-arginine vasopressin (OT-AVP) ligand-receptor family influences a variety of physiological, behavioral, and social behavioral processes in the brain and periphery. The OT-AVP family is highly conserved in mammals, but recent discoveries have revealed remarkable diversity in OT ligands and receptors in New World Monkeys (NWMs) providing a unique opportunity to assess the effects of genetic variation on pharmacological signatures of peptide ligands. The consensus mammalian OT sequence has leucine in the 8^th position (Leu⁸-OT), whereas a number of NWMs, including the marmoset, have proline in the 8^th position (Pro⁸-OT) resulting in a more rigid tail structure. OT and AVP bind to OT’s cognate G-protein coupled receptor (OTR), which couples to various G-proteins (G_i/o, G_q, G_s) to stimulate diverse signaling pathways. CHO cells expressing marmoset (mOTR), titi monkey (tOTR), macaque (qOTR), or human (hOTR) OT receptors were used to compare AVP and OT analog-induced signaling. Assessment of G_q-mediated increase in intracellular calcium (Ca²⁺) demonstrated that AVP was less potent than OT analogs at OTRs from species whose endogenous ligand is Leu⁸-OT (tOTR, qOTR, hOTR), relative to Pro⁸-OT. Likewise, AVP-induced membrane hyperpolarization was less potent at these same OTRs. Evaluation of (Ca²⁺)-activated potassium (K⁺) channels using the inhibitors apamin, paxilline, and TRAM-34 demonstrated that both intermediate and large conductance Ca²⁺-activated K⁺ channels contributed to membrane hyperpolarization, with different pharmacological profiles identified for distinct ligand-receptor combinations. Understanding more fully the contributions of structure activity relationships for these peptide ligands at vasopressin and OT receptors will help guide the development of OT-mediated therapeutics.

Engineering a Potent, Long-Acting, and Periphery-Restricted Oxytocin Receptor Agonist with Anorexigenic and Body Weight Reducing Effects

The effects of oxytocin on food intake and body weight reduction have been demonstrated in both animal models and human clinical studies. Despite being efficacious, oxytocin is enzymatically unstable and thus considered to be unsuitable for long-term use in patients with obesity. Herein, a series of oxytocin derivatives were engineered through conjugation with fatty acid moieties that are known to exhibit high binding affinities to serum albumin. One analog (OT-12) in particular was shown to be a potent full agonist at the oxytocin receptor (OTR) in vitro with good selectivity and long half-life (24 h) in mice. Furthermore, OT-12 is peripherally restricted, with very limited brain exposure (1/190 of the plasma level). In a diet-induced obesity mouse model, daily subcutaneous administration of OT-12 exhibited more potent anorexigenic and body weight reducing effects than carbetocin. Thus, our results suggest that the long-acting, peripherally restricted OTR agonist may offer potential therapeutic benefits for obesity.

Development of a Highly Potent Analogue and a Long-Acting Analogue of Oxytocin for the Treatment of Social Impairment-Like Behaviors

The nonapeptide hormone oxytocin (OT) has pivotal brain roles in social recognition and interaction and is thus a promising therapeutic drug for social deficits. Because of its peptide structure, however, OT is rapidly eliminated from the bloodstream, which decreases its potential therapeutic effects in the brain. We found that newly synthesized OT analogues in which the Pro⁷ of OT was replaced with N-( p-fluorobenzyl)glycine (2) or N-(3-hydroxypropyl)glycine (5) exhibited highly potent binding affinities for OT receptors and Ca²⁺ mobilization effects by selectively activating OT receptors over vasopressin receptors in HEK cells, where 2 was identified as a superagonist ( E_Max = 131%) for OT receptors. Furthermore, the two OT analogues had a remarkably long-acting effect, up to 16-24 h, on recovery from impaired social behaviors in two strains of CD38 knockout mice that exhibit autism spectrum disorder-like social behavioral deficits, whereas the effect of OT itself rapidly diminished.

Building upon Nature's Framework: Overview of Key Strategies Toward Increasing Drug-Like Properties of Natural Product Cyclopeptides and Macrocycles

The pharmaceutical industry has focused mainly in the development of small-molecule entities intended for oral administration for the past decades. As a result, the majority of existing drugs address only a narrow range of biological targets. In the era of post-genomics, transcriptomics, and proteomics, there is an increasing interest on larger modulators of proteins that can span larger surfaces, access new therapeutic mechanisms of action, and provide greater target specificity. Traditional drug-like molecules developed using "rule-of-five" (Ro5) guidelines have been proven ineffective against a variety of challenging targets, such as protein-protein interactions, nucleic acid complexes, and antibacterial modalities. However, natural products are known to be effective at modulating such targets, leading to a renewed focus by medicinal chemists on investigating underrepresented chemical scaffolds associated with natural products. Here we describe recent efforts toward identification of novel natural cyclopeptides and macrocycles as well as selected medicinal chemistry strategies to increase drug-like properties or further exploration of their activity.