Tetrahydrocannabinol and Its Major Metabolites Are Not (or Are Poor) Substrates or Inhibitors of Human P-Glycoprotein [ATP-Binding Cassette (ABC) B1] and Breast Cancer Resistance Protein (ABCG2).

(-)-Δ9-Tetrahydrocannabinol (THC) is the primary psychoactive constituent of cannabis. In humans, 11-hydroxy-THC (11-OH-THC) and 11-nor-9-carboxy-THC (THC-COOH) are psychoactive and nonpsychoactive circulating metabolites of THC, respectively. Whether these cannabinoids are substrates or inhibitors of human P-glycoprotein (P-gp) or breast cancer resistance protein (BCRP) is unknown. Previous animal studies suggest that THC and its metabolites could be substrates of these transporters. Therefore, we performed Transwell, cellular accumulation, and vesicular transport assays, at pharmacologically relevant concentrations of these cannabinoids, using Madin-Darby canine kidney (MDCK) II cells or plasma membrane vesicles overexpressing human P-gp or BCRP. Neither THC nor 11-OH-THC was found to be a substrate or inhibitor of P-gp or BCRP. The efflux ratio of THC-COOH in MDCKII-BCRP cells was 1.6, which was significantly decreased to 1.0 by the BCRP inhibitor Ko143. Likewise, cellular accumulation of THC-COOH was significantly increased 1.6-fold in the presence versus absence of Ko143. THC-COOH also significantly inhibited BCRP-mediated transport of Lucifer yellow, a BCRP substrate; however, THC-COOH was neither a substrate nor an inhibitor of P-gp. Collectively, these results indicate that THC and 11-OH-THC are not substrates or inhibitors (at pharmacologically relevant concentrations) of either P-gp or BCRP. THC-COOH is a weak substrate and inhibitor of BCRP, but not of P-gp. Accordingly, we predict that P-gp/BCRP will not modulate the disposition of these cannabinoids in humans. In addition, use of these cannabinoids will not result in P-gp- or BCRP-based drug interactions. SIGNIFICANCE STATEMENT: This study systematically investigated whether Δ9-tetrahydrocannabinol (THC) and its major metabolites, 11-hydroxy-THC and 11-nor-9-carboxy-THC, are substrates and/or inhibitors of human P-gp and BCRP at pharmacologically relevant concentrations. The results obtained are highly valuable for mechanistic understanding and prediction of the roles of P-gp and BCRP in determining the human pharmacokinetics, tissue distribution, and drug interactions of cannabinoids.

The Fungal Iron Chelator Desferricoprogen Inhibits Atherosclerotic Plaque Formation.

Hemoglobin, heme and iron are implicated in the progression of atherosclerosis. Therefore, we investigated whether the hydrophobic fungal iron chelator siderophore, desferricoprogen (DFC) inhibits atherosclerosis. DFC reduced atherosclerotic plaque formation in ApoE-/- mice on an atherogenic diet. It lowered the plasma level of oxidized LDL (oxLDL) and inhibited lipid peroxidation in aortic roots. The elevated collagen/elastin content and enhanced expression of adhesion molecule VCAM-1 were decreased. DFC diminished oxidation of Low-density Lipoprotein (LDL) and plaque lipids catalyzed by heme or hemoglobin. Formation of foam cells, uptake of oxLDL by macrophages, upregulation of CD36 and increased expression of TNF-α were reduced by DFC in macrophages. TNF-triggered endothelial cell activation (vascular cell adhesion molecule-1 (VCAM-1), intercellular adhesion molecules (ICAMs), E-selectin) and increased adhesion of monocytes to endothelium were attenuated. The increased endothelial permeability and intracellular gap formation provoked by TNF-α was also prevented by DFC. DFC acted as a cytoprotectant in endothelial cells and macrophages challenged with a lethal dose of oxLDL and lowered the expression of stress-responsive heme oxygenase-1 as sublethal dose was employed. Saturation of desferrisiderophore with iron led to the loss of the beneficial effects. We demonstrated that DFC accumulated within the atheromas of the aorta in ApoE-/- mice. DFC represents a novel therapeutic approach to control the progression of atherosclerosis.

Combination of a novel microtubule inhibitor MBRI-001 and gemcitabine synergistically induces cell apoptosis by increasing DNA damage in pancreatic cancer cell lines.

Pancreatic cancer (PC) is a highly malignant cancer with poor prognosis. Although gemcitabine (GEM; 2',2'-difluoro-deoxycytidine) has been used as the first-line chemotherapeutic agent in PC treatment for decades, its limited efficacy remains a significant clinical issue, which may be resolved by GEM combination therapy. In this study, we aimed to investigate the anti-tumor effects of MBRI-001 in combination with GEM in BxPC-3 and MIA PaCa-2 human PC cell lines. In vitro and in vivo results indicate that MBRI-001 showed synergistic activity with GEM. GEM induced apoptosis by increasing DNA damage (phosphorylated core histone protein H2AX (γ-H2AX)), MBRI-001 activated mitochondrial-apoptotic pathway (cleaved poly-ADP ribose polymerase (PARP)). Thus, the combination of the two intensified both apoptosis and DNA damage and showed significantly superior anti-tumor activity compared to each agent alone. The adoption of combination of MBRI-001 with GEM may be beneficial as they act synergistically and thus, can be a potential therapeutic choice for improving the prognosis of PC patients in the future.

Fluorescence-based discrimination of breast cancer cells by direct exposure to 5-aminolevulinic acid.

Protoporphyrin IX-fluorescence measurement is a powerful in situ approach for cancer detection after oral/topical administration of 5-aminolevulinic acid. However, this approach has not been clinically established for breast cancer, probably due to insufficient delivery of 5-aminolevulinic acid to the mammary glands. In the present study, we directly exposed breast cancer cells to 5-aminolevulinic acid to assess their discrimination via protoporphyrin IX-fluorescence. Fluorescence intensity (FI) was measured in the human breast cancer cell lines MCF7 and MDA-MB-231 and breast epithelial cell line MCF10A by confocal microscopy and flow cytometry. After 5-aminolevulinic acid exposure for 2 hours, protoporphyrin IX-FI in MCF7 and MDA-MB-231 cells significantly increased with marked cell-to-cell variability, whereas that in MCF10A cells increased moderately. Combined exposure of the cancer cells to 5-aminolevulinic acid and Ko143, a specific inhibitor of ATP-binding cassette transporter G2, further increased protoporphyrin IX-FI and alleviated the cell-to-cell variability in MCF7 and MDA-MB-231 cells, indicating improvement in the reproducibility and accuracy for fluorescence-based cancer detection. The increased FI by combined administration of these two drugs was also demonstrated in cells obtained via fine needle aspiration from mouse xenograft models inoculated with MDA-MB-231 cells. Furthermore, a cutoff value for increased protoporphyrin IX-FI ratio, before and after exposure to these drugs, clearly discriminated between cancer and noncancer cells. Taken together, direct exposure to 5-aminolevulinic acid and Ko143 may be a promising strategy for efficient fluorescence-based detection of breast cancer cells ex vivo using fine needle aspiration.

In vitro and in vivo pharmacokinetic and pharmacodynamic study of MBRI-001, a deuterium-substituted plinabulin derivative as a potent anti-cancer agent.

MBRI-001 was demonstrated preliminary better pharmacokinetics and antitumor effects than that of plinabulin in vivo. In this approach, we further carried out systematic pharmacokinetic and pharmacodynamic study of MBRI-001 in vitro and in vivo. MBRI-001 was tested stable in rat plasma and more stable in liver microsomes than plinabulin in vitro. In vivo, MBRI-001 could be distributed rapidly and widely in various tissues, especially the concentration of MBRI-001 in lung was remarkably higher than other tissues. Excretion study indicated that MBRI-001 might been decomposed and excreted as metabolites. Additionally, the combination treatment of MBRI-001 and gefitinib revealed better antitumor inhibition rate than monotherapy in vivo. Therefore, we suggest that MBRI-001 could be developed as a promising anti-cancer agent in near future.

Preclinical absorption, distribution, metabolism, excretion and pharmacokinetics of a novel selective inhibitor of breast cancer resistance protein (BCRP).

1. Breast cancer resistance protein (BCRP) plays an important role in drug absorption, distribution and excretion. It is challenging to evaluate BCRP functions in preclinical models because commonly used BCRP inhibitors are nonspecific or unstable in animal plasma. 2. In this work, in vitro absorption, distribution, metabolism and elimination (ADME) assays and pharmacokinetic (PK) experiments in Bcrp knockout (KO) (Abcg2-/-) and wild-type (WT) FVB mice and Wistar rats were conducted to characterize the preclinical properties of a novel selective BCRP inhibitor (ML753286, a Ko143 analog). 3. ML753286 is a potent inhibitor for BCRP, but not for P-glycoprotein (P-gp), organic anion-transporting polypeptide (OATP) or major cytochrome P450s (CYPs). It has high permeability, but is not an efflux transporter substrate. ML753286 has low to medium clearance in rodent and human liver S9 fractions, and is stable in plasma cross species. Bcrp inhibition affects oral absorption and clearance of sulfasalazine in rodents. A single dose of ML753286 at 50-300 mg/kg orally, and at 20 mg/kg intravenously or 25 mg/kg orally inhibits Bcrp functions in mice and rats, respectively. 4. These findings confirm that ML753286 is a useful selective inhibitor to evaluate BCRP/Bcrp activity in vitro and in rodent model systems.

Single- and Multiple-Day Dosing Studies to Investigate High-Dose Pharmacokinetics of Epelsiban and Its Metabolite, GSK2395448, in Healthy Female Volunteers.

Open-label single- and double-blind repeat-dose studies in healthy female volunteers were conducted to investigate the pharmacokinetics (PK) and safety/tolerability of epelsiban total daily doses ranging from 600 to 900 mg. In 1 study (n = 12), epelsiban was dosed at 300 or 450 mg twice daily (every 12 hours) for a single day. In the repeat-dose double-blind study, epelsiban and placebo were administered to 31 subjects as 200 mg 3 times daily, 300 mg 3 times daily (TID), or 450 mg twice daily (BID) for 14 days. After both single and 14 daily repeat doses, the PK profiles for epelsiban and its metabolite, GSK2395448, remained linear at all administered doses. The exposures at a given total daily dose were also similar between BID and TID dosing regimens. Exposure (AUC0-τ ), based on dosing intervals, for both epelsiban and GSK2395448 was similar. However, compared with morning dosing, Cmax was lower after evening dosing, possibly because of a food effect. The highest accumulation of epelsiban and GSK2395448 exposures (AUC0-τ ) was approximately 34% for each after repeat dosing, consistent with the short half-life. At total daily doses of 600 and 900 mg, epelsiban was generally well tolerated, and there were no significant safety concerns identified.

New diketopiperazines as vectors for peptide protection and brain delivery: Synthesis and biological evaluation.

New strategies allowing the transfer of molecules, especially peptides, through the blood-brain barriers are a major pharmacological challenge for the treatment of brain diseases. The present study aims at evaluating in vivo the cerebral bioavailability of carrier systems, based on small and functionalizable 2,5-diketopiperazine (DKP) motifs. We studied 2 different cyclo(Lys-Lys) DKP scaffolds alone and a cyclo(Lys-Gly) DKP carrier bearing as peptide model, the tau protein hexapeptide VQIVYK sequence. The different carrier systems were synthesized and radiolabeled using one of the free domains. The stability, biodistribution, and ability to cross blood-brain barrier were investigated in vivo in mice for 99m Tc-DKP scaffolds, 99m Tc-HVQIVYK peptide alone, and 99m Tc-DKP-VQIVYK. 125 I-labelled bovine serum albumin was used as negative control for brain uptake. Both radiolabeled DKPs scaffolds and 99m Tc-DKP-VQIVYK showed a high stability, while peptide 99m Tc-HVQIVYK alone was quickly degraded in vivo. The presence of 99m Tc-DKPs scaffolds and 99m Tc-DKP-VQIVYK was observed in the ventricular and subarachnoid spaces and to a lower extent in the brain parenchyma up to 45 minutes post-injection in mice. This work highlights the potentiality of DKP scaffolds as vectors to transport peptides into the brain by limiting proteolysis and favoring cerebral bioavailability.

Synthesis of a new inhibitor of breast cancer resistance protein with significantly improved pharmacokinetic profiles.

The design, synthesis, in vitro inhibitory potency, and pharmacokinetic (PK) profiles of Ko143 analogs are described. Compared to commonly used Ko143, the new breast cancer resistance protein (BCRP) inhibitor (compound A) showed the same potency and a significantly improved PK profile in rats (lower clearance [1.54L/h/kg] and higher bioavailability [123%]). Ko143 on the other hand suffers from poor bioavailability. Compared to Ko143, compound A would be a useful probe for delineating the role of BCRP during in vivo studies in animals.

A single- and multiple-dose study to investigate the pharmacokinetics of epelsiban and its metabolite, GSK2395448, in healthy female volunteers.

An open-label single- and repeat-dose study was conducted to investigate the pharmacokinetics, safety, and tolerability of ascending doses of epelsiban in healthy female volunteers (n = 48). The pharmacokinetics of the epelsiban metabolite, GSK2395448, were also assessed. Epelsiban was readily absorbed and parent and metabolite readily appeared in plasma. The parent drug's median tmax was approximately 0.5 hours, and the metabolite's median tmax ranged from 0.5 to 1.0 hours post-parent dosing. Both epelsiban and GSK2395448 had rapid elimination half-lives, ranging between 2.66 and 4.85 hours. The metabolite:parent ratios for exposure (AUC and Cmax ) ranged from approximately 70% to greater than 100%, and therefore, GSK2395448 is considered a major metabolite of epelsiban. Mean epelsiban and GSK2395448 AUC values increased in a dose-proportional manner following both single-dose administration from 10 to 200 mg and repeat administration from 10 to 150 mg following twice daily or 4-times-daily dosing. Single-dose epelsiban pharmacokinetics in women was similar to single-dose pharmacokinetics previously observed in men. Epelsiban was generally well tolerated, and no events of clinical concern were observed in volunteers dosed in this study. The safety findings were consistent with the previous study in men, with headache the most commonly reported adverse effect.

In search of the active metabolites of an anticancer piperazinedione, TW01003, in rats.

TW01003, a piperazinedione derivative designed as an antimitotic agent, exhibited potent anticancer and antiangiogenesis activities in mice. However, oral administration of this compound in rats led to poor systemic bioavailability which suggested that in vivo efficacy might come from its metabolites. This report describes the identification of TW01003 metabolites in pig and Wistar rats. Following intravenous administration of TW01003, pig urine samples were subjected to sulfatase and glucuronidase treatment to monitor the biotransformation products. Rats were given TW01003 both intravenously and orally, and blood samples were collected and then analyzed by HPLC to quantitatively determine the metabolic transformation of TW01003 to its metabolite. A sulfate conjugate, TW01003 sulfate, was identified as the major metabolite for TW01003 after intravenous injection in both pig and rats. However, in rats, the glucuronide conjugate became major metabolite 30 min after TW01003 oral dosing. Pharmacokinetic analysis after intravenous administration of TW01003 indicated that TW01003 sulfate had a systemic bioavailability 2.5 times higher, volume of distribution three times higher, residence time seven times longer, and clearance rate 2.3 times lower compared to TW01003. Our results indicate that the potent anticancer and antiangiogenesis activities of TW01003 might not come from TW01003 per se but from its metabolites TW01003 sulfate.

Safety and efficacy of epelsiban in the treatment of men with premature ejaculation: a randomized, double-blind, placebo-controlled, fixed-dose study.

AIM: To assess the efficacy and safety of the selective oxytocin receptor antagonist epelsiban in the treatment of premature ejaculation (PE). METHODS: Double-blind, randomized, parallel-group, placebo-controlled, stopwatch-monitored, phase 2, multicenter study (GSK557296; NCT01021553) conducted in men (N=77) 18-55 years of age, with PE defined as per International Society for Sexual Medicine consensus definition. Patients provided informed consent prior to a 4-week un-medicated run-in to determine baseline intravaginal ejaculatory latency times (IELT) recorded in an electronic diary. Patients needed to make a minimum of four intercourse attempts and have a mean IELT<65 seconds to be considered for randomization. Men with moderate-to-severe erectile dysfunction were excluded from the study. Eligible patients were randomized to placebo, epelsiban 50 mg, or 150 mg, taken 1 hour before sexual activity. Active treatment IELT times were recorded in an electronic diary, along with subjective measures of intercourse satisfaction, over an 8-week treatment period. The Modified Index of Premature Ejaculation and International Index of Erectile Function were completed at study visits. MAIN OUTCOME MEASURES: Stopwatch timed IELT recordings and a modified version of the patient-reported outcome questionnaire the IPE were used in this study to determine the effect of epelsiban when taken orally prior to intercourse in subjects diagnosed with PE. RESULTS: The baseline (mean) IELT for patients pretreatment was (0.52, 0.63, and 0.59 minutes) for placebo, epelsiban 50 mg and 150 mg, respectively. On-treatment, average geometric least squares means of the median IELT values (mean) were slightly higher in the 50 mg and 150 mg groups (0.72 and 0.69 minutes), respectively, vs. the placebo group (0.62 minutes). Headache was the most common adverse event, and rates were similar across all groups. CONCLUSIONS: Epelsiban 50 mg and 150 mg were well tolerated, but did not result in a clinically or statistically significant change in IELT in men with PE, compared with placebo.

Diketopiperazine alkaloids from a mangrove rhizosphere soil derived fungus Aspergillus effuses H1-1.

Effusin A (1), a spirobicyclic N,O-acetal derivative with an unprecedented 3',3a',5',6'-tetrahydrospiro[piperazine-2,2'-pyrano[2,3,4-de]chromene] ring system, and a spiro-polyketide-diketopiperazine hybrid dihydrocryptoechinulin D (2) were isolated from a mangrove rhizosphere soil derived fungus, Aspergillus effuses H1-1. Their structures were determined by detailed spectroscopic analysis. Effusin A (1) and dihydrocryptoechinulin D (2) occurred as racemates, the enantiomers of which were separated and characterized by online HPLC-ECD analysis and their absolute configurations were determined by the solution TDDFT ECD calculation approach. The cytotoxic effects of 1 and 2 were preliminarily evaluated and 2 showed potent activity on P388 cells with an IC(50) value of 1.83 µM. The target of racemic 2 was also investigated and the (12R,28S,31S)-2 enantiomer showed selectivity against topoisomerase I.

Pyridyl-2,5-diketopiperazines as potent, selective, and orally bioavailable oxytocin antagonists: synthesis, pharmacokinetics, and in vivo potency.

A six-stage stereoselective synthesis of indanyl-7-(3'-pyridyl)-(3R,6R,7R)-2,5-diketopiperazines oxytocin antagonists from indene is described. SAR studies involving mono- and disubstitution in the 3'-pyridyl ring and variation of the 3-isobutyl group gave potent compounds (pK(i) > 9.0) with good aqueous solubility. Evaluation of the pharmacokinetic profile in the rat, dog, and cynomolgus monkey of those derivatives with low cynomolgus monkey and human intrinsic clearance gave 2',6'-dimethyl-3'-pyridyl R-sec-butyl morpholine amide Epelsiban (69), a highly potent oxytocin antagonist (pK(i) = 9.9) with >31000-fold selectivity over all three human vasopressin receptors hV1aR, hV2R, and hV1bR, with no significant P450 inhibition. Epelsiban has low levels of intrinsic clearance against the microsomes of four species, good bioavailability (55%) and comparable potency to atosiban in the rat, but is 100-fold more potent than the latter in vitro and was negative in the genotoxicity screens with a satisfactory oral safety profile in female rats.

Phase 1 study of the novel vascular disrupting agent plinabulin (NPI-2358) and docetaxel.

BACKGROUND: Plinabulin (NPI-2358) is a vascular disrupting agent (VDA) that destabilizes tumor vascular endothelial cell architecture resulting in selective collapse of established tumor vasculature producing anti-tumor activity alone or in combination with cytotoxic agents. The objective of this study was to assess the recommended Phase 2 dose (RP2D) of plinabulin combined with docetaxel. PATIENTS AND METHODS: Patients received 75 mg/m(2) docetaxel on day 1 and plinabulin on days 1 and 8 intravenously in 21 day cycles. Plinabulin was escalated from the biologically effective dose (BED) of 13.5 mg/m(2) to the standard single agent dose of 30 mg/m(2) using a "3+3" design. RESULTS: Thirteen patients were enrolled. Adverse events were consistent with those of both agents alone. Fatigue, pain, nausea, diarrhea and vomiting were the most common events. One dose limiting toxicity of nausea, vomiting, dehydration and neutropenia occurred. The RP2D was 30 mg/m(2) of plinabulin with 75 mg/m(2) docetaxel. Pharmacokinetics did not indicate drug-drug interactions. Of the 8 patients with NSCLC evaluable for response, 2 achieved a partial response and 4 demonstrated lesser decreases in tumor measurements. CONCLUSIONS: The combination of full doses of plinabulin and docetaxel is tolerable. With encouraging antitumor activity, this supported further development of this combination.

2,5-diketopiperazines as neuroprotective agents.

2,5-diketopiperazines are the simplest cyclic peptides found in nature, commonly biosynthesized from amino acids by different organisms, and represent a promising class of biologically active natural products. Their peculiar heterocyclic structure confers high stability against the proteolysis and constitutes a structural requirement for the active intestinal absorption. Furthermore, the diketopiperazine-based motif is considered as a novel brain shuttle for the delivery of drugs with limited ability to cross the blood-brain barrier (BBB) and can be proposed as an ideal candidate for the rational development of new therapeutic agents. Although these cyclic peptides have been known since the beginning of the 20th century, only recently have they attracted substantial interest with respect to the wide spectrum of their biological properties, including antitumor, antiviral, antifungal, antibacterial and antihyperglycemic activities. In addition to these, the most challenging function of the diketopiperazine derivatives is related with their remarkable neuroprotective and nootropic activity. The aim of the present paper is to provide an overview of the two major classes of diketopiperazines, the TRH-related and the unsaturated derivatives both characterized by a significant ability to protect against neurotoxicity in several experimental models. The neuroprotective profile of these compounds suggests that they may have a future utility in the therapy of neuronal degeneration in vivo, potentially through several different mechanisms.

Discovery of 4-[4-({(3R)-1-butyl-3-[(R)-cyclohexyl(hydroxy)methyl]-2,5-dioxo-1,4,9-triazaspiro[5.5]undec-9-yl}methyl)phenoxy]benzoic acid hydrochloride: a highly potent orally available CCR5 selective antagonist.

Based on the original spirodiketopiperazine design framework, further optimization of an orally available CCR5 antagonist was undertaken. Structural hybridization of the hydroxylated analog 4 derived from one of the oxidative metabolites and the new orally available non-hydroxylated benzoic acid analog 5 resulted in another potent orally available CCR5 antagonist 6a as a clinical candidate. Full details of a structure-activity relationship (SAR) study and ADME properties are presented.

Spirodiketopiperazine-based CCR5 antagonist: discovery of an antiretroviral drug candidate.

Following the discovery that hydroxylated derivative 3 (Fig. 1) was one of the oxidative metabolites of the original lead 1, it was found that hydroxylated compound 4 possesses higher in vitro anti-HIV potency than the corresponding non-hydroxylated compound 2. Structural hybridation of 4 with the orally available analog 5 resulted in another orally-available spirodiketopiperazine CCR5 antagonist 6a that possesses more favorable pharmaceutical profile for use as a drug candidate.

The design of orally bioavailable 2, 5-diketopiperazine oxytocin antagonists: from concept to clinical candidate for premature labor.

A short, efficient and highly stereoselective synthesis has been developed for a series of 6-indanyl-3-alkyl-7-aryl/heterocyclic-(3R, 6R, 7R)-2, 5-diketopiperazine amides that are potent and selective oxytocin (OT) antagonists. Property-based design using an estimate of human oral absorption enabled focus to be directed to those templates with the greatest chance of delivering high bioavailability in humans. This led to the 2', 4'-difluorophenyl dimethylamide 40, a highly potent (pK(i) =9.2) and selective OT antagonist (>1,000-fold selectivity vs. the human vasopressin receptors V1a, V2, and V1b) with good oral bioavailability (>50%) in the rat and dog. Increased solubility and an improved Cyp450 profile was achieved with a range of 2'-substituted 7-(1',3'-oxazol-4'-yl)-(3R,6R,7R)-2,5-diketopiperazine amides and branching at the α-carbon of the 3-butyl group led to a superior rat pharmacokinetic profile that resulted in the discovery of the 2'-methyl-1',3'-oxazol-4'-yl morpholine amide derivative 74 GSK221149A (Retosiban), which had the best oral exposure and bioavailability in the rat. Retosiban has sub-nanomolar affinity (K(i) =0.65 nM) for the oxytocin receptor with >1400-fold selectivity over the closely related vasopressin receptors. It has good solubility, low protein binding and has a good Cyp450 profile with no significant inhibition IC(50) >100 µM. Retosiban is >15-fold more potent at the human oxytocin receptor than atosiban (a marketed i.v, peptide OT antagonist) and it has been shown to be an effective tocolytic by i.v. and by oral administration in rats, and was selected for progression as a potential clinical candidate for preterm labor.