Mechanism of Action

As a PEGylated derivative of naloxone, naloxegol binds to μ-opioid receptors in the gastrointestinal tract, thereby decreasing the constipating effects of opioid.

Naloxegol acts as an antagonist of μ- (K_i = 33.8 nM) and δ-opioid receptors (K_i = 53.5 nM), as well as a weak partial agonist at κ-opioid receptors (K_i = 186.5 nM).

Naloxegol is a substrate of the P-gp transporter, which reduces its ability to cross the blood-brain barriers, resulting in reduction of CNS effects.^[9]

Naloxegol had no significant activity in radioligand binding and enzyme assays in a panel of 327 receptors, ion channels, transporters and enzymes at 10 μM or in a panel of 7 cardiac ion channels up to 100 μM, except μ-, δ- and κ and other 7 receptors.^[9,10]

In Vitro Efficacy

In vitro antagonist at μ-opioid receptors expressed in cells:

●    In CHO-K1 cells: pA₂ = 7.95 and IC₅₀ = 11 nM.

●    In membranes from HEK-293 cells:

v  pIC₅₀ = 6.64 (DAMGO).

v  pIC₅₀ = 7.25 (morphine).

In vitro partial agonist at κ-opioid receptors:

●    Receptors expressed in HEK-293 cells: EC₅₀ = 47 nM.

●    Contraction of rabbit vas deferens: No effect at 10 µM.

In vitro antagonist at δ-opioid receptors expressed in CHO cells: IC₅₀ = 886 nM.

In Vivo Efficacy

Morphine-induced charcoal meal delay in gastrointestinal transit model in rats:

•    Reversal of the morphine effect: ED₅₀ = 23.1 mg/kg and complete at 90 mg/kg.

In an orphine-induced analgesia model in rats:

•    ED₅₀ = 55.4 nM and ED₅₀_Analgesia/ED₅₀_GI = 2.4.

Absorption of Naloxegol

The increase in AUC_inf appeared to be dose-proportional in humans in the dose range of 12.5 to 100 mg naloxegol, but C_maxwas slightly more than dose-proportional after oral dose administration.

Had low oral bioavailability in dogs (20.6%) and monkeys (2.26%).

Was absorbed rapidly with the T_max occurring 0.5 to 1 h in non-clinical species and humans.

Showed a half-life ranging between 6.1-8 h in humans, shorter than that in dogs (9.58 h), but longer than that in monkeys (2.77 h) after oral administrations.

Had high clearance in rats (2.65 L/h/kg), monkeys (1.46 L/h/kg) and dogs (2.65 L/h/kg), in contrast to liver blood flow, after intravenous administrations.  The clearance of naloxegol in humans was 158-174 L/h after oral administration.

Exhibited an extensive tissue distribution in rats, dogs and monkeys, with apparent volumes of distribution at 3.14, 4.66 and 2.44 L/kg, respectively, after intravenous administrations.  The apparent volume of distribution in humans was 1550 L after oral administration.

Showed a low permeability, with a Papp_(A→B) of 0.7 × 10^-6 cm/s in Caco-2 cell monolayer model, but exhibited moderate intestinal permeability.

Distribution of Naloxegol Oxalate

Exhibited low plasma protein binding in humans (4.2%), rats (14.1%), mice (20.8%), dogs (3.8%-53.3%) and monkeys (9.7%).

Naloxegol had a low CNS penetration with a much lower uptake rate into the brain (4.1 pmol/g brain/sec).

Pigmented male and albino female rats after single oral administrations:

•     The drug was well distributed into most tissues except for the central nervous system (CNS) since the blood-brain barrier was crossed by a very small extent.

•     Relatively higher concentration levels were observed in uveal tract, liver, kidneys and glandular tissues.

•    In pigmented rats, the concentration of naloxegol in skin, eyes and uveal tract persisted up to 504 h, indicating naloxegol was likely bound to melanin.

•    In albino females, radioactivity was widely distributed into all tissues including liver, kidneys, small intestine wall, placenta and uterus.

•    A sex difference was observed in the female rats, i.e. an earlier T_max and higher concentrations than males.

•    By 48 h, the concentrations of radioactivity were low in the majority of tissues.

Metabolism of Naloxegol Oxalate

The metabolism of naloxegol was high in human and non-clinical hepatocytes.

The major metabolites of naloxegol in human plasma were N-dealkylation (M1) and oxidative metabolism of the PEG chain (M7, M10, and M13), including partial cleavage but not complete removal of the chain.

All the metabolites found in humans could be found in other species.  Glucuronide conjugation at the phenol hydroxyl was also a common process in animals and man but the naloxegol glucuronide showed very low concentrations in the human plasma.

Naloxegol was predominantly metabolized by CYP3A4 with a minor contribution of CYP2D6.

Excretion of Naloxegol Oxalate

Was predominantly excreted in feces in all species, with parent as the major component in human and rat feces, but M10 in dog feces.

About 66% of naloxegol was recovered via biliary excretion in bile duct-cannulated (BDC) rats, with M1 and M2 as the major component in rat bile.

Drug-Drug Interaction

Naloxegol was a weak inhibitor of CYP2D6 (IC₅₀ = 84.7 μM).  Naloxegol had no inhibition for CYP1A2, 2C9 or 2C19.

Time-dependent inhibition was observed at 50 μM for CYP3A4/5 (%inhibition = 24.3), but no time-dependent inhibition for CYP1A2, 2C9, 2C19 or 2D6.

Naloxegol was not an inducer of CYP450 (CYP3A4, CYP2B6 or CYP1A2).

Naloxegol was a substrate of P-gp and BCRP, but had no inhibition for P-gp or BCRP.

Naloxegol was a substrate of OATP1B1, OATP1B3, but had no inhibition for OATP1B1, OATP1B3, OAT1, OAT3 or OCT2.

Single-Dose Toxicity

Single-dose toxicity studies in mice (p.o.), rats (p.o. and i.v.), and dogs (p.o. and i.v.):

•    In mice, naloxegol was tolerated at up to 2000 mg/kg with in-life observation of intolerability (e.g., decreased motor activity and hunched posture).

•    In rats, naloxegol was tolerated at up to 2000 mg/kg with slight in-life observation of intolerability (e.g., decreased motor activity, hunched posture, pilo-erection and respiration changes).

•    In dogs, naloxegol was well tolerated despite of sporadic soft feces, although doses in these acute pharmacokinetic studies did not exceed 20 mg/kg.

Repeated-Dose Toxicity

Repeated-dose toxicity studies in mice (up to 3 months), rats (up to 6 months), and dogs (up to 9 months):

•    By the longest studies employed in each species, the NOAELs were <400/600 mg/kg/day (male/female) in mice (<49/135 × MRHD), 50 mg/kg/day in rats (143 × MRHD) and 200 mg/kg/day in dogs (169 × MRHD).

•    The most observed were generally limited to effects on body weight and food consumption as well as stress-related findings which occurred at dose levels above the NOAEL and generally at doses of or close to the MTD.  A dose-related finding of soft stool/diarrhoea in the dog may reflect exaggerated pharmacological effects-inhibition of opioid receptor signaling.  Some findings, such as ataxia, tremors and hypoactive behavior observed at HD in the dog, were likely to be indicative of CNS exposure.  It was not considered to be relevant for naloxegol at the intended clinical dose.

The target organ of toxicity identified across all main toxicity species was the liver (weight increase and associated hypertrophy in rats, weight increase in dogs).  But the findings were slight, adaptive and reversible, and occurred at exposures sufficiently above that of MRHD, so were hence to be considered of little relevance to clinical use.

Safety Pharmacology

In vitro and in vivo safety pharmacology studies to evaluate effects on central nervous, cardiovascular, renal, gastrointestinal and respiratory system:

•     No effect on central nervous and respiratory system.

•     Cardiovascular effects: IC₅₀ of the hERG current was > 300 μM considering the in vitro studies, suggestive of low potential for QT prolongation, and naloxegol was inactive at 7 cardiac ion channels, and had no effect on contractility parameters.  However, in the conscious dog telemetry model, there were moderate decreases in ABP, LVSP, and indices of cardiac contractility and relaxation, the NOEL was considered to be 5 mg/kg, where C_max was close to that of MRHD.

•    Gastrointestinal effects: Striking increase of stomach weight, indicating decreased gastric emptying, and inhibited intestinal transport were noted in conscious rats, with NOEL of 30 mg/kg (15 × MRHD) and 300 mg/kg (113 × MRHD), respectively.

•    Renal effects: Dose-related changes in clinical chemistry parameters were observed, but findings were not considered adverse for the overall low degree and 113 × MRHD C_max of NOAEL.

Genotoxicity

Genotoxicity in the in vitro Ames, MLA and the in vivo bone marrow micronucleus assay:

•     Naloxegol was negative in MLA and bone marrow micronucleus assay, but was positive in Ames, likely due to its degradation to naloxone

Reproductive and Developmental Toxicity

Fertility and early embryonic development in rats:

•     Naloxegol did not impair fertility in rats, and the NOAELs were ≥1000 mg/kg/day for males and females.

Embryo-fetal development in rats and rabbits:

•     The NOAELs for maternal and fetal development were 750 mg/kg/day in rats and 450 mg/kg/day in rabbits, sufficiently in excess of the MRHD.

Pre- and postnatal development in rats:

•     The NOAELs were 500 mg/kg/day for F₀ and F₁.

Naloxegol was distributed into fetal brain, fetal liver, eye and uveal tract in pregnant rats.

Naloxegol could efficiently transfer into milk.

Carcinogenicity

2-year carcinogenicity in mice and rats:

•     For mice, there were no naloxegol-mediated clinical sign except for bodyweight changes and no neoplastic changes.

•     For rats, there were no clinical sign except for bodyweight changes but increase of Leydig cell hyperplasia and adenom.