To characterize the pharmacokinetics (PK) of naloxegol after single oral dose and through population PK in paediatric patients with opioid induced constipation (OIC) or at risk of OIC.

Main inclusion criteria, patient with:

• malignant or non-malignant pain who are receiving or (are about to receive) acute or chronic treatment with opioids
• newly diagnosed constatipation, with history of constipation treated with laxatives or expected to develop constipation after opioid treatment
• ability to be present in the clinic for at least 10 hours following the first study drug for blood sampling and to return at 24 hours for blood sampling.

Main exclusion criteria:

• Current acute or chronic use of methadone
• History of an neoplasm or an ongoing gastrointestinal-related issue
• Signs or symptoms of gastrointestinal obstruction
• History of prolonged neutropenia or thrombocytopenia with clinical sequelae.
• Patients currently receiving the first cycle of chemotherapy