The symptoms associated in advanced cancer patients and adverse events due to use of opioids have major influence on the state of health and quality of life of patients. The pain, in particular, is a symptom with severe negative impact and with a prevalence ranging between 33% and 64%, according to the stage of the disease, with values around 70-90% in advanced stages and metastatic.

The use of opioids, however, is usually associated with the appearance of common adverse events as drowsiness/sedation, constipation, nausea/vomiting, and dizziness. Some effects are self-limiting in the time for the appearance of tolerance while others, as constipation persist.

Several clinical studies have demonstrated that the association oxycodone-naloxone (OXN), which consists in the union between a molecule agonist and an antagonist of opioid receptors, reduced the constipation in the presence of unchanged analgesic efficacy compared to oxycodone alone.

Inclusion Criteria:

• Patients with diagnostic (histological or cytological) evidence of locally advanced or metastatic solid tumour;
• With average pain intensity > 4, measured with NRS and related to the last 24 hours, due to the cancer, requiring OXN for the first time;
• With life expectancy > one month;
• Strong opioid naïve;
• Eligible to take any of the medications under evaluation;
• With age ≥ 18 years.

Exclusion Criteria:

• With presence of other diseases, including psychiatric/mental illness, severe senile or other form of dementia, that can interfere with participation and compliance with the study protocol or can contraindicate the use of the investigational drugs;
• Diagnosis of primary brain tumor or leukaemia;
• Diagnosis of chronic renal failure;
• Patients with antalgic radiotherapy or radio-metabolic therapy in progress or completed less than 14 days before study;
• Patients starting a first line chemotherapy simultaneously to the beginning of the study;
• Other types of analgesic treatments, including local-regional anesthetic techniques or neurosurgical /ablative methods;
• Patients who cannot guarantee regular follow-up visits for logistic or geographic reasons.

This study involves a pain longitudinal evaluation in cancer patients through a follow-up of 4 weeks, which will examine in a dynamic clinical, biological and genetic aspects.