Patients with advanced prostate cancer are often treated with the chemotherapy drug docetaxel. The manufacturers of this drug suggest using the same dose of drug per m² body surface area for all patients. However we know that individuals end up with more or less of the drug circulating in their blood even after they have been given the same dose.

A software program (a "Dosing Tool") has been developed to provide information to doctors on how docetaxel will affect individual patients. The program may help doctors to make informed decisions about exactly how much drug to give to different people.

The purpose of the PARTNER study is to gather information from blood tests on patients being treated with docetaxel to help in the further development of the Dosing Tool. Some of these blood tests would have been taken anyway as part of patients' routine treatment. Others are extra for this study. Apart from the additional blood tests, everyone who enters the study is treated just as they would normally be if they were not taking part in the study.

Inclusion Criteria:

• Diagnosed with metastatic prostate cancer (hormone sensitive or castrate resistant) and histologically confirmed
• Metastatic at disease onset or relapsed disease
• A treatment decision has been made to start a course of docetaxel as part of their standard of care treatment in accordance with the drug's label and the treating clinician's judgement
• Patient has not received a previous cycle of docetaxel within 8 weeks of the date of enrolment
• Life expectancy > 12 weeks
• Participant is willing and able to give informed consent for participation in the study

Exclusion Criteria:

- Patients with autoimmune disease receiving active treatments Participant is unable to give informed consent

Any patient with metastatic prostate cancer due to start treatment with docetaxel