Introduction & Objectives: Novel local cytoreductive treatments (e.g. external beam radiotherapy, cytoreductive radical prostatectomy) and metastasis-directed therapy (e.g. stereotactic ablative body radiotherapy [SABR]) are now under trial evaluation. Such interventions may provide added survival benefit or delay the requirement for further systemic agents, and associated toxicity, but can confer additional harm. Understanding patients’ preferences for treatment options in this disease state is crucial for patients, clinicians, carers and future healthcare service providers.Materials & Methods: IP5-MATTER (ClinicalTrials.gov Identifier: NCT04590976) will use a prospective, multi-centre discrete choice experiment (DCE), to determine the attributes associated with treatment that are most important to men with mHSPC. Furthermore, we aim to determine men’s preferences for, and trade-offs between, the attributes (survival and side-effects) of different treatment options including systemic therapy, local cytoreductive approaches and metastases-directed therapies. All men with newly-diagnosed mHSPC within four months of commencing ADT and WHO performance status 0 to 2 are eligible. Men who have previously consented to a cytoreductive treatment or have developed castrate- resistant disease will be excluded. This study includes a qualitative analysis component, performed with patients with mHSPC and their healthcare professionals to develop the DCE. The main phase component planned recruitment is 300 patients over 30 sites.Results: Qualitative analysis components identified the key attributes associated with treatment options that would warrant trade-off evaluation as: overall survival, time until progression, risk of fatigue interfering with daily activities, risk of urinary incontinence and risk of erectile dysfunction. Using experimental design theory, three sets of 12-choice cards were developed and prospectively validated in ‘Think Aloud’ interviews with patients (Figure 1). Main phase component recruitment commenced across 8 of 30 centres in February 2021, with planned study completion by April 2022.