Inclusion Criteria:  signed informed consent. have an Eastern Cooperative Oncology Group (ECOG) status 0-2. have selected active surveillance as a management option. have an mpMRI (multi parametric Magnetic Resonance Imaging) detectable lesion with an M score of ≥ 3 using Likert scale OR PIRADS (Prostate Imaging Reporting and Data System (PIRADS) version 2.1) reporting criteria. have prostate cancer from a combination of image guided targeted + systematic biopsies and MRI lesion and biopsy are concordant for a prostate cancer diagnosis. not anticipated to require bladder outlet surgery during drug treatment or up to 12 months of Follow Up.  Meet all of the following clinical laboratory assessment criteria:  haemoglobin ≥ 9.0 g/dL, independent of transfusion and/or growth factors within 3 months prior to randomisation. platelet count ≥ 100 x 10^9/L independent of transfusion and/or growth factors within 3 months prior to randomisation. absolute neutrophil count (ANC) ≥ 1350/µL. serum albumin ≥ 3.0 g/dL. glomerular filtration rate (GFR) ≥ 30ml/min AND Serum creatinine ≤ 2 times the ULN concurrent with creatinine clearance ≥ 50mL/min (calculated by Cockcroft and Gault equation). serum potassium ≥3.5 mmol/L. aspartate aminotransferase (AST) or alanine aminotransferase (ALT) <2.5 × ULN AND Serum total bilirubin ≤1.5 × ULN (Note: In subjects with Gilbert's syndrome, if total bilirubin is >1.5 × ULN, measure direct and indirect bilirubin and if direct bilirubin is ≤1.5 × ULN, subject may be eligible). have prostate cancer with any one or more of the following: Cambridge Prognostic Group 2 (Favourable intermediate risk prostate cancer by AUA criteria) OR CPG1 (AUA Low risk) with PSA high density (PSAd>0.15). have a LIKERT or PIRADS 4/5 lesion (individual or combined) of ≥10mm and be at least CPG1 (no PSAd limit).  Exclusion Criteria:  Contraindications to apalutamide or its excipients. Pelvic metalwork interfering with MRI prostate interpretation. Any prior use of androgen deprivation therapy or androgen receptor targeting agents (not including 5-alpha reductase inhibitors). Any prior systemic therapy for prostate cancer. Inability for patient to have prostate mpMRI scan. Concurrent CTIMP involvement; participation in an observational study/ studies is acceptable. Seizure or known condition that may pre-dispose to seizure (including but not limited to prior stroke, transient ischemic attack, loss of consciousness within 1 year prior to randomisation, brain arteriovenous malformation; or intracranial masses such as schwannomas and meningiomas that are causing oedema or mass effect). Participant has history/is at risk of falls/fracture. Severe/unstable angina, myocardial infarction, symptomatic congestive heart failure, arterial or venous thromboembolic events (e.g., pulmonary embolism, cerebrovascular accident including transient ischemic attacks), or clinically significant ventricular arrhythmias within 6 months prior to randomisation. Cardiovascular risk factors should be optimised i.e. hypertension, diabetes, dyslipidaemia. Uncontrolled hypertension (SBP ≥ 160 mmHg or DBP≥90 mmHg). Patients with a history of uncontrolled hypertension are allowed provided blood pressure is controlled by anti-hypertensive treatment. Gastrointestinal disorder affecting absorption. Medications known to lower the seizure threshold must be discontinued or substituted at least 4 weeks prior to study entry. Medicinal products known to prolong the QT interval or medicinal products able to induce Torsade de pointes. Strong inhibitors of CYP2C8. Strong inhibitors of CYP3A4.