In the Phase I part of the study, the AZD5069 is started first and will be taken twice daily as an oral tablet at Cycle 1, Day -14 for 14 days. Two weeks later on Cycle 1 Day 1, patients will start taking 160mg enzalutamide once a day in addition to the AZD5069. The starting dose of AZD5069 will be 40mg taken orally twice daily with single dose escalations to 80mg, 120mg and 160mg taken orally twice daily to determine the MTD to take forward to a Phase II reversal of resistance cohort. The Phase II reversal of enzalutamide resistance study will explore whether the addition of AZD5069 to enzalutamide reverses resistance to enzalutamide alone. In the phase II reversal of enzalutamide resistance study patients will start taking the AZD5069 at the dose established in the Phase I safety run in part of the study in combination with 160mg of enzalutamide once a day and at the same time from Cycle 1 Day 1 onwards. Potential patients who previously progressed on enzalutamide (having received at least 12 weeks treatment) within 6 months of trial entry (first dose of IMP) will enter the Phase II reversal of enzalutamide resistance study immediately. However, those patients who progressed on enzalutamide (having received at least 12 weeks treatment) greater than 6 months before trial entry will first enter the Phase II enzalutamide resistance cohort to confirm resistance. Study patients will receive 160mg enzalutamide once a day until disease progression. Once enzalutamide resistance is confirmed in these patients, they will be eligible to enter the Phase II reversal of enzalutamide resistance cohort.