The purpose of this study is to find out the side effects and safety of a combination of the CXCR2 antagonist, AZD5069 in combination with the androgen receptor antagonist, enzalutamide in patients with metastatic castration resistant prostate cancer and to determine the most appropriate dose of this combination. In the Phase I part of this study groups of 3 to 6 patients will be treated with increasing doses of AZD5069 in combination with a fixed dose of enzalutamide (160mg once daily). Once Phase I has been completed the combination with the optimum safety and pharmacokinetic/pharmacodynamic profile will be taken forward to the Phase II part of the study. The Phase II part of the study will evaluate the optimized dose/schedule identified in Phase I of the study in patients with metastatic castration resistant prostate cancer.