Prostate cancer that has returned after local treatment usually responds to hormone blocking treatment, but most patients eventually experience disease progression. Further chemotherapy does not normally lead to a cure or dramatic improvement in the disease and there is a need to identify new drugs that are beneficial for these patients without unacceptable side effects. Prodrug chemotherapy is an approach in which an inactive non-toxic agent is administered to the patient and gets activated within the body at specific locations, resulting in a higher concentration of the cytotoxic form at a tumour location whilst avoiding general side effects. G-202 is an example of prodrug chemotherapy. It does not have many general side effects because it is converted to a cell toxin only at the tumour or other specific locations in the body. G-202 is activated by Prostate Specific Memory Antigen (PSMA), a substance expressed by prostate cancer cells and in the blood vessels of most solid tumours, but not by normal cells or blood vessels in normal tissue. It is believed that activation of the prodrug G-202 will allow the drug to kill cancer cells, particularly prostate cancer cells. This study will evaluate the activity and safety of G-202 in men with castration-resistant prostate cancer (CRPC), which means the cancer has progressed after hormone blocking treatment, but who have not yet received chemotherapy and who have no or only a few symptoms from their CRPC. The study will evaluate clinical activity and safety of G-202 administered on three consecutive days of a 28-day cycle.