In both arms, disease status will be evaluated with clinical and imaging evaluation. Baseline total tumor burden must be assessed by MRI within 2 weeks prior to randomization. For patients randomized to VAL-083, post-baseline tumor assessments are to be performed prior to every other 21-day cycle for VAL-083, i.e., prior to Cycles 3, 5, etc., while the patient is receiving VAL-083 treatment, and then approximately every 42 ± 7 days while remaining on study. For patients randomized to the Physician's choice salvage therapy arm, post-baseline tumor assessments are to be performed approximately every 42 ± 7 days while remaining on study, as long as the patient continues to demonstrate response or stable disease or comes off the study.