Patients will remain at the study center for 48 hours after infusion to allow adequate time for post-treatment safety observation, monoclonal antibody clearance and rhenium decay. Prior to the escalation of the dose of unlabeled PTI-6D2, safety data from the inpatient period and 2-week follow-up visit for the three patients at the current dose level will be reviewed. If there is no evidence of a safety risk, the dose of unlabeled PTI-6D2 will be escalated for the next cohort of patients. Dose escalation of unlabeled antibody will occur according to a specific scheme.