Background: The receptor for luteinizing hormone releasing hormone (LHRH-R) is highly expressed on CRPC cells and is a potential therapeutic target. AEZS-108 is an LHRH-cytotoxic hybrid that covalently couples an LHRH agonist with doxorubicin. We report the completed Phase I trial of AEZS-108 in men with taxane-resistant CRPC. We explored visualization of AEZS-108 internalization into circulating tumor cells (CTCs) exploiting the auto-fluorescence of doxorubicin and also tested LHRH-R expression on CTCs. Methods: This was a dose escalation Phase I trial in men with taxane-resistant CRPC to confirm the dose established in a phase I trial in women. Standard 3+3 design was used with planned expansion at the MTD to ensure 6 patients received 2+ courses without DLT. Eligibility criteria included progression of disease despite prior LHRH agonist and taxane therapy. Patients received AEZS-108 every 21 days until progression or unacceptable toxicity. The primary endpoint was safety. CTCs were captured with a novel slot microfilter and identified by PSA and DAPI staining. AEZS-108 internalization was visualized by fluorescence microscopy. Results: Eighteen men with a median of 2 prior chemotherapy regimens (range 1-5) and a median PSA of 106.4 ng/mL (range 8.4-1624.0) enrolled from November 2010 to August 2012. The dose was escalated from 160 mg/m² to 210 mg/m² then to 267 mg/m². There were 2 DLTs in the 7 men receiving 267 mg/m² (grade 4 neutropenia), prompting de-escalation to 210 mg/m² where 1 of 8 men experienced a DLT (grade 4 neutropenic fever), establishing 210 mg/m² as the MTD. Significant non-hematologic toxicities included a case of grade 3 nausea. No cardiotoxicity was seen on serial evaluation and 6 patients completed 6 cycles. Internalization of AEZS-108 was consistently visualized in CTCs 1-3 hours after dosing. Maximal PSA response was stable or decreased in 8 of 18 men. Conclusions: The MTD of AEZS-108 in men with taxane-resistant CRPC is 210 mg/m², which is below the MTD reported in women with refractory endometrial, ovarian and breast cancer. The activity of AEZS-108 was promising in this heavily pretreated population. The Phase II portion is currently accruing. Clinical trial information: NCT01240629.