Background: Sip-T is an autologous immunotherapy approved for asymptomatic or minimally symptomatic metastatic castration-resistant prostate cancer (mCRPC).Many therapeutic agents have been approved in a short time frame, and their optimal sequencing in mCRPC is under investigation. The sequencing of agents post–sip-T treatment on PROCEED (NCT01306890) and factors associated with a shorter time to first ACI (tACI) are described. Methods: The number of unique and most common sequences and sequence trends were analyzed. Post–sip-T ACIs evaluated were abiraterone (abi), enzalutamide (enz), docetaxel (D), and cabazitaxel (cbz), excluding radium-223 due to its late approval relative to PROCEED initiation. The tACI and factors associated with shorter tACI were assessed. Results: 1902 pts received ≥1 sip-T infusion, and an estimated 37.0% and 21.2%had not received an ACI at 1 and 2 y, respectively, post–sip-T. 1331 (70%) pts received abi, enz, D, or cbz after sip-T. 47 therapy sequences were recorded post–sip-T, with the most used therapies in order of prevalence: abi alone, enz alone, D alone, abi → enz, abi → D, D → enz, and D → abi. The most common post–sip-T sequences beginning with abi, enz, or D are below (Table). Only 25 pts received cbz as the first ACI post–sip-T. Pts who received abi prior to sip-T had a shorter median tACI (4.6 mos) compared with all pts (8.3 mos) or pts with D prior to sip-T (7.3 mos). Higher baseline alkaline phosphatase, lower hemoglobin, and prior radical prostatectomy were associated with shorter tACI. Conclusions: In the evolving mCRPC space, 47 unique ACI sequences were recorded post-sip-T. The most common sequences post–sip-T were abi, enz, and D as single agents. Prior abi was associated with a shorter tACI post–sip-T, which warrants longer follow-up. While many pts received subsequent therapy, ~20% had not received an ACI at 2 y post–sip-T. Clinical trial information: NCT01306890

*Many pts in PROCEED received their first ACI prior to FDA approval of enz pre-D.