Mononuclear cell recruitment during extracorporeal photopheresis: Partial results of a phase 1/2 randomized clinical trial in multiple sclerosis

Background: Despite the routine clinical use of extracorporeal photopheresis (ECP) for the last decades, there has been no sufficient investigation on the intra-apheresis dynamics of mononuclear cells (MNCs). Objective: This research aims to verify the "intra-apheresis recruitment" (IAR) of MNCs, which may be particularly useful in cases with low preapheresis MNC counts. Methods: Prospective data of ECP procedures for patients with multiple sclerosis (MS) in the context of the PHOMS study (NCT05168384) were analyzed. ECPs were performed with the use of the Therakos (R) CellEx (R) Photopheresis System, and the recruitment factor (RF), collection efficiency 1 (CE1), and fold enrichment (FE) for MNCs, lymphocytes, monocytes, and neutrophils were calculated at various intervals per the approved protocol. Results: A total of 252 ECP procedures were performed on nine patients with MS, and data from 45 ECPs were analyzed for this study. IAR (RF >= 1) of MNCs, lymphocytes, and monocytes was observed in the 93.3 %, 95.5 %, and 88.8 % of the ECP procedures, respectively. CE1 exceeded 100 % for these populations in 13.3 %, 11.1 %, and 24.4 % of ECPs, while FE >= 1 for MNCs and lymphocytes was found in all ECPs, and in 97.8 % of ECPs for monocytes. Additionally, RF correlated positively with CE1 for MNCs (r = 0.40, p = 0.006) and lymphocytes (r = 0.43, p = 0.003), as well as with FE for monocytes (r = 0.39, p = 0.01) and lymphocytes (r = 0.48, p = 0.001), further improving collection quality. Conclusions: Administering photopheresis with suboptimal MNC counts was plausible and did not worsen the anticipated cytopenias. Moreover, our findings revealed sustained IAR of MNCs in patients with MS who underwent ECP.