Antibodies to S-303 Treated RBC Prepared with the Original Treatment Process (OSRBC) for Pathogen Inactivation Do Not React with RBC Prepared with a Modified S-303 Treatment Process (MSRBC).
Authors: Conlan Maureen G. , Garratty George , Castro Grace , Erickson Anna , Schott Mary Ann , Arndt Pat , Cook David , Corash Laurence , Stassinopoulos Adonis 
Published: April 2020

Abstract
Background: S-303 (an acridine compound) treatment of RBCs inactivates a broad spectrum of pathogens. Two randomized, controlled Phase III trials of OSRBC to support patients (pts) undergoing cardiovascular surgery (CVS) or with hemoglobinopathies (HB) were in progress when antibodies (Ab) to OSRBC were detected. The specificity of these Ab was determined to be due to residual RBC-bound acridine. The S-303 treatment process was modified in an attempt to reduce immunoreactivity and immunogenicity. MSRBC show reduced RBC-bound S-303 and no reactivity with anti-S-303 Ab.
Methods: Retrospective central laboratory testing was performed on all available trial pt serum samples (multiple time points for each pt). Crossmatch (CM) testing (PEG CM) against a minimum of 3 OSRBC units and the paired pre-treatment RBC segment was performed for each sample. CM was defined positive when reactive with any OSRBC, and consistent with anti-S-303 Ab when reactive with all OSRBC, but none of the paired pre-treatment RBC. CM-positive sera were tested for S-303 specificity with a CM inhibition assay using S-303 analogues and also tested for CM reactivity against MSRBC with a gel card CM test. All available baseline sera from trial pts, as well as plasma from 200 healthy blood donors, were also tested by gel card CM against OSRBC and MSRBC to determine prevalence of Ab reactivity to SRBC in absence of prior SRBC exposure.
Results: Sera from 8 of 174 (4.6%) pts from both trials showed some CM reactivity with OSRBC. 4 were positive with all ORBC tested; and 4 were inconsistently positive, reactive with some but not all OSRBC. For the 4 pts (2.3%) with consistently positive CM, 3 had OSRBC exposure (HB trial) and 1 had only Control RBC exposure (CVS trial); 3 had positive CM with S-303 specificity. None of these 4 pt sera was CM positive with MSRBC. Only 1 of the other 4 pts with inconsistently positive CM tests had a CM test specific for SRBC (a Control CVS pt); sera from this pt were not reactive with MSRBC. 4 (1.8%) baseline sera from 220 trial pts (205 CVS and 15 HB) were CM-positive with OSRBC; only 2 (0.9%) were CM-positive with all units tested; none was positive with MSRBC. 3 of 200 (1.5%) healthy blood donor plasma were CM-positive with OSRBC; only 2 were positive with all units tested; none was positive with MSRBC.
Conclusions: OSRBC were immunogenic in HB pts with prior chronic RBC exposure. Inconsistently positive CM to OSRBC without antigenic specificity for S-303 were detected with some pt sera; these results are under further investigation. Ab to OSRBC was detectable in sera of pts never exposed to S-303 treated RBC. Approximately 1% of pts and healthy blood donors had Ab that reacted with OSRBC. However, no sera with either inconsistent or confirmed CM reactivity to OSRBC were reactive with MSRBC. The MSRBC process eliminated CM reactivity to RBC treated with S-303 and offers the potential for pathogen inactivation treatment of RBC without immunologic reactivity.