COVID-19 severity ranged from asymptomatic to moderate-II based on the Clinical Guidelines (Ver. activity against these virus variants to treat persistent SARS-CoV-2 infection in immunocompromised patients. Keywords:COVID-19, hematological malignancies, persistent infection, Imdevimab/Casirivimab, mRNA == Introduction == Patients with hematological disorders, especially those undergoing chemotherapy, are at particularly high risk of severe or fatal disease if they contract coronavirus disease 2019 (COVID-19) (1-3). One major reason for this is impaired immunity (4). To circumvent this drawback, vaccination against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is being conducted worldwide. Nevertheless, antibody responses to the COVID-19 mRNA vaccine are significantly diminished in patients with hematological malignancies, especially those treated with anti-CD20 monoclonal antibodies (mAbs) (5,6). Persistent infections in extremely immunocompromised patients are also concerning. We previously showed that patients with Amsilarotene (TAC-101) hematological diseases are less likely to develop antibodies against SARS-CoV-2 than are those with non-hematological diseases (7). Prolonged viral shedding in patients with B-cell depletion caused by anti-CD20mAb has been demonstrated, and prolonged COVID-19 pneumonia during anti-CD20mAb treatment has also been reported (8-10). Several antiviral drugs and mAbs are currently approved for the treatment of COVID-19 patients who are at a risk of severe disease. In Japan, two mAb therapeutics are available for patients with non-severe COVID-19. Although sotrovimab has completely lost neutralizing activity against BA.2 and BA.5, Imdevimab/Casirivimab (Imde/Casiri) has retained some efficacy against some variants, albeit reduced compared with that against the ancestral strains (11). Several cases of successful treatment of persistent SARS-CoV-2 infection using mAbs, including Imde/Casiri with antiviral agents, have been reported (12-14). Even though our country’s guidelines for COVID-19 recommend an infusion of mAb to patients who develop symptoms within eight days and do not require oxygen for respiratory distress, combination therapies are often applied to those with sustained, relatively severe cases requiring oxygen support (13,14). Even if mAbs should have sensitivity against SARS-CoV-2 by an early period of Omicron predominance, there is no information regarding a clinical efficacy of Imde/Casiri in the ongoing mAb-resistant Omicron era. Thus, we conducted a clinical trial of Imde/Casiri infusion after early treatment failure with conventional antiviral drugs. == Materials and Methods == From April 2022 to February 2023, during the sixth to seventh waves of the COVID-19 pandemic in Japan, patients with hematological malignancies who suffered from persistent SARS-CoV-2 infection even after antiviral drug treatment (remdesivir or molnupiravir) were enrolled. Patients who met these criteria [6 malignant with lymphoma (ML) and 3 with multiple myeloma (MM)] received Imde/Casiri infusion 11-49 days after the disease onset. The SARS-CoV-2 diagnosis was confirmed by real-time reverse-transcription polymerase chain reaction testing of nasopharyngeal swab samples at our institution; the viral load is shown as the cycle threshold value. The SARS-CoV-2 anti-spike IgG titer at diagnosis was determined using a chemiluminescent microparticle immunoassay (SARS-CoV-2 IgG II Quant; Abbott Laboratories, Chicago, USA. COVID-19 severity ranged from asymptomatic to moderate-II based on the Clinical Guidelines (Ver. 9) for COVID-19 by the Japanese Ministry of Health, Labour and Welfare. Thein vitro50% inhibitory concentration (IC50) values of Imde/Casiri were determined using a focus reduction neutralization Amsilarotene (TAC-101) test, as previously described (13). Serial dilutions of Imde/Casiri (starting concentration, 50,000 ng/mL) were mixed with 100-400 focus-forming units (FFUs) Amsilarotene (TAC-101) of virus/well and incubated for 1 h at 37C. The antibody-virus mixture (50 L) was then inoculated onto Vero E6-TMPRSS2-T2A-ACE2 cells in 96-well plates in triplicate. After a 1-h incubation at 37C, 100 L of 1 1.5% Methyl Cellulose 400 (FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan) in the culture medium was added to each well. The SAPK cells were incubated for 14-18 h at 37C and fixed with formalin. After formalin removal, the cells were immunostained with a mouse monoclonal antibody against SARS-CoV-2 nucleoprotein (N45; TAUNS Laboratories, Izunokuni, Japan), followed by horseradish peroxidase-labeled goat anti-mouse immunoglobulin (Jackson ImmunoResearch Laboratories, West Grove, USA). The infected cells were stained with TrueBlue Substrate (SeraCare Life Sciences, Milford, MA, USA) and washed with distilled water. After drying, the focus numbers were quantified using an ImmunoSpot S6 Analyzer, the ImmunoCapture software program, and the BioSpot software program (Cellular Technology, Cleveland, USA). The results are expressed as IC50values, which were calculated using the GraphPad Prism software program.