To measure the duration of B-cell depletion in a cohort of patients receiving ocrelizumab or rituximab for  MS or NMOSD.

B-cell depleting agents, such as ocrelizumab and rituximab, are increasingly being administered as disease modifying therapies for CNS demyelinating diseases. The manufacturers of these drugs, based on clinical trials, recommend a fixed reinfusion interval dosing regimen of every 6-months.  However, previous studies show that B-cell depletion may last longer than 6 months, with several rituximab-focused studies demonstrating a range of 9 to 26 months.

We retrospectively searched our database for patients diagnosed with MS or NMOSD, who were receiving ocrelizumab or rituximab and had available CD19 measurements. We paired each infusion with the subsequent CD19 measurements recorded before the next infusion, discarding measurements done during a washout period of 30 days after each infusion. We applied three definitions for B-cell depletion, the most stringent of which was an absolute B-cell count ≤20 cells/uL.

From 695 patients with demyelinating diseases in our database, over the period of January 1^st 2010 to March 1^st 2020, we identified 188 patients (178 with MS and 10 with NMOSD), who had received ocrelizumab or rituximab and had available CD19 measurements. 1054 CD19 measurements were captured. B-cell depletion, as defined above, was recorded as far out as 22.8 months after an ocrelizumab infusion, and 22.3 months after a rituximab infusion. Out of 90 B-cell measurements done ≥8 months (≥211 days) after ocrelizumab infusion, 45(50%) measurements showed B-cell depletion. Similarly for rituximab, out of 113 measurements, 49(43%) showed B-cell depletion.

This study demonstrates that B-cell depletion after ocrelizumab and rituximab continues beyond the 6-month re-infusion interval in many patients. Our report provides retrospectively gathered evidence that can support increasing the interval of re-infusion with ocrelizumab and rituximab beyond 6-months, as long as B-cell counts are monitored.