Hypogammaglobulinemia After a BCMA-Directed Bispecific: When to Replace
A BCMA-directed bispecific antibody for multiple myeloma has driven this patient's IgG far lower than her prior anti-CD20 therapy ever did, testing whether replacement should wait for the next infection or start at the number itself.
Eleanor P. is 67, and spent thirty of those years as the children's librarian at her town's public library, and still volunteers there most Saturdays reading aloud to the pre-school story hour, something she'd hate to give up. Her multiple myeloma was diagnosed four years ago and has moved through two prior lines of therapy — daratumumab-based induction, then lenalidomide maintenance — before relapsing again this spring, which is what brought her to teclistamab, a BCMA-directed bispecific antibody now three months into treatment. She has been hospitalized twice in that window for pneumonia, once requiring oxygen support, a pattern that is new for her; before starting teclistamab she had gone years without a serious infection.
Her IgG this week is 210 mg/dL, well below the roughly 400-500 mg/dL threshold most replacement-therapy guidance uses as a floor, and far below where it sat before treatment began. The mechanism matters here more than the number alone: teclistamab redirects a patient's own T cells to kill any cell expressing BCMA, a marker not just on malignant plasma cells but on the entire normal plasma cell compartment that manufactures antibody in the first place — a fundamentally different, more complete depletion than anti-CD20 therapies like rituximab produce, since those spare mature plasma cells and typically leave antibody production only partially impaired. The trial data behind teclistamab's approval reported high rates of serious infection specifically in this pattern, which is part of why the question in front of the team isn't whether her IgG is low, but whether to wait for the next infection to justify replacement or to treat the number itself as the trigger.
Her own course reads as a fairly literal illustration of what that trial data described in aggregate: a steep drop in IgG within the first month of dosing, sustained rather than transient across her three months on therapy, tracking closely with the two clinically significant respiratory infections that brought her back to the hospital. That correspondence between the mechanism, the trial-level infection signal, and her own individual course is what moves this from a hypothetical policy question to a decision that has to be made this visit, not deferred to a data review at her next scheduled myeloma follow-up.
Reading the IgG number the way this drug actually depletes it
My default with hypogammaglobulinemia after antibody-depleting therapy has always been reactive, not proactive — start replacement once someone's actually had a serious infection, not off a lab value alone, because most patients on anti-CD20 therapy tolerate low IgG without ever needing it. IVIG isn't free of its own burden either: infusion reactions, thrombotic risk, cost, and time she doesn't have much of between library shifts and treatment visits.
You're right that the reactive approach works for most anti-CD20 patients — but she isn't an anti-CD20 patient, and I don't think that reasoning transfers cleanly.
Rituximab depletes mature B cells and spares long-lived plasma cells, which is exactly why most of those patients keep making enough antibody to get by. Teclistamab redirects T cells against BCMA, which sits on the plasma cells themselves — the actual antibody factories — not just their B-cell precursors. The trials behind its approval, including MajesTEC-1, reported high rates of serious infection in this population and used an IgG threshold, not an infection history, to trigger replacement in their own safety monitoring. She's already had two pneumonias since starting; by the time a third qualifies as 'enough evidence,' the number has already done exactly what it was threatening to do.
If we're treating the number, subcutaneous replacement is worth considering over monthly IV dosing specifically for her — she's already tethered to the infusion center for teclistamab visits, and weekly or biweekly SCIG she can eventually self-administer gives her back some of the Saturdays this diagnosis keeps taking. It also avoids the larger single-dose IVIG volume load, which matters given how much fluid she's already receiving with each teclistamab cycle.
Either route treats the same number; the choice is really about which schedule fits the life she's still trying to hold onto.
Agreed: IgG-replacement therapy started now rather than deferred to the next infection, given the mechanistic argument that teclistamab depletes the plasma cell compartment directly rather than sparing it. Subcutaneous administration chosen over monthly IV dosing specifically to reduce her infusion-center burden.
Not addressed today, flagged for the next visit: whether replacement should continue indefinitely for as long as teclistamab does, or be reassessed each time her dosing interval extends, since the trial data the infectious disease physician cited describes risk during active treatment and says less about what happens once her disease responds and dosing spaces out.