Radiologic Pseudoprogression Associated With a Positive Response in a Patient With Multiple Myeloma Treated With a BCMA×CD3 Bispecific Antibody: A Case Report.

Pseudoprogression is characterised by an increase in tumour size, driven by an influx of inflammatory cells, followed by regression. This phenomenon has rarely been reported in multiple myeloma (MM), despite increased use of immunotherapy approaches that have been associated with pseudoprogression in other malignancies. We report a case of pseudoprogression in a female in her early seventies with relapsed/refractory MM who was treated with the BCMA×CD3 bispecific antibody linvoseltamab. At study Day 28, she was hospitalised with acute bone pain and a new fracture, and a positron emission tomography/computed tomography scan suggested disease progression. However, biochemical markers of disease burden (serum M-protein, kappa light chain) had decreased from baseline, and therefore linvoseltamab was continued. At Day 77, M-protein was absent and the patient had achieved complete biochemical response, but she had to discontinue linvoseltamab due to infectious complications. A subsequent scan on Day 86 demonstrated marked improvement in osseous lesions. Despite being off therapy, the disease did not progress for 2 years. Assessment of T-cell activation and proinflammatory cytokine production indicated increased T-cell activation concurrent with onset of radiologic pseudoprogression. To our knowledge, this is the first description of radiologic pseudoprogression in MM with anti-BCMA bispecific antibody therapy.