The purpose of this single-center retrospective study was to determine the 
incidence of decreased blood phosphorus levels and hypophosphatemia among 
multiple myeloma (MM) patients treated with elotuzumab. Hypophosphatemia, which 
is defined as a serum phosphorus concentration < 2.5 mg/dL, leads to 
complications ranging from muscle weakness and disorientation to seizures and 
heart failure. A total of 23 MM patients receiving care in a clinic specializing 
in treatment of MM from July 2018 to March 2020 and treated with an 
elotuzumab-containing therapy were evaluated, and 9 were investigated for this 
study. Elotuzumab was given at 10 mg/kg weekly for the first two treatment 
cycles (28 days/cycle), followed by 10 mg/kg every other week for all subsequent 
cycles. Four different elotuzumab combination therapies were administered: 1) 
elotuzumab and dexamethasone 2) elotuzumab, lenalidomide and dexamethasone 3) 
elotuzumab, pomalidomide and dexamethasone and 4) elotuzumab, carfilzomib, 
pomalidomide, and dexamethasone. Phosphorous levels were determined at a median 
of every 13 days at intervals ranging from once weekly to once monthly until a 
phosphate supplement was prescribed to the patient or when elotuzumab treatment 
was discontinued. We found that regardless of elotuzumab combination therapy, 
all patients treated showed decreased phosphorus levels after initiating 
elotuzumab treatment with reductions ranging from 12.5% to 44.1% below baseline. 
Six participants (67%) demonstrated an average serum phosphorus at or below 
2.5 mg/dL after starting elotuzumab therapy. This retrospective study suggests 
that hypophosphatemia commonly occurs among MM patients receiving 
elotuzumab-containing therapies.