Retrospective evaluation of the use of the International Myeloma Working Group response criteria in dogs with secretory multiple myeloma

BACKGROUND: Current recommendations for monitoring disease progression and response to treatment in humans with multiple myeloma include evaluation of serum paraprotein (M‐protein) concentration. Densitometry, species‐specific radial immunodiffusion (RID) and ELISA methods can be used to quantify M‐proteins. OBJECTIVE: Retrospectively evaluate use of the International Myeloma Working Group (IMWG) response criteria for humans in dogs with multiple myeloma. ANIMALS: Sixteen dogs with a diagnosis of multiple myeloma, M‐protein documented by serum protein electrophoresis (SPE) and immunofixation (IF) in an initial sample and subsequent electrophoretic evaluation of serial samples. METHODS: Retrospectively, densitometric M‐proteins, RID and globulins were measured and characterized according to IMWG criteria. Available clinical history was reviewed. Overall survival time (OST) was calculated from initial electrophoretic evaluation to death or last contact. RESULTS: All cases received some form of nonstandardized chemotherapy. Complete response (CR), a lack of detectable M‐protein by SPE and IF, was documented in 1 case. Median survival was longer for dogs that attained ≥90% densitometric M‐protein reduction (630 days) than for those that did not attain at least 50% reduction in densitometric M‐protein (284 days; log rank P = .006). Five dogs were defined as having progressive disease (M‐protein increase of >25% and at least 0.5 g/dL from nadir), which correlated with concurrent or subsequent clinical deterioration. Response criteria categorized by serum globulins or RID was not correlated with OST or clinical findings. CONCLUSIONS AND CLINICAL IMPORTANCE: Densitometric M‐protein characterized using IMWG response criteria correlated with OST and clinical findings. Densitometric M‐protein detection should be used to monitor dogs with multiple myeloma.