Effect of chemotherapy on the labelling index of myeloma cells

The labelling index, namely the percentage of bone marrow plasma cells incorporating tritiated thymidine after short in vitro exposures, was correlated with tumor mass reduction following chemotherapy in 37 patients with multiple myeloma. Tumor mass reduction was assessed from changes in myeloma protein production rate. Patients untreated or not responding to treatment had the lowest labelling index values. The median labelling index increased more than six-fold when tumor mass was reduced by more than 50%. These findings support the concept that an increased fraction of myeloma cells proliferates during remission and justify clinical trials with cell cycle-active drugs in selected patients with a high labelling index.