The growth fraction of human myeloma cells

Greater reductions of tumor load in patients with multiple myeloma may result from therapeutic strategies that are based on a better knowledge of growth kinetics. We have previously shown that the labeling index of myeloma cells remains unchanged when tumor mass is reduced and that the cells of relapsing patients have different biologic properties than the cells present before melphalan-prednisone therapy. This study investigated the growth fraction (GF) of myeloma cells at various disease stages using continuous i.v. infusions of tritiated thymidine. We studied 17 patients on 22 occasions (4 untreated, 2 unresponsive, 6 in remission, and 10 in relapse). All untreated and unresponsive patients and 5 of 6 patients in remission had a GF of less than 4%. GF was defined in these studies as the maximum percentage of labeled plasma cells exposed continuously to tritiated thymidine. Relapsing patients, with the most rapid tumor doubling times, had GF ranging from 14% to 83%. The plasma cell transit time through the proliferative compartment for all of the relapsing patients ranged from 6.6 to 11.9 days and the calculated intrinsic cell loss ranged from 50% to 86%. These findings support our model for the growth kinetics of multiple myeloma that assumes that the entire tumor mass issues from a small proportion of proliferating cells and that the growth kinetics of myeloma cells in relapsing patients differ from those in untreated and unresponsive patients. Therapeutic trials with cycle-active agents need further investigation in selected relapsing patients who are likely to have a high growth fraction.