Drug resistance in multiple myeloma associated with high in vitro incorporation of 3H-thymidine.

In multiple myeloma, tumor cell mass and labeling index correlate with subsequent survival duration, but do not predict for response to treatment. In the present study was have autoradiographically measured the incorporation of 3H-thymidine as determined by the number of grains over the myeloma nuclei in bone marrow aspiration samples. In 33/37 patients with less than 50% tumor regression or progressive disease, the pretreatment grain count was greater than or equal to 20/myeloma nucleus. Conversely, values of less than 20 were found in 27/29 patients who had greater than or equal to 50% cell mass reduction. Survival duration was significantly better than (p less than 0.001) in patients with grain counts less than 20. Sixty percent of the patients with both a low labeling index (less than or equal to 3%) and grain count (less than 20) were alive at 48 mo, whereas 15/17 patients with a high labeling index and grain count had a median survival of less than 6 mo. In a subset of 22 patients, there as a significant correlation between in vitro resistance to melphalan, adriamycin, and vincristine as tested in the myeloma stem cell colony assay system and a grain count of greater than 20. We can only speculate as to the reasons for the increased 3H-thymidine uptake by myeloma cells resistant to treatment, however, it could be associated with accumulation of excess DNA and /or increased unscheduled DNA synthesis following injury from alkylating agents.