Serum Thymidine Kinase as a Marker of Disease Activity in Patients With Multiple Myeloma

Serum thymidine kinase (STK) levels have recently been used to detect tumour regression and progression in a number of hematological malignancies. In this study, patients with myeloma were monitored longitudinally for STK and several other potentially useful tumour markers to determine which laboratory parameters are the most useful for differentiating between stable and progressive disease. STK was determined by radioenzyme assay, lymphocyte surface markers were analysed by flowcytometry, plasma cell labelling index (LI) by immunofluorescence with anti BU-1, serum B2-microglobulin (SB2M) by radioimmunoassay and M proteins by radial immunodiffusion. Detailed multiparameter longitudinal investigations of 5 patients and ongoing studies of 70 other patients suggest that STK is a more reliable marker of progressive disease than either SB2M, LI, M-protein or CD10 positive lymphocytes. A rise in STK during the emergence of progressive disease at least paralleled and usually preceded any change in the other parameters which often did not change at all. All samples from patients with progressive disease (n = 29) had a STK above the normal range (0-5U/l) whereas 76% of patients in clear stable disease had a STK within the normal range. All samples (n = 34) from patients with light chain isotype suppression (LCIS) had STK values of less than 12 U/L and 82% of samples (n = 33) from patients without LCIS had a STK above the normal range (0-5U/L). The correlation between STK and LI was r = 0.65; p less than 0.001 (n = 21). The radioenzyme assay for STK is simple, reproducible and a valuable tool for monitoring patients with myeloma and when used in conjunction with other clinical and laboratory investigations, aids in the separation of patients with stable myeloma from patients whose disease is progressive.