Lymphocyte thymidine kinase and treatment response in acute lymphocytic leukemia

The activity of thymidine kinase (TK) and the proportion of its isozymes (TK1/TK2) were studied in peripheral lymphoid cells of 37 children with acute lymphocytic leukemia (ALL). The high TK in 25 untreated subjects (31.5 +/- 8.9) decreased during chemotherapy-induced remission to uniformly low (5.3 +/- 0.4) normal values, and rose again during relapse to a mean of (24.8 +/- 8.1). The proportion of isozyme 1 followed the same pattern but TK was a more sensitive indicator of disease state. The lymphocyte fractions' TK (per mg protein) correlated with the number (per ml blood) of WBCs, blasts and lymphocytes. Although the higher TK of blasts than of apparently normal lymphocytes was confirmed in cases permitting clean physical separation, the lymphocyte fraction of several untreated subjects with minimal blast counts also exhibited elevated TK. Moreover, this elevation was also seen in relapsed cases even if their blood (unlike bone marrow) was devoid of blasts. The results indicate that quantification of TK can reveal a subpopulation of maldifferentiated lymphocytes which are microscopically normal and that it may provide an objective parameter of prognostic differences between ALL subjects with similar hematological characteristics.