Comparison of in Vivo and in Vitro Effects of Continuous Exposure of L1210 Cells to 6-Thioguanine

In this study the cytokinetic and antitumor effects of 12-h continuous treatment with 6-thioguanine (TG) were studied in L1210 cells in vivo and in vitro. Loss of clonogenicity in vitro was maximized at a drug concentration of 0.2 μM. Higher drug concentrations produced less cell kill, and the surviving fraction observed after exposure to 25 μM TG was 1 log higher than at 0.2 μM (2% versus 0.2% of control cloning efficiency, respectively). Delayed G₂ arrest in vitro was also found to be most pronounced at 0.2 μM with G₁ arrest more predominant at higher concentrations. Studies in vivo were conducted using C57BL x DBA/2 F₁ mice, with or without advanced L1210 ascites tumor. In initial experiments performed on animals without tumor, the 50% lethal dose for 12-h s.c. infusions of TG was approximately 0.8 μmol/kg/min. Correlation of steady-state TG plasma levels with infusion rate revealed a linear relationship up to 0.62 μmol/kg/min, above which the TG plasma concentration increased disproportionately to input rate. Total body clearance of TG, calculated from the linear portion of this curve, was 123 ml/kg/min. The antitumor effects of TG infusions were correlated with steady state plasma concentrations achieved in each individual animal, and it was found that dose rates yielding levels from 1 to 10 μM increased survival time by about 40%, with no apparent optimum plasma level in this range. Examination of the cytokinetic effects caused by TG infusions at the low and high ends of this maximally therapeutic range showed that, as was the case in vitro, lower concentrations of TG caused delayed G₂ arrest, while higher concentrations induced more rapid G₁ arrest. On the basis of these, as well as previous findings, we propose that the operative mechanism of cell kill by TG in vivo may be dose dependent and may be reflected by the relative degree of G₂versus G₁ arrest. We also suggest that the appropriate strategy for the clinical use of TG is to determine the drug concentration which produces maximum G₂ arrest of tumor cells, and to infuse continuously at a rate to achieve that level for the maximum time tolerated by the patient, rather than to select an arbitrary length of infusion followed by escalation to maximum tolerated drug concentration.