17-Allylamino-17-demethoxygeldanamycin-induced Changes in [¹⁸F]Fluorothymidine Uptake

The proliferation status of tumor cells can be imaged using [¹⁸F]fluorothymidine ([¹⁸F]FLT), which is trapped by cell cycle-dependent thymidine kinase 1 (TK1). Targeting of heat shock protein 90 (HSP90) disrupts multiple client proteins, leading to heterogeneous cell-cycle phenotypes. To investigate whether [¹⁸F]FLT uptake reflects the growth arrest caused by various mechanisms of HSP90 inhibition, we used HCT116 cells that were arrested at G₀/G₁ phase, and Hep3B cells at G₂/M phase, by the HSP90 inhibitor 17-AAG. In HCT116 cells, 17-AAG did not induce significant changes in [¹⁸F]FLT uptake despite the decreased expression of TK1, cyclin A, and cyclin B. The mRNA level of 5'3'-deoxynucleotidase 1 (NT5C), which antagonizes TK1 by de-phosphorylating [¹⁸F]FLT monophosphate, was also decreased in 17-AAG-treated HCT116 cells by 55.3±18.1% compared to vehicle-treated cells. In Hep3B cells, 17-AAG treatment did not induce TK1 expression, TK1 activity nor [¹⁸F]FLT uptake. Nucleoside transporter activity in the plasma membrane was unchanged in both cell lines. These results showed that a HSP90 inhibitor exerts multifaceted effects on [¹⁸F]FLT uptake before inducing cell death in a cell-cycle-independent manner. Therefore, the use of [¹⁸F]FLT-positron emission tomography for monitoring treatment by HSP90 inhibitor would be more appropriate when tumor cell death is induced after growth arrest.