Data from Deoxycytidine Kinase Expression Underpins Response to Gemcitabine in Bladder Cancer

Abstract

<div>Abstract<p><b>Purpose:</b> In a recent phase II clinical trial, low-dose (100 mg/m<sup>2</sup>) gemcitabine showed promise as a radiosensitizer in bladder cancer, but underlying mechanisms lack elucidation. Here, we investigated the mechanism of radiosensitization by low-dose gemcitabine in bladder cancer cell lines.</p><p><b>Experimental Design:</b> Four bladder cancer cell lines were screened for radiosensitization by low-dose gemcitabine using clonogenic assay, and gemcitabine-resistant RT112gem and CALgem cells created by exposure to increasing gemcitabine doses. Four key gemcitabine-regulatory genes were knocked down by transient siRNA. Nude mice carrying CALgem subcutaneous xenografts were exposed to 100 mg/kg gemcitabine ± ionizing radiation (IR) and response assessed by tumor growth delay.</p><p><b>Results:</b> Gemcitabine was cytotoxic in the low nanomolar range (10–40 nmol/L) in four bladder cancer cell lines and radiosensitized all four lines. Sensitizer enhancement ratios at 10% survival were: RT112 1.42, CAL29 1.55, T24 1.63, and VMCUB1 1.47. Transient siRNA knockdown of deoxycytidine kinase (dCK) significantly reduced radiosensitization by gemcitabine (<i>P</i> = 0.02). RT112gem and CALgem cells displayed robust decreases of dCK mRNA and protein levels; reexpression of dCK restored gemcitabine sensitivity. However, CALgem xenografts responded better to combination gemcitabine/IR than either treatment alone (<i>P</i> < 0.001) with dCK strongly expressed in the tumor vasculature and stroma.</p><p><b>Conclusions:</b> Gemcitabine resistance in bladder cancer cell lines was associated with decreased dCK expression, but gemcitabine-resistant xenografts were responsive to combination low-dose gemcitabine/IR. We propose that dCK activity in tumor vasculature renders it gemcitabine sensitive, which is sufficient to invoke a tumor response and permit tumor cell kill in gemcitabine-resistant tumors. <i>Clin Cancer Res; 20(21); 5435–45. ©2014 AACR</i>.</p></div>