Data from Retinoblastoma Tumor Suppressor Gene Expression Determines the Response to Sequential Flavopiridol and Doxorubicin Treatment in Small-Cell Lung Carcinoma

Abstract

<div>Abstract<p><b>Purpose:</b> Small-cell lung cancers (SCLC) are defective in many regulatory mechanisms that control cell cycle progression, i.e., functional retinoblastoma protein (pRb). Flavopiridol inhibits proliferation and induces apoptosis in SCLC cell lines. We hypothesized that the sequence flavopiridol followed by doxorubicin would be synergistic in pRb-deficient SCLC cells.</p><p><b>Experimental Design:</b> A H69 pRb-deficient SCLC cell line, H865, with functional pRb and H865 pRb small interfering RNA (siRNA) knockdown cells were used for <i>in vitro</i> and <i>in vivo</i> experiments. The <i>in vivo</i> efficiencies of various sequential combinations were tested using <i>nude/nude</i> athymic mice and human SCLC xenograft models.</p><p><b>Results:</b> Flavopiridol then doxorubicin sequential treatment was synergistic in the pRB-negative H69 cell line. By knocking down pRb with specific siRNA, H865 clones with complete pRb knockdown became sensitive to flavopiridol and doxorubicin combinations. pRb-deficient SCLC cell lines were highly sensitive to flavopiridol-induced apoptosis. pRb-positive H865 cells arrested in G<sub>0</sub>-G<sub>1</sub> with flavopiridol exposure, whereas doxorubicin and all flavopiridol/doxorubicin combinations caused a G<sub>2</sub>-M block. In contrast, pRb-negative SCLC cells did not arrest in G<sub>0</sub>-G<sub>1</sub> with flavopiridol exposure. Flavopiridol treatment alone did not have an <i>in vivo</i> antitumor effect, but sequential flavopiridol followed by doxorubicin treatment provided tumor growth control and a survival advantage in Rb-negative xenograft models, compared with the other sequential treatments.</p><p><b>Conclusions:</b> Flavopiridol and doxorubicin sequential treatment induces potent <i>in vitro</i> and <i>in vivo</i> synergism in pRb-negative SCLC cells and should be clinically tested in tumors lacking functional pRB.</p></div>