The retinoid X receptor agonist bexarotene (Targretin) synergistically enhances the growth inhibitory activity of cytotoxic drugs in non-small cell lung cancer cells
Introduction
Lung cancer is the leading cause of cancer death for both men and women [1]. The American Cancer Society estimates that in 2004 there will be about 173,770 new cases of lung cancer in the United States, and about 160,440 people will die of this disease. Non-small cell lung cancer (NSCLC) is the most common type, accounting for almost 80% of lung cancers. Unfortunately, most NSCLC patients already have advanced disease (stage IIIB/IV) at diagnosis and this is associated with a poor prognosis. Currently the major treatment modality for patients with unresectable NSCLC is chemotherapy with a platinum-based agent (e.g., cisplatin, carboplatin) in combination with a second cytotoxic agent such as paclitaxel, vinorelbine or docetaxel [2]. Although it has been demonstrated that modern two-drug combinations are more active than single agents, the benefits are still modest, and the current 5-year survival rate has not improved significantly since the implementation of these combination regimens [3]. This has motivated researchers to explore the possibility of adding a third agent to the two-drug regimens. While Phase II clinical trials of three-drug combination therapy in NSCLC have been encouraging, the 5-year survival rate has still not improved with this strategy and these regimens often come at the cost of a significant increase in toxicity [4], [5]. Recent data from clinical trials of a number of novel targeted agents administered in combination with chemotherapy unfortunately did not show significant survival benefit over chemotherapy alone [6], [7], [8]. These data emphasize that agents capable of enhancing the activity of the current treatment regimens while exhibiting better-tolerated, non-overlapping side effects are still desired.
Bexarotene (LGD1069, Targretin®) is a retinoid X receptor (RXR) selective agonist which binds and activates all three RXR isoforms (RXRα, RXRβ, and RXRγ) with equivalent affinity and potency [9]. Our previous work in solid tumor models has demonstrated that bexarotene is an efficacious chemopreventive and chemotherapeutic agent in a number of preclinical rodent models of human cancer and in the rat N-nitrosomethylurea (NMU)-induced mammary carcinoma model, where it was shown to be effective against both tamoxifen-resistant and tamoxifen-sensitive tumors [10], [11], [12]. In addition, we have recently demonstrated that the bexarotene/paclitaxel combination resulted in a synergistic growth inhibition (combination index, CI < 1) in a rat mammary tumor-derived cell line in vitro [13]. In the rat NMU-induced tumor model in vivo, the combination regimen resulted in a statistically significant decrease in total tumor burden and an increase in overall objective response without further intensifying the major side effects of paclitaxel. More recently, we have shown that bexarotene can prevent and overcome acquired paclitaxel resistance in human NSCLC cells [14].
These encouraging preclinical results with bexarotene led us to further examine its role in combination regimens with other cytotoxic agents.
The first goal of this study was to extend our observation on the combination effect of bexarotene and paclitaxel in the rat NMU-induced mammary carcinoma model to models of NSCLC and determine whether the addition of bexarotene could enhance the activity of chemotherapy agents commonly used in first-line treatment of advanced NSCLC. We evaluated combinations of bexarotene with several cytotoxic agents in a panel of 10 NSCLC cell lines that represent different histomorphologies (Table 1). In addition, we sought to extend the in vitro observations to in vivo xenograft tumor models of NSCLC. Our results show that bexarotene enhanced the growth inhibitory activity of both paclitaxel and vinorelbine. These observations were confirmed in vivo where the antitumor efficacy of the combination regimen was superior to that of the single agents.
Section snippets
Chemicals and reagents
RPMI 1640 medium, glutamine and gentamycin were obtained from Cambrex (Walkersville, MD). Fetal bovine serum was from HyClone (Logan, UT). Vinorelbine (10 mg/mL in sterile solution) and paclitaxel for injection (6 mg/mL in 1:1 (v/v) in Cremophor EL and dehydrated alcohol) were purchased through Pharmaceutical Buyers Inc. (Cedarhurst, NY). Bexarotene was synthesized at Ligand Pharmaceuticals (San Diego, CA). RXR-isoform specific binding affinity and activation data for bexarotene has been
Bexarotene monotherapy has modest growth inhibitory activity on human NSCLC cell lines
Before investigating the activity of bexarotene in combination with cytotoxic drugs, bexarotene's growth inhibitory activity as single agent was surveyed against a panel of human NSCLC cell lines. Table 1 lists the NSCLC cell lines used in this study; they represent the variety of NSCLC histomorphology typical for this disease. In general, bexarotene had modest growth inhibitory effect when used as single agent (up to 10 μM), and only in a subset of the lines tested (Fig. 1). The level of growth
Discussion
The purpose of this study was to evaluate the efficacy of combinations of bexarotene with cytotoxic chemotherapeutic drugs commonly used in the treatment of NSCLC. We examined various combinations of bexarotene and cytotoxic agents for growth inhibition, using a panel of NSCLC cell lines, and confirmed the in vitro observations in vivo, transitioning to NSCLC xenograft tumor models. Our in vitro data show that although bexarotene had little or only modest antitumor activity when used as a
Acknowledgement
The authors thank Mr. Manny R. Corpuz for conducting xenograft studies and Ms. Tracy A. Cooke for analysis of RXR expression profiles in cell lines.
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TWH and WCY contributed equally to this work and share first authorship.