PRECLINICAL STUDIES Purine analogs sensitize the multidrug resistant cell line (NCI-H460/R) to doxorubicin and stimulate the cell growth inhibitory effect of verapamil Milica Pešić & Ana Podolski & Ljubiša Rakić & Sabera Ruždijić Received: 8 April 2009 / Accepted: 25 May 2009 / Published online: 18 June 2009 # Springer Science + Business Media, LLC 2009 Summary The resistant cell line NCI-H460/R and its counterpart NCI-H460 were used to investigate the ability of purine analogs to overcome multidrug resistance (MDR) that seriously limit the efficacy of lung cancer regimens with chemotherapeutic agents. Two purine analogs, sulfinosine (SF) and 8-Cl-cAMP, exerted dose-dependent effects on cell growth in both parental and resistant cell lines. They significantly decreased mdr1 expression in NCI-H460/R cells. Low concentrations (1 μM) of SF and 8-Cl-cAMP in combination with doxorubicin (DOX) exerted synergistic growth inhibi- tion in both cell lines. Pretreatment with SF and 8-Cl-cAMP improved the sensitivity to DOX more than verapamil (VER), the standard modulator of MDR. The increased accumulation of DOX observed after the treatment with SF and 8-Cl-cAMP was consistent with the results obtained with VER. VER stimulated the effect of 8-Cl-cAMP on DOX cytotoxicity and mdr1 expression. Combinations of either SF or 8-Cl-cAMP with VER at clinically acceptable concentrations exhibited synergistic effects on cell growth inhibition in the resistant cell line. SF and 8-Cl-cAMP modulated MDR in NCI-H460/ R cells, especially when applied before DOX administration. This feature, together with their ability to reverse MDR, renders the purine analogs (in combination with VER) as potential candidates for improving the clinical activity of existing lung cancer therapeutics. Keywords Drug combination . Multidrug resistance . Sulfinosine . 8-Cl-cAMP . Doxorubicin . Verapamil Introduction MDR is the main cause of lung cancer treatment failure. The predictors of a poor outcome are increased expression of drug export proteins and drug inactivation as a result of increased enzyme activities [1]. MDR frequently correlates with overexpression of the ABC transporters in cell membranes that actively pump anticancer drugs out of cells, i.e. P-glycoprotein (P-gp) and/or MDR-associated protein 1 (MRP1) [2]. To obtain a better understanding of the mechanisms of drug resistance, many in vitro-selected cell lines have been produced in the presence of continuous or pulsed exposure to drugs [3–5]. A DOX-resistant cell line NCI-H460/R characterized previously [6] was employed in the present study. This cell line was established from large cell neuroendocrine carcinoma (LCNEC), the most aggressive form of non-small cell lung carcinoma (NSCLC). Acquisition of the MDR phenotype includes additional mechanisms aside from the emergence of increased P-gp expression. The inhibition of transport proteins are prerequisites for the effective reversion of the MDR phenotype. The potential of synthetic compounds and compounds originating from plants for MDR reversion are under scrutiny [7–10]. Combinations of two or more drugs and/or adjustments of the dosage and administration schedule can improve the efficacy of cancer therapy. Combined therapy with anti- metabolites, including purine analogs, and DNA-damaging agents and topoisomerase II inhibitors has had good results in generally incurable malignancies such as lung and colon Invest New Drugs (2010) 28:482–492 DOI 10.1007/s10637-009-9277-x M. Pešić : A. Podolski : S. Ruždijić (*) Institute for Biological Research, Department of Neurobiology, Laboratory of Molecular Neurobiology, University of Belgrade, Bulevar Despota Stefana 142, 11060 Belgrade, Republic of Serbia e-mail: sabir@ibiss.bg.ac.yu L. Rakić Serbian Academy of Sciences and Arts, Knez Mihailova 35, 11000 Belgrade, Republic of Serbia