Reversal of doxorubicin, etoposide, vinblastine, and taxol resistance in multidrug resistant human sarcoma cells by a polymer of spermine

Abstract

 We have previously descibed the synthesis of a cytotoxic polymeric conjugate of spermine (Poly-SPM) which is able to inhibit the transport of polyamines (spermine, spermidine, and putrescine) into normal and malignant cells. Recent studies examining the toxicity of Poly-SPM in parental and multidrug resistant (MDR) cancer cells have revealed a cross- resistance in the MDR variant Dx5 to the toxic effects of the conjugate in the MDR-positive cells. There were also differences in spermine and putrescine uptake rates between parental and MDR-positive cells with the MDR-positive cells having a lower V_max and a higher K_m. The ability of this Poly-SPM to reverse MDR was examined in MDR variants (Dx5 cells) of the human sarcoma cell line MES-SA. The cells express high levels of the mdr1 gene product, P-glycoprotein, and are 25- to 60-fold resistant to doxorubicin (DOX), etoposide (VP-16), vinblastine (VBL), and taxol (TAX). Cytotoxicity was measured by the MTT [3-(4,5-dimethyldiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay. Poly-SPM (50 μM) lowered the drug concentration IC₅₀ values in the Dx5 cells by 37-fold with VBL, 42-fold with DOX, 29-fold with VP-16, and 25-fold with TAX when compared to the control IC₅₀ values without Poly-SPM. This reversal of resistance was concentration dependent, decreasing 17-fold with DOX, 6.1-fold with VBL, 19-fold with VP-16, and 5-fold with TAX when 25 μM Poly-SPM was used. No modulation was observed in the parental cell line MES-SA, which does not express the mdr1 gene. Poly-SPM had no influence on the IC₅₀ of non-MDR chemotherapeutic agents such as cisplatin. The modulation studies correlated with the ability of Poly-SPM to reverse the cellular accumulation defect of [³H]-VBL and [³H]-TAX in the Dx5 but not MES-SA cells. Pretreatment of the Dx5 cells with α-difluoromethylornithine (DFMO at 2 and 5 μM) for 24 h increased the function of the MDR transporter to further decrease the cellular accumulation of VBL and TAX when compared to untreated cells. DFMO pretreatment is known to up-regulate the polyamine transporter(s). These findings show that, in addition to inhibiting polyamine transport, Poly-SPM reverses MDR in Dx5 cells, suggesting a potential relationship between the polyamine influx transporter and the MDR efflux pump. This potential functional link between the polyamine influx transporter(s) and the MDR efflux transporter (P-glycoprotein) offers a novel approach to inhibiting this form of drug resistance.