REVERSAL OF DOXORUBICIN, ETOPOSIDE, VINBLASTINE, AND TAXOL RESISTANCEIN MULTIDRUG-RESISTANT HUMAN SARCOMA-CELLS BY A POLYMER OF SPERMINE

We have previously descibed the synthesis of a cytotoxic polymeric con jugate of spermine (Poly-SPM) which is able to inhibit the transport o f polyamines (spermine, spermidine, and putrescine) into normal and ma lignant cells. Recent studies examining the toxicity of Poly-SPM in pa rental and multidrug resistant (MDR) cancer cells have revealed a cros s-resistance in the MDR variant Dx5 to the toxic effects of the conjug ate in the MDR-positive cells. There were also differences in spermine and putrescine uptake rates between parental and MDR-positive cells w ith 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 hi gh levels of the mdr1 gene product, P-glycoprotein, and are 25- to 60- fold resistant to doxorubicin (DOX), etoposide (VP-16), vinblastine (V BL), and taxol (TAX). Cytotoxicity was measured by the MTT -(4,5-dimet hyldiazol-2-yl)-2,5-diphenyltetrazolium bromide] assay. Poly-SPM (50 m u M) 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 r eversal 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 w hen 25 mu M Poly-SPM was used. No modulation was observed in the paren tal cell line MES-SA, which does not express the mdr1 gene. Poly-SPM h ad 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 [H-3]-VBL and [H-3 ]-TAX in the Dx5 but not MES-SA cells. Pretreatment of the Dx5 cells w ith alpha-difluoromethylornithine (DFMO at 2 and 5 mu M) for 24 h incr eased the function of the MDR transporter to further decrease the cell ular accumulation of VBL and TAX when compared to untreated cells. DFM O pretreatment is known to upregulate the polyamine transporter(s). Th ese findings show that, in addition to inhibiting polyamine transport, Poly-SPM reverses MDR in Dx5 cells, suggesting a potential relationsh ip between the polyamine influx transporter and the MDR efflux pump. T his potential functional link between the polyamine influx transporter (s) and the MDR efflux transporter (P-glycoprotein) offers a novel app roach to inhibiting this form of drug resistance.