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Cisplatin is one of the most potent cytotoxic drugs and in chemotherapy has ameliorated numerous tumors. Nevertheless, resistance to cisplatin is a problem that is encountered in the chemotherapy of urologic tumors, especially transitional cell carcinomas. In order to improve definition of the mechanisms of cisplatin-resistance we established a series of cisplatin-resistant sublines from the cell line RT 112 in increasing concentrations of cisplatin. The most resistant subline CP3 is approximately 10 times more resistant than the parental line and shows a 10-fold cross-resistance against methotrexate, whereas vinblastine and doxorubicin are equally effective in the parental and sublines. Combined treatment of CP3 cells with cisplatin and buthionine sulfoximine (BSO) does not result in enhanced cell kill, thereby ruling out glutathione as a resistance mechanism. However, in comparison with parental cells, CP3 cells are about 1.5 times more resistant against cadmium. On the protein level, the cisplatin-resistant cells reveal an enhanced expression of metallothionein II (MTII), but not MTI, suggesting that the cisplatin resistance we observed in these sublines is at least partly mediated by MTII. These sublines will in the future serve as valuable tools for the analysis of cisplatin resistance, especially in view of metallothionein-mediated resistance mechanisms.

Objective: We evaluated the cytotoxic activity of the three anthracyclines, doxorubicin, epirubicin and idarubicin, in different sublines of the Dunning rat prostate carcinoma as well as in multidrug-resistant KB cells, expressing a high amount of the human multidrug resistance gene product, P glycoprotein., Methods: The effectiveness of the three anthracyclines was tested in vitro in the Dunning rat prostate carcinoma sublines G, AT.1, AT.3.1, MatLu, and MatLyLu, as well as in multidrug-resistant KB cells, using an MTT assay., Results: All drugs were clearly more effective in the androgen-sensitive Dunning rat prostate carcinoma subline G than in the androgen-independent growing sublines AT.1, AT.3.1, MatLu, and MatLyLu. Idarubicin was much more effective than doxorubicin or epirubicin. To further elucidate the mechanism of action of idarubicin as compared with doxorubicin and epirubicin, we tested the cytotoxicity of these anthracyclines in highly multidrug-resistant KB-V1 cells, which express high amounts of P glycoprotein, as well as in the drug-sensitive parental KB-3-1 cells. KB-V1 cells proved to be highly resistant to doxorubicin and epirubicin with IC50 values of 2,300 and 1,000 ng/ml, respectively. Idarubicin, however, was about 57.5-fold and 25-fold more active, respectively, suggesting, that it is able to overcome P-glycoprotein-mediated multidrug resistance., Conclusion: The strong in vitro effectiveness of idarubicin in androgen-insensitive prostate carcinoma cells suggests that this drug might be useful in the treatment of hormone-refractory prostate carcinoma.

Prostate carcinomas are in general resistant against virtually all cytotoxic drugs. Up to now it has not been thoroughly evaluated whether specific resistance factors, such as the expression of the MDR1 gene, play a role in this multi-agent resistance and whether there is a link between drug resistance and hormone-independent growth. We investigated the resistance patterns of a hormone-sensitive and four hormone-independent Dunning rat carcinoma sublines against four drugs which are substrates of P-glycoprotein (vinblastine, taxol, doxorubicin, and etoposide) and two agents (methotrexate and cis-platinum) which are not transported by this efflux pump. All hormone-insensitive sublines, AT.1, AT. 3.1., MatLu and Mat LyLu, continuously showed a clearly enhanced resistance (3- to 26-fold) against the P-glycoprotein substrates, compared to the hormone-sensitive subline G. Only two of the androgen-independent sublines displayed enhanced resistance against methotrexate, whereas all of them were more sensitive against cisplatin than the androgen-sensitive G cells. By addition of verapamil the resistance against vinblastine (9- to 10-fold) and taxol (6.7- to 26.7-fold) in the hormone-insensitive cells could be almost totally reversed. Furthermore, the fluorescent P-glycoprotein substrate rhodamine-123 was effectively pumped out of the four tested hormone-independent cell lines, whereas the hormone-sensitive G cells were unable to extrude the dye. By reverse transcriptase polymerase chain reaction (RT-PCR) with primers specific for the rat mdr1b gene, the homologue to the human MDR1 gene, we could easily detect mdr1b expression in the androgen independent cell lines, but not in the G cells. Our results suggest that the product of the rat mdr1b gene is involved in the multidrug resistance of androgen-independent Dunning prostate carcinoma cells.

We investigated the presence of interleukin 6 (IL6) receptors in human prostate carcinomas and benign prostatic hyperplasias. Interleukin 6 receptor expression was measured at the mRNA level by slot blot analysis using a probe that recognizes mRNA encoding the 80-kDa subunit of the IL6 receptor. Significant expression was found in 29 of 37 (78%) samples of benign prostate hyperplasia (BPH) and in 17 of 17 prostate carcinomas. Quantitative analysis of the expression level revealed that 11 of the 29 positive hyperplasia tissues (38%) and 4 carcinoma samples (23.5%) expressed equal or higher levels of IL6 receptor mRNA than the human hepatoma cell line PLC/PRF5, which contains about 2300 IL6 receptors per cell. We also measured IL6 receptor mRNA levels in three human prostate carcinoma cell lines LNCaP, DU145 and PC3, which are known to contain IL6 receptors because they are sensitive to the cytotoxic action of an IL6-toxin fusion protein. We were not able to detect IL6 receptor expression with the slot blot procedure, but we were able to detect IL6 receptor mRNA using a very sensitive PCR assay. Our data provide evidence that IL6 may play a role in the growth of benign and malignant prostate tumors and suggest that the IL6 receptor could be a target for the delivery of therapeutic agents in prostate cancer.

The antifungal agent ketoconazole was found to overcome resistance to vinblastine and doxorubicin in multidrug resistant KB-V1 cells in vitro. These cells are several hundred-fold more resistant than the parental cell line KB-3-1. Ketoconazole had little or no effect on the parental KB-3-1 cells. The concentrations used to overcome drug resistance in vitro have already been safely used in vivo for treatment of fungal infections and in the monotherapy of hormone independent prostate carcinomas to block adrenal androgen production. Because of a possible beneficial effect of a combination of ketoconazole and a chemotherapeutic drug in multidrug resistant cancers, we examined a panel of 11 prostate carcinoma tissues for the expression of the MDR1 gene by an RNA-PCR assay. MDR1 expression was detectable, albeit at low levels, in 8 of the 11 tumors, suggesting a possible role of this gene in the drug resistance of prostate carcinomas. Our data suggest that ketoconazole might be useful in overcoming multidrug resistance in concentrations that are achievable in humans.