Cellular pharmacology of gemcitabine.

Gemcitabine (2',2'-difluoro 2'-deoxycytidine, dFdC) is the most important cytidine analogue developed since cytosine arabinoside (Ara-C). The evidence of its potent antitumor activity in a wide spectrum of in vitro and in vivo tumor models has been successfully confirmed in the clinical setting. Despite structural and pharmacological similarities to Ara-C, gemcitabine displays distinctive features of cellular pharmacology, metabolism and mechanism of action. Following influx through the cell membrane via nucleoside transporters, gemcitabine undergoes complex intracellular conversion to the nucleotides gemcitabine diphosphate (dFdCDP) and triphosphate (dFdCTP) responsible for its cytotoxic actions. The cytotoxic activity of gemcitabine may be the result of several actions on DNA synthesis. dFdCTP competes with deoxycytidine triphosphate (dCTP) as an inhibitor of DNA polymerase. dFdCDP is a potent inhibitor of ribonucleoside reductase, resulting in depletion of deoxyribonucleotide pools necessary for DNA synthesis and, thereby potentiating the effects of dFdCTP. dFdCTP is incorporated into DNA and after the incorporation of one more nucleotide leads to DNA strand termination. This extra nucleotide may be important in hiding the dFdCTP from DNA repair enzymes, as incorporation of dFdCTP into DNA appears to be resistant to the normal mechanisms of DNA repair. Gemcitabine can be effectively inactivated mainly by the action of deoxycytidine deaminase to 2,2'-difluorodeoxyuridine. Also, 5'-nucleotidase opposes the action of nucleoside kinases by catalysing the conversion of nucleotides back to nucleosides. Additional sites of action and self-potentiating effects have been described. Evidence that up- or down-regulation of the multiple membrane transporters, target enzymes, enzymes involved in the metabolism of gemcitabine and alterations in the apoptotic pathways may confer sensitivity/resistance to this drug, has been provided in experimental models and more recently also in the clinical setting. Synergism between gemcitabine and several other antineoplastic agents has been demonstrated in experimental models based on specific pharmacodynamic interactions. Knowledge of gemcitabine cellular pharmacology and its molecular mechanisms of resistance and drug interaction may thus be pivotal to a more rational clinical use of this drug in combination regimens and in tailored therapy.

Modulation of dihydrofolate reductase gene expression in methotrexate-resistant human leukemia CCRF-CEM/E cells by antisense oligonucleotides.

An increase in the cellular levels of dihydrofolate reductase (DHFR) is one of the most common mechanisms of tumor resistance to methotrexate (MTX), an antimetabolite that is widely used in the treatment of a variety of human malignancies. The MTX-resistant phenotype generally occurs as a consequence of DHFR gene amplification which in turn is responsible for DHFR gene overexpression. We have designed antisense oligodeoxynucleotides (aODNs) against the DHFR mRNA and tested their in vitro effect on human leukemia CCRF-CEM/E cells, overexpressing the DHFR gene about 20-fold in comparison with the CCRF-CEM/S parental cell line. An aODN complementary to a region encompassing the AUG translation start (DHFR1) of DHFR mRNA and a mixture of two aODNs complementary to the 5' untranslated region (DHFR2+DHFR3) have been used. A DHFR1 scrambled-sequence ODN and a fully degenerated ODN were the controls. All ODNs had a phosphodiester backbone. DHFR1 and the relevant scrambled ODN were also capped with two phosphorothioate derivatives at both the 5' and 3' ends in order to increase ODN stability against serum nucleases. ODNs were vehiculated with a cationic lipid, N-[1-(dioleoyloxy)propyl]-N,N,N-trimethylammonium methyl sulfate (DOTAP), known to enhance ODN cell uptake and biological activity. The effects of ODNs on DHFR gene expression were studied after a 4 day treatment by measuring both DHFR mRNA levels, using a semi-quantitative reverse transcription polymerase chain reaction method, and DHFR protein levels by flow cytometry. A marked reduction in DHFR mRNA levels (79.7 and 74.2%, respectively) was observed with both DHFR1 and DHFR2+DHFR3 aODNs, associated with a lower decrease in DHFR enzyme (44.8 and 61%, respectively). aODN effects on MTX cytotoxicity in CCRF-CEM/E cells were also assessed. No marked enhancement of in vitro MTX cytotoxicity was observed following co-exposure of cells with aODNs and the tested concentrations of the antifol (0.05 and 0.5 microM), indicating that no substantial reversal of the MTX-resistant phenotype was induced by the study aODNs.

Cytotoxicity, DNA damage, and cell cycle perturbations induced by two representative gold(III) complexes in human leukemic cells with different cisplatin sensitivity.

The gold(III) complexes [Au(phen)Cl2]Cl and [Au(dien)Cl]Cl2 were recently shown to exert important cytotoxic effects in vitro on human tumor cell lines. To elucidate the biochemical mechanisms leading to cell death, the effects produced by these gold(III) complexes on the leukemic CCRF-CEM cell line--either sensitive (CCRF-CEM) or resistant to cisplatin (CCRF-CEM/CDDP)--were analyzed in detail by various techniques. For comparison purposes the effects produced by equitoxic concentrations of cisplatin were also analyzed. First, the dependence of the IC50 values of either complex on the incubation time was investigated. Cytotoxicity experiments confirmed that both gold(III) compounds retain their efficacy against the cisplatin-resistant line: only minimal cross-resistance with cisplatin was detected. Notably, [Au(phen)Cl2]Cl is more cytotoxic than [Au(dien)Cl]Cl2, with IC50 values of 7.4 and 6.0 M at 24 and 72 h, respectively, on the resistant line. Results of the COMET assay point out that both gold(III) complexes directly damage nuclear DNA. Remarkably, DNA damage inferred by either gold(III) complex in the two cell lines is larger than that produced by equitoxic cisplatin concentrations. Finally, the effects that either gold(III) complex produces on the cell cycle were investigated by flow cytometry. It was found that both complexes cause only moderate and transient cell cycle perturbations. Larger cell cycle perturbations are induced by equitoxic concentrations of cisplatin. The implications of the present results for the mechanism of action of cytotoxic gold(III) complexes are discussed.

Interleukin-4 rapidly down-modulates the macrophage colony-stimulating factor receptor in murine macrophages.

The activation of macrophages interferes with their response to macrophage colony-stimulating factor (M-CSF), the main growth and differentiation factor for mononuclear phagocytes. We tested the rapid effects of interleukin-4 (IL-4), the alternative macrophage activator produced by Th2 helper lymphocytes, on the receptor for M-CSF (M-CSFR) expressed on the cell surface of murine macrophages. IL4 rapidly down-modulated M-CSFR in a dose-dependent fashion. This effect was unique to IL-4 among a number of Th2-produced cytokines, none of which, with the exception of IL4 itself, is able to activate macrophages. The down-modulation of M-CSFR by IL4 was partially prevented by the inhibition of the activity of phospholipase C or protein kinase C. The data are consistent with the hypothesis that the down-modulation of M-CSFR is a property common to, and exclusive of, macrophage activators, and is driven by different activators via a common mechanism.

Interleukin 2 down-modulates the macrophage colony-stimulating factor receptor in murine macrophages.

Macrophage colony-stimulating factor (M-CSF) is the main growth factor for mononuclear phagocytes. Responsiveness to growth factors is reduced in the course of functional activation of macrophages. We studied the interference of the macrophage activator interleukin 2 (IL-2) with the response to M-CSF, in macrophages of the M-CSF-dependent murine line BAC-1.2F5. Long-term effects of IL-2 on cell growth were determined, showing that IL-2 reduces the M-CSF-dependent proliferation of macrophages. Short-term effects of IL-2 on the expression of the receptor for M-CSF (M-CSF.R) were characterized in more detail. IL-2 rapidly down-modulated M-CSF.R in a dose-dependent fashion, and interferon-gamma and lipopolysaccharides synergized with IL-2 in this modulation. The IL-2-induced down-modulation of M-CSF.R was shown to require the activity of protein-kinase-C and phospholipase-C. The data are consistent with the hypothesis that the down-modulation of M-CSF.R is a general property of macrophage activators.