Semiquinones derived from anthraquinone-containing antitumor drugs can partition into phosphatidylcholine bilayers

Role of membrane charge and semiquinone structure on naphthosemiquinone derivatives and 1,4-benzosemiquinone disproportionation and membrane-buffer distribution coefficients

Semiquinone membrane/buffer partition coefficients have been determined for 1,2-naphthosemiquinone (ONQ.-), 1,4-naphthosemiquinone (NQ.-) and two of its hydroxylated derivatives, 5,8-dihydroxy-1,4-naphthosemiquinone (NZQ.-) and 5-hydroxy-1,4-naphthosemiquinone (JQ.-) as a function of membrane charge in multilamellar vesicles of phosphatidylcholine (PC) and equimolar mixtures of this lipid and phosphatidic acid (PC:PA) and cetyltrimethylammonium bromide (PC:CTAB) at physiological pH with the exception of values corresponding to PC:PA mixtures which were obtained at pH 9. These coefficients follow the order PC:PA < PC < PC:CTAB in agreement with the negative charge of the semiquinones. The disproportionation equilibria of the naphthosemiquinone derivatives are shifted to the semiquinone in the presence of neutral and positive membranes, being more pronounced in the latter. However, very low partition coefficients as well as small shifts in the semiquinone disproportionation equilibrium were observed for ONQ.- as compared to the other semiquinones. No partition of 1,4-benzosemiquinone (BQ.-) into the lipid phase was detected for either charged or neutral lipid membranes. The presence of lipid membranes decreases the BQ.- equilibrium concentration in the presence of all the types of membranes considered here.

Adriamycin and daunomycin semiquinone membrane/buffer partition constants using the spin-broadening technique

Membrane/buffer partition constants have been determined for the semiquinones of adriamycin, Adrsq, and daunomycin, Daunosq, as a function of ionic strength and cholesterol content in phosphatidylcholine multilamellar vesicles using the spin-broadening technique. The partition constants corresponding to Adrsq were essentially similar to those of Daunosq under any of the conditions studied. These constants increase with ionic strength increase and decrease with an increase in cholesterol concentration at the membrane. These observations could have implications in the Adr and Dauno cytotoxicities where their corresponding semiquinones are postulated as important intermediates and consequently their interactions with biological membranes should also be considered as important.

Is the drug-responsive NADH oxidase of the cancer cell plasma membrane a molecular target for adriamycin?

Enhanced growth inhibition and antitumor responses to adriamycin have been observed repeatedly from several laboratories using impermeant forms of adriamycin where entry into the cell was greatly reduced or prevented. Our laboratory has described an NADH oxidase activity at the external surface of plasma membrane vesicles from tumor cells where inhibition by an antitumor sulfonylurea, N-(4-methylphenylsulfonyl)-N'-(4-chlorophenyl)urea (LY181984), and by the vanilloid, capsaicin (8-methyl-N-vanillyl-6-noneamide) correlated with inhibition of growth. Here we report that the oxidation of NADH by isolated plasma membrane vesicles was inhibited, as well, by adriamycin. An external site of inhibition was indicated from studies where impermeant adriamycin conjugates were used. The EC50 for inhibition of the oxidase of rat hepatoma plasma membranes by adriamycin was several orders of magnitude less than that for rat liver. Adriamycin cross-linked to diferric transferrin and other impermeant supports also was effective in inhibition of NADH oxidation by isolated plasma membrane vesicles and in inhibition of growth of cultured cells. The findings suggest the NADH oxidase of the plasma membrane as a growth-related adriamycin target at the surface of cancer cells responsive to adriamycin. Whereas DNA intercalation remains clearly one of the principal bases for the cytotoxic action of free adriamycin, this second site, possibly related to a more specific antitumor action, may be helpful in understanding the enhanced efficacy reported previously for immobilized adriamycin forms compared to free adriamycin.

Inhibition of anthracycline semiquinone formation by ICRF-187 (dexrazoxane) in cells

The formation of semiquinone free radicals of doxorubicin, epirubicin, daunorubicin, and idarubicin was measured by electron paramagnetic resonance (EPR) spectroscopy in hypoxic suspensions of chinese hamster ovary (CHO) cells. The amount of semiquinone produced was in the order idarubicin >> doxorubicin > daunorubicin > epirubicin. The idarubicin semiquinone signal was both the fastest to be formed and to decay. Idarubicin, which was the most lipophilic of the anthracyclines studied, also displayed the fastest fluorescence-measured cellular uptake of drug. Thus, it was concluded that semiquinone formation was dependent upon the rate of cellular uptake. Lysed cell suspensions were also shown to be capable of producing the doxorubicin semiquinone in the presence of added NADPH. The cardioprotective agent dexrazoxane (ICRF-187) was observed to decrease the amount of doxorubicin semiquinone observed in cell suspensions. Dexrazoxane also decreased the amount of doxorubicin semiquinone observed in the NADPH-lysed cell suspension mixture. Neither bipyridine nor deferoxamine decreased NADPH-dependent doxorubicin semiquinone formation. These results suggest that dexrazoxane does not decrease doxorubicin semiquinone formation through an iron complex formed from hydrolyzed dexrazoxane. Dexrazoxane may be inhibiting an NADPH-dependent enzyme.

Response to adriamycin of transplasma membrane electron transport in adriamycin-resistant and nonresistant HL-60 cells

Adriamycin, 10(-8) - 10(-5) M, inhibited transplasma membrane electron transport of uninduced HL-60 cells susceptible to adriamycin and not in uninduced HL-60 cells resistant to adriamycin as measured by reduction of external ascorbate free radical. Electron flow across the plasma membrane was measured with the intact living cells by means of a simple assay procedure whereby the transported electrons were captured by ascorbate free radical to slow the rate of chemical oxidation of ascorbate. The response to adriamycin was rapid with maximum inhibition in less than 1 min. Preincubation was not required and the inhibition presumably was not mediated through effects on DNA replication or transcription. Except at the highest concentration tested of 10 microM, both transplasma membrane electron transport and growth were unaffected by adriamycin with a line of HeLa cells resistant to the drug. The findings provide evidence, using a physiological acceptor, ascorbate free radical, for a direct inhibition of transmembrane electron transport of HL-60 cells by adriamycin that correlates closely with adriamycin inhibition of cell growth. The lack of response with resistant cells suggests an alternative mechanism for adriamycin resistance not necessarily based on transport control.

Inhibition of transplasma membrane electron transport by transferrin-adriamycin conjugates

Transplasma membrane electron transport from HeLa cells, measured by reduction of ferricyanide or diferric transferrin in the presence of bathophenanthroline disulfonate, is inhibited by low concentrations of adriamycin and adriamycin conjugated to diferric transferrin. Inhibition with the conjugate is observed at one-tenth the concentration required for adriamycin inhibition. The inhibitory action of the conjugate appears to be at the plasma membrane since (a) the conjugate does not transfer adriamycin to the nucleus, (b) the inhibition is observed within three minutes of addition to cells, and (c) the inhibition is observed with NADH dehydrogenase and oxidase activities of isolated plasma membranes. Cytostatic effects of the compounds on HeLa cells show the same concentration dependence as for enzyme inhibition. The adriamycin-ferric transferrin conjugate provides a more effective tool for inhibition of the plasma membrane electron transport than is given by the free drug.