Prevention of vinblastine-induced cytotoxicity by ruthenium red

Reversal of natural resistance to bouvardin (NSC 259968) in sarcoma 180 cells in vitro and in vivo by verapamil

Reversal of natural resistance to bouvardin (NSC 259968) has been attained in vitro and in vivo, by the calcium influx blocker verapamil in sarcoma 180 cells insensitive to bouvardin. Verapamil increased the in vitro lethality of the tumor cells following exposure with cells for 1 and 3 h as a result of the cytotoxic effect of bouvardin. Similar results were observed in vivo when the tumor cells were exposed to verapamil and then treated with bouvardin, showing a significant percent increase in the lifespan to 30% and 45%. This suggested that this calcium channel blocker had interacted with tumor cell membrane to modulate the response of the cells, and make them more amenable to the drug action.

Effects of ruthenium red on accumulation and cytotoxicity of m-AMSA, vinblastine and daunorubicin in leukemia cells

Effects of ruthenium red on accumulation and cytotoxicity of m-AMSA, vinblastine and daunorubicin were examined in the P388 murine leukemia cell line and in an adriamycin-resistant subline, P388/ADR, which is cross-resistant to all three agents. Ruthenium red increased m-AMSA accumulation by both P388 and P388/ADR cells; the extent of this effect was a function of the concentration of both agents. Uptake enhancement occurred within 5 min of exposure of cells to ruthenium red and was readily reversed when cells were suspended in fresh medium. A 24-hr exposure to ruthenium red was needed to affect vinblastine or daunorubicin accumulation, and the effect was substantially less than that observed with m-AMSA. Ruthenium red protected P388 cells from m-AMSA toxicity. These data, together with reports indicating a protective effect of ruthenium red against vinblastine and anthracycline toxicity, suggest that the dye promotes tight binding of m-AMSA and other agents to cellular loci on which toxic effects are not exerted.

Cellular pharmacology of Vinca alkaloid resistance and its circumvention

Vinca alkaloid-resistant human leukemic cells express the multiple drug-resistant phenotype, characterized by cross resistance to natural product compounds of unrelated structure and action. While there are also distinct biochemical lesions associated with this phenotype, their role(s) in the expressions of resistance is not known at this time. More clear, however, is our understanding of the pharmacologic determinants of this resistance--apparent decreased drug uptake and decreased drug retention. This latter phenomenon has been attributed to the workings of an active efflux pump, but data presented here and elsewhere permit an alternative explanation--that of altered drug binding to an as yet unidentified target(s). Elucidation of the mechanism of multiple drug resistance is important in the design of new chemotherapeutic strategies to overcome it. In this regard, calcium channel blocking agents and calmodulin inhibitors can cause an apparent reversal of resistance by enhancing the cytotoxic effectiveness of the anticancer drugs, possibly by increasing the amount of drug retained by the tumor cells. The basis for this enhanced retention and cytotoxicity is not presently known, but it may be related to cellular calcium fluxes, calmodulin content or membrane fluidity and permeability. The meaning of these findings is unclear at the present time, but they may provide new insights into the mechanism of action and ultimate cellular target(s) for Vinca alkaloids. Whether these modifying drugs sensitize the cells to the action of the alkaloids or potentiate the oncolytic drug effect in the cell remains to be determined. Regardless of the mechanism, calcium channel blocking agents may have a role in the combination chemotherapy of the leukemias with Vinca alkaloids.

Role of calcium in triggering the release of transmitters at the neuromuscular junction

Calcium ions have a key role in triggering the release of packaged transmitter at the amphibian neuromuscular junction and of the chromaffin granules at the adrenal medulla. It is suggested that (i) proteins on the vesicle and plasma membranes are of particular importance in promoting membrane fusion and exocytosis (ii) they may be divalent cation-stimulated ATPases, which form the calcium-binding sites or have a specific calcium-binding protein in close molecular apposition (iii) these ATPases in synaptic vesicles and chromaffin granules also generate a protonmotive force which is associated with the uptake of transmitter (iv) the osmotic properties of the vesicle may be important during fission, but it is not suggested that chemiosmotic effects are involved in Ca2+-triggered fusion (v) the action of calcium is markedly co-operative (vi) the adrenal medullary cell and the n.m.j. may differ in the Ca2+-binding site; there is evidence for the involvement of calmodulin in granule-plasmalemma fusion in the chromaffin cells, but not at present (surprisingly) for a role of this Ca2+-binding protein at the n.m.j. (vii) exocytosis requires MgATP (viii) phosphorylation of the ATPase may well be involved; phosphorylation via cAMP does not seem to be involved in fusion in either system (ix) the ATPase may undergo configurational changes during exocytosis and is markedly sensitive to the physical state of its phospholipid environment and to the oxidation of its -SH groups.