ENHANCEMENT OF VINCRISTINE CYTOTOXICITY IN DRUG-RESISTANT CELLS BY SIMULTANEOUS TREATMENT WITH ONCONASE, AN ANTITUMOR RIBONUCLEASE

Background: Onconase, a protein isolated from oocytes and early embryo s of the frog Rana pipiens, shares extensive homology with bovine panc reatic ribonuclease (RNase A) and possesses similar enzyme activity, O nconase is cytotoxic toward cancer cells in vitro and exhibits antitum or activity in animal models, In addition, Onconase has been shown to enhance the cytotoxic activity of some chemotherapeutic agents in vitr o, Purpose: We studied interactions between the cytotoxic effects of O nconase and the chemotherapeutic agent vincristine (VCR) in the treatm ent of drug-sensitive and multidrug-resistant human colon carcinoma ce lls in vitro and in mice, Methods: Transplantable human colon carcinom a cells (HT-29(par) cells) were infected with a retrovirus containing human mdr1 (also known as MDR1 and PGY1) complementary DNA (encoding P -glycoprotein [P-gp]), and clones that were cross-resistant to colchic ine, doxorubicin, and vinblastine were selected (HT-29(mdr1) cells), D rug-resistant HT-29(mdr1) cells and drug-sensitive HT-29(par) parental cells were treated with Onconase and/or VCR in vitro at varying conce ntrations to measure the effects on protein synthesis and cell viabili ty, The impact of Onconase on VCR accumulation in both types of cells was determined in the presence or absence of MRK-16, an anti-P-gp mono clonal antibody capable of reversing the multidrug-resistant phenotype , The antitumor effects of Onconase and/or VCR treatment were assessed in nude mice bearing established HT-29(par) or HT-29(mdr1) intraperit oneal tumors, IC50 values (drug concentrations resulting in 50% inhibi tion of protein synthesis or cell viability) for Onconase and VCR were determined from semilogarithmic dose-response curves; interactions be tween the cytotoxic effects of these two agents were evaluated using d ata from protein synthesis inhibition experiments and a two-way analys is of variance, Survival distributions from in vivo experiments were c ompared using Cox proportional hazards models, Results: The combinatio n of Onconase and VCR yielded enhanced cytotoxicity in vitro that was independent of P-gp expression, Evaluation of the effects of these two compounds on protein synthesis over a wide range of drug concentratio ns indicated possible synergistic interactions (i.e., greater than add itive effects) in both drug-resistant and drug-sensitive cells, The en hancement of VCR cytotoxicity was dependent on Onconase enzyme activit y and was not associated with increased intracellular levels of VCR, S imultaneous treatment of mice bearing HT-29(par) tumors with Onconase and VCR did not extend their median survival time (MST) significantly (MST with VCR = 66 days; MST with VCR plus Onconase = 69 days; two-tai led P = .57); however, the MST of mice with HT-29(mdr1) tumors was ext ended significantly by this treatment (MST with VCR = 44 days; MST wit h VCR plus Onconase = 66 days; two-tailed P < .001). Conclusion: Combi ned administration of Onconase and VCR yields enhanced cytotoxicity in vitro and in vivo against human colon carcinoma cells that overexpres s the mdr1 gene.