ENHANCEMENT BY RECOMBINANT HUMAN INTERFERON-ALFA OF THE REVERSAL OF MULTIDRUG-RESISTANCE BY MRK-16 MONOCLONAL-ANTIBODY

Background: The anti-P-glycoprotein monoclonal antibody MRK-16 mediates the reversal of multidrug resistance. Recombinant human interferon alfa (rHuIFN alpha) enhances the cytotoxic activity of diverse chemotherapeutics and may modulate multidrug resistance. Purpose: Our purpose was to determine the outcome of combination treatment with MRK-16, rHuIFN alpha-2a, and cytotoxic agents on tumor cells that express P-glycoprotein (Pgp). Methods: Three Pgp-expressing, multidrug-resistant human tumor cell lines were used: the MDR1 retrovirus-infected HT-29 colon adenocarcinoma (HT-29(mdr1)), the doxorubicin (Adriamycin)-resistant MCF-7 (Adr(R) MCF-7) breast carcinoma, and the de novo Pgp-acquired, HCT-15 colon carcinoma. The parental cell lines HT-29(par) and MCF-7 were used as controls. The in vitro effects of MRK-16 and rHuIFN alpha-2a were studied on: (a) chemosensitivity of parental and multidrug-resistant cell lines to vincristine, doxorubicin, or paclitaxel (Taxol); (b) intracellular drug concentrations; and (c) Pgp expression, The efficacy of vincristine alone or in combination with MRK-16 and/or rHuIFN alpha-2a was assessed against HT-29(mdr1) cells in female, athymic NCr-nu/nu mice. Results: For vincristine, the IC50 (i.e., the concentration that causes 50% inhibition of cell growth) was 7.0 ng/mL in HT-29(mdr1) cells. Pretreatment of HT-29(mdr1) cells with MRK-16 partially restored vincristine sensitivity (IC50 = 4.8 ng/mL), which was enhanced by noncytotoxic concentrations of rHuIFN alpha-2a (IC50 = 2.9 ng/mL) via a mechanism independent of Pgp modulation or [H-3]vincristine efflux, rHuIFN alpha-2a potentiated MRK-16 reversal of multidrug resistance with both doxorubicin and paclitaxel on HT-29(mdr1) cells and with vincristine on Adr(R) MCF-7 and HCT-15 tumor cells. Treatment of mice with 1 mg/kg vincristine weekly for 3 weeks, beginning 10 days after tumor injection, significantly increased the median survival times of the HT-29(par) tumor-bearing mice (60 days versus 35 days; P<.0001) but was only marginally therapeutic for HT-29(mdr1) tumor-bearing mice (52 days versus 46 days), Pretreatment with MRK-16 (500 mu g) and rHuIFN alpha-2a (5 x 10(4) U), alone or in combination, 24 hours before vincristine therapy did not affect the survival of HT-29(par) tumor-bearing mice, In contrast, the survival of mice bearing HT-29(mdr1) tumors was significantly increased following treatment with MRK-16 before vincristine (80 days; P<.0001). Administration of a nontherapeutic dose of rHuIFN alpha-2a (5 x 10(4) U) with MRK-16 before vincristine treatment further increased the median survival times of HT-29(mdr1) tumor-bearing mice (116 days; P<.0001). Conclusions: MRK-16 used in combination with rHuIFN alpha-2a was significantly more effective than MRK-16 in overcoming multidrug resistance.