Verapamil and cyclosporin A potentiate the effects of chemotherapeutic drugs in the human medullary thyroid carcinoma TT cell line not expressing the 170 kDa P-glycoprotein.

The TT-cell line, derived from a patient with metastatic medullary thyroid carcinoma (MTC), was found to exhibit intrinsic resistance to vincristine (VCR) despite the absence of immunohistochemically detectable 170 kDa P-glycoprotein (PGP 170) associated with multidrug resistance (MDR). Verapamil and cyclosporin A, two well known resistance modifiers of MDR, were found to significantly potentiate the action of VCR (60-fold) and to a lesser degree also of VP-16 and daunorubicin (dnr). The present results suggests that resistance of MTC to chemotherapy may be at least partly circumvented by the addition resistance modifiers to chemotherapeutic regimens.