Multidrug resistance protein (MRP) expression in retinoblastoma correlates with the rare failure of chemotherapy despite cyclosporine for reversal of P-glycoprotein.

Failure of chemotherapy associated with expression of the multidrug resistance protein p170 frequently occurs in retinoblastoma (RB). Despite using cyclosporine, which inhibits p170 and improves our chemotherapy results, rare failures occur. In nonmetastatic primarily enucleated RBs, we show expression of p170 in 3 of 18 samples and expression of multidrug resistance protein (MRP), the second protein associated with resistance to chemotherapy, in 1 of 18 samples. All three RBs that failed chemotherapy without cyclosporine expressed MRP with p170. All three RBs that were enucleated immediately when chemotherapy failed despite the addition of cyclosporine expressed only MRP. One RB enucleated 2 years after failing chemotherapy with cyclosporine, despite radiation and salvage chemotherapy, expressed both p170 and MRP. Two metastatic RBs that expressed both p170 and MRP at diagnosis and at recurrence failed chemotherapy without cyclosporine, whereas one metastatic RB that expressed neither protein was cured by chemotherapy without cyclosporine. MRP may result in failure of chemotherapy despite the elimination of p170-expressing clones by cyclosporine.