Purpose: Adjuvant intravitreal daunomycin is frequently used for the management of proliferative vitreoretinopathy (PVR). In this study the authors examined whether daunomycin could induce multidrug resistance (MDR), mediated by the mdr-1 gene product P-glycoprotein, in the cells responsible for reproliferation in vivo and in human retinal pigment epithelial (RPE) cells in vitro.
Methods: Expression of P-glycoprotein was examined by immunohistochemistry in surgically removed epiretinal membranes. The cellular source of P-glycoprotein was examined by colabeling for cytokeratin, glial fibrillary acidic protein, and the macrophage marker EBM-11. P-glycoprotein expression by cultured RPE cells was assessed by reverse transcription-polymerase chain reaction and immunoblot analysis. Daunomycin toxicity was quantified by crystal violet assay.
Results: P-glycoprotein expression was detected in 10 of 10 patients pre-exposed to intravitreal daunomycin. In contrast, epiretinal membranes from only 2 of 13 patients never exposed to daunomycin showed faint P-glycoprotein expression. P-glycoprotein expression was strong within 8 months after daunomycin treatment and faded thereafter. Colocalization studies demonstrated predominant expression of P-glycoprotein by RPE cells. Pre-exposure of cultured human RPE cells to subtoxic concentrations of daunomycin induced resistance to daunomycin that was sensitive to the MDR inhibitor, verapamil. Induction of the MDR phenotype in RPE cells by daunomycin was associated with a minor increase in the mdr-1 mRNA level but a prominent increase in P-glycoprotein expression, thus suggesting a primarily translational mechanism of MDR development in human RPE cells.
Conclusions: Intravitreal daunomycin induced P-glycoprotein expression in PVR. Reproliferation in daunomycin-pretreated patients probably necessitates cotreatment with daunomycin and inhibitors of multidrug resistance such as verapamil or administration of antiproliferative drugs such as 5-fluorouracil, which act in a MDR-independent fashion.