ABCB1 and neoplasm: This compromised efficacy could not be explained by minor changes in FR copies/cell (6.31 × 106 vs 6.33 × 106 receptors/cell for KB-145-55 and parental KB cells, respectively), tumor growth rate (37 vs. 28 days to reach 1500 mm3 in volume for KB-145-55 and parental KB cells, respectively), or p-glycoprotein (P-gp) expression levels (Fig. 4).