MRPs are known to co-transport drugs with glucuronide, glutathione or sulphate. Amongst the nine members of this transporter family (MRP1–9), the first five (MRP-1, MRP-2, MRP-3, MRP-4 and MRP-5) are frequently associated with the efflux of therapeutic agents. MRP-1, MRP-2 and MRP-3, transport hydrophilic anionic compounds, large molecules and peptidomimetics (Dallas et al., 2006); however, both MRP-4 and MRP-5 transport small polar compounds such as nucleosides, cyclic nucleotides and nucleoside analogs (Schuetz et al., 1999; Wijnholds et al., 2000).