Molecular mechanism involved in the transport of a prodrug dopamine glycosyl conjugate

We have previously demonstrated that dopamine conjugation to glucose allows it to induce therapeutic effects against Parkinson's disease after intravenous administration. In this paper we demonstrate that, unlike dopamine, the prodrug glu-dopamine is a transportable substrate of glucose transporters. Towards this, the effect of glucose-conjugation on the affinity and uptake of dopamine have been assessed in vitro, using human retinal pigment epithelium (HRPE) cells. Glucose transporter-mediated uptake was measured using [³H]3-O-methylglucose ([³H]3-O-MG) as the tracer. The uptake was found to be rapid and hyperbolically related to its concentrations (K_t = 7.8 ± 1.2 mM and V_max = 54 ± 2 nmol/min mg protein). Inhibition experiments showed that dopamine was able to interact with glucose carriers only when conjugated to glucose (IC₅₀ = 2.6 ± 0.6 mM). HPLC analysis of HRPE cell extracts showed that both dopamine and the prodrug permeate the cell, but only the uptake of the prodrug is inhibitable by glucose. This confirms that glucose transporters mediate the transport of the prodrug glu-dopamine, but not of dopamine. HRPE cells is therefore proposed as a promising model for in vitro studies involving the glucose transporter-mediated transport of drugs and their conjugates.