EFFLUX TRANSPORTER (ABCG2) INHIBITION ENHANCES TOPOTECAN OCULAR DELIVERY IN VITRO

CARCABOSO A.M.; CHANTADA G.L.; BRAMUGLIA G.F.; CHIAPPETTA D.A.; DE DAVILA M.T.G.; LABRAGA M.; RUBIO M.C.; ABRAMSON D.H.

SIENA, ITALIA

Congreso; ISSO MEETING 2007; 2007

INTERNATIONAL SOCIETY OF OCULAR ONCOLOGY

The ATP-binding cassette transporter G2 (ABCG2/BCRP) has been recently described to play a role in the inner blood retinal barrier as efflux transporter of  light-sensitive toxins. Topotecan, a candidate agent for the treatment of ocular tumors, is a substrate for this transporter. We developed in vitro permeability experiments to assess the ability of topotecan to pass the posterior segment ocular barrier in the absence or presence of pantoprazole, a known ABCG2 inhibitor. Eyes from New-Zealand albino rabbits were dissected and cut in two equivalent halves. Neural retina was removed and tissues composed of sclera-choroid-RPE were mounted in specially designed polypropylene vertical diffusion cells maintained at 37 ºC. The receiver chamber was perfused with isotonic buffer at a continuous rate using an infusion pump. Pantoprazole (500 µM) was added to this buffer when required. The outflow tube was elevated 22 cm over the mounted tissue to provide a simmulated intraocular pressure of 15 mm Hg. The donor chamber was then filled with topotecan solution (100 µg) and the outflow samples were collected every 30 minutes and analyzed by an HPLC method. The permeability coefficient (Pc, cm/s) was calculated as the steady state flux of topotecan permeated through the sclera normalized to the effective surface area of the sclera and the initial topotecan concentration. The steady state flux of topotecan was calculated from the slope of the cummulative amount of permeated drug versus time plot in the 180 to 300 minute interval. Permeability coefficients with and without pantoprazole were 8.0 ± 1.6 cm/s vs 6.6 ± 1.5 cm/s, respectively, (n=10-13). Pantoprazole induces a statistically significant increase in topotecan permeability (P<0.05, t test). This effect was not observed when sclera was dissected from choroid-RPE and mounted alone in the perfusion chambers, showing a localized effect on the choroid-RPE layers. No alterations on tissue integrity were observed by histochemistry. Taken together, our results suggest that ABCG2 expression may not only be able to alter ocular chemotherapy disposition in cancer cells but also in normal ocular tissues.