Imaging Pgp transport in vivo with PET
Radiolabelled MDR-associated cytostatic agents can be used to study drug efflux pumps in vivo. One example is colchicine, a Pgp substrate which is a naturally occurring alkaloid (Ford & Hait 1990). Mehta et al (1992) studied the biodistribution of [3H]colchicine in mice xenografted with Pgp-negative and Pgp-positive human neuroblastoma tumours (Mehta et al 1992). The
pharmacokinetics of colchicine are possibly favourable for PET studies due to their limited metabolism (Hunter & Klaassen 1975). Using [14C]colchicine, the colchicine distribution and metabolism were studied (Mehta et al 1994). In vivo uptake of [14C]colchicine in tumour-bearing mice demonstrated that activity in sensitive tumours was twice as high as in resistant tumours at 60 min after [14C]colchicine injection. The biodistribution of [14C]colchicine in tumour-bearing mice was also studied with whole body quantitative autoradiography. The accumulation in sensitive tumours was twice as high as in resistant tumours (Mehta et al 1996). This study suggests that it is feasible to image Pgp functionality in tumours with [nC]colchicine and PET. However, biodistribution studies in the tumour-bearing mice showed a relatively high uptake of radioactivity in the liver and intestine, due to detoxification of colchicine via the bile (Hunter et al 1975). This makes radiolabelled colchicine less useful for monitoring Pgp drug efflux in abdominal tumours.
In our institute, we prepared [11C]verapamiland [11C]daunorubicinto study Pgp function non-invasively (Elsinga et al 1996). In vitro experiments in ovarian carcinoma cell lines (A2780) and its Pgp overexpressing counterpart (A2780AD) demonstrated that the accumulation of [11C]daunorubicin and [11C]verapamil were respectively 16-fold and fivefold increased in A2780 as compared to 2780AD and the accumulation of [11C]daunorubicin was increased in A2780AD cells after addition of unlabelled verapamil (Hendrikse et al 1998a).
Subsequently, the Pgp function in the BBB and the effects of a modulator on this function were imaged in Mdr/a-disrupted mice (Mdr/a7/1 mice) and wild-type mice (Mdr/a+/+ mice). Ex vivo biodistribution studies revealed 9.5-fold higher [11C]verapamil levels in the brain and 3.4-fold higher levels in the testes of Mdr/a1/1 mice than in Mdr/a+/+ mice. The [11C]verapamil levels were dose-dependently increased by the Pgp blocker cyclosporin A in Mdr/a+/+ mice. No modulating effects of cyclosporin A were found in the Mdr/a1/1 mice. Positron camera data showed lower [11C]verapamil levels in the brain of Mdr/a+/+ mice than in Mdr/a1/1 mice. Time—activity curves demonstrated that [11C]verapamil accumulation in the brain of Mdr/a+/+ mice was increased by cyclosporin A to levels comparable with those in Mdr/a1/1 mice, indicating that reversal of Pgp mediated efflux can be monitored by PET (Hendrikse et al 1998b).
We also studied Pgp transport of [11C]verapamil and [11C]daunorubicin in a two-sided tumour-bearing rat. Rats were inoculated with GLC4 cells in one flank and with the MDR/-transfected Pgp-overexpressing subline GLC4/Pgp in the other flank. Biodistribution studies demonstrated higher accumulation of [11C]verapamil and [11C]daunorubicin in GLC4 than in the GLC4/Pgp tumours. The decreased accumulation of radioactivity in the GLC4/Pgp tumours could be completely reversed by cyclosporin A. In vivo data of [11C]verapamil kinetics and the modulating effects of cyclosporin A showed that Pgp function and its reversal can be visualized non-invasively with a positron camera (Hendrikse et al 1999).
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