Monitoring the expression and activity of ABC transporters by radiopharmaceutical and molecular approaches

ATP binding cassette (ABC) transporters are a group of cellular efflux transporters, which are believed to play critical roles in drug distribution and elimination. Inflammation is a commonly occurring event and is often associated with various disease states. As medications are usually taken during disease conditions, the adequacy of transporter function under these conditions is of particular importance. Clinical cases have shown the impact of inflammation on the changes in pharmacokinetics and pharmacodynamics of drugs. Studies contained within this thesis focused on the detection of ABCB1 (P-glycoprotein; Pgp) functional activity using 99mTc-sestamibi imaging and biodistribution in a rodent endotoxin model of inflammation. The effect of inflammation on the expression of ABCG2 (breast cancer resistance protein; Bcrp) as well as several other key placental drug transporters was also evaluated along with the biodistribution of a hypoglycemic agent, glyburide. We also attempted to develop a radio-labeled antisense oligonucleotide to utilize as a probe for non-invasive in vivo detection of Pgp expression. Our studies demonstrated that 99mTc-sestamibi imaging and biodistribution can successfully monitor the inflammation-mediated changes in Pgp functional activity in brain, liver, heart and placenta, and the results corresponded to the mRNA levels. Endotoxin-induced inflammation imposed a significant downregulation in the expression of several influx and efflux drug transporters in placenta. Endotoxin was associated with significantly lower glyburide accumulation in fetal tissues, despite elevations in plasma glyburide concentrations. As glyburide is a recognized substrate for both Bcrp and the organic anion transport peptides (Oatps), this result suggests that downregulation of Oatp efflux transporters had more impact than Bcrp downregulation in the trans-placental transfer of glyburide in vivo. This highlighted the difficulty in the use of drug substrates to selectively examine changes in the activity of specific transporters. In vitro results with the antisense oligodeoxynucleotides indicated that radio-labeled antisense probes may be useful in selectively targeting the human mdr1 gene in Pgp overexpressing tumors. This thesis presented the significance and efficacy of evaluating the expression and activity of Pgp as well as Bcrp using imaging and molecular approaches. The results also provide valuable information in considering therapeutic drug disposition under certain disease circumstances.