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Detection of P-Glycoprotein Activity in Endotoxemic Rats by ^99mTc-Sestamibi Imaging

¹ Department of Pharmaceutical Sciences, University of Toronto, Toronto, Ontario, Canada
² Division of Nuclear Medicine, University Health Network, Toronto, Ontario, Canada

^99mTc-sestamibi is a widely used radiopharmaceutical agent for myocardial and oncologic imaging. Because of its unique role as a P-glycoprotein (Pgp)–specific substrate, this compound can be used to examine Pgp functional activity in vitro and in vivo under pathologic conditions. Our objective was to use ^99mTc-sestamibi as a tool to investigate whether systemic inflammation induced by Escherichia coli lipopolysaccharide (LPS) would affect in vivo Pgp function in the brain, heart, liver, and kidneys of rats. Moreover, we also wanted to examine LPS-mediated effects in the placenta of pregnant rats because of the limited amount of in vivo data on this tissue. Methods: Rats were injected intraperitoneally with LPS or an equal volume of saline as controls. After certain time periods (6 or 24 h), animals were administered 20 MBq of ^99mTc-sestamibi intravenously, and then images were taken at 0.5, 1, 2, and 3 h. Tissues of rats were excised for ^99mTc-sestamibi biodistribution analysis by -counting and messenger RNA (mRNA) analysis by reverse transcription–polymerase chain reaction. Western blot analysis with antibody C-219 was used to detect Pgp levels. Results: LPS treatment for 6 h caused a significant downregulation of mdr1a mRNA levels in the brain, heart, and liver, whereas 24 h of LPS treatment significantly reduced mdr1a mRNA levels only in the liver. A significant downregulation of mdr1a mRNA was seen in the brain, heart, and liver within 6 h after LPS administration. Imaging and biodistribution studies demonstrated a higher accumulation of ^99mTc-sestamibi in the brain, heart, and liver of LPS-treated rats. In the brain, LPS-imposed downregulation of mdr1a mRNA levels was transient, with significant suppression at 4, 6, and 12 h, and the levels recovered to nearly normal by 24 h. This time-dependent downregulation of mRNA correlated with protein levels determined by Western blot analysis. Biodistribution studies of pregnant rats demonstrated a 3.5-fold-higher accumulation of ^99mTc-sestamibi in the fetal tissues of LPS-treated pregnant rats than in saline-treated control rats. Furthermore, placental mdr1a and mdr1b mRNA levels were also significantly downregulated by LPS treatment. Conclusion: Our results indicate that LPS-induced systemic inflammation caused an increased retention of ^99mTc-sestamibi in the brain, heart, liver, and fetal tissues. These results correlated with a reduction in mdr1a mRNA levels in each organ.

Key Words: radiopharmaceuticals • multidrug resistance • ATP-binding cassette transporters • inflammation

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