Imaging the function of P-glycoprotein with radiotracers: pharmacokinetics and in vivo applications

Abstract

P-glycoprotein (P-gp), an efflux transporter, controls the pharmacokinetics of various compounds under physiological conditions. P-gp-mediated drug efflux has been suggested as playing a role in various disorders, including multidrug-resistant cancer and medication-refractory epilepsy. However, P-gp inhibition has had, to date, little or no clinically significant effect in multidrug-resistant cancer. To enhance our understanding of its in vivo function under pathophysiological conditions, substrates of P-gp have been radiolabeled and imaged using single-photon emission computed tomography (SPECT) and positron emission tomography (PET). To accurately quantify P-gp function, a radiolabeled P-gp substrate should be selective for P-gp, produce a large signal after P-gp blockade, and generate few radiometabolites that enter the target tissue. Furthermore, quantification of P-gp function via imaging requires pharmacological inhibition of P-gp, which requires knowledge of P-gp density at the target site. By meeting these criteria, imaging can elucidate the function of P-gp in various disorders and improve the efficacy of treatments.

Figure 1

Structural model of P-glycoprotein (P-gp) and a diagram of the mechanism by which it pumps substrates. (a) P-gp is a transmembrane protein located on the apical side of polarized cells that facilitates the translocation or prevents the ingress of molecules. Polarized cells are joined together by tight junctions that prevent paracellular diffusion and ensure that the passage of small molecules is transporter-regulated. (b) A model of P-gp in the lipid bilayer extruding doxorubicin (to scale). The binding and hydrolysis of ATP (shown bound during hydrolysis) initiate substrate extrusion. Substrates can be intercepted and extruded directly from the lipid bilayer or be drawn from the intracellular pool. The model of P-gp incorporated in the figure was kindly provided by Robert Rutledge.

Figure 2

Direction of substrate transport by P-glycoprotein (P-gp) located in various organs of the human body. The bold solid arrows indicate the known direction of transport, whereas the broken-line arrow indicates unclear direction of transport. P-gp is located in the lipid bilayer (thick black line) that forms a barrier between various organs; red indicates vasculature, blue represents tissue, and white indicates excreta. CSF, cerebrospinal fluid; MDR, multidrug resistance. Modified from ref. .

Figure 3

Schematic representation of a simplified model of two compartments (blood and tissue) used to quantify P-glycoprotein (P-gp) function. The two compartments are separated by a lipid bilayer in which P-gp is located. The kinetic parameter k₁ (ml · cm⁻³ · min⁻¹) is the influx rate of the radiolabeled P-gp substrate, and k₂ (min⁻¹) is the efflux rate of the substrate from tissue to blood.

Figure 4

Chemical structures of P-glycoprotein substrates radiolabeled for single-photon emission computed tomography and positron emission tomography imaging. The radiolabeled atom is indicated in red.

Figure 5

Representative positron emission tomography (PET) and magnetic resonance imaging (MRI) images of brain after injection of [¹¹C]-N-desmethyl-loperamide ([¹¹C]dLop) under baseline and blockaded conditions in three species. Under baseline conditions, uptake of [¹¹C]dLop in the brain is minimal in all species (column 1), except in the pituitary in monkeys (arrow) and humans and in the choroid plexus (arrowhead) of humans. The choroid plexus shown in this section is located near the medial surface of the lateral ventricles and the roof of the third ventricle. A P-glycoprotein (P-gp)-knockout mouse brain (circled) shows higher uptake of [¹¹C]dLop than that of a wild-type mouse (row 1). Under P-gp-blocked conditions (column 2), after injection of the P-gp inhibitor DCPQ (8 mg/kg, intravenously (i.v.)) in monkey, the uptake of [¹¹C]dLop in the brain markedly increases (row 2). Preliminary studies in humans show a small increase (20%) in the uptake of [¹¹C]dLop in the brain after injection of tariquidar (2 mg/kg, i.v.). MRI images of corresponding coronal slices are shown in the column on the right. DCPQ, (2R)-anti-5-{3-[4-(10,11-dichloromethanodibenzo-suber-5-yl)piperazin-1-yl]-2-hydroxypropoxy}quinoline trihydrochloride. PET and MRI images for mice kindly provided by Jeih-San Liow; images for monkeys reprinted from ref. with permission of the Society of Nuclear Medicine; and images for humans reprinted from ref. with permission of the Society of Nuclear Medicine.

Figure 6

P-glycoprotein (P-gp) inhibitor tariquidar (2 mg/kg, intravenously) increased the uptake of [^99mTc]sestamibi in a metastatic tumor mass in the left thigh (arrow) of a patient with renal cell carcinoma. Bladder accumulation of [^99mTc]sestamibi was lower after P-gp inhibition than at baseline, thereby supporting the hypothesis that P-gp excretes substrates into urine. Reprinted from ref. with permission of the American Association for Cancer Research.