n-demethylloperamide and tariquidar

The permeability-glycoprotein (P-gp) efflux transporter is densely expressed at the blood-brain barrier, and its resultant spare capacity requires substantial blockade to increase the uptake of avid substrates, blunting the ability of investigators to measure clinically meaningful alterations in P-gp function. This study, conducted in humans, examined 2 P-gp inhibitors (tariquidar, a known inhibitor, and disulfiram, a putative inhibitor) and 2 routes of administration (intravenous and oral) to maximally increase brain uptake of the avid and selective P-gp substrate (11)C-N-desmethyl-loperamide (dLop) while avoiding side effects associated with high doses of tariquidar.. Forty-two (11)C-dLop PET scans were obtained from 37 healthy volunteers. PET was performed with (11)C-dLop under the following 5 conditions: injected under baseline conditions without P-gp inhibition, injected 1 h after intravenous tariquidar infusion, injected during intravenous tariquidar infusion, injected after oral tariquidar, and injected after disulfiram. (11)C-dLop uptake was quantified with kinetic modeling using metabolite-corrected arterial input function or by measuring the area under the time-activity curve in the brain from 10 to 30 min.. Neither oral tariquidar nor oral disulfiram increased brain uptake of (11)C-dLop. Injecting (11)C-dLop during tariquidar infusion, when plasma tariquidar concentrations reach their peak, resulted in a brain uptake of the radioligand approximately 5-fold greater than baseline. Brain uptake was similar with 2 and 4 mg of intravenous tariquidar per kilogram; however, the lower dose was better tolerated. Injecting (11)C-dLop after tariquidar infusion also increased brain uptake, though higher doses (up to 6 mg/kg) were required. Brain uptake of (11)C-dLop increased fairly linearly with increasing plasma tariquidar concentrations, but we are uncertain whether maximal uptake was achieved.. We sought to increase the dynamic range of P-gp function measured after blockade. Performing (11)C-dLop PET during peak plasma concentrations of tariquidar, achieved with concurrent administration of intravenous tariquidar, resulted in greater P-gp inhibition at the human blood-brain barrier than delayed administration and allowed the use of a lower, more tolerable dose of tariquidar. On the basis of prior monkey studies, we suspect that plasma concentrations of tariquidar did not fully block P-gp; however, higher doses of tariquidar would likely be associated with unacceptable side effects.

Topics: Administration, Intravenous; Administration, Oral; Adult; ATP Binding Cassette Transporter, Subfamily B, Member 1; Biological Transport; Blood-Brain Barrier; Dose-Response Relationship, Drug; Female; Humans; Loperamide; Male; Permeability; Positron-Emission Tomography; Quinolines; Safety

Permeability-glycoprotein (P-gp), an efflux transporter in several organs, acts at the blood-brain barrier to protect the brain from exogenous toxins. P-gp almost completely blocks brain entry of the PET radiotracer (11)C-N-desmethyl-loperamide ((11)C-dLop). We examined the ability of (11)C-dLop to quantify P-gp function in humans after increasing doses of tariquidar, an inhibitor of P-gp.. Seventeen healthy volunteers had a total of 23 PET scans with (11)C-dLop at baseline and after increasing doses of tariquidar (2, 4, and 6 mg/kg intravenously). A subset of subjects received PET with (15)O-H(2)O to measure cerebral blood flow. Brain uptake of (11)C-dLop was quantified in 2 ways. Without blood data, uptake was measured as area under the time-activity curve in the brain from 10 to 30 min (AUC(10-30)). With arterial blood data, brain uptake was quantified with compartmental modeling to estimate the rates of entry into (K(1)) and efflux from (k(2)) the brain.. Brain uptake of radioactivity was negligible at baseline and increased only slightly (approximately 30%) after 2 mg of tariquidar per kilogram. In contrast, 4 and 6 mg of tariquidar per kilogram increased brain uptake 2- and 4-fold, respectively. Greater brain uptake reflected greater brain entry (K(1)), because efflux (k(2)) and cerebral blood flow did not differ between tariquidar-treated and untreated subjects. In the subjects who received the highest dose of tariquidar (and had the highest brain uptake), regional values of K(1) correlated linearly with absolute cerebral blood flow, consistent with high single-pass extraction of (11)C-dLop. AUC(10-30) correlated linearly with K(1).. P-gp function at the blood-brain barrier in humans can be quantified using PET and (11)C-dLop. A simple measure of brain uptake (AUC(10-30)) may be used as a surrogate of the fully quantified rate constant for brain entry (K(1)) and thereby avoid arterial sampling. However, to dissect the function of P-gp itself, both brain uptake and the influx rate constant must be corrected for radiotracer delivery (blood flow).

Topics: Adult; ATP Binding Cassette Transporter, Subfamily B; Biological Transport; Blood-Brain Barrier; Cerebrovascular Circulation; Dose-Response Relationship, Drug; Drug-Related Side Effects and Adverse Reactions; Female; Humans; Kinetics; Loperamide; Magnetic Resonance Imaging; Male; Positron-Emission Tomography; Quinolines; Radioactive Tracers

One mechanism that may be responsible for drug resistance in epilepsy is the upregulation of P-glycoprotein (P-gp), a drug efflux pump, at the epileptogenic focus. In this study, we sought to evaluate the potential of a recently developed P-gp PET radiotracer, [(11)C]N-desmethyl-loperamide ([(11)C]dLop), for measuring P-gp function in the rat brain.. The precursor to [(11)C]dLop was synthesized in two steps from commercially available starting materials and subsequently radiolabeled in one step using [(11)C]methyl iodide. [(11)C]dLop was then administered to two groups of rats, controls (n = 4) and those treated with a P-gp inhibitor (n = 8). Cyclosporin A (CsA, 50 mg/kg, n = 3) and tariquidar (TQ, 20 mg/kg, n = 5) were both used as P-gp inhibitors. MicroPET brain scans were performed for 120 min with arterial blood sampling. A one-tissue compartment model was used to estimate the distribution volume of radiotracer as the outcome measure of P-gp function.. Plasma levels of parent [(11)C]dLop decreased rapidly to <0.1 mean standardized uptake value (SUV) at 60 min. In controls, brain uptake of [(11)C]dLop was very low (<0.1 mean SUV). In contrast, the mean SUVs were significantly higher in rats treated with CsA (0.51) or TQ (0.22). Estimation of distribution volumes was stable by 70 min. Estimated distribution volumes were significantly larger after P-gp inhibition (CsA = 7.3, TQ = 4.7) compared to controls (no inhibitor = 2.1).. The rat brain demonstrates significantly increased uptake of [(11)C]dLop after P-gp inhibition. [(11)C]dLop is a substrate of P-gp, and will serve as a promising radiotracer for studying P-gp function in the future.

Topics: Animals; ATP Binding Cassette Transporter, Subfamily B; Brain; Cyclosporine; Kinetics; Loperamide; Male; Positron-Emission Tomography; Quinolines; Rats; Rats, Sprague-Dawley

To date, the in vitro-in vivo correlation (IVIVC) of P-glycoprotein (P-gp)-mediated drug-drug interaction (DDI) at the blood-brain barrier (BBB) in rats indicated that the cutoff value to significantly affect the brain penetration of digoxin was [I,unbound/Ki] of 1, where I,unbound is the unbound plasma concentration of P-gp inhibitors. On the basis of the IVIVC in rats, we speculated that clinically used P-gp inhibitors do not cause DDI at the human BBB, because none of the compounds studied was [I,unbound/Ki]>1 at therapeutic doses. Recently, positron emission tomography studies with P-gp substrates, such as [(11)C]verapamil, [(11)C]N-desmethyl loperamide, and [(11)C]loperamide, together with potent P-gp inhibitors, have indicated that increases in the influx rate constant for brain entry were observed in humans. Therefore, we aimed to retrospectively analyze the results of P-gp-mediated DDIs with in vitro P-gp inhibition assays and to confirm the appropriate cutoff value. In vitro P-gp inhibition assays using verapamil, N-desmethyl loperamide, and loperamide as P-gp probe substrates were performed in human multidrug resistance protein 1-expressing LLC-PK1 cells. The efflux ratios decreased in the presence of P-gp inhibitors, and the Ki of tariquidar was 10 nmol/L, regardless of probe substrates. Taking the in vitro Ki and unbound plasma concentrations in clinical DDI studies together, the criterion [I,unbound/Ki] of 1 was an appropriate cutoff limit to observe significant P-gp-mediated DDI at the BBB in humans. On the other hand, no significant DDI was observed in cases in which [I,unbound/Ki] was less than 0.1. This criterion was comparable to the previous IVIVC result in rats.

Topics: Animals; ATP Binding Cassette Transporter, Subfamily B, Member 1; Blood-Brain Barrier; Drug Interactions; Humans; LLC-PK1 Cells; Loperamide; Quinolines; Swine; Verapamil

The radiotracer [(11)C]N-desmethyl-loperamide (dLop) images the in vivo function of P-glycoprotein (P-gp), a transporter that blocks the entry of drugs that are substrates into brain. When P-gp is inhibited, [(11)C]dLop, a potent opiate agonist, enters and becomes trapped in the brain. This trapping is beneficial from an imaging perspective, because it amplifies the PET signal, essentially by accumulating radioactivity over time. As we previously demonstrated that this trapping was not caused by binding to opiate receptors, we examined whether [(11)C]dLop, a weak base, is ionically trapped in acidic lysosomes. To test this hypothesis, we measured [(3)H]dLop accumulation in human cells by using lysosomotropics. Because the in vivo trapping of dLop was seen after P-gp inhibition, we also measured [(3)H]dLop uptake in P-gp-expressing cells treated with the P-gp inhibitor tariquidar. All lysosomotropics decreased [(3)H]dLop accumulation by at least 50%. In P-gp-expressing cells, tariquidar (and another P-gp inhibitor) surprisingly decreased [(3)H]dLop uptake. Consequently, we measured [(11)C]dLop uptake before and after tariquidar preadministration in lysosome-rich organs of P-gp KO mice and humans. After tariquidar pretreatment in both species, radioactivity uptake in these organs decreased by 35% to 40%. Our results indicate that dLop is trapped in lysosomes and that tariquidar competes with dLop for lysosomal accumulation in vitro and in vivo. Although tariquidar and dLop compete for lysosomal trapping in the periphery, such competition does not occur in brain because tariquidar has negligible entry into brain. In summary, tariquidar and [(11)C]dLop can be used in combination to selectively measure the function of P-gp at the blood-brain barrier.

Topics: Animals; ATP Binding Cassette Transporter, Subfamily B, Member 1; Blood-Brain Barrier; Cell Line, Tumor; Humans; Isotope Labeling; Loperamide; Lysosomes; Mice; Positron-Emission Tomography; Quinolines; Radiography; Tritium