Use of imaging to assess the activity of hepatic transporters

Figures & data

Figure 1. Clinically relevant human drug transporters in the sinusoidal (blood-facing) and canalicular (bile-facing) membranes of hepatocytes. For illustrative purposes, two hepatocytes expressing different transporters are shown in order to depict the functional coupling between sinusoidal and canalicular membrane transporters.

Table 1. Characteristics of the covered imaging techniques to study hepatic transporter activity.

	PET	SPECT	DCE-MRI
Detection principle	Radioactive decay	Radioactive decay	Nuclear magnetic resonance
Employed nuclei	^11C (20.4 min), ^18F (109.8 min)	^99mTc (6.1 h), ^123I (13.3 h)	Gd
Spatial resolution	2 – 8 mm	7 – 10 mm	2 – 5 mm
Temporal resolution	Seconds – minutes	Minutes	Seconds – minutes
Detection sensitivity	10^−11 – 10^−12 mol/L	10^−10 – 10^−11 mol/L	10^−4 – 10^−5 mol/L
Quantification	Quantitative	(Semi)-quantitative	(Semi)-quantitative
Radiation exposure	Yes	Yes	No

Table 2. PET radiotracers to study the activity of hepatic transporters.

PET radiotracer	Hepatic transporters	Species	Application	Ref
[^11C]N-acetyl-cysteinyl-leukotriene E4	OATPs, MRP2	Rats, Monkeys	Probe validation	[74]
(15R)-[^11C]TIC-Me	OATPs, MRP2	Rats, Humans	Probe validation	[57,63]
[^11C]SC-62,807	OATP1B1/3, BCRP	Mice, Rats	Probe validation	[55,73]
[^11C]Telmisartan	OATP1B3	Rats, Humans	Mechanistic assessment of hepatic clearance; DDI study with rifampicin; assessment of dose dependency of hepatic disposition	[56,64–66]
[^11C]Cholylsarcosine	OATPs, NTCP, BSEP	Pigs, Humans	Probe validation and evaluation of altered hepatic transporter activity in liver disease	[87–90]
[^11C]Dehydropravastatin	OATP1B1/3, MRP2	Rats, Humans	Probe validation	[67–69]
[^11C]Rhodamine-123	OCT1, P-gp	Mice, Rats	Probe validation	[79]
[^11C]Glyburide	OATPs	Mice, Baboons	Mechanistic assessment of hepatic clearance; DDI study with rifampicin and cyclosporine A	[70]
[^11C]Metformin	OCT1, MATE1	Mice, Humans	Assessment of the effect of genetic polymorphisms or disease on liver distribution as pharmacological target tissue	[75–78,96]
[^11C]Rosuvastatin	OATPs, NTCP, BCRP, P-gp, MRP2	Rats, Humans	Mechanistic assessment of hepatic clearance; DDI study with rifampicin and cyclosporine A; validation of IVIVE	[37,94]
[^11C]Erlotinib	OATP2B1, BCRP, P-gp,	Mice, Rats, Humans	Mechanistic assessment of hepatic clearance; DDI study with rifampicin; assessment of dose dependency of hepatic disposition	[60,61,97,98]
[^18F]Pitavastatin	OATP1B1/3, BCRP, MRP2	Rats	Mechanistic assessment of hepatic clearance; DDI study with rifampicin	[58,71]
[^18F]PTV-F1	OATP1B1/3	Rats	Probe validation	[59,72]
3β-[^18F]Fluorocholic acid	OATPs, NTCP, BSEP	Mice	Probe validation and evaluation of altered hepatic transporter activity in mouse models of liver disease	[92,93]
[^11C]Sulpiride	OCT1	Mice, Humans	Mechanistic assessment of hepatic clearance; assessment of dose dependency of hepatic disposition	[80]
[^11C]Tariquidar	P-gp, BCRP	Mice, Humans	Probe validation; assessment of dose dependency of hepatic disposition	[84]

Table 3. SPECT and MR imaging agents used to study the activity of hepatic transporters.

Imaging agent		Hepatic transporters	Species	Application	Ref
	SPECT
[^99mTc]Mebrofenin		OATP1B1/3, MRP2, MRP3	Mice, rats, humans	Mechanistic assessment of hepatic clearance; assessment of the effect of genetic polymorphisms or disease on hepatic transporter activity	[106–112]
[^99mTc]PMT		OATP1B1/3, P-gp, MRP2	Rats, rabbits, humans	Probe validation	[116]
[^111In]EOB-DTPA		OATPs	Mice	Probe validation	[117]
[^99mTc]DTPA-CDCA		OATP1B1/3, MRP2	Mice	Probe validation	[121]
[^99mTc]DTPA-CA		OATP1B1/3, MRP2	Mice	Probe validation	[121]
[^131I]NP-59		OATP1B1/3, BCRP	Mice	Probe validation	[120]
	MRI
Gadoxetate		OATPs, MRP2	Mice, rats, humans	Mechanistic assessment of hepatic clearance; assessment of the effect of genetic polymorphisms or disease on hepatic transporter activity	[101,102,124–132]
BOPTA		OATPs, MRP2	Perfused rat livers	Mechanistic assessment of hepatic clearance	[133–137]

Figure 2. Pharmacokinetic models developed to study the hepatobiliary disposition of [¹¹C]erlotinib. (a) Four compartment model developed from the combination of two previously developed models [60,89]. This model includes two main regions of interest (ROIs): the liver and the extrahepatic bile duct and gall bladder (eBD/GB). The liver ROI includes compartments representing the amount of radioactivity in the blood fraction in the liver sinusoids used as the model input function (X_blood, 0.25% of the total liver volume), in the hepatocytes (X_hep) and in the intrahepatic bile (X_ih, 0.32% of the total liver volume). The extrahepatic ROI compartment represents the amount of radioactivity in the visible part of the extrahepatic bile duct and the gall bladder. The kinetic parameters define the exchange rate of radioactivity between blood and hepatocytes (k₁ and k₂), from hepatocytes to the intrahepatic bile duct (k₃), or from the intrahepatic bile duct to the extrahepatic bile duct and gall bladder (k₅). (b) The three compartment model includes compartments representing the blood in the liver sinusoids (used as the model input function), liver tissue (combination of the hepatocytes and the intrahepatic bile duct from the four compartment model) and excreted bile out of the liver ROI. The kinetic parameters define the exchange rate of radioactivity between the blood and the liver (k₁ and k₂) or from the liver to the excreted bile (k₃). The differential equations depicted under the kinetic models are implemented to fit the amount of radiotracer in each compartment to the obtained PET TACs and obtain the estimates of the kinetic parameters. In the equations, X represents the amount of radiotracer in the compartment and ks are the respective rate constants. Note that k₁ represents the basolateral uptake rate and does not discriminate between active or passive uptake mechanisms. According to PET pharmacokinetic modeling, the parameter K₁ is perfusion-dependent and can be related to blood flow (Q) as: K₁ = E x Q; where E represents the unidirectional first-pass extraction ratio [140]. In the present model, k₁ can be expressed by means of K₁ as k₁ = K₁ x (V_liver/V_blood), where V_liver corresponds to the volume of hepatocytes in (a) and the volume of liver tissue in (b), and V_blood corresponds to the volume of blood in the hepatic sinusoids. Therefore, the uptake rate constant k₁ can be expressed in terms of perfusion as: k₁ = (E x Q x V_liver)/V_blood.

Figure 3. Representation of [¹¹C]erlotinib radioactivity concentration in the radial artery (assumed to be equal to the concentration in the hepatic artery), in the mathematically-derived portal vein and in the estimated dual input function in one representative subject. The enlarged graph section shows only the first 10 min of the PET scan duration for which the difference in concentration is larger between arterial and venous hepatic blood. The equations depicted below the graph show the mathematical method implemented to estimate the radiotracer concentration in the portal vein. The concentration in the portal vein along time, C_PV(t), can be calculated as the convolution integral between the concentration in the hepatic artery, C_HA(t), and the impulse-response function, h(t) as described in the first equation. This impulse-response function (second equation) is mainly characterized by the parameter β which determines the mean transit time for the passage of radiotracer from the intestine to the portal vein. Finally, as indicated in the third equation, the total concentration in the liver sinusoids (dual input function, C_dual(t)) can be represented as a flow-weighted function which takes into account the hepatic artery flow fraction (f_HA, ~0.25) and the portal vein flow fraction (f_PV, ~0.75).

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