Measuring drug-on-target receptor occupancy
Receptor occupancy (RO) is calculated from PET data as the treatment-induced relative change in the concentration of available (not occupied) receptors (Bavail):
Measurement of Bavail (or ”Bmax”) is challenging, and other binding parameters are often used instead. If we can assume that treatment does not change the ligand-receptor affinity (1/KD), then useful binding parameters for RO estimation are:
Total volume of distribution (VT) is also related to binding potential (BP), but not linearly. If baseline BP>>(1+k5/k6), then VT can be used to measure receptor occupancy, otherwise VT from reference region must be measured and used to calculate BPND or BPP.
RO estimation based on binding potentials, or ratios, requires that a reference region (devoid of specific binding) is available. For several radioligands there is no true reference region, and in these cases the Lassen plot or its extension can be used ( Lassen et al., 1995; Cunningham et al., 2010; Kuwabara et al., 2016), even from SUV (Takano et al., 2014). However, Lassen plot does not always provide satisfactory results (Kågedal et al., 2013; Joshi et al., 2015).
In theory, high-affinity PET radioligands can be used to measure receptor occupancy. If there is a state of equilibrium between administrated or endogenous and tracer ligands, free receptor and receptor-ligand complexes, radioligand affinity should not affect measurement of occupancy by administrated or endogenous ligand. Yet, a frequent observation from several studies is that the lower affinity radiotracers appear to be more susceptible to competition by synaptic endogenous or administered ligands than radiotracers which have very high receptor affinity.
It has been suggested that the magnitude of the competition is not reduced by the relative difference in ligand affinities, but by failure of the receptor binding of high-affinity radioligands to rapidly attain equilibrium (Gatley et al., 2000; Laruelle 2000). It is important that equilibrium is achieved within the time scale of the in vivo binding experiment with PET. Under conditions in which the radiotracer binding is still far from reaching equilibrium with the tissue receptors, radiotracer accumulation in the tissue is determined mostly by delivery (perfusion and transport) rather than by density of available receptors.
Even radiotracers which bind their receptors with an affinity so high that the binding is nearly irreversible in the time available for PET can be used to monitor receptor blockade, if proper modelling is applied (Ishizu et al., 2000; Laruelle 2000).
Instead of different affinities, a possible explanation for differing competition results obtained with different tracers is matter of different ability to access the internalized receptors (Laruelle 2000).
Agonist versus antagonist
Agonist ligands may be more useful than antagonists for measuring receptor occupancy by endogenous synaptic neurotransmitters (Cumming et al., 2002).
Partial volume effect
Measured occupancy is independent of partial volume effect (Martinez et al, 2001), if it is similar in baseline and during medication. Notice that when occupancy is very high, image contrast is usually reduced, leading to lower partial volume effect.
Altered perfusion and peripheral clearance do not affect the receptor binding estimates calculated using graphical analysis or compartmental kinetic modelling (Laruelle, 2000). Sander et al. (2017) demonstrated this in a primate PET-fMRI study, where CO2 induced hypercapnia increased cerebral blood flow by ∼2.5-fold, as measured with fMRI, but no changes were observed in the uptake of [11C]raclopride or [18F]fallypride. However, these methods are vulnerable to variations in blood flow or clearance that occur during the PET scan (Laruelle, 2000).
Specific binding in reference region
Modelling receptor occupancy
Aim is to predict receptor occupancy (RO) at any time relative to the blocking drug dosing or when changing the dosage regimen. To achieve this
- conventional PK modelling is needed to relate the drug plasma concentration to drug dosage regime,
- RO as a function of time (measurements with PET) after drug dosing needs to be related to the plasma drug concentration (Zamuner et al., 2010), and
- these two models can be modelled together, using PK data as a link between RO and dose (Vandenhende et al., 2008).
Since PET studies can provide only sparse RO data relative to dosing, it will be challenging to develop a model that could reliably predict the RO. Any model should be validated by simulating with it the RO for dosage regimen that was not used to develop the model.
Receptor occupancy can be related to plasma concentration directly or indirectly.
Often the following Emax model is a reasonable approximation for a direct (sigmoidal) relationship between drug plasma concentration (C) and the level of RO.
Usually E0=0, and then the unconstrained one site binding hyperbola for RO as a function of drug plasma concentration,
, or as a function of drug dose (D),
, can be fitted to estimate Emax and EC50 or
ED50, respectively. Fitting can be done for example in
Also our command-line tool fit_sigm
can be used with options
-EC50 -n=1, or if Emax is constrained to 1 or 100%, with
-EC50 -n1 -A=1 or
-EC50 -n1 -A=100, respectively.
Reliability of the fitting may be improved by fitting all ROIs simultaneously with a common EC50 shared across regions (Graff-Guerrero et al., 2010).
Two site binding model may be necessary if PET tracer binds to two receptor subtypes, for example dopamine receptors D2 and D2 (Graff-Guerrero et al., 2010):
Indirect response model must be considered when the data suggest a delay in RO compared to plasma concentration. Examples of delayed RO have been reported for example by Tauscher et al. (2002) and Ingman et al. (2005).
Abanades S, van der Aart J, Barletta JAR, Marzano C, Searle GE, Salinas CA, Ahmad JJ, Reiley RR, Pampols-Maso S, Zamuner S, Cunningham VJ, Rabiner EA, Laruelle MA, Gunn RN. Prediction of repeat-dose occupancy from single-dose data: characterisation of the relationship between plasma pharmacokinetics and brain target occupancy. J Cereb Blood Flow Metab. 2011; 31: 944-952.
Asselin MC, Montgomery AJ, Grasby PM, Hume SP. Quantification of PET studies with the very high-affinity dopamine D2/D3 receptor ligand [11C]FLB 457: re-evaluation of the validity of using a cerebellar reference region. J Cereb Blood Flow Metab. 2007; 27(2): 378-392.
Black KJ, Koller JM, Miller BD. Rapid quantitative pharmacodynamic imaging by a novel method: theory, simulation testing and proof of principle. PeerJ 2013; 1:e117; DOI 10.7717/peerj.117
Bourdet DL, Tsuruda PR, Obedencio GP, Smith JAM. Prediction of human serotonin and norepinephrine transporter occupancy of duloxetine by pharmacokinetic/pharmacodynamic modeling in the rat. J Pharmacol Exp Therapeutics 2012; 341: 137-145.
Cumming P, Wong DF, Dannals RF, Gillings N, Hilton J, Scheffel U, Gjedde A. The competition between endogenous dopamine and radioligands for specific binding to dopamine receptors. Ann NY Acad Sci. 2002; 965: 440-450.
Cunningham VJ, Rabiner EA, Slifstein M, Laruelle M. Measuring drug occupancy in the absence of a reference region: the Lassen plot re-visited. J Cereb Blood Flow Metab. 2010; 30: 46-50.
Gatley SJ, Gifford AN, Carroll FI, Volkow ND. Sensitivity of binding of high-affinity dopamine receptor radioligands to increased synaptic dopamine. Synapse 2000; 38: 483-488.
Graff-Guerrero A, Redden L, Abi-Saab W, Katz DA, Houle S, Barsoum P, Bhathena A, Palaparthy R, Saltarelli MD, Kapur S. Blockade of [11C](+)-PHNO binding in human subjects by the dopamine D3 receptor antagonist ABT-925. Int J Neuropsychoparmacol. 2010; 13: 273-287.
Gunn RN, Rabiner EA. Imaging in central nervous system drug discovery. Semin Nucl Biol. 2017; 47:89-98.
Ingman K, Hagelberg N, Aalto S, Någren K, Juhakoski A, Karhuvaara S, Kallio A, Oikonen V, Hietala J, Scheinin H. Prolonged central mu-opioid receptor occupancy after single and repeated nalmefene dosing. Neuropsychopharmacology 2005; 30(12): 2245-2253.
Ishizu K, Smith DF, Bender D, Danielsen E, Hansen SB, Wong DF, Cumming P, Gjedde A. Positron emission tomography of radioligand binding in porcine striatum in vivo: haloperidol inhibition linked to endogenous ligand release. Synapse 2000; 38: 87-101.
Kim E, Howes OD, Kim B-H, Jeong JM, Lee JS, Jang I-J, Shin S-G, Turkheimer FE, Kapur S, Kwon JS. Predicting brain occupancy from plasma levels using PET: superiority of combining pharmacokinetics with pharmacodynamics while modeling the relationship. J Cereb Blood Flow Metab. 2012; 32: 759-768.
Kuwabara H, Gao Y, Stabin M, Coughlin J, Nimmagadda S, Dannals RF, Pomper MG, Horti AG. Imaging α4β2 nicotinic acetylcholine receptors (nAChRs) in baboons with [18F]XTRA, a radioligand with improved specific binding in extra-thalamic regions. Mol Imaging Biol. 2017; 19(2): 280-288. doi: 10.1007/s11307-016-0999-9.
Kågedal M, Cselényi Z, Nyberg S, Raboisson P, Ståhle L, Stenkrona P, Värnas K, Halldin C, Hooker AC, Karlsson MO. A positron emission tomography study in healthy volunteers to estimate mGluR5 receptor occupancy of AZD2066 - Estimating occupancy in the absence of a reference region. Neuroimage 2013; 82(15): 160-169.
Kågedal M, Karlsson MO, Hooker AC. Improved precision of exposure-response relationships by optimal dose-selection. Examples from studies of receptor occupancy using PET and dose finding for neuropathic pain treatment. J Pharmacokinet Pharmacodyn. 2015; 42: 211-224.
Laruelle M. Imaging synaptic neurotransmission with in vivo binding competition techniques: a critical review. J Cereb Blood Flow Metab. 2000; 20: 423-451.
Lassen NA, Bartenstein PA, Lammertsma AA, Prevett MC, Turton DR, Luthra SK, Osman S, Bloomfield PM, Jones T, Patsalos PN, O’Connell MT, Duncan JS, Andersen JV. Benzodiazepine receptor quantification in vivo in humans using [11C]flumazenil and PET: application of the steady-state principle. J Cereb Blood Flow Metab. 1995; 15: 152-165.
Martinez D, Hwang D-R, Mawlawi O, Slifstein M, Kent J, Simpson N, Parsey RV, Hashimoto T, Huang Y, Shinn A, Van Heertum R, Abi-Dargham A, Caltabiano S, Malizia A, Cowley H, Mann JJ, Laruelle M. Differential occupancy of somatodendritic and postsynaptic 5HT1A receptors by pindolol: a dose-occupancy study with [11C]WAY 100635 and positron emission tomography in humans. Neuropsychopharmacology 2001; 24:209-229.
Matthews PM, Rabiner I, Gunn R. Non-invasive imaging in experimental medicine for drug development. Curr Opin Pharmacol. 2011; 11: 501-507.
Passchier J, Gee A, Willemsen A, Vaalburg W, van Waarde A. Measuring drug-related receptor occupancy with positron emission tomography. Methods 2002; 27: 278-286.
Ridler K, Gunn RN, Searle GE, Barletta J, Passchier J, Dixson L, Hallett WA, Ashworth S, Gray FA, Burgess C, Poggesi I, Bullman JN, Ratti E, Laruelle MA, Rabiner EA. Characterising the plasma-target occupancy relationship of the neurokinin antagonist GSK1144814 with PET. J Psychopharmacol. 2014; 28(3): 244-253.
Salinas C, Weinzimmer D, Searle G, Labaree D, Ropchan J, Huang Y, Rabiner EA, Carson RE, Gunn RN. Kinetic analysis of drug-traget interactions with PET for characterization of pharmacological hysteresis. J Cereb Blood Flow Metab. 2013; 33: 700-707.
Slifstein M. When reversible ligands do not reverse, and other modelers’ dilemmas. J Nucl Med. 2010; 51(7): 1005-1008.
Takano A, Gulyás B, Varnäs K, Little PB Noerregaard PK, Jensen NE, Elling C, Halldin C. Low brain CB1 receptor occupancy by a second generation CB1 receptor antagonist TM38837 in comparison with rimonabant in nonhuman primates: A PET study. Synapse 2014; 68(3): 89-97.
Takano A, Varrone A, Gulyás B, Salvadori P, Gee A, Windhorst A, Vercouillie J, Bormans G, Lammertsma AA, Halldin C. Guidelines to PET measurements of the target occupancy in the brain for drug development. Eur J Nucl Med Mol Imaging 2016; 43: 2255-2262.
Tauscher J, Jones C, Remington G, Zipursky RB, Kapur S. Significant dissociation of brain and plasma kinetics with antipsychotics. Mol Psychiatry 2002; 7(3): 317-321.
Vandenhende F, Renard D, Nie Y, Kumar A, Miller J, Tauscher J, Witcher J, Zhou Y, Wong DF. Bayesian hierarchical modeling of receptor occupancy in PET trials. J Biopharm Stat. 2008; 18(2): 256-272.
van Waarde A. Measuring receptor occupancy with PET. Curr Pharm Des. 2000; 6: 1593-1610.
Wong DF, Tauscher J, Gründer G. The role of imaging in proof of concept for CNS drug discovery and development. Neuropsychopharmacology 2009; 34: 187-203.
Zamuner S, Di Iorio VL, Nyberg J, Gunn RN, Cunningham VJ, Gomeni R, Hooker AC. Adaptive-optimal design in PET occupancy studies. Clin Pharmacol Ther. 2010; 87(5): 563-571.
Zhang Y, Fox GB. PET imaging for receptor occupancy: meditations on calculation and simplification. J Biomed Res. 2012; 26(2): 69-76.
Created at: 2004-08-09
Updated at: 2018-01-12
Written by: Vesa Oikonen