Converting blood TAC to plasma TAC

Most PET tracers equilibrate relatively slowly between plasma and red blood cells (RBCs). In these cases only the tracer in plasma is available to tissue extraction during the blood passage through the capillary. Thus, for most tracers, the blood TAC measured using “blood pump” with on-line detector, or extracted from the PET image, must be converted to plasma TAC before it can be used as input function in quantitative analysis.

Plasma and blood TACs (PTACs and BTACs, respectively) are usually not similar; even if the equilibrium between blood cells and plasma is reached instantly, the water space is smaller inside the cells than in the plasma, or binding to plasma proteins or blood cell membrane lipids or haemoglobin (Paixao et al, 2009) affects the distribution of the tracer. Partition of a drug between plasma and RBC has even been utilized in measurement of the plasma protein binding.

Certain analysis methods require that the impact of vascular radioactivity must be considered. If only plasma TAC is measured, then the blood TAC must be calculated from it, e.g. using p2blood. Note that hematocrit in small vessels in tissue is usually lower than in large venous vessels, from where the sample for hematocrit determination is collected.

Measurement of concentration in blood cells

Radioactivity concentration in red blood cells cannot be measured directly from centrifuged blood samples, because RBC preparations always contain some plasma. If plasma is washed away, then part of the radioactivity inside the cells is also removed.

However, if the TACs of whole blood and plasma, and hematocrit (HCT) are measured, then the TAC of RBC can be calculated from Eq (1) in Excel or using program b2rbc. Blood-to-plasma or plasma-to-blood ratio curve can be converted to RBC-to-plasma with bpr2cpr.

New PET tracers

When a new tracer is introduced, hematocrit and radioactivity concentrations in both plasma and blood have to be measured during the length of the PET imaging.

Based on the plasma-to-blood ratio curve a decision can be made whether a conversion can be applied, e.g. based on hematocrit or measured one-point ratio, or whether both plasma and blood samples need to be individually measured also in future.

The rate of partitioning of the parent tracer between plasma and RBC determines whether PTAC or BTAC should be used as reference fluid in analysis; this does not only apply to PET studies but to all drug studies (Hinderling, 1997).

Note also that partitioning may be different in different animal species, due to different properties of RBC and plasma proteins.

Tracer distribution between plasma and blood cells

Radioactivity concentration in whole blood is the sum of concentrations in plasma and red blood cells, weighed by their volume fractions, represented by hematocrit (HCT).

Blood cell to plasma partition coefficient (rRBC/P) can be measured In vitro in +37 °C by adding a known amount of radioactivity into a known volume of fresh blood (providing CB), centrifuging the sample, and sampling a known volume of plasma. Radioactivity of the plasma sample is measured to calculate CP. Blood cell to plasma ratio is then calculated as (Bower, 1982):

There are four main types of tracer distribution between plasma and RBC:

  1. Tracer persists in plasma
  2. Tracer persists in red blood cells
  3. Tracer penetrates RBC membrane instantly
  4. Tracer penetrates RBC membrane slowly

Tracer persists in plasma

If PET tracer, or any radioactive metabolite in plasma, can not penetrate the red blood cell (RBC) membrane, or the concentration inside RBCs is negligible because of high plasma protein binding, then CRBC=0. The concentrations in plasma and blood are related by the equation

Plasma-to-blood ratio for tracers which persist in plasma

Figure. Plasma-to-blood ratio is only dependent on hematocrit, if tracer, and possible label-carrying metabolites, both persist in plasma.

These PET tracers include [11C]raclopride, [18F]FTHA, [11C]TMSX, [carbonyl-11C]WAY-100635, and [68Ga]citrate.

Radioactive metabolite penetrates RBC membrane

Although the authentic radioligand does not penetrate RBC membrane, it is possible that its radioactive metabolites do.

For example, Unchanged [11C]-R-PK11195 persists in plasma, but its radioactive metabolites equilibrate between RBC and plasma water spaces. In the case of [11C]palmitate, the unchanged tracer and most of its radioactive metabolites stay in plasma, but the first appearing metabolite, [11C]CO2 (or [11C]HCO3-) permeates the RBCs readily.

Average plasma-to-blood ratio in [11C]-R-PK11195 PET studies Average plasma-to-blood ratio in [11C]palmitate PET studies

Figure. Functions representing population average of plasma-to-blood ratio in [11C]-R-PK11195 and [11C]palmitate studies (unpublished data from TPC). For [11C]-R-PK11195 see also similar plot by Kropholler et al. (2009), Fig 2a.

[18F]Fluoride is used as a PET tracer, but it is also a common metabolite of many [18F]-labelled radioligands, including [18F]FMPEP-d2. Transport of [18F]F- between plasma and RBCs is very fast, but concentration inside RBCs is lower than in plasma.

Plasma-to-blood ratio in [18F]NaF PET study Plasma-to-blood ratio in [18F]FMPEP-d2 PET study

Figures. Plasma-to-blood ratio in [18F]NaF (Hawkins et al., 1992), and in [18F]FMPEP-d2 (unpublished data from TPC) studies.


Tracer persists in red blood cells

These tracers include [15O]O2 and [15O]CO. The plasma-to-blood ratio of authentic tracers is then zero.

Note that the concentration of metabolite of [15O]O2, [15O]H2O, is in equilibrium between blood and plasma water spaces.

Tracer penetrates RBC membrane instantly

The transport rate of a molecule across the red blood cell membrane is correlated with its lipophilicity (Schanker et al., 1961).

These tracers include [15O]H2O, [18F]EF5, [11C]FETNIM, [11C]HED ([11C]hydroxyephedrine), [11C]metomidate, [11C]MP4A and [11C]MP4B, [11C]FLB-457, [18F]CFT ([18F]WIN-35428).

Plasma-to-blood ratio for tracers which distribute equally in water
          spaces of plasma and red blood cells

Figure. Plasma-to-blood ratio is only dependent on hematocrit, if tracer, and possible label-carrying metabolites, are distributed instantly and equally in the water spaces of plasma and red blood cells.

Notice that even if the tracer can penetrate RBC membrane quickly, the concentration inside RBCs may still be negligible because of high plasma protein binding. In that case follow the case for tracers that persist in plasma.

Note that some radioactive metabolites may persist in the plasma, or are trapped in blood cells, and that the ratio at equilibrium may be very different from 1. In these cases, the blood-to-plasma ratio may have to be measured at several time points (for example [11C]L-deprenyl-D2), fitted to an appropriate function (for example a sigmoidal function), and thereafter blood-to-plasma conversion is done using the individual or population average function.

[11C]diprenorphine plasma-to-blood ratio is initially close to 1, and increases then slowly in linear fashion (Jones et al., 1994).

[11C]Metomidate is an example of tracer which, along with its radioactive metabolites, stays in the blood water spaces; the blood-to-plasma ratio is only dependent on blood hematocrit and water contents of blood cells (63%) and plasma (94%).

[11C]Carfentanil and other fentanyl analogs penetrate the RBC membrane easily. Its plasma-to-blood ratio is dependent on the relative binding to plasma proteins and proteins in RBCs. The radioactive metabolites of [11C]carfentanil seem to be also found inside the red blood cells, with less affinity to the plasma proteins:

Average plasma-to-blood ratio in [11C]carfentanil PET studies Average RBC-to-plasma ratio in [11C]carfentanil PET studies

Figure. “Hill type” functions representing population average of plasma-to-blood and RBC-to-plasma ratio in [11C]carfentanil studies (unpublished data from TPC).

[11C]PBR28 and its radioactive metabolites equilibrate between RBC and plasma water spaces, but in addition the parent tracer binds specifically to its target receptor TSPO in blood cells and plasma proteins. Therefore, the blood-to-plasma ratio is not only time-dependent, but also different in the high-, mixed-, and low affinity binding (HAB, MAB, and LAB, respectively) subpopulations.

Blood-to-plasma ratio in HAB, MAB, and LAB in [11C]PBR28 PET study Plasma-to-blood ratio in HAB, MAB, and LAB in [11C]PBR28 PET study

Figure. “Hill type” functions representing subpopulation averages of blood-to-plasma and plasma-to-blood ratio in [11C]PBR28 studies (unpublished data from TPC).

Tracer penetrates RBC membrane slowly

These PET tracers include [18F]FDOPA, [11C]DOPA, [18F]FBPA, [11C]MeAIB, and [11C]methionine, and probably also most other amino acid tracers. Distribution of glucose between plasma and blood cells differs between animal species (Andreen-Svedberg, 1933). In humans and pigs glucose tracer [18F]FDG equilibrates between RBC and plasma water spaces rapidly (less than one minute), but continues to be slowly accumulated in the RBCs; in rats and some other animals the transport may be slower (Gjedde, 1983; Buxton, 1999; Weber et al., 2002; Wu et al., 2007; Huang et al., 2017).

Average plasma-to-blood ratio in [18F]FDOPA PET studies Average plasma-to-blood ratio in [18F]FBPA PET studies

Figure. Functions representing population average of plasma-to-blood ratio in [18F]FDOPA and [18F]FBPA studies (unpublished data from TPC).

Also with these tracers, plasma-to-blood ratio often starts to increase during the PET scan. This is indicative of the appearance of labelled metabolites which do not penetrate the RBC membrane and therefore accumulate in the plasma.


Software for converting blood to plasma and plasma to blood

For at least the following tracers, b2plasma and p2blood can be used to make the conversions from blood to plasma or from plasma to blood:

Notice that (arterial) blood TAC calculated this way does not represent the average TAC of blood in tissue, which consists of arterial and venous blood in unknown proportion, and also the local venous blood TAC is unknown.

Applications fit_bpr and fit_sigm may be useful in fitting functions to blood-to-plasma or RBC-to-plasma ratio data.


See also:



References:

Buxton DB. Glucose permeability in nonprimate erythrocytes. J Nucl Med. 1999;40(12):2125-2126.

Gjedde A. Modulation of substrate transport to the brain. Acta Neurol Scand. 1983; 67: 3-25. doi: 10.1111/j.1600-0404.1983.tb04541.x.

Hinderling PH. Red blood cells: a neglected compartment in pharmacokinetics and pharmacokinetics. Pharmacol Rev. 1997; 49(3): 279-295.

Hinz R, Bhagwagar Z, Cowen PJ, Cunningham VJ, Grasby PM. Validation of a tracer kinetic model for the quantification of 5-HT2A receptors in human brain with [11C]MDL 100,907. J Cereb Blood Flow Metab. 2007; 27: 161-172.

Koeppe RA. Quantitative functional imaging using positron computed tomography and rapid parameter estimation techniques. Thesis (Ph.D.), The University of Wisconsin, Madison, 1984.

Lee J-S, Su K-H, Lin J-C, Chuang Y-T, Chueh H-S, Liu R-S, Wang S-J, Chen J-C. A novel blood-cell-two-compartment model for transferring a whole blood time activity curve to plasma in rodents. Comput Methods Programs Biomed. 2008; 92(3): 299-304.

Lubberink M, Boellaard R, Greuter HNJM, Lammertsma AA. Effect of uncertainty in plasma metabolite levels on kinetic analysis of [11C]flumazenil and [11C](R)-PK11195 PET studies. Neuroimage 2004; 22: T119-T120.

Nahmias C, Wahl LM, Amano S, Asselin M-C, Chirakal R. Equilibration of 6-[18F]fluoro-L-m-tyrosine between plasma and erythrocytes. J Nucl Med. 2000; 41: 1636-1641.

Paixão P, Gouveia LF, Morais JAG. Prediction of drug distribution within blood. Eur J Pharm Sci. 2009; 36: 544-554. doi: 10.1016/j.ejps.2008.12.011.

Weber B, Burger C, Biro P, Buck A. A femoral arteriovenous shunt facilitates arterial whole blood sampling in animals. Eur J Nucl Med. 2002; 29: 319-323.



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Updated at: 2018-11-21
Written by: Vesa Oikonen, Tuula Tolvanen, Kaisa Liukko, Pauliina Luoto, Anne Roivainen