# Correction of plasma TAC for metabolites

It is common that the PET tracer is metabolized in the liver, kidneys or other parts of the body already during the PET scan, and one or more of the metabolites is still carrying the isotope label. If labeled metabolites are found in the plasma in significant amounts, their proportion has to be subtracted from the plasma curve, because only the concentration of parent tracer can be used as input function in quantitative analysis of the tracer kinetics.

In brain studies the radioactive metabolites, that usually are
more polar than the authentic tracer, do not usually pass the
blood-brain barrier (BBB), but in other tissues,
especially in oncological studies, marked uptake of radioactive metabolite(s) can be observed.
In those cases the plasma concentrations of both the parent tracer and the radioactive metabolite
may have to included in the compartmental model or spectral analysis
(Tomasi et al., 2012; Ichise et al., 2016).
Small polar radiometabolites, such as [^{11}C]formaldehyde and
[^{11}C]CO_{2} can pass even the
BBB, and substantially affect the brain tissue concentrations and reduce the signal-to-background
ratio (Johansen et al., 2018).

## Metabolite correction in TPC

The fractions of authentic (parent) tracer in plasma must be written in an ASCII file (fraction data). A mathematical function can be fitted to these fractions. Total radioactivity in plasma (PTAC) is measured from arterial plasma samples. With that and the fitted parent fractions, metabolite corrected plasma curve can be calculated using metabcor. TACs of radioactive metabolites in plasma can also be saved, if necessary.

**Figure 1.** Example of plasma metabolite correction in [^{11}C]flumazenil
study: each plasma concentration concentration (black) is multiplied by the parent tracer fraction
at each sample time point; result is the curve of unchanged (parent) radioligand concentration in
plasma (red).

## Alternative metabolite correction methods

### Mathematical metabolite correction

For references, see Burger and Buck (1996), and Sanabria-Bohórquez et al. (2000).

### Population based methods

Ideally, fractions of plasma metabolites should be measured for each person participating in
a PET study. However, the measured fraction curves are sometimes noisy, or there are missing samples.
One alternative is to calculate population average curve of
the fractions of parent tracer in the plasma, if the inter-individual variation in the rate of
metabolism is small. Population average must be determined from a group that is comparable to the
study population by their age, sex, and body weight. For example, for rate of metabolism of
[^{18}F]FDPN a significant gender difference has been found (Henriksen et al., 2006).

The population average fraction curve can be fitted to a function, for example to the “Hill-type” or power or exponential functions, if there were only few samples or if the fraction curve must be extrapolated. In the fitting, use the weights that were written in the mean fraction curve.

## See also:

- Fractions of authentic tracer in plasma
- Converting percentage values to fractions in plasma parent fraction files
- Processing input data
- [
^{15}O]O_{2}metabolite correction - [
^{11}C]CO_{2}as a metabolite

## References:

Burger C, Buck A. Tracer kinetic modelling of receptor data with mathematical metabolite
correction. *Eur J Nucl Med.* 1996; 23(5): 539-545.

Henriksen G, Spilker ME, Sprenger T, Hauser AI, Platzer S, Boecker H, Toelle TR, Schwaiger M,
Wester H-J. Gender dependent rate of metabolism of the opioid receptor-PET ligand
[^{18}F]fluoroethyldiprenorphine. *Nuklearmedizin* 2006; 45: 197-200.

Huang SC, Barrio JR, Yu DC, Chen B, Grafton S, Melega WP, Hoffman JM, Satyamurthy N, Mazziotta JC,
Phelps ME. Modelling approach for separating blood time-activity curves in positron emission
tomographic studies. *Phys Med Biol.* 1991; 36(6): 749–761.

Ichise M, Kimura Y, Shimada H, Higuchi M, Suhara T. PET quantification in molecular brain imaging
taking into account the contribution of the radiometabolite entering the brain. In:
Kuge Y et al. (eds.), *Perspectives on Nuclear Medicine for Molecular Diagnosis and
Integrated Therapy*, Springer, 2016.

Nagar S, Argikar UA, Tweedie DJ (eds). *Enzyme Kinetics in Drug Metabolism - Fundamentals and
Applications.* Humana Press, 2014, ISBN 978-1-62703-757-0.

Nelissen N, Warwick J, Dupont P (2012). Kinetic modelling in human brain imaging. In:
*Positron Emission Tomography - Current Clinical and Research Aspects*,
Dr. Chia-Hung Hsieh (Ed.), ISBN: 978-953-307-824-3, InTech.
doi: 10.5772/30052.

Sanabria-Bohórquez SM, Labar D, Levêque P, Bol A, De Volder AG, Michel C, Veraart C.
[^{11}C]Flumazenil metabolite measurement in plasma is not necessary for accurate brain
benzodiazepine receptor quantification. *Eur J Nucl Med.* 2000; 27:1674-1683.

Sestini S, Halldin C, Mansi L, Castagnoli A, Farde L. Pharmacokinetic analysis of plasma curves
obtained after i.v. injection of the PET radioligand [11C] raclopride provides likely explanation
for rapid radioligand metabolism. *J Cell Physiol.* 2012; 227: 1663-1669.

Tomasi G, Kimberley S, Rosso L, Aboagye E, Turkheimer F. Double-input compartmental modeling and
spectral analysis for the quantification of positron emission tomography data in oncology.
*Phys Med Biol.* 2012; 57: 1889-1906.

Tags: Input function, Metabolite correction, Parent fraction, Plasma

Created at: 2008-03-02

Updated at: 2018-02-06

Written by: Vesa Oikonen