Quantification of fatty acid uptake with [18F]FTHA PET
Long-chain free fatty acids (FFA) are transported in blood bound to albumin. Passive diffusion and facilitated transport via specific membrane-associated and cytosolic fatty acid binding proteins (FABP) are involved in entry into the cells (Frohnert and Bernlohr, 2000). In cells acyl-CoA synthetase (ACS) activates FFA to the fatty acyl-CoA derivative. This is necessary for the major metabolic pathways of FFA metabolism, including β-oxidation and esterification, and therefore net flux through this reaction represents total fatty acid utilization.
The long-chain fatty acid analogue 14(R,S)-[18F]fluoro-6-thia-heptadecanoic acid ([18F]FTHA, DeGrado et al., 1991) enters the cells by the same mechanism as natural fatty acids and undergoes partial metabolism in mitochondria before being trapped. Tissue accumulation of 18F represents total fatty acid utilization, including both storage and β-oxidation. [18F]FTHA production can be automated, according to GMP (Savisto et al., 2017).
[18F]FTHA net influx rate (Ki) or fractional uptake rate (FUR) must be multiplied by FFA concentration in (arterial) plasma to calculate the FFA uptake (utilization) rate. There is a 100-fold range of possible FFA plasma concentrations. To account for the different kinetics of [18F]FTHA and palmitate and other FFAs, the rate index should also be be divided by ”lumped constant” (LC).
Fatty acids in plasma and blood
FFAs in plasma are normally more than 99.99% bound to circulating plasma proteins.
Although FFA transport across erythrocyte plasma membranes is fast, the high plasma protein binding leads to negligible FFA concentration inside the erythrocytes. Covalently bound fatty acids in erythrocytes slowly exchange (halflife 30 min or more) with fatty acids in plasma.
Metabolite correction of plasma TAC
Label-carrying metabolites of [18F]FTHA appear fast in the plasma, and 30 min after injection only about 20-30% of the total radioactivity is due to the parent radiotracer. Metabolism rate may differ between study groups (Supplemental Figure 1 by Labbé et al., 2011a). Fluoride ([18F]F-) is one of the metabolites, which may be seen as bone uptake in the late images. Fluoride distributes from plasma to other tissues, too, which may affect quantitation in case the FFA uptake is relatively low.
Multiple-time graphical analysis for irreversible uptake (Patlak plot) with metabolite corrected plasma input can be used to measure FFA oxidation and storage rate in the fasting state (Iozzo et al., 2003a; Hannukainen et al., 2007; Viljanen et al., 2009; Immonen, et al. 2017). The uptake kinetics of [18F]FTHA are not fully irreversible in the liver, but the slight downward curvature of Patlak plots does not prevent line-fitting to plot data measured 10-32 min after injection (Iozzo et al., 2003a). Uptake rate can be calculated regionally or a Ki image can be computed. If only static image or late scan with few time frames is available, FUR (retention index) image can be computed.
[18F]FTHA has also been administered orally to study organ-specific dietary fatty-acid uptake (Labbé et al., 2011b).
Quantification methods for skeletal muscle have been described by Mäki et al., 1998 and Turpeinen et al., 1999.
Using [18F]FTHA and Patlak graphical analysis has been validated in a pig study, suggesting that in skeletal muscle [18F]FTHA depicts FFA uptake, but not specifically β-oxidation (Takala et al., 2002). [18F]FTHA uptake has been quantified also from a single late PET frame (Hannukainen et al., 2006 and 2007).
Regulation of fatty acid utilization
It is not known what exactly are the roles of passive diffusion and facilitated transport via specific membrane-associated and cytosolic fatty acid binding proteins in entry into cells; at least in certain conditions the fatty acid uptake displays saturation kinetics. Regulation of fatty acid utilization in skeletal muscle during exercise may lie mainly within the entrance into the mitochondria or metabolism within the mitochondria (Kiens and Roepstorff, 2003), although there is evidence that plasmalemmal fatty acid transport in heart and skeletal muscle is regulated for example by contraction, insulin, and leptin (Bonen et al., 2003). It has been shown in humans that a marked storage of lipids in skeletal muscle can be increased by the combination of hyperinsulinemia and elevation of circulating FFAs, but not by one of these conditions alone (Brechtel et al., 2001).
Entry of long-chain fatty acids into adipocytes is mediated by facilitated transport. [18F]FTHA can be used to measure FFA uptake in white adipose tissue (Bucci et al., 2015; Dadson et al., 2017) and brown adipose tissue (Blondin et al., 2017).
Experimental validation studies have shown correlation between trapping of [18F]FTHA and fatty acid oxidation in various conditions (Ebert et al., 1994; Stone et al., 1998; Takala et al., 2002). However, in hypoxic conditions of myocardium [18F]FTHA may not be optimal for measuring changes in β-oxidation (Renstrom et al., 1998).
Patlak plot has been used in human studies to calculate the [18F]FTHA net influx rate (Ki), and further by assuming LC=1, it has been used to estimate myocardial β-oxidation rate of long chain fatty acids without (Taylor et al., 2001; Wallhaus et al., 2001) or with (Ebert et al., 1994; Mäki et al. 1997, 1998; Takala et al., 1999, 2002; Turpeinen et al., 1999; Hannukainen et al., 2007; Tuunanen et al., 2007; Viljanen et al., 2009) plasma metabolite correction. In rodents the [18F]FTHA uptake is not irreversible (DeGrado et al., 1991), preventing the use of Patlak plot in strictly quantitative analysis; Huber et al. (2017) estimated changes in influx rates in rat models with Patlak plot, using as input function the blood curve derived from LV cavity without metabolite correction.
A good review on fatty acid incorporation in the brain is written by Robinson et al. (1992).
The equilibrium unbound fraction of unacylated palmitate in plasma is less than 0.007%, and fatty acid-albumin complex cannot cross the blood-brain barrier. Yet, in rat brain the single-pass extraction of palmitate is about 5%. This means that fatty acids must be released from albumin during the passage through capillaries, and therefore protein-bound fraction is not needed to quantitate the fatty acid uptake in PET studies. Relatively low first-pass extraction also means that fatty acid uptake in the brain is independent of cerebral blood flow.FUR method can be used in quantification of [18F]FTHA uptake rate in the brain (Karmi et al., 2010). Patlak plot has been used in human studies to calculate the [18F]FTHA net influx rate (Ki) and fatty acid uptake (Honkala et al., 2018).
- PET imaging of fatty acid metabolism
- Quantification of fatty acid uptake with [11C]palmitate
- Lumped constant
- Metabolite correction
- Compartmental models
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Updated at: 2018-10-04
Created at: 2005-10-19
Written by: Vesa Oikonen