Quantification of the purinergic P2X₇ receptor with [¹¹C]SMW139 improves through correction for brain-penetrating radiometabolites

Abstract

The membrane-based purinergic 7 receptor (P2X7R) is expressed on activated microglia and the target of the radioligand [11C]SMW139 for in vivo assessment of neuroinflammation. This study investigated the contribution of radiolabelled metabolites which potentially affect its quantification. Ex vivo high-performance liquid chromatography with a radio detector (radioHPLC) was used to evaluate the parent and radiometabolite fractions of [11C]SMW139 in the brain and plasma of eleven mice. Twelve healthy humans underwent 90-min [11C]SMW139 brain PET with arterial blood sampling and radiometabolite analysis. The volume of distribution was estimated by using one- and two- tissue compartment (TCM) modeling with single ( VT) and dual ( VTp) input functions. RadioHPLC showed three major groups of radiometabolite peaks with increasing concentrations in the plasma of all mice and humans. Two radiometabolite peaks were also visible in mice brain homogenates and therefore considered for dual input modeling in humans. 2TCM with single input function provided VT estimates with a wide range (0.10–10.74) and high coefficient of variation (COV: 159.9%), whereas dual input function model showed a narrow range of VTp estimates (0.04–0.24; COV: 33.3%). In conclusion, compartment modeling with correction for brain-penetrant radiometabolites improves the in vivo quantification of [11C]SMW139 binding to P2X7R in the human brain.

Glial reactivity in a mouse model of beta-amyloid deposition assessed by PET imaging of P2X7 receptor and TSPO using [11C]SMW139 and [18F]F-DPA

“…Defining the time frame for quantifying [ 11 C]SMW139 in vivo uptake was complicated by the fast metabolic profile in mice, and the presence of radioactive metabolites in the brain tissue. In a previous study with [ 11 C]SMW139, we detected up to three brain penetrant radiometabolites of [ 11 C]SMW139 in mouse brain homogenates and one radiometabolite more in plasma already at 10 min after tracer injection [ 32 ]; similar findings were also reported by Brumberg and colleagues [ 42 ]. Accordingly, an appropriate time frame needs to be selected in which there is still a reasonable percentage of the parent fraction and as little interference as possible from the radiometabolite fraction.…”

P2X7-receptor binding in new-onset and secondary progressive MS – a [11C]SMW139 PET study

“…Furthermore, signi cantly lower 2TDI AIC scores have only been reported in association with cGM, and 1TDI was associated with robust ts across all examined regions, and with somewhat lower %SE [25]. A 2TDI model may be too complex to t reliably in all cases and regions of interest, while the 1TDI could be more sensitive to inaccuracies in plasma input data.…”

“…The plasma proteins were precipitated by adding 700 µL of acetonitrile to 500 µL of plasma, vortexing and centrifuging (3370 g, 3 min). The protein free supernatant was analyzed with high-performance liquid chromatography (HPLC) using a method described in the supplementary material of Brumberg et al [25] to obtain fractions of intact [ 11 C]SMW139 and its radioactive metabolites for correcting the plasma TAC. A radioactive standard was prepared by spiking the time point 0 plasma supernatant with [ 11 C]SMW139 in order to analyze the correct peak of the chromatograms to correspond to the parent.…”

“…However, the speci c tissue compartment of [ 11 C]SMW139 is small [23], and rapid metabolism results in a signi cant fraction of activity from brain-penetrant radiometabolites [21,24,25]. It is debatable whether a single-input 2TCM improves the t by correcting for unspeci c radiometabolite activity, rather than providing an accurate estimate of speci c parent tracer binding.…”

“…Using a dual-input (DI) function for [ 11 C]SMW139 kinetic modelling which, by also incorporating protein binding of the parent and its radiometabolites, provided estimates of unbound parent tracer V T . Compared to the 2TCM, the 2TDI model produced a narrow range of V T estimates, and it was concluded that correction for brain-penetrant radiometabolites improves the quanti cation of speci c [ 11 C]SMW139 binding [25].…”