Purpose: Positron emission tomography imaging of zirconium-89-labelled monoclonal antibodies (⁸⁹Zr-Immuno-PET) allows for visualisation and quantification of antibody uptake in tumours in vivo. Patlak linearization provides distribution volume (V_T) and nett influx rate (K_i) values, representing reversible and irreversible uptake, respectively. Standardised uptake value (SUV) and tumour-to-plasma/tumour-to-blood ratio (TPR/TBR) are often used, but their validity depends on the comparability of plasma kinetics and clearances. This study assesses the validity of SUV, TPR and TBR against Patlak K_i for quantifying irreversible ⁸⁹Zr-Immuno-PET uptake in tumours. Methods: Ten patients received 37 MBq 10 mg ⁸⁹Zr-anti-EGFR with 500 mg/m² unlabelled mAbs. Five patients received two doses of 37 MBq ⁸⁹Zr-anti-HER3: 8–24 mg for the first administration and 24 mg–30 mg/kg for the second. Seven tumours from four patients showed ⁸⁹Zr-anti-EGFR uptake, and 18 tumours from five patients showed ⁸⁹Zr-anti-HER3 uptake. SUV_peak, TPR_peak and TBR_peak values were obtained from one to six days p.i. Patlak linearization was applied to tumour time activity curves and plasma samples to obtain K_i. Results: For ⁸⁹Zr-anti-EGFR, there was a small variability along the linear regression line between SUV (− 0.51–0.57), TPR (− 0.06‒0.11) and TBR (− 0.13‒0.16) on day 6 versus K_i. Similar doses of ⁸⁹Zr-anti-HER3 showed similar variability for SUV (− 1.3‒1.0), TPR (− 1.1‒0.53) and TBR (− 1.5‒0.72) on day 5 versus K_i. However, for the second administration of ⁸⁹Zr-anti-HER3 with a large variability in administered mass doses, SUV showed a larger variability (− 1.4‒2.3) along the regression line with K_i, which improved when using TPR (− 0.38–0.32) or TBR (− 0.56‒0.46). Conclusion: SUV, TPR and TBR at late time points were valid for quantifying irreversible lesional ⁸⁹Zr-Immuno-PET uptake when constant mass doses were administered. However, for variable mass doses, only TPR and TBR provided reliable values for irreversible uptake, but not SUV, because SUV does not take patient and mass dose-specific plasma clearance into account.

Introduction: ⁸⁹Zr-immuno-PET (positron emission tomography with zirconium-89-labeled monoclonal antibodies ([⁸⁹Zr]Zr-mAbs)) can be used to study the biodistribution of mAbs targeting the immune system. The measured uptake consists of target-specific and non-specific components, and it can be influenced by plasma availability of the tracer. To find evidence for target-specific uptake, i.e., target engagement, we studied five immune-checkpoint-targeting [⁸⁹Zr]Zr-mAbs to (1) compare the uptake with previously reported baseline values for non-specific organ uptake (ns-baseline) and (2) look for saturation effects of increasing mass doses. Method: ⁸⁹Zr-immuno-PET data from five [⁸⁹Zr]Zr-mAbs, i.e., nivolumab and pembrolizumab (anti-PD-1), durvalumab (anti-PD-L1), BI 754,111 (anti-LAG-3), and ipilimumab (anti-CTLA-4), were analysed. For each mAb, 2–3 different mass doses were evaluated. PET scans and blood samples from at least two time points 24 h post injection were available. In 35 patients, brain, kidneys, liver, spleen, lungs, and bone marrow were delineated. Patlak analysis was used to account for differences in plasma activity concentration and to quantify irreversible uptake (K_i). To identify target engagement, K_i values were compared to ns-baseline K_i values previously reported, and the effect of increasing mass doses on K_i was investigated. Results: All mAbs, except ipilimumab, showed K_i values in spleen above the ns-baseline for the lowest administered mass dose, in addition to decreasing K_i values with higher mass doses, both indicative of target engagement. For bone marrow, no ns-baseline was established previously, but a similar pattern was observed. For kidneys, most mAbs showed K_i values within the ns-baseline for both low and high mass doses. However, with high mass doses, some saturation effects were seen, suggestive of a lower ns-baseline value. K_i values were near zero in brain tissue for all mass doses of all mAbs. Conclusion: Using Patlak analysis and the established ns-baseline values, evidence for target engagement in (lymphoid) organs for several immune checkpoint inhibitors could be demonstrated. A decrease in the K_i values with increasing mass doses supports the applicability of Patlak analysis for the assessment of target engagement for PET ligands with irreversible uptake behavior.

Background: Distribution of mAbs into tumour tissue may occur via different processes contributing differently to the ⁸⁹Zr-mAb uptake on PET. Target-specific binding in tumours is of main interest; however, non-specific irreversible uptake may also be present, which influences quantification. The aim was to investigate the presence of non-specific irreversible uptake in tumour tissue using Patlak linearization on ⁸⁹Zr-immuno-PET data of biopsy-proven target-negative tumours. Data of two studies, including target status obtained from biopsies, were retrospectively analysed, and Patlak linearization provided the net rate of irreversible uptake (K_i). Results: Two tumours were classified as CD20-negative and two as CD20-positive. Four tumours were classified as CEA-negative and nine as CEA-positive. K_i values of CD20-negative (0.43 µL/g/h and 0.92 µL/g/h) and CEA-negative tumours (mdn = 1.97 µL/g/h, interquartile range (IQR) = 1.50–2.39) were higher than zero. Median K_i values of target-negative tumours were lower than CD20-positive (1.87 µL/g/h and 1.90 µL/g/h) and CEA-positive tumours (mdn = 2.77 µL/g/h, IQR = 2.11–3.65). Conclusion: Biopsy-proven target-negative tumours showed irreversible uptake of ⁸⁹Zr-mAbs measured in vivo using ⁸⁹Zr-immuno-PET data, which suggests the presence of non-specific irreversible uptake in tumours. Consequently, for ⁸⁹Zr-immuno-PET, even if the target is absent, a tumour-to-plasma ratio always increases over time.

Introduction: The novel positron emission tomography (PET) imaging tracer, [¹⁸F]F-AraG, targets activated T-cells, offering a potential means to improve our understanding of immune-oncological processes. The aim of this study was to determine the optimal pharmacokinetic model to quantify tumour lesion [¹⁸F]F-AraG uptake in patients with non-small cell lung cancer (NSCLC), and to validate simplified measures at different time intervals against the pharmacokinetic uptake parameter. Methods: Ten patients with early-stage NSCLC and three patients with advanced NSCLC underwent a dynamic PET scan of minimal 60 min. Venous and/or arterial blood sampling was obtained at maximum seven time points. Tumour lesion time activity curves and metabolite-corrected input functions were analysed using single-tissue reversible (1T2k), two-tissue irreversible (2T3k) and two-tissue reversible (2T4k) plasma input models. Simplified uptake measures, such as standardised uptake value (SUV) and tumour-to-blood (TBR) or tumour-to-plasma ratio (TPR), were evaluated for different time intervals. Results: Whole-blood and plasma radioactivity concentrations showed rapid clearance of [¹⁸F]F-AraG. Metabolite analysis revealed a low rate of metabolism, at 70 min p.i., on average, 79% (SD = 9.8%) of the total radioactivity found in blood corresponded to intact [¹⁸F]F-AraG. The time activity curves were best fitted by the 2T3k model. Strong positive correlations were found for SUV (body weight (BW), lean body mass (LBM) or body surface area (BSA) corrected), TBR and TPR for any time interval between 20 and 70 min p.i. against the 2T3k-derived K _i . The correlation of TBR at 60–70 min p.i. with 2T3K-derived K _i (r (df = 20) = 0.87, p < 0.01), was stronger than for SUV_BW (r (df = 20) = 0.80, p < 0.01). Conclusion: Tumour lesion [¹⁸F]F-AraG uptake in patients with NSCLC is characterised by a 2T3k model. TBR and TPR show most potential for simplified quantification of tumour lesion [¹⁸F]F-AraG uptake in patients with NSCLC.

Purpose: Zirconium-89-immuno-positron emission tomography (⁸⁹Zr-immuno-PET) has enabled visualization of zirconium-89 labelled monoclonal antibody (⁸⁹Zr-mAb) uptake in organs and tumors in vivo. Patlak linearization of ⁸⁹Zr-immuno-PET quantification data allows for separation of reversible and irreversible uptake, by combining multiple blood samples and PET images at different days. As one can obtain only a limited number of blood samples and scans per patient, choosing the optimal time points is important. Tissue activity concentration curves were simulated to evaluate the effect of imaging time points on Patlak results, considering different time points, input functions, noise levels and levels of reversible and irreversible uptake. Methods: Based on ⁸⁹Zr-mAb input functions and reference values for reversible (V_T) and irreversible (K_i) uptake from literature, multiple tissue activity curves were simulated. Three different ⁸⁹Zr-mAb input functions, five time points between 24 and 192 h p.i., noise levels of 5, 10 and 15%, and three reference K_i and V_T values were considered. Simulated K_i and V_T were calculated (Patlak linearization) for a thousand repetitions. Accuracy and precision of Patlak linearization were evaluated by comparing simulated K_i and V_T with reference values. Results: Simulations showed that K_i is always underestimated. Inclusion of time point 24 h p.i. reduced bias and variability in V_T, and slightly reduced bias and variability in K_i, as compared to combinations of three later time points. After inclusion of 24 h p.i., minimal differences were found in bias and variability between different combinations of later imaging time points, despite different input functions, noise levels and reference values. Conclusion: Inclusion of a blood sample and PET scan at 24 h p.i. improves accuracy and precision of Patlak results for ⁸⁹Zr-immuno-PET; the exact timing of the two later time points is not critical.

Background: PET scans using zirconium-89 labelled monoclonal antibodies (⁸⁹Zr-mAbs), known as ⁸⁹Zr-immuno-PET, are made to measure uptake in tumour and organ tissue. Uptake is related to the supply of ⁸⁹Zr-mAbs in the blood. Measuring activity concentrations in blood, however, requires invasive blood sampling. This study aims to identify the best delineation strategy to obtain the image-derived blood concentration (IDBC) from ⁸⁹Zr-immuno-PET scans. Methods: PET imaging and blood sampling of two ⁸⁹Zr-mAbs were included, ⁸⁹Zr-cetuximab and ⁸⁹Zr-durvalumab. For seven patients receiving ⁸⁹Zr-cetuximab, PET scans on 1–2 h, 2 and 6 days post-injection (p.i.) were analysed. Five patients received three injections of ⁸⁹Zr-durvalumab. The scanning protocol for the first two injections consisted of PET scanning on 2, 5 and 7 days p.i. and for the third injection only on 7 days p.i. Blood samples were drawn with every PET scan and the sample-derived blood concentration (SDBC) was used as gold standard for the IDBC. According to an in-house developed standard operating procedure, the aortic arch, ascending aorta, descending aorta and left ventricle were delineated. Bland–Altman analyses were performed to assess the bias (mean difference) and variability (1.96 times the standard deviation of the differences) between IDBC and SDBC. Results: Overall, the activity concentration obtained from the IDBC was lower than from the SDBC. When comparing IDBC with SDBC, variability was smallest for the ascending aorta (20.3% and 17.0% for ⁸⁹Zr-cetuximab and ⁸⁹Zr-durvalumab, respectively). Variability for the other regions ranged between 17.9 and 30.8%. Bias for the ascending aorta was − 10.9% and − 11.4% for ⁸⁹Zr-cetuximab and ⁸⁹Zr-durvalumab, respectively. Conclusions: Image-derived blood concentrations should be obtained from delineating the ascending aorta in ⁸⁹Zr-immuno-PET scans, as this results in the lowest variability with respect to sample-derived blood concentrations.