Clinical validation of the novel HDAC6 radiotracer [18F]EKZ-001 in the human brain

Abstract

Purpose: Histone deacetylase 6 (HDAC6) is a cytoplasmic enzyme that modulates intracellular transport and protein quality control. Inhibition of HDAC6 deacetylase activity has shown beneficial effects in disease models, including Alzheimer's disease and amyotrophic lateral sclerosis. This first-in-human positron emission tomography (PET) study evaluated the brain binding of [¹⁸F]EKZ-001 ([¹⁸F]Bavarostat), a radiotracer selective for HDAC6, in healthy adult subjects.

Methods: Biodistribution and radiation dosimetry studies were performed in four healthy subjects (2M/2F, 23.5 ± 2.4 years) using sequential whole-body PET/CT. The most appropriate kinetic model to quantify brain uptake was determined in 12 healthy subjects (6M/6F, 57.6 ± 3.7 years) from 120-min dynamic PET/MR scans using a radiometabolite-corrected arterial plasma input function. Four subjects underwent retest scans (2M/2F, 57.3 ± 5.6 years) with a 1-day interscan interval to determine test-retest variability (TRV). Regional volume of distribution (V_T) was calculated using one-tissue and two-tissue compartment models (1-2TCM) and Logan graphical analysis (LGA), with time-stability assessed. V_T differences between males and females were evaluated using volume of interest and whole-brain voxel-wise approaches.

Results: The effective dose was 39.1 ± 7.0 μSv/MBq. Based on the Akaike information criterion, 2TCM was the preferred model compared to 1TCM. Regional LGA V_T were in agreement with 2TCM V_T, however demonstrated a lower absolute TRV of 7.7 ± 4.9%. Regional V_T values were relatively homogeneous with highest values in the hippocampus and entorhinal cortex. Reduction of acquisition time was achieved with a 0 to 60-min scan followed by a 90 to 120-min scan. Males demonstrated significantly higher V_T than females in the majority of cortical and subcortical brain regions. No relevant radiotracer related adverse events were reported.

Conclusion: [¹⁸F]EKZ-001 is safe and appropriate for quantifying HDAC6 expression in the human brain with Logan graphical analysis as the preferred quantitative approach. Males showed higher HDAC6 expression across the brain compared to females.

Keywords: First-in-human; HDAC6; Neuroimaging; Positron emission tomography; [18F]Bavarostat; [18F]EKZ-001.

Fig. 1

Whole-body time-activity distribution of [¹⁸F]EKZ-001 in subject number 1 (male, Table 1, cohort A), with representative coronal slices. PET image intensities are expressed as standard uptake value (SUV) and are visualized relative to the maximum color table values as indicated in the scale bar, to account for physical radiotracer decay. The times (min) indicate the start of the whole-body scan relative to radiotracer injection

Fig. 2

Metabolization rate of [¹⁸F]EKZ-001 for all subjects from cohorts B-C (mean with error bars ± SD) with average data points for males and females separately (a), one-tissue compartment model (1TCM) and two-tissue compartment model (2TCM) Akaike information criterion (AIC) values for model fitting to time activity curves (TACs) of different brain regions for dynamic [¹⁸F]EKZ-001 PET scans (n = 16 scans, 8M/8F inclusive of 2M/2F with two scans each, cohorts B–C) with a 120-min acquisition time (b) and representative 1TCM and 2TCM fitting results (subject 5, scan 1, see Table 1, cohort B) for the composite cortical TAC (c)

Fig. 3

Average parametric Logan graphical analysis (LGA) V_T datasets for a 120-min [¹⁸F]EKZ-001 PET scan (n = 6 subjects per group, cohorts B–C, only the first scan was considered for subjects with 2 scans) for males (a) and females (b)

Fig. 4

Multiple comparison plot of male and female Logan graphical analysis (LGA) V_T values (n = 6 subjects per group, cohorts B–C, only the first scan was considered for subjects with 2 scans), presenting the mean difference and 95% confidence interval of the difference

Fig. 5

Significant clusters of increased Logan graphical analysis (LGA) V_T values in males compared to females (n = 6 subjects per group, cohorts B–C, only the first scan was considered for subjects with 2 scans) with a threshold set at p_{height uncorr} < 0.001 and p_{cluster FWE-corr} < 0.05 (no threshold kext used for the cluster extent) shown by surface rendering (a) and axial, coronal and sagittal slices (b)

Fig. 6

Graphical representation of the time stability of Logan graphical analysis (LGA) V_T for a coffee-break protocol (0–60/90–120 min) and reduced acquisition times (0–[60–120] min) compared to a 120-min dynamic acquisition protocol. Test-retest variability (TRV) defined as $2\times \left(V_T^{\mathit{\text{test}}}-V_T^{\mathit{\text{retest}}}\right)/\left(V_T^{\mathit{\text{test}}}+V_T^{\mathit{\text{retest}}}\right)$ for test-retest datasets (4 subjects, 2M/2F, cohort B) with dotted lines corresponding to ± 10% TRV (a) and bias defined as the relative difference between LGA V_T values using a reduced and 120-min scan time (n = 12 subjects, 6M/6F, cohorts B–C, only the first scan was considered for subject with 2 scans) with dotted lines corresponding to ± 10% bias (b)