Have (R)-[11C]PK11195 challengers fulfilled the promise? A scoping review of clinical TSPO PET studies

Purpose The prototypical TSPO radiotracer (R)-[C-11]PK11195 has been used in humans for more than thirty years to visualize neuroinflammation in several pathologies. Alternative radiotracers have been developed to improve signal-to-noise ratio and started to be tested clinically in 2008. Here we examined the scientific value of these "(R)-[C-11]PK11195 challengers" in clinical research to determine if they could supersede (R)-[C-11]PK11195. Methods A systematic MEDLINE (PubMed) search was performed (up to end of year 2020) to extract publications reporting TSPO PET in patients with identified pathologies, excluding studies in healthy subjects and methodological studies. Results Of the 288 publications selected, 152 used 13 challengers, and 142 used (R)-[C-11]PK11195. Over the last 20 years, the number of (R)-[C-11]PK11195 studies remained stable (6 +/- 3 per year), but was surpassed by the total number of challenger studies for the last 6 years. In total, 3914 patients underwent a TSPO PET scan, and 47% (1851 patients) received (R)-[C-11]PK11195. The 2 main challengers were [C-11]PBR28 (24%-938 patients) and [F-18]FEPPA (11%-429 patients). Only one-in-ten patients (11%-447) underwent 2 TSPO scans, among whom 40 (1%) were scanned with 2 different TSPO radiotracers. Conclusions Generally, challengers confirmed disease-specific initial (R)-[C-11]PK11195 findings. However, while their better signal-to-noise ratio seems particularly useful in diseases with moderate and widespread neuroinflammation, most challengers present an allelic-dependent (Ala147Thr polymorphism) TSPO binding and genetic stratification is hindering their clinical implementation. As new challengers, insensitive to TSPO human polymorphism, are about to enter clinical evaluation, we propose this systematic review to be regularly updated (living review).