You can read up on PQQ here: https://www.lifeextension.com/vitamins-supplements/item01647/pqq-caps

Pyrroloquinoline-Quinone, PQQ, is a supplement that was developed for other uses. Howver, the Cleveland Clinic tested its use for Neuropathy, in conjunction with CoQ10. The National Institute of Health(NIH), has referenced other supplements being tested, but the Cleveland Clinic study seemed to show promising benefits. I once had the link to that study, but my laptop was stolen and it wasn’t on my backup. Haven’t been able to find it again since then.