Indole-13C is the 13C labeled Indole[1]. Indole is an endogenous metabolite.
References
[1] Russak EM, et al. Impact of Deuterium Substitution on the Pharmacokinetics of Pharmaceuticals. Ann Pharmacother. 2019 Feb;53(2):211-216. DOI:10.1177/1060028018797110
