Thus, metabolites of peptide hormones including GLP-1 that are often considered inactive may provide a means of manipulating key aspects of receptor function and a novel therapeutic strategy.
http://plosone.org/article/info:doi/10.1371/journal.pone.0047936#article1.front1.article-meta1.abstract1.p1

The authors would like to fully acknowledge the excellent and comprehensive study of Wootten et al (Mol Pharmacol, 82: 281-290, Allosteric Modulation of Endogenous Metabolites as an Avenue for Drug Discovery) that clearly demonstrates the phenomenon subsequently described in our own work. We apologise to the authors for missing the published ahead-of-print version, that preceded submission of our own work. We are delighted to support the work of such a large and prestigious group and importantly, hope that allosteric modulation of endogenous metabolites does indeed provide a very real hope for future drug development.