Delivery and Release of Small-Molecule Probes in Mitochondria Using Traceless Linkers

Mitochondria-penetrating peptides (MPPs) are specific targeting vectors for the localization of small molecules to the mitochondrial matrix. Mitochondrial targeting of small molecules has enabled the development of a number of potential therapeutics and chemical probes. However, the need for covalent conjugation of small molecules to MPPs can negatively affect the activity of the appended cargo against its cellular target. Here, we describe cleavable linkers designed for the traceless release of chemical cargo from MPPs following mitochondrial transit. The cleavage kinetics of a number of disulfides were investigated using a fluorescent reporter system in order to optimize linker stability for mitochondrial release. The stability of mono- and disubstituted disulfides was determined to be sufficient during transit through the cytosol while still allowing for release of the cargo within 24 h. This linker system successfully released the compound Luminespib, an HSP90 inhibitor, which was deactivated by direct MPP conjugation. The releasable conjugate regenerated Luminespib activity and induced mitochondrial phenotypes of HSP90 inhibition. This linker may prove useful in expanding the repertoire of small molecules that can be used with mitochondrial targeting vectors.