The Synthesis and kinetic evaluation of lysosomotropic substrate-based probes for cathepsins B and L

Abstract

Members of the cathepsin family of cysteine proteases are gaining interest as potential imaging biomarkers and drug targets due to their multifunctional roles in a range of diseases such as cancer, asthma and arthritis. Cathepsins are also involved in important regulatory processes such as cell recycling, prohormone activation, and wound healing. Developing imaging agents with the ability to assess cathepsin activity in vivo is important for identifying the specific roles these enzymes have in disease processes. We report here novel prodrug-inspired probes for Cathepsins B and L that employ weakly basic aminoquinoline reporter groups intended to be lysosomotropic once released from the enzyme, meaning they are expected to be retained in the acidic lysosome where the majority of cysteine cathepsins are primarily active. To evaluate this approach to probe design we first synthesized a series of prodrug inspired substrates using the self-immolative linker p-aminobenzyl alcohol conjugated to four different aminoquinolines as potential reporters of enzyme activity. We then developed a convenient HPLC method to estimate kcat/KM values for well-established fluorogenic substrates to first validate our new HPLC method. Once a reliable HPLC method was developed for fluorogenic substrates consistent with data obtained on a plate reader, we then determined kcat/KM values for all novel quinoline-based probe candidates. All compounds were excellent substrates of both CTB and CTL, and three candidates were hydrolyzed with particularly efficient kcat/KM values by CTL. This suggests that efficiently hydrolyzed prodrug-inspired probes bearing aminoquinoline reporters could potentially be adapted into substrate-based PET imaging agents which offer an amplification of signal and reporter immobilization in the lysosomes of cancer cells overexpressing CTL.