There is currently much interest in developing photoswitchable inhibitors and fluorogenic probes of medicinally important enzymes and biological targets as a means for targeted therapeutic intervention and to study the associated biology. Here we target tuberculosis (TB), the world’s most lethal infectious agent which causes about 2 million deaths annually (World Health Orginisation, 2018). Mycobacterium tuberculosis (Mtb), the causative agent of TB, evades host innate immunity and causes disease by replication within macrophages (Pieters, 2008). Once assimilated into macrophages, Mtb can modulate macrophage responses and interfere with dendritic cells (immune response cells), enabling the pathogen to escape early detection of an immunity response (Russell, 2001; Ehrt and Schnappinger, 2009). Hip1, a Mtb cell-wall associated serine protease, has very recently been linked to these immunity evasion pathways (Naffin-Olivos et al., 2014; Naffin-Olivos et al., 2017). Studies of mice infected with Mtb containing mutant Hip1 had increased survival rates and reduced lung pathology compared to those infected with wild type Mtb (Naffin-Olivos et al., 2017), indicating Hip1 as a valuable therapeutic target.

We have very recently identified (unpublished) several extremely potent (nM-pM K_i range) tripeptide peptidomimetic inhibitors of Hip1 and tested them in vitro against both Mtb-infected murine macrophage and human cell lines. One of these inhibitors demonstrates good growth inhibition of Mtb infected in macrophages, while displaying no measurable cytotoxicity towards macrophage and liver cells (HepG2). This is an exciting finding and we are now developing photoswitchable azobenzene-containing derivatives of our most potent inhibitors to selectively limit activity to the site of infection. We are also investigating the use of fluorogenic activity probes (Lentz et al., 2016) to probe cell-permeability and to guide the development of new inhibitors suitable for testing efficacy and cytotoxicity in vivo in mice.