A binding kinetics study of human adenosine A3 receptor agonists.

The human adenosine A3 (hA3) receptor has been suggested as a viable drug target in inflammatory diseases and in cancer. So far, a number of selective hA3 receptor agonists (e.g. IB-MECA and 2-Cl-IB-MECA) inducing anti-inflammatory or anticancer effects are under clinical investigation. Drug-target binding kinetics is increasingly recognized as another pharmacological parameter, next to affinity, for compound triage in the early phases of drug discovery. However, such a kinetics-driven analysis has not yet been performed for the hA3 receptor. In this study, we first validated a competition association assay for adenosine A3 receptor agonists to determine the target interaction kinetics. Affinities and Kinetic Rate Index (KRI) values of 11 ribofurano and 10 methanocarba nucleosides were determined in radioligand binding assays. Afterwards, 15 analogues were further selected (KRI <0.70 or KRI >1.35) for full kinetics characterization. The structure-kinetics relationships (SKR) were derived and longer residence times were associated with methanocarba and enlarged adenine N6 and C2 substitutions. In addition, from a kon-koff-KD kinetic map we divided the agonists into three subgroups. A residence time "cliff" was observed, which might be relevant to (N)-methanocarba derivatives' rigid C2-arylalkynyl substitutions. Our findings provide substantial evidence that, next to affinity, additional knowledge of binding kinetics is useful for developing and selecting new hA3R agonists in the early phase of the drug discovery process.

Proteasomal activity-based probes mark protein homeostasis in muscles.

BACKGROUND: Protein homeostasis, primarily regulated by the ubiquitin-proteasome system is crucial for proper function of cells. In tissues of post-mitotic cells, the impaired ubiquitin-proteasome system is found in a wide range of neuromuscular disorders. Activity-based probes (ABPs) measure proteasomal proteolytic subunits and can be used to report protein homeostasis. Despite the crucial role of the proteasome in neuromuscular pathologies, ABPs were not employed in muscle cells and tissues, and measurement of proteasomal activity was carried out in vitro using low-throughput procedures. METHODS: We screened six ABPs for specific application in muscle cell culture using high throughput call-based imaging procedures. We then determined an in situ proteasomal activity in myofibers of muscle cryosections. RESULTS: We demonstrate that LWA300, a pan-reactive proteasomal probe, is most suitable to report proteasomal activity in muscle cells using cell-based bio-imaging. We found that proteasomal activity is two-fold and three-fold enhanced in fused muscle cell culture compared with non-fused cells. Moreover, we found that proteasomal activity can discriminate between muscles. Across muscles, a relative higher proteasomal activity was found in hybrid myofibers whereas fast-twitch myofibers displayed lower activity. CONCLUSIONS: Our study demonstrates that proteasomal activity differ between muscles and between myofiber types. We suggest that ABPs can be used to report disease progression and treatment efficacy.