Structural insights into selective small molecule activation of PKG1α.

cGMP-dependent protein kinase I-α (PKG1α) is a target for pulmonary arterial hypertension due to its role in the regulation of smooth muscle function. While most work has focused on regulation of cGMP turnover, we recently described several small molecule tool compounds which were capable of activating PKG1α via a cGMP independent pathway. Selected molecules were crystallized in the presence of PKG1α and were found to bind to an allosteric site proximal to the low-affinity nucleotide binding domain. These molecules act to displace the switch helix and cause activation of PKG1α representing a new mechanism for the activation and control of this critical therapeutic path. The described structures are vital to understanding the function and control of this key regulatory pathway.

Optimization and Mechanistic Investigations of Novel Allosteric Activators of PKG1α.

Activation of PKG1α is a compelling strategy for the treatment of cardiovascular diseases. As the main effector of cyclic guanosine monophosphate (cGMP), activation of PKG1α induces smooth muscle relaxation in blood vessels, lowers pulmonary blood pressure, prevents platelet aggregation, and protects against cardiac stress. The development of activators has been mostly limited to cGMP mimetics and synthetic peptides. Described herein is the optimization of a piperidine series of small molecules to yield activators that demonstrate in vitro phosphorylation of vasodilator-stimulated phosphoprotein as well as antiproliferative effects in human pulmonary arterial smooth muscle cells. Hydrogen/deuterium exchange mass spectrometry experiments with the small molecule activators revealed a mechanism of action consistent with cGMP-induced activation, and an X-ray co-crystal structure with a construct encompassing the regulatory domains illustrated a binding mode in an allosteric pocket proximal to the low-affinity cyclic nucleotide-binding domain.

Optimization of Inhibitors of Mycobacterium tuberculosis Pantothenate Synthetase Based on Group Efficiency Analysis.

Ligand efficiency has proven to be a valuable concept for optimization of leads in the early stages of drug design. Taking this one step further, group efficiency (GE) evaluates the binding efficiency of each appendage of a molecule, further fine-tuning the drug design process. Here, GE analysis is used to systematically improve the potency of inhibitors of Mycobacterium tuberculosis pantothenate synthetase, an important target in tuberculosis therapy. Binding efficiencies were found to be distributed unevenly within a lead molecule derived using a fragment-based approach. Substitution of the less efficient parts of the molecule allowed systematic development of more potent compounds. This method of dissecting and analyzing different groups within a molecule offers a rational and general way of carrying out lead optimization, with potential broad application within drug discovery.

Optimization of the interligand Overhauser effect for fragment linking: application to inhibitor discovery against Mycobacterium tuberculosis pantothenate synthetase.

Fragment-based methods are a new and emerging approach for the discovery of protein binders that are potential new therapeutic agents. Several ways of utilizing structural information to guide the inhibitor assembly have been explored to date. One of the approaches, application of interligand Overhauser effect (ILOE) observations, is of particular interest, as it does not require the availability of a three-dimensional protein structure and is an NMR-based method that can be applied to targets that cannot be observed directly because of their size. Fragments, as small and often hydrophobic molecules, suffer from problems including compound aggregation in an aqueous environment and nonspecific binding contributions, especially when screened at higher concentrations suitable for ILOE observations. Here we report how this problem can be overcome by applying a step-by-step iterative procedure that includes the application of optimized probe molecules with known binding modes to elucidate the unknown binding modes of fragments. An enzyme substrate with well-characterized binding was used as a starting point, and the relative binding modes of modified fragments derived from ILOE observations were used to guide the fragment linking, leading to a potent inhibitor of our model system, Mycobacterium tuberculosis pantothenate synthetase, a potential drug target. We have supported our NMR data with crystal structures, thus establishing the guidelines for optimizing the ILOE observations. This model study should expand the application of the technique in drug discovery.