De novo design of tunable, pH-driven conformational changes.

The ability of naturally occurring proteins to change conformation in response to environmental changes is critical to biological function. Although there have been advances in the de novo design of stable proteins with a single, deep free-energy minimum, the design of conformational switches remains challenging. We present a general strategy to design pH-responsive protein conformational changes by precisely preorganizing histidine residues in buried hydrogen-bond networks. We design homotrimers and heterodimers that are stable above pH 6.5 but undergo cooperative, large-scale conformational changes when the pH is lowered and electrostatic and steric repulsion builds up as the network histidine residues become protonated. The transition pH and cooperativity can be controlled through the number of histidine-containing networks and the strength of the surrounding hydrophobic interactions. Upon disassembly, the designed proteins disrupt lipid membranes both in vitro and after being endocytosed in mammalian cells. Our results demonstrate that environmentally triggered conformational changes can now be programmed by de novo protein design.

Disruption of integrin-fibronectin complexes by allosteric but not ligand-mimetic inhibitors.

Failure of Arg-Gly-Asp (RGD)-based inhibitors to reverse integrin-ligand binding has been reported, but the prevalence of this phenomenon among integrin heterodimers is currently unknown. In the present study we have investigated the interaction of four different RGD-binding integrins (α5ß1, αVß1, αVß3 and αVß6) with fibronectin (FN) using surface plasmon resonance. The ability of inhibitors to reverse ligand binding was assessed by their capacity to increase the dissociation rate of pre-formed integrin-FN complexes. For all four receptors we showed that RGD-based inhibitors (such as cilengitide) were completely unable to increase the dissociation rate. Formation of the non-reversible state occurred very rapidly and did not rely on the time-dependent formation of a high-affinity state of the integrin, or the integrin leg regions. In contrast with RGD-based inhibitors, Ca2+ (but not Mg2+) was able to greatly increase the dissociation rate of integrin-FN complexes, with a half-maximal response at ~0.4 mM Ca2+ for αVß3-FN. The effect of Ca2+ was overcome by co-addition of Mn2+, but not Mg2+. A stimulatory anti-ß1 monoclonal antibody (mAb) abrogated the effect of Ca2+ on α5ß1-FN complexes; conversely, a function-blocking mAb mimicked the effect of Ca2+. These results imply that Ca2+ acts allosterically, probably through binding to the adjacent metal-ion-dependent adhesion site (ADMIDAS), and that the α1 helix in the ß subunit I domain is the key element affected by allosteric modulators. The data suggest an explanation for the limited clinical efficacy of RGD-based integrin antagonists, and we propose that allosteric antagonists could prove to be of greater therapeutic benefit.

The Health Technology Assessment of companion diagnostics: experience of NICE.

Companion diagnostics are used to aid clinical decision making to identify patients who are most likely to respond to treatment. They are becoming increasingly important as more new pharmaceuticals receive licensed indications that require the use of a companion diagnostic to identify the appropriate patient subgroup for treatment. These pharmaceuticals have proven benefit in the treatment of some cancers and other diseases, and also have potential to precisely tailor treatments to the individual in the future. However, the increasing use of companion diagnostics could place a substantial burden on health system resources to provide potentially high volumes of testing. This situation, in part, has led policy makers and Health Technology Assessment (HTA) bodies to review the policies and methods used to make reimbursement decisions for pharmaceuticals requiring companion diagnostics. The assessment of a pharmaceutical alongside the companion diagnostic used in the clinical trials may be relatively straightforward, although there are a number of challenges associated with assessing pharmaceuticals where a range of alternative companion diagnostics are available for use in routine clinical practice. The UK HTA body, the National Institute for Health and Care Excellence (NICE), has developed policy for considering companion diagnostics using its Technology Appraisal and Diagnostics Assessment Programs. Some HTA bodies in other countries have also adapted their policies and methods to accommodate the assessment of companion diagnostics. Here, we provide insight into the HTA of companion diagnostics for reimbursement decisions and how the associated challenges are being addressed, in particular by NICE. See all articles in this CCR Focus section, "The Precision Medicine Conundrum: Approaches to Companion Diagnostic Co-development."