Sylvie Lehmann

Structural Basis for the Exceptional in vivo Efficacy of Bisphosphonate Drugs

To understand the structural basis for bisphosphonate therapy of bone diseases, we solved the crystal structures of human farnesyl pyrophosphate synthase (FPPS) in its unliganded state, in complex with the nitrogen‐containing bisphosphonate (N‐BP) drugs zoledronate, pamidronate, alendronate, and ibandronate, and in the ternary complex with zoledronate and the substrate isopentenyl pyrophosphate (IPP). By revealing three structural snapshots of the enzyme catalytic cycle, each associated with a distinct conformational state, and details about the interactions with N‐BPs, these structures provide a novel understanding of the mechanism of FPPS catalysis and inhibition. In particular, the accumulating substrate, IPP, was found to bind to and stabilize the FPPS–N‐BP complexes rather than to compete with and displace the N‐BP inhibitor. Stabilization of the FPPS–N‐BP complex through IPP binding is supported by differential scanning calorimetry analyses of a set of representative N‐BPs. Among other factors such as high binding affinity for bone mineral, this particular mode of FPPS inhibition contributes to the exceptional in vivo efficacy of N‐BP drugs. Moreover, our data form the basis for structure‐guided design of optimized N‐BPs with improved pharmacological properties.

Allosteric non-bisphosphonate FPPS inhibitors identified by fragment-based discovery.

Bisphosphonates are potent inhibitors of farnesyl pyrophosphate synthase (FPPS) and are highly efficacious in the treatment of bone diseases such as osteoporosis, Pagets disease and tumor-induced osteolysis. In addition, the potential for direct antitumor effects has been postulated on the basis of in vitro and in vivo studies and has recently been demonstrated clinically in early breast cancer patients treated with the potent bisphosphonate zoledronic acid. However, the high affinity of bisphosphonates for bone mineral seems suboptimal for the direct treatment of soft-tissue tumors. Here we report the discovery of the first potent non-bisphosphonate FPPS inhibitors. These new inhibitors bind to a previously unknown allosteric site on FPPS, which was identified by fragment-based approaches using NMR and X-ray crystallography. This allosteric and druggable pocket allows the development of a new generation of FPPS inhibitors that are optimized for direct antitumor effects in soft tissue.

Discovery of Novel Allosteric Non-Bisphosphonate Inhibitors of Farnesyl Pyrophosphate Synthase by Integrated Lead Finding.

Farnesyl pyrophosphate synthase (FPPS) is an established target for the treatment of bone diseases, but also shows promise as an anticancer and anti‐infective drug target. Currently available anti‐FPPS drugs are active‐site‐directed bisphosphonate inhibitors, the peculiar pharmacological profile of which is inadequate for therapeutic indications beyond bone diseases. The recent discovery of an allosteric binding site has paved the way toward the development of novel non‐bisphosphonate FPPS inhibitors with broader therapeutic potential, notably as immunomodulators in oncology. Herein we report the discovery, by an integrated lead finding approach, of two new chemical classes of allosteric FPPS inhibitors that belong to the salicylic acid and quinoline chemotypes. We present their synthesis, biochemical and cellular activities, structure–activity relationships, and provide X‐ray structures of several representative FPPS complexes. These novel allosteric FPPS inhibitors are devoid of any affinity for bone mineral and could serve as leads to evaluate their potential in none‐bone diseases.

A General Strategy for Targeting Drugs to Bone.

Targeting drugs to their desired site of action can increase their safety and efficacy. Bisphosphonates are prototypical examples of drugs targeted to bone. However, bisphosphonate bone affinity is often considered too strong and cannot be significantly modulated without losing activity on the enzymatic target, farnesyl pyrophosphate synthase (FPPS). Furthermore, bisphosphonate bone affinity comes at the expense of very low and variable oral bioavailability. FPPS inhibitors were developed with a monophosphonate as a bone-affinity tag that confers moderate affinity to bone, which can furthermore be tuned to the desired level, and the relationship between structure and bone affinity was evaluated by using an NMR-based bone-binding assay. The concept of targeting drugs to bone with moderate affinity, while retaining oral bioavailability, has broad application to a variety of other bone-targeted drugs.

Blockade of activin type II receptors with a dual anti-ActRIIA/IIB antibody is critical to promote maximal skeletal muscle hypertrophy

Significance We recently reported that activin type II receptors (ActRIIs) blockade using bimagrumab could positively impact muscle wasting in mice and humans. However, the specific role of each individual ActRII at regulating adult muscle mass had not been clarified. Here, we highlight the importance of concomitant neutralization of both ActRIIs in controlling muscle mass. Through comparison with single specificity antibodies, we uncover unique features related to bimagrumab and its neutralizing interactions with both ActRIIA and ActRIIB at the structural and cellular levels and in vivo in adult mice. The need for simultaneous engagement and neutralization of both ActRIIs to generate a strong skeletal muscle response confers unique therapeutic potential to bimagrumab, in the context of muscle wasting conditions. The TGF-β family ligands myostatin, GDF11, and activins are negative regulators of skeletal muscle mass, which have been reported to primarily signal via the ActRIIB receptor on skeletal muscle and thereby induce muscle wasting described as cachexia. Use of a soluble ActRIIB-Fc “trap,” to block myostatin pathway signaling in normal or cachectic mice leads to hypertrophy or prevention of muscle loss, perhaps suggesting that the ActRIIB receptor is primarily responsible for muscle growth regulation. Genetic evidence demonstrates however that both ActRIIB- and ActRIIA-deficient mice display a hypertrophic phenotype. Here, we describe the mode of action of bimagrumab (BYM338), as a human dual-specific anti-ActRIIA/ActRIIB antibody, at the molecular and cellular levels. As shown by X-ray analysis, bimagrumab binds to both ActRIIA and ActRIIB ligand binding domains in a competitive manner at the critical myostatin/activin binding site, hence preventing signal transduction through either ActRII. Myostatin and the activins are capable of binding to both ActRIIA and ActRIIB, with different affinities. However, blockade of either single receptor through the use of specific anti-ActRIIA or anti-ActRIIB antibodies achieves only a partial signaling blockade upon myostatin or activin A stimulation, and this leads to only a small increase in muscle mass. Complete neutralization and maximal anabolic response are achieved only by simultaneous blockade of both receptors. These findings demonstrate the importance of ActRIIA in addition to ActRIIB in mediating myostatin and activin signaling and highlight the need for blocking both receptors to achieve a strong functional benefit.

The human IL-17A/F heterodimer: a two-faced cytokine with unique receptor recognition properties.

IL-17A and IL-17F are prominent members of the IL-17 family of cytokines that regulates both innate and adaptive immunity. IL-17A has been implicated in chronic inflammatory and autoimmune diseases, and anti-IL-17A antibodies have shown remarkable clinical efficacy in psoriasis and psoriatic arthritis patients. IL-17A and IL-17F are homodimeric cytokines that can also form the IL-17A/F heterodimer whose precise role in health and disease remains elusive. All three cytokines signal through the assembly of a ternary complex with the IL-17RA and IL-17RC receptors. Here we report the X-ray analysis of the human IL-17A/F heterodimer that reveals a two-faced cytokine closely mimicking IL-17A as well as IL-17F. We also present the crystal structure of its complex with the IL-17RA receptor. Unexpectedly in view of the much higher affinity of this receptor toward IL-17A, we find that IL-17RA is bound to the “F-face” of the heterodimer in the crystal. Using site-directed mutagenesis, we then demonstrate that IL-17RA can also bind to the “A-face” of IL-17A/F with similar affinity. Further, we show that IL-17RC does not discriminate between the two faces of the cytokine heterodimer either, thus enabling the formation of two topologically-distinct heterotrimeric complexes with potentially different signaling properties.