Alfred A. Reszka

Generation and selection of novel fully human monoclonal antibodies that neutralize Dickkopf-1 (DKK1) inhibitory function in vitro and increase bone mass in vivo

Wnt/LRP5 signaling is a central regulatory component of bone formative and resorptive activities, and the pathway inhibitor DKK1 is a suppressor of bone formation and bone mass accrual in mice. In addition, augmented DKK1 levels are associated with high bone turnover in diverse low bone mass states in rodent models and disease etiologies in human. However, examination of the precise role of DKK1 in the normal skeleton and in higher species requires the development of refined DKK1-specific pharmacological tools. Here, we report the strategy resulting in isolation of a panel of fully human anti-DKK1 antibodies applicable to studies interrogating the roles of mouse, rhesus, and human DKK1. Selected anti-DKK1 antibodies bind primate and human DKK-1 with picomolar affinities yet do not appreciably bind to DKK2 or DKK4. Epitopes mapped within the DKK1 C-terminal domain necessary for interaction with LRP5/6 and consequently effectively neutralized DKK1 function in vitro. When introduced into naïve normal growing female mice, IgGs significantly improved trabecular bone volume and structure and increased both trabecular and cortical bone mineral densities in a dose-related fashion. Furthermore, fully human DKK1-IgG displayed favorable pharmacokinetic parameters in non-human primates. In summary, we demonstrate here a rate-limiting function of physiologic DKK1 levels in the regulation of bone mass in intact female mice, amendable to specific pharmacologic neutralization by newly identified DKK1-IgGs. Importantly the fully human IgGs display a profile of attributes that recommends their testing in higher species and their use in evaluating DKK1 function in relevant disease models.

Bisphosphonate Mechanisms of Action

The nitrogen-containing bisphosphonates are potent and highly effective non-hormonal anti-osteoporotic agents for clinical use in the treatment of post menopausal or glucocorticoid-induced osteoporosis, Paget’s disease, metastatic bone disease and hypercalcemia of malignancy, among others. The Potency of the nitrogen-containing bisphosphonates can partially be attributed to their specific targeting to bone-associated osteoclasts with intermittent dosing at monthly or yearly intervals, the nitrogen-containing bisphosphonates may also label the osteoblast surface, where they remain insert, awaiting the next resorption cycles that will eventually initials at these sites. Through current or future resorption cycles these bisphosphonates can be liberated from the bone surface and taken into the osteoclast interior, where they exert their pharmacological effects as inhibitors of the isoprenoid biosynthetic enzyme, farnesyl diphosphate synthase. This chapter will discuss the utility of this class of drug as effective antifracture agents with a focus on their intriguing mechanism of action.

Chapter 81 – Bisphosphonates: Mechanisms of Action

Publisher Summary The bisphosphonates are a class of drugs used in various diseases of calcium metabolism. This chapter describes the history of the development, chemistry, biological actions, and molecular mechanisms of action of bisphosphonates. It also highlights the newer developments in the field of study of bisphosphonates. Great progress has been made over the past two decades in understanding the mechanism of action of the bisphosphonates. Bisphosphonates are widely used in the treatment of osteoporosis, Pagets disease, tumor-associated bone disease, with potential uses in several other skeletal conditions. Owing to their bone-binding characteristics, bisphosphonates target to the skeleton, where they primarily act by inhibiting osteoclastic bone resorption. Whether they directly affect other cell types such as osteoblasts, osteocytes, and tumor cells in vivo, is still a matter of debate. The simple bisphosphonates, clodronate, etidronate and tiludronate, are intracellularly metabolized to cytotoxic analogues of ATP, whereas the more potent, nitrogen containing bisphosphonates act by inhibiting the enzyme FPP synthase, thereby preventing the prenylation of small GTPases that are necessary for the normal function and survival of osteoclasts. With emerging differences between bisphosphonates in bone affinity and enzyme inhibitory potency, it is becoming apparent that each bisphosphonate may have a unique pharmacological profile. Unraveling the exact molecular mechanisms underlying differences in efficacy and adverse effects may help to expand the utility of bisphosphonates and ensure their overall safe use in the treatment of a variety of bone diseases.