A highly potent inhibitor of cathepsin K (relacatib) reduces biomarkers of bone resorption both in vitro and in an acute model of elevated bone turnover in vivo in monkeys

Abstract

Cathepsin K is an osteoclast-derived cysteine protease that has been implicated as playing a major role in bone resorption. A substantial body of evidence indicates that cathepsin K is critical in osteoclast-mediated bone resorption and suggests that its pharmacological inhibition should result in inhibition of bone resorption in vivo. Here we report the pharmacological characterization of SB-462795 (relacatib) as a potent and orally bioavailable small molecule inhibitor of cathepsin K that inhibits bone resorption both in vitro in human tissue and in vivo in cynomolgus monkeys. SB-462795 is a potent inhibitor of human cathepsins K, L, and V (K_{i, app} = 41, 68, and 53 pM, respectively) that exhibits 39–300-fold selectivity over other cathepsins. SB-462795 inhibited endogenous cathepsin K in situ in human osteoclasts and human osteoclast-mediated bone resorption with IC₅₀ values of ∼ 45 nM and ∼ 70 nM, respectively. The anti-resorptive potential of SB-462795 was evaluated in normal as well as medically ovariectomized (Ovx) female cynomolgus monkeys. Serum levels of the C- and N-terminal telopeptides of Type I collagen (CTx and NTx, respectively) and urinary levels of NTx were monitored as biomarkers of bone resorption. Administration of SB-462795 to medically ovariectomized or normal monkeys resulted in an acute reduction in both serum and urinary markers of bone resorption within 1.5 h after dosing, and this effect lasted up to 48 h depending on the dose administered. Our data indicate that SB-462795 potently inhibits human cathepsin K in osteoclasts, resulting in a rapid inhibition of bone resorption both in vitro and in vivo in the monkey. These studies also demonstrate the therapeutic potential of relacatib in the treatment of postmenopausal osteoporosis and serves to model the planned clinical trials in human subjects.

Introduction

Despite recent advances in the development of therapies to treat osteoporosis, approximately 40–50% of patients on current treatment regimes still have fractures [48]. Furthermore, with the advent of parathyroid hormone (PTH[1–34], Forteo), it is likely that anti-resorptive and bone formation therapies may be used in combination. While an exact combination or sequential treatment regimen for these therapies has not yet been developed, it will likely depend upon the specific properties of the various anti-resorptive and bone formation therapies. An ideal anti-resorptive agent for combination therapy will have a quick onset and termination of action as well as fast clearance upon treatment withdrawal so that various treatment sequences and modalities can be explored. Clearly, there is still a need for novel anti-resorptive therapeutics with improved efficacy. One such therapeutic target is cathepsin K, a cysteine proteinase that is highly and selectively expressed by osteoclasts [8], [10].

Cathepsin K is a cysteine protease having the capacity to degrade a large number of the key bone matrix proteins and is unique in its ability to cleave both the helical and telopeptide regions of type I collagen [1], [3], [13]. Non-selective cysteine proteinase inhibitors have been known for a number of years to inhibit bone resorption either in vitro or in vivo [7] and had been interpreted to involve cathepsin L. Following identification of cathepsin K, it was reported that elimination of cathepsin K in osteoclasts in vitro using anti-sense oligonucleotides resulted in inhibition of bone resorption [19]. It has been reported that the rare skeletal disorder, pycnodysostosis, is linked to loss of function mutations in the gene encoding cathepsin K [16]. Bone turnover is reduced in these patients, and the bone produced during development is very dense and of poor quality, leading to frequent fractures.

We and others have shown [17], [42] that knockout of cathepsin K in the mouse results in a similar osteopetrosis characterized by increased trabecular number and thickness. In addition, the cortices of the long bones in these animals are significantly thickened. In these mice, areas of demineralized matrix can be observed, suggesting that elimination of cathepsin K allows the removal of the mineral component of bone without the concomitant degradation of organic matrix; a mechanism which is unique to cathepsin K [17]. Elimination of cathepsin K does not appear to have any other developmental consequences. Conversely, overexpression of cathepsin K in mice has been shown to result in high turnover osteopenia of metaphyseal trabecular bone [23]. Thus, a substantial body of evidence indicates that cathepsin K plays a key role in osteoclast-mediated bone resorption and that the elimination of cathepsin K results in inhibition of bone resorption. Based on compelling data, several groups have targeted the inhibition of cathepsin K as a potential treatment for osteoporosis [6], [26].

Efforts at GlaxoSmithKline (GSK) have evolved from the discovery of potent, short acting, ketone-based inhibitors [51] to cyclic ketone-based inhibitors with improved pharmacokinetics [28], [30]. This template was further optimized to provide azepanone-based inhibitors which possessed good oral bioavailability in both rats [27] and monkeys [29]. Full optimization of all developability parameters for the azepanone template was realized in SB-462795 (relacatib) [50]. Herein, we report pharmacological evaluations of SB462795 in cynomolgus monkeys after both subcutaneous and oral dosing that establish in vivo efficacy and serve to model clinical studies in humans that are now under way with this compound. The molecular identity of mature monkey [31] and human cathepsin K enzymes assures the highest confidence in predictions generated in these studies for outcomes in the human trials. This would be in contrast to studies in rodents where large differences have been observed in the behavior of inhibitors with rodent and human cathepsins K [25].