By Preety Panwar, Postdoctoral Fellow in Bromme Lab, CBR

Excessive breakdown of collagen leads to the condition commonly referred to as osteoporosis. This condition affects ~50% of women aged 50 years or older, costing an estimated 17-20 billion dollars annually in the US alone. Excessive production of cathepsin K (catK) is a known cause of osteoporosis, as its primary role is to digest collagen, a matrix protein that forms 80% of all bone tissue. This makes the enzyme a very attractive target for drug design.

Various CatK inhibitors have been evaluated in clinical trials of osteoporosis. Some have shown effectiveness by increasing bone density and reducing fracture rates. However, they have all failed clinical trials because of unpredicted severe side effects, such as cardiovascular complications and skin fibrosis. The most thoroughly investigated CatK inhibitor to date is odanacatib (ODN). It is a highly specific CatK inhibitor that made it to Phase III clinical trials before being abandoned due to off-target effects. We do not know why these side effects arise, and understanding them is hindered by the absence of suitable animal models for drug evaluation.

Mice that have been genetically modified to be deficient in CatK have long been used to understand the role of CatK in bone breakdown. These mice do not however, display the typical symptoms seen in humans with inherited CatK deficiency and their response to ODN treatment is also much weaker. Thus, it is very important to understand how the structure of mouse CatK (mCatK) differs from that of humans (hCatK). This may help us to understand why hCatK inhibitors appear to bind differently in mice.

In an article recently published in the Biochemical Journal, Simon Law, a PhD student, and his colleagues in Dr. Bromme’s Lab at the CBR, were able to shed some light on the key differences. To begin with, they resolved the first inhibitor-free three-dimensional crystal structures of mouse and human CatK. This had been considered an impossible task due to catK propensity to digest itself quite rapidly. They then compared the two structures with a third structure of hCatK bound to the ODN inhibitor. This allowed them to identify sites and residues on the mCatK that acted to reduce the binding of the ODN inhibitor. By altering these residues, they were then able improve the binding of ODN to mCatK making it equivalent or similar to how ODN binds to the human version. They also tested the modified mCatK with a different inhibitor, balicatib, and saw similar results of improved binding.

These tests were essential to allow the scientific community to identify how to generate mice with genetically modified CatK, such that they more closely correspond to the results of human clinical trials. These mice will enable researchers to better evaluate the adverse effects of CatK inhibitors and to explore means of avoiding them before these drugs enter the expensive clinical pipeline. Hopefully, this breakthrough will lead to the future development of a more effective treatment for osteoporosis with minimal side effects.