Preclinical evaluation of [11 C]L-235 as a radioligand for Positron Emission Tomography cathepsin K imaging in bone.

The cathepsin K (CatK) enzyme is abundantly expressed in osteoclasts, and CatK inhibitors have been developed for the treatment of osteoporosis. In our effort to support discovery and clinical evaluations of a CatK inhibitor, we sought to discover a radioligand to determine target engagement of the enzyme by therapeutic candidates using positron emission tomography (PET). L-235, a potent and selective CatK inhibitor, was labeled with carbon-11. PET imaging studies recording baseline distribution of [11 C]L-235, and chase and blocking studies using the selective CatK inhibitor MK-0674 were performed in juvenile and adult nonhuman primates (NHP) and ovariectomized rabbits. Retention of the PET tracer in regions expected to be osteoclast-rich compared with osteoclast-poor regions was examined. Increased retention of the radioligand was observed in osteoclast-rich regions of juvenile rabbits and NHP but not in the adult monkey or adult ovariectomized rabbit. Target engagement of CatK was observed in blocking studies with MK-0674, and the radioligand retention was shown to be sensitive to the level of MK-0674 exposure. [11 C]L-235 can assess target engagement of CatK in bone only in juvenile animals. [11 C]L-235 may be a useful tool for guiding the discovery of CatK inhibitors.

Development of an LC-MS/MS assay for quantification of intact INSL3 in rat plasma.

Background: Relatively large disulfide-linked polypeptides can serve as signaling molecules for a diverse array of biological processes and may be studied in animal models to investigate their function in vivo. The aim of this work was to develop an LC-MS/MS assay to measure a model peptide, INSL3, in rat plasma. Results: A dual enrichment strategy incorporating both protein precipitation and solid phase extraction was utilized to isolate INSL3 from rat plasma, followed by targeted LC-MS/MS detection. The method was able to measure full-length INSL3 (6.1 kDa) down to 0.2 ng/ml with acceptable accuracy and precision. Conclusion: The final assay was applied to support an exploratory pharmacokinetic study to evaluate steady-state concentrations of dosed INSL3 in rat plasma.

High-resolution peripheral quantitative computed tomography and finite element analysis of bone strength at the distal radius in ovariectomized adult rhesus monkey demonstrate efficacy of odanacatib and differentiation from alendronate.

Translational evaluation of disease progression and treatment response is critical to the development of therapies for osteoporosis. In this study, longitudinal in-vivo monitoring of odanacatib (ODN) treatment efficacy was compared to alendronate (ALN) in ovariectomized (OVX) non-human primates (NHPs) using high-resolution peripheral computed tomography (HR-pQCT). Treatment effects were evaluated using several determinants of bone strength, density and quality, including volumetric bone mineral density (vBMD), three-dimensional structure, finite element analysis (FEA) estimated peak force and biomechanical properties at the ultradistal (UD) radius at baseline, 3, 6, 9, 12, and 18 months of dosing in three treatment groups: vehicle (VEH), low ODN (2 mg/kg/day, L-ODN), and ALN (30 µg/kg/week). Biomechanical axial compression tests were performed at the end of the study. Bone strength estimates using FEA were validated by ex-vivo mechanical compression testing experiments. After 18months of dosing, L-ODN demonstrated significant increases from baseline in integral vBMD (13.5%), cortical thickness (24.4%), total bone volume fraction BV/TV (13.5%), FEA-estimated peak force (26.6%) and peak stress (17.1%), respectively. Increases from baseline for L-ODN at 18 months were significantly higher than that for ALN in DXA-based aBMD (7.6%), cortical thickness (22.9%), integral vBMD (12.2%), total BV/TV (10.1%), FEA peak force (17.7%) and FEA peak stress (11.5%), respectively. These results demonstrate a superior efficacy of ODN treatment compared to ALN at the UD radii in ovariectomized NHPs.

Discovery of MK-3168: A PET Tracer for Imaging Brain Fatty Acid Amide Hydrolase.

We report herein the discovery of a fatty acid amide hydrolase (FAAH) positron emission tomography (PET) tracer. Starting from a pyrazole lead, medicinal chemistry efforts directed toward reducing lipophilicity led to the synthesis of a series of imidazole analogues. Compound 6 was chosen for further profiling due to its appropriate physical chemical properties and excellent FAAH inhibition potency across species. [(11)C]-6 (MK-3168) exhibited good brain uptake and FAAH-specific signal in rhesus monkeys and is a suitable PET tracer for imaging FAAH in the brain.