Hurdles in the drug discovery of cathepsin K inhibitors.

There were many hurdles in the drug discovery of cathepsin K inhibitors such as species differences not only in bone metabolism but also in amino acid sequences in the critical site of the target enzyme, discrepancies between PK/PD due to unique tissue distribution of the inhibitor affecting both efficacy and side effects originated from a characteristic intracellular or tissue distribution of some classes of compounds. The value of this new therapeutic approach over the launched indirect competitors should be further clarified from the efficacy and side effect point of view. The cathepsin K inhibitor drug discovery was initiated based on a strong and osteoclast-specific expression of this enzyme. However, the tissues and cells expressing cathepsin K have been expanding as the investigation on pathological conditions progressed with respect to side effects as well as new possible indications.

Effect of cathepsin K inhibitors on bone resorption.

On the basis of the pyrrolopyrimidine core structure that was previously discovered, cathepsin K inhibitors having a spiro amine at the P3 have been explored to enhance the target, bone marrow, tissue distribution. Several spiro structures were identified with improved distribution toward bone marrow. The representative inhibitor 7 of this series revealed in vivo reduction in C-terminal telopeptide of type I collagen in rats and monkeys.

New chemotypes for cathepsin K inhibitors.

Cyano pyrimidine acetylene and cyano pyrimidine t-amine, which belong to a new chemical class, were prepared and tested for inhibitory activities against cathepsin K and the highly homologous cathepsins L and S. The use of novel chemotypes in the development of cathepsin K inhibitors has been demonstrated by derivatives of compounds 1 and 8.

Novel scaffold for cathepsin K inhibitors.

Pyrrolopyrimidine, a novel scaffold, allows to adjust interactions within the S3 subsite of cathepsin K. The core intermediate 10 facilitated the P3 optimization and identified highly potent and selective cathepsin K inhibitors 11-20.

Lysophospholipase I identified as a ghrelin deacylation enzyme in rat stomach.

Ghrelin, discovered in rat stomach as an endogenous growth hormone secretagogue, is octanoylated at the Ser3 residue. Since this octanoylation is essential for the functions of ghrelin, the enzymes that catalyze acylation for ghrelin biosynthesis and deacylation (deactivation step) must be considered as important regulators. We found that rat stomach homogenate contained ghrelin deacylation activity, and we isolated the active fractions by column chromatography. After sequencing and expressing candidate proteins, the ghrelin deacylation enzyme in the stomach was identified as lysophospholipase I (LysoPLA I). The enzyme properties were examined using recombinant rat LysoPLA I expressed in Escherichia coli. K(m) and V(max) values were determined as 6.5 microM and 2.3 micromol/min/mg for ghrelin and 2.2 x 10(2) microM and 0.5 micromol/min/mg for lysophosphatidylcholine (LysoPC), respectively. The deacylation of both substrates was inhibited by methyl arachidonyl fluorophosphonate (MAFP), which is known as an irreversible inhibitor of LysoPLA I. These results reveal that LysoPLA I catalyzes the removal of n-octanoic acid from ghrelin to form des-acyl ghrelin. Identification of the ghrelin deacylation enzyme in the stomach and a deacylation inhibitor will be helpful in investigating ghrelin biosynthesis.