Bioactive conformation of luteinizing hormone-releasing hormone: evidence from a conformationally constrained analog.

An analog of luteinizing hormone-releasing hormone containing a gamma-lactam as a conformational constraint has been prepared with the use of a novel cyclization of a methionine sulfonium salt. The analog is more active as a luteinizing hormone-releasing hormone agonist that the parent hormone, and provides evidence for a bioactive conformation containing a beta-turn.

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.

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 approximately 45 nM and approximately 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.

The new partnership of genomics and chemistry for accelerated drug development.

Genomics and combinatorial chemistry are two methods that are revolutionizing drug discovery efforts. Analysis of gene sequences allows identification of novel proteins which are potential therapeutic targets. Recent advances relate to the rate of gene sequencing and data handling resulting from the enormous flow of new gene sequences. Cathepsin K, a cysteine protease involved in bone resorption, was recently identified from a bone cell cDNA library as a potential target for osteoporosis therapy. New sources of chemical agents will rely on advances in high throughput chemical synthesis and structure-based design.

Low levels of cobalamin, epidermal growth factor, and normal prions in multiple sclerosis spinal cord.

We have previously demonstrated that multiple sclerosis (MS) patients have abnormal cerebrospinal fluid (CSF) levels of the key myelin-related molecules cobalamin (Cbl), epidermal growth factor (EGF), and normal cellular prions (PrP(C)s), thus confirming that some CSF abnormalities may be co-responsible for remyelination failure. We determined the levels of these three molecules in post-mortem spinal cord (SC) samples taken from MS patients and control patients. The control SC samples, almost all of which came from non-neurological patients, did not show any microscopic lesions of any type. All of the samples were supplied by the U.K. MS Tissue Bank. The Cbl, EGF, and PrP(C) levels were determined using enzyme-linked immunosorbent assays. The SC total homocysteine level was determined using a competitive immunoenzymatic assay. CSF samples, taken from a further group of MS patients, were used for the assay of holo-transcobalamin (holo-TC) levels. The Cbl, EGF, and PrP(C) levels were significantly decreased in MS SCs in comparison with controls and, paradoxically, the decreased Cbl levels were associated with decreased SC levels of homocysteine, a biochemical marker of Cbl deficiency. The trends of EGF and PrP(C) levels paralleled those previously found in CSF, whereas that of Cbl was the opposite. There was no significant difference in CSF holo-TC levels between the MS patients and the controls. Given that we have previously demonstrated that Cbl positively regulates central nervous system EGF levels, it is conceivable that the low EGF levels in the MS SC may be causally related to a local decrease in Cbl levels. Only PrP(C) levels were invariably decreased in both the SC and CSF regardless of the clinical course of the disease. These findings suggest that the simultaneous lack of Cbl, EGF, and PrP(C)s may greatly hamper the remyelination process in MS patients, because they are key molecules of the machinery for remyelination.

Human osteoclast cathepsin K is processed intracellularly prior to attachment and bone resorption.

Cathepsin K is a member of the papain superfamily of cysteine proteases and has been proposed to play a pivotal role in osteoclast-mediated bone resorption. We have developed a sensitive cytochemical assay to localize and quantify osteoclast cathepsin K activity in sections of osteoclastoma and human bone. In tissue sections, osteoclasts that are distant from bone express high levels of cathepsin K messenger RNA (mRNA) and protein. However, the majority of the cathepsin K in these cells is in an inactive zymogen form, as assessed using both the cytochemical assay and specific immunostaining. In contrast, osteoclasts that are closer to bone contain high levels of immunoreactive mature cathepsin K that codistributes with enzyme activity in a polarized fashion toward the bone surface. Polarization of active enzyme was clearly evident in osteoclasts in the vicinity of bone. The osteoclasts apposed to the bone surface were almost exclusively expressing the mature form of cathepsin K. These cells showed intense enzyme activity, which was polarized at the ruffled border. These results suggest that the in vivo activation of cathepsin K occurs intracellularly, before secretion into the resorption lacunae and the onset of bone resorption. The processing of procathepsin K to mature cathepsin K occurs as the osteoclast approaches bone, suggesting that local factors may regulate this process.

Peptide aldehyde inhibitors of cathepsin K inhibit bone resorption both in vitro and in vivo.

We have shown previously that cathepsin K, a recently identified member of the papain superfamily of cysteine proteases, is expressed selectively in osteoclasts and is the predominant cysteine protease in these cells. Based upon its abundant cell type-selective expression, potent endoprotease activity at low pH and cellular localization at the bone interface, cathepsin K has been proposed to play a specialized role in osteoclast-mediated bone resorption. In this study, we evaluated a series of peptide aldehydes and demonstrated that they are potent cathepsin K inhibitors. These compounds inhibited osteoclast-mediated bone resorption in fetal rat long bone (FRLB) organ cultures in vitro in a concentration-dependent manner. Selected compounds were also shown to inhibit bone resorption in a human osteoclast-mediated assay in vitro. Chz-Leu-Leu-Leu-H (in vitro enzyme inhibition Ki,app = 1.4 nM) inhibited parathyroid hormone (PTH)-stimulated resorption in the FRLB assay with an IC-50 of 20 nM and inhibited resorption by isolated human osteoclasts cultured on bovine cortical bone slices with an IC-50 of 100 nM. In the adjuvant-arthritic (AA) rat model, in situ hybridization studies demonstrated high levels of cathepsin K expression in osteoclasts at sites of extensive bone loss in the distal tibia. Cbz-Leu-Leu-Leu-H (30 mg/kg, intraperitoneally) significantly reduced this bone loss, as well as the associated hind paw edema. In the thyroparathyriodectomized rat model, Cbz-Leu-Leu-Leu-H inhibited the increase in blood ionized calcium induced by a 6 h infusion of PTH. These data indicate that inhibitors of cathepsin K are effective at reducing osteoclast-mediated bone resorption and may have therapeutic potential in diseases of excessive bone resorption such as rheumatoid arthritis or osteoporosis.

Potent and selective inhibition of human cathepsin K leads to inhibition of bone resorption in vivo in a nonhuman primate.

Cathepsin K is a cysteine protease that plays an essential role in osteoclast-mediated degradation of the organic matrix of bone. Knockout of the enzyme in mice, as well as lack of functional enzyme in the human condition pycnodysostosis, results in osteopetrosis. These results suggests that inhibition of the human enzyme may provide protection from bone loss in states of elevated bone turnover, such as postmenopausal osteoporosis. To test this theory, we have produced a small molecule inhibitor of human cathepsin K, SB-357114, that potently and selectively inhibits this enzyme (Ki = 0.16 nM). This compound potently inhibited cathepsin activity in situ, in human osteoclasts (inhibitor concentration [IC]50 = 70 nM) as well as bone resorption mediated by human osteoclasts in vitro (IC50 = 29 nM). Using SB-357114, we evaluated the effect of inhibition of cathepsin K on bone resorption in vivo using a nonhuman primate model of postmenopausal bone loss in which the active form of cathepsin K is identical to the human orthologue. A gonadotropin-releasing hormone agonist (GnRHa) was used to render cynomolgus monkeys estrogen deficient, which led to an increase in bone turnover. Treatment with SB-357114 (12 mg/kg subcutaneously) resulted in a significant reduction in serum markers of bone resorption relative to untreated controls. The effect was observed 1.5 h after the first dose and was maintained for 24 h. After 5 days of dosing, the reductions in N-terminal telopeptides (NTx) and C-terminal telopeptides (CTx) of type I collagen were 61% and 67%, respectively. A decrease in serum osteocalcin of 22% was also observed. These data show that inhibition of cathepsin K results in a significant reduction of bone resorption in vivo and provide further evidence that this may be a viable approach to the treatment of postmenopausal osteoporosis.

A potent small molecule, nonpeptide inhibitor of cathepsin K (SB 331750) prevents bone matrix resorption in the ovariectomized rat.

Inhibition of the cyteine proteinase, cathepsin K (E.C. 3.4.22.38) has been postulated as a means to control osteoclast-mediated bone resorption. The preferred animal models for evaluation of antiresorptive activity are in the rat. However, the development of compounds that inhibit rat cathepsin K has proven difficult because the human and rat enzymes differ in key residues in the active site. In this study, a potent, nonpeptide inhibitor of rat cathepsin K (K(i) = 4.7 nmol/L), 5-(2-morpholin-4-yl-ethoxy)-benzofuran-2-carboxylic acid ((S)-3-methyl-1-(3-oxo-1-[2-(3-pyridin-2-yl-phenyl)-ethenoyl]-azepan-4-ylcarbanoyl)-butyl)-amide (SB 331750), is described, which is efficacious in rat models of bone resorption. SB 331750 potently inhibited human cathepsin K activity in vitro (K(i) = 0.0048 nmol/L) and was selective for human cathepsin K vs. cathepsins B (K(i) = 100 nmol/L), L (0.48 nmol/L), or S (K(i) = 14.3 nmol/L). In an in situ enzyme assay, SB 331750 inhibited osteoclast-associated cathepsin activity in tissue sections containing human osteoclasts (IC(50) approximately 60 nmol/L) and this translated into potent inhibition of human osteoclast-mediated bone resorption in vitro (IC(50) approximately 30 nmol/L). In vitro, SB 331750 partially, but dose-dependently, prevented the parathyroid hormone-induced hypercalcemia in an acute rat model of bone resorption. To evaluate the ability of SB 331750 to inhibit bone matrix degradation in vivo, it was administered for 4 weeks at 3, 10, or 30 mg/kg, intraperitoneally (i.p.), u.i.d. in the ovariectomized (ovx) rat. Both 10 and 30 mg/kg doses of compound prevented the ovx-induced elevation in urinary deoxypyridinoline and prevented the ovx-induced increase in percent eroded perimeter. Histological evaluation of the bones from compound-treated animals indicated that SB 331750 retarded bone matrix degradation in vivo at all three doses. The inhibition of bone resorption at the 10 and 30 mg/kg doses resulted in prevention of the ovx-induced reduction in percent trabecular area, trabecular number, and increase in trabecular spacing. These effects on bone resorption were also reflected in inhibition of the ovx-induced loss in trabecular bone volume as assessed using microcomputerized tomography (microCT; approximately 60% at 30 mg/kg). Together, these data indicate that the cathepsin K inhibitor, SB 331750, prevented bone resorption in vivo and this inhibition resulted in prevention of ovariectomy-induced loss in trabecular structure.

Acute hypotensive responses to peptide inhibitors of renin in conscious monkeys: an effect on blood pressure independent of plasma renin inhibition.

In order to investigate the hypotensive mechanisms of action of peptide renin inhibitors, blood pressure responses to five renin inhibitors were compared with those to the angiotensin converting enzyme inhibitor, enalaprilat, in conscious African green and rhesus monkeys. (3S-4S)-4-amino-5-cyclohexyl-3-hydroxy pentanoic acid (ACHPA)-containing renin inhibitory peptide (ACRIP) and enalaprilat both decreased blood pressure in euvolemic and volume-depleted African green monkeys. However, while a maximum dose of enalaprilat reduced blood pressure to 80 +/- 4 and 56 +/- 4 mmHg in the euvolemic and volume-depleted monkeys, respectively, ACRIP lowered pressure to life-threatening levels (less than 40 mmHg) under both conditions. The relative potencies of ACRIP and four other renin inhibitors for inhibiting in vitro plasma renin activity (PRA; IC50) were compared with their potencies in reducing blood pressure by 15 mmHg (ED15 mmHg) and lowering blood pressure more than enalaprilat in volume-depleted rhesus monkeys. All renin inhibitors lowered blood pressure significantly beyond the maximal response to enalaprilat. Despite a significant correlation (r = 0.99, P less than 0.05) between the in vitro PRA inhibitory potency and the in vivo ED15 mmHg, doses which lowered blood pressure beyond the maximal responses to enalaprilat were not significantly correlated (r = 0.53, P greater than 0.05) with the in vitro PRA IC50 values. Furthermore, the profound depressor responses to renin inhibitors in rhesus monkeys were accompanied by increases in the heart rate and decreases in pulse pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis of thyrotropin-releasing hormone analogues with selective central nervous system effects.

Thyrotropin-releasing hormone (TRH) analogues which show relative selectivity for action in the central nervous system have been recognized. Practical syntheses for three of these TRH analogues which show the greatest selectivity, less than Aad-His-Tzl-NH2 (5), less than Glu-His-Pip-OMe (2), and less than Aad-His-Pro-NH2 (6), are described. The first two were prepared by solution methods of peptide synthesis. Compound 6 was prepared by the solid-phase method. Problems of histidine racemization, facile diketopiperazine formation, and instability of acylated thiazolidine carboxylic acid derivatives under acidic conditions have been minimized in order to attain optimal yields. Physical properties such as pK, NMR shifts, and circular dichroism have been examined as they might relate to biological activity and peptide conformation.

Cholecystokinin antagonists. Synthesis and biological evaluation of 4-substituted 4H-[1,2,4]triazolo[4,3-a][1,4]benzodiazepines.

A series of 4-substituted 4H-[1,2,4]triazolo[4,3-a][1,4]benzodiazepines was prepared by standard methodology. These compounds were tested in vitro as antagonists of the binding of [125I]cholecystokinin (CCK) to rat pancreas and guinea pig brain receptors and of the binding of [125I]gastrin to guinea pig gastric glands. All compounds proved to have greater affinity for the peripheral CCK receptor with some analogues having IC50's in the subnanomolar range. In vivo activity of selected compounds in mice is presented and the structure/activity profile of this class of benzodiazepines as CCK antagonists is discussed.

Development of a potent thrombin receptor ligand.

The N-terminal thrombin receptor peptide H-Ser-Phe-Leu-Leu-Arg-Asn-Pro-Asn-Asp-Lys-Tyr-Glu-Pro-Phe-OH (1) fully activates the thrombin receptor with an EC50 of 10 microM. Structural features in the tetradecapeptide which are responsible for receptor activation have been elucidated. Agonist potency has been enhanced 1000-fold with the design of the shortened peptide H-Ala-Phe(p-F)-Arg-Cha-HArg-Tyr-NH2 (56). This analog exhibits an EC50 of 0.01 microM and is the most potent agonist for receptor activation reported to date. The monoiodinated derivative H-Ala-Phe(p-F)-Arg-Cha-HArg-Tyr(3-I)-NH2 (59) exhibits an EC50 of 0.03 microM, a level sufficient for development of a radioligand.

Cyclic peptides as selective tachykinin antagonists.

Twenty homodetic cyclic peptides based on the C-terminal sequence of substance P were prepared (Table I) by a combination of solid-phase techniques and cyclizations using azide coupling procedures. Incorporation of dipeptide mimics based on substituted gamma-lactams were used in some cases to restrict their conformational mobility. Five of these cyclic peptides were shown to have high tachykinin antagonist activity (pA2 > 6) at NK-2 receptors (rat vas deferens). The two most potent of this series, XVII, cyclo(Gln-Trp-Phe-Gly-Leu-Met) (pA2 = 8.1), and I cyclo(Gln-Trp-Phe(R)Gly[ANC-2]Leu-Met) (pA2 = 6.7), were selective for NK-2 receptors compared with the other tachykinin receptors (Table II).

Synthesis of a proposed thymic factor.

Less than Glu-Ala-Lys-Ser-Gln-Gly-Gly-Ser-Asn, a proposed serum thymic factor, has been synthesized. The protected precursor, less than Glu-Ala-Lys(i-Noc)-ser(Bzl)-Gln-Gly-Gly-Ser(Bzl)-Asn, was prepared by a combination of solid phase and solution methods. The benzyl blocking groups were removed by HF and the i-Noc blocking group was removed by catalytic hydrogenation.

Design of nonpeptidal ligands for a peptide receptor: cholecystokinin antagonists.

A series of 3-substituted 5-phenyl-1,4-benzodiazepines, nonpeptidal antagonists of the peptide hormone cholecystokinin (CCK), have been synthesized. Designed on the basis of facts regarding CCK, its natural-product antagonist asperlicin (3), and the antianxiety agent diazepam (4), these compounds represent a significant departure from existing CCK antagonists. They also constitute perhaps the first examples of simple, nonpeptidal ligands for a peptide receptor to arise by design rather than by screening. These compounds serve to illuminate the distinction between central and peripheral CCK receptors, as well as to provide orally effective CCK antagonists of potential pharmacological or therapeutic utility. One rationale for their receptor affinity has possible applications in the design of nonpeptidal ligands for other receptors, peptidal as well as nonpeptidal.

Cholecystokinin antagonists. Synthesis of asperlicin analogues with improved potency and water solubility.

Seventeen analogues of the selective, competitive cholecystokinin (CCK) antagonist asperlicin 1 were prepared. These compounds were tested as inhibitors of the binding of [125I]CCK to rat pancreas and guinea pig brain receptors. Compounds 4, 7, and 8 were more potent than asperlicin on the pancreatic CCK receptor. One analogue, 17, displayed potency equivalent to asperlicin on the pancreas CCK receptor and showed a marked improvement in aqueous solubility, thereby facilitating the use of this class of CCK antagonists in physiological and pharmacological studies.

Orally active, nonpeptide oxytocin antagonists.

The first nonpeptide antagonists of the neurohypophyseal hormone, oxytocin (OT) are described. Derivatives of the spiroindenepiperidine ring system, these compounds include L-366,509, an orally bioavailable OT antagonist with good in vivo duration. The potential use of these agents for treatment of preterm labor and their significance as new nonpeptide ligands for peptide receptors are discussed.

Methods for drug discovery: development of potent, selective, orally effective cholecystokinin antagonists.

3-(Acylamino)-5-phenyl-2H-1,4-benzodiazepines, antagonists of the peptide hormone cholecystokinin (CCK), are described. Developed by reasoned modification of the known anxiolytic benzodiazepines, these compounds provide highly potent, orally effective ligands selective for peripheral (CCK-A) receptors, with binding affinities approaching or equaling that of the natural ligand CCK-8. The distinction between CCK-A receptors on the one hand and CNS (CCK-B), gastrin, and central benzodiazepine receptors on the other is demonstrated by using the structure-activity profiles of the new compounds. Details of the binding of these agents to CCK-A receptors are examined, and the method of development of these compounds is discussed in terms of its relevance to the general problem of drug discovery.

Cholecystokinin-A receptor ligands based on the kappa-opioid agonist tifluadom.

Tifluadom, a kappa-opioid agonist and cholecystokinin-A (CCK-A) receptor antagonist, was utilized as a model to prepare a series of 2-(aminomethyl)- and 3-(aminomethyl)-1,4-benzodiazepines. These compounds were tested in vitro as inhibitors of the binding of [125I]CCK to rat pancreas and guinea pig brain receptors. All compounds with IC50's less than 100 microM proved to have greater affinity for the CCK-A receptor, with the most potent analogue, 6e, having an IC50 of 0.16 microM. The benzodiazepines described in this study are simultaneously CCK-A and opioid receptor ligands. The ramification of this dichotomy on current concepts of peptide hormone action are discussed. These results further demonstrate the versatility of the benzodiazepine core structure for designing nonpeptide ligands for peptide receptors and the ability to fine-tune the receptor interactions of these benzodiazepines by appropriate structure modifications.

Use of papain as a model for the structure-based design of cathepsin K inhibitors: crystal structures of two papain-inhibitor complexes demonstrate binding to S'-subsites.

Papain has been used as a surrogate enzyme in a drug design effort to obtain potent and selective inhibitors of cathepsin K, a new member of the papain superfamily of cysteine proteases that is selectively and highly expressed in osteoclasts and is implicated in bone resorption. Here we report the crystal structures of two papain-inhibitor complexes and the rational design of novel cathepsin K inhibitors. Unlike previously known crystal structures of papain-inhibitor complexes, our papain structures show ligand binding extending deep within the S'-subsites. The two inhibitor complexes, carbobenzyloxyleucinyl-leucinyl-leucinal and carbobenzyloxy-L-leucinyl-L-leucinyl methoxymethyl ketone, were refined to 2.2- and 2.5-A resolution with R-factors of 0.190 and 0. 217, respectively. The S'-subsite interactions with the inhibitors are dominated by an aromatic-aromatic stacking and an oxygen-aromatic ring edge interaction. The knowledge of S'-subsite interactions led to a design strategy for an inhibitor spanning both subsites and yielded a novel, symmetric inhibitor selective for cathepsin K. Simultaneous exploitation of both S- and S'-sites provides a general strategy for the design of cysteine protease inhibitors having high specificity to their target enzymes.