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J. Biol. Chem., Vol. 278, Issue 39, 37099-37111, September 26, 2003
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From the Department of Cardiovascular and Metabolic Diseases, Pfizer Global Research and Development, Groton Laboratories, Pfizer Inc., Groton, Connecticut 06340
Received for publication, April 29, 2003 , and in revised form, June 10, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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50 nM. Inhibition was reversible, uncompetitive with respect to ATP, and non-competitive with respect to bicarbonate, acetyl-CoA, and citrate, indicating interaction with the enzymatic carboxyl transfer reaction. CP-610431 also inhibited fatty acid synthesis, triglyceride (TG) synthesis, TG secretion, and apolipoprotein B secretion in HepG2 cells (ACC1) with EC50s of 1.6, 1.8, 3.0, and 5.7 µM, without affecting either cholesterol synthesis or apolipoprotein CIII secretion. CP-640186, also inhibited both isozymes with IC50sof 55 nM but was 2–3 times more potent than CP-610431 in inhibiting HepG2 cell fatty acid and TG synthesis. CP-640186 also stimulated fatty acid oxidation in C2C12 cells (ACC2) and in rat epitrochlearis muscle strips with EC50s of 57 nM and 1.3 µM. In rats, CP-640186 lowered hepatic, soleus muscle, quadriceps muscle, and cardiac muscle malonyl-CoA with ED50s of 55, 6, 15, and 8 mg/kg. Consequently, CP-640186 inhibited fatty acid synthesis in rats, CD1 mice, and ob/ob mice with ED50s of 13, 11, and 4 mg/kg, and stimulated rat whole body fatty acid oxidation with an ED50 of 30 mg/kg. Taken together, These observations indicate that isozyme-nonselective ACC inhibition has the potential to favorably affect risk factors associated with the metabolic syndrome. |
INTRODUCTION |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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24% of adults in the United States alone suffer from some form of metabolic syndrome (4).
Recent studies have suggested that abnormal fatty acid metabolism may be at the core of metabolic syndrome (12–14). Acetyl-CoA carboxylase (ACC) catalyzes the rate-limiting reaction in fatty acid biosynthesis (15–17). Malonyl-CoA, the product of the ACC-catalyzed reaction, inhibits mitochondrial fatty acid oxidation through feedback inhibition of carnitine palmitoyltransferase 1 (CPT-1; Refs. 18 and 19), and therefore plays key roles both in controlling the switch between carbohydrate and fatty acid utilization in liver and skeletal muscle and also in regulating insulin sensitivity in the liver, skeletal muscle, and adipose tissue (18–20). Malonyl-CoA may also play an important regulatory role in controlling insulin secretion from the pancreas (21, 22).
Thus, in addition to inhibition of fatty acid synthesis, reduction in malonyl-CoA levels through ACC inhibition may provide a mechanism for increasing fatty acid utilization that may reduce TG-rich lipoprotein secretion (very low density lipoprotein) by the liver, alter insulin secretion by the pancreas, and improve insulin sensitivity in liver, skeletal muscle, and adipose tissue. Additionally, by increasing fatty acid utilization and by preventing increases in de novo fatty acid synthesis, chronic administration of an ACC inhibitor may also deplete liver and adipose tissue TG stores in obese subjects consuming a low fat diet, leading to selective loss of body fat.
Therefore, a well tolerated ACC inhibitor could effectively and simultaneously treat the multiple risk factors associated with metabolic syndrome and could have a significant impact on the prevention and treatment of the cardiovascular morbidity and mortality associated with obesity, hypertension, diabetes, and atherosclerosis.
The first reported evidence that inhibition of ACC could favorably affect the etiology of the metabolic syndrome came from studies with the long-chain fatty acid analog, 5-(tetradecyloxy)-2-furancarboxylic acid (TOFA; Refs. 23–28). In these reports, investigators demonstrated that TOFA, when converted to the CoA analog intracellularly (24), inhibited ACC activity (24), reduced fatty acid synthesis and TG secretion both in cultured hepatic cells (24, 27) and in experimental animals (23, 26, 28), reduced plasma cholesterol and TGs in experimental animals including rhesus monkeys (23, 27), and reduced body weight (25). However, the similarity of TOFA to various long-chain fatty acids called into question the specificity of the compound for ACC inhibition relative to inhibition of other long-chain fatty acyl-CoA-utilizing enzymes and therefore complicated interpretation of the findings with respect to the benefits of ACC inhibition per se. In addition, the existence of multiple tissue-specific ACC isoforms (15, 29–33), one primarily involved in fatty acid synthesis in the liver and adipose tissue (ACC1) and the other primarily involved in regulating fatty acid oxidation in the liver and skeletal muscle (ACC2), was not known at the time of these studies, and therefore the role of ACC1 inhibition versus ACC2 inhibition in the actions of TOFA was unknown.
More recently, a report by Wakil and co-workers (34) demonstrated that knock-out mice lacking ACC2 activity but maintaining a fully functional ACC1 gene locus, presented with a phenotype demonstrating a reduction in skeletal muscle malonyl-CoA concentrations, an increase in rates of muscle fatty acid oxidation, a reduced hepatic fat content, a reduction in total body fat in the face of a somewhat greater food consumption, and a reduction in plasma glucose and free fatty acid concentration relative to wild-type litter-mates, while maintaining a normal growth rate, life expectancy, and breeding capacity. These observations add further support to the concept of the utility of ACC inhibition to the treatment of metabolic syndrome. However, in these studies, it was clear that the liver and possibly also adipose tissues were compensating to some degree for the lack of ACC2-mediated control of fatty acid oxidation by increasing ACC1-mediated fatty acid synthesis (34). Indeed, in previous reports in which the efficacy of TOFA analogs was evaluated (25), investigators demonstrated the need to inhibit fatty acid production in the adipose tissue as well as in the liver, because compounds that were hepatoselective, and not peripherally available, were less effective or not effective (35). Taken together, these observations suggest that an isozyme-nonselective ACC inhibitor that would inhibit ACC2 activity in oxidative tissues such as the liver and skeletal muscle, and at the same time inhibit the activity of ACC1 in lipogenic tissues such as the liver and adipose and prevent any compensatory increases in fatty acid synthesis from attenuating the ACC2 inhibition-mediated increases in fatty acid oxidation, would have the optimal therapeutic potential for treating the multiple sequelae of metabolic syndrome.
In mid-1996, we set out to identify an ACC-specific, isozyme-nonselective ACC inhibitor for use in increasing confidence in the hypothesis that ACC inhibition-mediated reductions in fatty acid synthesis and increases in fatty acid oxidation represent a potential therapeutic approach for the treatment of the combined dyslipidemia, visceral obesity, and insulin resistance that are the hallmarks of the metabolic syndrome. In this report we describe the biochemical an physiological properties of CP-610431(Table I, inset), an isozyme-nonselective ACC inhibitor identified using a 96-well multiplexed, high throughput screening paradigm and its more metabolically stable analog CP-640186, which equally inhibit mammalian ACC1 and ACC2, reduce malonyl-CoA concentration in liver, heart, and skeletal muscle tissue, reduce fatty acid synthesis in cultured liver cells and in the liver and adipose tissue of experimental animals, increase fatty acid oxidation in cultured skeletal muscle cells and tissue slices, and reduce respiratory quotient in experimental animals. In a subsequent report,2 we will describe the utility of CP-640186 for reducing liver, adipose, and skeletal muscle fat content, reducing whole body fat mass, and improving insulin sensitivity in chronic (up to 8-week) studies in dietarily induced rat models of the metabolic syndrome.
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EXPERIMENTAL PROCEDURES |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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Preparation of ACC1 from Liver and Adipose Tissue and ACC2 from Skeletal and Cardiac Muscle—ACC1 was prepared from rat liver or rat adipose tissue, and ACC2 was prepared from rat skeletal or cardiac muscle tissue essentially as previously described (38) with the following modifications. Male Sprague-Dawley rats weighing 150–200 g were fasted for 2 days and then fed a high sucrose diet (AIN76A) for 3 days (induces ACC1 activity 10-fold), at which time they were killed by CO2 asphyxiation. The livers (for ACC1 preparation), epididymal adipose tissue (for ACC1 preparation), skeletal muscle tissue (for ACC2 preparation), and cardiac muscle tissue (for ACC2 preparation) were removed, rinsed in ice-cold PBS, and homogenized in 5 volumes of homogenization buffer (50 mM potassium phosphate (pH 7.5) 10 mM EDTA, 10 mM 2-mercaptoethanol, 2 mM benzamidine, 0.2 mM PMSF, 5 mg/liter each leupeptin, aprotinin, and antitrypsin) in a Waring blender for 1 min at 4 °C. Adipose tissue was frozen in liquid N2 and then pulverized in liquid N2 prior to homogenization as described above. All subsequent operations were carried out at 4 °C. The homogenate was made 3% with respect to polyethylene glycol (PEG) by the addition of 50% PEG solution and centrifuged at 20,000 x g for 15 min. The resulting supernatant was adjusted to 5% PEG with the addition of 50% PEG solution and stirred for 5 min. The pellet (contains ACC activity) was collected by centrifugation at 20,000 x g for 20 min, rinsed with 4 °C double-distilled H2O to remove excess PEG, and resuspended in one-fourth the original homogenate volume with homogenization buffer. Ammonium sulfate (200 g/liter) was slowly added with stirring. After 45 min the enzyme was collected by centrifugation for 30 min at 20,000 x g, resuspended in 10 ml of 50 mM HEPES (pH 7.5) buffer containing 0.1 mM DTT, 1.0 mM EDTA, and 10% glycerol, and desalted on a G-25 column (2.5 cm x 50 cm) equilibrated with the same buffer. The desalted enzyme preparation was stored in aliquots at –70 °C. When isolated under these conditions, partially purified rat adipose tissue ACC was exclusively ACC1, partially purified rat heart ACC was exclusively ACC2, partially purified rat skeletal muscle ACC was exclusively ACC2, and partially purified rat liver ACC was a mixture of both ACC1 and ACC2 in a 85:15 ratio, as demonstrated by immunoblotting with specific polyclonal antibodies after PAGE. Immediately prior to use, frozen aliquots of the above fractions were thawed, diluted to 500 µg/ml in buffer containing 50 mM HEPES (pH 7.5), 10 mM MgCl2,10mM tripotassium citrate, 2.0 mM DTT, and 0.75 mg/ml fatty acid-free BSA and the ACC was citrate-activated by preincubation at 37 °C for 30 min.
Measurement of Acetyl-CoA Carboxylase Inhibitory Activity—The procedure for measuring ACC1 and ACC2 activity utilizes a radiochemical method (38) that measures incorporation of [14C]bicarbonate into [14C]malonyl-CoA and separates product from unused substrate at the end of the reaction through acidification, which serves to both quench the reaction and remove residual radiolabeled substrate as 14CO2. For measurement of ACC activity and assessment of ACC inhibition, test compounds were dissolved in Me2SO and 1.0 µl was placed in 0.5-ml polypropylene tubes. Control tubes contained 1.0 µlofMe2SO alone. All assay tubes received 139 µl of substrate buffer containing 50 mM HEPES (pH 7.5), 2.0 mM MgCl2, 2.0 mM tripotassium citrate, 2 mM DTT, 0.75 mg/ml BSA, 25 µM acetyl-CoA, 4.0 mM ATP, and 12.5 mM KH[14C]O3 (2 x 106 cpm). The reaction was initiated by the addition of 10 µl of citrate-activated ACC solution. After 7 min the reaction was stopped by the addition of 50 µl of 6 N HCl and 150 µl of the quenched reaction mixture was transferred to glass scintillation vials and evaporated to dryness at 90 °C for 1 h. The dried vials were cooled, 0.5 ml of water and 5.5 ml of Beckman Readisafe liquid scintillation fluid were added, and the radioactivity was determined in a Beckman LS6500 liquid scintillation counter. Tubes that received HCl before citrate-activated ACC solution served as blanks. ACC activity is expressed as nanomoles of KH[14C]O3 incorporated into malonyl-CoA per min of incubation at 37 °C per mg of protein.
ACC Inhibition High Throughput Screening Format—The method for measuring ACC activity and inhibition was adapted to high throughput (80 plate-per-assay scale) inhibitor screening format as follows: compounds dissolved in Me2SO in 96-well multiplexed format were added to wells of a 96-well plate containing 90 µl of assay buffer consisting of 50 mM HEPES (pH 7.5), 2.0 mM MgCl2, 2.0 mM tripotassium citrate, 2 mM DTT, 0.75 mg/ml BSA, 25 µM acetyl-CoA, and 4.0 mM ATP (final compound concentration 5.0 µM). Plates were then transferred to a fume hood and were supplied with 10 µl of a solution containing 12.5 mM NaH[14C]O3 (2 x 106 cpm). Reactions were then initiated by addition of 10 µl of partially purified, citrate-activated rat liver ACC solution (5 µg of protein) isolated from fasted, high sucrose diet-refed rats (85% ACC1, 15% ACC2) and incubated for 10 min at room temperature. After incubation, reactions were terminated by addition of 10 µlof6 M HCl and plates were permitted to stand in the hood for2hto allow 14CO2 liberation. Plates were then transferred to a 65 °C vacuum oven fitted with an elaborate system of NaOH traps and charcoal filters between the vacuum oven and house vacuum system to prevent both contamination by residual 14CO2 and corrosion by the evaporated HCl. Monitoring the entrapment of 14CO2 and HCl between the vacuum oven and house line revealed that all 14CO2 was trapped in the first of the NaOH traps and little or none reached the charcoal filter for final entrapment. The HCl was likewise trapped by the first several NaOH traps, and they were replaced routinely as their pH was reduced. Plates remained in the vacuum oven overnight. The next morning, residue in the dried wells (contains [14C]malonyl-CoA) was resuspended in 30 µl of water, dissolved in 145 µl of OptiPhase "Hi-Safe" 3 liquid scintillation fluid with vigorous shaking, sealed, and transferred to pre-programmed robotics for radioactivity assessment. By selecting for inhibitors that inhibited hepatic ACC activity by 75%, we selected for compounds that inhibited ACC1 activity. We then selected for those that also equally inhibited ACC2 in secondary specificity assays.
Measurement of HepG2 Cell Cholesterol and Fatty Acid Synthesis— Cholesterol and fatty acid synthesis were evaluated in HepG2 cells by measuring incorporation of [2-14C]acetate into cellular lipids as previously described (36, 37) with the following modifications to allow simultaneous assessment of both cholesterol and fatty acid synthesis. After collection and assessment of the hexane fraction containing cholesterol and nonsaponifiable lipids as previously described (36, 37), the remaining aqueous phase (containing fatty acid sodium salts) was acidified to pH < 2 by addition of 0.5 ml of 12 M HCl. The resulting mixtures were then transferred to glass conical tubes and extracted three times with 4.5 ml of hexane. The pooled organic fractions were dried under nitrogen, resuspended in 50 µl of chloroform:methanol (1:1, v/v), and applied to 1 x 20-cm channels of Silica Gel 60C TLC plates. Channels containing non-radioactive fatty acids were included on selected TLC plates as separation markers. TLC plates were developed in hexane:diethyl ether:acetic acid (70:30:2) and air-dried, and the region of chromatograms corresponding to fatty acid mobility were removed and assessed for radioactivity in Aquasol-2 using a Beckman LS6500 liquid scintillation counter. Cholesterol and fatty acid synthesis are expressed as dpm [2-14C]acetate/mg of cellular protein incorporated into either cholesterol or saponifiable lipids during the 6-h incubation at 37 °C.
Measurement of HepG2 Cell TG Synthesis and Secretion—HepG2 cells grown in T-75 flasks as previously described (36, 37) were seeded into 24-well plates at 4–6 x 105 cells/well and maintained in a humidified, 5% CO2 atmosphere at 37 °C for 48 h. Media were then removed and replaced with fresh media containing 0.2% Me2SO ± test compound. One hour after compound addition, 25 µl of media containing 50 µCi of [3H]glycerol was added to each incubation well. Plates were then sealed with parafilm to avoid evaporation, and cells were incubated at 37 °C for 6 h with gentle shaking. After incubation, the media was removed and the cells were washed three times with PBS and scraped from wells into PBS. Lipids were extracted from the media and the cell lysate with chloroform-methanol (2:1) and applied to 1 x 20-cm channels of Silica Gel 60C TLC plates. Channels containing non-radioactive TGs were included on selected TLC plates as separation markers. TLC plates were developed in petroleum ether:diethyl ether:acetic acid (75: 25:1), air-dried, and the region of chromatograms corresponding to TG mobility were removed and assessed for radioactivity in Aquasol-2 using a Beckman LS6500 liquid scintillation counter. TG synthesis and TG secretion are expressed as dpm [3H]glycerol/mg of cellular protein incorporated into cellular TGs or secreted into the culture medium during the 6-h incubation at 37 °C.
Measurement of HepG2 Cell ApoB and ApoCIII Secretion—HepG2 cells were grown in DMEM containing 10% fetal bovine serum in 96-well plates in a humidified, 5% CO2 atmosphere at 37 °C until they are 70% confluent. On the day of experimentation, media were removed from each well and replaced with fresh media containing 0.2% Me2SO ± test compound. Twenty-four hours later, growth medium was collected and assessed for apoB content as previously described (39, 40). Growth medium was also assessed for apoCIII content essentially as described for assessing apoB content, except that BioDesign K74140G affinity-purified polyclonal goat anti-human apoCIII antibody solution (5.0 mg/ml in PBS) was used as the capture antibody, BioDesign K74170B biotinylated, affinity-purified polyclonal goat anti-human apoCIII antibody (1:16,000 dilution in PBS containing 1% BSA and 0.05% Tween 20) was used as the secondary antibody, and Amersham RPN1234 streptavidin-alkaline phosphatase (1:600 dilution in PBS containing 1% BSA and 2 mM MgCl2) was used as the detection enzyme conjugate. ApoCIII concentrations were calculated from a standard curve for purified apoCIII standards (0.01–20 ng) run in parallel.
Measurement of C2C12 Cell and Epitrochlearis Muscle Fatty Acid Oxidation—C3H mouse myoblast (C2C12) cells were grown in DMEM containing 10% fetal bovine serum in 96-well plates in a humidified, 5% CO2 atmosphere at 37 °C until they were no more than 70% confluent. On the day of study, the medium was removed, cells were washed with 0.25% trypsin plus 0.03% EDTA solution, then incubated in 2.0 ml of trypsin-EDTA solution without shaking for 5 min to allow cells to detach from the flask. Trypsin-released cells were resuspended with 6–8 ml of pH 7.4 assay buffer (20 mM HEPES, 25 mM NaHCO3, 1.2 mM KH2PO4, 3 mM glucose, 114 mM NaCl, 4.7 mM KCl, 1.2 mM MgSO4, 2.5 mM CaCl2, 1% ultra fatty acid-free BSA) to a cell density of 1.5 million cells/ml. Cell suspension aliquots of 2.0 ml were added to 25-ml Erlenmeyer flasks and incubated for 30 min at 37 °C in a shaking water bath. Epitrochlearis muscle was removed from male 8-week-old Sprague-Dawley rats after cervical dislocation and placed into 4.5 ml of pH 7.4 assay buffer (see above) in a 25-ml Erlenmeyer flask. Muscle strips were then pre-incubated for 30 min at 37 °C in a shaking water bath. After pre-incubation, 0.5-ml aliquots of assay buffer containing various concentrations of test compound were added to each flask and were followed immediately by 80 µl (C2C12 cells) or 160 µl (muscle strips) of a solution containing 12% ultra fatty acid-free BSA and 12.7 mM [1-14C]palmitate (3.4 µCi; 1.0 µCi/µmol). Flasks were then filled with a 5% CO2 atmosphere, capped with a rubber stopper into which a center well was inserted, and incubated for 2 h at 37 °C in a shaking water bath. After incubation, the reaction was terminated by addition of 0.5 ml of a 60% perchloric acid solution, a 300-µl volume of benzethonium hydroxide was added to each center well, and flasks were incubated overnight at room temperature to allow for quantitative capture of the released 14CO2. After incubation, stoppers were removed from the flasks, the center well cups were cut into 7-ml scintillation vials containing 6.0 ml of Hionic-fluor liquid scintillation fluid, and the radioactivity assessed in a Beckman LS6500 liquid scintillation counter. Rates of fatty acid oxidation are expressed as nanomoles of 14CO2 released/min of incubation per 106 cells or per mg of tissue.
Studies Using Experimental Animals—All procedures using experimental animals were approved by the Institutional Animal Care and Use Procedures Review Board. Sprague-Dawley rats, C57BL/6J-ob/ob mice, or CD1 mice, were given food (RMH3200 or AIN76A) and water ad libitum and treated orally at a volume of 1.0 ml/200 g of body weight (rats) or 0.25 ml/25 g of body weight (mice) with either an aqueous solution containing 0.5% methyl cellulose (vehicle) or an aqueous solution containing 0.5% methyl cellulose plus test compounds.
Measurement of Hepatic Fatty Acid Synthesis—Hepatic incorporation of [2-14C]acetate into TGs was essentially as previously described for assessment of hepatic sterol synthesis (36, 41) with the following modifications to allow for assessment of fatty acid synthesis. Briefly, animals given food and water ad libitum were treated orally at a volume of 1.0 ml/200 g of body weight (rat) or at a volume of 0.25 ml/25 g of body weight (mice) with either an aqueous solution containing 0.5% methyl cellulose (vehicle) or an aqueous solution containing 0.5% methyl cellulose plus test compound. One to 4 h after compound administration, animals received an intraperitoneal injection of 0.5 ml (rat) or 0.2 ml (mouse) of [2-14C]acetate (64 µCi/ml; 56 mCi/mmol). One hour after radiolabel administration, animals were killed by CO2 asphyxiation and two 0.75-g liver pieces were removed and saponified at 70 °C for 120 min in 1.5 ml of 2.5 M NaOH. After saponification, 2.5 ml of absolute EtOH were added to each sample and the solutions were mixed and allowed to stand overnight. Petroleum ether (4.8 ml) was then added to each sample, and the mixtures were first shaken vigorously for 2 min and then centrifuged at 1000 x g in a benchtop Sorvall for 5 min. The resultant petroleum ether layers, which contain nonsaponifiable lipids, were removed and discarded. The remaining aqueous layer was acidified to pH < 2 by addition of 0.6 ml of 12 M HCl and extracted two times with 4.8 ml of petroleum ether. The pooled organic fractions were transferred to liquid scintillation vials, dried under nitrogen, dissolved in 7 ml of Aquasol liquid scintillation fluid, and assessed for radioactivity using a Beckman 6500 liquid scintillation counter. Inhibition of fatty acid synthesis is expressed as the concentration required to reduce by 50% the [2-14C]acetate incorporated into saponifiable lipids during the interval between [2-14C]acetate injection and CO2 asphyxiation.
Measurement of Tissue Malonyl-CoA Concentration—Direct assessment of malonyl-CoA production in tissues that either do (e.g. liver and adipose tissue) or do not synthesize fatty acids (e.g. skeletal muscle), through its stoichiometric conversion to radiolabeled palmitate in the presence of purified fatty acid synthetase and radiolabeled acetyl-CoA, can also be used to determine ACC inhibition in those tissues as previously described by McGarry et al. (42). Briefly, Sprague-Dawley rats given food and water ad libitum were treated orally at a volume of 0.5 ml/200 g of body weight with either an aqueous solution containing 0.5% methyl cellulose (vehicle) or an aqueous solution containing 0.5% methyl cellulose plus test compound. One hour after compound administration, rats were anesthetized with pentobarbital and tissue samples were removed, immediately freeze-clamped, placed in a cryotube, and immersed in liquid N2 in the following order to minimize blood-flow interruption during harvesting: complete soleus muscle from both legs (combined weight: 0.2–0.3 g), 2.0 g of gastrocnemius muscle, 2.0 g of quadriceps muscle, 2.0 g of adipose tissue, 2.0 g of liver, and the entire heart (0.5–1.0 g). Frozen tissues were first pulverized under liquid nitrogen, and then 1.0 g of the powdered tissues were extracted with 4 ml of 10% (w/v) HClO4, neutralized to pH 6.0 with 5 M KOH, and centrifuged at 3500 rpm for 10 min in a Sorval RT6000 benchtop centrifuge to remove particulate residue. For the soleus muscle, the entire tissue was homogenized in 1.2 ml of HClO4 and then neutralized and clarified as described above. Measurements of malonyl-CoA concentration in tissue extracts were carried out in duplicate in 15-ml polypropylene screw-capped tubes. Reaction mixtures, in a final volume of 1.025 ml of 250 mM potassium phosphate buffer (pH 7.0), containing 2.5 mM dithiothreitol, 2.0 mM EDTA, 0.2 mM NADPH, 1 mg/ml fatty acid-free bovine serum albumin, 0.68 µM [3H]acetyl-CoA (150,000 dpm/nmol), 2.0 nM malonyl-CoA standard, and either 100 µl of liver extract or 200 µl of skeletal muscle, heart, or adipose tissue extract, were mixed with a 10-µl aliquot containing 25 milliunits of purified (as previously described (Ref. 42)) fatty acid synthetase (5 µg of protein) and were incubated at 37 °C for 45 min. After incubation, reactions were terminated by addition of 25 µl of 70% (w/v) HClO4 and nascent palmitate was extracted by addition to each tube of 1 ml of absolute EtOH and then 5 ml of petroleum ether. After vigorous mixing for 30 s and centrifugation at 2000 rpm for 5 min in a Sorval RT6000 benchtop centrifuge to facilitate phase separation, 4.0 ml of the petroleum ether phase was transferred to a second tube containing 2 ml of water, shaken, recentrifuged, and 2.5 ml of the petroleum ether phase was transferred to liquid scintillation vials, dried, and assessed for radioactivity in a liquid scintillation counter after addition of 6 ml of Aquasol liquid scintillation fluid. Blanks containing no added malonyl-CoA or tissue extract are included with each series of determinations and subtracted from all values. Malonyl-CoA concentration per gram of tissue was calculated using the ratio of one molecule of [3H]acetyl-CoA incorporated into palmitate for each seven molecules of malonyl-CoA present in the tissue extract. Inhibition of malonyl-CoA production is expressed as the concentration required to reduce by 50% the dpm [3H]acetyl-CoA incorporated into palmitate during the 45-min incubation at 37 °C.
Measurement of Respiratory Quotient—Acute modulation of whole-body fatty acid utilization was assessed by measuring changes in the respiratory quotient (RQ, the ratio of CO2 production to O2 consumption; Ref. 43) using an open circuit, indirect calorimeter (Oxymax, Columbus Instruments, Columbus, OH). Briefly, male Sprague-Dawley rats (350–400 g) housed under standard laboratory conditions, either fed chow, fasted, or fasted and refed a diet high in sucrose for 2 days prior to experimentation were removed from their home cages, weighed, and placed into sealed chambers (43 x 43 x 10 cm) of the calorimeter (one rat per chamber). The chambers were placed in activity monitors. The calorimeter was calibrated before each use, air flow rate was adjusted to 1.6 liters/min, and the system settling and sampling times were set to 60 and 15 s, respectively. Base-line oxygen consumption, CO2 production, and ambulatory activity were measured every 10 min for up to 3 h before treatment. After collecting base-line data, the chambers were opened and rats were given a 1.0-ml oral bolus of either an aqueous 0.5% methylcellulose solution (vehicle control) or an aqueous 0.5% methylcellulose solution containing test compound and then returned to the Oxymax chambers. Measurements were made every 10 min for an additional 3–6 h after dose. Fed vehicle controls were used to assess effects produced by vehicle administration and by drift in the RQ measurement during the course of the experimentation (if any). Overnight-fasted, vehicle-treated controls were used to determine maximal potential RQ reduction. For comparison purposes one group of rats received a selective 
3-adrenergic receptor agonist (CL-316243; 1 mg/kg). Changes in RQ in response to compound administration were calculated by dividing the average of the post-dosing RQ values (excluding values obtained during time periods where ambulatory activity exceeds 100 counts) by the average of the pre-dosing base-line values (excluding the first five values and values obtained during time periods where ambulatory activity exceeds 100 counts) and were expressed as percentage change from base line.
Measurement of Plasma and Tissue Drug Levels—To 100-µl aliquots of plasma were added 10 µl of an internal standard (2.5 µg/ml CP-633196 in methanol), 1 ml of water, and 3 ml of methylene chloride. After vigorous mixing and centrifugation, the organic layers were removed, evaporated to dryness under N2, and the residue was reconstituted with 100 µl of mobile phase (50% methanol, 50% 10 mM ammonium acetate, pH 6). Liver tissue (1.0 g) was homogenized in 3 ml of water, and 100 µl of the homogenate was extracted as above except that a larger volume of methylene chloride (5 ml) was used. Skeletal muscle tissue (1.0 g) was homogenized in a mixture of 3 ml of 10% trichloroacetic acid and 3 ml of methanol, and 175 µl of the homogenate was extracted with 5 ml of methylene chloride as outlined above for liver. For determination of half-life in hepatic microsomes, 100-µl aliquots of liver microsome incubates were removed at various time intervals and extracted as outlined above. Aliquots (20–40 µl) of reconstituted samples were injected onto a Monitor column (5-µm, C18; 150 x 4.6 mm, inner diameter; Columns Engineering). Separation was achieved using an isocratic gradient with 85% methanol and 15% 10 mM ammonium acetate (pH 6) with a flow rate of 0.5 ml/min. Quantitation of CP-640186 and internal standard was done using a PerkinElmer Sciex API3000 (Applied Biosystems, Toronto, Canada) mass spectrometer. Detection was done using multiple reaction monitoring with turbo ion spray source in the negative ionization mode. The ion transitions were monitored at m/z 486.2 (M–H) to m/z 205 (product ion), and m/z 472.0 (M–H) to m/z 205 (product ion) for CP-640,186 and CP633,196, respectively. The linear dynamic range was between 5 ng/ml (lower limit of quantification) and 1 µg/ml (upper limit of quantification).
Measurement of Rat Pharmacokinetic Parameters—Four male Sprague-Dawley rats (250 g), surgically prepared with jugular vein catheters, received a 5 mg/kg intravenous dose of CP-640186 in PEG 400 solution and four animals received a 10 mg/kg oral dose of CP-640186 in 0.5% methylcellulose. Blood samples (200 µl) were collected from the indwelling jugular vein catheter up to 24 h after dose administration and placed into heparinized tubes. Samples were centrifuged at 3000 rpm for 10 min and the plasma removed and stored frozen before analysis. Pharmacokinetics parameters were estimated using WinNonlin (version 1.3). Terminal elimination rate constant (Kel) was determined by linear regression of the log plasma concentrations. The terminal elimination half-life (t
) was calculated from 0.693/Kel. The area under the plasma concentration time curve (AUC) was calculated to the last time point at which drug could be measured using the linear trapezoidal rule and extrapolated to infinity by using Kel. Clearance was calculated by the relationship dose/AUC. The volume of distribution was calculated by the relationship clearance/Kel. Oral bioavailability was calculated from the ratio of AUCs after oral and intravenous doses after normalizing for the dose.
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RESULTS |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL PROCEDURESRESULTSDISCUSSIONREFERENCES |
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Our search for ACC inhibitors focused on those that either antagonize citrate-mediated ACC activation or inhibit enzymatic activity by mimicking acetyl-CoA or acyl-CoA molecules. Therefore, concentrations of citrate and acetyl-CoA were employed that yielded half-maximal activation (2 mM citrate; Ka = 2.7 mM) and half-maximal activity (30 µM acetyl-CoA; Km = 33 µM), respectively, whereas saturating concentrations of ATP (4 mM; Km = 43 µM) and KHCO3 (12.5 mM; Km = 2.5 mM) were employed. Additionally, by using partially purified ACC isolated from the livers of rats that had been fasted and then refed a high carbohydrate diet (induces ACC1 activity 10-fold; isozyme distribution: 85% ACC1, 15% ACC2), and by identifying compounds that inhibited ACC activity by greater than 75%, we selected for compounds that inhibited ACC1 activity. Inhibition of ACC2 activity was then determined in secondary specificity assays. Under these assay conditions, the IC50 values of known inhibitors of ACC activity, stearoyl-CoA, palmitoyl-CoA, and myristoyl-CoA were 0.6, 1.5, and 23 µM, respectively, similar to that reported in the literature (44).
The activity of hits identified using this format were confirmed, and subsequent potency evaluations yielded a number of actives that were considered worthy of further pursuit. Substructure searches around core structures and subsequent inhibitory activity evaluation confirmed that all were bona fide active series. The most attractive of these series is exemplified by the prototype CP-497485, a racemic compound that exhibited an IC50 against the partially purified rat liver enzyme of 250 nM.
Inhibition of ACC1 and ACC2 by CP-497485 and Its R-Enantiomer, CP-610431—In mammals, ACC exists as two tissue-specific isozymes, a 265-kDa isozyme (ACC1) present in lipogenic tissues such as liver and adipose tissue and a second 280-kDa isozyme (ACC2) present in oxidative tissues such as liver, skeletal muscle, and cardiac muscle (15, 29–33). Using antibodies specific for ACC1 and ACC2, exclusivity of ACC1 in rat adipose tissue, ACC2 in rat heart, ACC2 in rat skeletal muscle, and the presence of both isozymes in an 85:15 ratio in fasted, high sucrose diet-refed rat liver was demonstrated by immunoblotting after PAGE. As shown in Fig. 1, CP-497485 inhibited ACC activity partially purified from liver, heart, skeletal muscle, and adipose tissue with IC50 values of 120, 90, 150, and 80 nM, respectively, indicating a lack of isozyme specificity for the compound.
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Resolution of CP-497485 into its two enantiomers revealed that the R-enantiomer, CP-610431, was considerably more potent than the racemate in inhibiting rat ACC1 (IC50 = 35.7 ± 9.3 (S.D.) ± 1.9 (S.E.; n = 16) nM) and ACC2 (IC50 = 55 nM), whereas the S-enantiomer, CP-610432, did not substantially inhibit either ACC isoform at concentrations of up to 3 µM (chiral separation enantiomer efficacy data shown in Table I).
CP-610431 also inhibited mouse and cynomolgus macaque ACC1 and ACC2 activities. As noted above for the rat, mouse and cynomolgus macaque liver also contained both ACC1 and ACC2, whereas mouse and cynomolgus macaque skeletal muscle contained only ACC2. Likewise, when mice and cynomolgus macaques were fasted and then re-fed a high sucrose diet for 2 days, the hepatic ACC1 to ACC2 ratio was 6. Under these isolation conditions, rat, mouse, and cynomolgus macaque liver ACC activity averaged 29, 31.3, and 2.5 nmol/min/mg, whereas their skeletal muscle ACC averaged 3.4, 3.5, and 0.8 nmol/min/mg, respectively. CP-610431 inhibited both liver and skeletal muscle ACC activity from all three species with essentially equal potency (rat, 36 versus 55 nM; mouse, 50 versus 63 nM; cynomolgus macaque, 70 versus 26 nM), further demonstrating the isozyme nonselectivity and pan-species efficacy of ACC inhibition by CP-610431.
CP-497485 and CP-610431 both showed a high degree of specificity for ACC inhibition relative to inhibition of pyruvate carboxylase or propionyl-CoA carboxylase, exhibiting no inhibitory activity toward either of these enzymes at concentrations of up to 100 µM. CP-497485 and CP-610431 also both showed a high degree of specificity for inhibition of mammalian ACC versus fungal ACC. For example, CP-497485 (IC50 = 250 nM versus rat ACC1) showed no inhibition of Candida albicans ACC at concentrations of up to 10 µM and CP-610431 (IC50 = 36 nM versus rat ACC1) only modestly inhibited the C. albicans (IC50 = 6.0 µM) and Aspergillus fumigatus (IC50 = 1.1 µM) enzymes. Consistent with the enantiomer-specific inhibition of mammalian ACC, the less active enantiomer, CP-610432, was also less active versus the fungal enzyme, demonstrating no inhibition of the C. albicans enzyme at concentrations of up to 100 µM and only weakly inhibiting the A. fumigatus enzyme with an IC50 of 62 µM.
Kinetics of ACC Inhibition by CP-610431—ACC-catalyzed carboxylation of acetyl-CoA to form malonyl-CoA proceeds via a two-step process, a biotin carboxylase reaction and a carboxyl transferase reaction (38, 45, 46). Fatty acyl-CoAs inhibit ACC activity by competing with acetyl-CoA for binding within the active center (47). Citrate allosterically activates ACC by competing with this fatty acyl-CoA mediated enzyme inhibition (48). Detailed kinetic analyses of the catalytic mechanism of action of the ACC enzyme previously reported in the literature (45, 46) have suggested that the substrates bicarbonate and acetyl-CoA and the allosteric effectors citrate and fatty acyl-CoA all bind to overlapping regions within an area of the active center that catalyzes the enzymatic second half-reaction, that of the carboxyl transfer reaction, and that this site is distinct from the ATP binding site involved in CO2 fixation in the enzymatic first half-reaction.
As shown in Fig. 2, CP-610431 inhibits ACC noncompetitively with respect to acetyl-CoA, bicarbonate, and citrate but uncompetitively with respect to ATP. The reaction is reversible, as demonstrated by the absence of time-dependent loss of enzyme activity upon treatment with CP-610431 in experiments conducted between 0.5 and 10 min of duration in which, at concentrations of 50 and 200 nM, maximal inhibition of enzymatic activity of 44 and 87%, respectively, occurred within 30 s of compound addition and was maintained throughout the remainder of the 10-min incubation, consistent with a rapid establishment of an equilibrium between E and the various EI complex formations. Together, these observations suggest that CP-610431 interacts with the ACC active center at the overlapping acetyl-CoA, citrate, and fatty acyl-CoA binding sites of the second partial reaction, with little interaction with the ATP binding site of the first partial reaction.
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Inhibition of Fatty Acid Synthesis, TG Synthesis, and TG Secretion in HepG2 Cells and in Rat Primary Hepatocytes by CP-610431—To assess the ability of CP-497485 to inhibit the cholesterologenic, fatty acid synthetic, TG synthetic, and very low density lipoprotein secretory pathways in cultured cells that express predominately the ACC1 isoform and also to assess its potential to inhibit the human enzyme, CP-497485 was evaluated for its ability to inhibit cholesterol and fatty acid synthesis from [14C]acetate, inhibit TG synthesis from [3H]-glycerol, and inhibit TG and apoB secretion in HepG2 cells. In these studies, CP-497485 inhibited HepG2 cell fatty acid synthesis, TG synthesis, TG secretion, and apoB secretion with respective EC50 values of 1.5, 3.0, 3.5, and 9.1 µM, under conditions in which cholesterol synthesis, apoCIII secretion, and cell viability were not altered.
Similar to its improved in vitro potency, the active enantiomer, CP-610431, was also considerably more potent than CP-497485 in inhibiting HepG2 cell fatty acid and TG synthesis and in inhibiting TG and apoB secretion. In these studies, CP-610431 dose-dependently inhibited HepG2 cell fatty acid synthesis with an IC50 of 1.6 µM, TG synthesis with an IC50 of 1.8 µM, TG secretion with an IC50 of 3.0 µM, and apoB secretion with an IC50 of 5.7 µM (Fig. 3). Similar to its vastly reduced in vitro activity, the inactive enantiomer, CP-610432, also did not inhibit HepG2 cell fatty acid synthesis at concentrations up to 100 µM, and showed only modest inhibition of TG synthesis, TG secretion, and apoB secretion, at concentrations of 30 µM.
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CP-610431 also inhibited fatty acid synthesis, TG synthesis, and TG secretion in mouse primary hepatocytes with EC50 values similar to those noted in HepG2 cells without altering cholesterol synthesis at concentrations of up to 30 µM. In these studies, CP-610431 inhibited mouse primary hepatocyte fatty acid and TG synthesis with IC50 values of 0.11 and 1.2 µM and inhibited TG secretion with an IC50 of 10 µM.
Inhibition of Mouse Hepatic and Adipose Fatty Acid Synthesis by CP-610431—CP-610431 also inhibited hepatic fatty acid synthesis in fasting CD1 mice by 64 ± 12% (S.D.; n = 6), and 77 ± 4% (S.D.; n = 6), respectively, 1 h after intraperitoneal administration at doses of 30 and 100 mg/kg. Similar inhibition of hepatic fatty acid synthesis was noted 1 h after intraperitoneal administration of CP-610431 to non-fasting animals (18, 51, and 75% inhibition at 10, 30, and 100 mg/kg), suggesting an ED50 of 30 mg/kg. The hydrochloride salt of CP-610431 also showed acute (1 h) oral efficacy, inhibiting hepatic fatty acid synthesis with an ED50 of 22 mg/kg. Consistent with the findings above, the inactive enantiomer, CP-610432, was virtually inactive in this model at concentrations of up to 100 mg/kg.
CP-610431 also inhibited hepatic fatty acid synthesis in ob/ob mice within 1 h after dose with an ED50 of 3.9 mg/kg. Rates of hepatic fatty acid synthesis in ob/ob mice were 10 times those of CD1 mice (e.g. 352 ± 113 (S.D.; n = 6) versus 39.7 ± 8.2 (S.D.; n = 6) dpm/mg/h), consistent with reports that leptin is a negative regulator of fatty acid synthesis (49, 50). In these studies, CP-610431 also inhibited visceral and subcutaneous adipose tissue fatty acid synthesis with an ED50 of 10 mg/kg, suggesting peripheral exposure suitable for exhibiting efficacy in other nonhepatic tissues. Indeed, at a dose of 10 mg/kg, for example, CP-610431 inhibited hepatic fatty acid synthesis by 64%, visceral adipose fatty acid synthesis by 46%, and subcutaneous adipose tissue fatty acid synthesis by 56%.
Metabolic Instability of CP-610431—Although CP-610431 demonstrated acute in vivo fatty acid synthesis inhibition efficacy in short term (1-h) studies, the duration of efficacy of CP-610431 administered intraperitoneally to CD1 mice at a dose of 100 mg/kg was limited to the first 2 h of exposure with no inhibitory activity noted four hours after administration. Evaluation of the hepatic microsomal metabolism of CP-610431 demonstrated average half-lives of 8.5, 7.5, and 7.2 min, respectively, in human liver, rat liver, and CD1 mouse liver microsomes, indicating a P450-mediated metabolic liability of the compound. Half-lives of the inactive enantiomer, CP-610432, in the presence of human and rat liver microsomes averaged 6.6 and 12.4 min, respectively, indicating the absence of enantiomer-specific microsomal metabolism.
In the presence of 1-aminobenzotriazole (ABT; a nonspecific microsomal CYP inhibitor that blocks CYP-mediated microsomal metabolism), given intraperitoneally to CD1 mice at 50 mg/kg, 30 min prior to compound addition, CP-610431 showed an 10-fold increase in inhibitory activity, inhibiting fatty acid synthesis by 66 and 94% following intraperitoneal administration of doses of 3 and 10 mg/kg, respectively. In these studies, CP-610431 exhibited both an ED50 of 3.1 mg/kg, as compared with 24 mg/kg in the absence of ABT treatment, and a marked increase in duration of action such that maximal inhibition of 97% occurred 4 h after treatment, with 90 and 47% inhibition noted 8 and 16 h, respectively, after treatment, consistent with CYP-mediated metabolism to a less potent compound. Indeed, biotransformation studies to be published elsewhere3 identified the main route of metabolism of CP-610431 to be rapid cleavage of an N-ethyl group followed by slower loss of the second N-ethyl group. Both metabolites were found to be significantly less potent than the parent (IC50 = 510 and 1770 nM, respectively), confirming metabolic conversion of CP-610431 to a less active metabolite. Furthermore, the unsubstituted amide and also the free acid (IC50 = 10 µM) appeared relatively stable (metabolic half-lives in human liver microsomes of 31 and >45 min, respectively), identifying the diethylamide moiety of CP-610431 as the primary site of metabolism and suggesting minimal microsomal metabolism of other portions of the molecule.
Identification of CP-610431 Analogs with Improved Metabolic Stability—A variety of analogs with presumably improved metabolic stability were prepared and evaluated for intrinsic activity. The activity, microsomal stability, and structure-activity relationships for this series of analogs will be reported elsewhere.4 Three of these, however, CP-640188 (diisopropylamide moiety; IC50 = 10.5 nM), CP-640189 (di-[2,2,2-trifluoro-ethyl]amide moiety; IC50 = 29 nM), and CP-640186 (morpho-linamide moiety; IC50 = 70.3 ± 26.1 (S.D.) ± 4.4 (S.E.; n = 35) nM), were subsequently evaluated for their ability to inhibit murine hepatic fatty acid synthesis in the presence and absence of ABT. Both CP-640188 and CP-640189 exhibited a reduced requirement for ABT for maximal efficacy, suggesting improved metabolic stability. For example, when CD1 mice were treated for 1 h at a dose of 3 mg/kg prior to measurement of fatty acid synthesis, CP-610431 reduced fatty acid synthesis by 33% in the absence of ABT and 65% in its presence, whereas, CP-640188 lowered fatty acid synthesis by 53 and 54% and CP-640189 lowered fatty acid synthesis by 59 and 62% in the absence and presence of ABT. Indeed, when evaluated for their rates of hepatic microsomal metabolism, CP-640188 and CP-640189 demonstrated increased average half-lives in the presence of human liver, rat liver, and CD1 mouse liver microsomes of 9.3, 18.9, and 10.6 min, and 9.0, 41.2, and 11.7 min, respectively. Continued improvements in metabolic stability led to identification of CP-640186, which exhibited further improved metabolic half-lives in human liver, rat liver, and mouse liver microsomes of 13.8, >45, and 21.8 min, respectively. Consistent with this improved metabolic stability, CP-640186 demonstrated significant in vivo efficacy improvements in mice in the absence of ABT, retaining activity for up to 8 h after administration (see below), and was subsequently selected for additional in vivo evaluation.
In Vitro and in Vivo Efficacy of CP-640186 —Table II summarizes the in vitro, in culture, and acute in vivo fatty acid synthesis inhibitory properties of CP-640186. CP-640186 inhibits both ACC1 and ACC2 with a potency of 55 nM, similar to that observed for CP-610431. However, as a consequence of its improved metabolic stability in hepatic microsomes, CP-640186 exhibited an 3-fold greater efficacy in inhibiting fatty acid synthesis in HepG2 cells (0.62 versus 1.6 µM), and a 2- and 3-fold greater efficacy in inhibiting fatty acid synthesis in CD1 mice (11 versus 22 mg/kg) and ob/ob mice (4 versus 12 mg/kg) when administered orally in the absence of ABT.
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In ob/ob mice, CP-640186 demonstrated acute efficacy for up to 8 h after oral administration, exhibiting ED50 values of 4.6, 9.7, and 21 mg/kg, at 1, 4, and 8 h, respectively, after treatment. Threshold efficacy of CP-640186 coincided with plasma drug concentrations of 150 ng/ml, whereas half-maximal efficacy coincided with plasma drug concentrations of 350 ng/ml. Peak plasma levels of 2 µg/ml (10 mg/kg oral dose) coincided with peak efficacy and occurred within 1 h after oral administration, at a time when the liver to plasma drug ratio was 5. Similarly, in the Sprague-Dawley rat, threshold efficacy occurred at plasma drug concentrations of 120 ng/ml and half-maximal efficacy occurred at plasma drug concentrations of 300 ng/ml. Likewise, peak plasma levels also occurred in the rat within 1 h of oral administration, at which time the liver to plasma drug ratio was between 4 and 6. Skeletal muscle concentrations, determined 2 h after oral administration at doses between 10 mg/kg (ED50 dose) and 100 mg/kg increased linearly with increasing dose, averaged 154 ng/g of tissue (10 mg/kg) to 1330 ng/g of tissue (100 mg/kg), and mirrored plasma drug concentrations, which averaged 145–1145 ng/ml, respectively.
Pharmacokinetic evaluation of CP-640186 in male Sprague-Dawley rats (intravenous dose, 5 mg/kg; oral dose, 10 mg/kg) yielded a plasma half-life of 1.5 h, a bioavailability of 39%, a Clp of 65 ml/min/kg, a Vdss of 5 liters/kg, an oral Tmax of 1.0 h, an oral Cmax of 345 ng/ml, and an oral AUC0-
of 960 ng·h/ml. At the same dose level, pharmacokinetic evaluation of CP-640186 in ob/ob mice yielded a plasma half-life of 1.1 h, a bioavailability of 50%, a Clp of 54 ml/min/kg, an oral Tmax of 0.25 h, an oral Cmax of 2177 ng/ml, and an oral AUC0- of 3068 ng·h/ml. These data indicate that drug exposure is significantly lower in the rat than the ob/ob mouse at equal doses under these conditions, and this may in part explain the greater fatty acid reducing efficacy of CP-640186 in the ob/ob mouse relative to the rat (Table II). Whether this improved exposure is the result of an accumulation of the compound in the substantially larger adipose pool in the ob/ob mouse, which further limits its availability for metabolic clearance, remains to be demonstrated experimentally.
Tissue Malonyl-CoA Lowering by CP-640186 —As a consequence of inhibition of ACC1 and ACC2 activity by CP-640186, reductions in fatty acid synthesis in lipogenic tissues such as the liver and adipose tissue, and increases in mitochondrial fatty acid oxidation in oxidative tissues such as the liver, skeletal muscle, and cardiac muscle should be preceded by reductions in tissue malonyl-CoA concentrations. To assess changes in malonyl-CoA concentration in these tissues in response to CP-640186 treatment, the method for assessing tissue malonyl-CoA concentration developed by McGarry et al. (42) was adapted to an up to 30 rat/experiment scale (6 groups of 5 animals each), with simultaneous harvesting of soleus muscle, gastrocnemius muscle, quadriceps muscle, adipose tissue, liver, and heart, in the order specified to minimize blood-flow interruption, as outlined under "Experimental Procedures." Tissue malonyl-CoA levels, determined using this modification to McGarry's procedure, expressed as mean ± S.E. (n = 10) were as follows: soleus muscle, 1.64 ± 0.13 nmol/g; gastrocnemius muscle, 1.81 ± 0.23 nmol/g; quadriceps muscle, 1.15 ± 0.18 nmol/g; adipose tissue, 0.51 nmol/g; liver, 6.7 ± 0.34 nmol/g; and heart, 1.94 ± 0.15 nmol/g, and compared closely with values reported in the literature (51–53).
The dynamics of the measurement were validated by assessing the ability to modulate tissue malonyl-CoA levels in response to physiological perturbations known to raise and lower tissue malonyl-CoA concentrations in liver and skeletal muscle tissues (16, 54, 55). For example, fasting overnight reduced liver, heart, soleus muscle, and quadriceps muscle malonyl-CoA concentrations by 60% (Table III), with no further reduction noted when animals were starved for 48 h (not shown), and refeeding either chow (Table III) or a semi-purified high carbohydrate diet (not shown) for 24 h after an overnight fast returned tissue malonyl-CoA concentrations to initial levels.
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Single-dose oral treatment with 100 mg/kg CP-640186 also reduced tissue malonyl-CoA concentrations in chow-fed animals such that liver levels were reduced by 58% (p = 0.009), heart levels were reduced by 63% (p = 0.0001), soleus muscle levels were reduced by 59% (p = 0.007), and quadriceps muscle levels were reduced by 80% (p = 0.002) within 1 h of administration (Table III). The reduction in malonyl-CoA concentration induced by CP-640186 in all tissues evaluated was dose-dependent with ED50 values of 6 mg/kg (soleus muscle), 8 mg/kg (cardiac muscle), 15 mg/kg (quadriceps muscle), and 55 mg/kg (liver) noted 1 h after administration (Fig. 4). Tissue malonyl-CoA levels were also reduced somewhat in fasted animals, but the degree of lowering observed in this already depleted state were considerably diminished in magnitude (Table III).
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Stimulation of Fatty Acid Oxidation in C2C12 Rat Skeletal Muscle by CP-640186 —The reduction in malonyl-CoA levels that occurs in lipogenic tissues after inhibition of ACC1 by CP-640186 leads to a reduction in fatty acid synthesis both in cultured hepatic cells and in lipogenic tissues of experimental animals (see above). The reduction in malonyl-CoA concentration that occurs in oxidative tissues after inhibition of ACC2 by CP-640186 theoretically should lead to a derepression of mitochondrial CPT-1 activity (malonyl-CoA allosterically inhibits CPT-1 (Refs. 18 and 19)) and an increase in fatty acid utilization in these tissues. In this regard, the acute effects of CP-640186 on skeletal muscle fatty acid metabolism ([14C]palmitate oxidation) were assessed in cultured mouse muscle (C2C12) cells and in isolated rat epitrochlearis muscle strips. In both C2C12 cells and muscle strips, CP-640186 increased fatty acid metabolism in a concentration-dependent manner. In C2C12 cells, CP-640186 stimulated [14C]palmitate acid oxidation with an EC50 of 57 nM and a maximal stimulation of 280% (Fig. 5, upper panel). Likewise, in isolated rat epitrochlearis muscle, CP-640186 stimulated [14C]palmitate acid oxidation with an EC50 of 1.3 µM and a maximal stimulation of 240% (Fig. 5, lower panel). The higher EC50 value noted in the epitrochlearis muscle presumably reflects tissue penetration differences between the intact muscle bundle and isolated cells. For comparison, in the same experiments, 2.0 mM 5-amino-4-imidazolecarboxamide, Z-riboside, which activates AMP-activated protein kinase, leading to phosphorylation and inactivation of ACC (53), stimulated [14C]palmitate acid oxidation in C2C12 cells and in isolated rat epitrochlearis muscle by 200 and 210%, respectively.
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Stimulation of Rat Fatty Acid Oxidation by CP-640186 —CP-640186 also increased whole body fatty acid oxidation acutely in chow-fed and in fasted/high carbohydrate diet-refed rats, shifting energy metabolism away from carbohydrate utilization and toward increased <
