Supplemental Material can be found at: http://dmd.aspetjournals.org/content/suppl/2013/04/12/dmd.112.050864 .DC1.html
1521-009X/41/7/1319–1328$25.00 DRUG METABOLISM AND DISPOSITION Copyright ª 2013 by The American Society for Pharmacology and Experimental Therapeutics
http://dx.doi.org/10.1124/dmd.112.050864 Drug Metab Dispos 41:1319–1328, July 2013
Mechanistic Pharmacokinetic-Pharmacodynamic Modeling of BACE1 Inhibition in Monkeys: Development of a Predictive Model for Amyloid Precursor Protein Processing s Xingrong Liu, Harvey Wong, Kimberly Scearce-Levie, Ryan J. Watts, Melis Coraggio, Young G. Shin, Kun Peng, Kristin R. Wildsmith, Jasvinder K. Atwal, Jason Mango, Stephen P. Schauer, Kelly Regal,1 Kevin W. Hunt, Allen A. Thomas, Michael Siu, Joseph Lyssikatos, Gauri Deshmukh, and Cornelis E. C. A. Hop Genentech Inc., South San Francisco, California (X.L., H.W., K.S.-L., R.J.W., M.C., Y.G.S., K.P., K.R.W., J.K.A., J.M., S.P.S., M.S., J.L., G.D., C.E.C.A.H.); and Array BioPharma Inc., Boulder, Colorado (K.R., K.W.H., A.A.T.)
ABSTRACT This study was conducted to determine the pharmacokinetics (PK) and pharmacodynamics (PD) of two novel inhibitors of b-site amyloid precursor protein (APP)–cleaving enzyme (BACE1), GNE-629 [(4S,4a9S,10a9S)-2-amino-89-(2-fluoropyridin-3-yl)-1-methyl-39,49,4a9,10a9tetrahydro-19H-spiro[imidazole-4,109-pyrano[4,3-b]chromen]5(1H)-one] and GNE-892 [(R)-2-amino-1,39,39-trimethyl-79-(pyrimidin5-yl)-39,49-dihydro-29H-spiro[imidazole-4,19-naphthalen]-5(1H)-one], and to develop a PK-PD model to predict in vivo effects based solely on in vitro activity and PK. GNE-629 and GNE-892 concentrations and PD biomarkers including amyloid b (Ab) in the plasma and cerebrospinal fluid (CSF), and secreted APPb (sAPPb) and secreted APPa (sAPPa) in the CSF were measured after a single oral administration of GNE-629 (100 mg/kg) or GNE-892 (30 or 100 mg/kg) in cynomolgus monkeys. A mechanistic PK-PD model was developed
to simultaneously characterize the plasma Ab and CSF Ab, sAPPa, and sAPPb using GNE-629 in vivo data. This model was used to predict the in vivo effects of GNE-892 after adjustments based on differences in in vitro cellular activity and PK. The PK-PD model estimated GNE-629 CSF and free plasma IC50 of 0.0033 mM and 0.065 mM, respectively. These differences in CSF and free plasma IC50 suggest that different mechanisms are involved in Ab formation in these two compartments. The predicted in vivo effects for GNE-892 using the PK-PD model were consistent with the observed data. In conclusion, a PK-PD model was developed to mechanistically describe the effects of BACE1 inhibition on Ab, sAPPb, and sAPPa in the CSF, and Ab in the plasma. This model can be used to prospectively predict in vivo effects of new BACE1 inhibitors using just their in vitro activity and PK data.
Introduction
APP is processed via two distinct pathways: namely, the amyloidogenic and nonamyloidogenic. The amyloidogenic pathway includes cleavage of APP by b-secretase, also known as b-site APP cleavage enzyme 1 (BACE1), producing a secreted ectodomain of APP (sAPPb) and a membrane-tethered C-terminal fragment with 99 amino acids, which is further cleaved by g-secretase and results in multiple Ab peptides of 38 to 42 amino acids long (Sinha and Lieberburg, 1999). The most abundantly produced variants are Ab1-40 and Ab1-42. Although Ab1-40 is more abundant, Ab1-42 is considered the most toxic peptide (Younkin, 1995; Lesné et al., 2006). In contrast, the nonamyloidogenic pathway includes cleavage of APP by a-secretase and formation of secreted APPa (sAPPa) and a C-terminal fragment with 83 amino acids. These peptides do not contribute to the formation of plaques in the brain (Kandalepas and Vassar, 2012). One approach to reduce the levels of the toxic amyloidogenic peptides is to inhibit BACE1 and thereby direct the APP cleavage
Alzheimer’s disease (AD) is characterized pathologically by the accumulation of extracellular amyloid b (Ab) plaques and intracellular neurofibrillary tangles in the brain, subsequently leading to neuronal loss in brain regions important for cognition and memory. These amyloid plaques are primarily composed of Ab peptides generated from amyloid precursor protein (APP) (Glenner and Wong, 1984; Masters et al., 1985). AD caused by the accumulation of Ab in the brain through either overproduction or reduced clearance is the leading hypothesis in AD research and is known as “the amyloid hypothesis” (Hardy and Selkoe, 2002). This research was supported by Genentech Inc. 1 Current affiliation: ProPharma Services, Superior, Colorado. dx.doi.org/10.1124/dmd.112.050864. s This article has supplemental material available at dmd.aspetjournals.org.
ABBREVIATIONS: Ab, amyloid b peptide; AD, Alzheimer’s disease; APP, amyloid precursor protein; AUC, area under concentration-time curve; AZD3839, (S)-1-(2-(difluoromethyl)pyridin-4-yl)-4-fluoro-1-(3-(pyrimidin-5-yl)phenyl)-1H-isoindol-3-amine hemifumarate; BACE, b-site APP cleavage enzyme; BBB, blood-brain barrier; BSA, bovine serum albumin; CNS, central nervous system; CSF, cerebral spinal fluid; FBS, fetal bovine serum; fu, unbound fraction; GNE-629, 4S,4a’S,10a’S)-2-amino-8’-(2-fluoropyridin-3-yl)-1-methyl-39,4’,4a’,10a’-tetrahydro-1’H-spiro[imidazole-4,10’pyrano[4,3-b]chromen]-5(1H)-one; GNE-892, (R)-2-amino-1,39,39-trimethyl-7’-(pyrimidin-5-yl)-39,4’-dihydro-2’H-spiro[imidazole-4,1’-naphthalen]5(1H)-one; LC-MS/MS, liquid chromatography with tandem mass spectrometry; LY2811376, (4S)-4-(2,4-difluoro-5-pyrimidin-5-ylphenyl)4-methyl-5,6-dihydro-4H-1,3-thiazin-2-amine; MDCK, Madin-Darby canine kidney cell line; MDR1, human multidrug resistance gene; P-gp, P-glycoprotein; PD, pharmacodynamics; PK, pharmacokinetics; sAPPa, secreted APPa peptide; sAPPb, secreted APPb peptide. 1319
Downloaded from dmd.aspetjournals.org at F. Hoffmann La Roche Ltd on August 11, 2013
Received December 21, 2012; accepted April 12, 2013
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through the nonamyloidogenic pathway (McConlogue et al., 2007; Citron, 2010). However, it has been particularly challenging to develop orally available and brain-penetrable small-molecule BACE1 inhibitors. Great progress has been made in the last few years to design potent BACE1 inhibitors that can reduce brain Ab after oral administration. There are numerous reports studying the effects of BACE1 inhibition on cerebral spinal fluid (CSF) and/or brain Ab and other biomarkers in mice, rats, dogs, monkeys, and humans (Hussain et al., 2007; Elvang et al., 2009; Sankaranarayanan et al., 2009; Zhu et al., 2009; Fukumoto et al., 2010; May et al., 2011; Dineen et al., 2012; Weiss et al., 2012). Sankaranarayanan et al. (2009) were the first to show in vivo reduction of plasma and CSF Ab after single or multiple oral administration of a BACE1 inhibitor, N-[4-{5-[(1R)1-amino-1-methyl-2-phenylethyl]-1,3,4-oxadiazol-2-yl}-3-chloro-6-((2methoxyethyl){[(1S,2S)-2-methylcyclopropyl]methyl}amino)pyridin2-yl]-Nmethylmethanesulfonamide, in nonhuman primates (rhesus monkeys). However, due to its extensive first-pass metabolism, coadministration of the cytochrome P450 inhibitor ritonavir was necessary to boost the oral exposure of N-[4-{5-[(1R)-1-amino-1methyl-2-phenylethyl]-1,3,4-oxadiazol-2-yl}-3-chloro-6-((2-methoxyethyl){[(1S,2S)-2-methylcyclopropyl]methyl}amino)pyridin-2-yl]-Nmethylmethanesulfonamide to achieve efficacious levels in the reported preclinical studies. May et al. (2011) reported that a potent BACE1 inhibitor LY2811376 [(4S)-4-(2,4-difluoro-5-pyrimidin-5-ylphenyl)4-methyl-5,6-dihydro-4H-1,3-thiazin-2-amine] reduced Ab levels in human CSF. The development of this compound was discontinued due to ocular toxicity in rats. Recently, Jeppsson et al. (2012) disclosed that a potent BACE1 inhibitor AZD3839 [(S)-1-(2-(difluoromethyl)pyridin4-yl)-4-fluoro-1-(3-(pyrimidin-5-yl)phenyl)-1H-isoindol-3-amine hemifumarate] with an IC50 of 0.0048 mM can reduce CSF Ab in monkeys. However, the reduction was not robust as AZD3839 has a very short half-life (18 minutes) in monkeys. Therefore, more potent and safe BACE1 inhibitors are needed to test the amyloid hypothesis in humans (Probst and Xu, 2012). In this article, we report the in vivo characterization of BACE1 inhibition in monkeys for the following two novel BACE1 inhibitors: GNE-629 [4S,4a’S,10a’S)-2-amino-8’-(2-fluoropyridin-3-yl)-1methyl-39,4’,4a’,10a’-tetrahydro-1’H-spiro[imidazole-4,10’-pyrano[4,3b]chromen]-5(1H)-one] and GNE-892 [(R)-2-amino-1,39,39-trimethyl7’-(pyrimidin-5-yl)-39,4’-dihydro-2’H-spiro[imidazole-4,1’-naphthalen]5(1H)-one] (Fig. 1). Mechanistic pharmacokinetic-pharmacodynamic (PK-PD) modeling can play an important role in the drug discovery and development process by providing an integrated understanding of relationships between compound plasma concentrations and biomarker responses (Gabrielsson and Green, 2009; Gibbs, 2010; van der Graaf and Benson, 2011; Wong et al., 2012a,b). In addition, PK-PD modeling is a useful tool that can facilitate the translation of preclinical data to humans. Recently, Lu et al. (2012) reported the use of an indirect response PK-PD model for Ab lowering in CSF and brain (Jusko and Ko, 1994). The reported PK-PD modeling efforts on in vivo BACE1 inhibition studies characterized the effects of BACE1 inhibition on Ab, and no models have been reported that examine the effects of BACE1 inhibition on overall APP processing. The development of such a model would aid in better understanding the interactions of BACE1 inhibitors with various products of APP processing and allow for predictions of how these potential PD markers would behave after BACE1 inhibition. Therefore, the objectives of the present study were to examine BACE1 inhibition in cisterna magna cannulated cynomolgus monkeys for two novel BACE1 inhibitors GNE-629 and GNE-892 and to use a modeling approach to study BACE1 inhibition in the monkeys.
Fig. 1. Structures of GNE-629 and GNE-892.
Materials and Methods Chemicals. GNE-629 and GNE-892 were synthesized at Array BioPharma Inc. (Boulder, CO). All chemicals used in the experiments were of the highest available commercial grade. BACE1 Enzyme Activity Assay. Recombinant human BACE1 expressed and isolated from Chinese hamster ovary cells was incubated with 0.15 mM peptide substrate (Biotin-KTEEISEVNLDAEFRHDSGYEVHHQKL) (American Peptide Company, Inc., Sunnyvale, CA) in 6 ml 50 mM sodium acetate, pH 4.4, and 0.1% 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate for 6 hours at room temperature. The reaction with various concentrations of BACE1 inhibitors was quenched with 5 ml of 200 mM Tris pH 8.0 buffer containing 20 mM EDTA, 0.1% bovine serum albumin (BSA), 0.8 M KF, the time-resolved fluorescence resonance energy transfer acceptor strepavidin-D2 (final concentration 0.043 mM; CisBio Bioassays, Codolet, France), and the time-resolved fluorescence resonance energy transfer donor Europium cryptate–labeled antipeptide antibody 6E10 (final concentration 0.0011 mM; CisBio). After 2 hours at room temperature, the ratio of the emissions at 665 and 615 nm was determined and the IC50 was calculated using a 4-parameter logistic fit. BACE1 Cellular Inhibition Assay. HEK-293 cells stably transfected with a DNA construct containing the coding sequence for the wild-type human APP695 sequence (293-hAPP cells) were grown in Dulbecco’s modified Eagle’s medium supplemented with 10% fetal bovine serum (FBS), penicillin/ streptomycin, and 150 mg/ml G418 [O-2-amino-2,7-didesoxy-D-glycero-aD-gluco-heptopyranosyl-(1→4)-O-(3-desoxy-4-C-methyl-3-(methylamino)-b-Larabinopyranosyl-(1→6))-D-streptamin]. Cells were plated in 96-well plates at 35,000 cells per well and allowed to attach for 8–12 hours. Media were changed to Dulbecco’s modified Eagle’s medium supplemented with 10% FBS and penicillin/streptomycin 15 minutes prior to compound addition. Compound was then added at a final concentration of 0.5% dimethylsulfoxide. After 48 hours, 4 ml media from each well was added to a corresponding well of a 384-well plate (Cat# 6008280; PerkinElmer, Waltham, MA) containing the homogeneous timeresolved fluorescence reagents. Homogeneous time-resolved fluorescence analysis was performed using the CisBio amyloid b1-40 peptide assay kit (Cat# 62B40PEC) following the manufacturer’s instructions. In Vitro Transporter Assay. Madin-Darby canine kidney cell line 1 (MDCKI) cells transfected with the human multidrug resistance gene 1
PK-PD Modeling of BACE1 Inhibition (MDR1) gene (MDCKI–MDR1) were maintained at 37°C, 95% humidity, and 5% CO2 in culture with Eagle’s minimum essential medium (0.1% nonessential amino acids, 1 mM sodium pyruvate, 2 mM L-glutamine, 1.5 g/l sodium bicarbonate) supplemented with 10% FBS. The MDCKI–MDR1 cell medium was further supplemented with 0.2 mM colchicine to sustain MDR1 expression. The monolayers were equilibrated for 30 minutes in transport buffer (Hanks’ balanced salt solution with 10 mM HEPES, pH 7.4) at 37°C with 5% CO2 and 95% humidity prior to the experiment. Drug concentrations were determined by liquid chromatography with tandem mass spectrometry (LC-MS/MS). The transport of GNE-629 or GNE-892 (5 mM) was examined in the apical-tobasolateral and basolateral-to-apical directions. The apparent permeability (Papp) was calculated using eq. 1: Papp ¼
dQ 1 1 • • dt C0 A
ð1Þ
where dQ/dt is the rate of compound appearance in the receiver compartment, C0 is the concentration in the donor compartment at time = 0, and A is the surface area of the insert. In Vivo Experiments. The PK and PD studies were conducted in male cynomolgus macaques monkeys (3–5 years, 2–3 kg) implanted with catheters in the cisterna magna that allowed for repeated sampling of CSF in conscious animals (Maccine Pte Ltd, Singapore). After the catheters were implanted, animals were maintained for at least 2 weeks on a 12-hour light/dark cycle in a temperature- and humidity-controlled environment with free access to food and water prior to dosing the inhibitors. Dosing solutions of each drug were prepared as a suspension in 1% carboxymethylcellulose sodium, 0.5% Tween 80, and 5 mM citrate buffer at pH 5.0. The GNE-629 PK-PD study was conducted in a two-way crossover design with vehicle and the 100 mg/kg dose being given orally with an 18-day washout between each dosing period (n = 11 animals). The GNE-892 PK-PD study was conducted in a three-way crossover design with the vehicle, 30 mg/kg, and 100 mg/kg doses being given orally with 18- to 20-day washout periods between each arm of the study (n = 13 animals). Whole blood was collected from the femoral vein. Baseline blood and CSF samples were collected from 216 to 24 hours prior to dosing. Serial blood and CSF samples were collected postdose from 0 to 72 hours for GNE-629 and from 0 to 48 hours for GNE-892. CSF samples were diluted 1:2 with 5% BSA solution in polypropylene tubes. Whole blood samples were stored on ice until centrifuged for the preparation of plasma. The plasma and CSF samples were immediately frozen on dry ice and then stored at 270°C. All studies were conducted in accordance with Genentech Animal Care and Use Procedures. Determination of Ab1-40 in Plasma. The concentrations of Ab1-40 in plasma were determined using an assay modified based on the MA6000 Human (6E10) Ab Kit (Cat# K111BVE-2; Meso Scale Discovery, Gaithersburg, MD). Assay procedures followed the manufacturer’s instructions. The capture antibody, specific for the C terminus of Ab1-40, was precoated on the 96-well plates (Meso Scale Discovery), and Sulfo-Tag anti-Ab monoclonal antibody 6E10 was used for detection. A detection substrate was added and electrochemical luminescence was measured with a Sector Imager (Meso Scale Discovery). The assay had a lower limit of quantification of 0.060 ng/ml in plasma. Determination of Ab 1-40 in CSF. The concentrations of total Ab1-40 in CSF samples were determined using a sandwich enzyme-linked immunosorbent assay. Briefly, rabbit polyclonal antibody specific for the C terminus of Ab1-40 (Millipore, Bedford, MA) was coated onto 96-well plates, and biotinylated anti-Ab1-40 monoclonal antibody 6E10 (Covance, Dedham, MA) was used for detection. The assay had a lower limit of quantification of 0.14 ng/ml in CSF. Determination of sAPPa and sAPPb in CSF. CSF concentrations of sAPPa and sAPPb were determined using the sAPPa/sAPPb multi-spot assay from Meso Scale Discovery. CSF was thawed on ice, and then diluted 1:20 into 1% BSA in Tris-buffered saline/Tween 20 (TBST) (10 mM Tris buffer, pH 8.0, 150 mM NaCl, 0.1% Tween-20). The assay was performed per the manufacturer’s protocol. Briefly, the multi-spot plate (Meso Scale Discovery) was blocked for 1 hour at room temperature with 3% BSA in TBST. Plates were rinsed three times with TBST, and the diluted CSF was added to the wells in duplicate for 1 hour. After analyte capture, plates were rinsed with TBST and detection antibody was added for 1 hour. After a final TBST rinse, a detection substrate (Sulfo-Tag; Meso Scale Discovery) was added and electrochemical
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luminescence was measured with a Sector Imager (Meso Scale Discovery). The assay had lower limit of quantification of 0.05 ng/ml for sAPPa and 0.03 ng/ml for sAPPb. Plasma Protein Binding. The unbound fractions in plasma for GNE-629 and GNE-892 were determined in a 48-well rapid equilibrium dialysis device using a dialysis membrane with a molecular mass cut-off value of 8000 (Pierce Biotechnology, Rockford, IL). Blank plasma was spiked with GNE-629 or GNE-892 at a final concentration of 5 mM. Three hundred microliters of plasma containing compound was added to the donor side and 500 ml of buffer was added to the receiver side. The plate was placed on a rocking platform at 400 rpm with 1 mm radius agitation for 4 hours at 37°C. Twenty microliters of buffer and plasma samples was transferred to a 96-well plate and 20 ml of blank plasma and buffer was added to the buffer and plasma samples, respectively. Plasma proteins were precipitated using 65% acetonitrile containing internal standard (0.1 mM propranolol), drug concentrations were quantitated by LC-MS/MS, and the unbound fraction (fu) was calculated as the ratio of the buffer concentration versus the plasma concentration. LC-MS/MS Analysis of GNE-629 and GNE-892 in Plasma and CSF. The plasma samples were prepared for analysis by adding 25 ml GNE-629 or GNE892 plasma into a 96-well plate followed by the addition of 25 ml internal standard solution and 200 ml acetonitrile to each sample for protein precipitation. The samples were vortexed and centrifuged for 5 minutes at 1500g. Fifty microliters of the resulting supernatant was mixed with 300 ml water and 10 ml was injected onto an LC-MS/MS for analysis. For the CSF samples, 25 ml of GNE-629 or GNE-892 CSF samples was mixed with 25 ml blank plasma and then 25 ml of each CSF-blank plasma mixture was extracted using the same protein precipitation procedure as described above for the plasma samples. Concentrations of GNE-629 and GNE-892 in plasma and CSF extracts were determined using an AB Sciex QTrap4000 mass spectrometer (AB Sciex, Foster City, CA) connected to a CTC PAL autosampler (Leap Technologies, Chapel Hill, NC). GNE-629 and GNE-892 were separated using an ACE 5 phenyl reverse phase (100 2.1 mm, 5 mm) column (Advanced Chromatography Technologies, Aberdeen, UK) at room temperature using a LC-10AD pump with a Shimadzu SCL-10A controller (Shimadzu, Columbia, MD). The mobile phase consisted of mobile phase A (water with 0.1% formic acid) and mobile phase B (acetonitrile with 0.1% formic acid). GNE-629 and GNE-892 were eluted using an initial condition of 20% mobile phase B followed by a linear gradient to 80% mobile phase B over 5 minutes with a flow rate of 0.5 ml/min. The mass spectrometer was used in the positive-ion mode using an electrospray interface at 400°C with nitrogen as both the nebulizing and heating gases. GNE-629 and GNE-892 were analyzed in the multiple reaction monitoring using the following transitions: m/z 383.2→299.1 and 336.2→252.2, respectively. The calibration curves for GNE-629 and GNE-892 were prepared by plotting the appropriate peak area ratios of analyte versus internal standard against the known concentrations of GNE-629 and GNE-892 in plasma or CSF using a quadratic regression with 1/x2 weighing. Concentrations of GNE-629 and GNE-892 in samples were determined by interpolation from their respective standard curves. The dynamic ranges for both compounds were 0.0027 to 52 mM in plasma and 0.00014 to 0.52 mM in CSF. A run was considered as acceptable when quality control samples as well as calibration curve samples were within 625% of the nominal concentration, except for the lowest quantification sample where 630% was acceptable. Data Analysis. The concentrations of all biomarkers are expressed as the percentage of the average baseline from 216 to 24 hours prior to the oral dose for each group. The plasma and CSF area under concentration-time curve (AUC) values were calculated using linear trapezoidal rule. The unpaired t test was used to determine the significance of the biomarker changes at each time point using Microsoft Excel (Redmond, WA). Mechanistic PK-PD Modeling. A mechanistic PK-PD model describing the pharmacokinetics and pharmacodynamics of GNE-629 in monkey was constructed using SAAM II v1.2.1 software (Saam Institute, University of Washington, Seattle, WA). All parameters were estimated by fitting the PK and PD models to the average of the observed data at each time point. Estimates of all parameters are presented as the parameter estimate followed by the percent standard error in parentheses. The configuration of the full mechanistic PK-PD model is shown in Fig. 2. An oral two compartment model with an additional compartment describing GNE-629 CSF concentrations (Fig. 2A) was fit to GNE-629 plasma and CSF concentrations as described by eqs. 2–5.
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Fig. 2. Schematic representation of the mechanistic PK-PD model for BACE1 inhibition. (A) PK model for GNE-629. (B) PK model for GNE-892. (C) PD model for CSF Ab, sAPPa, and sAPPb. (D) PD model for plasma Ab. Unbound plasma concentrations were linked to the plasma PD model and CSF concentrations were linked to the CSF PD model.
dX0 ¼ 2 k a X0 dt
ð2Þ
dX1 ¼ ka X0 þ k21 X2 þ kcsf 21 Xcsf 2 ðke þ k12 þ k12csf fu ÞX1 dt
ð3Þ
dX2 ¼ k12 X1 2 k21 X2 dt
ð4Þ
dXcsf ¼ k12csf fu X1 2 kcsf 21 Xcsf dt
ð5Þ
X0, X1, X2, and Xcsf are the amounts of compound (i.e., GNE-629) (mmol/kg) in the gastrointestinal, central, peripheral, and CSF compartments, respectively. ka and ke are the absorption and elimination rate constants (h21), respectively. k12 and k21 are the rate constants from the central to peripheral and from the peripheral to central compartment, respectively. k12csf and kcsf21 are the rate constants from the central to CSF and from the CSF to central compartment, respectively. The unbound fraction (fu) of GNE-629 in plasma was fixed to 0.39 as determined by equilibrium dialysis (Table 1). V1 is the apparent volume of the central compartment after oral dosing (l/kg) was fixed to 12.3 l/kg (calculated as Vc/F) based on a volume of the central compartment of 2.5 l/kg (Vc) estimated from an independent intravenous
cynomolgus monkey study and a bioavailability (F) of 0.21 at the 100 mg/kg oral dose (unpublished data). Vcsf is the volume of the CSF compartment (l/kg) and was fixed to a value of 2 ml/kg (Poplack et al., 1977; Muraszko et al., 1993; Rubenstein et al., 2003). Amounts in the central (or plasma) compartment and CSF compartment were scaled to concentrations by dividing by V1 and Vcsf, respectively. CL1-csf is the intercompartmental clearance between the central and the CSF compartment (l/h per kilogram) and CLcsf21 is the intercompartmental clearance between the CSF and central compartment (l/h per kilogram). They are secondary calculated parameters and were calculated using methods listed in Table 3. As noted in eqs. 3 and 5 above, unbound GNE-629 in the central compartment is assumed to have access to the CSF compartment. Parameter estimates from fitting the plasma and CSF concentrations were fixed prior to modeling the CSF and plasma PD models described below. The effects of GNE-629 on CSF APP processing are depicted by the CSF PD model in Fig. 2C and described by eqs. 6–9. The proposed PK-PD model assumes that the rate-limiting step for Ab formation is the cleavage of APP via b-secretase. This assumption is supported by the findings that the Ab profiles are similar to the sAPPb-time profiles for both BACE1 inhibitors in monkeys. Furthermore, it has been demonstrated in rats that brain Ab and C99 were also changed similarly after BACE1 inhibition (May et al., 2011). In addition, we assume the maximal inhibition is 100%.
TABLE 1 Molecular mass and in vitro parameters for GNE-629, GNE-892, and LY2811376 Parameter
GNE-629
GNE-892
LY2811376
Molecular mass BACE1 enzyme IC50 (mM) BACE1 cell IC50 (mM) MDR1 Pab (1026 cm/s) MDR1 Pba (1026 cm/s) MDR1 Pba/Pab ratio fu (monkey) fu (human)
382.4 0.010 6 0.003 0.0080 6 0.002 1.0 6 0.2 45 6 5 44 0.39 6 0.02 NA
335.4 0.048 6 0.041 0.027 6 0.014 1.2 6 0.1 31 6 3 25 0.55 6 0.02 NA
320.4 0.21 6 0.09 0.023 6 0.009 21 6 2 56 6 2 2.6 NA 0.44 6 0.01
NA, not available.
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PK-PD Modeling of BACE1 Inhibition dAPP Ccsf ¼ kinAPP 2 kAPPð1 2 Þ 2 ka APP dt IC50 þ Ccsf
ð6Þ
dAb Ccsf ¼ kAPPð1 2 Þ 2 kAb dt IC50 þ Ccsf
ð7Þ
dsAPPb Ccsf ¼ kAPPð1 2 Þ 2 ksAPPb dt IC50 þ Ccsf
ð8Þ
dsAPPa ¼ ka APP 2 ka sAPPa dt
ð9Þ
APP, sAPPa, sAPPb, and Ab are APP, sAPPa, sAPPb, and Ab1-40 levels in CSF (%), kinAPP is the formation rate of APP (%/h), k is a rate constant (h21) describing Ab and sAPPb formation from APP and their elimination, IC50 (mM) is the CSF concentration of a BACE1 inhibitor (i.e., GNE-629) that inhibits the formation rate of Ab and sAPPb from APP by 50%, and ka is the rate constant (h21) describing the formation and elimination of sAPPa. The initial values of APP, sAPPa, sAPPb, and Ab were set at 100%. Changes in plasma Ab concentrations are characterized using an indirect response model in Fig. 2D and described by eq. 10. dAbplasma fu C 1 ¼ kinplasma ð1 2 Þ 2 koutplasma Abplasma dt IC50ðplasmaÞ þ fu C1
ð10Þ
Abplasma is the plasma concentrations of Ab (%), kinplasma (%/h) is the formation rate of plasma Ab, IC50(plasma) (mM) is the unbound GNE-629 plasma concentration required for inhibiting the plasma Ab formation by 50%, and koutplasma (h21) is the first-order plasma Ab elimination rate constant. The initial value of Abplasma was set to 100%. The value of koutplasma was fixed to equal kinplasma/(initial value of Abplasma). Simulations of Ab, sAPPb, and sAPPa CSF Levels and Plasma Ab after Oral Administration of GNE-892 to Monkey. To test the performance of the mechanistic PD models, simulations of CSF levels of Ab, sAPPb, and sAPPa and plasma Ab levels were performed for the second BACE inhibitor, GNE-892. Prior to the simulation of the PD, the GNE-892 plasma and CSF concentrations were fit to PK model shown in Fig. 2B to provide GNE-892 concentrations to drive the PD responses. A CSF peripheral compartment was added to the GNE-629 PK model (Fig. 2B) to more accurately describe the CSF concentrations at the terminal phase. The model is described by eqs. 2–4 and eqs. 11 and 12 describe the additional CSF peripheral compartment. dXcsf ¼ k12csf fu X1 2 ðkcsf 21 þ kcsf 2 csfp ÞXcsf þ kcsfp 2 csf Xcsfp dt
ð11Þ
dXcsfp ¼ kcsf 2 csfp Xcsf 2 kcsfp 2 csf Xcsfp dt
ð12Þ
Xcsfp is the amount of GNE-892 in the CSF peripheral compartment (mmol/kg). kcsf2csfp and kcsfp2csf are the rate constants from the CSF to CSF peripheral and CSF peripheral to CSF compartments, respectively (h21). The fu of GNE-892 in plasma was fixed to 0.55 as determined by equilibrium dialysis (Table 1). Simulations of PD response in monkey were performed using the PK model for GNE-892 to provide GNE-892 plasma and CSF concentrations to drive the PD models. The in vivo plasma and CSF IC50 values for GNE-892 were calculated using the in vivo IC50 of GNE-629 (Table 3) after adjustment for the 3.4-fold decrease in potency for GNE-892 compared with GNE-629 in the BACE1 cell assay (Table 1). The in vivo CSF IC50 was set to 0.0111 mM and the plasma in vivo IC50(plasma) was set to 0.221 mM. Simulations of Ab, sAPPb, and sAPPa CSF Levels after Oral Administration of LY2811376 to Humans. CSF levels of Ab, sAPPb, and sAPPa after oral administration of 30 mg and 90 mg LY2811376 were simulated using the CSF PD model for GNE-629. LY2811376 CSF concentrations after a 90-mg oral dose of LY2811376 were simulated based on the following reported human values (May et al., 2011): CSF AUC of 4.74 mM*h. The CSF half-life of LY2811376 was assumed to be identical to the plasma half-life of 40 hours. CSF Tmax was assumed to either match reported plasma Tmax (2 hours) or to be slightly delayed (i.e., 5 hours). No substantial differences were observed in the CSF Ab, sAPPb, and sAPPa profiles when a CSF Tmax of 2 hours versus 5 hours was used; therefore, simulations were performed with a CSF Tmax of 2 hours.
Concentrations of LY2811376 in CSF after a 30-mg dose were extrapolated from the simulated CSF concentrations for the 90-mg dose, assuming linear pharmacokinetics. The simulated CSF concentrations of LY2811376 at the two doses were used to simulate the PD effects of LY2811376 using the GNE-629 PD model after making adjustments for the in vivo GNE-629 IC50 value to account for the 3-fold lower potency of LY2811376 relative to GNE-629 (Table 1). The adjusted in vivo CSF IC50 for LY2811376 was set to 0.010 mM.
Results In Vitro Data. GNE-629 and GNE-892 exhibited potent BACE1 inhibition in the recombinant human BACE1 enzymatic assay with IC50 values of 0.010 and 0.048 mM, and in an APP cellular assay with IC50 values of 0.0080 and 0.027 mM, respectively (Table 1). Both compounds showed relatively low protein binding with unbound fraction of 0.39 for GNE-629 and 0.55 for GNE-892. Their permeability efflux ratios (permeability from basolateral side to apical side versus permeability from apical to basolateral side, Pba/Pab) in P-glycoprotein (P-gp) transfected cells (MDCKI-MDR1) were 44 for GNE-629 and 25 for GNE-892, indicating that they are both P-gp substrates (Table 1). GNE-629 Plasma and CSF PK in Monkeys: Experimental Observation and PK Modeling. Similar concentration-time profiles were observed in monkey plasma and CSF after a single dose of GNE629 at 100 mg/kg (Fig. 3, A and B). The plasma and CSF Tmax values were approximately 2 hours. CSF concentrations of GNE-629 were notably lower relative to the corresponding unbound plasma concentrations, resulting in a CSF to unbound plasma AUC ratio of 0.0348 6 0.0017, indicating the blood-brain barrier (BBB) limits GNE-629 brain penetration as predicted by the in vitro P-gp assay (Table 2). As shown in Fig. 3, A and B, GNE-629 concentrations were well characterized by the PK model described in Fig. 2A. The predicted plasma and CSF concentrations matched the observed concentrations as demonstrated by a correlation coefficient of 0.99 (Supplemental Fig. 1A). The PK parameters associated with the GNE-629 PK modeling were estimated with good precision, generating a percent standard error for all parameters of less than 30% (Table 3). An approximately 30-fold higher distribution clearance from the CSF to central compartment relative to the reverse distribution clearance from the central compartment to CSF was observed. This result is consistent GNE-629 being effluxed by P-gp at the BBB. GNE-629 Plasma and CSF PD in Monkeys: Experimental Observation and Mechanistic PK-PD Modeling. The baseline for plasma Ab was 0.238 6 0.037 ng/ml and the baseline for CSF Ab, sAPPa, and sAPPb was 4.21 6 1.78 ng/ml, 32.0 6 14.5 ng/ml, and 34.5 6 16.8 ng/ml, respectively. GNE-629 significantly reduced the Ab in the plasma and CSF after a single oral dose at 100 mg/kg from 4 to 72 postdosing (P , 0.001) (Fig. 3, C and D). The maximal APPrelated biomarker changes in monkeys are listed in Table 2. Similar maximal Ab reduction was observed in the plasma and the CSF (31.7–35.1% of the baseline or 64.9–68.3% reduction from the baseline). The extent of reductions in CSF sAPPb and Ab were similar as well. The maximal sAPPa increased to 143% of the baseline. The average Tmax for biomarker changes ranged from 8 to 24 hours, which was notably longer than the observed Tmax for plasma and CSF concentrations of GNE-629 (2 hours) (Fig. 3). Inhibition of plasma Ab formation by GNE-629 was well characterized by an indirect response PD model (Figs. 2D and 3C) (Jusko and Ko, 1994). The mechanistic CSF PD model (Fig. 2C) was able to describe both the decrease in CSF Ab and sAPPb levels as well as the increase in CSF sAPPa levels (Fig. 3, D–F). PD parameters were estimated with good precision, all showing a percent standard error of less than 35% (Table 3). The estimated in vivo CSF IC50 for GNE-629 (0.0033
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Fig. 3. GNE-629 significantly inhibited BACE1 in cynomolgus monkeys. Monkey plasma (A) and CSF (B) concentrations of GNE-629, and levels of plasma Ab (C), CSF Ab (D), CSF sAPPb (E), and CSF sAPPa (F) after a single 100 mg/kg oral dose of GNE-629 are presented. The symbols represent the observations (mean 6 S.D.). n = 9–10 from 0–48 hours and n = 4–5 at 72 hours. *P , 0.05; **P , 0.01; and ***P , 0.001 in comparison with the corresponding vehicle controls. The lines in (A) and (B) represent the best fit of the PK model in Fig. 2A to the observed plasma and CSF concentrations, and lines in (C–F) represent the best fit of the PD model in Fig. 2, C and D, to the observed PD markers.
mM) was in the same range as the IC50 values from the cellular assay (0.0080 mM). The estimated unbound plasma IC50(plasma) (0.065 mM) was approximately 20-fold higher than the CSF IC50. Overall, the proposed mechanistic PK-PD model adequately described the plasma Ab, and CSF Ab, sAPPa, and sAPPb levels in cynomolgus monkeys. The predicted PD biomarkers in the plasma and CSF correlated well with the observed values showing a correlation coefficient of 0.86 (Supplemental Fig. 1B). GNE-892 Plasma and CSF PK in Monkeys: Experimental Observation and PK Modeling. Plasma and CSF concentration-time profiles for GNE-892 were similar after a single oral dose of 30 mg/kg or 100 mg/kg (Fig. 4, A and B). The plasma and CSF Tmax values were similar and ranged from 2 to 4 hours at 30 and 100 mg/kg. The CSF concentrations again were notably lower than the unbound plasma concentrations as demonstrated by the CSF to unbound plasma AUC ratios of 0.0408 6 0.0036 for 30 mg/kg and 0.0423 6 0.032 for
100 mg/kg, indicating that the BBB limits GNE-892 brain penetration as predicted by in vitro P-gp assay (Table 2). GNE-892 plasma and CSF concentrations (Fig. 4, A and B) were well described by its PK model as shown in Fig. 2B. The addition of a central nervous system (CNS) depot compartment was necessary to accurately capture the CSF concentrations of GNE-892 at the later time points as they were substantially underpredicted when the GNE629 PK model (Fig. 2A) was used (unpublished data). PK parameters for GNE-892 were estimated with good precision, generating a percent standard error for all parameters of less than 20% (Table 4). The predicted plasma and CSF concentrations correlated well with the experimentally measured results, with a correlation coefficient of 0.96 (Supplemental Fig. 1C). Distribution clearance from CSF to the central compartment was approximately 21-fold higher compared with the distribution clearance from the central compartment to CSF for GNE-892, similar to what was observed for GNE-629 (Table 4).
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PK-PD Modeling of BACE1 Inhibition TABLE 2 Summary of concentration and activity from monkey studies with GNE-629 and GNE-892 Data are presented as the mean 6 S.E. GNE-629
GNE-892
Parameter 100 mg/kg
Plasma AUC (mM*h) CSF AUC (mM*h) CSF/unbound plasma AUC ratio Minimal plasma Ab (% baseline) Minimal CSF Ab (% baseline) Maximal CSF sAPPa (% baseline) Minimal CSF sAPPb (% baseline)
63.5 0.870 0.0348 31.7 35.1 143 29.4
6 6 6 6 6 6 6
30 mg/kg
11.5 0.164 0.0017 2.46 3.5 7 4.0
GNE-892 Plasma and CSF PD in Monkeys: Experimental Observation and PK-PD Simulation. GNE-892 significantly reduced Ab in plasma and CSF after a single oral dose at 30 mg/kg or 100 mg/kg from 4 to 24 hours (Fig. 4, C and D) (P , 0.01). It also reduced sAPPb and increased sAPPa in CSF (Fig. 4, E and F) (P , 0.05). Similar maximal Ab reduction was observed in the plasma for the 30 and 100 mg/kg doses (23.4–24.6% of the baseline or 75.4–76.6% reduction from the baseline) (Table 2). The biomarker changes in the CSF were generally dose dependent. The maximal Ab reduction in CSF was 48.6% (51.4% reduction from the baseline) of the baseline at 30 mg/kg and 29.2% of the baseline (71.8% reduction from the baseline) at 100 mg/kg. The levels of CSF sAPPb and Ab after administration of GNE-892 were reduced to a similar extent. The maximal increases in sAPPa levels in the CSF were 119% of the baseline at 30 mg/kg and 148% of the baseline at 100 mg/kg. The Tmax values for the biomarker changes (8–24 hours) were notably longer than the Tmax values for the plasma and CSF drug concentrations (2–4 hours) (Fig. 4). Simulations of APP-related biomarker changes in plasma and CSF for GNE-892–treated monkeys were made using the mechanistic PK-PD model developed with GNE-629 data to validate its performance. As detailed in the Materials and Methods, the in vivo CSF and plasma IC50 values for GNE-892 were adjusted from the corresponding in vivo GNE629 IC50 values (0.00328 mM in CSF and 0.0654 mM in plasma; Table 3) to account for the decrease in cell potency for GNE-892 (0.0080 versus 0.027 mM; Table 1). The in vivo CSF IC50 for GNE-892 was set to 0.0111 mM and the plasma IC50 was set to 0.221 mM. The remaining system-dependent PD parameters were fixed to those obtained from the GNE-629 PD model parameters (Table 3). Simulations of monkey Ab, sAPPb, and sAPPa in CSF and Ab in plasma at doses of 30 and 100 mg/kg GNE-629 are shown in Fig. 4, C–F. The simulations captured the dose dependence of the PD responses for all of the PD markers; the decreases in CSF Ab, CSF sAPPb, and plasma Ab; and the increases in CSF sAPPa. Overall, the CSF and plasma PD levels from simulation were in good agreement with the observed concentrations in the monkey showing a correlation of coefficient of 0.85 (Supplemental Fig. 1D). LY2811376 CSF PD in Humans: Experimental Observation and Mechanistic PK-PD Simulation. The utility of the mechanistic PK-PD model developed for GNE-629 was further investigated by testing its ability to predict the PD biomarker changes observed in human CSF for LY2811376 after oral administration of 30 and 90 mg. The predicted maximum reductions of CSF Ab for the 30-mg and 90mg doses were approximately 61 and 81%, respectively, and were comparable with the observed average maximum changes of 53–59% at 30 mg and 82–91% at 90 mg (May et al., 2011). Discussion In the present study, we report that two novel and potent BACE1 inhibitors, GNE-629 and GNE-892, significantly reduced Ab levels in
25.2 0.571 0.0408 23.4 48.6 119 47.3
6 6 6 6 6 6 6
2.1 0.075 0.0036 4.4 5.0 25 6.3
100 mg/kg
138 2.95 0.0423 24.6 29.2 148 31.2
6 6 6 6 6 6 6
21 0.38 0.0032 5.4 2.5 14 3.6
plasma and CSF, and modulate other biomarkers of APP processing such as sAPPa and sAPPb after a single dose oral administration in monkeys. The observed changes in these biomarkers are consistent with selective BACE1 inhibition in the brain. Therefore, these two compounds can be used as tool compounds to study BACE1 inhibition in monkeys. For the first time, a mechanism-based PK-PD model has been developed to characterize plasma Ab and CSF Ab, sAPPa, and sAPPb simultaneously. The modeling revealed that the free plasma IC50 was approximately 20-fold of the CSF IC50, suggesting that Ab in the plasma and brain may be formed by different mechanisms. CSF Ab has been used as a biomarker for Ab in the brain. For GNE-629, we observed that reduction of Ab1-40 in CSF was associated with the reduction of Ab1-40 in the brain in guinea pigs but the extent of reduction was generally greater in the CSF than in the brain (unpublished data). Similar results were reported in mouse and rats after other BACE1 inhibitors or g-secretase inhibitors (Lu et al., 2011, 2012; Tai et al., 2012; Weiss et al., 2012). Recently, Lu et al. (2012) demonstrated that the PK/PD relationship for the Ab lowering in CSF was predictive of Ab lowering in the brain in mice, rats, and guinea pigs for BACE1 or g-secretase inhibitors and modulators and the extent of Ab lowering was greater in the CSF than in the brain. There are several Ab peptides resulting from the sequential cleavage
TABLE 3 Summary of PK-PD model parameters estimated from cynomolgus monkey studies with GNE-629 Parameter
PK Parameter ka (h21) ke (h21) k12 (h21) k21 (h21) k1-csf (h21) kcsf-1 (h21) Vcsf (l/kg) V1 (l/kg) fu CL1-csf (l/h per kilogram)b CLcsf-1 (l/h per kilogram)b Ratio CLcsf-1/CL1-csfb PD Parameter for CSF PD Model kinAPP (%/h) k (h21) ka (h21) IC50 (mM) PD Parameter for Plasma PD Model kinplasma (%/h) IC50(plasma) (mM)
Estimate (% S.E.)a
1.24 2.00 1.52 4.53
0.812 0.352 0.164 0.0827 1025 2.27 1023 12.3 0.39 1024 1023 29.7
(7.0) (3.4) (8.2) (28) (16) (16) (fixed) (fixed) (fixed)
55.7 0.140 0.411 0.00328
(26) (6.8) (34) (13)
17.7 (7.8) 0.0654 (13)
a Fitted parameters are expressed as the estimate followed by the % standard error in parentheses. b Parameters are calculated as follows: CL1-csf = V1 k1-csf; CLcsf-1 = Vcsf kcsf-1.
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of APP by b-secretase and g-secretase. A good concordance between CSF Ab1-40 and Ab1-42 has been demonstrated in the literature for other rodents, monkeys, and humans with BACE1 inhibitors (Hussain et al., 2007; Sankaranarayanan et al., 2009; Fukumoto et al., 2010; May et al., 2011). Therefore, we used Ab1-40 to represent the overall amyloid peptides based on its relative abundance. GNE-629 and GNE-892 Significantly Inhibit Brain BACE1 in Monkeys after a Single Oral Dose. The present study demonstrated that GNE-629 at 100 mg/kg inhibited monkey Ab by 64.9–68.3% in the plasma and CSF and GNE-892 at 30 or 100 mg/kg inhibited monkey Ab by 75.4–76.6% in the plasma and 51.4–71.8% in the CSF. Since sAPPb is a direct product of b-secretase cleavage for APP, it would be expected that BACE1 inhibition also would lead to a reduction of sAPPb levels and accumulation of full-length APP in brain, potentially an increase of sAPPa via a-secretase cleavage. Our data support this prediction because both GNE-629 and GNE-892 led to increased sAPPa levels, correlating with decreased sAPPb in
monkey CSF. The observed changes in multiple APP-related biomarkers demonstrate both inhibitors inhibit CNS BACE1 activity and can be used as tool compounds to study BACE1 inhibition in nonhuman primates. BACE1 Inhibition can be Characterized by a Mechanistic PKPD model. The development of new treatments for AD is challenging for many reasons, including the time lag between the disease onset and clinical symptoms, unreliability of outcomes measured in clinical trials, and the lack of understanding of relationships between drug effects and disease in preclinical models and in humans (Becker and Greig, 2008). Therefore, the discovery and development of new Alzheimer’s medicines will benefit from an improved understanding of PK-PD relationships through the development of mechanistic PKPD models. Several of the PK-PD models for APP processing largely rely on the effects of g-secretase inhibition on Ab levels (Craft et al., 2002; Lu et al., 2011; Tai et al., 2012). Recently, Lu et al. (2012) reported a PK-PD model for Ab lowering in CSF and brain in rodents.
Fig. 4. GNE-892 significantly inhibited BACE1 in cynomolgus monkeys. Monkey plasma (A) and CSF (B) concentrations of GNE-892, as well as levels of plasma Ab (C), CSF Ab (D), CSF sAPPb (E), and CSF sAPPa (F) after a 30 or 100 mg/kg oral dose of GNE-892 are presented. The symbols represent the observations (mean 6 S.D.) n = 12–13 for plasma Ab and CSF Ab, n = 7 for CSF sAPPb and sAPPa. *P , 0.05; **P , 0.01; ***P , 0.001 in comparison with the corresponding vehicle controls. The lines in (A) and (B) represent the best fit of the PK model in Fig. 2B to the plasma and CSF concentrations. The lines in (C–F) are the simulated levels of plasma Ab and CSF Ab, sAPPa, and sAPPb using PD models in Fig. 2, C and D.
PK-PD Modeling of BACE1 Inhibition TABLE 4 Summary of pharmacokinetic parameters estimated from cynomolgus monkey studies with GNE-892 PK Parameter
ka (h21) ke (h21) k12 (h21) k21 (h21) k1-csf (h21) kcsf-1 (h21) kcsf-csfp (h21) kcsfp-csf (h21) Vcsf (l/kg) V1 (l/kg) fu CL1-csf (l/h per kilogram)b CLcsf-1 (l/h per kilogram)b Ratio CLcsf-1/CL1-csfb
Estimate (% S.E.)a
1.87
2.00 1.17 2.49
1.00 0.608 0.0284 0.0783 1025 1.25 0.692 0.100 1023 6.23 0.55 1024 1023 21.3
(15) (8.1) (11) (3.6) (18) (12) (16) (5.5) (fixed) (8.6) (fixed)
a Fitted parameters are expressed as the estimate followed by the % standard error in parentheses. b Parameters are calculated as follows: CL1-csf = V1 k1-csf; CLcsf-1 = Vcsf kcsf-1.
To our knowledge, no models have been reported that detail the effects of BACE1 inhibition on overall APP processing including Ab, sAPPa, and sAPPb. Although the PK-PD modeling was based on the GNE-629 dataset, an exploratory modeling exercise showed that use of the GNE-892 dataset generated similar PK/PD model parameters. Our PK-PD model was able to characterize the responses of the APPrelated PD markers in both plasma and CSF. Our modeling work revealed the kinetics of Ab in the monkeys. The estimated elimination rate constant for the CSF and plasma Ab was 0.140 and 0.177 h21, respectively, resulting in an Ab half-life of 5 and 4 hours, respectively. These values are consistent with the reported Ab elimination rate constant (0.08 h21) and half-life (8.3 hours) in monkeys (Cook et al., 2010). A similar elimination rate constant was also observed in humans (Bateman et al., 2006). The model estimated an in vivo CSF IC50 of 0.0033 mM which is similar to GNE-629 cell-based BACE1 inhibition IC50 of 0.0080 mM. Interestingly, the free plasma IC50 for GNE-629 was 0.065 mM, a near 20-fold of the in vivo CSF IC50. In the PD model, it was assumed that 100% Ab in the plasma and brain can be inhibited by BACE1 inhibition. If we assume that only 60% of Ab in the plasma can be reduced via BACE1 inhibition, the free plasma IC50 for GNE-629 is 0.0056 mM, which is more in line with in vivo CSF and in vitro cellbased IC50s. These results suggest that Ab in the brain may be mainly generated by BACE1 but Ab in plasma may be generated by BACE1 and other mechanisms. This hypothesis is supported by recent observations that Ab in the brain of BACE1/2 double knockout mice was reduced by more than 90% relative to wild-type littermates, but Ab in the plasma was only reduced to 50–60% of wild-type levels (William J. Meilandt et al., submitted manuscript). Previous observations that only approximately 50% plasma Ab can be inhibited by high concentrations of an anti-BACE1 antibody in monkeys also support this hypothesis (Atwal et al., 2011). The general utility of this PD model built with GNE-629 data were tested by performing simulations of the in vivo PD markers for another BACE1 inhibitor, GNE-892, following simple adjustments to account for differences in compound in vitro potency and PK. For GNE-892, a CSF peripheral compartment (Fig. 2B) was added to the CSF compartment to accurately describe the CSF concentrations in the terminal phase. Potentially slower distribution between plasma, brain, and CSF compartments may contribute to this observation. A limitation of the GNE-892 PK-PD data set was that the last sampling time point was 48 hours where the PD biomarkers had not yet returned to
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the baseline. Despite this, the simulated biomarker changes in the plasma and CSF compartments matched the observed values reasonably well. We also evaluated whether the monkey PD model can predict the CSF biomarkers for LY2811376 in humans after potency adjustments using its cell IC50 generated under the same condition as GNE-629 in the authors’ laboratory, and its CSF concentrations reported by May et al. (2011). Our model predicted maximal CSF Ab reductions were 61% at 30 mg and 81% at 90 mg. These values are similar to the clinical observed maximal CSF Ab reductions of 53–59% at 30 mg and 82–91% at 90 mg. GNE-892 and GNE-629 Significantly Inhibit CNS BACE1 Despite P-gp Efflux in Monkeys. Both GNE-629 and GNE-892 are P-gp substrates as indicated by their high efflux ratios in P-gp transfected cells. P-gp at the BBB likely impaired brain penetration of these two compounds as demonstrated by the low CSF to free plasma concentration ratios in monkeys as well as approximately 21- to 30fold higher distribution clearance from the CSF compartment to the central compartment than that from the central compartment to the CSF compartment. This is supported by the observations that the brain-to-plasma AUC(0–1 h) ratios increased approximately 11-fold for GNE-629 and 7-fold for GNE-892, in the Mdr1a/1b knockout or Mdr1a/1b/Bcrp knockout mice compared with the wild-type mice. There were not significant changes observed in the Bcrp knockout mice (Supplemental Figs. 2 and 3). These observations are also in agreement with previous findings that P-gp substrates have low CSF to unbound plasma concentration ratios (Liu et al., 2006, 2008, 2009). Although P-gp at the BBB limits the brain penetration of GNE-629 and GNE-892, sufficient concentrations were achieved to inhibit BACE1 in the brain. Thus, P-gp is just one of many hurdles to overcome in designing CNS drugs. If a P-gp substrate is sufficiently potent and selective against its target, it still may be useful as a CNS drug (de Leon et al., 2010). In summary, two novel and potent BACE1 inhibitors, GNE-629 and GNE-892, are capable of eliciting significant inhibition of Ab formation in the monkey plasma and CSF after oral administration and can be used as tool compounds to study BACE1 inhibition. A novel mechanismbased PK-PD model was developed to provide a better understanding of the relationships between drug concentrations and BACE1 biomarkers in plasma and CSF. This modeling suggests the potential of different mechanisms being responsible for Ab formation in the plasma and brain. This work illustrates the utility of a mechanistic PK-PD model in quantitatively understanding the biologic processes as well as in the translation of in vitro activities to in vivo activities in drug discovery. Acknowledgments The authors thank Emile Plise, Jonathan Cheong, Quynh Ho, and Mary Geck Do for their support of in vitro studies, and Sonali Medakkar and Daniel Hutcheson at Maccine Pte Ltd and Eric Bourhis for their support of in vivo studies. Authorship Contributions Participated in research design: Liu, Scearce-Levie, Watts, Regal, Hunt, Siu, Lyssikatos, Hop. Conducted experiments: Liu, Coraggio, Shin, Peng, Wildsmith, Atwal, Mango, Schauer, Deshmukh. Contributed new reagents or analytic tools: Hunt, Thomas, Siu, Lyssikatos. Performed data analysis: Liu, Wong, Atwal, Deshmukh. Wrote or contributed to the writing of the manuscript: Liu, Wong, ScearceLevie, Watts, Regal, Hop. References Atwal JK, Chen Y, Chiu C, Mortensen DL, Meilandt WJ, Liu Y, Heise CE, Hoyte K, Luk W, and Lu Y, et al. (2011) A therapeutic antibody targeting BACE1 inhibits amyloid-b production in vivo. Sci Transl Med 3:84ra43.
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Address correspondence to: Dr. Xingrong Liu, Genentech Inc., MS 41-2A, 1 DNA Way, South San Francisco, CA 94080. E-mail:
[email protected]
