Edna F. Choo

Identification of functionally variant MDR1 alleles among European Americans and African Americans

MDR1 (P‐glycoprotein) is an important factor in the disposition of many drugs, and the involved processes often exhibit considerable interindividual variability that may be genetically determined. Single‐strand conformational polymorphism analysis and direct sequencing of exonic MDR1 deoxyribonucleic acid from 37 healthy European American and 23 healthy African American subjects identified 10 single nucleotide polymorphisms (SNPs), including 6 nonsynonymous variants, occurring in various allelic combinations. Population frequencies of the 15 identified alleles varied according to racial background. Two synonymous SNPs (C1236T in exon 12 and C3435T in exon 26) and a nonsynonymous SNP (G2677T, Ala893Ser) in exon 21 were found to be linked (MDR1ast;2) and occurred in 62% of European Americans and 13% of African Americans. In vitro expression of MDR1 encoding Ala893 (MDR1ast;1) or a site‐directed Ser893 mutation (MDR1ast;2) indicated enhanced efflux of digoxin by cells expressing the MDR1‐Ser893 variant. In vivo functional relevance of this SNP was assessed with the known P‐glycoprotein drug substrate fexofenadine as a probe of the transporters activity. In humans, MDR1ast;1 and MDR1ast;2 variants were associated with differences in fexofenadine levels, consistent with the in vitro data, with the area under the plasma level–time curve being almost 40% greater in the *1/*1 genotype compared with the *2/*2 and the *1/*2 heterozygotes having an intermediate value, suggesting enhanced in vivo P‐glycoprotein activity among subjects with the MDR1ast;2 allele. Thus allelic variation in MDR1 is more common than previously recognized and involves multiple SNPs whose allelic frequencies vary between populations, and some of these SNPs are associated with altered P‐glycoprotein function.

Antitumor Activity of Targeted and Cytotoxic Agents in Murine Subcutaneous Tumor Models Correlates with Clinical Response

Purpose: Immunodeficient mice transplanted with subcutaneous tumors (xenograft or allograft) are widely used as a model of preclinical activity for the discovery and development of anticancer drug candidates. Despite their widespread use, there is a widely held view that these models provide minimal predictive value for discerning clinically active versus inactive agents. To improve the predictive nature of these models, we have carried out a retrospective population pharmacokinetic–pharmacodynamic (PK–PD) analysis of relevant xenograft/allograft efficacy data for eight agents (molecularly targeted and cytotoxic) with known clinical outcome. Experimental Design: PK–PD modeling was carried out to first characterize the relationship between drug concentration and antitumor activity for each agent in dose-ranging xenograft or allograft experiments. Next, simulations of tumor growth inhibition (TGI) in xenografts/allografts at clinically relevant doses and schedules were carried out by replacing the murine pharmacokinetics, which were used to build the PK–PD model with human pharmacokinetics obtained from literature to account for species differences in pharmacokinetics. Results: A significant correlation (r = 0.91, P = 0.0008) was observed between simulated xenograft/allograft TGI driven by human pharmacokinetics and clinical response but not when TGI observed at maximum tolerated doses in mice was correlated with clinical response (r = 0.36, P = 0.34). Conclusions: On the basis of these analyses, agents that led to greater than 60% TGI in preclinical models, at clinically relevant exposures, are more likely to lead to responses in the clinic. A proposed strategy for the use of murine subcutaneous models for compound selection in anticancer drug discovery is discussed. Clin Cancer Res; 18(14); 3846–55. ©2012 AACR.

Differential in Vivo Sensitivity to Inhibition of P-glycoprotein Located in Lymphocytes, Testes, and the Blood-Brain Barrier

A major functional component of the blood-brain barrier is P-glycoprotein. In principle, inhibition of this efflux transporter would permit greater distribution of its substrates into the brain and increased central effects. Tariquidar and elacridar, potent and selective P-glycoprotein inhibitors, were investigated in this regard using the opioid loperamide as an in vivo probe in mice. Pretreatment with both inhibitors converted intravenous loperamide from a drug without central effects to one producing antinociception. Radiolabeled loperamide tissue distribution studies indicated that inhibition was associated with increased uptake into brain and testes in the absence of changes in plasma levels, along with enhanced efflux of rhodamine 123 from CD3e+ T-lymphocytes. However, with tariquidar, the loperamide dose-response curves for testes/plasma and brain/plasma concentration ratios were shifted 6- (p = 0.07) and 25-fold (p < 0.01) to the right, respectively (ED50 = 1.48 and 5.65 mg/kg), compared with the rhodamine 123 efflux curve (ED50 0.25 mg/kg). Less pronounced shifts were noted with elacridar where the brain/plasma ratio was shifted only 2-fold relative to the rhodamine 123 efflux data (ED50 = 2.36 versus 1.34 mg/kg, respectively; p 0.01). These results indicate that the P-glycoprotein localized in the blood-brain barrier and, to a lesser extent, the testes-blood barrier is more resistant to inhibition than at other tissue sites such as the lymphocyte; moreover, the extent of this effect depends on the inhibitor. Such resistance can be overcome by a sufficiently high dose of an inhibitor; however, whether this is safely attainable in the clinical situation remains to be determined.

Bridging the Gap between Preclinical and Clinical Studies Using Pharmacokinetic–Pharmacodynamic Modeling: An Analysis of GDC-0973, a MEK Inhibitor

Purpose: GDC-0973 is a potent and selective mitogen-activated protein (MAP)/extracellular signal–regulated kinase (ERK) kinase (MEK) inhibitor. Pharmacokinetic–pharmacodynamic (PK–PD) modeling was used to relate GDC-0973 plasma and tumor concentrations, tumor pharmacodynamics and antitumor efficacy to establish pharmacokinetic endpoints and predict active doses in the clinic. Experimental Design: A PK–PD model was used to characterize GDC-0973 tumor disposition and in vivo potency in WM-266-4 xenograft mice. Simulations were conducted using the PK–PD model along with human pharmacokinetics to identify a target plasma concentration and predict active doses. In vivo potency and antitumor efficacy were characterized in A375 melanoma xenograft mice, and a population-based integrated PK–PD-efficacy model was used to relate tumor pharmacodynamics (%pERK decrease) to antitumor activity. Results: GDC-0973 showed a sustained tumor pharmacodynamic response due to longer residence in tumor than in plasma. Following single doses of GDC-0973, estimated in vivo IC50 values of %pERK decrease based on tumor concentrations in xenograft mice were 0.78 (WM-266-4) and 0.52 μmol/L (A375). Following multiple doses of GDC-0973, the estimated in vivo IC50 value in WM-266-4 increased (3.89 μmol/L). Human simulations predicted a minimum target plasma concentration of 83 nmol/L and an active dose range of 28 to 112 mg. The steep relationship between tumor pharmacodynamics (%pERK decrease) and antitumor efficacy suggests a pathway modulation threshold beyond which antitumor efficacy switches on. Conclusions: Clinical observations of %pERK decrease and antitumor activity were consistent with model predictions. This article illustrates how PK–PD modeling can improve the translation of preclinical data to humans by providing a means to integrate preclinical and early clinical data. Clin Cancer Res; 18(11); 3090–9. ©2012 AACR.

Limited Sampling Strategy to Predict AUC of the CYP3A Phenotyping Probe Midazolam in Adults: Application to Various Assay Techniques

Midazolam clearance is used to phenotype hepatic CYP3A activity but requires multiple plasma samples following a single intravenous dose. The authors evaluated the use of a limited sampling scheme, using different assay techniques to determine the reproducibility of such a strategy in estimating midazolam AUC. Seventy‐three healthy adults received midazolam as a single intravenous bolus dose. At least eight plasma samples were collected from each subject and were assayed using either LC/MS/MS or electron capture gas chromatography. Eleven subjects were randomly selected for the training set using stepwise linear regression to determine relationships between midazolam plasma concentrations and AUC. Validation of the predictive equations was done using the remaining 62 subjects. Mean percent error (MPE), mean absolute error (MAE), and root mean square error (RMSE) were calculated to determine bias and precision. Based on the training set, five models were generated with coefficients of determination ranging from 0.87 to 0.95. Validation showed that MPE, MAE, and RMSE values were acceptable for three of the models. Int rasubject reproducibility was good. In addition, training set data from one institution were able to predict data from the other two institutions using other assay techniques. Minimized plasma sampling may provide a simpler method for estimating midazolam AUC for CYP3A phenotyping. A limited sampling strategy is more convenient and cost‐effective than standard sampling strategies and is applicable to more than one assay technique.

Pharmacodynamics of 2-{4-[(1E)-1-(Hydroxyimino)-2,3-dihydro-1H-inden-5-yl]-3-(pyridine-4-yl)-1H-pyrazol-1-yl}ethan-1-ol (GDC-0879), a Potent and Selective B-Raf Kinase Inhibitor: Understanding Relationships between Systemic Concentrations, Phosphorylated Mitogen-Activated Protein Kinase Kinase 1 Inhibition, and Efficacy

The Raf/mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase signaling pathway is involved in cellular responses relevant to tumorigenesis, including cell proliferation, invasion, survival, and angiogenesis. 2-{4-[(1E)-1-(Hydroxyimino)-2,3-dihydro-1H-inden-5-yl]-3-(pyridine-4-yl)-1H-pyrazol-1-yl}ethan-1-ol (GDC-0879) is a novel, potent, and selective B-Raf inhibitor. The objective of this study was to characterize the relationship between GDC-0879 plasma concentrations and tumor growth inhibition in A375 melanoma and Colo205 colon cancer xenografts and to understand the pharmacodynamic (PD) marker response requirements [phosphorylated (p)MEK1 inhibition] associated with tumor growth inhibition in A375 xenografts. Estimates of GDC-0879 plasma concentrations required for tumor stasis obtained from fitting tumor data to indirect response models were comparable, at 4.48 and 3.27 μM for A375 and Colo205 xenografts, respectively. This was consistent with comparable in vitro potency of GDC-0879 in both cell lines. The relationship between GDC-0879 plasma concentrations and pMEK1 inhibition in the tumor was characterized in A375 xenografts after oral doses of 35, 50, and 100 mg/kg. Fitting pMEK1 inhibition to an indirect response model provided an IC50 estimate of 3.06 μM. pMEK1 inhibition was further linked to A375 tumor volume data from nine different GDC-0879 dosing regimens using an integrated pharmacokinetic-PD model. A simulated PD marker response curve plot of K (rate constant describing tumor growth inhibition) versus pMEK1 inhibition generated using pharmacodynamic parameters estimated from this model, showed a steep pMEK1 inhibition-response curve consistent with an estimated Hill coefficient of ≅8. A threshold of >40% pMEK1 inhibition is required for tumor growth inhibition, and a minimum of ∼60% pMEK1 inhibition is required for stasis in A375 xenografts treated with GDC-0879.ABSTRACT The Raf/MEK/ERK signaling pathway is involved in cellular responses relevant to tumorigenesis, including cell proliferation, invasion, survival and angiogenesis. GDC-0879 is a novel, potent and selective B-Raf inhibitor. The objective of this study was to characterize the relationship between GDC-0879 plasma concentrations and tumor growth inhibition in A375 melanoma and Colo205 colon cancer xenografts, and to understand the pharmacodynamic (PD) marker response requirements (pMEK1 inhibition) associated with tumor growth inhibition in A375 xenografts. Estimates of GDC-0879 plasma concentrations required for tumor stasis obtained from fitting tumor data to indirect response models were comparable, at 4.48 and 3.27 µM for A375 and Colo205 xenografts, respectively. This was consistent with comparable in vitro potency of GDC-0879 in both cell lines. The relationship between GDC-0879 plasma concentrations and pMEK1 inhibition in the tumor was characterized in A375 xenografts following oral doses of 35, 50 and 100 mg/kg. Fitting pMEK1 inhibition to an indirect response model provided an IC

Pharmacodynamics of GDC-0879, a Potent and Selective B-Raf Kinase Inhibitor: Understanding Relationships between Systemic Concentrations, pMEK1 Inhibition, and Efficacy

The Raf/mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase signaling pathway is involved in cellular responses relevant to tumorigenesis, including cell proliferation, invasion, survival, and angiogenesis. 2-{4-[(1E)-1-(Hydroxyimino)-2,3-dihydro-1H-inden-5-yl]-3-(pyridine-4-yl)-1H-pyrazol-1-yl}ethan-1-ol (GDC-0879) is a novel, potent, and selective B-Raf inhibitor. The objective of this study was to characterize the relationship between GDC-0879 plasma concentrations and tumor growth inhibition in A375 melanoma and Colo205 colon cancer xenografts and to understand the pharmacodynamic (PD) marker response requirements [phosphorylated (p)MEK1 inhibition] associated with tumor growth inhibition in A375 xenografts. Estimates of GDC-0879 plasma concentrations required for tumor stasis obtained from fitting tumor data to indirect response models were comparable, at 4.48 and 3.27 μM for A375 and Colo205 xenografts, respectively. This was consistent with comparable in vitro potency of GDC-0879 in both cell lines. The relationship between GDC-0879 plasma concentrations and pMEK1 inhibition in the tumor was characterized in A375 xenografts after oral doses of 35, 50, and 100 mg/kg. Fitting pMEK1 inhibition to an indirect response model provided an IC50 estimate of 3.06 μM. pMEK1 inhibition was further linked to A375 tumor volume data from nine different GDC-0879 dosing regimens using an integrated pharmacokinetic-PD model. A simulated PD marker response curve plot of K (rate constant describing tumor growth inhibition) versus pMEK1 inhibition generated using pharmacodynamic parameters estimated from this model, showed a steep pMEK1 inhibition-response curve consistent with an estimated Hill coefficient of ≅8. A threshold of >40% pMEK1 inhibition is required for tumor growth inhibition, and a minimum of ∼60% pMEK1 inhibition is required for stasis in A375 xenografts treated with GDC-0879.ABSTRACT The Raf/MEK/ERK signaling pathway is involved in cellular responses relevant to tumorigenesis, including cell proliferation, invasion, survival and angiogenesis. GDC-0879 is a novel, potent and selective B-Raf inhibitor. The objective of this study was to characterize the relationship between GDC-0879 plasma concentrations and tumor growth inhibition in A375 melanoma and Colo205 colon cancer xenografts, and to understand the pharmacodynamic (PD) marker response requirements (pMEK1 inhibition) associated with tumor growth inhibition in A375 xenografts. Estimates of GDC-0879 plasma concentrations required for tumor stasis obtained from fitting tumor data to indirect response models were comparable, at 4.48 and 3.27 µM for A375 and Colo205 xenografts, respectively. This was consistent with comparable in vitro potency of GDC-0879 in both cell lines. The relationship between GDC-0879 plasma concentrations and pMEK1 inhibition in the tumor was characterized in A375 xenografts following oral doses of 35, 50 and 100 mg/kg. Fitting pMEK1 inhibition to an indirect response model provided an IC

Discovery of Novel Allosteric Mitogen-Activated Protein Kinase Kinase (MEK) 1,2 Inhibitors Possessing Bidentate Ser212 Interactions.

Using structure-based design, two novel series of highly potent biaryl amine mitogen-activated protein kinase kinase (MEK) inhibitors have been discovered. These series contain an H-bond acceptor, in a shifted position compared with previously disclosed compounds, and an adjacent H-bond donor, resulting in a bidentate interaction with the Ser212 residue of MEK1. The most potent compound identified, 1 (G-894), is orally active in in vivo pharmacodynamic and tumor xenograft models.

Single Plasma Concentrations of 1'-Hydroxymidazolam or the Ratio of 1'-Hydroxymidazolam:Midazolam Do Not Predict Midazolam Clearance in Healthy Subjects

The 30‐minute ratio of 1′‐hydroxymidazolam: midazolam plasma concentrations has been used as a measure of midazolam clearance in liver transplant patients. This study determined if a single concentration of 1′‐hydroxymidazolam or the ratio of 1′‐hydroxymidazolam: midazolam could be used to predict midazolam clearance in healthy subjects. Plasma midazolam and 1′‐hydroxymidazolam concentrations from three previous studies were used for analyses. Data obtained predose and at 5,30, 60,120,240, 300, and 360 minutes following intravenous doses of midazolam in 61 adults were divided and used to derive and validate equations to predict midazolam clearance. Equations were derived using linear regression and then validated by comparing predicted to observed clearance. Only one equation was related to midazolam clearance as a function of 1′‐hydroxymidazolam, but it did not predict midazolam clearance (r = 0.29, p = 0.31). Single sampling of 1′‐hydroxymidazolam or 1′‐hydroxymidazolam: midazolam plasma concentrations cannot be used to predict midazolam clearance in healthy adults.

Role of P-Glycoprotein on the Brain Penetration and Brain Pharmacodynamic Activity of the MEK Inhibitor Cobimetinib

Cobimetinib is a MEK inhibitor currently in clinical trials as an anticancer agent. The objectives of this study were to determine in vitro and in vivo if cobimetinib is a substrate of P-glycoprotein (P-gp) and/or breast cancer resistance protein (Bcrp1) and to assess the implications of efflux on cobimetinib pharmacokinetics (PK), brain penetration, and target modulation. Cell lines transfected with P-gp or Bcrp1 established that cobimetinib was a substrate of P-gp but not a substrate of Bcrp1. In vivo, after intravenous and oral administration of cobimetinib to FVB (wild-type; WT), Mdr1a/b(-/-), Bcrp1 (-/-), and Mdr1a/b(-/-)/Bcrp(-/-) knockout (KO) mice, clearance was similar in WT (35.5 ± 16.7 mL/min/kg) and KO animals (22.0 ± 3.6 to 27.6 ± 5.2 mL/min/kg); oral exposure was also similar between WT and KO animals. After an oral 10 mg/kg dose of cobimetinib, the mean total brain to plasma ratio (Kp) at 6 h postdose was 0.3 and 0.2 in WT and Bcrp1(-/-) mice, respectively. In Mdr1a/b(-/-) and Mdr1a/1b/Bcrp1(-/-) KO mice and WT mice treated with elacridar (a P-gp and BCRP inhibitor), Kp increased to 11, 6, and 7, respectively. Increased brain exposure in Mdr1a/b(-/-) and Mdr1a/1b/Bcrp1(-/-) KO and elacridar treated mice was accompanied by up to ∼65% suppression of the target (pErk) in brain tissue, compared to WT mice. By MALDI imaging, the cobimetinib signal intensity was relatively high and was dispersed throughout the brain of Mdr1a/1b/Bcrp1(-/-) KO mice compared to low/undetectable signal intensity in WT mice. The efflux of cobimetinib by P-gp may have implications for the treatment of patients with brain tumors/metastases.