John P. Harrelson

Prediction of CYP3A Mediated Drug-Drug Interactions Using Human Hepatocytes Suspended in Human Plasma

Cryopreserved human hepatocytes suspended in human plasma (HHSHP) represent an integrated metabolic environment for predicting drug-drug interactions (DDIs). In this study, 13 CYP3A reversible and/or time-dependent inhibitors (TDIs) were incubated with HHSHP for 20 min over a range of concentrations after which midazolam 1′-hydroxylation was used to measure CYP3A activity. This single incubation time method yielded IC50 values for the 13 inhibitors. For each CYP3A inhibitor-victim drug pair, the IC50 value was combined with total average plasma concentration of the inhibitor in humans, fraction of the victim drug cleared by CYP3A, and intestinal availability of the victim drug to predict the ratio of plasma area under the curve of the victim drug in the presence and absence of inhibitor. Of 52 clinical DDI studies using these 13 inhibitors identified in the literature, 85% were predicted by this method within 2-fold of the observed change, and all were predicted within 3-fold. Subsequent studies to determine mechanism (reversible and time-dependent inhibitors) were performed by using a range of incubation periods and inhibitor concentrations. This system differentiated among reversible inhibitors, TDIs, and the combination of both. When the reversible and inactivation parameters were incorporated into predictive models, 65% of 52 clinical DDIs were predicted within 2-fold of the observed changes and 88% were within 3-fold. Thus, HHSHP produced accurate DDI predictions with a simple IC50 determined at a single incubation time regardless of the inhibition mechanism; further if needed, the mechanism(s) of inhibition can be identified.

Multiple-Ligand Binding in CYP2A6: Probing Mechanisms of Cytochrome P450 Cooperativity by Assessing Substrate Dynamics†

The contribution of ligand dynamics to CYP allosterism has not been considered in detail. On the basis of a previous study, we hypothesized that CYP2A6 and CYP2E1 accommodate multiple xylene ligands. As a result, the intramolecular ( k H/ k D) obs values observed for some xylene isomers are expected to be dependent on ligand concentration with contributions from [CYP.xylene] and [CYP.xylene.xylene], etc. To explore this possibility and the utility of kinetic isotope effects in characterizing allosteric CYP behavior, steady state kinetics, product ratios, and ( k H/ k D) obs values for CYP2E1 and CYP2A6 oxidation of m-xylene-alpha- (2)H 3 and p-xylene-alpha- (2)H 3 were determined. Evidence is presented that CYP2A6 accommodates multiple ligands and that intramolecular isotope effect experiments can provide insight into the mechanisms of multiple-ligand binding. CYP2A6 exhibited cooperative kinetics for m-xylene-alpha- (2)H 3 oxidation and a concentration-dependent decrease in the m-methylbenzylalcohol:2,4-dimethylphenol product ratio (9.8 +/- 0.1 and 4.8 +/- 0.3 at 2.5 microM and 1 mM, respectively). Heterotropic effects were observed as well, as incubations containing both 15 microM m-xylene-alpha- (2)H 3 and 200 microM p-xylene resulted in further reduction of the product ratio (2.4 +/- 0.2). When p-xylene (60 microM) was replaced with deuterium-labeled d 6- p-xylene (60 microM), an intermolecular competitive inverse isotope effect on 2,4-dimethylphenol formation [( k H/ k D) obs = 0.49] was observed, indicating that p-xylene exerts heterotropic effects by residing in the active site simultaneously with m-xylene. The data indicate that there is a concentration-dependent decrease in the reorientation rate of m-xylene, as no increase in ( k H/ k D) obs was observed in the presence of an increased level of metabolic switching. That is, the accommodation of a second xylene molecule in the active site leads to a decrease in substrate dynamics.

Predictions of Cytochrome P450-Mediated Drug-Drug Interactions Using Cryopreserved Human Hepatocytes: Comparison of Plasma and Protein-Free Media Incubation Conditions

Cryopreserved human hepatocytes suspended in human plasma (HHSHP) have previously provided accurate CYP3A drug-drug interaction (DDI) predictions from a single IC50 that captures both reversible and time-dependent inhibition. The goal of this study was to compare the accuracy of DDI predictions by a protein-free human hepatocyte system combined with the fraction unbound in plasma for inhibitor(s) with those obtained with protein-containing incubations. Seventeen CYP3A, CYP2C9, or CYP2D6 inhibitors were incubated with hepatocytes in human plasma or hepatocyte maintenance medium (HMM) for 20 min over a range of concentrations after which midazolam 1′-hydroxylation, diclofenac 4′-hydroxylation or (R)-bufuralol 1′-hydroxylation were used to quantify the corresponding cytochrome P450 (P450) catalytic activities. Two methods were used to predict the human exposure ratio of the victim drug in the presence and absence of inhibitor. The HMM Ki, app values were combined with the free average systemic plasma concentration (“free [I] with HMM Ki, app”) and the plasma Ki, app values were combined with the total average systemic plasma concentration (“total [I] with plasma Ki, app”). Of 63 clinical DDI studies, the total [I] with plasma Ki, app method predicted 89% of cases within 2-fold of the reported interaction whereas the free [I] with HMM Ki, app method predicted only 59%. There was a general underprediction by the free [I] with HMM Ki, app method, which is consistent with an underestimation of in vitro inhibition potency in this system. In conclusion, the HHSHP system proved to be a simple, accurate predictor of DDIs for three major P450s and superior to the protein-free approach.

Expanding the view of breast cancer metabolism: Promising molecular targets and therapeutic opportunities

The changes in breast cancer cells that contribute to tumor evolution, heterogeneity, metastasis and ultimately drug resistance are shaped by numerous genetic changes including alterations in cellular metabolism. These include intermediary metabolic pathways such as glycolysis, the citric acid cycle oxidative phosphorylation, amino acid synthesis and lipid metabolism. However, cancer cells also exhibit key alterations in other metabolic pathways involved in drug metabolism such as cytochrome P450 enzymes, sulfotransferase and steroid sulfatases that are involved in the synthesis of estrogens and themselves serve as drug targets. In this review we bring together these two sides of metabolism, discuss the evidence underpinning their role in breast cancer development and bring to light promising therapeutic targets and up and coming pharmacologic agents.

Inactivation of CYP2A6 by the Dietary Phenylpropanoid trans-Cinnamic Aldehyde (Cinnamaldehyde) and Estimation of Interactions with Nicotine and Letrozole

Human exposure to trans-cinnamic aldehyde [t-CA; cinnamaldehyde; cinnamal; (E)-3-phenylprop-2-enal] is common through diet and through the use of cinnamon powder for diabetes and to provide flavor and scent in commercial products. We evaluated the likelihood of t-CA to influence metabolism by inhibition of P450 enzymes. IC50 values from recombinant enzymes indicated that an interaction is most probable for CYP2A6 (IC50 = 6.1 µM). t-CA was 10.5-fold more selective for human CYP2A6 than for CYP2E1; IC50 values for P450s 1A2, 2B6, 2C9, 2C19, 2D6, and 3A4 were 15.8-fold higher or more. t-CA is a type I ligand for CYP2A6 (KS = 14.9 µM). Inhibition of CYP2A6 by t-CA was metabolism-dependent; inhibition required NADPH and increased with time. Glutathione lessened the extent of inhibition modestly and statistically significantly. The carbon monoxide binding spectrum was dramatically diminished after exposure to NADPH and t-CA, suggesting degradation of the heme or CYP2A6 apoprotein. Using a static model and mechanism-based inhibition parameters (KI = 18.0 µM; kinact = 0.056 minute−1), changes in the area under the concentration-time curve (AUC) for nicotine and letrozole were predicted in the presence of t-CA (0.1 and 1 µM). The AUC fold-change ranged from 1.1 to 3.6. In summary, t-CA is a potential source of pharmacokinetic variability for CYP2A6 substrates due to metabolism-dependent inhibition, especially in scenarios when exposure to t-CA is elevated due to high dietary exposure, or when cinnamon is used as a treatment of specific disease states (e.g., diabetes).

Covalent Modification and Time-Dependent Inhibition of Human CYP2E1 by the meta-Isomer of Acetaminophen

The hypothesis that N-acetyl-m-aminophenol (AMAP), the meta isomer of acetaminophen, will covalently bind to and inhibit human CYP2E1 in a time- and NADPH-dependent manner was investigated. Liquid chromatography/electrospray ionization-mass spectrometry analysis indicated that AMAP metabolites (i.e., AMAP*) selectively and covalently modified CYP2E1 apoprotein in a ratio of 1.4:1 (AMAP*/CYP2E1) in a reconstituted system. The deconvoluted spectra of CYP2E1 apoprotein from incubations containing NADPH and AMAP displayed mass shifts of 167.2 ± 7.1 and 334.4 ± 6.5 Da, suggesting the addition of one and two hydroxylated AMAP metabolites to CYP2E1, respectively. Mass shifts in cytochrome P450 reductase, cytochrome b5, and heme from these samples were not observed. CYP2E1 inhibition by AMAP increased with time in the presence of NADPH; a reversible inhibition component was also observed. The results support a bioactivation process that involves formation of a hydroquinone metabolite that undergoes further oxidation to a quinone, which reacts with CYP2E1 nucleophilic residues. The data are consistent with evidence from previous studies that identified hydroxylated AMAP glutathione conjugates collected from mice and indicate that cysteine residues are the most likely sites for adduct formation. This study reports the first direct evidence of AMAP-derived hydroquinone metabolites bound to human CYP2E1.

Valproic acid as an adjunctive therapeutic agent for the treatment of breast cancer

Breast cancer is one of the leading causes of cancer-related death among women. A significant challenge in treating breast cancer is the limited array of therapeutic options and the rapid development of resistance to existing agents. Indeed, breast cancer patients, particularly those with hormone-receptor (HR)-positive breast cancer, initially respond to systemic treatment with cytotoxic, hormonal, and immunotherapeutic agents but frequently progress to a more advanced disease that is refractory to therapy. Thus, new agents are needed to improve the effectiveness of current agents, decrease the emergence of resistance, and increase disease-free survival. To this end, numerous agents have been investigated for use in combination with existing therapies. Histone deacetylase (HDAC) inhibitors are a class of potent epigenetic modulators that have been investigated recently for their potential use in the treatment of breast cancer. In this review, we will discuss the underlying molecular rationale for using HDAC inhibitors for the treatment of breast cancer. In particular, we will focus our discussion on the FDA approved HDAC inhibitor valproic acid (VPA) which has been shown to alter proliferation, survival, cell migration, and hormone receptor expression of breast cancer cells in both the pre-clinical and clinical settings. We also discuss the promising pre-clinical data suggesting that VPA can be repurposed as an adjunctive agent in combination with many cytotoxic, hormonal, and immunotherapeutic agents for the treatment of breast cancer. Finally, we will examine the current models used to study the actions of VPA on breast cancer alone and in tandem with other agents.

Forecasting academic success through implementation of an online prerequisite review tutorials program for first year pharmacy students

OBJECTIVE Online prerequisite review (OPR) tutorials were designed and implemented to reinforce foundational scientific material in order to protect in-class time, foster self-directed learning, and ensure all students have similar baseline knowledge. METHODS Twenty-one tutorials covering undergraduate prerequisite material were developed by faculty and organized into six core modules, comprising basic biology, chemistry, and physiology topics. A quiz on this material was given on the first day of each course. This score was correlated with the final exam score at course completion. Additional student and faculty feedback was collected through surveys. RESULTS 2372 quiz-exam pairings were collected over three consecutive fall semesters. A one point increase in the quiz score was associated with a 3.6 point (95% confidence interval 3.1-4.0) higher exam score, as well as a greater probability of passing the exam (P<0.0001). Furthermore, simple linear regression revealed a positive correlation between quiz and exam scores (P<0.0001). Three full years of student survey data revealed an overwhelmingly positive perception of the OPR tutorials, and surveyed faculty reported better use of class time and improved student competency and participation. CONCLUSIONS Implementation of OPR tutorials may give faculty more efficient use of class time, and their associated quizzes serve as an early indicator for students at-risk of not passing who are candidates for early interventions. Furthermore, the OPR tutorial design gives it great transferability to biomedical post-graduate programs.