Brain tissue binding of drugs: evaluation and validation of solid supported porcine brain membrane vesicles (TRANSIL) as a novel high-throughput method.

Estimating the unbound fraction of drugs in brain has become essential for the evaluation and interpretation of the pharmacokinetics and pharmacodynamics of new central nervous system drug candidates. Dialysis-based methods are considered to be accurate for estimating the fraction unbound in brain; however, these techniques are hampered by a low throughput. In this study, we present a novel, matrix-free, high-throughput method for estimating the unbound fraction, based on a sample pooling approach combining the TRANSIL brain absorption assay with liquid chromatography-mass spectrometry. The base measurement of the TRANSIL approach is the affinity to brain membranes, and this method is used directly to predict the free fraction in brain. The method was evaluated by comparing the free fraction of drugs in brain [f(u,brain) (%)] obtained using the TRANSIL brain absorption assay and equilibrium dialysis methods for a test set of 65 drugs (27 marketed and 38 proprietary drugs). A good correlation (r(2) > 0.93) of f(u,brain) (%) between the TRANSIL brain absorption assay and equilibrium dialysis was observed. Moreover, we compared the lipid composition of rat and porcine brain and analyzed the influence of the brain albumin content on brain tissue binding measurement. The comparison of the lipid composition indicated only minor differences between rat and porcine brain, and albumin appears to have a low impact on brain tissue binding measurements. The TRANSIL brain absorption assay with sample pooling methodology not only significantly reduces the biological matrix required but also increases the throughput, compared with the conventional dialysis methods.

Application of drug efficiency index in drug discovery: a strategy towards low therapeutic dose.

INTRODUCTION: The ultimate objective of optimizing adsorption, distribution, metabolism and excretion (ADME) parameters in drug discovery is to maximize the unbound concentration at the site of action for a given dose level. This has the added benefit of minimizing the efficacious dose, reducing the potential for attrition related to drug burden and direct organ toxicity. The concept of drug efficiency was formulated as a tool to obtain a balanced profile between target affinity and ADME properties during lead optimization. AREAS COVERED: The authors discuss how it is possible to maximize the in vivo pharmacological potential addressing whether drug efficiency adds value to the decision-making process and whether it is possible to introduce a single optimization parameter, the drug efficiency index (DEI), linking target affinity and ADME properties, as a marker of in vivo efficacy. EXPERT OPINION: In the absence of a clear hypothesis-driven approach at the beginning of the program (i.e., pharmacokinetic-pharmacodynamic link), the objective to select molecules with a low therapeutic dose is still a major hurdle in drug discovery. The authors believe that a greater strategic focus on mechanistically relevant measures of the determinants of receptor occupancy would help the optimization and selection process. In this respect, the introduction of the DEI, which can be seen as a correction of target affinity by the in vivo pharmacokinetic potential, may help drug discovery to select and promote those molecules with the highest probability to interact with the biological target and with the best balance between target affinity and ADME properties.

A comparative study of the CYP450 inhibition potential of marketed drugs using two fluorescence based assay platforms routinely used in the pharmaceutical industry.

Semi-automated high throughput screening for the inhibition of major human cytochrome P450 enzymes (1A2, 2C9, 2C19, 2D6 and 3A4) expressed in Escherichia Coli (Cypex bactosomes) or human lymphoblastoid cells (Gentest cDNA microsomes) using fluorescent probes has been evaluated using 68 marketed drugs. In general lower IC50 values were obtained with Cypex bactosomes compared with Gentest cDNA microsomes. This could be due to use of higherconcentration of protein and also the lower activity of Gentest cDNA microsomes. Notably, when compared with in vivo clinical drug-drug interactions (cDDIs) gathered from clinical studies reported in the scientific literature Cypex bactosome data was better at predicting in vivo cDDI. Consequently, from the data obtained in this comparative study, a fluorescence based assay using Cypex bactosomes is more suitable as a front-line screen for the prediction of potential downstream CYP450 driven cDDIs.

Identifying a higher throughput assay for metabolism dependent inhibition (MDI).

A higher throughput method of screening for the metabolism dependent inhibition of 56 marketed drugs was evaluated and compared data from the PHLM assay (using midazolam as probe) with Cypex assay (using diethoxyfluoresin (DEF) as probe) for CYP3A4 by using 96 well plate. Also 27 marketed drugs were selected to evaluate the reproducibility of Cypex assay using 7-benzyloxyquinoline (7-BQ) as second probe substrate for CYP3A4. Furthermore Cypex CYP2D6 was used to evaluate the reproducibility of this system using 4-methylaminomethyl-7-methoxycoumarin (MMC) as probe substrate with 15 marketed drugs. The fold change was estimated using the fold change obtained from triplicates experiments in same day or different days. All replicates in agreement (i.e. all positive or all negative) for >80% of compounds. The IC50 values for the two assays closely matched only for 13 compounds (23%). Only 5 of the variant 56 compounds had higher IC50 values with the recombinant enzymes, whereas 38 had lower IC50 values with the recombinant cypex CYP3A4 enzyme. The Cypex assay is comparable to PHLM assay in terms of predictivity and reproducibility. The Cypex assay therefore offers a higher throughput, reproducible alternative to PHLM for placement earlier in the lead optimisation process. In conclusion, the results obtained from a fluorescence-based method using Cypex CYP3A4 reflect mostly those obtained from conventional assay using human liver microsomes. This method provides more rapid and reliable detection of MDI inhibitors and may be useful in drug discovery.