Adapting ADME and Pharmacokinetic Analysis to the Next Generation of Therapeutic Modalities.

The development of multiple drug modalities over the past 20 years has dramatically expanded the therapeutic space for intervention in disease processes. Rather than being alternative therapeutic approaches, these modalities tend to be complimentary both in the scope of target space and the biological mechanisms harnessed for disease control. Realization of these therapeutic opportunities requires an understanding of the physiological, biochemical and biological barriers that control exposure to the drug target and resulting biological response. Consequently, successful application of ADME and PK/PD to characterization of novel therapeutics needs to consider the unique attributes conferred by the therapeutic modality and the desired and potential off-target biological responses. The discussion that follows provides examples of how barriers to exposure, and translation of exposure to efficacy can change across different modalities. Additionally, recommendations are made for ADME analysis in which biological barriers and mechanistic properties unique to specific modalities are used to focus ADME PK optimization and characterization.

siRNA-mediated knockdown of P450 oxidoreductase in rats: a tool to reduce metabolism by CYPs and increase exposure of high clearance compounds.

PURPOSE: To develop a tool based on siRNA-mediated knockdown of hepatic P450 oxidoreductase (POR) to decrease the CYP-mediated metabolism of small molecule drugs that suffer from rapid metabolism in vivo, with the aim of improving plasma exposure of these drugs. METHODS: siRNA against the POR gene was delivered using lipid nanoparticles (LNPs) into rats. The time course of POR mRNA knockdown, POR protein knockdown, and loss of POR enzyme activity was monitored. The rat livers were harvested to produce microsomes to determine the impact of POR knockdown on the metabolism of several probe substrates. Midazolam (a CYP3A substrate with high intrinsic clearance) was administered into LNP-treated rats to determine the impact of POR knockdown on midazolam pharmacokinetics. RESULTS: Hepatic POR mRNA and protein levels were significantly reduced by administering siRNA and the maximum POR enzyme activity reduction (~85%) occurred 2 weeks post-dose. In vitro analysis showed significant reductions in metabolism of probe substrates due to POR knockdown in liver, and in vivo POR knockdown resulted in greater than 10-fold increases in midazolam plasma concentrations following oral dosing. CONCLUSIONS: Anti-POR siRNA can be used to significantly reduce hepatic metabolism by various CYPs as well as greatly increase the bioavailability of high clearance compounds following an oral dose, thus enabling it to be used as a tool to increase drug exposure in vivo.

RNA-induced silencing complex-bound small interfering RNA is a determinant of RNA interference-mediated gene silencing in mice.

Deeper knowledge of pharmacokinetic and pharmacodynamic (PK/PD) concepts for RNA therapeutics is important to streamline the drug development process and for rigorous selection of best performing drug candidates. Here we characterized the PK/PD relationship for small interfering RNAs (siRNAs) targeting luciferase by examining siRNA concentration in plasma and liver, the temporal RNA-induced silencing complex binding profiles, mRNA reduction, and protein inhibition measured by noninvasive bioluminescent imaging. A dose-dependent and time-related decrease in bioluminescence was detected over 25 days after a single treatment of a lipid nanoparticle-formulated siRNA targeting luciferase messenger RNA. A direct relationship was observed between the degree of in vivo mRNA and protein reduction and the Argonaute2 (Ago2)-bound siRNA fraction but not with the total amount of siRNA found in the liver, suggesting that the Ago2-siRNA complex is the key determinant of target inhibition. These observations were confirmed for an additional siRNA that targets endogenously expressed Sjögren syndrome antigen B (Ssb) mRNA, indicating that our observations are not limited to a transgenic mouse system. Our data provide detailed information of the temporal regulation of siRNA liver delivery, Ago2 loading, mRNA reduction, and protein inhibition that are essential for the rapid and cost-effective clinical development of siRNAs therapeutics.

Effect of P-glycoprotein-mediated efflux on cerebrospinal fluid concentrations in rhesus monkeys.

Brain penetration of drugs which are subject to P-glycoprotein (Pgp)-mediated efflux is attenuated, as manifested by the fact that the cerebrospinal fluid concentration (C(CSF)), a good surrogate of the unbound brain concentration (C(ub)), is lower than the unbound plasma concentration (C(up)) for Pgp substrates. In rodents, the attenuation magnitude of brain penetration by Pgp-mediated efflux has been estimated by correlating the ratio of CSF to plasma exposures (C(CSF)/C(p)) with the unbound fraction in plasma (f(u)) upon the incorporation of the in vivo or in vitro Pgp-mediated efflux ratios (ERs). In the present work, we investigated the impact of Pgp-mediated efflux on C(CSF) in monkeys. Following intravenous administration to cisterna magna ported rhesus monkeys, the CSF and plasma concentrations were determined for 25 compounds from three discovery programs. We also evaluated their f(u) in rhesus plasma and ER in human and African green monkey MDR-transfected LLC-PK1 cells. These compounds varied significantly in the f(u) (0.025-0.73), and 24 out of 25 are considered Pgp substrates based on their appreciable directional transport (ER>2). The C(CSF)/C(p) was significantly lower than the corresponding f(u) (>or=3-fold) for 16 compounds regardless of a significant correlation (R(2)=0.59, p=4 x 10(-5)) when the C(CSF)/C(p) was plotted against the f(u). When the f(u) was normalized to the ER (f(u)/ER) the correlation was improved (R(2)=0.75, p=8 x 10(-8)). More importantly, only one compound showed the C(CSF)/C(p) that exceeded 3-fold of the normalized f(u). The results suggest that the impact of Pgp-mediated efflux in monkeys, similar to the case in rodents, is reasonably reflected by the gradient between the free concentrations in plasma and in CSF. Therefore, f(u) and Pgp ER may serve as useful measurements in estimating in vivo C(CSF)/C(p) ratios in monkeys, and potentially in humans.

First demonstration of cerebrospinal fluid and plasma A beta lowering with oral administration of a beta-site amyloid precursor protein-cleaving enzyme 1 inhibitor in nonhuman primates.

beta-Site amyloid precursor protein (APP)-cleaving enzyme (BACE) 1 cleavage of amyloid precursor protein is an essential step in the generation of the potentially neurotoxic and amyloidogenic A beta 42 peptides in Alzheimer's disease. Although previous mouse studies have shown brain A beta lowering after BACE1 inhibition, extension of such studies to nonhuman primates or man was precluded by poor potency, brain penetration, and pharmacokinetics of available inhibitors. In this study, a novel tertiary carbinamine BACE1 inhibitor, tertiary carbinamine (TC)-1, was assessed in a unique cisterna magna ported rhesus monkey model, where the temporal dynamics of A beta in cerebrospinal fluid (CSF) and plasma could be evaluated. TC-1, a potent inhibitor (IC(50) approximately 0.4 nM), has excellent passive membrane permeability, low susceptibility to P-glycoprotein transport, and lowered brain A beta levels in a mouse model. Intravenous infusion of TC-1 led to a significant but transient lowering of CSF and plasma A beta levels in conscious rhesus monkeys because it underwent CYP3A4-mediated metabolism. Oral codosing of TC-1 with ritonavir, a potent CYP3A4 inhibitor, twice daily over 3.5 days in rhesus monkeys led to sustained plasma TC-1 exposure and a significant and sustained reduction in CSF sAPP beta, A beta 40, A beta 42, and plasma A beta 40 levels. CSF A beta 42 lowering showed an EC(50) of approximately 20 nM with respect to the CSF [TC-1] levels, demonstrating excellent concordance with its potency in a cell-based assay. These results demonstrate the first in vivo proof of concept of CSF A beta lowering after oral administration of a BACE1 inhibitor in a nonhuman primate.

Strategies toward improving the brain penetration of macrocyclic tertiary carbinamine BACE-1 inhibitors.

This letter describes replacements for the P3 amide moiety present in previously reported tertiary carbinamine macrolactones. Although P-gp efflux issues associated with these amide-macrolactones were solved and full brain penetration was measured in one case, potency was compromised in the process.

Design, synthesis, and SAR of macrocyclic tertiary carbinamine BACE-1 inhibitors.

This Letter describes the design and synthesis of tertiary carbinamine macrocyclic inhibitors of the beta-secretase (BACE-1) enzyme. These macrocyclic inhibitors, some of which incorporate novel P2 substituents, display a 2- to 100-fold increase in potency relative to the previously described acyclic analogs while affording greater stability.

Discovery and SAR of isonicotinamide BACE-1 inhibitors that bind beta-secretase in a N-terminal 10s-loop down conformation.

A series of low-molecular weight 2,6-diamino-isonicotinamide BACE-1 inhibitors containing an amine transition-state isostere were synthesized and shown to be highly potent in both enzymatic and cell-based assays. These inhibitors contain a trans-S,S-methyl cyclopropane P(3) which bind BACE-1 in a 10s-loop down conformation giving rise to highly potent compounds with favorable molecular weight and moderate to high susceptibility to P-glycoprotein (P-gp) efflux.

Identification of novel, orally bioavailable spirohydantoin CGRP receptor antagonists.

A rapid analogue approach to identification of spirohydantoin-based CGRP antagonists provided novel, low molecular weight leads. Modification of these leads afforded a series of nanomolar benzimidazolinone-based CGRP receptor antagonists. The oral bioavailability of these antagonists was inversely correlated with polar surface area, suggesting that membrane permeability was a key limitation to absorption. Optimization provided compound 12, a potent CGRP receptor antagonist (K(i)=21nM) with good oral bioavailability in three species.

Development and characterization of LLC-PK1 cells containing Sprague-Dawley rat Abcb1a (Mdr1a): comparison of rat P-glycoprotein transport to human and mouse.

INTRODUCTION: P-glycoprotein is localized in numerous tissues throughout the body and plays an important role in the disposition of many xenobiotics. The contribution of P-glycoprotein-mediated drug transport is being evaluated in early drug discovery stages, particularly for compounds targeted to the central nervous system, using in vitro tools including cell lines expressing P-glycoprotein. Previous work in our laboratory suggests there are species differences in P-glycoprotein transport activity between humans and animals. The rat Abcb1a form of P-glycoprotein (formerly known as Mdr1a), the predominate isoform in the brain, has not been described in a functional cell system. Here, we describe the development and characterization of LLC-PK1 cells expressing rat Abcb1. METHODS: We cloned rat Abcb1a and generated a stable LLC-PK1 cell line. Expression and function of the cells were evaluated by immunoblot analysis, cytotoxicity analysis, cellular accumulation assays, and transcellular transport of probe substrates. The transport ratios of structurally diverse compounds obtained from parental cells or cells stably transfected with human ABCB1, mouse Abcb1a or rat Abcb1a were compared. RESULTS: Two forms of rat Abcb1a were cloned from Sprague-Dawley cDNA that differ by six amino acids and a base pair deletion. The intact form was stably transfected in LLC-PK1 cells. Immunoblot analysis demonstrated expression of the protein. The cells demonstrated P-glycoprotein-mediated function by directional transport of dexamethasone, ritonavir, and vinblastine in a transwell assay that was inhibited in the presence of cyclosporin A, verapamil, or quinidine. Likewise, the cells showed reduced cellular accumulation of Rh123 by FACS analysis that was reversed in the presence of cyclosporin A. These cells showed >or=350-fold resistance to colchicine, doxorubicin, vinblastine, and taxol and were sensitized in the presence of verapamil or cyclosporin A. Of 179 chemically diverse compounds evaluated, approximately 20% of the compounds evaluated were predicted to be substrates in one species but not in other species. DISCUSSION: Taken together, these data suggest these cells will be useful for evaluation of rat Abcb1a-mediated transport and for evaluation of species-specific P-glycoprotein-mediated transport.

Interactions of human P-glycoprotein with simvastatin, simvastatin acid, and atorvastatin.

PURPOSE: In this study, P-glycoprotein (P-gp) mediated efflux of simvastatin (SV), simvastatin acid (SVA), and atorvastatin (AVA) and inhibition of P-gp by SV, SVA, and AVA were evaluated to assess the role of P-gp in drug interactions. METHODS: P-gp mediated efflux of SV, SVA, and AVA was determined by directional transport across monolayers of LLC-PK1 cells and LLC-PK1 cells transfected with human MDR1. Inhibition of P-gp was evaluated by studying the vinblastine efflux in Caco-2 cells and in P-gp overexpressing KBV1 cells at concentrations of SV, SVA, and AVA up to 50 microM. RESULTS: Directional transport studies showed insignificant P-gp mediated efflux of SV, and moderate P-gp transport [2.4-3.8 and 3.0-6.4 higher Basolateral (B) to Apical (A) than A to B transport] for SVA and AVA, respectively. Inhibition studies did not show the same trend as the transport studies with SV and AVA inhibiting P-gp (IC50 -25-50 microM) but SVA not showing any inhibition of P-gp. CONCLUSIONS: The moderate level of P-gp mediated transport and low affinity of SV, SVA, and AVA for P-gp inhibition compared to systemic drug levels suggest that drug interactions due to competition for P-gp transport is unlikely to be a significant factor in adverse drug interactions. Moreover, the inconsistencies between P-gp inhibition studies and P-gp transport of SV, SVA, and AVA indicate that the inhibition studies are not a valid means to identify statins as Pgp substrates.

Acyl glucuronidation and glucosidation of a new and selective endothelin ET(A) receptor antagonist in human liver microsomes.

Compound A [(+)-(5S,6R,7R)-2-isopropylamino-7-[4-methoxy-2-((2R)-3-methoxy-2-methylpropyl)-5-(3,4-methylenedioxyphenyl) cyclopenteno [1,2-b] pyridine 6-carboxylic acid] is a new and selective endothelin ET(A) receptor antagonist. It underwent significant acyl glucuronidation and acyl glucosidation in human liver microsomes supplemented with UDP-glucuronic acid (UDPGA) and UDP-glucose (UDPG). These two conjugations were observed in a panel of human liver microsomal samples (n = 16) that gave rise to varying activities but with no significant correlation with each other in the native and activator-treated microsomal preparations (r(2) 0.05). The lack of correlation may be explained by the involvement of multiple UDP-glucuronosyltransferases (UGTs; UGT1A1, 1A3, 1A9, 2B7 and 2B15) in the glucuronidation but essentially solely UGT2B7 in the glucosidation. Both reactions conformed to monophasic Michaelis-Menten kinetics in human liver microsomes. The glucuronidation reaction exhibited apparent K(m) values (mean +/- S.E.) for compound A and UDPGA of 8.4 +/- 0.6 and 605 +/- 35 microM, respectively, whereas the values for the glucosidation reaction were 10.2 +/- 1.5 and 670 +/- 120 microM, respectively. In both pooled human liver microsomes and expressed UGT2B7, UDPG and UDPGA competitively inhibited their counterpart conjugations with K(i) values close to their K(m) values, indicating a comparable affinity of the enzyme toward these two nucleotide sugars. We herein report a drug acyl glucoside formed in human liver microsomes at a considerable turnover rate and provide the evidence for a UGT isoform (UGT2B7) capable of transferring both glucuronic acid and glucose from UDPGA and UDPG to an aglycone.

Evaluation of drug interactions with P-glycoprotein in drug discovery: in vitro assessment of the potential for drug-drug interactions with P-glycoprotein.

The pharmacological effects of a drug are highly dependent on the absorption, metabolism, elimination, and distribution of the drug. In the past few years it has become apparent that transport proteins play a major role in regulating the distribution, elimination and metabolism of some drugs. As a consequence of our new understanding of the influence of transport proteins on the pharmacokinetic and pharmacodynamic behavior of drugs, increasing attention has been focused on the potential for drug-drug interactions arising from interactions with drug transport proteins. The efflux transporter P-glycoprotein (P-gp) has received the most attention with regard to its role in restricting drug absorption and distribution and as a potential source for variability in drug pharmacokinetics and pharmacodynamics. This review will focus on the evaluation of drug candidates to assess the potential for drug interactions at the level of P-gp. We will discuss the role of P-gp in drug disposition, the biochemistry of P-gp efflux as it relates to model systems to study drug interactions with P-gp, and the implementation of P-gp assay models within the drug discovery process.