Preclinical characterization of a non-peptidomimetic HIV protease inhibitor with improved metabolic stability.

Protease inhibitors (PIs) remain an important component of antiretroviral therapy for the treatment of HIV-1 infection due to their high genetic barrier to resistance development. Nevertheless, the two most commonly prescribed HIV PIs, atazanavir and darunavir, still require co-administration with a pharmacokinetic boosting agent to maintain sufficient drug plasma levels which can lead to undesirable drug-drug interactions. Herein, we describe GS-9770, a novel investigational non-peptidomimetic HIV PI with unboosted once-daily oral dosing potential due to improvements in its metabolic stability and its pharmacokinetic properties in preclinical animal species. This compound demonstrates potent inhibitory activity and high on-target selectivity for recombinant HIV-1 protease versus other aspartic proteases tested. In cell culture, GS-9770 inhibits Gag polyprotein cleavage and shows nanomolar anti-HIV-1 potency in primary human cells permissive to HIV-1 infection and against a broad range of HIV subtypes. GS-9770 demonstrates an improved resistance profile against a panel of patient-derived HIV-1 isolates with resistance to atazanavir and darunavir. In resistance selection experiments, GS-9770 prevented the emergence of breakthrough HIV-1 variants at all fixed drug concentrations tested and required multiple protease substitutions to enable outgrowth of virus exposed to escalating concentrations of GS-9770. This compound also remained fully active against viruses resistant to drugs from other antiviral classes and showed no in vitro antagonism when combined pairwise with drugs from other antiretroviral classes. Collectively, these preclinical data identify GS-9770 as a potent, non-peptidomimetic once-daily oral HIV PI with potential to overcome the persistent requirement for pharmacological boosting with this class of antiretroviral agents.

Assessing Pleiotropic Effects of a Mixed-Mode Perpetrator Drug, Rifampicin, by Multiple Endogenous Biomarkers in Dogs.

Rifampicin (RIF) is a mixed-mode perpetrator that produces pleiotropic effects on liver cytochrome P450 enzymes and drug transporters. To assess the complex drug-drug interaction liabilities of RIF in vivo, a known probe substrate, midazolam (MDZ), along with multiple endogenous biomarkers were simultaneously monitored in beagle dogs before and after a 7-day treatment period by RIF at 20 mg/kg per day. Confirmed by the reduced MDZ plasma exposure and elevated 4ß-hydroxycholesterol (4ß-HC, biomarker of CYP3A activities) level, CYP3A was significantly induced after repeated RIF doses, and such induction persisted for 3 days after cessation of the RIF administration. On the other hand, increased plasma levels of coproporphyrin (CP)-I and III [biomarkers of organic anion transporting polypeptides 1b (Oatp1b) activities] were observed after the first dose of RIF. Plasma CPs started to decline as RIF exposure decreased, and they returned to baseline 3 days after cessation of the RIF administration. The data suggested the acute (inhibitory) and chronic (inductive) effects of RIF on Oatp1b and CYP3A enzymes, respectively, and a 3-day washout period is deemed adequate to remove superimposed Oatp1b inhibition from CYP3A induction. In addition, apparent self-induction of RIF was observed as its terminal half-life was significantly altered after multiple doses. Overall, our investigation illustrated the need for appropriate timing of modulator dosing to differentiate between transporter inhibition and enzyme induction. As further indicated by the CP data, induction of Oatp1b activities was not likely after repeated RIF administration. SIGNIFICANCE STATEMENT: This investigation demonstrated the utility of endogenous biomarkers towards complex drug-drug interactions by rifampicin (RIF) and successfully determined the optimal timing to differentiate between transporter inhibition and enzyme induction. Based on experimental evidence, Oatp1b induction following repeated RIF administration was unlikely, and apparent self-induction of RIF elimination was observed.

Ontogeny of Human Liver Aldehyde Oxidase: Developmental Changes and Implications for Drug Metabolism.

Despite the increasing importance of aldehyde oxidase (AO) in the drug metabolism of clinical candidates, ontogeny data for AO are limited. The objective of our study was to characterize the age-dependent AO content and activity in the human liver cytosolic fraction (HLC) and human hepatocytes (HH). HLC (n = 121 donors) and HH (n = 50 donors) were analyzed for (1) AO protein content by quantitative proteomics and (2) enzyme activity using carbazeran as a probe substrate. AO activity showed high technical variability and poor correlation with the content in HLC samples, whereas hepatocyte samples showed a strong correlation between the content and activity. Similarly, AO content and activity showed no significant age-dependent differences in HLC samples, whereas the average AO content and activity in hepatocytes increased significantly (∼20-40-fold) from the neonatal levels (0-28 days). Based on the hepatocyte data, the age at which 50% of the adult AO content is reached (age50) was 3.15 years (0.32-13.97 years, 95% CI). Metabolite profiling of carbazeran revealed age-dependent metabolic switching and the role of non-AO mechanisms (glucuronidation and desmethylation) in carbazeran elimination. The content-activity correlation in hepatocytes improved significantly (R2 = 0.95; p < 0.0001) in samples showing <10% contribution of glucuronidation toward the overall metabolism, confirming that AO-mediated oxidation and glucuronidation are the key routes of carbazeran metabolism. Considering the confounding effect of glucuronidation on AO activity, AO content-based ontogeny data are a more direct reflection of developmental changes in protein expression. The comprehensive ontogeny data of AO in HH samples are more reliable than HLC data, which are important for developing robust physiologically based pharmacokinetic models for predicting AO-mediated metabolism in children.

Dissecting Parameters Contributing to the Underprediction of Aldehyde Oxidase-Mediated Metabolic Clearance of Drugs.

We investigated the effect of variability and instability in aldehyde oxidase (AO) content and activity on the scaling of in vitro metabolism data. AO content and activity in human liver cytosol (HLC) and five recombinant human AO preparations (rAO) were determined using targeted proteomics and carbazeran oxidation assay, respectively. AO content was highly variable as indicated by the relative expression factor (REF; i.e., HLC to rAO content) ranging from 0.001 to 1.7 across different in vitro systems. The activity of AO in HLC degrades at a 10-fold higher rate in the presence of the substrate as compared with the activity performed after preincubation without substrate. To scale the metabolic activity from rAO to HLC, a protein-normalized activity factor (pnAF) was proposed wherein the activity was corrected by AO content, which revealed up to sixfold higher AO activity in HLC versus rAO systems. A similar value of pnAF was observed for another substrate, ripasudil. Physiologically based pharmacokinetic (PBPK) modeling revealed a significant additional clearance (CL; 66%), which allowed for the successful prediction of in vivo CL of four other substrates, i.e., O-benzyl guanine, BIBX1382, zaleplon, and zoniporide. For carbazeran, the metabolite identification study showed that the direct glucuronidation may be contributing to around 12% elimination. Taken together, this study identified differential protein content, instability of in vitro activity, role of additional AO clearance, and unaccounted metabolic pathways as plausible reasons for the underprediction of AO-mediated drug metabolism. Consideration of these factors and integration of REF and pnAF in PBPK models will allow better prediction of AO metabolism. SIGNIFICANCE STATEMENT: This study elucidated the plausible reasons for the underprediction of aldehyde oxidase (AO)-mediated drug metabolism and provided recommendations to address them. It demonstrated that integrating protein content and activity differences and accounting for the loss of AO activity, as well as consideration of extrahepatic clearance and additional pathways, would improve the in vitro to in vivo extrapolation of AO-mediated drug metabolism using physiologically based pharmacokinetic modeling.

Comparison of Tissue Abundance of Non-Cytochrome P450 Drug-Metabolizing Enzymes by Quantitative Proteomics between Humans and Laboratory Animal Species.

The use of animal pharmacokinetic models as surrogates for humans relies on the assumption that the drug disposition mechanisms are similar between preclinical species and humans. However, significant cross-species differences exist in the tissue distribution and protein abundance of drug-metabolizing enzymes (DMEs) and transporters. We quantified non-cytochrome P450 (non-CYP) DMEs across commonly used preclinical species (cynomolgus and rhesus monkeys, beagle dog, Sprague Dawley and Wistar Han rats, and CD1 mouse) and compared these data with previously obtained human data. Aldehyde oxidase was abundant in humans and monkeys while poorly expressed in rodents, and not expressed in dogs. Carboxylesterase (CES) 1 abundance was highest in the liver while CES2 was primarily expressed in the intestine in all species with notable species differences. For example, hepatic CES1 was 3× higher in humans than in monkeys, but hepatic CES2 was 3-5× higher in monkeys than in humans. Hepatic UDP-glucuronosyltransferase (UGT) 1A2 abundance was ∼4× higher in dogs compared with rats, whereas UGT1A3 abundance was 3-5× higher in dog livers than its ortholog in human and monkey livers. UGT1A6 abundance was 5-6× higher in human livers compared with monkey and dog livers. Hepatic sulfotransferase 1B1 abundance was 5-7× higher in rats compared with the rest of the species. These quantitative non-CYP proteomics data can be used to explain unique toxicological profiles across species and can be integrated into physiologically based pharmacokinetic models for the mechanistic explanation of pharmacokinetics and tissue distribution of xenobiotics in animal species. SIGNIFICANCE STATEMENT: We characterized the quantitative differences in non-cytochrome P450 (non-CYP) drug-metabolizing enzymes across commonly used preclinical species (cynomolgus and rhesus monkeys, beagle dogs, Sprague Dawley and Wistar Han rats, and CD1 mice) and compared these data with previously obtained human data. Unique differences in non-CYP enzymes across species were observed, which can be used to explain significant pharmacokinetic and toxicokinetic differences between experimental animals and humans.

18O-Enabled High-Throughput Acyl Glucuronide Stability Assay.

Acyl glucuronides (AGs) are common metabolites of carboxylic acid-containing compounds. In some circumstances, AGs are suspected to be involved in drug toxicity due to formation of acyl migration products that bind covalently to cellular components. The risk of this adverse effect has been found to be correlated with the chemical stability of the AG, and assays have been described that monitor acyl migration by liquid chromatography coupled with mass spectrometry (LC-MS). This analysis can be challenging as it requires baseline chromatographic separation of the unmigrated 1-ß-acyl glucuronide from the migrated isomers and thus needs to be individually optimized for each aglycone. Therefore, a high-throughput assay that eliminates LC method development is desirable. Herein, we report an improved acyl glucuronide stability assay based on the rate of 18O-incorporation from [18O] water, which is compatible with high-throughput bioanalytical LC-MS workflows. Synthetic AGs with shorter migration half-lives showed faster incorporation of 18O. The level of differential incorporation of 18O following a 24 h incubation correlates well with the migration tendency of AGs. This assay was developed further, exploring in situ generation of AGs by human hepatic microsomal fraction. The results from 18 in situ-formed acyl glucuronides were similar to those obtained using authentic reference standards. In this format, this new 18O-labeling method offers a simplified workflow, requires no LC method development or AG reference standard, and thus facilitates AG liability assessment in early drug discovery.

Human and nonclinical disposition of [14C]bictegravir, a potent integrase strand-transfer inhibitor for the treatment of HIV-1 infection.

Bictegravir (BIC) is a potent small-molecule integrase strand-transfer inhibitor (INSTI) and a component of Biktarvy®, a single-tablet combination regimen that is currently approved for the treatment of human immunodeficiency virus type 1 (HIV-1) infection. The absorption, metabolism, distribution, and elimination (ADME) characteristics of BIC were determined through in vivo nonclinical and clinical studies (IND 121318).[14C]BIC was rapidly absorbed orally in mice, rats, monkeys and human. The cumulative dose recovery was high in nonclinical species (>80%) and humans (95.3%), with most of the excreted dose recovered in faeces. Quantifiable radioactivity with declining concentration was observed in rat tissues suggesting reversible binding. Unchanged BIC was the most abundant circulating component in all species along with two notable metabolites M20 (a sulphate conjugate of hydroxylated BIC) and M15 (a glucuronide conjugate of BIC). BIC was primarily eliminated by hepatic metabolism followed by excretion of the biotransformed products into faeces. In vitro drug-drug interaction (DDI) studies with M15 and M20 demonstrated that no clinically relevant interactions were expected.Overall, BIC is a novel and potent INSTI with a favourable resistance, PK, and ADME profile that provides important improvements over other currently available INSTIs for the treatment of HIV-1.

Human pharmacokinetics prediction with an in vitro-in vivo correction factor approach and in vitro drug-drug interaction profile of bictegravir, a potent integrase-strand transfer inhibitor component in approved biktarvy® for the treatment of HIV-1 infection.

Bictegravir (BIC) is a potent small-molecule integrase strand-transfer inhibitor (INSTI) and a component of Biktarvy®, a single-tablet combination regimen that is currently approved for the treatment of human immunodeficiency virus type 1 (HIV-1) infection. The in vitro properties, pharmacokinetics (PK), and drug-drug interaction (DDI) profile of BIC were characterised in vitro and in vivo.BIC is a weakly acidic, ionisable, lipophilic, highly plasma protein-bound BCS class 2 molecule, which makes it difficult to predict human PK using standard methods. Its systemic plasma clearance is low, and the volume of distribution is approximately the volume of extracellular water in nonclinical species. BIC metabolism is predominantly mediated by cytochrome P450 enzyme (CYP) 3A and UDP-glucuronosyltransferase 1A1. BIC shows a low potential to perpetrate clinically meaningful DDIs via known drug metabolising enzymes or transporters.The human PK of BIC was predicted using a combination of bioavailability and volume of distribution scaled from nonclinical species and a modified in vitro-in vivo correlation (IVIVC) correction for clearance. Phase 1 studies in healthy subjects largely bore out the prediction and supported the methods used. The approach presented herein could be useful for other drug molecules where standard projections are not sufficiently accurate. .

Key Metabolic Enzymes Involved in Remdesivir Activation in Human Lung Cells.

Remdesivir (RDV; GS-5734, Veklury), the first FDA-approved antiviral to treat COVID-19, is a single-diastereomer monophosphoramidate prodrug of an adenosine analogue. RDV is taken up in the target cells and metabolized in multiple steps to form the active nucleoside triphosphate (TP) (GS-443902), which, in turn, acts as a potent and selective inhibitor of multiple viral RNA polymerases. In this report, we profiled the key enzymes involved in the RDV metabolic pathway with multiple parallel approaches: (i) bioinformatic analysis of nucleoside/nucleotide metabolic enzyme mRNA expression using public human tissue and lung single-cell bulk mRNA sequence (RNA-seq) data sets, (ii) protein and mRNA quantification of enzymes in human lung tissue and primary lung cells, (iii) biochemical studies on the catalytic rate of key enzymes, (iv) effects of specific enzyme inhibitors on the GS-443902 formation, and (v) the effects of these inhibitors on RDV antiviral activity against SARS-CoV-2 in cell culture. Our data collectively demonstrated that carboxylesterase 1 (CES1) and cathepsin A (CatA) are enzymes involved in hydrolyzing RDV to its alanine intermediate MetX, which is further hydrolyzed to the monophosphate form by histidine triad nucleotide-binding protein 1 (HINT1). The monophosphate is then consecutively phosphorylated to diphosphate and triphosphate by cellular phosphotransferases. Our data support the hypothesis that the unique properties of RDV prodrug not only allow lung-specific accumulation critical for the treatment of respiratory viral infection such as COVID-19 but also enable efficient intracellular metabolism of RDV and its MetX to monophosphate and successive phosphorylation to form the active TP in disease-relevant cells.

JAK selectivity and the implications for clinical inhibition of pharmacodynamic cytokine signalling by filgotinib, upadacitinib, tofacitinib and baricitinib.

OBJECTIVE: Janus kinase inhibitors (JAKinibs) are efficacious in rheumatoid arthritis (RA) with variable reported rates of adverse events, potentially related to differential JAK family member selectivity. Filgotinib was compared with baricitinib, tofacitinib and upadacitinib to elucidate the pharmacological basis underlying its clinical efficacy and safety. METHODS: In vitro JAKinib inhibition of signal transducer and activator of transcription phosphorylation (pSTAT) was measured by flow cytometry in peripheral blood mononuclear cells and whole blood from healthy donors and patients with RA following cytokine stimulation of distinct JAK/STAT pathways. The average daily pSTAT and time above 50% inhibition were calculated at clinical plasma drug exposures in immune cells. The translation of these measures was evaluated in ex vivo-stimulated assays in phase 1 healthy volunteers. RESULTS: JAKinib potencies depended on cytokine stimulus, pSTAT readout and cell type. JAK1-dependent pathways (interferon (IFN)α/pSTAT5, interleukin (IL)-6/pSTAT1) were among the most potently inhibited by all JAKinibs in healthy and RA blood, with filgotinib exhibiting the greatest selectivity for JAK1 pathways. Filgotinib (200 mg once daily) had calculated average daily target inhibition for IFNα/pSTAT5 and IL-6/pSTAT1 that was equivalent to tofacitinib (5 mg two times per day), upadacitinib (15 mg once daily) and baricitinib (4 mg once daily), with the least average daily inhibition for the JAK2-dependent and JAK3-dependent pathways including IL-2, IL-15, IL-4 (JAK1/JAK3), IFNγ (JAK1/JAK2), granulocyte colony stimulating factor, IL-12, IL-23 (JAK2/tyrosine kinase 2) and granulocyte-macrophage colony-stimulating factor (JAK2/JAK2). Ex vivo pharmacodynamic data from phase 1 healthy volunteers clinically confirmed JAK1 selectivity of filgotinib. CONCLUSION: Filgotinib inhibited JAK1-mediated signalling similarly to other JAKinibs, but with less inhibition of JAK2-dependent and JAK3-dependent pathways, providing a mechanistic rationale for its apparently differentiated efficacy:safety profile.

Regional Proteomic Quantification of Clinically Relevant Non-Cytochrome P450 Enzymes along the Human Small Intestine.

Current challenges in accurately predicting intestinal metabolism arise from the complex nature of the intestine, leading to limited applicability of available in vitro tools as well as knowledge deficits in intestinal physiology, including enzyme abundance. In particular, information on regional enzyme abundance along the small intestine is lacking, especially for non-cytochrome P450 enzymes such as carboxylesterases (CESs), UDP-glucuronosyltransferases (UGTs), and sulfotransferases (SULTs). We used cryopreserved human intestinal mucosa samples from nine donors as an in vitro surrogate model for the small intestine and performed liquid chromatography tandem mass spectrometry-based quantitative proteomics for 17 non-cytochrome P450 enzymes using stable isotope-labeled peptides. Relative protein quantification was done by normalization with enterocyte marker proteins, i.e., villin-1, sucrase isomaltase, and fatty acid binding protein 2, and absolute protein quantification is reported as picomoles per milligram of protein. Activity assays in glucuronidations and sequential metabolisms were conducted to validate the proteomics findings. Relative or absolute quantifications are reported for CES1, CES2, five UGTs, and four SULTs along the small intestine: duodenum, jejunum, and ileum for six donors and in 10 segments along the entire small intestine (A-J) for three donors. Relative quantification using marker proteins may be beneficial in further controlling for technical variabilities. Absolute quantification data will allow for scaling factor generation and in vivo extrapolation of intestinal clearance using physiologically based pharmacokinetic modeling. SIGNIFICANCE STATEMENT: Current knowledge gaps exist in intestinal protein abundance of non-cytochrome P450 enzymes. Here, we employ quantitative proteomics to measure non-cytochrome P450 enzymes along the human small intestine in nine donors using cryopreserved human intestinal mucosa samples. Absolute and relative abundances reported here will allow better scaling of intestinal clearance.

Characterization of Differential Tissue Abundance of Major Non-CYP Enzymes in Human.

The availability of assays that predict the contribution of cytochrome P450 (CYP) metabolism allows for the design of new chemical entities (NCEs) with minimal oxidative metabolism. These NCEs are often substrates of non-CYP drug-metabolizing enzymes (DMEs), such as UDP-glucuronosyltransferases (UGTs), sulfotransferases (SULTs), carboxylesterases (CESs), and aldehyde oxidase (AO). Nearly 30% of clinically approved drugs are metabolized by non-CYP enzymes. However, knowledge about the differential hepatic versus extrahepatic abundance of non-CYP DMEs is limited. In this study, we detected and quantified the protein abundance of eighteen non-CYP DMEs (AO, CES1 and 2, ten UGTs, and five SULTs) across five different human tissues. AO was most abundantly expressed in the liver and to a lesser extent in the kidney; however, it was not detected in the intestine, heart, or lung. CESs were ubiquitously expressed with CES1 being predominant in the liver, while CES2 was enriched in the small intestine. Consistent with the literature, UGT1A4, UGT2B4, and UGT2B15 demonstrated liver-specific expression, whereas UGT1A10 expression was specific to the intestine. UGT1A1 and UGT1A3 were expressed in both the liver and intestine; UGT1A9 was expressed in the liver and kidney; and UGT2B17 levels were significantly higher in the intestine than in the liver. All five SULTs were detected in the liver and intestine, and SULT1A1 and 1A3 were detected in the lung. Kidney abundance was the most variable among the studied tissues, and overall, high interindividual variability (>15-fold) was observed for UGT2B17, CES2 (intestine), SULT1A1 (liver), UGT1A9, UGT2B7, and CES1 (kidney). These differential tissue abundance data can be integrated into physiologically based pharmacokinetic (PBPK) models for the prediction of non-CYP drug metabolism and toxicity in hepatic and extrahepatic tissues.

Pharmacokinetics and Disposition of Momelotinib Revealed a Disproportionate Human Metabolite-Resolution for Clinical Development.

Momelotinib (MMB), a small-molecule inhibitor of Janus kinase (JAK)1/2 and of activin A receptor type 1 (ACVR1), is in clinical development for the treatment of myeloproliferative neoplasms. The pharmacokinetics and disposition of [14C]MMB were characterized in a single-dose, human mass-balance study. Metabolism and the pharmacologic activity of key metabolites were elucidated in multiple in vitro and in vivo experiments. MMB was rapidly absorbed following oral dosing with approximately 97% of the radioactivity recovered, primarily in feces with urine as a secondary route. Mean blood-to-plasma [14C] area under the plasma concentration-time curve ratio was 0.72, suggesting low association of MMB and metabolites with blood cells. [14C]MMB-derived radioactivity was detectable in blood for ≤48 hours, suggesting no irreversible binding of MMB or its metabolites. The major circulating human metabolite, M21 (a morpholino lactam), is a potent inhibitor of JAK1/2 and ACVR1 in vitro. Estimation of pharmacological activity index suggests M21 contributes significantly to the pharmacological activity of MMB for the inhibition of both JAK1/2 and ACVR1. M21 was observed in disproportionately higher amounts in human plasma than in rat or dog, the rodent and nonrodent species used for the general nonclinical safety assessment of this molecule. This discrepancy was resolved with additional nonclinical studies wherein the circulating metabolites and drug-drug interactions were further characterized. The human metabolism of MMB was mediated primarily by multiple cytochrome P450 enzymes, whereas M21 formation involved initial P450 oxidation of the morpholine ring followed by metabolism via aldehyde oxidase.

Discovery of potent and selective inhibitors of calmodulin-dependent kinase II (CaMKII).

We hereby disclose the discovery of inhibitors of CaMKII (7h and 7i) that are highly potent in rat ventricular myocytes, selective against hERG and other off-target kinases, while possessing good CaMKII tissue isoform selectivity (cardiac γ/δ vs. neuronal α/ß). In vitro and in vivo ADME/PK studies demonstrated the suitability of these CaMKII inhibitors for PO (7h rat F = 73%) and IV pharmacological studies.

Contribution of the organic anion transporter OAT2 to the renal active tubular secretion of creatinine and mechanism for serum creatinine elevations caused by cobicistat.

Many xenobiotics including the pharmacoenhancer cobicistat increase serum creatinine by inhibiting its renal active tubular secretion without affecting the glomerular filtration rate. This study aimed to define the transporters involved in creatinine secretion, applying that knowledge to establish the mechanism for xenobiotic-induced effects. The basolateral uptake transporters organic anion transporter OAT2 and organic cation transporters OCT2 and OCT3 were found to transport creatinine. At physiologic creatinine concentrations, the specific activity of OAT2 transport was over twofold higher than OCT2 or OCT3, establishing OAT2 as a likely relevant creatinine transporter and further challenging the traditional view that creatinine is solely transported by a cationic pathway. The apical multidrug and toxin extrusion transporters MATE1 and MATE2-K demonstrated low-affinity and high-capacity transport. All drugs known to affect creatinine inhibited OCT2 and MATE1. Similar to cimetidine and ritonavir, cobicistat had the greatest effect on MATE1 with a 50% inhibition constant of 0.99 µM for creatinine transport. Trimethoprim potently inhibited MATE2-K, whereas dolutegravir preferentially inhibited OCT2. Cimetidine was unique, inhibiting all transporters that interact with creatinine. Thus, the clinical observation of elevated serum creatinine in patients taking cobicistat is likely a result of OCT2 transport, facilitating intracellular accumulation, and MATE1 inhibition.

Structure-activity relationships of diamine inhibitors of cytochrome P450 (CYP) 3A as novel pharmacoenhancers, part I: core region.

Ritonavir (RTV), an HIV-1 protease inhibitor (PI), is also a potent mechanism-based inhibitor of human cytochrome P450 3A (CYP3A) and has been widely prescribed as a pharmacoenhancer. As a boosting agent for marketed PIs, it reduces pill burden, and improves compliance. Removal of the hydroxyl group from RTV reduces, but does not eliminate HIV PI activity and does not affect CYP3A inhibition. Herein we report the discovery of a novel series of CYP3A inhibitors that are devoid of antiviral activity. The synthesis and evaluation of analogs with extensive modifications of the 1,4-diamine core along with the structure activity relationships with respect to anti-HIV activity, CYP3A inhibitory activity, selectivity against other CYP enzymes and the human pregnane X receptor (PXR) will be discussed.

Structure-activity relationships of diamine inhibitors of cytochrome P450 (CYP) 3A as novel pharmacoenhancers. Part II: P2/P3 region and discovery of cobicistat (GS-9350).

The HIV protease inhibitor (PI) ritonavir (RTV) has been widely used as a pharmacoenhancer for other PIs, which are substrates of cytochrome P450 3A (CYP3A). However the potent anti-HIV activity of ritonavir may limit its use as a pharmacoenhancer with other classes of anti-HIV agents. Ritonavir is also associated with limitations such as poor physicochemical properties. To address these issues a series of compounds with replacements at the P2 and/or P3 region was designed and evaluated as novel CYP3A inhibitors. Through these efforts, a potent and selective inhibitor of CYP3A, GS-9350 (cobicistat) with improved physiochemical properties was discovered.

Preclinical characterization of GS-9669, a thumb site II inhibitor of the hepatitis C virus NS5B polymerase.

GS-9669 is a highly optimized thumb site II nonnucleoside inhibitor of the hepatitis C virus (HCV) RNA polymerase, with a binding affinity of 1.35 nM for the genotype (GT) 1b protein. It is a selective inhibitor of HCV RNA replication, with a mean 50% effective concentration (EC(50)) of ≤ 11 nM in genotype 1 and 5 replicon assays, but lacks useful activity against genotypes 2 to 4. The M423T mutation is readily generated clinically upon monotherapy with the thumb site II inhibitors filibuvir and lomibuvir, and it is notable that GS-9669 exhibited only a 3-fold loss in potency against this variant in the genotype 1b replicon. Rather than M423T, resistance predominantly tracks to residues R422K and L419M and residue I482L in GT 1b and 1a replicons, respectively. GS-9669 exhibited at least additive activity in combination with agents encompassing four other direct modes of action (NS3 protease, NS5A, NS5B via an alternative allosteric binding site, and NS5B nucleotide) as well as with alpha interferon or ribavirin in replicon assays. It exhibited high metabolic stability in in vitro human liver microsomal assays, which, in combination with its pharmacokinetic profiles in rat, dog, and two monkey species, is predictive of good human pharmacokinetics. GS-9669 is well suited for combination with other orally active, direct-acting antiviral agents in the treatment of genotype 1 chronic HCV infection. (This study has been registered at ClinicalTrials.gov under registration number NCT01431898.).

Cobicistat boosts the intestinal absorption of transport substrates, including HIV protease inhibitors and GS-7340, in vitro.

The experimental pharmacoenhancer cobicistat (COBI), a potent mechanism-based inhibitor of cytochrome P450 3A enzymes, was found to inhibit the intestinal efflux transporters P-glycoprotein and breast cancer resistance protein. Consistent with its transporter inhibition, COBI significantly increased the absorptive flux of potential candidates for clinical coadministration, including the HIV protease inhibitors atazanavir and darunavir and the lymphoid cell- and tissue-targeted prodrug of the nucleotide analog tenofovir, GS-7340, through monolayers of Caco-2 cells in vitro.

In vitro characterization of GS-8374, a novel phosphonate-containing inhibitor of HIV-1 protease with a favorable resistance profile.

GS-8374 is a novel bis-tetrahydrofuran HIV-1 protease (PR) inhibitor (PI) with a unique diethylphosphonate moiety. It was selected from a series of analogs containing various di(alkyl)phosphonate substitutions connected via a linker to the para position of a P-1 phenyl ring. GS-8374 inhibits HIV-1 PR with high potency (K(i) = 8.1 pM) and with no known effect on host proteases. Kinetic and thermodynamic analysis of GS-8374 binding to PR demonstrated an extremely slow off rate for the inhibitor and favorable contributions of both the enthalpic and entropic components to the total free binding energy. GS-8374 showed potent antiretroviral activity in T-cell lines, primary CD4(+) T cells (50% effective concentration [EC(50)] = 3.4 to 11.5 nM), and macrophages (EC(50) = 25.5 nM) and exhibited low cytotoxicity in multiple human cell types. The antiviral potency of GS-8374 was only moderately affected by human serum protein binding, and its combination with multiple approved antiretrovirals showed synergistic effects. When it was tested in a PhenoSense assay against a panel of 24 patient-derived viruses with high-level PI resistance, GS-8374 showed lower mean EC(50)s and lower fold resistance than any of the clinically approved PIs. Similar to other PIs, in vitro hepatic microsomal metabolism of GS-8374 was efficiently blocked by ritonavir, suggesting a potential for effective pharmacokinetic boosting in vivo. In summary, results from this broad in vitro pharmacological profiling indicate that GS-8374 is a promising candidate to be further assessed as a new antiretroviral agent with potential for clinical efficacy in both treatment-naïve and -experienced patients.