Discovery of Novel HIV Protease Inhibitors Using Modern Computational Techniques

The human immunodeficiency virus type 1 (HIV-1) has continued to be a global concern. With the new HIV incidence, the emergence of multi-drug resistance and the untoward side effects of currently used anti-HIV drugs, there is an urgent need to discover more efficient anti-HIV drugs. Modern computational tools have played vital roles in facilitating the drug discovery process. This research focuses on a pharmacophore-based similarity search to screen 111,566,735 unique compounds in the PubChem database to discover novel HIV-1 protease inhibitors (PIs). We used an in silico approach involving a 3D-similarity search, physicochemical and ADMET evaluations, HIV protease-inhibitor prediction (IC50/percent inhibition), rigid receptor-molecular docking studies, binding free energy calculations and molecular dynamics (MD) simulations. The 10 FDA-approved HIV PIs (saquinavir, lopinavir, ritonavir, amprenavir, fosamprenavir, atazanavir, nelfinavir, darunavir, tipranavir and indinavir) were used as reference. The in silico analysis revealed that fourteen out of the twenty-eight selected optimized hit molecules were within the acceptable range of all the parameters investigated. The hit molecules demonstrated significant binding affinity to the HIV protease (PR) when compared to the reference drugs. The important amino acid residues involved in hydrogen bonding and п-п stacked interactions include ASP25, GLY27, ASP29, ASP30 and ILE50. These interactions help to stabilize the optimized hit molecules in the active binding site of the HIV-1 PR (PDB ID: 2Q5K). HPS/002 and HPS/004 have been found to be most promising in terms of IC50/percent inhibition (90.15%) of HIV-1 PR, in addition to their drug metabolism and safety profile. These hit candidates should be investigated further as possible HIV-1 PIs with improved efficacy and low toxicity through in vitro experiments and clinical trial investigations.

Decoding molecular mechanism underlying binding of drugs to HIV-1 protease with molecular dynamics simulations and MM-GBSA calculations

HIV-1 protease (PR) is thought to be efficient targets of anti-AIDS drug design. Molecular dynamics (MD) simulations and multiple post-processing analysis technologies were applied to decipher molecular mechanism underlying binding of three drugs Lopinavir (LPV), Nelfinavir (NFV) and Atazanavir (ATV) to the PR. Binding free energies calculated by molecular mechanics generalized Born surface area (MM-GBSA) suggest that compensation between binding enthalpy and entropy plays a vital role in binding of drugs to PR. Dynamics analyses show that binding of LPV, NFV and ATV highly affects structural flexibility, motion modes and dynamics behaviour of the PR, especially for two flaps. Computational alanine scanning and interaction network analysis verify that although three drugs have structural difference, they share similar binding modes to the PR and common interaction clusters with the PR. The current findings also confirm that residues located interaction clusters, such as Asp25/Asp25', Gly27/Gly27', Ala28/Ala28', Asp29, Ile47/Ile47', Gly49/Gly49', Ile50/Ile50', Val82/Val82' and Ile84/Ile84, can be used as efficient targets of clinically available inhibitors towards the PR.

Drug binding dynamics of the dimeric SARS-CoV-2 main protease, determined by molecular dynamics simulation

We performed molecular dynamics simulation of the dimeric SARS-CoV-2 (severe acute respiratory syndrome corona virus 2) main protease (Mpro) to examine the binding dynamics of small molecular ligands. Seven HIV inhibitors, darunavir, indinavir, lopinavir, nelfinavir, ritonavir, saquinavir, and tipranavir, were used as the potential lead drugs to investigate access to the drug binding sites in Mpro. The frequently accessed sites on Mpro were classified based on contacts between the ligands and the protein, and the differences in site distributions of the encounter complex were observed among the ligands. All seven ligands showed binding to the active site at least twice in 28 simulations of 200 ns each. We further investigated the variations in the complex structure of the active site with the ligands, using microsecond order simulations. Results revealed a wide variation in the shapes of the binding sites and binding poses of the ligands. Additionally, the C-terminal region of the other chain often interacted with the ligands and the active site. Collectively, these findings indicate the importance of dynamic sampling of protein-ligand complexes and suggest the possibilities of further drug optimisations.

Application of a novel regulatable Cre recombinase system to define the role of liver and gut metabolism in drug oral bioavailability

The relative contribution of hepatic compared with intestinal oxidative metabolism is a crucial factor in drug oral bioavailability and therapeutic efficacy. Oxidative metabolism is mediated by the cytochrome P450 mono-oxygenase system to which cytochrome P450 reductase (POR) is the essential electron donor. In order to study the relative importance of these pathways in drug disposition, we have generated a novel mouse line where Cre recombinase is driven off the endogenous Cyp1a1 gene promoter; this line was then crossed on to a floxed POR mouse. A 40 mg/kg dose of the Cyp1a1 inducer 3-methylcholanthrene (3MC) eliminated POR expression in both liver and small intestine, whereas treatment at 4 mg/kg led to a more targeted deletion in the liver. Using this approach, we have studied the pharmacokinetics of three probe drugs--paroxetine, midazolam, nelfinavir--and show that intestinal metabolism is a determinant of oral bioavailability for the two latter compounds. The Endogenous Reductase Locus (ERL) mouse represents a significant advance on previous POR deletion models as it allows direct comparison of hepatic and intestinal effects on drug and xenobiotic clearance using lower doses of a single Cre inducing agent, and in addition minimizes any cytotoxic effects, which may compromise interpretation of the experimental data.

Toxicology and carcinogenesis studies of mixtures of 3'-azido-3'-deoxythymidine (AZT), lamivudine (3TC), nevirapine (NVP), and nelfinavir mesylate (NFV) (Cas Nos. 30516-87-1, 134678-17-4, 129618-40-2, 159989-65-8) in B6C3F1 Mice (transplacental exposure studies)

BACKGROUND

Antiretroviral drugs are used to treat patients positive for the human immunovirus HIV-1, and increasingly treatments include a combination of such drugs. The noninfected children of women who are pregnant and receiving such treatment may also be exposed to the drugs by transplacental exposure. We studied the long-term effects of such transplacental exposure in mice by exposing pregnant mice to combinations of four such antiretroviral drugs for seven days and then observing their pups for two years following birth. The four drugs studied were 3′-azido-3′-deoxythymidine (AZT), lamivudine (3TC), nevirapine (NVP), and nelfinavir mesylate (NFV).

METHODS

Four different sets of exposure studies were performed: exposure to AZT; to AZT plus 3TC; to AZT, 3TC, and NVP; or to AZT, 3TC, and NFV. In each of these studies, groups of pregnant females were given one of three concentrations of the drug combinations seven times though a tube directly into their stomachs, and after birth their pups were maintained with no further exposure for two years. The offspring of another group of pregnant females not treated with the drugs served as controls. At the end of the study, tissues from more than 40 sites were examined for every animal.

RESULTS

Survival of pups whose mothers were exposed to AZT or AZT plus 3TC was similar to their controls, while the survival rates for offspring of mice exposed to AZT, 3TC, and NVP or AZT, 3TC, and NFP were lower than for controls. In most cases the body weights of pups from mothers exposed were slightly less than those of the controls. There were slight increases in the incidences of thyroid gland tumors and skin tumors in the female pups of mothers exposed to AZT alone and of lung tumors in female pups of mothers exposed to AZT plus 3TC. For offspring of mothers exposed to AZT, 3TC, and NVP there were increased incidences of skin tumors in both male and female pups, and more so in the males.

CONCLUSIONS

We conclude that exposure to the combination of AZT, 3TC, and NVP during pregnancy caused an increase in skin tumors in the male offspring and possibly also to the female offspring. Exposure to AZT alone during pregnancy may have been related to thyroid gland or skin tumors in female offspring, and exposure to AZT plus 3TC may have been related to lung tumors in female offspring.

Effect of transporter modulation on the emergence of nelfinavir resistance in vitro

BACKGROUND

HIV drug resistance is of increasing concern and could result from inadequate drug potency, poor therapy adherence and the existence of pharmacological sanctuary sites for viral replication. One contributing factor to the generation of such sites could be drug efflux transporters, which have been shown capable of effluxing HIV protease inhibitors from cells.

METHODS

In this 'proof-of-concept' study, the ability of the efflux transport inhibitor verapamil to modulate the intracellular accumulation of radiolabelled nelfinavir (NFV) and the antiviral effect of NFV was assessed in MT4 cells. Wild-type virus was then serially passaged with increasing concentrations of NFV with and without verapamil and resistance mutations monitored by sequencing of the viral protease gene.

RESULTS

The cellular accumulation ratio of 3H-NFV was 116.8 +/- 9.7 in controls and was significantly increased to 149.8 +/- 24.5 following incubation with verapamil (P < 0.05, n=4). The EC50 of NFV was decreased in MT4 cells in the presence of verapamil from 8.5 11.3 nM to 4.4 +/- 0.8 nM (P < 0.001, n=6). Of the 24 isolates passaged without verapamil, 21 carried the D30N mutation at detectable levels. Of the 23 passaged with verapamil, 14 carried the mutation (odds ratio = 4.5; P < 0.05).

CONCLUSIONS

These results suggest that 'intracellular boosting' of PIs is achievable through inhibition of drug efflux proteins in vitro and that such boosting has the ability to enhance suppression of viral replication, slowing the emergence of resistance mutations.

Lack of pharmacokinetic drug interaction between tenofovir disoproxil fumarate and nelfinavir mesylate

A study explored the pharmacokinetics of tenofovir (300 mg administered once daily) and nelfinavir (1,250 mg twice daily) when coadministered in 29 healthy volunteers. Tenofovir, nelfinavir, and M8 pharmacokinetics was unaltered when tenofovir and nelfinavir were coadministered, and tenofovir administration did not affect the M8/nelfinavir area under the concentration-versus-time curve over the dosing interval (AUC(tau)) ratio. No interaction between tenofovir and nelfinavir was observed.

Influence of single-nucleotide polymorphisms in the multidrug resistance-1 gene on the cellular export of nelfinavir and its clinical implication for highly active antiretroviral therapy

Protease inhibitors (PIs) such as nelfinavir (NFV) suppress HIV replication. PIs are substrates of P-glycoprotein (P-gp), the product of the multidrug-resistance-1 (MDR1) gene. Three single-nucleotide polymorphisms (SNPs) are present in exons of the MDR1 gene: MDR1 1236, MDR1 2677 and MDR1 3435. We speculated that these genetic polymorphisms affected PI concentration in the cell. To verify this hypothesis, we first genotyped these SNPs in 79 Japanese patients by the SNaPshot method and found incomplete linkage disequilibrium between the SNPs. Because the SNP at MDR1 3435 has been reported to be associated with P-gp expression, we evaluated the effect of that SNP on the export of NFV from HIV-positive patients' lymphoblastoid cell lines by measuring time-dependent decrease in the amount of intracellular NFV by high-performance liquid chromatography. We found the intracellular concentration of NFV in lymphoblastoid cell lines (LCLs) with the homozygous T/T genotype at MDR1 3435 were higher than that with C/C genotype with statistical significance. This suggests that the activity of P-gp in patients' LCL cells with the MDR1 3435 T/T genotype was lower. In a retrospective study we evaluated the effect of the SNPs on CD4 cell count recovery in response to antiretroviral treatment with PIs, and obtained statistically significant evidence that suggested marginal association of the SNP at MDR1 1236 but not at MDR1 2677 or MDR1 3435. As in vitro results were not consistent with the clinical evaluation, clinical importance of MDR1 genotyping for antiretroviral therapy remains to be investigated in a larger, case-controlled study.

Conversion of the HIV protease inhibitor nelfinavir to a bioactive metabolite by human liver CYP2C19

Antiretroviral therapy for human immunodeficiency virus (HIV) infection includes treatment with both reverse transcriptase inhibitors and protease inhibitors, which markedly suppress viral replication and circulating HIV RNA levels. Cytochrome P450 (P450) enzymes in human liver, chiefly CYP3A4, play a pivotal role in protease inhibitor biotransformation, converting these agents to largely inactive metabolites. However, the protease inhibitor nelfinavir (Viracept) is metabolized mainly to nelfinavir hydroxy-t-butylamide (M8), which exhibits potent antiviral activity, and to other minor products (termed M1 and M3) that are inactive. Since indirect evidence suggests that CYP2C19 underlies M8 formation, we examined the role of this inducible, polymorphic P450 enzyme in nelfinavir t-butylamide hydroxylation by human liver. Rates of microsomal M8 formation were 50.6 +/- 28.3 pmol of product formed/min/nmol P450 (n = 5 subjects), whereas kinetic analysis of the reaction revealed a KM of 21.6 microM and a Vmax of 24.6 pmol/min/nmol P450. In reconstituted systems, CYP2C19 catalyzed nelfinavir t-butylamide hydroxylation at a turnover rate of 2.2 min(-1), whereas CYP2C9, CYP2C8, and CYP3A4 were inactive toward nelfinavir. Polyclonal anti-CYP2C9 (cross-reactive with CYP2C19) and monoclonal anti-CYP2C19 completely inhibited microsomal M8 production, whereas monoclonal CYP2C9 and polyclonal CYP3A4 antibodies were without effect. Similarly, the CYP2C19 substrate omeprazole strongly inhibited (75%) hepatic nelfinavir t-butylamide hydroxylation at a concentration of only 12.5 microM. Our study shows that CYP2C19 underlies formation in human liver of M8, a bioactive nelfinavir metabolite. The inducibility of CYP2C19 by agents (e.g., rifampicin) often taken concurrently with nelfinavir, together with this P450's known polymorphic nature, may thus be important determinants of nelfinavir's antiviral potency.

Maternal-fetal transfer and amniotic fluid accumulation of protease inhibitors in pregnant women who are infected with human immunodeficiency virus

OBJECTIVE

The purpose of this study was to investigate placental transfer and amniotic fluid concentrations of protease inhibitors when they are given to pregnant women who are infected with human immunodeficiency virus.

STUDY DESIGN

Fifty-eight mothers who received antiretroviral therapy that included > or =1 protease inhibitors for clinical indications at the time of delivery were enrolled in the study. Maternal blood samples and amniotic fluid were obtained during delivery or cesarean delivery, and paired cord blood samples were obtained by venipuncture immediately after the delivery. Drug concentrations were measured with high performance liquid chromatography.

RESULTS

Most maternal protease inhibitor plasma concentrations (38/66 concentrations) were below the trough concentrations that are recommended for therapeutic drug monitoring. Cord blood concentrations were below the assay limit of detection in 10 of 40 samples for nelfinavir and 25 of 40 samples for its metabolite M8, 9 of 11 samples for ritonavir, 4 of 6 samples for indinavir, 5 of 6 samples for saquinavir but were detectable in 3 of 3 samples for amprenavir. Among the 24 amniotic fluid samples that were available, the concentrations below the detection limit were 10 of 16 samples for nelfinavir, 11 of 16 samples for M8, 1 of 3 samples for indinavir, 4 of 4 samples for ritonavir, and 0 of 1 samples for amprenavir. There were significant correlations between cord blood and maternal concentrations of nelfinavir and its metabolite M8.

CONCLUSION

Placental transfer of the human immunodeficiency virus protease inhibitors is generally low; however, it may differ greatly according to the molecule.

HIV-1 drug resistance in subjects with advanced HIV-1 infection in whom antiretroviral combination therapy is failing: a substudy of AIDS Clinical Trials Group Protocol 388

We evaluated phenotypic and genotypic markers of drug resistance in human immunodeficiency virus type 1 (HIV-1) at the time of virologic failure (VF) in subjects in the AIDS Clinical Trials Group Protocol 388 (ACTG 388) who received lamivudine-zidovudine (or lamivudine-stavudine) and either indinavir, efavirenz-indinavir, or nelfinavir-indinavir. At VF, phenotypically susceptible HIV-1 was found in 55% of subjects in the nelfinavir-indinavir arm, compared with 22% in the indinavir arm (P=.006). Phenotypic resistance to lamivudine was less common in the efavirenz-indinavir arm (33% of subjects; P=.002) and the nelfinavir-indinavir arm (43%; P=.003), compared with the indinavir arm (78%). Isolated phenotypic resistance to efavirenz at VF occurred in HIV-1 recovered from 33% of subjects in the efavirenz-indinavir arm; 24% of the subjects had HIV-1 with both efavirenz and lamivudine resistance. Results of genotypic tests were similar. The lower frequency of resistance in the nelfinavir-indinavir arm likely reflects decreased drug exposure that is due to intolerance, which is consistent with the lower potency and tolerability of this combination in ACTG 388. The lower frequency of lamivudine resistance in the efavirenz-indinavir arm is consistent with reports in other studies of potent regimens. Thus, although dual resistance to efavirenz and lamivudine occurred at VF in the efavirenz-indinavir arm, this risk was relatively low when evaluated in the context of the potency and tolerability of this regimen.

Intracellular and plasma pharmacokinetics of nelfinavir and M8 in HIV-infected patients: relationship with P-glycoprotein expression

One of the targets of antiretroviral therapy is within cells infected with HIV. In order to improve therapeutic efficacy, it is therefore important that the intracellular pharmacokinetics of drugs, such as nelfinavir mesylate and its active metabolite M8, are studied in addition to plasma pharmacokinetics. Previously, the intracellular accumulation of protease inhibitors has been reported in vivo, displaying the following hierarchy: nelfinavir > saquinavir > ritonavir > indinavir. Multidrug resistance transporters, such as P-glycoprotein (P-gp), may result in a lower intracellular concentration of drug via an efflux mechanism, thus contributing to sanctuary site formation. The objective of this study was to determine concentrations of nelfinavir and M8 in plasma and peripheral blood mononuclear cells from HIV-infected patients, and to ascertain the relationship between intracellular accumulation and lymphocyte P-gp expression. Venous blood samples from 12 HIV-infected patients (viral load <50 copies/ml) receiving nelfinavir (1250 mg twice daily) and dual nucleoside reverse transcriptase inhibitor therapy were collected over a full dosage interval (0, 2, 4, 8 and 12 h). Plasma and intracellular (cell-associated) drug concentrations were measured by HPLC-MS/MS. Drug exposure in plasma and cells was expressed as the area under the concentration-time curve (AUC(0-12h)), derived from non-compartmental modelling. The ratio of intracellular AUC(0-12h)/total plasma AUC(0-12h) was calculated to determine cellular drug accumulation. P-gp expression on lymphocytes was determined by flow cytometry. The median (range) AUC(0-12h) of nelfinavir in plasma and cellular compartments was 21.8 mg x h x l(-1) (5.64-50.8) and 104.6 mg x h x l(-1) (23.1-265.7), respectively. Corresponding values for M8 in plasma and cells were 6.60 mg x h x l(-1) (2.16-17.3) and 19.6 mg x h x l(-1) (5.14-60.8). A ratio of plasma M8/plasma nelfinavir (AUC(0-12h)) and intracellular M8/intracellular nelfinavir (AUC(0-12h)) gave median values of 0.32 and 0.17, respectively. The cellular accumulations [median; (range)] of nelfinavir and M8 were 5.30 (2.28-16.2) and 2.32 (1.01-10.7), respectively. A significant correlation between plasma and intracellular nelfinavir minimum concentration (Cmin) (r2=0.34; P=0.049), but not between plasma and intracellular M8 Cmin was observed. C(0h) concentrations were higher than C(12h) for both nelfinavir and M8. No relationship was observed between nelfinavir or M8 accumulation and lymphocyte cell surface expression of P-gp. This study illustrates that intracellular concentrations were higher than plasma concentrations for both nelfinavir and M8, suggesting lymphocyte accumulation. The mechanism of differential intracellular accumulation of nelfinavir and M8 remains to be elucidated. It may be that affinities for influx transporters or fundamental drug characteristics play a major role in the greater accumulation of nelfinavir than M8.

Prodrugs of HIV protease inhibitors—saquinavir, indinavir and nelfinavir—derived from diglycerides or amino acids: synthesis, stability and anti-HIV activity

With the aim of improving the pharmacological properties of current protease inhibitors (PIs), the synthesis of various acyl and carbamate amino acid- or diglyceride-containing prodrugs derived from saquinavir, indinavir and nelfinavir, their in vitro stability with respect to hydrolysis and their anti-HIV activity in CEM-SS and MT4 cells have been investigated. l-Leucine (Leu) and l-phenylalanine (Phe) were connected through their carboxyl to the PIs while l-tyrosine (Tyr) was conjugated through its aromatic hydroxyl via various spacer units. Hydrolysis of the prodrug with liberation of the active free drug was crucial for antiviral activity. The Leu- and Phe-PI prodrugs released the active free drug very rapidly (half-lives of hydrolysis in buffer at 37 degree C of 3-4 h). The Tyr-PI conjugates with a -C(O)(CH(2))(4)- linker exhibited half-lives in the 40-70 h range and antiviral activities in the 21-325 nM range (from 2 to 22 nM for the free PIs). The chemically very stable carbamate "peptidomimetic" Tyr-PI prodrugs (no hydrolysis detected after 7 days in buffer) displayed a very low anti-HIV activity or were even inactive (EC(50) from 2300 nM to >10 microM). A very low antiviral activity was measured for the diglyceride-substituted saquinavir and for all of the disubstituted indinavir and nelfinavir prodrugs. All these prodrugs probably released the active parent PI too slowly under the antiviral assay conditions. These results combined with those from transepithelial transport studies (Rouquayrol et al., Pharm. Res., 2002, 19, 1704-1712) indicate that conjugation of amino acids (through their carboxyl) to the PIs constitutes a most appealing alternative which could improve the intestinal absorption of the PIs and reduce their recognition by efflux carriers.

Secondary Mutations M36I and A71V in the Human Immunodeficiency Virus Type 1 Protease Can Provide an Advantage for the Emergence of the Primary Mutation D30N

Development of resistance mutations in enzymatic targets of human immunodeficiency virus 1 (HIV-1) hampers the ability to provide adequate therapy. Of special interest is the effect mutations outside the active site of HIV-1 protease have on inhibitor binding and virus viability. We engineered protease mutants containing the active site mutation D30N alone and with the nonactive site polymorphisms M36I and/or A71V. We determined the K(i) values for the inhibitors nelfinavir, ritonavir, indinavir, KNI272, and AG1776 as well as the catalytic efficiency of the mutants. Single and double mutation combinations exhibited a decrease in catalytic efficiency, while the triple mutant displayed catalytic efficiency greater than that of the wild type. Variants containing M36I or A71V alone did not display a significant change in binding affinities to the inhibitors tested. The variant containing mutation D30N displayed a 2-6-fold increase in K(i) for all inhibitors tested, with nelfinavir showing the greatest increase. The double mutants containing a combination of mutations D30N, M36I, and A71V displayed -0.5-fold to +6-fold changes in the K(i) of all inhibitors tested, with ritonavir and nelfinavir most affected. Only the triple mutant showed a significant increase (>10-fold) in K(i) for inhibitor nelfinavir, ritonavir, or AG-1776 displaying 22-, 19-, or 15-fold increases, respectively. Our study shows that the M36I and A71V mutations provide a greater level of inhibitor cross-resistance combined with active site mutation D30N. M36I and A71V, when present as natural polymorphisms, could aid the virus in developing active site mutations to escape inhibitor binding while maintaining catalytic efficiency.

Simultaneous determination of the HIV drugs indinavir, amprenavir, saquinavir, ritonavir, lopinavir, nelfinavir, the nelfinavir hydroxymetabolite M8, and nevirapine in human plasma by reversed-phase high-performance liquid chromatography

A reversed-phase high-performance liquid chromatography method for the simultaneous quantitative determination of the currently available HIV protease inhibitors amprenavir, indinavir, lopinavir, nelfinavir, ritonavir, saquinavir, the active nelfinavir metabolite M8, and the nonnucleoside reverse transcriptase inhibitor nevirapine in human plasma is described. The method involved liquid-liquid extraction from plasma, followed by high-performance liquid chromatography with an OmniSpher 5 C18 column and ultraviolet detection set at a wavelength of 215 nm for the protease inhibitors and 280 nm for nevirapine. The runtime was 25 minutes. The assay has been validated over the concentration range of 0.05 to 30 mg/L for indinavir, nelfinavir, ritonavir, and saquinavir, 0.07 to 30 mg/L for amprenavir and lopinavir, and 0.05 to 15 mg/L for M8 and nevirapine. This method proved to be simple, accurate, and precise and is useful for the therapeutic drug monitoring of protease inhibitors and the nonnucleoside reverse transcriptase inhibitor nevirapine on a routine basis.

Reverse phase high-performance liquid chromatography method for the analysis of amprenavir, efavirenz, indinavir, lopinavir, nelfinavir and its active metabolite (M8), ritonavir, and saquinavir in heparinized human plasma

The increasing interest in applying therapeutic drug monitoring (TDM) to antiretroviral therapy is related to the observed interindividual variation in antiretroviral pharmacokinetics that results in a wide range of drug exposure from fixed-dosing regimens and the rapid evolution in the availability of phenotypic assays that generate a target 50% inhibitory concentration (e.g., IC(50)) as a basis for adjusting individual antiretroviral dosages. To facilitate the application of TDM, a method for the simultaneous determination of eight species has been developed. This method is used to quantitate efavirenz and the following protease inhibitors: amprenavir, indinavir, lopinavir, nelfinavir and its active metabolite (M8), ritonavir, and saquinavir. The method using reversed-phase high-performance liquid chromatography (RP-HPLC) was validated. Detection is effected using a photodiode-array detector (PDA) scanning at four different wavelengths. This method allows for detection of all analytes to a lower limit of quantitation of 0.1 to 0.2 microg/mL with an interday variation in CV ranging from 3.5% to 10.4%. The method is being applied to a TDM program that is currently being implemented in the authors' laboratory.

Pharmacokinetics of nelfinavir and its active metabolite, hydroxy-tert-butylamide, in infants perinatally infected with human immunodeficiency virus type 1

BACKGROUND

In children younger than 2 years of age vertically infected with HIV-1, the recommended pediatric dosing regimen for nelfinavir (20 to 30 mg/kg three times a day) provides insufficient drug exposure. This study was conducted to determine the steady state pharmacokinetics of nelfinavir and its active metabolite, M8, in this population.

METHODS

Fourteen infants (2.3 to 8.5 months) underwent 18 intensive pharmacokinetic studies of nelfinavir and M8 at steady state. Nelfinavir and M8 concentrations were measured by high performance liquid chromatography coupled with mass spectrometry, and individual pharmacokinetic values were determined.

RESULTS

A mean nelfinavir daily dose of 135.7 +/- 18.8 mg/kg (twice or three times a day) resulted in median C(min), C(max), area under the plasma concentration-time curve (AUC(0-24 h)) and CL/ for nelfinavir of 0.627 mg/l, 2.39 mg/l, 30.6 mg*h/l and 4.2 liters/h/kg, respectively. When normalized for a daily dose of nelfinavir of 150 mg/kg/day, 16.7% of C(max) and 27.8% of AUC(0-24 h) values were below the tenth percentile for adult values.

CONCLUSIONS

During the first year of life, nelfinavir requirement is much higher than in older children and adults to obtain similar drug exposure. The mechanisms underlying such differences may involve higher first past metabolism and/or drug interactions or might be related to feeding conditions.

Effects of nelfinavir and its M8 metabolite on lymphocyte P-glycoprotein activity during antiretroviral therapy

The efflux pump P-glycoprotein decreases drug penetration into cells and tissues. To determine whether nelfinavir or its metabolites inhibit P-glycoprotein in lymphocytes from a healthy volunteer, whole blood cells from human immunodeficiency virus-negative donors were incubated either in human plasma to which nelfinavir or its M8 metabolite were added ex vivo or in plasma from human immunodeficiency virus-positive patients receiving nelfinavir. The 50% P-glycoprotein inhibitory concentrations of purified nelfinavir and M8 were 10.9 micromol/L and 29.5 micromol/L, respectively, for CD4(+) T cells and 19.3 micromol/L and >48 micromol/L, respectively, for CD8(+) T cells. Significant inhibitory activity was present in plasma from 27 of 46 patients (59%) receiving nelfinavir. Plasma nelfinavir concentrations correlated with percent inhibition on CD4(+) (rho = 0.85, P <.0001) and CD8(+) (rho = 0.83, P <.0001) T cells. The M8 concentrations correlated weakly with both inhibition and nelfinavir concentrations. On the basis of our findings in lymphocytes from a healthy volunteer exposed to plasma from human immunodeficiency virus-positive patients, we believe it is likely that CD4(+) and CD8(+) lymphocytes in patients receiving nelfinavir as therapy for human immunodeficiency virus may have P-glycoprotein inhibited by plasma concentrations of nelfinavir.

Impact of human immunodeficiency virus type 1 subtypes on virologic response and emergence of drug resistance among children in the Paediatric European Network for Treatment of AIDS (PENTA) 5 trial

The association between virologic response and human immunodeficiency virus type 1 (HIV-1) subtype was investigated in 113 HIV-1-infected children randomly assigned to receive zidovudine plus lamivudine, zidovudine plus abacavir, or lamivudine plus abacavir in the Paediatric European Network for Treatment of AIDS (PENTA) 5 trial. Symptomatic children (n=68) also received nelfinavir; asymptomatic children (n=45) were randomly assigned to receive nelfinavir or placebo. HIV-1 subtypes A, B, C, D, F, G, H, A/E, and A/G were found in 15%, 41%, 16%, 9%, 5%, 2%, 1%, 5%, and 7% of the children, respectively. Resistance assay failure rates were higher for non-B subtypes than for B subtypes (genotype, P=.01; phenotype, P=.02). HIV-1 subtype was not associated with virologic response at 24 and 48 weeks after initiation of treatment. No differences were observed in the frequency of development of resistance mutations L90M (P=1.00) and D30N (P=.61) in B and non-B viruses. In conclusion, no evidence that subtype determined virologic response to therapy was found.

P-Glycoprotein and transporter MRP1 reduce HIV protease inhibitor uptake in CD4 cells: potential for accelerated viral drug resistance?

BACKGROUND

The multidrug transporters P-glycoprotein (P-gp) and MRP1 are functionally expressed in several subclasses of lymphocytes. HIV-1 protease inhibitors interact with both; consequently the transporters could reduce the local concentration of HIV-1 protease inhibitors and, thus, influence the selection of viral mutants.

OBJECTIVES

To study the effect of the expression of P-gp and MRP1 on the transport and accumulation of HIV-1 protease inhibitors in human lymphocytes and to study the effects of specific P-gp and MRP1 inhibitors.

METHODS

The initial rate and the steady-state intracellular accumulation of radiolabelled ritonavir, indinavir, saquinavir and nelfinavir was measured in three human lymphocyte cell lines: control CEM cells, CEM-MDR cells, which express 30-fold more P-gp than CEM cells, and CEM-MRP cells, which express fivefold more MRP1 protein than CEM cells. The effect of specific inhibitors of P-gp (GF 120918) and MRP1 (MK 571) was also examined.

RESULTS

Compared with CEM cells, the initial rates of uptake and the steady-state intracellular concentrations of all protease inhibitors are significantly reduced in CEM-MDR cells. The intracellular concentrations of the protease inhibitors are increased upon co-administration with GF 120918, in some cases to levels approaching those in CEM cells. The intracellular concentrations of the protease inhibitors are also significantly reduced in CEM-MRP cells. Co-administration with MK -571 can partially overcome these effects.

CONCLUSIONS

The overexpression of multidrug transporters significantly reduces the accumulation of protease inhibitors at this major site of virus replication, which, potentially, could accelerate the acquisition of viral resistance. Targeted inhibition of P-gp may represent an important strategy by which this problem can be overcome.

Analysis of variation in plasma concentrations of nelfinavir and its active metabolite M8 in HIV-positive patients

OBJECTIVE

To characterize sources of variation in plasma concentrations of nelfinavir and its active metabolite M8 and to evaluate the use of therapeutic drug monitoring for nelfinavir treatment.

METHODS

Plasma samples and patient's characteristics were obtained from outpatient clinic. Differences between groups of patients were studied by comparing the observed plasma concentrations with the corresponding concentration on a pharmacokinetic population curve based on median plasma levels.

RESULTS

Plasma samples (618) were available from 355 patients taking 1250 mg nelfinavir twice daily. The median ratio between M8 and nelfinavir concentrations was 0.29. This ratio appeared to be independent of the time after ingestion. Statistically significantly lower M8 concentrations were found in Black and Asian patients, or when comedication with CYP3A4 inducers was used. Coadministration of CYP2C19 inhibitors, such as omeprazole, decreased the median M8/nelfinavir ratio. Nevertheless, nelfinavir concentrations and summed concentrations of nelfinavir and M8 were only marginally affected in these patients. Diarrhoea was identified as a cause for lower nelfinavir concentrations, without changing the M8/nelfinavir ratio. In a number of patients with suspected therapy failure or intoxication, abnormal nelfinavir plasma concentrations were found. Dose adjustments based on nelfinavir plasma levels were helpful in a number of patients.

CONCLUSION

This study shows that the total concentration of nelfinavir and M8 together is not significantly influenced when variation in M8 levels occurs. Consequently, measuring M8 concentrations in addition to nelfinavir concentrations is not required for the purpose of therapeutic drug monitoring for this drug.

Determination of interaction kinetic constants for HIV-1 protease inhibitors using optical biosensor technology

The interaction between HIV-1 protease and inhibitors has been studied with optical biosensor technology. Optimized experimental procedures and mathematical analysis permitted determination of association and dissociation rate constants. A sensor surface with native enzyme was unstable and exhibited a drift that was influenced by the binding of inhibitor. This was hypothesized to be due to a specific mechanism involving autoproteolysis and/or dimer dissociation. The use of a mutant predicted to be less susceptible to autoproteolysis (Q7K) than wild-type enzyme resulted in a minor effect on surface stability, while a completely stable surface was obtained by treatment of the immobilized enzyme with N-ethyl-N'-(dimethylaminopropyl)-carbodiimide and N-hydroxysuccinimide; the most stable surface was achieved by chemically modifying the Q7K enzyme. The stabilized surface was enzymatically active and the interaction with inhibitors was similar to that for native enzyme. Several of the inhibitors had very high association rates, and estimation of kinetic constants was therefore performed with a binding equation accounting for limited mass transport. Of the clinical inhibitors studied, saquinavir had the highest affinity for the enzyme, a result of the lowest dissociation rate. Although the dissociation rate for ritonavir was sixfold faster, the affinity was only twofold lower than that for saquinavir since the association rate was threefold faster. Nelfinavir and indinavir exhibited lower affinities relative to the other inhibitors, a consequence of a slower association for nelfinavir and a relatively fast dissociation for indinavir. These results show that biosensor-based interaction studies can resolve affinity into association and dissociation rates, and that these are characteristic parameters for the interaction between enzymes and inhibitors.

Differences in the intracellular accumulation of HIV protease inhibitors in vitro and the effect of active transport

OBJECTIVES

To investigate the intracellular accumulation of HIV protease inhibitors (PI) and to assess the effect of active transport on this accumulation.

METHODS

CEM cells were incubated with a PI for 18 h and the intracellular concentration determined using cell number and radioactivity. The effect of active transport was investigated using cells expressing P-glycoprotein (CEM(VBL)) and cells expressing multidrug resistance-associated protein 1 (MRP1; CEM(E1000)). Incubations were also carried out at 4 degrees C and in the presence of 2-deoxyglucose plus rotenone to examine the effect of inhibiting active transport.

RESULTS

Nelfinavir (NFV) accumulated to the greatest extent (> 80-fold) followed by saquinavir (SQV; approximately 30-fold), ritonavir (RTV; 3-7-fold) and finally indinavir (IDV; extracellular equivalent to intracellular). In CEM(VBL) cells there was a significant reduction in the intracellular accumulation of NFV, SQV and RTV and in CEM(E1000) cells there was reduced accumulation of SQV and RTV. Inhibition of active transport processes caused a reduction in SQV and RTV accumulation but had no effect on IDV accumulation in all cell types. NFV accumulation was increased in CEM(VBL) cells as a result of inhibition of active transport.

CONCLUSIONS

Marked differences can be detected in the intracellular accumulation of HIV PI drugs in vitro. Both P-glycoprotein and MRP1 may play a role in limiting the intracellular concentration of the PI and active influx mechanisms may contribute to drug accumulation.

A study of the pharmacokinetics of azithromycin and nelfinavir when coadministered in healthy volunteers

A two-way, open-label, crossover study in 12 subjects was undertaken to study the potential for azithromycin to alter the pharmacokinetics of nelfinavir and/or its active metabolite, M8. A secondary objective was to characterize any potential interaction that nelfinavir may have with azithromycin. During one dosing arm, subjects received a single 1200 mg oral dose of azithromycin. During the other, subjects received 11 days of nelfinavir 750 mg q8h with a single 1200 mg oral dose of azithromycin given concurrently with the Day 9 morning nelfinavir dose. Serum samples were collected after each azithromycin dose for 168 hours and after the Day 8 and 9 morning nelfinavir doses for 8 hours to characterize azithromycin, nelfinavir, and M8 pharmacokinetic parameters during both control and test periods. Both dosing regimens were well tolerated, with only mild to moderate GI side effects being the most frequently reported. Azithromycin was found to cause a statistically, though not clinically, significant decrease in nelfinavir and M8 exposures. In contrast, nelfinavir caused azithromycin Cmax and exposure (AUC) values to increase by > 100%. Inhibition of p-glycoprotein by nelfinavir may be responsible for this significant interaction. This increase in azithromycin exposure has the potential to increase clinical antibacterial efficacy without significantly increasing gastrointestinal side effects, though the impact on other systemic sites needs to be studied.

In vitro metabolism of trazodone by CYP3A: inhibition by ketoconazole and human immunodeficiency viral protease inhibitors

BACKGROUND

Pharmacologic treatment of emotional disorders in HIV-infected patients can be more easily optimized by understanding of potential interactions of psychotropic drugs with medications used to treat HIV infection and its sequelae.

METHODS

Biotransformation of the antidepressant trazodone to its principal metabolite, meta-chlorophenylpiperazine (mCPP), was studied in vitro using human liver microsomes and heterologously expressed individual human cytochromes. Interactions of trazodone with the azole antifungal agent, ketoconazole, and with human immunodeficiency virus protease inhibitors (HIVPIs) were studied in the same system.

RESULTS

Formation of mCPP from trazodone in liver microsomes had a mean (+/- SE) K(m) value of 163 (+/- 21) micromol/L. Ketoconazole, a relatively specific CYP3A inhibitor, impaired mCPP formation consistent with a competitive mechanism, having an inhibition constant (K(i)) of 0.12 (+/- 0.01) micromol/L. Among heterologously expressed human cytochromes, only CYP3A4 mediated formation of mCPP from trazodone; the K(m) was 180 micromol/L, consistent with the value in microsomes. The HIVPI ritonavir was a potent inhibitor of mCPP formation in liver microsomes (K(i) = 0.14 +/- 0.04 micromol/L). The HIVPI indinavir was also a strong inhibitor, whereas saquinavir and nelfinavir were weaker inhibitors.

CONCLUSIONS

CYP3A-mediated clearance of trazodone is inhibited by ketoconazole, ritonavir and indinavir, and indicates the likelihood of pharmacokinetic interactions in vivo.

Kinetic analysis of the interaction between HIV-1 protease and inhibitors using optical biosensor technology

The interaction between HIV-1 protease and reversible inhibitors was studied by surface plasmon resonance biosensor technology. The steady-state binding level and the time course of association and dissociation could be observed by measuring the binding of inhibitors injected in a continuous flow of buffer to the immobilized enzyme. Fourteen low molecular weight inhibitors (500-700 Da), including the four clinically used HIV-1 protease inhibitors (indinavir, nelfinavir, ritonavir, and saquinavir), were analyzed. Affinities were estimated as B(50) values from a series of sensorgrams at different concentrations of inhibitors. These values were found to be correlated with inhibition constants (K(i)) determined by an enzyme inhibition assay (r(2) = 0.84, logarithmic values). Dissociation rates were estimated at a single saturating concentration of the inhibitors as t(1/2,obs), but these values did not correlate with K(i) (r(2) = 0.26, logarithmic values). Indinavir had the highest affinity (B(50) = 11 nM) and the fastest dissociation (t(1/2,obs) = 500 s) among the clinically used inhibitors while saquinavir had a lower affinity (B(50) = 25 nM) and the slowest dissociation rate (t(1/2,obs) = 6500 s). Since these two inhibitors have similar K(i) values, the differences in dissociation rates reveal important characteristics in the interaction that cannot be obtained by the inhibition studies. The biosensor data are expected to be of greater in vivo relevance since the experiments were performed in a buffer more similar to physiological conditions.

Nelfinavir: an update on its use in HIV infection

Nelfinavir is one of several currently available protease inhibitors used to limit viral replication and improve immune function in HIV-infected individuals. It is administered in combination with other antiretroviral agents. Nelfinavir has been evaluated as first-line therapy with nucleoside reverse transcriptase inhibitors (NRTIs) in treatment-naive patients, or as an additional antiretroviral agent in protease inhibitor-naive patients already receiving NRTIs. These studies have shown good efficacy in terms of HIV viral load reduction and increased CD4+ cell counts. When used in combination with NRTIs, nelfinavir 1250 mg twice daily produced similar results to 750 mg 3 times daily. The more convenient twice-daily dosage schedule, which is now approved in the US, may be beneficial in improving patient adherence to therapy. Nelfinavir has also been used successfully in combination with non-nucleoside reverse transcriptase inhibitors and/or other protease inhibitors, with or without NRTIs. Resistance to nelfinavir has been observed in vitro and in clinical isolates from patients experiencing insufficient or waning viral suppression during treatment. Nelfinavir primarily selects for the D30N mutation, which is not seen with other protease inhibitors, and alone does not cause resistance to other protease inhibitors in vitro. Several studies have shown that patients who experience virological failure while receiving nelfinavir can respond to salvage therapy with other protease inhibitors. Diarrhoea is the most frequent adverse event in patients receiving nelfinavir-based combination therapy, but was generally mild and resulted in minimal discontinuation of therapy in clinical trials. Diarrhoea can usually be controlled with drugs that slow gastrointestinal motility. Metabolic disturbances associated with protease inhibitor use (hypercholesterolaemia, hyperglycaemia and lipodystrophy) have also been reported with nelfinavir. Nelfinavir is associated with a number of clinically significant drug interactions and coadministration of some drugs (e.g. astemizole, cisapride, triazolam) is contraindicated. Coadministration of nelfinavir with other protease inhibitors generally resulted in favourable pharmacokinetic interactions (usually increased area under the concentration-time curve for both drugs).

CONCLUSION

Nelfinavir, in combination with reverse transcriptase inhibitors and/or other protease inhibitors, is effective in limiting HIV replication and increasing CD4+ cell counts in HIV-infected adults and children. The convenience of its dosage administration, the low incidence of adverse events, and the potential for salvage therapies indicate that nelfinavir (as part of combined antiretroviral therapy regimens) should be considered as a first-line option in protease inhibitor-naive patients and in those unable to tolerate other protease inhibitors.

Interaction of anti-HIV protease inhibitors with the multidrug transporter P-glycoprotein (P-gp) in human cultured cells

The anti-HIV protease inhibitors represent a new class of agents for treatment of HIV infection. Saquinavir, ritonavir, indinavir, and nelfinavir are the first drugs approved in this class and significantly reduce HIV RNA copy number with minimal adverse effects. They are all substrates of cytochrome P450 3A4, and are incompletely bioavailable. The drug transporting protein, P-glycoprotein (P-gp), which is highly expressed in the intestinal mucosa, could be responsible for the low oral bioavailability of these and other drugs which are substrates for this transporter. To determine whether these protease inhibitors are modulators of P-gp, we studied them in cell lines which do and do not express P-gp. Saquinavir, ritonavir and nelfinavir significantly inhibited the efflux of [3H]paclitaxel and [3H]vinblastine in P-gp-positive cells, resulting in an increase in intracellular accumulation of these drugs. However, similar concentrations of indinavir did not affect the accumulation of these anticancer agents. In photoaffinity labeling studies, saquinavir and ritonavir displaced [3H]azidopine, a substrate for P-gp, in a dose-dependent manner. These data suggest that saquinavir, ritonavir, and nelfinavir are inhibitors and possibly substrates of P-gp. Because saquinavir has a low bioavailability, its interaction with P-gp may be involved in limiting its absorption.

Characterization of the selectivity and mechanism of human cytochrome P450 inhibition by the human immunodeficiency virus-protease inhibitor nelfinavir mesylate

In vitro studies with human liver microsomes and P450 probe substrates were performed to characterize selectivity and mechanism of cytochrome P450 inhibition by nelfinavir mesylate. At therapeutic concentrations (steady-state plasma concentrations approximately 4 microM), nelfinavir was found to be a competitive inhibitor of only testosterone 6beta-hydroxylase (CYP3A4) with a Ki concentration of 4. 8 microM. At supratherapeutic concentrations, nelfinavir competitively inhibited dextromethorphan O-demethylase (CYP2D6), S-mephenytoin 4-hydroxylase (CYP2C19), and phenacetin O-deethylase (CYP1A2) with Ki concentrations of 68, 126, and 190 microM, respectively. Nelfinavir did not appreciably inhibit tolbutamide 4-hydroxylase (CYP2C9), paclitaxel 6alpha-hydroxylase (CYP2C8), or chlorzoxaxone 6beta-hydroxylase (CYP2E1) activities. The inhibitory potency of nelfinavir toward CYP3A4 suggested the possibility of in vivo inhibition of this isoform, whereas in vivo inhibition of other P450s was considered unlikely. In a one-sequence crossover study in 12 healthy volunteers, nelfinavir inhibited the elimination of the CYP3A substrate terfenadine and the carboxylate metabolite of terfenadine. The 24-hr urinary recoveries of 6beta-hydroxycortisol were reduced by an average of 27% during nelfinavir treatment, consistent with CYP3A inhibition by nelfinavir. Inhibition of CYP3A4 by nelfinavir in vitro was NADPH-dependent requiring the catalytic formation of a metabolite or a metabolic intermediate. The catechol metabolite of nelfinavir (M3) was considered unlikely to be responsible for inhibition as the addition of catechol O-methyl transferase, S-adenosyl methionine, and ascorbic acid to the preincubation mixture did not protect against the loss of testosterone 6beta-hydroxylase activity. Also, the addition of M3 to human liver microsomes did not inhibit CYP3A4. Although incubations with nelfinavir showed a time- and concentration-dependent loss of CYP3A4 activity, the partial or complete recovery of enzyme activity upon dialysis indicated that inhibition was reversible. Microsomal incubations with nelfinavir and NADPH did not result in a loss of spectral P450 content compared with the NADPH control. Glutathione, N-acetylcysteine, and catalase did not attenuate CYP3A4 inhibition by nelfinavir. Collectively, these results suggest that the probable mechanism for CYP3A4 inhibition by nelfinavir is a transient metabolic intermediate or stable metabolite that coordinates tightly but reversibly to the heme moiety of the P450.