The binding of HIV-1 protease inhibitors to human serum proteins

The investigation of the complex population-drug-drug interaction between ritonavir-boosted lopinavir and chloroquine or ivermectin using physiologically-based pharmacokinetic modeling

OBJECTIVES

Therapy failure caused by complex population-drug-drug (PDDI) interactions including CYP3A4 can be predicted using mechanistic physiologically-based pharmacokinetic (PBPK) modeling. A synergy between ritonavir-boosted lopinavir (LPVr), ivermectin, and chloroquine was suggested to improve COVID-19 treatment. This work aimed to study the PDDI of the two CYP3A4 substrates (ivermectin and chloroquine) with LPVr in mild-to-moderate COVID-19 adults, geriatrics, and pregnancy populations.

METHODS

The PDDI of LPVr with ivermectin or chloroquine was investigated. Pearson's correlations between plasma, saliva, and lung interstitial fluid (ISF) levels were evaluated. Target site (lung epithelial lining fluid [ELF]) levels of ivermectin and chloroquine were estimated.

RESULTS

Upon LPVr coadministration, while the chloroquine plasma levels were reduced by 30, 40, and 20%, the ivermectin plasma levels were increased by a minimum of 425, 234, and 453% in adults, geriatrics, and pregnancy populations, respectively. The established correlation equations can be useful in therapeutic drug monitoring (TDM) and dosing regimen optimization.

CONCLUSIONS

Neither chloroquine nor ivermectin reached therapeutic ELF levels in the presence of LPVr despite reaching toxic ivermectin plasma levels. PBPK modeling, guided with TDM in saliva, can be advantageous to evaluate the probability of reaching therapeutic ELF levels in the presence of PDDI, especially in home-treated patients.

Species differences in plasma protein binding of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) main protease inhibitor nirmatrelvir

Plasma protein binding (PPB) studies on the SARS-CoV-2 main protease inhibitor nirmatrelvir revealed considerable species differences primarily in dog and rabbit, which prompted further investigations into the biochemical basis for these differences.The unbound fraction (f_u) of nirmatrelvir in dog and rabbit plasma was concentration (2-200 µM)-dependent (dog f_u,p 0.024-0.69, rabbit f_u,p 0.010-0.82). Concentration (0.1-100 µM)-dependent binding in serum albumin (SA) (f_u,SA 0.040-0.82) and alpha-1-acid glycoprotein (AAG) (f_u,AAG 0.050-0.64) was observed in dogs. Nirmatrelvir showed minimal binding to rabbit SA (1-100 µM: f_u,SA 0.70-0.79), while binding to rabbit AAG was concentration-dependent (0.1-100 µM: f_u,AAG 0.024-0.66). In contrast, nirmatrelvir (2 µM) revealed minimal binding (f_u,AAG 0.79-0.88) to AAG from rat and monkeys. Nirmatrelvir showed minimal-to-moderate binding to SA (1-100 µM; f_u,SA 0.70-1.0) and AAG (0.1-100 µM; f_u,AAG 0.48-0.58) from humans across tested concentrations.Nirmatrelvir molecular docking studies using published crystal structures and homology models of human and preclinical species SA and AAG were used to rationalise the species differences to plasma proteins. This suggested that species differences in PPB are primarily driven by molecular differences in albumin and AAG resulting in differences in binding affinity.

Pharmacokinetic and Chemical Synthesis Optimization of a Potent d-Peptide HIV Entry Inhibitor Suitable for Extended-Release Delivery

Peptides often suffer from short in vivo half-lives due to proteolysis and renal clearance that limit their therapeutic potential in many indications, necessitating pharmacokinetic (PK) enhancement. d-Peptides, composed of mirror-image d-amino acids, overcome proteolytic degradation but are still vulnerable to renal filtration due to their small size. If renal filtration could be slowed, d-peptides would be promising therapeutic agents for infrequent dosing, such as in extended-release depots. Here, we tether a diverse set of PK-enhancing cargoes to our potent, protease-resistant d-peptide HIV entry inhibitor, PIE12-trimer. This inhibitor panel provides an opportunity to evaluate the PK impact of the cargoes independently of proteolysis. While all the PK-enhancing strategies (PEGylation, acylation, alkylation, and cholesterol conjugation) improved in vivo half-life, cholesterol conjugation of PIE12-trimer dramatically improves both antiviral potency and half-life in rats, making it our lead anti-HIV drug candidate. We designed its chemical synthesis for large-scale production (CPT31) and demonstrated that the PK profile in cynomolgous monkeys supports future development of monthly or less frequent depot dosing in humans. CPT31 could address an urgent need in both HIV prevention and treatment.

A Reappraisal of Testosterone's Binding in Circulation: Physiological and Clinical Implications

In the circulation, testosterone and other sex hormones are bound to binding proteins, which play an important role in regulating their transport, distribution, metabolism, and biological activity. According to the free hormone hypothesis, which has been debated extensively, only the unbound or free fraction is biologically active in target tissues. Consequently, accurate determination of the partitioning of testosterone between bound and free fractions is central to our understanding of how its delivery to the target tissues and biological activity are regulated and consequently to the diagnosis and treatment of androgen disorders in men and women. Here, we present a historical perspective on the evolution of our understanding of the binding of testosterone to circulating binding proteins. On the basis of an appraisal of the literature as well as experimental data, we show that the assumptions of stoichiometry, binding dynamics, and the affinity of the prevailing models of testosterone binding to sex hormone-binding globulin and human serum albumin are not supported by published experimental data and are most likely inaccurate. This review offers some guiding principles for the application of free testosterone measurements in the diagnosis and treatment of patients with androgen disorders. The growing number of testosterone prescriptions and widely recognized problems with the direct measurement as well as the computation of free testosterone concentrations render this critical review timely and clinically relevant.

Characterization, biomedical and agricultural applications of protease inhibitors: A review

This review describes Protease Inhibitors (PIs) which target or inhibit proteases, protein digesting enzymes. These proteases play a crucial task in many biological events including digestion, blood coagulation, apoptosis etc. Regardless of their crucial roles, they need to be checked regularly by PIs as their excess may possibly damage host organism. On basis of amino acid composition of PIs where Protease-PI enzymatic reactions occur i.e. serine, cysteine, and aspartic acid, they are classified. Nowadays, various PIs are being worked upon to fight various parasitic or viral diseases including malaria, schistosomiasis, colds, flu', dengue etc. They prevent an ongoing process begun by carcinogen exposure by keeping a check on metastasis. They also possess potential to reduce carcinogen-induced, increased levels of gene amplification to almost normal levels. Some PIs can principally be used for treatment of hypertension and congestive heart failure by blocking conversion of angiotensin I to angiotensin II for example Angiotensin-converting enzyme inhibitors (ACEIs). Also PIs target amyloid β-peptide (Aβ) level in brain which is prime responsible for development of Alzheimer's Disease (AD). Also, PIs inhibit enzymatic activity of HIV-1 Protease Receptor (PR) by preventing cleavage events in Gag and Gag-Pol that result in production of non-virulent virus particles.

Pharmacokinetic Interactions Between Quinine and Lopinavir/Ritonavir in Healthy Thai Adults

This study aimed to investigate the pharmacokinetic interactions between quinine and lopinavir boosted with ritonavir (LPV/r) in healthy Thai adults (8 males and 12 females). Period 1 (day 1): subjects received a single oral dose of 600 mg quinine sulfate. Period 2: subjects received LPV/r (400/100 mg) twice daily. Period 3: subjects received a single quinine sulfate dose plus LPV/r twice a day. Intensive blood sampling was performed during each phase. Quinine AUC0-48h (area under the plasma concentration-time curve from time 0 to 48 hours), AUC0-∞ (area under the plasma concentration-time curve from time 0 to infinity), and Cmax (maximum concentration over the time-span specified), were 56%, 57%, and 47% lower, respectively, in the presence of LPV/r. 3-Hydroxyquinine AUC0-48h, AUC0-∞, and Cmax were significantly lower and the metabolite-to-parent ratio was significantly reduced. Lopinavir and ritonavir exposures were not significantly reduced with quinine coadministration, but Cmax of both drugs were significantly lower. The geometric mean ratio (GMR) and 90% CI of AUC0-48h, AUC0-∞, and Cmax for quinine, 3-hydroxyquinine, lopinavir, and ritonavir lay outside the bioequivalent range of 0.8-1.25. Drug treatments during all periods were generally well tolerated. The reduction in systemic exposure of quinine and 3-hydroxyquinine with concomitant LPV/r use raises concerns of suboptimal exposure. Studies in HIV/malaria coinfection patients are needed to determine the clinical impact to decide if any change to the quinine dose is warranted.

A novel tricyclic ligand-containing nonpeptidic HIV-1 protease inhibitor, GRL-0739, effectively inhibits the replication of multidrug-resistant HIV-1 variants and has a desirable central nervous system penetration property in vitro

We report here that GRL-0739, a novel nonpeptidic HIV-1 protease inhibitor containing a tricycle (cyclohexyl-bis-tetrahydrofuranylurethane [THF]) and a sulfonamide isostere, is highly active against laboratory HIV-1 strains and primary clinical isolates (50% effective concentration [EC50], 0.0019 to 0.0036 μM), with minimal cytotoxicity (50% cytotoxic concentration [CC50], 21.0 μM). GRL-0739 blocked the infectivity and replication of HIV-1NL4-3 variants selected by concentrations of up to 5 μM ritonavir or atazanavir (EC50, 0.035 to 0.058 μM). GRL-0739 was also highly active against multidrug-resistant clinical HIV-1 variants isolated from patients who no longer responded to existing antiviral regimens after long-term antiretroviral therapy, as well as against the HIV-2ROD variant. The development of resistance against GRL-0739 was substantially delayed compared to that of amprenavir (APV). The effects of the nonspecific binding of human serum proteins on the anti-HIV-1 activity of GRL-0739 were insignificant. In addition, GRL-0739 showed a desirable central nervous system (CNS) penetration property, as assessed using a novel in vitro blood-brain barrier model. Molecular modeling demonstrated that the tricyclic ring and methoxybenzene of GRL-0739 have a larger surface and make greater van der Waals contacts with protease than in the case of darunavir. The present data demonstrate that GRL-0739 has desirable features as a compound with good CNS-penetrating capability for treating patients infected with wild-type and/or multidrug-resistant HIV-1 variants and that the newly generated cyclohexyl-bis-THF moiety with methoxybenzene confers highly desirable anti-HIV-1 potency in the design of novel protease inhibitors with greater CNS penetration profiles.

X-ray structural and biological evaluation of a series of potent and highly selective inhibitors of human coronavirus papain-like proteases

Structure-guided design was used to generate a series of noncovalent inhibitors with nanomolar potency against the papain-like protease (PLpro) from the SARS coronavirus (CoV). A number of inhibitors exhibit antiviral activity against SARS-CoV infected Vero E6 cells and broadened specificity toward the homologous PLP2 enzyme from the human coronavirus NL63. Selectivity and cytotoxicity studies established a more than 100-fold preference for the coronaviral enzyme over homologous human deubiquitinating enzymes (DUBs), and no significant cytotoxicity in Vero E6 and HEK293 cell lines is observed. X-ray structural analyses of inhibitor-bound crystal structures revealed subtle differences between binding modes of the initial benzodioxolane lead (15g) and the most potent analogues 3k and 3j, featuring a monofluoro substitution at para and meta positions of the benzyl ring, respectively. Finally, the less lipophilic bis(amide) 3e and methoxypyridine 5c exhibit significantly improved metabolic stability and are viable candidates for advancing to in vivo studies.

Characterization of binding of raltegravir to plasma proteins

The objective of this study was to characterize raltegravir (RAL) binding to albumin and alpha-1-acid glycoprotein (AAG). Unbound and bound RAL were separated by ultrafiltration. The association constant (Ka) was estimated by a graphical method. In HIV-infected patients, the average plasma protein binding is 76%. RAL did not bind to AAG but bound to nonsaturable, low-affinity albumin sites with an n (number of sites) · Ka product of 9.8 × 10(2) liters/mol. A pH increase of 0.2 U led to a 2% increase in the bound fraction.

A physiologically based pharmacokinetic (PBPK) approach to evaluate pharmacokinetics in patients with cancer

Potential differences in pharmacokinetics (PK) between healthy subjects and patients with cancer were investigated using a physiologically based pharmacokinetic approach integrating demographic and physiological data from patients with cancer. Demographic data such as age, sex and body weight, and clinical laboratory measurements such as albumin, alpha-1 acid glycoprotein (AAG) and hematocrit were collected in ~2500 patients with cancer. A custom oncology population profile was built using the observed relationships among demographic variables and laboratory measurements in Simcyp® software, a population based ADME simulator. Patients with cancer were older compared with the age distribution in a built-in healthy volunteer profile in Simcyp. Hematocrit and albumin levels were lower and AAG levels were higher in patients with cancer. The custom population profile was used to investigate the disease effect on the pharmacokinetics of two probe substrates, saquinavir and midazolam. Higher saquinavir exposure was predicted in patients relative to healthy subjects, which was explained by the altered drug binding due to elevated AAG levels in patients with cancer. Consistent with historical clinical data, similar midazolam exposure was predicted in patients and healthy subjects, supporting the hypothesis that the CYP3A activity is not altered in patients with cancer. These results suggest that the custom oncology population profile is a promising tool for the prediction of PK in patients with cancer. Further evaluation and extension of this population profile with more compounds and more data will be needed.

GRL-0519, a novel oxatricyclic ligand-containing nonpeptidic HIV-1 protease inhibitor (PI), potently suppresses replication of a wide spectrum of multi-PI-resistant HIV-1 variants in vitro

We report that GRL-0519, a novel nonpeptidic human immunodeficiency virus type 1 (HIV-1) protease inhibitor (PI) containing tris-tetrahydrofuranylurethane (tris-THF) and a sulfonamide isostere, is highly potent against laboratory HIV-1 strains and primary clinical isolates (50% effective concentration [EC50], 0.0005 to 0.0007 μM) with minimal cytotoxicity (50% cytotoxic concentration [CC50], 44.6 μM). GRL-0519 blocked the infectivity and replication of HIV-1NL4-3 variants selected by up to a 5 μM concentration of ritonavir, lopinavir, or atazanavir (EC50, 0.0028 to 0.0033 μM). GRL-0519 was also potent against multi-PI-resistant clinical HIV-1 variants isolated from patients who no longer responded to existing antiviral regimens after long-term antiretroviral therapy, highly darunavir (DRV)-resistant variants, and HIV-2ROD. The development of resistance against GRL-0519 was substantially delayed compared to other PIs, including amprenavir (APV) and DRV. The effects of nonspecific binding of human serum proteins on GRL-0519's antiviral activity were insignificant. Our analysis of the crystal structures of GRL-0519 (3OK9) and DRV (2IEN) with protease suggested that the tris-THF moiety, compared to the bis-THF moiety present in DRV, has greater water-mediated polar interactions with key active-site residues of protease and that the tris-THF moiety and paramethoxy group effectively fill the S2 and S2' binding pockets, respectively, of the protease. The present data demonstrate that GRL-0519 has highly favorable features as a potential therapeutic agent for treating patients infected with wild-type and/or multi-PI-resistant variants and that the tris-THF moiety is critical for strong binding of GRL-0519 to the HIV protease substrate binding site and appears to be responsible for its favorable antiretroviral characteristics.

Effects of ritonavir-boosted lopinavir on the pharmacokinetics of quinine

The centuries-old antimalarial drug, quinine, continues to play a critical role in the treatment of severe falciparum malaria and uncomplicated malaria in pregnant women. It shares cytochrome P450 (CYP )-mediated metabolic pathways with several commonly used antiretroviral drugs, raising the potential for clinically important drug–drug interactions. A phase I pharmacokinetic study was conducted to assess the impact of long-term use of ritonavir-boosted lopinavir (LPV/r) on quinine pharmacokinetics in healthy volunteers. LP V/r significantly decreased the exposure of quinine and its major active metabolite, 3-hydroxyquinine, in both total and free (unbound) forms. These findings highlight the complex nature of the influence exerted by LPV/r on several of the drug-metabolizing enzymes involved in quinine disposition,including CYP 3A4, UDP-glucuronosyltransferase (UG T), and P-glycoprotein (P-gp). A decline in quinine exposure may compromise clinical efficacy. Further studies are warranted to assess changes in quinine pharmacokinetics and treatment outcomes in patients with acute malaria receiving antiretroviral therapy that includes LPV/r.

Activity of human immunodeficiency virus type 1 protease inhibitors against the initial autocleavage in Gag-Pol polyprotein processing

Inhibitors of HIV protease have proven to be important drugs in combination anti-HIV therapy. These inhibitors were designed to target mature protease and prevent viral particle maturation by blocking Gag and Gag-Pol processing by mature protease. Currently there are few data assessing the ability of these protease inhibitors to block the initial step in autoproteolytic processing of Gag-Pol. This unique step involves the dimerization of two Gag-Pol polyproteins and autocleavage of the Gag-Pol polyprotein by the embedded dimeric protease. We developed a plasmid encoding a modified form of Gag-Pol that can undergo autoprocessing only at the initial cleavage site between p2 and nucleocapsid. Using an in vitro transcription/translation system, we assessed the ability of six different approved protease inhibitors (darunavir, indinavir, nelfinavir, ritonavir, saquinavir, and tipranavir) to block this initial autocleavage step. Of these inhibitors, darunavir and saquinavir were the most effective. Darunavir and saquinavir were also the most effective at blocking the initial autoprocessing of full-length Gag-Pol in HIV-1-infected T cells. Thus, we have identified at least two HIV-1 protease inhibitors that have activity against the primary autocatalytic step of the embedded HIV-1 protease in Gag-Pol at concentrations that may be attained in HIV-1-infected patients. Due to unique aspects of the initial processing step, it may be possible to develop inhibitors with greater potency against this step, thus halting viral maturation at the earliest stages. The transcription/translation assay could be used to develop more potent inhibitors of this essential first step in viral maturation.

Microcalorimetry: a response to challenges in modern biotechnology

Almost any process in life is accompanied by heat changes which can be monitored by isothermal titration calorimetry (ITC) and differential scanning calorimetry (DSC). Both techniques are now established tools in fundamental research but over the last decade a clear tendency towards more problem-driven applications is noted. This review aims at summarizing these problem-oriented applications of microcalorimetry and the solutions both techniques can provide to problems in biotechnology. The biotechnological issues to which microcalorimetry has been successfully applied are as diverse as rational drug design, overcoming drug resistance, optimization of long-term stability of proteins, estimation of the bioavailability of drugs, control of complex pharmaceutical products or the optimization of gene delivery efficiency. The main limitation of microcalorimetry, which is the relatively large amounts of sample necessary for analysis, is less important in the biotechnology sector which frequently uses large-scale produced bulk products for analysis. The recently developed high-throughput DSC and ITC microcalorimeters will additionally reduce the labour intensity of these techniques. Due to the precision of microcalorimetric analyses and the versatility of processes which can be studied, it is expected that ITC and DSC will soon be key technologies in biotechnological research.

Pharmacologic aspects of new antiretroviral drugs

The biggest challenge facing highly antiretroviral-experienced patients and their caregivers is the diminishing number of therapeutic options available that sustain activity despite increasing numbers of drug-resistance mutations. New options in antiretroviral treatment have been introduced: two new members of traditional antiretroviral classes (darunavir and etravirine) and two drugs with novel mechanisms of action (raltegravir and maraviroc). Each was approved for use in treatment-experienced patients. A fifth drug-containing efavirenz, tenofovir, and emtricitabine (Atripla; Bristol-Myers Squibb, New York, NY, and Gilead Sciences, Foster City, CA)-is a novel coformulation of existing drugs from two different classes, simplifying administration with the intent of increasing adherence. Because successful management of HIV infection requires the simultaneous use of three or more drugs, understanding the pharmacologic aspects of coadministration is critical. This review summarizes the pharmacokinetic properties affecting the administration of these recently approved drugs in light of highly active antiretroviral treatment guidelines.

Age-related changes in plasma concentrations of the HIV protease inhibitor lopinavir

The advent of highly active antiretroviral therapy in the treatment of HIV disease has substantially extended the lifespan of individuals infected with HIV resulting in a growing population of older HIV-infected individuals. The efficacy and safety of antiretroviral agents in the population are important concerns. There have been relatively few studies assessing antiretroviral pharmacokinetics in older patients. Thirty-seven subjects aged 18-30 years and 40 subjects aged 45-79 years, naive to antiretroviral therapy, received lopinavir/ritonavir (400/100) bid, emtricitibine 200 mg qd, and stavudine 40 mg bid. Trough lopinavir concentrations were available for 44 subjects, collected at 24, 36, and 96 weeks. At week 24, older age was associated with higher lopinavir trough concentrations, and a trend was observed toward older age being associated with higher lopinavir trough concentrations when all time points were evaluated. In the young cohort, among subjects with two or more measurements, there was a trend toward increasing intrasubject trough lopinavir concentrations over time. Using a nonlinear, mixed-effects population pharmacokinetic model, age was negatively associated with lopinavir clearance after adjusting for adherence. Adherence was assessed by patient self-reports; older patients missed fewer doses than younger patients (p = 0.02). No difference in grade 3-4 toxicities was observed between the two age group. Older patients have higher trough lopinavir concentrations and likely decreased lopinavir clearance. Age-related changes in the pharmacokinetics of antiretroviral drugs may be of increasing importance as the HIV-infected population ages and as older individuals comprise an increasing proportion of new diagnoses.

Effects of obesity induced by high-fat diet on the pharmacokinetics of nelfinavir, a HIV protease inhibitor, in laboratory rats

The effect of obesity induced by a high-fat diet on the pharmacokinetics (PK) of nelfinavir (NFV) was investigated, focusing on the change of distribution and elimination caused by dyslipidemia and hepatic steatosis.The plasma unbound fraction (f(u)) of NFV in obese rats (0.61+/-0.03%) was significantly lower than in the control (1.10+/-0.09%), caused by increasing the plasma triglyceride-rich lipoprotein level. After intravenous (i.v.) administration of NFV, the marked decrease of the distribution volume and slower total clearance (39.5% and 69.1% of the control, respectively) caused by the lower f(u) were the main reasons for the significantly higher area under the blood concentration versus time curve (AUC) in obese rats (145.3% of the control). The absorption of NFV after intraduodenal (i.d.) administration in obese rats was significantly greater than in the control (AUC; 170.4% of the control). The increased bile in obese rats was the main reason for the increasing absorption of NFV, and the lower expression of intestinal P-glycoprotein was also considered. On the other hand, although higher AUCs in obese rats were shown, unbound AUCs in the obese rats were slightly lower than in the control, namely, the plasma NFV concentration in obese rats to obtain the same pharmacological effect was higher than in the control, suggesting the difficulty of drug monitoring. These results suggest that it is necessary to pay further attention to therapeutic drug monitoring of NFV in patients manifesting metabolic syndrome, such as dyslipidemia and visceral fat accumulation, including hepatic steatosis.

HSA binding of HIV protease inhibitors: a high-performance affinity chromatography study

The binding of HIV protease inhibitors, drugs important for anti-HIV chemotherapy, to HSA was examined by high-performance affinity chromatography. Frontal analysis was first used to determine the amount of anchored protein and the binding capacity for selected markers on this column. Zonal elution experiments then ranked the HSA bound fraction of the examined compounds. Information on the binding region was obtained by competitive zonal elution experiments using probe compounds with known sites on HSA. An allosteric competition between HIV protease inhibitors (PIs) and valproate (a probe for the bilirubin site) was detected, consistent with a noncooperative binding mechanism. No significant competition was observed between the examined compounds and salicylate or ibuprofen, probes for sites I and II, respectively. The observations were confirmed by circular dichroism spectroscopy, based on the change in the induced circular dichroism signals of selected markers for the main binding sites of HSA when ritonavir was added as the competitor. These results were in good agreement with previous literature reports and provide more details on how PIs are transported in plasma and how they may compete with other drugs in the body.

Influence of pharmacogenetics on indinavir disposition and short-term response in HIV patients initiating HAART

AIMS

To assess the relationship between genetic polymorphisms and indinavir pharmacokinetic variability and to study the link between concentrations and short-term response or metabolic safety.

METHODS

Forty protease inhibitor-naive patients initiating highly active antiretroviral therapy (HAART) including indinavir/ritonavir and enrolled in the COPHAR 2-ANRS 111 trial were studied. At week 2, four blood samples were taken before and up to 6 h following drug intake. A population pharmacokinetic analysis was performed using the stochastic approximation expectation maximization (SAEM) algorithm implemented in MONOLIX software. The area under the concentration-time curve (AUC) and maximum (C(max)) and trough concentrations (C(trough)) of indinavir were derived from the population model and tested for their correlation with short-term viral response and safety measurements, while for ritonavir, these same three parameters were tested for their correlation with short-term biochemical safety

RESULTS

A one-compartment model with first-order absorption and elimination best described both indinavir and ritonavir concentrations. For indinavir, the estimated clearance and volume of distribution were 22.2 L/h and 97.3 L, respectively. The eight patients with the *1B/*1B genotype for the CYP3A4 gene showed a 70% decrease in absorption compared to those with the *1A/*1B or *1A/*1A genotypes (0.5 vs. 2.1, P = 0.04, likelihood ratio test by permutation). The indinavir AUC and C(trough) were positively correlated with the decrease in human immunodeficiency virus RNA between week 0 and week 2 (r = 0.4, P = 0.03 and r = -0.4, P = 0.03, respectively). Patients with the *1B/*1B genotype also had a significantly lower indinavir C(max) (median 3.6, range 2.1-5.2 ng/mL) than those with the *1A/*1B or *1A/*1A genotypes (median 4.4, range 2.2-8.3 ng/mL) (P = 0.04) and a lower increase in triglycerides during the first 4 weeks of treatment (median 0.1, range -0.7 to 1.4 vs. median 0.6, range -0.5 to 1.7 mmol/L, respectively; P = 0.02). For ritonavir, the estimated clearance and volume of distribution were 8.3 L/h and 60.7 L, respectively, and concentrations were not found to be correlated to biochemical safety. Indinavir and ritonavir absorption rate constants were found to be correlated, as well as their apparent volumes of distribution and clearances, indicating correlated bioavailability of the two drugs.

CONCLUSION

The CYP3A4*1B polymorphism was found to influence the pharmacokinetics of indinavir and, to some extent, the biochemical safety of indinavir.

Methadone metabolism and clearance are induced by nelfinavir despite inhibition of cytochrome P4503A (CYP3A) activity

BACKGROUND

Methadone plasma concentrations are decreased by nelfinavir. Methadone clearance and the drug interactions have been attributed to CYP3A4, but actual mechanisms of methadone clearance and the nelfinavir interaction are unknown. We assessed nelfinavir effects on methadone pharmacokinetics and pharmacodynamics, intestinal and hepatic CYP3A4/5 activity, and intestinal P-glycoprotein transport activity. CYP3A4/5 and transporters were assessed using alfentanil and fexofenadine, respectively.

METHODS

Twelve healthy HIV-negative volunteers underwent a sequential crossover. On three consecutive days they received oral alfentanil plus fexofenadine, intravenous alfentanil, and intravenous plus oral methadone. This was repeated after nelfinavir. Plasma and urine analytes were measured by mass spectrometry. Opioid effects were measured by pupil diameter change (miosis).

RESULTS

Nelfinavir decreased intravenous and oral methadone plasma concentrations 40-50%. Systemic clearance, hepatic clearance, and hepatic extraction all increased 1.6- and 2-fold, respectively, for R- and S-methadone; apparent oral clearance increased 1.7- and 1.9-fold. Nelfinavir stereoselectively increased (S>R) methadone metabolism and metabolite formation clearance, and methadone renal clearance. Methadone bioavailability and P-glycoprotein activity were minimally affected. Nelfinavir decreased alfentanil systemic and apparent oral clearances 50 and 76%, respectively. Nelfinavir appeared to shift the methadone plasma concentration-effect (miosis) curve leftward and upward.

CONCLUSIONS

Nelfinavir induced methadone clearance by increasing renal clearance, and more so by stereoselectively increasing hepatic metabolism, extraction and clearance. Induction occurred despite 50% inhibition of hepatic CYP3A4/5 activity and more than 75% inhibition of first-pass CYP3A4/5 activity, suggesting little or no role for CYP3A in clinical methadone disposition. Nelfinavir may alter methadone pharmacodynamics, increasing clinical effects.

Chemogenomics with protein secondary-structure mimetics

During molecular recognition of proteins in biological systems, helices, reverse turns, and beta-sheets are dominant motifs. Often there are therapeutic reasons for blocking such recognition sites, and significant progress has been made by medicinal chemists in the design and synthesis of semirigid molecular scaffolds on which to display amino acid side chains. The basic premise is that preorganization of the competing ligand enhances the binding affinity and potential selectivity of the inhibitor. In this chapter, current progress in these efforts is reviewed.

Darunavir: Pharmacokinetics and Drug Interactions

Darunavir (TMC114) is a new HIV protease inhibitor that has demonstrated substantial antiretroviral activity against wild-type HIV-1 virus and multidrug-resistant strains. Darunavir inhibits and is primarily metabolized by cytochrome P450 3A (CYP3A) isoenzymes and is coadministered with low-dose ritonavir (darunavir/r); ritonavir is an inhibitor of CYP3A isoenzymes and pharmacologically enhances darunavir, resulting in increased plasma concentrations and allowing for a lower daily dose. The t1/2 (terminal elimination half-life) of darunavir is 15 h in the presence of ritonavir. An extensive darunavir/r drug-drug interaction programme has been undertaken, covering a wide range of therapeutic areas. Studies conducted in HIV-negative healthy volunteers and in HIV-infected patients show that the potential for interactions is well characterized and the interactions are manageable. For most drugs investigated, no dose adjustments of darunavir/r or the co-administered drug are required. This article reviews all the pharmacokinetic and drug-drug interaction studies conducted to date for darunavir/r, providing guidance on how to co-administer darunavir/r with many other antiretroviral or non-antiretroviral medications commonly used in HIV-infected individuals.

Perturbation of the dimer interface of triosephosphate isomerase and its effect on Trypanosoma cruzi

BACKGROUND

Chagas disease affects around 18 million people in the American continent. Unfortunately, there is no satisfactory treatment for the disease. The drugs currently used are not specific and exert serious toxic effects. Thus, there is an urgent need for drugs that are effective. Looking for molecules to eliminate the parasite, we have targeted a central enzyme of the glycolytic pathway: triosephosphate isomerase (TIM). The homodimeric enzyme is catalytically active only as a dimer. Because there are significant differences in the interface of the enzymes from the parasite and humans, we searched for small molecules that specifically disrupt contact between the two subunits of the enzyme from Trypanosoma cruzi but not those of TIM from Homo sapiens (HTIM), and tested if they kill the parasite.

METHODOLOGY/PRINCIPAL FINDINGS

Dithiodianiline (DTDA) at nanomolar concentrations completely inactivates recombinant TIM of T. cruzi (TcTIM). It also inactivated HTIM, but at concentrations around 400 times higher. DTDA was also tested on four TcTIM mutants with each of its four cysteines replaced with either valine or alanine. The sensitivity of the mutants to DTDA was markedly similar to that of the wild type. The crystal structure of the TcTIM soaked in DTDA at 2.15 A resolution, and the data on the mutants showed that inactivation resulted from alterations of the dimer interface. DTDA also prevented the growth of Escherichia coli cells transformed with TcTIM, had no effect on normal E. coli, and also killed T. cruzi epimastigotes in culture.

CONCLUSIONS/SIGNIFICANCE

By targeting on the dimer interface of oligomeric enzymes from parasites, it is possible to discover small molecules that selectively thwart the life of the parasite. Also, the conformational changes that DTDA induces in the dimer interface of the trypanosomal enzyme are unique and identify a region of the interface that could be targeted for drug discovery.

Male genital tract pharmacology: developments in quantitative methods to better understand a complex peripheral compartment

A substantial health burden results from medical problems affecting the male genital tract, including chronic morbidity conditions affecting a large proportion of men, such as benign prostatic hypertrophy and prostatitis, and potentially lethal conditions, such as prostate cancer and human immunodeficiency virus transmission. Rational approaches to therapeutics in these conditions should benefit from understanding local pharmacokinetics and pharmacodynamics of active drugs within the male genital tract. However, the description of drug distribution into the male genital tract has been largely limited to total, rather than protein-free drug concentrations in the whole ejaculate at one or two time points within a dosing interval, which may be misleading in understanding local drug action. Recent innovations enable a quantitative understanding of protein-free drug kinetics into semen and drug distribution into individual male genital tract glands. These methods may benefit therapeutics through optimization of targeted drug delivery and facilitate drug development for diseases related to these glands.

Darunavir (TMC114): a new HIV-1 protease inhibitor

Effective combination therapy for HIV/AIDS is now available and has made a major impact on HIV-related mortality and morbidity. The effects of even the most active of antiretroviral drugs are hampered by drug resistance and tolerability issues. Darunavir (TMC114), coadministered with low-dose ritonavir (darunavir/r), is a new HIV-1 protease inhibitor that has been designed to be active against both wild-type and multi-resistant virus. Darunavir/r 600/100 mg b.i.d. in a combination antiretroviral regimen in the POWER trials has provided treatment-experienced patients with substantially greater virological and immunological benefits compared with standard of care. This article reviews the presently available data on darunavir, its pharmacology, pharmacokinetics, drug-drug interactions and clinical trial results, as well as examining darunavir from a health economic perspective.

Indinavir/ritonavir remains an important component of HAART for the treatment of HIV/AIDS, particularly in resource-limited settings

For over a decade, indinavir has been approved for the treatment of HIV/AIDS; however, following the introduction of new protease inhibitors (PIs) with improved safety and pharmacologic profiles, its use in developed countries has become almost obsolete. In contrast, in resource-limited settings where the majority of people living with HIV/AIDS reside, indinavir is part of the most affordable PI-based highly active antiretroviral treatment regimen. A major drawback of indinavir use is renal toxicity, but low-dose indinavir plus ritonavir (400/100 mg) twice daily is both efficacious and tolerable. Similar low dosing levels in children have also proven successful, but data in pregnant women remains limited. Due to its low cost and proven efficacy indinavir remains a key component of HIV/AIDS treatment in resource-limited settings.

Pharmacokinetics of experimental antiretroviral agents

Many novel agents are being developed for the management of HIV-1 infection. In currently available classes, there are new protease inhibitors, non-nucleoside and nucleotide reverse transcriptase inhibitors, all in various stages of development. Novel classes include co-receptor entry antagonists (e.g., CCR5 inhibitors) as well as integrase and maturation inhibitors. Data on the pharmacodynamics and the pharmacokinetics of these new agents continue to generate interest. This article will report on the known data of the pharmacokinetics of experimental antiretrovirals, particularly concentrating on recently presented data, and describe the main drug–drug interactions studied to date.

QSAR Modeling of Human Serum Protein Binding with Several Modeling Techniques Utilizing Structure−Information Representation

Four modeling techniques, using topological descriptors to represent molecular structure, were employed to produce models of human serum protein binding (% bound) on a data set of 1008 experimental values, carefully screened from publicly available sources. To our knowledge, this data is the largest set on human serum protein binding reported for QSAR modeling. The data was partitioned into a training set of 808 compounds and an external validation test set of 200 compounds. Partitioning was accomplished by clustering the compounds in a structure descriptor space so that random sampling of 20% of the whole data set produced an external test set that is a good representative of the training set with respect to both structure and protein binding values. The four modeling techniques include multiple linear regression (MLR), artificial neural networks (ANN), k-nearest neighbors (kNN), and support vector machines (SVM). With the exception of the MLR model, the ANN, kNN, and SVM QSARs were ensemble models. Training set correlation coefficients and mean absolute error ranged from r2=0.90 and MAE=7.6 for ANN to r2=0.61 and MAE=16.2 for MLR. Prediction results from the validation set yielded correlation coefficients and mean absolute errors which ranged from r2=0.70 and MAE=14.1 for ANN to a low of r2=0.59 and MAE=18.3 for the SVM model. Structure descriptors that contribute significantly to the models are discussed and compared with those found in other published models. For the ANN model, structure descriptor trends with respect to their affects on predicted protein binding can assist the chemist in structure modification during the drug design process.

Differential protein binding of indinavir and saquinavir in matched maternal and umbilical cord plasma

AIMS

To determine whether lower umbilical cord than maternal binding of indinavir and saquinavir contributed to the low cord : maternal (C : M) total concentration ratios reported previously.

METHODS

Indinavir and saquinavir unbound fraction (fu) was determined using equilibrium dialysis. Buffer solutions of human serum albumin (HSA) (20.0, 30.0, 40.0 g l(-1)) and alpha(1)-acid glycoprotein (AAG) (0.20, 0.60, 2.00 g l(-1)) were spiked with indinavir (1.00 and 8.00 mg l(-1)) or saquinavir (0.15 and 1.50 mg l(-1)). Matched maternal and umbilical cord plasma was spiked with 1.00 mg l(-1) indinavir (n = 12) or 0.15 mg l(-1) saquinavir (n = 20). Spiked protein/plasma solutions were dialyzed against isotonic phosphate buffer, at 37 degrees C. At equilibrium, indinavir and saquinavir concentrations were quantified, and the f(u) determined.

RESULTS

Indinavir and saquinavir demonstrated protein concentration-dependent binding in buffer solutions of HSA and AAG. Indinavir f(u) was significantly higher in umbilical cord (0.53 +/- 0.12) compared with maternal (0.36 +/- 0.11) plasma (95% CI of the difference -0.26, -0.097). Similarly, saquinavir fu was different between umbilical cord (0.0090 +/- 0.0046) and maternal plasma (0.0066 +/- 0.0039) (95% CI of the difference -0.0032, -0.0016). The transplacental AAG concentration gradient contributed significantly to the binding differential of both drugs.

CONCLUSIONS

The differential plasma binding of both drugs, which was largely the result of the transplacental AAG concentration gradient, would contribute to the low C : M total plasma concentration ratios observed previously. Unbound concentrations of indinavir and saquinavir are likely to be substantially lower in umbilical cord than maternal plasma.

Allosteric modulation of anti-HIV drug and ferric heme binding to human serum albumin

Human serum albumin (HSA), the most prominent protein in plasma, is best known for its exceptional capacity to bind ligands (e.g. heme and drugs). Here, binding of the anti-HIV drugs abacavir, atazanavir, didanosine, efavirenz, emtricitabine, lamivudine, nelfinavir, nevirapine, ritonavir, saquinavir, stavudine, and zidovudine to HSA and ferric heme-HSA is reported. Ferric heme binding to HSA in the absence and presence of anti-HIV drugs was also investigated. The association equilibrium constant and second-order rate constant for the binding of anti-HIV drugs to Sudlow's site I of ferric heme-HSA are lower by one order of magnitude than those for the binding of anti-HIV drugs to HSA. Accordingly, the association equilibrium constant and the second-order rate constant for heme binding to HSA are decreased by one order of magnitude in the presence of anti-HIV drugs. In contrast, the first-order rate constant for ligand dissociation from HSA is insensitive to anti-HIV drugs and ferric heme. These findings represent clear-cut evidence for the allosteric inhibition of anti-HIV drug binding to HSA by the heme. In turn, anti-HIV drugs allosterically impair heme binding to HSA. Therefore, Sudlow's site I and the heme cleft must be functionally linked.

Dependence of nelfinavir brain uptake on dose and tissue concentrations of the selective P-glycoprotein inhibitor zosuquidar in rats

Most reverse transcriptase and protease inhibitors used in highly active antiretroviral therapy for treating human immunodeficiency virus (HIV) infections exhibit poor penetration into the brain, raising the concern that the brain may be a sanctuary site for the development of resistant HIV variants. This study explores the relationship between the dose and plasma and brain concentrations of zosuquidar and the effect of this selective P-glycoprotein inhibitor on central nervous system penetration of the HIV protease inhibitor nelfinavir maintained at steady state by intravenous infusions in rats. Nelfinavir was infused (10 mg/kg/h) for up to 10 h with or without concurrent administration of an intravenous bolus dose of 2, 6, or 20 mg/kg zosuquidar given at 4 h. Brain tissue and plasma were analyzed for both drug concentrations. Brain tissue/plasma nelfinavir concentration ratios (uncorrected for the vascular contribution) increased nonlinearly with zosuquidar dose from 0.06 +/- 0.03 in the absence of zosuquidar and 0.09 +/- 0.02 between 2 and 6 h after 2 mg/kg zosuquidar to 0.85 +/- 0.19 after 6 mg/kg and 1.58 +/- 0.67 after 20 mg/kg zosuquidar. Zosuquidar brain tissue/plasma concentration ratios exhibited a similar abrupt increase from 2.8 +/- 0.3 after a 2 mg/kg dose to approximately 15 after the 6 and 20 mg/kg doses. The apparent threshold in the plasma concentration of zosuquidar necessary to produce significant enhancement in brain uptake of nelfinavir appears to be close to the plasma concentrations associated with the maximum tolerated dose reported in the literature after repeated dosing of zosuquidar in patients.

Adaptive inhibitors of the HIV-1 protease

A significant obstacle to the efficacy of drugs directed against viral targets is the presence of amino acid polymorphisms in the targeted molecules. Amino acid polymorphisms may occur naturally due to the existence of variations within and between viral strains or as the result of mutations associated with drug resistance. An ideal drug will be one that is extremely effective against a primary target and maintains its effectiveness against the most important variations of the target molecule. A drug that simultaneously inhibits different variants of the target will lead to a faster suppression of the virus, retard the appearance of drug-resistant mutants and provide more efficacious and, in the long range, more affordable therapies. Drug molecules with the ability to inhibit several variants of a target with high affinity have been termed adaptive drugs (Nat. Biotechnol. 20 (2002) 15; Biochemistry 42 (2003) 8459; J. Cell. Biochem. S37 (2001) 82). Current drug design paradigms are predicated upon the lock-and-key hypothesis, which emphasizes shape complementarity as a way to attain specificity and improved binding affinity. Shape complementarity is accomplished by the introduction of conformational constraints in the drug molecule. While highly constrained molecules do well against a unique target, they lack the ability to adapt to target variations like those originating from naturally occurring polymorphisms or drug-resistant mutations. Targeting an array of closely related targets rather than a single one while still maintaining selectivity, requires a different approach. A plausible strategy for designing high affinity adaptive inhibitors is to engineer their most critical interactions (for affinity and specificity) with conserved regions of the target while allowing for adaptability through the introduction of flexible asymmetric functionalities in places facing variable regions of the target. The fundamental thermodynamics and structural principles associated with this approach are discussed in this chapter.

Early Absorption and Distribution Analysis of Antitumor and Anti-AIDS Drugs: Lipid Membrane and Plasma Protein Interactions

The interactions of a set of compounds of potential importance for anticancer and AIDS chemotherapy with lipid membranes and plasma proteins were studied with a surface plasmon resonance (SPR) based optical biosensor, giving valuable information on the absorption and distribution of the compounds. The technique allowed both effective screening of compounds and more detailed kinetic and mechanistic analysis of specific interactions. The interaction with two different types of lipid membranes could be reliably measured at a drug concentration as low as 20 microM, allowing analysis of poorly soluble compounds. Distribution was evaluated by investigation of the interactions with two human plasma proteins, human serum albumin (HSA) and alpha(1)-acid glycoprotein (AGP). Two apparent binding sites were clearly defined for HSA: one with rapid and one with slow association and dissociation rates. The sites appear to differ in accessibility and recognition characteristics rather than in their capacities to form strong complexes with drugs.

Design of HIV-1 Protease Inhibitors Active on Multidrug-Resistant Virus

On the basis of structural data gathered during our ongoing HIV-1 protease inhibitors program, from which our clinical candidate TMC114 9 was selected, we have discovered new series of fused heteroaromatic sulfonamides. The further extension into the P2' region was aimed at identifying new classes of compounds with an improved broad spectrum activity and acceptable pharmacokinetic properties. Several of these compounds display an exceptional broad spectrum activity against a panel of highly cross-resistant mutants. Certain members of these series exhibit favorable pharmacokinetic profiles in rat and dog. Crystal structures and molecular modeling were used to rationalize the broad spectrum profile resulting from the extension into the P2' pocket of the HIV-1 protease.

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.

Molten-globule like partially folded states of human serum albumin induced by fluoro and alkyl alcohols at low pH

Human serum albumin (HSA) exists in a molten-globule like state at low pH (pH 2.0). We studied the effects of trifluoroethanol (TFE) and hexafluoroisopropanol (HFIP) on the acid-denatured state of HSA by far-UV circular dichroism (CD), near-UV CD, tryptophan fluorescence, and 1-anilinonaphthalene-8-sulfonic acid (ANS) binding. At pH 2.0, these alcohols induced the formation of alpha-helical structure as evident from the increase in mean residue ellipticity (MRE) value at 222 nm. On addition of different alcohols, HSA exhibited a transition from the acid-denatured state to the alpha-helical state and loss of ANS-binding sites reflected by the decrease in ANS fluorescence at 480 nm. However, the concentration range required to bring about the transition varied greatly among different alcohols. HFIP was found to have highest potential whereas methanol was least effective in inducing the transition. The order of effectiveness of alcohols was shown to be: HFIP > TFE > 2-chloroethanol > tert-butanol > isopropanol > ethanol > methanol as evident from the Cm values. The near-UV CD spectra and tryptophan fluorescence showed the differential effects of halogenated alcohols with those of alkanols. A comparison of the m values, showing the dependence of Delta GH on alcohol concentration, suggests that the helix stabilizing potential of different alcohols is due to the additive effect of different constituent groups present whereas remarkably higher potential of HFIP involves some other factor in addition to the contribution of constituent groups.

A survey of the year 2003 literature on applications of isothermal titration calorimetry

Over the last decade isothermal titration calorimetry (ITC) has developed from a specialist method which was largely restricted in its use to dedicated experts, to a major, commercially available tool in the arsenal directed at understanding molecular interactions. The number of those proficient in this field has multiplied dramatically, as has the range of experiments to which this method has been applied. This has led to an overwhelming amount of new data and novel applications to be assessed. With the increasing number of publications in this field comes a need to highlight works of interest and impact. In this overview of the literature we have attempted to draw attention to papers and issues for which both the experienced calorimetrist and the interested dilettante hopefully will share our enthusiasm.