Induction and autoinduction properties of rifamycin derivatives: a review of animal and human studies

Rifampicin and its derivatives: stability, disposition, and affinity towards pregnane X receptor employing 2D and 3D primary human hepatocytes

Rifampicin is a model ligand of the pregnane X receptor (PXR), the nuclear receptor involved in the regulation of cytochrome P450 3A4 (CYP3A4). Rifampicin forms several degradation products and metabolites of which 25-desacetylrifampicin is the most abundant in vivo. Here, we aimed to study both the stability and metabolism of rifampicin in media and 2D and 3D primary human hepatocytes (PHHs). Additionally, we analyzed interactions of rifampicin derivatives with PXR. We described that rifampicin gradually degrades by more than 50 % in the medium partly into quinone over 72 h. We observed 25-desacetylrifampicin in 2D PHHs but not in 3D PHHs. Contrary, rifampicin was converted into quinone in a one-direction process in media of 3D PHHs. The potency of rifampicin and its derivatives to activate human PXR was arranged as follows: 3-formylrifamycin SV > rifampicin quinone > rifampicin > rifampicin N-oxide > 25-desacetylrifampicin, respectively, but none activates mouse and rat PXR. The binding differences between rifampicin and 25-desacetylrifampicin were modeled in silico. Finally, we showed that overexpressed uptake organic anion transporting polypeptide 1B1 (OATP1B1) potentiated activation of PXR by rifampicin and rifampicin quinone, but overexpressed efflux multidrug resistance protein 1 (MDR1) decreased PXR activation by all derivatives.

Chitosan nanoparticle toxicity: A comprehensive literature review of in vivo and in vitro assessments for medical applications

Topic definition

This literature review aims to update the current knowledge on toxicity of chitosan nanoparticles, compare the recent findings and identify the gaps with knowledge that is present for the chitosan nanoparticles.

Methods

The publications between 2010 and 2020 were searched in Science Direct, Pubmed.gov, Google Scholar, Research Gate, and ClinicalTrials.gov, according to the inclusion and exclusion criteria. 30 primary research studies were obtained from the literature review to compare the in vitro in vivo toxicity profiles among the chitosan nanoparticles.

Major highlights

Chitosan nanoparticles and other types of nanoparticles show cytotoxic effects on cancer cells while having minimal toxicity on normal cells. This apparent effect poses some considerations for use in incorporating cancer therapeutics into chitosan nanoparticles as an administration form. The concentration, duration of exposure, and pH of the solution can influence nanoparticle cytotoxicity, particularly in zebrafish. Different cell lines exhibit varying degrees of toxicity when exposed to nanoparticles, and of note are liver cells that show toxicity under exposure as indicated by increased alanine transaminase (ALT) levels. Aside from ALT, platelet aggregation can be considered a toxicity induced by chitosan nanoparticles. In addition, zebrafish cells experience the most toxicity, including organ damage, neurobehavioral impairment, and developmental abnormalities, when exposed to nanoparticles. However, nanoparticles may exhibit different toxicity profiles in different organisms, with brain toxicity and liver toxicity being present in zebrafish but not rats. Different organs exhibit varying degrees of toxicity, with the eye and mouth apparently having the lowest toxicity, while the brain, intestine, muscles and lung showing mixed results. Cardiotoxicity induced by chitosan nanoparticles was not observed in zebrafish embryos, and nanoparticles may reduce cardiotoxicity when delivering drug. Toxicity found in an organ may not necessarily mean that it is toxic towards all the cells found in that organ, as muscle toxicity was present when tested in zebrafish but not in C2C12 myoblast cells. Some of the studies conducted may have limitations that need to be reconsidered to account for differing results, with some examples being two experiments done on HeLa cells where one study concluded chitosan nanoparticles were toxic to the cells while the other seems to have no toxicity present. With regards to LD50, one study has stated the concentration of 64.21 mg/ml was found. Finally, smaller nanoparticles generally exhibit higher toxicity in cells compared to larger nanoparticles.

Scope for future work

This literature review did not uncover any published clinical trials with available results. Subsequent research endeavors should prioritize conducting clinical trials involving human volunteers to directly assess toxicity, rather than relying on cell or animal models.

A Phase 1, Open-Label, Fixed-Sequence, Drug-Drug Interaction Study of Zanubrutinib with Rifabutin in Healthy Volunteers

Zanubrutinib is a second-generation Bruton tyrosine kinase inhibitor that is primarily metabolized by CYP3A enzymes. Previous drug-drug interaction (DDI) studies have demonstrated that co-administration of zanubrutinib with rifampin, a strong CYP3A inducer, reduces zanubrutinib plasma concentrations, potentially impacting activity. The impact of the co-administration of zanubrutinib with less potent CYP3A inducers is unclear. This phase 1, open-label, fixed-sequence DDI study evaluated the pharmacokinetics, safety, and tolerability of zanubrutinib when co-administered with steady-state rifabutin, a known CYP3A inducer less potent than rifampin, in 13 healthy male volunteers (NCT04470908). Co-administration of zanubrutinib with rifabutin resulted in a less than 2-fold reduction of zanubrutinib exposures. Overall, zanubrutinib was well tolerated. The results of this study provide useful information for the evaluation of the DDI between rifabutin and zanubrutinib. In conjunction with safety and efficacy data from other clinical studies, these results will be taken into consideration to determine the appropriate dose recommendation of zanubrutinib when co-administered with CYP3A inducers.

Physiologically-based pharmacokinetic modelling to predict intragastric rifabutin concentrations in the treatment of Helicobacter pylori infection

BACKGROUND

Sustained intragastric antibiotic exposure is important for Helicobacter pylori eradication, yet little is known about gastric pharmacology of commonly used H. pylori regimens. For rifabutin, differing intragastric concentrations based on dosing regimen may account for differences in reported eradication rates.

AIM

To compare intragastric rifabutin concentrations between low-dose rifabutin (50 mg three time daily; as in RHB-105) and generically dosed rifabutin 150 mg once daily, 150 mg twice daily, and 300 mg once daily using a validated Physiologically-based pharmacokinetic (PBPK) model.

METHODS

We obtained plasma pharmacokinetic data from the RHB-105 clinical development programs and used it to develop and validate a whole-body PBPK model using PK-SIM software. We modified the existing rifabutin model to include the impact of omeprazole on gastric pH and emptying time. Modelled intragastric rifabutin exposure was expressed as the time that each regimen maintained its concentration ≥MIC90 .

RESULTS

Rifabutin 50 mg three times daily achieved significantly longer times with intragastric concentration above MIC90 (22.3 ± 1.1 h) than 150 mg once daily (8.3 ± 1.7 h), 150 mg twice daily (16.3 ± 2.3 h), or 300 mg once daily (8.5 ± 1.9 h) while providing the lowest mean maximal plasma concentration and mean area under the plasma concentration-time curve of all regimens studied.

CONCLUSIONS

PBPK modelling showed rifabutin 50 mg three times daily had higher intragastric exposure times than 150 mg once daily or twice daily, or 300 mg once daily. This low-dose rifabutin regimen provides the highest potential for H. pylori eradication while minimising systemic rifabutin exposure.

Determinants of adverse reactions to first-line antitubercular medicines: a prospective cohort study

BACKGROUND

The success of tuberculosis treatment relies on patients adhering to their medication regimen consistently. However, adherence levels tend to decrease among patients who experience adverse drug reactions to antitubercular medications, leading to suboptimal treatment outcomes. Hence, this study aimed to examine the types, incidence rates, and severity of adverse reactions caused by first-line antitubercular drugs. Additionally, it aimed to identify factors associated with the development of these reactions. By doing so, the study aimed to facilitate the provision of personalized and effective treatment to patients, ultimately improving treatment outcomes.

METHODS

Newly diagnosed patients with active tuberculosis were monitored from the start of their treatment until the completion of therapy. Any adverse reactions to anti-TB drugs that they encountered were carefully recorded. The collected data were analyzed using appropriate statistical methods such as analysis of variance, Chi-squared test, Fisher's exact test, and independent t-tests. Logistic regression was employed to assess the association between adverse drug reactions and various socio-demographic and clinical factors of the patients, using odds ratios as a measure of association.

RESULTS

Among the 378 patients included in the study, 181 individuals (47.9%) reported experiencing at least one adverse drug reaction, with an incidence rate of 1.75 events per 100-person months. The majority of these reactions occurred during the intensive phase of treatment. The gastrointestinal tract was the most commonly affected system, followed by the nervous system and skin. Patients aged over 45 years (OR = 1.55, 95% CI 1.01-2.39, p = 0.046) and those with extrapulmonary tuberculosis (OR = 2.41, 95% CI 1.03-5.64) were more likely to develop gastrointestinal reactions. Female gender was a significant predictor of both skin (OR = 1.78, 95% CI 1.05-3.02, p = 0.032) and nervous system (OR = 1.65, 95% CI 1.07-2.55, p = 0.024) reactions. Additionally, alcohol use and HIV infection were identified as independent predictors of adverse drug reactions affecting all three systems.

CONCLUSION

Significant risk factors for developing antitubercular drug adverse reactions include alcohol consumption, cigarette smoking, being HIV positive, female gender and extrapulmonary tuberculosis.

Drug-level change and optimal dose adjustment of tacrolimus with the use of rifabutin for treating mycobacterial disease in solid organ transplant recipients

BACKGROUND

Little is known about the change in drug level and the need for dose adjustment of calcineurin inhibitor when it is used with rifabutin in solid organ transplant (SOT) recipients. We aimed to analyze whether the drug level of tacrolimus significantly reduced after the use of rifabutin and to assess optimal adjustment of tacrolimus dose in SOT recipients.

METHODS

Of the SOT recipients in a tertiary referral center in South Korea in 2000-2019, 50 patients who maintained an unchanged dose of tacrolimus after the use of rifabutin for treating mycobacterial disease were enrolled. Their medical records were reviewed retrospectively.

RESULTS

The mean age of the patients was 53.9 ± 11.5 years. The most commonly transplanted organ was the liver (66.0%). The most common indication of rifabutin use was for treating active tuberculosis (78.0%). After rifabutin initiation, the trough level of tacrolimus decreased significantly to the subtherapeutic range in 38 (76.0%) patients. The drug levels of these 38 patients dropped from 7.2 to 3.8 ng/mL (p < 0.001) after rifabutin treatment. In these patients, the median 1.5-fold increase in the tacrolimus dose was required to restore the drug level to the within-therapeutic range.

CONCLUSIONS

These findings indicate that careful tacrolimus drug-level monitoring and dose adjustment are necessary for most SOT recipients when rifabutin is administered for the treatment of mycobacterial disease. This article is protected by copyright. All rights reserved.

Clinical Pharmacokinetics and Pharmacodynamics of Rifampicin in Human Tuberculosis

The introduction of rifampicin (rifampin) into tuberculosis (TB) treatment five decades ago was critical for shortening the treatment duration for patients with pulmonary TB to 6 months when combined with pyrazinamide in the first 2 months. Resistance or hypersensitivity to rifampicin effectively condemns a patient to prolonged, less effective, more toxic, and expensive regimens. Because of cost and fears of toxicity, rifampicin was introduced at an oral daily dose of 600 mg (8-12 mg/kg body weight). At this dose, clinical trials in 1970s found cure rates of ≥ 95% and relapse rates of < 5%. However, recent papers report lower cure rates that might be the consequence of increased emergence of resistance. Several lines of evidence suggest that higher rifampicin doses, if tolerated and safe, could shorten treatment duration even further. We conducted a narrative review of rifampicin pharmacokinetics and pharmacodynamics in adults across a range of doses and highlight variables that influence its pharmacokinetics/pharmacodynamics. Rifampicin exposure has considerable inter- and intra-individual variability that could be reduced by administration during fasting. Several factors including malnutrition, HIV infection, diabetes mellitus, dose size, pharmacogenetic polymorphisms, hepatic cirrhosis, and substandard medicinal products alter rifampicin exposure and/or efficacy. Renal impairment has no influence on rifampicin pharmacokinetics when dosed at 600 mg. Rifampicin maximum (peak) concentration (C_max) > 8.2 μg/mL is an independent predictor of sterilizing activity and therapeutic drug monitoring at 2, 4, and 6 h post-dose may aid in optimizing dosing to achieve the recommended rifampicin concentration of ≥ 8 µg/mL. A higher rifampicin C_max is required for severe forms TB such as TB meningitis, with C_max ≥ 22 μg/mL and area under the concentration-time curve (AUC) from time zero to 6 h (AUC₆) ≥ 70 μg·h/mL associated with reduced mortality. More studies are needed to confirm whether doses achieving exposures higher than the current standard dosage could translate into faster sputum conversion, higher cure rates, lower relapse rates, and less mortality. It is encouraging that daily rifampicin doses up to 35 mg/kg were found to be safe and well-tolerated over a period of 12 weeks. High-dose rifampicin should thus be considered in future studies when constructing potentially shorter regimens. The studies should be adequately powered to determine treatment outcomes and should include surrogate markers of efficacy such as C_max/MIC (minimum inhibitory concentration) and AUC/MIC.

In Vitro Assessment of the Interaction Potential of Ocimum basilicum (L.) Extracts on CYP2B6, 3A4, and Rifampicin Metabolism

Ocimum basilicum L. or basilicum is a common culinary herb, used as a traditional medicine for various medical conditions including HIV/AIDS and tuberculosis, in Africa. The objective of this study was to evaluate the effect of methanol, ethanol, aqueous and ethyl acetate extracts of the dried leaves and inflorescence of O. basilicum, on the activity of cytochrome P450 enzymes (CYPs) CYP2B6 and 3A4, as well as esterase-mediated metabolism of rifampicin to 25-O-desacetyl rifampicin (25ODESRIF). Human liver microsomes (HLM) were used to evaluate inhibition and CYP2B6/3A4 mRNA expression HepG2 assays were used to measure induction. Furthermore, the phytoconstituents likely involved in causing the observed effect were analyzed using biochemical tests and LC-MS. The aqueous and methanolic extracts showed reversible and time-dependent inhibition (TDI) of CYP2B6 with TDI-IC₅₀s 33.35 μg/ml (IC₅₀ shift-fold >1.5) and 4.93 μg/ml (IC₅₀ shift-fold >7) respectively, while the methanolic and ethanolic extracts inhibited 25ODESRIF formation (IC₅₀s 31 μg/ml, 8.94 μg/ml). In HepG2 assays, the methanolic and ethanolic extracts moderately induced CYP2B6, 3A4 mRNA with 38%-, 28%-fold shift, and 22%-, 44%-fold shift respectively. LC-MS full scans identified phenols rosmarinic acid [m/z 359 (M-H)^-, approximately 2298 mg/L in aqueous extract] and caftaric acid along with flavones salvigenin [m/z 329 (M+H)⁺, approximately 1855 mg/L in ethanolic extract], eupatorin [m/z 345 (M+H)⁺, 668.772 mg/L in ethanolic extract], rutin [m/z 609 (M-H)^-] and isoquercetin [m/z 463 (M-H)^-] and other compounds—linalool [m/z 153 (M-H)^-], hydroxyjasmonic acid [m/z 225 (M-H)^-], eucommiol [m/z 187 (M-H)^-] and trihydroxy octadecenoic acid [m/z 329 (M-H)^-, 530 mg/L in ethanolic extract]. The putative gastrointestinal tract (GIT) concentration for all extracts was calculated as 2,400 μg/ml and hepatic circulation concentrations were estimated at 805.68 μg/ml for the aqueous extract, and 226.56 μg/ml for methanolic extract. Based on the putative GIT concentration, estimated hepatic circulation concentration [I] and inhibition constant K_i, the predicted percentile of inhibition in vivo was highest for the aqueous extract on CYP2B6 (96.7%). The observations indicated that O. basilicum extracts may have the potential to cause clinically relevant herb-drug interactions (HDI) with CYP2B6 and rifampicin metabolism in vivo, if sufficient hepatic concentrations are reached in humans.

Metabolism and interactions of antileprosy drugs

Leprosy is a chronic infectious disease caused my Mycobacterium leprae that primarily affects peripheral nervous system and extremities and is prevalent in tropical countries. Treatment for leprosy with multidrug regimens is very effective compared to monotherapy especially in multibacillary cases. The three major antileprosy drugs currently in use are 4, 4'-diaminodiphenyl sulfone (DDS, dapsone), rifampicin, and clofazimine. During multidrug therapy, the potent antibiotic rifampicin induces the metabolism of dapsone, which results in decreased plasma half-life of dapsone and its metabolites. Furthermore, rifampicin induces its own metabolism and decreases its half-life during monotherapy. Rifampicin upregulates several hepatic microsomal drug-metabolizing enzymes, especially cytochrome P450 (CYP) family that in turn induce the metabolism of dapsone. Clofazimine lacks significant induction of any drug-metabolizing enzyme including CYP family and does not interact with dapsone metabolism. Rifampicin does not induce clofazimine metabolism during combination treatment. Administration of dapsone in the acetylated form (acedapsone) can release the drug slowly into circulation up to 75 days and could be useful for the effective treatment of paucibacillary cases along with rifampicin. This review summarizes the major aspects of antileprosy drug metabolism and drug interactions and the role of cytochrome P450 family of drug metabolizing enzymes, especially CYP3A4 during multidrug regimens for the treatment of leprosy.

Effects of Rifampin on the Pharmacokinetics of a Single Dose of Istradefylline in Healthy Subjects

Istradefylline, a selective adenosine A_2A inhibitor, is under development for the treatment of Parkinson's disease. The effect of oral steady‐state rifampin 600 mg/day, a potent cytochrome P450 (CYP) 3A4 inducer, on the disposition of a single oral dose of istradefylline 40 mg was determined in a crossover study in 20 healthy subjects by measuring plasma concentrations of istradefylline and its M1 and M8 metabolites and their derived pharmacokinetic parameters. Based on the geometric mean ratio of log‐transformed data, rifampin reduced istradefylline exposure: C_max, 0.55 (90%CI, 0.49–0.62); AUC_last, 0.21 (90%CI, 0.19–0.22); and AUC_inf, 0.19 (90%CI, 0.18–0.20), indicating nonequivalence. These changes were primarily because of the effect of rifampin on the elimination parameters of istradefylline; mean CL/F was increased from 4.0 to 20.6 L/h, and mean t_1/2 was reduced from 94.8 to 31.5 hours. The effect of rifampin coadministration on the disposition of the istradefylline M1 and M8 metabolites was inconsistent and variable. Furthermore, as exposure of the istradefylline M1 and M8 metabolites in plasma was generally <9% of total drug exposure, it would be expected to have a negligible impact on the pharmacodynamic effect of istradefylline. Caution should be exercised when istradefylline is administered concurrently with strong CYP3A4 inducers and dose adjustment considered.

Kinetic and thermodynamic study of bovine serum albumin interaction with rifampicin using surface plasmon resonance and molecular docking methods

The interaction of bovine serum albumin (BSA) with various drugs, such as antibiotics, due to the importance of BSA in drug delivery has attracted increasing research attention at present. Therefore, the aim of this study was investigation of BSA interaction with rifampicin using surface plasmon resonance (SPR) and molecular docking methods under the imitated physiological conditions ( pH = 7.4 ). BSA immobilization on carboxymethyl dextran hydrogel chip has been carried out after activation with N-hydroxysuccinimide/N-ethyl-N-(3-diethylaminopropyl) carbodiimide. The dose-response sensorgrams of BSA upon increasing concentration of refampicin were attained in SPR analysis. The high affinity of rifampicin to BSA was demonstrated by a low equilibrium constants ( K D ) value ( 3.46 × 10 ? 5 at 40°C). The process of kinetic values changing shows that affinity of BSA to rifampicin decreased with rising temperature. The positive value of both enthalpy change ( ? H ) and entropy change ( ? S ) showed that hydrophobic force plays major role in the BSA interaction with rifampicin. The positive value of ? G was indicative of nonspontaneous and enthalpy-driven binding process. In addition, according to the molecular docking study, hydrogen binding has some contributions in the interaction of rifampicin with BSA.

ISONIAZID AND RIFAMPIN PHARMACOKINETICS IN TWO ASIAN ELEPHANTS (ELEPHAS MAXIMUS) INFECTED WITH MYCOBACTERIUM TUBERCULOSIS

This report describes the pharmacokinetic profiles of chronically administered oral isoniazid and rifampin in one adult male and one adult female Asian elephant ( Elephas maximus ) that were asymptomatically infected with Mycobacterium tuberculosis . Rifampin's half-life was reduced when compared to previous single-dose pharmacokinetic profiles of healthy uninfected Asian elephants. Both elephants experienced delayed absorption of isoniazid and rifampin as compared to previous pharmacokinetic studies in this species. The altered pharmacokinetics of both drugs in repeated-dosing clinical situations underscores the need for individual therapeutic drug monitoring for tuberculosis treatment.

Optimizing the clinical pharmacology of tuberculosis medications

Tuberculosis (TB) treatment has changed little in the past 40 years. The current standard therapy requires four drugs for 2 months followed by two drugs for 4 months. This "short-course" regimen is not based on optimized pharmacokinetic and pharmacodynamic properties, but empiric evidence. A review of existing data suggests that pharmacokinetic variability with isoniazid and rifampin is greater than previously thought, and that efficacy is not as good as traditionally reported. Adding new drugs to the current regimen will be costly and time-consuming. Maximizing the efficacy of the current medications is a less expensive and more feasible option. This article reviews the current potential of the first-line TB drugs (rifamycins, isoniazid, pyrazinamide, and ethambutol) as well as the fluoroquinolones to introduce a true short-course TB regimen.

Interaction between rifampicin, amodiaquine and artemether in mice infected with chloroquine resistant Plasmodium berghei

Background

Artemisinin-based combination therapy (ACT) remains the most effective chemotherapeutic strategy in the management of malaria. However, reports of reduced susceptibility of Plasmodium falciparum to the ACT justify the need for continued search for alternative anti-malarial drugs. The use of antibiotics with anti-malarial properties represents a potentially valuable chemotherapeutic option for the management of drug resistant infections. Thus, the intrinsic anti-malarial activity of the combination of clinical doses of rifampicin with amodiaquine and artemether was evaluated in an animal model using Plasmodium berghei.

Methods

A modification of the suppressive tests in vivo was employed. The anti-malarial activity of standard doses of amodiaquine (AQ) with or without artemether (ART) and combined with varying doses of rifampicin (RIF 15 mg/kg or RIF 30 mg/kg body weight) was evaluated in 40 mice sub-divided into eight groups and inoculated intraperitoneally with 1 × 10⁷ red blood cells infected with chloroquine-resistant P. berghei ANKA strain. There were two control groups of animals, one group received amodiaquine alone while the other group received saline. Parasiticidal activity and survival of the animals were assessed over 21 days.

Results

Parasitaemia in the control animals peaked at 38% on day 9 and all animals died by day 10. The combination of amodiaquine with rifampicin 15 mg/kg body weight was the most effective of all the combinations and more efficacious than amodiaquine alone. The order of superiority of anti-malarial efficacy of the combinations was as follows; AQ + RIF 15 > AQ > AQ + ART + RIF 30 > AQ + ART + RIF 15 > AQ + RIF 30.

Conclusion

The combination of the clinical dose of rifampicin (15 mg/kg) with amodiaquine represents a potentially valuable treatment option in management of drug resistant malaria. In addition, the role of pharmacokinetic interaction in multiple drug therapy cannot be over-emphasized.

Expression Profile of Markers of Apoptosis, Injury and Oxidative Stress in Human Lung Epithelium Cells-A5449 Receiving Chronic Exposure of Potential Anti-Tubercular Drug-trans-Cyclohexane-1, 4-Diamine Derivative-"9u"

Earlier, we had reported the synthesis of a library of symmetrical trans-cyclohexane-1,4-diamine derivatives and evaluated them for their anti-mycobacterium activity in the H37RV strain of Mycobacterium tuberculosis. A range of efficacy was recorded in different derivatives and the compound "9u" having an i-propyl group substitution at the p-position was found to be the most effective. The compound "9u" was found to be safe for cytotoxic and genotoxic responses in human lung epithelium cells-A549, even up to many fold higher than that required to kill M. tuberculosis. Hence, compound "9u" was inferred to be a potential anti-tuberculosis drug of choice. However, the biological safety of compound "9u" upon chronic exposure was still to be answered because anti-tuberculosis (TB) treatment requires a minimum of 6 months' exposure of host systems and most of the available anti-TB drugs are known to induce apoptosis, oxidative stress and injury during such exposures. Thus, the present investigations were aimed to study the alterations, if any, in the expression profile (mRNA and protein) of markers associated with apoptosis, injury and oxidative stress in human lung epithelium cells-A549 receiving a chronic exposure of the potential anti-TB compound "9u." Our findings demonstrate that there was a statistically insignificant transient shift (until 3 weeks) in the markers of apoptosis, injury and oxidative stress, after which expression changes were similar to baseline, when compared with unexposed cells of respective time periods. The studied markers showed linearity in the trend at both mRNA and protein level, indicating the suitability of the test system selected in the study. The data confirm the therapeutic potential of compound "9u" for even long-term treatment against M. tuberculosis without having any significant apoptosis, injury and oxidative stress.

Cyto-genotoxicity Assessment of Potential Anti-tubercular Drug Candidate Molecule-trans-cyclohexane-1, 4-diamine Derivative-9u in Human Lung Epithelial Cells A549

Increasing incidences of multiple drug-resistance (MDR) in Mycobacterium tuberculosis are emerging as one among the serious public health threats and socio-economic burden to the third world countries including India. Last couples of decades are witnesses of the dedicated and sustained efforts made toward the development of target specific and cost-effective antimicrobial agents against MDR-M. tuberculosis. However, the drugs in use are still incapable of controlling the upsurge of MDR. Thus, in order to address the issue, we synthesized a library of symmetrical trans-cyclohexane-1, 4-diamine derivatives and evaluated their anti-mycobacterium activity in H₃₇RV strain of M. tuberculosis. A range of efficacy has been recorded in different derivatives of synthesized compounds and compound “9u” having i-propyl group substitution at p-position, was found to have more significant detrimental effects against the tested strain of M. tuberculosis. The present investigations were aimed to study whether the effective anti-mycobacterium concentrations of “9u” are biologically safe to human cells or not? The human lung epithelial cell line-A549 were exposed to a range of concentrations, i.e., at and above the anti-mycobacterium effective dose of “9u” for a period of 0-96 h. The standard endpoints of cytotoxicity viz., tetrazolium bromide salt (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide), neutral red uptake, lactate dehydrogenase release, trypan blue dye exclusion assays; and genotoxicity viz., micronucleus and chromosomal aberrations assays were used to evaluate the bio-safety of test compound. The compound “9u” shows no significant cytotoxicity and genotoxicity in A549 cells exposed to 10^-5 M for 72 h, a concentration substantially higher than the concentration kill the H₃₇Rv strain of M. tuberculosis. The compound 9u was found to be safe up to 10^-4 M if given for 24 h. The data reveal the therapeutic potential of compound 9u against M. tuberculosis without any having any cytotoxicity and genotoxicity responses.

Single and multiple dose intravenous and oral pharmacokinetics of the hedgehog pathway inhibitor vismodegib in healthy female subjects

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

While recent publications have suggested the pharmacokinetics (PK) of vismodegib appear to be non-linear, there has not been a report describing the mechanisms of non-linearity.

WHAT THIS STUDY ADDS

This study provides evidence that two separate processes, namely, solubility-limited absorption and concentration-dependent plasma protein binding, can explain the non-linear PK of vismodegib. This study provides quantitative results which can account for the lower than expected accumulation of vismodegib with continuous daily dosing.

AIM

Vismodegib has demonstrated clinical activity in patients with advanced basal cell carcinoma. The pharmacokinetics (PK) of vismodegib are non-linear. The objective of this study was to determine whether vismodegib PK change following repeated dosing by administering a tracer intravenous (i.v.) dose of (14) C-vismodegib with single and multiple oral doses.

METHODS

Healthy post menopausal female subjects (n= 6/group) received either a single or daily 150 mg vismodegib oral dose with a (14) C-labelled 10 µg i.v. bolus dose administered 2 h after the single or last oral dose (day 7). Plasma samples were assayed for vismodegib by LC-MS/MS and for (14) C-vismodegib by accelerator mass spectrometry.

RESULTS

Following a single i.v. dose, mean clearance, volume of distribution and absolute bioavailability were 43.4 ml h(-1) , 16.4 l and 31.8%, respectively. Parallel concentration-time profiles following single oral and i.v. administration of vismodegib indicated elimination rate limited PK. Following i.v. administration at steady-state, mean clearance and volume of distribution were 78.5 ml h(-1) and 26.8 l, respectively. Comparison of i.v. PK parameters after single and multiple oral dosing showed similar half-life, increased clearance and volume of distribution (81% and 63% higher, respectively) and decreased bioavailability (77% lower) after repeated dosing. Relative to single dose, the unbound fraction of vismodegib increased 2.4-fold with continuous daily dosing.

CONCLUSION

Vismodegib exhibited a long terminal half-life after oral and i.v. administration, moderate absolute bioavailability and non-linear PK after repeated dosing. Results from this study suggest that the non-linear PK of vismodegib result from two separate, non-linear processes, namely solubility limited absorption and high affinity, saturable plasma protein binding.

Sucralose, a synthetic organochlorine sweetener: overview of biological issues

Sucralose is a synthetic organochlorine sweetener (OC) that is a common ingredient in the world's food supply. Sucralose interacts with chemosensors in the alimentary tract that play a role in sweet taste sensation and hormone secretion. In rats, sucralose ingestion was shown to increase the expression of the efflux transporter P-glycoprotein (P-gp) and two cytochrome P-450 (CYP) isozymes in the intestine. P-gp and CYP are key components of the presystemic detoxification system involved in first-pass drug metabolism. The effect of sucralose on first-pass drug metabolism in humans, however, has not yet been determined. In rats, sucralose alters the microbial composition in the gastrointestinal tract (GIT), with relatively greater reduction in beneficial bacteria. Although early studies asserted that sucralose passes through the GIT unchanged, subsequent analysis suggested that some of the ingested sweetener is metabolized in the GIT, as indicated by multiple peaks found in thin-layer radiochromatographic profiles of methanolic fecal extracts after oral sucralose administration. The identity and safety profile of these putative sucralose metabolites are not known at this time. Sucralose and one of its hydrolysis products were found to be mutagenic at elevated concentrations in several testing methods. Cooking with sucralose at high temperatures was reported to generate chloropropanols, a potentially toxic class of compounds. Both human and rodent studies demonstrated that sucralose may alter glucose, insulin, and glucagon-like peptide 1 (GLP-1) levels. Taken together, these findings indicate that sucralose is not a biologically inert compound.

Rifapentine is not more active than rifampin against chronic tuberculosis in guinea pigs

Rifamycins are key sterilizing drugs in the current treatment of active tuberculosis (TB). Daily dosing of rifapentine (P), a potent rifamycin with high intracellular accumulation, in place of rifampin (R) in the standard antitubercular regimen significantly shortens the duration of treatment needed to prevent relapse in a murine model of active TB. We undertook the current study to compare directly the activities of human-equivalent doses of P and R in a guinea pig model of chronic TB, in which bacilli are predominantly extracellular within human-like necrotic granulomas. Hartley strain guinea pigs were aerosol infected with ~200 bacilli of Mycobacterium tuberculosis H37Rv, and treatment given 5 days/week was initiated 6 weeks later. R at 100 mg/kg of body weight and P at 100 mg/kg were given orally alone or in combination with isoniazid (H) at 60 mg/kg and pyrazinamide (Z) at 300 mg/kg. Culture-positive relapse was assessed in subgroups of guinea pigs after completion of 1 and 2 months of treatment. Human-equivalent doses of R and P showed equivalent bactericidal activity when used alone and in combination therapy. In guinea pigs treated with rifampin, isoniazid, and pyrazinamide (RHZ) or PHZ, microbiological relapse occurred in the lungs of 8/10 animals treated for 1 month and in 0/10 animals treated for 2 months. Substitution of P for R in the standard antitubercular regimen did not shorten the time to cure in this guinea pig model of chronic TB. Data from ongoing clinical trials comparing the activity of these two drugs are awaited to determine the relevance of the guinea pig TB model in preclinical drug screening.

A semimechanistic pharmacokinetic-enzyme turnover model for rifampin autoinduction in adult tuberculosis patients

The currently recommended doses of rifampin are believed to be at the lower end of the dose-response curve. Rifampin induces its own metabolism, although the effect of dose on the extent of autoinduction is not known. This study aimed to investigate rifampin autoinduction using a semimechanistic pharmacokinetic-enzyme turnover model. Four different structural basic models were explored to assess whether different scaling methods affected the final covariate selection procedure. Covariates were selected by using a linearized approach. The final model included the allometric scaling of oral clearance and apparent volume of distribution. Although HIV infection was associated with a 30% increase in the apparent volume of distribution, simulations demonstrated that the effect of HIV on rifampin exposure was slight. Model-based simulations showed close-to-maximum induction achieved after 450-mg daily dosing, since negligible increases in oral clearance were observed following the 600-mg/day regimen. Thus, dosing above 600 mg/day is unlikely to result in higher magnitudes of autoinduction. In a typical 55-kg male without HIV infection, the oral clearance, which was 7.76 liters · h⁻¹ at the first dose, increased 1.82- and 1.85-fold at steady state after daily dosing with 450 and 600 mg, respectively. Corresponding reductions of 41 and 42%, respectively, in the area under the concentration-versus-time curve from 0 to 24 h were estimated. The turnover of the inducible process was estimated to have a half-life of approximately 8 days in a typical patient. Assuming 5 half-lives to steady state, this corresponds to a duration of approximately 40 days to reach the induced state for rifampin autoinduction.

Lack of a clinically meaningful pharmacokinetic effect of rifabutin on raltegravir: in vitro/in vivo correlation

Raltegravir is an HIV-1 integrase strand transfer inhibitor with potent activity against HIV-1. A prior investigation of raltegravir coadministered with rifampin demonstrated a decrease in plasma concentrations of raltegravir likely secondary to induction of UGT1A1, the enzyme primarily responsible for the metabolism of raltegravir. Little is known regarding the induction of UGT1A1 by rifabutin, an alternate rifamycin. In vitro characterization of the induction potency of rifampin and rifabutin on UGT1A1 was performed. In vitro studies indicate that rifabutin is a less potent inducer of UGT1A1 messenger RNA expression than is rifampin. A fixed-sequence, 2-period, clinical crossover study was conducted to assess the effect of rifabutin on plasma levels of raltegravir: period 1, 400 mg of raltegravir every 12 hours for 4 days; period 2, 400 mg of raltegravir every 12 hours and 300 mg of rifabutin once daily for 14 days. Geometric mean ratio (GMR) (coadministration of rifabutin and raltegravir vs raltegravir alone) of raltegravir area under the concentration-time curve from 0 to 12 hours post dose (AUC(0-12h)) and the 90% confidence interval (CI) was 1.19 (0.86-1.63); GMR of concentration at 12 hours (C(12h)) and 90% CI was 0.80 (0.68-0.94); and GMR of time to maximal concentration (C(max)) and 90% CI was 1.39 (0.87-2.21). Overall, coadministration of rifabutin did not alter raltegravir pharmacokinetics to a clinically meaningful degree.

In vivoinduction of human cytochrome P450 3A4 by rifabutin in chimeric mice with humanized liver

The induction of human cytochrome P450 enzymes (CYPs) often poses a serious problem in clinical practice. The induction of CYP3A leads to a decrease in the pharmacological potency of drugs, since many drugs are substrates of CYP3A. The present study examined the in vivo induction potency of human CYP3A in chimeric mice with humanized liver, recently established in Japan, by a specific inducer of human CYP3A enzyme activity in this experimental condition, rifabutin, which is an analogue of rifampicin. The chimeric mice were treated intraperitoneally daily for 4 days with rifabutin (50 mg kg(-1) day(-1)). The mRNA, protein and enzyme activity in liver of the chimeric mice were measured by reverse-transcriptase polymerase chain reaction, Western blot analysis and high-performance liquid chromatography, respectively. In the chimeric mice, the human CYP3A4 mRNA expression, CYP3A4 protein content, testosterone 6ss-hydroxylase activity and dexamethasone 6-hydroxylase activity were increased 7.4-, 3.0-, 2.4- and 1.9-fold, respectively, by treatment with rifabutin. The mRNA expression of other human CYPs, transporters and nuclear receptors was not significantly changed by rifabutin. On the other hand, rifabutin was demonstrated not to increase the murine Cyp3a enzyme activities in the control mice. It was demonstrated that human CYP3A4 expressed in the chimeric mice with humanized liver was induced by rifabutin, suggesting that human CYP3A4 in the chimeric mice had induction potency. This chimeric mouse model may be a useful animal model to estimate and predict the in vivo induction of CYPs in human.

Repeated administration of high-dose intermittent rifapentine reduces rifapentine and moxifloxacin plasma concentrations

Moxifloxacin- and rifapentine-based regimens are under investigation for the treatment of tuberculosis. However, rifapentine may induce enzymes that metabolize moxifloxacin, resulting in decreased moxifloxacin concentrations. In this phase I, two-period, sequential-design study, 13 subjects received 400 mg moxifloxacin daily for 4 days followed by daily moxifloxacin coadministered with 900 mg rifapentine thrice weekly. Pharmacokinetic analyses were performed after the 4th and 19th doses of moxifloxacin and after the 1st and 7th doses of rifapentine. For moxifloxacin, the mean area under the concentration-time curve from 0 to 24 h (AUC(0-24)) decreased by 17.2% (P = 0.0006) when the drug was coadministered with rifapentine, and the mean half-life (t(1/2)) decreased from 11.1 to 8.9 h (P = 0.0033). For rifapentine, the mean AUC(0-48) after seven thrice-weekly doses decreased by 20.3% (P = 0.0035) compared to the AUC(0-48) after the first dose, and the mean t(1/2) decreased from 18.5 to 14.8 h (P = 0.0004). The AUC(0-48) for the 25-desacetyl-rifapentine metabolite diminished 21%. Two days after completing the study drugs, one subject developed a fever and hepatitis, and another developed a flu-like illness with a rash. In conclusion, rifapentine modestly reduced moxifloxacin concentrations. Changes consistent with rifapentine autoinduction of metabolism were seen. Adverse reactions in two subjects may have represented rifamycin hypersensitivity syndrome, although some features were atypical.

Expression and induction by rifampicin of CAR- and PXR-regulated CYP2B and CYP3A in liver, kidney and airways of pig

The transcript levels of CYP2B22, 3A22, 3A29, 3A46, CAR, PXR and HNF4alpha were investigated in liver, kidney and airways from control and rifampicin-treated male pigs. The presence and induction of CYP genes transcription were studied by RT-PCR, real-time PCR, Western blotting and enzymatic activity whereas the expression of receptors was studied by RT-PCR or real-time PCR. Pretreatment with rifampicin resulted in a transcriptional activation, although to different extents, of all the CYP3A genes in liver but not in kidney, lung, bronchi or trachea. In the hepatic microsomes, the induction of CYP3A genes was accompanied by an increase of CYP3As marker activities and of two protein bands immunoreactive with anti-human CYP3A4. The CYP2B22 transcript was found to be markedly induced only in liver and kidney. In parallel, a protein band immunoreactive with anti-rat CYP2B1 was elevated while enhanced CYP2B marker activities were observed in hepatic and renal microsomes. As expected, based on human data, the basal expression of CAR, PXR and HNF4alpha was found to be high in liver and low in airways and not susceptible to induction by rifampicin. A significant expression of these transcriptional factors was also demonstrated in kidney. Thus, it is likely that rifampicin induced CYP2B22 both in liver and kidney of pig, not via activation of CAR, but via PXR, through a cross-talk mechanism, as previously observed in human liver. Taken together, our results demonstrated a differential expression and regulation of three individual CYP3As, CYP2B22, CAR, PXR and HNF4alpha genes in liver, kidney and airways of pig.

Relative and absolute quantitative expression profiling of cytochromes P450 using isotope-coded affinity tags

The development of a novel method for absolute quantification of proteins based on isotope-coded affinity tagging using ICAT reagents is described. The method exploits synthetic peptide standards to determine protein content at the femtomole level in biological samples. The approach is generally applicable to any subset of proteins, but is particularly appropriate for quantitative analysis of multiple, closely related isoforms, and for hydrophobic proteins that are poorly represented in 2-D gels. Relative and absolute quantification techniques are applied to an important group of microsomal metabolic enzymes, the cytochromes P450 (P450), which are critical in determining the disposition, safety and efficacy of drugs in man. Measurement of the P450 induction profile in response to chemicals is a fundamental aspect of drug safety evaluation and is currently achieved by low-throughput methods employing poorly discriminatory antibodies or substrates. Tagging technology is shown to supersede conventional methods for P450 profiling in terms of discriminatory power and throughput, exemplified by the simultaneous detection of distinct induction profiles for cyp2c subfamily members in response to phenobarbitone: cyp2c29 expression, but not cyp2c40 or cyp2c50, was induced threefold by treatment. This technology should abbreviate the drug development pathway, and provide a widely applicable, rapid means of quantifying proteins.

Roles of rifampicin in drug-drug interactions: underlying molecular mechanisms involving the nuclear pregnane X receptor

Rifampicin, an important drug in the treatment of tuberculosis, is used extensively despite its broad effects on drug-drug interactions, creating serious problems. The clinical importance of such interactions includes autoinduction leading to suboptimal or failed treatment. The concomitantly administered effects of rifampicin on other drugs can result in their altered metabolism or transportation that are metabolised by cytochromes P450 or transported by p-glycoprotein in the gastrointestinal tract and liver. This review paper summarises recent findings with emphases on the molecular mechanisms used to explain these broad drug-drug interactions. In general, rifampicin can act on a pattern: rifampicin activates the nuclear pregnane X receptor that in turn affects cytochromes P450, glucuronosyltransferases and p-glycoprotein activities. This pattern of action may explain many of the rifampicin inducing drug-drug interactions. However, effects through other mechanisms have also been reported and these make any explanation of such drug-drug interactions more complex.

A Toxicologist's Guide to the Preclinical Assessment of Hepatic Microsomal Enzyme Induction

The assessment of hepatic microsomal enzyme induction at the completion of preclinical toxicology studies in rodents and large mammals provides a wealth of information to the toxicologist and pharmacokineticist regarding how the drug-metabolizing system of the hepatocyte endoplasmic reticulum responded to high-dose levels of a xenobiotic designed for a specific pharmacological target in any of several target organs. The interpretation of these data can be greatly enhanced by a clear understanding of how this system functions and what the immediate and long-term ramifications are to organs and organ systems. This review focuses on how drugs modify the hepatic cytochrome P450 system, how those modifications are detected, the various consequences of these modifications, and some differences in the induction response among species.

Induction of proteins involved in multidrug resistance (P-glycoprotein, MRP1, MRP2, LRP) and of CYP 3A4 by rifampicin in LLC-PK1 cells

P-glycoprotein, multidrug resistance-related proteins (MRPs) and lung resistance-related protein (LRP) are involved in multidrug resistance in tumor cells but are also expressed in normal tissues. In the LLC-PK(1) tubular renal cell line, a 15-day treatment with 25 microM rifampicin significantly increased the mRNA levels of P-glycoprotein, MRP1, MRP2, LRP and cytochrome P450 3A4 (CYP 3A4). Western blot analysis confirmed a moderate increase in the expression of P-glycoprotein and MRP2, but not MRP1 also at the protein level. The intracellular uptake of doxorubicin was significantly lower in rifampicin pretreated cells. A pretreatment with 6-[82S,4R,6E)-4-methyl-2-(methylamino)-3-oxo-6-octenoic acid]cyclosporin D, valspodar (PSC 833), a specific inhibitor of P-glycoprotein, with (3-(3-(2-(7-chloro-2-quinidinyl)ethenyl-phenyl)((3-diimethyl amino-3oxo propyl)thio)methyl)thio)propanoic acid, sodium salt (MK-571), a specific inhibitor of MRP1, and with verapamil, that inhibits both proteins, significantly increased doxorubicin cell accumulation in rifampicin pretread cells. In rifampicin treated cells cultured on porous membranes, doxorubicin showed a polarized transport, that was reduced by a pretreatment with PSC 833. A chronic treatment with rifampicin induces the expression of transport proteins and of CYP 3A4 and could therefore alter the renal elimination kinetics of drugs that are their substrates.

Assessment of temporal biochemical and gene transcription changes in rat liver cytochrome P450: utility of real-time quantitative RT-PCR

PURPOSE

A conventional approach to assess cytochrome P450 (CYP) induction in preclinical animal models involves daily dosing for a least a week followed by Western blot and/or enzyme activity analysis. To evaluate the potential benefit of a third more specific and sensitive assay, real-time quantitative reverse transcription polymerase chain reaction (qRT-PCR), with the objective of reducing the duration of the conventional 1-week study, we simultaneously assessed gene expression by qRT-PCR along with Western blots and enzyme activity assays as a time course in an in vivo model.

METHODS

Rats were dosed daily for 8 days with model inducers of CYP1A, CYP2B, CYP3A, or CYP4A. Liver P450 levels were measured after 0.5, 1, 2, 4, and 8 days of dosing by qRT-PCR, Western blot, and enzyme activity.

RESULTS

CYP1A, CYP3A, and CYP4A genes were maximally induced very rapidly (0.5-1 day), whereas the CYP2B gene was maximally induced after a lag time of 4 days. In all cases, fold changes in induction detected by qRT-PCR were greater than fold changes in protein levels and enzyme activities.

CONCLUSIONS

Maximal persistent and larger fold changes observed by qRT-PCR either preceded or occurred simultaneously with maximal sustained fold changes in protein levels as measured by Western blots and enzyme activity assays. Our data show that qRT-PCR provides increased sensitivity and specificity over conventional assays and may be key information for reliable assessment of drug-related changes in CYP induction during the transition from discovery to toxicology studies.

Semi-automatic quantitative RT-PCR to measure CYP induction by drugs in human hepatocytes

An assay has been developed for the quantitative measurement of CYP mRNA content of the major human isoforms (1A1, 1A2, 2A6, 2B6, 2C9, 2C19, 2D6, 2E1, 3A4 and 3A5) in human hepatocytes. The method is based on the conversion of mRNAs into their corresponding cDNAs, followed by PCR amplification using appropriate primers. Making use of appropriate internal and external standards it is possible to estimate changes in CYP mRNA content of hepatocytes. The technique has been standardised to run semi-automatically. This procedure can be used to assess the CYP induction potential of new pharmaceuticals at a pre-clinical stage of development. To this aim, human hepatocytes obtained from functional liver tissue are incubated with the drugs for 50 h. Total RNA is extracted from culture and the cDNA is prepared by reverse transcription using high fidelity reverse transcriptase. Using appropriate primers, selective amplification of each CYP cDNA is achieved and real-time quantified by SYBR-Green fluorescence measurement. The extent of CYP induction obtained with the tested compounds is compared with the induction obtained with CYP model inducers (methylcholantrene, phenobarbital and rifampicin). This technique can be of value, to considerably simplify the identification of drug candidates with potential CYP inducing ability in man.

Strategies and molecular probes to investigate the role of cytochrome P450 in drug metabolism: focus on in vitro studies

Drug metabolism is the major determinant of drug clearance and, because of polymorphic or inducible expression of drug-metabolising cytochrome P450s (CYPs), is the factor most frequently responsible for interindividual differences in pharmacokinetics. A number of well characterised CYP substrates and inhibitors have been identified that allow precise measurements of individual CYP isoforms. Their use, alone or in combination, facilitates the phenotype characterisation of hepatocytes in vitro and in vivo. Two procedures are used for in vitro investigation of the metabolic profile of a drug: incubation with microsomes and incubation with metabolically competent cells. The major limitation of microsomes is that they express phase I activities, but only part of phase II activities, and can only be used for short incubation times. When intact cells are used, gene expression, metabolic pathways, cofactors/enzymes and plasma membrane are largely preserved, but fully differentiated cells such as primary cultured hepatocytes need to be used, since hepatoma cell lines have only very low and partial CYP expression. CYP-engineered cells or their microsomes ('supersomes') have made the identification of the CYPs involved in the metabolism of a drug candidate straightforward and easier. Inhibition of CYP is an undesirable feature for a drug candidate, and needs to be addressed by examining whether the drug candidate inhibits the metabolism of other compounds or whether other compounds inhibit the metabolism of the drug candidate. Such experiments can be conducted both with microsomes and in cells. The major limitation of microsomes is that inhibition parameters may not accurately reflect the situation in vivo, since the contribution of drug transport is not considered. The best picture of a potential drug-drug interaction can be obtained in metabolically competent hepatocytes. Screening of CYP inducers cannot be done in microsomes. It requires the use of a cellular system fully capable of transcribing and translating CYP genes, and can be monitored in vitro as an increase in enzyme mRNA or activity. Human hepatocytes in primary culture respond well to enzyme inducers during the first few days; this ability is lost thereafter. Rat hepatocytes are much less stable and soon become unresponsive to inducers. Hepatoma cell lines respond poorly to inducers, although the induction of a few isoenzymes has been reported. Primary cultured hepatocytes are still the unique in vitro model that allows global examination of the inductive potential of a drug.

Interaction between saquinavir soft-gel and rifabutin in patients infected with HIV

AIMS

To evaluate the potential pharmacokinetic interaction between the HIV protease inhibitor saquinavir and rifabutin.

METHODS

Fourteen HIV-infected patients provided full steady-state pharmacokinetic profiles following administration of rifabutin alone (300 mg once daily) or saquinavir soft-gel formulation (1200 mg three times daily) plus rifabutin (300 mg once daily) in this open label, partially randomized study.

RESULTS

Coadministration of saquinavir and rifabutin resulted in a reduction in saquinavir AUC(0,8 h) and C(max)(0,8 h) of 47% (95% CI 30, 60%) and 39% (95% CI 11, 59%), respectively. Rifabutin AUC(0,24 h) and C(max)(0,24 h) was increased by an average of 44% (95% CI 17, 78%) and 45% (95% CI 14, 85%), respectively. Saquinavir in combination with rifabutin was well tolerated. Gastrointestinal intolerance and asymptomatic increases in liver enzymes were the only adverse events of note.

CONCLUSIONS

Administration of rifabutin with saquinavir may decrease the efficacy of this HIV protease inhibitor.

Evaluation of gene induction of drug-metabolizing enzymes and transporters in primary culture of human hepatocytes using high-sensitivity real-time reverse transcription PCR

In the present study, the induction of drug-metabolizing enzymes and transporters was evaluated by analyzing mRNA expression in human hepatocytes after exposure to various compounds. The compounds tested included typical enzyme inducers, rifampicin and omeprazole, and controls. All experiments were performed in the presence of 0.1% DMSO. Analysis was performed by the real-time reverse-transcription polymerase chain reaction method (RT-PCR) in the presence of TaqMan probes using an ABI PRISM 7700 Sequence Detector system. A new analytic method to quantify mRNA levels in small numbers of human hepatocytes has been developed for phase I enzymes, phase II enzymes, and transporters. The levels of CYP1A1, CYP2B6, CYP2C8, CYP3A4, CYP3A5, ADH3, and ABCG1 mRNA in human hepatocytes increased after exposure to rifampicin. The levels of CYP1A1, CYP2B6, CYP3A4, CYP3A5, and ABCG1 mRNA recovered after a change to media without rifampicin. The levels of CYP1A1, CYP1A2, CYP1B1, ALDH3, and ALDH6 mRNA increased after exposure to omeprazole, and recovered after a change to media without omeprazole. On the other hand, the levels of ADH3 and ABCB4 mRNA decreased after exposure to omeprazole, and recovered after a change to media without omeprazole. In conclusion, these results demonstrate the applicability of quantitative real-time RT-PCR to the evaluation of the gene induction and recovery of drug-metabolizing enzymes and transporters after exposure to drugs in human hepatocytes.

Comparative pharmacokinetics and pharmacodynamics of the rifamycin antibacterials

The rifamycin antibacterials, rifampicin (rifampin), rifabutin and rifapentine, are uniquely potent in the treatment of patients with tuberculosis and chronic staphylococcal infections. Absorption is variably affected by food; the maximal concentration of rifampicin is decreased by food, whereas rifapentine absorption is increased in the presence of food. The rifamycins are well-known inducers of enzyme systems involved in the metabolism of many drugs, most notably those metabolised by cytochrome P450 (CYP) 3A. The relative potency of the rifamycins as CYP3A inducers is rifampin > rifapentine > rifabutin; rifabutin is also a CYP3A substrate. The antituberculosis activity of rifampicin is decreased by a modest dose reduction from 600 to 450mg. This somewhat surprising finding may be due to the binding of rifampicin to serum proteins, limiting free, active concentrations of the drug. However, increasing the administration interval (after the first 2 to 8 weeks of therapy) has little effect on the sterilising activity of rifampicin, suggesting that relatively brief exposures to a critical concentration of rifampicin are sufficient to kill intermittently metabolising mycobacterial populations. The high protein binding of rifapentine (97%) may explain the suboptimal efficacy of the currently recommended dose of this drug. The toxicity of rifampicin is related to dose and administration interval, with increasing rates of presumed hypersensitivity with higher doses combined with administration frequency of once weekly or less. Rifabutin toxicity is related to dose and concomitant use of CYP3A inhibitors. The rifamycins illustrate the complexity of predicting the pharmacodynamics of treatment of an intracellular pathogen with the capacity for dormancy.

Induction of hepatic and presystemic metabolism of antipyrine in the mice: rifampicin versus rifabutin

The effects of hepatic and presystemic enzyme induction on the bioavailability (F) and disposition of antipyrine after repeated rifampicin (RFM) and rifabutin (RBT) exposure were studied in mice. ICR mice were divided to receive 4 daily oral dosing of either the dosing vehicle or 50 mg/kg of REM or RBT. At the completion of rifamycin dosing, the pharmacokinetics of antipyrine were assessed following either a single 50 mg/kg oral dose or a 20 mg/kg intravenous dose. Blood samples were collected (n=4/timepoint) over a 6 h period. The content of P450 in the liver and small intestine (GI) was also assessed in parallel. Systemic antipyrine clearance (CL) increased from 31.8 ml/min/kg (controls) by 64% and 42% following repeated exposure to RFM and RBT, respectively. Estimate of F for antipyrine decreased from 0.97 in controls to 0.58 and 0.82 in animals treated with RFM and RBT, respectively. The content of P450 (nmol/mg protein) in the liver increased from 0.61 (control) to 1.36 following RFM and 0.82 for RBT, while no significant changes were observed for the GI tract. The i.v. dosing data confirmed the induction of antipyrine metabolism in the liver by both rifamycins yet the induction potential was approximately 1/3 lower for RBT. This difference was consistent with the changes observed in the hepatic P450 protein content, but this alone could not account for the reduction in the F for antipyrine. Therefore, predictions for changes in F of an interacting agent should not be judged solely on the basis of the metabolic status of the liver. The relative contribution of metabolic induction and presystemic drug loss to bioavailability/absorption should also be further delineated for its relevance to poly-pharmacy in patients likely to receive long-term rifamycin based treatment.

Effects of repeated rifabutin administration on the pharmacokinetics of intravenous and oral ciprofloxacin in mice

The combination of rifabutin and ciprofloxacin is potentially useful for the treatment of disseminated Mycobacterium avium-intracellulare (MAC) diseases in HIV-infected patients. Rifabutin is a metabolic enzyme inducer structurally similar to its predecessor, rifampin. Using a mouse model, the effects of repeated exposure of rifabutin on the pharmacokinetics of ciprofloxacin after intravenous (i.v.) and oral (p.o.) dosing were investigated in the present study. Results showed that repeated exposure of rifabutin, relative to control, caused a 16% increase in the plasma clearance (CL) of ciprofloxacin after i.v. dosing (4.19 vs. 4.87 L/h/kg). Estimates of elimination half-life (T1/2) were not affected by rifabutin (control: 0.81 vs. rifabutin pretreated: 1.18 h). The data obtained after oral dosing showed that repeated rifabutin dosing caused a significant reduction in the maximal plasma concentration (Cmax: 1.34 vs. 0.91 microg/mL) and a longer time to Cmax (Tmax: 0.17 vs. 0.33 h). These changes might be in part attributable to the increase in oral clearance (CL/F) by 18%. With or without rifabutin pretreatment, the T1/2 estimates of ciprofloxacin for p.o. dosing were similar (2.37-2.58 h) and were approximately twice as long as those obtained after i.v. dosing. Since the changes in systemic exposure as a result of rifabutin pretreatment were similar after i.v. and p.o. dosing, the oral bioavailability (F) of ciprofloxacin remained unaffected by rifabutin at approximately 38%. The effects of rifabutin on the pharmacokinetics of ciprofloxacin appear to be moderate in the mouse model which might be attributable to the absorption and distribution behavior of the quinolone antibiotic. The therapeutic implications of this interaction, if any, remain to be defined.

Species comparison in P450 induction: effects of dexamethasone, omeprazole, and rifampin on P450 isoforms 1A and 3A in primary cultured hepatocytes from man, Sprague-Dawley rat, minipig, and beagle dog

Induction of P450 isoforms 1A (CYP1A) and 3A (CYP3A) by model inducers dexamethasone, omeprazole and rifampin was evaluated in primary cultured hepatocytes from man and laboratory animals. Inducer-specific species-differences were observed. Results with human hepatocytes from six human donors consistently show that both rifampin and dexamethasone were inducers of CYP3A activity (measured as testosterone 6beta-hydroxylase activity), with rifampin being more potent. Conversely, in rat hepatocytes, dexamethasone was a potent CYP3A inducer while rifampin was not an inducer. Rifampin but not dexamethasone induced CYP3A in minipig and beagle dog hepatocytes. Omeprazole was a potent inducer of CYP1A activity (measured as ethoxyresorufin-O-deethylase activity) in human, beagle dog and minipig hepatocytes, and not an inducer in rat hepatocytes. The species-differences observed suggest that human hepatocytes represent the most appropriate preclinical experimental system for the evaluation of P450 induction in human.

Rat pregnane X receptor: molecular cloning, tissue distribution, and xenobiotic regulation

An orphan nuclear receptor, termed the pregnane X receptor (PXR), has recently been cloned from mouse and human and defines a novel steroid signaling pathway (Cell 92, 73-82, 1998; Proc. Natl. Acad. Sci. USA 95, 12208-122313, 1998). Transient cotransfection experiments demonstrate that the PXR responds to structurally dissimilar compounds and confers the induction of cytochrome P4503A (CYP3A), a subfamily of enzymes that involve the metabolism of two-thirds of drugs and other xenobiotics. In this report, we describe the molecular cloning, tissue distribution, and xenobiotic regulation of a rat PXR designated rPXR-1. rPXR-1 exhibits a 95% sequence identity with the mouse PXR, but only 79% identity with the human PXR, providing the molecular basis that rats and mice have a similar CYP3A induction profile but differ from humans. rPXR-1 gene was expressed abundantly in liver, intestine, and, to a lesser extent, kidney, lung, and stomach. The tissue distribution and the relative abundance of rPXR-1 mRNA among these tissues resemble those of CYP3A, suggesting that PXR is important not only for induction but also for constitutive expression of these enzymes. Xenobiotics known to induce liver microsomal enzymes showed differential effects on the rPXR-1 expression as determined by Northern blot analysis. Dexamethasone, for example, increased the accumulation of rPXR-1 mRNA, whereas troleandomycin slightly suppressed it. Compounds that increase PXR expression (inducers) and compounds that interact with PXR (ligands) likely have synergistic effects on CYP3A induction, which provides a novel molecular explanation for drug-drug interactions.

Induction of zidovudine glucuronidation and amination pathways by rifampicin in HIV-infected patients

AIMS

The objective of the study was to determine the effect of multiple doses of rifampicin on the steady-state pharmacokinetics of zidovudine and its 5'-glucuronosyl (GZDV) and 3'-amino (AMT) metabolites.

METHODS

Eight asymptomatic HIV-infected patients (seven male, one female) participated in this three-period longitudinal study. Each patient received zidovudine (200 mg every 8 h) for 14 days (period 1), followed by rifampicin (600 mg every 24 h) with zidovudine for 14 days (period 2), and then zidovudine alone for a further 14 days (period 3). Blood and urine samples were collected over 6 h on the last day of each period for measurements of zidovudine and GZDV by h.p.l.c.-u.v. and AMT by h.p.l.c.-m.s-m.s.

RESULTS

Compared with zidovudine-alone values in period 1, 14 days of coadministration with rifampicin significantly increased zidovudine oral clearance (89%) and formation clearances to GZDV (100%) and AMT (82%). Correspondingly, there were decreases in maximum plasma concentration (43%), AUC (47%) and urine recovery (37%) of zidovudine. GZDV/zidovudine and AMT/zidovudine AUC ratios increased by 99% and 36%, respectively, despite a significant 29% decrease in AMT AUC. After stopping rifampicin for 14 days, values of these pharmacokinetic parameters returned to within 26% of baseline. Over the three periods AMT plasma levels were <18 ng ml-1 (n=6) and <40 ng ml-1 (n=2), and molar AMT/zidovudine AUC ratios ranged from 1.7% to 4.5%.

CONCLUSIONS

Rifampicin induced zidovudine glucuronidation and amination pathways resulting in decreased plasma and urine exposures to zidovudine. AMT plasma exposure decreased because induction was more pronounced for the major GZDV metabolite. The magnitude of the residual inductive effect was minimal at 14 days after stopping rifampicin.

Multiple dose pharmacokinetics of artemether in Chinese patients with uncomplicated falciparum malaria

Multiple dose pharmacokinetics of artemether and dihydroartemisinin were investigated in chinese patients treated for malaria. They received over 2 days either 4 x 80 mg artemether orally (n = 48) or 4 x 80-480 mg co-artemether (n = 40), a combination of artemether and lumefantrine (benflumetol). Lag time = 0.48 h (mean), Cmax after first dose = 157 ng/ml, t(max) = 1.73 h and elimination half-life = 1.16 h. The lag and absorption times were 0.5 h longer for co-artemether compared with artemether. Dihydroartemisinin paralleled artemether pharmacokinetics. Artemether Cmax after the last dose was one-third of the Cmax after the first dose while, inversely, dihydroartemisinin Cmax increased over time. We suggest that auto-induction of gut mucosa enzymes and/or liver enzymes causes a time-dependent increase in first-pass metabolisation of artemether.

Comparative effects of rifabutin and rifampicin on cytochromes P450 and UDP-glucuronosyl-transferases expression in fresh and cryopreserved human hepatocytes

The aim of this study was to evaluate rifabutin (RBT) and rifampicin (RIF) capabilities in inducing various xenobiotic metabolizing enzymes such as cytochromes P450 (CYPs) and UDP-glucuronosyl-transferases (UGTs) in cultured fresh and cryopreserved human hepatocytes. Enzyme induction was assessed through the use of several diagnostic markers, i.e. testosterone, midazolam (MDZ), diazepam (DZP) and 7-ethoxyresorufin for CYP-dependent enzyme reactions; and AZT for UGT-dependent enzyme reactions. RBT concentrations (0.118, 0.708 microM) were selected according to previously published pharmacokinetic data in patients. The known CYP3A4 inducer in humans, RIF, was used as a positive control. At the concentrations used, no sign of cytotoxicity was evidenced. Both compounds were able to dose-dependently induce the overall metabolism of testosterone (approximately 2-fold for RBT, 4-fold for RIF) and the formation of the 6beta-hydroxylated-derivative (up to approximately 4-fold over control for RBT and approximately 10-fold for RIF), which is CYP3A4 dependent. The other hydroxylated metabolites (16alpha-OH and 2alpha-OH) were also enhanced. The metabolism of MDZ, which is specifically metabolized by CYP3A4 in humans, was also investigated following drug's exposure to hepatocytes. DZP one, which is governed by various CYPs, including CYP3A, was also investigated. RBT was shown to increase the biotransformation of both benzodiazepines (approximately 1.9-fold over control). Moreover, the effects of both drugs on ethoxyresorufin O-deethylase activity (EROD), which is representative of CYPIA1/2 isoforms, were tested. Results showed only a moderate induction of this marker (approximately 2-fold over control) when compared to the high effect observed after hepatocyte exposure to 3-methylcholantene (approximately 14-fold over control). Finally, the action of RBT and RIF on UGTs expression was investigated by using AZT as diagnostic substrate: glucuronides formation was not significantly affected by the two rifamycin derivatives. On the whole, exposure of fresh or cryopreserved human hepatocytes to RBT dose-dependently affected the levels of drug metabolizing enzymes in a dose-dependent manner. However, as already demonstrated by in vivo pharmacokinetic studies, its inducing properties towards CYPs, CYP3A in particular, are less pronounced than RIF.

Effects of rifabutin and rifampicin on the pharmacokinetics of ethinylestradiol and norethindrone

This open-label, randomized, three-way crossover study of 28 healthy premenopausal women was conducted to compare the impact of concomitant rifabutin and rifampicin on the safety, pharmacokinetics, and pharmacodynamics of the oral contraceptives ethinylestradiol and norethindrone (Ortho-Novum 1/35; Ortho Pharmaceutical, Raritan, NJ). Each participant received oral contraceptives daily for 21 days for the first control cycle, then was randomized to one of two sequences to receive oral contraceptives with concomitant rifampicin and rifabutin at equal doses of 300 mg/day for 10 days. Ethinylestradiol, norethindrone, follicle stimulating hormone (FSH), luteinizing hormone (LH), progesterone, rifampicin, and rifabutin (and metabolite) were measured in plasma over the same time frames in all three cycles. Safety was assessed from before the beginning to the end of each cycle. Twenty-two subjects completed all three cycles. Compared with the control cycle, rifabutin and rifampicin significantly altered the disposition of the oral contraceptive. Area under the concentration-time curve from 0 to 24 hours (AUC0-24) and maximum plasma concentration (Cmax) of ethinylestradiol decreased by 64% and 42%, respectively, after coadministration with rifampicin and by 35% and 20%, respectively, after coadministration with rifabutin. The AUC0-24 of norethindrone decreased by 60% and 20% after coadministration with rifampicin and rifabutin, respectively. Unlike progesterone levels, FSH and LH levels increased during coadministration with rifampicin and rifabutin. The incidence of spotting was significantly higher after coadministration with rifampicin (36.4%) and rifabutin (21.7%) than during the control cycle (3.7%). Although both rifampicin and rifabutin affected the pharmacokinetics of ethinylestradiol and norethindrone, the magnitude of this effect was more pronounced with rifampicin. Likewise, the fact that the highest incidence of spotting occurred with rifampicin was consistent with higher metabolic induction by rifampicin. Despite the fact that there was no change in progesterone levels, it is recommended that patients be advised to use additional contraceptive methods while receiving rifabutin or rifampicin with oral contraceptives to prevent inadvertent pregnancy.

Inhibition and induction of cytochrome P450 and the clinical implications

The cytochrome P450s (CYPs) constitute a superfamily of isoforms that play an important role in the oxidative metabolism of drugs. Each CYP isoform possesses a characteristic broad spectrum of catalytic activities of substrates. Whenever 2 or more drugs are administered concurrently, the possibility of drug interactions exists. The ability of a single CYP to metabolise multiple substrates is responsible for a large number of documented drug interactions associated with CYP inhibition. In addition, drug interactions can also occur as a result of the induction of several human CYPs following long term drug treatment. The mechanisms of CYP inhibition can be divided into 3 categories: (a) reversible inhibition; (b) quasi-irreversible inhibition; and (c) irreversible inhibition. In mechanistic terms, reversible interactions arise as a result of competition at the CYP active site and probably involve only the first step of the CYP catalytic cycle. On the other hand, drugs that act during and subsequent to the oxygen transfer step are generally irreversible or quasi-irreversible inhibitors. Irreversible and quasi-irreversible inhibition require at least one cycle of the CYP catalytic process. Because human liver samples and recombinant human CYPs are now readily available, in vitro systems have been used as screening tools to predict the potential for in vivo drug interaction. Although it is easy to determine in vitro metabolic drug interactions, the proper interpretation and extrapolation of in vitro interaction data to in vivo situations require a good understanding of pharmacokinetic principles. From the viewpoint of drug therapy, to avoid potential drug-drug interactions, it is desirable to develop a new drug candidate that is not a potent CYP inhibitor or inducer and the metabolism of which is not readily inhibited by other drugs. In reality, drug interaction by mutual inhibition between drugs is almost inevitable, because CYP-mediated metabolism represents a major route of elimination of many drugs, which can compete for the same CYP enzyme. The clinical significance of a metabolic drug interaction depends on the magnitude of the change in the concentration of active species (parent drug and/or active metabolites) at the site of pharmacological action and the therapeutic index of the drug. The smaller the difference between toxic and effective concentration, the greater the likelihood that a drug interaction will have serious clinical consequences. Thus, careful evaluation of potential drug interactions of a new drug candidate during the early stage of drug development is essential.