Serum Rifampicin Levels in Patients with Tuberculosis

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.

Rifampicin tolerance and growth fitness among isoniazid-resistant clinical Mycobacterium tuberculosis isolates from a longitudinal study

Antibiotic tolerance in Mycobacterium tuberculosis reduces bacterial killing, worsens treatment outcomes, and contributes to resistance. We studied rifampicin tolerance in isolates with or without isoniazid resistance (IR). Using a minimum duration of killing assay, we measured rifampicin survival in isoniazid-susceptible (IS, n=119) and resistant (IR, n=84) isolates, correlating tolerance with bacterial growth, rifampicin minimum inhibitory concentrations (MICs), and isoniazid-resistant mutations. Longitudinal IR isolates were analyzed for changes in rifampicin tolerance and genetic variant emergence. The median time for rifampicin to reduce the bacterial population by 90% (MDK90) increased from 1.23 days (IS) and 1.31 days (IR) to 2.55 days (IS) and 1.98 days (IR) over 15-60 days of incubation, indicating fast and slow-growing tolerant sub-populations. A 6 log10-fold survival fraction classified tolerance as low, medium, or high, showing that IR is linked to increased tolerance and faster growth (OR = 2.68 for low vs. medium, OR = 4.42 for low vs. high, p-trend = 0.0003). High tolerance in IR isolates was associated with rifampicin treatment in patients and genetic microvariants. These findings suggest that IR tuberculosis should be assessed for high rifampicin tolerance to optimize treatment and prevent the development of multi-drug-resistant tuberculosis.

Visualizing and quantifying antimicrobial drug distribution in tissue

The biodistribution and pharmacokinetics of drugs are vital to the mechanistic understanding of their efficacy. Measuring antimicrobial drug efficacy has been challenging as plasma drug concentration is used as a surrogate for tissue drug concentration, yet typically does not reflect that at the intended site(s) of action. Utilizing an image-guided approach, it is feasible to accurately quantify the biodistribution and pharmacokinetics within the desired site(s) of action. We outline imaging modalities used in visualizing drug distribution with examples ranging from in vitro cellular drug uptake to clinical treatment of microbial infections. The imaging modalities of interest are: radio-labeling, magnetic resonance, mass spectrometry imaging, computed tomography, fluorescence, and Raman spectroscopy. We outline the progress, limitations, and future outlook for each methodology. Further advances in these optical approaches would benefit patients and researchers alike, as non-invasive imaging could yield more profound insights with a lower clinical burden than invasive measurement approaches used today.

Effect of efavirenz-based ART on the pharmacokinetics of rifampicin and its primary metabolite in patients coinfected with TB and HIV

Objectives

To evaluate the effects of concomitant efavirenz-based ART and genetic polymorphism on the variability in rifampicin and 25-desacetylrifampicin pharmacokinetics.

Patients and methods

Plasma concentrations of rifampicin and 25-desacetylrifampicin from 63 patients coinfected with TB and HIV were analysed by LC-MS/MS followed by non-linear mixed-effects modelling. Patients were genotyped for SLCO1B1 (463 C>A, 388 A>G, 11187 G>A, rs4149015, 521 T>C and 1436 G>C) and SLCO1B3 (334 T>G).

Results

One-compartment disposition models described the observations adequately. The oral clearances of rifampicin and 25-desacetylrifampicin were 140% and 110% higher, respectively, in patients on concomitant efavirenz-based ART. Rifampicin bioavailability was also lower in patients on concomitant ART. Further, although not included in the final model, a lower relative bioavailability in carriers of WT SLCO1B3 334 T>G compared with carriers of mutations in the genotype was estimated.

Conclusions

The results presented indicate both pre-systemic and systemic induction by efavirenz-based ART affecting rifampicin pharmacokinetics. The described drug–drug interaction has a clinical impact on rifampicin exposure prior to steady state and may impact the early bactericidal activity in patients on efavirenz-based ART.

Altered expressions of circulating microRNAs 122 and 192 during antitubercular drug induced liver injury indicating their role as potential biomarkers

Drug induced liver toxicity is a serious health complication leading to high mortality rates and post marketing withdrawal of drugs. Although considered to be the gold standard biomarkers; aspartate aminotransferase, alanine aminotransferase, total bilirubin and alkaline phosphatase have been found to have specificities beyond liver, therefore more specific and predictive markers for the detection of antitubercular drug mediated liver damage are required. Unfortunately, the effectiveness of currently used first line antitubercular drugs namely isoniazid, rifampicin, pyrazinamide is often accompanied with liver injury, impeding the cure of patients. Keeping in view, the prognostic and diagnostic applications of microRNAs in various diseases, we tried to assess the importance of microRNAs 122 and 192 in antitubercular drug associated liver injuries. The study included subjects having tuberculosis of any type with antitubercular drug induced liver injury; naïve or newly diagnosed tuberculosis patients, tuberculosis patients on drugs not having toxicity and healthy controls. Observations from this study revealed that expression levels of miR-122 and miR-192 were significantly decreased in the serum of antitubercular drug induced liver injury patients only. Therefore, these microRNAs or the pathways associated with them can be used as a tool to predict or cure antitubercular drug associated liver injury in future.

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.

Prevalence of p-glycoprotein (PGP) expression, function and its effect on efficacy of rifampicin in patients with lymph node tuberculosis

OBJECTIVE

P-glycoprotein (PGP) overexpression may be one of the operating mechanisms of suboptimal responses to antitubercular treatment (ATT) in patients with lymph node tuberculosis. This might become responsible for the development of drug resistance later due to exposure of subtherapeutic concentrations to the mycobacteria. In this study we aim to study the prevalence of PGP expression and function and its relationship with serum concentrations of Rifampicin in consecutive patients with lymph node tuberculosis.

METHODS

All newly diagnosed treatment naïve subjects with a confirmed diagnosis of tubercular lymphadenopathy were included in the study and the expression and function of PGP in blood was determined by flowcytometry at baseline and after two months of treatment. Serum levels of Rifampicin was measured at 2 months by high performance liquid chromatography (HPLC). The mean net PGP expression expressed as percent and relative fluorescence indices (RFI) of PGP expression and function respectively was compared at baseline at 2 months and was also correlated with serum rifampicin levels.

RESULTS

The mean net PGP expression, RFI of PGP expression and RFI of PGP function were significantly higher in patients with lymph node tuberculosis as compared to healthy controls and the mean net PGP expression and RFI of PGP expression were significantly higher at 2 months as compared to baseline (25.64 ± 5.18% vs. 27.68 ± 4.89%, 4.34 ± 1.09% vs. 4.95 ± 1.55). There was no significant difference in RFI of PGP expression and RFI of PGP function between the poor-responders and responders at baseline and 2 months however there was a trend towards significantly higher net PGP expression amongst poor responders at baseline. The mean serum rifampicin levels were 10.74 ± 2.36 μg/ml in the responder group and 7.86 ± 1.21 μg/ml in the non-responder group and the difference between the two was statistically significant (p = 0.004).

CONCLUSIONS

Overexpression of PGP is common in patients with lymph node tuberculosis and leads to lower concentrations of Rifampicin in blood which subsequently may give rise to development of drug resistance. This is also responsible for poor therapeutic responses in these patients. Nonspecific inhibitors of PGP may be used in conjunction with ATT to augment therapeutic response in such cases.

A Population Pharmacokinetic Model Incorporating Saturable Pharmacokinetics and Autoinduction for High Rifampicin Doses

Accumulating evidence suggests that increasing doses of rifampicin may shorten tuberculosis treatment. The PanACEA HIGHRIF1 trial assessed safety, pharmacokinetics, and antimycobacterial activity of rifampicin at doses up to 40 mg/kg. Eighty-three pulmonary tuberculosis patients received 10, 20, 25, 30, 35, or 40 mg/kg rifampicin daily over 2 weeks, supplemented with standard doses of isoniazid, pyrazinamide, and ethambutol in the second week. This study aimed at characterizing rifampicin pharmacokinetics observed in HIGHRIF1 using nonlinear mixed effects modeling. The final population pharmacokinetic model included an enzyme turnover model accounting for time-dependent elimination due to autoinduction, concentration-dependent clearance, and dose-dependent bioavailability. The relationship between clearance and concentration was characterized by a Michaelis-Menten relationship. The relationship between bioavailability and dose was described using an E_max relationship. The model will be key in determining exposure-response relationships for rifampicin and should be considered when designing future trials and when treating future patients with high-dose rifampicin.

Population pharmacokinetics of rifampicin in adult patients with osteoarticular infections: interaction with fusidic acid

AIMS

Rifampicin represents the key antibiotic for the management of osteoarticular infections. An important pharmacokinetic variability has already been described, particularly for absorption and metabolism. All previous pharmacokinetic studies have been focused only on patients treated for tuberculosis. The objective of the present study was to describe a population pharmacokinetic model of rifampicin in patients with staphylococcal osteoarticular infections, which has not been investigated to date.

METHOD

Rifampicin concentrations were collected retrospectively from 62 patients treated with oral rifampicin 300 mg three times daily. Plasma concentration-time data were analysed using NONMEM to estimate population pharmacokinetic parameters. Demographic data, infection characteristics and antibiotics taken in addition to rifampicin antibiotics were investigated as covariates.

RESULTS

A one-compartment model, coupled to a transit absorption model, best described the rifampicin data. Fusidic acid coadministration was identified as a covariate in rifampicin pharmacokinetic parameters. The apparent clearance and apparent central volume of distribution mean values [95% confidence interval (CI)] were 5.1 1 h^-1 (1.2, 8.2 1 h^-1 )/23.8 l (8.9, 38.7 l) and 13.7 1 h^-1 (10.6, 18.0 1 h^-1 )/61.1 1 (40.8, 129.0 1) for patients with and without administration of fusidic acid, respectively. Interindividual variability (95% CI) in the apparent clearance and apparent central volume of distribution were 72.9% (49.5, 86.0%) and 59.1% (5.5, 105.4%), respectively. Residual variability was 2.3 mg l^-1 (1.6, 2.6 mg l^-1 ).

CONCLUSION

We developed the first population pharmacokinetic model of rifampicin in patients with osteoarticular infections. Our model demonstrated that fusidic acid affects rifampicin pharmacokinetics, leading to potential high drug exposure. This finding suggests that fusidic acid dosing regimens should be reconsidered.

Population Pharmacokinetics of Rifampicin in Chinese Patients With Pulmonary Tuberculosis

Rifampicin (RIF) induces cytochrome P450, which in turn catalyzes drug metabolism; however, pharmacokinetic studies on this phenomenon in the Chinese population, especially in the context of disease, are limited. Therefore, we sought to establish population-based pharmacokinetic models of RIF in a Chinese population with pulmonary tuberculosis (TB). Clinical data were retrospectively collected from 54 patients with pulmonary TB and analyzed alongside RIF blood levels from 95 samples collected prior to RIF administration and between 2 and 12 hours after treatment. HPLC was used to measure serum RIF concentrations. A nonlinear mixed model used to characterize RIF pharmacokinetics and the data generated from the present study were validated using a bootstrap method. Covariates, including demographics, as well as hematological and biological indicators were analyzed. We observed a 1-compartment model with first-order absorption. Typical population values of apparent clearance (CL/F) and apparent volume of distribution (VD /F) were 4.02 L/h and 57.8 L, respectively. No covariate significantly changed the parameters of CL/F and VD . The present study may serve as a foundation for individualized therapy and offer a basis for pharmacokinetic-pharmacodynamic (PK-PD) analysis.

Limited sampling strategies to predict the area under the concentration-time curve for rifampicin

BACKGROUND

Rifampicin (RMP) is the most effective first-line antituberculosis drug. One of the most critical aspects of using it in fixed-drug combination formulations is to ensure it reaches therapeutic levels in blood. The determination of the area under the concentration-time curve (AUC) and appropriate dose adjustment of this drug may contribute to optimization of therapy. Even when the maximal concentration (Cmax) of RMP also predicts its sterilizing effect, the time to reach it (Tmax) takes 40 minutes to 6 hours. The aim of this study was to develop a limited sampling strategy (LSS) for therapeutic drug monitoring assistance for RMP.

METHODS

Full concentration-time curves were obtained from 58 patients with tuberculosis (TB) after the oral administration of RMP in fixed-drug combination formulation. A validated high-performance liquid chromatographic method was used. Pharmacokinetic parameters were estimated with a noncompartmental model. Generalized linear models were obtained by forward steps, and bootstrapping was performed to develop LSS to predict AUC curve from time 0 to the last measured at 24 hours postdose (AUC0-24). The predictive performance of the proposed models was assessed using RMP profiles from 25 other TB patients by comparing predicted and observed AUC0-24.

RESULTS

The mean AUC0-24 in the current study was 91.46 ± 36.7 mg·h·L, and the most convenient sampling time points to predict it were 2, 4 and 12 hours postdose (slope [m] = 0.955 ± 0.06; r = 0.92). The mean prediction error was -0.355%, and the root mean square error was 5.6% in the validation group. Alternate LSSs are proposed with 2 of these sampling time points, which also provide good predictions when the 3 most convenient are not feasible.

CONCLUSIONS

The AUC0-24 for RMP in TB patients can be predicted with acceptable precision through a 2- or 3-point sampling strategy, despite wide interindividual variability. These LSSs could be applied in clinical practice to optimize anti-TB therapy based on therapeutic drug monitoring.

Pharmacokinetics of isoniazid, rifampicin, pyrazinamide and ethambutol in Indian children

BACKGROUND

The available pharmacokinetic data on anti-tubercular drugs in children raises the concern of suboptimal plasma concentrations attained when doses extrapolated from adult studies are used. Also, there is lack of consensus regarding the effect of malnutrition on pharmacokinetics of anti-tubercular drugs in children. We conducted this study with the aims of determining the plasma concentrations of isoniazid, rifampicin, pyrazinamide and ethambutol achieved with different dosage of the anti-tubercular drugs so as to provide supportive evidence to the revised dosages and to evaluate the effects of malnutrition on the pharmacokinetics of these drugs in children. We also attempted to correlate the plasma concentrations of these drugs with clinical outcome of therapy.

METHOD

Prospective drug estimation study was conducted in two groups of children, age 6 months to 15 years, with tuberculosis, with or without severe malnutrition, receiving different dosage of daily anti- tubercular therapy. The dosage (range) of isoniazid was 5 (4-6) and 10 (7-15) mg/kg in the two groups, respectively, that of rifampicin-10 (8-12) and 15 (10-12) mg/kg, respectively, both the groups received same dose of pyrazinamide (30-35 mg/kg) and ethambutol (20-25 mg/kg). All four drugs were simultaneously estimated by liquid chromatography-mass spectrometry (LC-MS/MS).

RESULTS AND CONCLUSION

The median (IQR) Cmax of isoniazid increased significantly from 0.6 (0.3,1.2) μg/mL to 3.4 (1.8, 5.0) μg/mL with increase in the dose. Plasma rifampicin concentrations increased only marginally on increasing the dose [median (IQR) Cmax: 10.4 (7.2, 13.9) μg/mL vs. 12.0 (6.1, 24.3) μg/mL, p=0.08]. For ethambutol, 55.9% of the children had inadequate 2-hour concentrations. Two-hour plasma concentrations of at least one drug were low in 59 (92.2%) and 54 (85.7%) children in the two dosing regimen, respectively. We did not observe any effect of malnutrition on pharmacokinetic parameters of the drugs studied. We did not observe an association between low plasma drug concentrations and poor outcome. We may have to be cautious while increasing the doses and strive to asses other factors influencing the drug concentrations and treatment outcomes in children.

Drug-resistant tuberculosis can be predicted by Mycobacterial interspersed repetitive unit locus

It is unknown whether MIRU-VNTR (Mycobacterial Interspersed Repetitive Unit-Variable Number of Tandem Repeat) is associated with drug resistance of Mycobacterium tuberculosis. The purpose of this study was to explore the ability of 24 MIRU loci to predict the drug resistance of Isoniazid (INH), Rifampicin (RFP), Streptomycin (SM), Ethambutol (EMB) and Pyrazinamide (PZA). We collected the drug resistance and MIRU loci information of 109 strains of M. tuberculosis from an open database. The results of multivariate logistic regression showed that the VNTR polymorphism of MTUB04 was related to INH resistance [odds ratio (OR) = 2.82, P = 0.00], RFP resistance (OR = 1.91, P = 0.02), SM resistance (OR = 1.98, P = 0.01) and EMB resistance (OR = 1.95, P = 0.03). MIRU40 was associated with INH resistance (OR = 2.22, P = 0.00). MTUB21 was connected with INH resistance (OR = 1.63, P = 0.02) and SM resistance (OR = 1.69, P = 0.01). MIRU26 was correlated with SM resistance (OR = 1.52, P = 0.04). MIRU39 was associated with EMB resistance (OR = 4.07, P = 0.02). The prediction power of MIRU loci were 0.84, 0.70, 0.85, and 0.74 respectively for INH (predicted by MTUB04, MIRU20, and MTUB21), RFP (predicted by MTUB04), SM (predicted by MTUB21 and MIRU26) and EMB (MTUB04 and MIRU39) through ROC analysis. Our results showed that MIRU loci were related to anti-tuberculosis drug and could predict the drug resistance of tuberculosis.

Pharmacokinetics of rifampicin in Mexican patients with tuberculosis and healthy volunteers

OBJECTIVE

The aim of this study was to compare the pharmacokinetics (PK) of rifampicin (RIF) between healthy volunteers and patients with tuberculosis (TB).

METHODS

RIF was administered as a single 600-mg dose to 24 healthy volunteers and 24 TB patients, followed by serial blood sampling. Plasma concentrations were analysed using a chromatographic method, and the PK parameters were estimated using WinNonlin software.

KEY FINDINGS

Peak plasma concentration ranged from 6.4 to 19.9 mg/l, which was subtherapeutic for 15% of the study participants in both groups, mostly in men (71.4%). The mean area under the concentration-time curve (AUC0-24h ) did not show differences between these groups (P > 0.05). The absorption rate was slower in TB patients and the volume of distribution normalized by total body weight (Vd/kg) was greater than healthy volunteers (P < 0.05). A greater Vd and clearance were found in male subjects. The lag time (tlag) and the time before reach Cmax (Tmax) were longer for female TB patients (P < 0.05).

CONCLUSION

The main differences in PK parameters of RIF between Mexican TB patients and healthy volunteers were demonstrated in absorption and distribution processes. In addition, differences in PK parameters observed by sex should be considered for further dosing recommendations.

Population pharmacokinetics of rifampicin in Mexican patients with tuberculosis

WHAT IS KNOWN AND OBJECTIVE

Rifampicin (RIF) shows wide variability in its pharmacokinetics. The purpose of this study was to develop and validate a population pharmacokinetic model to characterize the inter- and intra-individual variability in pharmacokinetic parameters of RIF in Mexican patients.

METHODS

Ninety-four patients receiving antituberculosis therapy participated in this prospective study. Plasma concentration-time data were described using a one-compartment model with lag time, absorption and first-order elimination. The potential influence of demographic and clinical characteristics of the patients, and the pharmaceutical formulation (A, B, C and D) on the pharmacokinetics parameters, was evaluated by non-linear mixed-effect modelling (nonmem). Seventy-seven additional patients participated in the validation of the model.

RESULTS AND DISCUSSION

The final population pharmacokinetic model obtained was as follows: apparent clearance CL/F = 8·17 L/h (1·40 as high for males), apparent distribution volume V(d)/F = 50·1 L (1·29 as high for males), absorption rate constant K(aA) = 0·391/h, K(aB,C,D) = 2·70/h, relative bioavailability F(A) = 0·468, F(B,C,D) = 1, lag time in the absorption phase T(lag) = 0·264 h. The final model improved the precision on the parameter estimates (CL/F, V(d) /F and K(a) by 31·9%, 16·7% and 92·9%, respectively). The residual variability was 27·3%.

WHAT IS NEW AND CONCLUSION

Gender was associated with changes in CL/F and V(d) /F whereas the pharmaceutical formulation was associated with changes in F and altered the K(a) . The validation data set showed that the model could be used in clinical practice for Bayesian dose adjustment of RIF in TB patients.

The pharmacokinetics and pharmacodynamics of rifampicin in adults and children in relation to the dosage recommended for children

The dosages of antituberculosis agents recommended for treatment of childhood tuberculosis often reflect those for adult patients with similar mg/kg body weight dosages and ranges advised. Literature relating to the pharmacokinetics and pharmacodynamics of rifampicin (RMP) is reviewed and the serum concentrations reached by adults, both patients and healthy volunteers and children, established or not established on RMP, compared. Straight line regression of maximum RMP serum concentrations (C(max)) on dosage, weighted for the number of individuals, found slopes (SE) of 1.025 (0.067) and 0.881 (0.046) respectively for adult volunteers not established and established on RMP (P = 0.076), and similarly 0.748 (0.057) and 0.684 (0.038) respectively for adult patients (P < 0.001) and 0.622 (0.050) and 0.368 (0.041) respectively for children (P < 0.001). These results indicate that for equivalent RMP dosages adult patients reach a lower C(max) than adult volunteers and that adults, both volunteers and patients established on RMP reach higher C(max) values than children; children established on RMP require approximately twice the mg/kg body weight dosage of RMP to reach serum concentrations equivalent to those of adults. It is noteworthy that many adult patients receiving currently recommended RMP dosages also do not reach the often recommended RMP 2 h serum concentration of 8 μg/mL.

Chemotherapeutic Efficacy of Nanoparticle Encapsulated Antitubercular Drugs

Our objective was to evaluate the chemotherapeutic potential of oral poly lactide-co-glycolide (PLG, a synthetic polymer) nanoparticle encapsulated ethambutol in combination with PLG-nanoparticle encapsulated-(rifampicin + isoniazid + pyrazinamide) in a murine tuberculosis (TB) model. Our formulation was prepared by the multiple emulsion technique and administered orally to mice for the biodistribution, pharmacokinetic, and chemotherapeutic studies. A single oral administration of the formulation to mice could maintain sustained drug levels in the plasma for 5 days and in the organs (lungs, liver, spleen) for 7-9 days. There was a striking improvement in the pharmacokinetic parameters such as half-life and mean residence time as compared with free drugs. The relative/absolute bioavailability of the 4 antituberculosis drugs was enhanced several fold. Repeated administration of the formulation did not produce any hepatotoxicity as assessed on a biochemical basis. In M. tuberculosis H37Rv infected mice, just 3 oral doses of the 4-drug formulation administered at every 10th day resulted in undetectable bacilli in the organs replacing 28 conventional doses of free drugs. We concluded that polymeric nanoparticle based oral 4-drug combination bears significant potential to shorten the duration of TB chemotherapy, besides reducing the dosing frequency.

Evaluation of the modulation of P-glycoprotein (P-gp) on the intraocular disposition of its substrate in rabbits

PURPOSE

To evaluate the functional role of P-gp and ocular tissue distribution of intravitreally injected Rhodamine-123 (Rho-123) in the presence of P-gp specific blocker (GF 120918) in normal as well as rifampicin-fed rabbits using microdialysis and direct sampling technique.

METHODS

Intravitreal pharmacokinetics of Rho-123 were conducted in male New Zealand albino rabbits. Direct sampling and microdialysis were employed to study the disposition of Rho-123 in normal as well as rifampicin-fed conditions. Control animals received Rho-123 at the concentration of 350 ng in PBS (0.05 ml) intravitreally, and the blocker-treated group received GF 120918 intravenously at the dose of 3.5 mg/kg 30 min prior to intravitreal injection of Rho-123. In case of direct sampling, four eyes were enucleated at different time points, and ocular tissues and humors were stored at -86 degrees C until analysis by HPLC with fluorescence detection.

RESULTS

In direct sampling, the blocker group showed significant increase (2.6 fold) in the mean vitreous concentration of Rho-123. Other tissues like ret-choroid, iris, and cornea also showed significant increase in their mean concentration. Microdialysis did not significantly predict the changes observed with direct sampling. Rifampicin-fed rabbits showed a vitreous pharmacokinetic profile comparable with non-fed (control) animals, and the pharmacokinetic parameters were unaffected by the blocker pretreatment.

CONCLUSION

Intravenously injected blocker significantly altered the ocular disposition of intravitreally injected P-gp substrate. Rifampicin pretreatment did not upregulate P-gp transporters of the retina to the extent to affect the intravitreal kinetics of Rho-123 significantly.

Evidence of Efflux-Mediated and Saturable Absorption of Rifampicin in Rat Intestine Using the Ligated Loop and Everted Gut Sac Techniques

This study was carried out to explore whether efflux-mediated and saturable mechanisms play any role toward poor and variable intestinal absorption of rifampicin. In situ segmental permeability of rifampicin at various residence times was determined in rat gastrointestinal tract using the ligated loop technique. The involvement of efflux-mediated and saturable absorption of rifampicin was studied in rat intestine using the everted sac method. The samples were analyzed by a validated HPLC method. Rifampicin showed decreased permeability in jejunum and ileum with an increase in residence time. The permeation of rifampicin from the serosal to the mucosal side (secretion) was significantly higher than permeation from the mucosal to the serosal side (absorption) of jejunum and ileum. This indicated the involvement of efflux-mediated transport. Addition of verapamil, an inhibitor for P-glycoprotein (Pgp), multidrug resistance associated protein-2 (MRP-2), and other related transporters, increased absorption of rifampicin in jejunum and ileum by 2-3-fold and decreased secretion by almost 4-fold. The permeation rate (flux) of rifampicin through duodenum increased with concentration up to 300 microg/mL, becoming constant thereafter, indicating the existence of saturable absorption. There was no saturable permeation in jejunum and ileum. Thus the present study indicates the involvement of efflux-mediated and saturable absorption mechanisms of rifampicin in rat intestine, which act as barriers to the absorption of the drug. This explains the drug's poor absolute bioavailability. As Pgp varies from person to person to an extent of 2-8-fold, it can be one direct reason for the interindividual variable bioavailability shown by rifampicin.