Using therapeutic drug monitoring to dose the antimycobacterial drugs

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.

Pharmacokinetics of oral antimycobacterials and dosing guidance for Mycobacterium avium complex treatment in cystic fibrosis

BACKGROUND

Treatment failure of Mycobacterium avium complex (MAC) pulmonary disease occurs in about 30% of people with cystic fibrosis (CF) and may be a result of abnormal drug concentrations.

METHODS

Prospective, cross-over, single-dose PK study of 20 pancreatic insufficient individuals with CF and 10 healthy controls (HC). CF subjects received simultaneous doses of oral azithromycin, ethambutol, and rifampin in the fasting state and with food and pancreatic enzymes, separated by two weeks. HC received fasting doses only. A non-compartmental model was used to estimate PK parameters of drugs and metabolites.

RESULTS

Azithromycin maximum concentration (C_max ) was higher and rifampin C_max was lower in fasting CF subjects compared to HC, while other PK measures, including those for ethambutol, were similar. Addition of food and enzymes did not improve the C_max of the antimycobacterial drugs. Nineteen of 20 CF subjects had one or more abnormal C_max z-scores in either the fasting or fed state (or both), when compared to HC.

CONCLUSION

PK profiles of azithromycin and ethambutol were similar between CF and HC, except azithromycin C_max was slightly higher in people with CF after a single dose. Rifampin PK parameters were altered in persons with CF. Addition of food and enzymes in CF subjects did not improve PK parameters. Standard dosing guidelines should be used as a starting point for people with CF initiating MAC therapy and therapeutic drug monitoring should be routinely performed to prevent the possibility of treatment failure due to abnormal drug concentrations.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT02372383 Prior abstract publication: 1. Martiniano S, Wagner B, Brennan L, Wempe M, Anderson P, Nick J, Sagel S. Pharmacokinetics of oral MAC antibiotics in cystic fibrosis. Am J Resp Crit Care Med A4842-A4842, 2017. 2. Martiniano SL, Wagner BD, Brennan L, Wempe MF, Anderson PL, Nick JA, Sagel SD. Pharmacokinetics of oral MAC antibiotics in cystic fibrosis. J Cyst Fibros 16: S52-53, 2017.

Development of a limited sampling strategy for the estimation of isoniazid exposure considering N-acetyltransferase 2 genotypes in Korean patients with tuberculosis

A limited sampling strategy (LSS) to estimate the exposure to isoniazid was developed considering N-acetyltransferase 2 (NAT2) genotypes in Korean patients with tuberculosis. The influence of the genotypes on the pharmacokinetics of isoniazid was also evaluated. A total of 33 participants participated in the study and received isoniazid 300 mg once daily. Evaluable participants consist of ten slow (SA), fourteen intermediate (IA) and six rapid acetylators (RA). As expected, isoniazid exposure was higher (mean AUC, 28.4 versus 7.6 mg*h/L) and systemic clearance lower (mean apparent clearance, 14.8 versus 50.6 L/h) in SAs than RAs. The formulas to estimate isoniazid exposure were constructed using one or more concentration-time points that correlate with the area under the concentration-time curve (AUC). The LSS using a formula of single concentration-time point at 4 h post dose (C₄) is applicable for all acetylators to the therapeutic drug monitoring (TDM) of isoniazid in patients with tuberculosis when evaluated using the Deming regression and Bland-Altman plot (AUC = 1.53 + 10.03*C₄, adjusted r² = 0.95, p < 0.001). Considering that SAs are more prone to adverse effects, pre-dose NAT2 genotyping would be valuable for optimal isoniazid dosing in conjunction with TDM.

Intra-individual effects of food upon the pharmacokinetics of rifampicin and isoniazid

Background

Poor response to TB therapy might be attributable to subtherapeutic levels in drug-compliant patients. Pharmacokinetic parameters can be affected by comorbidities or the interaction of drugs with food.

Objectives

This study aimed to determine the effect of food intake upon pharmacokinetics of rifampicin and isoniazid in a Peruvian population with TB.

Methods

Rifampicin and isoniazid levels were analysed at 2, 4 and 6 h after drug intake in both fasting and non-fasting states using LC-MS methods.

Results

Sixty patients participated in the study. The median rifampicin Cmax and AUC0-6 were higher during fasting than non-fasting: 7.02 versus 6.59 mg/L (P = 0.054) and 28.64 versus 24.31 mg·h/L (P = 0.002). There was a statistically significant delay overall of non-fasting Tmax compared with the fasting state Tmax (P = 0.005). In the multivariate analysis, besides the effect of fasting, Cmax for females was 20% higher than for males (P = 0.03). Concerning isoniazid, there were significant differences in the Cmax during non-fasting (median = 3.51 mg/L) compared with fasting (4.54 mg/L). The isoniazid dose received had an effect upon the isoniazid levels (1.26, P = 0.038). In the multivariate analysis, isoniazid exposure during fasting was found to be 14% higher than during non-fasting (CI = 1.02-1.28, P < 0.001). Neither radiological extent of the disease nor consumption of food with drug intake nor pharmacokinetics of rifampicin or isoniazid was associated with a poorer treatment outcome.

Conclusions

Rifampicin in particular and isoniazid pharmacokinetics were significantly affected by the intake of the drug with food between and within individuals.

Encapsulating Rifampicin into SLNs: A Viable Option for Managing its Bioavailability Issues Upon Co-Delivery with Isoniazid

BACKGROUND

Rifampicin is known to degrade at the acidic pH of the stomach, especially in the presence of isoniazid. Although isoniazid also degrades partially, its degradation is reversible.

OBJECTIVE

Presently, we provide a proof of the fact that the simultaneous oral administration of rifampicin (RIF), upon incorporation into solid lipid nanoparticles (RIF-SLNs), with isoniazid (INH) overcomes its INH-induced degradation and improves its oral bioavailability in rats.

METHODS

Solid lipid nanoparticles of RIF (RIF-SLNs) were prepared using a novel and patented method. The effect of INH was investigated on in vivo bioavailability of RIF both in its free and encapsulated (RIF-SLNs) form, after oral administration to rats.

RESULTS

C_max and AUC_0-∞ of RIF increased 158 % and 125 %, respectively, upon incorporation into SLNs versus free RIF when combined with INH. The T_max decreased from 5.67 h to 3.3 h, and the plasma concentration of RIF remained above its MIC (8 μg/ml) at all the tested time points starting with 15 min, when administered as RIF-SLNs in combination with INH.

CONCLUSION

The results confirm the scope of combining RIF-SLNs with INH to overcome the bioavailability of free RIF when combined with INH, especially in fixed dose combinations.

Recommendations for Optimizing Tuberculosis Treatment: Therapeutic Drug Monitoring, Pharmacogenetics, and Nutritional Status Considerations

Although tuberculosis is largely a curable disease, it remains a major cause of morbidity and mortality worldwide. Although the standard 6-month treatment regimen is highly effective for drug-susceptible tuberculosis, the use of multiple drugs over long periods of time can cause frequent adverse drug reactions. In addition, some patients with drug-susceptible tuberculosis do not respond adequately to treatment and develop treatment failure and drug resistance. Response to tuberculosis treatment could be affected by multiple factors associated with the host-pathogen interaction including genetic factors and the nutritional status of the host. These factors should be considered for effective tuberculosis control. Therefore, therapeutic drug monitoring (TDM), which is individualized drug dosing guided by serum drug concentrations during treatment, and pharmacogenetics-based personalized dosing guidelines of anti-tuberculosis drugs could reduce the incidence of adverse drug reactions and increase the likelihood of successful treatment outcomes. Moreover, assessment and management of comorbid conditions including nutritional status could improve anti-tuberculosis treatment response.

Peak Plasma Concentration of Azithromycin and Treatment Responses in Mycobacterium avium Complex Lung Disease

Macrolides, such as azithromycin (AZM) and clarithromycin, are the cornerstones of treatment for Mycobacterium avium complex lung disease (MAC-LD). Current guidelines recommend daily therapy with AZM for cavitary MAC-LD and intermittent therapy for noncavitary MAC-LD, but the effectiveness of these regimens has not been thoroughly investigated. This study evaluated associations between microbiological response and estimated peak plasma concentrations (Cmax) of AZM. The AZM Cmax was measured in patients receiving daily therapy (250 mg of AZM daily, n = 77) or intermittent therapy (500 mg of AZM three times weekly, n = 89) for MAC-LD and daily therapy for Mycobacterium abscessus complex LD (MABC-LD) (250 mg of AZM daily, n = 55). The AZM Cmax was lower with the daily regimen for MAC-LD (median, 0.24 μg/ml) than with the intermittent regimen for MAC-LD (median, 0.65 μg/ml; P < 0.001) or daily therapy for MABC-LD (median, 0.53 μg/ml; P < 0.001). After adjusting for confounding factors, AZM Cmax was independently associated with favorable microbiological responses in MAC-LD patients receiving a daily regimen (adjusted odds ratio [aOR], 1.58; 95% confidence interval [CI], 1.01 to 2.48; P = 0.044) but not an intermittent regimen (aOR, 0.85; 95% CI, 0.58 to 1.23, P = 0.379). With the daily AZM-based multidrug regimen for MAC-LD, a low AZM Cmax was common, whereas a higher AZM Cmax was associated with favorable microbiologic responses. The results also suggested that the addition of rifampin may lower AZM Cmax When a daily AZM-based multidrug regimen is used for treating severe MAC-LD, such as cavitary disease, the currently recommended AZM dose might be suboptimal. (This study has been registered at ClinicalTrials.gov under identifier NCT00970801.).

Review of evidence for measuring drug concentrations of first-line antitubercular agents in adults

Measurement of drug concentrations and performing therapeutic drug monitoring (TDM) are widely used to optimize efficacy and safety of many commonly used drugs today. Although TDM of first-line antitubercular drugs is used during the treatment of tuberculosis, the extent of any benefit achieved is currently unknown. This review summarizes the available literature describing TDM of first-line treatment agents in patients with tuberculosis and describes clinical associations with achievement of target drug concentrations, including data from special populations. A literature review was conducted for articles describing drug concentration and TDM outcomes for first-line tuberculosis agents in adults. A total of 40 studies were included in the review. Studies were a mixture of controlled trials, observational studies, cross-sectional studies, and case reports. The majority of the studies showed standard dosing does not consistently achieve target concentrations for the first-line antitubercular drugs; however, the clinical implications of this finding are still unclear. Presence of HIV and diabetes mellitus appeared to indicate achievement of lower than target concentrations and this warrants further study in prospective studies. Current published data neither prove nor disprove the utility of TDM for general tuberculosis populations but evidence does not currently support routine measurement of drug concentrations.

Non-adherence and drug-related interruptions are risk factors for delays in completion of treatment for tuberculosis

SETTING

A key program performance objective established by the Centers for Disease Control and Prevention (CDC) is that ≥93% of tuberculosis (TB) cases complete treatment within 12 months.

OBJECTIVE

To determine the rate of and risk factors for delay in anti-tuberculosis treatment completion.

DESIGN

Nested case-control study among TB cases reported to the Tennessee Department of Health between 1 January 2000 and 31 December 2010. Time to complete treatment was calculated using treatment start and stop dates documented in the Tuberculosis Information Management System (TIMS).

RESULTS

Of 2627 cases, 261 (9.9%) required >12 months to complete treatment. In adjusted conditional logistic regression analyses, cavitary disease and positive cultures after 2 months of therapy (OR 5.85, 95%CI 1.98-17.32, P = 0.001), non-adherence (OR 4.13, 95%CI 1.76-9.72, P < 0.001), and interruptions in treatment due to drug-related issues (OR 6.91, 95%CI 3.76-12.70, P < 0.001) were independently associated with delay in completion of TB treatment.

CONCLUSION

From 2000 to 2010, the proportion of TB cases completing treatment within 12 months increased from 84.6% to 94.9%, and remained above the CDC target during 2009-2010. Efforts to improve patient adherence and reduce interruptions in treatment due to anti-tuberculosis drug-related issues could improve the proportion of TB cases completing treatment within 12 months.

Pharmacokinetics of rifampin in Peruvian tuberculosis patients with and without comorbid diabetes or HIV

For drug-compliant patients, poor responses to tuberculosis (TB) treatment might be attributable to subtherapeutic drug concentrations. An impaired absorption of rifampin was previously reported for patients with diabetes mellitus (DM) or HIV. The objectives of this study were to determine whether TB drug pharmacokinetics differed in Peruvian TB patients with DM or HIV. In this cross-sectional study, TB patients, recruited from health centers in Lima, Peru, had blood samples taken at 2 and 6 h after directly observed TB drug ingestion, to determine plasma concentrations of rifampin. Of 105 patients, 50 had TB without a comorbidity, 26 had coexistent DM, and 29 had coexistent HIV. Unexpectedly, the overall median 2- and 6-h levels of rifampin were 1.6 and 3.2 mg/liter, respectively, and the time to the peak concentration was 6 h (slow absorber) instead of 2 h (fast absorber) for 61 patients (62.2%). The geometric mean peak concentration of drug in serum (C(max)) was significantly higher in fast absorbers than in slow absorbers (5.0 versus 3.8 mg/liter; P = 0.05). The rifampin C(max) was significantly lower in male patients than in female patients (3.3 versus 6.3 mg/liter; P < 0.001). Neither slow nor fast absorbers with comorbidities (DM or HIV) had significantly different C(max) results compared to those of TB patients without comorbidities. An analysis of variance regression analysis showed that female gender (P < 0.001) and the time to maximum concentration of drug in serum (T(max)) at 2 h (P = 0.012) were independently correlated with increased exposure to rifampin. Most of this Peruvian study population exhibited rifampin pharmacokinetics different from those conventionally reported, with delayed absorption and low plasma concentrations, independent of the presence of an HIV or DM comorbidity.

Clinical pharmacology and lesion penetrating properties of second- and third-line antituberculous agents used in the management of multidrug-resistant (MDR) and extensively-drug resistant (XDR) tuberculosis

Failure of first-line chemotherapy to cure tuberculosis (TB) patients occurs, in part, because of the development of resistance to isoniazid (INH) and rifampicin (RIF) the two most sterilizing agents in the four-drug regimen used to treat primary infections. Strains resistant to both INH and RIF are termed multidrug-resistant (MDR). Treatment options for MDR patients involve a complex array of twenty different drugs only two classes of which are considered to be highly effective (fluoroquinolones and aminoglycosides). Resistance to these two classes results in strains known as extensively drug-resistant (XDR) and these types of infections are becoming increasingly common. Many of the remaining agents have poorly defined pharmacology but nonetheless are widely used in the treatment of this disease. Several of these agents are known to have highly variable exposures in healthy volunteers and little is known in the patients in which they must be used. Therapeutic drug monitoring (TDM) is infrequently used in the management of MDR or XDR disease yet the clinical pharmacokinetic studies that have been done suggest this might have a large impact on disease outcome. We review what is known about the pharmacologic properties of each of the major classes of second- and third-line antituberculosis agents and suggest where judicious use of TDM would have the maximum possible impact. We summarize the state of knowledge of drug-drug interactions (DDI) in these classes of agents and those that are currently in clinical trials. Finally we consider what little is known about the ability of TB drugs to reach their ultimate site of action--the interior of a granuloma by penetrating the diseased lung area. Careful consideration of the pharmacology of these agents is essential if we are to avoid further fueling the growing epidemic of highly drug-resistant TB and critical in the development of new antituberculosis drugs.

Treatment of active pulmonary tuberculosis in adults: current standards and recent advances. Insights from the Society of Infectious Diseases Pharmacists

Tuberculosis is a global pandemic, with 9 million new cases of the disease and approximately 2 million deaths each year. More than 98% of patients treated for tuberculosis in the United States between 1993 and 2007 had drug-susceptible strains. The standard treatment regimen for drug-susceptible tuberculosis has not changed in decades and was developed on the basis of empiric observations of different treatment regimens. Only recently has the veracity of the scientific basis for standard therapy been examined. The backbone of therapy is still isoniazid, rifampin, and pyrazinamide, although fluoroquinolones are being investigated as a replacement for isoniazid. Recent population pharmacokinetic studies have demonstrated the importance of individualized dosing of isoniazid, pyrazinamide, and rifampin. Isoniazid serum clearance differs depending on the patient's number of N-acetyltransferase 2 gene *4 (NAT2*4) alleles. Pyrazinamide serum clearance has been shown to increase with increases in body weight. Rifampin's volume of distribution, clearance, and absorption have wide between-patient and within-patient variability. Microbial pharmacokinetic-pharmacodynamic (PK-PD) indexes and targets to optimize microbial killing and minimize resistance have been identified for rifampin, isoniazid, pyrazinamide, and the fluoroquinolones. These PK-PD indexes suggest that different doses and dosing schedules than those currently recommended could optimize therapy and perhaps shorten duration of therapy. Efflux pump inhibition is also being investigated to enhance first-line antituberculosis drug therapy. Comorbid conditions such as diabetes mellitus and genetically determined iron overload syndromes have been associated with significantly worse patient outcomes. Therapy for these and other patient groups needs further improvement. These patient factors, the covariates for pharmacokinetic variability, and PK-PD factors suggest the need to individualize therapy for patients with tuberculosis in order to optimize outcomes and reduce the duration of therapy.

Therapeutic drug monitoring of antimycobacterial drugs in patients with both tuberculosis and advanced human immunodeficiency virus infection

STUDY OBJECTIVE

To determine the feasibility of therapeutic drug monitoring for adjusting low serum antimycobacterial concentrations in patients with both tuberculosis and advanced human immunodeficiency virus (HIV).

DESIGN

Retrospective cohort study.

DATA SOURCE

De-identified dataset from a tuberculosis clinic.

PATIENTS

Twenty-one patients (median age 38 yrs, range 25-68 yrs) with advanced HIV infection (CD4(+) cell count < 100 cells/mm(3)) who received treatment for active tuberculosis between March 2002 and September 2007.

MEASUREMENTS AND MAIN RESULTS

We evaluated data based on the practices performed at the tuberculosis clinic. After the daily doses of isoniazid and rifamycins (rifampin or rifabutin) were ingested, serum concentrations were obtained at 2 hours for isoniazid and rifampin, at 3 hours for rifabutin, and, when possible, at 6 hours for all three drugs to detect delayed absorption. Antimycobacterial drug concentrations were compared with published reference levels, and dosages were adjusted to achieve desired concentrations. Costs of monitoring were recorded for all patients. Of the 21 patients, 18 (86%) had low serum concentrations of at least one drug 2 hours after ingestion: 2 (10%) had low isoniazid concentrations, 5 (24%) had low rifamycin concentrations, and 11 (52%) had low serum concentrations of both drugs. The median number of dosage adjustments to attain normal concentrations was 1 (range 0-4 adjustments). The median cost/patient for therapeutic drug monitoring was $619 (range $230-1948). The median final doses to achieve normal concentrations were isoniazid 600 mg/day (range 300-1500 mg/day), rifampin 1050 mg/day (range 600-1200 mg/day), and rifabutin 300 mg (range 150-450 mg) 3 times/week. No patient demonstrated any adverse effects attributed to these higher doses.

CONCLUSION

Low serum concentrations of antituberculous drugs, which suggest malabsorption, are common among patients with advanced HIV who also have tuberculosis but can be overcome with higher doses. Therapeutic drug monitoring may be an effective tool to optimize therapy, but needs further study.

Evolution of central nervous system multidrug-resistant Mycobacterium tuberculosis and late relapse of cryptic prosthetic hip joint tuberculosis: complications during treatment of disseminated isoniazid-resistant tuberculosis in an immunocompromised host

We report a case of disseminated isoniazid-resistant tuberculosis in an immunocompromised patient with evolution of rifampin (rifampicin) resistance in the central nervous system. This was cured with intraventricular and oral treatment but was followed by a late relapse of the original infection in a prosthetic hip joint. We provide drug levels in cerebrospinal fluid and serum.

A microbiological assessment of novel nitrofuranylamides as anti-tuberculosis agents

OBJECTIVES

Nitrofuranylamides (NFAs) are nitroaromatic compounds that have recently been discovered and have potent anti-tuberculosis (TB) activity. A foundational study was performed to evaluate whether this class of agents possesses microbiological properties suitable for future antimycobacterial therapy.

METHODS

Five representative compounds of the NFA series were evaluated by standard microbiological assays to determine MICs, MBCs, activity against anaerobic non-replicating persistent Mycobacterium tuberculosis, post-antibiotic effects (PAEs), antibiotic synergy and the basis for resistance.

RESULTS

The antimicrobial activity of these compounds was restricted to bacteria of the M. tuberculosis complex, and all compounds were highly active against drug-susceptible and -resistant strains of M. tuberculosis, with MICs 0.0004-0.05 mg/L. Moreover, no antagonism was observed with front-line anti-TB drugs. Activity was also retained against dormant bacilli in two in vitro low-oxygen models for M. tuberculosis persistence. A long PAE was observed, which was comparable to that of rifampicin, but superior to isoniazid and ethambutol. Spontaneous NFA-resistant mutants arose at a frequency of 10(-5)-10(-7), comparable to that for isoniazid (10(-5)-10(-6)). Some of these mutants exhibited cross-resistance to one or both of the nitroimidazoles PA-824 and OPC-67683. Cross-resistance was associated with inactivation of the reduced F(420)-deazaflavin cofactor pathway and not with inactivation of the Rv3547, the nitroreductase for PA-824 and OPC-67683.

CONCLUSIONS

Based on these studies, NFAs have many useful antimycobacterial properties applicable to TB chemotherapy and probably possess a unique mode of action that results in good activity against active and dormant M. tuberculosis. Therefore, the further development of lead compounds in this series is warranted.

Determinants of rifampin, isoniazid, pyrazinamide, and ethambutol pharmacokinetics in a cohort of tuberculosis patients

Evaluation of sources of pharmacokinetic variation can facilitate optimization of tuberculosis treatment regimens by identification of avoidable sources of variation and of risk factors for low or high drug concentrations in patients. Our objective was to describe the pharmacokinetics of rifampin, isoniazid, pyrazinamide, and ethambutol in a cohort of tuberculosis patients established on first-line treatment regimens and to evaluate the determinants of pharmacokinetic variation. Plasma concentration-time profiles were determined for each of the drugs in 142 patients with drug-sensitive pulmonary tuberculosis after 2 months of daily treatment in hospital. Pharmacokinetic measures were described by noncompartmental analysis. Multiple linear regression was used to evaluate the patient and the treatment factors associated with variation of the area under the concentration-time curve from 0 to 8 h. Several factors independently associated with variations in antituberculosis drug concentrations were identified: human immunodeficiency virus infection was associated with 39% and 27% reductions for rifampin and ethambutol, respectively; formulation factors were determinants of rifampin and isoniazid bioavailability; female patients had increased rifampin and isoniazid concentrations but reduced ethambutol concentrations; older patients had higher levels of isoniazid and ethambutol; patients with a history of previous antituberculosis treatment had lower ethambutol concentrations; and the dose per kilogram of body weight was associated with the concentrations of all four agents. Further studies are required to assess the implications of variations in antituberculosis drug concentrations for efficacy and safety before decisions are made to change the dosing strategy in patients at risk.

Population pharmacokinetics of isoniazid in the treatment of Mycobacterium tuberculosis among Asian and African elephants (Elephas maximus and Loxodonta africana)

We recently described the clinical presentation and treatment of 18 elephants from six herds infected with TB. Treatment protocols and methods varied between herds to include both oral and rectal dosing using multiple drug doses and formulations. In this paper we present information regarding the pharmacokinetics (PK) of isoniazid (INH) in elephants and provide suggestions regarding initial treatment regimens. Forty-one elephants received INH daily by either oral or rectal administration with different formulations. Population PK analysis was performed using Non-linear Mixed Effect Modeling (NONMEM). Results of oral administration indicated that compared with premixed INH solution, the drug exposure was highest with a suspension prepared freshly with INH powder. When INH was concomitantly given as an admixture over food, Tmax was delayed and variability in drug absorption was significantly increased. Compared with oral administration, similar drug exposures were found when INH was dosed rectally. The data generated suggest that a starting dose of 7.5 mg/kg of INH is appropriate for initial TB treatment in elephants when premixed solution is administered directly into the oropharynx or rectal vault and 4 mg/kg are when INH is administered following immediate suspension from powdered form.

Use of therapeutic drug monitoring for multidrug-resistant tuberculosis patients

STUDY OBJECTIVES

Therapeutic drug monitoring (TDM) is the process of obtaining the serum concentration of a medication and modifying the dose based on the results. Little is known about the application of TDM in the treatment of patients with multidrug-resistant (MDR) tuberculosis (TB) in clinical practice. This study characterized how TDM was applied in the management of MDR TB patients, and examined the clinical indications for ordering TDM, the process for obtaining drug concentrations, and the clinician response to the drug concentrations.

DESIGN

In a retrospective study, we compared the clinical and demographic characteristics of MDR TB patients who received TDM with those who did not. The clinical application of TDM also was described in patients who received TDM.

SETTING

A municipal TB control program.

PATIENTS OR PARTICIPANTS

Patients in whom TB was diagnosed that was caused by an isolate resistant to at least isoniazid and rifampin, and who received treatment for TB in one of the health department chest clinics between July 1, 1993, and August 31, 1997, were studied.

RESULTS

Forty-nine patients receiving TDM had a longer time to culture conversion and treatment duration, more pulmonary TB in combination with an extrapulmonary site, drug resistance, and visits to the health department clinics (p < 0.05) than the 60 patients without TDM. Of the 49 patients who had initial TDM, 73.5% of them had the reason for being tested specified. A total of 85.7% of initial TDM results were collected at the appropriate time of blood sampling. Clinician response to TDM results varied with the drug that was being tested.

CONCLUSIONS

The use of TDM depended largely on the patient's clinical presentation. Site-specific guidelines on the use of TDM for managing TB patients may maximize the benefit of TDM.

In vitro postantibiotic effects of rifapentine, isoniazid, and moxifloxacin against Mycobacterium tuberculosis

Postantibiotic effects (PAEs) of rifapentine, isoniazid, and moxifloxacin against Mycobacterium tuberculosis ATCC 27294 were studied using a radiometric culture system. Rifapentine at 20 mg/liter gave the longest PAE (104 h) among the drugs used alone. The combinations of rifapentine plus isoniazid, rifapentine plus moxifloxacin, and isoniazid plus moxifloxacin gave PAEs of 136.5, 59.0, and 8.3 h, respectively.

Malabsorption of rifampin and isoniazid in HIV-infected patients with and without tuberculosis

The absorption of rifampin, isoniazid, and D-xylose in patients with human immunodeficiency virus (HIV) infection and diarrhea, in patients with HIV infection and tuberculosis (TB), in patients with pulmonary TB alone, and in healthy subjects was studied. Percentage of dose of the drugs, their metabolites, and D-xylose excreted in urine were calculated. A significant reduction in the absorption of drugs and D-xylose in both the HIV infection/diarrhea and HIV infection/TB groups was observed (P<.05), and the correlation between them was significant. Our results indicate that patients with HIV infection and diarrhea and those with HIV infection and TB have malabsorption of rifampin and isoniazid.

Therapeutic drug monitoring in the treatment of tuberculosis

Therapeutic drug monitoring (TDM) is a standard clinical technique used for many disease states, including many infectious diseases. As for these other conditions, the use of TDM in the setting of tuberculosis (TB) allows the clinician to make informed decisions regarding the timely adjustment of drug therapy. Such adjustments may not be required for otherwise healthy individuals who are responding to the standard, four-drug TB regimens. However, some patients are slow to respond to treatment, have drug-resistant TB, are at risk of drug-drug interactions or have concurrent disease states that significantly complicate the clinical situation. Such patients may benefit from TDM and early interventions may preclude the development of further drug resistance. It is not possible to collect multiple blood samples in the clinical setting for logistical and financial reasons. Therefore, one typically is limited to one or two time points. When only one sample can be obtained, the 2-hour post-dose concentrations of isoniazid, rifampin, pyrazinamide and ethambutol are usually most informative. Unfortunately, low 2-hour values do not distinguish between delayed absorption (late peak, close to normal range) and malabsorption (low concentrations at all time points). A second sample, often collected at 6-hour post-dose, can differentiate between these two scenarios. The second time point can also provide some information about clearance and half-life, assuming that drug absorption was nearly completed by 2 hours. TDM requires that samples are promptly centrifuged, and that the serum is promptly harvested and frozen. Isoniazid and ethionamide, in particular, are not stable in human serum at room temperature. Rifampin is stable for more than 6 hours under these conditions. During TB treatment, isoniazid causes the greatest early reduction in organisms and is considered to be one of the two most important TB drugs, along with rifampin. Although isoniazid is highly active against TB, low isoniazid concentrations were associated with poorer clinical and bacteriological outcomes in US Public Health Services (USPHS) TB Trial 22. Several earlier trials showed a clear dose-response for rifampin and pyrazinamide, so low concentrations for those two drugs also may correlate with poorer treatment outcomes. At least in USPHS TB Trial 22, the rifampin pharmacokinetic parameters were not predictive of the outcome variables. In contrast, low concentrations of unbound rifapentine may have been responsible, in part, for the worse-than-anticipated performance of this drug in clinical trials. The 'second-line' TB drugs, including p-aminosalicylic acid, cycloserine and ethionamide, are relatively weak TB drugs. Under the best conditions, treatment with these drugs takes over 2 years, as opposed to 6 to 9 months with isoniazid- and rifampin-containing regimens. Therefore, TB centres such as National Jewish Medical and Research Center in Denver, CO, USA, measure serum concentrations of the 'second-line' TB drugs early in the course of treatment. That way, poor drug absorption can be dealt with in a timely manner. This helps to minimise the time that patients are sputum smear- and culture-positive with multidrug-resistant TB, and may prevent the need for even longer treatment durations. Patients with HIV are at particular risk for drug-drug interactions. Because the published guidelines typically reflect interactions only between two drugs, these guidelines are of limited value when the patient is treated with three or more interacting drugs. Under such complicated circumstances, TDM often is the best available tool for sorting out these interactions and placing the patient the necessary doses that they require. TDM is only one part of the care of patients with TB. In isolation, it is of limited value. However, combined with clinical and bacteriological data, it can be a decisive tool, allowing the clinician to successfully treat even the most complicated TB patients.

Population pharmacokinetics of ethionamide in patients with tuberculosis

SETTING

Three US referral hospitals.

OBJECTIVE

Determine the population pharmacokinetic (PK) parameters of ethionamide (ETA) following multiple oral doses.

DESIGN

Fifty-five patients with tuberculosis (TB) participated. Patients received multiple oral doses of ETA as part of their treatment. They also received other anti-tuberculosis medications based upon in vitro susceptibility data. Serum samples were collected over 12 h post-dose, and concentrations were determined using a validated high-performance liquid chromatography (HPLC) assay. Concentration-time data were analyzed using population methods.

RESULTS

ETA areas under the concentration-versus-time curve (AUCs) increased linearly with increasing oral doses from 250 to 1000 mg. Compared to the population pattern, delayed absorption was seen at least once in 15% of patients. ETA PK parameter estimates were independent of age, weight, height, gender, and creatinine clearance. TB patients appeared to have larger volumes of distribution (3.22 l/kg) and clearance values (1.88 l/h/kg) compared to previously studied healthy volunteers. This resulted in lower AUC values (3.95 mcg h/ml) in the TB patients. ETA displayed a short elimination half-life (1.94 h). The effect of different dosing strategies on calculated pharmacodynamic parameters was explored. Simulated doses of 250 mg BID to TID failed to achieve serum concentrations above the MIC.

CONCLUSION

ETA PK parameters differed between TB patients and healthy volunteers, possibly due to differences in the completeness of absorption. Doses of at least 500 mg appear to be required to achieve serum concentrations above the typical ETA MIC. Additional research is needed to determine the optimal dosing of ETA.

Drug-drug interactions in inmates treated for human immunodeficiency virus and Mycobacterium tuberculosis infection or disease: an institutional tuberculosis outbreak

The use of rifamycins is limited by drug interactions in human immunodeficiency virus (HIV)-infected persons who are receiving highly active antiretroviral therapy (HAART). During a tuberculosis (TB) outbreak at a prison housing HIV-infected inmates, rifabutin was used to treat 238 men (13 case patients and 225 contacts). Steady-state peak plasma rifabutin concentrations were obtained after rifabutin dosages were adjusted for men receiving single-interacting HAART (with either 1 protease inhibitor [PI] or efavirenz), multi-interacting HAART (with either 2 PIs or > or =1 PI with efavirenz), and for noninteracting HAART (>1 nucleoside reverse-transcriptase inhibitor or no HAART) without rifabutin dose adjustments. Low rifabutin concentrations occurred in 9% of those receiving noninteracting HAART, compared with 19% of those receiving single-interacting and 29% of those receiving multi-interacting HAART (chi2, 3.76; P=.05). Of 225 contacts treated with rifabutin-pyrazinamide, 158 (70%) completed treatment while incarcerated. Rifabutin-pyrazinamide therapy was difficult to implement, because of the need for dosage adjustments and expert clinical management.

Bactericidal activity of increasing daily and weekly doses of moxifloxacin in murine tuberculosis

Moxifloxacin (MXF) is a new 8-methoxyquinolone with potent activity against Mycobacterium tuberculosis and a half-life of 9 to 12 h in humans. Previous in vivo studies using daily doses of 100 mg/kg of body weight have demonstrated bactericidal activity comparable to that of isoniazid (INH) in a murine model of tuberculosis (TB). Recent pharmacokinetic data suggest that MXF may have been underadministered in these studies and that a 400-mg/kg dose in mice better approximates the area under the concentration-time curve obtained in humans after a 400-mg oral dose. Therefore, the bactericidal activity of MXF in doses up to 400 mg/kg given daily or weekly for 28 days was assessed in mice infected intravenously with 5 x 10(6) CFU of M. tuberculosis. INH was used as a positive control. After 3 days of daily therapy, the CFU counts from splenic homogenates for mice treated with MXF in doses of 100 to 400 mg/kg/day were lower than those from pretreatment controls. No significant differences in CFU counts were seen when mice receiving INH or MXF at 50 mg/kg/day were compared to pretreatment controls. After 28 days of therapy, dose-dependent reductions in CFU counts in splenic homogenates were seen for daily MXF therapy. The maximum bactericidal effect was seen with daily doses of 400 mg/kg, which resulted in a reduction in CFU counts of 1 log(10) greater than that with INH treatment, although the difference was not statistically significant. CFU counts from lung homogenates after 28 days of therapy were significantly lower in all treatment groups than in untreated controls. The weekly administration of MXF in doses ranging from 50 to 400 mg/kg resulted in no significant bactericidal activity. Mice receiving daily MXF doses of 200 and 400 mg/kg/day failed to gain weight and appeared ill after 28 days of therapy, findings suggestive of drug toxicity. In conclusion, MXF has dose-dependent bactericidal activity against M. tuberculosis in the mouse when given in doses up to 400 mg/kg, where its pharmacokinetic profile better approximates that of standard human dosages. Combination regimens which take advantage of the enhanced pharmacodynamic profile of MXF at these doses have the potential to shorten the course of antituberculous therapy or allow more intermittent (i.e., once-weekly) therapy and should be evaluated in the mouse model of TB.

Pharmacokinetics of antituberculosis medications delivered via percutaneous gastrojejunostomy tube

We treated a 23-year-old aboriginal woman with drug-resistant pulmonary tuberculosis (TB). She experienced intolerance to her oral anti-TB medications, had subtherapeutic drug levels, and failed to respond to treatment. She then was effectively treated with percutaneous gastrojejunostomy tube (PGJT) administration of drugs. We present our data on the serum drug levels of rifampin, para-aminosalicylic acid, and levofloxacin after PGJT administration, and compare these values to published levels for oral administration of these drugs. In our patient, serum drug levels peaked and began to decline earlier than in the published data for oral administration of the same drugs.

Pharmacological issues in the treatment of tuberculosis

A thorough review of the clinical trial data combined with new in vitro experimental information may make the optimization of dosages of TB drugs possible. Several factors can affect the selection and dosage of TB drugs including hepatic and renal impairment, pregnancy, duration of disease before treatment, and extent of debilitation. Drug interactions and pharmacodynamics must be considered, and their roles are discussed.

Postantibiotic effects of antituberculosis agents alone and in combination

The postantibiotic effects (PAEs) of seven antimycobacterial agents, tested at their respective peak concentrations in serum alone and in different combinations, against Mycobacterium tuberculosis ATCC 27294 were studied with a radiometric culture system in parallel with the viable count method. Rifampin gave the longest PAE (67.8 h) among the drugs used alone, and combinations of first-line drugs generally gave PAEs longer than 120 h. The data obtained might help provide a better understanding of the scientific basis of intermittently administered antituberculosis chemotherapy.

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.

Therapeutic drug monitoring in antituberculosis chemotherapy: clinical perspectives

BACKGROUND

Therapeutic drug monitoring (TDM) represents an area where multidisciplinary service is rendered for optimization of clinical care, through enabling maximum drug therapeutic efficacy and minimum drug toxicity. This is a recently developed field for antituberculosis chemotherapy. The other goals of TDM principally include study and management of pharmacokinetic drug-drug and drug-disease interactions as well as the evaluation of new fixed-dose combinations (FDC) developed for treatment of tuberculosis.

CONCLUSION

In the years to come, with the advancement in knowledge, technology and expertise, it is hoped that TDM, as a novel paradigm of care in tuberculosis, may gain expanding indications and reach a new horizon.

Mycobacterial infections in patients with chronic renal disease

In this article, the authors have provided a comprehensive review of TB and MOTT infections in patients on renal dialysis and receiving kidney transplants. Because most published series are small retrospective studies or case reports, there are several uncertainties still involved in the diagnosis and treatment of such patients. Unanswered questions include selection of optimal dosage and duration of therapeutic agents; the best tests for screening and diagnosis, especially in high prevalence areas; and the best management of MOTT infections because of unavailability of highly effective therapy.

Population pharmacokinetics of intravenous and intramuscular streptomycin in patients with tuberculosis

STUDY OBJECTIVES

To determine population pharmacokinetic parameters of streptomycin after administration of multiple intramuscular and intravenous doses.

DESIGN

Prospective, unblinded clinical study.

SETTING

Two medical centers in Denver, Colorado.

PATIENTS

Thirty patients with tuberculosis.

INTERVENTION

Patients received multiple doses of streptomycin as part of their tuberculosis treatment. They received concurrent drugs based on in vitro susceptibility data.

MEASUREMENTS AND MAIN RESULTS

Serum samples were collected over a 10-hour period and assayed by validated high-performance liquid chromatography Concentration-time data were analyzed using population methods. Streptomycin concentrations increased linearly with increasing intravenous doses. The intramuscular doses did not produce as linear a relationship, presumably because of variability in rates of and, potentially, completeness of absorption. Streptomycin elimination decreased with declining renal function. Higher, intermittent doses were well tolerated and appeared to maximize the peak concentration:minimal inhibitory concentration ratio.

CONCLUSION

Overall, pharmacokinetic parameters of streptomycin were comparable with those previously published for streptomycin and other aminoglycosides. Higher, intermittent doses maximize pharmacodynamic parameter estimates and might have advantages for treatment of tuberculosis.

The role of fluoroquinolones in tuberculosis today

Tuberculosis is a growing international health concern; it is the leading infectious cause of death in the world today. The fluoroquinolones are the most recent class of drugs offering hope in the fight against this disease. Ciprofloxacin, ofloxacin, levofloxacin and sparfloxacin are currently the most commonly used agents used against Mycobacterium tuberculosis (TB), with in vitro minimum inhibitory concentrations (MICs) of 0.1 to 4 mcg/ml. Resistance in TB to fluoroquinolones may occur spontaneously or may be acquired, especially when these agents are used inappropriately. Cross-resistance among the fluoroquinolones has been shown in TB. The fluoroquinolones offer a favourable pharmacokinetic profile for the treatment of TB. Most demonstrate excellent oral bioavailability and achieve maximum (peak) serum concentrations well above the MIC. They are also distributed widely, including intracellularly. The fluoroquinolones are cleared renally and/or hepatically, with varying serum half-lives. Fluoroquinolones are most effective when the peak concentration (Cmax) to MIC ratio is maximised. Fluoroquinolones such as ciprofloxacin and ofloxacin have been used in regimens for the prevention of TB, but have been poorly tolerated when used in combination with pyrazinamide. Favourable responses with fluoroquinolones in regimens used in the treatment of clinical TB disease have been seen. They, however, are not to be considered as equal replacements for isoniazid or rifampicin (rifampin) and should be used with at least 2 other antituberculous agents. Therapeutic drug monitoring of fluoroquinolones is beneficial in assuring that maximum Cmax to MIC ratios are being achieved, especially in patients at risk for malabsorption, such as those infected with HIV. Higher, once-daily doses of most fluoroquinolones are becoming more common in treating TB. Fluoroquinolones are generally well tolerated with long term use in treating TB, but rare, serious adverse effects have been reported with general fluoroquinolone use. The most common drug interactions with fluoroquinolones in TB therapy include the malabsorption interactions associated with multivalent cations and cytochrome P450 interactions with ciprofloxacin. An increased risk of central nervous system effects with concomitant cycloserine has been reported and seen clinically. When using fluoroquinolones to treat TB, careful consideration of individual susceptibility patterns, pharmacokinetic and toxicity profiles should be taken. The aid of a TB expert may also be warranted. The exact role of the fluoroquinolones in treating TB remains to be determined.

Novel treatment of meningitis caused by multidrug-resistant Mycobacterium tuberculosis with intrathecal levofloxacin and amikacin: case report

We report the case of a 25-year-old HIV-negative man with disseminated multidrug-resistant tuberculosis (MDRTB), who-on a retreatment regimen-experienced total resolution of TB miliary disease, but progressive TB meningitis. Therefore, intrathecal treatment with amikacin and levofloxacin was instituted, with successful clinical and microbiological results.

Tuberculosis recurrence: multivariate analysis of serum levels of tuberculosis drugs, human immunodeficiency virus status, and other risk factors

We examined risk factors for tuberculosis recurrence in patients admitted to a tuberculosis hospital in Florida in 1996 and 1997. Recurrence of tuberculosis was not significantly associated with tuberculosis drug levels or HIV status, which indicates that routine drug monitoring may not be beneficial in general patient management.

Use of rifabutin with protease inhibitors for human immunodeficiency virus-infected patients with tuberculosis

Drug interactions between rifamycins and highly active antiretroviral therapy (HAART) have raised concerns in the treatment of human immunodeficiency virus (HIV)-infected patients with tuberculosis. We conducted a study of this interaction by measuring serum drug levels of all HIV-infected patients with tuberculosis who were admitted to A. G. Holley State Tuberculosis Hospital (Florida) from October 1997 through December 1998, who were concomitantly treated with rifabutin and HAART. All 25 patients studied became culture-negative within 2 months of initiation of therapy for tuberculosis and remained negative for a median of 13 months follow-up after completion of therapy. HIV viral loads (mean+/-SEM) decreased significantly from 4.95+/-0.21 log10 copies/mL before initiation of HAART to 2.77+/-0.07 log10 copies/mL before discharge (P<.001); 20 of 25 patients achieved viral loads of <500copies/mL. In summary, the concomitant use of rifabutin and HAART can lead to successful treatment of HIV-infected patients with tuberculosis without increased side effects.