Mycobacterium abscessus HelR interacts with RNA polymerase to confer intrinsic rifamycin resistance.

Rifampicin (RIF), the frontline drug against M. tuberculosis, is completely ineffective against M. abscessus, partially due to the presence of an ADP-ribosyltransferase (Arr) that inactivates RIF. Using RNA-seq, we show that exposure of M. abscessus to sublethal doses of RIF and Rifabutin (RBT), a close analog of RIF, results in an ∼25-fold upregulation of Mab_helR in laboratory and clinical isolates. An isogenic deletion in Mab_helR results in RIF/RBT hypersensitivity, and overexpression of Mab_helR confers RIF tolerance in M. tuberculosis. We demonstrate an increased HelR-RNAP association in RIF-exposed bacteria and a MabHelR-mediated dissociation of RNAP from stalled initiation complexes in vitro. Finally, we show that the tip of the PCh-loop of Mab_helR, present in proximity to RIF, is critical for conferring RIF resistance but dispensable for dissociation of stalled RNAP complexes, suggesting that HelR-mediated RIF resistance requires a step in addition to displacement of RIF-stalled RNAP.

Population pharmacokinetics of rifabutin among HIV/TB co-infected children on lopinavir/ritonavir-based antiretroviral therapy.

In adults requiring protease inhibitor (PI)-based antiretroviral therapy (ART), replacing rifampicin with rifabutin is a preferred option, but there is lack of evidence to guide rifabutin dosing in children, especially with PIs. We aimed to characterize the population pharmacokinetics of rifabutin and 25-O-desacetyl rifabutin (des-rifabutin) in children and optimize its dose. We included children from three age cohorts: (i) <1-year-old cohort and (ii) 1- to 3-year-old cohort, who were ART naïve and received 15- to 20-mg/kg/day rifabutin for 2 weeks followed by lopinavir/ritonavir (LPV/r)-based ART with 5.0- or 2.5 mg/kg/day rifabutin, respectively, while the (iii) >3-year-old cohort was ART-experienced and received 2.5-mg/kg/day rifabutin with LPV/r-based ART. Non-linear mixed-effects modeling was used to interpret the data. Monte Carlo simulations were performed to evaluate the study doses and optimize dosing using harmonized weight bands. Twenty-eight children were included, with a median age of 10 (range 0.67-15.0) years, a median weight of 11 (range 4.5-45) kg, and a median weight-for-age z score of -3.33 (range -5.15 to -1.32). A two-compartment disposition model, scaled allometrically by weight, was developed for rifabutin and des-rifabutin. LPV/r increased rifabutin bioavailability by 158% (95% confidence interval: 93.2%-246.0%) and reduced des-rifabutin clearance by 76.6% (74.4%-78.3%). Severely underweight children showed 26% (17.9%-33.7%) lower bioavailability. Compared to adult exposures, simulations resulted in higher median steady-state rifabutin and des-rifabutin exposures in 6-20 kg during tuberculosis-only treatment with 20 mg/kg/day. During LPV/r co-treatment, the 2.5-mg/kg/day dose achieved similar exposures to adults, while the 5-mg/kg/day dose resulted in higher exposures in children >7 kg. All study doses maintained a median Cmax of <900 µg/L. The suggested weight-band dosing matches adult exposures consistently across weights and simplifies dosing.

Rifabutin but not rifampicin can partly out-balance P-glycoprotein induction by concurrent P-glycoprotein inhibition through high affinity binding to the inhibitory site.

Physiology-based pharmacokinetic modeling suggests that rifabutin can out-balance P-glycoprotein (P-gp) induction by concurrent P-gp inhibition. However, clinical or experimental evidence for this Janus-faced rifabutin effect is missing. Consequently, LS180 cells were exposed to a moderately (2 µM) and strongly (10 µM) P-gp-inducing concentration of rifampicin or rifabutin for 6 days. Cellular accumulation of the fluorescent P-gp substrate rhodamine 123 was evaluated using flow cytometry, either without (induction only) or with adding rifamycin drug to the cells during the rhodamine 123 efflux phase (induction + potential inhibition). Rhodamine 123 accumulation was decreased similarly by both drugs after 6-day exposure (2 µM: 55% residual fluorescence compared to non-induced cells, P < 0.01; 10 µM: 30% residual fluorescence compared to non-induced cells, P < 0.001), indicating P-gp induction. Rhodamine 123 influx transporters mRNA expressions were not affected, excluding off-target effects. Acute re-exposure to rifabutin, however, considerably re-increased rhodamine 123 accumulation (2 µM induction: re-increase by 55%, P < 0.01; 10 µM induction: 49% re-increase, P < 0.001), suggesting P-gp inhibition. In contrast, rifampicin only had weak effects (2 µM induction: no re-increase; 10 µM induction: 16% re-increase; P < 0.05). Molecular docking analysis eventually revealed that rifabutin has a higher binding affinity to the inhibitor binding site of P-gp than rifampicin (ΔG (kcal/mol) = -11.5 vs -5.3). Together, this study demonstrates that rifabutin can at least partly mask P-gp induction by P-gp inhibition, mediated by high affinity binding to the inhibitory site of P-gp.

Lack of CYP3A4 protein induction despite mRNA induction in primary hepatocytes exposed to rifabutin as a possible explanation for its low interaction risk in vivo.

Rifampicin is a strong inducer of cytochrome P450 (CYP3A4) and P-glycoprotein (P-gp/ABCB1), leading to profound drug-drug interactions. In contrast, the chemically related rifabutin does not show such pronounced induction properties in vivo. The aim of our study was to conduct a comprehensive analysis of the different induction potentials of rifampicin and rifabutin in primary human hepatocytes and to analyze the mechanism of potential differences. Therefore, we evaluated CYP3A4/ABCB1 mRNA expression (polymerase chain reaction), CYP3A4/P-gp protein expression (immunoaffinity-liquid chromatography-mass spectrometry, IA-LC-MS/MS), CYP3A4 activity (testosterone hydroxylation), and considered intracellular drug uptake after treatment with increasing rifamycin concentrations (0.01-10 µM). Furthermore, rifamycin effects on the protein levels of CYP2C8, CYP2C9, and CYP2C19 were analyzed (IA-LC-MS/MS). Mechanistic analysis included the evaluation of possible suicide CYP3A4 inhibition (IC50 shift assay) and drug impact on translational efficiency (cell-free luminescence assays). Rifabutin accumulated 6- to 15-fold higher in hepatocytes than rifampicin, but induced CYP3A4 mRNA comparably to rifampicin (e. g. rifampicin 61-fold vs. rifabutin 44-fold, 72 h). While rifampicin for example enhanced protein (10 µM: 21-fold) and activity levels considerably (53-fold), rifabutin only slightly increased CYP3A4 protein expression (10 µM: 3.3-fold) or activity (11-fold) compared to rifampicin after 72 h. Both rifamycins similarly influenced expression of other eliminating proteins. A potential CYP3A4 suicide inhibition by a specific rifabutin metabolite or disruption of ribosome function were excluded experimentally. In conclusion, the lack of protein enhancement, could explain rifabutin's weaker induction-related drug-drug interaction risk in vivo.

Antibiotic Adjuvant 4-Hexylresorcinol Enhances the Efficiency of Antituberculosis Drugs.

We studied the possibility of using 4-hexylresorcinol to increase the efficiency of anti-mycobacterial chemotherapy. In an in vitro experiment, 4-hexylresorcinol increased the efficiency of rifampicin, kanamycin, and isoniazid against Mycobacterium smegmatis by 3-5 times. Experiments in sanitation of BALB/c mice infected with M. smegmatis showed the best efficacy of the isoniazid and 4-hexylresorcinol combination in comparison with isoniazid monotherapy. The growth-inhibiting activity of the combination of antibiotic rifabutin with 4-hexylresorcinol was shown on 6 strains of M. tuberculosis. A 2-fold decrease in the minimum inhibitory concentration of this antibiotic in the presence of half-minimum inhibitory concentration of 4-hexylresorcinol was demonstrated for monoresistant strain M. tuberculosis 5360/42Hr. On the mouse model of experimental tuberculosis caused by M. tuberculosis H37Rv, a 5-fold decrease in lung contamination and more rapid complete cure were achieved in animals treated with the combination of rifabutin and 4-hexylresorcinol in comparison with rifabutin monotherapy.

Rifabutin-Containing Triple Therapy Versus Bismuth Quadruple Therapy for Helicobacter pylori Rescue Treatment: A Multicenter, Randomized Controlled Trial.

BACKGROUND: We compared the efficacy and safety of rifabutin-containing triple therapy with bismuth quadruple therapy for rescue treatment of Helicobacter pylori. METHODS: This was a noninferiority study trial of H. pylori treatment for subjects who had failed at least 2 prior treatments. Subjects were randomly assigned to receive rifabutin triple therapy with 14-day esomeprazole (20 mg), amoxicillin (1.0 g), and rifabutin (150 mg) twice a day; or bismuth quadruple therapy with esomeprazole (20 mg) and bismuth (220 mg) twice a day, plus metronidazole (400 mg) and tetracycline (500 mg) 4 times a day. Antimicrobial susceptibility was assessed by agar dilution and E-test. RESULTS: From May 2021 to October 2022, a total of 364 subjects were randomized. The eradication rates by intention-to-treat, per-protocol, and modified intention-to-treat were 89.0% (162/182; 95% confidence interval [CI], 83.6%-92.8%), 94.0% (157/167; 95% CI, 89.3%-96.7%), and 93.6% (162/173; 95% CI, 89.0%-96.4%) for rifabutin triple group. For bismuth quadruple group, they were 89.6% (163/182; 95% CI, 84.3%-93.2%), 95.3% (143/150; 95% CI, 90.7%-97.7%), and 93.7% (163/174; 95% CI, 89.0%-96.4%). CONCLUSIONS: The rifabutin triple therapy is an alternative to classical bismuth quadruple therapy for the rescue treatment of H. pylori with fewer side effects and higher compliance. CLINICAL TRIALS REGISTRATION: NCT04879992.

Using Dynamic Oral Dosing of Rifapentine and Rifabutin to Simulate Exposure Profiles of Long-Acting Formulations in a Mouse Model of Tuberculosis Preventive Therapy.

Administration of tuberculosis preventive therapy (TPT) to individuals with latent tuberculosis infection is an important facet of global tuberculosis control. The use of long-acting injectable (LAI) drug formulations may simplify and shorten regimens for this indication. Rifapentine and rifabutin have antituberculosis activity and physiochemical properties suitable for LAI formulation, but there are limited data available for determining the target exposure profiles required for efficacy in TPT regimens. The objective of this study was to determine exposure-activity profiles of rifapentine and rifabutin to inform development of LAI formulations for TPT. We used a validated paucibacillary mouse model of TPT in combination with dynamic oral dosing of both drugs to simulate and understand exposure-activity relationships to inform posology for future LAI formulations. This work identified several LAI-like exposure profiles of rifapentine and rifabutin that, if achieved by LAI formulations, could be efficacious as TPT regimens and thus can serve as experimentally determined targets for novel LAI formulations of these drugs. We present novel methodology to understand the exposure-response relationship and inform the value proposition for investment in development of LAI formulations that have utility beyond latent tuberculosis infection.

Drug-drug interaction between rifabutin and dolutegravir: A population pharmacokinetic model.

Rifampicin, a potent enzyme inducer, causes marked reduction of dolutegravir exposure. Rifabutin, a less potent enzyme inducer, may offer an alternative to rifampicin. We aimed to characterize the population pharmacokinetics of dolutegravir when co-administered with rifabutin. We extended an existing dolutegravir model to include data from volunteers co-administered with dolutegravir 50 mg and rifabutin 300 mg once daily. We ran simulations of dolutegravir with and without rifabutin co-administration and compare dolutegravir trough concentrations with the IC90 and EC90 of 0.064 and 0.3 mg/L, respectively. Rifabutin decreased dolutegravir's volume of distribution by 33.1% (95% confidence interval 25.1%-42.3%) but did not affect the area under the concentration-time curve. Simulations showed that when 50 mg dolutegravir is co-administered with rifabutin once daily, the probability to attain trough concentrations above the IC90 of 0.064 mg/L is more than 99%. Therefore, there is no need for dolutegravir dose adjustment. Rifabutin may offer an alternative to rifampicin for the treatment of HIV/tuberculosis co-infected individuals.

Comprehensive in vitro analysis evaluating the variable drug-drug interaction risk of rifampicin compared to rifabutin.

Compared to rifampicin (600 mg/day), standard doses of rifabutin (300 mg/day) have a lower risk of drug-drug interactions due to induction of cytochrome P450 3A4 (CYP3A4) or P-glycoprotein (Pgp/ABCB1) mediated by the pregnane X receptor (PXR). However, clinical comparisons with equal rifamycin doses or in vitro experiments respecting actual intracellular concentrations are lacking. Thus, the genuine pharmacological differences and the potential molecular mechanisms of the discordant perpetrator effects are unknown. Consequently, the cellular uptake kinetics (mass spectrometry), PXR activation (luciferase reporter gene assays), and impact on CYP3A4 and Pgp/ABCB1 expression and activity (polymerase chain reaction, enzymatic assays, flow cytometry) were evaluated in LS180 cells after treatment with different rifampicin or rifabutin concentrations for variable exposure times and eventually normalized to actual intracellular concentrations. In addition, inhibitory effects on CYP3A4 and Pgp activities were investigated. While rifampicin is poorly taken up by LS180 cells, it strongly activates PXR and leads to enhanced expression and activity of CYP3A4 and Pgp. In contrast, rifabutin is a significantly less potent and less efficient PXR activator and gene inducer, despite sixfold to eightfold higher intracellular accumulation. Finally, rifabutin is a potent inhibitor of Pgp (IC50 = 0.3 µM) compared to rifampicin (IC50 = 12.9 µM). Together, rifampicin and rifabutin significantly differ by their effects on the regulation and function of CYP3A4 and Pgp, even when controlled for intracellular concentrations. Rifabutin's concurrent Pgp inhibitory action might partly compensate the inducing effects, explaining its weaker clinical perpetrator characteristics.

The Inappropriateness of Using Rifampicin E-Test to Predict Rifabutin Resistance in Helicobacter pylori.

BACKGROUND: The aim of this study was to evaluate the rifamycin cross-resistance in Helicobacter pylori, and whether the use of rifampicin E-test strips to screen H. pylori rifabutin resistance is appropriate. METHODS: A total of 89 H. pylori isolates were included. Rifampicin minimum inhibitory concentrations (MICs) were obtained by E-test, while the MICs for rifapentine, rifaximin, and rifabutin were determined by agar dilution method. The rifamycin resistance rates based on different breakpoints were compared. Isolates with high-level rifampicin resistance were subjected to whole-genome sequencing. RESULTS: A wide distribution of MICs (mostly in the range 0.125-8 mg/L) was observed for rifampicin, rifapentine, and rifaximin. Using MIC >1, ≥ 4, and > 4 mg/L as the breakpoints, resistance rates to rifampicin/rifapentine/rifaximin were 60.4%/48.3%/38.2%, 28.1%/25.8%/23.6%, and 15.7%/16.9%/7.9%, respectively. However, the rifabutin MICs of all the tested H. pylori isolates were extremely low (≤0.016 mg/L). Applying MIC ≥ 0.125 mg/L as the breakpoint, rifabutin resistance was nil. No mutation was found in the rpoB gene sequences of the 2 isolates with high-level rifampicin resistance. CONCLUSIONS: There is a lack of cross-resistance between rifabutin and other rifamycins in H. pylori. The use of rifampicin E-test to predict H. pylori rifabutin resistance is inappropriate.

Novel Regimens of Bedaquiline-Pyrazinamide Combined with Moxifloxacin, Rifabutin, Delamanid and/or OPC-167832 in Murine Tuberculosis Models.

A recent landmark trial showed a 4-month regimen of rifapentine, pyrazinamide, moxifloxacin, and isoniazid (PZMH) to be noninferior to the 6-month standard of care. Here, two murine models of tuberculosis were used to test whether novel regimens replacing rifapentine and isoniazid with bedaquiline and another drug would maintain or increase the sterilizing activity of the regimen. In BALB/c mice, replacing rifapentine in the PZM backbone with bedaquiline (i.e., BZM) significantly reduced both lung CFU counts after 1 month and the proportion of mice relapsing within 3 months after completing 1.5 months of treatment. The addition of rifabutin to BZM (BZMRb) further increased the sterilizing activity. In the C3HeB/FeJ mouse model characterized by caseating lung lesions, treatment with BZMRb resulted in significantly fewer relapses than PZMH after 2 months of treatment. A regimen combining the new DprE1 inhibitor OPC-167832 and delamanid (BZOD) also had superior bactericidal and sterilizing activity compared to PZM in BALB/c mice and was similar in efficacy to PZMH in C3HeB/FeJ mice. Thus, BZM represents a promising backbone for treatment-shortening regimens. Given the prohibitive drug-drug interactions between bedaquiline and rifampin or rifapentine, the BZMRb regimen represents the best opportunity to combine, in one regimen, the treatment-shortening potential of the rifamycin class with that of BZM and deserves high priority for evaluation in clinical trials. Other 4-drug BZM-based regimens and BZOD represent promising opportunities for extending the spectrum of treatment-shortening regimens to rifamycin- and fluoroquinolone-resistant tuberculosis.

Efficacy and safety of low-dose rifabutin-based 7-day triple therapy as a third- or later-line Helicobacter pylori eradication regimen.

BACKGROUND: Rifabutin-based regimens are used as rescue therapy for refractory Helicobacter pylori infection; however, the duration for which treatment is required and side effects are concerning. This study assessed the efficacy and safety of 7-day rifabutin, amoxicillin, and vonoprazan triple therapy as third- or later-line treatment for H. pylori infection. MATERIALS AND METHODS: Patients who did not respond to second-line therapy were enrolled. After H. pylori infection was confirmed with the culture method, the patients received rifabutin-containing triple therapy (20 mg vonoprazan b.i.d., 500 mg amoxicillin q.i.d., and 150 mg rifabutin q.d.) for 7 days. Twelve weeks after the eradication therapy, successful eradication was confirmed using a 13 C urea breath test or the H. pylori stool antigen test. The results obtained from our previous study that reported a 10-day or 14-day esomeprazole based rifabutin-containing triple therapy as a third- or fourth-line rescue therapy treated patients were used as historical control. We determined the minimum inhibitory concentrations of amoxicillin and rifabutin. We also evaluated whether the patients were positive for the mutation of the rpoB gene. RESULTS: Intention-to-treat and per-protocol analyses showed that our regimen resulted in a high eradication rate (91.2%, 95% CI: 84%-99% and 92.7%, 95% CI: 86%-100%, respectively). Adverse events occurred in 31.6% of the patients, and two patients discontinued the therapy. CONCLUSIONS: This is the first study to evaluate the efficacy and safety of a 7-day low-dose rifabutin-based triple therapy with vonoprazan and amoxicillin. Our results suggest that our regimen was effective and safe as a third- or later-line H. pylori eradication regimen. To clarify what component in this regimen are critical, subsequent studies using a factorial design (comparing vonoprazan-amoxicillin dual therapy vs. vonoprazan-rifabutin triple therapy) will be needed.

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 < .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.

Pitfalls of Physician-Directed Treatment of Helicobacter pylori: Results from Two Phase 3 Clinical Trials and Real-World Prescribing Data.

BACKGROUND: Helicobacter pylori (H. pylori) infects ~ 35% of Americans and can lead to serious sequelae if left untreated. Growing evidence indicates that clarithromycin-based therapies (CBT) are becoming increasingly ineffective for treating H. pylori infection. RHB-105 was approved by the US Food and Drug Administration in 2019 for the treatment of H. pylori infection in adults. AIMS: The primary aim of this study was to assess prescribing patterns and associated cure rates of physician-directed therapy for subjects with persistent H. pylori infection after participation in one of two Phase 3 clinical trials (ERADICATE Hp and ERADICATE Hp2). METHODS: We reviewed study reports to identify specific physician-directed regimens selected for subjects whose H. pylori infection was not eradicated. We also conducted a CYP2C19 genotype analysis of subjects who were prescribed CBT. Finally, we analyzed real-world H. pylori retail prescription data and compared these with to the physician-directed therapies in the clinical trials studies. RESULTS: Following ERADICATE Hp, CBT was prescribed for 27/31 (87%) subjects achieving a 59.3% cure rate. Following ERADICATE Hp2, CBT was prescribed for 48/94 (51%) subjects achieving a 60.4% cure rate. Rapid CYP2C19 metabolizers (2/11) had a cure rate of 18.2% with CBT. Real-world prescription data from IQVIA showed more than 80% of prescriptions for H. pylori infection were for CBT. CONCLUSIONS: Rates of CBT use persist despite sub-optimal eradication rates. Since RHB-105 does not contain clarithromycin, it can be prescribed first-line without concerns about clarithromycin resistance or CYP2C19 status. NCT03198507 & NCT01980095.

In vitro activity of rifabutin against Mycobacterium abscessus, clinical isolates.

The antibiotic options available for Mycobacterium abscessus (M. abscessus) infection are limited and no definitive therapeutic strategies have been formulated. The recent discovery that rifabutin is active against M. abscessus has raised interest in using rifabutin to treat this intractable disease. In this study, we evaluated the in vitro activity of rifabutin against 194 M. abscessus clinical isolates collected during 2012 January to 2017 December. As expected, rifabutin demonstrated considerably lower MICs against M. abscessus, with an MIC50 of 2 µg/mL and MIC90 of 4 µg/mL, respectively. Notably, the anti-M.abscessus activity was even stronger among clarithromycin-insusceptible strains. In addition, M. abscessus isolates with a rough morphotype were more sensitive to rifabutin compared with those forming smooth colonies when considered as a whole or in separate subspecies. Results from synergistic experiments revealed that the in vitro activity of rifabutin was significantly enhanced by the addition of amikacin, suggesting a promising strategy for M. abscessus infection combination treatment. Finally, five and three mutation patterns in rpoB and arr, respectively, were identified among the 194 strains through whole genome sequencing. However, none of them conferred rifabutin resistance. Our study is among the first to report the susceptibility of M. abscessus to rifabutin in vitro with a large amount of clinical isolates, suggesting that rifabutin is active, both alone and in combination, against M. abscessus and is worth considering as part of a combination treatment regimen for M. abscessus infections.

Low-Dose Rifabutin Increases Cytotoxicity in Antimitotic-Drug-Treated Resistant Cancer Cells by Exhibiting Strong P-gp-Inhibitory Activity.

The cytotoxicity of various antibiotics at low doses in drug-resistant cancer cells was evaluated. Low doses of rifabutin were found to markedly increase the cytotoxicity of various antimitotic drugs, such as vincristine (VIC), to P-glycoprotein (P-gp)-overexpressing antimitotic-drug-resistant KBV20C cells. Rifabutin was also found to exert high levels of P-gp-inhibitory activity at 4 and 24 h posttreatment, suggesting that the cytotoxicity of VIC + rifabutin was mainly due to the direct binding of rifabutin to P-gp and the reduction of VIC efflux by P-gp. The combination of VIC + rifabutin also increased early apoptosis, G2 arrest, and the DNA damaging marker, pH2AX protein. Interestingly, only the combination of VIC + rifabutin induced remarkable levels of cytotoxicity in resistant KBV20C cells, whereas other combinations (VIC + rifampin, VIC + rifapentine, and VIC + rifaximin) induced less cytotoxicity. Such finding suggests that rifabutin specifically increases the cytotoxicity of VIC in KBV20C cells, independent of the toxic effect of the ansamycin antibiotic. Only rifabutin had high P-gp-inhibitory activity, which suggests that its high P-gp-inhibitory activity led to the increased cytotoxicity of VIC + rifabutin. As rifabutin has long been used in the clinic, repositioning this drug for P-gp-overexpressing resistant cancer could increase the availability of treatments for patients with drug-resistant cancer.

Safety and Pharmacokinetics of Double-Dose Lopinavir/Ritonavir + Rifampin Versus Lopinavir/Ritonavir + Daily Rifabutin for Treatment of Human Immunodeficiency Virus-Tuberculosis Coinfection.

BACKGROUND: Protease inhibitor-based antiretroviral therapy may be used in resource-limited settings in persons with human immunodeficiency virus and tuberculosis (HIV-TB). Data on safety, pharmacokinetics/pharmacodynamics (PK/PD), and HIV-TB outcomes for lopinavir/ritonavir (LPV/r) used with rifampin (RIF) or rifabutin (RBT) are limited. METHODS: We randomized adults with HIV-TB from July 2013 to February 2016 to arm A, LPV/r 400 mg/100 mg twice daily + RBT 150 mg/day; arm B, LPV/r 800 mg/200 mg twice daily + RIF 600 mg/day; or arm C, LPV/r 400 mg/100 mg twice daily + raltegravir (RAL) 400 mg twice daily + RBT 150 mg/day. All received two nucleoside reverse transcriptase inhibitors and other TB drugs. PK visits occurred on day 12 ± 2. Within-arm HIV-TB outcomes were summarized using proportions and 95% CIs; PK were compared using Wilcoxon tests. RESULTS: Among 71 participants, 52% were women; 72% Black; 46% Hispanic; median age, 37 years; median CD4+ count, 130 cells/mm3; median HIV-1 RNA, 4.6 log10 copies/mL; 46% had confirmed TB. LPV concentrations were similar across arms. Pooled LPV AUC12 (157 203 hours × ng/mL) and Ctrough (9876 ng/mL) were similar to historical controls; RBT AUC24 (7374 hours × ng/mL) and Ctrough (208 ng/mL) were higher, although 3 participants in arm C had RBT Cmax <250 ng/mL. Proportions with week 48 HIV-1 RNA <400 copies/mL were 58%, 67%, and 61%, respectively, in arms A, B, and C. CONCLUSIONS: Double-dose LPV/r+RIF and LPV/r+RBT 150mg/day had acceptable safety, PK and TB outcomes; HIV suppression was suboptimal but unrelated to PK. Faster RBT clearance and low Cmax in 3 participants on RBT+RAL requires further study.

Model-Based Comparative Analysis of Rifampicin and Rifabutin Drug-Drug Interaction Profile.

Rifamycins are widely used for treating mycobacterial and staphylococcal infections. Drug-drug interactions (DDI) caused by rifampicin (RIF) are a major issue. We used a model-based approach to predict the magnitude of DDI with RIF and rifabutin (RBT) for 217 cytochrome P450 (CYP) substrates. On average, DDI caused by low-dose RIF were twice as potent as those caused by RBT. Contrary to RIF, RBT appears unlikely to cause severe DDI, even with sensitive CYP substrates.

Synergistic Rifabutin and Colistin Reduce Emergence of Resistance When Treating Acinetobacter baumannii.

Recently, we reported rifabutin hyperactivity against Acinetobacter baumannii We sought to characterize potential interactions between rifabutin and colistin, the last-resort drug for carbapenem-resistant infections. Rifabutin and colistin were synergistic in vitro and in vivo, and low-dose colistin significantly suppressed emergence of resistance to rifabutin. Thus, this combination is a promising therapeutic option for highly resistant A. baumannii infections.

Differential In Vitro Activities of Individual Drugs and Bedaquiline-Rifabutin Combinations against Actively Multiplying and Nutrient-Starved Mycobacterium abscessus.

Current treatment options for lung disease caused by Mycobacterium abscessus complex infections have limited effectiveness. To maximize the use of existing antibacterials and to help inform regimen design for treatment, we assessed the in vitro bactericidal activity of single drugs against actively multiplying and net nonreplicating M. abscessus populations in nutrient-rich and nutrient-starvation conditions, respectively. As single drugs, bedaquiline and rifabutin exerted bactericidal activity only against nutrient-starved and actively growing M. abscessus, respectively. However, when combined, both bedaquiline and rifabutin were able to specifically contribute bactericidal activity at relatively low, clinically relevant concentrations against both replicating and nonreplicating bacterial populations. The addition of a third drug, amikacin, further enhanced the bactericidal activity of the bedaquiline-rifabutin combination against nutrient-starved M. abscessus Overall, these in vitro data suggest that bedaquiline-rifabutin may be a potent backbone combination to support novel treatment regimens for M. abscessus infections. This rich data set of differential time- and concentration-dependent activity of drugs, alone and together, against M. abscessus also highlights several issues affecting interpretation and translation of in vitro findings.