Scientists based in Minneapolis have developed new rifamycin analogs that show effectiveness against drug-resistant Mycobacterium abscessus pulmonary disease (Mab-PD). This breakthrough marks a significant advancement in infectious disease therapeutics. Novel C25-substituted rifabutin derivatives present potent antibacterial properties with optimized pharmacokinetic behavior and low cytochrome P450 3A4 (CYP3A4) enzyme interaction making them suitable for administering to patients requiring multiple medications.
Tuberculosis (TB) therapy relies on rifamycins, which include rifampicin, rifabutin, and rifapentine. The introduction of rifampicin in the 1960s transformed TB treatment through by reducing the duration of therapy from 24 months to just 6 months. Rifabutin later became the preferred option for patients on antiretroviral therapy, due to its lower induction of the cytochrome P450 3A4 (CYP3A4) enzyme. Following its development rifapentine made it possible to shorten treatment duration specifically for patients with latent TB.
The resistance of Mab-PD to rifamycin therapy occurs because bacterial enzymes including Arr cause ADP-ribosylation-based inactivation of rifamycins. The new rifabutin analogs prevent inactivation by Arr enzyme through C25 position modifications which maintain their antibacterial properties.
The preclinical research examined GM-CSF knockout mice infected with M. abscessus to evaluate two compounds UMN-120 and UMN-121 which demonstrated extraordinary outcomes. The combination of daily oral treatment with UMN-121 and UMN-120 at a dosage of 25 mg/kg achieved complete bacterial clearance from lungs within seven days of administration. UMN-121 exhibited superior sterilization capability, by reducing bacterial levels below detectable limits. The antibacterial properties of rifabutin remained unchanged.
The design of these analogs focused on optimizing pharmacokinetic/pharmacodynamic (PK/PD) parameters. Rifabutin maintains an AUC/MIC ratio range of 500-600 against M. tuberculosis yet this ratio decreases to 2-3 against M. abscessus thereby rendering it insufficient for treating Mab-PD. The newly developed analogs showcase a similar rifabutin-like PK profile against TB yet maintain their effectiveness against Mab and achieve essential therapeutic coverage.
MIC testing on wild-type (WT) and Δarr Mab strains, combined with macrophage uptake assays and plasma protein binding experiments, formed the basis for analog screening. The evaluation process for new candidates required minimum inhibitory concentration (MIC) < 0.05 µM and MICWT/MICΔarr ratios below 1.5 to continue progress to in-vivo assessment. Macrophage retention demonstrating potential lung lesion penetration displayed a correlation with compound lipophilicity (cLogP) resulting in UMN-34 and UMN-123 as preferred analogs.
In-vitro testing using human hepatocytes confirmed the analogs had limited potential to interfere with other drugs because they induce minimal CYP3A4 mRNA activity. The rifabutin compound achieved 33–43-fold increases in CYP3A4 expression while new candidate compounds maintained less than 14-fold changes and four candidates exhibited less than 2-fold induction. For Mab-PD patients with cystic fibrosis and human immunodeficiency virus (HIV) who take CYP3A4-metabolized medicines this capability proves essential.
The bactericidal activity testing for Mab populations included both extracellular and intracellular stages in addition to both replicating and non-replicating conditions. All candidates exhibited MBC90 values (90% minimum concentration required to kill bacteria) at equivalent levels to their MICs while intracellular efficacy surpassed rifabutin in every experiment.
Comprehensive analysis demonstrated that the compounds displayed high metabolic stability in human liver microsomes while maintaining acceptable toxicity levels measured by CC50 values between 14 and 175 µM. The calculated selectivity indexes exceeded those of rifabutin, indicating improved therapeutic potential.
The analogs demonstrated effective activity towards several non-tuberculous mycobacterial strains including M. fortuitum, M. chelonae, and M. simiae yet retained their potency against M. tuberculosis. Among the analogs UMN-120 demonstrated the most potent AUC/MIC ratio alongside the best bacterial lung burden reduction despite lacking an individual PK-PD indicator for outcome prediction which shows that the mechanisms of action are complex.
The absence of biofilm-specific potency tests for Mab in the study still highlights significant progress in developing next-generation rifamycin compounds. These analogs show reduced pregnane X Receptor (PXR) activation linked to minimal P-glycoprotein interaction which may enable avoidance of rifampicin’s well-known absorption and resistance challenges.
Future investigations will examine the minimum effective doses of these analogs while testing them against persistent Mab strains with necrosis. Antimicrobials UMN-120 and UMN-121 demonstrate superior potency against both replicating and persistent Mab populations which marks them as leading candidates in their new therapeutic class.
References
Dartois V, Lan T, Ganapathy US, et al. Next-generation rifamycins for the treatment of mycobacterial infections. Proc Natl Acad Sci U S A. 2025;122(18):e2423842122. doi:10.1073/pnas.2423842122
