Spectinamide MBX-4888A exhibits favorable lesion and tissue distribution and promotes treatment shortening in advanced murine models of tuberculosis.

The spectinamides are novel, narrow-spectrum semisynthetic analogs of spectinomycin, modified to avoid intrinsic efflux by Mycobacterium tuberculosis . Spectinamides, including lead MBX-4888A (Lee-1810), exhibit promising therapeutic profiles in mice, as single drugs and as partner agents with other anti-tuberculosis antibiotics including rifampin and/or pyrazinamide. To demonstrate that this translates to more effective cure, we first confirmed the role of rifampin, with or without pyrazinamide, as essential to achieve effective bactericidal responses and sterilizing cure in the current standard of care regimen in chronically infected C3HeB/FeJ mice compared to BALB/c mice. Thus, demonstrating added value in testing clinically relevant regimens in murine models of increasing pathologic complexity. Next we show that MBX-4888A, given by injection with the front-line standard of care regimen, is treatment shortening in multiple murine tuberculosis infection models. The positive treatment responses to MBX-4888A combination therapy in multiple mouse models including mice exhibiting advanced pulmonary disease can be attributed to favorable distribution in tissues and lesions, retention in caseum, along with favorable effects with rifampin and pyrazinamide under conditions achieved in necrotic lesions. This study also provides an additional data point regarding the safety and tolerability of spectinamide MBX-4888A in long-term murine efficacy studies.

Drug distribution and efficacy of the DprE1 inhibitor BTZ-043 in the C3HeB/FeJ mouse tuberculosis model.

BTZ-043, a suicide inhibitor of the Mycobacterium tuberculosis cell wall synthesis decaprenylphosphoryl-beta-D-ribose 2' epimerase, is under clinical development as a potential new anti-tuberculosis agent. BTZ-043 is potent and bactericidal in vitro but has limited activity against non-growing bacilli in rabbit caseum. To better understand its behavior in vivo, BTZ-043 was evaluated for efficacy and spatial drug distribution as a single agent in the C3HeB/FeJ mouse model presenting with caseous necrotic pulmonary lesions upon Mycobacterium tuberculosis infection. BTZ-043 promoted significant reductions in lung and spleen bacterial burdens in the C3HeB/FeJ mouse model after 2 months of therapy. BTZ-043 penetrates cellular and necrotic lesions and was retained at levels above the serum-shifted minimal inhibitory concentration in caseum. The calculated rate of kill was found to be highest and dose-dependent during the second month of treatment. BTZ-043 treatment was associated with improved histology scores of pulmonary lesions, especially compared to control mice, which experienced advanced fulminant neutrophilic alveolitis in the absence of treatment. These positive treatment responses to BTZ-043 monotherapy in a mouse model of advanced pulmonary disease can be attributed to favorable distribution in tissues and lesions, retention in the caseum, and its high potency and bactericidal nature at drug concentrations achieved in necrotic lesions.

Lung microenvironments harbor Mycobacterium tuberculosis phenotypes with distinct treatment responses.

Tuberculosis lung lesions are complex and harbor heterogeneous microenvironments that influence antibiotic effectiveness. Major strides have been made recently in understanding drug pharmacokinetics in pulmonary lesions, but the bacterial phenotypes that arise under these conditions and their contribution to drug tolerance are poorly understood. A pharmacodynamic marker called the RS ratio® quantifies ongoing rRNA synthesis based on the abundance of newly synthesized precursor rRNA relative to mature structural rRNA. Application of the RS ratio in the C3HeB/FeJ mouse model demonstrated that Mycobacterium tuberculosis populations residing in different tissue microenvironments are phenotypically distinct and respond differently to drug treatment with rifampin, isoniazid, or bedaquiline. This work provides a foundational basis required to address how anatomic and pathologic microenvironmental niches may contribute to long treatment duration and drug tolerance during the treatment of human tuberculosis.

ADP-ribosylation-resistant rifabutin analogs show improved bactericidal activity against drug-tolerant M. abscessus in caseum surrogate.

Necrotic lesions and cavities filled with caseum are a hallmark of mycobacterial pulmonary disease. Bronchocavitary Mycobacterium abscessus disease is associated with poor treatment outcomes. In caseum surrogate, M. abscessus entered an extended stationary phase showing tolerance to killing by most current antibiotics, suggesting that caseum persisters contribute to the poor performance of available treatments. Novel ADP-ribosylation-resistant rifabutin analogs exhibited bactericidal activity against these M. abscessus persisters at concentrations achievable by rifamycins in caseum.

A Novel Tool to Identify Bactericidal Compounds against Vulnerable Targets in Drug-Tolerant M. tuberculosis found in Caseum.

Caseous necrosis is a hallmark of tuberculosis (TB) pathology and creates a niche for drug-tolerant persisters within the host. Cavitary TB and high bacterial burden in caseum require longer treatment duration. An in vitro model that recapitulates the major features of Mycobacterium tuberculosis (Mtb) in caseum would accelerate the identification of compounds with treatment-shortening potential. We have developed a caseum surrogate model consisting of lysed and denatured foamy macrophages. Upon inoculation of Mtb from replicating cultures, the pathogen adapts to the lipid-rich matrix and gradually adopts a nonreplicating state. We determined that the lipid composition of ex vivo caseum and the surrogate matrix are similar. We also observed that Mtb in caseum surrogate accumulates intracellular lipophilic inclusions (ILI), a distinctive characteristic of quiescent and drug-tolerant Mtb. Expression profiling of a representative gene subset revealed common signatures between the models. Comparison of Mtb drug susceptibility in caseum and caseum surrogate revealed that both populations are similarly tolerant to a panel of TB drugs. By screening drug candidates in the surrogate model, we determined that the bedaquiline analogs TBAJ876 and TBAJ587, currently in clinical development, exhibit superior bactericidal against caseum-resident Mtb, both alone and as substitutions for bedaquiline in the bedaquiline-pretomanid-linezolid regimen approved for the treatment of multidrug-resistant TB. In summary, we have developed a physiologically relevant nonreplicating persistence model that reflects the distinct metabolic and drug-tolerant state of Mtb in caseum. IMPORTANCE M. tuberculosis (Mtb) within the caseous core of necrotic granulomas and cavities is extremely drug tolerant and presents a significant hurdle to treatment success and relapse prevention. Many in vitro models of nonreplicating persistence have been developed to characterize the physiologic and metabolic adaptations of Mtb and identify compounds active against this treatment-recalcitrant population. However, there is little consensus on their relevance to in vivo infection. Using lipid-laden macrophage lysates, we have designed and validated a surrogate matrix that closely mimics caseum and in which Mtb develops a phenotype similar to that of nonreplicating bacilli in vivo. The assay is well suited to screen for bactericidal compounds against caseum-resident Mtb in a medium-throughput format, allowing for reduced reliance on resource intensive animal models that present large necrotic lesions and cavities. Importantly, this approach will aid the identification of vulnerable targets in caseum Mtb and can accelerate the development of novel TB drugs with treatment-shortening potential.

Adaptation to the intracellular environment of primary human macrophages influences drug susceptibility of Mycobacterium tuberculosis.

As a facultative intracellular pathogen, M. tuberculosis (Mtb) is highly adapted to evading antibacterial mechanisms in phagocytic cells. Both the macrophage and pathogen experience transcriptional and metabolic changes from the onset of phagocytosis. To account for this interaction in the assessment of intracellular drug susceptibility, we allowed a 3-day preadaptation phase post-macrophage infection prior to drug treatment. We found that intracellular Mtb in human monocyte-derived macrophages (MDM) presents dramatic alterations in susceptibility to isoniazid, sutezolid, rifampicin and rifapentine when compared to axenic culture. Infected MDM gradually accumulate lipid bodies, adopting a characteristic appearance reminiscent of foamy macrophages in granulomas. Furthermore, TB granulomas in vivo develop hypoxic cores with decreasing oxygen tension gradients across their radii. Accordingly, we evaluated the effects of hypoxia on preadapted intracellular Mtb in our MDM model. We observed that hypoxia induced greater lipid body formation and no additional shifts in drug tolerance, suggesting that the adaptation of intracellular Mtb to baseline host cell conditions under normoxia dominates changes to intracellular drug susceptibility. Using unbound plasma concentrations in patients as surrogates for free drug concentrations in lung interstitial fluid, we estimate that intramacrophage Mtb in granulomas are exposed to bacteriostatic concentrations of most study drugs.

CinA mediates multidrug tolerance in Mycobacterium tuberculosis.

The ability of Mycobacterium tuberculosis (Mtb) to resist and tolerate antibiotics complicates the development of improved tuberculosis (TB) chemotherapies. Here we define the Mtb protein CinA as a major determinant of drug tolerance and as a potential target to shorten TB chemotherapy. By reducing the fraction of drug-tolerant persisters, genetic inactivation of cinA accelerated killing of Mtb by four antibiotics in clinical use: isoniazid, ethionamide, delamanid and pretomanid. Mtb ΔcinA was killed rapidly in conditions known to impede the efficacy of isoniazid, such as during nutrient starvation, during persistence in a caseum mimetic, in activated macrophages and during chronic mouse infection. Deletion of CinA also increased in vivo killing of Mtb by BPaL, a combination of pretomanid, bedaquiline and linezolid that is used to treat highly drug-resistant TB. Genetic and drug metabolism studies suggest that CinA mediates drug tolerance via cleavage of NAD-drug adducts.

A Rabbit Model to Study Antibiotic Penetration at the Site of Infection for Nontuberculous Mycobacterial Lung Disease: Macrolide Case Study.

Nontuberculous mycobacterial pulmonary disease (NTM-PD) is a potentially fatal infectious disease requiring long treatment duration with multiple antibiotics and against which there is no reliable cure. Among the factors that have hampered the development of adequate drug regimens is the lack of an animal model that reproduces the NTM lung pathology required for studying antibiotic penetration and efficacy. Given the documented similarities between tuberculosis and NTM immunopathology in patients, we first determined that the rabbit model of active tuberculosis reproduces key features of human NTM-PD and provides an acceptable surrogate model to study lesion penetration. We focused on clarithromycin, a macrolide and pillar of NTM-PD treatment, and explored the underlying causes of the disconnect between its favorable potency and pharmacokinetics and inconsistent clinical outcome. To quantify pharmacokinetic-pharmacodynamic target attainment at the site of disease, we developed a translational model describing clarithromycin distribution from plasma to lung lesions, including the spatial quantitation of clarithromycin and azithromycin in mycobacterial lesions of two patients on long-term macrolide therapy. Through clinical simulations, we visualized the coverage of clarithromycin in plasma and four disease compartments, revealing heterogeneous bacteriostatic and bactericidal target attainment depending on the compartment and the corresponding potency against nontuberculous mycobacteria in clinically relevant assays. Overall, clarithromycin's favorable tissue penetration and lack of bactericidal activity indicated that its clinical activity is limited by pharmacodynamic, rather than pharmacokinetic, factors. Our results pave the way toward the simulation of lesion pharmacokinetic-pharmacodynamic coverage by multidrug combinations to enable the prioritization of promising regimens for clinical trials.

On-Slide Heat Sterilization Enables Mass Spectrometry Imaging of Tissue Infected with High-Threat Pathogens Outside of Biocontainment: A Study Directed at Mycobacterium tuberculosis.

Mass spectrometry imaging investigations of tissues infected with agents that require high-security biocontainment, such as Mycobacterium tuberculosis, have been limited due to incompatible sterilization techniques. Here we describe an on-slide heat sterilization method that enables mass spectrometry imaging investigations of pharmaceuticals, lipids, and metabolites in infected tissue samples outside of biocontainment. An evaluation of different temperatures and incubation times determined that 100 °C for 1 h was essential to sterilize 5 times the bacterial burden observed in tuberculosis (TB) cavity sections. Laser-capture microdissection combined with liquid chromatography with tandem mass spectrometry quantitation, in addition to mass spectrometry imaging, showed that no degradation was observed following the on-slide heat sterilization protocol for a variety of drug classes covering a range of physicochemical properties. Utilizing the tissue mimetic model, we demonstrated that the detection of lipid and metabolite ions was not impacted by heat sterilization and that, for several metabolites, the on-slide heat sterilization method improved the sensitivity when compared to control samples. An application of the on-slide heat sterilization to M. tuberculosis infected tissue enabled the first detection and spatial distribution of lipids indicative of a lysosomal storage disease phenotype within TB granuloma macrophages, in addition to the differential distribution of metabolites central to the fatty acid oxidation pathway. These initial investigations detected a pronounced heterogeneity within the cellular regions and necrotic cores of individual TB granulomas and across different evolving granulomas. This study provides the framework for mass spectrometry imaging investigations of high-threat pathogens outside of biocontainment.

Structure, In Vivo Detection, and Antibacterial Activity of Metabolites of SQ109, an Anti-Infective Drug Candidate.

SQ109 is a drug candidate for the treatment of tuberculosis (TB). It is thought to target primarily the protein MmpL3 in Mycobacterium tuberculosis, but it also inhibits the growth of some other bacteria. SQ109 is metabolized by the liver, and it has been proposed that some of its metabolites might be responsible for its activity against TB. Here, we synthesized six potential P450 metabolites of SQ109 and used these as well as 10 other likely metabolites as standards in a mass spectrometry study of M. tuberculosis-infected rabbits treated with SQ109, in addition to testing all 16 putative metabolites for antibacterial activity. We found that there were just two major metabolites in lung tissue: a hydroxy-adamantyl analog of SQ109 and N'-adamantylethylenediamine. Neither of these, or the other potential metabolites tested, inhibited the growth of M. tuberculosis or of M. smegmatis, Bacillus subtilis, or E. coli, making it unlikely that an SQ109 metabolite contributes to its antibacterial activity. In the rabbit TB model, it is thus the gradual accumulation of nonmetabolized SQ109 in tissues to therapeutic levels that leads to good efficacy. Our results also provide new insights into how SQ109 binds to its target MmpL3, based on our mass spectroscopy results which indicate that the charge in SQ109 is primarily localized on the geranyl nitrogen, explaining the very short distance to a key Asp found in the X-ray structure of SQ109 bound to MmpL3.

Pharmacokinetics and Target Attainment of SQ109 in Plasma and Human-Like Tuberculosis Lesions in Rabbits.

SQ109 is a novel well-tolerated drug candidate in clinical development for the treatment of drug-resistant tuberculosis (TB). It is the only inhibitor of the MmpL3 mycolic acid transporter in clinical development. No SQ109-resistant mutant has been directly isolated thus far in vitro, in mice, or in patients, which is tentatively attributed to its multiple targets. It is considered a potential replacement for poorly tolerated components of multidrug-resistant TB regimens. To prioritize SQ109-containing combinations with the best potential for cure and treatment shortening, one must understand its contribution against different bacterial populations in pulmonary lesions. Here, we have characterized the pharmacokinetics of SQ109 in the rabbit model of active TB and its penetration at the sites of disease-lung tissue, cellular and necrotic lesions, and caseum. A two-compartment model with first-order absorption and elimination described the plasma pharmacokinetics. At the human-equivalent dose, parameter estimates fell within the ranges published for preclinical species. Tissue concentrations were modeled using an "effect" compartment, showing high accumulation in lung and cellular lesion areas with penetration coefficients in excess of 1,000 and lower passive diffusion in caseum after 7 daily doses. These results, together with the hydrophobic nature and high nonspecific caseum binding of SQ109, suggest that multiweek dosing would be required to reach steady state in caseum and poorly vascularized compartments, similar to bedaquiline. Linking lesion pharmacokinetics to SQ109 potency in assays against replicating, nonreplicating, and intracellular M. tuberculosis showed SQ109 concentrations markedly above pharmacokinetic-pharmacodynamic targets in lung and cellular lesions throughout the dosing interval.

Blocking Bacterial Naphthohydroquinone Oxidation and ADP-Ribosylation Improves Activity of Rifamycins against Mycobacterium abscessus.

Rifampicin is an effective drug for treating tuberculosis (TB) but is not used to treat Mycobacterium abscessus infections due to poor in vitro activity. While rifabutin, another rifamycin, has better anti-M. abscessus activity, its activity is far from the nanomolar potencies of rifamycins against Mycobacterium tuberculosis. Here, we asked (i) why is rifabutin more active against M. abscessus than rifampicin, and (ii) why is rifabutin's anti-M. abscessus activity poorer than its anti-TB activity? Comparative analysis of naphthoquinone- versus naphthohydroquinone-containing rifamycins suggested that the improved activity of rifabutin over rifampicin is linked to its less readily oxidizable naphthoquinone core. Although rifabutin is resistant to bacterial oxidation, metabolite and genetic analyses showed that this rifamycin is metabolized by the ADP-ribosyltransferase ArrMab like rifampicin, preventing it from achieving the nanomolar activity that it displays against M. tuberculosis. Based on the identified dual mechanism of intrinsic rifamycin resistance, we hypothesized that rifamycins more potent than rifabutin should contain the molecule's naphthoquinone core plus a modification that blocks ADP-ribosylation at its C-23. To test these predictions, we performed a blinded screen of a diverse collection of 189 rifamycins and identified two molecules more potent than rifabutin. As predicted, these compounds contained both a more oxidatively resistant naphthoquinone core and C-25 modifications that blocked ADP-ribosylation. Together, this work revealed dual bacterial metabolism as the mechanism of intrinsic resistance of M. abscessus to rifamycins and provides proof of concept for the repositioning of rifamycins for M. abscessus disease by developing derivatives that resist both bacterial oxidation and ADP-ribosylation.

Caseum: a Niche for Mycobacterium tuberculosis Drug-Tolerant Persisters.

Caseum, the central necrotic material of tuberculous lesions, is a reservoir of drug-recalcitrant persisting mycobacteria. Caseum is found in closed nodules and in open cavities connecting with an airway. Several commonly accepted characteristics of caseum were established during the preantibiotic era, when autopsies of deceased tuberculosis (TB) patients were common but methodologies were limited. These pioneering studies generated concepts such as acidic pH, low oxygen tension, and paucity of nutrients being the drivers of nonreplication and persistence in caseum. Here we review widely accepted beliefs about the caseum-specific stress factors thought to trigger the shift of Mycobacterium tuberculosis to drug tolerance. Our current state of knowledge reveals that M. tuberculosis is faced with a lipid-rich diet rather than nutrient deprivation in caseum. Variable caseum pH is seen across lesions, possibly transiently acidic in young lesions but overall near neutral in most mature lesions. Oxygen tension is low in the avascular caseum of closed nodules and high at the cavity surface, and a gradient of decreasing oxygen tension likely forms toward the cavity wall. Since caseum is largely made of infected and necrotized macrophages filled with lipid droplets, the microenvironmental conditions encountered by M. tuberculosis in foamy macrophages and in caseum bear many similarities. While there remain a few knowledge gaps, these findings constitute a solid starting point to develop high-throughput drug discovery assays that combine the right balance of oxygen tension, pH, lipid abundance, and lipid species to model the profound drug tolerance of M. tuberculosis in caseum.

Tissue Distribution of Doxycycline in Animal Models of Tuberculosis.

Doxycycline, an FDA-approved tetracycline, is used in tuberculosis in vivo models for the temporal control of mycobacterial gene expression. In these models, animals are infected with recombinant Mycobacterium tuberculosis carrying genes of interest under transcriptional control of the doxycycline-responsive TetR-tetO unit. To minimize fluctuations of plasma levels, doxycycline is usually administered in the diet. However, tissue penetration studies to identify the minimum doxycycline content in food achieving complete repression of TetR-controlled genes in tuberculosis (TB)-infected organs and lesions have not been conducted. Here, we first determined the tetracycline concentrations required to achieve silencing of M. tuberculosis target genes in vitro Next, we measured doxycycline concentrations in plasma, major organs, and lung lesions in TB-infected mice and rabbits and compared these values to silencing concentrations measured in vitro We found that 2,000 ppm doxycycline supplemented in mouse and rabbit feed is sufficient to reach target concentrations in TB lesions. In rabbit chow, the calcium content had to be reduced 5-fold to minimize chelation of doxycycline and deliver adequate oral bioavailability. Clearance kinetics from major organs and lung lesions revealed that doxycycline levels fall below concentrations that repress tet promoters within 7 to 14 days after doxycycline is removed from the diet. In summary, we have shown that 2,000 ppm doxycycline supplemented in standard mouse diet and in low-calcium rabbit diet delivers concentrations adequate to achieve full repression of tet promoters in infected tissues of mice and rabbits.

Extreme Drug Tolerance of Mycobacterium tuberculosis in Caseum.

Tuberculosis (TB) recently became the leading infectious cause of death in adults, while attempts to shorten therapy have largely failed. Dormancy, persistence, and drug tolerance are among the factors driving the long therapy duration. Assays to measure in situ drug susceptibility of Mycobacterium tuberculosis bacteria in pulmonary lesions are needed if we are to discover new fast-acting regimens and address the global TB threat. Here we take a first step toward this goal and describe an ex vivo assay developed to measure the cidal activity of anti-TB drugs against M. tuberculosis bacilli present in cavity caseum obtained from rabbits with active TB. We show that caseum M. tuberculosis bacilli are largely nonreplicating, maintain viability over the course of the assay, and exhibit extreme tolerance to many first- and second-line TB drugs. Among the drugs tested, only the rifamycins fully sterilized caseum. A similar trend of phenotypic drug resistance was observed in the hypoxia- and starvation-induced nonreplicating models, but with notable qualitative and quantitative differences: (i) caseum M. tuberculosis exhibits higher drug tolerance than nonreplicating M. tuberculosis in the Wayne and Loebel models, and (ii) pyrazinamide is cidal in caseum but has no detectable activity in these classic nonreplicating assays. Thus, ex vivo caseum constitutes a unique tool to evaluate drug potency against slowly replicating or nonreplicating bacilli in their native caseous environment. Intracaseum cidal concentrations can now be related to the concentrations achieved in the necrotic foci of granulomas and cavities to establish correlations between clinical outcome and lesion-centered pharmacokinetics-pharmacodynamics (PK-PD) parameters.

An In Vitro Caseum Binding Assay that Predicts Drug Penetration in Tuberculosis Lesions.

The eradication of tuberculosis disease requires drug regimens that can penetrate the multiple layers of complex pulmonary lesions. Drug distribution in the caseous cores of cavities and lesions is especially crucial because they harbor subpopulations of drug-tolerant bacteria also commonly referred to as persisters. Existing methods for the measurement of drug penetration in tuberculosis lesions involve costly and time-consuming in vivo pharmacokinetic studies coupled to bioanalytical or imaging techniques. The in vitro measurement of drug binding to caseum macromolecules was proposed as an alternative to such techniques since this binding hinders the passive diffusion of drug molecules through caseum. Rapid equilibrium dialysis is a fast and reliable system for performing plasma protein and tissue binding studies. In this protocol, we used a rapid equilibrium dialysis (RED) device to measure drug binding to homogenates of caseum that is excised from the lesions and cavities of tuberculosis-infected rabbits. The protocol also describes how to generate a surrogate matrix from lipid loaded THP-1 macrophages to use in place of caseum. This caseum/surrogate binding assay is an important tool in tuberculosis drug discovery and can be adapted to help study drug distribution in lesions or abscesses caused by other diseases.

Pyrazinamide Resistance Is Caused by Two Distinct Mechanisms: Prevention of Coenzyme A Depletion and Loss of Virulence Factor Synthesis.

Pyrazinamide (PZA) is a critical component of first- and second-line treatments of tuberculosis (TB), yet its mechanism of action largely remains an enigma. We carried out a genetic screen to isolate Mycobacterium bovis BCG mutants resistant to pyrazinoic acid (POA), the bioactive derivative of PZA, followed by whole genome sequencing of 26 POA resistant strains. Rather than finding mutations in the proposed candidate targets fatty acid synthase I and ribosomal protein S1, we found resistance conferring mutations in two pathways: missense mutations in aspartate decarboxylase panD, involved in the synthesis of the essential acyl carrier coenzyme A (CoA), and frameshift mutations in the vitro nonessential polyketide synthase genes mas and ppsA-E, involved in the synthesis of the virulence factor phthiocerol dimycocerosate (PDIM). Probing for cross resistance to two structural analogs of POA, nicotinic acid and benzoic acid, showed that the analogs share the PDIM- but not the CoA-related mechanism of action with POA. We demonstrated that POA depletes CoA in wild-type bacteria, which is prevented by mutations in panD. Sequencing 10 POA-resistant Mycobacterium tuberculosis H37Rv isolates confirmed the presence of at least 2 distinct mechanisms of resistance to the drug. The emergence of resistance through the loss of a virulence factor in vitro may explain the lack of clear molecular patterns in PZA-resistant clinical isolates, other than mutations in the prodrug-converting enzyme. The apparent interference of POA with virulence pathways may contribute to the drug's excellent in vivo efficacy compared to its modest in vitro potency.

Prediction of Drug Penetration in Tuberculosis Lesions.

The penetration of antibiotics in necrotic tuberculosis lesions is heterogeneous and drug-specific, but the factors underlying such differential partitioning are unknown. We hypothesized that drug binding to macromolecules in necrotic foci (or caseum) prevents passive drug diffusion through avascular caseum, a critical site of infection. Using a caseum binding assay and MALDI mass spectrometry imaging of tuberculosis drugs, we showed that binding to caseum inversely correlates with passive diffusion into the necrotic core. We developed a high-throughput assay relying on rapid equilibrium dialysis and a caseum surrogate designed to mimic the composition of native caseum. A set of 279 compounds was profiled in this assay to generate a large data set and explore the physicochemical drivers of free diffusion into caseum. Principle component analysis and modeling of the data set delivered an in silico signature predictive of caseum binding, combining 69 molecular descriptors. Among the major positive drivers of binding were high lipophilicity and poor solubility. Determinants of molecular shape such as the number of rings, particularly aromatic rings, number of sp(2) carbon counts, and volume-to-surface ratio negatively correlated with the free fraction, indicating that low-molecular-weight nonflat compounds are more likely to exhibit low caseum binding properties and diffuse effectively through caseum. To provide simple guidance in the property-based design of new compounds, a rule of thumb was derived whereby the sum of the hydrophobicity (clogP) and aromatic ring count is proportional to caseum binding. These tools can be used to ensure desirable lesion partitioning and guide the selection of optimal regimens against tuberculosis.