A New Strategy Enabling Combined Fluorescence Imaging of Individual Tuberculous Granulomas and Precise Photothermal Therapy of Tuberculosis With Lesion- and Pathogen-Targeting Capabilities of the Nanoparticles

Abstract

Tuberculosis (TB) infection, caused by Mycobacterium tuberculosis, reveals tens of millions of new cases and causes millions of deaths each year. The traditional clinical treatment for TB is long-term and combined usage of antibiotics but suffers from poor lesion and pathogen targeting, insufficient efficacy, high systemic toxicity, and ineffectiveness against drug-resistant M. tuberculosis. As a characteristic lesion of TB, granulomas have a compact construction, which greatly limits the access of small-molecule antibiotics to the necrotic area to kill M. tuberculosis [1]. Moreover, M. tuberculosis bacilli, which are quiescent within the granulomas, are phenotypically resistant to traditional antibiotics. These factors contribute to the poor outcomes of conventional antibiotic regimens. So far, several types of nanodrugs including PEGylated liposomes, PEGylated quantum dots, and polymer micelles have been studied with the aim of granuloma targeting [2, 3]. However, these nanoparticles (NPs) cannot achieve specific targeting of M. tuberculosis. Therefore, the development of lesion–pathogen dual-targeting formulations that can successfully track granulomas, penetrate their compact structures to reach the necrotic region, and bind specifically to the internal M. tuberculosis is highly desirable to enhance anti-TB efficiency.

Wang and coworkers [4, 5] have recently reported an ingenious and simple design for the precise theranostic treatment of TB, that is, a mycobacterium pre-activated macrophage-like biomimetic nanoparticle (BNP) strategy. As depicted in Figure 1a, macrophages display abundant expression of pathogen-specific receptors on their surface after mycobacterium stimulation; thus, M. tuberculosis targeting can be realized through the specific binding of these receptors to the corresponding ligands of M. tuberculosis. Next, cell membranes are isolated from these pre-activated macrophages and coated onto drug-loaded polymer NPs to form advanced BNPs called BBTD@PM NPs. The propeller-shaped tetraphenylethylene substituents of TPE-BT-BBTD ensure that the drug does not suffer from aggregation-caused quenching of emission; instead, encapsulated BBTD@PM NPs are aggregation-induced emission (AIE)-active with near-infrared region IIb (NIR-IIb) fluorescence from the extended heteroaromatic π-system upon excitation by 1064-nm laser light. Moreover, the NPs have strong photothermal (light-to-heat) conversion efficiency (33.8%) which facilitates fluorescence imaging of lesions/pathogens, as well as photothermal killing of M. tuberculosis. Photoinduced heating of NPs in the NIR region is an effective targeted treatment using simple procedures with minimal damage to the surrounding healthy tissues.

The authors applied BBTD@PM NPs to a mouse model of pulmonary TB. Upon intravenous injection into TB mice, BBTD@PM NPs rapidly accumulate in granulomas and reside in situ for a long period of time through the enhanced permeability and retention (EPR) effect unique to nanomaterials (Figure 1b). Meanwhile, these BNPs emit bright NIR-IIb fluorescence under 1064-nm stimulation and light up individual granulomas in mice lungs. It is noteworthy that the fluorescence imaging resolution of BBTD@PM NPs for the visualization of single granulomas is up to 0.2 mm, which is dramatically higher than the computed tomography (CT) technology (resolution ≈ 1.0 mm) that is used for the clinical imaging of TB. Importantly, this study reports for the first time fluorescence imaging of in situ tuberculous granulomas in lungs.

Moreover, BBTD@PM NPs penetrate the dense structure of granulomas and enter the necrotic areas to target the internal M. tuberculosis through specific receptor–ligand binding (Figure 1b). Upon irradiation of the whole chest from the outside of the thoracic cavity using a 1064-nm laser, the photothermal effect of BBTD@PM NPs is activated, resulting in the targeted photothermal elimination of M. tuberculosis. After treatment, the pathological damage to the lungs is notably attenuated, and the level of inflammation in the lung tissue is dramatically decreased. This “BBTD@PM NPs + 1064 nm laser” regimen is superior to the combined administration of traditional antibiotics.

In summary, the lesion–pathogen dual-targeting and NIR-IIb fluorescence imaging-guided precise photothermal modality developed by Wang and coworkers has been demonstrated to be a first-of-its-kind theranostic strategy for TB, which may guide future viable strategies for TB management in clinical trials. Furthermore, this work provides new perspectives on the potential applications of nanomedicines in the management of drug-susceptible and drug-resistant TB either by the targeted photothermal bactericidal strategy or by providing an alternative modality that circumvents traditional antibiotic resistance mechanisms. Challenges include the design of NPs with even higher photothermal conversion efficiency, translating the new methodology into clinical trials, and the application to other infectious diseases. The authors identify automated or semi-automated synthesis techniques, such as microfluidic chips, to enhance reproductivity and scalability for translational research. Additionally, to streamline the fabrication steps, the design of amphiphilic NIR-II AIE-active molecules that can self-assemble into water-dispersible NPs are a viable approach [4].

The authors declare no conflict of interest.