Old Drug, New Immune Trick: Benztropine 'Teaches' Macrophages to Crush Tuberculosis

Scientists have found an unexpected antibacterial effect in benztropine, a long-standing drug for the symptoms of Parkinson's disease. It does not kill the tuberculosis bacteria directly, but rewires the host's macrophages via the histamine H1 receptor, making the bacteria less able to survive inside the cells. In mice with aerosolized tuberculosis, oral benztropine reduced the bacterial load in the lungs by up to 70%; in a model of Salmonella skin abscesses, local injection reduced the size of the lesion by 71% and reduced the number of bacteria by about a logarithm. The work is published in npj Antimicrobials and Resistance.

Background

In 2023, WHO recorded about 8.2 million new diagnoses and about 1.25 million deaths — tuberculosis is again the main infectious killer. A special pain is drug-resistant forms (MDR/RR-TB), where treatment is long, toxic and often unavailable. This pushes for strategies that strengthen the host's immunity, rather than hitting only the mycobacterium.

• What is host-directed therapy (HDT) for TB? These are supplements to the standard regimen that target host cells: they enhance the bactericidal mechanisms of the macrophage, trigger autophagy/phagosome acidification, suppress destructive inflammation, and help repair lung tissue. Their advantage is a lower risk of resistance and an effect on several pathways at once. Candidates include metformin, statins, imatinib, vit. D, NSAIDs; some have already reached early clinical trials.
• Macrophage as a "battlefield". M. tuberculosis lives inside the macrophage, blocking phagosome acidification, ROS response, and fusion with lysosomes. There is evidence that the bacterium uses the host's histamine pathway: activation of the H1 receptor (HRH1) on macrophages inhibits NOX2-dependent ROS and slows down acidification, facilitating Mtb survival (GRK2–p38MAPK signaling). This means that HRH1 blockade is a logical HDT target.
• Why benztropine was interesting. It is an old anti-Parkinsonian drug with antimuscarinic and antihistamine activity; its profile allows it to inhibit HRH1 and at the same time it has already been studied for safety/pharmacokinetics - i.e. a candidate for repositioning. (These properties are described in pharmacology references.)
• Are there any precedents for successful HDT in TB? There are signals, but the picture is mixed. For example, in an RCT, metformin did not accelerate sputum sterilization, but it reduced excessive inflammation and improved X-ray dynamics — that is, it affected the “quality” of recovery. As for statins, there is convincing preclinical evidence (autophagy via AMPK–mTOR–TFEB), but there is little clinical evidence so far. This sets realistic expectations: HDT is not a replacement for antibiotics, but an adjuvant with a chance to improve outcomes.
• What remains unresolved in HDT. Answers are needed about dose and delivery (is the concentration in alveolar macrophages sufficient), safety in long courses, drug interactions with anti-TB drugs, and proper endpoints (not just CFU, but also restoration of lung function and reduction of post-TB damage).
• How does this new work contribute to the field? It adds a mechanistic and in vivo argument for HRH1-directed HDT: it shows that pharmacological inhibition of H1 in macrophages enhances phagosome acidification and limits Mtb growth, while the oral drug reduces the bacterial load in mouse lungs. This opens the way for: (1) small clinical trials of adjuvant benztropine/its HRH1-selective analogues, (2) the search for biomarkers of response (HRH1-axis activity in monocytes, phagosome acidity), (3) careful assessment of the side-effect profile taking into account the anticholinergic effect.

What exactly did they do?

• A high-throughput screening of the COVID Box (MMV) library was performed on macrophages infected with Mycobacterium tuberculosis (Mtb). Among the “hit the target” were several known drugs - and benztropine stood out because it worked inside the cells, but did not work in broth against Mtb (up to 100 μM), i.e. it is a host-directed therapy (HDT), targeting the host mechanisms.
• Benztropine activity in human and mouse macrophages was confirmed; IC₅₀ was ~15 μM (THP-1) and 4 μM (RAW264.7). In terms of dynamics inside the cells, the drug acted bacteriostatically - it restrained the reproduction of Mtb, but did not "mow it down" instantly.
• In a mouse model of tuberculosis (low-dose aerosol infection), a two-week course of benztropine (10–20 mg/kg orally) reduced lung CFU; a dose of 20 mg/kg gave a ≈70% reduction—comparable to rifampin 10 mg/kg (≈80%). There was no significant effect on the liver/spleen; synergy with rifampin was not observed in this model either.
• In a Salmonella Typhimurium abscess model, a single local injection of benztropine (5 mg/kg) reduced lesion diameter by 71% and decreased bacterial load by approximately 1 log; clinical signs of disease were also reduced.

How does this work

The key is that benztropine blocks the histamine H1 receptor (HRH1) on macrophages. In tuberculosis, histamine and HRH1, according to the authors' data and previous studies, weaken the cell's defense mechanisms. If HRH1 is inhibited (with benztropine or the classic antihistamine pyrilamine, or the HRH1 CRISPR/siRNA gene is turned off), then phagosomes with Mtb are more acidified, and the mycobacterium survives worse. Thus, benztropine indirectly reduces the intracellular growth of Mtb - through the host, and not by directly attacking the microbe.

Why is this important?

• A new class of targets. HDT (host-targeted) approaches have the potential to generate less resistance than direct antibiotics and may enhance standard regimens, especially in multidrug-resistant TB.
• Repositioning speeds up the path to the clinic. Benztropine has been known since the 1950s: it has pharmacokinetics and safety data. In mice, oral administration worked without visible toxicity at the doses tested - this is an argument "for" further research. (The estimated human equivalent may be higher than the usual "Parkinsonian" doses - this remains to be tested.)
• Not just tuberculosis. The effect against Salmonella in abscess suggests that HRH1 modulation may help in other intracellular bacterial infections.

Important details and limitations

• In broth culture of Mtb, benztropine is almost inactive (up to 100 μM) - that is, it is not a substitute for antibiotics, but a candidate for addition (or for special clinical situations), namely as HDT.
• In the mouse model, synergy with rifampicin was not seen, possibly due to dose, timing, or organ specificity. This does not rule out the benefit of combinations in other regimens, but requires a separate trial design.
• The path to the patient goes through the pharmacology of HRH1: it is necessary to clarify the dose dependence, permeability into the lung tissue, the side effect profile and, probably, to develop HRH1-selective benztropine analogues with less penetration into the CNS (to reduce anticholinergic/dopamine effects). The authors have already described the first developments in the structure-activity.

What's next?

• Phase I/IIa: safety and pharmacodynamics (biomarkers of macrophage activation, phagosome acidity) in patients with TB, options - adjuvant to standard therapy.
• Response biomarkers: HRH1 expression/function in monocytes/macrophages, intracellular bacterial load dynamics based on transcript fingerprints.
• Chemistry: Generation of HRH1-selective benztropine derivatives based on the authors' SAR prompts.

Source: Sahile HA et al. The Parkinson's drug benztropine possesses histamine receptor 1-dependent host-directed antimicrobial activity against Mycobacterium tuberculosis. npj Antimicrobials and Resistance, 4 August 2025. doi.org/10.1038/s44259-025-00143-x