An intriguing research explored the repurposing of existing drugs that could block viral entry or mitigate the hyperinflammatory response. Among these, the combination of bromhexine—a widely available mucolytic with TMPRSS2 inhibitory activity—and spironolactone—an anti-androgenic mineralocorticoid receptor antagonist with immunomodulatory properties—was investigated in a pioneering open-label clinical trial led by Mareev and colleagues. The study, conducted early in the pandemic, sought to determine whether this dual approach could accelerate clinical recovery in hospitalized patients with moderate COVID-19. Its results added an important piece to the evolving puzzle of host-directed therapies for SARS-CoV-2 infection.
The Rationale for Bromhexine and Spironolactone in COVID-19
Early in the pandemic, researchers rapidly identified that SARS-CoV-2 relies on two key host proteins to enter human cells: the angiotensin-converting enzyme 2 (ACE2) receptor and the type II transmembrane serine protease TMPRSS2. While ACE2 acts as the docking station, TMPRSS2 primes the viral spike protein, enabling membrane fusion and viral entry. Inhibiting TMPRSS2 therefore emerged as a promising strategy to block infection at its earliest stage. Bromhexine, an inexpensive over-the-counter mucolytic, was known from in vitro studies to inhibit TMPRSS2, and its active metabolite ambroxol had demonstrated anti-viral properties against other respiratory viruses. This made bromhexine an attractive candidate for rapid clinical testing against COVID-19.
Spironolactone, on the other hand, was included for a different reason. While primarily used as a potassium-sparing diuretic, spironolactone also inhibits androgen receptor signalling and has been shown to down-regulate ACE2 expression in certain tissues. Additionally, it possesses anti-inflammatory and anti-fibrotic effects that could theoretically dampen the cytokine storm and prevent lung damage in severe COVID-19. By combining bromhexine with spironolactone, the investigators hypothesized a dual benefit: reducing viral entry while simultaneously modulating the host’s harmful immune response. This dual-pathway approach set the stage for the Mareev trial, formally titled “Bromhexine and spironolactone for coronavirus infection: an open-label non-randomized comparative clinical trial” (BISC).
Trial Design and Patient Selection
The Mareev trial was designed as a prospective, open-label, non-randomized comparative study. It enrolled hospitalized adult patients with PCR-confirmed COVID-19 and clinical signs of moderate pneumonia—characterised by fever, respiratory symptoms, and radiological evidence of lung involvement—but not requiring mechanical ventilation at the time of enrollment. A tot patients were allocated to one of three treatment groups, based on the decision of the attending physician and patient consent. While non-randomized designs are inherently susceptible to bias, they can provide rapid feasibility and safety data, which were crucial at a time when therapeutic options were limited.
The three groups consisted of: (1) a control group receiving standard of care therapy as per local guidelines, which included anticoagulants, oxygen support, and, in some cases, hydroxychloroquine or azithromycin; (2) a bromhexine group receiving standard of care plus oral bromhexine hydrochloride at a dose of 8 mg four times daily; and (3) a combination group receiving standard of care, bromhexine 8 mg four times daily, and spironolactone 25 mg twice daily. Treatment was continued for a tot days or until hospital discharge, whichever came first. Baseline characteristics, including age, sex, body mass index, and severity of lung involvement, were broadly comparable across the groups, though the lack of randomization meant that unrecognized confounders could not be entirely ruled out.
Primary and Secondary Endpoints
The primary endpoint of the Mareev trial was the time to clinical improvement, defined as a two-point reduction on a seven-category ordinal scale of clinical status. This scale ranged from 1 (not hospitalised, no limitations) to 7 (death). The use of such ordinal scales became a standard approach in COVID-19 trials to capture meaningful transitions in patient condition. Secondary endpoints included the proportion of patients achieving clinic and day 28, time to viral clearance measured by serial nasopharyngeal swab PCR, changes in inflammatory biomarkers such as C-reactive protein (CRP) and interleukin-6 (IL-6), and safety outcomes including any adverse events potentially attributable to the study medications.
Collecting data on both clinical and virological outcomes allowed the researchers to assess not only how quickly patients felt better but also whether the intervention was capable of accelerating the elimination of the virus—a crucial piece of evidence for any antiviral strategy. Elevations in inflammatory markers were tracked to determine whether the addition of spironolactone would translate into a measurable anti-inflammatory effect, as its preclinical profile suggested.
Reported Efficacy Findings
The results of the Mareev trial, published in late 2020, offered signals of benefit, particularly in the combination arm. According to the study, patients treated with bromhexine alone experienced a shorter time to clinical improvement compared to the standard-of-care group. More strikingly, those receiving both bromhexine and spironolactone showed the most pronounced acceleration, with the median time to improvement dropping markedly. The proportion of patients achieving improvement within 10 days was also higher in the treatment arms, though the small sample size and non-randomized design caution against over-interpretation. Viral clearance, as assessed by the time to a negative PCR test, appeared to trend favourably in the bromhexine-containing groups, lending credibility to the TMPRSS2 inhibition hypothesis.
“The combination of a TMPRSS2 inhibitor and an immunomodulator represents a logical two-pronged approach against COVID-19, targeting the virus at the point of entry while calming the host’s own damaging response.”
Inflammatory markers such as CRP and IL-6 declined more rapidly in the combination group, a finding that aligned with spironolactone’s proposed anti-inflammatory actions. However, given the open-label design, subjective endpoints like clinical improvement could be influenced by patient and clinician expectations, so these results must be viewed as hypothesis-generating rather than definitive. Nevertheless, the Mareev trial provided some of the earliest clinical data that bromhexine, especially when paired with spironolactone, could positively influence the course of moderate COVID-19 and justified further investigation in larger, randomized trials.
Safety and Tolerability
Safety is a paramount concern when repurposing existing drugs, and the Mareev trial carefully documented adverse events. Overall, bromhexine was well tolerated at the 32 mg daily dose. The most commonly reported side effects were mild gastrointestinal disturbances, such as nausea and diarrhoea, which are known class effects of oral mucolytics. No patient discontinued treatment due to bromhexine-related adverse events. Spironolactone, used at a modest dose of 50 mg daily, did not lead to significant hyperkalaemia or hypotension in this short-term treatment setting. The trial’s investigators noted that monitoring of serum potassium was prudent but did not identify any clinically concerning elevations.
These safety findings are consistent with the long-established safety profiles of both medications. Bromhexine has been marketed for decades as a cough remedy, and its adverse event profile is mild. Spironolactone is widely used in heart failure and hypertension with a manageable safety profile. The Mareev trial thus supported the notion that these drugs could be deployed safely in the acute COVID-19 setting, a critical prerequisite for any larger-scale testing or potential clinical use.
Placing the Mareev Trial in Context
The Mareev study did not exist in isolation. Around the same time, multiple groups around the world were investigating bromhexine for COVID-19. For instance, Tolouian and colleagues examined bromhexine in hospitalized patients with COVID-19 and reported mixed outcomes, while Mikhaylov et al. explored bromhexine as post-exposure prophylaxis for medical personnel with encouraging results. Other studies, such as the Li trial on bromhexine hydrochloride tablets for moderate COVID‐19 and the Ansarin study assessing clinical outcomes and mortality, contributed further data. The overall picture from these early-phase trials suggested that bromhexine might offer modest benefits, particularly in mild to moderate disease, but that larger randomized controlled trials were necessary to confirm efficacy.
What set the Mareev trial apart was its use of spironolactone as an adjunct. The observed improvement in inflammatory markers and the apparent synergistic effect on recovery time pointed toward a potential advantage of combining host-directed therapies. However, because the trial was not placebo-controlled and lacked randomization, the true magnitude of any benefit remains uncertain. Subsequent larger trials of TMPRSS2 inhibitors like camostat mesylate did not replicate the early promise, and bromhexine itself failed to show a clear mortality benefit in some studies, underscoring the difficulty of translating in vitro findings into clinical practice.
Nevertheless, the Mareev trial remains an important part of the COVID-19 research tapestry. It demonstrated scientific agility—testing a logical drug combination rapidly—and provided hypotheses that could inform future pandemic preparedness. The notion that a cheap, widely available mucolytic could block viral entry, especially when reinforced by an immunomodulator, continues to invite research interest, even if the window for large COVID-19 trials has narrowed with the advent of antivirals and vaccines.
Conclusion
The Mareev open-label non-randomized comparative clinical trial was a bold early attempt to test a dual-target drug strategy in moderate COVID-19. Its findings suggested that the addition of bromhexine, and particularly the combination of bromhexine with spironolactone, might accelerate clinical improvement and reduce inflammation, with an acceptable safety profile. While the study’s design limitations prevent definitive conclusions, it contributed valuable real-world data to the growing body of evidence on host-directed therapies. For clinicians and researchers, the Mareev trial serves as a reminder that in the face of a novel pathogen, well-reasoned repurposing of existing safe drugs can rapidly generate hypotheses and inform the direction of future trials, even if those hypotheses eventually require validation in more rigorous, placebo-controlled settings.