The mammalian target of rapamycin (mTOR) signaling pathway senses and responds to nutrient availability, energy sufficiency, stress, hormones and mitogens to modulate protein synthesis. Rapamycin is a bacterial product that can inhibit mTOR via the PI3K/AKT/mTOR pathway. mTOR signaling is necessary for the development of influenza and modulates the antibody response to provide cross-protective immunity to lethal infection with influenza virus. In one human study, it was found that the treatment of severe H1N1 influenza-related pneumonia with rapamycin and steroids improved the outcome. However, in other studies, immunosuppression with systemic steroids, and possibly rapamycin as well, was associated with an increased morbidity/mortality and a prolonged viral replication. In order to avoid the systemic side-effects, some investigators have postulated that the inhalation of rapamycin would be desirable. However, the inhalation of rapamycin, with its well-documented lung toxicity, could be contraindicated. Another class of drug, biguanides, can also inhibit mTOR, but have no lung toxicity. Biguanides are widely used small molecule drugs prescribed as oral anti-diabetics that have exhibited considerable promise in oncology. During the 1971 outbreak of influenza, diabetic patients treated with the biguanides, phenformin and buformin, had a lower incidence of infection than diabetics treated with sulfonylureas or insulin. Both buformin and phenformin reduce the mortality of influenza in mice; phenformin is less effective than buformin. The inhalation of buformin or phenformin for influenza may be an effective novel treatment strategy that would limit the risk of systemic side-effects associated with biguanides due to the low inhaled dose. Coronavirus disease 2019 (COVID-19) is an infectious disease caused by SARS-CoV-2, a virus closely related to the SARS virus. The disease is the cause of the 2019-2020 coronavirus outbreak. It is primarily spread between individuals via small droplets emitted from infected individuals when breathing or coughing. PI3K/AKT/mTOR signaling responses play important roles in MERS-CoV infection and may represent a novel drug target for therapeutic intervention strategies. The present review article discusses the effects of biguanides on influenza and coronavirus.
Influenza develops in approximately 20% of the world's population each year. In the US, 30,000 to 100,000 deaths occur annually due to influenza. The pandemic of 1918-1919 resulted in 50 million to 100 million deaths.
Vaccination is the primary strategy for the prevention of influenza; however, it is not always adequate. The effectiveness of the seasonal influenza vaccine varies by season. For example, during the period between November 23, 2018 to February 2, 2019, the overall adjusted vaccine effectiveness against all influenza virus infection associated with medically attended acute respiratory illness was 47%. For children aged 6 months to 17 years, the overall vaccine effectiveness was 61% (
Five drugs are currently available for the treatment or prophylaxis of influenza infections: The adamantanes (amantadine and rimantadine) and the neuraminidase inhibitors (zanamivir and oseltamivir). In 2019, the FDA approved baloxavir marboxil (trade name, Xofluza), a new class of drug which targets the endonuclease function of the viral PA polymerase subunit and prevents the transcription of viral mRNA (
The mammalian target of rapamycin (mTOR) signaling pathway senses and responds to nutrient availability, energy sufficiency, stress, hormones and mitogens to modulate protein synthesis. The mTOR pathway is dysregulated in human diseases, particularly in cancers. Rapamycin (sirolimus) is a bacterial product that can inhibit mTOR via AMPK activation and the inhibition of the PI3K/AKT/mTOR pathway (
mTOR signaling is necessary for the development of influenza and modulates the antibody response to provide cross-protective immunity to lethal infection with influenza virus. In animal studies, rapamycin was shown to promote cross-strain protection against lethal infection with influenza virus of various subtypes when administered during immunization with influenza virus subtype H3N2(
In human studies, the treatment of severe H1N1 influenza-related pneumonia with rapamycin and steroids was shown to improve the outcome (
In order to avoid the systemic side-effects, some investigators have postulated that the inhalation of rapamycin would be desirable. Inhalable rapamycin preparations have been formulated and tested on rats (
Another class of drug, biguanides, can also inhibit mTOR activation but has no lung toxicity. Biguanides are widely used small molecule drugs prescribed as oral anti-diabetics. They include the following: i) Metformin; ii) phenformin, withdrawn from US market because of its propensity to cause lactic acidosis; iii) buformin (1-butylbiguanide), an oral antidiabetic drug of the biguanide class, chemically related to metformin and phenformin; buformin was marketed by the German pharmaceutical company, Grünenthal, as Silubin; and iv) benfosformin, etoformin, tiformin
Metformin activates the 5' AMP-activated protein kinase (AMPK) pathway through liver kinase B1 (LKB1), eventually causing the inhibition of the mTOR pathway and thus, a reduction in protein synthesis and cellular proliferation. Metformin also appears to indirectly reduce AKT activation, through the AMPK-mediated phosphorylation of insulin receptor substrate 1 (IRS-1), causing the inhibition of the mTOR pathway (
Biguanides have no known lung toxicity after decades of use in millions of patients. Biguanides are cell proliferation inhibitors, and their use in oncology holds considerable promise (
During the 1971 outbreak of influenza (1968 Hong-Kong H3N2 strain), 110 diabetic patients treated with phenformin or buformin (group A) and 79 diabetic patients treated with insulin or sulfonylurea derivatives (group B) were observed (
A smaller number of complications following influenza in group A (1/110, 0.9%) as compared with group B (4/79, 5%) was not statistically significant (P=0.16, Fisher's exact test) (
Biguanides act against other viruses, apart from influenza. For example, polyhexamethylene biguanide exposure has been shown to lead to the viral aggregation of MS2 bacteriophage (
Denys and Bocian examined the protective effect of buformin (Silubin retard) against influenza in mice (
The buformin preparation was dissolved in phosphate-buffered 0.9% NaCl solution. A total of 110 white BALB/C mice were used for the experiments, weighing 18-20 g. Mice were infected with 0.05 ml influenza virus intranasally following mild anesthesia. An LD50 infectious dilution assay for mice was carried out and estimated to be 10-4 (
The buformin preparation was injected once daily subcutaneously, at a dose of 20 mg/kg, beginning 24 h after 40 animals had been infected. Treatment was carried out over a period of 4 days. Influenza-infected animals in the control group (40 animals) received 0.9% NaCl. The observations were carried out over a period of 10 days (
Inhaling a biguanide for influenza would limit the risk of systemic side-effects associated with biguanides due to the low inhaled dose. Lactic acidosis is the main biguanide systemic side-effect (
Precedence exists for inhaled drug use in influenza. The neuraminidase inhibitor, zanamivir, is administered by inhalation. Inhaled zanamivir requires 10 mg twice a day. The dose of typical inhaled asthma medications is 10-100 µg day.
Oral metformin, then known as flumamine, was examined as an anti-influenza and anti-malarial drug in the Philippines during the late 1940s. Another biguanide anti-malarial drug, biguanil, is still in use (
Metformin is taken orally twice daily by diabetic patients, with a maximum total dose of 2.5 g/day. Reducing the inhaled dose of an oral drug by a factor of 10-20 typically results in the same local concentration in the airways as by oral administration. Thus, just to equal what the oral dose of metformin would deliver to the airways, a subject would need to inhale 125-250 mg metformin per day; or, if broken into 3 doses/day, 40-80 mg/dose. Delivering this amount of metformin powder to the lungs is at the upper limit of acceptability, and would result in reduced compliance, bronchospasm and cough (
Buformin has eight times the potency of metformin. The inhalation of buformin as opposed to metformin, could reduce the dose by a factor of eight. The usual maximum oral dose of buformin is 300 mg/day. Decreasing the inhaled dose by a factor of 10 to 20, 3 doses per day inhaled buformin could be administered at 5 to 10 mg per dose, much less than metformin. This dose of buformin, 15 to 30 mg per day, would be highly unlikely to produce lactic acidosis, the main biguanide complication. In a previous study, the toxic oral buformin dose was 329±30 mg/day in 24 patients who developed lactic acidosis while using buformin. Another group of 24 patients administered 258±25 mg/day buformin did not develop lactic acidosis (
Inhaled buformin has a relatively long lung residence time. Buformin has an octanol/water partition coefficient (log P) of -1.2 and is hydrophilic. Hydrophilic small molecules with a log P-value <0 have a mean lung half-life (t½) of approximately 1 h (
A final advantage of buformin over phenformin is that it improves survival of influenza-infected mice with higher efficiency than phenformin (
Coronavirus disease 2019 (COVID-19) is an infectious disease caused by SARS-CoV-2, a virus closely related to the SARS virus. The disease is the cause of the 2019-2020 coronavirus outbreak. It is primarily spread between individuals by small droplets emitted from infected individuals when they breathe or cough. The PI3K/AKT/mTOR signaling responses play important roles in MERS-CoV infection and may represent a novel drug target for therapeutic intervention strategies (
The repurposing of old drugs as antivirals holds considerable promise. Statins are a prime example. A randomized placebo-controlled phase II clinical trial (NCT02056340) aimed at evaluating the potential effect of atorvastatin to reduce the severity of illness in influenza-infected patients is currently underway (
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SL as the sole author of the present review article was responsible for the conception and design of this article, as well as for the literature search, writing and manuscript preparation and revisions. The author has read and approved the final manuscript.
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The authors declare that they have no competing interests.
Survival of mice following influenza virus infection with three viral dilutions, 10 mice per dilution group. The effect of dilution was significant (P<0.001, log-rank test). The data shown are from the study by Denys and Bocian (
Survival of influenza-infected mice. A total of 40 mice were treated with buformin and 40 mice were used as the untreated controls. The effect of buformin was significant (P<0.001, log rank test). The mean survival time for the treated mice was 9.4 days (95% CI 8.9-9.9). Mean survival of the untreated controls was 7.6 days (95% CI 6.6-8.7). The data shown are from the study by Denys and Bocian (
Buformin dosage. The inhaled buformin dose can be increased 10-fold above what would be needed to treat influenza and would still be well below the systemic toxic dose. This is a key strength of buformin.
Five dilutions of influenza virus tested on 8 allantoic sections.
10-2 | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes |
10-3 | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes |
10-4 | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes |
10-5 | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes |
10-6 | No | No | No | No | No | No | No | No |
Five dilutions of influenza virus were tested on 8 allantoic sections (40 tests in total). The maximal dilution of influenza virus causing hemagglutination (+) was 10-5. Allantoic sections contain an inhibitor of hemagglutination, and this method is a standard assay of virus infectivity. The data shown are from the study by Denys and Bocian (