Orally available antivirals against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are still scarce and the emergence of new variants challenging immunized individuals, suggests that mutant viruses might also emerge because of antiviral pressure. Therefore, beyond the recently alleged positive antiviral clinical results with molnupiravir™ and paxlovid™, the continuous search for drugs against 2019 coronavirus disease (COVID-19) is necessary. Because severe COVID-19 is a virus-triggered immune and inflammatory disfunction, molecules endowed with both antiviral and anti-inflammatory activity are highly desirable. We identified here that N6-furfurylaminopurine (kinetin, MB-905) inhibits the in vitro replication of SARS-CoV-2 at the sub-micromolar range in human hepatic and pulmonary cell lines. On infected monocytes, MB-905 reduced virus replication, IL-6 and TNFα levels. As a pro-drug, MB-905 is converted into its triphosphate nucleotide to inhibit viral RNA synthesis and induce an error-prone virus replication. Consistently, co-inhibition of SARS-CoV-2 exonuclease, a proofreading enzyme that corrects erroneously placed nucleotides during viral RNA replication, potentiated the inhibitory effect of MB-905. SARS-CoV-2-infected transgenic mice expressing human ACE2 were treated with MB-905 and decreased viral replication of the gamma variant was observed, along with reduced lung necrosis, hemorrhage and inflammation, together with increasedmice survival. MB-905 showed good oral absorption, its metabolites were stable and achieved long-lasting plasma concentrations exceeding those required for the in vitro inhibition. Besides, MB-905 was neither mutagenic, toxic during chronic treatment, nor cardiotoxic. Because kinetin has already been clinically investigated for a rare genetic disease at regimens that are beyond the predicted concentrations of antiviral/anti-inflammatory inhibition demonstrated here, our investigation strongly suggests the opportunity for a rapid clinical development of a new and orally available antiviral substance for the treatment of COVID-19.

The nucleoside guanosine is an endogenous neuromodulator associated with neuroprotection. The roles of guanosine during aging are still not fully elucidated. Guanosine modulates SUMOylation in neurons and astrocytes in vitro, but it is not known whether guanosine can modulate SUMOylation in vivo and improve cognitive functions during aging. SUMOylation is a post‐translational protein modification with potential neuroprotective roles. In this follow‐up study, we investigated whether guanosine could modulate SUMOylation in vivo and behavior in young and aged mice. Young (3‐month‐old) and aged (24‐month‐old) C57BL/6 mice were treated with guanosine (8 mg/kg intraperitoneal) daily for 14 days. Starting on day 8 of treatment, the following behavioral tests were performed: open field, novel object location, Y‐maze, sucrose splash test, and tail suspension test. Treatment with guanosine did not change the locomotor activity of young or aged mice in the open‐field test. Treatment with guanosine improved short‐term memory only for young mice but did not change the working memory of either young or aged mice, as evaluated using object recognition and the Y‐maze tests, respectively. Depressive‐like behaviors, such as impaired grooming evaluated through the splash test, did not change in either young or aged mice. However, young mice treated with guanosine increased their immobility time in the tail suspension test, suggesting an effect on behavioral coping strategies. Global SUMO1‐ylation was significantly increased in the hippocampus of young and aged mice after 14 days of treatment with guanosine, whereas no changes were detected in the cerebral cortex of either young or aged mice. Our findings demonstrate that guanosine also targets hippocampal SUMOylation in vivo, thereby contributing to a deeper understanding of its mechanisms of action. This highlights the involvement of SUMOylation in guanosine's modulatory and neuroprotective effects.image