**6. Outlook**

Given the unique mechanism in mitigating the cytokine storm syndrome, the calcineurin/NF-AT signaling pathway presents an attractive target for therapeutic drug development for prevention of severe COVID-19. Currently the research of CNI for SARS-CoV-2 infection and prevention of severe COVID-19 disease is still limited. Therefore, the results of the clinical TACROVID trial from Barcelona, Spain are urgently awaited and currently pending. This trial investigates the clinically important question of tacrolimus in patients with COVID-19, with in one arm treating patients with methylprednisolone pulses 120 mg/day for three consecutive days (if they were not previously administered) with tacrolimus at the necessary dose to achieve plasma levels of 8–10 ng/mL, versus the other arm with usual care including all necessary treatments with the exception of CNIs [65].

Recruitment has also started for the study in which cyclosporine is clinically tested in patients with COVID-19 requiring oxygen supplementation but not requiring ventilator support [60]. This trial is a phase 1 safety study to determine the tolerability, clinical effects, and changes in laboratory parameters of short course oral or IV cyclosporine administration [66].

Major questions that remain open should be addressed in research, and the TACROVID trial, the American cyclosporine study (and probably an alispirovir trial in the near feature) will likely shed more light on this issue [65,66]. In our opinion, clinically relevant questions comprise those mentioned in Table 2.


**Table 2.** Proposed research questions on treatment with calcineurin inhibitors.

From a general point of view there is a vast experience with CNI in transplantation medicine including dosing regimens and experience with achieving specific drug levels by therapeutic drug monitoring. This experience can be beneficial when, within a short time frame, a CNI-based immunosuppression should be established, which also takes into account comorbidity (renal function, other medication/interactions). Which target drug levels should be used is another open question. Based on the experience with SOT recipients, similar drug levels should probably be targeted as for maintenance of immunosuppression in such patients. Whether in certain situations augmentation of immunosuppression may be wise as a second step would have to be evaluated as well. In addition to the immunosuppressive strategy the most appropriate marker for disease activity measurement in these COVID-19 patients will have to be determined in the context of the immunosuppressive therapy (CRP, Procalcitonin (PCT), certain Interleukins, differential white blood cell count, etc). Besides dose and monitoring of the immunosuppression, the duration of continuation of this therapy will have to be evaluated in addition to the effects on viral load and potentially observed prolonged viral shedding. The best evidence is probably derived from dual or triple immunosuppressive regimens in SOT, therefore the combination of two immunosuppressive drugs which certainly includes a CNI because of its pleiotropic effects (including antiviral effects) is likely to be a promising pharmacological strategy to prevent severe COVID-19. As a potential predictor of severe disease course, the AIFELL

score may be considered [67]. The triage score relies on disease markers that at an early stage indicate whether a more severe disease progression may be expected.

Although there remain many open questions, CNI should be investigated as a first treatment option, based on evidence of direct antiviral effects and its properties preventing CSS, as has been observed in COVID-19 with predominantly immunopathological changes of the respiratory tract.

**Author Contributions:** Conceptualization, R.H.; M.M.S.; writing—original draft preparation, R.H.; writing review and editing, R.H.; C.S.; F.G.; M.M.S.; supervision M.M.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
