*3.2.2 Zika virus*

Zika virus (ZIKV) is another virus that is propagated by mosquitoes and belongs to the genus of flaviviruses. ZIKV infection generally causes only mild symptoms, including fever, rash, conjunctivitis, muscle and joint pain, and headache. However, it has shown severe tetatogenic impacts, being able to cause a range of neurological complications, such as Guillain-Barre syndrome and microcephaly, in the fetuses of infected pregnant women [142].

There are no currently approved specific therapies for ZIKV infection [143]. However, a screening study utilizing 774 approved drugs has shown promising results. In vitro studies showed that ivermectin (anthelmintic), mycophenolic acid (an immunosuppressant), and daptomycin (a lipopeptide antibiotic) can inhibit ZIKV, resulting in reduced infection rates [144].

#### *3.2.3 Ebola virus*

Ebola virus is one of numerous hemorrhagic fever viruses, which was first discovered in 1976. It can cause severe viral haemorrhagic fever with case fatality rates vary from 25 to 90% [145]. It is characterized as a non-specific febrile illness (symptoms may include anorexia, arthralgia, headache, malaise, myalgia and rash) in the early infection and progresses to severe gastrointestinal symptoms (nausea, vomiting and high-volume diarrhea) in the first week [146]. To date, a monoclonal antibody (mAb114) and a cocktail of three antibodies (REGN-EB3) have been approved for the treatment of Ebola [147, 148]. Besides these biologics there has also been attempts at drug repurposing for this disease. Several drugs such as Amiodarone (anti-arrhythmia), bepridil (anti-angina pectoris), teicoplanin (antibiotic), amiodarone (ventricular fibrillation/tachycardia) and favipiravir (RNA polymerase inhibitor) have shown therapeutic potential for Ebola, but their efficacy requires further confirmation [149–151].

#### *3.2.4 MERS-CoV*

A warning of the potential for a coronavirus pandemic was provided by the Middle East respiratory syndrome coronavirus (MERS-CoV). While the impact of this outbreak was significantly less than that of the current SARS-CoV2 outbreak the urgent need for MERS-CoV treatments was recognized, also including a focus on repurposing approaches and a call for the development of pan-corona virus inhibitors [152]. Suggested repurposing agents included GS-5734, which has previously demonstrated antiviral against multiple viral families, including *Coronaviridae*. GS-5734 activity *in vitro* was supported by reduced disease effects in mouse models and, while resistance mechanisms emerged, they were associated with a loss in viral fitness *in vitro* and *in vivo*—supporting the further analysis of GS-5734 as a pan-corona inhibitor [153, 154].

Similarly, lopinavir-ritonavir (a molecule designed as an inhibitor of the HIV-1 protease inhibitor) was proposed as a repurposing target of the 3CLpro of both SARS-CoV and MERS-CoV during their respective outbreaks [155, 156]. Combination therapy approaches in both cases resulted in improved patient outcomes, thereby offsetting the lacking of designed affinity that is a hallmark of repurposed compounds. In the example of SARS-CoV, a study on a combined therapy with ribarivin (a guanosine analog with activity against multiple viral families that inhibits viral RNA synthesis by RdRp) demonstrated both reduced viral load and improved clinical outcomes [157]. Whereas, a clinical trial of lopinavir-rotonavir in combination with IFN-β1b targeted therapies was proposed for MERS-CoV patients in Saudi Arabia [158]. Ribarivin itself was also a focus for repurposing during the SARS-CoV and MERS-CoV outbreaks. However, while efficacy of ribarivin alone could be demonstrated *in vitro* the doses required for a clinical response could not be supported by patients [159, 160].

Screening of an FDA-approved compound subset against viral replication in culture identified lopinavir and an additional 3 compounds with IC50 values in the low micromolar range (chloroquine, chlorpromazine, and loperamide) [156]. This again demonstrates not only the potential for experimentally based repurposing screens

to identify potential agents, but also suggests that the relatively limited potency of the agents identified may require the assessment of combination therapies to provoke a clinical response. This additional limitation of identifying appropriate combination therapies may well represent a common theme as a complicating factor in repurposing strategies.
