## Part 2 Filters to obtain an effective broad-spectrum anti-enterovirus drug.
### Candidates' Selection for MTDL Strategy
#### 2.5.2 Combination of Three Compounds  
Some assays were performed by combining three compounds in vitro to improve the efficacy of compound combinations and avoid viral resistance.  
Some triple consecutive alternating administration (CAA) combination assays were performed by the [PERSON] Bulgarian team, namely one drug per day in 3-day cycles. They combined disoxaril, guanidine hydrochloride, and oxoglaucine (DGO) against CVB1 and CVB3 [121; 122; 123; 124]. Other CAA combination studies were led by the same team substituting disoxaril with pleconaril (PGO combination) or guanidine-hydrochloride by MDL-860 (PMO combination) against the same viruses [121; 122; 123; 124; 125; 126; 127]. These first assays were performed to decrease the resistance established with a combination of disoxaril and enviroxime [128]. In this type of experiment, order of administration seems essential: first, a capsid binder, then a 2C protease inhibitor, and finally, an active compound in the early stage of viral replication. Other orders of administration showed less or no inhibition of viral proliferation. Likewise, adding PTU-23 to make a 4-day cycle suppresses the antiviral potency. DGO combinations are more effective than PGO combinations. Substituting guanidine-hydrochloride (45 mg/kg) by MDL-860 (75 mg/kg) increases the anti-enterviral effect. The concentration of MDL-860 should be compulsory to maintain the potency without developing toxicity due to higher doses. CAA combinations keep the potency of the most effective drugs among the three inhibitors. Moreover, combination with CAA should reduce the risk of resistance apparition but also increase drug sensitivity in comparison with every drug treatment used alone or administrated simultaneously. This is an exciting approach, but it remains to be seen whether it is effective and can be implemented in humans, where the time of diagnosis does not necessarily coincide with the onset of enteroviral infection.  
Recently, a new study combined remdesivir (RNA-dependent-RNA-polymerase inhibitor), rupintrivir (3C protease), and pleconaril (capsid binder) with a higher in vitro efficacy than monotherapy or two-drug cocktail [129]. This combination was successfully evaluated on a broad spectrum of enterovirus, including five cosasdexiveiruses, three echoviruses, encephalopathy virus, EV-A71, EV-D68, foot-and-mouth viruses, three polioviruses, and nine rhinoviruses. Moreover, this three-drug cocktail also delayed the emergence of antiviral drug resistance for any compound in the combination.  
Even if clinical trials of capsid binders were stopped, the hydrophobic pocket inside the VP1 capsid canyon remains the easiest target with conserved domains between species (except for RV-C) [112; 113; 114]. However, non-structural proteins are also prime targets for the weak fluctuation of their constitution between species, even if double-membrane vesicles protect them.  
An MTDL strategy is based on combination therapies, which result in a synergistic effect with only one compound, not a cocktail of drugs. According to the most recent combination therapies tested, capsid binders have a strong synergistic effect with 3C protease inhibitors and a moderate or weak effect with 3A protease inhibitors. PI4 KIII8 inhibitors also have an additive synergy with capsid binders.