Study to Evaluate the Efficacy and Safety of Plitidepsin in Adults with Post-COVID-19 Condition (PCC)

"Receive 1.5 mg/day of plitidepsin intravenously (IV) over a 1-hour infusion for 3 consecutive days every 15 days during 4 treatment periods.~The participant will receive the following pre-medication before receiving the study treatment:~* Palonosetron 0.25 mg IV~* Dexchlorpheniramine maleate 5 mg IV (or equivalent to H1 receptor antihistamines)~* Famotidine 40 mg oral (60 minutes before starting the plitidepsin/placebo infusion)~* Dexamethasone phosphate 8 mg IV (equivalent to 6.6 mg of dexamethasone)~Premedication should be completed 20-30 minutes before starting the plitidepsin/placebo infusion."

"Receive placebo intravenously (IV) over a 1-hour infusion for 3 consecutive days every 15 days during 4 treatment periods.~The participant will receive the following pre-medication before receiving the study treatment:~* Palonosetron 0.25 mg IV~* Dexchlorpheniramine maleate 5 mg IV (or equivalent to H1 receptor antihistamines)~* Famotidine 40 mg oral (60 minutes before starting the plitidepsin/placebo infusion)~* Dexamethasone phosphate 8 mg IV (equivalent to 6.6 mg of dexamethasone)~Premedication should be completed 20-30 minutes before starting the plitidepsin/placebo infusion."

"Receive placebo intravenously (IV) over a 1-hour infusion for 3 consecutive days every 15 days during two treatment periods and receive plitidepsin 1.5mg/day during the last two treatment periods.~The participant will receive the following pre-medication before receiving the study treatment:~* Palonosetron 0.25 mg IV~* Dexchlorpheniramine maleate 5 mg IV (or equivalent to H1 receptor antihistamines)~* Famotidine 40 mg oral (60 minutes before starting the plitidepsin/placebo infusion)~* Dexamethasone phosphate 8 mg IV (equivalent to 6.6 mg of dexamethasone)~Premedication should be completed 20-30 minutes before starting the plitidepsin/placebo infusion."