Nucleoside and nucleotide analogs
are an essential class of antivirals
for COVID-19 treatment. Several nucleoside/nucleotide analogs have
shown promising effects against SARS-CoV-2
in vitro
; however, their
in vivo
efficacy is limited. Nucleoside/nucleotide
analogs are often formed as ester prodrugs to improve pharmacokinetics
(PK) performance. After entering cells, the prodrugs undergo several
enzymatic metabolism steps to form the active metabolite triphosphate
nucleoside (TP-Nuc); prodrug activation is therefore associated with
the abundance and catalytic activity of the corresponding activating
enzymes. Having the activation of nucleoside/nucleotide prodrugs occur
at the target site of action, such as the lung, is critical for anti-SARS-CoV-2
efficacy. Herein, we conducted an absolute quantitative proteomics
study to determine the expression of relevant activating enzymes in
human organs related to the PK and antiviral efficacy of nucleoside/nucleotide
prodrugs, including the lung, liver, intestine, and kidney. The protein
levels of prodrug-activating enzymes differed significantly among
the tissues. Using catalytic activity values reported previously for
individual enzymes, we calculated prodrug activation profiles in these
tissues. The prodrugs evaluated in this study include nine McGuigan
phosphoramidate prodrugs, two cyclic monophosphate prodrugs, two
l
-valyl ester prodrugs, and one octanoate prodrug. Our analysis
showed that most orally administered nucleoside/nucleotide prodrugs
were primarily activated in the liver, suggesting that parenteral
delivery routes such as inhalation and intravenous infusion could
be better options when these antiviral prodrugs are used to treat
COVID-19. The results also indicated that the
l
-valyl ester
prodrug design can plausibly improve drug bioavailability and enhance
effects against SARS-CoV-2 intestinal infections. This study further
revealed that an octanoate prodrug could provide a long-acting antiviral
effect targeting SARS-CoV-2 infections in the lung. Finally, our molecular
docking analysis suggested several prodrug forms of favipiravir and
GS-441524 that are likely to exhibit favorable PK features over existing
prodrug forms. In sum, this study revealed the activation mechanisms
of various nucleoside/nucleotide prodrugs relevant to COVID-19 treatment
in different organs and shed light on the development of more effective
anti-COVID-19 prodrugs.