20-F-20-C-Methyl Nucleosides and Nucleotides for the Treatment of Hepatitis C Virus: from Discovery to the Clinic - Accounts in Drug Discovery Case Studies in Medicinal Chemistry - page 242

potency and safety advantages. As previously mentioned, when PSI-6130 was
tested against other members of the Flaviviridae family, including BVDV, little
or no antiviral activity was detected. 12 PSI-6130 was also shown not to be active
against HIV or HBV. 12 This was in contrast to the previously reported 2 0 -C-
methylcytidine (1)and2 0 -C-methyladenosine (4) analogs, which showed
activity against other viruses. The 2 0 -F-2 0 -methyl combination also had unan-
ticipated affects on the safety characteristics of PSI-6130. We observed that 2 0 -
deoxycytidine nucleosides mono- or disubstituted with a fluorine atom at the
2 0 -position showed non-specific inhibitory activity against HCV in the sub-
genomic replicon assay, and were also shown to be substantially cytotoxic when
assessed against a panel of cell lines (Table 11.1). 13 It was also observed that 2 0 -
deoxy-2 0 -b-methylcytidine (7) showed activity against HCV but only at levels at
which it was cytotoxic. So, it was surprising when a cytidine nucleoside analog
(PSI-6130) that combined a 2 0 -b-methyl group and a 2 0 -a-F group exhibited
exceptional potency, selectivity, and lack of both cytotoxicity and mitochon-
drial toxicity.
In efforts to assess the SAR around the 2 0 -F-2 0 -C-methyl nucleoside class of
HCV inhibitors and potentially identify compounds with improved char-
acteristics, we undertook an exhaustive analog development effort. This effort
studied substitution variations at each of the positions on the ribose ring,
including base modifications (Figure 11.2). One line of investigation main-
tained the 2 0 -F-2 0 -C-methyl substitution intact and varied the 3 0 ,4 0 -substitution
and also varied the nature of the base. In addition, a carbocyclic analog was
prepared in which the ring oxygen of PSI-6130 was replaced with a carbon

2 0 -Substitution: HCV activity vs. cytotoxicity

Table 11.1

NH 2

N

N

O

HO

O

X

HO Y

HCV activity
replicon EC 90
(mM)

Cytotoxicity (CC 50 , mM)

Compound X Y Clone A Hep G2 BxPC3 CEM

5 F F o 1 o 0.1 o 1 o 1 o 1

2 a H F 5.66 4100 400 10 6

6 F H o 1 o 50 200 5 5

7 CH 3 H 9.73 10.47 40

o 1 o 1

3 (PSI-6130) CH 3 F 4.5 4100 41000 41000 41000

a Demonstrates cytostatic effects.