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GS9148: A Novel Nucleotide Active against HIV-1 Variants with Drug-resistance Mutations in Reverse Transcriptase
Tomas Cihlar*1, A Ray1, D Boojamra1, L Zhang1, H Hui1, D Grant1, K White1, M Desai1, N Parkin2, and R Mackman1
1Gilead Sci, Foster City, CA, US and 2Monogram Biosci, South San Francisco, CA, US
Background: Long-term clinical benefit of nucleoside(tide) reverse transcriptase inhibitors (NRTI) can
be limited by adverse effects, development of resistance, drug-drug
interactions, and sub-optimal efficacy against viruses harboring drug
resistance mutations. A nucleotide analog GS9148 (phosphonomethoxy-2’-fluoro-2’,3’-dideoxydidehydroadenosine) was selected in a search for
novel NRTI with improved pharmacological and resistance profiles.
Methods: GS9148 and its prodrug were subjected to an
extensive in vitro evaluation of
antiviral potency, toxicity, and resistance. PhenoSense HIV assay was used to
compare the activity of GS9148 and 6 approved NRTI—zidovudine
(ZDV), didanosine (ddI), stavudine (d4T), emtricitabine (FTC),
abacavir (ABC), and tenofovir
(TFV))—against HIV-1 variants with all major types of NRTI-resistance
mutations. An ethylalaninyl phenyl ester was identified as a prodrug of GS9148
with the most favorable in vivo
pharmacokinetic profile.
Results: The prodrug of
GS9148 exhibits potent antiretroviral activity both in primary cells and T cell
lines (EC50 = 25 to 200 nM). Low cytotoxicity (CC50 >100
mM)
was observed in multiple cell types including renal cells. The active
diphosphate metabolite of GS9148 acts as an obligatory DNA chain terminator and
a competitive inhibitor of HIV-1 RT (Ki = 0.8 mM).
Unlike ddI, d4T, or d4FC, neither GS9148 nor its prodrug showed an effect on
mitochondrial DNA or lactate production in HepG2 liver cells. In the PhenoSense
assay, the activity of GS9148 was not affected by the presence of K65R, L74V,
M184V or their various combinations (EC50 fold change <1).
Viruses with 4 or more thymidine analog mutations showed 0.74- to 2.0-fold
change in the susceptibility to GS-9148, a shift that was smaller than any
other tested NRTI. Passaging of HIV-1 in the presence of GS9148 selected for a
primary K70E mutation in RT that conferred <3-fold reduced susceptibility to
GS9148. In dogs, the oral bioavailability of GS9148 prodrug exceeded 20%,
resulting in an effective and prolonged accumulation of GS9148 diphosphate in
blood lymphocytes.
Conclusions: GS9148 exhibits a favorable in vitro pharmacological profile
including lower levels of resistance than approved NRTIs, an indication of
potential efficacy against existing NRTI-resistant viruses. The amidate prodrug
of GS9148 is an attractive candidate for further development with potential for
once daily dosing and activity in both treatment-naïve and NRTI-experienced
patients.
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