The Discovery of GS-9131, an Amidate Prodrug of a Novel Nucleoside Phosphonate HIV Reverse Transcriptase Inhibitor - Accounts in Drug Discovery Case Studies in Medicinal Chemistry - page 219

worth noting is that the antiviral potency of the N(t)RTIs or NRTIs is reflected
by the levels of diphosphophosphonate or triphosphate that can be maintained
over time inside the target cells, and the ability of the metabolite to compete
with deoxynucleotides and incorporate into the nascent DNA (determined in
vitro as IC 50 for RT). This can be contrasted with the other classes of HIV
inhibitors, including NNRTI 8, where the free fraction of drug in plasma is
often used as a predictor for in vivo antiviral ecacy. For N(t)RTIs, antiviral
activity results from an intracellular, charged metabolite species and is, there-
fore, not subject to these same free fraction correlations.
10.2 Discovery of the Novel Nucleoside Phosphonate
GS-9148
10.2.1 Medicinal Strategy
The research program focused exclusively on the discovery of a novel N(t)RTI
rather than NRTI for several reasons: (i) the expected long intracellular half-
lives of the active metabolites would favor qd administration; (ii) the freedom to
operate in the N(t)RTI space was much greater than NRTIs; (iii) prodrugs that
target the N(t)RTI to the desired lymphoid cells could be developed to reduce
therapeutic dose and in vivo toxicites; (iv) only two intracellular metabolism
steps would be required, allowing the first and often rate-limiting step of NRTIs
to be bypassed; and (v) cross resistance to a novel phosphonate may be less
likely since only one N(t)RTI, prodrug 2, is currently in clinical use for HIV.
There are many N(t)RTI-resistant mutations observed clinically, but initial
evaluation of the new inhibitors focused primarily on the RT mutants that
reduce susceptibility to 1, 3, 4, and 7 due to their more abundant clinical use.
Therefore, new analogs were profiled toward wild-type (wt) HIV-1 in cell
culture, and also three recombinant RT mutants harboring K65R, M184V, and
multiple thymidine analog mutations (6TAMs), respectively. The design of the
novel N(t)RTIs was also structure based, using binding models of proposed
compounds in the active site of wt RT. These models were based on the
reported X-ray structure of the wt RT complex. 8 Particular attention was also
paid to the characterization of potential N(t)RTI-related toxicities, such as
mitochondrial DNA depletion and renal accumulation. Once a suitable
N(t)RTI was identified, the focus would shift to prodrug development for
ecient oral delivery. Although the disoproxil prodrug 2 was effective in the
delivery of 1, it was envisaged that a different class of prodrugs could be
developed based on amidate 11 (GS-7340), shown in Figure 10.3.
During the early stages of the program, 11 was dosed in dogs and the
[AUC] 0-24h levels of 1 inside peripheral blood mononuclear cells (PBMCs) and
the plasma compartment evaluated. Compared to the disoproxil prodrug 2, the
exposure ratio inside PBMCs of 1 following oral dosing was improved by 35-
fold for an equivalent molar dose. 9 Furthermore, in proof-of-concept Phase 1
clinical studies, dosing of 11 at lower equivalent doses than 2 resulted in an