Science - USA (2020-08-21)

RESEARCH ARTICLE SUMMARY

◥

DRUG DEVELOPMENT

An orally available non-nucleotide STING agonist

with antitumor activity

Bo-Sheng Pan, Samanthi A. Perera†, Jennifer A. Piesvaux, Jeremy P. Presland,
Gottfried K. Schroeder†, Jared N. Cumming, B. Wesley Trotter*†, Michael D. Altman,
Alexei V. Buevich, Brandon Cash, Saso Cemerski, Wonsuk Chang, Yiping Chen, Peter J. Dandliker,
Guo Feng, Andrew Haidle, Timothy Henderson, James Jewell, Ilona Kariv, Ian Knemeyer,
Johnny Kopinja, Brian M. Lacey, Jason Laskey, Charles A. Lesburg, Rui Liang, Brian J. Long, Min Lu,
Yanhong Ma, Ellen C. Minnihan, Greg O’Donnell, Ryan Otte, Laura Price, Larissa Rakhilina,
Berengere Sauvagnat, Sharad Sharma, Sriram Tyagarajan, Hyun Woo, Daniel F. Wyss, Serena Xu,
David Jonathan Bennett†, George H. Addona†

INTRODUCTION:Activation of the STING (stimu-
lator of interferon genes) protein by its natural
ligand, cyclic guanosine monophosphate–
adenosine monophosphate (cGAMP), triggers
signaling responses, inducing the release of
type I interferons and other proinflammatory
cytokines. STING-controlled interferon pro-
duction is involved in antiviral defense as well
as antitumor immunity. Pharmacological ac-
tivation of STING is considered a promising
therapeutic strategy for cancer.

RATIONALE:First-generation STING agonists
are cyclic dinucleotide (CDN) analogs of cGAMP.
When administered systemically in animal
models, they induce inflammatory cytokines
equipotently in tumor and normal tissues,
owing to ubiquitous STING expression. Thus,

CDN-based STING agonists currently under-

going clinical trials are dosed by direct intra-

tumor injection, which limits their application

to a narrow set of tumors. To address a broad

spectrum of cancers, STING agonists that are

suitable for systemic administration and pref-

erentially target tumors are needed. We iden-

tified a previously unknown compound (MSA-2)

that exhibits such behavior through its dis-

tinctive mechanism of action. Moreover, MSA-2

is amenable to oral administration, a desir-

able delivery route because of convenience and

low cost.

RESULTS:MSA-2 was identified in a phenotypic

screen for chemical inducers of interferon-b

secretion (see the figure, top). In cell-free as-

says, MSA-2 binds human and mouse STING.

MSA-2 is orally available, manifesting similar

oral and subcutaneous exposure in mice. In

tumor-bearing mice, MSA-2 induced elevations

of interferon-bin plasma and tumors by both

routes of administration. Well-tolerated regi-

mens of MSA-2 induced tumor regressions in

mice bearing MC38 syngeneic tumors. Most

mice that exhibited complete regression were

resistant to reinoculation of MC38 cells, sug-

gesting establishment of durable antitumor

immunity. In tumor models that were moder-

ately or poorly responsive to PD-1 blockade,

combinations of MSA-2 and anti–PD-1 anti-

body were superior in inhibiting tumor growth

and prolonging survival over monotherapy

(see the figure, right).

Structural studies showed that MSA-2 was

bound as a noncovalent dimer to STING in a

“closed-lid”conformation (see the figure, left).

Each bound MSA-2 interacted with both mono-

mers of the STING homodimer (depicted in blue

and orange). The simplest model that can ac-

count for all observed equilibrium and kinetic

behaviors of MSA-2 is as follows: MSA-2 in

solution exists as monomers and noncovalent

dimers in an equilibrium that strongly favors

monomers; MSA-2 monomers cannot bind

STING, whereas the noncovalent MSA-2 dimers

bind STING with nanomolar affinity (see the

figure, center). The model was further sup-

ported by findings that covalently tethered

dimers of MSA-2 analogs exhibited nanomolar

affinity for STING.

Simulations and experimental analyses pre-

dicted that MSA-2, a weak acid, would exhibit

substantially higher cellular potency in an

acidified tumor microenvironment than nor-

mal tissue, owing to increased cellular entry

and retention combined with the inherently

steep MSA-2 concentration dependence of

STING occupancy (see the figure, bottom). It

is likely that preferential activation of STING

by MSA-2 in tumors substantially contrib-

utes to the observed favorable in vivo anti-

tumor activity and tolerability profile of this

compound.

CONCLUSION:In this work, we describe the

identification, in vivo antitumor properties,

and mechanism of action of MSA-2, an orally

available human STING agonist. MSA-2 could

prove valuable for the discovery and design of

human STING agonists suitable for systemic

administration in the clinic.▪

RESEARCH

Panet al.,Science 369 , 935 (2020) 21 August 2020 1of1

The list of author affiliations is available in the full article online.

*These authors contributed equally to this work.

†Corresponding author. Email: samanthi_perera@merck.

com (S.A.P.); [email protected] (G.K.S.);

[email protected] (B.W.T.); jonathan.bennett@

merck.com (D.J.B.); [email protected] (G.H.A.)

Cite this article as B.-S. Panet al.,Science 369 , eaba6098

(2020). DOI: 10.1126/science.aba6098

READ THE FULL ARTICLE AT

https://doi.org/10.1126/science.aba6098

STING agonist MSA-2.Identified in a cell-based screen, MSA-2 is bound to STING as a noncovalent dimer.
Extensive experimental analysis indicates that MSA-2 predimerization is required for binding. Acidic tumor
microenvironments favor permeable, uncharged MSA-2. Intracellular MSA-2 is“trapped”(deprotonated) and
accumulation drives MSA-2 dimerization, preferentially activating STING intratumorally. Orally dosed MSA-2
is well tolerated in mice, exhibiting STING-dependent antitumor activity, as monotherapy and combined with
antibodies against PD1 (anti-PD1). Me, methyl group; IFNb, interferon-b.