ANTIMICROBIAL AGENTS AND CHEMOTHERAPY, Oct. 2006, p. 3289–3296
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Vol. 50, No. 10
Potent Antiviral Synergy between Monoclonal Antibody and
Small-Molecule CCR5 Inhibitors of Human
Immunodeficiency Virus Type 1
Jose D. Murga, Michael Franti, Daniel C. Pevear,† Paul J. Maddon, and William C. Olson*
Progenics Pharmaceuticals, Inc., 777 Old Saw Mill River Road, Tarrytown, New York 10591
Received 6 June 2006/Returned for modification 13 July 2006/Accepted 28 July 2006
The chemokine receptor CCR5 provides a portal of entry for human immunodeficiency virus type 1
(HIV-1) into susceptible CD4?cells. Both monoclonal antibody (MAb) and small-molecule CCR5 inhib-
itors have entered human clinical testing, but little is known regarding their potential interactions. We
evaluated the interactions between CCR5 MAbs, small-molecule CCR5 antagonists, and inhibitors of
HIV-1 gp120, gp41, and reverse transcriptase in vitro. Inhibition data were analyzed for cooperative effects
using the combination index (CI) method and stringent statistical criteria. Potent, statistically significant
antiviral synergy was observed between the CCR5 MAb PRO 140 and the small-molecule CCR5 antago-
nists maraviroc (UK-427,857), vicriviroc (SCH-D), and TAK-779. High-level synergy was observed con-
sistently across various assay systems, HIV-1 envelopes, CCR5 target cells, and inhibition levels. CI values
ranged from 0.18 to 0.64 and translated into in vitro dose reductions of up to 14-fold. Competition binding
studies revealed nonreciprocal patterns of CCR5 binding by MAb and small-molecule CCR5 inhibitors,
suggesting that synergy occurs at the level of receptor binding. In addition, both PRO 140 and maraviroc
synergized with the chemokine RANTES, a natural ligand for CCR5; however, additive effects were
observed for both small-molecule CCR5 antagonists and PRO 140 in combination with other classes of
HIV-1 inhibitors. The findings provide a rationale for clinical exploration of MAb and small-molecule
CCR5 inhibitors in novel dual-CCR5 regimens for HIV-1 therapy.
The armamentarium for human immunodeficiency virus
type 1 (HIV-1) infection currently includes 22 antiretroviral
agents drawn from four mechanistic treatment classes: nucle-
oside reverse transcriptase inhibitors (NRTI), nonnucleoside
reverse transcriptase inhibitors (NNRTI), protease inhibitors,
and fusion inhibitors. The standard of care for HIV-1 infection
involves combination use of three or more antiretroviral
agents. Where available, such therapies have markedly re-
duced HIV-1 morbidity and mortality (34). However, current
therapies are limited by the emergence of multidrug-resistant
virus, by treatment-related toxicities, by unfavorable drug-drug
interactions, and by often-complex dosing regimens that can
reduce adherence to therapy. Consequently, many patients
eventually exhaust their treatment options, and there is an
urgent need for new agents that can be deployed in novel
In 1996, we and others demonstrated that the chemokine
receptor CCR5 serves as an entry coreceptor for HIV-1 (1, 10,
12). HIV-1 entry proceeds through a cascade of events medi-
ated by the HIV-1 envelope glycoproteins gp120 and gp41:
gp120 sequentially binds CD4 and then CCR5 or another co-
receptor molecule, thereby triggering gp41-mediated fusion of
the viral and cellular membranes. CCR5 has emerged as an
important target for novel HIV-1 therapies (reviewed in ref-
erence 35). Both small-molecule and monoclonal antibody
(MAb) inhibitors of CCR5 have entered human testing, and
the first of these has demonstrated potent antiviral effects in
HIV-infected individuals (14, 21).
PRO 140 is a humanized CCR5 MAb that has entered phase
1b testing for HIV-1 therapy. PRO 140 and the parent mouse
MAb (PA14) broadly and potently block CCR5-mediated
HIV-1 entry in vitro (32, 33, 45). Although PRO 140 and
small-molecule CCR5 antagonists target the same protein,
their properties are complementary in a number of important
respects. Whereas the available small-molecule CCR5 inhibi-
tors potently block the natural activity of CCR5 (11, 39, 40, 48),
antiviral concentrations of PRO 140 do not block CCR5 func-
tion in vitro (33). In addition, preliminary studies indicate that
PRO 140 is highly active against viruses that are resistant to
small-molecule CCR5 antagonists (20, 27). These functional
differences are likely related to the distinct differences in
CCR5 binding. Small-molecule CCR5 antagonists bind a hy-
drophobic pocket formed by the transmembrane helices of
CCR5 and inhibit HIV-1 via allosteric mechanisms (13, 30, 47,
48), while PRO 140 binds an extracellular epitope on CCR5
and appears to act as a competitive inhibitor (33).
Given the mechanistic differences between PRO 140 and
small-molecule CCR5 antagonists in clinical development and
the need for novel combination regimens, we examined the
interactions between these agents in vitro. PRO 140, structur-
ally diverse small-molecule CCR5 antagonists, and other
classes of HIV-1 inhibitors were tested alone and in combina-
tion for the ability to inhibit HIV-1 membrane fusion and viral
entry. Surprisingly, we observed potent antiviral synergy for
PRO 140 in combination with each of several small-molecule
CCR5 antagonists but not for PRO 140 in combination with
* Corresponding author. Mailing address: Progenics Pharmaceuti-
cals, Inc., 777 Old Saw Mill River Road, Tarrytown, NY 10591. Phone:
(914) 789-2800. Fax: (914) 789-2807. E-mail: email@example.com.
† Present address: Novartis Institute for Biomedical Research, 500
Technology Square, Cambridge, MA 02139.
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3296MURGA ET AL.ANTIMICROB. AGENTS CHEMOTHER.