Entry inhibitors, a prominent new class of antiretroviral drugs, were
a focus of numerous presentations at the 7th Conference on Retroviruses
and Opportunistic Infections (CROI), with several groups reporting pre-clinical
and clinical data on candidates.
For an in-depth report on all the news from CROI, see "Conference
Coverage" in this issue.
The Need for New Antiretroviral
Classes
Antiretroviral drugs are the current backbone of HIV/AIDS treatment,
in spite of existing and emerging side effects associated especially
with long-term use, and in spite of persistent questions about the best
ways to use them including when to start (antiretroviral treatment),
what to start with, and when to switch.
All 16 antiretroviral formulations approved by the U.S. Food and Drug
Administration (FDA) fall into three classes: nucleoside analogs, non-nucleoside
analogs, and protease inhibitors. Approved drugs to date target the
HIV-1 enzymes reverse transcriptase (the nucleoside and non-nucleoside
inhibitors) and protease; the goal of current antiretrovirals is to
impede those two enzymes at the point in the HIV replication cycle where
they are needed, thereby slowing or stopping the (replicative) cycle.
During the process of cell infection, HIV attaches to a target cell
such as a CD4 cell and, via a series of complex interactions, enters
it. Once inside the cell, HIV's RNA genome is transcribed (process
early in protein synthesis and replication by which genetic information
is "converted" for use as a template for the production of new proteins)
into DNA by reverse transcriptase (RT). The genetic product of that
step is transcribed to more RNA, which is translated into proteins,
some of which must at that stage be cut by protease enzymes in order
to be active. Again, all currently available antiretroviral agents work
by attacking intracellular HIV in order to prevent its replication.
It has long been thought that a combination of drugs targeted at different
steps in the viral life cycle or targeted at different aspects of the
same step will result in a more effective therapeutic approach to inhibiting
viral growth and replication.
Today, especially as more and more people develop HIV resistance to
existing antiretrovirals from all three currently available classes,
there is a pressing need to discover new drugs that combat HIV in novel
ways, i.e., new drug classes, including those that will act at new steps
in the infection process. The development of new anti-HIV medications
is a major goal of research in both the public (government-funded) and
the private (the pharmaceutical industry) sectors.
Novel Agents to Block the
Infection of New Cells
Entry inhibitors belong to a new class of antiretroviral drugs and
target a new step(s) in the life cycle of HIV. Specifically, they are
compounds designed to disrupt HIV-1 replicative functions not yet targeted
by approved antiretrovirals: most prominently, HIV-1 cell fusion and
entry. A broad category of drugs under investigation, entry inhibitors
include fusion inhibitors (the focus of this article), attachment inhibitors,
and co-receptor inhibitors. Only the fusion inhibitors have advanced
to clinical trials; candidates from the other two classes are still
in preclinical (laboratory) studies.
All drugs in the class of entry inhibitors work by blocking the ability
of HIV to successfully enter and thereby infect a target cell. Fusion
inhibitors work by attacking the virus itself; some of the less advanced
candidates work with the cells that HIV targets.
HIV and Cell Fusion: The Rationale
The rationale for these drugs derives from x-ray crystallographic,
structural studies that permit improved understanding of the interactions
between HIV and host (human) cells. Cells are by design resistant to
infection by viruses and other microbes; the steps a virus undergoes
to successfully infect a cell are many and complex.
Successful infection of a cell requires HIV to bind and to fuse with
cell receptors in a series of steps, followed by entry. Binding and
fusion involve interactions between HIV glycoproteins (gp) 120 and 41,
CD4 cell receptors, and secondary receptors on the target cell, often
referred to as chemokine receptors; the two primary co-receptors
are CCR5 (R5) and CXCR4 (R4). CD4 cells exhibit ("express") both CCR5
and CXCR4. Other key immune system cells, monocytes and macrophages,
which are also frequent targets of HIV, generally express CXCR4 but
not CCR5.
In the initial binding process, HIV gp120 binds to the CD4 cell receptor,
inducing changes in gp120 that cause it to bind to the chemokine receptor,
either CCR5 or CXCR4.
Current knowledge of binding and entry processes has permitted the
identification of a number of new, potential targets for antiretroviral
drugs, including CD4 cell binding, chemokine receptor binding, and gp41-mediated
fusion. A number of candidate drugs have been developed, including soluble
CD4, fusion inhibitors, and chemokine receptor antagonists.
The theory behind soluble CD4 was that it would inhibit viral binding
on CD4 cell receptors on target cells by blocking a specific, gp120-CD4
interaction between HIV and the target cell. While this approach did
well in studies using laboratory strains of HIV, it did less well when
tested with clinical HIV strains in vivo (in the body),
where it became clear that extremely high--too high--concentrations
of the substance would be needed to inhibit viral entry into cells and
thus replication.
Fusion Inhibitors T-20 and
T-1249 Lead the Way
To date, fusion inhibitors are the type of entry inhibitor that have
advanced furthest in the research and development (R&D) process.
Trimeris, a biopharmaceutical company called based in Durham, NC, has
two fusion inhibitors in development. (Last July, Roche joined forces
with Trimeris for development and marketing of the agents.) Both candidates
successfully inhibit HIV fusion with host cells and have advanced to
clinical trials. In addition, both agents have been granted "fast track"
status by the FDA for treating HIV positive individuals.
The candidate out in front, T-20, has demonstrated promising
anti-HIV activity throughout its development so far. (See the January
1999 issue of BETA for an earlier report on the drug.) T-20
is a 36-amino acid synthetic peptide that binds to the HIV protein gp41.
In studies to date, T-20 consistently inhibits both wild-type (the normal
type of a virus or other organism before genetic mutation takes place)
virus and virus that is resistant to other approved antiretrovirals.
In laboratory studies, T-20 appears to be synergistic (when combined,
the effects of component parts are enhanced) with other anti-HIV agents.
T-20 is administered as a subcutaneous (under the skin) self-injection
twice daily in a dose of 50 mg. T-20 is reportedly well tolerated; adverse
events observed include irritation at the injection site, headache,
nausea, fever, weakness, diarrhea, and dizziness. Thus far, two clinical
trials have been completed (TRI-100 and TRI-300), and a Phase III trial
is being planned and should begin enrolling sometime this year.
This past September Jay Lalezari, MD, of Quest Clinical Research in
San Francisco presented 16-week data at the 39th Interscience Conference
on Antimicrobial Agents and Chemotherapy (ICAAC) on T-20's utility in
a group of heavily pretreated (with antiretrovirals) people.
Participants had taken a median of 11 prior anti-HIV drugs; 93% had
taken drugs from all three anti-HIV drug classes. After genotypic testing,
participants began a new multi-drug salvage regimen that included T-20.
At study entry, participants had a median viral load of 4.9 log copies/mL
and a median CD4 cell count of 70 cells/mm3; 93% had resistance to at
least one protease inhibitor and 87% had resistance to NNRTIs or NRTIs.
In this study, T-20 at a dose of 50 mg was self-injected subcutaneously
twice daily, as an addition to participants' established drug regimens
(which were chosen after receiving the results of genotypic resistance
testing).
At 16 weeks, 20 people had an undetectable viral load (fewer than 400
copies/mL) and 13 others had had a one-log copies/mL decrease in viral
load. Overall the drug was well tolerated; the most common side effect
was a reaction at the injection site. Serious side effects occurred
in 7% but no one dropped out due to adverse reactions. According to
investigators, the groups' overall response was comparable to the response
to "mega-HAART" seen in heavily treatment-experienced persons.
At the 7th CROI, Samuel Hopkins, MD, of Trimeris presented 32-week,
follow-up results from the same cohort. (Again, all participants had
extensive prior antiretroviral experience.) Forty-six people were available
for analysis at 32 weeks; those on treatment at that point showed approximately
one log copies/mL reduction in viral load. According to Hopkins, T-20
continued to be safe and effective for most people at all dose levels.
Martin Hirsch, MD, of Massachussetts General Hospital in Boston led
a team that studied the combination of T-20 and the CXCR4 inhibitor
AMD 3100 (see below) in vitro (in test tube models). Clinical
studies will evaluate whether the high level of synergism observed in
vitro will also occur in vivo.
Data from cumulative studies of T-20 provide clinical evidence that
the strategy behind fusion inhibitors (blocking membrane fusion and
viral entry) is sound. Currently, a protocol for a Phase III trial is
being drafted, with enrollment expected to begin by mid-2000. Trimeris
is working on a new formulation of T-20 that they believe will be twice
as concentrated as the earlier formulation and that might therefore
require a person to receive fewer injections. The new formulation also
is reported to cause less irritation.
On March 14, Trimeris and Hoffmann-La Roche announced that a Phase
II clinical study (T20-208) of two novel alternative formulations had
begun; up to 60 people will receive daily subcutaneous injections of
the new formulations, in addition to their (oral) antiretroviral regimens.
Sponsored by the National Institute of Allergy and Infectious Disease
(NIAID), a Phase I/II dose-escalating study of T-20 in children 3-12
years of age is currently enrolling at several sites around the nation.
A Trimeris-sponsored Phase II trial assessing three doses of T-20 in
combination with abacavir (Ziagen), amprenavir (Agenerase), ritonavir
(Norvir), and efavirenz (Sustiva) in adults is also still enrolling
at several sites; for more information including eligiblity criteria
and sites, contact Trimeris at 919/419-6050 or ACTIS at 1-800-TRIALS-A.
Trimeris' second fusion inhibitor, T-1249, is similar to T-20 in many
ways; it is a 39-amino acid synthetic peptide and it also binds to gp41,
although in a different region than T-20. Laboratory studies suggest
that T-1249 is a potent inhibitor of cell fusion in both laboratory
and clinical viral strains. The agent also appears active against HIV
that is resistant to AZT (Retrovir), saquinavir (Fortovase), and also
to T-20. T-1249 appears likely to have some activity against HIV-2,
as well as HIV-1. Like T-20, T-1249 is taken either by either intravenous
or subcutaneous injection. Compared with T-20, T-1249 has double the
half-life (the time it takes for an original amount of the drug to be
decreased by half).
Currently, a Phase I/II trial is underway that is comparing once-daily
vs twice-daily subcutaneous injections at five dose levels. Seventy-two
people will be enrolled. Dose assignment will escalate in response to
safety data gathered first at the lowest level. The treatment phase
of the study lasts for two weeks, during which the drug is given on
an outpatient basis. Eligibility criteria include an HIV viral load
greater than 5,000 copies/mL. Sites are Birmingham, AL; Boston, MA;
Chapel Hill, NC; Denver, CO; Houston, TX; New York, NY (two sites);
Stanford, CA; and Tampa, FL.
At the 39th ICAAC in San Francisco this past autumn, Progenics Pharmaceuticals
presented results of Phase I trials of its fusion inhibitor, PRO 542.
PRO 542 binds to HIV's gp120. In the study presented at ICAAC, 15 participants
received a single injection of the drug in one of four doses. According
to investigator Jeffrey Jacobson, MD, from Mt. Sinai Medical Center
in New York, a single dose of PRO 542 was well tolerated and achieved
significant antiviral activity; a large, Phase II trial is being planned.
Chemokine Receptor Antagonists
Drugs designed to prevent HIV from binding to the chemokine receptors
on target cells, a crucial aspect of cell entry and infection, are also
being developed. CCR5 and CXCR4, the two main co-receptors on human
cells used by HIV, are the two that experimental agents primarily seek
to block. Inhibitors aimed at CCR5 and CXCR4 are currently in preclinical
development.
In primary and early infection, HIV seems to preferentially use the
CCR5 chemokine receptor to enter cells, i.e., early on, CCR5-using virus
predominates. Over time, with HIV disease progression, a switch occurs
and the virus becomes syncytium-inducing (a more virulent form) and
skilled at using both co-receptors for cell entry and infection. Thus,
one obstacle in the development of CCR5 inhibitors is the presence of
alternative pathways for virus entry, i.e., "R5" viruses can evolve
their receptor specificity and enter cells via other chemokine receptors--CXCR4,
in particular.
Although these concerns are still hypothetical, since R4 viruses are
thought to be more virulent, this limitation may prove significant,
and researchers are scrutinizing ways to prevent their CCR5 inhibitors
from causing HIV to switch from a CCR5- to a CXCR4-type receptor. Phase
I/II studies should provide some elucidation.
Another R&D challenge relates to the fact that there are natural
chemokines or ligands (molecules that attach to HIV co-receptors) that
help block HIV infection by themselves binding to the receptors. Natural
ligands for CCR5 include the chemokines RANTES, MIP-1alpha, and MIP-1beta.
A derivative of the chemokine RANTES, called AOP-RANTES, has even been
shown to inhibit HIV in vitro (but was inactive in a small clinical
trial). The challenge to researchers is to design drugs that will not
inhibit natural ligands, which not only play a role in the body's natural
immune defense against HIV but may be needed to fulfill other physiological
functions.
Progenics Pharmaceuticals is developing PRO 140, a monoclonal antibody
that binds to CCR5. In vitro studies of PRO 140 indicate that
it can effectively inhibit viral replication in a variety of cell lines.
Schering-Plough is actively developing a CCR5 antagonist, still in
preclinical studies, known as SCH-C. This candidate's oral bioavailability
eliminates the need for injection or intravenous infusion. In vitro
studies suggest that the agent is very potent and highly active. At
the 7th CROI, Bahige Baroudy of the Schering-Plough Research Institute
in Kenilworth, NJ, described the progress of the agent thus far. SCH-C
appears to inhibit HIV-1 in mononuclear cells and to prevent HIV from
entering into CD4 and CCR5-expressing cells. SCH-C also seems to be
potentiated (i.e., it becomes even more potent) when used with AZT (Retrovir)
and indinavir (Crixivan).
AnorMED, a Canadian company, is developing a substance designed to
prevent HIV from binding to the CXCR4 receptor. At the 7th CROI, Erik
De Clercq, MD, from the Rega Institute in Leuven, Belgium, reviewed
the development of bicyclams or AMD 3100. AMD 3100 is not a brand- new
compound; ten years ago it was described in the literature as having
antiviral activity against both HIV-1 and HIV-2. Since then, more has
been learned about its mechanism of action and it is now known to be
an antagonist of CXCR4 that works by interrupting the interaction between
gp120 and CXCR4. From in vitro studies, resistance of HIV to
AMD 3100 appears to develop slowly.
Human trials of AMD 3100 have already begun. A Phase I single-dose,
dose-escalation trial has been completed, in which 13 subjects took
24 doses intravenously, subcutaneously, or orally, at doses ranging
from 10 to 80 micrograms/kg. Investigators found that the drug was well
tolerated. Currently a Phase Ib/IIa dose-escalating trial is underway,
in which people with greater than 50 CD4 cells/mm3 and greater than
5,000 HIV RNA copies/mL are receiving 10 days of continuous infusion
of AMD 3100.
Other candidates at early preclinical stages of R&D include ALX40-4C
and T-22, both of which block CXCR4-using virus. A CCR5 receptor antagonist
drug called TAK-779 is also under development by Takeda Chemical Industries.
On March 27, Consensus Pharmaceuticals, a Massachusetts biotechnology
company, was awarded a Phase I Small Business Research grant from NIAID
to fund the continuance of a drug discovery program designed to target
the CCR5 receptor--an indication of broad support for this approach
from the respected scientific community.
Conclusion
These new agents offer new treatment options for people who have exhausted
current options. Progress thus far is encouraging, although even for
those relatively advanced candidates, more data are needed on long-term
potency and efficacy and on such issues as side effects and drug resistance.
One looming drawback may well be the high cost of manufacturing these
agents, which will undoubtedly be transmitted to consumers. Another
detriment for consumers is that several of the candidates are large
molecules that require administration by injection. Some researchers
believe that the identification of these potentially beneficial molecules
may provide clues to finding smaller, more manageable molecules which
when developed could be taken orally. At present, there is widespread
agreement that the principles behind these agents' development are sound.
Leslie Hanna is Editor of BETA.
