Topotecan, CPT-11 (Irinotecan), Camptothecin, and Other Topoisomerase I Inhibitors

We sent the following memos to the National Task Force on
AIDS Drug Development, for their meeting on April 14 and 15.
They describe a new class of potential drugs, which are not
well known in the AIDS world but are being actively developed
for cancer. Because a major meeting on this subject occurred
on April 13 in San Francisco, we send a last-minute update
memo to the National Task Force, and we also delayed the
printing of this issue of AIDS TREATMENT NEWS.

We will continue to follow this subject, and would appreciate
any information about it.

* * *



The following is a revised version of the memo we sent on
April 11 to the National Task Force on AIDS Development:

A new class of anti-cancer compounds, generating wide
interest in cancer research and already in human trials with
hundreds of patients, also shows major promise for treating
HIV. But almost nothing is being done to test or develop this
possibility; there are no plans for any HIV clinical trial
anywhere in the world, as far as we can determine. The main
obstacle seems to be that some of the key information is new,
and almost all of the work has been done by cancer, not AIDS,
researchers; therefore the HIV medical/scientific community
is not yet aware of the promise. The National Task Force
should pay attention and make sure that an important
opportunity for immediate, practical progress in treating HIV
disease is not delayed or overlooked.

While no one knows if topoisomerase I inhibitors will prove
successful in treating HIV disease, these drugs do have the
following advantages:

* Laboratory tests have suggested that the doses required for
HIV inhibition may be a small fraction of those needed for
treating cancer. One laboratory study found 80 percent
inhibition of HIV at a dose 200 times less than cancer
patients have tolerated; another found an anti-HIV
concentration 400 times less than that required to kill
cells. Therefore, the toxicity seen in cancer trials might
be greatly reduced, if much lower doses can be used for
treating HIV disease.

* These drugs have been found to be highly active against HIV
in both acutely and chronically infected cells -- unlike
drugs now in use.

* They have been found to be equally active against HIV
strains which are highly resistant to AZT, as against
susceptible strains. (See reference in Li, Wang, and Pardee,
below.)

* Although no long-term resistance studies have yet been
done, theory suggests that the development of viral drug
resistance may be much slower with these drugs than with
other antivirals.

* The mechanism of action of these substances is entirely
different from that of any other anti-HIV agents -- either
treatments in use, or potential treatments in trials or in
laboratory testing. A new class of drugs would open new
worlds of therapeutic options -- for example, new kinds of
combinations to target different steps in the HIV life cycle.

* These potential drugs are also effective against at least
some other retroviruses -- a finding which may offer a number
of research and clinical advantages.

* Because of the large cancer trials, the problems of
manufacture and quality control have been largely worked out;
and extensive human pharmacokinetics and toxicity data is
already available. This means that clinical trials to test
for anti-HIV activity in people could start almost
immediately. All that is needed would be to give a low dose
to a handful of people, and measure virological and clinical
markers (HIV RNA, T-helper counts, etc.) before, during, and
after the treatment.

To the best of our knowledge, no person with HIV has ever
taken any of these drugs. Therefore, it is possible that a
very important advance is being overlooked. (Note: drugs
which inhibit the closely-related enzyme topoisomerase II do
not have anti-HIV activity.)

We should also point out that these are not underground
drugs. Topoisomerase I inhibitors can be very toxic, and
difficult to dissolve and administer in an effective form.
The first known compound, camptothecin, was abandoned as an
experimental cancer treatment about 20 years ago, although it
recently re-entered human cancer testing in the U.S.
(Although camptothecin was discovered in a Chinese tree, it
was never used in traditional Chinese medicine.) These
obstacles could easily be overcome by qualified researchers,
however, due to the human experience in the cancer trials.
What is needed is serious mainstream attention -- and moving
quickly on a small, practical trial to measure anti-HIV
activity in humans.

Some Potential Topoisomerase I Inhibitors

* Topotecan. A camptothecin derivative being developed by
SmithKline Beecham, topotecan is now in phase II trials in a
number of countries as a potential treatment for many kinds
of cancers. It might be the most feasible drug to test
against HIV. Laboratory work showed that it inhibited HIV at
a concentration 200 times lower than cancer patients have
tolerated.

* CPT-11. This drug has now been approved in Japan, for
certain cancers.

* Camptothecin. A major review published in 1993 (see below)
noted that the sodium salt of camptothecin, which was
abandoned 20 years ago, is now known to be poorly effective,
because of the open-ring structure used to make the drug
soluble. The article also reported that a new phase I trial
of camptothecin in the lactone form, as an oral preparation,
was being planned. [Note: based on the results of that trial,
a U.S. phase II trial has recently been approved.]

* There are a number of other camptothecin derivatives in
early development.

* Some topoisomerase I inhibitors (for example, b-lapachone)
are not camptothecin derivatives, and may have different
mechanisms of action.

Drug Resistance

In cancer, resistance to topoisomerase I inhibitors is
already well known. In AIDS, there is no data on resistance.
However, the two situations are entirely different.

In cancer, the drug is used to kill cells, so resistant cells
tend to survive and multiply. In AIDS, the drug appears to
target cellular, not viral, proteins. Cellular genes mutate
far less rapidly than HIV. Perhaps more importantly,
supporting HIV replication is hardly a survival advantage for
the cell, so the selective pressures are to remove, not
preserve, any cells resistant to the drug's antiviral action.
And in any case, the occasional resistant cell is not
multiplying, so its impact on the disease process is likely
to be negligible.

Independently, the fact that topoisomerase I inhibitors
appear to be broadly effective against retroviruses, not only
HIV, suggests that we are unlikely to see the very rapid
resistance found with drugs highly targeted against HIV-1
gene products. (Priel, Showalter, and Blair, reference below,
"found that camptothecin inhibits equine infectious anemia
virus (EIAV) replication in chronically infected cells and
Moloney murine leukemia virus replication [manuscript in
preparation], suggesting that camptothecin may act at a
common, but as yet unidentified, step in the life cycle of
retroviruses.")

In addition, the finding, published only last month, that
camptothecin inhibits the Tat pathway of LTR (long terminal
repeat) activation of HIV, recalls the case of Ro 24-7429,
the Hoffmann-La Roche Tat inhibitor, to which HIV developed
no resistance at all despite a two-year-long attempt to
create such resistance in laboratory cultures (see reference,
below).

Annotated Bibliography

The information above is based on the following articles:

* Li CJ, Zhang LJ, Dezube BJ, Crumpacker CS, and Pardee AB.
Three inhibitors of type 1 human immunodeficiency virus long
terminal repeat-directed gene expression and virus
replication. Proceedings of the National Academy of Sciences,
USA. March 1993; volume 90, pages 1839-1842.

This paper described the use of a well-known research test to
screen for LTR inhibitors as a new class of potential AIDS
drugs. Three LTR inhibitors were identified: topotecan (the
most powerful), b-lapachone, and curcumin (the least
powerful). The results of the screening were confirmed by
live-virus tests. "For comparison, topotecan can be tolerated
in cancer patients at a plasma concentration that is 200
times the concentration that inhibits 80% of p24 antigen
production in chronically infected cells."

* Li CJ, Wang C, and Pardee AB. Camptothecin inhibits Tat-
mediated transactivation of type 1 human immunodeficiency
virus. The Journal of Biological Chemistry. March 11, 1994;
volume 269, number 10, pages 7051-7054.

This research, using engineered cells but not live HIV, found
that camptothecin worked as an LTR inhibitor at a very low
concentration -- about 0.03 micromolar, an order of magnitude
lower than for the Roche Tat inhibitor Ro-7429. Camptothecin
seemed to work as a Tat inhibitor: "Experiments with HIV-1
LTR mutants suggest that transactivation response sequence
but not NF-kB is responsible for the inhibition by
camptothecin. The target for camptothecin may be a cellular
factor that is important for the activation of HIV-1 LTR by
Tat and thus may offer a potential target for therapy of HIV-
1 infection."

The Tat mechanism of action is important. The other main
mechanism for inhibiting activation of the HIV LTR (namely,
NF-kB), is (1) already achievable by readily available drugs,
for example, pentoxifylline, and (2) can only be partially
inhibited in any case, because cellular activation by NF-kB
is needed by the body. Effective inhibition of HIV through
the LTR (to keep the virus in latency) will probably require
inhibition of both the Tat and the NF-kB mechanisms of LTR
activation; but of the two, Tat-inhibitor drugs are most
needed now.

* Priel E, Showalter SD, and Blair DG. Inhibition of human
immunodeficiency virus (HIV-1) replication in vitro by
noncytotoxic doses of camptothecin, a topoisomerase I
inhibitor. AIDS Research and Human Retroviruses. 1991;
volume 7, number 1, pages 65-72.

This early paper showed inhibition of live HIV by
camptothecin, at concentrations of 0.01 to 0.02 micromolar
when H9 cells were treated for one hour at the time of
infection. This is important in providing live-virus
confirmation of the work by Li, Wang, and Pardee (cited
above), which used a screening test for LTR inhibitors, but
did not use live HIV in their study of camptothecin. "The
results suggest that the inhibitory effect of camptothecin on
HIV replication is due to its effect on the virus replication
and is not a result of its cytotoxic character. Moreover, the
anti-HIV doses are 400 times lower than the cytotoxic doses
of camptothecin."

Also, this paper reported that two topoisomerase II
inhibitors (m-Amsa, and VP-16) had no effect on HIV
replication.

* Slichenmyer WJ, Rowinsky EK, Donehower RC, and Kaufmann SH.
The current status of camptothecin analogues as antitumor
agents. Journal of the National Cancer Institute. February
17, 1993; volume 85, number 4, pages 271-291. This article
reviews clinical and pre-clinical research with camptothecin
itself, and with its derivatives, especially topotecan and
CPT-11.

"...The previous clinical trials assessing the efficacy of
sodium camptothecin as an antitumor agent were probably
inadequate. The insolubility of camptothecin lactone makes it
difficult to devise a suitable formulation for further
clinical testing. Nonetheless, the decision to reopen a
limited clinical trial of camptothecin lactone (a single-
institution phase I trial of an oral preparation) represents
an interesting first step in re-evaluating camptothecin."

* Wall ME and Wani MC. Camptothecin. In Anticancer Agents
Based on Natural Product Models, Academic Press, 1980. This
paper describes the discovery of camptothecin, as a result of
screening thousands of plants by the U.S. National Cancer
Institute in the late 1950s. Only one of the plants screened
in that program, C acuminata, a small tree native to China,
showed significant activity. The article also noted that the
plant had not been used in Chinese traditional medicine.
China, however, has done considerable research with
camptothecin since its anticancer activity was discovered.

* Hsu MC, Dhingra U, Earley JV, and others. Inhibition of
type 1 human immunodeficiency virus replication by a tat
antagonist to which the virus remains sensitive after
prolonged exposure in vitro. Proceedings of the National
Academy of Sciences, USA. July 15, 1993; volume 90, number
14, pages 6395-6399.

This paper reported that HIV developed no resistance to Ro
24-7429, the Hoffmann-La Roche Tat inhibitor. "After 2 years
of continued weekly passage of the virus in fresh CEM cells
grown in the presence of the compound at 1 or 10 micromolar,
the virus did not develop resistance to the drug. These
results indicate that the compound's action might involve a
cellular factor."

* * *

We sent the following update to the National Task Force on
AIDS Drug Development, on April 14, 1994.

Update on topoisomerase I inhibitors

Here is more information on topoisomerase I inhibitors --
some from papers we had missed, some from a major symposium
on the subject yesterday, at the American Association for
Cancer Research annual conference in San Francisco.

* CPT-11 (also called irinotecan; like topotecan, it is a
camptothecin derivative, with the molecule modified to make
it water-soluble) was approved in Japan on January 18, 1994,
for lung, cervical, and ovarian cancers. This is the first
topoisomerase I inhibitor approved anywhere. CPT-11 is a pro-
drug with a long half life in humans, which can simplify
administration; the active form of all the compounds has a
shorter half life.

Also, Dr. Arthur Pardee's group at the Dana Farber Cancer
Institute has tested CPT-11 (using their HIV LTR screening
system, not live HIV), and found results comparable to those
for the similar drugs, topotecan and camptothecin (private
communication).

* Camptothecin has recently been approved for a phase II
cancer trial in the U.S.

* In animal tests, camptothecin prevented the development of
two different retroviral diseases, when given one, two, or
sometimes three days after exposure to the infection. (E.
Priel and others, Inhibition of Retrovirus-Induced Disease in
Mice by Camptothecin, Journal of Virology, June 1993, pages
3624-3629.)

* An Australian group failed to find anti-HIV activity of
camptothecin, except at doses that killed cells (D Jardine
and others, Cellular Topoisomerase I Activity Associated with
HIV-1, AIDS Research and Human Retroviruses, December 1993, p
1245-1250).

At this time, two separate groups (Pardee's in Boston, and
Esther Priel and others at Ben-Gurion University in Israel)
did find anti-HIV activity at doses hundreds of times less
than the cell-killing dose, in tests with live HIV (with
topotecan in Boston, and with camptothecin in Israel). We do
not know why the Australian finding is different.

In any case, the risk and cost of a small, low-dose trial is
slight; and the need for a new class of HIV drug is great.
Unless a specific antiviral effect can be largely ruled out,
a low dose of one of the three drugs (CPT-11, topotecan, or
camptothecin) should be given to a few people, with careful
monitoring of viral and clinical markers such as HIV RNA, to
see if there is anti-HIV activity in humans.