oncolytic reovirus sensitizes cancer cells to chemotherapy

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hermit_crab67 - 07 Dec 2003 06:26 GMT

I am an investor in a small biotech company called Oncolytics Biotech.
Their core technology is centered around the unmodified human
reovirus. The reovirus was discovered by Matt Coffey, Patrick Lee, et
al., to have cytolytic effects on cancer cells which have their RAS
gene expression stuck in the ON position. Since RAS is related to
growth signalling mechanisms, this is a common mutation among cancer
cells -- approximately 2/3 of cancer cells are claimed to possess this
mutation.

Here is where it gets interesting, and where my question comes in.
Recently, it has been discovered by the Oncolytics researchers that
the reovirus had some unexpected effects on non-RAS activated cancer
cells: it makes them more susceptible to treatment by chemotherapy. I
don't have a research paper, the only thing I can provide is a patent:

http://v3.espacenet.com/textdoc?AB=reovirus&sf=q&FIRST=1&CY=ep&LG=en&DB=EPODOC&s
t=AB&kw=reovirus&F=0&IDX=EP1361884

"SUMMARY OF THE INVENTION
The present invention is directed to a method of sensitizing drug
resistant cells to chemotherapeutic agents by the use of reovirus.
Reovirus has recently been discovered as a selective anticancer agent
which kills ras-activated neoplastic cells but not normal cells, due
to selective replication of reovirus in cells with an activated ras
pathway (U. S. Patent No. 6,136,307; Coffey et al., 1998; Strong et
al., 1998). Unexpectedly, it was further discovered in the present
invention that reovirus increased the sensitivity of cells to
chemotherapeutic agents as well.

Thus, a tumor which is refractory to cisplatin was treated with a
combination of cisplatin and reovirus, and the results indicate that
the combination was more effective than reovirus alone. Since
cisplatin had no effect on the tumor when administered in the absence
of reovirus, the effect of the combination was not simply an additive
or synergistic result of the individual effects. Instead, reovirus
sensitized the tumor to a chemotherapeutic agent to which the tumor is
normally refractory."

(end quote)

I've been driven by curiousity to try to solve the mystery of the
mechanisms at work. According to Mel Graeves' "Cancer: The
evolutionary legacy" (pp 240-242), cancer cells are so resistent to
chemotherapy because they have supressed expression of the p53 gene.
The p53 gene is responsible for detecting changes in the DNA and
taking the cell through apoptosis when it finds any. However, since
cancer cells have suppressed the p53 gene, the mutations caused by
chemotherapy no longer cause the cell to be killed. So, according to
Graeves, anything that *increased* expression of the p53 gene would
have the effect of making the cell more vulnerable to chemotherapy.

So going off these assumptions, it seems logical to assume that the
reovirus is somehow -- directly or indirectly -- causing cancer cells
to increase their p53 expression. Since a reovirus' replication cycle
is stunted very early in non-RAS cells, I don't think the direct route
is as likely as the indirect route.

As far as the indirect route, two possible mechanisms I see:

1. The nearby RAS+ cells in the tumor that are being destroyed by the
reovirus are causing large amounts of interferon to be released,
bringing in large numbers of NK cells to the area. The NK cells
"notice" that nearby cells are missing p53 proteins from the MHC
molecules, and send the cells signals to cause them to express p53
proteins. (no idea if this is in the NK cells' job descriptions, just
a guess)

2. The nearby RAS+ cells in the tumor that are being destroyed by the
reovirus are sending waves of distress signals of the following
nature: DANGEROUS VIRUS IN REGION. GENOME UNDER POTENTIAL ATTACK.
SECURE DNA BY INCREASING P53 EXPRESSION.

So, any feedback and/or corrections would be welcome -- in particular
alternate speculations of the mechanisms behind the observed results.

Reply to this Message

Helen Stanbro - 08 Dec 2003 00:07 GMT

> I am an investor in a small biotech company called Oncolytics Biotech.
> Their core technology is centered around the unmodified human
[quoted text clipped - 69 lines]
> So, any feedback and/or corrections would be welcome -- in particular
> alternate speculations of the mechanisms behind the observed results.

------------------------------------
Very interesting observations! Your guesses sound like things that
could certainly be investigated. Not much is really known about
mechanisms of cisplatin resistance, but there seem to be several.
Which ones might tie in with reovirus physiology are anyone's guess,
but here is one review that might give you some other directions to
explore:
Drugs. 2000;59 Suppl 4:1-8; discussion 37-8.

Preclinical perspectives on platinum resistance.

Kelland LR.

CRC Centre for Cancer Therapeutics, The Institute of Cancer Research,
Sutton, Surrey, England.

In the 30 years since the introduction of cisplatin into the clinic,
laboratory studies have provided considerable information as to both
how the drug exerts its antitumour effects and how some tumours are,
or become, resistant. Once inside a cell, the chlorine groups of
cisplatin are exchanged for water (aqua) species, which are more
chemically reactive. The intracellular target for cisplatin is DNA,
where a variety of adducts are formed, some on the same strand of DNA
(intrastrand adducts) and others between strands (interstrand
adducts). Of the 4 bases, guanine is the preferred site for binding
and the most common adduct involves linkages on 2 adjacent guanines on
the same strand of DNA. It remains uncertain which of the various
types of adduct is the most important in terms of producing antitumour
effects. Resistance to cisplatin has been studied extensively using
tumour cells repeatedly exposed to the drug in vitro. In these cell
models, resistance is generally due to a combination of mechanisms,
some resulting in reduced damage to DNA and others following DNA
damage. Resistance due to inadequate binding to DNA has been shown to
be caused by reduced drug uptake (influx rather than efflux) and
inactivation by thiol-containing species such as glutathione and
metallothioneins. Resistance occurring post-DNA binding may be due to
changes in DNA repair pathways [an increase in nucleotide excision
repair (NER) or a loss of DNA mismatch repair (MMR)]. Conversely, the
hypersensitivity of some cell lines to cisplatin has been shown to be
due to defective NER, through loss or reduced expression of NER
proteins such as XPG and XPA. Resistance may also be mediated through
alterations in proteins involved in programmed cell death (apoptosis)
such as p53 and the BCL2 family. A basic understanding of cisplatin
resistance pathways has made a major impact in the development of new
platinum analogues capable of circumventing resistance. Examples
(which are now undergoing clinical trial) include ZD0473 (which,
relative to cisplatin, possesses a reduced reactivity towards
inactivating thiol-containing molecules) and the trinuclear platinum
BBR3464 (which has markedly different DNA binding properties compared
with cisplatin).

Publication Types:
Review
Review Literature

PMID: 10864225
---------
It would be nice to know what tumor system the investigators were
using (cultured cell line? xenograft in mice? human or animal tumor
and what histological type, etc.?) and whether this finding was
replicated in any other tumor system. It would be easy enough to do
some preliminary testing in tumor cell lines whose mechanism of
cisplatin resistance is known (MRP2 protein hyperexpression,
glutathione-related resistance, cMOAT-protein-mediated resistance,
etc.) and see which ones, if any, become sensitive after reovirus
infection. That would at least give you a sense of where to look for
reovirus effects on resistance mechanisms.
Let us know what you find out!
Helen S.

Reply to this Message

matdu1 - 13 Dec 2003 15:51 GMT

Hi Hermit, sorry to take this long to respond. I have been extremely
busy as of late. I'll try to make couple points now and will try to
respond in greater details later.

(1) Over 50% of cancer cells have mutation defects in the p53 gene. In
fact this mutation occurs more common among late stage cancers. In p53
mutated cancer cells, you cannot recoup p53 activity since the active
gene does not exist in the first place. So the theory proposed by
Graeves, while plausible, can only be true in cancer cells that have a
functional p53 gene.

(2) I suggested the possible involvement of DNA repair enzymes for the
following reasons:

a. A company of which I was a board member has identified the enhanced
expression of a DNA repair enzyme in most of the cancer cells they had
screened, including biosy samples. By using antisense and SiRNAs, they
were able to sensitize these cancer cells to radiation and chemo
treatments.

b. A biotech called Kudos is developing small organic molecules to
inhibit some DNA repair enzymes. They have found that some of these
inhibitors were able to sensitize cancer cells to radiation and
chemos.

(3) Your suggestion of some signals sent by Ras-activated cancer cells
to adjacent non-Ras cells is interesting and can be proved as follows:
-Grow cancer cells that are non Ras-activated and radiation or chemo
resistant in vitro and infect them with reovirus and then see if they
become sensitized.
-Do the same experiment as above except co-culture both Ras-activated
and non Ras cells side by side. This shows whether the presence of
Ras-activated cells is needed.
-Grow a tumor in mice that consists only of chemo resistant and non
Ras cells and inject them with reovirus. See if these cells can be
sensitized in the absence of Ras-activated cells.
-Do the experiment as above but now grow a second tumor with
Ras-activated cells. This will determine if the presence of
Ras-activated cells is required.

> > I am an investor in a small biotech company called Oncolytics Biotech.
> > Their core technology is centered around the unmodified human
[quoted text clipped - 139 lines]
> Let us know what you find out!
> Helen S.

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