oncogene] translocation in CML [chronic
myelogenous leukemia]. In those cases,
we’re finding that even a single drug can be
effective. But in other carcinomas, the majority
of tumors are evolving over time, after
multiple mutations, from environmental hits
in a particular germline background, and
these are so different that the chance of having
single‑drug treatments or even two- or
three‑drug treatments, I think, is low.
For the majority of tumors, we’re already
thinking combinatorial treatments only
because it matches the heterogeneity of the
tumor. But the successes to date are with
single drug agents like Gleevec [imatinib;
Novartis, Basel]. But CML and retinoblastoma
are special cases and rare tumors where
you have a predefined genetic abnormality
and [in the case of CML] it happens to be
druggable. The majority of tumors are much
too heterogeneous, and something I worry
about a lot is whether the tumors in a patient
we’re treating even matter. And what I mean
by that is, if you take a small-cell carcinoma,
they’ll melt away with conventional chemotherapy,
but a few months later, they’ll all
come back and be deadly. We can treat the
bulk of the tumor, but clearly, there’s something
else underlying that. And its not clear
whether these are stem cells or we’re selecting
for de novo resistant clones. I think that’s
an unanswered question that needs to be
answered. But if it’s stem cells, then we can
more or less ignore most of the tumor we see
by imaging or anything else; it’s the cells we
can’t see yet that are the problem.
So the question in my mind is, what is the
heterogeneity at that level? Because that’s
probably all that matters. I think a lot of what
we do in oncology, we treat bulk disease,
but it’s not clear to me that the bulk of disease
we’re treating, when there’s a response,
whether it matters.
RMH: To me, the most important heterogeneity
is between dividing cells and nondividing
cells because most drugs target only
dividing cells. So I think this maybe could
explain why we get variable results with
current chemotherapy treatments: when it
works, you’re killing off the dividing cells.
But then the non‑dividing cells are relatively
resistant to the chemotherapy and eventually
they begin to cycle, especially as they get
near the surface of the tumor as the other
dividing cells are killed by the chemotherapy.
The tumor starts growing again. So to me,
the most important heterogeneity is between
dividing and non‑dividing cells in a tumor.
I think the goal is to find drugs that target
non‑dividing cells.