Originally written April 15th, 2015
MolMed today exercised an option on a CD44v6 CAR-T:
I’ll primarily be referring to this paper describing the CAR construct:
This CAR targets a non-mutated tumor antigen, CD44v6. Non-mutated self
antigens have previously been a dangerous class of antigens to target
(such as Her2). Mesothelin is another one, and we will see some of the
data from this target at the upcoming AACR, which I would expect will
discuss trying to mitigate toxicity. As a guess, perhaps the Mesothelin
mRNA CAR-Ts last a short enough time that you don’t get intolerable
On to CD44v6 – this is an antigen expressed rarely in normal cells,
mainly keratinocytes and monocytes, but seems to be expressed in a range
of tumor types, such as AML, MM, pancreatic, and head & neck
cancer. CD44v6 was found to be expressed in a high percentage of AML
& MM samples. Importantly, they corroborated previous work
suggesting expression is important for tumor growth, as knockdown of
CD44v6 reduced tumor seeding and progression. This suggests that it
might be more difficult for the tumor to lose CD44v6 expression as a way
of becoming resistant to the CAR.
They made the CAR construct based off of a previously used antibody in
clinical trials that is specific for CD44v6, called bivatuzumab.
Injection of the CAR-T cells successfully suppressed growth of human AML
& MM xenografts, although it looks like the first time point for
evaluation was 5 weeks after T-cell injection. However, this doesn’t
model potential on-target off-tumor effects of the CAR, which is the big
concern for this type, and really any type, of CAR.
They looked at expression of CD44v6 in normal tissue and found it
expressed, as expected, primarily in keratinocytes & monocytes, both
at seemingly similar levels (maybe keratinocytes slightly less), and
both considerably less than expression in AML samples.
First they tested skin (keratinocyte) toxicity, and surprisingly found
the CAR-T cells did not react against keratinocytes. I was surprised the
CAR didn’t have skin toxicity, although this was not tested in vivo,
only in co-culture of CAR-T cells with keratinocytes. The antibody used
to make the CD44v6 recognition portion (scFv) of the CAR construct comes
from bivatuzumab. Previously, this antibody had been conjugated to
radionuclides to deliver radiation to patients' head & neck tumors.
Some responses were seen, toxicity was myelosuppression (presumed to be
due to the circulating radionuclide) & grade 2 oral mucositis
(potentially on-target toxicity). A second trial in head & neck
cancer tested bivatuzumab conjugated to mertanzine (an anti-mitotic
drug). Preclinical work suggested reversible skin toxicities, presumed
to be due to expression of CD44v6 in skin, and reversible ones were seen
in the first patients enrolled. There was some evidence of efficacy,
some stable disease and minor tumor responses. However, at the highest
dose, the first patient developed a fatal skin toxicity – toxic
epidermal necrolysis, even though this patient had only a slightly
higher dose than the previous patients, and a lower cumulative dose.
There wasn’t evidence of deconjugation of the toxin from the antibody,
as the free toxin has other toxicities that weren’t observed.
The authors of the CAR-T study propose that for somewhat unknown reasons
the CAR-T may have a wider therapeutic index than the antibody
conjugates. This would be somewhat surprising to me, as it generally
seems that CAR-T cells can recognize very few molecules of the target on
cells, and that they have one of the narrowest therapeutic indices of
antibody targeted therapeutics. Perhaps playing with the affinity of the
scFv could allow for broader therapeutic index, but this is the same
antibody used here as in the previous clinical studies. Maybe there is
something structural about the scFv on the surface of T-cell versus
floating & conjugated to toxin, or perhaps keratinocytes can
suppress T-cells, but it is surprising there seems to be little to no
reactivity of the CAR-T cells against keratinocytes.
The CAR-T cells did react against monocytes, and when injected in mice
reconstituted with a human hematopoietic system, the CD14+ monocytes
appeared to be depleted relatively rapidly, in ~10 days. While the
accompanying preview by Richard Morgan suggested that short term
monocytopenia is tolerable, long term it is not tolerable: http://www.bloodjournal.org/content/122/20/3392.long?sso-checked=true
The authors dealt with this toxicity by introducing suicide gene
constructs to be able to deplete the T-cells after a period of time, to
allow reversibility of the monocytopenia. This seemed to work well,
however I’m not sure at what duration monocytopenia become intolerable,
and if the CAR-T cells will have significant enough activity by that
point. From what I can tell, based on the xenograft, at 5 weeks there
was some clear activity, but it only took 10 days for the CD14+ cells to
be depleted, so it might be difficult to separate activity from
toxicity. Although for the CD19 CAR-T cells in humans, significant
anti-tumor activity appears to occur very rapidly, so CD44v6 CAR
anti-tumor activity could occur more rapidly in humans.
Some of the most impressive data was with the suicide gene, iC9 (I
believe this is the same as Bellicum’s suicide gene construct) – which
was much better than expression of the other suicide construct thymidine
kinase. iC9 killed both activated & resting T-cells and did so in
only a few hours.
So with all these concerns, I would take a wait & see attitude about
the application of this CAR-T in the clinic. It is differentiated,
which is nice, considering the paucity of targets, however, I am
concerned about the therapeutic index, specifically if skin toxicity
shows up, and even the monocyte toxicity might be too difficult to
separate from the anti-tumor activity. If everything works like in the
paper, it could be a useful CAR in the clinic, however, for me, the
above are some reasons to be cautious.
Let me know any feedback or questions/comments you have, happy to continue discussion.
Phase I Bivatuzumab Mertanzine trial: http://www.ncbi.nlm.nih.gov/pubmed/17062682
CD44(v6) as therapeutic target: http://www.ncbi.nlm.nih.gov/pubmed/20303742
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Biotechr is written by Dr. Robert Kruse (@RobertLKruse), who holds a PhD and is currently completing his MD. His research work focused on infectious disease and immunology. This blog is focused on analyzing the latest developments in biotechnologies being developed in academia and industry, with a particular focus on biomedical therapeutics. I hope that the posts are interesting and useful, and hope you join in the discussion with guest posts on the site!
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