In this series
of articles we have been reviewing the available information regarding the
parathyroid hormone (PTH) actions in the bone and cartilage to understand if
PTH could turn to be an appropriate therapeutic option for achondroplasia. We
have briefly visited this hormone’s metabolic properties and learned that it is
part of a small family, as there is a very close correlate protein produced by
the body, called PTH-related protein (PTHrP). Both molecules produce quite
similar effects acting through the same cell membrane receptor, the PTHR.
We also
learned that while PTH circulates in the blood, released by the parathyroid
glands, PTHrP is found within the tissues which produce it; in other words, in
normal situations, PTHrP is a local agent, it doesn’t circulate. PTHrP is
important for us (and by extension PTH) in the context of achondroplasia,
because this protein exerts a crucial role in bone growth.
If you have
been following this blog, you might have already read about the complexities of
the cartilage
growth plate and know that PTHrP is part of a very intricate symphony where many
different agents control bone growth in children. While PTHrP is a growth
promoter, fibroblast growth factor receptor type 3 (FGFR3), the protein which
is mutated (altered) in achondroplasia, works like a growth brake. As in
achondroplasia FGFR3 is working too much, bone growth is severely impaired. We
saw that there is evidence that the growth arrest caused by the mutated FGFR3
may cause reduction of the availability of PTHrP within the growth plate, and
this in turn may account for part of the FGFR3 effects in the cartilage (a
vicious cycle).
We have also
seen that PTH is not like a simple on/off button, with just one single
activity. Its molecule contains segments that when cleaved (sliced) may have
distinct metabolic functions. The two examples given in the last article are
worth to repeat here because they may help us to choose the right PTH form for
potential tests in achondroplasia:
- The N-terminal part of PTH, constituted by the last 34 amino acids of the hormone chain seems to be responsible for the strong anabolic action of this hormone in bone (and is called PTH 1-34);
- The C-terminal part of PTH seems to be responsible for a modulation of this anabolic activity, and probably is responsible for the differences of the anabolic effects of PTH 1-34 compared to the integral hormone, which is also called PTH 1-84 (1).
To answer these
questions we will need to examine the results of the toxicity studies performed
with PTH when it was being evaluated as an experimental drug for osteoporosis.
Is PTH risky?
For any
experimental drug to become a medicine it not only need to prove it works
(efficacy) but also must pass a series of exhaustive studies both in vitro (in the lab) and in vivo (in animal and human) to test if
it is safe. So, let’s check what happened with one of the PTH analogues, teriparatide,
during the obligatory tests made to assess its safety. The findings of those
tests, which we will be reviewing below brought concern about its long term use
in osteoporosis. You see, osteoporosis is a chronic, slow evolving bone
disorder and therapies for this condition must be also long term. In summary, tests
made in animals, looking for a carcinogen effect (capability of causing
cancer), showed that mice submitted to lifelong high dose exposure to
teriparatide developed osteosarcomas and other kinds of bone cancer
(2).
What does
explain those findings? PTH has a physiological pro-proliferative role (it
stimulates the multiplication of cells) so the results of those tests were
interpreted as the cancer observed in the animals being a consequence of the
continuous pro-proliferative stimulation of PTH (2).
These findings
caused the Food and Drug Administration (FDA) to impose a black box warning in
the teriparatide label with two main restrictions to its use in the clinic (3):
1. Prohibition of its
administration in patients with open epiphyses (a synonym of growth plates;
growing children and adolescents);
2. Continuous use up
to 2 years only.
FDA determined
that children and adolescents could not use teriparatide because they are under
a natural growing pace and cells are multiplying in great speed, so there is a
fear about the risk of having a booster (PTH) working in these fast developing
organisms that could cause a cancer transformation in the exposed cells and
tissues.
These
restrictions reflect concerns over risks not certainties and currently there is
no clear evidence that long term use of PTH analogues in fact induce
osteosarcoma or other kinds of cancer in man. On the contrary, the subsequent
studies made in animals (mice and monkeys) to confirm the cancer increase
failed to demonstrate so. These subsequent studies concluded that the oncogenic
potential (property of causing cancer) of PTH is linked to the dose used,
several times higher than the approved dose in humans (4-6).
Furthermore, as
part of an agreement with the manufacturer, FDA requested also a long term
follow up of teriparatide use after is approval. The epidemiological
surveillance about the risk of osteosarcoma in patients treated with
teriparatide showed that it is similar to what is found in the general
population. For instance, in 2007 there were already about 300,000 patients
treated with teriparatide and there was no clear evidence that in the recommended
doses it increased the rate of bone cancer compared to the unexposed population
(7). In summary, pre-clinical and surveillance studies with PTH (the full form)
and teriparatide have not been showing any tendency of increased risk of cancer
in treated patients in defined therapeutic doses. Furthermore, as we have
already reviewed, PTH analogues have been tested in adults and children with
hypoparathyroidism and have been showing to be safe.
Is there a
place for PTH therapy in achondroplasia?
This is the
key question. Now it’s time to address it.
First thing we
need to know is that PTH and/or PTHrP are not FGFR3 antagonists. They work
quite independently one from each other, although we already know that FGFR3
could reduce PTHrP availability in the growth plate (8).
And then,
could these two proteins be used to treat achondroplasia? You see, as PTH or
PTHrP could not block FGFR3, they would theoretically only overcome one of the
main consequences of the FGFR3 actions in the growth plate, which is the chondrocyte proliferation arrest.
This is an
interesting concept, treating a disorder without working directly in the
causative agent. However, this is exactly the idea being applied in the CNP
(BMN-111) development. The goal here would be to rescue the proliferation of chondrocytes,
which is impaired because of the FGFR3 excessive activity.
The idea of
using PTH in achondroplasia is not new. Back on 2004, the group of Amizuka published an
interesting work, where they tested mouse models to study the consequences of
switching off FGFR3 or PTHrP or both at the same time. You should look at the
published Figures 1 and 2 in their paper, which I don’t reproduce here to
respect the copyright (access should be free, after registering at the editor’s
website).
Figure 1 shows
the differences among the animal models they created. Pay attention to the size
of the long bones in the figures 1A (normal mouse, called wild type), 1B (no
FGFR3, PTHrP positive) and 1C (FGFR3 positive, no PTHrP). Figure 1C resembles
figures published in other studies showing mice models of achondroplasia.
Figure 1B highlights the proliferative effect of PTHrP: the long bones are
longer than those of the wild type mouse.
Figure 2 shows
microscopic cuts of the growth plates corresponding to those of the Figure 1.
Pay attention to the same sequence of figures 1A, 1B and 1C. Figure 2B shows a
larger proliferative chondrocyte layer compared to the wild type in 2A, while
Figure 2C highlights the reduction of the thickness of this layer.
The study by
Amizuka et al. is important because it shows the natural effects of both FGFR3
and PTHrP. And then, is there anything done with PTH? The answer is yes.
In 2007, Koso Ueda and
cols. (10) published a
study where they tested the use of a recombinant (synthetic) form of PTH in bone explants of an achondroplasia mouse
model (these were mouse long bones excised from the animal and kept in a
culture medium). The administration of PTH rescued the proliferative layer of
chondrocytes despite the presence of the mutant, overactive FGFR3. PTH works.
Look at the figures showing the differences perceived among the different
chondrocyte layers. Ueda’s group also published a small work in a medical
meeting in 2009, showing further experiments with the use of PTH in a mouse
model of achondroplasia, again with positive results (Ueda K et al.
2009).
Recently, the
group of Chen, who has also been very interested in the growth plate
cartilage and achondroplasia, published a large study where they tested long
term therapy of achondroplastic mice with an intermittent injection of a PTH analogue (12).
Results are striking in terms of chondrocyte proliferation and in the growth
plate proliferative layer, although authors pointed out that the rescue was not
complete. Importantly, they also found that FGFR3 was downregulated (had its
production inhibited or reduced) in PTH-treated animals. You see, the cycle is
closing, isn’t it? Excessive FGFR3 inhibits PTHrP production; introduction of
PTH in achondroplasia reduces FGFR3 production.
Let’s see a
bit more about the study by the Chen group (12), which brings several very
interesting insights in terms of design, findings and conclusions. The authors
used an intermittent scheme of PTH administration, resembling the strategy used
for the treatment of osteoporosis in humans with the commercial available PTH
analogues. Intermittent use of PTH has been showed to build bone in
osteoporosis. When used continuously, PTH causes osteoporosis among other
metabolic disturbances and we surely wouldn´t want it to happen if we were treating a child with achondroplasia. So, by using intermittent administration the authors
very likely simulated the way PTH could be used in humans to treat the bone growth arrest in achondroplasia.
In summary
In summary,
PTH and its analogues represent a true potential therapy for achondroplasia. There
are several steps to be performed in terms of understanding its mechanism of
action and the effects in the animal model and also to decide which analogue should be safer and to explore the potential
undesired effects before it could be used in clinic. For instance, in children
with hypoparathyroidism, PTH has been showed to be safe in long term, with few
and manageable side effects. However, they have low PTH from the beginning, which
would not be true about a child with achondroplasia. PTH use could cause renal
stones, an issue we would not like to cause in the patient. How can we manage
these effects? What we need are diligent tests, focused researchers and
resources.
A final note
In this series
of articles about PTH we approached a true boundary. To decide to work on a
known molecule in a new indication, especially if it is a rare condition
affecting children is not an easy step to do. There is a lot of risks involved,
from the affected child’s health to the money to be invested. However, if we
just sit down and wait for the lower hanging fruit – as this is the common behavior
within the industry – little will be achieved. In the last two years I have made contact with four different pharma industries or biotechs which are developing PTH or PTHrP analogues. I received polite answers explaining that it would be difficult to address the indication at that time or simply received no answer at all. Well, I don’t think
achondroplasia is untreatable, but it is the kind of condition that won’t have
true attention from major health sponsors easily. It is up to those directly or
indirectly affected to change the future.
I realize I
have extended a lot this article series. The goal is always to inform, to share
knowledge while trying not to be excessively technical. It is the scientific
language which prevents many interested people to understand what is really
going on with achondroplasia. The lack of scientific background could be a kind
of barrier which makes people in front of the inevitable to just sit and wait. I
wish these articles I am publishing help the achondroplasia community to get more
conscience about the science ongoing and become stronger. It will be this strength that can make the
difference for our children. We can do more.
Well, this is far
from the end. There are other potential therapeutic solutions waiting for
review. I will be bringing a new one in the next article. Not really new since I have already addressed it in one of the first reviews in this blog.
References
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region of parathyroid hormone on bone-derived cells: determinants of
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2. Vahle JL et al. Skeletal Changes in Rats Given Daily
Subcutaneous Injections of Recombinant Human Parathyroid Hormone (1-34) for 2
Years and Relevance to Human Safety. Toxicol Pathol 2002; 30: 3312-21.
4. Vahle JL et al.
Bone neoplasms in F344 rats given teriparatide [rhPTH(1-34)] are dependent on
duration of treatment and dose. Toxicol Pathol 2004;32(4):426-38.
5. Vahle JL et
al. Lack of bone
neoplasms and persistence of bone efficacy in cynomolgus macaques after long-term treatment with teriparatide [rhPTH(1-34)].
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Defining a noncarcinogenic dose of recombinant human parathyroid hormone 1–84
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7. Harper KD et
al. Osteosarcoma and teriparatide? (letter) J Bone Miner Res 2007; 22:334.
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fibroblast growth factor receptor 3 in mouse downregulates Ihh/PTHrP signals
and causes severe achondroplasia. Hum Mol Genet 2001; 10(5): 457-65.
9. Amizuka et al. Signalling by fibroblast growth factor receptor 3 and parathyroid hormone-related peptide coordinate cartilage and bone development.
Bone 2004; 34(1): 13-25.
10. Ueda
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11. Ueda
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