There have passed 20 years since the identification of the cause of achondroplasia. We now understand that achondroplasia is caused by a modification in the structure (a mutation) of the gene that carries the chemical information for the production of the protein named fibroblast growth factor receptor 3 (FGFR3) (1,2) (Figure 1). This reactive protein (so also called enzyme) is responsible for reducing the ability of a particular kind of cells, the chondrocytes of the cartilage growth plates, to proliferate (multiply) and hypertrophy (mature) (3).
Figure 1. FGFR3 structure
The growth plate is a thin, very specialized region located in both extremities of long bones and is the tissue responsible for bone growth (4) (Figure 2). FGFR3 is a key factor for bone growth, but it is not the only one: there is a large number of other chemical and mechanical factors acting positively or negatively to modulate the pace of bone development in the growing body (5). So, any disruption in this complex network can compromise the normal bone growth. Why is this important to know? Because when we are trying to find therapies for achondroplasia we must pay attention to the drugs' mechanisms of action: we don't want to use a drug that could block FGFR3 but also cause impact in other agents there in the growth plate.
Figure 2. The growth plate cartilage.
We know that the natural role of FGFR3 in the chondrocytes is to reduce the speed of their proliferation and maturation, and that the mutation causing achondroplasia makes it more active: chemical reactions inside the chondrocyte that were mildly activated by the normal enzyme are intensified by the mutated one. Some think that these chemical reactions drive the chondrocyte to enter in a state of paralysis, called cell senescence, when the cell doesn't die but stops most of its normal functions (6). With less chondrocytes multiplying and maturing, the growth plate doesn't achieve its full potential and the bones grow short.
We have already reviewed all the above concepts before, here in the blog. You can read more about the way chondrocytes work under FGFR3 influence in several previous articles, which you can find in your preferred language page accessible on the top of this page.
Treating achondroplasia
Is achondroplasia treatable? The answer is yes, although the way to achieve it may be a hard one. Achondroplasia has some characteristics that makes it a fairly "drugable" disorder from medical, pharmacological and economic standpoints:
1. The FGFR3 mutation causing achondroplasia is almost always the same in all cases: G380R (an amino acid called glycine is substituted by an arginine in the position 380 of the protein chain), so it is highly foreseeable. This is quite different from other disorders caused by gene mutations, where the mutation may be more unpredictable;
2. FGFR3 mechanisms of action in the chondrocyte are now reasonably known, and there have been no major concerns expressed in the specialized literature regarding unexpected functionality, although now and then new interactions between FGFR3 and other proteins and its influence in other cells such as osteoblasts are being published.
This is important from the drug development standpoint, because one of the questions that the researcher of new medications needs to answer is whether the identified target (here, FGFR3) really has a cause & effect relationship with the clinical condition. In other words, we need to answer if there is clear evidence that FGFR3 is the cause of achondroplasia. Clearly, there is much evidence to that effect;
3. FGFR3, a very important agent during the development of the new life in utero, stops to be expressed (produced) by most of the body cells in a significant way after birth. The main exception is exactly the growth plate chondrocyte. This pattern makes FGFR3 a very interesting target: the chance that a drug aiming FGFR3 to cause effects in other cells or tissues is low;
4. FGFR3 is a protein that can be reached outside the cell, so a potential therapy wouldn't have to enter the chondrocyte to beat the overactive receptor. Theoretically, this makes FGFR3 an easier target for drug development;
5. The target population is predictable, with a known prevalence rate (around 1 in 25000 births). Achondroplasia is, most of the time, caused by a spontaneous event, what is technically called de novo mutation. So, for drug developers, there will be new patients in the future;
6. FGFR3, in its growth regulatory function in chondrocytes, has an expiry date: the growth plate closes by the end of puberty, so the enzyme is not a lifelong target. This is important to be understood by health care/reimbursement systems, governments and payers in general, who will be balancing potential cost of therapies against budgets, disorder health costs if left untreated (costs vs. benefits) and social impact.
The list can be extended but the idea here is just to enumerate some of the main topics. Now, what does make achondroplasia difficult to treat? There are at least four special concerns:
1. The chondrocyte within the growth plate is not easy to reach. The cartilage growth plate is a dense, electrically charged environment and does not have direct blood supply, so any therapeutic agent will need to have the ability to traffic through this maze to reach the target. It must have the right size and molecular properties. Previous studies have shown that large molecules such as antibodies may not have this ability, because of their size. Other potential agents, such as aptamers, are fragile and may need transport systems (let's say a taxi) to reach their destiny, but again this could compromise their final size. Or else, changes in their structure, such as the one which has been applied to the C-type natriuretic peptide (CNP) analogue BMN-111.
2. Developing drugs to beat a protein is not easy. The current technology has been able to create drugs to inhibit many receptor proteins including FGFR3. The problem resides in the fact that they are not specific enough. These drugs, many already in the pharmacy, have been usually designed to block a single point in the structure of these proteins located inside the cell and called ATP pockets (the tyrosine kinase domain pointed out in the Figure 1 above). Well, these structures of the receptor proteins are very similar (what is called homology), so the drugs aiming this region may inhibit several receptors at the same time. Take as an example dovitinib, a pan-FGFR inhibitor. We must have in mind that while treating a kid with achondroplasia, Inhibiting other normal proteins in a developing body is far from desirable.
With a target so difficult to reach, drug discovery development risks escalate and this is a good reason for drug developers not focusing in potential therapeutic solutions for achondroplasia. Rather than that, research in the area is small and usually driven by independent, academic investigators. As examples, we may cite the cases of meclizine and the ligand trap sFGFR3.
3. The patient is a kid. When it comes to explore solutions for disorders that specially affect children, drug development becomes even tougher. There is a fairly reasonable yet strict regulation over clinical research in children, a special population. Ethical standards must be high. At the same time, today it is not possible anymore to infer the pediatric dose of a drug extrapolating from the adult one. The drug developer must find the adequate pediatric dose performing clinical studies in children, so risks are higher.
4. The less frequent the clinical condition to treat the more expensive the new therapy tends to be when approved. This indeed is the major challenge in terms of granting access to those individuals that will require or need the new therapy. For conditions which have similar prevalence rates as achondroplasia, available therapies easily reach more than USD300K per patient/year.
Currently, there is a strong debate among involved stakeholders regarding drug pricing and reimbursement. The great question is how to make drug development sustainable at the same time the new therapies coming keep the way to the patients in need? It is natural and pretty fair to seek reward for the good work done but, as an example of how things are going, take a look in the current debate about the price of the new breakthrough therapy for Hepatitis C, sofosbuvir (7-11). Hepatitis C is not exactly a rare disease, there are millions of people around the world candidates for the new treatment, so the logic of the low prevalence vs. high cost doesn't fit in this case.
Current available potential strategies for the treatment of achondroplasia
No matter the challenges for developing a therapy for achondroplasia are, we are in fact watching new strategies being proposed and explored, with one of them already in clinical development. So, let's briefly review the available information about them. Remember that you can find more specific information about everything written here in previous articles of the blog.
Meclizine
Meclizine is an old drug used today to treat motion sickness. The Japanese group leaded by Dr. Ohno started to explore its potential action in bone development under the new regulatory policy of reassessing old medicines for new therapeutic indications (12). In their experiments they found that meclizine was capable to reduce FGFR3 signaling in chondrocytes (Figure 3). This is exactly what a drug intended to treat achondroplasia must do. The tests were performed in cells and bone tissue extracted from animals (explants).
Figure 3. Sites of action of Meclizine, CNP and the tyrosine kinase inhibitors NF449 and A31.
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| Matsushita M et al. (2013). PLoS ONE 8(12): e81569. doi:10.1371/journal.pone.0081569. Reproduced here for illustration purposes only |
If meclizine is proven to work in an appropriate animal model of achondroplasia, then there will be evidence to explore it in the clinical setting. Meclizine has at least two fundamental advantages over new compounds. Being available for decades in the market, its safety profile is reasonably known, although questions about the appropriate dose for the target population and its use for long term in children are yet to be answered. The second advantage is its low cost and immediate availability. You can read more about meclizine here.
sFGFR3 and other ligand trap strategies
The study performed by the French investigators leaded by Dr. Gouze (13) has received a lot of attention by the midia around the world. This compound is a copy of the natural FGFR3 made without the cell membrane anchor the original receptor has. Instead of being inserted across the chondrocyte cell membrane it can circulate freely. But, how does it work?
The concept is simple and interesting: FGFR3 doesn't work alone, it needs a FGF to light it on. Inside the growth plate, FGFs are polipeptides that are found in the vicinity of the chondrocytes and activate the receptors under certain conditions. By floating freely in the same environment, sFGFR3 could capture these FGFs before they could reach the natural receptor in the cell membrane. That's why this is called a ligand trap or decoy strategy.
The results of the sFGFR3 are impressive and exciting (Figure 4). However, how to transpose the great barrier between the academic lab to the real world drug development settings? This will need a pharma industry to take over the next steps, since the continuity of the research towards the clinical development elicits a lot of further work, from testing the drug in larger animals to learn with more precision the chemical characteristics of sFGFR3.
Figure 4. Treatment with sFGFR3 rescues growth in an achondroplasia mouse model.
There are at least two other ligand trap strategies targeting FGFRs disclosed in the literature, one of them is being investigated for the treatment of some kinds of cancer where FGFRs play an important role (FP-1039/GSK3052230) (14). This one is made of the fusion of the basic part of an antibody (the Fc part) with the ligand binding domain (the extracellular part) of FGFR1. It is interesting that this molecule was shown to bind mainly to the same FGFs that are thought to be more relevant in achondroplasia (FGF9 and FGF18). It also has a great advantage at this moment: it is already being tested in humans, in clinical studies for cancer. So, major questions needing answers in the pre-clinical phase of drug development have already been cleared. The developer would have to get interested in exploring this other potential indication.
The third one has been disclosed in a patent registry and comes from Florida, US (15). Dr Ghivizzani describes a soluble form of FGFR3 and claims it can be used for the treatment of achondroplasia. I couldn't find any published study exploring this molecule.
BMN-111 and other CNP analogues
So far the most advanced potential therapy being explored to treat achondroplasia, BMN-111 is being tested now in children with achondroplasia in a phase 2 clinical trial. Phase 2 means that the goals are to prove the concept of the therapy with the new drug (does it really work?), to find the most adequate therapeutic dose for the next steps of research and to learn about the short term safety of the drug. Take a look at the Figure 3 to see how BMN-111, a CNP analogue, works.
BMN-111 and other CNP analogues
So far the most advanced potential therapy being explored to treat achondroplasia, BMN-111 is being tested now in children with achondroplasia in a phase 2 clinical trial. Phase 2 means that the goals are to prove the concept of the therapy with the new drug (does it really work?), to find the most adequate therapeutic dose for the next steps of research and to learn about the short term safety of the drug. Take a look at the Figure 3 to see how BMN-111, a CNP analogue, works.
It is very important to understand that CNP does not block FGFR3 directly. It works indirectly, by reducing the activation of the main chemical cascade driven by FGFR3 in the chondrocyte, the MAPK pathway (Figure 3). It is not wrong to infer that CNP and FGFR3 have antagonistic functions in the chondrocytes.
The figure 5, extracted from one of the last public presentations from Biomarin, the BMN-111 developer, shows the design of the phase 2 study.
Figure 5. BMN-111 phase 2 study design.
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| From the Biomarin's presentation at the Barclays Global Healthcare Conference Mar 12th 2014. |
This phase 2 study has started on last January and is still ongoing, according to information available from the conference held by Wells Fargo and Biomarin on June 17th. This means that to date there has been no major concern in terms of safety.
Biomarin estimates that it will have the complete results of the phase 2 study by 2Q15. If they are positive, then a larger and longer phase 3 study will be necessary to confirm the results obtained. Given the normal timeframe needed for the development and approval of a new therapy, if BMN-111 proves to be safe and efficient one could estimate that it will take about three years more from now before becoming available to patients.
Recently, another CNP analogue had been tested in an animal model of neurofibromatosis, a genetic disorder that coincidently produces the same kind of overactivity of the main chemical cascade ruled by FGFR3 in achondroplasia (the MAPK pathway), and also leading to restricted bone growth (16). By blocking the MAPK cascade in the same way CNP does (look at Figure 3), the molecule called NC-2 could turn to be an option as a therapy for achondroplasia. We know that it is in the portfolio of a biotech industry (17). However, since NC-2 was used by an independent researcher, in another context, it is reasonable to conclude that this developer has abandoned further research with this compound (risks?).
Concluding
There have been also other new approaches published in the last years. For instance, at least two groups of researchers designed peptides that are able to bind the external part of FGFR3 and block its function (18,19). The recently described above mentioned anti-FGFR3 tyrosine kinase inhibitors NF449 (20) and A31 (21) have shown improvements in terms of molecular design towards better specificity for the target, but it seems they are no longer being developed. An investigator from Florida, US, has been proposing that other natriuretic peptides than CNP, such as the one called Vessel Dilator (22) could be used to treat achondroplasia, but there has been no direct work in animal models published to confirm this hypothesis. The list is even longer but I will stop here for now. You can explore the blog to learn about other potential approaches (try aptamers).
With all these new paths being explored it is likely that no so far in the future there will be more than one option to treat achondroplasia and other related disorders. The question then will no longer be if there is a therapy for achondroplasia, but which one to choose.
I hope this short summary has been useful for the interested reader to understand where the investigation for therapies to treat achondroplasia stands now.
With all these new paths being explored it is likely that no so far in the future there will be more than one option to treat achondroplasia and other related disorders. The question then will no longer be if there is a therapy for achondroplasia, but which one to choose.
I hope this short summary has been useful for the interested reader to understand where the investigation for therapies to treat achondroplasia stands now.
References
7. Knox R. $1,000 pill for Hepatitis C spurs debate over drug prices. NPR News. Accessed 18th June 2014. Free access.
8. Medecins Sans Frontiers. MSF responds to reports on Gilead pricing for hepatitis C drug sofosbuvir in developing countries. Accessed 18th June 2014. Free access.
9. Hill A et al. Minimum costs to produce Hepatitis C direct acting antivirals. Presented at the 64th Annual Meeting of AASLD, Washington DC, USA, November 2013 [Poster 1097]. Published as: Hill A et al. Minimum production costs of direct acting antivirals, for use in large-scale HCV eradication programmes in developing countries. Hepatology 2013; 58 (suppl.1): 740A. Free access.
10. Waxman HA et al. Congress of the United States. Letter to John C Martin, CEO Gilead Sciences Inc. 2014 Mar 20th. Free Access.
11. Senior M. Sovaldi makes blockbuster history, ignites drug pricing unrest. Nature Biotech 2014; 32 (6): 501-2.
12. Matsushita M et al. Meclozine facilitates proliferation and differentiation of chondrocytes by attenuating abnormally activated FGFR3 signaling in achondroplasia. PLoS ONE 2013 8(12): e81569. doi:10.1371/journal.pone.0081569. Free access.
13. Garcia S et al. Postnatal soluble FGFR3 therapy rescues achondroplasia symptoms and restores bone growth in mice. Sci Transl Med 2013;5:203ra124.
14. Tolcher A et al. Preliminary results of a dose escalation study of the fibroblast growth factor(FGF) “trap” FP-1039 (FGFR1:Fc) in patients with advanced malignancies. 22nd EORTC-NCI-ACR symposium on molecular targets and cancer therapeutics, November 16-19, 2010. Berlin, Germany. Free access.
15. Ghivizzani SC. Delivery of soluble FGFR3 as a treatment for achondroplasia. National Institute of Arthritis and Musculoskeletal and Skin Diseases. 2013; Project Number: 5R01AR057422-04.
16. Ono K et al. The ras-GTPase activity of neurofibromin restrains ERK-dependent FGFR signaling during endochondral bone formation. Hum Mol Genet 2013;22(15):3048-62.
17. Alexion Pharma International. Compositions comprising natriuretic peptides and methods of use thereof.Patent US 20120164142 A1. Jun, 28th 2012. Free access.
18. Jin M et al. A novel FGFR3-binding peptide inhibits FGFR3 signaling and reverses the lethal phenotype of mice mimicking human thanatophoric dysplasia. Hum Mol Genet 2012; 21(26):5443-55. Free access.
19. Yissum Research Development Company. The original link offering their peptides has been put down. Refer to the following article of the blog to read about this approach: Treating achondroplasia with peptides specifically against FGFR3.
7. Knox R. $1,000 pill for Hepatitis C spurs debate over drug prices. NPR News. Accessed 18th June 2014. Free access.
8. Medecins Sans Frontiers. MSF responds to reports on Gilead pricing for hepatitis C drug sofosbuvir in developing countries. Accessed 18th June 2014. Free access.
9. Hill A et al. Minimum costs to produce Hepatitis C direct acting antivirals. Presented at the 64th Annual Meeting of AASLD, Washington DC, USA, November 2013 [Poster 1097]. Published as: Hill A et al. Minimum production costs of direct acting antivirals, for use in large-scale HCV eradication programmes in developing countries. Hepatology 2013; 58 (suppl.1): 740A. Free access.
10. Waxman HA et al. Congress of the United States. Letter to John C Martin, CEO Gilead Sciences Inc. 2014 Mar 20th. Free Access.
11. Senior M. Sovaldi makes blockbuster history, ignites drug pricing unrest. Nature Biotech 2014; 32 (6): 501-2.
12. Matsushita M et al. Meclozine facilitates proliferation and differentiation of chondrocytes by attenuating abnormally activated FGFR3 signaling in achondroplasia. PLoS ONE 2013 8(12): e81569. doi:10.1371/journal.pone.0081569. Free access.
13. Garcia S et al. Postnatal soluble FGFR3 therapy rescues achondroplasia symptoms and restores bone growth in mice. Sci Transl Med 2013;5:203ra124.
14. Tolcher A et al. Preliminary results of a dose escalation study of the fibroblast growth factor(FGF) “trap” FP-1039 (FGFR1:Fc) in patients with advanced malignancies. 22nd EORTC-NCI-ACR symposium on molecular targets and cancer therapeutics, November 16-19, 2010. Berlin, Germany. Free access.
15. Ghivizzani SC. Delivery of soluble FGFR3 as a treatment for achondroplasia. National Institute of Arthritis and Musculoskeletal and Skin Diseases. 2013; Project Number: 5R01AR057422-04.
16. Ono K et al. The ras-GTPase activity of neurofibromin restrains ERK-dependent FGFR signaling during endochondral bone formation. Hum Mol Genet 2013;22(15):3048-62.
18. Jin M et al. A novel FGFR3-binding peptide inhibits FGFR3 signaling and reverses the lethal phenotype of mice mimicking human thanatophoric dysplasia. Hum Mol Genet 2012; 21(26):5443-55. Free access.
19. Yissum Research Development Company. The original link offering their peptides has been put down. Refer to the following article of the blog to read about this approach: Treating achondroplasia with peptides specifically against FGFR3.
20. Krejci P et al. NF449 is a novel inhibitor of fibroblast growth factor receptor 3 (FGFR3) signaling active in chondrocytes and multiple myeloma cells. J Biol Chem 2010; 285(27): 20644-53. Free access.
21. Jonquoy A et al. A novel tyrosine kinase inhibitor restores chondrocyte differentiation and promotes bone growth in a gain-of-function Fgfr3 mouse model. Hum Mol Genet 2012; 21(4):841-51. Free access.
22. Vesely DL. Method of treating skeletal dysplasias using vessel dilator. Patent US 20130096061 A1. April 18th, 2013.
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