According to Biomarin, the developer of BMN-111, the phase 2 study that is currently ongoing with the first group of kids with achondroplasia, will have it outcomes published by the end of the first half of 2015, just three months from now. It is noteworthy two other news released by the lab. The first one is the updated information posted in clinicaltrials.gov, mentioning a long term extension study for the kids participating in the phase 2 trial. The second is the recently published history of the development of BMN-111, which we will be reviewing here.
A view of the bone growth
The cartilage growth plates (Figure 1) are narrow cartilaginous layers located in both extremities of the long bones, and are the structures responsible for the bone formation and growth. The growth plate is a dense, electrically charged tissue where there is no direct blood supply, so nutrients and all other agents need to traffic within a formidable chemical barrier to reach the master cells of the growth plate, the chondrocytes. Large molecules are not welcome there.
Figure 1. The cartilage growth plate
Dozens of proteins, enzymes, peptides, hormones and other factors play important roles in the growth plate, acting together to allow a timely and organized rhythm of bone growth that will last until the end of the puberty when, responding to several hormone stimuli, the growth plates will fuse and bones will stop to grow (figure 2). These agents may have positive and negative actions that will counteract the effects of each other in order to let the chondrocytes develop in a well organized system to build more bone. When all these agents work in balance, it is like a wonderful Mozart's symphony.
Figure 2. Fate of the growth plate
When we look at the growth plate through a microscope what we see is that the growth plate itself is divided in a number of layers or zones, each of them with chondrocytes in several stages of development (Figure 3).
Figure 3. Growth plate organization
Chondrocytes in the resting zone have few signs of activity. Answering to those several stimuli, they start to rapidly multiply (proliferate) and organize themselves in piles (proliferative zone). Always acting in response to those agents, they stop to proliferate and begin to enlarge (hypertrophic zone) and finally they are substituted by osteoblasts (bone forming cells) when the cartilage begins to calcify.
Achondroplasia and the bug in the symphony
Let's talk a bit now about the two agents that interest us the most in this review.
FGFR3
Achondroplasia is caused by a mutation in the gene FGFR3, which encodes (or carries the information for the production of) the active protein (or enzyme) called fibroblast growth factor receptor 3, or FGFR3. This enzyme is located across the cell wall of the chondrocyte and functions like a TV antenna, receiving signals from outside the cell, in this case given by FGFs that are present in the neighborhood. When it receives the signal from a FGF, it is turned on and starts a chemical chain reaction in cascade, activating a neighbor protein which will activate another one and this one a third and so on, like in a domino chain (Figure 4). The signal will be carried up to the cell nucleus and will make the cell to respond accordingly. While most of the agents I mentioned above play positive roles in the bone growth process, FGFR3 is a natural brake for bone growth. If there was no FGFR3, bones would probably grow without control, leading to gigantism. In fact, the literature describes cases where FGFR3 is inactivated, causing overgrowth (1). In achondroplasia, the mutation makes FGFR3 more active than normal, or in other words, FGFR3 signals excessively to the nucleus after being activated. Since its natural role is to reduce the bone growth pace, when it is overactive, it causes bone growth arrest. Now, look at the Figure 4 and pay attention to the cascade to the right, which begins with RAS and goes on with RAF, MEK and ERK. These enzymes form what is called the mitogen-activated protein kinase (MAPK) pathway, which is of top importance for our review.
Figure 4. The FGFR3 signaling pathways
C-type Natriuretic Peptide (CNP)
CNP is a peptide naturally produced by our body as are other two main sister molecules, called ANP and BNP. The natriuretic peptides have been already reviewed in several articles of the blog, and I would recommend this one for you to read first. CNP is produced by several tissues in the body, but usually doesn't circulate in the blood as it is rapidly inactivated by circulating enzymes. One of the tissues where CNP has a preponderant action is exactly the cartilage growth plate. CNP is produced in the vicinity of the chondrocytes and these cells also produce the receptor for CNP, called NPRB (natriuretic peptide receptor B) (Figure 5). When CNP binds to its receptor, this also leads to a chemical cascade reaction inside the cell. The interesting aspect of the CNP cascade is that it "crosses" the MAPK cascade activated by FGFR3, inhibiting it (2). By inhibiting the MAPK cascade, CNP acts as a regulator of the effect of FGFR3, so we can say that they have antagonistic roles in the bone growth simphony. Accordingly, there is a lot of evidence that CNP has a positive role in bone growth, given both by reports of cases where NPRB has an overactive mutation leading to overgrowth (3) or the opposite, when it is not working, causing a bone dysplasia with similarities with achondroplasia (4).
Figure 5. CNP signals inhibit the MAPK pathway in the growth plate chondrocyte
CNP in achondroplasia
A Japanese group is responsible for a wealth of information about the biological role of CNP in the growth plate (5). The group leaded by Kazua Nakao developed a strategy where they were able to cause continuous production of CNP in a mouse model of achondroplasia. Exposed to large volumes of CNP, those animals not only had rescued the achondroplasia phenotype but also exhibited signs of overgrowth (6). Nakao's group showed us that CNP could be a solution for the treatment of achondroplasia, but the model they developed, in which we could foresee that CNP would have to be given through a vein, continuously, didn't sound practical and/or feasible.
So, what happened then? Let's see why CNP can't be used, in a practical way, to treat achondroplasia and the further developments of this compelling strategy.
Developing CNP analogues
CNP is a molecule made of amino acids, like proteins, but much shorter than a typical protein. Proteins and peptides are often active and cause chemical reactions, so the body developed measures that, in normal conditions, control the amount and time allowed to them to cause chemical reactions. CNP is a natural victim of some blood circulating enzymes called neutral endopeptidases. CNP lasts only two minutes after an injection in the vein due to the action of those enzymes. On the contrary BNP, another member of natriuretic peptides family, has a natural resistance to those digesting enzymes so it can circulate for more time (and for that it has a relevant role in some cardiac diseases and has been extensively explored in that context). This resistance is thought to be caused by the extended tail of BNP (Figure 6).
Figure 6. Main natriuretic peptides.
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| From http://www.heartfailure.com/ |
BMN-111 is a modified CNP bearing an extended tail and some amino acid modifications that made it more resistant against the peptidases (8). The practical result is that it lasts up to 20 minutes in the blood, after a subcutaneous injection. So, let's see the lab tests made with this CNP analogue.
BMN-111 development
The article by Wendt et al. (8) describes many of the preclinical (lab) studies performed with molecules related to CNP that ended up to identify BMN-111 as the candidate for further development into the clinical settings. This will be a brief overview of the article.
In summary, the reseachers applied several approaches to enlongate or modify the CNP structure, without modifying its active part, to see how the changes would affect the affinity of the digesting enzymes. They created several analogues that were more resistant to degradation and then started to test if they were able to exert effects in cells and in the growth plate. Only part of these molecules showed positive effects and were further compared up until the researchers decided that only a small number of molecules, including BMN-111, had appropriate characteristics to continue development.
The researchers then tested these analogues in wild type (non-affected) mice to see the extension of their effect in the growth plate. BMN-111 was found to be the analogue causing more growth effect. They further tested BMN-111 in a mouse model of achondroplasia (mild presentation) and saw that by the end of the study long bone and the nasal-anal lengths were restored.
In sequence, they tested BMN-111 in healthy monkeys and found that it had expected mild effects in cardiovascular parameters. They also found that the treated animals grew more and faster than the control animals. They further tested the bone quality of the treated animals and saw that the new bone formed had normal physiological properties.
The study offers much more detail about the tests performed but the idea here was to give you a snapshot of the whole history. The cumulative positive results the researchers found entitled BMN-111 to be tested in the clinical settings, after evaluation by the Food and Drug Administration (FDA).
In a separate study, Biomarin also tested BMN-111 in a much more severe model of FGFR3-related chondrodysplasias, resembling the human tanatophoric dysplasia (9). In this study, BMN-111 was capable to partially but significantly restore the bone growth in the treated animals, providing more evidence to the concept of using BMN-111 for the treatment of achondroplasia.
In 2012, with FDA approval, Biomarin performed a phase 1 study in healthy adults, specially to check the cardiovascular effects of BMN-111(NCT01590446). In several public hearings, Biomarin informed that the phase 1 study was conducted without surprises, and that they found the expected mild effects of BMN-111 on blood pressure. The results were submitted to FDA, which finally approved the phase 2 study in children with achondroplasia (NCT02055157), which started in the beginning of 2014. In the meanwhile, to allow appropriate comparisons on some key growth parameters, Biomarin also started an observational study (a study with no pharmacological interventions) following children with achondroplasia, to learn about their natural growth pace (NCT01603095). Biomarin has released some of the clinical findings from this study in public hearings (you can access the last one here; pages 133,134).
Reading the inclusion criteria section for the participation in the phase 2 study, we learn that previous enrollment in this observational study is required. This was important to allow the researchers to compare the natural growth rate before and after starting the therapy with BMN-111 in the phase 2 study.
BMN-111 phase 2 study
As I said in the beginning of this article, Biomarin has been declaring in public hearings that the outcomes of the phase 2 study will be released by the end of the first half of 2015. In these meetings, the developer has also been mentioning that they were working with FDA in order to test another BMN-111 dose in the phase 2 study (read this previous short article of the blog). Also, they have recently updated the information of the phase 2 study to include a long term extension study for those kids participating in that trial. These are signs that allow us to imply that the drug is not showing relevant safety issues and that it could be exerting the expected effects in bone growth.
Next (possible) step: the phase 3 study?
Assuming that the phase 2 study with BMN-111 will be successful, then the next natural step, after submitting the results to the FDA, is to get the approval for the phase 3 study. A phase 3 study is key for any compound in development to become a medicine in the future. In this phase the drug will be tested to confirm its safety and efficacy in a larger population of affected volunteers. Normally, a phase 3 study in the rare condition context will require no less than 100 volunteers. One can foresee that more sites and possibly more countries would be required to attain the number of volunteers needed for the study.
Given the current model required by the FDA for phase 3 studies, there is a chance that the study would have a placebo controlled design. My opinion is that this kind of design could be questioned on ethical grounds. Children have limited time to grow (of course), so having them enrolled in a study with a drug that has already been showing positive effects in bone growth (again, assuming that the phase 2 study will be successful) may impose to the volunteers exposed to placebo a disadvantage that could not be recovered afterwards. Although the question to be answered in the phase 3 study is not known from the beginning: is this drug safe and efficient in a significant sample of the affected population?, instead of comparing the candidate drug against placebo, a more adequate design could be to use the same one applied for the phase 2 study: to compare the individual growth rate after the beginning of the treatment with that one registered before.
How long should last a phase 3 study with BMN-111? Based on the pre clinical experience and on the design of the phase 2 study published in clinicaltrials.gov, it is likely it will have to last a minimum of 6 months. A longer placebo-controlled study could again be questioned on ethical grounds, for the same reason exposed above.
Well, this is just a personal view of what is possibly coming. We will need to wait for the results of the phase 2 study and see what really happens. Only three months ahead...
References
1. Makrythanasis P et al. A novel homozygous mutation in FGFR3 causes tall stature, severe lateral tibial deviation, scoliosis, hearing impairment, camptodactyly, and arachnodactyly. Hum Mutat 2014;35(8):959-63.
2. Yasoda A et al. Overexpression of CNP in chondrocytes rescues achondroplasia through a MAPK-dependent pathway. Nat Med 2004;10(1):80-6.
3. Miura K et al. An overgrowth disorder associated with excessive production of cGMP due to a gain-of-function mutation of the natriuretic peptide receptor 2 gene. PLoS One 2012;7(8):e42180. doi:10.1371/journal.pone.0042180. Free access.
5. Yasoda A & Nakao K. Translational research of C-type natriuretic peptide (CNP) into skeletal dysplasias. Endocr J 2010;57(8):659-66. Free access.
6. Kake T et al. Chronically elevated plasma C-type natriuretic peptide level stimulates skeletal growth in transgenic mice. Am J Physiol Endocrinol Metab 2009;297(6):E1339-48. Free access.
8. Wendt DJ et al. Neutral endopeptidase-resistant C-type natriuretic Peptide variant represents a new therapeutic approach for treatment of fibroblast growth factor receptor 3-related dwarfism. J Pharmacol Exp Ther 2015;353(1):132-49. Free access.
9. Lorget F et al. Evaluation of the therapeutic potential of a CNP analog in a Fgfr3 mouse model recapitulating achondroplasia. Am J Hum Genet 2012;91(6):1108-14. Free access.
9. Lorget F et al. Evaluation of the therapeutic potential of a CNP analog in a Fgfr3 mouse model recapitulating achondroplasia. Am J Hum Genet 2012;91(6):1108-14. Free access.
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