Step by step, it seems that the first medicine made to help bones grow in achondroplasia is advancing in its development, an exciting perspective. But, before lighting fireworks, it would be interesting to learn more about the CNP and what we should expect about its use in the treatment of ACH children.

 The Natriuretic Peptide Family

Peptides are molecules made of amino acids, like proteins, but they are smaller and, like their larger cousins, they are also encoded in genes. Natriuretic means a property of something that causes sodium to be eliminated in the urine. The name natriuretic came after the description of one of the first recognized properties of these peptides, which is exactly promoting the elimination of sodium in urine.

The history of the natriuretic peptides begins about 30 years ago when the first peptide of the family was discovered in extracts of rat atria and for this was called atrial natriuretic peptide(ANP). Not much longer, BNP was identified in extracts of pig brains and then CNP, and as it was the third in the row, received the C-type NP name (this is a link to a good review on CNP by Olney RC). While ANP and BNP are most found in cardiac tissue and are linked to cardiac physiology and related diseases, CNP is expressed in a number of other body tissues and remarkably found within the cartilage growth plate, where it exerts the most important of its actions. This link will take you to a figure showing the three peptides.

CNP

CNP is a known positive player in the growing bone according to many studies made in animal models and also in related mutations in spontaneous human cases. For instance, genetic mutations in the CNP gene causing its overexpression lead to overgrowth. The research also showed that slight changes in the natriuretic peptide receptor type C structure (NPR-C, one NP receptor that is thought to serve as a CNP clearance system) may be responsible for the higher final height found in people from some of the Northern European countries (Estrada K et al.;Bocciardi et al.).

This peptide is expressed (produced) locally in the growth plate. When it binds to its preferential receptor enzyme, NPR-B, located across the cell membrane (in the same way FGFR3 is) of the chondrocyte, it activates this receptor, which in turn causes the activation of other enzymes in the cell cytoplasm. Interestingly, this CNP cascade of chemical reactions will then cross with one of the most important cascades responding to FGFR3 activation, the RAF-RAS-MAPK pathway (discussed here).

activated by FGFR3, the RAS-RAF-MAPK pathway will lead to one of the most well characterized consequences of achondroplasia, which is slowing down the rate the chondrocytes enlarge (hypertrophy) and mature, thus impairing the entire cartilage growth pace. By the other side, when CNP activates its receptor, the chemical messages emitted by its cascade will turn off or reduce the RAS-RAF-MAPK cascade activity, so causing an inverse action in terms of bone growth. The main observed characteristic of growth plates of ACH mice models treated with CNP is an enlargement of the hypertrophic zone of the growth plate. Take a look in this article (free access) by Drs Yasoda and Nakao, two of the most prominent researchers of CNP in achondroplasia. They tell the history and results of their research, which has strongly contributed to the understanding of CNP in achondroplasia. This article also has a very didactic graphic showing the two cascades described above.

With the reassuring results of the research by scientists like Drs. Yasoda and Nakao, it became clear that CNP could be used somehow to rescue the bone growth arrest in achondroplasia. But how?

The Japanese group developed a mouse model where the CNP was naturally produced in large quantities by the animal body, in a strategy to simulate a situation where the peptide would be given continuously to the patient. This was necessary because of the nature of CNP. Being a small peptide, it is a usual target of several enzymes present in the blood stream calledpeptidases. This is so true that CNP, after a single intravenous injection, would last less than five minutes circulating. With such a short half-life (the way scientists describe the interval of time half the quantity of a drug will take to be processed by the body) giving multiple injections would not be a clever strategy to treat any situation. So they probably thought about a therapeutic scheme where their CNP would be given trough a continuous pump infusion, in the same way other clinical conditions have been treated in the past. Their work showed CNP indeed cause bone growth in an achondroplasia +/CNP+ composite mouse model, rescuing the bone growth arrest.

However, this solution, although feasible, has a lot of practical challenges easy to foresee. Then, is there any other way we could give CNP to a child to treat achondroplasia? The answer is yes. Given the strategy announced by Biomarin, in which their CNP analogue will be given subcutaneously once a day, there are other ways. As there is no publicly available information about the compound formula or structure we can only speculate about the solution they found, but it might be related to the knowledge we have about the metabolism of the NPs. Let’s talk a little bit about this.

As mentioned above, CNP and the other related peptides are natural victims of peptidases present in the blood and other tissues. However, the most relevant of these enzymes, neprilysin, does not cleave (cut) the NPs in the same way. Neprilysin has different affinities with the NPs, being ANP and CNP more easily cut than BNP. If you visited the figure I presented above, you may have already identified the structural differences among the three NPs. BNP has two “legs” or branches leaving the main ring while CNP has only one. Evidence exists that the longer BNP branch would be the responsible to its relative resistance to neprilysin (Potter LR, free access). So, there is a chance Biomarin could have developed a CNP analogue bearing a slight modification in its only branch structure (like in BNP) that would give it more resistance to neprilysin activity. This change could give this CNP analogue more time to circulate and diffuse into the tissues and especially into the cartilage growth plate.

It looks like a very smart solution. Tests made with animals have been showing positive results (links above) and, given the FDA authorization to let them proceed to clinical trials, results have been robust enough in terms of efficacy and safety in those animal models.

Testing CNP in clinical trials

Now is the time to test the CNP analogue in humans. What we should expect about these experiments in terms of safety and efficacy?

Safety

First, as CNP is closely related to the other NPs, and that both ANP and BNP have significant effects in the blood pressure and other circulatory parameters, a strict oversight on cardiologic and other circulatory indexes must be carried on. Biomarin has showed during a public presentation in December that the CNP analogue did cause a decrease in the blood pressure in monkeys after each injection.

Second, CNP is found in other tissues throughout the body, including the brain. A recently published study by Dr Nakao and colleagues showed that CNP can influence the body weight, possibly by acting directly in the brain. The mouse model used by the Japanese group does not reproduce the real life, so their results must be understood under this context. Nevertheless, it will be important to follow patients using CNP chronically to understand this aspect of CNP.

Third, bones are not equal, some are thin others thicker. Furthermore, achondroplasia is described as a rhizomelic (rhizo means root) bone dysplasia. This means that it is recognized that proximal (to the trunk) bones are more affected that the distal ones (those in the extremities). There is a theory this could be caused by distinct influences FGFR3 would have across the skeleton, with some bones being more affected than others by the mutation. In some of the papers published by the Dr. Nakao’s group, pictures of mice treated with continuous CNP could cause the impression that had thinner spines and longer feet and tails than the control (normal, non-treated) animals. Again, here the kind of exposure those animals had was quite different of what we would expect in the real life or with a single CNP shot a day. Nevertheless, this could be a good aspect to be observed throughout future studies in affected patients.

Fourth, another aspect to be taken in account is the kind of effect the extra CNP would have in other cartilaginous tissues such as the joints, ears, nose and trachea. Although having some specific patterns, chondrocytes tend to behave similarly to the same stimuli wherever they are located, so this is also a question that will need an answer, too.

Efficacy

How will the efficacy of the treatment of CNP be measured? Growth is not a parameter easy to measure in the short term. However, there are some indexes which can be used to monitor the growth rate in children under treatment. For instance the average growth speed rates can be derived from the NCHS series. You can see how this was made by examining this Brazilian Ministry of Health guideline directed to pediatric health care which uses these derived curves (sorry, it is in Portuguese, but look at the page 21 to see the derived graphic.

Growth tends to be fast in the first year after birth and then it starts to slow down up until puberty. It is likely that in children with achondroplasia, taking in account the intrinsic growth impairment, the growth pace could be similar (actually, this would have to be proved. I am not aware of any study addressing this feature).

The idea could be to plot the already known child heights over the years and create an individual graphic. With the exposure to CNP it would be expected that the growth speed would increase and this can be better measured comparing to the previous pace and to the expected ongoing pace. This is more than just measuring the absolute height.

Another possible marker of growth could be to take measures of the four member bones or, in other words, the lengths of the arms and forearms and thighs and legs could be taken. Then, during the treatment these measures could be readdressed to look for trends in the growth pace in the different member segments. Achondroplasia is a rhizomelic dysplasia, so it would be interesting to learn about the response of the proximal bones to the treatment and also this measure would help to spot earlier any tendency for overgrowth of the extremities.

We must remember that everything in a child with achondroplasia is normal but the workaholic FGFR3. So, if the mutated receptor is compensated what we can expect in terms of bone growth? Doctors know, for a long time, the ‘catch up growth’ phenomenon, seen in several distinct clinical conditions. When the reason for growth impairment is resolved, the affected child tends to grow faster than the average for the age until an individual mark is reached and the growth normalizes. Could the catch-up growth phenomenon happen to children with achondroplasia treated with CNP? This is difficult to say, because in this case the receptor would still be active (so, in a future therapy with a FGFR3 inhibitor, the catch up growth could be expected). Nevertheless, measuring the growth speed would give an insight about this phenomenon in the context of the treatment of achondroplasia with CNP.

arrival of the CNP analogue as the first potential therapy to help children with achondroplasia to rescue, at least partially, the bone growth, is remarkable. There are several steps to be taken in this phase of its development; the drug must prove to be safe and to have the expected efficacy.  Growing more, affected children could be spared from suffering the many common interventions seen in achondroplasia, from removal of tonsils and adenoids to serious orthopedic and neurological complications. At this moment we must be rational, not presuming that the bone growth will be restored to its full potential. However, in the case of this first possible therapy succeed, a better quality of life could be expected for children with achondroplasia.