Reducing FGFR3 influence in achondroplasia, part 1

As we have already reviewed (see previous articles), the key features impairing bone growth in achondroplasia are caused by the excessive activity presented by the fibroblast growth factor receptor 3 (FGFR3) due to a single point mutation in this protein.

How can we manage this workaholic receptor? How can we reduce or abolish FGFR3 activity or counteract its effects in the chondrocytes, the cells ultimately responsible for creating the scaffold where bones are built? If they don’t multiply (proliferate) and enlarge (hypertrophy) there will less base for the bone to grow. 

We have taken a look in the way FGFR3 works, how it is produced and how it is driven for deactivation. Let’s start now to see the many possibilities to deal with it beginning by looking how to directly block or inhibit FGFR3 activity. When someone thinks about treating achondroplasia, the first and natural thought is, well, we have to stop this receptor. It must stop to exert its actions. 

Targeting FGFR3 from outside the cell

These were very likely the first thoughts of the researchers since the basic defect of achondroplasia was identified. However, from the beginning, the task also showed to be complex. You may remember we mentioned that the growth plate is a very well protected environment. There are no blood vessels inside it so there is no direct blood flow to bring oxygen and nutrients to cells. The local environment we call cartilage matrix is a dense terrain and everything directed to the chondrocytes must pass through this territory. A paper published in 2006 (Farnum et al.) showed that molecules with low molecular weight can traffic within the growth plate and that molecules  larger than 40kDa will probably suffer increased resistance to diffuse in this environment (kDa means kilodalton, where dalton is a molecular weight measure unit).

Antibodies

Learning about the growth plate trafficking may help to explain a lot about the first interesting experiments made with specific antibodies against FGFR3. Proteins have the property of eliciting immune system reactions. When a non-self protein (a protein that does not pertain to the body) is exposed to the immune system, the body defense usually produces specific antibodies to bind and block the ‘invader’. This protein property is largely used in the creation of therapeutic antibodies to treat many diseases. We also must keep in mind that antibodies are like proteins, so they can also provoke immune responses against them.

In 2003, an Israeli group (Aviezer et al.) published the results of tests made with a specific antibody against FGFR3, called PRO-001. The tests were performed in rat bones and the results were promising. However, no new developments have been published since then and the reason (we are not sure why) could be that a conventional antibody normally weighs 150 kDa or more, so it cannot traffic freely through the cartilage matrix and is probably cleared from the blood before it can reach its target in a therapeutic concentration. It is probably the size of the antibodies’ molecules that prevents their therapeutic use for achondroplasia. You can find more technical information about using antibodies to block FGFR3 reading the editorial by Dr Joseph Schlessinger, where he comments the findings of a study with another anti-FGFR3 antibody called R3Mab here. So, here goes a first recommendation to one looking for a therapeutic approach for achondroplasia: the molecule must be small, probably weighing less than 50kDa.

Aptamers

Aptamers are very specific to their targets in a degree similar to conventional antibodies. Apart from the similar specificities, aptamers have a number of advantages. First, they are far less immunogenic than antibodies (this means they don’t cause the same level of immune defense reactions as therapeutic antibodies do). The second and important positive feature is that aptamers use to weigh 15 to 45kDa.


In conclusion, aptamers are a potential approach that could be explored in achondroplasia for several reasons:

• They work outside the cell

• They are small molecules

• They are very specific

• They are not immunogenic

• They have fair safety profiles

During the last decades, researchers have been learning a lot about the interaction of proteins and the nucleic acids (DNA, RNA), let’s call them nucleotides. There are several proteins capable of binding directly to the DNA (for instance, the tailor proteins I mentioned in a previous post) and RNA. Well, why not making the reverse, by designing nucleotides-made molecules that could directly bind to proteins? This has already been done, and the molecules made of nucleotides (we cite them as oligonucleotides) with this purpose are called aptamers.

You can read more about aptamers here. For more technical information, there is a good review reachable here.

In summary, we have reviewed some of the challenges to treat achondroplasia. The first one is to make the drug reach the target. We also briefly discussed two options to block FGFR3 from outside the cell. One is already available, which are specific antibodies against the receptor. The second is a potential one yet to be explored, the aptamers.

For instance, antibodies against receptor enzymes like FGFR3 are currently being used to treat some types of cancer. One good example is trastuzumab, which is an antibody designed to block the activity of a cell receptor enzyme called epidermal growth factor receptor (or EGFR) to treat breast cancer. EGFR is, like FGFR3, positioned across the cell membrane, with an extracellular portion (or domain) in contact with the environment. The antibody leaves the circulation and binds to the receptor, blocking its docking site so the activator molecule (the EGF) can´t make the connection with the receptor. Thus, an antibody works outside the cell. The main difference here is that the breast tissue is well covered by a blood vessel web. Cartilage is not. You can have a view of the way trastuzumab works watching this short animation (no audio).

Maintaining the view from outside the cell, we still have this target, which is the outer part of the FGFR3. Is there any other way we could block the docking of FGFR3 activators (or ligands, the FGFs) so the receptor is kept turned off? The answer is yes.

There is at least one therapeutic aptamer already in the market (pegaptanib), designed to treat an ophthalmologic condition and there are others in clinical trials for several indications. To date, there is no published research of aptamers against FGFR3. A simple query in Pubmed retrieved no papers (keywords: aptamer, FGFR3; 29^th Dec 2011). Aptamers represent a new technology and there are many questions to be answered regarding their utility in the therapy for many diseases. For instance, the cartilage is not the preferential tissue for aptamer deposit. We don´t know if a specific aptamer against FGFR3 would reach the target. There is only one way to know, which is testing it.

In the next article, we will continue our review on potential therapies for achondroplasia aiming the direct blockage or inhibition of FGFR3, this time looking for approaches from inside the cell.