quarta-feira, 13 de junho de 2012

Challenges for the treatment of achondroplasia: from drug development to drug delivery, part 2

In the last article, we briefly reviewed the marathon a drug must face before it can reach patients. We talked about many questions scientists must answer when designing a potential new medicine to treat a disease. Then, we also talked about the clinical development phase, which is the part of the research when the drug is given to humans to observe its effects. Now, we will drive back to the phase when the potential new drug is still in the lab and see how researchers find the more adequate way to make it reach the target cell or tissue. This is a fast developing area of pharmacological science, one that is commonly described as drug delivery.

The goal of this article is to give a bird’s-eye view of some of the many strategies being developed to allow the delivery of the next generation of drugs, especially those that are made of nucleic acids or designed to change one or more aspects of gene transcription (the chemical reactions that copy the DNA code into RNA) and translation (the chemical reactions that translates the RNA code into a new protein). The focus is on how these delivery systems could be applied to help drugs find the way to the chondrocyte living in the growth plate, a region we know it is not exactly the easiest place to reach. There are several potential therapies waiting to be explored for the treatment of achondroplasia. Identifying the more adequate means to make them reach the growth plate may help developers to expedite the research.

Designing a new drug

Thanks to the information technology advances, it has been increasingly easier (if you could ever say this) to study the complex molecular structures formed by each of the thousands of proteins the body produces. Why should we study them? Proteins are the molecules that govern all aspects of biological reactions, or in other words, they are the final responsible agents that allow us to be what we are. We have already reviewed this. As they take part in almost all chemical reactions within the body, learning how they are mounted and where in their structures a given reaction is taking place, allows researchers to design compounds that can interfere with those places either causing that reaction or preventing it to occur. If you have been following the articles in this series you will possibly feel much is being repeated here. But, you see, there is always a piece more of detail included…

To simplify and make it easier to understand the chemical world, we usually illustrate proteins as 1D or 2D structures, as they were simple straight strips of atoms arranged one after another. Well, the real life is not so simple and the molecule of the fibroblast growth factor receptor type 3 (FGFR3), the protein which, when altered by a mutation in the gene FGFR3, causes achondroplasia, will spin, flatten, curl up, truncate and do many other moves and accommodations to reach a final 3D structure and become ready to make what FGFR3 is designed to do. With the aid of computers, scientists can create 3D models of FGFR3 to study how it makes what it makes and where we can interfere to block it. The study of the structures of molecules is called crystallography.

 Delivering drugs to the right place

How do we know a drug given by mouth will reach its target inside the body? To learn how a drug will behave in the body and how the body will deal with the drug, researchers perform pharmacokinetics and pharmacodynamics studies. While the first deals with what is the path taken by the drug from the moment it is administered, the other is about how the body deals with it.

Theoretically, most of the drugs that enter the blood will be able to reach any tissue and organ that receive direct blood flow. So, for many old, classical medicines, there is no major concern about drug delivery, even to places without vasculature. The same is true for some classes of the new drugs used in cancer; there is no relevant concern about them reaching most types of tumors. I mention this because achondroplasia has been benefiting from cancer research. The pharma industry is devoting a lot of effort to design drugs to block proteins thought to be linked to cancer growth and progression in such a scale that cancer research is currently the largest among all therapeutic areas (Berggren et al., 2012).

FGFR3 is one of those proteins, so a drug designed to fight a cancer dependent of FGFR3 actions could be used (theoretically) to treat achondroplasia. Take a look in this table provided in the previous article. You will find a number of compounds with action against FGFR3 and pertaining to these new classes of drugs. Some of the papers published about them actually show that they reach the growth plate and work in chondrocytes (ex.: Brown A et al., 2005).

Nevertheless, for the new generation of drugs being developed to work in the protein production machinery, it is unlikely the absorption patterns of the classical drugs can be applied. Compounds made of amino acids, like CNP, or made of oligonucleotides, like the aptamers or siRNAs, will not be able to reach their intended targets unprotected. This happens because of their electrochemical nature.

Disguising drugs

Hepatitis C is a major health problem around the world, since millions of people are estimated to be infected by the causing virus. The current standard therapy for hepatitis C includes a drug called ribavirine and a protein called interferon (INF). Thanks to INF, the history of hepatitis C has changed and many patients get cured with the right treatment. INF is a very important protein naturally occurring in the body and a crucial active participant of the immune system. It is called a cytokine, a messenger between cells that trigger cell responses against a foreign invasion.

INF is so powerful that the body makes sure it has a short life by producing enzymes that degrades it and after some hours circulating in the blood it is broken by them. You can guess that the short half-life of INF posed a challenge for one trying to use it in a therapy. In the beginning of its use in the treatment of hepatitis C, the first commercial forms of INF had to be given three times a week. The treatment was difficult because INF must be given by shots and, with this frequency, causes a lot of (natural) side effects and consequently, many patients gave up and didn’t finish it (24 to 48 week therapy), leading to therapy failure.

Masks, coats and transporters

To overcome this natural limitation, researchers developed a system that makes INF to last up to a week circulating in the blood, thus allowing a therapy where it is administered just once a week. The molecule developed to ‘protect’ INF from degradation, giving it time to exert its actions, is called polyethylene glycol or PEG. You can imagine how successful has been the therapies for hepatitis C today compared to the older ones, although INF keeps giving hard time to patients in treatment due to its natural effects.

Molecules like PEG are one of those we could call masks.  Bearing the right electric charges they reduce the speed of the degradation rate of the drug they are linked to; they disguise the drug. Other delivery systems, using molecules that can facilitate the entrance of the drug into the target cells, have been also developed. Some of them are based in compounds that imitate the composition of the cell membrane, so they are called lipid transport systems. It is common to call a complex formed by a drug and its carrier as a nanoparticle (literally meaning very small piece). 

Researchers are able to cover the drug entirely with a layer of lipids (like a coat) and make it reach a cell. When the drug-transporter complex (the nanoparticle) reaches the cell membrane, the lipids of the transporter are incorporated to the membrane and the drug enters the cell to exert its effects. Systems like these are clever solutions to enhance the cell uptake of drugs. One of the problems with these transport systems is that they are not specific enough to warrant that only the right cell will receive the drug. For a very recent comprehensive technical update about lipid nanoparticles you can read the paper by Battaglia and Gallarate (2012).

Fortunately, the history doesn’t stop here. Thinking about how to increase the specificity of the delivery, researchers started to attach other small compounds to the nanoparticle. For instance, knowing which kind of cell surface receptors that are produced by the target cell, they can incorporate to the nanoparticle a compound that can bind to one of those receptors. Of course, the best receptor is that one that is expressed (produced) only by the target cell, which is something not easily found.  Let´s say finding an exclusive membrane receptor makes the cell looks like as having a concrete address where a postman could deliver a letter. 

So, let’s see if we could find an ‘address’ specific enough within the cartilage that we could target to increase the delivery of a drug against FGFR3. Actually, there are very few researchers working on cartilage drug delivery and recent articles in the field describe systems aiming only the articular cartilage, through local administration. This won’t work in achondroplasia because all bones in a child are growing and they will need to receive the therapy at the same time, in a stable manner. So, for achondroplasia, we need a systemic (whole body) therapy.

However, as I said, these studies are also looking for ways to ensure the drugs they want to deliver into the articular cartilage will be due absorbed. One way to do this is exactly finding an address within the tissue, to target the chondrocytes.

Knowing that a molecule called hyaluronic acid has great affinity for another cell membrane marker called CD44, which is expressed (produced) by chondrocytes, a group of researchers has developed a system where hyaluronic acid is attached to the ‘coat’ of the nanoparticle. They were able to prove that using this strategy, delivery of the drug within the nanoparticle was far greater than with a comparator without hyaluronic acid (Laroui et al., 2007).

Another group (Rothenfluh et al, 2008) has described a system where they involve the drug in a PEG-like coat and attach a molecule that has great affinity with the cartilage matrix (reviewed in a previous article), the tissue that surrounds the chondrocytes. Because of the system used, the matrix retains the nanoparticle, leading to increased exposure of the drug within the cartilage.

These are only two examples. There are many other delivery systems under development. Several of them might be useful for the administration of drugs based in nucleic acids (oligonucleotides, aptamers) to enhance their uptake by the chondrocytes. However, we do need more research looking for the growth plate in order to find smart solutions to pass this cartilage challenge. A broad review of delivery systems was published in 2011, covering many aspects of this field (Villaverde A, Ed.; Nanoparticles in Translational Science and Medicine, 2011).
I mentioned before that this article would be like a panoramic view of the field of drug delivery. The main goal was to show that even for a genetic condition such as achondroplasia, where the target for treatment is difficult to reach, there are potential solutions to get there. An investigator working in therapeutic strategies for achondroplasia should not feel overwhelmed about the cartilage barrier.

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