Introduction to Pharmaceuticals

A cure for psoriasis

Before the actual development of a new drug can begin, it is necessary to decide which disease you want to treat. In the following, we will take as our starting point how a drug was developed for the disease psoriasis. Psoriasis is a skin disease in which the cells in the skin begin to divide more than normal. This forms a thicker skin that peels heavily.

Once you have decided which disease you want to treat, you need to find a target. A target is the place in the body where a drug must bind to in order to produce the desired effect. A target can be many things, but virtually all targets are proteins in the body. These can be, for example, transport proteins, enzymes or receptors. How a drug binds to its target and thereby exerts a function in the body is described by the drug’s pharmacodynamics. Pharmacodynamics is the part of drug development that describes what a drug does to the body. You can read more about this in the article “Identification of Target and its Structure”. (Conversely, pharmacokinetics is the description of what the body does to the drug.)

When developing new drugs, a wide range of different substances are systematically examined for their binding to the target found. In the 1980s, it was discovered that vitamin D had a positive effect on a special type of cancer cells. By giving vitamin D to the cells, it was seen that they did not grow as quickly. The theory was developed that cancer patients who had the special cancer cells in their bodies could be treated by vitamin D. However, this did not prove possible due to a serious side effect. Vitamin D also regulates the amount of calcium in the blood, and when the substance is dosed in the relatively large amount necessary for effective treatment, the concentration of calcium increases so much in the blood that calcium precipitates in the kidneys, among other things. This is called a calcemic effect. It was therefore necessary to develop new substances that had the good growth-inhibiting effect on the cancer cells, but had no calcemic effect – more on this later.

In order for vitamin D to act on a cell, the vitamin D receptor (target for Vitamin D) must be present, as it was in the given cancer cells. Figure 1 shows the vitamin D receptor bound to a ligand.

 

Figure 1. Shows the vitamin D receptor with calcipotriol bound to it [Source: www.pdb.org; H.M. Berman, J. Westbrook, Z. Feng, G. Gilliland, T.N. Bhat, H. Weissig, I.N. Shindyalov, P.E. Bourne (2000) The Protein Data Bank Nucleic Acids Research, 28: 235-242].

Vitamin D binds to the vitamin D receptor via a series of chemical bonds in the right positions. This activates the receptor and acts as an agonist on the receptor. In order to develop new candidates for testing, it is fundamental to know something about the bonds between target and drug. This can be read about in the article “Organic Chemistry and Pharmaceuticals”. When vitamin D binds to its receptor, hydrogen bonds are formed, as can be seen in Figure 2, and some hydrophobic interactions, as can be seen in Figure 3.

The desire to develop new substances with a good effect on cell growth and minimal calcemic effect was coupled with knowledge of the vitamin D receptor. They began to optimize the structure of vitamin D, i.e. produce analogues that affected the target as an agonist, but which did not have a calcemic effect. First of all, it was necessary to find out which functional groups on the vitamin D molecule are individually necessary for the “agonistic” activity. This can be done via a SAR study (Structure Activity Relationship study), where the different functional groups are removed individually from the molecule, and tested for activity against the target – in this case, the growth of the cancer cells. In this way, it is possible to either rule out or confirm in stages whether the group in question is necessary to maintain the effectiveness of the medicine. You can read more about this in the article “Optimisation of the medicine”. For vitamin D, it was found that the inhibitory effect on cell growth was preserved when the basic structure of vitamin D was preserved. If you instead changed a side group, it resulted in a reduced calcemic effect (Fig. 4). A special analogue was found that was a good agonist and had no calcemic effect. This analogue was called calcipotriol.

It turned out that calcipotriol did not show a calcemic effect because the substance is very quickly broken down/metabolized in the liver. Therefore, the drug was only available in the blood for a limited period of time. Unfortunately, the time period was so short that it did not have a sufficient effect on the cancer cells. It was decided that calcipotriol was not suitable as an oral drug. (At LEO Pharma, work continued to find a good vitamin D analogue, and the substance seocalcitol was developed. This substance was tested in cancer patients and had a positive effect, but unfortunately, the calcemic effect appeared after treatment for a long time, and the development of seocalcitol had to be stopped. )

Figure 2. Shows calcipotriol and the amino acids from the vitamin D receptor that form hydrogen bonds together [Source: www.pdb.org; H.M. Berman, J. Westbrook, Z. Feng, G. Gilliland, T.N. Bhat, H. Weissig, I.N. Shindyalov, P.E. Bourne (2000) The Protein Data Bank Nucleic Acids Research, 28: 235-242].

Figure 3. Shows calcipotriol and the amino acids from the vitamin D receptor that form hydrophobic interactions [Source: www.pdb.org; H.M. Berman, J. Westbrook, Z. Feng, G. Gilliland, T.N. Bhat, H. Weissig, I.N. Shindyalov, P.E. Bourne (2000) The Protein Data Bank Nucleic Acids Research, 28: 235-242].

Shortly after calcipotriol was discarded as an oral drug, it was observed that a Japanese patient with osteoporosis who was treated with vitamin D as a “positive side effect” had his psoriasis significantly reduced. Independently of this, it was also found that the vitamin D receptor is found in the cells of the skin. These two findings were linked by LEO Pharma and they therefore tried to use calcipotriol instead for topical treatment of psoriasis. In vitro experiments were conducted with calcipotriol on skin cells containing the vitamin D receptor. Calcipotriol was found to have the same positive effects as vitamin D on skin cells. There was therefore a basis for continuing to work with calcipotriol. They began to investigate how calcipotriol was metabolized. The metabolism of calcipotriol is part of pharmacokinetics, and you can read more about this in the article “The drug’s path through the body”. In vitro, they tested how calcipotriol was metabolized by adding an extract extracted from a liver. Figure 5 shows the metabolites of calcipotriol that were identified.

Figure 4. Shows vitamin D3 and the side chain that has been altered to form the drug Calcipotriol.

LEO Pharma’s showed that calcipotriol’s metabolites are not toxic. However, this is not always the case, and you therefore have to take a step back in the development and change the structure of the drug again. In this way, you will many times in the development of a drug. Whether it is to change the toxic metabolites, or because the drug does not have the desired effect, you have to go back and change the structure of the drug again and again until the right analogue is produced. This is one of the reasons why the development of a drug takes many years from the start of research until you can put a drug on the market.

When it has been shown through various in vitro experiments that the drug binds well to the target and how it is metabolized, it is necessary to conduct experiments with animals (in vivo experiments). The vast majority of substances are discarded after in vitro tests and only a fraction are tested in vivo. The in vivo experiments are carried out to confirm that the results obtained in the in vitro experiments also apply to a living organism. After LEO Pharma had done in vitro metabolism experiments (with the liver extract), they switched to in vivo metabolism experiments. Rats and mini-pigs were given radioactive calcipotriol, which could be followed in the body. It turned out that exactly the same metabolites were formed in the animals as were found in the in vitro experiments. It is also important to test for metabolites in animals because some of the metabolites formed can have an undesirable side effect. Metabolism tests are not the only in vivo tests that are being done. It is also tested to see if the drug has the right pharmacodynamic effect in a living organism.

Once it has been shown that the drug has the right effect in animals, and it has no unwanted side effects, you will start testing the drug on humans. There are three different phases of what are called clinical studies: phase I, II and III. In phase I studies, the drug is tested on healthy volunteers to see if there are any side effects in humans that were not seen in the test animals. When calcipotriol was tested on the skin of healthy people, it was found that there were no serious side effects of the drug – in particular, no calcemic effect was observed. In phase II, the trials are expanded to include patients, to be able to prove that the drug relieves the disease, and to test which dose should be used. Finally, you go to phase III, where you use a large group of patients.

Calcipotriol was also tested on a large group of psoriasis patients. A great improvement in the skin was seen and it was now possible to market calcipotriol.

Figure 5. Calcipotriol is metabolized in the body by first turning an alcohol into a ketone, and then removing a double bond.

Figure 6. Before and after picture when treating psoriasis with calcipotriol [Source: LEO Pharma].