Designer pharmaceuticals

Ritonavir is a medicine used to treat HIV/AIDS. It inhibits one of the enzymes of the HIV, but is most often used to boost the effectiveness of other HIV medicines. Ritonavir was developed during the late 1980s and, after a series of successful clinical trials, was approved for use in the USA in March 1996, one of only two similar medicines at the time. Over the next two years, the use of these medicines saw the US HIV death rate drop by two-thirds, yet by the end of that two year period ritonavir use had been suspended. But why? The halt in prescribing the medicine was not because any new side-effects were discovered, nor because there were great numbers of superior treatments.

No, ritonavir use was stopped because it was found that ritonavir was changing as it sat on the shelves of the pharmacy. In order to actually make a capsule of ritonavir, the molecules of the drug were crystallised into a solid. But in 1998, scientists discovered that this wasn’t the only crystal that ritonavir could make, there was a second form – and when it was in this form, it was much less soluble, and far harder for the human body to absorb. Even worse, this form was actually lower in energy; it was more stable than the active form.

Even the tiniest amount of this second form would, over time, cause the whole capsule to change, rendering it ineffective – and when stocks were tested, this ineffective form was found even in some of the production lines. This halted production in its tracks. The ability of ritonavir to form more than one crystal is called polymorphism, and it is quite common in pharmaceuticals. So today, pharmaceutical companies use quantum mechanics to predict the chemical bonding in drug molecules, and try to determine all the possible polymorphs that could form. Polymorphism is so important, there’s a competition held every few years to see who can predict molecular crystal structures most reliably. This is not an easy task, and competition is fierce.

The weak intermolecular forces are particularly challenging to predict accurately, and this is an active area of development in physics. Our team at York have developed advanced quantum mechanical software, including modelling these intermolecular forces, which is used by almost all pharmaceutical companies. Every year our understanding deepens, and our methods get better and better. In the case of ritonavir, there’s a happy ending. Because the active form was more soluble, it could be stabilised in a liquid suspension or gel. These were approved in 1999, and have been used ever since, boosting the effectiveness of HIV treatment and saving countless lives.