At the Society for Biomolecular Sciences meeting in Lille in April 2009, time was set aside for a symposium on drug repurposing. In addition to Numedicus, there were talks from Didier Leroy for the Medicines for Malaria Venture, and from Chris Lipinski, of Melior Discovery.
Melior operate a two-part strategy, to provide screening services for new indication discovery for clients, and to identify their own therapeutic switch projects. Their model for indication switch discovery revolves around phenotypic screening, which brings to mind the old Irwin test.
This test, first described by Irwin in 1968, evaluated the effect of a test substance on the behaviour and physiological fuction of a rodent, at a range of doses. Of course, it was also able to give some guide to the pharmacodynamics of the test drug, or time course of these changes. The key thing though is the characteristics that are measured.
Irwin had a standard observational grid, including (among others) convulsions, fore-paw treading, exophthalmia, rectal temperature and pupil diameter. Many of these result from CNS effects.
It is somewhat odd to think that in the new millennium, this century of biology, we are reverting to methods which were put in place over 40 years ago, and have almost died out. Actually, I am sure Melior’s methods are more sophisticated than just a replica of Irwin’s screening battery. But many of the points about phenotypic screening are perfectly valid: despite being an old method, it is tried and tested, and takes into account a range of whole animal parameters that in vitro testing ignores.
There is evidential support for the this approach in the success of Paul Janssen’s work. Janssen’s company (which became part of Johnson and Johnson) relied heavily on behavioural screening to identify active compounds, and was phenomenally successful in getting original molecules to market. Out of a total of 70,000 synthesised compounds, over 70 have been marketed, a success rate far in excess of the industry average. The most important aspect of phenotypic screeening is that it allows multiple outcomes to be tested in one experiment. It is the in vivo equivalent of the in vitro high content screen, with some ADME addded in.
To take it further, the phenotypic screen is rather like the Fourier Transform of biological testing. In case a reminder is needed, FT-NMR revolutionised the speed and sensitivity of the previous method of taking a NMR spectrum. The earliest NMR spectra were obtained by varying the radiofrequency in a constant magnetic field or by varying the field with a fixed frequency and recording the individual resonances. Continuous wave spectrometry was slow and laborious, and has now been entirely replaced by Fourier Transform methods, in which all the nuclei are excited a the same time by a single radiofrequency pulse, and the resulting spectrum taken in a single step, often less than a second long.
The physics behind FT-NMR is not straightforward, and it is best explained by the analogy (Derome, 1987) of the process of tuning a church bell. In principle you could measure the resonance frequency of the bell with a series of tuning forks, although it would be a long process. Far better is to hit the bell with an enormous hammer, exciting all the resonances at once, and analyse the resulting complex wave pattern (BOIIING!!) to identify the spectrum of frequencies.
Retrograde it may seem at first sight, phenotypic testing is actually a modern method of quickly assessing a compound’s ability to act in an number of possible ways. It has been claimed that up to 30% of the compounds subjected to phenotypic screening reveal unexpected activities, despite them having previously been through a vast range of biochemical screens.
The key point remains: what kind of bell hammer are you going to use, how hard will you whack, and to what will you listen? Then, beyond the identification of a new indication, how could it become a new product?