We are constantly reviewing documents targeting the FDA and EMA. On reviewing a recent pre-IND briefing book for the U.S. FDA, we were struck by some translational gaps in the story board. While these were not significant, we see these translational gaps frequently enough that we thought it warranted a comment.
The briefing book was excellently authored. The questions were well-worded and appropriate for the development phase of the program. All primary pharmacology values were presented in molar concentrations and, rightfully, so. Secondary pharmacology and receptor binding assays were also reported in molar concentrations. Calculating a selectivity index was easy. So far so good!! It was unclear, though, whether these molar concentrations were in the presence or absence of protein. Meanwhile, all drug assays were reported, as is customary, in micrograms. Converting these to molar concentrations was not easy. There were two molecular weights reported – one for the free base and the other for the salt. On reading further, it became clear that the assay was measuring the free base. Finally, tucked elsewhere, was the protein binding of the drug. All this was back-extrapolated to express the in vitro primary and secondary pharmacology concentrations of the drug in micrograms for the free base (consistent with the assay), including the adjustment for protein binding in vitro.
Suddenly, things fell into place. In non-clinical toxicology studies, the limiting adverse effect was related to the secondary pharmacology of the drug. The concentrations at which toxicity was noted in vivo was similar to the in vitro derived protein bound concentrations of the base expressed in micrograms for the secondary pharmacology of the drug.
Voila!! In our experience, connecting the dots from in vitro molar concentrations, protein binding, to understanding the assay are common gaps in translation. The first thing we tend to do is, therefore, express all in vitro concentrations in micrograms corrected for protein binding and the right molecular weight. Though, like any real scientist, we would rather our friends who develop the assay, toxicologists, clinical pharmacologists and physician scientists express all values, instead, in molar concentrations. But that is a topic for another discussion.
For more blogs, please visit: https://www.rxmd.com/insights