In the first part of this blog series, Dr Johanna Haglund introduced what are the first steps in the Metabolites in Safety Testing evaluation (MIST). The second part advised how to proceed after in vitro cross-species comparison between human and safety species. This third part is about in vivo human MetID.
Human Met ID, why and when?
What is known after in vivo MetID in toxicity species?
The in vivo plasma MetID is one step further towards the truth. Although major metabolic pathways are obtained from in vitro data, the in vivo data in toxicity species can confirm or reject in vitro observed phenomena such as low turnover, less active enzymes, or lack of sequential metabolism. Still, after in vivo MetID in the tox species, the only human data is in vitro based at this stage.
How to proceed?
The ultimate goal of the process is to understand the metabolism in human through metabolite profiling and characterization in plasma and urine. The first human samples for this become available after the SAD (single ascending dose) study. Despite the single dose used in SAD study it is worth to perform metabolite characterisation and establish the human metabolite profile over the dose interval (e.g. 0-24 h), particularly if the MAD (multiple ascending dose) study is not run close to SAD. Another reason for starting MetID in human in SAD samples is the possibility to access higher dose levels than in the MAD samples, which allows better characterisation of metabolites.
How SAD MetID data can be used?
At this stage a rather good picture is available concerning human metabolism based on plasma (circulating metabolites) and urine (excretory) MetID. From a MIST perspective, the metabolite profile in plasma is established, although as a result of a single dose.
There are two major actions that should be done at this stage. First, the relative levels of metabolites as part of total drug related material detected in the 0–24 h dose interval profile should be evaluated. Despite the assumption of equal MS response of each analyte and the single dose, the data will support to understand which metabolites, if any, might be of concern (i.e. ≥10% of total drug related material). Secondly, the metabolite profile in human plasma should be compared to those of toxicity species to confirm similarities or discrepancies between species from a qualitative perspective. This allows to identify where and if there is a potential risk of a disproportionate human metabolite.
To conclude, by taking this approach it is possible to act proactively during the project, although the human metabolite profile in the repeated dose study at steady state of parent (MAD) provides the so-called worst scenario in terms of relative levels and which metabolites that might be of concern. The MAD samples will also be used for the ultimate quantitative comparison of human metabolites between human and toxicity species, to be described in part 4. You could also check the part 2 and part 1.
Written by Johanna Haglund