It has several limitations, however, which limit its applicability to the real world. These limitations, discussed below, suggest that these results imply that interactions of these four compounds in live animals at a minimum would be additive. They might be much more. First, this pioneering work tells us that some compounds interact additively, but work of this nature is too rare, as yet, to know whether this is a general pattern to expect with other compounds. They designed the experiment that way to limit the mechnisms of action to the one they could measure directly.
Many compounds have more than one mechanism of action.
Other mechanisms of action by the same compound might provoke other responses simultaneously that could alter the pattern of interaction. Mixtures in real animals interact with many different cells, organs and tissues simultaneously, with the potential for myriad positive and negative feedback loops.