What is the most likely reaction between minocycline and tramadol in physiological conditions?

[Minocyline](https://en.wikipedia.org/wiki/Minocycline) (left) and [Tramadol](https://en.wikipedia.org/wiki/Tramadol) (right) are two commonly prescribed medications containing typical organic functional groups.


What would an expert chemist surmise as the first or most-likely reaction involving these drugs in the human body, if any? For example, in the stomach, in a highly acidic environment?


I realize the chemistry here is obviously complex, so an answer excluding biological components is fine. e.g. what would these compounds do in a solution of $\ce{HCl}$ alone?


I'm not an organic chemist, but my guess is that $\ce{HCl}$ can protonate an amine to encourage some kind of reaction.


In any case, some research has shown that the drugs may have a synergistic effect for pain relief: <https://www.ncbi.nlm.nih.gov/pubmed/22964800>.


[![Minocyline](https://i.stack.imgur.com/e9SPB.png)](https://i.stack.imgur.com/e9SPB.png)
[![Tramadol](https://i.stack.imgur.com/vt0Uc.png)](https://i.stack.imgur.com/vt0Uc.png)


