i have data from a "negative" clinical trial and would like to run an analysis to see if there is a certain group of patients (theoretically) that respond to the drug that was otherwise overlooked. What is the best way to go about doing this? The sample size is not too big (109) so power will likely be an issue.
Your reference to "a certain group of patients" [emphasis mine] suggests to me that maybe you are starting with a definite hypothesis that is informed by pathophysiology or maybe observational research. In this case, your approach should not attempt to "see if there is" evidence for the hypothesized response, but rather to attempt to falsify your hypothesis.
Thus, you ought to consider all the ways this data set (and others!) might falsify, constrain or reshape your hypothesis. If you think of yourself as a blacksmith, you ought to be using the data to hammer on your hypothesis, and not vice versa!