Off the top of my head, I can think of four outright violations of the Prime Directive in TOS. One may surprise folks:
1. John Gill in "Patterns of Force".
2. Ronald Tracey in "The Omega Glory".
3. R.M. Merik in "Bread and Circuses".
4. Jim Kirk in "Miri".
I think the only time Kirk actually broke the PD was in "Miri", when McCoy's antitidote to the virus was not only given to the Enterprise crewmembers, but to the children. Strictly speaking, the children shouldn't have been treated. It would've made for an interesting debate within the episode, but I don't think any writers had created the PD, yet.
It can be argued April isn't violating the PD in this case if the Klingons were there first. Once the genie is out of the bottle, how can the PD really apply? However, that doesn't mean April can't be committing other criminal acts in carrying out his cause.