I thought I read in an article a while back that KHMX in Houston was sort of technically the first station with a "Hot AC" format back around 1991. I could be wrong on that.
Now 97.5 in Philly failed because there were too many changes. They started as Mainstream AC with song tags, then quickly went all-Christmas, then morphed into a CHR-ish Hot AC, then into an 80s 90s and Now Hot AC. That's WAY too much change for under a year.
People wonder why Hot AC doesn't work in Philly. There's two factors:
1.) Fringe market signals. WSTW in Wilmington, DE is an Adult Leaning CHR, and WPST in Trenton is a Hot AC. Both stations put decent signals into the Philly market and get some ratings.
2.) No one who does Hot AC STARTS with Hot AC. Look back at all the Philly Hot AC stations. Star 104.5 morphed from AC to CHR to Hot AC (or some variation of it.) Then failed. Alice 104.5 started as a before-the-curve Jack FM, then morphed into Hot AC and flipped. 96.5 The Point started as all 80's, morphed into Hot AC, and flipped. Now 97.5, started as Mainstream AC, morphed, and flipped. The only two stations to START as Hot AC were Max 95.7 and Mix 95.7. Max was killed to beat CC to the Jammin Gold format, and Mix 95.7 went through too many changes to build an audience. If someone started as a Hot AC, and kept with it... it could work.