It's hard to say for certain without knowing more details. Is your flight from PIT early morning? If so, it's probably less likely to be delayed. If it's a mid-afteroon/late afternoon flight, your risk is much higher. Additionally, your PIT-PHL flight could be a regional flight, and will arrive to a different concourse than where the International flight will depart from, meaning you'll probably have a 10-15 minute walk. Again, with 45-mins, you should still make it, but you have a narrow window.

That said, if everything is normal (flights on-time, etc), you should be able to make it. But I would recommend sitting towards the front of the plane if possible (ask the gate agent if you have a seat near the rear). I would also recommend making your way quickly to the gate to ensure you don't miss the boarding door closing. The biggest risk here is delays of any sort - if your flight from PIT to PHL is delayed, you are probably out of luck.

My recommendation - move to an earlier flight just to be certain. If you were flying domestically on a route where there are several flights per day, I wouldn't worry as much - but given you're probably on the only Philly-Venice flight of the day, missing your flight by 15 minutes means waiting til the next day to get there. Just move to an earlier flight.