Hi Kathleen,
I have an RX450 (so not an IS300), but I would suggest that 12,000 miles is a fairly low figure for a set of tyres.
Understandably, tyre wear is contingent on many factors, the road surfaces, the volume of city stop/start driving, the tyre itself, maintenance of tyre pressure, check your tracking regularly, (every year)and, not the least - one's driving style.
I've only driven 6,000 miles on my current set of tyres in this car, but my previous motor was a heavy (and fast!) Mercedes S500, and I'd never achieved less than 30,000 miles out of a set on that.
Mind you, I normally accelerate gently, and apply the brakes even more gently, (and being retired means I don't have to join the rush-hour Grand prix every day)