Possible? Yes, that is roughly 10 pounds a month, 2.5 pounds a week... if you stick to 1200 calories a day and work your behind off it can be done.
The trick to that kind of hardcore sustained weight loss is going to be exercise exercise exercise. If your not willing to hit the gym HARD, choose a less ambitious goal.
You won't make that through diet alone. Your looking at TV show competitive weight loss type numbers.
Ladada, she says how big she is. She is trying to go from overweight to low normal weight so she won't lose 4 pounds a week by diet alone... she will need to exercise too.
Your caloric need is currently about 2200 calories a day with light exercise (a normal daily routine basically, with walking etc.). You can diet to eat 1200 calories, running a 1000 calorie deficit per day. A pound is 3600 calories. You need to lose 2.5 pounds a week. So without additional exercise you can lose 7000 calories (1000 every day) a week. This is just under 2 pounds. To lose that additional 0.5 pounds per week you will need to add in exercise... a lot of it. Also, as you lose your daily caloric need will decrease (at 180 pounds it will only be about 1800 calories), so as you lose you will need to seriously step up the exercise to maintain that rate of weight loss.
You could cut calories as low as 800, but that will make it hard to stick to, not allow you to get enough nutrients in, not teach you the proper way to eat etc.