Hi there

It is possible to turn rats around i have hand tamed a few in the 16 years i've been keeping them.

If they get a reaction to them biting you that benefits them eg your hand comes out of the cage they will continue what works.
What you need to do is be able to get them used to being appraoched and touched without you feeling the pain.

I used to use a thick leather or gardening glove once i'd learnt to prevent myself reacting to them attempting to bite it worked with constant and regular attempts i bought them round.
the other thing i use is a fake hand (one of those from a joke shop) rub your scent on every time you use it that way it will help with the transition to your hand. Once comfortable with this then try your own hand.

Just remember to take it slow. daily handling is the key and not giving them the reaction that rewards them for the effort.

The longest its taken me to bring one round was a month but it was worth the time and effort
Let us now how you get on