Medicines and medical procedures have to be tested on living creatures in order to determine whether or not they are safe. If they are not tested on animals, then they must be tested on humans. I think a human being's life is far more important than any animal's life, so it's better to test on animals than on humans.
Yes, it is justifiable to perform medical experiments on animals in order to provide medical benefits to humanity. Our long-term survival has more of a moral imperative than that of animals. With that said, it is important to follow best practices of humane handling when running the experimental processes on the animals.
Yes, humans should respect animals and animal life. However, even the Bible says that humans should have dominion over the animals of the Earth. The life of an animal isn't worth that of a human. If there's a rabbit in the middle of the highway and I'm traveling at 70 MPH, I'm not swerving my car to avoid it because there is a risk I could hurt human beings while doing so. It's that kind of concept that makes this a very simple question to answer.
Yes, experimentation on animals due to medical incentive is morally justifiable, because it is better for a human to find relief, even if an animal has to suffer. Experimentation on animals is a good way to filter out problems before the medicine or the remedy is tried on a human. Problems for humans can be avoided.
Given that the alternative is to test directly on humans, I believe it is justifiable to experiment on animals for medical incentive, especially when considering pharmaceuticals. I understand people don't like this because they don't want the animals to be harmed, but the alternative is far more drastic and unreasonable given this option.