What Causes Rash-turned-hole In Armpit?

I have had a rash in my armpit for the last 7 months. It has now morphed into a 1 inch hole. I was put on prednisone and cipra which has eroded the inflammation and bacteria, but my armpit hole is not healing. My dermatologist thinks, as he did 5 months ago, that I have pyoderma gangrenosum. First biopsy was negative. Awaiting second biopsy taken 5 days ago. I am a 70 year old senior, in otherwise good health and at a complete loss as to the cause. Do you or your colleagues have any suggestions? Since my problem may be very educational for the medical community, I will not, at this time, make any payments; I will respond to questions that may help your education.